Simultaneous measurements of X-ray diffraction–differential scanning calorimetry

Journal of Thermal Analysis and Calorimetry - Ganciclovir (GCV) is an antiviral drug for treating cytomegalovirus infections. This drug is reported to exist in four crystal forms in the solid...     

Thermal stability and crystallization kinetics of Se–Te–Sn alloys using differential scanning calorimetry

The present article deals with the differential scanning calorimetric (DSC) study of Se?CTe glasses containing Sn. DSC runs are taken at four different heating rates (10, 15, 20 and 25?K?min^?1). The crystallization data are examined in terms of modified Kissinger, Matusita equations, Mahadevan method and Augis and Bennett approximation for the non-isothermal crystallization. The activation energy for crystallization (E _c) is evaluated from the data obtained at different heating rates. Activation energy of glass transition is calculated by Kissinger??s relation and Moynihan theory. The glass forming tendency is also calculated for each composition. The glass transition temperature and peak crystallization temperature increases with the increase in Sn % as well as with the heating rate.     

Thermal and flame retardant properties of epoxy resin cured by a novel phosphorus-containing 4,4′-bisphenol novolac curing agent

A novel flame retardant curing agent for epoxy resin (EP), i.e., a DOPO (9,10-dihydro-9-oxa-10-phosphaphenan-threne-10-oxide)-containing 4,4'-bisphenol novolac (BIP-DOPO) was synthesized and characterized by Fourier transform infrared (FTIR), ¹H NMR, ³¹P NMR spectroscopy, and gel permeation chromatography. The epoxy resin cured by BIP-DOPO itself or its mixture with a commonly used bisphenol A-formaldehyde novolac resin (NPEH720) was prepared. The flame retardancy of the cured EP thermosets were studied by limiting oxygen index (LOI), UL 94 and cone calorimeter test (CCT), and the thermal properties by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results show that the cured epoxy resin EPNP/BI/3/1, which contains 2.2% phosphorus, possesses a value of 26.2% and achieves the UL 94 V-0 rating. The data from cone calorimeter test demonstrated that the peak release rate, average heat release rate, total heat release decline sharply for the flame retarded epoxy resins, compared with those of pure ones. DSC results show that the glass-transition temperatures of cured epoxy resins decrease with increasing phosphorus content. TGA indicates that the incorporation of BIP-DOPO promotes the decomposition of epoxy resin matrix ahead of time and leads to higher char yield. The surface morphological structures of the char residues reveal that the introduction of BIP-DOPO benefits to the formation of a continuous and solid char layer on the epoxy resin material surface during combustion.     

Study of the compositional heterogeneity of ethylene–hexene-1 copolymers by thermal fractionation technique by means of differential scanning calorimetry

The successive self-nucleation/annealing technique (SSA) by differential scanning calorimetry has been applied to study the heterogeneity of ethylene–hexene-1 copolymers produced with supported catalytic systems of different compositions: highly active supported Ziegler–Natta (Z–N) catalysts—a titanium–magnesium catalyst TiCl₄/MgCl₂ (TMC) and a vanadium–magnesium catalyst VCl₄/MgCl₂ (VMC), a supported zirconocene catalyst Me₂Si(Ind)₂ZrCl₂/SiO₂ (MAO), and a chromium-oxide catalyst CrO₃/SiO₂. Comparative data by SSA technique with the same temperature program were obtained for copolymers differed by MWD from narrow to very broad (Mw/Mn = 2.4–54) and short chain branching distribution from narrow (zirconocene catalyst) to very broad (TMC and chromium oxide catalysts). It is demonstrated that copolymers produced with the zirconocene catalyst have the narrowest melting range and do not contain thick lamellae. The widest lamella thickness distribution has been found for a copolymer produced with the chromium-oxide catalyst. Copolymers produced with the supported Z–N catalysts are ranked in the middle with a more narrow lamella thickness distribution for copolymer prepared with VMC as compared with the one produced with TMC. The SSA results are compared with the data on copolymer fractionation by TREF. It is shown that these methods give a good correlation for copolymers with narrow short-chain branching distribution produced with the supported zirconocene catalyst. In the case of copolymers produced with TMC, TREF yields a higher content of the high-branched fractions.     

