Polymerization of para-xylylene derivatives (parylene polymerization). III. Heat effects during deposition of parylene N at different temperatures

Thermal effects accompanying the vacuum deposition of poly-para-xylene (Parylene N) at different temperatures have been studied by following the changes in the temperature of the substrate. Similarly to the case of polychloro-para-xylylene (Parylene C), two distinct exothermic effects were observed; one discrete, resulting in sharp exothermic spikes and the other continuous, resulting in the shift of the baseline. The spike effect, attributed to the solid-state polymerization of para-xylylene, is observed at the low-temperature range, the upper limit of which moves higher for higher deposition rates. The shift of a baseline as a function of deposition temperature exhibits two maxima, one independent of deposition rate and the second moving toward higher temperatures (that is, toward the first maximum) for higher deposition rates. First maximum falls at about ? 72°C and is attributed to the melting point of para-xylylene crystals. X-ray diffraction studies of polymer samples have shown that the existence of the second maximum is always followed by the appearance of an additional crystalline phase in the respective range of deposition temperatures. When the deposition rate is high enough, the second maximum merges with the first one, or virtually disappears. Under such conditions the new crystalline phase is no more detectable. It is postulated that the evolution of the additional amount of heat resulting in the appearance of the second maximum is due to the cyclization reaction and the formation of cyclic oligomers. This reaction very likely requires a particular spatial arrangement of monomer molecules and, therefore, it is suggested to take place in certain domains of the crystalline lattice.     

An extension of Goldie's result and further results in infinite divisibility

Summary It is shown that gamma random variables satisfy a certain factorisation property. Using this property, Goldie's result on mixtures of the exponential distributions is extended. The note also establishes that the generalized inverse Gaussian distribution and the generalized hyperbolic distribution considered by Barndorff-Nielsen and Halgreen (1977) are indeed self-decomposable.     

Dielectric relaxation, thermodynamic and theoretical studies on hydrogen bonded polar binary mixtures of N-methyl aniline with propan-1-ol and isopropyl alcohol

The dielectric relaxation studies on polar binary mixtures of N-methyl aniline with, alcohols, propan-1-ol and isopropyl alcohol have been carried out, at different temperatures and mole fractions, using — LF impedance analyzer, Plunger method and Abbe’s refractometer in radio, microwave and optic frequency regions respectively. Kirkwood effective correlation factor, corrective Kirkwood correlation factor, Bruggeman parameter, relaxation time and thermodynamical parameters are calculated using the experimental data. Conformational analysis of the formation of hydrogen bond in the equi molar binary mixture systems of N-methyl aniline with propan-1-ol and isopropyl alcohol is supported by experimental and theoretical FT-IR values.     

Image analysis method to study electro-optic parameters of nematogens

This paper demonstrated the image analysis technique for the investigation of electro-optic behaviour of homogeneously aligned nematic liquid crystals p-n alkyl benzoic acids (nBA) where n = 5 and 6 under the application of direct current (DC) electric field. Textures of the liquid crystals in the mesophase region of particular temperature were recorded in three monochromatic image planes at the wavelengths of 635 (red), 530 (green) and 475 nm (blue) as a function of applied DC voltage. Electro-optic properties such as transmittance, birefringence, phase retardation and contrast ratio were measured by analysing the optical intensities of the recorded textures using MATLAB software. The changes in the textural features and formation of new textures as a function of voltage provided information for better understanding of the electro-optic characteristics of liquid crystals. Results of this study were compared with the data of the other standard techniques: laser source (633 nm) experiment and United Detector Technology (UDT) sensor of model UV- 35P silicon photo diode which uses white light as source and filters the wavelengths using three colour filters of wavelengths 635, 530 and 475nm.     

