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

Mappingcopolymer properties to composition and microstructureenriches our understanding of how these features are correlated andestablishes a pathway to tune the structural characteristics of thematerial to meet the performance demands of its end-use application.Here, racemic & beta;-butyrolactone (B) and & beta;-valerolactone(V) are polymerized by ring-opening to afford a series of syndio-enriched(co)polymers that cover the entire compositional range. The effectof copolymer composition on the thermal properties, crystallinity,morphology, and mechanical properties is reported. A monotonic decreasein elastic modulus with increasing V content is observed, with melttransitions following a similar functional form. Tensile properties,including toughness and tensile strength, show sharp transitions ata V content of 36%, coinciding with changes in crystallinity and crystalstructure at the same composition. The copolymer microstructure wasinvestigated using C-13 NMR spectroscopy enabling a partialassignment of resonances at the tetrad level. The results of thisstudy show that at 27% V content, nearly 50% of the copolymer is comprisedof syndiotactic tetrads that have at least three B units. At 36% Vcontent, a significant compositional shift is observed whereby themajority of tetrads are syndiotactic with at least two V units orare iso-enriched.