Critical fluctuations and static inhomogeneities in polymer gel volume phase transitions

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.

Connecting Elasticity and Effective Interactions of Neutral Microgels: The Validity of the Hertzian Model

“…Such a behavior might be explained, at least from a qualitative standpoint, by noting that K is the inverse of the isothermal compressibility χ T . Even though it is still not clear whether the VPT in microgels should be modeled as a proper phase transition or as a crossover, 92,93 χ T should always display a maximum at the transition (or crossover) temperature. Nevertheless, we do not observe any minimum in K. One possible reason for this behavior could be the small size of the investigated microgels, whereas another reason could be the fact that we use an implicit solvent.…”

Inner structure and dynamics of microgels with low and medium crosslinker content prepared via surfactant-free precipitation polymerization and continuous monomer feeding approach

“…The obtained values correspond to the microgel mesh sizes reported in the literature. 11,38,60 Thus, the polymers in the network of f-MG x are distributed more uniformly and behave like Gaussian chains in solution (exponents of the fractal Porod law of 1.5-2.5). The crosslinker in b-MG x is mostly localized in the core part of the particle.…”

Deswelling and deformation of microgels in concentrated packings

“…For example, they can be compressed to packing density in excess of random close packing and their increase in viscosity as they approach the liquid-solid boundary shows significant deviations from the behaviour of hard spheres 6 , 7 . Moreover, microgels exhibit a rich phase behaviour 8 – 10 , which can be tailored by their degree of crosslinking 11 , the presence of charges 12 or inherent network inhomogeneities 13 .…”