STRUCTURE AND MOLECULAR-CONFORMATION OF TUSSAH SILK FIBROIN FILMS TREATED WITH WATER-METHANOL SOLUTIONS - DYNAMIC-MECHANICAL AND THERMOMECHANICAL BEHAVIOR

The thermal response of tussah (Antheraea pernyi) silk fibroin films t reated with different water-methanol solutions at 20 degrees C was stu died by means of dynamic mechanical (DMA) and thermomechanical (TMA) a nalyses as a function of methanol concentration and treatment time. Th e DNLA. curves of alpha-helix films (treated with greater than or equa l to 80% v/v methanol for 2 min and 100% methanol for 30 min) showed t he sharp fall of storage modulus at about 190 degrees C, and the loss peak in the range 207-213 degrees C. The TMA curves were characterized by a thermal shrinkage at 209-211 degrees C, immediately followed by an abrupt extension leading to film failure. Both storage and loss mod ulus curves significantly shifted upwards for beta-sheet films, obtain ed by treatment with less than or equal to 60% methanol for 30 min. Th e loss peak exhibited a maximum at 236 degrees C. Accordingly, the TMA shrinkage at above 200 degrees C disappeared. The films broke beyond 330 degrees C, failure being preceded by a broad contraction step. Int ermediate DMA and TMA patterns were observed for the other solvent-tre ated films. The loss peak shifted to higher temperature (219-220 degre es C), and a minor loss modulus component appeared at about 230 degree s C. This coincided with the onset of a plateau region in the storage modulus curve. The TMA extension-contraction events in the range 200-3 00 degrees C weakened, and the samples displayed a final broad contrac tion (peak temperature 326-338 degrees C) before breaking. The DMA and TMA response of these films was attributed to partial annealing by so lvent treatment, which resulted in the formation of nuclei of beta-she et crystallization within the film matrix. The increased thermal stabi lity was probably due to the small beta-sheet crystals formed, which a cted as high-strength junctions between adjacent random coil and a-hel ix domains. (C) 1998 John Wiley & Sons, Inc.