Sequence-Enhanced Self-Healing in “Lock-and-Key” Copolymers

Abstract

Van der Waals-driven self-healing in copolymers with “lock-and-key” architecture has emerged as a concept to endow engineering-type polymers with the capacity to recover from structural damage. Complicating the realization of “lock-and-key”-enabled self-healing is the tendency of copolymers to form nonuniform sequence distributions during polymerization reactions. This limits favorable site interactions and renders the evaluation of van der Waals-driven healing difficult. Here, methods for the synthesis of lock-and-key copolymers with prescribed sequence were used to overcome this limitation and enable the deliberate synthesis of “lock-and-key” architectures most conducive to self-healing. The effect of molecular sequence on the material’s recovery behavior was evaluated for the particular case of three poly(n-butyl acrylate/methyl methacrylate) [P(BA/MMA)] copolymers with similar molecular weights, dispersity, and overall composition but with different sequences: alternating (alt), statistical (stat), and gradient (grad). They were synthesized using atom transfer radical polymerization (ATRP). Copolymers with alt and stat sequence displayed a 10-fold increase of recovery rate compared to the grad copolymer variant despite a similar overall glass transition temperature. Investigation with small-angle neutron scattering (SANS) revealed that rapid property recovery is contingent on a uniform microstructure of copolymers in the solid state, thus avoiding the pinning of chains in glassy MMA-rich cluster regions. The results delineate strategies for the deliberate design and synthesis of engineering polymers that combine structural and thermal stability with the ability to recover from structural damage.

Copolymer Brush Particle Hybrid Materials with “Recall-and-Repair” Capability

“…Figure reveals two pertinent observations: First, the recovery of both modulus and toughness was more efficient in the case of the statistical sequence structure of the copolymer. This bears similarity to the self-healing behavior of linear copolymer analogs for which more rapid recovery was found in the case of statistical (and alternating) sequence and attributed to a more uniform microstructure. , Irrespective of sequence, the recovery of modulus was more rapid as compared to toughness for which only partial recovery was observed. This was rationalized as a consequence of the more long-range diffusion processes required to re-establish the entanglement network structures that determine the fracture toughness of polymers. − Unexpectedly, SiO 2 -M 49 - grad -B 51 , with a soft BA-rich region in the periphery of the brush (i.e., BA-out), displayed the slowest recovery of both modulus and toughness.…”

“…Heat flow and derivative heat flow curves, shown in Figure and Figure S1, revealed the impact of sequence on glass transition temperature for symmetric compositions. Like linear analogs, statistical sequence systems exhibited the narrowest T g range (22 °C), compared to MMA-out (45 °C) and BA-out (35 °C) gradient systems, suggesting a more uniform microstructure (Figure b). , Note that, contrary to linear analogs, the thermal analysis revealed a sensitive dependence of the glass transition on the brush orientation. The highest T g was observed for MMA-out gradient SiO 2 -B 51 - grad -M 49 (9 °C) as compared to the statistical SiO 2 -B 52 - stat -M 48 (6 °C), and BA-out gradient SiO 2 -M 49 - grad -B 51 (−7 °C).…”

“…Such “self-healing” materials could provide a path toward more sustainable polymer technologies with increased reliability and prolonged service life. , Common approaches to realizing “self-heal ability” rely on the implementation of bio-inspired features such as embedded monomer reservoirs or polymer chemistries involving reversible primary and/or secondary bond network structures. − However, while such “bio-inspired” approaches have shown promise, the cost of fabricating complex structured systems as well as the sensitivity of reversible bond chemistries to external parameters such as humidity remains a challenge. − A novel concept for realizing self-heal ability in more commonly used engineering polymer platforms relies on the synthesis of copolymers with pendant groups capable of forming toothed wheel-like interlocked microstructures that promote van-der-Waals-driven structure reconstitution. , This was first shown by Urban and coworkers who demonstrated that poly­( n -butyl acrylate- co -methyl methacrylate) (P­(BA- co -MMA)) copolymers with sufficient content of BA-MMA dyads featured self-heal ability . The deliberate variation of sequence structure in the case of P­(BA- co -MMA) copolymers using controlled and free radical methods further confirmed that alternating or statistical sequence copolymers featured a significant acceleration of property recovery as compared to gradient analogs. , Despite these advances, a common challenge to both, bio-inspired and van-der-Waals-driven self-healing, has been that the healing process is contingent on sufficient chain mobility to allow for the re-establishment of a bond network structure. This implies that structure recovery is tied to sufficiently high chain dynamics and thus low cohesive energy density and elastic modulus (<1 GPa) to avoid impractically high healing temperatures. − …”

Composition-Orientation Induced Mechanical Synergy in Nanoparticle Brushes with Grafted Gradient Copolymers

“…Apart from the broad glass transitions, gradient copolymers were also expected to show microstructural heterogeneity. ,, This unique property was anticipated to provide the material with enhanced Young’s modulus and toughness compared with copolymers with a statistical sequence of similar compositions, evidenced by small-angle neutron scattering (SANS) referred to in our previous study . Mechanical analysis by uniaxial tension testing was conducted on the spontaneous gradient copolymer grafted from particle brushes under ambient conditions (23 °C) (Figure a).…”

“…The gradual compositional change along the chain imparts intriguing physical properties, including broad glass transition, phase transition, and interfacial activity. − These unique features make gradient copolymers suitable for applications, such as compatibilizers for immiscible polymer blends, stabilizers for emulsions/dispersions, thermoplastic elastomers, adhesives, and wetting agents . Recently, there has been a growing interest in self-healable poly­(methyl methacrylate/ n -butyl acrylate) [P­(MMA/BA)] copolymers. − Compared to the alternating and statistical copolymer architectures, the gradient P­(MMA/BA) demonstrated superior mechanical performance attributed to the segregation of segments to form nanoscopic PMMA-rich cluster regions. , Gradient P­(MMA/BA) copolymers thus hold promise as a platform for self-healing materials due to the simple synthesis and superior mechanical properties . When applied to brush particle systems, P­(MMA/BA) gradient copolymers have shown to endow hybrid materials with dual “self-heal” and “shape memory” ability, thus providing a path to multifunctional materials …”