Modification Of Chemical Structure Research Articles

Zintl Chemistry for Designing High Efficiency Thermoelectric Materials

Zintl phases and related compounds are promising thermoelectric materials; for instance, high zT has been found in Yb14MnSb11, clathrates, and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors.

Self-assembled monolayers of benzenethiol and benzenemethanethiol on Au(1 1 1): Influence of an alkyl spacer on the structure and thermal desorption behavior

The surface structures, adsorption conditions, and thermal desorption behaviors of benzenethiol (BT) and benzenemethanethiol (BMT) self-assembled monolayers (SAMs) on Au(1 1 1) were examined by means of scanning tunneling microscopy (STM), X-ray photoelectron microscopy (XPS), and thermal desorption spectroscopy to understand the effects of the alkyl spacer between the phenyl group and the sulfur atom. Although XPS spectral shapes in the S 2p region for both SAMs are similar, the surface structures and thermal desorption behaviors differ significantly. BT SAMs on Au(1 1 1) were composed of disordered phases, whereas BMT SAMs have well-ordered phases containing vacancy islands. The strong desorption peak for parent mass species ( m/z = 110, C 6H 5SH +) was observed in BT SAMs at about 500 K, whereas no desorption peak ( m/z = 124, C 6H 5CH 2SH +) was observed from BMT SAMs. Interestingly, the dominant TD peak for the benzyl fragments ( m/z = 91, C 6 H 5 CH 2 + ) formed via C–S bond cleavage was observed in BMT SAMs at around 400 K. From this study, we clearly revealed that the small modification in chemical structure by inserting a methylene spacer between the phenyl group and the sulfur atom affects 2D SAM structures, adsorption conditions, and thermal desorption behaviors and stability. The results obtained here will be very useful in designing and fabricating aromatic thiol SAMs for further applications.

Polymer hydrophilicity and hydrophobicity induced by femtosecond laser direct irradiation

Controlled modification of surface wettability of polymethyl methacrylate (PMMA) was achieved by irradiation of PMMA surface with femtosecond laser pulses at various laser fluences and focus distances. Fluences from 0.40 to 2.1 J/cm2 produced a hydrophobic surface and 2.1 to 52.7 J/cm2 (maximum investigated) produced a hydrophilic surface. Fluences less than 0.31 J/cm2 had no effect on the wettability of the raw PMMA. This change in wettability was caused dominantly by laser induced chemical structure modification and not by a change in surface roughness.

Investigation of Asphaltene Chemical Structural Modification Induced by Thermal Treatments

In the conversion of heavy oil into valuable fuels by thermal processes, one of the main problems is the formation of soft coke-like substances that can cause equipment fouling and catalyst deactivation. Asphaltenes are present in large quantity in heavy crude oil and are known to be coke precursors. The objective of the present study was to obtain chemical structure information about the asphaltene molecules when they undergo a thermal treatment and to investigate the mechanism of coke formation. Asphaltenes were separated from two industrial thermal treated heavy crude oils and characterized by elemental analysis, NMR, FTIR, and FT-ICR MS. Analytical data were then compared with that obtained from asphaltene samples after thermal treatment at 400 °C. From complementary and comparative use of the different analytical techniques, we demonstrated that asphaltenes thermally treated at 400 °C tend to aromatize to form coke precursors. We found that the species rich in saturated rings and/or alkyl chains are ...

Characterization of Microscale Wear in a Polysilicon-Based MEMS Device Using AFM and PEEM–NEXAFS Spectromicroscopy

Mechanisms of microscale wear in silicon-based microelectromechanical systems (MEMS) are elucidated by studying a polysilicon nanotractor, a device specifically designed to conduct friction and wear tests under controlled conditions. Photoelectron emission microscopy (PEEM) was combined with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) to quantitatively probe chemical changes and structural modification, respectively, in the wear track of the nanotractor. The ability of PEEM–NEXAFS to spatially map chemical variations in the near-surface region of samples at high lateral spatial resolution is unparalleled and therefore ideally suited for this study. The results show that it is possible to detect microscopic chemical changes using PEEM–NEXAFS, specifically, oxidation at the sliding interface of a MEMS device. We observe that wear induces oxidation of the polysilicon at the immediate contact interface, and the spectra are consistent with those from amorphous SiO2. The oxidation is correlated with gouging and debris build-up in the wear track, as measured by AFM and scanning electron microscopy (SEM).

