Mechanical properties of tetra-PEG gels with supercoiled network structure.

We investigate the effects of swelling and deswelling on the mechanical properties of tetra-polyethylene glycol gels with the precisely tuned polymerization degree of network strand (Nc) and polymer volume fraction at preparation (ϕ0) by varying the fraction of interest (ϕm). The ϕm-dependence of the elastic modulus exhibits a crossover at ϕc due to large contraction of the network strands (supercoiling) accompanying deswelling. The Obukhov model successfully describes the ϕm-dependence of the elastic modulus. We estimate the fractal dimension of network strands (Df) by analyzing the stress-elongation relationships at high stretching using Pincus blob concept. In the supercoiling region, Df increases with an increase in ϕm, which suggests that the gyration radius of network strands decreases with deswelling in affine manner. The extensibility increases with an increase in ϕm because the deswelling reduces the distance between the neighboring junctions. These findings will help to understand the structure and formation mechanism of supercoiling.