Mechano-optical behavior of loosely crosslinked double-network hydrogels: Modeling and real-time birefringence measurement during uniaxial extension

A Review on Novel Channel Materials for Particle Image Velocimetry Measurements-Usability of Hydrogels in Cardiovascular Applications

Particle image velocimetry (PIV) is an optical and contactless measurement method for analyzing fluid blood dynamics in cardiovascular research. The main challenge to visualization investigated in the current research was matching the channel material’s index of refraction (IOR) to that of the fluid. Silicone is typically used as a channel material for these applications, so optical matching cannot be proven. This review considers hydrogel as a new PIV channel material for IOR matching. The advantages of hydrogels are their optical and mechanical properties. Hydrogels swell more than 90 vol% when hydrated in an aqueous solution and have an elastic behavior. This paper aimed to review single, double, and triple networks and nanocomposite hydrogels with suitable optical and mechanical properties to be used as PIV channel material, with a focus on cardiovascular applications. The properties are summarized in seven hydrogel groups: PAMPS, PAA, PVA, PAAm, PEG and PEO, PSA, and PNIPA. The reliability of the optical properties is related to low IORs, which allow higher light transmission. On the other hand, elastic modulus, tensile/compressive stress, and nominal tensile/compressive strain are higher for multiple-cross-linked and nanocomposite hydrogels than single mono-cross-linked gels. This review describes methods for measuring optical and mechanical properties, e.g., refractometry and mechanical testing.

Nonlinear photoelasticity of rubber-like soft materials: comparison between theory and experiment

Photoelasticity often refers to the birefringence effect of materials induced by elastic deformation. Recently, many experiments on the photoelasticity of soft materials have been reported. However, the experimental results are mainly qualitative observations and lack any theoretical analysis. In this paper, we revisit Treloar's and Arruda's models of nonlinear photoelasticity for rubber-like materials. Both models establish the intrinsic relationship between stretch and birefringence, based on the statistics of chain polarizability and a network theory. We discuss the difference of the two models and build an experimental setup to measure the birefringence of PDMS samples as a function of stress/stretch. We vary the curing ratio of PDMS to study the effect of chain density on birefringence and compare with Treloar's theory. We further use experimental data of double-network hydrogels in the literature to compare with theory and find that when the deformation is large compared with the limiting stretch of the material, Arruda's model fits the experimental data much better than Treloar's model. This work presents a basis of using the theory of nonlinear photoelasticity to guide the analysis of experiments.

Molecular Characterization of Polymer Networks

Polymer networks are complex systems consisting of molecular components. Whereas the properties of the individual components are typically well understood by most chemists, translating that chemical insight into polymer networks themselves is limited by the statistical and poorly defined nature of network structures. As a result, it is challenging, if not currently impossible, to extrapolate from the molecular behavior of components to the full range of performance and properties of the entire polymer network. Polymer networks therefore present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. A barrier to sophisticated molecular approaches to polymer networks is that the techniques for characterizing the molecular structure of networks are often unfamiliar to many scientists. Here, we present a critical overview of the current characterization techniques available to understand the relation between the molecular properties and the resulting performance and behavior of polymer networks, in the absence of added fillers. We highlight the methods available to characterize the chemistry and molecular-level properties of individual polymer strands and junctions, the gelation process by which strands form networks, the structure of the resulting network, and the dynamics and mechanics of the final material. The purpose is not to serve as a detailed manual for conducting these measurements but rather to unify the underlying principles, point out remaining challenges, and provide a concise overview by which chemists can plan characterization strategies that suit their research objectives. Because polymer networks cannot often be sufficiently characterized with a single method, strategic combinations of multiple techniques are typically required for their molecular characterization.