Polymer-nanoparticle Interface Research Articles

Dielectric study of Poly(styrene-co-butadiene) Composites with Carbon Black, Silica, and Nanoclay

Dielectric spectroscopy is used to measure polymer relaxation in styrene–butadiene rubber (SBR) composites. In addition to the bulk polymer relaxation, the SBR nanocomposites also exhibit a slower relaxation attributed to polymer relaxation at the polymer–nanoparticle interface. The glass transition temperature associated with the slower relaxation is used as a way to quantify the interaction strength between the polymer and the surface. Comparisons were made among composites containing nanoclay, silica, and carbon black. The interfacial relaxation glass transition temperature of SBR–clay nanocomposites is more than 80 °C higher than the SBR bulk glass transition temperature. An interfacial mode was also observed for SBR–silica nanocomposites, but the interfacial glass transition temperature of SBR–silica nanocomposite is somewhat lower than that of clay nanocomposites. An interfacial mode is also seen in the carbon black filled system, but the signal is too weak to analyze quantitatively. The interfacial polymer relaxation in SBR–clay nanocomposites is stronger compared to both SBR–carbon black and SBR–silica composites indicating a stronger interfacial interaction in the nanocomposites containing clay. These results are consistent with dynamic shear rheology and dynamic mechanical analysis measurements showing a more pronounced reinforcement for the clay nanocomposites. Comparisons were also made among clay nanocomposites using different SBRs of varying styrene concentration and architecture. The interfacial glass transition temperature of SBR–clay nanocomposites increases as the amount of styrene in SBR increases indicating that styrene interacts more strongly than butadiene with clay.

The effect of TiO 2 nanoparticles on gas transport properties of Matrimid5218-based mixed matrix membranes

In this study, the effect of TiO 2 nanoparticles addition on mixed matrix membranes (MMMs) based on Matrimd5218 prepared by using solution-casting method has been investigated. The morphology of MMM was evaluated by Scanning Electron Microscopy (SEM). TiO 2 nanoparticles distribution in matrimid matrix and top surface of membrane films was observed by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM), respectively. It was found that at low particle loadings (less than 15%) nanoparticles were mainly dispersed individually forming nanoscale aggregates. However, at high volume fractions (more than 20%) the size of aggregates increases and micro-scale aggregates form. The permeability of N 2, CH 4, O 2, CO 2 and He have been measured in order to determine the effect of TiO 2 on gas separation performance of Matrimid5218. The results demonstrated that inclusion of TiO 2 increases the gas permeability of MMMs possibly due to chain packing disruption, void formation at polymer–nanoparticle interface as well as within nanoparticle aggregates. For example, in MMMs containing 15 vol.% TiO 2, the permeability of N 2, CH 4 and CO 2 increased up to 2.76, 3.3 and 1.86 times higher than pure matrimid respectively. Diffusivity coefficients of larger molecules obtained by time-lag method show that permeability enhancement is mainly affected by diffusivity increment. Finally, the results reveal that use of TiO 2 nanoparticles improves membrane performance in CO 2/CH 4 separation and presents a trade-off line with similar slope compare to Robeson upper insertion bound.

Relaxation processes in polymers filled with nanoparticles

We report on the influence of filling poly( n-alkyl methacrylates) with 12 nm in diameter Aerosil nanoparticles on relaxation processes and glass transitions, as investigated by dielectric spectroscopy and differential scanning calorimetry. The agglomeration of Aerosil particles in polymer formed a three-dimensional network dividing the polymer into random domains. The filling of polymers with nanoparticles had at least two effects: random field effects imposed by the network and a large particle surface area that imposed an interfacial effect on the polymer. In order to study the role of interfacial effects on the nanoparticle–polymer interface we used two types of Aerosil particles: with hydrophilic and hydrophobic surfaces. We found that the relaxation times of α-relaxation process (sensitive to glass transition) in polymers filled with both hydrophilic and hydrophobic particles were faster than those of bulk polymers measured at the same temperatures. This might be interpreted as reduction of the glass transition temperature in the filled polymers. Semi quantitatively this reduction is in accordance with the Vogel–Fulcher data analysis of the temperature dependencies of the α-process relaxation times. The β-relaxation process was slower for bulk than for filled polymers, but the activation energies of this process were about the same for pure and filled polymers.

