title: Enthalpy-Driven Stabilization of Dispersions of Polymer-Grafted Nanoparticles in High-Molecular-Weight Polymer Melts.
abstract: Phase stability of polymer nanocomposites (PNCs) composed of polymer-grafted SiO2nanoparticles (NPs) blended with high-molar-mass host polymer chains is investigated. We focus on blends in which the particle-grafted polymer, polyethylene glycol (PEG), and the host-atactic poly(methylmethacrylate) (PMMA) or PMMA/oligo-PEG blends-exhibit favorable enthalpic interactions. Small-angle X-ray scattering measurements are used to evaluate the phase stability of the blends and to report on the structure of the materials at intermediate and long length scales. By exploring SiO2-PEG/PMMA and SiO2-PEG/PMMA-PEG systems covering a wide range of molecular weights (Mw) of PMMA (1.1 kDa ≤ Mw,PMMA≤ 1.1 × 103kDa) and tethered PEG (0.5 kDa ≤ Mw, PEG ≤ 2 kDa), we are able to develop a comprehensive stability map for PNCs based on hairy NPs. At low Mw,PEG, the phase behavior is dominated by entropic effects and the negative Flory-Huggins χ parameter between PEG and PMMA plays no role in phase stability. For higher Mw,PEOand intermediate Mw,PMMA, a crossover from entropy- to enthalpy-dominated behavior is observed, which leads to the phase stability in PNCs well beyond the conventional limits reported for SiO2-PEG/PEG mixtures. This enhanced mixing ceases above a critical Mw,PMMA, where it is found that PMMA chains wet a sufficiently large number of SiO2-PEG particles to bridge and thereby destabilize the composites.
authors.affiliation: Robert F. Smith School of Chemical and Biomolecular Engineering and ‡Sibley School of Mechanical and Aerospace Engineering, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Rahul
authors.initials: R
authors.lastname: Mangal
authors.affiliation: Robert F. Smith School of Chemical and Biomolecular Engineering and ‡Sibley School of Mechanical and Aerospace Engineering, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Pooja
authors.initials: P
authors.lastname: Nath
authors.affiliation: Robert F. Smith School of Chemical and Biomolecular Engineering and ‡Sibley School of Mechanical and Aerospace Engineering, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Mukul
authors.initials: M
authors.lastname: Tikekar
authors.affiliation: Robert F. Smith School of Chemical and Biomolecular Engineering and ‡Sibley School of Mechanical and Aerospace Engineering, Cornell University , Ithaca, New York 14853, United States.
authors.forename: Lynden A
authors.initials: LA
authors.lastname: Archer