Study On Relation Between Enthalpy Relaxation And Structure Of Glassy Polymer

Material property reflects molecular movement from a macroscopic view. The study on the molecular movement is the basis of the relation between structure and property of the polymer. Molecular movement shows obvious relaxation characteristics, so the structure and properties of polymer also show relaxation characteristics. Structural relaxation of polymers around the glass transition has great effect on mechanical properties, the problem of glass transition has not been resolved until now, so it is significant for the glass transition to study enthalpy relaxation kinetic of polymer. Enthalpy relaxation is the mostly used method to study relaxation of polymer. It is an important content of this article to explore the relation between enthalpy relaxation and structure of polymer.First of all, a free volume expression, Free-Volume(FV) equation, that describing the relaxation time is formulated based on the free volume theory of glass transition using Adam-Gibbs concepts. Polystrene was studied using Vogel-Fulcher-Tammann(VFT), Macedo, Adam-Gibbs-Vogel(AGV), Tools-Narayanaswamy-Moynihan(TNM) model and the new model. The result shows that the variance of the FV model decreased significantly compared with that of the VFT model, and is little difference than that of the AGV and TNM models. Therefore, it is a new way to the study of enthalpy relaxation kinetic based on the free volume theory of glass transition. On the basis, when the model of equilibrium state was modified with assuming the existence of an intermediate aging plateau, the fitting result is improved with the variance reducing to some extent.On the basis, this thesis discusses the influences of different molecular weight and molecular weight distribution on kinetics parameters of different relaxation time models. For the polystyrene(PS) samples with narrow molecular weight distributions, the experimental heat capacity data are fitted using the AGV, TNM, Macedo, FV model and the models modified with assuming the existence of an intermediate aging plateau. The obtained kinetics parameters of different relaxation time models with different molecular weight are compared and analyzed. The kinetics parameters of the FV model are dependent on molecular weight and are similar with those of other relaxation time models. When the model of equilibrium state was modified with assuming the existence of an intermediate aging plateau, the kinetics parameters of different relaxation time models with different molecular weight are also dependent on molecular weight.Finally we try to use the enthalpy relaxation kinetic to investigate the variation of mechanical properties of the materials with aging time during physical aging. The mechanical properties of polycarbonate under different treatments were tested. The limit fictive temperature of polycarbonate under different treatments was obtained using relaxation time models(FV, AGV and TNM models). After the results of mechanical properties testing were matched with limit fictive temperatures predicted with relaxation time models, the mechanical properties of materials were described by limit fictive temperature. It shows that enthalpy relaxation kinetic could predict the change of mechanical properties of materials with different environmental conditions.