Anomalous bulk viscosity of polymer-nanocomposite melts

Nanoparticles dispersed in polymer melts have recently been shown to decrease the bulk viscosity. This contradicts expectations based on Einstein's well-known theory for effective viscosity of dilute, random dispersions of rigid spheres in Newtonian fluids. In this paper, we examine a continuum hydrodynamic model where a layer of polymer at the nanoparticle–polymer interface has a different viscosity and density than the bulk polymer. When the layer thickness is greater than the nanoparticle radius, and the layer viscosity is lower than that of the bulk polymer, the intrinsic viscosity is comparable to the unexpectedly large, negative values reported experimentally by Mackay and coworkers. Accordingly, our continuum hydrodynamic model attributes a bulk viscosity reduction to a lower melt viscosity at the nanoparticle–polymer interface. Such a reduction has been ascribed to the Rouse dynamics of polymer chains at the nanoparticle–polymer interface. Fitting the theory to experimental data reveals a simple correlation between the reduced viscosity layer thickness, polymer chain size, entanglement tube diameter, and nanoparticle size and concentration. Our calculations support the arguments of Mackay and coworkers that nanoparticle inclusions significantly influence the polymer chain conformation in melts, even when the inclusion volume fraction is very small.

Tunable variation of optical properties of polymer capped gold nanoparticles

Optical properties of polymer capped gold nanoparticles of various sizes (diameter 3–6 nm) have been studied. We present a new scheme to extract size dependent variation of total dielectric function of gold nanoparticles from measured UV-Vis absorption data. The new scheme can also be used, in principle, for other related systems as well. We show how quantum effect, surface atomic co-ordination and polymer-nanoparticle interface morphology leads to a systematic variation in interband part of the dielectric function of gold nanoparticles, obtained from the analysis using our new scheme. Careful analysis enables identification of the possible changes to the electronic band structure in such nanoparticles.

Experimental Evidence for a New Parameter to Control the Glass Transition of Confined Polymers

Polymer nanocomposites can be used to study confinement effects on the polymer glass transition temperature (Tg) in a controlled manner by varying interparticle spacing. Using gold nanoparticles in polymethylmethacrylate, we show how the polymer Tg can be tuned by variation of the nanoparticle-polymer interface width (sigma), keeping interparticle spacing fixed. We report the first experimental observation of a crossover in the sign of Tg deviation for confined polymers by variation of sigma and propose a model to explain the dependence of crossover width on the spatial extent of cooperatively rearranging regions.

A molecular dynamics simulation study of the viscoelastic properties of polymer nanocomposites

We have carried out molecular dynamics simulations of model polymer–nanoparticle composites (PNCs) consisting of coarse-grained bead-necklace polymer chains and roughly spherical nanoparticles comprised of like beads for the purpose of gaining understanding of the influence of the nanoparticle–polymer interface on the viscoelastic properties of PNCs. The dynamic shear modulus Gc(t) and viscosity ηc of the PNCs were determined as a function of nanoparticle volume fraction, specific nanoparticle–polymer interfacial area, and the nature of the nanoparticle–polymer interaction. The viscoelastic properties of the PNCs were well described as a product of a polymer matrix dynamic shear modulus or viscosity and a particle volume fraction dependent effect independent of particle size, analogous to treatments of conventional particle composites. In contrast to many conventional composites, however, the viscoelastic properties of the polymer matrix were strongly perturbed by the nanoparticles and depended upon the nature of the nanoparticle–polymer interactions.