Study On The Crystallization Behavior Of Amorphous PET Glass And Large-Scale Preordering Phenomenon In The Induction Period

Polymer crystallization is a kind of first order phase transition, a process that macromolecular system changes from disordered amorphous state into ordered crystalline state. Two main mechanisms have been proposed till now to explain this phenomenon. One is the classical Nucleation-Growth mechanism (NG) based on the concept that crystallization should first overcome the nucleation barrier in order to form nucleus with critical size, and this step can only be realized through a localized order fluctuation. And only if nucleus with critical size appearcan crystalline grow further. The second mechanism, the Spinodal-assisted crystallization mechanism, originates from experimental results of small-angel X-ray scattering method (SAXS) performed on amorphous polymer glass, that a small scattering peak appears at low angle during the induction period of crystallization and its development, follows the Spinodal decomposition rule in liquid-liquid phase separation theory. This mechanism indicates that a large-scale ordering process must happen prior to crystallization and this kind of fluctuation was later proved as an orientational fluctuation through Small-Angle Light Scattering method (SALS).The Spinodal-assisted crystallization mechanism was proposed to explain the experiment fact that cannot be interpreted by classical NG theory. According to classical NG mechanism, the localized order fluctuation is so small and cannot be detected by SAXS method, let alone at low angle. But some problem still remain unresolved in the Spinodal-asssisted crystallization mechanism, they are:1. Is the large-scale ordering phenomenon a common process that will appear at the whole temperature range, or it just happens when molecular chains have small mobility, i.e., when crystallization temperature is low enough, as reported by some researchers?2. Is the orientational fluctuation the only kind of fluctuation happens prior to crystallization process? Will the fluctuation style have temperature dependence?3. In a scale beyond the SAXS method's resolution, does the ordering process still follow the Spinodal Decomposition rule?4. Is the fluctuation process a preparatory step for the later crystallization, or is it a metastable state that happens for its fast kineticspath?This thesis aims at making a tentative study to resolve the problems mentioned above. A Time-Resolved Small-Angle Light Scattering device was used to investigate the crystallization process, especially the large-scale preordering phenomenon of amorphous PET glass annealed under various temperatures. The large-scale preordering process' kinetics character was studied in detail; its consequent structure information was analyzed by using fractal theory. And the metastability concept was introduced to interpret the experimental facts.