Investigations on rheological properties and gelation of tasar regenerated silk fibroin solution

Tasar silk is a variety of non‐mulberry silk indigenous to the Indian subcontinent. We present the measured frequency‐dependent viscoelastic moduli of Tasar regenerated silk fibroin (RSF) solution using optical tweezers at two concentrations (0.16% and 0.25% w/v) and extend these measurements to the low frequency regime using a video microscopy technique. We extend the investigation on the rheological behavior of Tasar RSF for four more RSF concentrations, viz., 0.50%, 1.00%, 2.50% and 5.00% using video microscopy. In all the RSF samples, both storage and loss moduli are found to increase with frequency. At lower frequencies the loss modulus is more than the storage modulus and exhibit similar behavior until a crossover frequency beyond which the storage modulus exceeds the loss modulus at all frequencies. The relaxation time which is inversely related to the crossover frequency is found to rise sharply at 5% w/v, indicating the onset of gelation in the sample. These results are examined in relation to the viscoelastic parameters of mulberry silk, wherein the larger crossover frequencies at the same higher concentrations indicate relaxation times that are an order of magnitude smaller than those measured for Tasar RSF. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40104.