Crystallization and melting studies of branched isotactic polypropylenes

Crystallization, melting and structural studies were conducted on iostactic polypropylenes treated with varying dosages of electron beam radiation and an untreated iPP. Through FTIR methods, all specimens were found to be greater than 99% isotactic. Crystallization and melting studies were performed using light depolarizing microscopy (LDM) and other melting experiments were conducted using differential scanning calorimetry (DSC). Structural studies were conducted by use of a wide-angle x-ray diffractometer (WAXD). Through isothermal crystallization studies it was found that at the highest supercoolings all specimens had approximately the same half-time of crystallization values, t_½, attributed to increased nucleation by increased supercooling. At higher temperatures of crystallization, T_c, it was observed that t_½ varied for the specimens. This was attributed to the effects of branching on primary nucleation and to the size of the spherulites. All specimens were observed to nucleate in the heterogeneous mode, meaning that nuclei density stayed constant throughout the isothermal crystallization process. Average spherulite growth geometry (Avrami) exponent, n, values were in the range of 2.2 and 2.5. These low values were a consequence of the amount of branching and stereoregularity of the polymer chains and secondary crystallization. The spherulite growth rates, k, for all the samples decreased with decreasing supercooling, resulting from the decrease in the number of nuclei forming into spherulites. Through x-ray studies the predominant crystal form was found to be of the α modification, with some β and γ modifications observed. No structural changes at the crystal lattice level were detected. The degree of crystallization was seen to decrease as a result of increased branching in the treated specimens and attributed to thermal degradation in the untreated one. From the DSC endotherms small melting peaks in the range of 140 °C to approximately 145 °C was noticed in some of the specimens and attributed to the β modification as a consequence of nucleating agent(s) and stresses induced during sample film preparations. The equilibrium melting points taken from the highest peak and the return to baseline of the endothermic curves showed that the treated samples had lower points than the untreated one. This was due to branching and degradation from the irradiation process. The melting ranges of the treated specimens were shifted to lower values as compared to the untreated specimen, as a consequence of branching and degradation The temperature ranges for the irradiated specimens were broader than the melt range of the untreated sample The α peak also showed broadening as a result of branching.