Harnessing entropy to enhance toughness in reversibly crosslinked polymer networks

TL;DR

This paper demonstrates how reversible crosslinks in polymer networks can increase toughness without compromising elasticity by leveraging entropy-driven binding near permanent crosslinks, supported by theoretical and simulation models.

Contribution

It introduces a theoretical framework showing reversible crosslinks enhance toughness through entropy-driven localization, maintaining linear elasticity in polymer networks.

Findings

Reversible crosslinks tend to bind near permanent crosslinks due to entropy maximization.

The network's toughness increases while linear elasticity remains largely unchanged.

Guidelines for experimental optimization of reversible crosslinking are proposed.

Abstract

Reversible crosslinking is a design paradigm for polymeric materials, wherein they are microscopically reinforced with chemical species that form transient crosslinks between the polymer chains. Besides the potential for self-healing, recent experimental work suggests that freely diffusing reversible crosslinks in polymer networks, such as gels, can enhance the toughness of the material without substantial change in elasticity. This presents the opportunity for making highly elastic materials that can be strained to a large extent before rupturing. Here, we employ Gaussian chain theory, molecular simulation, and polymer self-consistent field theory for networks to construct an equilibrium picture for how reversible crosslinks can toughen a polymer network without affecting its linear elasticity. Maximisation of polymer entropy drives the reversible crosslinks to bind preferentially near the permanent crosslinks in the network, leading to local molecular reinforcement without significant alteration of the network topology. In equilibrium conditions, permanent crosslinks share effectively the load with neighbouring reversible crosslinks, forming multi-functional crosslink points. The network is thereby globally toughened, while the linear elasticity is left largely unaltered. Practical guidelines are proposed to optimise this design in experiment, along with a discussion of key kinetic and timescale considerations.