While the phase separation of binary mixtures of chemically different polymer-grafted nanoparticles (PGNPs) is observed to superficially resemble conventional polymer blends, the presence of a “soft” polymer-grafted layer on the inorganic core of these nanoparticles qualitatively alters the phase separation kinetics of these “nanoblends” from the typical pattern of behavior seen in polymer blends and other simple fluids. We investigate this system using a direct immersion annealing method (DIA) that allows for a facile tuning of the PGNPs phase boundary, phase separation kinetics, and the ultimate scale of phase separation after a sufficient “aging” time. In particular, by switching the DIA solvent composition from a selective one (which increases the interaction parameter according to Timmerman’s rule) to an overall good solvent for both PGNP components, we can achieve rapid switchability between phase-separated and homogeneous states. Despite a relatively low and non-classical power-law coarsening exponent, the overall phase separation process is completed on a time scale on the order of a few minutes. Moreover, the roughness of the PGNP blend film saturates at a scale that is proportional to the in-plane phase separation pattern scale, as observed in previous blend and block copolymer film studies. The relatively low magnitude of the coarsening exponent n is attributed to a suppression of hydrodynamic interactions between the PGNPs. The DIA method provides a significant opportunity to control the phase separation morphology of PGNP blends by solution processing, and this method is expected to be quite useful in creating advanced materials.

Linear poly(n-butyl acrylate)-gradient-poly(methyl methacrylate) (PBA-grad-PMMA) copolymers and SiO 2 -g-PBA-grad-PMMA particle brushes were synthesized by activator regenerated by electron transfer atom transfer radical polymerization via a semibatch copolymerization method in which the methyl methacrylate (MMA) monomer was slowly fed to the polymerizing n-butyl acrylate (BA) solutions. The effects of initial BA concentration and the MMA feeding rate on the polymerization were investigated. Controlled gradient compositions were achieved at relatively low conversion, <20%. Two gradient copolymer particle brushes, with grafting densities of 0.55 and 0.126 chains/nm 2 , and one linear gradient copolymer were prepared with the same composition. Significant changes were observed after attaching the gradient copolymer ligands onto the surface of SiO 2 nanoparticles, in both thermal and mechanical properties. Greater heterogeneity and microphase separation were introduced after the addition of SiO 2 nanoparticles, and the nanocomposites displayed more complex glass transition temperature (T g ) behavior and a broader T g range. An improvement in mechanical properties (strength and stiffness) was observed as the SiO 2 nanoparticle content increased in gradient copolymer particle brushes; however, the damping property was compromised with increasing stiffness of the materials, especially under low frequency conditions.

A facile synthetic route toward silica brush particles with random and gradient copolymer grafts comprised of n-butyl acrylate (BA) and methyl methacrylate (MMA) using surface-initiated atom transfer radical polymerization (SI-ATRP) was developed. The effect of chain architecture on the structure and thermomechanical properties of copolymer-tethered brush particle films was investigated and compared to linear copolymer analogues. Random copolymer brush systems displayed a uniform and narrow glass transition, and the dependence of the glass-transition temperature followed a similar trend as for linear copolymers. In contrast, gradient brush materials featured a splitting of the glass-transition temperature with increasing MMA content along with a step-like increase of Young’s modulus and a decrease in ductility. The results were interpreted in terms of the effect of brush architecture on the microstructure of films. The results suggested a “molecularly uniform” microstructure in films of the random brush as well as linear random and linear gradient copolymer systems. In contrast, gradient brush systems featured a heterogeneous microstructure in which MMA-rich regions acted as “traps” that reduced chain mobility. The formation of compositional heterogeneity was rationalized by the “prescribed orientation” of gradient copolymer chains near the particle interface that amplified composition fluctuations and hence local segregation of repeat units. The results highlight the need for a better understanding of the role of “geometric constraints” on the physical properties of copolymer brush particle-based hybrid materials and the opportunity for multiple property enhancement by control of segment distribution in addition to the overall composition.

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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.