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

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

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

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…

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

Intrinsic Viscosity

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

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)

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

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

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

The PL-BV 400 Series

Viscometer Operation T

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

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

Typical Chromatograms

Analysis

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

Only small differences in the MWD of the two samples

The Mark-Houwink plots indicate the materials are structurally similar

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

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

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

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

Large differences in the MWD of the two samples

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

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