Active camouflage is widely recognized as a soft-tissue feature, and yet the ability to integrate adaptive coloration and tissuelike mechanical properties into synthetic materials remains elusive. We provide a solution to this problem by uniting these functions in moldable elastomers through the self-assembly of linear-bottlebrush-linear triblock copolymers. Microphase separation of the architecturally distinct blocks results in physically cross-linked networks that display vibrant color, extreme softness, and intense strain stiffening on par with that of skin tissue. Each of these functional properties is regulated by the structure of one macromolecule, without the need for chemical cross-linking or additives. These materials remain stable under conditions characteristic of internal bodily environments and under ambient conditions, neither swelling in bodily fluids nor drying when exposed to air.

Despite the versatility of synthetic chemistry, certain combinations of mechanical softness, strength, and toughness can be difficult to achieve in a single material. These combinations are, however, commonplace in biological tissues, and are therefore needed for applications such as medical implants, tissue engineering, soft robotics, and wearable electronics. Present materials synthesis strategies are predominantly Edisonian, involving the empirical mixing of assorted monomers, crosslinking schemes, and occluded swelling agents, but this approach yields limited property control. Here we present a general strategy for mimicking the mechanical behaviour of biological materials by precisely encoding their stress-strain curves in solvent-free brush- and comb-like polymer networks (elastomers). The code consists of three independent architectural parameters-network strand length, side-chain length and grafting density. Using prototypical poly(dimethylsiloxane) elastomers, we illustrate how this parametric triplet enables the replication of the strain-stiffening characteristics of jellyfish, lung, and arterial tissues.

We use a combination of the coarse-grained molecular dynamics simulations and scaling analysis to study conformations of bottlebrush and comb-like polymers in a melt. Our analysis shows that a crossover between comb and bottlebrush regimes is controlled by the crowding parameter, Φ, describing overlap between neighboring macromolecules. In comb-like systems characterized by a sparse grafting of side chains (Φ < 1), the side chains and backbones belonging to neighboring macromolecules interpenetrate. However, in bottlebrushes with densely grafted side chains (Φ ≥ 1), the interpenetration between macromolecules is suppressed by steric repulsion between side chains. In this regime, bottlebrush macromolecules can be viewed as filaments with diameter proportional to size of the side chains. For flexible side chains, the crowding parameter is given by Φ ≈ [v/(lb)3/2]­[(n sc/n g + 1)/n sc 1/2], which depends on both the architectural parameters (degree of polymerization of the side chains, n sc, and number of backbone bonds between side chains, n g) and chemical structure of monomers (bond length l, monomer excluded volume v, and Kuhn length, b). Molecular dynamics simulations corroborate this classification of graft polymers and show that the effective macromolecule Kuhn length, b K, and the mean-square end-to-end distance of the backbone, ⟨R e,bb 2⟩, are universal functions of the crowding parameter Φ for all studied systems.

A combination of scaling analysis and rheological experiments was used to study correlations between the entanglement plateau modulus and grafting density of graft polymers in a melt. Using the crowding parameter Φ, which describes overlap of side chains belonging to neighboring macromolecules, we identified two classes of graft polymerscombs and bottlebrushesthat demonstrate distinct conformational and rheological behaviors. In comb systems, both the backbones and sparsely grafted side chains are coiled that allow side chains of neighboring macromolecules to overlap (Φ < 1). In bottlebrush systems, steric repulsion between densely grafted side chains causes chain extension and inhibits side chain interpenetration (Φ ≥ 1). The ratio G e,gr /G e,lin ≅ φ 3 (1 + (Φ/ 0.7) 3 ) of the plateau modulus of a graft polymer melt, G e,gr , to that of a melt of linear chains, G e,lin , is a universal function of the crowding parameter Φ ≅ φ −1 n sc −1/2 and graft polymer composition φ = n g /(n g + n sc ), where n sc and n g are the degrees of polymerization of side chains and a spacer separating two consecutive side chains along the polymer backbone, respectively. This dependence of the plateau modulus is verified for poly(n-butyl acrylate) combs and other graft polymer systems reported in the literature. In a special case of graft polymers with long entangled side chains, the G e,gr /G e,lin ratio is proportional to φ 2 .

We have studied properties of comb-like and bottlebrush-like graft copolymers made of chemically different backbones and side chains in a melt. A diagram of states for this class of copolymers was calculated in terms of architectural parameters (degree of polymerization of the side chains, n sc, and the number of backbone bonds between grafting points of side chains, n g) and structural parameters (backbone and side-chain monomer projection lengths, excluded volumes, and Kuhn lengths). We apply the concept of the crowding parameter, Φ, describing overlap between neighboring macromolecules for classification of graft copolymers into combs and bottlebrushes. In this classification, the sparsely grafted side chains of comb-like macromolecules allow interpenetration of both side chains and backbones belonging to neighboring macromolecules (Φ < 1). However, in the case of bottlebrush-like macromolecules, the densely grafted side chains preclude molecular interpenetration because of steric repulsion, resulting in Φ ≥ 1. Coarse-grained molecular dynamics simulations corroborate this classification of graft copolymers and show that the effective macromolecule Kuhn length, b K, is a function of the crowding parameter, Φ. An increase in the backbone Kuhn length or monomer size results in a shift of the boundaries between different regimes in the diagram of states in comparison with those obtained for graft homopolymers with chemically identical backbones and side chains. Graft copolymers with backbones stiffer than side chains are comb-like in a broader range of parameter space. However, by increasing the excluded volume of the backbone monomers, one expands the parameter space where macromolecules demonstrate bottlebrush-like behavior.