Coamorphous Systems of Valsartan: Thermal Analysis Contribution to Evaluate Intermolecular Interactions Effects on the Structural Relaxation.

Coamorphous formation in binary systems of valsartan (Val) with 4,4'-bipyridine (Bipy) and trimethoprim (Tri) was investigated for mixtures with a mole fraction of 0.16~0.86 of valsartan and evaluated in terms of the glass transition temperature. The glass transition of the systems had a behavior outside the values predicted by the Gordon-Taylor equation, showing that Val-Bipy (hydrogen bonding between the components) had a lower deviation and Val-Tri (ionic bonding between the components) had a higher deviation. Mixtures of compositions 2:1 Val-Bipy and 1:1 Val-Tri were selected for further investigation and verified to be stable, as no crystallization was observed during subsequent heating and cooling programs. For these systems, the effective activation energy during glass transition was evaluated. Compared to pure valsartan, the system with the lower glass transition temperature (Val-Bipy) presented the highest effective activation energy, and the system with the higher glass transition temperature (Val-Tri) presented a lower effective activation energy. The results presented a good correlation between the data obtained from two different techniques to determine the fragility and effective activation energy: non-isothermal kinetic analysis by DSC and TSDC.

Space Charge Characteristics at the Interface of Laminated Epoxy Resin.

In the design and manufacturing of epoxy resin insulation components, complex structures can be achieved through multiple pours, thereby forming the structure of interface of laminated epoxy resin. This type of interface structure is often considered a weak link in performance which can easily accumulate charges and cause electric field distortion. However, research on the interlayer interface of epoxy resin has received little attention. In this study, epoxy samples with and without interlayer interfaces were prepared, and the space charge accumulation characteristics and trap characteristics of the samples were analyzed via pulsed electro-acoustic (PEA) and thermally stimulated depolarization current (TSDC) methods. The experimental results indicate that the Maxwell-Wagner interface polarization model cannot fully explain the charge accumulation at the interface. Due to the influence of the secondary curing, the functional groups in the post-curing epoxy resin can move and react with the partially reacted functional groups in the prefabricated epoxy resin layer, resulting in a weak cross-linking network at the interface. With the increase in temperature, the molecular chain segments in the weak cross-linked region of the interface become more active and introduce deep traps at the interface, thereby exacerbating the accumulation of interface charges. In addition, due to the influence of interface polarization and weak cross-linking, the ability of the interface charges to cause field strength distortions decreases with the increase in applied field strength. This research study can provide a theoretical reference for the interfacial space charge transport characteristics of epoxy-cured cross-linked layers and provide ideas for regulating interfacial cross-linking to suppress interfacial charge accumulation.

Surface Modification of Fumed Silica by Plasma Polymerization of Acetylene for PP/POE Blends Dielectric Nanocomposites.

Novel nanocomposites for dielectric applications-based polypropylene/poly(ethylene-co-octene) (PP/POE) blends filled with nano silica are developed in the framework of the European 'GRIDABLE' project. A tailor-made low-pressure-plasma reactor was applied in this study for an organic surface modification of silica. Acetylene gas was used as the monomer for plasma polymerization in order to deposit a hydrocarbon layer onto the silica surface. The aim of this modification is to increase the compatibility between silica and the PP/POE blends matrix in order to improve the dispersion of the filler in the polymer matrix and to suppress the space charge accumulation by altering the charge trapping properties of these silica/PP/POE blends composites. The conditions for the deposition of the acetylene plasma-polymer onto the silica surface were optimized by analyzing the modification in terms of weight loss by thermogravimetry (TGA). X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray fluorescence spectroscopy (EDX) measurements confirmed the presence of hydrocarbon compounds on the silica surface after plasma modification. The acetylene plasma modified silica with the highest deposition level was selected to be incorporated into the PP/POE blends matrix. X-ray diffraction (XRD) showed that there is no new crystal phase formation in the PP/POE blends nanocomposites after addition of the acetylene plasma modified silica. Differential scanning calorimetry results (DSC) show two melting peaks and two crystallization peaks of the PP/POE blends nanocomposites corresponding to the PP and POE domains. The improved dispersion of the silica after acetylene plasma modification in the PP/POE blends matrix was shown by means of SEM-EDX mapping. Thermally stimulated depolarization current (TSDC) measurements confirm that addition of the acetylene plasma modified silica affects the charge trapping density and decreases the amount of injected charges into PP/POE blends nanocomposites. This work shows that acetylene plasma modification of the silica surface is a promising route to tune charge trapping properties of PP/POE blend-based nanocomposites.

