ABSTRACT
Two crystal modifications of PBT are known [10, 17] - the reverse transition of a-form into 13-form under external stress and the transition of 13-form
into a-form after its removal [ 18-21]. At the same time the authors revealed a change in a/13 ratio during stretching of PBT samples, a-crystallites being more
perfect compared to 13-form. It is suggested [21] that the stress-induced transition of a crystalline form into 13 crystalline form in PBT is accompanied mainly by
conformational changes of the terephthaloyl part, whereas the tetramethylene part
of the polymer in a and 13 crystalline forms exists only in zigzag trans-
conformation. However, other invesigations [23-27] suggest that in crystalline
areas of non-stressed a-form the conformation of methylene groups does not
match the fully extended zigzag (trans-) conformation. This fact is supported by the value of identity period along the fiber axis that is equal to ~ 11.6 A, which is much lower than the identity period for a fully extended chain at applied external stress (127 A). Therefore, one could say that the changes in conformation of methylene groups are connected with the phase transitions in crystalline regions.
