Tuning the Morphology, Thermal Behavior, and Toughness of Poly(β-butyrolactone-co-β-valerolactone) Thermoplastics

Mapping copolymer properties to composition and microstructure enriches our understanding of how these features are correlated and establishes a pathway to tune the structural characteristics of the material to meet the performance demands of its end-use application. Here, racemic β-butyrolactone (B) and β-valerolactone (V) are polymerized by ring-opening to afford a series of syndio-enriched (co)polymers that cover the entire compositional range. The effect of copolymer composition on the thermal properties, crystallinity, morphology, and mechanical properties is reported. A monotonic decrease in elastic modulus with increasing V content is observed, with melt transitions following a similar functional form. Tensile properties, including toughness and tensile strength, show sharp transitions at a V content of 36%, coinciding with changes in crystallinity and crystal structure at the same composition. The copolymer microstructure was investigated using 13C NMR spectroscopy enabling a partial assignment of resonances at the tetrad level. The results of this study show that at 27% V content, nearly 50% of the copolymer is comprised of syndiotactic tetrads that have at least three B units. At 36% V content, a significant compositional shift is observed whereby the majority of tetrads are syndiotactic with at least two V units or are iso-enriched.