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.
Highlights
Silks are protein fibers produced by many types of arthropods.[1,2] Those produced by spiders and some lepidopteran larvae are well-known and have been researched extensively,[2−8] while the silks produced by other insects[9−11] and myriapods[12,13] have received much less attention
Gi and τi for the relaxation modes were evaluated by simultaneously fitting these equations to the G′ and G′′ data, using the “Solver” routine in Excel (Microsoft Office) software
Strong relations were observed between the shear viscosity and G′(ω), G′′(ω), or |η*(ω)|, measured at ω = 1 rad s−1 (Figure 2), the values of GX or ωX at the crossover (Figure 3) and the values of gi and τi obtained by model fitting the oscillatory data (Figure 4)
Summary
Silks are protein fibers produced by many types of arthropods.[1,2] Those produced by spiders and some lepidopteran larvae (i.e., caterpillars) are well-known and have been researched extensively,[2−8] while the silks produced by other insects[9−11] and myriapods (i.e., centipedes and millipedes)[12,13] have received much less attention. The fibers from different species have different protein compositions, serve a wide range of uses, and may be produced from different glands, suggesting multiple cases of convergent evolution.[10] in spite of this diversity, the fibers are all produced in a similar way: an aqueous protein feedstock is synthesized and stored in special glands inside the body, extruded “on demand” at relatively high speeds (from around 4 mm s−1 for silkworm cocoon fibers, up to 500 mm s−1 for forcibly spun spider dragline[14]) This distinguishes silk from other animal fibers, such as hair, which grow continuously but at much slower rates (e.g., around 0.3− 0.4 mm day−1 for sheep’s wool[15]). One may expect that fiber production methods similar to the natural spinning of silks could offer very large energy savings over conventional industrial methods.[20]
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