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Highly thermostable low-k polymer films with potential applications as dielectric materials in microelectronic industry were synthesized starting from 9,9-bis [4-(3,4-dicarboxyphenoxy)phenyl]fluorene dianhydride and various diamines. A polyetherimide/silica nanocomposite film was obtained using methyltriethoxysilane as precursor of inorganic phase. The chemical structure was confirmed by FTIR and [sup.1] NMR spectroscopy. Water vapor's sorption capacity, thermal stability, glass transition temperature, thermal diffusivity, specific heat, thermal conductivity, and dielectric characteristics of the films were determined. All the films exhibited excellent thermal stability, with an initial decomposition temperature in the range of 500-530[degrees]C. They showed low dielectric constant of 1.98-2.86 and low dielectric loss of 0.0037-0.011, at a frequency of 1 Hz and room temperature. The subglass [gamma]- and [beta]-relaxations, primary [alpha]-relaxation, and conductivity relaxation processes were discussed according to the chemical structure of the samples. Quantitative structure-property relationship (QSPR) study was conducted, and linear regression models were formulated to describe the causal relationships between different parameters and polyetherimide properties. KEYWORDS dielectric properties, fluorene groups, polyetherimides, QSPR study, thermal characteristics, 1 | INTRODUCTION Low dielectric constant (low-k) aromatic polyimides represent an important class of thermostable heterocyclic polymers that can be used in electronic industry as dielectrics between layers in thin [...]

This paper aims to study the possibility of improving the chemical and surface characteristics of the Ti6Al4V alloy by depositing phosphate layers on its surface. Accordingly, an innovative phosphating solution was developed and used in a chemical conversion process to obtain Ca–Zn phosphate layers on the base material surface. Moreover, the chemical composition of the phosphate solution was chosen considering the biocompatibility of the chemical elements and their possibility of contributing to the formation of phosphate compounds. The obtained layer was characterized by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and potentiodynamic polarization tests. The wetting of the Ca–Zn sample surface was also investigated using water and two liquids similar to body fluids, namely, Ringer and Dulbecco solutions. According to the surface energy study, the polar component is almost two times larger compared with the dispersive one. The SEM and EDS tests revealed a uniformly coated surface with intercalated crystals leading to a rough surface. Furthermore, the XRD results showed not only the presence of hopeite and scholzite but also of phosphophyllite. By the vibrations of the PO4−3 groups, the FTIR test confirmed the presence of these phases. The potentiodynamic tests revealed that the samples coated with the Ca–Zn phosphate layer present better corrosion resistance and a lower corrosion rate compared with the uncoated ones. [ABSTRACT FROM AUTHOR]

The degradation of a quenching cooling medium is a particularly important technical aspect in the technology of primary and final thermal treatments. This paper studied the effect of the thermal cycles of heating and cooling on a tempering medium type of 2.5% carboxymethyl cellulose solution in water. The main characteristic of a cooling medium is the absorption of heat from the part, which is influenced by the physico-chemical characteristics of the cooling liquid according to the degree of thermal fatigue. For this, the main properties of the environment are analyzed, such the chemical composition, corrosion capacity, wetting capacity and cooling characteristics. To analyze the change in properties during the operation, we studied the effect of thermal cycles on the physico-chemical characteristics of the quenching medium to determine the optimal period when the quenching medium could function in good parameters without the necessary corrections. For this purpose, cyclic and linear corrosion tests, FTIR compositional analyses and contact angle measurements were conducted. [ABSTRACT FROM AUTHOR]

Determining the phase transition temperature of different types of liquid crystals based on their structural parameters is a complex problem. The experimental work might be eliminated or reduced if prediction strategies could effectively anticipate the behavior of liquid crystalline systems. Neuro-evolutive modeling based on artificial neural networks (ANN) and a differential evolution (DE) algorithm was applied to predict the phase transition temperatures of bent-core molecules based on their resorcinol core. By these means, structural parameters such as the nature of the linking groups, the position, size and number of lateral substituents on the central core or calamitic wings and the length of the terminal chains were taken into account as factors that influence the liquid crystalline properties. A number of 172 bent-core compounds with symmetrical calamitic wings were selected from the literature. All corresponding structures were fully optimized using the DFT, and the molecular descriptors were calculated afterward. In the first step, the ANN-DE approach predicted the mesophase presence for the analyzed compounds. Next, ANN models were determined to predict the transition temperatures and whether or not the bent-core compounds were mesogenic. Simple structural, thermophysical and electronic structure descriptors were considered as inputs in the dataset. As a result, the models determined for each individual temperature have an R2 that varied from 0.89 to 0.98, indicating their capability to estimate the transition temperatures for the selected compounds. Moreover, the impact analysis of the inputs on the predicted temperatures showed that, in most cases, the presence or not of liquid crystalline properties represents the most influential feature. [ABSTRACT FROM AUTHOR]

