1994 рік

A novel thermosetting resin based on vinylic end groups was prepared and its network formation investigated. 2,2-Bis(4-vinylbenzyloxyphenyl)hexafluoropropane was prepared by the condensation of vinylbenzyl chloride with Bisphenol-AF in the presence of K2CO 3. This distyrenic monomer was cured thermally to produce a highly crosslinked network with <0.5% sol fraction. The networks prepared by this reactive monomer were brittle. However, their fracture toughness could be modified with the incorporation of vinylbenzyl end-capped perfluoroalkylene aryl ether oligomers. The thermoplastic component phase separated into discrete composite particles in the continuous network phase. The thermoplastic modified network showed improved fracture toughness without sacrificing the modulus or good dielectric characteristics.

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Fourier transform infra-red spectroscopy coupled with photoacoustic detection (PA-FTi.r.) has proved to be a useful tool for finding out about quantitative structural changes in glass-fibre-reinforced poly(ethylene terephthalate) (PET) obtained by compression moulding and subsequently annealed. Analysis of the photoacoustic parameters indicates that the photoacoustic characteristics of the composite are very similar to those of the isolated matrix. It was demonstrated that glass-fibre-reinforced PET satisfies the same two-phase conformational model as the matrix. In contrast to the annealed matrix, the primary and secondary isomerizations cannot be detected in the composite. Therefore, the thermal treatment on the composite samples does not show further conformational changes. As a result, the manufacturing process seems to determine the composite microstructure, and its effect is quite similar to that observed in annealed matrix at the highest temperatures. Consequently, it can be considered that the trans isomer content in the amorphous phase is negligible in the composite

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Doubly oriented iodine-doped nylon 6 (iodine-nylon 6 complex) was prepared by immersing nylon 6 in an I2-KI aqueous solution and was studied using X-ray diffraction. The intensities in the X-ray diffraction pattern from the iodine-doped nylon 6 vary with ageing time after doping. The variation is attributed to the rearrangements of polyiodides in the complex. In the complex two kinds of 'one-dimensional' ordered structures give streaks on the layer lines, and a three-dimensional ordered structure (complex crystal) gives Bragg reflections. Just after doping an unstable 'one-dimensional' structure emerges. This subsequently transforms into a stable 'one-dimensional' structure and/or the three-dimensional complex crystal with ageing. The arrangement of polyiodides in the complex has been determined. The unstable 'one-dimensional' structure is incommensurate with the complex crystal; a structural model based on co-ordination of 12 with I~ is proposed on the basis of the intensities of the unstable streaks vanishing with ageing. The complex crystal is concluded to include I~ ions inclining at an angle to the chain axis of nylon 6. The stable 'one-dimensional' structure contains I~ ions which are oriented parallel to the chain axis.

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The mechanical properties of poly-p-phenylene and its simplest oligomer, biphenyl, have been explored by ab initio and semiempirical molecular orbital methods. The contribution of zero-point energy to the torsional barrier is negligible, but the variation of vibrational entropy with torsion increases the height of the free energy barriers to rotation through the D2h and Dza conformations. The computed Hessians argue for a backbone that becomes 14% stiffer when strained to planarity. Cluster calculations of the polymer using periodic boundary conditions illuminate the role of bowing in compression and the importance of intermolecular forces to compressive mechanical performance of rigid-rod polymers.

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We investigated thermal, rheological, morphological and mechanical properties of an in situ composite of poly(ether imide) (PEI) and thermotropic liquid crystalline polymer (TLCP). Ultem 1000 was used as a matrix and Vectra B950 was used as the in situ reinforcing TLCP. Fibre-spinning of the blends was performed on a capillary rheometer. Differential scanning calorimetry thermograms of extruded fibres indicated that the thermal properties of PEI did not change noticeably with the amount of TLCP but thermogravimetric analysis showed that thermal stability of the blend was decreased with the amount of TLCP. Immiscibility was checked with thermal data. The rheological properties of the blends changed remarkably with temperature and composition. The tensile strength and modulus of blend fibre increased with TLCP content and spin draw ratio. The increase of tensile strength was more striking for the fibre of the blend containing more TLCP. Wide angle X-ray patterns suggested that the increase in tensile strength was due to the enhanced molecular orientation and resultant fibrillation of TLCP. A modified Tsai-Halpin equation was used to predict the aspect ratio of microfibrils for these blends. Morphology of the blend showed that PEI/TLCP fibres contained fine fibrils of almost infinite aspect ratio and nearly perfect orientation in the flow direction. The draw ratio effect on the mechanical properties was remarkable at low draw ratio, but levelled off soon. The amount of TLCP influenced the fibril formation. Morphological observation showed the effect of thermal history of the blend and its effect on mechanical performance. The blend showed maximum aspect ratio and aspect ratio change when TLCP content was 25 wt%. TLCP orientation and its steric effect seem to induce the optimum TLCP amount for fibril formation.

