Impact of Long-Chain Branching on Polypropylene Nucleation and Crystallization over a Wide Temperature Range without the Influence of Shear

Abstract

Even though the crystallization of long-chain branched polypropylene (LCB-PP) has been widely studied, the role of long-chain branches within isotactic polypropylene (iPP) under shear-free conditions and a comprehensive understanding of its isothermal crystallization kinetics over a wide crystallization temperature (T c ) range are still lacking. The samples in this work were prepared by solution casting to eliminate any possible shear effects. Then, the impact of LCB on the crystallization behavior of iPP with varying content of long-chain branches (LCB) was investigated in the entire crystallization window range. Polarized light optical microscopy (PLOM) and scanning electron microscopy (SEM) revealed that the iPP nucleation density increased with branching content. Standard differential scanning calorimetry (DSC) and ultrafast scanning chip calorimetry (or FSC, fast scanning calorimetry) showed that the overall crystallization rates had different dependence on the branching content across a wide temperature range (5−136 °C). Above 95 °C, only the αphase of iPP formed, with an increasing crystallization rate with branching content. In addition to the typical heterogeneous nucleation, the LCBs provoke a significant increase in the nucleation density through a homogeneous nucleation process triggered by the topological constraints of the segments linked to the branch point. These constraints stabilize the PP chain's helical structure and facilitate the self-assembly of these neighboring chains into primary nuclei. Between 45 and 95 °C, the crystallization rates with varying branching content did not show a specific trend, as there was a competition between primary nucleation and diffusion. Below 45 °C, where supercooling was high and homogeneous nucleation became dominant, the crystallization rates were similar for LCB-PP and neat linear iPP. This work deepens the understanding of the role of LCB in iPP crystallization, providing critical insights into both the nucleation mechanisms and crystallization kinetics of polymers over a broader T c range.