Dynamics of a-Relaxation and Structure Formation during Crystallization Process of Polymers:

We made time-resolved simultaneous measurements of dielectric relaxation and small­ angle X-ray scattering (SAXS) on poly (ethylene terephthalate) (PET) in order to investigate the relationship between the a-relaxation and the structure formation during the crystalliza­ tion process. We found that the a-relaxation changes to the a'-relaxation via a coexistent state of the two relaxations. The change in dynamics of a-relaxation occurs before the for­ mation of higher-ordered structure observed by SAXS. The results suggest the existence of a precursor phenomenon in polymer crystallization, which is related to the dynamics in the amorphous phase. Extensive researches have been made to elucidate the nature of dynamics in amorphous materials. l) The a-relaxation is the most prominent process of dynamics in amorphous materials, which is observed above the glass transition temperature. It is believed that the freezing of the a-relaxation is directly related to the glass tran­ sition. In crystalline polymers, when an amorphous sample obtained by quenching the samples after melting is heated above the glass transition temperature T 9 , the a­ relaxation appears and crystallization begins. 2 ) In this case·, the mobility of polymer chains which is observed as the a-relaxation is enhanced to induce the crystallization of amorphous samples, and the dynamics of the a-relaxation can also be changed by the appearance of crystalline phase. It is clear that there is a close relationship between the a-relaxation and the structure formation in crystalline polymers. In this paper, we made time-resolved simultaneous measurements of dielectric relaxation and small-angle X-ray scattering to investigate the time evolution of a­ relaxation and structural formation during the isothermal crystallization process. §2. Experimental The sample used in this study is poly (ethylene terephthalate) (RT580, Mn = 5.9 x 10 4 , Mw/Mn=2.51, kindly supplied by Toyobo Co., Ltd.). Amorphous films of PET (about O.lmm thick) are obtained by quenching into ice-water after melting the samples at c.a. 290°C for several minutes under an oil press. Gold is evaporated as electrodes onto the film surfaces, except the center of the films where an incident X-ray passes. In a measuring cell, the film is heated from room temperature to a crystallization temperature (Tc=97.5°C) above the glass transition temperature