Cross-Linked “Matrix-Free” Nanocomposites from Reactive Polymer−Silica Hybrid Nanoparticles

Abstract

Nanoparticles have been used to reinforce polymers for at least 150 years, beginning perhaps with the development of carbon black filled elastomers for the tire industry. The incorporation of nanoparticles generally brings about significant improvements in a number of important polymer properties. Silica nanoparticles dispersed homogeneously within polymer matrices, for example, have been reported to enhancemechanical properties including modulus, flexural, tensile, and impact strength up to a silica content of about 2.5%. As a result, fracture toughness and thermal stability were also improved. More recently, the advent of strategies for the synthesis of nanoparticles with unique photonic, magnetic, electrical, and catalytic properties has brought excitement and promise for new nanoparticle applications in a broad range of emergent areas including sensors, optics, membranes, biology, medicine, and microelectronics. While these new properties and applications have garnered great interest, it has not been widely appreciated that these nanoparticles must generally be combined with some organic material, usually polymeric in nature, in order to realize macroscopic materials with useful properties. The direct dispersion of nanoparticleswithin a polymermatrix, however, is a process prone to aggregation, a phenomenon that cannot be readily overcome by more intensive mixing and that negates many desired property enhancements. Much effort has therefore focused on the modification of nanoparticlenanoparticle interactions to improve dispersion quality. For example, decorating the nanoparticle surface with a polymer brush screens particle-particle interactions and creates a polymer-inorganic hybrid that disperses more readily into a homopolymermatrix.Unfortunately, nanoparticle aggregation, a deleterious effect that negates many of the desirable nanocomposite properties, is only avoided when the brush molecular weight exceeds that of the polymermatrix, a condition that does not lead to optimal physical properties. We report herein a supramolecular building block approach for the preparation of a new family of nanocomposites that comprise nanoparticles cross-linked by polymer bridges but that do not require a polymer matrix. These “matrix-free” nanocomposites (MFNs) are not prone to the nanoparticle aggregation effects that plague conventional nanocomposites and hold promise to provide exceptionally high strength and toughness owing to the formation of covalent polymer bridges linking nanoparticles. Our novel modular approach is based upon the construction of complementary reactive supramolecular building blocks: nanoparticles decorated with heterobifunctional polymer brushes that provide reactive functionality at their periphery as depicted in Figure 1. These hybrid building blocks are somewhat analogous to block copolymers, specifically star block copolymers, wherein covalent bonding between different polymer sequences of the block copolymer prevents the aggregation or phase separation that usually occurs in the physical blend of the corresponding two homopolymers. In a similar fashion, covalent bonding between the polymer and a nanoparticle can prevent nanoparticle aggregation in aMFN. In the present case, nanoparticles with covalently bound azide-terminated polymer brushes and nanoparticles with covalently bound alkyne-terminated polymer brushes constitute hybrid building blocks that are simply “clicked” together like molecularLegos orTinkerToys to formcross-linkednanoparticle assemblies as shown in Figure 1. Reinforcement is provided by polymer bridges joining adjacent nanoparticles, yet aggregation is avoided because a polymer matrix is not employed. “Click” chemistry, specifically the 1,3-dipolar cycloaddition of alkyne and azide end groups to produce triazole cross-links, is chosen as the curing chemistry because its quantitative and chemoselective nature allows nanoparticles to be decoratedwith virtually any polymer. Polymer brushes on the azideand alkyne-functional supramolecular building blocks need not be the same polymer species (two different polymers are illustrated inFigure 1), and the nanoparticles may also differ in chemical nature (identical nanoparticles are shown). MFNs offer unique gelation behavior because the supramolecular building blocks from which they are formed provide exceptionally high reactive functionality. Conventional gels, typically based on cross-linkers with a functionality of 3-4, require a reaction conversion in the range of 50-75% to reach the gel point. The cross-linkers in MFNs, in contrast, are hybrid nanoparticles decorated with hundreds of end-functional polymer brushes, potentially lowering the conversion required for gelation to 1%or less andminimizing the formationof any sol fraction. While cross-linked nanoparticle assemblies have been prepared previously, e.g., by adding bifunctional cross-linkers