Session R30: Polymer Nanocomposites

Bridging, Steric Stabilization, Aggregation and Phase Separation in Polymer Nanocomposites

The polymer reference interaction site model theory is employed to study the potential of mean force (PMF) between nano-sized spherical particles in a dense polymer melt. A simple monomer-filler attractive potential is used to probe the competition between packing entropy and interfacial enthalpic effects. As interfacial cohesion increases the nanoparticle organization varies from contact depletion aggregation, to well dispersed sterically stabilized behavior corresponding to a thermodynamically stable polymer coating, to tightly bridged particles. Near linear scaling of the PMF with particle/monomer size asymmetry ratio is found, and the spatial range of the interfacial attraction is crucial in determining particle organization. Spinodal demixing calculations suggest two types of phase separated states and a miscibility window. The latter is rapidly destroyed as particle size and/or interfacial attraction strength increase. The presence of bound polymer layers in miscible nanocomposites is predicted to result in microphase separation like features in the collective polymer structure factor.   

Glass transition behavior of polystyrene/silica nanocomposites.

The change in thermomechanical properties of nano-filled polymers is of considerable scientific and technological interest. The interaction between the nanofillers and the matrix polymer controls the nanocomposite properties. We will present the results from recent and ongoing DSC experiments on polystyrene/silica nanocomposites. Polystyrene of different molecular weights (and from different sources) and silica nanoparticles 10-15 nm in diameter (both as received from Nissan and surface modified by grafted or physisorbed polystyrene) are being used to process the nanocomposites. We are studying trends in the glass transition behavior by changing the matrix molecular weights and the silica weight fractions. Recent data indicate that the glass transition temperature can both decrease and increase depending on the polymer-nanofiller combination as well as the thermal treatment of the nanocomposites prior to the DSC runs.   

Molecular model for the rheology of polymer nanocomposites

In this work we develop a molecular model for the rheology of entangled monodisperse homopolymers filled with nanoparticles at low filling volume fraction. This research is the continuation of our ongoing effort towards establishing a connection between the macroscopic time-dependent behavior of polymer nanocomposites and their molecular structure. The model is developed based on insight gained from molecular simulations, regarding the structure and dynamics of polymeric chains confined between nanoparticles. These simulations provide physically relevant parameters for the rheological model. The model accounts for reptation, chain stretch and contour length fluctuations. It also accounts for the short-range energetic interactions between polymers and fillers. Its predictions are compared with experimental data. The rheological model is implemented in a FEM package to simulate nanocomposite processing.   

Comparison of the Glass Transition Temperature (Tg)-Confinement Effect in Real and Model Polymer Nanocomposites.

Addition of nanoparticles to polymers leads to enhancement of Tg when attractive interactions (e.g., hydrogen bonding) are present at the nanoparticle-polymer interface. Nanoparticle concentration and dispersion play major roles in determining the Tg enhancement. Unfortunately, characterization of dispersion by transmission electron microscopy is difficult and tedious. Here we show by determination of the Tg-confinement effect in ``model'' polymer-silica nanocomposites (NCs), i.e., a polymer film of known thickness with two silica substrates supporting both sides of the film, that it is possible to characterize the effect of interparticle spacing on Tg and the approximate interparticle spacing in real polymer-silica NCs. Studies of model poly(2-vinyl pyridine) (P2VP)-silica NCs with 200-900 nm interlayer spacing reveal that a significant Tg enhancement is observed at a 500-nm interlayer spacing and that the Tg enhancement exceeds 20 K at a 200-nm interlayer spacing. Studies of model poly(methyl methacrylate) (PMMA)-silica NCs exhibit lesser Tg changes at smaller interlayer spacing. By comparison of Tg enhancements in real and model NCs, a 5 K Tg enhancement in a 0.4 vol{\%} silica-PMMA NC relates to a 100-130 nm interparticle spacing while a 10 K Tg enhancement in a 4 vol{\%} silica-P2VP NC relates to a 300 nm interparticle spacing.   

Polymer-Nanoparticle Interfacial Interactions in Polymer Nanocomposites: Confinement Effects on Glass Transition Temperature and Suppression of Physical Aging.

