Write an essay on the applications of IR Spectroscopy to Polymers. You may include the

following structural characterization such as molecular weight, composition of copolymers,

crosslinking, conformation, stereoisomerism, mechanically stressed polymer systems and

intermolecular interactions.

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1.0 IR spectroscopy as a means to determine end group and molecular weight of a

polymer

IR spectroscopy is a technique which could be utilize to measure number-average

molecular weight of a polymer. This is done on the assumption that the chain of the molecule is

linear. This limit the number of end-groups of a given polymer to a few molecules only. For

polymer which posses branching (polymers which mainly produce from addition

polymerization), IR measurement is only a relative number-average molecular weight of the

system. To determine the molecular weight of fluorocarbons, IR spectroscopy is the standard

method as IR spectroscopy is particularly useful for measuring the number-average molecular

weight of an insoluble system.

In determination of end-group and molecular weight of polybutylenes terephtalete,

molecular weight, Mn, of the polymer is

Mn = 2

(E 1+E 2)

Where

E1 = equivalents per gram of alcoholic end groups

E2 = equivalents per gram of acid end groups

This equation presumes that no other end groups are present and that no grafting or degradation

has occurred.

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2.0 Composition and microstructure of copolymer

The composition of copolymer can be measured by ratioing an infrared band of A-

copolymer (which is not influenced by sequencing effects) to an internal thickness band (which

corrects for variations in thickness). The same measurement also is performed on B-copolymer

component though not compulsory. For example, the composition of polyethylene-co-vinyl

acetate, EVA can be determined by ratioing the 1740 cm-1 band for vinyl acetate to the 1460 cm-1

internal thickness band.

Vibrational coupling complicates the IR analysis of copolymers. The difficulty of the IR

analysis is a function of the size of the comonomer repeat units. The magnitude of the vibrational

coupling between units is a function of the intramolecular distance. When the repeat units are

small, the coupling is large and is a function of the type of vibrational motion. When the repeat

units are large, the coupling between units will be zero or very small. For example, the spectrum

of the trimer of poly(ethylene terephthalate) (PET) is essentially the same as that of high-

molecular-weight PET because the length of the PET repeat unit is so long that the

intramolecular interactions between repeat units is essentially zero. When the repeat units are

small (i.e., contain only two carbons, as is the case for vinyl monomers), vibrational coupling can

occur between the repeat units. This coupling results in spectral bands that have frequency and

absorptivity depending on the length of the ordered sequence. This intensity perturbation by the

coupling factor must be taken into consideration when developing a method of spectral analysis.

For large comonomer repeating units, the IR analysis is simplified because the analysis of only

the composition is possible, and the copolymer can be considered, from an IR point of view, to

be simply a mixture of the two comonomers. Unfortunately, for the case of large repeating units,

IR spectroscopy will not yield information about the copolymer sequence distribution. Consider

the case when vibrational coupling occurs; in this case, IR spectroscopy can yield connectivity

information about the microstructure of the copolymer. For the spectra of ethylene-propylene (E-

P) copolymers (Drushel et al, 1968). The structure of the various sequences of this copolymer is

shown in the following scheme, Figure 2.1 illustrating the expected origin of the different

segments of methylene units.

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Figure 2.1; An IR spectrum of the methylene rocking region of an ethylene-propylene

copolymer (54.3 wt% C2)

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3.0 Crosslinking

The extent of crosslinking in the Nadic methyl anhydride isomers or NMA and diglycidyl

ether of bisphenol-A based epoxy, EPON 828 (with 0.5 wt% benzyldimethyl amine as a curative

in each mixture) can be determined by using IR spectroscopy (Antoon et al, 1981). The least-

squares method was used to fit the spectra of the mixture of NMA and EPON 828. In order to

determine the degree of crosslinking, a difference spectrum was calculated by subtracting the

spectrum of a stoichiometric mixture of NMA and EPON 828 crosslinked for 37 min at 80°C

from the spectrum of the same reactant mixture crosslinked for 83 min at 80°C

Figure 3.1; Generation of difference spectrum to characterize crosslinking at 80~ of a stoichiometric mixture of NMA-EPON 828. The samples were crosslinked for 83 rain (top spectrum) and 37 min (middle spectrum). The bottom spectrum is the difference spectrum (top- middle).

4.0 Measurement of conformation

The relative geometric arrangement of the chemical groups along the polymer chain, that is, the

conformation of the chain is also of interest. Polymer chains have a number of possible rotational

isomers, depending on the temperature and thermal history of the sample. Rotational potentials for

single bonds joining chemical groups such as methylene units are necessarily threefold and symmetric

(Fig. 4.1). The energy minima occur when the substituents of the groups, hydrogens in this case, are in

the staggered conformations, but maxima occur at the eclipsed conformations. In molecules possessing

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C-C bonds, the rotation angle values near the potential minima are strongly favored over those near the

maxima. However, rapid interconversion is possible among the various forms. The trans form is a

staggered conformation in which the internal rotation angle X-C-C-X is 180 °C

Polymers generally have several different conformations in the solid state depending on their

thermal history. In the crystalline state, the regular structure results in a repeating polymer conformation.

This repeating conformation is usually determined by X-ray or electron diffraction methods, but for

amorphous and glassy polymers, these diffraction methods are not applicable. Vibrational spectroscopic

methods are useful because the vibrational modes are sensitive to differences in internal bond angles.

When the polymer chains exhibit extended order, specific vibrational selection rules apply, and these

rules can be used to determine the conformation of the isomer

To study rotational isomerism in polymers, the spectroscopic technique must be able to

distinguish the spectral features of each identifiable conformer. The vibrational spectrum of a mixture of

conformers will exhibit bands arising from the molecular vibrations of all conformers. In general, the

conformer bands in a polymer system are observed in pairs that correspond to modes of similar form but

slightly different frequencies in the high- and low-energy conformers. When the spectra are obtained as

a function of temperature, pairs of bands with one band intensity increasing and the other band intensity

decreasing will often be observed. For polymers, there is usually considerable overlap of these band

pairs.