With a significant number of features (namely being multipurpose, inexpensive and durable), thermoplastic polymers, most often named plastics, are part of our daily routine, with an increasing production over the last decade. Among them, polyethylene terephthalate (PET), high-density polyethylene (HDPE) and polypropylene (PP) are distinguished as the five most commonly used plastics in various fields, mainly in the packaging industry. Even if it is difficult to imagine the world without plastics, the boosted plastic assembly comes with huge plastic waste, creating a number of challenges, as the most important threat for our environment, but also opportunities for recycling. Currently, a special attention is dedicated on how to improve the current recycling methods or to find new ones, since the quality of recycled plastics and potential chemical or biological contaminations are two problematic aspects. Understanding the properties of each thermoplastic polymer and the interaction with possible contaminants may be the key for an efficient recycling process. The aim of this paper was to evaluate the surface behaviour of different composite supports based on recycled PET before and after interaction with collagen (used as a biological contaminant). The surface contamination bias of PET supports was studied through different techniques: scanning electron microscopy (SEM), water uptake through swelling studies, contact angle measurements and attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR). [ABSTRACT FROM AUTHOR]

The plasma nitriding edge effect phenomenon is characteristic of parts with sharp edges. The intersection for the discharge of negative light of the two adjacent faces causes the apparition of this effect. In some cases, this effect causes disturbance to the general process. In this work, a sample with different angles of 30°, 60°, and 90° was analyzed. The sample was subjected to ion nitriding with and without the cathode grid to highlight the reduction of the edge effect on the non-uniformity appearing on the edges and corners of the parts. The effect of the active screen was also analyzed by hardness measurements in the area of the nitride edges and by SEM and EDX analyses in the mentioned areas. Additionally, the influence of active screens was studied by nanoindentation and scratch tests and by measuring the contact angle of coolants and liquid lubricants on the nitride surfaces with both methods. [ABSTRACT FROM AUTHOR]

High-performance supramolecular polyimide systems were synthesized via a simple and innovative approach using two types of azo-chromophores, leading to concomitant special properties: high thermostability, the ability to be processed in the form of films with high flexibility, adequate morphological features, and good structuring capacity via phase mask ultraviolet (UV) laser irradiation, induced by the presence of the azo groups (–N=N–). The dimension and the anisotropy degree of the micro/nano patterns obtained on the surface of the flexible films (determined by atomic force microscopy) depend on the azo-dye type used in the supramolecular azopolyimide synthesis, which were higher when the azo-chromophore containing a –cyano group (–C≡N) was used. The molecular dynamics method, an excellent tool for an in-depth examination of the intermolecular interactions, was used to explain the morphological aspects. Energetic, dynamic and structural parameters were calculated for the two systems containing azo-chromophores, as well as for the pristine polymer system. It was highlighted that the van der Waals forces make a major contribution to the intermolecular interactions. The results from the combination of the dynamic analysis and the concentration profile explain the better mobility of the polyimide chains with a maximum content of azo groups in the cis configuration compared to the other systems. Taking all these data into account, the surfaces of the films can be tuned as required for the proposed applications, namely as substrates for flexible electronis. [ABSTRACT FROM AUTHOR]

Designing and self-assembly of unconventional star-shaped mesogens or 'hekates' are of interest to materials science and molecular electronics. This work presents the synthesis of several symmetric trimeric liquid crystalline compounds comprising 1,3,5-trihydroxybenzene (phloroglucinol) as central core. The liquid crystalline behaviour was induced by attaching three rod-like azo-aromatic semi-flexible units to the central core via esteric linkages. Based on differential scanning calorimetry and polarising optical microscopy, the mesophase stability mainly depends on the alkyl chain length. The compounds showed monotropic or enantiotropic behaviour, except the compound with nine atoms on the chain end which is non-mesomorphic. In addition to the synthesis and mesophase characterisation, the self-assembled structures and individual properties have been characterised by molecular simulations. [ABSTRACT FROM AUTHOR]

Novel systems based on modified azo-polysiloxanes and gelatin with potential biological applications were prepared and characterized. Simultaneous rheological and UV irradiation studies allowed evidencing structural modification inside the gelatin matrix. For all analyzed samples a shift in the temperature corresponding to the physical network destructuration, from 30°C (corresponding to gelatin) to 40°C (for the composite) was noticed. The experimental results proved the existence of interactions between gelatin and polysiloxanes intensified after UV irradiation. The increase in the values of G' and G" is a consequence of system restructuration leading to more arranged architectures able to release the included active principle. As a function of the azopolysiloxane structure, the destructuration temperature of the composite can be tuned in the domain 30-40°C. [ABSTRACT FROM AUTHOR]

