CONFORMATION OF BLOCK COPOLYMERS IN DILUTE-SOLUTION .3. DETERMINATION OF THE CENTER-TO-CENTER DISTANCE BETWEEN THE 2 BLOCKS BY LIGHT-SCATTERING

The conformation of an AB diblock copolymer in dilute solution may be characterized by the mean-square radii (S2)A and (S2)B of the two blocks and the mean-square distance (G2) between the centers of mass of them. It was previously established that (S2)K (K = A or B) is almost identical with the radius (S2)H-k of the equivalent K homopolymer, unless the A-B interactions are attractive. In such a situation, it is possible to determine (G2) by light scattering with a solvent having large refractive index increments for the two homopolymers. Experiments were conducted on polystyrene-poly(methyl methacrylate) diblock copolymers in 2-butanone. The parameter representing the extent of “segregation” of the two blocks, σ=(G2)/(2(S2)A + 2(S2)B), was evaluated with due regard to the sample heterogeneity. The influence of heterogeneity was found to be unexpectedly large even for a fairly homogeneous sample, say Mw/Mn < 1.1, and without adequate correction to this effect, the conclusion drawn from such an analysis could be misleading. It was found that lies σ very close to 1.2. This shows that the conformation of the block copolymer is almost the same as that of the homopolymers. Models like “segregated” and “core-in-shell” conformations are utterly at variance with reality. © 1979, American Chemical Society. All rights reserved.