(544e) 2-Ethylhexyl Acrylate Comonomer Units Eliminate the Free-Surface-Induced Tg-Confinement Effect in Polymer Films and Provides Robust Self-Healing of Polymer Interfaces

A free surface or polymer-air interface, whether present by design in films or by mishap in damaged bulk polymers, plays a significant role in perturbing polymer chain dynamics in nanoconfined polymer films and provides a challenging barrier to achieving self-healing, autonomous or otherwise, of a fracture. Here, we demonstrate that 2-ethylhexyl acrylate (EHA) units in have the ability to eliminate these perturbations of chain dynamics and to achieve autonomous self-healing behavior in styrene-based copolymers.

Due to the free-surface effect, the glass transition temperature (Tg) decreases significantly with decreasing film thickness below ~ 100 nm for many polymer films, e.g., polystyrene (PS) and poly(4-methylstyrene) (P4MS). We show that such effects can be eliminated down to 15 nm thickness for styrene (S)-based and 4-methylstyrene (4MS)-based copolymers with low levels (2-6 mol%) of EHA. The elimination of the free-surface effect obtained with EHA comonomer is not generalizable to n-alkyl acrylate comonomers, e.g., n-butyl acrylate and n-oxyl acrylate. Furthermore, studies of S/EHA and 4MS/EHA copolymers with up to 6 mol% EHA content on both piranha-treated and dichlorodimethylsilane-treated silicon wafers show that hydrogen bonding plays no significant role in eliminating the effect. Instead, we find a strong correlation between Tg-confinement and fragility-confinement effects for these copolymers. Two EHA units can form special association via interdigitation of the ethylhexyl side groups to reduce the bulk fragility and thus suppress the fragility-confinement effect, and we hypothesize it is this interdigitation with van der Waals interactions that is the cause of the reduced bulk fragility and the elimination of both fragility-confinement and Tg-confinement effects in ultrathin films.

The ability of EHA comonomer units to undergo interdigitation led us to hypothesize that copolymers containing substantial levels of EHA units may be able to undergo autonomous self-healing. The Marek Urban research group has recently demonstrated that low molecular weight (unentangled) copolymers of methyl methacrylate and n-butyl acrylate in a very narrow composition range (nearly 50/50) can exhibit interdigitation of n-butyl acrylate groups leading to self-healing capability. Here, we address these limitations of copolymer composition (and thus properties such as Tg) and low molecular weight with a random copolymer, poly(styrene-r-2-ethylhexyl acrylate) (P(S/EHA)), which showcases room-temperature self-healing across a wide, 45/55 to 70/30 composition range, with Tg values ranging from -25 degrees C to 14 degrees C, and for copolymers that are either unentangled, low molecular weight or entangled, high molecular weight materials. Infrared spectroscopy highlights the reorientation and reconfiguration of EHA units during self-healing, and thermal analysis underscores enhanced dynamic heterogeneity in P(S/EHA) but not in other styrene/acrylate copolymers, which can be explained by the special associations between EHA units. These analyses support the point that EHA units in copolymers can interdigitate, forming ‘key-and-lock’ associations between chains, which can lead to unusual capabilities regarding autonomous self-healing. Such outcomes have important implications for making polymers more resilient in the face of damage, extending the “in use” lifetime of such materials and contributing to sustainability.