Phase Transition in End-Functionalized Polystyrene-block-polyisoprene-block-polystyrene Copolymers

Abstract

Phase transition in polystyrene-block-polyisoprene-block-polystyrene (SIS triblock) copolymers end-capped with carboxylic acid group (-COOH) or sodium carboxylate group (-COONa) was investigated using oscillatory shear rheometry and transmission electron microscopy (TEM). For the study, three SIS triblock copolymers having almost an equal block length ratio, but varying molecular weights, were synthesized via sequential anionic polymerization. The number-average molecular weights of the three block copolymers synthesized were 1.8 x 10 4 g/mol (SIS1), 2.3 x 10 4 g/mol (SIS2), and 3.4 x 10 4 g/mol (SIS3). It was found that both SIS1 and SIS2 were homogeneous block copolymers, while SIS3 had lamellar microdomains, and it had an order-disorder transition temperature (T ODT ) of 180 °C. Each of the triblock copolymers was end-capped with a -COOH group to obtain SIS-COOH using high-purity, gaseous carbon dioxide. Subsequently, part of the SIS-COOH was neutralized with sodium hydroxide to obtain SIS-COONa. It was found that SIS1-COOH, SIS1-COONa, and SIS2-COOH remained homogeneous, while SIS2-COONa microphase-separated into a lamella-forming triblock copolymer, as determined from TEM, which has a Tool of 123 °C as determined from oscillatory shear rheometry. The T ODT of lamella-forming SIS3 was found to increase from 180 to 182 °C after it was end-capped with a -COOH group and to 228 °C after it was end-capped with a -COONa group. Cloud point measurements show that (i) the upper critical solution temperature (UCST) of the mixtures composed of polyisoprene (PI) and polystyrene (PS) end-capped with a -COONa group, PI/(PS-COONa) mixtures, is much higher than that of the mixtures composed of PI and PS end-capped with a -COOH group, PI/(PS-COOH) mixtures, and PIPS mixtures, and (ii) the UCST of PI/(PS-COOH) mixtures is only slightly higher than that of PI/PS mixtures. It is concluded that the formation of lamellar microdomains in SIS2-COONa from homogeneous SIS2 after it was end-capped with a -COONa group and a very high T ODT observed for SIS3-COONa are attributable to strong ionic associations and significantly increased repulsive segment-segment interactions between the PI and PS-COONa phases in the block copolymers.