Polymer-rich Layer Research Articles

Direct imaging of current paths in multiwalled carbon nanofiber polymer nanocomposites using conducting-tip atomic force microscopy

Using conducting-tip atomic force microscopy (C-AFM), we study the spatial distribution of current paths and local electrical properties in carbon nanofiber/polymer nanocomposites. Previous studies of similar systems were hindered by a polymer-rich skin layer that exists at the nanocomposite surfaces. We present an experimental technique using oxygen plasma etching to controllably remove this polymer skin layer. After this treatment, we can directly probe the microscopic transport characteristics of the nanocomposite using C-AFM. The C-AFM results show that the electrical transport is solely carried by the carbon nanofiber (CNF) networks in the nanocomposites. In addition, high-resolution C-AFM maps show nonuniform distribution of current along the length of some CNFs, suggesting the presence of a heterogeneously distributed adsorbed polymer layer around nanofibers. Finally, two probe conductivity measurements in which one electrode (the C-AFM tip) is contacting a single constituent conducting particle were performed to study local conductivity. Results indicate that Ohmic pathways exist in the conducting network of the nanocomposite to the lowest measured nanofiber concentrations. However, non-Ohmic behavior indicating tunneling transport may also be present, especially near the percolation threshold.

Generation of surface features in films deposited by matrix-assisted pulsed laser evaporation: the effects of the stress confinement and droplet landing velocity

Coarse-grained molecular dynamics simulations are applied to investigate the origins of the surface features observed in films deposited by the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. The formation of transient balloon-like structures with a polymer-rich surface layer enclosing matrix vapor, observed in earlier simulations of slow heating of polymer-matrix droplets, has been explored in this work at higher rates of thermal energy deposition. Tensile stresses generated in the regime of partial stress confinement are found to induce an internal boiling in the overheated droplets and associated generation of “molecular balloons” at thermal energy densities at which no homogeneous boiling takes place without the assistance of tensile stresses. Simulations of the dynamic processes occurring upon the collision of a polymer-matrix droplet with a substrate provide the molecular-level pictures of the droplet impact phenomenon and reveal the connections between the droplet landing velocity and the shapes of the polymer features observed in scanning electron microscopy images of films deposited in MAPLE experiments. The distinct types of surface features observed in MAPLE experiments, namely, wrinkled “deflated balloons,” localized arrangements of interconnected polymer filaments, and elongated “nanofibers,” are shown to emerge from different scenarios of droplet landing and/or disintegration observed in the simulations.

Transparent flexible conductor of poly(methyl methacrylate) containing highly-dispersed multiwalled carbon nanotube

Multiwalled carbon nanotubes (MWCNTs) were incorporated into a poly(methyl methacrylate) (PMMA) solution to develop a transparent and flexible composite conductive film. Monitoring the MWCNT-granule size in the PMMA solutions as well as after the film casting, the self-aggregation of MWCNTs was thoroughly investigated to provide a highly-dispersed polymeric conductor. In addition to the degree of acid treatment of MWCNTs, the dipole moment of solvent and the random-coil length of polymer were considered to be the key factors for the MWCNTs to retain the highly-dispersed state in the polymer matrix after solidification. Investigating several solvent systems, dimethylformamide was found to have the best dispersing capability for the MWCNT/PMMA system to give a surface electrical conductivity up to 10−2S/cm at ca. 3.0wt% of MWCNT, which was considered to be well above what had been reported for such a low level of MWCNT loading, with a light transmittance over 95%. Finally, the polymer-rich layer, which is usually formed on the coating surface due to the surface tension and wetting characteristics of the MWCNT/PMMA mixture, was mechanically peeled off to give an increase in electrical conductivity of nearly two orders of magnitude.

Quantification of Liquid Crystal Concentrations in Periodically Stratified Polymer-Dispersed Liquid Crystal Films by Dynamic Secondary Ion Mass Spectrometry and Multivariate Statistical Analysis

Liquid crystal (LC) concentrations in stratified holographic polymer-dispersed liquid crystal (PDLC) films have been quantified from depth profiles obtained by time-of-flight secondary ion mass spectrometry (SIMS). The volatile nature of the LCs was hindered during SIMS analysis by capping the PDLC samples with poly(vinyl alcohol) and cooling to cryogenic temperatures. It remains difficult to gain quantitative results from SIMS analysis due to matrix effects yielding complex SIMS fingerprint spectra. With the multivariate statistical method discriminant function analysis the LC contents in the stratified holographic PDLC films were quantified. The concentration of the LC-rich layers was determined to be 32.9 ± 3.4 wt % LC, and for the polymer-rich layers it was 28.8 ± 2.7 wt % LC. The low concentration difference was supported by imaging analysis and modeling results.