Dipolar Reorientations in Amorphous Nimesulide: A TSDC and DSC Study.

BACKGROUND: The preparation of APIs in the amorphous solid form can be a means of circumventing problems arising from poor solubility and low dissolution rate of the crystalline drugs. However, molecular mobility can be responsible for the glass instability, so that the kinetic characterization of the different relaxations that subsist in the amorphous solid is useful to allow define the conditions for greater stability of the glassy pharmaceutical. Our purpose is to use the experimental techniques of differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) to study the thermal behavior of the pharmaceutical drug nimesulide and its slow molecular mobility in the amorphous solid state. METHODS: DSC provides us a general view of the thermal behaviour of nimesulide and allows a general kinetic characterization of its glass transition relaxation. TSDC allows isolating the individual modes of motion present in nimesulide (in the temperature range between -150ºC and +15ºC). From the experimental output of the TSDC experiments, the kinetic parameters associated with the different mobility modes of motion were obtained, which allowed a detailed characterisation of the distribution of relaxation times of the complex relaxations. RESULTS: No molecular mobility was detected below ∼ -30ºC. A sub-Tg relaxation, or secondary process, was found by TSDC in the temperature region between ∼ -15ºC and ∼ +7ºC; this is a local mobility that is affected by physical aging, and was attributed to a slow ß-relaxation (Johari-Goldstein). The analysis by DSC and TSDC of the α-relaxation showed that nimesulide is a moderately fragile glass former. The dynamic fragility obtained by DSC was mDSC = 52 while that obtained by TSDC was mTSDC = 70. CONCLUSIONS: From the DSC study of the thermal behaviour we concluded that nimesulide has a moderate glass forming ability and a week glass stability. The fact that the cold crystallization occurs only some few tens of degrees above the glass transition temperature, and shows a slow kinetics, allowed the study of the mobility by TSDC. TSDC thus proved to be an adequate technique to study the molecular mobility in the amorphous nimesulide. However, the study by spectroscopic dielectric relaxation is probably impossible under these conditions.

Li2CO3-Coated Ni Particles for the Inner Electrodes of Multilayer Ceramic Capacitors: Evaluation of Lifetime.

In previous work, it was demonstrated that using Li2CO3-coated Ni particles in the manufacturing of multilayer ceramic capacitor (MLCC) devices could improve both the permittivity and dissipation factors. However, adding Li+ ions to the system gave rise to the concern that ions could migrate under sustained electrical fields and thereby increase the degradation rates of the insulation resistance in MLCCs. In this paper, thermally stimulated depolarization current and highly accelerated lifetime testing were both utilized to evaluate the oxygen vacancy space-charge regions and migration in MLCCs. The results suggested that three parameters (the sintering schedule, Li2CO3 coatings, and oxygen flow during sintering) determine the overall resilience to the degradation. The Li+ ions did not migrate during degradation, as verified by time-of-flight secondary-ion mass spectrometry mapping; however, the Li ions enter the perovskite structure as an acceptor and, if ionically compensated for, could introduce more oxygen vacancies to the system and decrease the lifetime of the MLCCs. Nevertheless, it was demonstrated that the relative lifetimes of the newly designed MLCCs significantly improve relative to the conventional samples.

The Slow Relaxation Dynamics in the Amorphous Pharmaceutical Drugs Cimetidine, Nizatidine, and Famotidine.

The slow molecular mobility in the amorphous solid state of 3 active pharmaceutical drugs (cimetidine, nizatidine, and famotidine) has been studied using differential scanning calorimetry and the 2 dielectric-related techniques of dielectric relaxation spectroscopy and thermally stimulated depolarization currents. The glass-forming ability, the glass stability, and the tendency for crystallization from the equilibrium melt were investigated by differential scanning calorimetry, which also provided the characterization of the main relaxation of the 3 glass formers. The chemical instability of famotidine at the melting temperature and above it prevented the preparation of the amorphous for dielectric studies. In contrast, for cimetidine and nizatidine, the dielectric study yielded the main kinetic features of the α relaxation and of the secondary relaxations. According to the obtained results, nizatidine displays the higher fragility index of the 3 studied glass-forming drugs. The thermally stimulated depolarization current technique has proved useful to identify the Johari-Goldstein relaxation and to measure τßJG in the amorphous solid state, that is, in a frequency range which is not easily accessible by dielectric relaxation spectroscopy.