MECHANISMS FOR AXIAL THERMAL CONTRACTION IN POLYMER CRYSTALS - POLYETHYLENE VS ISOTACTIC POLYPROPYLENE

We show that the mechanism for the negative axial thermal expansion is fundamentally different in polyethylene (PE) and isotactic polypropyl ene (iPP) crystals. For PE, axial contraction increases the entropy, t hereby leading to negative axial thermal expansion. In contrast, for i PP, axial expansion increases the entropy, which alone would lead to p ositive axial thermal expansion. The negative axial thermal expansion in iPP occurs as an elastic response to the positive transverse therma l expansion, and is based on potential energy effects.