The article describes the liquid-crystalline properties of some 2,5-asymmetric disubstituted [1,3,4]oxadiazole derivatives containing an azo and an ester linkage obtained through esterification of 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4]oxadiazole with a series of 4-(4-alkoxyphenyl)-benzoic acids containing 6-10 and 18 aliphatic carbon atoms. All reported compounds present liquid-crystalline properties, evidenced by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), with nematic and smectic C type structures, with very large range of stability of mesophases (between 130°C and 198°C on heating and 134°C and 214°C on cooling). [ABSTRACT FROM AUTHOR]

The glass transition temperature (Tg) is an important decision parameter when synthesizing polymeric compounds or when selecting their applicability domain. In this work, the glass transition temperature of more than 100 homopolymers with saturated backbones was predicted using a neuro-evolutive technique combining Artificial Neural Networks with a modified Bacterial Foraging Optimization Algorithm. In most cases, the selected polymers have a vinyl-type backbone substituted with various groups. A few samples with an oxygen atom in a linear non-vinyl hydrocarbon main chain were also considered. Eight structural, thermophysical, and entanglement properties estimated by the quantitative structure–property relationship (QSPR) method, along with other molecular descriptors reflecting polymer composition, were considered as input data for Artificial Neural Networks. The Tg's neural model has a 7.30% average absolute error for the training data and 12.89% for the testing one. From the sensitivity analysis, it was found that cohesive energy, from all independent parameters, has the highest influence on the modeled output. [ABSTRACT FROM AUTHOR]

The operability of liquid crystal displays is strongly impacted by the orientation aspects of nematics, which in turn are affected by the alignment layer surface features. In this work, two polyimide (PI) structures are obtained based on a cycloaliphatic dianhydride and aromatic or aliphatic diamines with distinct flexibility. The attained PI films have high transmittance (T) for visible radiations, i.e., at 550 nm T > 80%. Here, a novel strategy for creating surface anisotropy in the samples that combines rubbing with a cloth and stretching via pressing is reported. Birefringence and atomic force microscopy (AFM) scans reveal that the generated orientation of the chains is affected by the chemical structure of the polymer and order of the steps involved in the surface treatment. Molecular modeling computations and wettability tests show that the PI structure and produced surface topography are competitive factors, which are impacting the intensity of the interactions with the nematic liquid crystals. The achieved results are of great relevance for designing of reliable display devices with improved uniform orientation of liquid crystals. [ABSTRACT FROM AUTHOR]

Bent-core liquid crystals (BCLC) have been widely studied as a result of their unusual polar and chiral properties. Similar to calamitic and discotic molecules, BCLC molecules also exhibit nematic phases, besides other higher order mesophases. The aim of this work is to comparatively analyze the mesomorphic behavior of some bent-core 1,3-disubstituted benzene core compounds derived from resorcinol and isophthalic acid. Thus, the two classes of compounds differ in the nature of the orientation of the ester bond between the benzene central core and the two branches attached to the core. The mesomorphic behavior was elucidated by polarized light optical microscopy and differential calorimetry. Given the relatively high isotropic points of the compounds, confirmation of the thermal stability in the domains manifesting liquid crystalline properties was performed by thermogravimetric studies. The theoretical explanation of the difference in mesomorphic behavior for the two classes was based on molecular modeling studies. [ABSTRACT FROM AUTHOR]

Aromatic polyimides containing side azo-naphthalene groups have been investigated regarding their capacity of generating surface relief gratings (SRGs) under pulsed UV laser irradiation through phase masks, using different fluencies and pulse numbers. The process of the material photo-fluidization and the supramolecular re-organization of the surface were investigated using atomic force microscopy (AFM). At first, an AFM nanoscale topographical analysis of the induced SRGs was performed in terms of morphology and tridimensional amplitude, spatial, hybrid, and functional parameters. Afterward, a nanomechanical characterization of SRGs using an advanced method, namely, AFM PinPoint mode, was performed, where the quantitative nanomechanical properties (i.e., modulus, adhesion, deformation) of the nanostructured azo-polyimide surfaces were acquired with a highly correlated topographic registration. This method proved to be very effective in understanding the formation mechanism of the surface modulations during pulsed UV laser irradiation. Additionally to AFM investigations, confocal Raman measurements and molecular simulations were performed to provide information about structured azo-polyimide chemical composition and macromolecular conformation induced by laser irradiation. [ABSTRACT FROM AUTHOR]