BIMODAL DISTRIBUTION OF RELAXATIONAL MODES FOR THE HELIX COIL TRANSITION IN POLY(OXYMETHYLENE)

The helix-coil transition in poly(oxymethylene) has been investigated by using the recently developed dynamic rotational isomeric state (DRIS) model and the related matrix multiplication technique. The conditional probability P-tau-(h/h) that a chain originally in the perfect helical form will preserve its state unchanged at a later time tau is evaluated for various chain lengths and temperatures.The transition to the coiled structure in an originally fully ordered helix is found to exhibit a bimodal relaxation, indicated by a plateau in the time decay curves of the conditional probabilities, before restoration of equilibrium distribution. This intermediate quasi-steady state preceding full relaxation is attributed to an equilibrium between perfect helices and chains possessing a single defect in the form of a trans bond, interrupting sequences of gauche bonds of the same sign.