Chain aggregation in dilute solutions of poly(methyl methacrylate) below the phase-separation temperature

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.