Effect of Side-Chain gem-dialkyl Substitution on the Crystallization of poly(δ-valerolactone): Competition between Steric Effects and Chain Flexibility

The performance of renewable and biodegradable polyesters can be modified by introducing side-chain alkyl substituents into their repeating units. This work examines the impact of gem-alkyl disubstitution (two alkyl substituents on the same carbon) in poly(δ-valerolactone) (PVL) on its thermal properties and crystallization kinetics for the first time. The introduction of alkyl side-chain substituents first increases glass (T_g) and melting (T_m) transition temperatures until a specific number of carbon atoms in the side chain is introduced, after which these first-order transitions decrease. Isothermal crystallization experiments indicate that the spherulitic growth rate is governed by secondary nucleation or chain diffusion, depending on the alkyl chain length. Melt memory has also been assessed, revealing a maximum in the temperature region for self-nuclei persistence at intermediate alkyl chain lengths, as shown for T_g and T_m, reflecting the role of chain mobility. Molecular dynamics simulations demonstrate that the side-chain length in PVL derivatives regulates the glass transition and diffusion through a balance of chain stiffness and free volume. A nonmonotonic trend is observed in both T_g and mobility, underscoring the complex interplay of structural and dynamic factors, consistent with the experimental observations.