A. Habicht, W. Schmolke, G. Goerigk, F. Lange, K. Saalwächter, M. Ballauff, S. Seiffert.
Critical Fluctuations and
Static Inhomogeneities in Polymer Gel
Volume Phase Transitions.
J. Polym. Sci., Part B: Polym. Phys. 53, 1112-1122 (2015).
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.