| Variations of stress with temperature for stretched networks of trans-polyisoprene networks in the crystallization region are reported. Such profiles are characteristically U-shaped with the stress at lower temperatures often climbing to high levels. Valley widths of the profiles narrow as annealing temperature increases up to about 50;Studies of the crystallization kinetics of stretched TPI networks (in simple extensions) are also given. A detailed mechanism of crystallization of networks under stress is proposed. Finally, the thermal expansion coefficient of oriented TPI networks, crystallized at constant length and constant load, are investigated. Upon melting, oriented TPI networks first display a slight increase in length (i.e., a positive thermal expansion), then decrease. A new theory of stress-induced crystallization seems to provide a semiquantitative application of crystallization in oriented system without resort to morphological phase transitions. |