Making Molecular Balloons in Laser-Induced Explosive Boiling of Polymer Solutions

The effect of the dynamic molecular rearrangements leading to compositional segregation is revealed in coarse-grained molecular dynamics simulations of short pulse laser interaction with a polymer solution in a volatile matrix. An internal release of matrix vapor at the onset of the explosive boiling of the overheated liquid is capable of pushing polymer molecules to the outskirts of a transient bubble, forming a polymer-rich surface layer enclosing the volatile matrix material. The results explain unexpected "deflated balloon" structures observed in films deposited by the matrix-assisted pulsed laser evaporation technique.

Morphological Characteristics of Electrospun Poly(Methyl Methacrylate) Nanofibers Containing Multi-Walled Carbon Nanotubes

The electrospinning process was investigated to embed multi-walled carbon nanotubes (MWNTs) in the electrospun nanofibers of poly(methyl methacrylate) (PMMA) for anisotropic alignment and effective dispersion of carbon nanotubes in the transparent polymer substrate. The morphology of MWNT-incorporated electrospun nanofibers was substantially influenced by the types of solvent and the content of carbon nanotubes exhibiting various forms of beads, undulated fibers, or uniform fibers. In the fibrous forms of electrospun PMMA, the MWNTs were mostly attached on the surface of polymer by physical interlocking. On the other hand, when the bead-shaped PMMA was produced by the electrospinning process, most of the MWNTs were embedded in the PMMA matrix without being exposed on the polymer surfaces. Using the O2 plasma treatment for cutting-off the polymer-rich surface layer of the beads, the embedded MWNTs were exposed out of the beads and subsequently an urchin-shaped structure was obtained. The protruded MWNTs of the urchin-shaped beads could stabilize the polymer beads by the steric stabilization in the solution state and provide facile electronic pathway and percolation in the condensed solid state.

Electro-optical characterization of switchable Bragg gratings based on nematic liquid crystal–photopolymer composites with spatially ordered structure

The composite materials with spatially alternating polymer-rich and nematic liquid crystal-rich layers have been obtained and optical effects in these materials studied. The periodic structure of the composites and small sizes of their structural elements enable improved operating parameters of electro-optical devices: increased contrast ratio, shorter switching times, and lower reorientation fields. The composite materials obtained may also find applications in switchable diffractive optics.

On the mechanical properties, deformation and fracture of a natural fibre/recycled polymer composite

A composite laminate based on natural flax fibre and recycled high density polyethylene was manufactured by a hand lay-up and compression moulding technique. The mechanical properties of the composite were assessed under tensile and impact loading . Changes in the stress–strain characteristics, of yield stress, tensile strength , and tensile (Young's) modulus, of ductility and toughness, all as a function of fibre content were determined experimentally. A significant enhancement of toughness of the composite can be qualitatively explained in terms of the principal deformation and failure mechanisms identified by optical microscopy and scanning electron microscopy. These mechanisms were dominated by delamination cracking, by crack bridging processes, and by extensive plastic flow of polymer-rich layers and matrix deformation around fibres. Improvements in strength and stiffness combined with high toughness can be achieved by varying the fibre volume fraction and controlling the bonding between layers of the composite.

Density functional study of surface forces in athermal polymer solutions with additive hard sphere interactions: Solvent effects, capillary condensation, and capillary-induced surface transitions

A density functional theory for polymer solutions is generalized to cases where the monomers have a different diameter to the solvent. An appropriate free energy functional is obtained by integration of the generalized Flory equation of state for such systems. This functional predicts that entropic demixing may occur in polymer solutions in which the solvent particles are smaller than the monomers. Demixing is promoted not only by a large size disparity, but also by a high pressure as well as by polymer length. The existence of two separate phases in the bulk solution suggests the possibility of capillary-induced phase transitions, even when the confining surfaces are hard, but otherwise inert. We examine such phase transitions and their relation to surface forces and colloidal stability. The density functional theory also predicts that under certain conditions, layering transitions will occur at hard and flat surfaces. A transition from a thin to a thick polymer-rich surface layer may take place as the separation between two surfaces is decreased, and we study the concomitant change on the surface force. Stable thick phases are predicted even at very large undersaturations, and they give rise to a profound increase of the range and strength of the surface force. We furthermore include comparisons with predictions from a model in which the solvent only enters the description implicitly. Responses of the surface forces to changes in monomer diameter, solvent diameter, polymer density, and chain length are investigated.

