Insight into the Microscopic Structure of Module-Assembled Thermoresponsive Conetwork Hydrogels

The microscopic structure of module-assembled thermoresponsive conetworks was systematically investigated as a function of both temperature <i>T</i> and the mole fraction of the thermoresponsive modules <i>r</i> using small-angle neutron scattering (SANS). The conetworks were prepared by end-linking of hydrophilic modules and LCST-type thermoresponsive modules in water by the molar ratio of (1 – <i>r</i>):<i>r</i>. When the hydrogels with 0.02 ≤ <i>r</i> ≤ 0.10 were heated above certain <i>T</i>, nanometer-scale spherical domains were formed by aggregation of several prepolymer modules, whereas for the hydrogel with <i>r</i> = 0.01 such domain formation was not detected in the <i>T</i> range investigated. The size of spherical domains increased with increasing <i>r</i> and <i>T</i>. The observed <i>r</i> dependence of the domain size was theoretically explained by considering the free energy of domain formation, from which we concluded that the equilibrium domain size was determined mainly by the balance between two free energy contributions: the interfacial free energy of domain–matrix interface Δ<i>G</i><sub>interf</sub> and the conformational free energy of the matrix network Δ<i>G</i><sub>conf</sub>.