Native Silk Feedstock as a Model Biopolymer: A Rheological Perspective

Abstract

Variability in silk's rheology is often regarded as an impediment to understanding or successfully copying the natural spinning process. We have previously reported such variability in unspun native silk extracted straight from the gland of the domesticated silkworm Bombyx mori and discounted classical explanations such as differences in molecular weight and concentration. We now report that variability in oscillatory measurements can be reduced onto a simple master-curve through normalizing with respect to the crossover. This remarkable result suggests that differences between silk feedstocks are rheologically simple and not as complex as originally thought. By comparison, solutions of poly(ethylene-oxide) and hydroxypropyl-methyl-cellulose showed similar normalization behavior; however, the resulting curves were broader than for silk, suggesting greater polydispersity in the (semi)synthetic materials. Thus, we conclude Nature may in fact produce polymer feedstocks that are more consistent than typical man-made counterparts as a model for future rheological investigations.

The Rheology behind Stress-Induced Solidification in Native Silk Feedstocks

“…This was similar to the behavior reported previously [59,60,61]. The initial peak was ascribed to “stress overshoot”, which is a non-linear rheological effect commonly observed with polymeric systems and generally ascribed to the changes in coil shape at the onset of steady flow [83,84,85,86,87,88].…”

“…The elastic modulus ( G ’) dominated at higher frequencies, while the viscous modulus ( G ”) dominated at lower frequencies. As reported previously [59,60,61], the dynamic modulus data could be fitted well using a Maxwellian model of conceptual springs and dash-pots based on only two relaxation modes: $G^{\prime }={\displaystyle \sum _{i=3}^4{g}_i\frac{{\omega }^2{\tau }_i^2}{1+{\omega }^2{\tau }_i^2}}$ $G^{″}={\displaystyle \sum _{i=3}^4{g}_i\frac{\omega {\tau }_i}{1+{\omega }^2{\tau }_i^2}}$ where ω is the angular frequency (equal to 2π f ), τ i is the relaxation time and g i represents the contribution of that mode to the dynamic modulus. These modes were designated 3 and 4, in line with previous reports [59,60,61], as two slower modes (designated 1 and 2) were revealed by quasi-static stress relaxation measurements.…”

“…Nevertheless, the importance of a flow-induced phase change at a critical shear rate or stress has motivated a considerable number of rheological investigations [36,37,59,60,61,62,63,64,65,66,67,68,69,70,71,72]. Amongst these, recent work has demonstrated that B. mori silk feedstock behaves as a typical polymer solution [59,60,61].…”

“…Amongst these, recent work has demonstrated that B. mori silk feedstock behaves as a typical polymer solution [59,60,61]. Its flow behaviour was found to be dominated by a relatively small number of relaxation modes and followed an Arrhenius-type temperature dependence [59].…”

“…Its flow behaviour was found to be dominated by a relatively small number of relaxation modes and followed an Arrhenius-type temperature dependence [59]. Moreover, although considerable sample-to-sample variations have been observed, normalisation with respect to the cross-over frequency (ω X ) and modulus ( G X ) appeared to reduce the oscillatory data onto a master-curve [60]. …”