Influence of ultraviolet-irradiated oxygen on depolymerization of chitosan

In the present paper, the depolymerization of chitosan was carried out by the ultraviolet-irradiated oxygen treatment. Influence of reaction conditions on depolymerization of chitosan was investigated. The chemical structure of the depolymerized chitosan was characterized by FT-IR and 13C NMR spectra. The FT-IR and 13C NMR spectra suggested that there was no obvious modification of chemical structure of the depolymerized chitosan. The X-ray diffraction analysis showed that crystalline structure of chitosan can be destroyed by ultraviolet-irradiated oxygen. The use of depolymerization of chitosan by ultraviolet-irradiated oxygen treatment can be a convenient, timesaving, and cost-efficient method for replacing the expensive and time-consuming enzymatic or chemical methods that are currently used to depolymerize chitosan.

Effects of Dyeing Conditions on Discoloration Behavior for Oxidative Dyed Wool Fabrics

The effect of air function on discoloration behavior of oxidation dyes in wool fibers at ordinary temperature and humidity in dark room was investigated. The samples dyed under three different dyeing conditions and dyed with the different mixed ratios of precursor and copular were used in experiment. In the color change defined by chromaticity diagram (a* and b*) and color tone diagram (L* and C*), the color tone of samples dyed under dyeing system including alkali and hydrogen peroxide changed more significant than that of samples dyed in the performed oxidation dye solution. The discoloration of samples dyed with the different mixed rations of precursor and copular is scarcely dependent on mixed ratio, except for the couple system of p-diaminotoluene/p-amino-o-cresol. The modifications of physical and chemical structure of wool fibers had no effect on the decreasing rate of K/S value for the samples dyed using three precursor / copular systems.

Effect of H+ and N+ Irradiation on Structure and Permeability of the Polyimide Matrimid®

This paper presents a comparison of the impact of H+ and N+ ion irradiation on the chemical structure, microstructure, and gas permeation properites of the polyimide, Matrimid®. While irradiation with both ions resulted in evolution of chemical structure with loss of functional groups and crosslink formation, there was greater modification of polyimide structure following N+ irradiation at similar total deposited energy. Irradiation with N+ resulted in simultaneous large increases in permeance and permselectivity at comparatively low ion fluences or irradiation time. For example, irradiation at 4 × 1014 N+/cm2 resulted in a 2.5 fold increase in He permeance with a selectivity of He/CH4 of 340. Much higher H+ fluences were required to achieve similar total deposited energy and combined increases in permeance and permselectivity. The larger modification in chemical structure and gas permeation properties following N+ irradiation was attributed to the relatively large energy loss and damage from the nuclear energy relative to electronic energy loss.

Studies on chemical structure modification and biology of a natural product, Gambogic acid (I): Synthesis and biological evaluation of oxidized analogues of gambogic acid

Gambogic acid (GA), a natural product, exhibits high potency in inhibiting cancer cell growth through the effective induction of apoptosis. In order to investigate the structure–activity relationships of GA derivatives, 11 oxidized derivatives of GA were synthesized. Some of them showed strong inhibitory effects on HT-29, Bel-7402, BGC-823, A549, and SKOV 3 cell lines. Moreover, in this paper the cellular growth inhibitor 39-hydroxy-6-methoxy-gambogic acid methyl ester ( 10) was identified as a HepG2 cell apoptosis inhibitor through Annexin-V/PI double staining assay and the expression of the related apoptotic proteins (Bax and Bcl-2). Compound 10 may serve as a potential lead compound for the development of new anticancer drugs. Further SAR studies of GA derivatives indicated that modification of carbon–carbon double bond at C-32/33 or C-37/38 and of the methyl groups at C-39/C-35 can improve antitumor activity.

