Application of the Thermal Flash Technique for Low Thermal Diffusivity Micro/Nanofibers

The thermal flash method was developed to characterize the thermal diffusivity of micro/nanofibers without concern for thermal contact resistance, which is commonly a barrier to accurate thermal measurement of these materials. Within a scanning electron microscope, a micromanipulator supplies instantaneous heating to the micro/nanofiber, and the resulting transient thermal response is detected at a microfabricated silicon sensor. These data are used to determine thermal diffusivity. Glass fibers of diameter 15 mu m had a measured diffusivity of 1.21x10(-7) m(2)/s; polyimide fibers of diameters 570 and 271 nm exhibited diffusivities of 5.97x10(-8) and 6.28x10(-8) m(2)/s, respectively, which compare favorably with bulk values

Mechanisms of structural organizational processes as revealed by real time mechano optical behavior of PET film during sequential biaxial stretching

The main focus of this research is to identify the structural mechanisms that are responsible for changes observed in various stages of coupled mechano-optical (stress and strain optical) relationships as they are influenced by the deformation mode, level and rate. This involves in situ real time studies of the true-stress-strain-birefringence of sequential biaxially stretched PET films using a highly instrumented biaxial stretching machine and ex situ structural studies. These ex-situ studies include wide angle X-ray diffraction, Raman spectroscopy and DSC thermal analysis to examine the orientation, conformation and crystallization behaviors as well as formation of long range connectivity through physical network. The relationship between the stress and birefringence exhibited three Regimes. Regime I, linear relationship with a stress optical constant of 5.8 GPa(-1). Regime II, non-linear relationship with the establishment of chain tautness and nematic like two-dimensional order. Regime III, where the chains reach their finite extensibility. (C) 2014 Elsevier Ltd. All rights reserved

Instrumented Film-Insert Injection Compression Molding for Lens Encapsulation of Liquid Crystal Displays

A film-insert injection compression molding process was introduced to encapsulate cholesteric liquid crystal displays with flexible and rigid lens for full protection of displays to replace the currently used time consuming hand lamination technique. For this purpose, a new interchangeable cavity instrumented hot runner mold was designed and constructed. This complex method was carefully optimized considering challenges arising from an insert multilayer display with +80% liquid crystal content as well as different thermal expansion coefficients between the layers and the lens material as a high potential of delamination and warpage. Concerning the desired physical properties including transparency, low melt viscosity and melting temperature as well as a wide range of hardness grades from soft (flexible) to hard (rigid), three different hardness grades of thermoplastic polyurethanes were found to be the best candidates for this lens application. During proposed lens encapsulation, the pressure changes were evaluated with screw and mold movements using position detection via displacement transducers attached to track the mold closure and screw forward motion. The quality of encapsulation and shrinkage related problems, as well as their elimination, were all discussed. Display substrate material selection criteria for lowered warpage were defined with supporting thermal characterizations. Among the process parameters, tested also by applying the design of experiments with Taguchi method, mold temperature was found to be the most influential parameter on warpage, followed by pin gate opening time, packing pressure, and cooling time

Dynamic Assembly of Electrically Conductive PEDOT:PSS Nanofibers in Electrospinning Process Studied by High Speed Video

Electrospinning process is used to generate micrometer to nanometer sized fibers to form nonwoven mats, which is of great interest to produce functional materials exhibiting very high surface area needed to boost efficiency of devices such as sensors, catalyst carriers and drug delivery. Intrinsic conductive polymer materials, like PEDOT:PSS, offer unique material design pathways for a range of emerging flexible electronics applications, including flexible transparent electrodes, LEDs and photovoltaics. In these applications, conductivity and interfacial area of the intrinsic conductive polymer strongly affect the efficiency of the final assembled device. High conductivity increases the efficiency of the device by reducing the resistance. Large interfacial area provides more location for electron hole generation or recombination. This study provides a simple and easy way to generate highly conductive nonwoven nanomat of commercially available intrinsic conductive polymers. Spinnability and conductivity are achieved by using a very small amount of very high molecular weight PEO that provides stability in electrospinning process without interfering the percolation path of PEDOT:PSS within nanofibers. High speed video observations revealed a unique spinning pattern of fiber standing at the collecting plate in electrospinning. This was solved by introducing an air stream flowing along the direction deposition. Effect of humidity, viscosity and electrospinning voltage on electrospun fiber diameters was also investigated

A Real Time Study on Drying and the Mechano-Optical Behavior of Polyvinyl Alcohol Films in Solid and Swollen State

In this paper, we investigate film drying followed by mechano-optical behavior of PVA films in solid and swollen state, respectively. During drying the real time evolution of thickness, weight, and birefringence (both in and out of plane) are captured using a highly instrumented measurement system. During drying, initially isotropic solution rapidly loses water and beyond a critical stage it develops optical anisotropy in the thickness direction as reflected in rapid development of out of plane birefringence while in-plane isotropy is maintained. Stretching the dried films in the solid state yields three regimes of non-linear stress and strain-optical behavior. The temperature shows a positive effect on the birefringence value at given true strain levels. Swelling the PVA films with water, facilitate room temperature stretchability, hence providing a route of stretching that avoids decomposition and discoloration that occurs at elevated temperatures. The effects of moisture content on mechano-optical properties of PVA films are mapped and opposite dependences of photolelastic and strain optical constants on moisture content are observed

