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Abstract  

The space charge trapping/detrapping in thermally charged polyimide (PI) samples have been studied by means of “Final Thermally Stimulated Discharge Current” (FTSDC). We investigate the effects associated with space charge behavior in PI near glass transition region. The FTSDC spectra consist of space charge peak attributed trapping of charges at different trapping levels. The peak positions are found to be very sensitive with poling time, discharging time, and heating rate. The apparent activation energy distributed in the range from 0.51 to 1.12 eV, and the charge released has been found to be varying with poling temperature (T p) and poling time (t c)/discharging time (t d). The decay of space charge (Q) resembled an agreement with interfacial polarization.

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The method of comparative thermal analysis has been used to determine the thermal stability of polyimides and their model compounds. The extent of defects in the investigated polyimides was determined by gas chromatography. The dependence of the extent of the defects on the chemical structure of the initial components and the supermolecular structure of the polyimides was established.

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Thermogravimetry of polyimides based on dianhydrides of pyromellitic, diphenyl- and diphenyloxidetetracarboxylic acids and a series of diamines has been used to determine their temperature characteristics and calculate their activation energies. The dependence of the thermal stability of the polyimides on the structure of their diamine component was established. It was shown that the effect of the thermal stability of the polyimides progressively weakens with the increase in the rigidity of their chemical structure.

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Abstract  

The separation efficiency of hollow-filament polyimide membranes for 3H and 41Ar is preliminarily examined for a potential application to continuous gas monitoring systems for analysis of stack emission from accelerator facilities. The basic gas separation characteristics of the membranes are experimentally investigated, and a preliminary gas monitor design is proposed. The results indicate that the membranes are capable of selectively enriching hydrogen by more than 25 times, with negligible variation with respect to the species of isotope.

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Abstract  

Two aromatic polyimides and the corresponding poly(amic acid)s, with oxadiazole and para/meta phenoxyphenylene rings in the backbone, were synthesized and the structure — thermal properties correlation was followed by dynamic mechanical analysis. Concerning the poly(amic acid)s, the glass transition domain was emphasized only for the compound with meta-oriented rings because the process of imidization takes place with increasing temperature. A multiplex experiment was performed to calculate the activation energy of the transition localized under 200°C. Consecutive heating-cooling-heating cycles were accomplished. All phenomena are discussed by cross-examination of the storage modulus (E′), loss modulus (E″) and loss factor tanδ variation with temperature.

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Three aromatic polyimides based on 3,3′,4,4′-biphenyl-tetracarboxylic dianhydride (BPDA) and three different diamines 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), 2,2′-dimethyl-4, 4′-diaminophenyl (DMB) or 3,3′-dimethylbenzidine (OTOL) have been synthesized. These polyimides are soluble in hotp-chlorophenol,m-cresol or other phenolic solvents. Fibers have been spun from isotropic solutions using a dry-jet wet spinning method. The as-spun fibers generally exhibit low tensile properties, and can be drawn at elevated temperatures (>380° C) up to a draw ratio of 10 times. Remarkable increases in tensile strength and modulus are achieved after drawing and annealing. The crystal structures of highly drawn fibers were determinedvia wide angle X-ray diffraction (WAXD). The crystal unit cell lattices have been determined to be monoclinic for BPDA-PFMB and triclinic for both BPDA-DMB and BPDA-OTOL. Thermomechanical analysis (TMA) was used to measure thermal shrinkage stress and strain. A selfelongation has been found in the temperature region around 450°C. This phenomenon can be explained as resulting from the structural development in the fibers as evidencedvia WAXD observations.

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Preparation and investigations of polyimides III

Thermal investigation of the imidization of polyamide acid powders

Journal of Thermal Analysis and Calorimetry
Authors: J. Varga, T. Meisel, K. Belina, and J. Balla

Polyamide acid powders of pyromellitic dianhydride and 4,4′-diaminodiphenyl ether base were prepared in tetrahydrofuran, in the heterogeneous phase. The imidization of these powders was investigated by thermogravimetric, calorimetric and mass-spectrometric methods.

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