J. Niezette

Effect of tacticity on the transition behaviour of poly(methyl methacrylate)/poly(vinyl chloride) blends. Thermally stimulated depolarization study

Abstract The transition behaviour of blends of poly(vinyl chloride) (PVC) and poly(methyl methacrylate) isotactic (i-PMMA) and syndiotactic (s-PMMA) was determined in the temperature range −150°C to +130°C by the thermally stimulated depolarization currents method (TSDC). The evolution of the current spectra was analysed as a function of blend composition. From the variation of properties of the peaks (presence or not of two Tg peaks, shifting or not of their positions, existence or not of interfacial components, regular or complex variation of the peak amplitudes ...), it was concluded that i-PMMA and PVC form an incompatible system over the entire concentration range while in s-PMMA/PVC blends, some compatibility probably exists but only for concentrations in s-PMMA not higher than 10 wt%. By referring to literature data, this value is much smaller than the compatibility range found from d.s.c. and mechanical measurements but is close to that determined from optical and electronic spectroscopy methods. These results emphasize the important role played by the type of method used to find out compatibility of a given polymer pair and show that the TSDC technique may be particularly useful for studying the interfacial phenomena associated with phase separation owing to its exceptional ability for easily detecting Maxwell-Wagner-Sillars polarization generated by the trapping of charge carriers at phase boundaries.

Influence of physical aging processes on electrical properties of amorphous polymers

The influence of sub-T/sub g/ (the glass transition temperature) physical aging processes on the electrical properties of amorphous polymethylmethacrylate (PMMA) and polydiancarbonate was investigated by means of the thermally stimulated depolarization currents, isothermal transient charging currents, and AC low-frequency dielectric methods. A marked decrease in polarization and loss factor corresponding to the main alpha relaxation (glass transition) was observed with increasing aging times, especially for aging temperatures ranging from 10 to 30 degrees C below T/sub g/. This behavior is compatible with the free-volume concept and can be explained by a gradual decrease in segmental mobility following a shift of the relaxation time spectrum of molecular motions to longer time scales. In PMMA, the secondary beta relaxation is also significantly affected, which is probably an indication of cooperation of the main chain. The experimental evidence suggests that proper consideration should be given to thermal history and aging affects in standard electrical testing of polymer materials. >

Contribution to the ultrasonic degradation of polystyrene solutions

Abstract Narrow fractions of polystyrene and more heterogeneous samples were subjected to ultrasonic irradiation in tetrahydrofuran and toluene. Degraded products were investigated by gel permeation chromatography. Compared with Ovenalls results, the kinetic analysis of the degradation process shows an additional effect of molecular weight on rate. Experimental chromatograms obtained with classical styragel columns do not reveal clearly the non-random character of the degradation. However, bimodal distribution curves characteristic of a chain scission near the centre of the molecule are observed for degraded products of very narrow fractions analysed by high resolution microstyragel packings. It appears that the breakage does not occur exclusively near the centre but the probability of chain rupture decreases very rapidly out of the central part of the chain.

The Effect of the Temperature Dependence of the Dielectric Constant on Thermally Stimulated Currents in Polymers

A dipolar theory of thermally stimulated currents taking into account the thermal dependence of the equilibrium polarization has been tested in the glass transition range of a series of polymers selected for their well-resolved relaxation spectra and the absence of any significant bulk conductivity (triblock, diblock, and star-shaped styreneisoprene copolymers). Systematic studies of the effect exercized on thermally stimulated polarization currents (TSPC) and thermally stimulated depolarization currents (TSDC) by varying parameters such as field strength, electrode material, field polarity, and type of heating-cooling sequence, have been carried out and full qualitative agreement with the predictions of the theory has been found. In particular, these results imply that the variations of equilibrium polarization occurring during the nonisothermal steps of TSDC and TSPC experiments cannot be a priori neglected, as is still commonly done in classical treatments of experimental data.

Conductivity and Thermally Srimulated Processes I Model Ionic Pplymers. I. Halato Telechelic Polybutadienes.

The relaxation and conductivity properties of model ioncontaining macromolecules, i. e. halato-telechelic polybutadienes with various degrees of neutralization, were investigated from 100 to 350 K by means of thermally stimulated depolarization and polarization methods, stressing the comparison of materials based on alkaline (Na, K, Li) and alkalineearth (Ba, Ca, Mg) cations. It was shown that the behavior is largely determined by the thermal stability of ionic associations and their ability to trap impurity charge carriers. With alkaline-earth cations, the associations are thermally stable at least up to 350 K (i. e. more than 150 K above the glass transition temperature), which allows to observe a conductivity level smaller than that of the precursor polymer by several orders of magnitude. An opposite result is observed with alkaline cations as a consequence of the thermal breaking of unstable ionic associations. This dissociation first gives rise to a large relaxation process involving isolated carboxylate groups and then to a rapidly growing number of mobile ions which induce an important increase in conductivity. Generally speaking, and taking into account similar results previously obtained in various ionomers, it can be concluded that introducing a few percent ionic groups in polymers can modify their conductivity by several orders of magnitude (either decrease or increase depending on the type of cation involved).

Space charge due to contact effects in Al-polyethylene terephthalate-Al systems: modeling and study by the thermally stimulated current method

The Thermally Stimulated Depolarization Current method allowed us to emphasize the role played by aluminum-polyester contacts on the electrical properties of Metal-Polyethylene Terephthalate films-Metal systems. Assuming the existence of Schottky barriers at the Al-PET interfaces after heat treatment, the electronic band model permitted us to account for the nature and properties of the space charge in annealed PET films (1.5 to 12 /spl mu/m), in a wide range of poling temperatures, times and fields.