Network dynamics in nanofilled polymers

Abstract

It is well accepted that adding nanoparticles (NPs) to polymer melts can result in significant property improvements. Here we focus on the causes of mechanical reinforcement and present rheological measurements on favourably interacting mixtures of spherical silica NPs and poly(2-vinylpyridine), complemented by several dynamic and structural probes. While the system dynamics are polymer-like with increased friction for low silica loadings, they turn network-like when the mean face-to-face separation between NPs becomes smaller than the entanglement tube diameter. Gel-like dynamics with a Williams–Landel–Ferry temperature dependence then result. This dependence turns particle dominated, that is, Arrhenius-like, when the silica loading increases to ∼31 vol%, namely, when the average nearest distance between NP faces becomes comparable to the polymer's Kuhn length. Our results demonstrate that the flow properties of nanocomposites are complex and can be tuned via changes in filler loading, that is, the character of polymer bridges which ‘tie' NPs together into a network.

Free volume and lamellar structure of poly vinyl alcohol-nanosized BaTiO3 composite: Positron annihilation and small angle X-ray scattering study

“…The PALS and SAXS results supported by DSC measurements are found to be consistent with the recently proposed theory of nanocomposites i.e. system dynamics at low loading is polymer like with enhanced friction [25].…”

“…Differential scanning calorimetry (DSC) is one of the primary techniques to study the glass transition process of polymer matrices in their nanocomposites [25]. The changes such as shift in glass transition temperature (T g ), broadening of the glass transition temperature region and presence of two T g are indirectly assigned to the changes occurring in the molecular packing and segmental dynamics of polymer chains in the nanocomposites.…”

“…The changes such as shift in glass transition temperature (T g ), broadening of the glass transition temperature region and presence of two T g are indirectly assigned to the changes occurring in the molecular packing and segmental dynamics of polymer chains in the nanocomposites. In general, these changes are ascribed to the existence of a dynamically frozen 'bound' layer (constrained interphase) due to polymer-filler interfacial interaction [25]. Fig.…”

Nanoparticle amount, and not size, determines chain alignment and nonlinear hardening in polymer nanocomposites

“…Using scaling arguments, we arrive at a mechanism for chain alignment that only depends on filler volume fraction via a compensatory effect between individual chain alignment and number of (bridging) chains aligning to the load. Although our work highlights the importance of chains bridging filler aggregates over a length scale comparable with the end-to-end distance of a chain, Baeza et al (45) recently related the linear elasticity in nanocomposites to network formation among overlapping tightly bound chains in close proximity (approximately angstroms to nanometers) to filler surfaces. Along with our results showing that nonlinear elasticity of nanocomposites is insensitive to filler size, the longer length scale we identify underscores the different physicochemical origin of the linear and nonlinear elasticity in these materials.…”

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

“…These effects, which are primarily probed through rheology, are of continuing interest in this field. 19 ,27,40,136,202,216,264-270 We summarize one result, from the work of Baeza et al, 271 that highlights our current understanding. Rheological measurements on favorably interacting mixtures of spherical silica NPs and poly(2-vinylpyridine) show that system dynamics are polymer-like but with increased friction for low silica loadings.…”