Microscopic aspects of polymer-metal epitaxy

The textured oriented overgrowth (epitaxy) of certain metals evaporated on to substrates consisting of highly oriented ultra-thin thermoplastic polymer films has been known for a few years. However, the origin of the observed epitaxy was not clear: the formation of a chemical interface layer, classic epitaxy or graphoepitaxy (artificial epitaxy) all seemed to be possible explanations for the observed orientations. We have used the complementary methods of transmission electron microscopy (TEM) and scanning force microscopy (SFM) to investigate aspects of the polymer-metal epitaxy. Our investigations show that the bulk morphologies of polymer substrates determine their surface topographic properties. Highly oriented surface steps serve as suitable locations for an oriented growth of the evaporated metals. The results of the investigations suggest artificial epitaxy (graphoepitaxy) as an effective orientation mechanism for the oriented metallic growth on polymer substrates.     

Causes of extremal variations in the strength of polymer films under the influence of water

Phenomena taking place in oriented polymers (polyethylene terephthalate, polystyrene, etc.) under the influence of water lead to substantial changes in their physical and mechanical properties including strength, optical anisotropy, and density. To elucidate these phenomena and to determine the part played by each of them, we carried out tests in which heat-treated polymer-film specimens were subjected to long-term action of water. It was established that at least two processes take place simultaneously in polymer films under these conditions. One of them is associated with the relaxation of local overstresses in polymer regions characterized by a low degree of order and is accompanied by a rearrangement of structural elements in the direction of vectors of local overstresses. The other process is associated with the plasticizing action of water which weakens the forces of interaction between structural elements in the regions of a low degree of order without affecting the polymer skeleton itself. The intensity of each process is different, and this is the cause of the extremal character of the variation in physical and mechanical properties of films.Thanks are owed to N. F. Bakeev for his help in carrying out this investigation and interpreting its results.     

Orientation-independent spectra for biaxial systems in polarized Raman microspectrometry

From an extension of the scattering intensity expressions in polarized Raman macrospectrometry to the case of micro-Raman backscattering experiments, we have established theoretical expressions to calculate orientation-independent intensity sums. This approach makes use of the K2 Raman invariant and of correction coefficients due to integration of the Raman scattering over the cone of collection of the objective lens, and it may be applied to uniaxial as well as to biaxial symmetry systems. The intensity sums thus obtained are expected to be orientation insensitive and allow one to compare conformational changes in various biaxially oriented polymer samples, either amorphous and/or semicrystalline thin films. As application examples we have compared the polarized Raman results obtained for various biaxially oriented amorphous and semicrystalline polystyrene (PS), poly(ethylene terenaphthalate) (PEN), and poly(ethylene terephthalate) (PET) thin film samples with their respective "isotropic spectrum" and have considered the different main conformational changes in related polymer systems. The method is thus tested on these macromolecular systems and appears quite successful in providing evidence for the molecular conformational changes without interferences from spectral contributions due to orientation.     

High-performance liquid-crystalline polymer films for monolithic “composites”

Highly oriented, free-standing films of a thermotropic liquid-crystalline polymer (LCP) were prepared using a new processing technique, referred to as “foil-spintrusion”. These films underwent solid-state fusion at elevated temperatures to yield homogeneous, self-reinforced monolithic “composites” without the use of additional adhesives. In this study, unidirectional LCP monoliths were evaluated for their mechanical performance. The monoliths exhibited highly anisotropic behavior, with stiffness and tensile strength along the direction of orientation of around 65 GPa and 1.7 GPa, respectively, combined with excellent damping properties in the viscoelastic regime. In addition, experiments showed that the material has a high tensile strength (>2.2 GPa) at high strain rates, indicating promising behavior under high impact conditions. As expected for such highly oriented polymer systems, the monoliths were found to exhibit poor compressive and shear strengths due to low internal coherence of the material itself. However, it was shown that the LCP monoliths could be useful for applications requiring high specific stiffness, for example as facings for composite sandwich panels.     

Epoxy resin-based nanocomposite films with highly oriented BN nanosheets prepared using a nanosecond-pulse electric field

Facile orientation of boron nitride (BN) with high anisotropy in epoxy resin-based nanocomposite films was performed in a polyepoxide matrix using a nanosecond-pulse electric field to generate a high electric flux. Control of the BN anisotropy was achieved in the polymer without damaging the composite films or requiring surface modification of the BN. The degree of BN orientation perpendicular to the nanocomposite film plane, which was parallel to the electric flux, could be controlled by applying the nanosecond pulse for different lengths of time before cross-linking. The resulting composite films with oriented BN nanosheets manifested improved thermal diffusivity compared to a composite prepared without orientation.     

