Solvent-based synthesis, structural elucidation and thermal characterisation of free radical grafted PHBV
Samuel Lawless, Bronwyn Laycock, Paul Lant, Steven Pratt
Abstract
• Solvent-based free radical grafting of PHBV confirmed and quantified via NMR spectroscopy up to 5.8 mol% of monomer units. • The grafted PHBV exhibited a depression in melting temperature and changes in crystallisation behaviour, potentially widening the processing window. • Some grafted products showed less severe molecular weight reductions compared to similar chemistries observed previously. This study investigates the solvent-based free-radical graft copolymerization of poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV) with C 11 (undecenoic acid, UDA), C 12 (dodecene, DD), and C 18 (octadecene, OD) alkene substrates using dicumyl peroxide as an initiator. Comprehensive Nuclear Magnetic Resonance (NMR) spectroscopy was employed to confirm and quantify grafting, which was achieved at up to 5.8 mol% of monomer units. Thermal analysis revealed a significant reduction in melting temperature for all modified PHBVs, ranging from 10-20°C decrease in comparison to virgin PHBV. A decrease in crystallisation temperature and a reduction in glass transition temperature was measured for the UDA and DD grafted PHBV. Additionally, molecular weight analysis showed a marked decrease in weight average molecular weight ( M ¯ w ) and number average molecular weight ( M ¯ n ) for PHBV grafted with UDA, which was attributed to the presence of carboxylic acid functionality, which accelerates chain scission. The molecular weight profiles of the dodecene and octadecene grafted PHBV were only marginally reduced (32% and 18% reductions in M ¯ w , respectively), which is a much less severe reduction than has been previously observed for similar chemistries. Overall, this work demonstrates that solvent-based free-radical graft copolymerization can reduce the melting temperature and disrupt the crystallinity of PHBV, hence improving its processability and progressing the field of PHA modification and application.