Advanced GPC Part 1 – GPC and Viscometry

2

Introduction

The GPC experiment with a single concentration detector is called conventional GPC

This is by far the most common form of GPC

However there are some limitations to this technique

Recently, developments in detector technology have made viscometers more widely available

These detectors avoid some of the problems associated with conventional GPC

This presentation outlines GPC viscometry as an analysis methodology

3

Gel permeation chromatography separates polymers on the basis of size in solution

Separation occurs through the partitioning of polymer molecules into the pore structure of beads packed in a column

Re-cap - Gel Permeation Chromatography (GPC)

4

Conventional GPC

Calibrate the column by chromatographing a number of narrow standard polymers of known molecular weight, correlating MW with molecular size

For unknown samples slice the peak into components of weight Mi and height/area Ni, sum to determine molecular weight averages

5

Column separates on basis of molecular size NOT molecular weight

two different polymers will interact differently with solvent

At any molecular weight, the two polymers will have different sizes in solution

Molecular weights from conventional GPC are dependent on a comparison in size between the standards and the sample

The result – practically speaking the majority of conventional GPC experiments give the wrong results!

Viscometers get round this problem…

Limitations with Conventional GPC

6

Viscosity of Polymers

All polymers increase the viscosity of solutions by increasing the resistance to flow

Different types of polymers have differing viscosities depending on the interactions with the solvent

Viscometers are used to determine intrinsic viscosity, IV or [ŋ]

Intrinsic viscosity can be though of as the inverse of the molar density

At any given MW, a high IV means the sample is a large diffuse molecule, a small IV means a compact, dense molecule

7

Intrinsic Viscosity

8

Effect of Solvent and Temperature on Intrinsic Viscosity

Polystyrene

Solvent affects the intrinsic viscosity of polymers by altering how well solvated they are

Large changes occur in solvents of different polarities

Temperature has less of an effect

9

So Why do Viscometry? – The Universal Calibration

Ref : Grubisic, Rempp, Benoit, J. Polym. Sci., Part B, Polym. Lett., 5:753 (1967)

If a calibration of size versus retention time could be generated then one true calibration would hold for all sample types

Hydrodynamic volume = [] M

A Universal Calibration plot of log[]M versus RT holds true for all polymer types

Can use measured intrinsic viscosity and retention time to get accurate molecular weights

10

Accurate Molecular Weights

As a result of using the viscometer, a universal calibration can be set up that gives the same calibration line regardless of the type of standards employed

The chemistry of the sample is also unimportant – the column is separating on size and that is the parameter we have calibrated

Therefore the GPC/viscometer experiment will give accurate molecular weights for any samples regardless of their or the standard’s chemistry assuming that pure SEC takes place

We are still doing chromatography – the column must be calibrated

11

Comparisons of Conventional and Universal Calibrations

Conventional calibrations are offset due to differences in the molecular size of polystyrene and polyethylene

Universal calibrations account for the offset to the calibrations overlay

Discrepancy at low molecular weight is due to a conformation change in polyethylene

12

The Mark-Houwink Plot

A Mark-Houwink plot of log IV versus log M should give a straight line as long as the Universal Calibration is obeyed (i.e no interactions occur)

K and alpha vary between different solvents and polymers

Alpha is an indication of the shape of the polymer in solution

IV

13

The PL-BV 400 Series

14

Viscometer Operation

T

15

A viscometer that measures specific viscosity

A concentration detector that tells us how much material is eluting from the column

Can be any type that gives a response proportional to concentration

Typically a differential refractive index detector is used

DRI detector response proportional to concentration

Operation identical to conventional GPC, determines the concentration of material eluting from a GPC column

RIsignal = KRI (dn/dc) C

Measuring Intrinsic Viscosity - What do we need?…

16

Calibration with a series of narrow standards of known Mp and concentration

Calculate detector constant (Kvisc) using one standard for which IV is known

For the remainder of the standards, calculate [ from the viscometer response

Plot log M[ versus retention time to generate the Universal Calibration

For unknown sample, for each slice across the distribution determine [ from the viscometer, and then convert to molecular weight via the Universal Calibration curve

GPC/Viscometry Experimentation

17

Typical Chromatograms

18

Analysis

19

Columns: 2 x PLgel 5µm MIXED-C Eluent: TetrahydrofuranFlow rate: 1.0 ml/min Temperature: 40˚CDetector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer

Example chromatograms of one sample

Analysis of Poly(styrene-co-butadiene)

20

Only small differences in the MWD of the two samples

21

The Mark-Houwink plots indicate the materials are structurally similar

22

Columns: 2 x PLgel 5µm MIXED-D Eluent: TetrahydrofuranFlow rate: 1.0 ml/min Temperature: 40˚CDetector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer

Example chromatograms of one sample

Analysis of Polylactide and Poly(lactide-co-glycolide)

23

The copolymer (red) has a considerably lower molecular weight than the homopolymer (blue)

24

Structurally the co-polymer is very different to the homopolymer across the molecular weight range

25

Columns: 3 x PLgel 10µm MIXED-B Eluent: Dimethyl sulphoxide + 0.1% lithium bromideFlow rate: 1.0 ml/min Temperature: 50˚CDetector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer

Example chromatograms of one sample

Analysis of Cornflour

26

Large differences in the MWD of the two samples

27

Large differences in the Mark-Houwink plot indicate the samples are structurally dissimilar

28

Conventional GPC has limitations in that the results obtained are purely comparative

The situation can be remedied by adding a viscometer to the system

The viscometer allows calibrations of retention time as a function of molecular size to be generate

This give accurate molecular weight information regardless of the type of standards used in the analysis

The Mark-Houwink plot allows the change in density of the polymers as a function of molecular weight to be analysed

Summary