DSC-TG studies on kinetics of curing and thermal decomposition of epoxy–ether amine systems

The cure behavior of diglycidyl ether of bisphenol A with a simple ether amine (4,7,10, trioxa -1,13, tridecane diamine), system I and a polyether amine (polypropylene glycol block polyethylene glycol block polypropylene glycol bis 2 amino propyl ether), system II was compared by Differential Scanning Calorimetry. The exothermicity of the curing reaction of system I is higher than that of system II (316 ± 15 J g^?1 for System I and 230 ± 15 J g^?1 for system II). Kinetic parameters viz., activation energy, pre-exponential factor, and rate constant for curing were evaluated by Kissinger method and Kissinger–Akahira–Sunose isoconversion method. Both systems showed low glass transition temperatures and System II shows a much lower T _g (?38 °C) than system I (26 °C). The thermogravimetric analysis of the two cured epoxy amine systems showed comparable thermal stability.     

Effect of functionalized multiwall carbon nanotubes on the curing kinetics and reaction mechanism of bismaleimide–triazine

Functionalized multiwall carbon nanotubes (f-MWCNTs) with varying functionalization degrees were prepared by chemical methods. The effect of f-MWCNTs on the cure kinetics of bismaleimide–triazine (BT) resin was studied through nonisothermal differential scanning calorimetry (DSC) methods. The reaction activation energy (E _α ) was determined by Flynn–Wall–Ozawa method. The results show that f-MWCNTs have more acceleration ability than pristine MWCNTs, due to more groups on the surface of f-MWCNTs than that of pristine MWCNTs. The activation energy was decreased from a value of 91.3 kJ mol^?1 for the neat BT resin to 74.2 kJ mol^?1 at the small mass loading (1.0 %) of f7-MWCNTs. The effect of f-MWCNTs on the reaction mechanism has been investigated. It shows that the f-MWCNTs accelerate the cure reaction of BT resin by providing the Lewis acids (H⁺) to make the “Diels–Alder” reaction and “ENE” reaction of BT resins more efficient. These findings offer useful insights into the cure technology of thermosetting resin filled with f-MWCNTs, without negative effect on the cure reaction.     

Inclusion complex formation of β-cyclodextrin and Naproxen: a study on exothermic complex formation by differential scanning calorimetry

Inclusion complex formation between β-cyclodextrin and Naproxen was investigated using differential scanning calorimetry (DSC) as a function of the β-cyclodextrin-to-Naproxen molar ratio, ranging from 0:5:1 to 5:1. When these mixtures are heated above the melting temperature of Naproxen, an exothermic peak is observed at a temperature slightly higher than the melting peak of Naproxen. This peak, which has not been previously reported, has been interpreted as an exothermic energy of inclusion complex formation. The magnitude of this complex formation peak was found to be dependent upon the composition of the β-cyclodextrin and Naproxen mixture and increased in magnitude to a maximum value at a β-cyclodextrin:Naproxen molar ratio of 2:1. In addition, Naproxen recrystallization and re-melting peaks seen in the cooling and re-heating scans, respectively, decreased in magnitude with increasing molar ratio and totally disappeared for the mixture with 5:1 of β-cyclodextrin to Naproxen ratio indicative of complete inclusion of Naproxen in the cyclodextrin cavities. Complete inclusion was further reflected by the disappearance of key Naproxen peaks in Fourier transform infrared spectra of samples recovered from DSC experiments. The large excess of β-cyclodextrin needed to fully complex the Naproxen was found to be due to slow kinetics. Increasing the hold time after the initial melting led to inclusion efficiencies up to 95 % even for the 2:1 mixture. These experiments suggest that ratios of β-cyclodextrin:Naproxen 2:1 or greater facilitate the process by increasing the presence of cyclodextrin molecules in the close proximity of the drug molecules and lead to high inclusion efficiencies.     

