The coil-globule transition was studied by static light scattering measurements on poly͑methyl methacrylate͒ with the molecular weight M w ϫ10 Ϫ6 ϭ2.35 and 4.4 in isoamyl acetate. Since the phase separation of the dilute solution occurred very slowly, the measurements could be made in the broad temperature range from near the ⌰ temperature 61°C to 0°C 30 min after a quench of the solution. The observed expansion factor ␣ 2 for the radius of gyration was represented as a function only of M 1/2 and showed a constant value at large ϪM 1/2 with being 1Ϫ⌰/T. A quantitative comparison between a recent theory for a contracted coil and the data of ␣ 2 revealed the coil-globule crossover phenomena. The behavior of plot of 1/␣ 3 versus ϪM 1/2 was distinctly different in the three ranges, i.e., coil range, globule range, and range of a constant ␣. The plot of the observed second virial coefficient A 2 against temperature yielded a minimum as predicted from a theory of A 2 below the ⌰ temperature. ͓S1063-651X͑97͒11809-8͔

The coil-globule transition was studied by static light scattering measurements on poly͑methyl methacrylate͒ with the molecular weight M w ϫ10 Ϫ6 ϭ2.35 and 4.4 in isoamyl acetate. Since the phase separation of the dilute solution occurred very slowly, the measurements could be made in the broad temperature range from near the ⌰ temperature 61°C to 0°C 30 min after a quench of the solution. The observed expansion factor ␣ 2 for the radius of gyration was represented as a function only of M 1/2 and showed a constant value at large ϪM 1/2 with being 1Ϫ⌰/T. A quantitative comparison between a recent theory for a contracted coil and the data of ␣ 2 revealed the coil-globule crossover phenomena. The behavior of plot of 1/␣ 3 versus ϪM 1/2 was distinctly different in the three ranges, i.e., coil range, globule range, and range of a constant ␣. The plot of the observed second virial coefficient A 2 against temperature yielded a minimum as predicted from a theory of A 2 below the ⌰ temperature.

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 doublelogarithmic 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.

For dilute solutions of poly(methyl methacrylate) in isoamyl acetate with the molecular weight Mw=2.35×106, the phase separation process was observed as an aggregation process of polymer chains by light-scattering measurements. The aggregation process was measured for a period of hours at four polymer concentrations and at about 15 K below the phase separation temperature. The light-scattering data analysis by Guinier plot yielded the average molecular weight 〈M〉w and radius 〈R2〉z1/2 for polymer aggregates as a function of time t and revealed the exponential growth of 〈M〉w∼egt and 〈R2〉z∼eht. The coefficients g and h were proportional to the polymer concentration. A shape of the observed scattering function was independent of the concentration and time. The fractal dimension for 〈M〉w∼〈R2〉zD/2 was determined to be D=2.86±0.03. These characteristic features of the polymer aggregation were represented by the Smoluchowski equation for cluster–cluster aggregation with the collision kernel (i+j) for imer and jmer. The observed scattering function and fractal dimension were analyzed by the Smoluchowski equation with the assumed value D=3 for monodisperse clusters.

For dilute solutions of poly(methyl methacrylate) in isoamyl acetate with the molecular weight M(w)=4.4 x 10(6), the phase-separation process was studied by static light-scattering measurements. The dilute solutions in the concentration range from 1.4 x 10(-4) to 3.8 x 10(-4) g/cm(3) were quenched to about 16 K below the phase-separation temperature, and the aggregation processes of polymer chains were measured over a period of several hours. By analyzing the light-scattering data with the Guinier plot, the weight-averaged molecular weight (w) and z-averaged square radius (1/2)(z) for clusters of polymer chains were determined as a function of time t(min) and concentration c (g/cm(3)). The growth of clusters was represented by the exponential forms (w)/M(0)=e(gct) and (z)/R(2)(0)=e(hct) as a function of ct, where M(0) and R2(0) were the values at t=0, and the constants g and h were determined to be g=11.6 and h=7.5. A double-logarithmic plot of (w) versus (1/2)(z) yielded a straight line with the slope D=3.06+/-0.02. These characteristic features of the chain aggregation process were compared with the Smoluchowski equation for cluster-cluster aggregation with the collision kernel (i+j) for i-mer and j-mer. The observed slow growth of clusters was attributed to the reaction- limited cluster aggregation. The chain density in a cluster was found to increase with an increase of the cluster size, resulting in the slope D exceeding 3.