Styrene-Based Copolymers Exhibiting Unusual Glass Transition Breadth, Facile Autonomous Self-Healing over a Broad Composition Range, and Elimination of the Tg-Confinement Effect: Styrene/2-Propylheptyl Acrylate Random Copolymers

Statistical copolymers, also called random copolymers, of styrene (S) and 2-ethylhexyl acrylate (EHA) have recently been reported to exhibit unusual glass transition breadth and facile autonomous self-healing character over a broad composition range. Very low levels of EHA incorporated into styrene-based polymers have also been reported to eliminate the glass transition temperature (Tg)-confinement effect in supported styrene-based polymer films down to 15-20 nm thicknesses. Here, we show that these remarkable outcomes in S-based copolymers are not unique to EHA but are also present when 2-propylheptyl acrylate (PHA) is the comonomer. Like EHA, PHA has a branched, alkyl side chain capable of undergoing van der Waals interactions, which may result in interdigitation between 2-propylheptyl side groups on different chains. Such interactions are associated with enhanced Tg breadth and are considered the underlying cause of the facile self-healing character. 58/42 mol% and 62/38 mol% S/PHA copolymers exhibit Tg-breadths of 31-32 C, twice that of polystyrene (PS), and undergo 100% recovery after damage by room-temperature self-healing in time frames of 1 h and 6 h, respectively. Relative to bulk homo PS, supported PS films begin to exhibit significant deviations in Tg from bulk response at 50-60 nm thickness and show a Tg reduction of ~10 C in 15-nm-thick films. In contrast, 97/3 mol% and 94/6 mol% S/PHA copolymers show no Tg-confinement effect down to supported film thicknesses of 20 and 15 nm, respectively.