Bridging-Controlled Network Microstructure and Long-Wavelength Fluctuations in Silica–Poly(2-vinylpyridine) Nanocomposites: Experimental Results and Theoretical Analysis

Abstract

We have performed small-angle X-ray scattering (SAXS) measurements to study the evolution of length-scale-dependent nanoparticle (NP) correlations over a wide range of loadings in miscible silica–poly­(2-vinylpyridine) polymer nanocomposites (PNC) characterized by strong interfacial attraction. The local cage and intermediate-scale correlations evolve in a commonly observed manner with increasing silica concentration, while long-wavelength concentration fluctuations exhibit a complex behavior. Higher-loading PNCs show a nonmonotonic change in the structure factor amplitude with wavevector because of an upturn on the longest length scales, which is the most intense for the highest NP concentration sample. These observations suggest that the PNC is approaching a spinodal demixing transition of an unusual polymer bridging-induced network type. PRISM integral equation theory is quantitatively applied, captures the key features of the SAXS data, and provides a theoretical basis for a network-like phase separation analogous to polyelectrolyte coacervation. The theory with validated parameters is then used to make predictions of real-space pair correlation functions between all species, the small- and large-wavevector collective polymer structure factor, spatially resolved NP coordination numbers, the interfacial cohesive energy density, and a measure of an enlarged effective NP radius because of polymer adsorption. With increasing NP loading, intensification of tight secondary bridged NP configurations, but weakening of interpolymer and polymer–NP correlations due to packing frustration, is predicted. This local reorganization of the polymer structure coexists with macro- and microphase separation such as features at low wavevectors which vary distinctively with NP loading. The predictions for the collective polymer structure are potentially testable using scattering experiments. Our results provide an important starting point for building an understanding of collective NP dynamics.

Collective Nanoparticle Dynamics Associated with Bridging Network Formation in Model Polymer Nanocomposites

“…NPs with R N = 9.1 nm were used with the P2VP38k sample, while R N = 11.8 nm was used with the P2VP9k sample. A series of PNCs with increasing volume fraction of NPs (ϕ = 0.01, 0.06, 0.16, and 0.27 ) were prepared by dropcasting and subsequent solvent evaporation . We performed rheology experiments on the P2VP9K and P2VP38k series from 120–176 °C at approximately 10 °C increments.…”

“…The consensus is that the thicknesses of the BPL in various PNCs are commensurate with the radius of gyration ( R g ) of adsorbed polymer chains − (unless the distance to another interface is smaller than R g ). At a high NP volume fraction or loading (ϕ) typically required for commercial applications, the interparticle separation distance ( ID ) , between neighboring NPs decreases and chain confinement induced by neighboring NPs emerges. , At large enough interfacial polymer–NP attraction, the discrete BPL becomes thermodynamically disfavored, and relatively tight polymer-mediated bridges between neighboring NPs emerge, leading to a network-like microstructure that reinforces the PNCs as a large-scale skeleton . The formation of a percolating NP network has been reported based on mechanical property experiments ,,− or structural characterization ( i .…”

Improving Gas Selectivity in Membranes Using Polymer-Grafted Silica Nanoparticles

“…Bridge formation and NP clustering can potentially be initiated during ARGET-ATRP polymerization . Excessive radical concentration during surface-initiated polymerization and relatively highly concentrated NPs could increase the probability for a radical to connect polymer chains attached to different NPs. , Another mechanism of bridge formation is similar to that known to occur in regular composites, where chain penetrations to the surface of different NP and bridge formation are possible with low polymer surface coverage . This situation presumably occurs in PGNs with low G.D. With an increase in G.D., bridge formation would be less probable owing to the increased surface coverage with polymer chains, which presents an obstacle to diffusion of the polymer chains.…”

Direct Structural Evidence for Interfacial Gradients in Asymmetric Polymer Nanocomposite Blends

“…At high filler fractions and increasing interfacial attraction, large-scale heterogeneities that may be associated with "polymer-mediated bridging" are precursor of spinodal decomposition with a noticeable low-q increase in the polymer structure factor. 48 In addition to the upturn, a structure factor peak at intermediate q has been observed and backed up by theoretical work in the framework of the polymer reference interaction site model (PRISM). [48][49][50][51] Both aspects will be discussed with our results.…”

“…48 In addition to the upturn, a structure factor peak at intermediate q has been observed and backed up by theoretical work in the framework of the polymer reference interaction site model (PRISM). [48][49][50][51] Both aspects will be discussed with our results.…”