Chain Aggregation and Chain Collapse of Poly(methyl methacrylate) in a Mixed Solvent

Chain aggregation processes were studied for dilute solutions of PMMA with the molecular weight m = 1.57 × 10 6 g/mol in the mixed solvent tert -butyl alcohol + water (2.5 vol %) at 25.0 and 30.0 °C in the concentration range from 1.4 × 10 -4 to 5.7 × 10 -4 g/cm 3 . The weight-average molecular weight 〈 M 〉 w and z -average squared radius 〈 R 2 〉 z of clusters of polymer chains were determined as a function of time by static light-scattering measurements. The phase separation temperature increased from 35.7 to 36.7 °C in the concentration range. The aggregation process near the concentration c = 1.46 × 10 -4 g/cm 3 was observed for a time period of about 10 000 min at 25.0 °C and 2000 min at 30.0 °C. At each temperature both the plots of ln〈 M 〉 w and ln〈 R 2 〉 z vs t (min) were represented by curved lines. The initial slopes of the lines were proportional to the concentration c . Accordingly, the scaled relations of 〈 M 〉 w / M (0) = exp( gct ) and 〈 R 2 〉 z / R 2 (0) = exp( hct ) were obtained at small ct with g = 5.01 cm 3 /(g min) and h = 3.97 cm 3 /(g min) at 25.0 °C and with g = 16.9 and h = 13.2 at 30.0 °C, where M (0) and R 2 (0) are the values extrapolated to t = 0. The reciprocals of gc and hc give the characteristic times of the chain aggregation. The longer characteristic times at 25.0 °C than at 30.0 were attributed to the effect of the chain collapse. The exponential growth indicated a reaction-limited cluster aggregation of polymer chains. The double-logarithmic plot of 〈 M 〉 w vs 〈 R 2 〉 z 1/2 at each temperature was independent of concentration and represented by a single straight line with the slope D = 2.53 ± 0.02 at 25.0 °C and 2.66 ± 0.01 at 30.0 °C.