Conformational studies of intermolecular hydorgen bonding through induced crystal G phase in nBA:7HB

This paper exploits the physical investigation on liquid crystal complexes obtained by self-organisation of p-n-alkyl benzoic acid (nBA) mesogens with non-mesogenic materials heptyl p-hydroxy benzoate (7HB). Intermolecular interactions of the molecules result the hydrogen bond between the proton donor (COOH) of nBA and proton acceptor (OH) of 7HB. The formation of hydrogen bond is attributed to the quenching of the nematic phase and inducement of crystal G phase in liquid crystal complex. A comparative study of phase abundance is presented with respect to the pure nBAs and other hydrogen bonded liquid crystal complexes of nBAs. Thermal and phase behaviour of the complexes are determined by polarising optical microscope (POM), differential scanning calorimetry (DSC) and image moments approach. Intermolecular interactions which result the hydrogen bond in complexes are investigated using Fourier transform infrared (FTIR) spectroscopy. Molecular structure of the liquid crystal complexes in the solid phase was elucidated using powder X-ray diffraction and proton nuclear magnetic resonance (¹HNMR).     

Preparation and characterisation of fluorapatite whiskers

Fluorapatite (FAp) whiskers were prepared by using Molten Salt Synthesis (MSS) technique. β‐tricalcium phosphate (TCP) and various fluorine salts, along with potassium sulphate as a flux salt were used as starting materials for preparing FAp whiskers. Effects of flux to powder weight ratio, temperature and soaking time on the morphology were studied. The optimum temperature to obtain FAp whiskers was found to be 1175°C. The phase purity of the prepared FAp whiskers were confirmed by powder X‐ray diffraction and FTIR spectra, and the whiskers were further characterized by laser Raman spectra, chemical analysis and DTA/TGA thermal analysis. The morphology of the FAp whiskers were investigated by scanning electron microscope (SEM) analysis. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Triplet Generation Through Singlet Fission in Metal-Organic Framework: An Alternative Route to Inefficient Singlet-Triplet Intersystem Crossing

High quantum yield triplets, populated by initially prepared excited singlets, are desired for various energy conversion schemes in solid working compositions like porous MOFs. However, a large disparity in the distribution of the excitonic center of mass, singlet-triplet intersystem crossing (ISC) in such assemblies is inhibited, so much so that a carboxy-coordinated zirconium heavy metal ion cannot effectively facilitate the ISC through spin-orbit coupling. Circumventing this sluggish ISC, singlet fission (SF) is explored as a viable route to generating triplets in solution-stable MOFs. Efficient SF is achieved through a high degree of interchromophoric coupling that facilitates electron super-exchange to generate triplet pairs. Here we show that a predesigned chromophoric linker with extremely poor ISC efficiency (k_ISC) but form triplets in MOF in contrast to the frameworks that are built from linkers with sizable k_ISC but . This work opens a new photophysical and photochemical avenue in MOF chemistry and utility in energy conversion schemes.     

Decoupling Redox Hopping and Catalysis in Metal-Organic Frameworks -based Electrocatalytic CO2 Reduction

Traditional MOF e-CRR, constructed from catalytic linkers, manifest a kinetic bottleneck during their multi-electron activation. Decoupling catalysis and charge transport can address such issues. Here, we build two MOF/e-CRR systems, CoPc@NU-1000 and TPP(Co)@NU-1000, by installing cobalt metalated phthalocyanine and tetraphenylporphyrin electrocatalysts within the redox active NU-1000 MOF. For CoPc@NU-1000, the e-CRR responsive Co^I/0 potential is close to that of NU-1000 reduction compared to the TPP(Co)@NU-1000. Efficient charge delivery, defined by a higher diffusion (D_hop=4.1×10⁻¹² cm² s⁻¹) and low charge-transport resistance ( =59.5 Ω) in CoPC@NU-1000 led FE_CO=80 %. In contrast, TPP(Co)@NU-1000 fared a poor FE_CO=24 % (D_hop=1.4×10⁻¹² cm² s⁻¹ and =91.4 Ω). For such a decoupling strategy, careful choice of the host framework is critical in pairing up with the underlying electrochemical properties of the catalysts to facilitate the charge delivery for its activation.