Effects of branching characteristics and copolymer composition distribution on non-isothermal crystallization kineics of metallocene LLDPEs

The effects of branch content (BC) and copolymer composition distribution (CCD) on the non-isothermal crystallization kinetics of metallocene rn-LLDPEs were studied using modified Avrami analysis, modulated differential scanning calorimetry (MDSC), and Crystaf. Several m-LLDPEs and an m-HDPE - all having comparable M-w and PDI - were experimented. In addition, a ZN-LLDPE was used for comparison purposes. The branch content, unlike the used cooling rates (2-6 degrees C/min), significantly affected the crystallization behavior. Crystallization peak temperature, T-c(peak), decreased linearly with increasing BC. All the m-LLDPEs showed primary and secondary crystallizations. The secondary crystallization showed to be more pronounced at high BC. The primary crystallization Avrarmi parameter n for m-HDPE ranged between 3.72 and 4.50, indicating spherulitic crystal growth whereas that for the rn-LLDPEs, varied from 2.02 to 5.70. The ZN-LLDPE (having broader composition distribution) offered higher values of T-c(onset) and T-c(peak) than the m-LLDPEs with similar BC, M-w, and PDI. It, unlike the m-LLDPEs and m-HDPE, fairly agreed with the crystallization kinetic model proposed by Liu et al. The lamella thickness of the m-LLDPEs, L, calculated as per Gibbs-Thomson equation, showed to be in the range 2-16 nm, depending on BC and it decreased approximately following the relationship: L (nm) = 15.0 e((-0.0498BC)). (c) 2006 Elsevier Ltd. All rights reserved.