infrared and raman spectroscopy of polymers

Infrared and Raman Spectroscopy of Polymers

Report 134

Volume 12, Number 2, 2001

J.L. Koenig

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Item 1Macromolecules33, No.6, 21st March 2000, p.2171-83EFFECT OF THERMAL HISTORY ON THE RHEOLOGICALBEHAVIOR OF THERMOPLASTIC POLYURETHANESPil Joong Yoon; Chang Dae HanAkron,University

The effect of thermal history on the rheological behaviour of ester- andether-based commercial thermoplastic PUs (Estane 5701, 5707 and 5714from B.F.Goodrich) was investigated. It was found that the injectionmoulding temp. used for specimen preparation had a marked effect on thevariations of dynamic storage and loss moduli of specimens with timeobserved during isothermal annealing. Analysis of FTIR spectra indicatedthat variations in hydrogen bonding with time during isothermal annealingvery much resembled variations of dynamic storage modulus with timeduring isothermal annealing. Isochronal dynamic temp. sweep experimentsindicated that the thermoplastic PUs exhibited a hysteresis effect in theheating and cooling processes. It was concluded that the microphaseseparation transition or order-disorder transition in thermoplastic PUs couldnot be determined from the isochronal dynamic temp. sweep experiment.The plots of log dynamic storage modulus versus log loss modulus variedwith temp. over the entire range of temps. (110-190C) investigated. 57 refs.GOODRICH B.F.USA

Accession no.771897

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Previous Titles Still AvailableVolume 1Report 3 Advanced Composites, D.K. Thomas, RAE, Farnborough.Report 4 Liquid Crystal Polymers, M.K. Cox, ICI, Wilton.Report 5 CAD/CAM in the Polymer Industry, N.W. Sandland

and M.J. Sebborn, Cambridge Applied Technology.Report 8 Engineering Thermoplastics, I.T. Barrie, Consultant.Report 11 Communications Applications of Polymers,

R. Spratling, British Telecom.Report 12 Process Control in the Plastics Industry,

R.F. Evans, Engelmann & Buckham Ancillaries.

Volume 2Report 13 Injection Moulding of Engineering Thermoplastics,

A.F. Whelan, London School of Polymer Technology.Report 14 Polymers and Their Uses in the Sports and Leisure

Industries, A.L. Cox and R.P. Brown, RapraTechnology Ltd.

Report 15 Polyurethane, Materials, Processing and Applications,G. Woods, Consultant.

Report 16 Polyetheretherketone, D.J. Kemmish, ICI, Wilton.Report 17 Extrusion, G.M. Gale, Rapra Technology Ltd.Report 18 Agricultural and Horticultural Applications of

Polymers, J.C. Garnaud, International Committee forPlastics in Agriculture.

Report 19 Recycling and Disposal of Plastics Packaging,R.C. Fox, Plas/Tech Ltd.

Report 20 Pultrusion, L. Hollaway, University of Surrey.Report 21 Materials Handling in the Polymer Industry,

H. Hardy, Chronos Richardson Ltd.Report 22 Electronics Applications of Polymers, M.T.Goosey,

Plessey Research (Caswell) Ltd.Report 23 Offshore Applications of Polymers, J.W.Brockbank,

Avon Industrial Polymers Ltd.Report 24 Recent Developments in Materials for Food

Packaging, R.A. Roberts, Pira Packaging Division.

Volume 3Report 25 Foams and Blowing Agents, J.M. Methven, Cellcom

Technology Associates.Report 26 Polymers and Structural Composites in Civil

Engineering, L. Hollaway, University of Surrey.Report 27 Injection Moulding of Rubber, M.A. Wheelans,

Consultant.Report 28 Adhesives for Structural and Engineering

Applications, C. OReilly, Loctite (Ireland) Ltd.Report 29 Polymers in Marine Applications, C.F.Britton,

Corrosion Monitoring Consultancy.Report 30 Non-destructive Testing of Polymers, W.N. Reynolds,

National NDT Centre, Harwell.Report 31 Silicone Rubbers, B.R. Trego and H.W.Winnan,

Dow Corning Ltd.Report 32 Fluoroelastomers - Properties and Applications,

D. Cook and M. Lynn, 3M United Kingdom Plc and3M Belgium SA.

Report 33 Polyamides, R.S. Williams and T. Daniels,T & N Technology Ltd. and BIP Chemicals Ltd.

Report 34 Extrusion of Rubber, J.G.A. Lovegrove, NovaPetrochemicals Inc.

Report 35 Polymers in Household Electrical Goods, D.Alvey,Hotpoint Ltd.

Report 36 Developments in Additives to Meet Health andEnvironmental Concerns, M.J. Forrest, RapraTechnology Ltd.

Volume 4Report 37 Polymers in Aerospace Applications, W.W. Wright,

University of Surrey.Report 39 Polymers in Chemically Resistant Applications,

D. Cattell, Cattell Consultancy Services.Report 41 Failure of Plastics, S. Turner, Queen Mary College.Report 42 Polycarbonates, R. Pakull, U. Grigo, D. Freitag, Bayer

AG.Report 43 Polymeric Materials from Renewable Resources,

J.M. Methven, UMIST.Report 44 Flammability and Flame Retardants in Plastics,

J. Green, FMC Corp.Report 45 Composites - Tooling and Component Processing,

N.G. Brain, Tooltex.Report 46 Quality Today in Polymer Processing, S.H. Coulson,

J.A. Cousans, Exxon Chemical International Marketing.Report 47 Chemical Analysis of Polymers, G. Lawson, Leicester

Polytechnic.

Volume 5Report 49 Blends and Alloys of Engineering Thermoplastics,

H.T. van de Grampel, General Electric Plastics BV.Report 50 Automotive Applications of Polymers II,

A.N.A. Elliott, Consultant.Report 51 Biomedical Applications of Polymers, C.G. Gebelein,

Youngstown State University / Florida Atlantic University.Report 52 Polymer Supported Chemical Reactions, P. Hodge,

University of Manchester.Report 53 Weathering of Polymers, S.M. Halliwell, Building

Research Establishment.Report 54 Health and Safety in the Rubber Industry, A.R. Nutt,

Arnold Nutt & Co. and J. Wade.Report 55 Computer Modelling of Polymer Processing,

E. Andreassen, . Larsen and E.L. Hinrichsen, Senter forIndustriforskning, Norway.

Report 56 Plastics in High Temperature Applications,J. Maxwell, Consultant.

Report 57 Joining of Plastics, K.W. Allen, City University.Report 58 Physical Testing of Rubber, R.P. Brown, Rapra

Technology Ltd.Report 59 Polyimides - Materials, Processing and Applications,

A.J. Kirby, Du Pont (U.K.) Ltd.Report 60 Physical Testing of Thermoplastics, S.W. Hawley,

Rapra Technology Ltd.

Volume 6Report 61 Food Contact Polymeric Materials, J.A. Sidwell,

Rapra Technology Ltd.Report 62 Coextrusion, D. Djordjevic, Klckner ER-WE-PA GmbH.Report 63 Conductive Polymers II, R.H. Friend, University of

Cambridge, Cavendish Laboratory.Report 64 Designing with Plastics, P.R. Lewis, The Open University.Report 65 Decorating and Coating of Plastics, P.J. Robinson,

International Automotive Design.Report 66 Reinforced Thermoplastics - Composition, Processing

and Applications, P.G. Kelleher, New Jersey PolymerExtension Center at Stevens Institute of Technology.

Report 67 Plastics in Thermal and Acoustic Building Insulation,V.L. Kefford, MRM Engineering Consultancy.

Report 68 Cure Assessment by Physical and ChemicalTechniques, B.G. Willoughby, Rapra Technology Ltd.

Report 69 Toxicity of Plastics and Rubber in Fire, P.J. Fardell,Building Research Establishment, Fire Research Station.

Report 70 Acrylonitrile-Butadiene-Styrene Polymers,M.E. Adams, D.J. Buckley, R.E. Colborn, W.P. Englandand D.N. Schissel, General Electric Corporate Researchand Development Center.

Report 71 Rotational Moulding, R.J. Crawford, The QueensUniversity of Belfast.

Report 72 Advances in Injection Moulding, C.A. Maier,Econology Ltd.

Volume 7Report 73 Reactive Processing of Polymers, M.W.R. Brown,

P.D. Coates and A.F. Johnson, IRC in Polymer Scienceand Technology, University of Bradford.

Report 74 Speciality Rubbers, J.A. Brydson.

Report 75 Plastics and the Environment, I. Boustead, BousteadConsulting Ltd.

Report 76 Polymeric Precursors for Ceramic Materials,R.C.P. Cubbon.

Report 77 Advances in Tyre Mechanics, R.A. Ridha, M. Theves,Goodyear Technical Center.

Report 78 PVC - Compounds, Processing and Applications,J.Leadbitter, J.A. Day, J.L. Ryan, Hydro Polymers Ltd.

Report 79 Rubber Compounding Ingredients - Need, Theoryand Innovation, Part I: Vulcanising Systems,Antidegradants and Particulate Fillers for GeneralPurpose Rubbers, C. Hepburn, University of Ulster.

