Interfacial Improvements in a Green Biopolymer Alloy of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and Lignin via in Situ Reactive Extrusion

Green biopolymer alloys based on the bacterial polyester poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and softwood Kraft lignin were successfully prepared via dicumyl peroxide (DCP) initiated free radical grafting during melt extrusion to improve interfacial adhesion. It is postulated that lignin was grafted onto PHBV to form a cross-linked copolymer gel. The gel fraction of the biopolymer alloy grafted at four different loading levels of DCP was determined. At an optimal total concentration of 2 wt % DCP, tensile strength, Young's modulus, and storage modulus, by dynamic mechanical analysis (DMA), of the biopolymer alloy showed a maximum, coinciding with the highest gel yield. The presence of both lignin and PHBV characteristic bands by Fourier transform infrared spectroscopy (FTIR) in the extracted biopolymer alloy gel confirmed lignin was successfully grafted onto PHBV. Adhesion factor calculated from DMA data also indicated improved interfacial interaction. The crystallinity degree in the grafted alloy was reduced while crystallization temperature was increased as determined by FTIR, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses. Hot stage polarized optical microscopy observation confirmed that DCP induced grafting significantly reduced the spherulite size and increased nucleation density of PHBV. Glass transition temperature, thermal stability, and melt strength of the biopolymer alloy were all enhanced as a result of better molecular interaction by grafting. This study opens up a pathway to utilize effectively the low-cost and renewable lignin as a component in a biopolymer alloy based on sustainable materials.