Inverse mechanical-swelling coupling of a highly deformed double-network gel

Mechanical behaviors of a polymer gel are coupled with its swelling behavior. It has been known that typical hydrogels display extension-induced swelling and drying-induced stiffening, called normal mechanical-swelling coupling. In this study, we experimentally found that highly extended double-network (DN) hydrogels exhibit abnormal inverse mechanical-swelling coupling such as extension-induced deswelling and drying-induced softening. We established theoretical hyperelastic and swelling models that reproduced all the complicated mechanical and swelling trends of the highly deformed DN hydrogels. From these theoretical analyses, it is considered that the inverse mechanical-swelling coupling of a DN gel is derived from the extreme nonlinear elasticity of its first network at its ultimate deformation state. These findings contribute toward the understanding of the mechanics of rubber-like materials up to their ultimate deformation and fracture limit.

Application of a Clapeyron-Type Equation to the Volume Phase Transition of Polymer Gels

The applicability of the Clapeyron equation to the volume phase transition of cylindrical poly(N-isopropylacrylamide)-based gels under external force is reviewed. Firstly, the equilibrium conditions for the gels under tension are shown, and then we demonstrate that the Clapeyron equation can be applied to the volume phase transition of polymer gels to give the transition entropy or the transition enthalpy. The transition enthalpy at the volume phase transition obtained from the Clapeyron equation is compared with that from the calorimetry. A coefficient of performance, or work efficiency, for a gel actuator driven by the volume phase transition is also defined. How the work efficiency depends on applied force is shown based on a simple mechanical model. It is also shown that the force dependence of transition temperature is closely related to the efficiency curve. Experimental results are compared with the theoretical prediction.

Pronounced effects of the densities of threaded rings on the strain-dependent Poisson's ratio of polyrotaxane gels with movable cross-links

The density of threaded ring molecules (fCD) in polyrotaxane (PR) chains has pronounced effects on the strain-induced swelling of PR gels where the cross-linked ring molecules are slidable along the network strands. The equilibrium Poisson's ratio (μ∞), which is a measure of the strain-induced volume change, for the PR gel increases with an increase in elongation (λ) at moderate λ but becomes a constant value () at sufficiently large λ. When the modulus exceeds a threshold value (Ec), the λ dependence of μ∞ disappears due to the loss of the slidability of the cross-links. The fraction fCD significantly influences the values of and Ec. When fCD is sufficiently small (<14%), (≈0.25) agrees with the values of μ∞ for the classical gels in good solvents. When fCD is high (>25%), varies over a wide range (0.22 < < 0.33) depending on fCD and the cross-link concentration in a complicated way. The modulus Ec at fCD = 25% is more than twice as high as that at fCD = 5% due to the finite contribution of the larger amount of uncross-linked ring molecules via combinatorial entropy in the axial polymers. The origin of the markedly small values of μ∞ (less than 0.1) at small λ is also considered on the basis of the magnitude of the accompanying force reduction caused by the slidable function of the cross-links.

Reliable Hydrogel with Mechanical "Fuse Link" in an Aqueous Environment

A robust hydrogel with a reliable deformation region in an aqueous environment is proposed. The gel has a homogeneous network where hydrophilic/hydrophobic components are uniformly distributed. In an aqueous environment, aggregated hydrophobic segments serve as "mechanical fuse links," inhibiting sudden macroscopic fracture. The gel endures threefold stretching for more than 100 cycles in water without mechanical hysteresis.

Loading effect on swelling of nematic elastomers

Externally imposed loading has substantially different effects on the swelling of nematic elastomers in the high-temperature isotropic and low-temperature nematic states. In the isotropic state, the stretching drives a considerably large degree of further swelling, whereas the stretching-induced volume change in the nematic state is significantly suppressed. In the isotropic phase that favors the less anisotropic state, the further swelling occurs to reduce the shape anisotropy caused by the imposed elongation. In the nematic phase, no significant swelling is induced because further swelling decreases the nematic order enhanced by the applied stretching. These different loading effects in the isotropic and nematic states observed in the experiments are qualitatively described by a mean field theory.

Stress relaxation of deformed gel in a good solvent

A method for obtaining elastic moduli and frictional properties of gel chains, diagonal and off-diagonal elements of a friction tensor, from a tensile force relaxation of cylindrical gel that is stretched in a good solvent is presented. The theory to describe the relaxation is developed on the assumption that an off-diagonal element of the friction tensor is nonzero and compared with experimental results obtained for the poly(acrylamide) gel in water. The experiments revealed that the tensile force relaxation, F(t) at a long t, was described by a function: F(t)=A(Fi-FS) x exp(-t/tau)+FS, where Fi, FS, and A, respectively, were tensile forces at the initiation and the steady state, and a constant. The tau was found to be proportional to a square of radius of the gel. The experiments also revealed that the elastic force relaxation was synchronized with a change of the gel diameter. The theory well elucidates the experimental results to reveal the bulk and shear moduli, and the diagonal and off-diagonal elements of friction tensor.

The stress diffusion coupling in the swelling dynamics of cylindrical gels

The swelling dynamics of long cylindrical gel is analyzed by the stress diffusion coupling model [T. Yamaue and M. Doi, Phys. Rev. E 69, 41402 (2004)]. Two situations are analyzed: (i) stress induced swelling, where the swelling is caused by an elongational force applied on the gel, and (ii) free swelling, where the swelling is caused by thermodynamic force. The relaxation times characterizing these processes are calculated. It is shown that earlier calculations for the relaxation time, which are based on some physical or mathematical approximations, give results surprisingly close to the rigorous calculation, but the difference can be still seen experimentally.

Kinetics of shrinking of polymer gels induced by ultracentrifugal fields

The kinetics of the shrinking of polymer gels induced by ultracentrifugal fields is investigated. A theory is proposed to describe the diffusion process of polymer networks under centrifugal fields. The initial shrinking rate is proportional to the ratio of the centrifugal force to the frictional force of networks. The shrinking attains the stationary state as a result of the balance between the centrifugal force and the swelling force of networks. The characteristic time for shrinking is of the order of a2/D where a and D are the stationary displacement and diffusion constant, respectively. We also present the experimental data for the shrinking of the poly(acrylamide) (PAAm) gels under ultracentrifugal fields. The shrinkage increases linearly with time in the initial stage whereas it reaches the steady state in the long time limit as expected by the theory. Each of longitudinal elastic modulus and friction coefficient of the PAAm gels is evaluated from the data on the basis of the theory.