Strain-dependent microstructure evolution of ABA triblock gels captured using RheoSAXS*

ABA triblock copolymer gels dissolved in a B-block selective solvent are being investigated in many applications, from personal products to ballistic applications. Here, we present the transient microstructure of two ABA triblock copolymer gels captured using combined rheology and scattering (RheoSAXS) experiments. These two gels are poly(methyl methacrylate)-poly(n-butyl acrylate)-poly(methyl methacrylate) [PMMA-PnBA-PMMA] dissolved in 2-ethyl-1-hexano1, and poly(styrene)−poly(isoprene)−poly(styrene) [PS−PI−PS] dissolved in mineral oil.  The large-amplitude oscillatory strain was applied to these gels, and the microstructural evolution in a given strain cycle was captured. An isotropic scattering profile, typical to gels with A-block aggregates, was observed when no strain was applied. A polydispersed core hard sphere model was used to quantify the microstructural features, such as the size of A-aggregates and the distance between them. In a strain cycle, microstructure oriented in the applied strain direction resulting in an elliptical pattern, and the affine deformation model could capture the data adequately. However, at large strain amplitude, the PS-PI-PS gel displayed both circular and elliptical patterns, indicating some midblocks (B blocks) did not participate in load-bearing, likely caused by strain localization. The polymer concentration, solvent quality, and midblock length play a role in these gels' rheology and microstructure evolution.