The effect of pressure on the volume and the dielectric relaxation of linear polyethylene

Abstract

SynopsisThe effects of pressure on the a (ca. 7OoC, 1 kHz) and y (ca. -lOO°C, 1 kHz) relaxations of linear polyethylene were studied dielectrically between 0 and 4 kbar. Equation of state (PVT) data were also determined in the range of interest of these relaxations. The sample was rendered dielectrically active through oxidation (0.8 C=O per 1000 CH2). The a process (which occurs in the crystalline fraction) could he studied over a much wider temperature range than heretofore possible due to the effect of pressure in increasing the melting point. Examination of relaxation strength from 50 to 15OOC showed that there must be two crystalline relaxation processes: the well-known a relaxation plus a competing one. The a relaxation is believed to be due to a chain twist-rotation-translation mechanism that results in rotation-translation of an entire chain in the crystal. The relaxation strength of the a process decreases and therefore indicates the presence of a second (faster and not directly observed) process that increases in intensity with increasing temperature. It is postulated that the second process is due to motion of defects that become more numerous through thermal injection a t higher temperatures. Analysis of the relaxation data along with the PVT data allowed the constant volume activation energy for the a relaxation to be determined. It was found to be 19.4 f 0.5 kcal/mole. The constant volume activation energy is important in modeling calculations of the crystal motions and is significantly smaller than the atmospheric constant pressure activation energy of 24.9 kcal/mole. The effect of pressure on the activation parameters and shape of the y process was also determined. There has been controversy over whether the y process occurs only in the amorphous fraction or in both the amorphous and crystalline phases. Since the two phases have quite different compressibilities, increasing the pressure should change the shape of the loss curves (versus frequency and temperature) if the process occurs in both phases. The shape (but not location) of the loss curves was found to be remarkably independent of pressure. This finding strengthens the view that the y process is entirely amorphous in origin.

Effect of sub-Tg relaxations on chromophore reorientation in corona-poled polymers

“…In previous elevated pressure studorder parameters, typically Ç 0.025), and the ies on polymers, other researchers have found that samples were aged at atmospheric pressure for 48 similar silicone oils do not penetrate the exposed h after poling to allow the majority of the injected polymer at pressures up to 3000 atm. [22][23][24][25] Our DSC analysis confirmed these results in that no measur- not be removed from the substrate due to sample cracking; however, consistent data were still obtained. and Figure 4 it is clear that a negligible amount The temporal decay of the SHG signal from the of chromophore reorientation occurred during poled samples was measured using the optical cell heating at ú 3000 atm and the subsequent 10 s placed in the standard SHG configuration with pressure release period.…”

Molecular mechanisms of the chain diffusion between crystalline and amorphous fractions in polyethylene

“…First of all, our estimates are made at constant pressure, while the molecular mechanics (MM) minima of the listed works are to be compared with the constant volume experimental value. The experimental constant pressure activation energy of the a process in PE is 24.9 kcal/ mol, while the constant volume one is 19.4 AE 0.5 kcal/mol [13]. The earliest works [30,29] give the values 11.8 and 10.5 kcal/mol, for the conformational (of the type tg þ tg À tg þ t) twistecompression and the smooth (in a chain consisting of 22 CH 2 groups) twistetension defects, respectively.…”

“…The most striking draw ratio of 400 is obtained in dry gel films of PE [2], the simplest polymer which always served as a 'model system'. The presence of the chain diffusion through crystallites of PE is verified by direct 13 C NMR experiments in common melt-crystallized samples [3] and in samples of uncommon morphologies: fibers [3], electron beam irradiated samples [4] and even in single crystals [5].…”

“…The activation energy of the H-process does not practically depend on the mode of sample preparation [12], crystallinity [10], or pressure [13] at available temperatures. So the H-process seems to be very promising for molecular model development.…”

The dependence of dielectric response of an elastomer on hydrostatic pressure

“…To a lesser extent dielectric techniques have been used to study the effect of pressure on the relaxations of phase materials [2][3][4][5][6][7][8][9]. By varying pressure, temperature and frequency, one can separate the activation energy and activation volume contributions to the activation enthalpy [4].…”