Millisecond structural dynamics during the piezoelectric cycle of silk fibroin by synchrotron Xray scattering & comparison with DFT calculation
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Date
2021-08-14
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Abstract
While simple associations exist between piezoelectric properties and processing history, there is considerable scope for design of
materials based on a more detailed molecular understanding of the re-arrangements that underpin the piezoelectric phenomenon in silk
fibroin.[1] Crystallinity, and the two-phase model of semi-crystalline model of polymers, are often used to understand the properties
of protein based materials where there is considerable thermodynamic drive to short range ordering of polymer chains, folding, which
is not present in melt processed thermoplastics. Our investigations aim to probe the relationship between structure and dynamics in
silk fibroin based materials and correlate these with the piezo-electric signal.
Recently, we have used a triggered and summative data acquisition scheme to synchronise X-ray scattering data collection with a
piezoelectric cycle of a compressed electro-spun fibroin mat.[2] This mode enabled a steady perturbed state to be sampled. The
summation of the scattering patten from this state across multiple cycles provides superior statistics than could be achieved by
sampling a single cycle. The setup is shown in Figure 1A. At rest this poorly ordered system exhibits a limited number of very broad
peaks but quite a high degree of chain folding.[3] With compression there is marked increase in the scattered intensity, both in the
small (SAXS) and the wide (WAXS) angle regimes, as well as a shift and reduction in broadness of the WAXS peaks (Figure 1B). We
interpret the increase in the SAXS signal as an increase in scattering from grain boundaries and the WAXS as the formation of new
crystalline domains. However, the limited number of very broad diffraction peaks make these data unsuitable for structural
determination. In order to provide an alternative, but complementary, perspective on the structural dynamics and the nature of the potential surface
along which the polymer folds, during the piezo cycle we have turned to a computation approach. Density functional theory (DFT)
calculations were performed within the framework of the projector augmented wave potentials parametrised by Perdew et al., [3] and
the Tkatchenko-Scheffler correction [4] with a self-consistent screening to account for the weak correlations. Full structural
optimisation of orthorhombic C20O8N8H32 was performed. The calculated lattice parameters (a = 9.409 Å, b = 6.984 Å, c = 9.221 Å)
and the corresponding diffraction pattern (black vertical lines) are compared with the experimental data in Figure 1C. © The Authors
Description
Keywords
Piezoelectricity, Polymers, Synchrotrons, Scattering, Diffraction, Materials, Thermoplastics
Citation
Garvey, C. J., Mudie, S. T., Music, D., Olsson, P. A. T., & Sencadas, V. (2021). Millisecond structural dynamics during the piezoelectric cycle of silk fibroin by synchrotron X-ray scattering and comparison with DFT calculation. Paper presented to the IUCr 2021, 25th Congress of the International Union of Crystallography, 14-22 August 2021, Prague, Czech Republic. In Acta Crystallographica Section A, 77(a2), C368. doi:10.1107/S010876732109317X