Swelling-Induced Instabilities of Polymeric Hydrogels with Periodic Microstructures

Hydrogels are soft, hydrophilic materials which can absorb a large volume of solvent and undergo finite volumetric deformations known as swelling. The swelling of a hydrogel can be a driving mechanism for complex material responses such as pattern transformation which lead to change of periodicity as a result of a microscopic instability in periodic materials. In the present contribution, we deal with the computational analysis of swelling-induced instabilities in periodic hydrogels. The stability analysis based on the Bloch-Floquet theory is carried out within a transient two-field minimization-type variational principle. The presented formulation and methodology for the stability analysis are computationally efficient, since the computations are carried out on the smallest representative volume element of the microstructure. Within this framework, we study swelling-induced microscopic instabilities for various perforated hydrogels. Our findings are consistent with experimental observations and show that the so-called diamond plate patterns are the critical buckling mode for voided microstructures. Moreover, we observe long-wavelength instabilities for certain volume fractions of voids

Electrochemical strain microscopy time spectroscopy: Model and experiment on LiMn2O4

Electrochemical Strain Microscopy (ESM) can provide useful information on ionic diffusion in solids at the local scale. In this work, a finite element model of ESM measurements was developed and applied to commercial lithium manganese (III,IV) oxide (LiMn2O4) particles. ESM time spectroscopy was used, where a direct current (DC) voltage pulse locally disturbs the spatial distribution of mobile ions. After the pulse is off, the ions return to equilibrium at a rate which depends on the Li diffusivity in the material. At each stage, Li diffusivity is monitored by measuring the ESM response to a small alternative current (AC) voltage simultaneously applied to the tip. The model separates two different mechanisms, one linked to the response to DC bias and another one related to the AC excitation. It is argued that the second one is not diffusion-driven hut is rather a contribution of the sum of several mechanisms with at least one depending on the lithium ion concentration explaining the relaxation process. With proper fitting of this decay, diffusion coefficients of lithium hosts could be extracted. Additionally, the effect of phase transition in LiMn2O4 is taken into account, explaining some experimental observations. (C) 2015 AIP Publishing LLC

Two-scale computational homogenization of electro-elasticity at finite strains

This contribution addresses a two-scale computational homogenization framework for the simulation of electro-active solids at finite strains. A generalized form of the Hill-Mandel condition is employed for the derivation of energetically consistent transition conditions between the scales. The continuum mechanical formulation is implemented into a two-scale finite element environment, in which we attach a microscopic representative volume element at each integration point of the macroscopic domain. In order to allow for an efficient solution of the macroscopic boundary value problem an algorithmically consistent tangent of the macroscopic problem is derived. The method will be applied to the analysis of dielectric polymer-ceramic composites, where we determine the effective actuation of composites with different microstructures. Furthermore, we show the applicability of the proposed method to the computation of two-scale electro-mechanically coupled boundary value problems in consideration of large deformations

Affine full network model for strain-induced crystallization in rubbery polymers

We propose a micro-mechanically motivated material model for strain-induced crystallization in rubbers. Our point of departure is constructing a micro-mechanical model for a single crystallizing polymer chain. A thermodynamically consistent evolution law describing the kinetics of crystallization in the chain level is then proposed. The chain model is incorporated into the affine full network model. Finally, the numerical performance of the model is compared to the experimental dat