Monodisperse silica particles (SiPs) were surface-modified with a newly designed reversible addition–fragmentation chain transfer (RAFT) agent having a triethoxysilane moiety, 6-(triethoxysilyl) 2-(((methylthio)carbonothioyl)thio)-2-phenylacetate (EHT). Surface-initiated RAFT polymerization of styrene was carried out with the EHT-modified SiPs in the presence of a free RAFT agent. The polymerization proceeded in a living manner, producing SiPs coated with well-defined polystyrene of a target molecular weight with a graft density as high as 0.3 chains/nm2. Similarly, polymerizations of methyl methacrylate (MMA), N-isopropylacrylamide, and n-butyl acrylate were conducted, providing SiPs grafted with concentrated (high-density) polymer brushes. In all examined cases, the hybrid particles were highly dispersible in solvents for graft polymers, without causing any aggregations. Owing to exceptionally high uniformity and perfect dispersibility, these hybrid particles formed two- and three-dimensionally ordered arrays at the air–water interface and in suspension, respectively. In addition to the surface-grafting on SiPs, the versatility of this technique was demonstrated by carrying out surface-initiated RAFT polymerization of styrene from iron oxide nanoparticles modified with EHT.
There are limited theoretically predicted phase diagrams for polymer nanocomposites (PNCs) because conventional modeling techniques are largely unable to predict the macroscale phase behavior of PNCs. Here, we show that recent field-based methods, including PNC field theory (PNC-FT) and theoretically informed Langevin dynamics, can be used to calculate phase diagrams for polymer-grafted nanoparticles (gNPs) incorporated into a polymer matrix. We calculate binodals for the transition from the miscible, dispersed phase to the macrophase separated state as functions of important experimental parameters, including the ratio of the matrix chain degree of polymerization (P) to the grafted chain degree of polymerization (N), the enthalpic repulsion between the matrix and grafted chains, the grafting density (σ), the size of the NPs, and the NP volume fraction. We demonstrate that thermal and polymer conformational fluctuations enhance the degree of phase separation in gNP-PNCs, a result of depletion interaction effects. We support this conclusion by experimentally investigating the phase separation of poly(methyl methacrylate)-grafted silica NPs in a polystyrene matrix as a function of P/N. The simulations only agree with experiments when fluctuations are included because fluctuations are needed to properly capture the depletion interactions between the gNPs. We clarify the role of conformational entropy in driving depletion interactions in PNCs and suggest that inconsistencies in the literature may be due to polymer chain length effects since conformational entropy increases with increasing chain length.
An organotellurium chain transfer agent (CTA) bearing a triethoxysilyl group at one end and a 2-methyltellanyl-2-methylpropionate group at the other was prepared and immobilized on the surface of a silicon wafer and silica nanoparticle (SiP). Surface-initiated organotellurium-mediated living radical polymerization (SI-TERP) from the immobilized CTA in the presence of nonimmobilized (free) organotellurium CTA was examined. Concentrated polymer brushes (CPBs) having surface occupancies above 0.1 were prepared by polymerization of various monomers, including styrene, methyl methacrylate, butyl acrylate, N-isopropyl acrylamide, N-vinyl pyrrolidone (NVP), and N-vinyl carbazole (NVC). All CPBs were formed in a controlled manner, with number-average molecular weights close to the theoretical values and low polydispersity indices (<1.41). Structurally well-controlled CPBs comprising unconjugated monomers, NVP and NVC, were prepared for the first time. Atomic force microscopy and transmission electron microscopy analyses of the CPBs revealed the highly stretched and anisotropic structure of the grafted polymer chain in a good solvent.
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