Report 80 Anti-Corrosion Polymers: PEEK, PEKK and OtherPolyaryls, G. Pritchard, Kingston University.

Report 81 Thermoplastic Elastomers - Properties and Applications,J.A. Brydson.

Report 82 Advances in Blow Moulding Process Optimization,Andres Garcia-Rejon,Industrial Materials Institute,National Research Council Canada.

Report 83 Molecular Weight Characterisation of SyntheticPolymers, S.R. Holding and E. Meehan, RapraTechnology Ltd. and Polymer Laboratories Ltd.

Report 84 Rheology and its Role in Plastics Processing,P. Prentice, The Nottingham Trent University.

Volume 8Report 85 Ring Opening Polymerisation, N. Spassky, Universit

Pierre et Marie Curie.Report 86 High Performance Engineering Plastics,

D.J. Kemmish, Victrex Ltd.

Report 87 Rubber to Metal Bonding, B.G. Crowther, RapraTechnology Ltd.

Report 88 Plasticisers - Selection, Applications and Implications,A.S. Wilson.

Report 89 Polymer Membranes - Materials, Structures andSeparation Performance, T. deV. Naylor, The SmartChemical Company.

Report 90 Rubber Mixing, P.R. Wood.

Report 91 Recent Developments in Epoxy Resins, I. Hamerton,University of Surrey.

Report 92 Continuous Vulcanisation of Elastomer Profiles,A. Hill, Meteor Gummiwerke.

Report 93 Advances in Thermoforming, J.L. Throne, SherwoodTechnologies Inc.

Report 94 Compressive Behaviour of Composites,C. Soutis, Imperial College of Science, Technologyand Medicine.

Report 95 Thermal Analysis of Polymers, M. P. Sepe, Dickten &Masch Manufacturing Co.

Report 96 Polymeric Seals and Sealing Technology, J.A. Hickman,St Clair (Polymers) Ltd.

Volume 9Report 97 Rubber Compounding Ingredients - Need, Theory

and Innovation, Part II: Processing, Bonding, FireRetardants, C. Hepburn, University of Ulster.

Report 98 Advances in Biodegradable Polymers, G.F. Moore &S.M. Saunders, Rapra Technology Ltd.

Report 99 Recycling of Rubber, H.J. Manuel and W. Dierkes,Vredestein Rubber Recycling B.V.

Report 100 Photoinitiated Polymerisation - Theory andApplications, J.P. Fouassier, Ecole Nationale Suprieurede Chimie, Mulhouse.

Report 101 Solvent-Free Adhesives, T.E. Rolando, H.B. FullerCompany.

Report 102 Plastics in Pressure Pipes, T. Stafford, RapraTechnology Ltd.

Report 103 Gas Assisted Moulding, T.C. Pearson, Gas Injection Ltd.Report 104 Plastics Profile Extrusion, R.J. Kent, Tangram

Technology Ltd.

Report 105 Rubber Extrusion Theory and Development,B.G. Crowther.

Report 106 Properties and Applications of ElastomericPolysulfides, T.C.P. Lee, Oxford Brookes University.

Report 107 High Performance Polymer Fibres, P.R. Lewis,The Open University.

Report 108 Chemical Characterisation of Polyurethanes,M.J. Forrest, Rapra Technology Ltd.

Volume 10Report 109 Rubber Injection Moulding - A Practical Guide,

J.A. Lindsay.

Report 110 Long-Term and Accelerated Ageing Tests on Rubbers,R.P. Brown, M.J. Forrest and G. Soulagnet,Rapra Technology Ltd.

Report 111 Polymer Product Failure, P.R. Lewis,The Open University.

Report 112 Polystyrene - Synthesis, Production and Applications,J.R. Wnsch, BASF AG.

Report 113 Rubber-Modified Thermoplastics, H. Keskkula,University of Texas at Austin.

Report 114 Developments in Polyacetylene - Nanopolyacetylene,V.M. Kobryanskii, Russian Academy of Sciences.

Report 115 Metallocene-Catalysed Polymerisation, W. Kaminsky,University of Hamburg.

Report 116 Compounding in Co-rotating Twin-Screw Extruders,Y. Wang, Tunghai University.

Report 117 Rapid Prototyping, Tooling and Manufacturing,R.J.M. Hague and P.E. Reeves, Edward MackenzieConsulting.

Report 118 Liquid Crystal Polymers - Synthesis, Properties andApplications, D. Coates, CRL Ltd.

Report 119 Rubbers in Contact with Food, M.J. Forrest andJ.A. Sidwell, Rapra Technology Ltd.

Report 120 Electronics Applications of Polymers II, M.T. Goosey,Shipley Ronal.

Volume 11Report 121 Polyamides as Engineering Thermoplastic Materials,

I.B. Page, BIP Ltd.

Report 122 Flexible Packaging - Adhesives, Coatings andProcesses, T.E. Rolando, H.B. Fuller Company.

Report 123 Polymer Blends, L.A. Utracki, National ResearchCouncil Canada.

Report 124 Sorting of Waste Plastics for Recycling, R.D. Pascoe,University of Exeter.

Report 125 Structural Studies of Polymers by Solution NMR,H.N. Cheng, Hercules Incorporated.

Report 126 Composites for Automotive Applications, C.D. Rudd,University of Nottingham.

Report 127 Polymers in Medical Applications, B.J. Lambert andF.-W. Tang, Guidant Corp., and W.J. Rogers, Consultant.

Report 128 Solid State NMR of Polymers, P.A. Mirau,Lucent Technologies.

Report 129 Failure of Polymer Products Due to Photo-oxidation,D.C. Wright.

Report 130 Failure of Polymer Products Due to Chemical Attack,D.C. Wright.

Report 131 Failure of Polymer Products Due to Thermo-oxidation,D.C. Wright.

Report 132 Stabilisers for Polyolefins, C. Krhnke and F. Werner,Clariant Huningue SA.

Volume 12Report 133 Advances in Automation for Plastics Injection

Moulding, J. Mallon, Yushin Inc.

Titles Available in the Current Volume

Infrared and RamanSpectroscopy of Polymers

ISBN: 1-85957-284-7

J. L. Koenig(Case Western Reserve University)


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Contents

1. Introduction .............................................................................................................................................. 5

2. Elementary Theory of Vibrational Spectroscopy ................................................................................. 5

2.1 Selection Rules for IR Spectroscopy .............................................................................................. 6

2.2 Selection Rules for Raman Spectroscopy ....................................................................................... 6

3. Basis of Vibrational Spectroscopy as a Structural Tool ....................................................................... 6

3.1 Structural Dependence of Vibrational Frequencies ........................................................................ 6

4. Vibrational Spectroscopic Instrumentation .......................................................................................... 7

4.1 Types of IR Instrumentation ........................................................................................................... 74.1.1 Dispersive IR Instrumentation ............................................................................................ 74.1.2 FTIR Instrumentation ......................................................................................................... 74.1.3 Microscopic FTIR Instrumentation .................................................................................... 74.1.4 FTIR Imaging Instrumentation ........................................................................................... 84.1.5 Filter IR Instrumentation .................................................................................................... 84.1.6 Fibre-Optic IR Instrumentation .......................................................................................... 9

4.2 Types of Raman Instrumentation .................................................................................................... 94.2.1 Dispersive Raman Instrumentation .................................................................................... 94.2.2 Fourier Transform Raman Instrumentation ........................................................................ 94.2.3 Resonance Raman Instrumentation .................................................................................. 104.2.4 Surface-Enhanced Raman Instrumentation ...................................................................... 104.2.5 Raman Mapping and Imaging Instrumentation................................................................ 114.2.6 Fibre-Optic Raman Instrumentation................................................................................. 11

5. Sampling for Vibrational Spectroscopy ............................................................................................... 11

5.1 IR Sampling .................................................................................................................................. 115.1.1 Transmission Spectroscopy .............................................................................................. 125.1.2 Internal Reflection (or Attenuated Total Reflection (ATR)) (a.20) .................................. 125.1.3 External Reflection Spectroscopy .................................................................................... 135.1.4 Diffuse Reflectance FTIR (DRIFT) Spectroscopy........................................................... 135.1.5 Emission Spectroscopy ..................................................................................................... 135.1.6 Microsampling .................................................................................................................. 13

5.2 Sampling for Raman Spectroscopy ................................................................................................... 135.2.1 Microscopic Sampling in Raman Spectroscopy and Imaging ......................................... 14

6. Measuring Polymer Orientation with IR and Raman Spectroscopy ............................................... 14

6.1 IR Dichroism................................................................................................................................. 14

6.2 Raman Characterisation of Polymer Orientation ......................................................................... 15

7. IR and Raman Applications to Polymer Characterisation ................................................................ 16

7.1 Material Identification .................................................................................................................. 16


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7.1.1 Additive Analysis ............................................................................................................. 167.1.2 Chromatographic Separation and Identification .............................................................. 17

