Kinetics of Crystallization Mechanisms in High Density Polyethylene and Isotactic Polypropylene

This paper reports a detailed kinetic investigation on crystallization mechanisms occurring in two familiar industrial polyolefins, high-density polyethylene (HDPE) and isotactic polypropylene (iPP), under isothermal as well as non-isothermal conditions. Kinetic analysis by advanced kinetic approach to polymer crystallization processes suggests that the crystallization of HDPE and iPP goes to completion by following fairly invariable nucleation/diffusion phenomena. Nevertheless, crystallization mechanisms of both HDPE and iPP show dependence on mode of experiment. Under non-isothermal conditions, HDPE follows a crystallization mechanism intermediate between random nucleation followed by isotropic one-dimensional (1D) growth of spherulites and two-dimensional (2D) growth of spherulites. Under isothermal conditions, it predominantly follows 2D growth of spherulites. On the other hand, while iPP follows a crystallization mechanism analogous to HDPE under non-isothermal conditions, it pursues three-(3D) dimensional growth of spherulites under isothermal conditions. In general, pre-exponential factor remains independent of temperature in all the crystallization cases studied. Moreover, polymeric chains of HDPE demonstrate relatively slower transport rate when compared with iPP. The obtained kinetic parameters are interpreted in terms of their probable physical meanings and practical worth of the present study is taken into account and discussed.