Experiments and modeling of the chemo-mechanically coupled behavior of polymeric gels

Polymeric materials consist of mutually entangled or chemically crosslinked long njitmolecular chains which form a polymer network. Due to their molecular structure, the njitpolymeric materials are known to undergo large deformation in response to various njitenvironmental stimuli, such as temperature, chemical potential and light. When a polymer network is exposed to a suitable chemical solvent, the solvent molecules are able to diffuse inside the network, causing it to undergo a large volumetric deformation, known as swelling. In addition to volumetric deformation, this process involves the chemical mixing of the polymer network and solvent molecules, and is typically environmentally responsive. A polymeric material in this mixed and swollen state is known as a polymeric gel. Swollen polymers, or polymeric gels, find their application in the oil industry, soft robotics, drug delivery and microfluidic channels. Moreover, most of the organs inside our body are gel-like in structure, which makes this class of materials important for biomedical applications and tissue engineering. An important distinction between biological tissues and much of the previous literature on the mechanics of polymeric gels is that most biological tissues contain fibers. The existence of these fibers embedded in the material, causes the properties to be significantly different along the fiber direction. Recent years have seen the development of a vast number of large deformation continuum-level constitutive models aimed to capture the coupled diffusion-deformation behavior of polymeric gels. However, there is an insufficient amount of experimental data to complement such theoretical research. Thus, despite numerous potential applications, many aspects of polymeric gel behavior remain elusive. In addition, the diffusion-deformation behavior is known to be affected by the external stimuli. In the current state of the art there is a lack of theoretical models and robust simulation capabilities to account for the influence of such stimuli, hindering further advances in technologies involving polymeric gels. The purpose of this research is to bridge the gap between the experimental and theoretical studies, and provide reliable finite element simulation capabilities for polymeric gels. More specifically, the aim is to (i) experimentally characterize the behavior of polymeric gels, (ii) develop new experimentally motivated constitutive models and (iii) implement the models numerically for use in a finite element software. The final result of this research is a robust finite element method (FEM) code that can be used for simulations in the commercial software package Abaqus. Towards the goal, an experimental procedure is designed to thoroughly investigate the behavior of polymeric gels, and provide a direction for the development of novel constitutive models. The procedure involves mechanical testing of dry polymeric material, free swelling with suitable solvents, and mechanical testing when fully swollen. The experimental observations provide transformational insights in the mechanical behavior of polymeric gels, and are utilized to develop a continuum-level constitutive model. Further, the presence of embedded fibers in a swellable polymer matrix leads to anisotropy in the overall behavior. In order to capture this response, a constitutive model for fiber-reinforced polymeric gels is developed, that explicitly takes into account anisotropy in both the mechanical and diffusive behavior. The constitutive model is implemented as user element subroutine (UEL) in the commercial finite element software package Abaqus/Standard. Numerical simulations are performed to show the behavior of the model, and qualitative comparisons are made to experiments of a soft robotic gripper. In addition, many polymeric gels are known to respond or activate when exposed to a light stimulus. This light-driven alteration of the behavior is known to be caused by the photochemical reactions occurring inside the polymer network. Thus, the overall response of light-activated polymeric gels is affected by the mechanical stress, solvent content, and the extent of photochemical reaction caused by light irradiation. To account for such response of a polymeric gel, a continuum level constitutive model is developed and numerically implemented in Abaqus/Standard as a user element (UEL) subroutine