Roll-to-Roll Continuous Manufacturing Multifunctional Nanocomposites by Electric-Field-Assisted “Z” Direction Alignment of Graphite Flakes in Poly(dimethylsiloxane)

A roll-to-roll continuous process was developed to manufacture large-scale multifunctional poly­(dimethylsiloxane) (PDMS) films embedded with thickness direction (“Z” direction) aligned graphite nanoparticles by application of electric field. The kinetics of particle “Z” alignment and chain formation was studied by tracking the real-time change of optical light transmission through film thickness direction. Benefiting from the anisotropic structure of aligned particle chains, the electrical and thermal properties of the nanocomposites were dramatically enhanced through the thickness direction as compared to those of the nanocomposites containing the same particle loading without electrical field alignment. With 5 vol % graphite loading, 250 times higher electrical conductivity, 43 times higher dielectric permittivity, and 1.5 times higher thermal conductivity was achieved in the film thickness direction after the particles were aligned under electrical field. Moreover, the aligned nanocomposites with merely 2 vol % graphite particles exhibit even higher electric conductivity and dielectric permittivity than those of the nonaligned nanocomposites at random percolation threshold (10 vol % particles), as the “electric-field-directed” percolation threshold concentration is substantially decreased using this process. As the graphite loading increases to 20 vol %, the aligned nanocomposites exhibit thermal conductivity as high as 6.05 W/m·K, which is 35 times the thermal conductivity of pure matrix. This roll-to-roll electric field continuous process provides a simple, low-cost, and commercially viable method to manufacture multifunctional nanocomposites for applications as embedded capacitor, electromagnetic (EM) shielding, and thermal interface materials

Roll to Roll Electric Field “Z” Alignment of Nanoparticles from Polymer Solutions for Manufacturing Multifunctional Capacitor Films

A roll to roll continuous processing method is developed for vertical alignment (“Z” alignment) of barium titanate (BaTiO₃) nanoparticle columns in polystyrene (PS)/toluene solutions. This is accomplished by applying an electric field to a two-layer solution film cast on a carrier: one is the top sacrificial layer contacting the electrode and the second is the polymer solution dispersed with BaTiO₃ particles. Flexible Teflon coated mesh is utilized as the top electrode that allows the evaporation of solvent through the openings. The kinetics of particle alignment and chain buckling is studied by the custom-built instrument measuring the real time optical light transmission during electric field application and drying steps. The nanoparticles dispersed in the composite bottom layer form chains due to dipole–dipole interaction under an applied electric field. In relatively weak electric fields, the particle chain axis tilts away from electric field direction due to bending caused by the shrinkage of the film during drying. The use of strong electric fields leads to maintenance of alignment of particle chains parallel to the electric field direction overcoming the compression effect. At the end of the process, the surface features of the top porous electrodes are imprinted at the top of the top sacrificial layer. By removing this layer a smooth surface film is obtained. The nanocomposite films with “Z” direction alignment of BaTiO₃ particles show substantially increased dielectric permittivity in the thickness direction for enhancing the performance of capacitors

Large-Scale Roll-to-Roll Fabrication of Ordered Mesoporous Materials using Resol-Assisted Cooperative Assembly

Roll-to-roll (R2R) processing enables the rapid fabrication of large-area sheets of cooperatively assembled materials for production of mesoporous materials. Evaporation induced self-assembly of a nonionic surfactant (Pluronic F127) with sol–gel precursors and phenolic resin oligomers (resol) produce highly ordered mesostructures for a variety of chemistries including silica, titania, and tin oxide. The cast thick (\u3e200 μm) film can be easily delaminated from the carrier substrate (polyethylene terephthalate, PET) after cross-linking the resol to produce meter-long self-assembled sheets. The surface areas of these mesoporous materials range from 240 m2/g to \u3e1650 m2/g with these areas for each material comparing favorably with prior reports in the literature. These R2R methods provide a facile route to the scalable production of kilograms of a wide variety of ordered mesoporous materials that have shown potential for a wide variety of applications with small-batch syntheses

Enhanced Impact Resistance of Three-Dimensional-Printed Parts with Structured Filaments

Net-shape manufacture of customizable objects through three-dimensional (3D) printing offers tremendous promise for personalization to improve the fit, performance, and comfort associated with devices and tools used in our daily lives. However, the application of 3D printing in structural objects has been limited by their poor mechanical performance that manifests from the layer-by-layer process by which the part is produced. Here, this interfacial weakness is overcome using a structured, core–shell polymer filament where a polycarbonate (PC) core solidifies quickly to define the shape, whereas an olefin ionomer shell contains functionality (crystallinity and ionic) that strengthen the interface between the printed layers. This structured filament leads to improved dimensional accuracy and impact resistance in comparison to the individual components. The impact resistance from structured filaments containing 45 vol % shell can exceed 800 J/m. The origins of this improved impact resistance are probed using X-ray microcomputed tomography. Energy is dissipated by delamination of the shell from PC near the crack tip, whereas PC remains intact to provide stability to the part after impact. This structured filament provides tremendous improvements in the critical properties for manufacture and represents a major leap forward in the impact properties obtainable for 3D-printed parts