Associating Behavior of Sulfonated Polyisoprene Block Copolymers with Short Polystyrene Blocks at Both Chain Ends

Abstract

Self-association of highly asymmetric triblock copolymers of sulfonated isoprene (SI) with short styrene (St) blocks at both chain ends (St−SI−St) was studied. The studies were carried out using fluorescence probe, quasielastic light scattering, and static light scattering (SLS) techniques. Three block copolymers having different degrees of polymerization of the St block (DPSt) and SI block (DPSI) were employed, i.e., polymers with DPSt = 6 and DPSI = 350 (St−SI−St-5), DPSt = 11 and DPSI = 320 (St−SI−St-10), and DPSt = 19 and DPSI = 240 (St−SI−St-20). The formation of hydrophobic microdomains in water was indicated by fluorescence probe experiments using 1,6-diphenyl-1,3,5-hexatriene and pyrene. The presence of a critical polymer concentration for the onset of the hydrophobic domain formation (cmc) was indicated by excitation spectra of pyrene fluorescence. Apparent cmc values, obtained by a method that does not take into account the partition of pyrene between the aqueous and polymer phases, are on the order of 10-1−10-2 g/L. The existence of cmc suggests that micelles are formed via a closed association in equilibrium. The formation of micelles with increasing polymer concentration (Cp) is preceded by the formation of oligomeric aggregates in solutions of all the three polymers as suggested by analysis of pyrene fluorescence spectra. The hydrodynamic radii (Rh) of unimers and the oligomeric aggregates are on the order of a few nanometers. Values of Rh for micelles are 40−50 nm for St−SI−St-5 (Cp = 1−4 g/L) and 28−33 nm for St−SI−St-20 (Cp = 0.1−2.25 g/L), being independent of Cp in these Cp ranges. In the St−SI−St-20 system at Cp higher than 2.25 g/L, Rh of micelles grows as a result of bridging. In the case of St−SI−St-10, however, the presence of both micelles (Rh = 20−30 nm at Cp = 0.25−2.0 g/L) and bridged micelles (Rh = ca. 100 nm at Cp = 0.75−1.75 g/L) was found above cmc. The size of these bridged micelles showed a strong dependence on Cp. Taken together with weight-average masses of the micelles and aggregation numbers estimated from SLS, flower-type micelles were proposed as a hypothetical model for the triblock copolymer aggregates.