Microphase-separated structures of poly(4-tert-butylstyrene-block-4-tert- butoxystyrene) upon gradual changes in segregation strength through hydrolysis reaction

The morphological change of poly(4-tert-butylstyrene-block-4-tert- butoxystyrene)s (BO) upon hydrolysis reaction was investigated by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Poly(4-tert-butoxystyrene) (O) can be converted into poly(4-hydroxystyrene) (H) through hydrolysis reaction. Four BOs having number-average molecular weight, M_n, from 38K to 148K were synthesized, and copolymers with various conversion rates of O into H, f_Hs, were prepared. No microphase-separated structure is observed for nonhydrolyzed samples, while all hydrolyzed ones exhibit predominantly lamellar structures with a small portion of cylindrical one. These results indicate that the repulsive force between blocks becomes stronger upon hydrolysis reaction. Domain spacing of the lamellar structure, D, is found to increase with an increase in f_H, where it abruptly increases at a critical conversion rate, f_H,c, indicating that the chains stretched perpendicularly to the lamellar interface. The normalized domain spacings, D/D₀, where D₀ is the correlation length at f_H = 0, were scaled by segregation strength, πN, where π is the interaction parameter and N is the degree of polymerization. Three regimes can be distinguished in the plot of D/D ₀ vs πN: (I) the weak segregation regime with D/D₀ ∼ 1 that associates with the scaling behavior of D ∼ N^0.55, (II) the intermediate segregation regime with the scaling behavior of D/D ₀ ∼ (πN)^0.34, and (III) the strong segregation regime with D/D₀ ∼ 2.3 which corresponds to D ∼ N ^0.67. Rough computation of D based on strong segregation limit theory shows that hydrogen bonding from self-association of H compensates for the repulsive force among components leading to a constant D after f_H,c.