Influence of the crosslinks density on the crystallization of water in PAA gels

Water state, thermal transition behavior and structure of hydrated gelatin films

The state of water, thermal transition behaviors, molecular interactions, crystalline structure, and mechanical performance of hydrated gelatin films were studied by differential scanning calorimetry (DSC), infrared spectroscopy (FTIR), X-ray diffraction, and universal testing instruments. The DSC results showed that with increase of the water content, two types of water, including unfreezable bound water and freezable water, appeared in turn. Below a critical water content of 30%, the glass transition temperature (Tg) of the hydrated gelatin films decreased notably with an increase in water content, which leveled off at water content higher than this threshold. This observation suggests that only the unfreezable water exhibits a plasticizing effect. In addition, the melting temperature (Tm) of hydrated gelatin films decreased continuously with an increase in water content, whereas the melting enthalpy showed a non-monotonic dependence on hydration level. Structural analysis showed that at medium hydration levels up to 13.4% water content, the unfreezable water facilitated the formation of additional triple helices, confirmed by DSC results. Spectral data revealed that the -OH groups of unfreezable water molecules interacted with the -NH groups of the protein via hydrogen bonds. Moreover, the mechanical properties of the hydrated gelatin films were sensitive to their hydration level, and the tensile strength was dominated by the helix content of the protein films. These results show the feasibility of using hydration to regulate the microstructure and properties of biopolymers.

Mixed Protein Blends Composed of Gelatin andBombyx moriSilk Fibroin: Effects of Solvent-Induced Crystallization and Composition

Novel protein blends have been prepared by mixing gelatin (G) with Bombyx mori silk fibroin (SF) and using aqueous methanol (MeOH) to post-induce SF crystallization. When co-cast from solution, amorphous blends of these polymers appear homogeneous, as discerned from visual observation, microscopy, and Fourier-transform infrared (FTIR) spectroscopy. Upon subsequent exposure to aqueous MeOH, SF undergoes a conformational change from random coil to beta sheet. This transformation occurs in pure SF, as well as in each of the G/SF blends, according to X-ray diffractometry and thermal calorimetry. The influence of MeOH-induced SF crystallization on structure and property development has been ascertained in terms of preparation history and blend composition. Thermal gravimetric analysis reveals that the presence of beta sheets in SF and G/SF blends improves thermal stability, while extensional rheometry confirms that SF crystallization enhances the tensile properties of the blends. By preserving a support scaffold above the G helix-to-coil transition temperature, the formation of crystalline SF networks in G/SF blends can be used to stabilize G-based hydrogels for biomaterial and pharmaceutical purposes. The present study not only examines the properties of G/SF blends before and after SF crystallization, but also establishes the foundation for future research into thermally responsive G/SF bioconjugates.