7.2 Determination of the Chemical Structure of a Repeating Unit .................................................... 17

7.3 Crystalline and Conformational Structural Order in Polymers .................................................... 187.3.1 Introduction ...................................................................................................................... 187.3.2 Crystal Phases of Polymers .............................................................................................. 187.3.3 Complex Formation between Polymers and Complexing Agents ................................... 197.3.4 Conformational Analysis .................................................................................................. 197.3.5 Nematic Ordering in Polymer Dispersed Liquid Crystals ............................................... 19

7.4 Quantitative Analysis .................................................................................................................... 207.5 Copolymer Composition and Structure ........................................................................................ 21

7.6 Polymerisation Kinetics and Mechanism in Multicomponent Systems ....................................... 217.6.1 Approach .......................................................................................................................... 217.6.2 Examples Using IR Fibre Optics ...................................................................................... 217.6.3 Examples Using Raman Fibre Optics .............................................................................. 227.6.4 Examples Using Rapid Scan FTIR Spectroscopy ............................................................ 22

7.7 Polymer Blends.................................................................................................................................. 237.7.1 Utility of Polymer Blends ................................................................................................ 237.7.2 Miscible Blends ................................................................................................................ 23

7.7.2.1 Compatibilised Blends ........................................................................................ 237.7.2.2 Reactive Blends ................................................................................................... 23

7.8 Conducting Polymers .................................................................................................................... 24

7.9 Emulsion Polymers ....................................................................................................................... 24

7.10 Graft Polymers .............................................................................................................................. 25

7.11 Degradation of Polymers .............................................................................................................. 267.11.1 Degradation with Ionising Radiation ............................................................................... 267.11.2 Degradation by Photooxidation ........................................................................................ 267.11.3 Degradation by Thermal Oxidation Processes ................................................................. 277.11.4 Degradation by Chemical Exposure ................................................................................. 277.11.5 Analysis of Evolving Gases from Polymers .................................................................... 28

7.12 Orientation of Polymer Chains Due to External Perturbations .................................................... 287.12.1 Approach using Vibrational Spectroscopy ....................................................................... 287.12.2 Orientation Functions for Mechanical Deformation ........................................................ 297.12.3 Strain-Induced Crystallisation .......................................................................................... 297.12.4 Deformation-Induced Conformational Changes .............................................................. 307.12.5 Relaxation Processes after Drawing ................................................................................. 307.12.6 Electric Field Induced Reorientation of Polymers ........................................................... 307.12.7 Laser-Induced Orientation ................................................................................................ 30

7.13 Time-Resolved Spectroscopy ....................................................................................................... 30

7.14 Rheooptical FTIR Spectroscopy ................................................................................................... 31

7.15 Two-Dimensional IR (2D-IR) Spectroscopy ................................................................................ 317.16 Recycling of Polymers .................................................................................................................. 33


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The views and opinions expressed by authors in Rapra Review Reports do not necessarily reflect those ofRapra Technology Limited or the editor. The series is published on the basis that no responsibility orliability of any nature shall attach to Rapra Technology Limited arising out of or in connection with anyutilisation in any form of any material contained therein.

7.17 Depth Profiling from Surfaces and Interfaces .............................................................................. 347.17.1 Optical Depth Profiling Using ATR-FTIR Spectroscopy................................................. 347.17.2 Depth Profiling Using Photoacoustic IR Spectroscopy ................................................... 347.17.3 Depth Profiling Using Confocal Raman Microspectroscopy .......................................... 35

7.18 FTIR Microspectroscopy .............................................................................................................. 36

7.19 FTIR Imaging ............................................................................................................................... 37

7.20 Raman Microimaging ................................................................................................................... 38

Additional References ................................................................................................................................... 39

Abbreviations ................................................................................................................................................. 41

References from the Rapra Abstracts Database .............................................................................................. 43

Subject Index ...................................................................................................................................................... 129


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1 Introduction

Vibrational spectroscopy represents two physicallydifferent, yet complementary spectroscopictechniques: infrared (IR) and Raman spectroscopy.Vibrational spectroscopy is advantageous as ananalytical tool for polymers, because it provides awealth of information about complex macromoleculeswith respect to composition, structure, conformation,and intermolecular interactions utilising simplesampling techniques. Although both spectroscopicmethods have been successfully utilised for manyyears, recent advances in electronics, computertechnologies and sampling techniques make Fouriertransform IR (FTIR) and laser-excited Ramanspectroscopy powerful and versatile analytical tools.These spectroscopic methods utilise a broad range ofsampling techniques and do not require priorseparation techniques such as solvent extraction andthey do not require high vacuum. Low instrument cost,speed and simplicity make them cost effective forpolymer analysis. Additionally, the techniques aresuitable for quality and process control applications.

IR spectroscopy is one of the most powerfulspectroscopic tools available for the analysis of polymersystems (a.1). IR spectroscopy is molecularly specificwith high sensitivity. It is based on the absorption orattenuation by matter of electromagnetic radiation of aspecified motion of chemical bonds. Through quantumphysics, nature defines the absorption modes, theirlocations in the frequency spectrum and the amount ofenergy absorbed by each molecule. The absorbance at acharacteristic frequency is a measure of the concentrationof the chemical species being probed in the sample.

The Raman effect occurs when a sample is irradiatedby monochromatic light, causing a small fraction ofthe scattered radiation to exhibit shifted frequenciesthat correspond to the samples vibrational transitions.Ground-state molecules produce lines shifted toenergies lower than the source, while the slightlyweaker lines at higher frequency are due to moleculesin excited vibrational states. These lines, the result ofthe inelastic scattering of light by the sample, are calledStokes and anti-Stokes lines, respectively. Elasticcollisions result in Rayleigh scattering and appear asthe much more intense, unshifted component of thescattered light. The ratio of the intensities of the Stokesand anti-Stokes lines can be used to determine thetemperature of the sample.

In normal Raman scattering, a molecule is excited to avirtual state, which corresponds to a quantum levelrelating to the electron cloud distortion created by the

electric field of the incident light. A virtual state doesnot correspond to a real eigenstate (vibrational orelectronic energy level) of the molecule, but rather is asum over all eigenstates of the molecule.

Raman spectroscopy is now coming of age as a routineanalytical method. Advances in Raman technology havemeant that robust, user-friendly equipment can bemanufactured at a reasonable cost. Key features of theinstruments include high stability and optical efficiency,large frequency scan ranges, confocal and spatialresolution, mapping capability, rapid imaging, highresolution, and near IR (NIR) excitation to reduceinterfering fluorescence. These advances allow manyapplications of Raman spectroscopy and Ramanmicroscopic systems to polymer analysis.

2 Elementary Theory of VibrationalSpectroscopy

Vibrational spectra including IR and Raman result fromthe interactions of the vibrational motions of a moleculewith electromagnetic radiation (a.2). A simple harmonicoscillator model can describe these vibrationalinteractions. After separating the electroniccontributions, each molecule has an internal vibrationalenergy, U, which can be expressed in terms of thecoordinates and interbond forces between the atomsconstituting the molecule. A nonlinear moleculeconsisting of N atoms has 3N6 degrees of freedom.Thus, a set of 3N6 generalised coordinates, Gi, canbe found that completely describes the internal motionsof this nonlinear molecule. The internal energy of themolecule can be written as:

U = U(G1Gi)

where i = 3N6.

It can be shown that the internal energy can be written:

U U Qo ii

ii= + 1 2 2where Qi are a set of ith normal coordinates and ii isa diagonal matrix. Thus, we have a system of weaklycoupled harmonic oscillators. The first term, Uo, is theinternal potential energy of the molecule in theequilibrium state. The second term represents thecontribution to the potential energy from thefundamental collective vibrational bands, while thecubic and higher terms are responsible for combination,difference, and overtone bands.


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The decomposition of coupled harmonic oscillators intoa collection of independent oscillators is known as anormal mode expansion and the independent oscillatorsare called normal modes. Normal modes are definedas modes of vibration where the respective atomicmotions of the atoms are in harmony, i.e., they allreach their maximum and minimum displacements atthe same time. These normal modes can be expressedin terms of bond stretches and angle deformation(termed internal coordinates) and can be calculated byusing a procedure called normal coordinate analysis.

2.1 Selection Rules for IR Spectroscopy

A normal vibrational mode in a molecule may give riseto resonant IR absorption (or emission) ofelectromagnetic radiation only when the transition isinduced by the interaction of the electric vector, E, ofthe incident beam with the electric dipole moment, i,of the molecule. That is, the dynamic dipole momentof the ith normal mode, i/qi or i, is nonzero. Theintensity of the transition is proportional to the squareof the transition dipole moment, i.e., the matrix elementof the electric dipole moment operator between the twoquantised vibrational levels involved.

2.2 Selection Rules for Raman Spectroscopy

Raman scattering is envisaged as the process ofreradiation of scattered light by dipoles induced (P)in the molecules by the incident light and modulatedby the vibrations of the molecules (a.3). In normalRaman scattering by molecules in isotropic media,the dipoles are simply those that result from the actionof the electric field component, E, of the incident lighton the molecules,

P = E

where is the molecular (dipole) polarisability.

