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

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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 α2 for the mean-square radius of gyration was obtained as a function of time for various molecular weights. For the molecular weights Mw × 10-6 = 1.57, PMMA chains collapsed to an equilibrium globule within 30 min after quench. For Mw × 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 α2 = α2∞ + (1 − α2∞) exp[−(t/τ*)β], where α2∞ 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 α3 − α − C(α -3 − 1) = B(1 − θ/T)M1/2. The constants B and C were determined to be B = 0.0164 and C = 0.049 independent of the molecular weight.