Confinement effects on glass transition temperature (Tg) and physical aging are measured via fluorescence in polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(2-vinyl pyridine) (P2VP) nanocomposites (NCs) containing 10-15 nm diam. silica nanospheres or 47 nm diam. alumina nanospheres. At 0.1-10 vol{\%} nanofiller, Tgs can be enhanced or depressed relative to neat, bulk Tg or invariant with nanofiller content. For alumina NCs, Tg increases relative to bulk Tg by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. These results are explained by wetted P2VP-nanofiller interfaces with attractive interactions, non-wetted PMMA-nanofiller interfaces, and wetted PS-nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or non-wetted interfaces is controlled by choice of solvent; silica/PMMA NCs exhibit Tg enhancements or reductions relative to bulk Tg when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 10 reduction of physical aging rate relative to that of neat P2VP is shown in a 4 vol{\%} alumina/P2VP NC. Hence, a strategy for achieving non-equilibrium, glassy polymer systems that are nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer.   

Relaxation and Glass Transition in Confined and Filled Polymers.

We report the results of the investigations of the influence of confinement and filling of polymers with nanosize aerosil particles on the glass transition and dynamics of the $\alpha $- and the $\beta $-relaxation processes in poly(octylmethacrylate) (POMA) by dielectric spectroscopy dynamic light scattering and differential scanning calorimetry. The polymer was synthesized directly in pores of the porous glass matrix with interconnected and randomly oriented pores with an average pore size of 100 nm by free radical polymerization of the monomer. Both the $\alpha $- and the $\beta $- processes were observed in confined polymer. We found that confinement is resulted in the reduction of the glass transition temperature T$_{g}$ of the polymer. This change in T$_{g}$ was mainly due to the existence of a developed pore wall-polymer interface and difference in the dynamic behavior of polymer in the surface layers compared to that in the bulk. The random structure of the host media was less important. Variations of size of filling particles and their concentration are helpful in understanding of relaxation properties at polymer -- solid interface as well as a role of surface interactions in glass transition.   

Directed Assembly of Nanoparticle Arrays using Block Copolymer Templates

CdSe nanoparticles, functionalized with tetradecyl phosphonic acid (TDPA), were synthesized so as to preferentially segregate into the polystyrene (PS) domains of polystyrene-\textit{block}-poly(methyl methacrylate) (PS-$b$-PMMA). Nanocomposites, composed with ternary blends (PS-$b$-PMMA/PS/PMMA) and CdSe, could be directed to assemble into defect-free and registered periodic and non-regular structures on chemically patterned substrates. The location of nanoparticles in the PS differs depending on the blend composition and the commensurability between the chemical surface pattern and the bulk lamellar period of the composite and can be interpreted in terms of the interplay between the phase behavior of the composite systems and surface and interfacial interactions. CdSe nanoparticle arrays, replicating the block copolymer patterns, were obtained by removing the polymer using O$_{2}$ plasma.   

Surface segregation of silver nanoparticles in the in-situ synthesized Ag/PMMA nanocomposites.

Silver nanoparticles were synthesized in-situ, by thermal decomposition of (1,1,1,5,5,5 Hexafluoroacetylacetonato)silver(I), Ag(hfac), precursor at 185 $^{o}$C, in thin films of Poly(methyl methacryalate), PMMA. Rutherford backscattering spectrometry, RBS, was used to observe and quantify the silver distribution along the thickness of the sample. Strong surface segregation of nanoparticles to the polymer surface and substrate was observed in PMMA thin films by RBS and also confirmed by cross-sectional transmission electron microscopy (TEM). Surface segregation mechanism has been attributed to simultaneous thermal decomposition and diffusion of precursor to polymer surface and substrate. It has been shown that the particles present near the surface could be initially exposed and then are eventually covered by the polymer leading to particle sinking as observed by TEM. Atomic force microscopy (AFM) showed bigger nanoparticles and wider size distribution on increasing the nanoparticle loading from 5 to 20-wt {\%}. UV/Visible spectroscopy on PMMA/Ag nanocomposite films showed plasmon resonance peaked at 422 nm due to the presence of silver nanoparticles.   

Selective Metallization of Well Aligned PS-$b$-P2VP Block Copolymers in Thin Films and in Confined Geometries

Well aligned, microphase-separated structures of styrene-2-vinylpyridine block copolymers are being used as templates for macromolecule-metal nanocomposites. These composites are either prepared as thin films or confined in nanoporous aluminum oxide membranes. Under optimal conditions, templates are prepared as thin films or confined nanorods and metallized without disturbing the ordered structure. We have developed a procedure that deposits metal within the polymer using supercritical carbon dioxide-soluble metal precursors. The use of supercritical carbon dioxide allows for selective metallization of the polymer at or below the glass transition, without disrupting the morphology. In addition, similar procedures have been investigated using metal salts and acids. Using these techniques, metals and metal-sulfides including silver, gold, platinum and zinc sulfide have been selectively deposited.   