The molecule will scatter light at the incident frequency.However, the molecule vibrates with its own uniquefrequencies. If these molecular motions produce changesin the polarisability, , the molecule will further interactwith the light by superimposing its vibrationalfrequencies on the scattered light at either higher or lowerfrequencies. Raman scattering stems from the oscillatingpolarisability within a molecule as a function of vibrationand not from the permanent dipole of the molecule. Asa result, Raman spectra are less affected by dipoleinteractions and as a result show sharp, stable lines.

3 Basis of Vibrational Spectroscopyas a Structural Tool

Our primary interest is in determining the structureof polymers so we need to understand the molecularbasis of vibrational spectroscopy as a structural tool(a.4). The vibrational energy levels can be calculatedfrom first principles by using a technique callednormal coordinate analysis (a.5), and as a result someof the factors influencing spectra have beendiscovered.

The vibrational frequencies of a molecule depend on:

Nature of the motion, Mass of the atoms, Detailed geometric arrangements, Nature of the chemical bonding, and Chemical and physical environment.

Vibrational techniques are particularly sensitive to thestructure of the macromolecules. A vast literatureexists from which group frequencies have beencorrelated with structural components of themolecules (a.6).

3.1 Structural Dependence ofVibrational Frequencies

A vibrational spectrum either is ordinarily recorded inwavenumbers (cm-1), the number of waves percentimetre. The relationship between and thewavelength, (m), is

(cm-1) = (104) / (m)

which can also be written

(cm-1) = 3 x 1010 Hz

The wavenumber scale is directly proportional to theenergy and the vibrational frequency of the molecule.

In wavenumbers

Evib = h cs (cm-1)

where Evib is the vibrational energy level separation,h is Plancks constant (6.62 x 10-34 J.S), and cs is thespeed of light (3 x 1010 cm/s).


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The fundamental IR region arbitrarily extends from4,000 cm-1 to approximately 300 cm-1. The far-IRregion extends from 300 to 10 cm-1, and the observedbands are due to molecular torsional motions as wellas lattice and intermolecular modes. The low IR sourceenergy makes this region generally inaccessible exceptwith special instrumentation. The NIR region extendsfrom 14,000 to 4,000 cm-1 (0.7-2.5 m) and theobserved bands consist of overtones and combinationsof fundamental mid-IR bands (a.7). The NIR isbecoming an important IR method, particularly inquality control.

4 Vibrational SpectroscopicInstrumentation

4.1 Types of IR Instrumentation

IR spectroscopic instrumentation is grouped intomultiplex (Fourier transform) and nonmultiplex(dispersive) methods. These two categories reflect themeans by which the spectra are acquired.

4.1.1 Dispersive IR Instrumentation

A dispersive IR instrument (such as a scanningspectrometer) utilises a grating or prism as awavelength separation device to resolve the IRradiation into individual wavelength components(referred to as spectral resolution elements). A meansis provided in the instrument, such as an exit slit, toisolate specific spectral resolution elements forpassage to the detector. The IR spectrum is obtainedby moving (scanning) the grating over a givenwavenumber region after passing through the sample.The intensity of the spectral resolution element isdetermined by the intensity of the source and thesensitivity of the detector.

Scanning IR spectrometers have a rich history of usebut they have a number of disadvantages arisingprimarily from the step-wise nature in which thespectra are acquired. The optical dispersion process andthe entrance and exit slits limit the amount of energyfalling on the detector to a small fraction of the totalIR energy. Additionally, moving the grating or prismimposes strict mechanical tolerances on the opticalcomponents. With scanning spectrometers it is not easyto increase the signal-to-noise ratio (SNR) by multiplescanning, as it is difficult to reproduce precisely theposition of the grating.

4.1.2 FTIR Instrumentation

IR absorbance spectra have traditionally been recordedusing dispersive instruments but, since 1970, FTIRinstruments have been available that are capable ofcollecting high-quality spectra in a fraction of the timepreviously required with enhanced SNR andwavenumber accuracy. The key component of thesespectrometers is a Michelson interferometer, whichoperates on the principle of amplitude division of theincoming light. The mechanism is simple. Theincoming light is split inside an interferometer, onebeam going to an internal fixed mirror and the other tothe moving mirror. After reflection, the beamsrecombine inside the interferometer, undergoingconstructive and destructive interference, producing theinterferogram. The spectral information is containedin the interferogram. After the data are detected andstored, it is necessary to perform a mathematicaltransform operation (a Fourier transform) on the dataset to convert it into a conventional spectrum.

The primary advantage of multiplexing the spectralresolution elements in the IR is the improvement inthe SNR. The improvement arises because the detectorirradiance for each measurement is increased. Becausethe detector noise is constant and independent of thesource, the SNR is improved because the multiplexingdistributes the detector noise over the intensities ofmany spectral resolution elements. This distributionprocess lowers the noise in each spectral resolutionelement more so than if the spectral resolution elementis measured individually by step scanning.

Mid-IR measurements are typically performed usinga linear scan interferometer and appropriate samplingaccessory to guide the light to and from the sampleof interest.

4.1.3 Microscopic FTIR Instrumentation

FTIR microspectroscopy is a microanalyticaltechnique, which interfaces an FTIR spectrometer toan optical microscope. Regions of interest in the sampleare spatially isolated using the microscopes apertures.It enables the IR spectrum of sampling regions downto about 10 m resolution to be taken. Consequently,FTIR microscopy is ideal for compositional mappingand analysis of heterogeneous samples whose domainsizes are in the tens of micrometre range.

FTIR microscopes use x15 and x10 condensor lensesof the non-IR absorbing Cassegrainian type. They areconstructed from front surface mirrors and are mounted


8

on-axis so that the path for the visible light is parafocaland colinear with the IR light. The microscope stageis computer controlled with 1 m steps. The sampleis not physically aperatured since the sample isspatially isolated from the microscope aperatures. ForIR microscopy, two apertures take up positions suchthat their images concide in the sample plane(redundant aperturing). The aperatures are a set ofcrossed knife blades in the form of a rectangle whichis of adjustable size.

The principal problem in IR microscopy is thepresence of scattered light due to diffraction whichlimits the spatial resolution by increasing the noise.Diffraction is significant when the aperturedimensions approach the wavelength of the IRradiation. The main effect of diffraction is that at smallaperature sizes, light spreads outside the specified areainto the surrounding region. As higher spatialresolution is sought, the problem increases, as theapertures are smaller, ultimately leading to loss ofspectral quality and photometric accuracy.

4.1.4 FTIR Imaging Instrumentation

FTIR imaging (as opposed to FTIR mapping) is amethod using two-dimensional array detectors and theimage is obtained in the snap-shot mode, i.e., allelements of the field of view of the microscope arerecorded simultaneously. FTIR imaging is anoutstanding new tool for online materials monitoring.A spectroscopic imaging system consists of threecomponents: image capturing, image processing, anda visualisation system.

The spectral data obtained from IR imaging areanalysed to produce a variety of response factors whichare spatially expressed in terms of the individual datagrid coordinates. The data collection associated withthe spectroscopic mapping contains all of theinformation required to deconvolute the contributionsof the different species in the sample. Using a computer,one can analyse the entire spectral range at eachpositional coordinate or, for a given frequency, developa spatial image demonstrating the spatial distributionof an object-specific spectral feature (a.8).

The advantages of FTIR imaging are:

Micro-spatial (~7 m) chemical mapping ofheterogeneous complex samples,

High sensitivity (ppm in many cases),

High selectivity (IR spectra are fingerprints ofmolecules with many bands available),

Rapid acquisition of data,

Sample preparation is simple (optical microscopicmethods are generally useful),

Experiment can be automated to trigger acquisitionas required and

Images can be computer-enhanced for visualisationand interpretation.

FTIR imaging with a focal plane array (FPA) detectoris a new state-of-the-art multichannel (a.9) method ofsimultaneously recording the spectral image of asample as shown in Figure 1 (a.10).

The IR chemical imaging system measures chemically-specific IR spectra using a mercury cadmium telluride(HgCdTe) FPA detector which provides broadfrequency response (out to ~ 18 m), high sensitivity(2 x 1011 cm Hz 1/2/Watt), and an operating temperatureof 40-60 K.

4.1.5 Filter IR Instrumentation

One IR spectroscopic system often used for industrialmonitoring is the selective-wavelength spectrometer

Figure 1Diagram of an FPA camera imaging system

(x and y refer to axis)


9

or a filtometer (a contraction of the terms filter andphotometer). These devices consist of a source ofradiation, energy collecting optics, a beam chopper, asample compartment, another set of collection opticsand a radiation detector or detectors. Interference filtersprovide wavelength selection.

The basic function of band-pass optical filters is toefficiently transmit light within a chosen wavelengthband and effectively reject the wavelengths lyingoutside of this band. The ratio of the in-band (desired)transmitted energy to the out-of-band (undesired)transmitted energy is the single most importantparameter to be considered.

The electronics of the filtometer include power suppliesthat operate the source, control temperature and poweramplifiers and related circuits that convert detectorsignals to concentration values. The limitation of thistype of control instrumentation is primarily sensitivityparticularly when remote sensing, via emission, is thesampling method.

Filter IR instruments have found broad applications inprocess control in the past (a.11). By utilising a portableIR spectrometer, the characteristic chemical sensitivityand selectivity of IR measurements can be performedin the field to interrogate the composition of industrialsamples (a.12).

