Study On Theory And Method Of Crystallization Kinetics Of Polymers

It is one of the main processes of the structural evolvement that crystallization of crystalline polymers during process. The aggregation structure and properties of polymers depend to large extent on their crystallization behaviour. It is of important to investigate the theory and the method of kinetics of polymer crystallization for characterization and technological process of polymer materials.The derivative of the relative degree of crystallinity with respect to crystallization time were simulated by using the Monte Carlo method under the nonisothermal condition at different scanning rates. The common nonisothermal models of polymer crystallization were verified by using the data of the simulated experiments. The results show that the Ozawa model obtained based on the Evans theory is suitable to describe the nonisothermal crystallization behavior under ideal conditions, that is , taking no account of the effect of the secondary crystallization. The Avrami exponent and the rate function F(T) can be obtained according to the Ozawa equation, but the parameter characterizing the rate of crystallization can not be estimated due to the complexity of the rate function. The Jeziorny method is not suitable to calculate the kinetic parameters of polymer crystallization. The values of the Avrami exponent obtained from the Jeziorny method are higher than real one and vary with the scanning rate. The value of the Avrami exponent would approach the ideal value adopted in the simulated experiment only if the scanning rate approaches zero. The Kissinger model is unsuitable to obtain the parameter of activation energy of crystallization from the cooling DSC curves at different cooling rates, but likely to obtain one from the heating DSC curves at different heating rates. The Kissinger method can not be considered to be an accurate method because the parameter obtained depends to some extent on heating rates.An differential equation is derived based on the classical Avrami theory. The equation can be transformed into the Ozawa form, which has a different rate function F(T). A new method that the rate parameter of crystallization can be calculated from the DSC curves at different scanning rates is proposed in terms of the new equation. The rate parameter obtained from the new method is quite well consistent with that adopted in the simulated experiments.An equation describing nonisothermal cold crystallization of polymers is proposed based on the differential equation mentioned above. The kinetic parameterscan be derived from a heating DSC curve according to the equation, including the Avrami exponent, the activation energy of crystallization, and the rate constant. The validity of the equation and the method was verified by using the Monte Carlo simulation and the crystallization of poly(ethylene terephthalate)(PET).The cold crystallization of uniaxially oriented PET fibers was investigated. The preferred condition for preparation of uniaxially oriented amorphous PET fibers was explored in terms of the draw ratio of as-spun PET fibers at different temperatures. The multi-peaks of crystallization of PET fibers appear in the heating DSC curves when the as-spun fibers are drawn at draw ratios of 2 to 3 and at temperature of 40 ℃. The parameters of crystallization kinetics of PET fibers, such as the Avrami exponent and activation energy of crystallization, were derived and then the rate constant of crystallization was estimated. The dependence of the Avrami exponent on the nucleation mode and the growth geometry of the entities was established by assuming that diffusion of polymer segments predominate over crystallization rate in DSC heating process. The structural evolvement model of uniaxially oriented non-crystalline PET fibers was proposed in terms of the change of Avrami exponent, peak position, and peak area at different draw ratios.The isothermal crystallization of polymers in a cylindrically confined volume was simulated by using the Monte Carlo method. The crystallization in the given system was confined in one-dimension and two-dimensions and the new turning point, named as the primary turning point, was found. The further investigation indicates that the primary turning point is one that the entities grow from three-dimensional geometry to low-dimensional one. A mathematical model of crystallization in a confined volume was proposed. The validity of the model was verified by using PEO experiment, suggesting that the model can predict the emerging condition of the primary turning point.