CRYSTALLIZATION KINETICS OF ISOTACTIC POLY(1-HEXADECENE)

The crystallization kinetics of the isotactic content of three poly(1-hexadecene) (PHD) samples with different tacticity has been analysed by differential scanning calorimetry (d.s.c.), using both isothermal and non-isothermal treatments. Two d.s.c. peaks are found in both the melting and cooling of the three PHDs. The high- and low-temperature peaks are attributed to the crystallites formed by the isotactic and atactic chains, respectively. Additional small-angle X-ray diffraction (SAXD) experiments, using synchrotron radiation, have been performed, indicating that the isotactic crystals exhibit appreciably smaller spacings than the atactic ones. Because of overlapping problems, only the isothermal crystallization kinetics of the isotactic content can be studied. The corresponding analysis in the range of crystallization temperatures from 36 to 45-degrees-C showed that the process can be described by the Avrami equation up to relatively high conversions. The rate constant k of this equation increases very rapidly with undercooling (about four orders of magnitude in only 8 degrees), indicating that the process is controlled by the nucleation. The value of the Avrami exponent is approximately 4, suggesting a homogeneous nucleation followed by three-dimensional growth. The melting temperature of the isotactic content is independent of the crystallization temperature and was found to be 58-degrees-C. The study of the non-isothermal crystallization of PHDs shows that it can be analysed in terms of an Avrami-type equation and similar mechanisms of nucleation and growth apply to all temperatures and cooling rates in the ranges studied. Moreover, the Avrami exponent in this analysis is of the same order as in the case of isothermal treatment.