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Advanced GPC Part 1 – GPC and Viscometry

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

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Advanced GPC Part 1 – GPC and Viscometry

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  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. IV 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

  13. The PL-BV 400 Series

  14. Viscometer Operation T

  15. 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

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