Dielectric relaxations in PEEK by combined dynamic dielectric spectroscopy and thermally stimulated current

The molecular dynamics of a quenched poly (ether ether ketone) (PEEK) was studied over a broad frequency range from 10-3 to 106 Hz by combining dynamic dielectric spectroscopy (DDS) and thermo-stimulated current (TSC) analysis. The dielectric relaxation losses e00 KK has been determined from the real part e0 T(x) thanks to Kramers–Kronig transform. In this way, conduction and relaxation processes can be analyzed independently. Two secondary dipolar relaxations, the c and the b modes, corresponding to non-cooperative localized molecular mobility have been pointed out. The main a relaxation appeared close to the glass transition temperature as determined by DSC; it has been attributed to the delocalized cooperative mobility of the free amorphous phase. The relaxation times of dielectric relaxations determined with TSC at low frequency converge with relaxation times extracted from DDS at high frequency. This correlation emphasized continuity of mobility kinetics between vitreous and liquid state. The dielectric spectroscopy exhibits the ac relaxation, near 443 K, which has been associated with the rigid amorphous phase confined by crystallites. This present experiment demonstrates coherence of the dynamics of the PEEK heterogeneous amorphous phase between glassy and liquid state and significantly improve the knowledge of molecular/dynamic structure relationships

Relation between structure and gas transport properties of polyethylene oxide networks based on crosslinked bisphenol A ethoxylate diacrylate

Poly(ethylene oxide) (PEO) networks prepared from the photopolymerization of bisphenol A ethoxylate diacrylate (BPA-EDA) have been investigated as a function of crosslinker molecular weight and copolymer composition. Dynamic mechanical and dielectric methods have been used to elucidate the thermal relaxation characteristics of the polymers as a function of network composition and architecture, and these properties were related to measured gas transport for CO2 separations. Copolymerization strategies involving the insertion of flexible PEG side chains along the network backbone proved effective in enhancing network free volume and increasing permeability. The gas transport performance of rubbery amorphous membranes based on the n=15 BPA-EDA crosslinker (i.e., crosslinker encompassing 30 ethylene oxide repeat units between crosslinks) compared favorably to model polymers synthesized from poly(ethylene glycol) diacrylate. © 2008 Elsevier Ltd. All rights reserved

Pervaporative Separation of Aromatic/Aliphatic Mixtures with Poly(Siloxane-<i>co</i>-Imide) and Poly(Ether-<i>co</i>-Imide) Membranes

Aromatic random copolyimides were synthesized and tested as membrane materials for the separation of a mixture of aromatic and aliphatic hydrocarbons by pervaporation. The polymers were synthesized by a two-step polycondensation route with a total of 4 aromatic dianhydrides, 4 aromatic diamines, and 3 diamino-terminated aliphatic oligomers containing either ether or siloxane units. Pervaporation experiments were conducted at two temperatures with toluene/<i>n</i>-heptane and benzene/<i>n</i>-heptane mixtures as feed streams. All polymers were selective toward the aromatic hydrocarbon. Introduction of siloxane units in the polymer generally led to very high hydrocarbon permeability coefficients, but caused a reduction in selectivity relative to that of the aromatic homopolyimide. Incorporation of ether units, on the other hand, did not generally cause such large increases in permeability, nor large decreases in selectivity. The performance of these materials was compared with previous results reported in the literature for other polymers