# Maximally Stiffening Composites Require Maximally Coupled Rather Than Maximally Entangled Polymer Species

## TL;DR

This study combines simulations and microrheology to analyze DNA-actin composites, revealing that maximal stiffening occurs with maximally coupled species rather than maximally entangled ones, and uncovers dynamic microstructural oscillations.

## Contribution

It demonstrates that optimal stiffening in polymer composites is achieved through maximal coupling, not maximal entanglement, and introduces novel microstructural oscillation phenomena.

## Key findings

- Non-monotonic stress-stiffening is robust and tunable.
- Maximal stiffening occurs with maximally coupled species.
- Discovery of dynamic microstructural oscillations.

## Abstract

Polymer composites are ideal candidates for next generation biomimetic soft materials because of their exquisite bottom-up designability. However, the richness of behaviours comes at a price: the need for precise and extensive characterisation of material properties over a highly-dimensional parameter space, as well as a quantitative understanding of the physical principles underlying desirable features. Here we couple large-scale Molecular Dynamics simulations with optical tweezers microrheology to characterise the viscoelastic response of DNA-actin composites. We discover that the previously observed non-monotonic stress-stiffening of these composites is robust, yet tunable, in a broad range of the parameter space that spans two orders of magnitude in DNA length. Importantly, we discover that the most pronounced stiffening is achieved when the species are maximally coupled, i.e. have similar number of entanglements, and not when the number of entanglements per DNA chain is largest. We further report novel dynamical oscillations of the microstructure of the composites, alternating between mixed and bundled phases, opening the door to future investigations. The generic nature of our system renders our results applicable to the behaviour of a broad class of polymer composites.