Effect of the degree of neutralization on the micellization of block ionomers

Abstract

The effect of neutralization on the micellization of polystyrene-b-poly(acrylic acid) (PS-b-PAA) was investigated by static light scattering (SLS) and small-angle X-ray scattering (SAXS). The block copolymers, present initially in single chain form in either dioxane or benzene–methanol (90/10 (v/v)), were neutralized to different degrees by the addition of cesium hydroxide dissolved in methanol. The solutions were simultaneously monitored by SLS. It was found that micelle formation began near 5% neutralization. The normalized scattered light intensity at 90° (I90/c) increased dramatically in the neutralization range between ca. 10 and 60%. For neutralization degrees between 60 and ca. 100%, I90/c did not change significantly with a further increase in the percent of neutralization. In the second part of the study, two block copolymers, PS(600)-b-PAA(34) and PS(600)-b-PAA(45) (the number of monomer units is given in parentheses), were prepared from benzene–methanol (90/10 (v/v)) with different degrees of neutralization: 5, 25, 60, 100, and 150%. The resulting solutions were freeze-dried and the dried samples were redissolved in toluene. These samples were investigated by SLS and SAXS. By SLS, the aggregation numbers (f) were found to be the same between 5 and 60% neutralization. An increase in f was observed in going from 60 to 100%; i.e., f increased from 82 to 92 for PS(600)-b-PAA(34), and from 79 to 110 for PS(600)-b-PAA(45). The f values for the two block copolymers at 150% neutralization were found to be the same as those for 100% neutralization. The core radii values (Rcore) were measured for the neutralized PS(600)-b-PAA(45) samples at 0.05 g/mL in toluene by SAXS. The Rcore values increased from 5 to 60% neutralization, remained constant from 60 to 100%, and increased again in the range from 100 to 150%. The results are explained by dynamic considerations. For low percent neutralization, between 5 and 60%, the micelles and single chains are proposed to be in a dynamic equilibrium state. For higher percent neutralization, between 60 and 100%, the dynamics are probably much slower. Keywords: block copolymers, micelles, ionomers, light scattering, small angle X-ray scattering (or SAXS).