4.1.6 Fibre-Optic IR Instrumentation

IR-transmitting optical fibres are evanescent wavesensors using a mathematical deconvolution techniqueto extract the absorbances and follow theconcentrations of the components as they occur in bothlaboratory scale and process production. The fibre-opticprobe used can be placed at specific locations withinthe samples or at the surface. The specificity of thetechnique, the speed of data acquisition and theportability of equipment make this method ideal as atool to fundamentally probe polymer reactions andprocesses. Chalcogenide optical fibres are used to directIR radiation from an FTIR spectrometer through anattenuated total reflection (ATR) probe immersed in areactor and back to the spectrometer.

4.2 Types of Raman Instrumentation

The evolution of Raman instrumentation has beendramatic. After making difficult measurements withuncertain sources, the laser came along and Raman

became a revitalised field. This was followed by theavailability of computers, two-dimensional detectorsand filter devices to replace monochromaters. Only afew brief remarks will be made here with referencesgiven for the interested reader.

4.2.1 Dispersive Raman Instrumentation

The configuration for a Raman dispersive experimentbasically consists of five parts (laser, sample, dispersingelement, detector and computer) (a.13). Amonochromatic laser beam (excitation radiation) isfocused on the scattering sample. The scattered lightfrom the sample is focused on the entrance slit of amonochromator and dispersed. The dispersion elementdiscriminates between the strong elastic scattering(Rayleigh scattering) and the weak inelasticallyscattered light (Raman scattering) with differentfrequencies. A typical single monochromator providesstray light rejections of 10-5-10-6 (as a fraction of theRayleigh light that enters the spectrometer) limited bythe imperfections on the optical surfaces such asgratings. Double and triple monochromators are oftenrequired to obtain adequate stray light rejection.

The Raman spectrum is given by the detection of theintensity of the scattered, frequency-shifted light by aphotoelectric system. The resulting signal of thedetector is amplified and converted to a formappropriate for plotting as a function of frequency.

4.2.2 Fourier Transform Raman Instrumentation

The basic problem in Raman spectroscopy is theinefficiency of the Raman scattering; roughly only oneout of 108 incident photons is Raman scattered. This,coupled with the fact that most polymeric systems giverise to an interfering fluorescent background, leads toa real problem of detectibility. For example, if a systemcontains one part per million of an interfering systemwhich fluoresces under visible excitation, the sameincident flux of 108 photons will produce 100fluorescent photons which will completely mask theRaman signal. This fluorescence does not occur whenthe excitation frequency is below the electronic energylevels of the molecules. The recent development ofFourier transform Raman (FT-Raman) spectroscopy,using NIR excitation, drastically reduces the problemof fluorescent interference.

FT-Raman also benefits from advantages inherent tointerferometry: high collection efficiency, excellent


10

wavelength precision, easily variable resolution andspectral coverage, software developments in FTIR(a.14). The restrictions of FT-Raman arise from the lowSNR. The SNR is limited by detector noise rather thansignal shot noise. Detectors in the 1 to 2 m region arenoisy. The result is an SNR comparable to that for asingle-channel dispersive system with equalmeasurement time (a.15).

FT-Raman spectroscopy became possible due to thecommercial availability of the Nd:YAG laser output at9395 cm-1. The Stokes-shifted Raman scattering occursin the 5000-9000 cm-1 spectral range, coincident withthe NIR region. Although the photon energy at 1076 nmis insufficient to cause fluorescence (which canoverwhelm the weak Raman scattering spectrum), theintensity of Raman scattering is diminished by a factorof ~22, in comparison to excitation at 488.0 nm (thelaser frequency of the Ar+ laser); this factor can becompensated for by increasing the excitation power.

To obtain good Raman spectra using the FT-Ramanmethod, the unwanted Rayleigh scattering must beremoved. Because of the nature of the Michelsoninterferometer, both Rayleigh and Raman-scatteredlight enter the system. Since the Rayleigh componentis from 106 to 108 times more intense than the Ramanscattering, it must be removed before it reaches thedetector, because a signal of this intensity drives thedetector into the distributive noise regime, resulting inhigh-frequency noise spread throughout thetransformed spectrum.

4.2.3 Resonance Raman Instrumentation

Resonance Raman spectroscopy occurs when the energyof the exciting beam is close to an electronic energy levelof the molecule and the excitation makes one term inthe sum of eigenstates dominate over all others.Resonance Raman spectroscopy takes place when thefrequency of the exciting line is similar to the absorptionfrequency of a chromophore in the sample. When thisoccurs, the electron cloud surrounding the molecule ismore readily distorted by the electric field of the incidentlight. The transition has a higher probability, and thereis an enhancement of the Raman process. The Ramansignal from the chromophore can be enhanced by sevenor eight orders of magnitude. The vibrational modesenhanced are dominated by vibrational bands that arepart of the chromophore responsible for the resonance.The resonance lines are generally totally symmetric anddistort the molecule along directions of electron densitychanges between the ground and the resonant electronicexcited state.

Ultraviolet Raman resonance (UVRR) spectroscopyprovides for chemical species identification from boththe characteristic vibrational structure and electronicspectra. The resonance enhancement also increases theabsolute sensitivity of detection, making it easier todetect the structures. The advantages of UVRRspectroscopy are high sensitivity, lack of fluorescenceand suitability for use in aqueous solutions.

Resonance Raman spectroscopy combines bothvibrational and electronic spectroscopies. Thevibrational spectrum at a particular excitationwavelength provides the first dimension. Theexcitation spectrum, the intensity of each vibrationalband as a function of the excitation wavelength,provides the second dimension. Since mostmolecules have resonance enhancement in the UV,this approach is quite general, but not universal. Theavailability of a range of excitation frequencies fromthe laser source makes exploitation of this form ofRaman scattering possible.

Detection of UVRR spectra is complicated by a numberof experimental difficulties: the photooxidation andphotodestruction of molecules, distortion of spectralinformation due to optical saturation phenomena andphotoinduced transients, and fluctuations of thescattered light intensity in inhomogeneous samples.

4.2.4 Surface-Enhanced Raman Instrumentation

In surface-enhanced Raman spectroscopy (SERS),increased Raman signals are observed from moleculesattached to metallic clusters ranging in size of the orderof tens of nanometres (a.16). Enhancements as high as1014 have been observed. These enhancement factorscan lead to single-molecule Raman spectroscopy (a.17).Using a Raman microscope the probe volume can beas small as 10 picolitres. Spectra can be measured witha one-second collection time.

In order to achieve large surface Raman signals, it isnecessary that the metal surfaces are specially preparedin one of several ways, which renders them rough orfinely divided on a length scale comparable to opticalwavelengths. Under favourable conditions, SERS-active metals enhance the surface Raman intensity byup to 105-fold, giving strong Raman signals.

The enhancement of Raman scattering at noble metal(gold, platinum, etc.) surfaces can be attributed to twofactors. One is an electromagnetic effect (discussedabove) observed for molecules near roughened noblemetal surfaces, and the second is a chemical interaction


11

between the absorbed molecule and the metal surface.The electromagnetic enhancement observed by SERSarises from the unique optical properties of noble metals.The zero oxidation electronic configurations of thesemetals are d10s1. The single s electron is well shieldedby the inner shell electrons and behaves as a free electron.In the visible region of the spectrum, this free electroncauses the dielectric constant of the metal to have a largenegative real component and a small imaginarycomponent. The large negative component of the realpart means that the electrons react very strongly toincident radiation and their displacement is such thatthe field due to the polarisation of the particles resistsfurther polarisation. The result is a resonance conditionthat absorbs energy from the incident light and creates alocally intense electric field within the metal particle.

When the incident radiation interacts with the surface,it causes the free electrons to oscillate with the incidentelectric field and polarises the noble metal particles.This creates a strong local electric field at the particlesurface known as a surface plasmon. When a moleculeis in close proximity to a noble metal particle themolecule is polarised by the electric field of the noblemetal particle. This leads to an enhancement of theRaman signal because the Raman scattering isproportional to the square of the local electric field.

4.2.5 Raman Mapping and ImagingInstrumentation

The ideal Raman spectrometer would consist of a high-dispersion, low-stray-light (where the scattered lightis low) single monochromator with a multichanneldetector. In the new generation of Raman instruments,a polychromator or spectrograph and a multichanneldetector are used instead of the monochromator andphotomultiplier. The entire spectrum is collectedsimultaneously using an array of detectors in whicheach element of the multichannel detector iscomparable in sensitivity to a single photomultiplier.The assumption is that all light incident on the detectoris correctly positioned with a single wavelengthincident upon each single detector. In this way, the timeneeded to record a spectrum is reduced markedly.Additionally, the simultaneous detection of thespectrum increases the accuracy of intensitymeasurements of different Raman bands and avoidserrors in interpreting accidental changes in thebackground due to laser fluctuations as Raman bands.Furthermore, multichannel detectors offer newpossibilities for investigation of photolabile systems(i.e., systems that degrade by photoirradiation), time-resolved experiments and chemical images.

