the thermal properties , such as heat distortion temperature of biodegradable polymers , such as polylactic acid , polyglycolides , polybutyric acid and copolymers of butyric and valeric acid , may be increased by blending them with a polymer having tg from 75 ° c . to 200 ° c . and a hildebrand parameter of not more than 3 mpa 1 / 2 different from the biopolymer .