Kinetics of the Coil−Globule Transition of Poly(methyl methacrylate) in a Mixed Solvent

The coil−globule transition and chain collapse process were studied for poly(methyl methacrylate) in the mixed solvent tert -butyl alcohol + water (2.5 vol %) by static light-scattering measurements. The expansion factor α 2 for the mean-square radius of gyration was obtained as a function of time for various molecular weights. For the molecular weights M w × 10 -6 = 1.57, PMMA chains collapsed to an equilibrium globule within 30 min after quench. For M w × 10 -6 = 4.0 and 12.2, chain collapse processes were observed for time periods of hours and days, respectively. The time dependence of the observed expansion factor was represented by the stretched exponential form α 2 = α 2 ∞ + (1 − α 2 ∞ ) exp[−( t /τ*) β ], where α 2 ∞ is the equilibrium expansion factor sufficiently long time after quench. β was obtained in the range from 0.1 to 0.4 and decreased with decreasing temperature and increasing molecular weight. From the behavior of β, the stretched exponential process was suggested to be intrinsic to each polymer chain rather than due to a superposition of exponentially collapse processes of polymer chains with various decay times. The equilibrium expansion factor obtained as a function of temperature and molecular weight was represented by the theoretical prediction α 3 − α − C (α -3 − 1) = B (1 − θ/ T ) M 1/2 . The constants B and C were determined to be B = 0.0164 and C = 0.049 independent of the molecular weight.