No abstract

Polymer segmental dynamics, center-of-mass chain diffusion, and nanoparticle (NP) diffusion are directly measured in a series of polymer nanocomposites (PNC) composed of very small (radius ≈ 0.9 nm) octa­(aminophenyl) polyhedral oligomeric silsesquioxane (OAPS) NPs and poly­(2-vinylpyridine) (P2VP) of varying molecular weight. With increasing OAPS concentration, both the segment reorientational relaxation rate (measured by dielectric spectroscopy) and polymer chain center-of-mass diffusion coefficient (measured by elastic recoil detection) are substantially reduced, with reductions relative to bulk reaching ∼80% and ∼60%, respectively, at 25 vol % OAPS. This commensurate slowing of both the segmental relaxation and chain diffusion process is fundamentally different than the case of PNCs composed of larger, immobile nanoparticles, where the motion of most segments remains relatively unaltered even though chain diffusion is significantly reduced. Next, using Rutherford backscattering spectrometry to probe the NP diffusion process, we find that small OAPS NPs diffuse anomalously fast in these P2VP-based PNCs, reaching diffusivities 10–10000 times faster than predicted by the Stokes–Einstein relation assuming the melt zero-shear viscosity. The OAPS diffusion coefficients are found to scale very weakly with molecular weight, M w –0.7±0.1, and our analysis shows that this characteristic OAPS diffusion rate occurs on intermediate microscopic time scales, lying between the Rouse time of a Kuhn monomer τ0 and the Rouse time of an entanglement strand τe. Our findings suggest that transport of these very small, attractive nanoparticles through well-entangled polymer melts is consistent with the recently reported vehicle mechanism of nanoparticle diffusion.

We present a systematic study of segmental dynamics in model attractive polymer nanocomposites comprising poly(2-vinylpyridine) (P2VP) and 26 nm diameter colloidal silica nanoparticles (NPs) using quasi-elastic neutron scattering (QENS). Unlike most dynamic measurements, QENS provides both spatial and temporal information about small length scales (∼1 nm) and fast time scales (∼1 ns) and therefore at temperatures far above the glass transition. We find that on these length and time scales P2VP segmental motion is well-described by classic translational diffusion even under extreme confinement, where the average interparticle spacing is on the order of the Kuhn length. The average segmental diffusion coefficient decreases monotonically with increasing NP concentration by up to a factor of ∼5 at the highest NP concentrations (50 vol %). Interestingly, this reduction in segmental dynamics is very weakly dependent on P2VP molecular weight spanning the unentangled (10 kg/mol) to the highly entangled regimes (190 kg/mol). This stands in contrast to the well-documented molecular weight effect on segmental dynamics in attractive polymer nanocomposites at lower temperatures.

We use single-particle tracking (SPT) to explore the role of nanoparticles/polymer interactions and polymer molecular weight on nanoparticle (NP) diffusion in unentangled polymer melts. The very dilute NP concentrations (∼10 −7 wt %) in SPT measurements enable tuning NP/polymer interactions so that the systems with unfavorable or neutral NP/polymer interactions in polymer melts can be studied without nanoparticle aggregation. Here, the diffusion coefficients of weakly interacting (methyl-capped, CH 3 QDs) and strongly interacting (carboxylic acid-capped, COOH QDs) nanoparticles (radius = 6.6 nm) in poly(propylene glycol) (PPG) melts were measured via SPT. Mean-squared displacements and van Hove distributions of nanoparticle motion are consistent with Brownian motion of single nanoparticles in the long-time diffusion regime. The effective COOH QD size increases with PPG molecular weight as M w 0.5 , indicating a long-lived bound layer. However, for weakly interacting CH 3 QDs, the effective nanoparticle radius is independent of PPG M w due to the absence of a bound layer. In contrast to ensemble average methods (i.e., X-ray photon correlation spectroscopy), SPT methods directly detect spatial and temporal diffusion behavior of individual nanoparticles and provide previously inaccessible information about nanoparticle diffusion in polymer melts.

The interfacial regions between nanoparticles (NPs) and polymers in polymer nanocomposites (PNCs) underlie enhanced properties, and the temporal stability of these bound polymer layers is necessary for extended control on PNC performance. Using ion scattering techniques and poly­(2-vinyl pyridine) (P2VP) mixed with 26 nm silica NPs, we investigate the lifetime of the bound polymer layer by separating and directly measuring the fraction of free polymer and polymer adsorbed to attractive NPs entirely in the melt state. By annealing thin PNC films deposited on bulk polymer matrices, free polymer from the PNC rapidly diffuses into the underlying matrix while the spontaneously formed bound polymer in the melt remains with the NPs. By correlating the fraction of bound chains with the NP surface area, our analysis shows that bound polymer chains extend ∼R g from the NP surface into the melt. The calculated average NP surface area occupied by adsorbed chains in the melt is much smaller than predicted for an isolated chain or measured in an NP–polymer solution. The bound polymer fraction decreases as a function of annealing time and decreases more rapidly at higher temperatures and for lower molecular weights. This work demonstrates that ion scattering methods can quantitatively measure the chain-scale structure and dynamics of polymers bound to NPs in the melt state. This new information provides fundamental insights and enables the design of PNCs with greater thermal stability during fabrication and use.