The wavelength at a particular detector element isdetermined by the position of that element at the focalplane of the monochromator (the grating position). Inanother mode of operation, an acousto-optic tunablefilter (AOTF) is used rather than a monochromater. InRaman applications, the AOTF is used to spectrallyfilter the source (a.18). Under computer control, theAOTF is swept through a wavelength range and, atpredetermined intervals, images are recorded. For morerapid operation, the AOTF may be digitally tuned topredetermined frequencies where single frames maybe collected (a.19).

In practice, the AOTF is placed between the Ramanscatterer and a Si FPA detector yielding a no-movingparts Raman spectroscopic imager. The AOTF replacesthe dispersive monochromator and is operable between400 nm and 1900 nm. At each discrete wavelength acharge collection detector (CCD) frame is collected,digitised and stored. The size, wavelength range,wavelength increment and exposure time for eachimage frame maybe modified under computer control.Potentially, one can record one frame/second.

4.2.6 Fibre-Optic Raman Instrumentation

A dual-fibre optical probe has been developed forRaman spectroscopic monitoring of a number ofpolymer processes. In the dual-fibre configuration theinternal bundles transmit the light to the sample andthe outer fibres carry the signals back to thespectrometer for processing.

5 Sampling for VibrationalSpectroscopy

5.1 IR Sampling

One of the most attractive aspects of FTIR spectroscopyis its ability to deal with a great number of samplingtechniques, making it a versatile method which can beadapted to the study of a wide range of materials.

For IR spectroscopy, sample preparation has been alabour-intensive operation requiring some measure ofskill and experience. Conventional IR spectroscopy ismainly based on transmission measurements except forsamples for which the preparation of a thin layer isproblematic, inadequate, or prohibited. In these casesspecial sample preparation is required as indicated inSection 5.1.1, 5.1.2, etc. The main sampling techniquesmay be seen schematically in Figure 2.


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5.1.1 Transmission Spectroscopy

Transmission spectroscopy is the simplest and mostpopular mode for quantitative analysis. Radiation passesthrough the sample and is absorbed by a length equal tothe thickness of the sample in the direction of IRpropagation. The single beam intensity can be ratioedagainst a background collected without the sample andan absorbance spectrum obtained. The absorbance hasa simple linear relation to the concentration under mostconditions and can be used for quantitative analysis. Ingeneral, it delivers the highest SNR of all samplingtechniques and does not affect the spectral data in anymanner. No special sample handling techniques arerequired and quantitative data processing isstraightforward. However, films usually have to be thin,i.e., 1 to 100 m. Such films can be prepared by solventcasting or compression moulding.

5.1.2 Internal Reflection (or Attenuated TotalReflection (ATR)) (a.20)

Attenuated reflection may be used for samples with noor low transmission or when sample geometry demands

it. The sample is brought into contact with an IR-transparent crystal of high refractive index. The IRbeam then traverses the length of the crystal whilepenetrating the sample with multiple internal reflectionsat the free crystal surface. Thus, depending on therefractive index and angle of incidence, the region ofthe sample examined is typically a 0.1-10 m layerclose to the crystal. Spectral data is different from thatobtained in transmission and the signal does not varylinearly with concentration. Hence, calibration isrequired for quantitative studies.

Depth profiling is also a possibility using the ATRtechnique (a.21) and probed depth (i.e., how deep inthe sample the light probes) varies as

dn n

po

=

( )

2 2 2 2 1 2sin / (1)

where dp is the probed depth, no and n are the refractiveindices of the ATR crystal and the sample respectively, is the angle of incidence and the wavelength ofradiation. Algorithms to extract depth of penetrationindependent of wavenumber have been developed(a.22, a.23).

Figure 2IR sampling techniques for polymers


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5.1.3 External Reflection SpectroscopyReflection-absorption spectroscopy is an externalreflection technique that utilises an IR beam incident ata grazing angle. This allows for a convenient method tostudy polymers in contact with reflective (e.g., metal)surfaces. When IR light impinges on a metal surface, anelectrical field arises near the surface. The metallicsurface imposes a strict dipole moment selection rule,which demands that the incident light should have acomponent parallel to the plane of incidence (p- orparallel polarisation). The interaction between the sampleand the incident light is strong when the angle ofincidence is between 75 and 89 and approaches amaximum at 88. For parallel-polarised incidentradiation, only the dipole moments of the surface speciesnormal substrate interact with the electric field. A detaileddiscussion of the technique has been published (a.24,a.25). It has been established that the sensitivity ofreflection absorption IR spectroscopy is greatly improvedwhen using a grazing incidence (65 or more). In theparticular case of thin layers (of organic or inorganicmaterial) deposited on a good reflecting surface such asa metal, a lower detection limit of a few nanometres inthickness can be achieved. The drawbacks include theappearance of asymmetric absorption modes and shiftingof mode frequencies that may be corrected by the use ofthe Kramers-Kronig transformation.

5.1.4 Diffuse Reflectance FTIR (DRIFT)Spectroscopy

Light impinging on a solid or powdered surface isdiffusively scattered in all directions and may bedirected to an IR detector. This technique is particularlyuseful if the sample is strongly scattering and weaklyabsorbing. A general theory for powdered samples wasdeveloped by Kubelka and Munk (a.26) and relates thesample concentration to the scattered radiationintensity. DRIFT spectroscopy requires little samplepreparation, is ideal for particulate substances and hasfast data collection with good SNR. However, someinformation is lost in the highly absorbing regions andthere is small band shape distortion. Quantitativeanalysis should be performed on the same particle size.DRIFT spectroscopy has been used to probe polymerlayers, fibres and polymers on metals (a.27).

5.1.5 Emission Spectroscopy

If collected correctly, emission spectra mirrorabsorbance spectra and can be collected even fromopaque samples. The material must not have thermalgradients, readsorption of emission, or self-adsorption.

5.1.6 Microsampling

As far as microspectroscopy is concerned, onlytransmission and reflection under near-normalincidence are initially implemented. The recent launchof specialised objectives provide the user withadditional techniques such as external reflection undergrazing angle and ATR.

5.2 Sampling for Raman Spectroscopy

The sampling techniques used in Raman spectroscopyare shown in Figure 3. A sample in any state can beexamined without difficulty by using Ramanspectroscopy. The laser beam is narrow, collimated, andunidirectional, so it can be manipulated in a variety ofways depending on the configuration of the sample.

Figure 3Raman sampling techniques for polymers


14

For liquids, a cylindrical cell of glass or quartz withan optically flat bottom is positioned vertically inthe laser beam. For solids, the particular method useddepends on the transparency of the sample. For clearpellets or samples, right angle scattering is used.With translucent samples, it is helpful to drill a holein the sample pellet. Powdered samples can beanalysed by using front-surface reflection from asample holder consisting of a hole in the surface ofa metal block inclined at 60 with respect to thebeam. Injection moulded pieces, pipes, and tubing,blown films, cast sheets, and monofilaments can beexamined directly. One of the advantages of Ramansampling is that glass containers can be used whichcan be sealed if desired (a.29).

5.2.1 Microscopic Sampling in RamanSpectroscopy and Imaging

The incorporation of high-resolution optics in aRaman spectrometer allows sampling from areas ofless than 1 x 10-6 m in diameter. The addition of aconfocal microscope improves the axial resolution toa couple of microns. Those developments, along withthe introduction of notch filters and holographictransmission gratings, allow the reduction of theacquisition time of Raman spectra from minutes toseconds. The fast data collection combined with thehigh lateral and vertical resolutions makes possiblethe collection of Raman images with several micronspatial resolutions.

6 Measuring Polymer Orientationwith IR and Raman Spectroscopy

Orientation can be defined as the preferential alignment(either uniaxial or biaxial) of polymer chains orsegments when submitted to an external force. Theorientation induces significant changes in the propertiesboth positively and negatively.

A high degree of alignment is the basis of improvedmechanical, optical, and electrical properties in almostall polymers. Many factors influence the orientationbehaviour of polymer chains, including crystallinity,phase separation, free volume, chain rigidity, molecularfriction, interchain interactions and chainentanglements. Consequently, it is necessary tounderstand the correlations among molecular order,material properties, and fabrication procedures.Experimental characterisation of the degree and

direction of alignment is required in order to understandhow molecular design and processing strategies leadto the ultimate state of alignment. There are severalmethods available, including IR dichroism, X-raydiffraction, nuclear magnetic resonance (NMR),birefringence, polarised fluorescence, ultrasonicmeasurements, and polarised vibrational spectroscopies(IR and Raman). Of all these techniques, IR dichroismis one of the most frequently applied, since it isapplicable to many polymer systems, it can be used tomake measurements on the microscopic andmacroscopic scales, and it can specifically measureorientation of different phases (i.e., amorphous orcrystalline) in the same sample.

6.1 IR Dichroism

In general, IR absorption is caused by the interactionbetween the IR electric field vector and the moleculardipole transition moments related to the molecularvibrations. Absorption is at a maximum when theelectric field vector and the dipole transition momentare parallel to each other. In the case of perpendicularorientation, the absorption is zero. Directionalabsorptions are measured using polarised light. Theterms parallel and perpendicular refer to the orientationof the polarised beam with respect to a reference axis.For deformation studies, the reference axis correspondsto the stretching direction.