Conformational energetics of interpolyelectrolyte complexation between ι-carrageenan and poly(methylaminophosphazene) measured by high-sensitivity differential scanning calorimetry

The interaction of poly(methylaminophosphazene) hydrochloride (PMAP·HCl) of varying degrees of ionization (f) with the potassium salt of ι-carrageenan was studied by high-sensitivity differential scanning calorimetry at a KCl concentration of 0.15 M, which is included for the purpose of stabilizing the helix conformation of the polysaccharide up to 55 °C. The conditions of strong (pH 3.8, I = 0.15), moderate (pH 7.4, I = 0.15), and weak (pH 7.4, I = 0.25) electrostatic interactions of the polyelectrolytes were considered. The thermodynamic parameters of the helix-coil transition of ι-carrageenan were determined as a function of the polycation/polyanion ratio. We show that the interpolyelectrolyte reaction between PMAP·HCl and ι-carrageenan results in a complete unfolding of the polysaccharide helix under conditions of strong electrostatic interaction and increases its stability under conditions of medium and weak electrostatic interactions. The formation of stoichiometric PMAP-carrageenan interpolyelectrolyte complexes proceeded via a cooperative mechanism at pH 3.8 (f = 0.5) and pH 7.4 (f = 0.2) at an ionic strength of 0.15. In contrast, the complexation at pH 7.4 and an ionic strength of 0.25 could be considered to be a consecutive competitive binding of charged units of poly(methylaminophosphazene) to the oppositely charged polysaccharide matrix in the helix or coil conformation. Binding constants of the polycation to the helix and coil forms of ι-carrageenan were estimated. They revealed a preferential binding of the polycation to the helix form of the polysaccharide.     

Investigation of the electrical resistance of single-walled carbon nanotube films in the temperature range 4.2–290 K

Electrical resistance of films made of the source material and purified HiPCO and Arc single-walled carbon nanotubes (SWCNTs) with a thickness of 20−40 μm is 2.4 to 45 Ω (electrical conductivity of 0.42 × 10³ to 5.03 × 10³ S/m) at room temperature. The films have been formed by vacuum microfiltration of SWCNT suspensions in toluene and characterized by Raman and X-ray photoelectron spectroscopy and scanning electron microscopy. The conductivity of the films at room temperature depends on the type and degree of purity of the material of nanotubes. The resistance of the films decreases with the increasing temperature over the range of 4.2–290 K, and the rate of the step-down decreases with increasing purity of the material of the nanotubes. The conductivity of the films is semiconducting in character, and the electron transport is consistent with three-dimensional hopping conductivity.     

Fast-scan vs conventional differential scanning calorimetry (DSC) techniques in detection of crystallization events of tolbutamide–polyethylene glycol composite

This study investigated the capacity of fast-scan (400 °C min^?1) against conventional (10 °C min^?1) differential scanning calorimetry (DSC) techniques to track crystallization phenomenon in tolbutamide–polyethylene glycol 3000 composites prepared by hot melt method (mass ratios 1:1, 1:5, and 1:9) and stored at 25 and 75 % relative humidities. Drug crystallization in composites was indicated by X-ray diffractometry (XRD) and scanning electron microscopy characterization over 40 days storage. With reference to XRD as gold measurement standard, fast-scan DSC could not map the crystallization events of composites (Pearson correlation: fast-scan DSC peak temperature and enthalpy versus XRD peak intensity and area, p > 0.05). Conventional DSC was able to indicate marked drug crystallization through an increase in endothermic enthalpy value of peaks at high temperature regimes between 250 and 360 °C due to formation of high melting point crystal form.     

Synthesis and electrochemical characterization of sol–gel-derived RuO2/carbon nanotube composites

Ruthenium oxide was coated on multiwalled carbon nanotubes (MWCNTs) to obtain nanocomposite electrode which has a good response to the pH. To synthesize this electrode, gold and cobalt were coated on a stainless steel 304 substrates, respectively, and then, vertically aligned carbon nanotubes were grown on the prepared substrates by chemical vapor deposition. Gold reduced activity of the stainless steel, while cobalt served as a catalyst for growth of the carbon nanotube. Ruthenium oxide was then coated on MWCNTs via sol–gel method. At last, different techniques were used to characterize the properties of synthesized electrode including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction, and cyclic voltammetry. SEM results showed that the length of the carbon nanotubes varied with reaction time, and in this research, it was maintained around 9 μm with a diameter about 100 nm. Electrochemical analysis revealed that optimum sol concentration and heat treatment temperature to meet the best pH sensing response were 0.1 M RuCl₃ sol and 200 °C, respectively. Moreover, the obtained electrode represented a linear and near-Nernstian response (about ?63 mV/pH) throughout the whole pH range (2–12) of Britton–Robinson buffer solutions.