Crystallization Behavior and Properties of Polyolefins

An overview of thermodynamic properties and crystallization kinetics of linear chains and random copolymers based on polypropylenes is given. The details of the discrete isothermal thickening of monodisperse low molecular weight species are discussed in relevance to the interpretation of the crystallization kinetics. Emphasis is given to the need to distinguish the crystallites formed from the initial melt from those transformed from them during thickening. When this distinction is made the nucleation process of either extended or folded crystallites is found to have the same temperature coefficient.The effect of adding structural irregularities to the chain on the crystallization kinetics and lamellar morphology of high molecular weight species, is discussed by analyzing the behavior of well characterized metallocene isotactic polypropylenes. The analysis of the growth rates indicates the general behavior observed for random copolymers. However, the break found in the analysis of the kinetics according to classical nucleation theory is primarily related to a unique morphological change and may not be specific of a II – III regime transition. A dynamic morphological change due to the progressive development of the crosshatched morphology with time has been investigated as a function of crystallization temperature, crystallization time and concentration of defects.