Unusual swelling behavior of core-shell microgels built from polymers exhibiting lower critical solubility temperature

Using a combination of two LCST-type polymers a set of new core-shell microgels was obtained. Interestingly, the swelling behavior of obtained core-shell microgels was characteristic for the polymeric gels that possess both UCST-type and LCST-type properties. The cores were based on cross-linked poly(N-isopropylacrylamide-sodium acrylate) and were obtained by employing the precipitation polymerization. Then, the shells, based on cross-linked poly(N-isopropylacrylamide-sodium acrylate), were built over the cores via the seed polymerization. The swelling behaviors of the microgels were investigated by doing DLS experiments in a temperature range of 20–50 °C. This microgel appeared to be the first relatively simple system that can exist in the swollen state only in a narrow temperature range. The swelling of the microgels took place at temperature below the volume phase transition temperature (VPTT). The maximum value of radius appeared at VPTT of the core. This phenomenon was observed for various compositions of the core and the shell and in a wide range of pH. Differential scanning calorimetry measurements showed that the swelling process occurring at a temperature below VPTT is endothermic. The reported particle properties may open new possibilities in application of such materials in the fields of controlled release of various molecules, construction of sensors and chemical separations.