Polymer studies via gel permeation chromatographic analysis

Abstract

Studies involving polymer degradation have been of significant interest to industry. Although much work had been conducted in polymer degradation, it was impossible to study molecular weight distributions of degraded samples with any degree of accuracy. The development of gel permeation chromatography (gpc) has made a great contribution to the investigation of molecular weight distribution in polymer systems. The degradation of narrow-range molecular weight samples of polystyrene has been carried out by ultrasonic radiation and by benzoyl peroxide decomposition. The changes in molecular weight distribution during the course of degradation have been followed as a function of time. For ultrasonic degradation, the change in molecular distribution clearly showed a preferential breaking of the polymer molecule near the center. For the peroxide degradation, the distribution curves were those which resulted from random cleavage along the polymer chains. Under the conditions used, about 6 X 10^3 peroxide molecules decomposed for every polymer bond broken. A correlation of these results with current theories on ultrasonic degradation has been considered. A simple model was proposed which seemed to account quite well for calculating the change in molecular weight distribution by ultrasonics and by peroxide degradation. In order to study the molecular weight distribution of a polymer using gpc analysis, a suitable calibration curve must be developed. A conventional polystyrene calibration curve does not work for all polymer systems. To develop an effective calibration curve for poly(vinyl acetate), two methods of preparing samples were used. First, relatively high molecular weight samples of poly(vinyl acetate) were prepared by careful polymerization of the monomer in the presence of known quantities of benzoyl peroxide to low conversion. The second method involved polymerization of the monomer under conditions which are known to produce branching in the polymer. High conversion samples produced in this manner were hydrolyzed in base and reacetylated. Partial fractionation of these samples provided low molecular weight poly(vinyl acetate). The number-average molecular weights of these samples were determined independently by osmometry. Agreement between molecular weight parameters determined by osmometry usually agreed within +- 10% of the number-average molecular weights as calculated from the polystyrene calibration curve. The close agreement between these values indicated that a reasonably good calibration curve had been arrived at.