Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation

The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 and 20 wt % of poly(styrene)-poly(isoprene)-poly(styrene) [PS-PI-PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step strain. The presence of loosely bounded PS aggregates at temperatures higher than the rheologically determined gelation temperature (Tgel) signifies the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures sufficiently lower than Tgel. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low strain. However, at large strain, the oriented structure splits because of strain localization suggesting that only a fraction of PI blocks participates in load bearing. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step strain have been captured using a stretched-exponential model. However, the microstructure recovery time has been found to be 2 orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of stress relaxation and microstructure recovery processes involving midblock relaxation, endblock pullout, and reassociation. Due to their viscoelastic nature, these gels' mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural change as a function of these parameters will assist these gels' real-life applications and design new gels with improved properties.