Multiscale Structure, Interfacial Cohesion, Adsorbed Layers, and Thermodynamics in Dense Polymer-Nanoparticle Mixtures

Abstract

We establish the existence and size of adsorbed polymer layers in miscible dense nanocomposites and their consequences on microstructure and the bulk modulus. Using contrast-matching small-angle neutron scattering to characterize all partial collective structure factors of polymers, particles, and their interface, we demonstrate qualitative failure of the random phase approximation, accuracy of the polymer reference site interaction model theory, ability to deduce the adsorbed polymer layer thickness, and high sensitivity of the nanocomposite bulk modulus to interfacial cohesion. DOI: 10.1103/PhysRevLett.107.225504 PACS numbers: 61.46.Df, 61.05.fg, 61.41.+e Particle aggregation in concentrated polymer solutions, melts, and cross-linked elastomers profoundly alters the mechanical, optical, and electrical properties of nanocomposites [1][2][3][4][5]. Mechanisms of controlling the state of particle aggregation in these complex mixtures remain elusive and poorly understood [6]. Conceptual frameworks for achieving good dispersion generally rely on chemically or physically bound polymer layers [7,8] to induce a repulsive interparticle potential of mean force between nanoparticles [9,10]. Nonetheless, understanding the nature of, and what controls, the structure and properties of these layers remains an outstanding challenge in soft matter of broad importance in polymer science, colloid science, and even biological systems. Obstacles limiting progress include (i) developing experimental tools and physics-based strategies for measuring and controlling the material-specific strength of polymer segments and particle surface attraction, (ii) differentiating polymer segments adsorbed on the nanoparticle surface from bulk polymer in the dense polymer solutions or melts, and (iii) measuring the packing structure of both segments and particles over a wide range of length scales and volume fractions.In this Letter, we present experimental results addressing the above issues using contrast-matching small-angle neutron scattering techniques in conjunction with carefully designed, thermodynamically stable (miscible) concentrated ternary solutions of short-chain polymers (oligomers), nanoparticles, and solvent. Measuring the intensity of scattered neutrons as a function of particle and polymer scattering contrast allows determination of all three partial collective structure factors that quantify spatially resolved segment-segment, particle-particle, and interfacial concentration fluctuations [11]. We employ this experimental knowledge to (i) determine the adsorbed layer thickness, (ii) quantitatively test at an unprecedented level the microscopic polymer reference interaction site model (PRISM) theory [12,13], and (iii) discover strong limitations of the incompressible random phase approximation (IRPA) [14]. How interfacial cohesion can qualitatively modify the effect of particle addition on the nanocomposite bulk modulus is addressed based on the experimentally validated theory.Silica nanoparticles of diameter D ¼ 40 nm are s...

Pressure‐volume‐temperature and glass transition behavior of silica/polystyrene nanocomposite

“…This increase is consistent with the results from Roland and coworkers 18 for a PVAc nanocomposite. The result is also consistent with the prediction by Schweizer and coworkers, 12 where they reported that the bulk modulus can either increase or decrease depending on the interaction of the nanoparticle with the polymer, with increases predicted for weaker interfacial attraction strength, which seems to be the case for our nanocomposite sample. In fact, in another report by Schweizer and coworkers, 53 they specifically show that in aggregated fluid systems, the bulk modulus is predicted to be higher than the hard sphere system at low volume fraction.…”

“…[9][10][11][12] The results show that the pressure-dependent liquid bulk modulus increases 14% for the nanocomposite compared to the neat unfilled system. This increase is consistent with the results from Roland and coworkers 18 for a PVAc nanocomposite.…”

“…More recently, Schweizer and coworkers 12 show a high sensitivity of the nanocomposite bulk modulus to interfacial attraction strength, that is, K decreases for favorable interactions and increases for unfavorable interactions. Furthermore, a molecular dynamics study of Brown et al 13 has also reported that the bulk modulus of a nanocomposite could be lower than that of the pure polymer for a perfectly dispersed sample.…”

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

“…We particularly highlight the work by Zukoski and Schweizer which shows excellent agreement between theory and experimentally determined structure factors in the case of polymer solutions containing NPs. 171 Computer simulations with explicit representation of NP and polymer degrees of freedom (in contrast to the hybrid approach discussed above) have also been used. For instance, de Pablo et al have developed a number of MC techniques to study the structure and properties of polymer nanoparticle mixtures.…”

Small Particle Driven Chain Disentanglements in Polymer Nanocomposites

“…[4][5][6][7] It is now understood that the motion of NPs smaller than the entanglement mesh size is dictated by local Rouse dynamics of the chains while larger particles are significantly slowed down by the entanglements resulting in a sub-diffusive motion [8,9]. The NP effect on the dynamics of the polymer in the nanocomposites has also been the focus of many experimental and theoretical works mostly considering large NPs [10][11][12][13][14][15][16][17]. A recent molecular dynamics simulation [17] and primitive path analysis [16] emphasizes the particle size effects on the local segmental dynamics in attractive and repulsive systems.…”