The biocompatibility, processing ease, and mechanical properties of freeze-thawed poly(vinyl alcohol) (PVA)-based hydrogels have encouraged significant research toward developing this material for various biomedical applications. Crystallization that occurs during the freeze-thawing process is cited in the literature as the primary mechanism responsible for the resultant mechanical properties. Further analysis, however, shows the presence of two unique mechanisms that contribute to PVA's mechanical properties. During freeze–thaw cycling water freezes causing phase separation, which facilitates crystallization. The impact of phase separation during freeze–thaw cycling was investigated by comparing freeze-thawed and aged PVA hydrogels. Aged hydrogels were not prepared by freezing and, therefore, did not exhibit significant phase separation. The amount of phase separation was discerned using optical microscopy in the hydrated state. Crystallinity and mechanical properties were also evaluated as a function of the number of cycles (for freeze-thawed gels) and aging time (for aged gels). For freeze-thawed hydrogels, crystallinity deviated significantly from the trend observed in compressive modulus, indicating that crystallinity was not the only factor determining the hydrogel's mechanical properties. Phase separation was found to occur during freeze–thaw cycling independently of crystallization, especially at later freeze–thaw cycles (after the third). The trends observed for both crystallinity and modulus for aged hydrogels, however, were in better agreement with each other. Further evaluation of the mechanical properties of aged and freeze-thawed hydrogels with similar crystallinities indicated that freeze-thawed hydrogels have significantly higher modulus values (p < 0.05). As a result, phase separation, independently of crystallization, was determined to have a significant effect on gelation during freeze–thaw cycling. In particular, PVA-rich regions that are formed during phase separation, without additional cross-linking, are believed to have a significant effect on the resultant mechanical properties.

The highly crosslinked molecular architecture of thermosets makes this class of material strong but brittle. It is advantageous to enhance the ductility without sacrificing strength, glass transition temperature (T_g), and modulus. The hypothesis was tested that without altering the chemical structure, the network topology of a dense thermoset can be engineered to dissipate more energy before rupturing covalent bonds, producing a tougher material without sacrificing ultimate tensile strength, density, or glass transition temperature. A processing technique termed “Reactive Encapsulation of Solvent/Drying” (RESD) was used in which epoxy curing was conducted in the presence of varying amounts of inert small-molecule solvent, followed by a drying/annealing process in which the solvent was removed. Density measurements and freeze-fracture surface analysis revealed that the resulting RESD materials are not porous in their relaxed state after annealing. Comparing the dynamic mechanical response of the modified (RESD) to the unmodified (conventional) structures revealed no significant differences in glassy modulus and T_g. However, quasi-static mechanical testing showed that upon stretching, the modified structures have a volumetric energy capacity of up to 2.5 times that of the unmodified ones. SEM micrographs of fracture surfaces of RESD specimens indicated nano-sized cavities on the surface of the modified thermosets, which were not present before breaking. Therefore, the presence of distinct topological features in the modified network is likely the origin of the large improvement in ductility. Topology-based toughening is potentially an important step toward developing better high performance network polymers and composites.