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 (MW) and synthesis temperature (Tsyn). The dynamics and structure of the synthesized microgels below and above the LCST of the polymer (Tc∼41oC) 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. Tsyn dependence was studied with 1000kDa polymer. Increasing ΔT = Tsyn -- TC yields smaller microgels with a smaller swelling ratio up to ΔT = 8.5oC, after which the trend is reversed. The amphiphilic nature of the polymer may explain this trend. Tsyn also affects the structure of microgels; low Tsyn yields elongated particles, while high Tsyn microgels are more spherical. Polymer MW directly effects microgel polydispersity and temperature response. While microgels synthesized with 1000kDa polymer are relatively monodisperse, synthesis with low MW polymers (80-370kDa) yields systems with a large population (Rh∼1000nm) precipitating out of solution and a smaller population (Rh ∼300nm) staying in suspension. MW also influences the temperature response of microgels; high MW microgels show a gradual shrinkage with increasing temperature while low MW microgels display a delayed and sudden shrinkage at high temperatures