Polyethylene (PE) is the most-studied example of rotational isomerism. The crystalline domains

are made up of the all-trans conformer structure. Bands that are characteristic of rotamers in the

amorphous phase are also present. The most intense of the amorphous absorptions are the methylene 6

Figure 4.1; Potential energy curve for internal rotation about the C-C bond of butane. (Source: H. Tadokoro, Structure of Crystalline Polymers. Wiley-Interscience. New York, NY, 1979, pp. 9-10

wagging modes at 1303, 1353 and 1369 cm -1 . The TG conformation is correlated with the bands

observed at 1303 and 1369 cm -1. The 1353-cm -1 absorption band is assigned to the wagging of the GG

structure. When PE is heated through its melting point, the concentration of the TG and GG

conformations increases. However, the concentration of the TG conformation increases well below the

melting point, and this increase indicates the formation of localized conformational defects in the

crystalline polymer.

5.0 Mechanically stressed polymer system

IR spectroscopy can be used to study the effects of applied mechanical stress on highly oriented

samples. The goal is to obtain the molecular stress distribution function, which is an important quantity

for determining the stress relaxation moduli or creep compliances. Shifts in the peak frequencies are

observed and an attempt is made to determine the molecular stress distribution by deconvolution

(Bretzlaff et al, 1983). The shifts as a function of stress for the 1168-cm -1 band of oriented isotactic

polypropylene (Lee et al, 1984) are shown in Fig. 5.1.

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Figure 4.2; Several possible conformations of a single-bonded carbon chain. (Source: H. Tadokoro, Structure of Crystalline Polymers. Wiley-Interscience. New York, NY, 1979, pp. 9-10)

6.0 Intermolecular interaction in polymer

Vibrational spectroscopy can be used to study intermolecular effects in the solid state and the

changes produced by temperature effects. If the intermolecular forces are sufficient, the fundamental

modes of the single chain are split into different spectral components in the crystal. The number of

theoretically expected bands depends on the number of molecules in the unit cell. Polyethylene (PE) has

a planar conformation and an orthorhombic unit cell containing two molecules. Each group mode of the

isolated PE molecule is predicted to be split into two components for crystalline PE. This crystal field

splitting has been observed for the methylene rocking mode at 720 cm -1 and for the methylene bending

mode at 1460 cm-1 in spectra of crystalline PE. Although other modes should also exhibit such splitting,

their inherent bandwidth prevents the observation of separate components. When PE is melted, the

crystal field splitting disappears. Consequently, a measure of the relative intensities of the 720- to 730-

cm -1 bands can be used to rank the relative crystallinity of PE samples.

The problem is that very few polymers exhibit the crystal field splittings observed for PE. The

explanation is that the crystal field splitting is very sensitive to the distance of separation of the polymer

chains. The intermolecular interaction forces fall off at a rate of r 6, where r is the distance between

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Figure 5.1; Parallel-polarized 1168-cm -1 band of oriented isotactic polypropylene at successive stress levels. The average stress application rate was 0.5 (kg/mm2)/min. The spectra were obtained at 4-cm -1 resolution, and the profiles of successive levels were vertically offset by 0.09 absorbance units. The peak frequencyvaries from 1167.72 cm -1 at o- -- 0, to 1164.90 cm -1 at ~ - 22 kg/mm 2.

chains. A very small difference in separation results in a large difference in the magnitude of the

interactions, and consequently, in the frequency separation of the bands. In fact, the PE chains are closer

together than chains of any other polymer, so the crystal field splitting is the largest for PE. For

polypropylene (PP), on the other hand, it has not been possible to observe any IR splittings resulting

from intermolecular or crystalline packing. In fact, there are many features in the PP spectrum that look

like pairs of bands, and indeed they are. However, the band pairs arise from intramolecular helical

splitting, not from intermolecular crystalline splitting.

Actually, intermolecular packing influences the IR spectrum of PP very little. In Fig. 4.42, the X-ray

patterns of the monoclinic ot phase and the smectic 6 phase are shown. The monoclinic oe phase

consists of well-ordered crystalline 31 helices, while the smectic 6 phase consists of 31 helices that are

out of register with each other. The smectic phase will convert with time to the monoclinic oe phase at a

rate that is a function of temperature. The IR spectra of these two phases are also shown in Fig. 6.1, and

the spectra are strikingly similar. Only minor differences in intensities are observed in the IR spectra, yet

from the X-ray point of view, the smectic phase is noncrystalline. The reason for these small differences

is that the helices are far apart (relative to the distance in PE), and consequently, the intermolecular

forces are much lower, and the intermolecular splitting is below the resolution of the instrument.

Unfortunately, the spectral results for PP are typical for most semicrystalline polymers.

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Figure 6.1; A comparison of the IR spectra and the X-ray diffraction patterns of the monoclinic ot phase and smectic ~ phase of isotactic polypropylene.

Reference

Antoon, M.K., Zehner, B.E., Koenig, J.L., Polym. Compos. 2 (1981) 81

Bretzlaff, R.S., Wool, R.E, Macromolecules 16 (1983) 1907

Drushel, H.V., Ellerbe, J.J., Cos, R.C., Love, L.H., Anal. Chem. 40 (1968) 370

Koenig, J. L.,Spectroscopy of Polymer 2nd Ed.(1999)147-206

Lee, Y.-L., Bretzlaff, R.S., Wool, R.E, J. Polym. Sci., Part B: Polym. Phys. 22 (1984) 681

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