MASS-SPECTROMETRIC INVERSE GAS-CHROMATOGRAPHY - INVESTIGATION OF POLYMERIC PHASE-TRANSITIONS

An improved inverse gas chromatographic method involving the use of a mass-specific detector for the determination of the glass transition t emperature of polymeric materials is described, The new method allows the use of several probe solutes simultaneously with an automated, clo sed-loop injector and stepped temperature programming, The result is a single continuous chromatogram for each probe solute over a range of temperatures encompassing the glass transition temperature, T-g. Sever al different methods for the exact determination of T-g from the chrom atogram were investigated, including the classical van't Hoff-type plo ts with retention volumes calculated from both the peak maximum and fi rst moment values of the elution peaks, Two new methods are also propo sed for the evaluation of T-g from either the temperature dependence o f the second moments of the elution peaks for probe solutes or simple inspection of the variation of elution peak height (width) with temper ature, All four methods for the determination of T-g are evaluated wit h three probe solutes and four different polymers, viz., poly(methyl m ethacrylate), poly(ethylene terephthalate), polycarbonate, and two bat ches of polystyrene with different molecular weights and T-g values, T hree phenomenological models were used to interpret the chromatographi c retention mechanisms of the solute probes in glassy and rubbery poly mers. These are (i) the classical adsorption/absorption model for glas s and rubber polymers, (ii) the single absorption mechanism model, and (iii) a dual-mode model previously used to explain the sorption of ga ses, such as CO2, in glassy polymers. It is concluded that no single a pproach is adequate to interpret the experimental results for all of t he systems, although each model is adequate for some individual solute /polymer combinations.