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

Abstract

The addition of nanoparticles (NPs) to polymers is a powerful method to improve the mechanical and other properties of macromolecular materials. Such hybrid polymer–particle systems are also rich in fundamental soft matter physics. Among several factors contributing to mechanical reinforcement, a polymer-mediated NP network is considered to be the most important in polymer nanocomposites (PNCs). Here, we present an integrated experimental–theoretical study of the collective NP dynamics in model PNCs using X-ray photon correlation spectroscopy and microscopic statistical mechanics theory. Silica NPs dispersed in unentangled or entangled poly­(2-vinylpyridine) matrices over a range of NP loadings are used. Static collective structure factors of the NP subsystems at temperatures above the bulk glass transition temperature reveal the formation of a network-like microstructure via polymer-mediated bridges at high NP loadings above the percolation threshold. The NP collective relaxation times are up to 3 orders of magnitude longer than the self-diffusion limit of isolated NPs and display a rich dependence with observation wavevector and NP loading. A mode-coupling theory dynamical analysis that incorporates the static polymer-mediated bridging structure and collective motions of NPs is performed. It captures well both the observed scattering wavevector and NP loading dependences of the collective NP dynamics in the unentangled polymer matrix, with modest quantitative deviations emerging for the entangled PNC samples. Additionally, we identify an unusual and weak temperature dependence of collective NP dynamics, in qualitative contrast with the mechanical response. Hence, the present study has revealed key aspects of the collective motions of NPs connected by polymer bridges in contact with a viscous adsorbing polymer medium and identifies some outstanding remaining challenges for the theoretical understanding of these complex soft materials.

Nonlinear Architectures Can Alter the Dynamics of Polymer–Nanoparticle Composites

“…Also note that, in the systems where the NP bridging is effective, the NPs form locally heterogeneous NP distribution at the intermediate concentrations, which is evident in the low-Q upturn in SAXS results. 38 Such an inhomogeneity in particle structure is clearly not seen in this work (see Figure 1c). All of these suggest that, bridging does not play a primary role in the system studied here, rather the dynamics is governed by the interfacial polymer layer and its topological interaction with the surrounding chains.…”

“…The change in the trend from diffusive at short length scales and subdiffusive at longer length scales was attributed to the isolated single-particle dynamics of NPs at a high Q and their collective (network-like) dynamics at a low Q. 38 In the most compact polymer (the eight-arm star), the NPs at 30% loading are as fast as at 2.5% loading. Four-arm star and HB matrix cause mild increases (between linear and eight-arms) in τ at 30% loading.…”

“…In the attractive polymer nanocomposites, the effect of NP bridging on mechanical reinforcement and collective NP dynamics was studied in the case of linear matrices and spherical NPs. ,, From the rheological standpoint, well-dispersed NPs are bridged by flexible polymer chains at high volume fractions (when ID ∼ 2 R g ), resulting in a colloidal gel-like soft solid character. At extremely high loadings (ID ∼ segment length), glassy bridges can also form. , Typically, soft bridges occur when a particular polymer chain is adsorbed on surfaces of different NPs.…”

“…This is in contrast to the bridging effect, which would cause the elastic reinforcement to diverge at high concentrations due to percolation. Also note that, in the systems where the NP bridging is effective, the NPs form locally heterogeneous NP distribution at the intermediate concentrations, which is evident in the low- Q upturn in SAXS results . Such an inhomogeneity in particle structure is clearly not seen in this work (see Figure c).…”

Structural and Dynamical Roles of Bound Polymer Chains in Rubber Reinforcement

“…In addition, the existence of an interphase with the properties between those of the BR and the bulk has been indicated. − There is general agreement that the BR − and/or interphase are critical parameters for rubber reinforcement. It should be noted that the BR layer corresponds to the “bound polymer layer” observed in other PNCs whose thickness is commensurate with the radius of gyration ( R g ) of adsorbed polymer chains. − At high filler loadings (such as those required for commercial tire products), where the interparticle distance between neighboring fillers is on the order of the thickness of the BR chains, the BR chains begin to interfere and overlap with each other. ,, This in turn results in the formation of a filler network structure via the BR chains as polymer “bridges” between neighboring fillers. − Among several factors contributing to mechanical property enhancement, this filler–filler network formed via polymer bridges is considered the most significant. , However, a molecular understanding of the basic mechanisms that are important for the reinforcement in rubber, a topic of utmost importance for the molecular design of tires, is still being debated in this comparatively mature field . The main reason for this problem is the lack of experimental tools to directly probe the bound chains and/or interphase buried in a polymer matrix composed of the same polymer.…”