Microstructure and mechanical properties of AlN films obtained by plasma enhanced chemical vapor deposition

AlN films were prepared with a microwave plasma enhanced chemical vapor deposition reactor working at different process temperatures in order to obtain polycrystalline 〈0001〉 oriented films for piezoelectric applications. The films developed were characterized in terms of microstructure, composition, and mechanical properties. Crystalline development and a single orientation were obtained at high temperatures, where at the same time an increase in mechanical intrinsic stresses was observed. Well crystallized 〈0001〉 films were obtained at temperature as low as 500 °C. Furthermore, the evolution of microstructure with thickness at higher temperatures showed a single 〈0001〉 orientation with progressive increase of the texture as the thickness increased. This fact was related with changes in the observed microstructure along the film z-axis, evaluated by high resolution transmission electronic microscopy and selected area electron diffraction. Although orientation dispersion of these films, evidenced by the rocking curves FWHM, remained relatively high (>9°), they can be regarded as promising for piezoelectric applications. Annealing tests conducted at relatively high temperatures with films deposited at low temperature indicated that thermal effects have only a major effect during the film growth for the temperature values employed.     

Measuring the elastic properties of high-modulus fibres

The measurement of the elastic constants of several highly oriented thermoplastic polymer fibres is described. The method makes use of the hot-compaction process, developed and patented in this laboratory, which enables a solid section of highly oriented polymer to be produced from an aggregate of highly oriented fibres. As only a small fraction of the original fibre is melted and recrystallized during the process, the compacted materials offer a unique opportunity for measuring fibre properties in the bulk. An ultrasonic immersion technique is used to measure the elastic properties of the compacted materials, from which the properties of the polymer fibres are inferred. The experimentally determined fibre elastic properties have been compared with other oriented polymer materials to assess any similarities in elastic anisotropy between different methods for producing fibre orientation, and compared with theoretical upper limits for the fibre elastic properties based on theoretical estimates for the polymer crystal unit cell appropriately averaged for hexagonal symmetry using the aggregate model.     

Laser writing on oriented polymer films

Polymer films are suitable substrates for optical data storage due to their mechanical and optical properties. Furthermore, oriented polymer films show an additional effect to laser writing. In this review the effects of laser writing on oriented polymer films are investigated by atomic force microscopy. A change in surface topography of the film is observed. Different polymer types and different draw ratios are used. The dependence of topography effects on the draw ratio is shown. A model of the writing effect is framed.     

Structural studies of poly(p-phenylene benzbisthiazole) films

Poly(p-phenylene benzbisthiazole) (PBT) is one member of a new class of highly-rigid, linear, thermally-stable aromatic heterocyclic polymers. The role of heat-treatment in the improvement of the perfection of crystallinity and mechanical properties of oriented films is discussed. Part of the heat-treatment process seems to be to increase the conjugation length of the polymer chain by increasing the planarity of the molecule, as revealed by visual colour changes and by differential scanning calorimetry. This may in turn account for the improved quality of crystallinity. Considerable detail can be seen in the electron diffraction patterns of heat-treated films. With the exception of the equatorial diffraction peaks this scatter can be accounted for by the detailed molecular transform of the PBT polymer, suitably cylindrically averaged, indicating that the crystal structure is essentially two-dimensional, that is the chains while closely and regularly packed lack longitudinal register. A two-dimensional unit cell with the corresponding molecular packing is proposed which can satisfactorily account for the observed density and for the equatorial diffraction peaks.     

Conjugated Carbon Cyclic Nanorings as Additives for Intrinsically Stretchable Semiconducting Polymers

Molecular additives are often used to enhance dynamic motion of polymeric chains, which subsequently alter the functional and physical properties of polymers. However, controlling the chain dynamics of semiconducting polymer thin films and understanding the fundamental mechanisms of such changes is a new area of research. Here, cycloparaphenylenes (CPPs) are used as conjugated molecular additives to tune the dynamic behaviors of diketopyrrolopyrrole‐based (DPP‐based) semiconducting polymers. It is observed that the addition of CPPs results in significant improvement in the stretchability of the DPP‐based polymers without adversely affecting their mobility, which arises from the enhanced polymer dynamic motion and reduced long‐range crystalline order. The polymer films retain their fiber‐like morphology and short‐range ordered aggregates, which leads to high mobility. Fully stretchable transistors are subsequently fabricated using CPP/semiconductor composites as active layers. These composites are observed to maintain high mobilities when strained and after repeated applied strains. Interestingly, CPPs are also observed to improve the contact resistance and charge transport of the fully stretchable transistors. ln summary, these results collectively indicate that controlling the dynamic motion of polymer semiconductors is proved to be an effective way to improve their stretchability.     

