Thermomechanical Properties Of Polymer Nanocomposites Research Articles

Temperature-affected mechanical properties of polymer nanocomposites from glassy-state to glass transition temperature

In this study, the effects of nanoparticle content, polymer/particle interphase, aggregation/agglomeration, and temperature variation on the thermo-mechanical properties of polymer nanocomposites, containing spherical nanoparticles, were investigated. The temperature distribution in the samples was evaluated using a new proposed model on the thermal conductivity of nanocomposites (error < 4%) which also represented unique information about the thermal and physical characteristics of the interphase region. Consequently, the temperature dependency of all physical/mechanical parameters (tensile modulus, yield strength, density, specific heat capacity, etc.) was defined and the results were used to simulate the response mechanism of under-stress nanocomposite samples at a specific thermal condition. PS and PMMA nanocomposites samples containing 1, 2, 3 and 4 vol % of the surface-modified silica nanoparticles were prepared via melt mixing and subjected to tensile, rheology, heat conduction and DMA tests whose results were used to evaluate the accuracy and analyzing the theoretical obtained data. It was revealed that the increment of the nanoparticle content increases the thermal conduction coefficients of the polymer/particle interphase region and the entire system while it decreases the interphase thickness. This led to more significant negative effects of the temperature on the mechanical properties of the nanocomposites. Also, at temperatures about Tg, the under-stress samples with the nanoparticle content of 4 vol% experienced the plastic deformation sooner than the samples with nanoparticle content of 1 vol%.

Role of Grafting Mechanism on the Polymer Coverage and Self-Assembly of Hairy Nanoparticles.

It is now well-accepted that controlling the spatial dispersion of nanoparticles (NPs), which can be achieved by grafting them with polymers of different chain lengths and grafting densities, is central to optimizing the thermomechanical properties of the resulting polymer nanocomposites. In general, there are two methods for creating such polymer-grafted NPs: "grafting to" and "grafting from". The conventional wisdom is that the "grafting from" mechanism, where monomer-sized initiator/functional groups are attached to the surface followed by growing the chains, allows for higher polymer grafting densities and hence a more uniform polymer coverage of the NP surface. Here, we perform calculations and instead show that the "grafting to" strategy surprisingly leads to a more uniform polymer coverage of the NP surface at a given grafting density since the brush is formed while respecting the excluded volume constraints of the previously grafted chains. This conclusion is especially clear in the limit of low-to-moderate grafting density. Thus, at a given grafting density, the "grafting to" mechanism leads to an enhanced miscibility of the NPs in the matrix (which has the same chemistry as the grafts) and lower propensity to create self-assembled structures. Another important factor is that the dispersity in the number of grafted chains on the NPs is also smaller in the case of "grafting to" systems, thus leading to better defined materials. These two conclusions imply that the "grafting to" mechanism may provide better control over the NP dispersion state and hence the thermomechanical properties of polymer nanocomposites.

Controlling the thermomechanical behavior of nanoparticle/polymer films.

We show that the mesoscale (∼200 nm) thermomechanical properties of polymer nanocomposites formed from silica nanoparticles (NPs) and poly(2-vinylpyridine) (P2VP) critically depend on their interfacial structure, which can be controlled by the casting solvent. The composite films are solvent cast from either pyridine (PYR) or methylethylketone (MEK), with uniform NP spatial distribution obtained in both cases. In the films cast from MEK, our previous work has shown that a bound layer of P2VP is formed at the NP surfaces, while no such bound layer is formed when PYR is used as the casting solvent. In PYR as-cast films, Brillouin light scattering reveals a single acoustic phonon with its longitudinal sound velocity increasing with NP loading. This implies a homogeneous mixture of the NP and the polymer on the mesoscopic scales for all compositions examined. However, in the MEK as-cast films, two longitudinal and two transverse acoustic phonons are observed at NP loadings above ∼20 wt % (or ∼11 vol %), reminiscent of two metastable microscopic phases. The dense microphase is attributed to the bridging of NPs by P2VP chains, whereas for the softer medium, we conjecture that there exists an interfacial lower density P2VP layer whose longitudinal sound velocity barely changes with NP loading. These solvent-induced differences in the (elastic) mechanical behavior disappear upon thermal annealing, suggesting that these nanocomposite interfacial structures in the as-cast state (far from equilibrium) locally approach equilibrium (i.e., near equilibrium after annealing). Consistent with these conclusions, the abrupt decrease of the longitudinal sound velocity with temperature occurs at a single glass transition temperature for the annealed nanocomposites irrespective of the casting solvent used, which assumes only a slightly higher (∼5 K at 45 wt % or ∼29 vol %) value than that in bulk P2VP. The results emphasize the important role of solvent in determining the interfacial structure of nanocomposites, which can be used to tailor their thermomechanical behavior.

