The excess enthalpy of polymeric glasses was determined by the use of a differential scanning calorimeter (DSC). The obtained values of excess enthalpy were used for analyzing the enthalpy relaxation process. From the relationship between enthalpy relaxation time and excess enthalpy, the relaxation times at zero (τ
0) and infinite (τ
∞) annealing time were obtained. At glass transition temperature (
Tg), the activation energy calculated from τ
∞ showed a good agreement of that from WLF equation. In order to compare the rate of enthalpy relaxation, the relaxation time at half of initial excess enthalpy (τ
1/2) was employed. The effects of primary and secondary structures on the enthalpy relaxation process of polymeric glasses were analyzed quantitatively. It was concluded that the enthalpy relaxation in polymeric glasses proceeded as a result of a cooperative segmental motion of a few or less than 10 repeating units. The effect of enthalpy relaxation on the physical properties of polymeric glasses were estimated. The mechanical properties of polymeric glasses, such as Young's modulus, tensile strength and the deformation form were found to be affected by the molecular mobility and the deformation rate. It was found that the diffusion process was affected by the enthalpy relaxation of polymeric glasses, due to the control of molecular mobility and the stability of glassy structure which occurred with enthalpy relaxation.
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