Exploiting photoelasticity to characterize dynamics of polymer networks

The development of internal stresses within polymer networks is a major factor to consider when utilizing this class of soft materials, particularly when compared to their linear analogues. As an example, internal stresses that develop during network polymerizations often lead to macroscopic failures such as delamination or cracking when networks are constrained at one or multiple interfaces. To better understand and mitigate stress behavior, macroscopic mechanical characterization techniques (e.g. tensile testing, rheometry) have typically been employed. However, these methods provide little to no insight as to the dynamics of stress evolution on a local level, nor how internal stresses evolve in response to environmental changes. In this work, we exploit the photoelastic behavior of polymer networks to characterize internal stresses that develop during swelling and subsequent rupture of hydrogels. Compared to the majority of studies into mechanics of hydrogels, which typically characterize materials in one of two separate states (dry or swollen), this approach enables a non-invasive, real-time assessment of internal stress development during swelling. We discuss how this approach can better inform soft materials developed for dynamic environments.