Deformation of Brush Gels and Hidden Length

Brush gels demonstrate unique mechanical properties when compared to gels of linear strands. Using a combination of molecular dynamics simulations and theoretical analysis, we study mechanical properties of brush gels which strands are made of solvophobic backbones and solvophilic side chains. In poor solvent conditions for the brush backbones, backbones collapse creating reservoirs of the hidden lengths. The degree of backbone collapse is determined by a fine interplay between surface energy of the collapsed backbone and steric repulsion between side chains. The conformations of the brush strands are studied as a function of the degree of polymerization and grafting density of the side chains and solvent quality for the brush backbone. It is shown that the brush strand conformations are directly related to the gel swelling ability. For bottlebrush-like strands, the side chains screen and dilute backbones resulting in the large gel swelling ratios and weakening sensitivity to changes in solvent quality for the backbone. In contrast, for gels with comb-like strands, solvent quality has a pronounced effect on the backbone conformations creating reservoirs of hidden lengths and allowing such gels to sustain extremely large reversible deformations.