Measurement of transient behavior of the thermal diffusivity of flowing polymer melt

Molecular orientation of polymer molecules created by shear force due to the flow of the polymer melt in a duct rapidly relaxes after the change in the shear rate. This relaxation causes the changes in anisotropic behavior of mechanical. thermal, and optical properties which have strong effect in precision forming of casted polymer products. Among these properties, thermal diffusivity is one of the most difficult properties to measure by the conventional techniques. The present paper describes and discusses the subsecond measurement of relaxation characteristics in thermal dilfusivity anisotropy of flowing polymer melt in conjunction with measurements of some typical flow characteristics. The method used was the forced Rayleigh scattering method, an optical method which was developed and modified by the authors' group for applying to oriented polymer materials. The measuring lime of I ms can trace the relaxation occurred in the order of several seconds. Measured results of thermal diffusivity anisotropy were compared with transient characteristics of the now and quantitatively showed a good agreement with estimated behavior of oriented polymer molecules.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–39, 1995, Köln, Germany.     

An alternative approach for the oriented growth of ZnO nanostructures

An original two-step process was developed to control the oriented growth of ZnO films on SiO₂ substrates. This process combines a chemical bath deposition technique for the seeding and nucleation site generation of self-assembled nanorods and the pyrosol process for the oriented growth of high-crystalline quality ZnO films. Both preferred out-of-plane orientation along the polar < 0001> direction and crystallinity can be enhanced on seed layers. In addition, denser internal structures can be achieved.     

Adhesive properties of polypropylene (PP) and polyethylene terephthalate (PET) film surfaces treated by DC glow discharge plasma

In this study, the adhesive properties of the plasma modified polypropylene (PP) and polyethylene terephthalate (PET) film surfaces have been investigated. Hydrophilicity of these polymer film surfaces was studied by contact angle measurements. The surface energy of the polymer films was calculated from contact angle data using Fowkes method. The chemical composition of the polymer films was analyzed by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) was used to study the changes in surface feature of the polymer surfaces due to plasma treatment. The adhesion strength of the plasma modified film was studied by T-peel strength test. The results showed a considerable improvement in surface wettability even for short exposure times. The AFM and XPS analyses showed changes in surface topography and formation of polar groups on the plasma modified PP and PET surfaces. These changes enhanced the adhesive properties of polymer film surfaces.     

Influence of Aging on the Physical-Mechanical Properties of Acrylic Resin Films Cast from Aqueous Dispersions and Organic Solutions

The physical-mechanical properties of the enteric copolymers, poly(methacrylic acid, ethyl acrylate) Eudragit® L100-55, and Eudragit® L30D have been investigated. Free films of the copolymer containing varying levels of glyceryl triacetate (triacetin) and citrate ester (Citroflex®) plasticizers were prepared by both aqueous and solvent casting techniques. Conditioned films were stored at different humidities and temperatures for predetermined time periods prior to testing. Free films with plasticizer concentration ranging from 0 to 30% by weight of the polymer demonstrated that physical aging at room temperature resulted in physical-mechanical changes as the stress-strain curves indicated a decrease in the percent elongation with increases in the tensile strength. Films prepared from the aqueous latex approached a constant state of equilibrium at a faster rate than films prepared from isopropyl alcohol, where the mechanical properties approach a relatively constant value. Free films containing varying amounts of the plasticizer, triethyl citrate, were stored under different humidity and temperature conditions. In addition, a headspace-gas chromatography (HSGC) method was developed to correlate solvent residues with mechanical properties. The results indicated that an increase in temperature and humidity accelerated the rate of solvent evaporation as a function of time. High vacuum and low humidity conditions were demonstrated to be ineffective conditions for solvent removal. Solvent residues were found in all films after 48 hours of exposure to temperatures up to 60°C and/or exposure to relatively high humidity conditions. A direct relationship between plasticizer content and rate of solvent evaporation from the film was established. Higher concentrations of the plasticizer were found to augment the evaporation of the solvents. This enhanced evaporation was due to the stronger molecular interaction forces between the polymer and the plasticizer than between the polymer and the solvent.     