High performance liquid chromatographic enantioseparation of chiral bridged polycyclic compounds on chiralcel OD‐H and chiralpak OT(+)

The HPLC enantiomeric separation of 29 racemic bridged polycyclic compounds was examined on commercially available Chiralcel OD-H and Chiralpak OT(+) columns. The separations were evaluated under normal-phase mode (hexane containing mobile phase) for Chiralcel OD-H and under normal-phase as well as under reversed-phase mode (pure MeOH, temperature 5 degrees C) for Chiralpak OT(+). Almost all compounds were resolved either on Chiralcel OD-H or on Chiralpak OT(+), in some cases on both. The use of trifluoroacetic acid (TFA), as modifier of the hexanic mobile phase, had a beneficial effect on the enantioseparation of some polar and acidic compounds on Chiralcel OD-H. The influence of small chemical structural modifications of the analytes on the enantioseparation behavior is discussed. A structure-retention relationship has been observed on both stationary phases. This chromatographic evaluation may provide some information about the chiral recognition mechanism: in the case of Chiralcel OD-H, hydrogen bonding, pi-pi and distereoselective repulsive are supposed to be the major analyte-CSP interactions. In the case of Chiralpak OT(+), a reversed-phase enantioseparation could take place through hydrophobic interactions between the aromatic moiety of the analytes and the chiral propeller structure of the CSP. The synthesis of some unknown racemic bromobenzobicyclo[2.2.1] analytes is also described.

Structural modifications of polymers under the impact of fast neutrons

Fast neutron irradiations were performed at room temperature on a series of 13 polymer matrixes having great practical and academic interest. Polymers were irradiated with very low doses of 14 MeV neutrons deposited at reduced dose rate. The modifications of chemical structure of the polymeric matrixes resulting from irradiation were analyzed at the molecular scale by infrared and UV–visible spectroscopy. Particular attention was given to the oxidation occurring in irradiated matrixes during storage in the absence of light at room temperature. In addition the effects of irradiation on the chain arrangements in the polymers were analyzed by Differential Scanning Calorimetry. The most significant result obtained in this preliminary work was evidence that neutron irradiation with very low doses in the range ≈130–320 cGy could provoke structural changes. Indeed, depending on the matrixes, the formation of oxidation products along the macromolecular chains or the evolution of the polymer architecture, evidenced by changes of the T g or of the crystallinity, was observed. The perspectives of this preliminary work are discussed.

Physicochemical characterization of a modified cellulose acetate membrane for the design of oil-in-water emulsion disruption devices

In this paper, a new membrane separation process is presented. This process is based on a multiple chamber cell that allows the separation of oil from water by placing two membranes of different hydrophobicity in contact with the emulsion. Further chemical modifications of the membranes have been performed in order to vary their hydrophobicity in the perspective of optimizing the fractionation process via specific interactions between membrane and emulsion components. For this purpose, methylcellulose was modified with 1,2-epoxydodecane and incorporated in the membrane elaboration process. The modification of membrane surface chemical structure was demonstrated by ATR-FTIR and XPS experiments. Preliminary filtration tests are presented as well as future perspectives to optimize the membrane modification procedure.

Development of a new generation of propofol

The current advance in development of a new generation of propofol and its clinical implications are reviewed. All currently available formulations of propofol have side effects related to propofol itself, as well as to the emulsion delivery systems. A search for better and safer propofol continues. Improvement of the emulsion delivery systems is one of approaches, which can be achieved by identifying better inactive components, enhancing stability, and applying new technologies such as nanotechnology. Modification of chemical structure is another approach, particularly with the development of propofol prodrugs. There is a continuous need to develop a better and safer propofol. The development of a new propofol should be focused on minimizing the unwanted side effects, while preserving the beneficial profiles. Hopefully, just such a new propofol will be available in the near future for clinical use.

