Effect of heat treatment on the dielectric relaxation of polyacrylonitrile. III. Role of duration of heat treatment

Dielectric relaxation of polyacrylonitrile (PAN) heat-treated at 120°C in vacuum for durations varying from 4 to 24 h is studied in order to investigate the effect of the duration of heat treatment. Decrease in dielectric constant increment and loss peak height and an increase in relaxation temperature on heat treatment are observed at all durations of heat treatment. At the studied temperature of heat treatment the observed changes in dielectric relaxation were found to increase with duration of heat treatment showing maxima at a 16 h duration and then a decrease at a higher duration of heat treatment. The results are discussed in terms of the variation of degree of intermolecularly bound nitrile groups with the duration of heat treatment. Glass transition temperature calculated from the dielectric data, through WLF theory, showed a variation, similar to that of the degree of bound nitrile groups, with duration of heat treatment. The observed variation of T_g is attributed to the interlinking effect produced by the bound nitrile group pairs on the segmental mobility.     

Distribution of molecular weights and branching of high-density polyethylene

Samples of industrial high-density polyethylenes (Liten Macro, Liten FB 29, and Hostalen GM 9255 F) were divided into fractions by precipitation fractionation. The original samples and their fractions were characterized by light scattering, GPC, and viscometry. For all samples the molecular weight distribution is of logarithmic-normal type. High-density polyethylenes under investigation contain also high-molecular-weight fractions of pronouncedly branched structure, the content of branched molecules being, of course, considerably smaller than that in low-density polyethylenes.     

Optical constants of Mo x W1−x Se2 single crystal solid solutions

Single crystal solid solutions in the series Mo _x W_1−x Se₂ (0≤x≤1) have been studied by reflectivity measurements at oblique angles of incidence. The optical constants at a few wavelengths have been determined.     

Reactance-loaded short-backfire arrays: new results

The letter presents experimental data for modified reactance-loaded short-backfire arrays. It is shown that the use of a rectangular subreflector, in conjunction with a half-wave-length rim around the edge of the main reflector, results in a smaller and simpler structure having larger power gain, smaller beamwidth, and lower sidelobe levels.     

Electrical conduction in solution-grown polycarbonate films

The electrical conduction in solution-grown polycarbonate films was studied at different voltages between 1 and 10⁴V as a function of thickness from 25 to 65 μm at a constant temperature of about 385 K and as a function of temperature in the range 328–430 K at a constant thickness of about 50 μm. The results show two distinct regions corresponding to different types of conduction: region I at low or moderate fields where the slope of the current-voltage characteristics is about 2 and region II at high fields where the slope is about 4. The Richardson-Schottky effect was suggested as the dominant mode of conduction at moderate fields whereas at high fields the currents were found to be space charge limited.     

Behaviour of a two-unit standby redundant system with imperfect switching and delayed repair

A 2-unit standby redundant system with imperfect switchover is considered. A repair facility is assumed to be available only at a fixed proportion of time. Failure-time distributions of units are exponential whereas repair-time distributions, distribution of time for the repairman to become available are general. The system has been investigated in detail by the help of semi-Markov process and closed form results are obtained for mean-time-to-system-failure, steady-state availability, expected number of occurrences of a state, expected profit and second moment of time in up state. Behaviour of several parameters has also been studied and several earlier models are included as particular cases.     

Changes in carbohydrate metabolism in various organs of the snail, Lymnaea acuminata following exposure to trichlorfon

Several aspects of carbohydrate metabolism following 24 hr and 48 hr treatment with 10 and 20 mg/l of trichlorfon, were studied in hepatopancreas, mantle, intestine and foot of the snail. Lymnaea acuminata. Following treatment with the pesticide, the rate of oxygen consumption and concentration of glycogen were reduced, while the levels of lactic acid and reducing sugars were enhanced. Withdrawal of pesticide for 7 days following trichlorfon treatment (10 mg/l for 48 hrs) could not reverse these changes.     

Molecular characterization and effect of shear on the distribution of long branching in poly(vinyl acetate)

Poly(vinyl acetate) (PVAc) of M?_w 750,000 M?_w/M_n 3.85 and B?_n (the number of long branches per molecule) 2.2 was subjected to chain scission by mechanical (high-speed stirring) as well as chemical (saponification and reacetylation) methods to investigate the effect of shear on the branching distribution. The extent of long branching was measured by gel permeation chromatography on-line with low-angle laser light scattering photometry. It was concluded that (i) the branches through the acetate group are long and are ruptured preferentially on shearing, (ii) the branches through the α- and β-carbons are not broken on shearing, (iii) the extent of long branching through the acetate group is about 67% of total branching, and (iv) the poly(vinyl alcohol) derived from branched PVAc contains a smaller but nevertheless significant amount of branching.     

Dislocation network patterns in tungstenite single crystals

TEM studies of tungstenite crystals grown by sublimation method have been investigated. Different types of dislocation network patterns were observed. The observed network patterns in bright field are compared with weak beam dark field pictures and the results are discussed.