Advanced GPC Part 1 GPC and Viscometry Introduction
- Slides: 28
Advanced GPC Part 1 – GPC and Viscometry
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 2
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 3
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 4
Limitations with Conventional GPC § Column separates on basis of molecular size NOT molecular weight § two different polymers differently with solvent will interact § 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… 5
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 6
Intrinsic Viscosity 7
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 8 § Temperature has less of an effect
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) 9
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 10
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 polyethylene 11 in
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 12
The PL-BV 400 Series 13
Viscometer Operation T 14
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 material eluting from a GPC column RIsignal = KRI (dn/dc) C 15 of
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 [ ] 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 16 the viscometer, and then convert to molecular weight via the Universal Calibration curve
Typical Chromatograms 17
Analysis 18
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 400 RT viscometer § Example chromatograms of one sample 19
§ Only small differences in the MWD of the two samples 20
§ The Mark-Houwink plots indicate the materials are structurally similar 21
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 400 RT viscometer § Example chromatograms of one sample 22
§ The copolymer (red) has a considerably lower molecular weight than the homopolymer (blue) 23
§ Structurally the co-polymer is very different to the homopolymer across the molecular weight range 24
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 400 RT viscometer § Example chromatograms of one sample 25
§ Large differences in the MWD of the two samples 26
§ Large differences in the Mark-Houwink plot indicate the samples are structurally dissimilar 27
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 28
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