Ultrathin Solid Polyelectrolyte−Surfactant Complex Films: Structure and Wetting

A fluorinated polyelectrolyte−surfactant complex is shown to form highly ordered lamellar structures in the bulk material and in spin-cast films with a long period of 3.75 nm consisting of alternating fluorinated and polymer-rich layers. The smectic B-like structure was found to be stable up to temperatures of 185 °C. The wetting behavior of complex films with thicknesses in the range of 5.7−370 nm were investigated by using axis-symmetric drop shape analysis of contact angle measurements. Surface energies were calculated using the equation derived by Neumann and Li (ca. 11 mN/m), the Zisman critical surface tension (7 mN/m), and the Girifalco−Good−Fowkes dispersive surface energy (10 mN/m). The surface energies were found to be independent of the thickness of the films. By use of X-ray reflectivity it was found that up to 100 double layers were oriented macroscopically on silicon wafers when prepared by the spin-coat technique.

Thermochromism of a liquid crystalline dialkoxy substituted poly(1,4-phenylene-1,3,4-oxadiazol-2,5-diyl)

The thermotropic liquid crystalline behavior of poly[2′,5′-bis(hexadecyloxy)-1,4-phenylene-1,3,4-oxadiazol-2,5-diyl] C16pod was investigated using differential scanning calorimetry, small- and wide-angle X-ray scattering techniques. An order–order transition was found at about 120°C, which was attributed to the transition from a smectic H phase at a low temperature to a smectic A phase at a high temperature. The repeat unit of the smectic H phase is d = 3.70 nm which is composed of a polymer-rich layer (d1 = 1.70 nm) and an alkyl chain rich layer (d2 = 2.00 nm). For the smectic A phase we calculated d = 3.85 nm, d1 = 1.75 nm and d2 = 2.10 nm. The lamellae of the smectic A phase are assumed to show undulations with a periodicity of 6.3 nm. By contrast, the smectic H phase has no periodic undulations. The C16pod changes its color from yellow–green at 25°C to blue at 130°C. UV-vis and fluorescence spectroscopy were used for a quantitative determination of the thermochromicity of C16pod. It was found that the optical properties change continuously within a temperature range of 25 to 140°C. Even at the order–order transition the UV-vis and fluorescence intensities change continuously. It was therefore concluded that the effective conjugation length of the C16pod reduces continuously with increasing temperature. The order–order transition is caused predominantly by the melting of the side-chains.

Young’s Modulus of Dry-pressed Ceramics: The Effect of the Binder

Measurements of Young’s modulus of green compacts prepared from spray-dried alumina powders containing two binders: polyvinyl alcohol or polyethylene glycol are reported. The variations of Young’s modulus with forming pressure are compared to those of mechanical strength and discussed in terms of glass transition temperature ( T g ). When the T g of the polymer is lower than the pressing temperature (case of PEG and moisture-plasticized PVA), the variation of the Young’s modulus is related to the evolution of the binder film covering the surface of primary particles inside the granules. Microcracking of the brittle external polymer-rich layer of granules seems to be responsible for a different evolution of Young’s modulus of green compact in the case of a binder with a T g higher than the forming temperature (dry PVA).

Binder Distribution in Spray-Dried Alumina Agglomerates

Organic binders, especially those which are water-soluble, tend to migrate with the solvent flow which occurs during the spray-drying process of ceramic suspensions. The adsorption isotherm of PVA on the surface of alumina particles shows a rather low affinity of the polymer when compared to that of the ammonium polymethacrylate. Only 6% of the added PVA (3wt% on a dry weight basis of alumina) was adsorbed onto the alumina surface, and the main fraction of ‘free’, non-adsorbed, binder was able to migrate during drying. Images of the distribution of the polyvinyl alcohol in the alumina agglomerates and in the green compacts indicate an inhomogeneous distribution of the binder. The PVA segregates to the surface of the granules and leads to the formation of a polymer-rich layer. The thickness of this layer, which contains about 50% of the binder introduced, was estimated to be a few percent of the granule radius.

Mechanical Properties of Dry‐Pressed Ceramic Green Products: The Effect of the Binder

The strength of alumina green samples prepared by uniaxial pressing of powders spray‐dried with two binders such as poly(ethylene glycol) or poly(vinyl alcohol) is discussed in terms of (i) the adhesive properties of the polymer‐rich external layer of spray‐dried granules and (ii) the mechanical properties of this layer. A transition from intra‐ to intergranular fracture is observed when the glass transition temperature of the binder increases. The influence of the binder on the defect size distribution in the green products is investigated using the classical Weibull statistics.