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The effects of counter-anions on the microphase-separated structure and the properties of elastomeric ionene polymers (PTV) containing poly(tetrahydrofuran) (poly(THF)) soft-segment blocks and viologen hard ionic segments were investigated by thermal and mechanical analyses and by a small-angle X-ray scattering (SAXS) technique. Three sorts of PTVs having CI-, Br-, or I- halide ions as counter-anions for the viologen groups were synthesized. All PTVs showed good elastomeric properties, i.e. low tensile modulus, high tensile strength and high elongation at break. The PTV with C1- anions showed a higher capability of crystallization of the poly(THF) block chains than those with Br- or I- anions, in the cooling process prior to successive thermal and dynamic viscoelastic measurements, and exhibited a higher glass transition temperature of the amorphous poly(THF) chains in the heating process of these measurements. In the SAXS experiments, all PTVs exhibited multiple scattering peaks, indicating that microphase separation between the poly(THF) block chains and the viologen units was highly ordered. On the basis of these results, we considered a lamellar-type microphase-separated structure for PTVs with halide counter-anions.

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The effects of glass-fibre reinforcement on the fracture toughness, Kxc, of nylon 6,6 were examined and the deformation mechanisms of unreinforced nylon 6,6 were studied by varying the deformation rate, by dilatational measurements and by i.r. spectroscopy. In the unreinforced nylon 6,6 a flow stress plateau was observed in the stress-strain behaviour prior to the onset of necking. Of the 25-30°/'0 inelastic strains stored in this plateau a substantial portion appears to be related to a crystallographic plastic deformation due to the crystalline segments in nylon 6,6. In glass-fibre-reinforced nylon 6,6 a brittle to ductile transition was found to occur when the mean fibre-end spacing was less than a critical value. The observed brittle--ductile transition was found to originate from an observed enhanced matrix plasticity at fibre ends when the glass fibres are sufficiently closely spaced. Such enhanced localized plasticity at fibre ends was suggested to result from interactions of stress fields with nearby fibre ends when the fibre-end spacing is less than the critical value. It is further postulated that the enhanced localized fibre-end plasticity is made possible due to the ability of the matrix to exhibit a large degree of crystallographic plasticity. The toughening behaviour of fibre-reinforced nylon 6,6 was also compared qualitatively to that of rubber-toughened nylon 6,6 and a general principle for microstructural toughening in nylon 6,6 was addressed. Strategies for fibre toughening in fibre-reinforced nylon 6,6 were also discussed.

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Changes in structure and properties due to mechanical fatigue for segmented polyurethanes containing a block segment of poly(ethylene oxide)-block-poly(dimethyl siloxane)-block-poly(ethylene oxide) (PES) were investigated by mechanical and thermal analyses and a small-angle X-ray scattering (SAXS) technique. Six types of specimens designated KP-x-y were studied, where x and y denote, respectively, the fraction of PES component (x = 0 or 13 wt%) and the molecular weight of the soft-segment block, poly(tetramethylene glycol) (PTMG) (y= 1000, 2000 or 3000). The SAXS experiments revealed that dissociation of the hard-segment microdomains was induced by mechanical fatigue. The changes in the mechanical properties due to fatigue were discussed in relation to the PTMG soft-segment crystallization which can be triggered by the dissociation of the hard-segment microdomains. As already reported, KP-13-2000 has better antithrombogenicity and more suitable viscoelastic properties for use as a biomedical material than any of the others. It was found in this study that KP-13-2000 has strong resistance against mechanical fatigue as well, which is one of the most important characteristics required for biomedical materials. Although the microdomain structure was fractured by fatigue, the mechanical property of KP-13-2000 was not changed because the PES component prevents the PTMG soft segments from crystallizing.

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The phase transformations of a novel liquid crystalline epoxy, digiycidyl ether of 4,4'-dihydroxy-~tmethylstilbene (DGEDHMS), from isotropic phase to liquid crystalline phase during curing have been studied using a cross-polarized optical microscope equipped with a hot-stage. Time-temperaturetransformation (TTT) diagrams were constructed following the isothermal curing of the DGEDHMS in the isotropic phase with a difunctional amine, the aniline adduct of the DGEDHMS, and a tetrafunctional sulfonamido amine, sulfanilamide. Our results show that curing with the difunctional amine results in a nematic-like texture, while curing with the tetrafunctional sulfonamido amine leads to a smectic-like texture. Furthermore, curing with the difunctional amine gives rise to a faster development of the liquid crystalline phase, but with a less stable mesophase as compared to the tetrafunctional curing agent. These results are explained in terms of intermolecular interaction and the chain conformation anisotropy resulting from the curing reactions. The TTT diagram will be useful for providing guidelines for processing these new liquid crystalline epoxy systems.

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The deformation and fracture behaviour of nylon-6/EPR (ethylene-propylene rubber) blends is studied as a function of strain rate and rubber content. Therefore, tensile experiments are conducted on notched specimens over a broad range of draw speeds (including strain rates as encountered in normal tensile tests and in impact tests). The blends with a high rubber content show super-tough behaviour at low and high speeds. In the intermediate-speed regime the fracture energy reaches a minimum (with a level comparable with that of unmodified nylon). The blends with a low rubber content show a transition from tough to brittle behaviour with increasing strain rate. The sudden rise in fracture energy with rising strain rate is believed to be caused by a transition from isothermal to adiabatic deformation. In the adiabatic regime the rise in temperature is high enough to melt the material around the crack tip. This thermal blunting mechanism causes crack propagation to be stable at high strain rates. This mechanism is affirmed by scanning electron microscope studies of the deformation zone.

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