STRUCTURAL-ANALYSIS OF METHACRYLAMIDE-GRAFTED SILK FIBERS

The structural changes and the thermal behavior of silk fibers grafted with methacrylamide (MAA) were investigated as a function of the weig ht gain. The refractive index parallel to the fiber axis decreased wit h increasing weight gain, whereas that perpendicular remained almost u nchanged. Accordingly, birefringence decreased with a steeper slope in the weight gain range 0-80%, suggesting a lower degree of average mol ecular orientation. Only small changes in the isotropic refractive ind ex were detected, suggesting that the crystallinity of the fibers rema ined essentially unaffected by MAA grafting, as confirmed by the X-ray diffraction data. The molecular orientation in the crystalline region s remained unchanged in the weight gain range 0-60%, then sharply decr eased. The strength and the initial tensile resistance of grafted silk fibers decreased both in the dry and wet states, while elongation at break increased in the dry state and remained almost constant in the w et state. The results of the thermal behavior, investigated by differe ntial scanning calorimetry, thermomechanical and thermogravimetric ana lysis, and dynamic mechanical measurements, were consistent with an in creased thermal stability conferred on silk fibers by MAA grafting. Th e cross-sectional area of MAA-grafted silk fibers increased. Moreover, ion-etched cross sections of the grafted silk fiber showed the presen ce of fibrils with a diameter larger than that of the untreated contro l. (C) 1993 John Wiley & Sons, Inc.