Viscoelastic Behavior of Semicrystalline Thermoplastic Polymers during the Early Stages of Crystallization

Abstract

Using rheological techniques, we investigate the evolution of the microstructure evolution during the early stages of quiescent crystallization of poly(1-butene). In performing the measurements, use is made of an innovative experimental protocol, called inverse quenching, which allows stopping the crystallization process and producing a stable biphasic (crystalline/amorphous) system. In this way, very low frequency measurements at fixed degrees of crystallization are made possible. We find that crystallization, evidenced as a liquid-to-solid transition (LST) under isothermal conditions, with characteristics of critical gel behavior, takes place at surprisingly low degrees of crystallinity (below 1.5%). The critical gel properties, which are found to depend on both crystallization temperature and molecular weight, can be reduced to a single master curve when the gel strength is plotted as a function of the relaxation exponent. More importantly, the LST is preceded by the development of a long relaxation process. This latter process, although not fully understood, brings analogies to the slow dynamics observed in hybrid colloid-polymer systems (block copolymer micelles or multiarm star polymers) as well as the recently suggested presence of dormant nuclei. It is clear, however, that the connectivity among crystallites, apparently via the amorphous segments, plays a key role in this new process.