Effect of alkyl side chain length on microscopic structures and mechanical properties of ionically-functionalized block polymer-based thermoplastic elastomers

Abstract

Recently, the demand for thermoplastic elastomers (TPEs) with high tensile strength and toughness has grown. In this study, we synthesized ionically-functionalized polystyrene-b-polyisoprene-b-polystyrene (i-SIS(n)) containing 7.1 mol% of monosodium succinate mono-alkyl amide units in the middle I block of SIS and investigated the effects of the alkyl side chain length on the formation of microscopic ionic aggregates and the mechanical properties, where n represents the carbon number of the alkyl side chains. Both neat SIS and i-SIS(n) formed cylindrical structures, while the microscopic structures in i-SIS(n) exhibited slightly smaller domain spacing and poor orientation of domains compared to those in neat SIS. Furthermore, i-SIS(n) formed microscopic ionic aggregates, and the core dimensions of these aggregates increased as the alkyl side chain length shortened. The relaxation behavior of i-SIS(n) was associated with the disassembly/re-assembly of the ionic aggregates. The time scale of this relaxation behavior tended to be longer for i-SIS(n) with the shorter alkyl side chains, suggesting that the ionic aggregates in i-SIS(n) with shorter alkyl side chains were stronger than those in i-SIS(n) with longer alkyl side chains. These results were attributed to the easier approach among ion pairs, facilitated by the reduced steric hindrance associated with the shorter alkyl chain length. Moreover, the ionic aggregates acted as physical cross-links, leading to superior tensile properties of i-SIS(n) compared to neat SIS. Additionally, the shorter alkyl side chains contributed to greater toughness as compared to the longer alkyl side chains.