1. 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. Re-cap - Gel Permeation Chromatography (GPC)
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

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. Limitations with Conventional GPC
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…

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

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 IV
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

13. The PL-BV 400 Series

14. Viscometer Operation T

15. RIsignal = KRI (dn/dc) C
Measuring Intrinsic Viscosity - What do we need?…
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

16. GPC/Viscometry Experimentation
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 [h] from the viscometer response
Plot log M[h] versus retention time to generate the Universal Calibration
For unknown sample, for each slice across the distribution determine [h] from the viscometer, and then convert to molecular weight via the Universal Calibration curve

17. Typical Chromatograms

18. Analysis

19. Analysis of Poly(styrene-co-butadiene)
Columns: 2 x PLgel 5µm MIXED-C Eluent: Tetrahydrofuran
Flow rate: 1.0 ml/min Temperature: 40˚C
Detector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer
Example chromatograms of one sample

20. Only small differences in the MWD of the two samples

21. The Mark-Houwink plots indicate the materials are structurally similar

22. Analysis of Polylactide and Poly(lactide-co-glycolide)
Columns: 2 x PLgel 5µm MIXED-D Eluent: Tetrahydrofuran
Flow rate: 1.0 ml/min Temperature: 40˚C
Detector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer
Example chromatograms of one sample

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. Analysis of Cornflour
Columns: 3 x PLgel 10µm MIXED-B Eluent: Dimethyl sulphoxide + 0.1% lithium bromide
Flow rate: 1.0 ml/min Temperature: 50˚C
Detector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer
Example chromatograms of one sample

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. Summary
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