Habicht, A.; Schmolke, W.; Goerigk, G.J.; Lange, F.; Saalwächter, K.; Ballauff, M.; Seiffert, S.: Critical Fluctuations and Static Inhomogeneities in Polymer Gel Volume Phase Transitions. Journal of Polymer Science A: Polymer Chemistry 53 (2015), p. 1112–1122
10.1002/polb.23743

Abstract:
Thermoresponsive polymer gels exhibit pronounced swelling and deswelling upon changes in temperature, accompanied by dynamic concentration fluctuations that have been interpreted as critical opalescence. These fluctuations span lengthscales similar to that of static structures in the gels, such as the gel polymer-network meshsize (1–10 nm) and static polymer-network crosslinking inhomogeneities (10– 1000 nm). To systematically investigate this overlay, we use droplet-based microfluidics and fabricate submillimeter-sized gel particles with varying static heterogeneity, as revealed on a molecular scale by proton NMR. When these microgels are probed by small-angle neutron scattering, the detection of dynamic fluctuations during the volume phase transitions is strongly perturbed by the co-existing static inhomogeneity. Depending of the type of data analysis employed, the temperature-dependent evolution of the correlation length associated to the dynamic fluctuations does or does not agree with predictions by the critical scaling theory. Only the most homogeneous sample of this study, prepared by controlled polymer crosslinking in droplet microfluidics, shows a diverging correlation length in agreement to the critical scaling theory independent of the specific approach of data analysis. These findings suggest that care must be taken about polymernetwork heterogeneity when gel volume phase transitions are evaluated as critical phenomena.