The IR dichroic ratio may be considered to becharacteristic of the directional orientation of thesegments of the molecule. For a polymer whosemolecular axis is oriented parallel to the spectrometersampling plane, the dichroic ratio, R, is defined as

R AA

=

||

where A|| is the absorbance parallel to the chain axis,and A is absorbance perpendicular to the chain axis.For highly oriented samples, the dichroic ratio mayapproach either infinity or zero, depending on thealignment of the transition-moment vector withrespect to the molecular chain axis. The alignmentof the chain segments can be determined fromdichroic ratio measurements if the inherentpolarisations are known (a.29). In general, A|| andA are determined successively by using a polariserthat is aligned parallel and perpendicular to thestretching direction. For samples that have a lowlevel of orientation, the magnitude of the dichroicratio is close to 1. In these cases of minimal


15

orientation, it is better to measure the dichroicdifference A||A because it is a more sensitivemeasurement under these conditions.

One of the practical problems in the case of IRdichroism measurements arises from the requirementof low band absorbance (roughly lower than 0.7absorbance units) in order to permit use of the Beer-Lambert law. This condition implies use of very thinfilms in the range of 1 to 200 m. NIR measurementsalleviate this problem.

6.2 Raman Characterisation of PolymerOrientation

Raman scattering arises from the interaction betweenradiation induced oscillating electric dipoles andmolecular vibrational modes. In general, the inducedpolarisability is not necessarily in the direction of theincident beam, and they are related by a second rangetensor, , thus:

P = E

or written in terms of the components:

Px = xxEx + xyEy + xzEz

Py = yxEx + yyEy + yzEz

Pz = zxEx + zyEy + zzEz

where xx, yy and zz are the components of theprincipal axes of the polarisability ellipsoid and xy,yz and xz are the other components. A single crystalcan give up to six different spectra, depending on itsorientation towards the two directions of polarisationof the incident laser light.

Consequently, the Raman scattered light emanatingfrom even a random sample is polarised to a greater orlesser extent. For randomly oriented systems, thepolarisation properties are determined by the two tensorinvariants of the polarisation tensor, i.e., the trace andthe anisotropy. The depolarisation ratio is always lessthan or equal to 3/4. For a specific scattering geometry,this polarisation is dependent upon the symmetry ofthe molecular vibration giving rise to the line.

In solids, the problem of polarisation is morecomplicated, but the results are more rewarding (a.30).In solids, the molecular species are oriented withrespect to each other. Therefore, the molecular

polarisability ellipsoids are also oriented along definitedirections in the solid. Since the electric vector of theincident laser beam is polarised, the directionality inthe crystal can be utilised to excite and obtain Ramandata from each element of the polarisability ellipsoid.With the laser polarisation along z and collection alongz, a spectrum from the zz component of the tensor isobtained. By rotating the analyser 90, therebycollecting x polarised light while still exciting along z,zx is obtained.

In the usual Raman experiment, the observations aremade perpendicular to the direction of the incidentbeam, which is plane polarised. The depolarisationratio is defined as the intensity ratio of the twopolarised components of the scattered light which areparallel and perpendicular to the direction of thepropagation of the (polarised) incident light. Thepolarisation of the incident beam is perpendicular tothe plane of propagation and observation. For thisgeometry, the depolarisation ratio is defined as theintensity ratio:

= VH/VV

where, for the right angle scattering experiment, V isperpendicular to the scattering plane and H is in thescattering plane. An alternate notation expressed interms of the laboratory coordinate system is:

A(BC)D

where A is the direction of travel of the incident beam,B and C are the polarisation of the incident and scatteredlight, respectively, and D is the direction in which theRaman scattered light is observed. Generally, theincoming beam is along the x axis, the scattered beamis along the z axis and the y axis is perpendicular to thescattering plane.

One of the major problems is the difficulty ofdetermining that the polarisation is scrambled whetherby scattering inhomogeneities in the sample itself orby the birefringence of the sample itself. Many spectrashow evidence of extensive depolarisation of both theincident and scattered light, and thus each spectrum isan indeterminate mixture of four different polarisations:the intended polarisation, the one resulting fromdepolarisation of the incident light, the one resultingfrom depolarisation of the scattered light, and (to alesser extent) the one resulting from depolarisation ofboth the incident and scattered light. When this occurs,little useful information is available from the individualpolarised spectra.


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7 IR and Raman Applications toPolymer Characterisation

7.1 Material Identification

IR and Raman spectroscopy are the best methods forthe identification of unknown compounds if the spectraof the reference compounds are known. The ideaunderlying spectroscopic identification is that a patternof frequencies provides a fingerprint of a particularmolecule and that this pattern of frequencies can berecognised when a spectral database is searched.Spectral comparison of an unknown sample foridentification is generally done by visual comparison(often with the computer as a tool) or by using cross-correlation techniques. With cross-correlationtechniques, an identification is performed bycalculating a matching score for the spectra of theunknown and an IR spectrum of a known compoundin the database. Additional spectra in the database areranked according to the matching score determined andthe spectroscopist must make a final determinationbased on his knowledge of the properties of theunknown sample relative to the chemicals with highmatching scores.

Independent of the type of scoring system used, thereis always a risk of obtaining a false identification result.Each measured spectra can match the frequencies ofseveral different molecules in addition to the matchwith the molecule actually present in the sample. Falseresults are caused by the accidental matching of thefrequencies of a chemical with no properties similar tothe unknown. An incorrect result is also obtained whenthe score due to random matching cannot be discernedfrom the score match due to matching of the real spectrain the sample. It is therefore useful to include otherinformation when interpreting the quality of the match,such as the chemistry involved in the unknown sample.The likelihood of accidental matches (and hence thelikelihood of false identification) increases if thespectral reproducibility is reduced.

The Sadtler division of Bio-Rad Laboratories (http://www.sadtler.com) has a product on compact disccontaining 175,000 IR and 3,300 Raman spectra. Fora subscription price the user gets the disc and an e-mailed code from Sadtler that opens the disc forsearches on one computer for a year. Once the codeopens the disc, the user may perform unlimited lookupson one computer for one year. A lookup retrievesspectra by names or structures for comparison. Inaddition, the user may perform unlimited searches toidentify or classify unknowns. Other databases for

polymer and additives are also available (166, 357)including one for fibres (376). For Ramanidentification, the databases are smaller because of thelimited history of collection of spectra but attempts arebeing made to bridge the gap (a.31).

7.1.1 Additive Analysis

Polymer additives are materials designed to enhanceor upgrade the performance or capabilities of basepolymers to achieve the optimal properties for a specificapplication (a.32). Plastics would not be able to performtheir diverse functions without the assistance of a verybroad range of plastics additives (a.33). Without them,some plastics would degrade during processing and,over time, the polymers would lose impact strength,discolour, and become statically charged, to list just afew problems. Additives not only overcome these andother limitations, but can also impart improvedperformance properties to the final product.

It is necessary to be able to identify and quantify theadditives in polymers and vibrational spectroscopyis a particularly useful approach to this problem.Compared with traditional chemical analyses,vibrational methods are nondestructive and are time-and cost-effective as well as more precise. A largenumber of examples exist in the literature. Forexample, antistatic agents (polyethylene glycol (PEG)in polyethylene (PE)) can be detected directly usingFTIR sampling (367). An IR spectroscopic techniquefor the analysis of stabilisers (2, 6-di-tert-butyl-4-methylphenol) in PE and ethylene-vinyl acetate (EVA)copolymer has been described (368). It is possible toquantify the amount of external and internal lubricants(stearic acid in polystyrene (PS)) (371). Fillers inpolymers can also be analysed (white rice husk ash(predominantly silica in polypropylene (PP)) (268).Raman spectroscopy has been used to detect residualmonomer in solid polymethyl methacrylate (PMMA)samples (326).

It is also possible to determine multiadditives in thesame sample using FTIR and multivariate analysismethods. For example, the simultaneous determinationof the concentrations of silica, erucamide andbutylhydroxytoluene in PE were measured using IRspectroscopy and suitable calibration models. Theconcentrations were between 20 and 1100 wt/ppm(366). A multiadditive method was developed forcharacterisation of an antiblocking agent (silica) andlubricant (erucamide) in molten low densitypolyethylene (LDPE) samples (324).


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A method for the determination of vulcanised rubberadditives by FTIR spectroscopy using partial least-squares regression (PLSR) for multivariate calibrationwas developed (264). Interestingly enough,photoacoustic FTIR spectroscopy was used tocharacterise tread lugs sectioned from two worn off-the-road tyres. The tyres had experienced similarservice but displayed significantly differentperformance in that chipping/chunking was visiblein one tyre brand. The technique was also used toexamine a variety of rubber compounding ingredients,such as polymers and fillers, and model tyre compoundscontaining different levels of these ingredients (341).A long standing problem in analysing natural rubber(NR) is the determination of attached nitrogenouscompounds. Bands at 3280 and 1540 cm-1 diminishedwhen a fresh field latex was treated with enzymefollowed by washing in the presence of surfactant. Aband at 3320 cm-1 that remained after the treatmentindicated the presence of residual amino acids bondedto the hevea rubber particles (349).

