1992 рік

The effect of creep and recovery on the Young's modulus of different aramid fibres was investigated by an acoustic method. Creep leads to the growth of the Young's modulus of up to 215 GPa. Some of the difference between moduli values obtained by the acoustic and mechanical methods can be traced to fibre viscoelastic deformation. The dependence of the reciprocal Young's modulus on the creep strain is described by a straight line, the slope of which is practically independent of stress and temperature. Creep and recovery may be described as a thermoactivated transition between two energy states that have different elasticities of the polymer chain. These states are supposed to be the straightened molecule and a molecular kink.

Детальніше...  

Effects of lattice energy of the additive salt on ionic species and ion conductivity characteristics were studied in the solid polymer electrolyte. Polyroligo(oxyethylene) methacrylate] and several lithium salts were used as matrix and additive salt for solid polymer electrolytes, respectively. Salt and salt content deeply affected the ion conductivity characteristics as well as morphology even for amorphous type solid polymer electrolytes. The relationship between ion conductivity at constant reduced temperature (T- Tg = 90°C) and lattice energy of the additive salt was evaluated at each salt content. A different relationship was found for each salt content. Multiple ion aggregate formation was considerably affected by the salt, and was revealed to be effective to increase the ion conductivity.

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Multiblock copolymers were prepared by anionic copolymerization of deuterated and classical styrene monomers. The use of bifunctional initiators gives copolymers with an odd number of sequences. Neutron scattering experiments were performed on these samples in order to check the existing theories which are extended in the first part to multiblock copolymers with an odd number of blocks. The agreement between theory and experiment is excellent and, surprisingly, the height of the maximum of the curves : scattered intensity versus scattering angle, does not depend on the number of blocks.

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Novel imide-aryl ether benzothiazole copolymers were prepared and their morphology and mechanical properties investigated. Diamines containing preformed benzothiazole rings were prepared and used in conventional polyimide syntheses. The preparation of these diamines involved the nucleophilic aromatic substitution reaction of 2,6-bis (4-fluorophenyl)benzo [ 1,2,4,5-1 bisthiazole with either 3- or 4-aminophenol in the presence of K2CO3 in N-methyl-2-pyrrolidone to afford 2,6-bis(4-aminophenoxy-4-phenyl)benzo- [1,2,4,5]dithiazole and 2,6-bis(3-aminophenoxy-4-phenyl)benzo[1,2,4,5]dithiazole, respectively. The monomers were each reacted with pyromellitic dianhydride to synthesize the desired poly(amic acids). Films were cast and cured to 350°C to effect imidization, producing films with good mechanical properties. The copolymers exhibited good dimensional (Tg > 400°C) and thermal stability. Wide angle X-ray diffraction measurements showed that in one case a semicrystalline morphology was obtained producing a film with both a high modulus and low thermal expansion coefficient.

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Oxidative and catalytic polymerizations of DEBP (p-diethynylbiphenyl) were investigated. The characterization of the reaction products, ox-PDEBP and c-PDEBP (originating from the oxidative and catalytic reactions respectively), was performed by means of i.r., XPS and t.g.-d.t.a, techniques. The different behaviour of ox-PDEBP and c-PDEBP in the presence of oxygen and humidity was examined. The variations of the electrical properties upon doping with FeCI 3 and 12 are also reported

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The transitions/relaxations of polycarbonate around its glass transition temperature, T~, have been investigated by differential scanning calorimetry and thermally stimulated current spectroscopy. A sub-glass transition/relaxation has been observed with higher intensity in the quenched sample. This precursor of Tg is characterized by relaxation times following a Fulcher-Vogel equation. It might be explained by the diffusion of conformational defects all along the chain. The glass transition/relaxation is constituted of two components: (i) The lower-temperature component is associated with a distribution of relaxation times obeying a compensation law. It corresponds to cooperative movements of sequences of various length in the 'true' amorphous regions. (ii) The upper-temperature component is enhanced by annealing above T~. It has been attributed to the 'constrained' amorphous regions. Note that such domains with high local order have been previously observed by electron microscopy and X-ray diffraction

