Current issues in research on structure–property relationships in polymer nanocomposites

Developing descriptors to predict mechanical properties of nanotubes

Descriptors and quantitative structure property relationships (QSPR) were investigated for mechanical property prediction of carbon nanotubes (CNTs). 78 molecular dynamics (MD) simulations were carried out, and 20 descriptors were calculated to build quantitative structure property relationships (QSPRs) for Young's modulus and Poisson's ratio in two separate analyses: vacancy only and vacancy plus methyl functionalization. In the first analysis, C(N2)/C(T) (number of non-sp2 hybridized carbons per the total carbons) and chiral angle were identified as critical descriptors for both Young's modulus and Poisson's ratio. Further analysis and literature findings indicate the effect of chiral angle is negligible at larger CNT radii for both properties. Raman spectroscopy can be used to measure C(N2)/C(T), providing a direct link between experimental and computational results. Poisson's ratio approaches two different limiting values as CNT radii increases: 0.23-0.25 for chiral and armchair CNTs and 0.10 for zigzag CNTs (surface defects <3%). In the second analysis, the critical descriptors were C(N2)/C(T), chiral angle, and M(N)/C(T) (number of methyl groups per total carbons). These results imply new types of defects can be represented as a new descriptor in QSPR models. Finally, results are qualified and quantified against experimental data.

Characterization of Polymer Brush Membranes via HF Etch Liftoff Technique

Surface modification using end-tethered polymer brushes is an attractive, versatile, and effective method of tailoring the surface properties of a material. However, because the chains are covalently attached, characterization of these films is limited. When polymer brushes are detached in their native state, as opposed to fabricating a cross-linked initiator support, additional analytical techniques can be employed. We report lifting off patterned polymer brush membranes from a silicon oxide surface via a hydrofluoric acid etch. This method allows examination of polymer brushes via TEM and thus provides information regarding the perfection of initiator self-assembled monolayer formation and brush growth, as well as the effect of different cross-linking procedures.

Fragility and cooperative motion in a glass-forming polymer-nanoparticle composite

Polymer-nanoparticle composites play a vital role in ongoing materials development. The behavior of the glass transition of these materials is important for their processing and applications, and also represents a problem of fundamental physical interest. Changes of the polymer glass transition temperature Tg due to nanoparticles have been fairly well catalogued, but the breadth of the transition and how rapidly transport properties vary with temperature T - termed the fragility m of glass-formation - is comparatively poorly understood. In the present work, we calculate both Tg and m of a model polymer nanocomposite by molecular dynamics simulations. We systematically consider how Tg and m vary both for the material as a whole, as well as locally, for a range of nanoparticle (NP) concentrations and two polymer-NP interactions. We find large positive and negative changes in Tg and m that can be interpreted in terms of the Adam-Gibbs model of glass-formation, where the scale of the cooperative motion is identified with the scale of string-like cooperative motion. This provides a molecular perpective of fragility changes due to the addition of NPs and for glass formation more generally. We also contrast the behavior along isobaric and isochoric approaches to Tg , since these differing paths can be important to compare experiments (isobaric) and simulations (very often isochoric). Our findings have practical implications for understanding the properties of nanocomposites and fundamental significance for understanding the properties glass-forming materials more broadly.

Thermal annealing induced enhancements of electrical conductivities and mechanism for multiwalled carbon nanotubes filled poly(ethylene-co-hexene) composites

Thermal annealing-induced enhancements of electrical conductivities at the temperature higher than the melting point of poly(ethylene-co-hexene) matrix for multiwalled carbon nanotubes filled poly(ethylene-co-hexene) (MWCNTs/PEH) composites were investigated by electrical conductivity measurements. Two types of MWCNTs with low and high aspect ratios (4 and 31) were added as fillers into PEH matrix, respectively for comparison study purpose. The morphological changes due to annealing for MWCNTs/PEH composites were observed by SEM. The formation of MWCNT networks in the composites were clearly demonstrated by rheological measurements. It is surprisingly found that the electrical conductivity for MWCNTs/PEH composites with high MWCNT concentrations increases obviously with annealing time of 40 min and the maximum increment approaches about 3 orders of magnitude with annealing time of 120 min. The increase of electrical conductivity of MWCNTs/PEH composites depends on MWCNT content, MWCNT aspect ratio and annealing time. SEM results clearly reveal that micrometer-sized MWCNT aggregates are broken down and more loosely packed MWCNT networks form due to annealing. Different types of networks in the composites are responsible for the evolutions of rheological (MWCNT network and PEH chain-MWCNT combined network) and electrical conductivity properties (tube-tube contacting MWCNT network). The reconstruction of MWCNT network during annealing is attributed to rotational diffusion of MWCNTs in PEH matrix at high temperature and the length of MWCNTs shows significant effect on this. The obvious enhancements of electrical conductivities can be ascribed to the thermal annealing-induced formation of loosely packed more homogeneous networks through non-Brownian motions.

