Synthesis And Characterization Of Silk-Protein Like Polymers And Their Blends With Silk Fibroin

In this work, on the secondary structure level, a series of well-defined structure polymers which had the similar secondary structure with animal silk proteins (i.e. silkworm fibroin and spidroin) were synthesized by the condensed polymerization of the functional oligopeptides and diisocyanate. These silk-protein-like polymers could be used as the basic materials for understanding the effect of chain folding models of fibroin and their three-dimensional order which be formed in the spinning process on the mechanical properties of silk fibers.Firstly, four functional oligopeptide monomers (M1~M4), containing the amino sequence [GlyAlaGlyAla, AlaGlyAlaGly, (Ala)4] which derived from the crystal region of silkworm (Bombyx mori) silk and spider dragline silk, were synthesized with the traditional liquid-phase peptide synthesis method. Then the well-defined structure multiblock polymers (P1~P4) were obtained from the polymerization of the functional oligopeptides and hexamethylene diisocyanate (HDI) with the base as catalyst. These simulated polymers were the alternating copolymers formed by oligopeptide segment and non-peptide segment instead of the pure protein.The silk-protein-like polymers could be dissolved in hexafluroisopropanol and strong organic acid (e.g., formic acid, dichloroacetic acid and trifluroacetic acid), while they showed a poor solubility in common solvents. According to the intrinsic viscosity in dichloroacetic acid at 25, the molecular weight of polymers were estimated to be 20,000~50,000. Because of the thermal instability of the urea bond inmolecular chain, all synthetic polymers began to degrade around 220, and had no melting point. In addition, only P4 which had a higher molecular weight relative to the others showed an apparent glass transition at 107. Therefore, it's reasonable that the differences of thermal properties between the simulated polymers and animal silk protein were attributed to their different primary structures.The FT-IR, CP/MAS Solid-state C-NMR and Wide Angle X-ray Diffraction measurements on P3 and P4 revealed that a major -sheet conformation as well as other conformations coexisted in the polymers. In solid-state P3 and P4, the molecular chain in p-sheet conformation spontaneously aggregated into some ordered regions, and then formed the crystals which be similar to that in natural silk. Namely the silk-protein like polymers had the similar solid-state structures with animal silk proteins. These results meant our design for the simulated synthesis of the silk-protein like polymers on the secondary structure level was feasible and successful.In addition, the blends of the silk-protein like polymer (P3, P4) and silkworm fibroin were studied. The experimental results revealed that the P-sheet and random coil/a-helix conformation coexisted in the SF/P3 and SF/P4 blend films, while the predominant conformation in the pure P4 and fibroin film were random coil/a-helix. The intermolecular hydrogen-bond interaction, which be formed between the molecular chain of fibroin and the oligopeptide segments in the silk-protein like polymers, induced a partial random coil/a-helix conformation transfer to P-sheet conformation after blending, and some ordered regions were formed by the aggregation of the molecular chain in P-sheet conformation. The crystal properties of the SF/P3 and SF/P4 blend films were dependent on the amino acid sequence of the oligopeptide in P3 and P4. The cross-section morphology of the blend films indicated silkworm fibroin and these simulated polymers were miscible in their blend films. These conclusions would be important for searching the spinning condition of the silk-protein like polymers as well as producing artificial fibers of animal silk protein.