Preparation and characterization of acetylated corn starch–(PVOH)/clay nanocomposite films

Acetylated corn starch (ACS)-based clay (NaMMT) nanocomposite films, with or without addition of polyvinyl alcohol (PVOH), were prepared by casting with glycerol as a plasticizer. The obtained nanocomposite structure was ascertained by XRD study for all polymer–clay nanocomposites. XRD patterns are indicative of an intercalated nanocomposite structure. Mechanical and thermomechanical properties of polymer nanocomposites were studied. The addition of clay induces significant reinforcing effects in the thermoplastic ACS systems. Replacement of glycerol with PVOH in the ACS–NaMMT system results in superior mechanical strength, due to the creation of hydrogen bonds between the ACS and the PVOH chains. Enhancement in water barrier properties was observed for all nanocomposite films, which reaches up to 67% for acetylated starch–PVOH–clay nanocomposites in comparison to acetylated thermoplastic starch, as indicated by water vapor transmission measurements.

Thermomechanical Properties of Bimodal Brush Modified Nanoparticle Composites

The enthalpic incompatibility of organic polymer matrices and high surface energy inorganic nanoparticles often leads to phase separation in polymer nanocomposites (PNC) precluding the realization of anticipated property enhancements. The grafting of polymer chains to nanoparticles holds promise as a means for controlling dispersion. A single population of polymer chains with tunable graft density (σ) and molecular weight (N) is observed to have antithetical enthalpic and entropic effects on interface compatibility. We report the use of bimodal polymer brushes with fundamentally decoupled enthalpic and entropic parameters. Bimodal polystyrene brushes were grafted from 15 nm colloidal silica nanoparticles by a previously reported consecutive RAFT (reversible addition–fragmentation chain transfer) polymerization technique. The combination of a high graft density short brush and a low graft density long brush was found to cause improved nanoparticle dispersion in a polystyrene matrix when compared to single populations of long and short brushes of corresponding graft densities and molecular weights. A new quantitative model was developed to understand these results and was found capable of predicting dispersions in grafted nanoparticle composites in the allophobic dewetting regime. The bimodal-brush-graft particles were also found to be remarkably well dispersed in an entropically unfavorable higher molecular weight matrix. This facilitated a study of the role of matrix–brush entanglement on the thermomechanical properties of PNCs, isolated from the effects of particle dispersion. The best enhancements in glassy properties resulted from improved matrix–brush entanglement, attained by lowering the long chain graft density and increasing the long chain to matrix molecular weight ratio.

Bound Polymer Layer in Nanocomposites

There has been considerable interest in characterizing the polymer layer that is effectively irreversibly bound to nanoparticles (NPs) because it is thought to underpin the unusual thermomechanical properties of polymer nanocomposites (PNC). We study PNCs formed by mixing silica nanoparticles (NPs) with poly-2-vinylpyridine (P2VP) and compare the bound layer thickness δ determined by three different methods. We show that the thickness obtained by thermogravimetric analysis (TGA) and assuming that the bound layer has a density corresponding to a dense melt clearly underestimates the real bound layer thickness. A more realistic extent of the bound layer is obtained by in situ measurements of the interaction pair potential between NPs in PNCs via analysis of TEM micrographs; we verify these estimates using Dynamic Light Scattering (DLS) in θ solvent. Our results confirm the existence of long-ranged interactions between NPs corresponding roughly in size to the radius of gyration of the bound chains.

Thermomechanical modeling of polymer nanocomposites by the asymptotic homogenization method

There is much current interest to incorporate nano-scale fillers into polymer matrices to achieve potentially unique properties. Compared with traditional microcomposites, a nanocomposite has a significant large ratio of interface area to volume that results in improved thermomechanical properties. Desired thermomechanical properties of polymer nanocomposites, to achieve the ever-increasing performance requirements, can be obtained by tailoring their microstructures. To this end, computational analyses of the relations between the thermomechanical properties, e.g., Young’s modulus, shear modulus, Poisson’s ratio, yield strength, coefficient of thermal expansion and coefficient of thermal conductivity, in different directions and the microstructures of polymer nanocomposites are performed. The asymptotic homogenization method based on the finite element analysis is used to model the thermomechanical behaviors of different polymer nanocomposites with periodic microstructures. The effects of adding silica, rubber, and clay nanoparticles to epoxy resin as a polymer matrix are analyzed. Mixtures of the nano-particles which differ in volume fraction, material type, size and/or geometry are considered. Some predictions of the thermomechanical properties are compared with experimental data in order to verify the applied modeling technique as an effective design tool to tailor optimal microstructures of polymer nanocomposites.

Controlling the thermomechanical properties of polymer nanocomposites by tailoring the polymer–particle interface

AbstractWe show that the thermomechanical properties of polymer nanocomposites are critically affected by polymer‐particle wetting behavior. Silica nanoparticles grafted with dense polystyrene brushes of degree of polymerization 1050 are blended with polystyrene melts to form nanocomposites. It was found that low molecular weight (MW) polystyrene melts with lengths &lt;880 wet these particles. Concurrently, the glass transition temperature (Tg) of the nanocomposite increases. At higher MW, the matrix does not wet the particles and the Tg decreases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2944–2950, 2006