Hydrophobic-hydrophilic comb-type quaternary ammonium-functionalized SEBS copolymers for high performance anion exchange membranes

Abstract

Poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) triblock copolymers are prepared with 50% hexyl quaternary ammonium (HQA) and 20% alkyl chains of different chain lengths: C-4 (50HQA-20C(4)-SEBS) and C-12 (50HQA-20C(12)-SEBS). Their physicochemical and electrical properties are investigated for use as novel alkaline anion exchange membranes. In most other comb-type polymers, the alkyl chain is introduced as a hydrophilic spacer unit between the polymer backbone and conducting head group. In the current study, additional alkyl chains are grafted onto the SEBS polymer as hydrophobic side chains as well as typical hydrophilic side chains. Introduction of this extra hydrophobic alkyl spacer group is done to obtain well-connected morphology, increased free volume, enhanced water uptake and related conductivity, together with improved physicochemical properties of the membranes obtained. The results are compared with HQA-functionalized SEBS without the hydrophobic alkyl spacer group (50HQA-20C(0)-SEBS), and the effects of the alkyl chain lengths on the structure of polymers and physicochemical properties of the corresponding membranes are investigated.