Strain-dependent fractional molecular diffusion in humid spider silk fibres

Abstract

Spider silk is a material well known for its outstanding mechanical properties, combining elasticity and tensile strength. The molecular mobility within the silk's polymer structure on the nanometre length scale importantly contributes to these macroscopic properties. We have therefore investigated the ensemble-averaged single-particle self-dynamics of the prevailing hydrogen atoms in humid spider dragline silk fibres on picosecond time scales in situ as a function of an externally applied tensile strain. We find that the molecular diffusion in the amorphous fraction of the oriented fibres can be described by a generalized fractional diffusion coefficient K a that is independent of the observation length scale in the probed range from approximately 0.3-3.5 nm. K a increases towards a diffusion coefficient of the classical Fickian type with increasing tensile strain consistent with an increasing loss of memory or entropy in the polymer matrix.

Stress-induced long-range ordering in spider silk

“…It does rely, however, on some pre-existing fluctuations in the distribution of crystals, such that order should not arise from a fiber with a perfectly homogenous crystal distribution. The observed ordering effect is in line with the very recent observation by quasi-elastic neutron scattering experiments of spider silk that the diffusive mobility of the amorphous phase increases with tensile strain 31 . We propose that tensile forces present during the spinning process of silk or other synthetic fibers enhance long-range pre-order, which in turn boosts further ordering during tensile loading.…”

“…Small-angle neutron scattering (SANS) was undertaken to test our computational prediction of stress-induced ordering in silk fibers. Deuterated fiber bundles using H/D exchange of Nephila edulis spider silk 23 , 31 were subjected to increasing values of constant strains, for each of which a SANS pattern was recorded. An increase in scattering peak intensity on the nanometer length scale upon fiber stretch would reflect a long-range order of crystalline units within the flexible and thus more strongly deuterated amorphous phase, which also includes a partially ordered phase around the non-deuterated crystallites 32 .…”

“…A value < 1 is associated with deformation by fibril slip 34 , which is consistent with our simulation results assuming lateral friction within the amorphous phase of spider silk 29 . The lateral friction within the amorphous phase is connected to the increased unraveling of the random polymer chains upon strain, which is be a process associated with friction and thus reflected in the spectroscopic response 29 , 31 , 35 .…”

Spider Silk‐Inspired Artificial Fibers

“…[ 95 ] The mechanical properties of dragline silk are optimal in a suitable ambient environment, which changes drastically if the fibers are wet or in a high‐moisture environment. [ 96 ] Studies have found that water can disrupt the H‐bonds to increase the molecular mobility, so that the molecular entropy could drive the non‐crystalline regions to rearrange to lower energetic configurations. [ 13 , 97 ] When the relative humidity of spider silk exceeded 60%, the diameter of the spider dragline silk increased and the fiber length decreased by more than 50%, which is commonly referred to as supercontraction behavior.…”

Neutron spectroscopy on protein solutions employing backscattering with an increased energy range

“…where R is the spectrometer resolution function, β, β L(Γ(q) + γ(q), ω) may be replaced by a more general function such as a generalized model of internal diffusivity [5,43], the Fourier transform of a Kohlrausch [4,16,44,45] or Mittag-Leffler [46,47,48] function, or the result of a molecular dynamics simulation, if applicable. In reported backscattering experiments on protein solutions, the apparent center-of-mass diffusion was consistent with simple Brownian diffusion, γ(q) = D q 2 [9,15,17,19].…”