以PVA为溶剂ZnO薄膜的液相法制备及其机理研究

以聚乙稀醇(PVA)水溶液为溶剂,采用液相法制备了高度c轴取向的ZnO薄膜。采用XRD、Raman以及AFM分析了退火温度与涂层厚度对ZnO薄膜的影响。结果表明,随着退火温度的提高,ZnO薄膜的结晶度及其均方根粗糙度有所提高;同时厚度的增加使得ZnO薄膜的单一取向性减弱。其生长机理可表述为:在每一层涂层中一致或不一致的成核同时产生,通过层内与层间晶粒的聚合、联并,最后形成具有(002)取向的柱状与颗粒状并存的ZnO连续膜。     

Improved Properties of Polymers Under Hydrostatic Pressure

The role of hydrostatic pressure in strengthening of polymer materials was studied. It was established that the orientation crystallization of fibers and films in a high-pressure chamber leads to significant increase of their mechanical properties. The optimal temperature and pressure for the crystallization of polyethylene were determined, providing a static strength of 490 MPa, that is higher than the strength of a type 3 steel. A strength of 100 MPa was achieved for fluorinoplast-4 oriented under pressure at a stretch ratio of about 6. This opens the possibility of obtaining high strength fibers and fabrics using this material.     

Plasmavorbehandlung und Beschichtung von Kunststoffolien. Teil 1: Beschichtung von unbehandelten Kunststoffolien

Plasma Pretreatment and Coating of Polymer Films. Part 1: Coating of Non-treated Polymer Films The food packaging industry demands cheap polymer films possessing a high barrier against permeation of gases, moisture and flavor. Candidates for the most successful materials fulfilling these requirements are vacuum web coated biaxial oriented polypropylene (BOPP) films containing a thin inorganic barrier layer. For a good adhesion of the barrier layer on the BOPP films, the polymer film must be pretreated. The industry uses the Corona atmosphere plasma. This work is separated in three parts. The first part describes the experimental setup and the properties of vacuum web coated layers on polymer films. The next part contains the results of the systematic modification of polymer surfaces by atmosphere and low pressure plasmas. The influence of the surface properties on the final functionality of the coated films is given. In the last part, the discussion of the results of the first and second part reveals systematic relations between the production parameters of the high barrier films and their final functionality. These results firstly reveal the adhesion mechanism of the inorganic barrier layers of the polymer films and the necessary surface properties of the polymer films, in order to get cheap high barrier films by vacuum web coating.     

Orienting MoS2 flakes into ordered films

Layered transition metal di-chalcogenide (TMD) materials exhibit a unique combination of structural anisotropy combined with rich chemistry that confers controllability over physical properties such as bandgap and magnetism. Most research in this area is focused on single layers that are technologically challenging to produce, especially when trying to dope and alloy the host lattice. In this work, we use MoS₂ flakes as a model system for the production of deliberately oriented films for practical applications in which anisotropic materials are required. The proposed production method combines ball milling with exfoliation in solution of MoS₂ flakes, followed by their arrangement on a large centimeter-scale substrate by a simple and non-expensive procedure. The results show that the level of orientation achieved using the proposed system is as good as that of materials that were pressed and subjected to thermal treatment. The ball milling and exfoliation processes maintain the original crystalline structure of the MoS₂ flakes, and the XRD results show that additional crystallographic phases were not produced. Lattice parameters are preserved, which verifies that other species such as water molecules did not intercalate into the MoS₂ molecules. The proposed method of producing oriented films is universal, and as such, it is useful both for pure materials and for mixtures of compounds, the latter of which can be used to produce films with specifically tailored physical properties.     

Nanocomposite based on modified multiwalled carbon nanotubes: Fabrication by an oriented spinning process and electrical conductivity

We have produced nanocomposite films consisting of modified multiwalled carbon nanotubes (MWCNTs) and a polymer (MWCNT/polymer weight ratio of 95/5). The nanocomposite has been applied to a paper substrate by an oriented spinning process from a liquid phase. The resistivity of the films has been measured as a function of temperature in the range 20–140°C along and across the preferential orientation direction of the nanotubes in the nanocomposite. The results point to an irreversible transition from semiconducting to metallic behavior of the conductivity of the films.     

Effect of rapid thermal annealing on the crystal quality and the piezoelectric response of polycrystalline AlN films

This work analyzes the effect of post-deposition rapid thermal annealing (RTA) on the crystal quality and the piezoelectric response of sputtered polycrystalline aluminium nitride (AlN) thin films. AlN films with mixed crystal texture were not significantly affected by RTA processing. However, in films exhibiting clear c-axis preferred orientation, the annealing produced a crystallization process, characterized by an increase in the grain size of the original crystallites, the growth of new small grains, and the reduction of defects. The improvement in the crystal quality was more evident in highly textured c-axis oriented films. However, the enhanced crystal quality of the films due to RTA was not accompanied by a significant improvement in the piezoelectric response. This is attributed to the presence of grains with opposite polarities that could not be rearranged through the RTA treatment.