New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. II. Study of the transition temperatures of poly(methyl methacrylate) at various tacticities and of poly(methyl methacrylate) adsorbed on alumina and silica

Some Irregularities in the Evaluation of Surface Parameters of Solid Materials by Inverse Gas Chromatography

The London dispersive component of the surface energy of solid particles, their specific interactions, and Lewis acid-base parameters were usually calculated by using the inverse gas chromatography technique at infinite dilution. Serious irregularities were committed by some authors when using the Schultz method or Dorris-Gray relation. We proved that these methods cannot be used because they did not consider the important role of the temperature on variations in the surface area of solvents. We corrected in this paper some recent results by using new relations of the surface area of organic probes depending on the temperature and by using different molecular models using different geometries and mathematical calculations. The application of our new thermal model gave more precise results of the dispersive energy and Lewis's acid-base constants of solid particles. The new thermal model was applied to several solid surfaces such as Ni-MOF-74, MgO, ZnO, TiO2, and Zn(OH)2. The obtained results showed a stronger Lewis basicity of MgO solid substrate and higher Lewis acidity of the Ni-MOF-74 surface.

Flexible, low-voltage, and low-hysteresis PbSe nanowire field-effect transistors

We report low-hysteresis, ambipolar bottom gold contact, colloidal PbSe nanowire (NW) field-effect transistors (FETs) by chemically modifying the silicon dioxide (SiO(2)) gate dielectric surface to overcome carrier trapping at the NW-gate dielectric interface. While water bound to silanol groups at the SiO(2) surface are believed to give rise to hysteresis in FETs of a wide range of nanoscale materials, we show that dehydration and silanization are insufficient in reducing PbSe NW FET hysteresis. Encapsulating PbSe NW FETs in cured poly(methyl) methacrylate (PMMA), dehydrates and uniquely passivates the SiO(2) surface, to form low-hysteresis FETs. Annealing predominantly p-type ambipolar PbSe NW FETs switches the FET behavior to predominantly n-type ambipolar, both with and without PMMA passivation. Heating the PbSe NW devices desorbs surface bound oxygen, even present in the atmosphere of an inert glovebox. Upon cooling, overtime oxygen readsorption switches the FET polarity to predominantly p-type ambipolar behavior, but PMMA encapsulation maintains low hysteresis. Unfortunately PMMA is sensitive to most solvents and heat treatments and therefore its application for nanostructured material deposition and doping is limited. Seeking a robust, general platform for low-hysteresis FETs we explored a variety of hydroxyl-free substrate surfaces, including silicon nitride, polyimide, and parylene, which show reduced electron trapping, but still large hysteresis. We identified a robust dielectric stack by assembling octadecylphosphonic acid (ODPA) on aluminum oxide (Al(2)O(3)) to form low-hysteresis FETs. We further integrated the ODPA/Al(2)O(3) gate dielectric stack on flexible substrates to demonstrate low-hysteresis, low-voltage FETs, and the promise of these nanostructured materials in flexible, electronic circuitry.

Tough nanocomposites: the role of carbon nanotube type

Unusually large deformation is observed in poly(methyl metacrylate) (PMMA) electrospun fibers under tension when multiwall or single-wall carbon nanotubes (MWCNTs and SWCNTs) are included as a second phase in the fibers. These distortions are virtually absent in pure PMMA fibers and stem from markedly different energy dissipation mechanisms and necking modes arising from the dissimilar nanotube morphologies. Thus, both nanotubes types are effective tougheners of PMMA fibers, with an advantage for MWCNTs over SWCNTs.

Determination of glass temperature of polymers by inverse gas chromatography

Inverse gas chromatography (IGC) is an attractive technique for polymer characterization due to possible simultaneous determination of various physicochemical properties of polymer systems merely from retention times of selected sorbates. The technique is especially advantageous to polymers that cannot be characterized by conventional methods. In this review, the utilization of the method for glass transition determination of homopolymers, copolymers and polymer blends is described. Advantages and drawbacks of the IGC method over traditionally used methods for glass transition temperature determination is discussed, along with the most important parameters that influence the precision and accuracy of the glass transition temperature (T(g)) measurements.

Changes in surface properties by granulation and physicochemical stability of granulated amorphous cefditoren pivoxil with additives

The evaluation of the physicochemical stability of granules of amorphous cefditoren pivoxil (CDTR-PI), alone or with polymers, demonstrated that granulated amorphous CDTR-PI with hydroxypropyl methylcellulose was the most stable. We measured glass transition temperature by differential scanning calorimetry (DSC). The molecular mobility of the whole granules did not change, and it was not consistent with the results of the evaluation of physicochemical stability. Peak shifts were observed in IR spectra of amorphous CDTR-PI with polymers after granulation, and the shifts were similar to those observed for spray-dried samples. Furthermore, the shifts were not observed after the granules were ground. Acid-base parameters, which were also measured by inverse gas chromatography (IGC), changed after granulation. These results suggested that on the surface of the granules, CDTR-PI and the polymers would be mixed monomolecularly, as in the spray-dried samples. The changes in the molecular state of a drug when mixed monomolecularly with a polymer on the surface of granules were successfully confirmed by IGC and IR.