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Swelling and dissolution of silk fibroin (Bombyx mori) in N-methyl morpholine N-oxide
Abstract:
Bombyx mori silk fibers were dissolved in N-methyl morpholine N-oxide (MMNO), an organic cyclic amine oxide used for the solvent spinning of regenerated cellulosic fibers. The commercial MMNO monohydrate used in this study as a solvent for silk is a hygroscopic compound crystalline at room temperature, which becomes an active solvent after melting at 76 degrees C. The degree of hydration of MMNO was checked by DSC measurements. The solvation power of MMNO towards silk fibroin drastically decreased at a water content > or = 20-21% w/w. Dissolution of silk required both thermal and mechanical energy. The optimum temperature was 100 degrees C. At lower temperatures dissolution proceeded very slowly. At higher temperatures, rapid depolymerization of silk fibroin occurred. The value of the Flory-Huggins interaction parameter chi for the MMNO-H2O-silk fibroin system was -8.5, suggesting that dissolution is a thermodynamically favored process. The extent of degradation of silk fibroin was assessed by measuring the intrinsic viscosity and determining the amino acid composition of silk after regeneration with an aqueous methanol solution, which was effective in removing the solvent and coagulating silk. Regenerated silk fibroin membranes were characterized by infrared spectroscopy, differential scanning calorimetry and scanning electron microscopy. The prevailing molecular conformation of silk fibroin chains was the beta-sheet structure, as shown by the intense amide I-III bands at 1704, 1627, 1515, 1260, and 1230 cm(-1). The value of the I1260/I1230 intensity ratio (crystallinity index) was 0.68, comparable to that of the fibers. The DSC thermogram was characteristic of a silk fibroin material with unoriented beta-sheet crystalline structure, with an intense decomposition endotherm at 294 degrees C. The SEM examination of fractured surfaces showed the presence of a dense microstructure with a very fine texture formed by densely packed roundish particles of about 100-200 nm diameter.
Insights
N-methyl morpholine N-oxide (MMNO) effectively dissolves Bombyx mori silk fibroin, with optimal conditions at 100°C. Regenerated silk retains its beta-sheet structure, demonstrating a thermodynamically favored dissolution process.
Area of Science:
- Materials Science
- Polymer Chemistry
- Biomaterials Engineering
Background:
- Bombyx mori silk fibroin is a natural protein fiber with unique properties.
- Traditional silk processing often involves harsh chemicals or limited dissolution methods.
- Developing efficient and controlled dissolution methods is crucial for silk-based material applications.
Purpose of the Study:
- To investigate the dissolution of Bombyx mori silk fibroin using N-methyl morpholine N-oxide (MMNO).
- To determine the optimal conditions for silk dissolution and regeneration.
- To characterize the structural and morphological properties of the regenerated silk fibroin.
Main Methods:
- Silk fibroin dissolution in MMNO at varying temperatures and hydration levels.
- Differential Scanning Calorimetry (DSC) for MMNO hydration and silk characterization.
- Intrinsic viscosity measurements and amino acid analysis to assess degradation.
- Regeneration of silk using aqueous methanol.
- Characterization of regenerated membranes using Infrared Spectroscopy (IR), DSC, and Scanning Electron Microscopy (SEM).
Main Results:
- MMNO monohydrate acts as an active solvent for silk above 76°C.
- Optimal dissolution temperature is 100°C; higher temperatures cause depolymerization.
- Solvation power decreases significantly with MMNO water content ≥ 20-21% w/w.
- The Flory-Huggins interaction parameter (χ) of -8.5 indicates thermodynamically favorable dissolution.
- Regenerated silk predominantly exhibits a beta-sheet structure, with a crystallinity index of 0.68.
- SEM revealed a dense microstructure with fine, roundish particles (100-200 nm).
Conclusions:
- MMNO is a viable solvent for Bombyx mori silk fibroin, enabling regeneration with preserved secondary structure.
- Controlled dissolution and regeneration are achievable, yielding materials with desirable microstructural features.
- The findings support the potential of MMNO for advanced silk processing and biomaterial development.