Crystallization kinetics of PETand PET/polylefin blends

Abstract

A new crystallization kinetic equation was derived considering the decrease in growth rate during the crystallization process. The crystallization kinetics of neat ploy(ethylene terephthalate) (PET), PET with different shear history, and PET/Polyolefin blends were studied by the dynamic and isothermal differential scanning calorimetry (DSC). The kinetic data were analyzed using the newly derived kinetic equation. The average linear growth rate of spherulite was assumed to be proportional to the m-th order of the uncrystallized fraction of the crystallizing material. A modified Avrami equation, $1-V_c=exp[-Kf(t)^n]$, was obtained where f(t) is the integral of the growth function, $(1-V_c)^m$. This kinetic equation could interpret both the free (m=0) and restricted (m ＞ 0) growth of spherulites. The asymmetry and tailing in the DSC exotherms under isothermal crystallization was described well by the restricted growth. The equation suggested that the growth rate decrease caused decrease of the Avrami exponent in the conventional Avrami analysis though the dimensionality of the growth did not change. In the crystallization kinetics of neat PET analyzed by DSC and fitted by the newly derived equation, the Avrami exponent, n, decreased in the range of 2.9-3.5 with increasing crystallization temperature when crystallized from the glassy state. The Avrami exponent, n, was nearly constant at 3 when crystallized from the melt. The growth function parameter, m, was around 0.33 implying the dependence of growth rate decrease on the average linear distance of growing fronts. The crystallization process was considered to be the combined process of crystallization under thermal nucleation (rate constant $K_t$) and athermal nucleation (rate constant $K_a$). By evaluating the temperature effect on $K_a$, it was observed that the maximum growth rate was attained at 180℃. A new dimensionless parameter, $R=K_a/(K_t \cdot t_{1/2})$, was suggested to account for the relative c...