The influences of the transfer method and particle surface chemistry on the dispersion of nanoparticles in nanocomposites

The synthesis via in situ polymerization and characterization of nanocomposites (NCs) made from silica (SiO(2)) nanoparticles in a methyl methacrylate (MMA) monomer matrix is reported. We first investigate the transfer of well-defined spherical silica nanoparticles (NPs) (average radius R = 24.2 ± 3.2 nm) into the monomer solvent. We study the influence of two transfer methods and different surface chemistries on the resulting colloidal stability. The first transfer method consists of drying the silica particles into powder before dispersing them via ultrasonication in the MMA matrix. The second is based on repetitive centrifugations to exchange NPs from their synthetic milieu to pure MMA, avoiding the dried powder state. These transfer methods are compared for two kinds of NP surface chemistry, natural silanol (Si-OH) groups of the silica NPs and an additional silane coupling agent, namely 3-(trimethoxysilyl)propyl methacrylate (TPM), which mimics the monomeric group of the MMA dispersing milieu. We then characterize the morphology of the resulting nanocomposites prepared via in situ polymerization at a fixed amount of dispersed NPs (2.2 wt%; 1 vol%) using a combination of transmission electron microscopy (TEM) from ultramicrotomed nanocomposite films, thermogravimetry (TGA) and small angle X-ray scattering (SAXS).

Enhanced epoxy/silica composites mechanical properties by introducing graphene oxide to the interface

Controlling the interface interaction of polymer/filler is essential for the fabrication of high-performance polymer composites. In this work, a core-shell structured hybrid (SiO(2)-GO) was prepared and introduced into an epoxy polymer matrix as a new filler. The incorporation of the hybrid optimized the modulus, strength and fracture toughness of the composites simultaneously. The ultrathin GO shells coated on silica surfaces were regarded as the main reason for the enhancement. Located at the silica-epoxy interface, GO served as an unconventional coupling agent of the silica filler, which effectively enhanced the interfacial interaction of the epoxy/SiO(2)-GO composites, and thus greatly improved the mechanical properties of the epoxy resin. We believe this new and effective approach that using GO as a novel fillers surface modifier may open a novel interface design strategy for developing high performance composites.

Universal features of the melt elasticity of interacting polymer nanocomposites

We study the structure and linear viscoelasticity of interacting polymer nanocomposites based on mixtures of poly(ethylene oxide) and fumed silica particles. The filler is dispersed within the polymer using two different techniques which lead to different dispersion states. The analysis of the dynamic response of our systems highlights the formation of a stress-bearing network above a critical volume fraction, Φ(c). Extending a two-phase model used to describe weakly interacting systems, we show that above Φ(c) the melt-state elasticity of the composites arises from the independent contributions of a polymer-particle network and a viscous matrix. We also find that, although Φ(c) depends on the initial state of dispersion, the network elasticity scales with volume fraction following a universal power-law, with an exponent ν ≈ 1.8. Such a scaling law has been recently predicted for the stress-bearing mechanism governed by polymer-mediated interactions.

Dielectric studies on the heterogeneity and interfacial property of composites made of polyacene quinone radical polymers and sulfonated polyurethanes

Sulfonated polyurethane (PUI, matrix) is synthesized and composited with polyacene quinone radical polymers (PAQRs, filler). The polarization mechanism of these polymers and composites were investigated in terms of their frequency, temperature, and filler-concentration-dependent dielectric properties. We found that PUI/PAQR composites have a high permittivity, which is attributed to the filler-matrix interfacial polarization and the contact effect. The PAQR-concentration-dependent permittivity of different PUI/PAQR composites reveals a percolation threshold at 20-30 wt % with scaling exponents that indicate the intercluster polarization. The frequency dependence of dielectric response is well-fitted by using the Debye and Cole-Cole functions on the basis of the structural diagrams and equivalent circuit, leading to a detailed evaluation on heterogeneous structures of different PUI/PAQR composites.