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nerated silk fibroin (RSF) fibers were directly dry-spun from RSF aqueous solutions into air. To improve mechanical properties of fiber, the as-spun fibers were postdrawn in 80 vol.% ethanol aqueous solution, in which an immersion process was performed subsequently. With the increase in draw ratio, the fibers show substantial improvements of orientation and mechanical properties. Quantitative analysis of Fourier transform infrared spectroscopy indicates that the ratio of b-sheet to a-helix conformation increases sharply at the beginning of immersion process, then approaches a constant value after 90 min of immersion. All fibers exhibit very smooth surfaces. There is no obvious relationship between the pH of the spinning dope and the mechanical properties of the regenerated fibers. The breaking stress of the posttreated fiber is improved up to 301 MPa, which approaches that of degummed silk. The posttreated fiber is over three times the breaking energy of degummed silk.

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.47

showed poor mechanical properties. Um18 used phosphoric acid/formic acid mixture as solvent and spun RSF fibers in methanol coagulation bath. The resultant fibers had a breaking strength of 180 MPa and a breaking elongation of 16%. RSF fibers were also obtained by spinning a concentrated RSF aqueous solution into a hot coagulation bath of ammonium sulfate [(NH4)2SO4] solution.19 Taken together, these observations suggest that most artificial fabrications of RSF belong to wet spinning process, which is much different from the natural dry spinning process of silkworm extruding aqueous solution at ambient temperature and low hydrostatic pressure in air. In our previous work, RSF fibers were dry-spun from its aqueous solutions by mimicking the dry spinning process of silkworm.20 However, the obtained fiber showed a low breaking strength of only 45.5 MPa, which is much lower than that of natural silk. To improve the mechanical properties of the dry-spun fiber, we then tried different agents and methods to preliminarily posttreat the as-spun fiber.21 This study extends our earlier work on dry spinning of silk solution. We emphasized on 80 vol.% ethanol aqueous solution and a method to posttreat the as-spun fiber. The effects of draw ratio, immersion time in ethanol aqueous solution on the conformation, and the mechanical properties of the fibers were studied. The fibers spun from the dopes with different pHs were also posttreated, characterized, and compared. The aim of this work is to further improve the mechanical properties of our dryspun fibers and to discuss the possibility of establishing an environmentally friendly process for preparing highperformance silk fiber.

1100

Ó Materials Research Society 2011

I. INTRODUCTION

Over the past few decades, silkworm silk has attracted considerable interests for its outstanding mechanical properties, comparable to high performance synthetic materials and natural fibers like cellulose fibers.1–5 As a natural protein fiber, silk is safe, nontoxic