Static and Dynamic Neutron Scattering Studies of Polystyrene-Silica Nanocomposites.

Studies of polymer chain conformation and dynamics in the presence of nanofillers are critical in the context of the mechanical properties of polymer nanocomposites. We will present the results from recent and ongoing SANS experiments on polystyrene-silica nanocomposites containing polystyrene matrices of different molecular weights and silica nanofillers (10 -- 15 nm in diameter) over a range of weight fractions. Preliminary neutron backscattering fixed-window scans of these nanocomposites indicate a similarity between the dynamics of the nanocomposites and thin polystyrene films in the sub-glass region. These results and results from ongoing backscattering experiments will also be discussed.   

Structure and properties of poly(methyl methacrylate) (PMMA)-fullerene (C$_{60}$) nanocomposites

We examined the rheological and dynamical mechanical (DMA) properties of PMMA-C$_{60}$ nanocomposite materials with relatively low concentrations of C$_{60}$, 0.1-5wt{\%}. Decreases of the glass transition temperature (T$_{g})$, $\sim $7 $^{o}$C, were observed in samples with 0.5 and 1 wt{\%} C$_{60}$. The decreases in T$_{g}$ are connected to decreases of the longest relaxation time measured for the system. The plateau modulus of the composites was enhanced relative to the homopolymer but did not increase monotonically with C$_{60}$ concentration. The relative changes of the relaxation time, the plateau modulus and T$_{g}$ cannot be explained by the Doi-Edwards model and appear to be connected to the distribution of the nanoparticles, as determined by electron microscopy.   

Self-Corralling of nanorods under electric fields.

Orienting semiconductor nanorods normal to a surface is highly desirable for applications including photovoltaic, spintronics, etc. We have achieved this using solutions of mixture of alkane-covered CdSe nanorods with PMMA by solvent evaporation under an electric field. The electric field aligns the nanorods parallel to the field lines whereas incompatibility between the PMMA and alkane ligands causes a phase separation that tends to segregate the nanorods. This phase separation generates line tension that further compacts the packing of nanorods. Furthermore the nanorods are being synthesized that are covered with photoactive polymer such as poly (3-hexyl thiophene). This may lead to enhancement in efficiency of organic-inorganic heterojunction photovoltaic devices.   

Theoretical and Experimental Studies of New Polymer-Metal High-Dielectric Constant Nanocomposites

High-dielectric-constant (high-K) gate materials are important for the needs of electronics industry. Most polymers have dielectric constant in the range 2 $<$ K $<$ 5; thus to create materials with K $>$ 10 it is necessary to combine polymers with ceramic or metal nanoparticles. Several formulations based on functionalized Au-nanoparticles (R $\sim$ 5 -— 10 nm) and PMMA matrix polymer are prepared. Nanocomposite films are subsequently cast from solution. We study the morphology of those nanocomposites using theoretical (Self-Consistent Mean-Field Theory [SCMFT]) and experimental (Transmission Electron Microscopy [TEM]) techniques. Good qualitative agreement between theory and experiment is found. The study validates the utility of SCMFT as screening tool for the preparation of stable (or at least metastable) polymer/nanoparticle mixtures.   

Epoxy-silica composites for micro-electronic applications: effect of filler

The epoxy composites filled with silica fillers having various amounts and sizes were prepared and a detailed study of the structure-property relationships of the resulting composites is reported. Scanning electron microscopy (SEM), thermomechanical analyzer (TMA), rheological behavior, and stress-strain analysis were used to evaluate the epoxy-silica composite morphology and physical properties. The stiffness and thermal expansion were improved with increasing filler amount. Whereas, the ductility decreased significantly as the filler content increased. Scanning electron micrographs of fractured composites revealed that the dispersion of the filler in the matrix is highly dependent on the size and amount of fillers and has a strong effect on the mechanical, thermal, and rheological properties of the composites. The processibility of the composites for micro-electronic applications such as printed circuit board (PCB), is briefly discussed.   

Quantum dots tailored with water soluble conjugated polymer

Placing the conjugated polymer (CP) in direct contact with the quantum dot (QD) offers advantages over cases where QD aggregation dominates. Such quantum dot\textbf{- }conjugated polymer nanocomposite (QD\textbf{-}CP) possesses a well-defined interface, thereby significantly promoting the charge or energy transfer between these two components. Here we demonstrate an approach to graft water soluble, negatively charged conjugated polymer, MPS-PPV from CdSe QD surfaces. The conjugation length of the MPS-PPV is adjustable by varying the ratio of co-solvents used. The photophysical properties of the nanocomposites in nanoscopic confined geometries are studied.