Impact of Chain Length on PCL Dielectric β-Relaxation: Decoupling the Roles of Chain Ends, Main Chains, Entanglements, and Crystallinity

The structure of semicrystalline polymers is quite complex due to the presence of interconnected amorphous and crystalline regions. Extended or folded chain crystals, along with entanglements and tie molecules, are formed depending on the chain length. The material’s structure determines its final performance, so understanding the impact of chain length is essential. Broadband dielectric spectroscopy is highly sensitive to the molecular dynamics of polar polymers. In this work, a systematic study of the impact of chain length on PCL’s dielectric relaxation is conducted, decoupling for the first time the role of chain length, chain ends, entanglements, and crystallinity on the β relaxation. PCL samples have been analyzed within a number-average molar mass range of 2–173 kg/mol. The results are modeled by considering the contributions of end groups, the main chain segments, and the degree of crystallinity, which effectively describes the experiments. As the chain length increases, the contribution of the amorphous main chain is altered due to entanglements that may affect the chain dynamics. Our results suggest a strong correlation between changes in the local dynamics of polymer segments and the presence of a rigid amorphous fraction in semicrystalline polymers. Overall, this study contributes to the understanding of the microstructure of semicrystalline polymers, which can be applied to other polymers. The work offers tools for utilizing broadband dielectric spectroscopy to isolate complex chain parameters.