1999 рік

The homopolymerizations of bisphenol-A-bis(2-hydroxypropyl methacrylate) (BisGMA) and 2-hydroxyethyl methacrylate (HEMA) were monitored using FT-Raman spectroscopy and those of BisGMA also by FT-i.r. photoacoustic spectroscopy. Raman spectroscopy was applied to quantify the conversion of CyC double bonds of the monomers. Changes of intensity and wavenumber of the carbonyl band observed in the Raman spectra during the polymerizations were complementally studied by use of FT-i.r. photoacoustic spectroscopy. The carbonyl vibrational band at 1700 cm -¹ was identified as the vibrational mode of the carbonyl group with hydrogen bonds and those at 1717 cm -¹ (Raman) as well as 1725 cm -¹ (i.r.) as the modes of the 'free’ carbonyl group. Localization of the p-electrons of the carbonyl group and decreasing strength of the hydrogen bonds caused by the polymerization reaction result in a decrease of the Raman intensity and an increase of the i.r. intensity as well as of the wavenumber of the carbonyl i.r. vibrational mode at 1700 cm -¹.

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In this paper, the curing of an unsaturated polyester resin catalysed with methyl ethyl ketone peroxide (MEKP) and cobalt octoate as the promoter, is studied at different heating rates. Given the non-symmetrical shape of differential scanning calorimetry (DSC) curves, it has been assumed that they would represent two independent reactions. The objective of this work has been to obtain a set of kinetic parameters for each DSC peak, which describe the overall cure process, using an empirical kinetic model of the form: da ¼ yk1 f (a1 ) þ (1 ¹ y)k2 f (a2 ) dt where y is the fraction corresponding to the heat released during the first reaction, and k i represents the rate constant for each process whose temperature dependence is an Arrhenius-type equation. With regard to f(a), two different kinetic functions have been employed: (1 ¹ a) n and a m(1 ¹ a) n. The corresponding software to compute the degree of conversion a 1 and a 2, and the kinetic parameters, has been developed. The kinetic parameters obtained considering two independent reactions and an autocatalysed function f(a), fit better than the DSC experimental data (da/dt,T) exp, than those obtained when a single kinetic process is considered. The activation energies for each process are in accordance with tabulated values for typical free-radical polymerizations, induced by redox and thermal decomposition of the peroxides.

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The synthesis and gas transport properties of aromatic polyisophthalamides (PIPAs) based on isophthaloyl chloride (IPC) derivatives bearing a pendent phenyl group and a hexafluoroisopropylidene (6F) linkage in the main chain are reported. The properties of these polymers are compared with the properties of similar PIPAs containing sulfonyl (SO2) rather than 6F in the main chain. Polymers containing a phenyl pendent group at the five position of the isophthaloyl linkage are more permeable than those bearing only a hydrogen atom at this position, although increases in permeability are generally accompanied by decreases in selectivity. In the SO2-bearing polymer, the addition of a phenyl pendent group hinders chain packing more than in the 6F containing PIPAs. Consequently, permeability coefficients increase more upon addition of a pendent phenyl group in SO2-containing rather than 6F-containing PIPAs. The effect of amide linkage reversal on the gas transport properties of a polymer containing 6F linkages in the chain backbone and a hydrogen atom at the five position of the isophthaloyl linkage was minimal. All the PIPAs considered in this study were more permeable to nitrogen than to methane, some with nitrogen/methane selectivities of more than two.

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Four dispersing methods were used to disperse increasing amounts of Polypyrrole (PPy), synthesized either via dispersion (PPy d) or suspension polymerization (PPy s), in an insulating epoxy matrix in order to make it electrically conductive. As shown in SEM micrographs and by the physical characteristics, PPy d particles exhibit higher density and smaller particle size than PPy s. In addition, optical micrographs of the blends show that PPy d form compact agglomerates and PPy s aerated clusters. It is deduced from conductivity measurements, that energy intensive methods or larger particle size (PPy s) provide low percolation threshold. Indeed, energy intensive methods, by breaking the PPy d agglomerates, favour the formation of conductive network of fine particles and therefore lower the PPy concentration at percolation. Similarly, larger PPy s particles are arranged in clusters allowing a more homogeneous distribution of PPy particles in the resin and a smaller percolation threshold. The microstructure of the blends are observed by TEM.