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Accurate dynamic mechanical measurements were performed on poly(monocyclohexyl itaconate) at different frequencies over a wide temperature range. Three main relaxations could be observed at different temperatures, which were labelled as 7, fl and ~ relaxations. The 7 relaxation was attributed to the motions of the cyclohexyl group, specifically to the chair-chair flipping of the ring; and the fl relaxation to the rotation of the side carboxylic groups. In order to analyse ~ relaxation a mechanical model was used which considered an isotropic continuum containing spherical isotropic non-interacting particles. Qualitative validity of this hypothesis was found and the results were consistent with the dielectric data obtained for the same polymer.

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Flory and Mayo have proposed alternative mechanisms to explain the spontaneous polymerization of styrene. Both involve the formation of reactive styrene dimers capable of generating initiating radicals. However, the dimer intermediate proposed in the Mayo mechanism has defied all attempts at isolation. Since the Flory dimer would be expected to be stable to acid while the Mayo dimer would not, spontaneous styrene polymerization was studied in an acid environment to determine the effects on oligomer structure, polymerization rate, and polymer molecular weight. The results clearly show that the initiating Mayo dimer is deactivated by acid, forming high levels of inactive dimer. The decreased initiation results in a lower concentration of growing polymer chains, subsequently lowering the rate of termination (by chain coupling ). The end result is a large shift of the spontaneous rate molecular weight relation for polystyrene

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A liquid crystal copolymer containing poly(ethylene terephthalate) (PET) and 0.6 mole fraction of p-hydroxybenzoic acid (PHB) was studied. There is a large amount of information available for this material, which has been obtained by techniques other than thermally stimulated depolarization (t.s.d.). Hence the usefulness of this latter method can be evaluated for the characterization of polymer liquid crystals. First, by applying the same field of 13.3 MV m- 1 in each case, effects caused by varying the polarization temperature were determined. Second, for a constant polarization temperature of 90°C, the polarizing field was varied between 3.3 MV m-1 and 13.3 MV m 1. Third, for the same polarization temperature and fixed field, the storage time was varied between 0 h and 125 h. In the temperature range studied (20-190°C) the PET-rich phase contributes more to t.s.d, than the PHB-rich phase. High temperatures and strong fields produce similar effects of randomization of dipole orientations, or more generally of destruction of low-temperature structures. There is a competing effect of removal of restrictions on angular motions of dipoles along with increasing free volume. Longer storage times enable relaxation of electric stress created during electret formation. T.s.d. is one of several methods used for the determination of locations of phase transitions. However, in contrast to most other characterization techniques, t.s.d, permits the determination of effects of annealing on resulting structures at lower (e.g. ambient) temperatures. In particular, it is found that annealing shifts the cold crystallization process to higher temperatures.

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Blends of isotactic polypropylene and hydrogenated oligo(cyclopentadiene) were prepared by meltingmixing in a Brabender-like apparatus. Sheets of such blends were moulded under pressure, quenched in water and then annealed for 24 h at 140°C. Wide angle X-ray scattering was used to examine the effect of the annealing treatment on the quenched specimens. Thermal properties were analysed using a differential scanning calorimeter. Melting points and overall crystallinity content decreased linearly up to a content of 50% in water of HOCP. Beyond such a value a further decrease was observed. Dynamic-mechanical tests were performed at various frequencies (in the range 1 x 10-2-75 Hz) and at different temperatures (from -40°C to 120°C). The two components appeared to be compatible in the amorphous regions of the system, as already suggested in the literature. In fact a unique peak of the loss modulus versus temperature (which is representative of the glass transition temperature), was always present. Moreover the maximum of the peak was shifted to higher temperatures with increasing oligomer content. The most interesting feature, however, was the change in shape of the whole curve. In fact the peak broadened more and more whereas the height increased only slightly as the oligomer percentage was increased. A tentative explanation is given in the present paper in terms of particular features of the blend morphology, and of a phase inversion at high HOCP content in the blends

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