Low-voltage scanning electron microscopy of multilayer polymer systems

The possibilities of low-voltage scanning electron microscopy for visualization of specific features of the microstructure in internal layers of multilayer polymer films are demonstrated by the example of “chitosan–polyelectrolyte complex–alginic acid” composite. The process of electron beam interaction with a sample at low electron energies is considered. The key parameters of low-voltage electron microscopy, which make it possible to increase the resolution of SEM images of polymer systems, are discussed.     

Study of polyelectrolyte complexes of chitosan and sulfoethyl cellulose

The complexing of polycation chitosan and polyanion sulphoethyl cellulose during the formation of polyelectrolyte simplex membranes using the layer-by-layer deposition of a solution of one polyion on a gel-like film of another one has been studied. The structural characteristics of the multilayer composites and their components have been analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray microanalysis. A technique is proposed for studying the structure of surface layers of thin polymer films (15–20 μm) using a portable DIFREI-401 diffractometer. It is shown that the sequence of layer deposition during the formation of membrane films does not affect their structural characteristics. The interaction between positively charged chitosan groups (-NH ₃ ⁺ ) and negatively charged sulfoethyl cellulose groups (-SO ₃ ^? ) during the growth of polyelectrolyte complexes results in a packing of chitosan chains in the multilayer film.     

Low-Voltage Scanning Electron Microscopy and Energy-Dispersive X-Ray Microanalysis of the Interface of Multilayer Polymer Composite

The morphology of the layer of polyelectrolyte complex, which is formed in the composite based on chitosan and sulfoethyl cellulose during the contact of a solution of one of them with a gel film of the other one, has been investigated by the low-voltage (≤3 kV) scanning electron microscopy. A technique for eliminating the effects related to charge accumulation on the dielectric film surface is proposed.     

Polystyrene Blend Alignment Layers for Pretilt Angle Control of Liquid Crystal

We prepared blend alignment layer based on polystyrene containing 7-hydroxycoumarin (P7COU) and polystyrene containing 2-naphthoxy (P2NAP) side groups for liquid crystal (LC) alignment. The LC cells made from the rubbed P7COU/P2NAP blended films exhibited a shift of the LC alignment direction by approximately 20° compared with those made from a polystyrene film. Stable and homogeneous planar LC cells having pretilt angles adjustable from 0° to 5° were obtained from these polymer films having molar contents of P7COU and P2NAP in the ranges of 100–80 and 0–20 mole%, respectively. The LC alignment behavior was well correlated with the wettability of the polymer films.     

Magnetic films of negative Poisson’s ratio in rotating magnetic fields

Mechanical properties of a thin magnetic film subjected to an external, rotating magnetic field have been studied by computer simulations. The film is modeled by a two-dimensional bead-spring model of a non-magnetic polymer matrix with inclusions of magnetic nanoparticles (nanograins) of uniaxial anisotropy. The isotropic polymer matrix is represented as a hexagonal structure with sites decorated with small hexagonal particles containing magnetic nanograins. The beads correspond to polymer segments and the springs between them imitate chemical bonds, approximated by harmonic interactions. The beads also interact by the Lennard–Jones potential whose role is to mimic the core of non-bonded polymer segments.The Poisson ratio of the constructed polymer matrix is negative, i.e. the system expands laterally under longitudinal stretching force. It is shown that the magneto-elastic coupling can be used to control the expansion and contraction of the polymer matrix. Depending on the applied magnetic field, the Poisson ratio of the magnetic film may assume positive or negative values.     

Crosslinked Cholesteric Network from the Acrylic Acid Ester of (Hydroxypropyl)Cellulose

The acrylic acid ester of (hydroxypropyl)cellulose was prepared from (hydroxypropyl)cellulose and acryloyl chloride. The resultant polymer, with 2.2 ester groups per anhydroglucose unit, formed a thermotropic cholesteric mesophase with visible reflection hands at temperatures between ambient and 60°C. By exposing a thin layer of the mesophase to UV light, the mesophase structure was stabilized to give a crosslinked cholesteric film.     

Preparation of various morphological films at nanoscale by phase separation method

Abstract

A film consisting of nanopore and nanopillar structures was produced from a binary immiscible polymer solution of poly(amic acid) (PAA) and polyimide (PI), which can be used as a potential light extraction layer or flexible substrate in organic light-emitting diode (OLED) devices. A phase separation method was applied to create uniform and random pattern structures in nanoscale via a simple spin-coating technique. Firstly, the binary immiscible polymer solutions of PAA and PI whose precursor is carbazole-based dianiline were prepared with various volume ratios, and then the polymer mixtures were spin-coated onto substrates to form transparent films with various morphologies and dimensions, as observed by a field emission scanning electron microscope technique. In addition, after annealing PAA/PI films at 300?°C, the homogenous and flexible characteristic of nanopore and nanopillar structures could be achieved.     