XPS Investigation of Plasma‐Deposited Polysiloxane Films Irradiated with Helium Ions

AbstractThis work describes an XPS investigation of plasma‐deposited polysiloxane films irradiated with 170 keV He+ ions at fluences, Φ, ranging from 1 × 1014 to 1 × 1016 cm−2. Modifications in the atomic concentrations of the surface atoms with Φ were revealed by changes in the [O]/[Si], [O]/[C] and [C]/[Si] atomic ratios. Surface chemical structure modifications were evidenced by the increasing C1s peak width and asymmetry as Φ was increased, due to the formation of ether and carboxyl functionalities. Moreover, structural transformations were indicated by the positive binding energy shift of the Si2p peaks, due to the increasing Si oxidation. Correlations of the XPS data with other results from previous work on polysiloxanes illustrate the role of ion beam‐induced bond breaking on the structural modifications.magnified image

Stability of atmospheric-pressure plasma induced changes on polycarbonate surfaces

Polycarbonate films are subjected to plasma treatment in a number of applications such as improving adhesion between polycarbonate and silicon alloy in protective and optical coatings. The surfaces that undergo changes in surface properties due to plasma treatment have a tendency to revert back to their original states. Thus, the stability of the plasma induced changes on polymer surfaces over a desired time period is an important issue. The objective of this study was to examine the effect of ageing on atmospheric-pressure helium-plasma treated polycarbonate (PC) sample as a function of treatment time. The ageing effects were studied over a period of 10 days. The samples were plasma treated for 0.5, 2, 5 and 10 min. Contact angle made by water droplet on polymer surfaces were measured to study surface energy changes. Modification of surface chemical structure was examined using X-ray photoelectron spectroscopy (XPS). Contact angle decreased from 93° for untreated samples to 30° for samples treated with plasma for 10 min. After 10 days the contact angle for the 10 min plasma treated sample increased to 67°, but it never reverted back to that of the untreated surface. Similarly, the oxygen-carbon (O:C) ratio increased from 0.136 for untreated samples to 0.321 for 10 min plasma-treated samples, indicating an increase in surface energy.

Optimization of culture conditions for production of antimalarial menisporopsin A by the seed fungusMenisporopsis theobromaeBCC 4162

The aim of this work was to optimize the production of a novel antimaralial menisporopsin A by the seed fungus Menisporopsis theobromae BCC 4162. Fungal cultures were grown in shake flasks at 25 degrees C in the basal medium with varying carbon and nitrogen sources, aeration rates and initial pH levels. The optimal carbon and nitrogen sources that improved the production of menisporopsin A were 1% fructose and 2.5% meat extract respectively. The production was further enhanced when the culture incubated on a shaker at 200 rev min(-1) with an initial pH of 8. The yield of menisporopsin A cultured under the optimized conditions was increased from 348.30 (obtained from basal medium) to 889.02 mg l(-1), and the cultivation time was reduced from 28 to only 4 days. As a result, the productivity of menisporopsin A was greatly enhanced to 222.26 mg l(-1) day(-1) which is 18-fold higher than that of basal conditions. Larger scale production in a fermenter was also achieved, yielding menisporopsin A at a maximal level of 594.32 mg l(-1) in 4 days. The optimized culture conditions for menisporopsin A production by M. theobromae BCC 4162 was the cultivation under shaking or agitation at 25 degrees C in fructose-meat extract medium with an initial pH of 8. The production of menisporopsin A in a fermenter with a relatively short incubation period could be valuable for further utilization for chemical structure modification and derivatization.

Chemical Extractions Affect the Structure and Phenanthrene Sorption of Soil Humin

Humin is a major fraction of soil organic matter and strongly affects the sorption behavior and fate of organic contaminants in soils and sediments. This study evaluated four different extraction methods for soil humins in terms of their organic carbon structural changes and the consequent effects on phenanthrene sorption. Solid-state 13C NMR demonstrated that 0.1 M NaOH exhaustively extracted humin and humin extracted with 6 M HF/HCl at 60 degrees C had a relatively high amount of aliphatic components as compared with 1 M HF-extracted humin. The treatment of 6 M HF/HCl at 60 degrees C greatly reduced carbohydrate components (50-108 ppm) from humin samples, i.e., more than 50% reduction. In addition, the humin from this 6 M HF/HCl treatment contained relatively more amorphous poly(methylene) domains than the humins extracted by other methods. With the respect to phenanthrene sorption, the linearity of sorption isotherm (N) and sorption affinity (Koc) varied markedly among the humin samples extracted by different methods. The NaOH exhaustively extracted humin had the most nonlinear sorption isotherm and the HF-extracted humin had the lowest Koc. It is concluded that humin samples from different extraction procedures exhibited substantial differences in their organic carbon structure and sorption characteristics, even though they were from the same soil. Therefore, one needs to be cautious when comparing the structural and sorption features of soil humins, especially when they are extracted differently. The 6 M HCl/HF extraction at elevated temperature is not encouraged, due to the modifications of chemical structure and physical conformation of organic matter.

