A new route to nanostructured thermosets with block ionomer complexes

We report a novel approach to prepare nanostructured thermosets using block ionomer complexes. Neither block copolymer polystyrene-<i>block</i>-poly(ethylene-<i>ran</i>- butylene)-<i>block</i>-polystyrene (SEBS) nor block ionomer sulfonated SEBS (SSEBS) is miscible with diglycidyl ether of bisphenol A (DGEBA) type epoxy resin. It is thus surprising that the block ionomer complex of SSEBS with a tertiary amine-terminated poly(3-caprolactone) (PCL), denoted as SSEBS-<i>c</i>-PCL, can be used to prepare nanostructured epoxy thermosets. The block ionomer complex SSEBS-<i>c</i>-PCL is synthesized <i>via </i>neutralization of SSEBS with 3-dimethylamino- propylamine-terminated PCL. Sulfonation of SEBS yields the block ionomer SSEBS which is immiscible with epoxy. But the block ionomer complex SSEBS-<i>c</i>-PCL can be easily mixed with DGEBA. When the curing agent 4,4'-methylenedianiline (MDA) is added and the epoxy cures, the system retains the nanostructure. In cured epoxy thermosets containing up to 30 wt% SSEBS-<i>c</i>-PCL, the exclusion of the poly(ethylene-<i>ran</i>-butylene) (EB) phase forms spherical micro-domains surrounded by separated sulfonated polystyrene phase while the PCL side-chains of SSEBS-c-PCL are dissolved in the cured epoxy matrix. The spherical micro-domains are highly aggregated in the epoxy thermosets containing 40 and 50 wt% SSEBS-<i>c</i>-PCL. The existence of epoxy-miscible PCL side-chains in the block ionomer complex SSEBS-<i>c</i>-PCL avoids macro-phase separation. Hence, the block ionomer complex can act as an efficient modifier to achieve nanostructured epoxy thermosets.<br />