Thermoresponsiveness\nAcross the Physiologically Accessible\nRange: Effect of Surfactant, Cross-Linker, and Initiator Content on\nSize, Structure, and Transition Temperature of Poly(<i>N</i>‑isopropylmethacrylamide) Microgels

The influence of surfactant, cross-linker, and initiator\non the\nfinal structure and thermoresponse of poly(<i>N</i>-isopropylmethacrylamide)\n(pNIPMAM) microgels was evaluated. The goals were to control particle\nsize (into the nanorange) and transition temperature (across the physiologically\naccessible range). The concentration of the reactants used in the\nsynthesis was varied, except for the monomer, which was kept constant.\nThe thermoresponsive suspensions formed were characterized by dynamic\nlight scattering, small-angle X-ray scattering, atomic force microscopy,\nand rheology. Increasing surfactant, sodium dodecyl sulfate content,\nproduced smaller microgels, as expected, into the nanorange and with\ngreater internal entanglement, but with no change in phase transition\ntemperature (LCST), which is contrary to previous reports. Increasing\ncross-linker, <i>N</i>,<i>N</i>-methylenebis acrylamide,\ncontent had no impact on particle size but reduced particle deformability\nand, again contrary to previous reports of decreases, progressively\nincreased the LCST from 39 to 46 °C. The unusual LCST trends\nwere confirmed using different rheological techniques. Initiator,\npotassium persulfate, content was found to weakly influence the outcomes.\nAn optimized content was identified that provides functional nanogels\nin the 100 nm (swollen) size range with controlled LCST, just above\nphysiological temperature. The study contributes chemistry-derived\ndesign rules for thermally responsive colloidal particles with physiologically\naccessible LCST for a variety of biomedical and soft robotics applications.