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Theoretical rheo-physics of silk: Intermolecular associations reduce the critical specific work for flow-induced crystallization

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JournalJournal of Rheology
DateAccepted/In press - 15 Mar 2022
DatePublished (current) - 1 May 2022
Issue number3
Volume66
Number of pages20
Pages (from-to)515-534
Original languageEnglish

Abstract

Silk is a semidilute solution of randomly coiled associating polypeptide chains that crystallize following the stretch-induced disruption, in the strong extensional flow of extrusion, of the solvation shell around their amino acids. We propose that natural silk spinning exploits both the exponentially broad stretch distribution generated by associating polymers in extensional flow and the criterion of a critical concentration of sufficiently stretched chains to nucleate flow-induced crystallization. To investigate the specific-energy input needed to reach this criterion in start-up flow, we have coupled a model for the Brownian dynamics of a bead-spring-type chain, whose beads represent coarse-grained Gaussian chain segments, to the stochastic, strain-dependent binding and unbinding of their associations. We have interpreted the simulations with the aid of analytic calculations on simpler, tractable models with the same essential physical features. Our simulations indicate that the associations hamper chain alignment in the initial slow flow, but, on the other hand, facilitate chain stretching at low specific work at later, high rates. We identify a minimum in the critical specific work at a strain rate just above the stretch transition (i.e., where the mean stretch diverges), which we explain in terms of analytical solutions of a two-state master equation. We further discuss how the silkworm appears to exploit the chemical tunability of the associations to optimize chain alignment and stretching in different locations along the spinning duct: this delicate mechanism also highlights the potential biomimetic industrial benefits of chemically tunable processing of synthetic association polymers.

Bibliographical note

Funding Information:
This research was funded by the Engineering and Physical Sciences Research Council (Grant No. EPSRC EP/N031431/ 1). C.S. is grateful for the computational support from the University of York high-performance computing service (the Viking Cluster) and thanks Daniel J. Read for his suggestion to pair closed stickers of different chains. C.S. and T.C.B.M. thank Pete Laity and Chris Holland for useful discussions on the mechanism of silk self-assembly.

Publisher Copyright:
© 2022 Author(s).

    Research areas

  • Polymer melt dynamics, silk spinning, Biophysical Processes, POLYELECTROLYTES

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