Effect of Polymer Molecular Weight and Synthesis Temperature on Structure and Dynamics of Microgels

Environmentally-sensitive microgels have been synthesized under varying conditions to study the dependences on polymer molecular weight (M$_{W})$ and synthesis temperature (T$_{syn})$. The dynamics and structure of the synthesized microgels below and above the LCST of the polymer (T$_{c}\sim $41$^{o}$C) were studied using dynamic and static light scattering spectroscopy. All microgels exhibit a volume phase transition above the LCST of the polymer and undergo a reversible 15-50-fold volume shrinkage. The size distribution, structure, deswelling ability, and temperature response of microgels strongly depend on synthesis conditions. T$_{syn}$ dependence was studied with 1000kDa polymer. Increasing $\Delta $T = T$_{syn}$ -- T$_{C}$ yields smaller microgels with a smaller swelling ratio up to $\Delta $T = 8.5$^{o}$C, after which the trend is reversed. The amphiphilic nature of the polymer may explain this trend. T$_{syn}$ also affects the structure of microgels; low T$_{syn}$ yields elongated particles, while high T$_{syn}$ microgels are more spherical. Polymer M$_{W}$ directly effects microgel polydispersity and temperature response. While microgels synthesized with 1000kDa polymer are relatively monodisperse, synthesis with low M$_{W}$ polymers (80-370kDa) yields systems with a large population (R$_{h} \quad \sim $1000nm) precipitating out of solution and a smaller population (R$_{h}$ $\sim $300nm) staying in suspension. M$_{W}$ also influences the temperature response of microgels; high M$_{W}$ microgels show a gradual shrinkage with increasing temperature while low M$_{W}$ microgels display a delayed and sudden shrinkage at high temperatures.