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.
| Original language | English |
|---|---|
| Pages (from-to) | 11501-11513 |
| Number of pages | 13 |
| Journal | ACS Nano |
| Volume | 15 |
| Issue number | 7 |
| DOIs | |
| State | Published - Jul 27 2021 |
Funding
The Donors of the American Chemical Society Petroleum Research Fund is acknowledged for support of this research (T.K.). T.K. also acknowledges partial financial support from Henkel Corporation and Brookhaven National Laboratory and National Science Foundation (NSF DGE 1922639). Y.Z., V.B., A.P.S., and K.S.S. acknowledge support for samples preparation, data analysis, and theory development by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. This work used resources of the Center for Functional Nanomaterials and the National Synchrotron Light Source II (Beamlines 11-ID and 11-BM), which are U.S. DOE Office of Science Facilities, at Brookhaven National Laboratory under contract no. DE- SC0012704. The authors thank Ruipeng Li, Masafumi Fukuto, Andrei Fluerasu, and Yugang Zhang for their help performing the SAXS and XPCS experiments and Dmytro Nykypanchuk and Shiwang Cheng for their help conducting the rheology experiments and analyzing the data.
| Funders | Funder number |
|---|---|
| Henkel Corporation | |
| National Synchrotron Light Source II | |
| National Science Foundation | DGE 1922639 |
| U.S. Department of Energy | DE- SC0012704 |
| Office of Science | |
| Basic Energy Sciences | |
| Brookhaven National Laboratory | |
| American Chemical Society Petroleum Research Fund | |
| Division of Materials Sciences and Engineering |
Keywords
- PRISM theory
- collective nanoparticle dynamics
- mode-coupling theory
- polymer bridges
- polymer nanocomposites
- x-ray photon correlation spectroscopy