A surface method of measurement of a sizing agent(partly hydrolysed polyvinyl alcohol) on the warp yarnsubstrate (polyester/cotton) using NIR diffusereflectance spectroscopy has been described (337). Apartial least-squares (PLS) modelling procedure useda frequency segment of the NIR spectrum that is mostsensitive to changes in size concentration relative tothe warp yarn.

7.1.2 Chromatographic Separation andIdentification

The combination of chromatography and IRspectroscopy provides a versatile tool for thecharacterisation of additives. High performance liquidchromatography-FTIR (HPLC-FTIR) interface systemsdeposit the output of a chromatograph on an IR opticalmedium, which is then scanned to provide data as atime-ordered set of spectra of the chromatogram (138).A spray-jet interface is used to deposit the effluent froma narrow-bore liquid chromatography (LC) column ona zinc selenide window. The deposited additives areanalysed by FTIR transmission microscopy, yieldingidentification limits in the low-nanogram range (215).Although it is not a real time analysis, it has theadvantages of total solvent removal before IR analysis,analysis of the total LC effluent as opposed toincremental fractions, and the ability to produce IRspectra at high SNR.

A powerful new method of additive extraction has beendeveloped termed supercritical fluid extraction/FTIR

spectroscopy (SFE/FTIR) (303). The SFE/FTIRtechnique has been applied to the analysis of fibrefinishes on fibre/textile matrices. Three different fibrepolymer types were examined (polyurethane (PU),polyamide (PA) and aramid), each requiring a differentfinish. Finishes ranged from a single-componentpolydimethylsiloxane (PDMS) oil to more complexmulticomponent finishes that included varioussurfactants, fatty acid esters and soaps, antioxidantsand oils.

7.2 Determination of the Chemical Structure ofa Repeating Unit

The most common type of structure determination isstructure verification. In this case, enough informationis available (perhaps on the basis of well-knownsynthetic reaction paths) to propose a probablestructure. The structure information which is achievedusing IR and Raman spectroscopy is usually sufficienthere. One of the fundamental problems in polymerscience is the determination of the chemical structureof the repeat unit; this moiety determines all of thechemical properties such as reactivity, stability andweatherability. Vibrational spectroscopic techniques areadvantageous as a method of determining structurebecause the methods are applicable to all polymersregardless of the state of order. The IR and Ramanspectroscopy methods are accurate, faster than chemicalanalysis and reduce exposure to irritating, toxic andcorrosive chemicals.

Bisphenol A phenoxy polymers and bisphenol Apolycarbonates were blended in solution. Chemicalchanges from aromatic to aliphatic carbonate groupswere monitored by FTIR as a function of time andtemperature (348).

The network structure of an epoxy resin system andthe course of reaction of bisphenol A diglycidyl etherwith 1-cyanoguanidine as curing agent dissolved indimethyl formamide in the presence of differentaccelerators were studied. Fractions of reactive groups,such as oxirane rings, primary and secondary hydroxylgroups, imide and nitrile groups, were detected asfunctions of epoxy consumption and reactionconditions. Structural tautomerism of the curing agent,the influence of water, the reaction of nitrile groupsand other structural features of the curing system wereexamined (354).

The compound n-butyllithium was the initiator andcyclohexane the solvent for high-vinyl polybutadienesproduced from anionic polymerisation. Complexing


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agents were used to change the mode of addition to1,2 or vinyl, 1,2-dipiperidinoethane and diglyme werevery effective, although their effectiveness decreasedwith temperature. The microstructure of the polymerswas characterised by Raman spectroscopy using theRaman active carbon-carbon double bond stretchingbands of cis and trans vinyl structures at 1640, 1650and 1664 cm-1, respectively. The amount of eachcomponent was measured using band areas (369).

An IR method was developed for determining thehydroxyl number in samples of polyesters formed bythe copolymerisation of a hydroxy-functional acrylicoligomer with adipic acid and 1, 6-hexanediol (370).

Oxazolidone-isocyanurate polymers were prepared byreacting five glycidyl ethers with 4, 4-diisocyanato-diphenylmethane in the presence of 2-ethyl-4-methylimidazole. The degree of cure of the resins wasfollowed by IR spectroscopy by measuring the fractionof the isocyanurate to oxazolidone linkages (377).

7.3 Crystalline and Conformational StructuralOrder in Polymers

7.3.1 Introduction

IR and Raman spectra are particularly sensitive to theconfigurations and conformations of the polymermolecules. The variations in the conformational andconfigurational structures are reflected in specificobservable frequencies. Consequently, it is possible todetect and quantify the amounts of the conformationalisomers whether they be crystalline, liquid crystalline,or disordered (amorphous). The vibrationalspectroscopic approach to structural elucidation inpolymers relies on the comparison of vibrationalspectra of polymers containing specific conformationalstructures (incorporated into the polymer duringpolymerisation or by thermal or chemicalmodification), with spectra of models (polymers andsmall molecules) containing similar structurespresumed to be present in the polymers.

7.3.2 Crystal Phases of Polymers

Structural changes in the orthorhombic-to-hexagonalphase transition of PE crystals were investigated in thecourse of heating to the melting point. The IR andspectral patterns characteristic of the hexagonal phasewere confirmed. In particular, the bands characteristicof the disordered short trans segments (shorter than

five methylene units) and the bands of the kink ordouble gauche linkages were detected. The observationof the trans and gauche bands was confirmed as wellas the existence of conformationally disordered chainsin the hexagonal phase (195).

The defect density of states of the tight (110) fold inPE is calculated. Modes of the (110) fold calculated at1348, 1342 and 1288 cm-1 and are assigned to IR bandsat 1346-1347 cm-1 and 1342-1343 cm-1 and near 1295cm-1. The (110) folds (approximately ggggtg) alsocontribute to the gtg/gtg bands at 1368 cm-1, whereg = +60 and g = -60 from the plane of the C-Cbackbone and t = trans. In contrast to the (200) fold,the (110) fold does not exhibit a localised gap modenear 700 cm-1. The observed IR bands are assigned tothe defect modes of the (110) fold (373). The (110)folds are folds in the crystal plane and (200) folds liealong the (200) crystal plane.

The phase transition in an ethylene-tetrafluoroethylenealternating copolymer from the orthorhombic to thehexagonal structure is a result of the generation andpropagation of conformational collective defects (182).

Syndiotactic poly-p-methylstyrene (PPMS) exhibitsvarious crystalline forms and clathrate structures. Bandsdue to the syndiotactic stereostructure and bands typicalof the two different chain conformations are observedin the crystalline structures as well as bands sensitiveto intermolecular interactions typical of the differentmodes of chain packing (350).

In polyamide 66, normalised absorbances of thebands at 924 and 1136 cm-1 were plotted againstdensity and the intercept at zero absorbance provideda density of 1.26 g/cm3, which was very close to thecrystalline density. The 924 and 1136 cm-1 bandsare assigned to the conformation in the amorphousphase. The assignments of the bands at 936 and 1200cm-1 to the conformation in the crystalline phasewere confirmed (31).

FTIR analysis provided a direct method for theevaluation of the amount of alpha and beta formcrystalline phases in syndiotactic PS, although bothcontain chains in the same conformation (trans-planar)(186). Syndiotactic PS was found to exhibit two distinctvibrational peaks in the Raman spectrum. The presenceof long all-trans sequences gives rise to a peak at about773 cm-1, whereas trans/gauche conformations resultin a separate peak at 798 cm-1. With increasing levels ofcrystallinity, the integrated intensity of the 773 cm-1grows at the expense of the 798 cm-1. The relative area


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under the all-trans fundamental band can be directlyrelated to the crystalline volume fraction (173).

Cyclohexa(p-phenylene sulfide) (CHPS) is a precursorof poly(p-phenylene sulfide) (PPS). The FTIR andRaman spectra of CHPS resemble those of amorphousPPS. Specific modes indicative of the crystal structureand band shifts are reported and a crystal structurechange in CHPS which was induced by pressure orheat is identified (84).

7.3.3 Complex Formation between Polymers andComplexing Agents

Interactions between polyvinyl chloride (PVC) andcyclohexanone or n-methyl-2-pyrrolidone generatedthe occurrence of local configurational changes. Theresults suggest that the interaction is restricted to adefinite number of polymer sites although noinformation as to the exact type of these sites can bedrawn. The number of interaction sites is higher forcyclohexanone than for N-methyl-2-pyrrolidone (181).

The IR spectrum of the delta phase of the syndiotacticPS/ethylbenzene complex predominantly involvedbands in the 920 to 960 cm-1 region arising from helicalstructures. The 934 and 943 cm-1 peaks behaved as adoublet, primarily due to the delta phase, with thesplitting increasing on annealing. The 940 cm-1 peakwas solely due to the gamma phase helix. Theseassignments provide the opportunity for using thesebands for further studies of complexation/decomplexation in systems of this type (89).

7.3.4 Conformational Analysis

A study of annealed polycarbonate showed that theheating process was primarily a modification oftrans-trans and trans-cis population of the carbonategroup, leading to a more sta

infrared and raman spectroscopy of polymers

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