Relation between Glass Transition Temperature and Composition of Ethylene Propylene Copolymers

Abstract

Abstract Vulcanizate properties and processability of ethylene propylene copolymers (EPR) are known to be sensitive to variations in polymer microstructure. A basic understanding of the practical performance of EPR requires a knowledge of this microstructure and means for evaluating it. This study was concerned with determining the utility of a previously proposed method for quantitative analysis of microstructure in EPR. The influence of crystallinity effects on the glass transition temperature (Tg) of this polymer was also examined. A major objective of this work was to determine whether, as suggested by literature data, a minimum can exist in the relation between Tg and composition of EPR. If so, it would seriously impair the proposed method for quantitative microstructure analysis. Data relating Tg and composition were obtained by differential thermal analysis (DTA) and infrared analysis (IR) of EPR fractions obtained by gradient elution fractionation. An apparent minimum was observed in the curve defined by this data. Analysis of this minimum suggests that it is due to nonrandom microstructure. In the samples examined, it was evident in the 25 to 40 mole per cent propylene range. Low temperature annealing studies indicate that the minimum is not from crystallinity although there is some indication that small amounts of crystallinity can be induced in these systems by annealing. Analysis of the main Tg interval in a variety of EPR samples showed that three types can be distinguished. These appear to correspond to random, stereoblock, and random-block polymers. Further, a main Tg near −58° C. was observed over the 20 to 74 mole per cent propylene range in some nonrandom systems. Because of this, it is concluded that the proposed system for quantitative microstructure analysis in EPR is generally inapplicable to nonrandom systems. However, the data indicate that useful information about EPR microstructure can be obtained by the techniques described.