FREE-VOLUME PROPERTIES OF STYRENE-MALEIC ANHYDRIDE AND STYRENE-ACRYLONITRILE COPOLYMERS

Positron annihilation lifetime measurements were performed on amorphou s styrene-maleic anhydride and styrene-acrylonitrile copolymers of dif ferent compositions. Glass transition temperatures of the copolymers w ere determined by differential scanning calorimetry and densities and specific volumes were obtained with a gas pycnometer. The average free -volume hole size measured by the positron lifetime technique was almo st constant with composition in styrene-acrylonitrile, whereas in styr ene-maleic anhydride it first increased up to 28% maleic anhydride con tent and then decreased at higher concentrations. This is caused by di fferences in copolymer constitution, which changes from random to alte rnating with increasing concentration of maleic anhydride, leading to differences in the packing of the polymer chains. The results also sho w a clear decrease in the orthopositronium yield (I-3) with increasing maleic anhydride or acrylonitrile content of the copolymer, which is interpreted as inhibition of positronium formation by these components . An equation previously used to describe the inhibiting effect of sca vengers in non-polar solvents and of halogenated compounds added to po lycarbonate could be fitted to the experimental values. (C) 1998 Elsev ier Science Ltd. All rights reserved.