Surface structures tailoring of hexamethyldisiloxane films by pulse rf plasma polymerization

The surface structures of hexamethyldisiloxane (HMDSO) films prepared by rf plasma polymerization are investigated by Fourier transform infrared, solid-state 29Si CP/MAS nuclear magnetic resonance and scanning electron microscope. The reactive species in HMDSO plasma and the mechanism of HMDSO plasma synthesis are studied by optic emission spectrum. The effect of continuous wave discharge and different pulse discharge parameters especially plasma “high power” time on chemical structure and physical morphology modifications of HMDSO polymer film is compared elaborately. The results show that HMDSO-continuous-wave polymer is an organic film without Si O group. Its surface consists of several microscale islands with many clustered rods. While HMDSO-pulse-polymer is a network of interconnected primary methylsiloxane units and minor hydroxyl, oxymethylene, hydrogen and carbonyl groups. The ratio of Si O Si to Si (CH 3) n and new functional groups of OH, C O and C O are effectively “tailored” by changing plasma “high power” time. Moreover, HMDSO-pulse-polymer is a continuous, regular and compact film with a few asteroid protuberances. This deposited film becomes more regular after increasing plasma “high power” time and gradually presents perfect “straw-mat” morphology. It is indicated that pulse plasma polymerization is an effective way to tailor the surface structures of plasma polymer films through controlling plasma “high power” time.

Interactions of sodium montmorillonite with poly(acrylic acid)

The chemical–structural modifications of the natural clay sodium montmorillonite during interaction with poly(acrylic acid) were studied mainly by X-ray photoemission spectroscopy. Samples of modified montmorillonite were prepared from the reaction of sodium montmorillonite (∼0.5 g) and an aqueous solution of poly(acrylic acid) ( pH ∼ 1.8 , 50 g) at varying temperatures. X-ray diffraction indicated that the montmorillonite interlayer space (∼13 Å), formed by regular stacking of the silicate layers (dimension ∼ 1 × 1000 nm ), expanded to ∼16 Å as the reaction was carried out at room temperature and at 30 °C. At 60 °C, the interlayer space further expanded to ∼20 Å. The results of X-ray photoemission spectroscopy indicated that poly(acrylic acid) molecules exchange sodium ions on the surface of the silicate layers. These combined results allowed development of a reaction model that explains the dependency of the interlayer expansion with temperature. Information concerning the surface chemical reactions and systematic increases in the interlayer distances is particularly useful if montmorillonite and poly(acrylic acid) are to be used for formation of nanocomposite materials.

Investigation of the drastic change in the sputter rate of polymers at low ion fluence

The polymer sputter rate dependence on ion fluence and ion chemistry (Ar, N2, O2) at 1keV energy was investigated using a quartz crystal microbalance (QCM) which allowed to do real time etch rate measurements and to study kinetics of sputtering. The obtained sputter rates differed drastically from polymer to polymer showing, that the chemical structure of polymer is an important factor in the polymer etch yield. A decrease in the sputter rate was observed up to ion fluence of 5×1014–5×1015cm−2 (depending on the polymer type and ion chemistry) followed by the saturation in the rate at prolonged ion bombardment. Polymer removal was accompanied by the formation of degradation products, cross-linking or branching, modification of the surface chemical structure, which was studied in situ using XPS. The dependence of the surface glass transition temperature, Tgs on the ion fluence was studied using the method based on the embedding of metallic nanoparticles. The correlation between chemical yield data and ablation rate is discussed.