STRUCTURAL-ANALYSIS OF METHYL METHACRYLATE-GRAFTED SILK FIBERS

The structural changes of silk fibers grafted with methyl methacrylate (MMA) were investigated in relation to the weight gain. Both the refr active indices, parallel and perpendicular to the fiber axis, decrease d with increasing weight gain, although the former showed a steep drop in the weight gain range of 0-30%. Accordingly, birefringence and the isotropic refractive index also decreased, suggesting a lower degree of molecular orientation and crystallinity of MMA-grafted silk fibers. However, X-ray diffraction curves demonstrated that the crystalline s tructure remained unchanged, regardless of MMA grafting, implying that the MMA polymer attached preferentially within the amorphous regions of the fiber. Tensile strength and elongation at break of the grafted silk fibers, measured in dry and wet states, gradually decreased over the weight gain range examined. MMA-grafted silk fibers in wet state e xhibited comparatively lower values of strength and elongation, though the hydrophobicity, induced by the grafted polymer, resulted in an at tenuation of the differences, especially at high weight gains. TMA ana lysis demonstrated that the onset temperature of the final extension s hifted to a higher value, in the weight gain range 30-50%. Dynamic mec hanical measurements showed that the thermally induced molecular movem ent of MMA-grafted silk increased, as illustrated by the trend of both dynamic storage and loss modulus curves. (C) 1993 John Wiley & Sons, Inc.