Mechanical modeling and simulation of porous polymer networks: Load induced loss of saturation in isotropic elastomers and pressure driven seepage in directionally reinforced elastomers.

摘要

Elastomeric gels are high molecular weight crosslinked polymer networks immersed in a low molecular weight liquid medium. In the liquid environment, they could undergo a large deformation associated with swelling or shrinking in response to environmental stimuli, such as change in temperature, chemistry of the liquid bath, and light exposure. This valuable property makes them useful in a wide range of applications in drug delivery, surgical dressings, artificial tissue, and control material in engineering, which motivate the desire to better understand their underlying mechanical response.;In gels the polymer and liquid components mix in definite proportions as determined primarily by entropic and enthalpic effects. Mechanical loading can also alter the mixture proportions by absorbing or driving out the liquid. Gel swelling in the absence of mechanical loading is often described by a generalized Flory-Huggins equation, which accounts for the effects between such a treatment and the broader hyperelastic theory which accounts for the effect of mechanical loading. In this study we consider loadings that can lead to both fluid gain (swelling increase) and fluid loss (swelling reduction). For loadings that give fluid gain, we then consider a situation in which the amount of available fluid is limited. In this case, increased loading may reach a point at which no additional fluid is available for uptake into the gel system. This results in a transition of the gel from a state of liquid saturation to a state in which it is no longer saturated. This transition is first considered in the context of homogeneous deformation where an appropriate hyperelastic analysis shows that the transition from saturation to nonsaturation gives rise to an abrupt mechanical stiffening. Then two kinds of inhomogeneous deformation problems are investigated, including everting an axially loaded tube and twisting a hollow tube that originally swells freely in the liquid bath. Various boundary displacements and traction conditions are applied so as to study how these alter the original fluid distribution. It is found that certain boundary conditions generate an overall volume increase after free swelling, which results in a stiffer mechanical response after loss of saturation.;These static problems describe equilibrium situations in which both the fluid component and the polymer matrix component of the system are at rest. However, a more complicate phenomenon - which attracts abundant research interests - fluid diffusion through polymer networks requires further study of the relative motion between the fluid and the polymer network. A mixture theory is then invoked to specifically deal with separate mechanical balance principles for each component. Pressure driven fluid seepage problems for both isotropic and anisotropic (fiber reinforced) gels are discussed based on this framework.