Modeling the mechanisms for formation of helices and perversions in elastic nanofilaments through molecular dynamics

JPT Lopes and FV de Abreu and R Simoes, POLYMER BULLETIN, 79, 1929-1947 (2022).

DOI: 10.1007/s00289-021-04013-0

Helices made of polymeric elastic filaments have been the target of considerable and growing interest in past years, given several potential applications. Although the underlying mechanisms responsible for the formation of such helices are still not sufficiently understood, recent results suggest they may result from buckling instabilities emerging from torsion in the extremities or when there is asymmetry across the filament's cross section. Also, the occurrence of perversions (regions where the helical handedness changes) has attracted considerable interest in a number of theoretical works, but the possibility of creating more than a single perversion, and thus control the geometry of helices and perversions in the resulting filament, has been given much less attention, despite its clear importance. In this paper, we present coarse-grained Molecular dynamics (MD) simulations that show it is possible to replicate the formation of helices and perversions within certain conditions, and which complement information available from experimental approaches. We show how the helical radius can depend on the strength and the asymmetry of the pairwise interactions, the filament's aspect ratio, and the strain rate of recovery, and we discuss in detail how perversions occur. The bonding potential parameters were found to have a small effect on the number of perversions, while the strain rate exhibited a significant effect, namely, an increase in 200-fold of the strain rate can induce as many as eight times more perversions for an aspect ratio of 200 (and three times more perversions for an aspect ratio of 50). The increase in the pair-wise interaction stiffness leads to lower loop diameters and higher number of loops, while an increase in the pair-wise equilibrium distance leads to larger loop diameters and consequently a lower number of loops; however, both these parameters exhibit a strong dependence on the aspect ratio. It was also found that an increase in the surface modification by 30% leads to an increase in circa 2.3 times the number of formed loops, while the average loop diameter decreases by circa 40%. From these results emerges a better understanding of how to tailor the geometry of the studied polymer elastic filaments, vital information for the design of next- generation nano-mechanical systems, such as those obtained by nano- patterning of soft materials. GRAPHICS .

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