Nucleation density and growth rate of polypropylene measured by calorimetric experiments
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Nucleation density and growth rate of polypropylene measured by calorimetric experiments Felice De Santis • Roberto Pantani
Received: 14 February 2012 / Accepted: 24 September 2012 / Published online: 13 October 2012 Ó Akade´miai Kiado´, Budapest, Hungary 2012
Abstract A novel experimental protocol was adopted, based on calorimetric measurements, but varying each time the minimum temperature before the isothermal crystallization step. This method allows to measure nucleation density and growth rate for commercial semicrystalline polymers, for which the nucleation phenomenon is essentially heterogeneous. The protocol was applied to a commercial grade isotactic polypropylene, well characterized in the literature. Crystallization kinetics was evaluated using the Avrami model, through nucleation and growth, describing all experimental results. Morphological characterizations were modeled using the classical Lauritzen– Hoffman theory. Detailed comprehensive description of the crystallization kinetics of i-PP is provided, predicting nucleation and growth with the tailored thermal history. Keywords Crystallization kinetics Nucleation and growth Differential thermal analysis (DTA) Polypropylene
Introduction The crystallization is a key process affecting the physical properties of polymers. The crystallization behavior of polyolefins has been extensively investigated in recent decades. Generally, the physical properties of thermoplastic polymers such as thermal and mechanical properties are considerably influenced by the crystalline structure and
F. De Santis (&) R. Pantani Department of Industrial Engineering, University of Salerno, Via Ponte don Melillo, 84084 Fisciano, SA, Italy e-mail: [email protected]
morphology that can be manipulated by changing the crystallization conditions. Isotactic polypropylene (i-PP) has become a commodity polymer with numerous grades for specific end uses. At present, the physical properties of polypropylene should be tailored to the requirements with respect to processing and structure. This material has attracted much attention from both academic and practical viewpoints in the last decades, since the invention of isotactic polypropylene (1954), which was rewarded with a Nobel prize to Natta [1, 2] and Ziegler in 1963. Numerous investigations on the crystallization kinetics of i-PP have been carried out, and most of them are based on works by Kolmogorov and Avrami [3–7], i.e., theoretical treatments of processes occurring by nucleation and growth. Standard analysis of experimental studies, mainly by means of calorimetry, on the crystallization kinetics have been performed [8–10]. Calorimetry is a lumped analysis, so that it is commonly considered that this technique cannot provide information concerning the morphology of the material. However, several research groups have designed experimental protocols to evidence ‘‘self nucleation’’ in crystallizing polymers, and these methods have been named differently [11]. In particular, Prof. Muller and co-workers adopted thermal fracti
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