Changes in physical properties of methacrylonitrile (MAN)‐grafted silk fibers

Abstract

AbstractChanges in physical properties of silk fibers, grafted with methacrylonitrile (MAN), were investigated as a function of the weight gain. The weight gain increased steadily during the first 60 min of reaction and gradually attained an equilibrium value (60%) after about 4 h. The initial tensile resistance of silk fibers decreased by MAN grafting. The crystalline structure of silk fibers remained unchanged, regardless of MAN grafting, however. a minor and broad peak appeared in the X‐ray diffraction curves of MAN‐grafted silk fibers with a weight gain of 60%, corresponding to the unoriented MAN polymer attached inside the fibers. Molecular orientation of silk fibroin chains in the crystalline regions, evaluated from X‐ray diffraction curves, did not change significantly, while both birefringence and isotropic refractive index decreased as the weight gain increased, implying that MAN polymer attached preferentially to the amorphous and not to the crystalline regions. Dynamic vis‐coelastic measurements showed that the position at which the E′ value began to decrease shifted to a lower temperature as the weight gain increased. These findings suggest that the thermal movement of silk fibroin molecules was accelerated by the presence of the poly‐MAN chains attached to the amorphous regions of silk fibroin fibers. © 1993 John Wiley & Sons, Inc.