Role of Spin States of Polymethine Dyes in the Generation of Electron-Hole Pairs

The spectral and luminescent properties of film composites based on photoconductive poly-N-epoxypropylcarbazole (PEPC) and electrically neutral polyvinylbutyral (PVB) with admixtures of polymethine dyes with various ionicities are studied. The external magnetic field effect on the spectral and luminescent properties of film composites based on PEPC with cationic polymethine and merocyanine dyes is studied. The magnetic field effect in the photogeneration of electron-hole pairs is explained by the participation of the singlet–triplet intersystem crossing of excited dye molecules.     

Efficiency Enhancement of Polymer Solar Cells with Three-Component Active Layer

The photvoltaic performance of polymer solar cell (PSC) with a three-component active layer was studied. The incorporation of 4-cyano-4’-octylbiphenyl (8CB) as an additive to a P3HT [poly(3-hexylthiophene)]:PC₆₁BM [[6,6]-phenyl-C61-butyric acid methyl ester] blend film led to a higher absorbance, larger crystal size, closer packing of P3HT, and hence enhanced hole mobility. The power conversion efficiency of the PSC with the three-component active layer (P3HT: PC₆₁BM:8CB blend film) was improved by over 30% compared to that of the reference device without 8CB, due to an enhancement in all parameters such as short circuit current, open circuit voltage and fill factor.     

Effect of oxygen content on barrier properties of silicon oxide thin film deposited by dual ion-beam sputtering

A silicon oxide thin film barrier was prepared with various oxygen contents and its chemical composition, surface morphology and optical and barrier properties were related to the deposition conditions used. Our study showed that under Ar and O₂ assisted process conditions, a stoichiometric silicon oxide thin film formed at a critical oxygen content during deposition of 40-50%. The thin films deposited at the critical condition showed the lowest surface roughness giving similar or higher optical transmittance than that of the bare polycarbonate (PC) substrate. The boiling and tensile strength test performed on the thin film deposited with assisted ions before the deposition process showed improvement in the adhesion between the oxide layer and the polymer substrate. In addition, interface modification to improve for improving the barrier layer properties of the silicon oxide thin film was achieved through the introduction of dual ion beam sputtering without pre-treatment.     

Dielectric properties of natural rubber/chitosan blends: Effects of blend ratio and compatibilization

The electrical properties of blends from natural rubber and chitosan have been studied in the frequency range of 10²-10⁶ Hz. Blending of chitosan with a non-polar polymer such as natural rubber is expected to improve the dielectric performance of chitosan, by reducing the polarity. This paper presents the results of dielectric properties of natural rubber/chitosan blends. Special attention has been paid to analyze the effect of blend composition, vulcanization of the major phase and the influence of compatibilization using maleic anhydride on the dielectric properties of natural rubber/chitosan blends. The temperature dependence of dielectric properties on the blends has also been studied.     

Diamond film growth on the Mo–Re alloy foil

Nanocrystalline diamond film was deposited on the substrate of Mo–Re alloy foil by using a hot filament chemical vapor deposition (HFCVD) method. The morphology, band structures and crystalline structure of the film were analysed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffractometer (XRD), respectively. The results show that the thickness of the diamond film is about 300 nm after 1 h deposition. There is a 2H-Mo₂C layer between the diamond film and the Mo–Re substrate. The values of a and the ratio c/a of Mo₂C are 3.003 and 1.579 Å, respectively. This Mo₂C layer might be formed due to carbon atoms in the gas phase diffusing into the Mo–Re alloy.     

Properties of Some Polarized Chromophores in Polyvinylpyrrolidone

Poor miscibility of highly polarized chromophores to polymer matrices is a crucial issue for EO polymer development. By using poly(N-vinylpyrrolidone) (PVP) as a matrix polymer, we were able to prepare the uniform film containing ammonium-borate type zwitterions or Disperse Red 1 (DR1). For zwitterions, corona poling was unsuccessful probably because of hygroscopic property of PVP and/or strong intermolecular interaction to form the antiparallel dimers. For DR1, we found apparent color change originated from the H-aggregate formation assisted by moisture, which is usable to detect humidity. On the other hand, a passivation layer is necessary to use PVP for the EO applications.     

Facile preparation of graphene-based chitosan films: Enhanced thermal,mechanical and antibacterial properties

Chitosan is considered a model polymer because of its excellent biocompatibility, biodegradability, antibacterial property and metal binding ability. Despite the favorable properties, the poor mechanical strength and the loss of structural integrity limits the applications of chitosan. Graphene's intrinsic low weight with excellent thermal stability makes it an ideal filler for reinforcing polymers. In this work, we prepared graphene oxide (GO) via Hummer's method and simplified Hummer's method using graphite powder and graphite flakes as starting materials, respectively. The GO obtained using Hummer's method and simplified Hummer's method had a small area of less than 50 μm² and a large area of about 7000 μm², respectively. The small area GO and large area GO were reduced by sodium hydroxide, in which the reduced GO (rGO) with small area and large area were incorporated into a chitosan matrix, respectively, using a simplistic drop-casting technique to produce a thin film. Glass transition temperature (T_g) and mechanical strength of chitosan/large area rGO at concentrations of 0.3, 0.6 and 0.9 wt.% of GO were found to be better than chitosan/small area rGO. We also investigated the T_g and mechanical strength between chitosan/small area GO and chitosan/small area rGO, where we discovered that the former had better thermal and tensile properties. By comparing the T_g and mechanical strength of chitosan/small area GO against chitosan/large area GO, we found that the latter displayed superior thermal and tensile properties. Antibacterial tests were performed on the graphene-based chitosan composites and their ability to act as bactericide was manifested in the retardation of the growth of Pseudomonas aeruginosa. These composite materials with excellent thermal, tensile and antimicrobial properties find real-life applications in the physical, chemical, mechanical, electrical and bioengineering fields.     

Bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification

The purpose of this work was to investigate bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification. The experiments were performed in succinonitrile-based transparent alloys (SCN-1.5 wt%ACE). While the solid–liquid interface is cellular, the solid–liquid interface is separated into two layers by the bubble. Experimental results show that a cellular–planar–cellular transition for solid–liquid interface occurs on the lower layer and the stability of the tubular bubble is determined by local microstructure on the upper layer. When the interface is dendritic, morphological instability occurs on the solid thin film attached to the bubble after the solid–liquid interface hits the bubble. We analyzed the evolution of such cells (some cells become dendrites with time) as a function of the angle between the opposite growth direction of dendritic array and the small cell growth direction. It is demonstrated that the relative position between the existing bubble and the dendritic tip influences on the local growth pattern of dendritic array.     

Model of packing of cellulose acetomyristinate in Langmuir-Blodgett films

The variation of the structural characteristics in the transition from a block polymer to a Langmuir-Blodgett film has been investigated for cellulose acetomyristinate by the methods of X-ray diffraction analysis. The effect of length and number of acid residues on the diffraction pattern of block cellulose acetomyristinate is studied. The role of acyl substituent in the formation of an ordered structure of multilayer Langmuir-Blodgett films is established. A model of packing of cellulose acetomyristinate molecules in the Y-type films is suggested. The model is based on a two-domain structure in which close-packed myristic-acid residues are located either normally or at a certain angle to the plane of glucoside rings on one side of the polymer chain.     

A Thin Film Process to Improve Off Axis Viewing of Liquid Crystal Displays

A well known technique to obtain homogeneous alignment of nematic liquid crystals with positive dielectric anisotropy for use in twisted nematic displays is to deposit a silicon monoxide film onto the aligning substrate surface at an oblique angle between 3–6°. This range of deposition angles provides excellent normal contrast ratio but is poor when viewed off axis. This paper describes an alternative approach utilizing a double SiO deposition that improves off axis viewing. The first SiO layer (350 Å) was deposited at 6°. The second layer (20–50 Å) was deposited at 30°. The second layer seems to have a smoothing effect on the 6° structure, as substantiated by TEM studies, thus lowering the tilt angle.     

Real structure of the ZnO epitaxial films on (0001) leucosapphire substrates coated by ultrathin gold layers

The real structure of ZnO films formed by magnetron sputtering on (0001) leucosapphire substrates coated by an ultrathin (less than 0.7 nm) Au buffer layer has been studied by high-resolution microscopy. It is shown that modification of the leucosapphire substrate surface by depositing ultrathin Au layers does not lead to the formation of Au clusters at the film–substrate interface but significantly improves the structural quality of ZnO epitaxial films. It is demonstrated that the simplicity and scalability of the technique used to modify the substrate surface in combination with a high (above 2 nm/s) film growth rate under magnetron sputtering make it possible to obtain high-quality (0001) ZnO epitaxial films with an area of 5–6 cm².     

Supramolecular structure formation of Langmuir-Blodgett films of comblike precursor and polyimide

The surface structure of Langmuir-Blodgett films of a comblike polyimide precursor—a rigid-chain polyamic acid alkylamine salt bearing multichains of tertiary amine—and films of the corresponding polyimide were studied by atomic force microscopy (AFM). An analysis of the images of the surface of three-layer films revealed a domain structure. It was found that the Langmuir-Blodgett film formation of the precursor occurs as a result of the layer-by-layer deposition of two-dimensional domains (composed of polyamic acid salt molecules on the water surface) onto a substrate. The formation of domains in a monolayer is associated with the chemical structure of the precursor, to be more precise, with the rigidity of the main chain and the presence of closely spaced aliphatic side chains in the polymer chain, whose total cross-section area is close to the surface area of the projection onto the plane of the repeating unit of the main chain. Polyimide films inherit the domain structure of the precursor films; the inhomogeneity of the film thickness substantially decreases, whereas the domain size and character of their distribution in the film remain unchanged.     

High Stability and Uniformity Polymeric Thin-Film Transistor Arrays Fabricated by Directly Subtractive Photolithography

We prepared polymer thin film on a diffraction grating by electrochemical polymerization in cholesteric liquid crystal (ChLC). The polymer thin film synthesized in ChLC shows diffraction function due to periodic structure produced by imprinting of the matrix ChLC. Combination of diffractions from the substrate (prepared by top-down technique) and the polymer having fingerprint structure (prepared by bottom-up technique) provides double diffraction grating function.