Addition of Short Polymer Chains Mechanically Reinforces Glassy Poly(2-vinylpyridine)–Silica Nanoparticle Nanocomposites

Abstract

The addition of hard fillers to a polymer matrix is a well-known process for achieving mechanical reinforcement. With a decrease in the size of the fillers, the contribution from polymer–particle nanometer-sized interfaces becomes significant, and these interfaces affect the mechanical performance of polymer nanocomposites (PNCs) beyond the limits established for conventional composites. However, the molecular mechanisms underlying the improvement in the mechanical performance of glassy PNCs remain unresolved, necessitating a deeper understanding of the structure–property relationships in these intrinsically heterogeneous systems. In this effort, by using Brillouin light scattering (BLS) and dynamic mechanical analysis (DMA), we demonstrated that adding shorter chains to a PNC prepared with high molecular weight polymers significantly improved the mechanical properties of the PNC in the glassy state. The strongest enhancement of mechanical properties occurred at an optimum concentration of short chains. This is in contrast to the behavior of the glass transition temperature of PNCs which shows a monotonic decrease with an increase in the concentration of shorter chains. Using experimental data and coarse-grained molecular dynamics (MD) simulations, we have identified the molecular mechanism leading to the observed nonmonotonic changes in mechanical reinforcement. This mechanism includes changes in the nanoscale organization at the interface combined with chain stretching amplified by the addition of the short chains. Overall, our approach paves a simple and cost-effective pathway to fabricating glassy PNCs with significantly improved mechanical properties that will fill various practical needs.

A coupled MD-FE methodology to characterize mechanical interphases in polymeric nanocomposites

“…Another handy experimental method is Brillouin light scattering (BLS), which has been commonly used to examine interphases [23,24,25] and, in the case of nanocomposites, allows the measurement of mechanical quantities such as Poisson's ratio and Young's modulus in the submicron range [26,27,28]. Holt et al [29] used BLS to determine the bulk and shear moduli of P2VP-silica composites and compared the results for grafted and physically absorbed filler particles at comparable molecular weights.…”

Rationalizing the Composition Dependence of Glass Transition Temperatures in Amorphous Polymer/POSS Composites

“…Predicting the T g of nanocomposites remains challenging due to the complexity introduced by polymer/filler interfaces not captured by current models . In polymer nanocomposites, attractive nanoparticle surfaces significantly slow down the segmental dynamics of neighboring polymer chains, leading to a position-dependent increase in T g . − The relationships between nanocomposite composition and T g become even more complicated when the convoluted effects of nanofiller size, ,, polymer chain length, ,,,− and chemical structure ,, are considered.…”

Direct Structural Evidence for Interfacial Gradients in Asymmetric Polymer Nanocomposite Blends

“…Having characterized both symmetric and asymmetric silica-free blends, and found no signs of incompatibility, we can now investigate the effect of the presence of NP surfaces on the polymer structure (see section 4 for the PNC formulation). Our rationale is that differences observed in dynamics have been related to the presence of polymer surface layers with different properties than the bulk, like a different composition in terms of chain mass, 35 or a different mass density close to the surface. 2 The latter very subtle effect has been revealed by a SAXS analysis, and it will be shown below that it is too weak to affect our SANS data.…”

“…Following this idea, binary blends of otherwise the same short and long chains have been recently studied showing that the presence of a small fraction of short chains leads to an improved mechanical reinforcement. 35 Swelling of the interface with the short chains tends to favor stretching of the long chains as deduced from molecular dynamics simulations.…”