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 b y differential scanning calorimetry (d.s.c.), using both isothermal an d non-isothermal treatments. Two d.s.c. peaks are found in both the me lting and cooling of the three PHDs. The high- and low-temperature pea ks are attributed to the crystallites formed by the isotactic and atac tic chains, respectively. Additional small-angle X-ray diffraction (SA XD) experiments, using synchrotron radiation, have been performed, ind icating that the isotactic crystals exhibit appreciably smaller spacin gs than the atactic ones. Because of overlapping problems, only the is othermal crystallization kinetics of the isotactic content can be stud ied. The corresponding analysis in the range of crystallization temper atures from 36 to 45-degrees-C showed that the process can be describe d by the Avrami equation up to relatively high conversions. The rate c onstant k of this equation increases very rapidly with undercooling (a bout four orders of magnitude in only 8 degrees), indicating that the process is controlled by the nucleation. The value of the Avrami expon ent is approximately 4, suggesting a homogeneous nucleation followed b y three-dimensional growth. The melting temperature of the isotactic c ontent is independent of the crystallization temperature and was found to be 58-degrees-C. The study of the non-isothermal crystallization o f PHDs shows that it can be analysed in terms of an Avrami-type equati on and similar mechanisms of nucleation and growth apply to all temper atures and cooling rates in the ranges studied. Moreover, the Avrami e xponent in this analysis is of the same order as in the case of isothe rmal treatment.