The effects of comonomer content and tacticity on the nonisothermal crystallization kinetics of polypropylene

The quiescent non isothermal crystallization kinetics of six polypropylene resins varying in tacticity and ethylene content were investigated by light depolarizing microscopy (LDM) and differential scanning calorimetry (DSC). Crystallization behavior was defined for cooling rates in the range of 5 - 4000°C/min. The overall transformation kinetics were found to be strongly dependent on three parameters: tacticity, comonomer content, and sample purity. In general, the homopolymers crystallized faster than the copolymers. The homopolymers and copolymers alike showed increased crystallization rates with increasing isotactic content. Furthermore, the effect of ethylene comonomer functioned to decrease the crystallization rate. Nucleating impurities existed in one homopolymer and two copolymer samples, causing significant differences in nucleation densities between these and the other resins. A purification process was introduced to remove these impurities; the resultant nucleation density values of all samples then fell in the same range. This purification procedure affected no changes in the observed crystal growth geometry or growth rates. The growth rate data were analyzed in terms of the established regime theory and were found to follow the Hoffman-Lauritzen equation originally developed for isothermal crystallization. Here, the separation of the growth curves was again attributed to the rather independent influence of tacticity and comonomer content. The effect of these parameters on melting behavior and degree of crystallinity was also analyzed. Bulk crystallization data were analyzed according to Avrami theory and a comparison between DSC and LDM results was made. The Avrami exponent was found to be fairly independent of technique. Some discrepancies between the techniques were observed, however, in the form of unexpected crystallization rate trends given by preliminary DSC experiments. These trends were seen to follow those defined by LDM once the nucleating impurities were removed; thus, the initial discrepancies were attributed to a lesser degree of sensitivity of the LDM technique due to the significantly higher cooling rates employed.