Biodegradable Films and Foam of Poly(3-Hydroxybutyrate-<i>co</i>-3-hydroxyvalerate) Blended with Silk Fibroin

Solvent-cast films of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and silk fibroin (SF) blends were investigated to evaluate miscibility and mutual impacts on thermal properties and morphology to inform how SF may impact the cell microstructure of PHBV foam. It was determined through modulated differential scanning calorimetry (MDSC) and thermal gravimetric analysis (TGA) that the blends were immiscible at all compositions studied. Using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), we found that SF morphology changed from amorphous to 38.2-47.6% crystallinity in the presence of PHBV. Additionally, we observed that SF acts as a crystal nucleating agent for PHBV, but subsequently is excluded to the interspherulitc regions during PHBV crystal growth. After melting and fast cooling in the DSC, we found that SF reduces melting temperature and crystallinity of PHBV films, except for 1 and 40 wt% SF, which exhibited anomalous behavior. It is likely that SF nucleates new PHBV crystals, but in regions of higher SF content. The immobile SF causes more disorder of PHBV crystals near the nuclei resulting in overall lower crystallinity. To produce SF powder for foam extrusion, three 1-day freeze-thaw cycles reproducibly yielded SF aerogels from tough hydrogels after drying in a vacuum oven. These aerogels were ground and added to PHBV and foamed with 0.5, 1, 2, 3, and 4 phr azodicarbonamide (AZ) for 1 wt% SF and 4 phr AZ for 5 wt% SF. Surprisingly, SF actually led to poorer cell density above 0.5 phr AZ, especially for 5 wt% SF due to cell coalescence and greater foam shrinkage during cooling.