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An aromatic polyimide containing pendent hydroxyl groups ortho to the heterocyclic imide nitrogen was found to rearrange to a polybenzoxazole, with quantitative loss of carbon dioxide, upon heating above 400°C in an inert atmosphere. A hydroxy-containing polyimide film based on 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 3,3'-dihydroxy-4,4'-diaminobiphenyl (HAB) was prepared and converted to a fully aromatic polybenzoxazole by heating at 500°C for 1 h under nitrogen. The resulting polybenzoxazole film was found to be amorphous by small angle X-ray scattering (SAXS). The film also displayed excellent solvent resistance and good thermal stability by thermogravimetric analysis with 5% weight loss in nitrogen occurring at 625°C. A hydroxy-containing imide model compound was also found to undergo thermal conversion to a bisbenzoxazole when heated above 400°C under nitrogen.

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Polyaniline (PAn) was doped with phosphonic acid containing hydrophilic tails. The solubility of the doped PAn in water was controlled by changing the length of hydrophilic chain in the dopant. When poly(ethylene glycol) monomethyl ether (PEGME) with molecular weight Mw ˆ 550 was used as the hydrophilic chain of the dopant, the doped PAn was entirely soluble in water. The film cast from aqueous solution showed good electrochemical redox reversibility. Aqueous solution blending of PAn with poly(ethylene glycol) (PEG, Mw ˆ 20 000) and poly(N-vinyl pyrrolidone) (PVP, Mw ˆ 360 000) was achieved. Percolation threshold of the composite film was lower than 3 wt.%. Electrical conductivity of the composite film was in the range of 10 21 –10 25 S cm 21, depending on molecular weight of the acid and the content of PAn in the composite.

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In the present paper, an electrochemical method to obtain the true transmissibilities and permeabilities of hydrogels to oxygen is proposed. The method involves the measurement of the electric current arising from the flow of oxygen through a potentiostat cell integrated by a top water layer, the hydrogel membrane, and a thin water layer between the membrane and the cathode, where the oxygen is reduced. Varying the thickness of the top water layer, the true permeation characteristics of the hydrogels are obtained. A parallel study was carried out in which the transmissibilities and permeation coefficients were obtained from the variation of the steady state current with the thickness of the membranes. The permeation results obtained by the two methods agree very satisfactorily. The method also permits to determine the values of the transmissibility of the water layers of the experimental set-up.

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Crystal structure of uniaxially oriented polyethylene has been analysed as a function of temperature by using an X-ray imaging plate system equipped with software useful for the structure analysis of polymer crystals. The cell parameters a and b, the anisotropic temperature factors of the carbon atom, and the setting angle of the planar–zigzag chain measured from the b axis, etc., were found to show a deflection point around þ 10°C. The structure and thermal motion below this characteristic point could be interpreted in terms of harmonic vibration. But, above this deflection point, the thermal motion became more remarkable, requiring an introduction of an idea of anharmonic vibration. The temperature dependence of these structural parameters is different between the samples used in the measurement. The sample oriented at relatively low draw ratio showed larger temperature dependence and a clearer deflection point around 10°C, compared with the case of the ultradrawn sample. This difference may reflect a difference in the remnant strain in the samples.

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Emulsion polymerizations of dodecyl and octadecyl methacrylate were carried out using a conventional water soluble initiator and sulphonate surfactant in the presence of b cyclodextrin. For the first time stable latices that were taken to high conversion of monomer were produced. It is thought that b cyclodextrin enhances the water solubility of these highly water insoluble monomers, so that the ease of transport of the monomer across the aqueous phase to the loci of polymerization is improved. Absence of sulphonate surfactant resulted in gross coagulation during polymerization.

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Studies of the states of water, hydrazine and hydrated hydrazine in ethylcellulose membrane were made for various mass fractions of solvents using differential scanning calorimetry (d.s.c.). Four or five energetically distinct states of these species in the membrane were identified and their quantitative estimations made. A new peak of enthalpy of mixing was identified at higher temperatures. Such a significant peak has not been previously recorded or discovered by d.s.c.

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