The mechanical properties of gels present qualitatively contradictory behavior; they are commonly soft but also notoriously brittle. We investigate the elasticity and fracture behavior of swollen polymer networks using a simple experimental method to induce cavitation within a gel and adapt scaling theories to capture the observed transition from reversible to irreversible deformations as a function of polymer volume fraction. It is shown quantitatively that the transition from reversible cavitation to irreversible fracture depends on the polymer volume fraction and an initial defect length scale. The use of cavitation experiments permits characterization of network properties across length scales ranging from mm to mm. We anticipate that these results may significantly enhance the understanding of mechanical properties of soft materials, both synthetic and biological.

Enzymes immobilized on solid supports are increasingly used for greener, more sustainable chemical transformation processes. Here, we used microreactors to study enzyme-catalyzed ring-opening polymerization of ε-caprolactone to polycaprolactone. A novel microreactor design enabled us to perform these heterogeneous reactions in continuous mode, in organic media, and at elevated temperatures. Using microreactors, we achieved faster polymerization and higher molecular mass compared to using batch reactors. While this study focused on polymerization reactions, it is evident that similar microreactor based platforms can readily be extended to other enzyme-based systems, for example, high-throughput screening of new enzymes and to precision measurements of new processes where continuous flow mode is preferred. This is the first reported demonstration of a solid supported enzyme-catalyzed polymerization reaction in continuous mode.

Gels are increasingly being used in many applications, and it is important to understand how these gels fail subjected to mechanical deformation. Here, we investigate the failure behavior of a thermoplastic elastomer gel (TPEG) consisting of poly(styrene)-poly(isoprene)-poly(styrene) in mineral oil, in tensile mode, under constant stress, and in fracture tests, where the fracture initiates from a predefined crack. In these gels, the poly(styrene) endblocks associate to form spherical aggregates, as captured using SAXS. Shear-rheology experiments indicate that the poly(isoprene) midblocks connecting these aggregates are loosely entangled. The relaxation behavior of these gels has been captured by time-temperature superposition of frequency sweep data and stress-relaxation experiments. The relaxation process in these gels involves endblock pullout from the aggregates and subsequent relaxation of the chains. An unfavorable enthalpic interaction between the endblock and mineral oil results in a significantly large relaxation time. These gels display rate dependent mechanical properties, likely due to the midblock entanglements. Fracture and creep failure tests provide insights into the gel failure mechanism. Creep experiments indicate that these gels fail by a thermally activated process. Fracture experiments capture the energy release rate as a function of crack-tip velocity. The critical energy release rate is estimated by incorporating the friction force the polystyrene chains are subjected to, as those are pulled out of aggregates, and the enthalpic cost to overcome unfavorable interaction between poly(styrene) and mineral oil. Our results provide further insights to the failure behavior of the self-assembled TPEGs.

Mechanical properties including the failure behavior of physically assembled gels or physical gels are governed by their network structure. To investigate such behavior, we consider a physical gel system consisting of poly(styrene)-poly(isoprene)-poly (styrene)[PS-PI-PS] in mineral oil. In these gels, the endblock (PS) molecular weights are not significantly different, whereas, the midblock (PI) molecular weight has been varied such that we can access gels with and without midblock entanglement. Small angle X-ray scattering data reveals that the gels are composed of collapsed PS aggregates connected by PI chains. The gelation temperature has been found to be a function of the endblock concentration. Tensile tests display stretch-rate dependent modulus at high strain for the gels with midblock entanglement. Creep failure behavior has also been found to be influenced by the entanglement. Fracture experiments with predefined cracks show that the energy release rate scales linearly with the crack-tip velocity for all gels considered here. In addition, increase of midblock chain length resulted in higher viscous dissipation leading to a higher energy release rate. The results provide an insight into how midblock entanglement can possibly affect the mechanical properties of physically assembled triblock copolymer gels in a midblock selective solvent.