Nonlinearity In A Crosslinked Polyelectric Polypeptide

TL;DR

This study investigates the nonlinear elastic and viscoelastic properties of crosslinked polypeptide materials under varying humidity, revealing strain-dependent softening and hardening behaviors with potential for engineering tailored mechanical properties.

Contribution

It provides the first detailed characterization of the nonlinear elasticity and viscoelasticity of crosslinked polypeptides, highlighting humidity effects and strain thresholds for mechanical behavior.

Findings

Nonlinear elasticity observed at high humidity with strain softening and hardening.

Neo-Hookean model fitting yields elastic modulus between 40 and 300 kPa.

Viscoelastic relaxation times vary with strain and humidity levels.

Abstract

Youngs modulus of soft solids composed of crosslinked synthetic polypeptides has been determined under different conditions. Co-poly-(L-glutamic acid$_4$, L-tyrosine$_1$) [PLEY (4:1)] was crosslinked with poly-L-lysine (PLK) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). Elasticity was assessed by subjecting samples to a compressive strain. Cross-linked material at high relative humidity, RH 75-85%, exhibited non-linear elasticity. Stress-strain response was approximately linear at low strain but nonlinear above a threshold strain. Analysis of the secant modulus revealed apparent softening of samples at low strain and hardening at high strain, as in biological soft tissues. Fitting stress-strain data with a neo-Hookean model yielded approximately 40 $\le E \le$ 300 kPa at high RH. Viscoelasticity was nonlinear at low RH. The average viscosity-driven relaxation time was 13 min at high strain and 6 min at low strain. Analysis of the derivative of the secant modulus for non-linear elastic materials revealed a transient response up to a strain of $\varepsilon \approx$ 0.18-0.20. Above this range, oscillations tended to zero. Non-linear viscoelastic materials showed lower-amplitude oscillations than samples at high RH up to $\varepsilon \approx$ 0.06 and strong damping thereafter. The data suggest that it will be possible to engineer mechanical properties of polypeptide materials.