1999 рік

The high thermal stability of this class of polybenzoxazines is a combined result of polymerization of acetylene terminal functional group and oxazine ring-opening polymerization. The high char yield achieved for this class of materials is in the range of 71–81% by weight at 8008C in a nitrogen atmosphere and 30% by weight at 7008C in air as it is determined by thermogravimetric analysis (TGA). Temperature at 10% weight loss (T10%) is in the range of 520–6008C. The polymerization and post-cure reactions of these benzoxazines are studied by differential scanning calorimetry (DSC). The char formation of the blend of an acetylene-functional benzoxazine with an analogous benzoxazine without acetylene group is also studied by TGA.

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Introducing crystallinity in polyimides offers the advantage of increased solvent resistance and retention of mechanical properties above the glass transition temperature (T g). This work examines the thermal stability, semicrystalline morphology and adhesive properties of a new polyimide based on 1,3-bis(4-aminophenoxy) benzene (TPER diamine) and 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA). The polymer has a T g of approx. 210°C and a T m of 395°C. Polarized light microscopy was used to evaluate the morphology and spherulitic growth rates from the melt for M n ¼ 15 000 dalton and M n ¼ 30 000 dalton molecular weight polymer. Lap-shear specimens were made using Ti–6Al–4V alloy coupons using a melt process. Three surface treatments: gritblasting, chromic acid anodization and sodium hydroxide etch were evaluated and it was found that macroroughness provided by gritblasting was sufficient in providing a strong interface. The bonding conditions were optimized with respect to bonding temperature, bonding time and pressure, and adhesive bonds made using the optimized conditions displayed a bondline thickness of approx. 2–3 mils. High room temperature lap-shear strengths of more than 6000 psi were obtained and the fracture surface investigation using the SEM revealed a very rough and plastically deformed surface. Durability studies on lap-shear specimens were conducted for aging and testing temperatures of ambient, 177°C and 232°C. Significant stability was observed for all these aging and testing temperatures for a period up to seven weeks. The bond strengths were found to be affected by testing temperatures rather than the aging conditions for the duration of the study. The influence of various solvents on the lap-shear strengths was investigated by immersing the specimens in these solvents for 9 days. None of the solvents had any major effect on the strength of lap-shear specimens.

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The excited state characteristics of 13 novel 1-chloro-4-oxy/acyloxy derivatives of thioxanthone were determined using micro-, nano- and picosecond flash photolysis techniques. Triplet energy levels have also been determined using phosphorescence analysis while photoinitiated polymerisation activities were measured using photocalorimetry. All the initiators exhibit high photopolymerisation activity except the 4hydroxy model and 2-methyl-4-n-propoxy derivatives. The triplet energies are found to be fairly insensitive to solvent polarity with an observed spectral broadening from non-polar to polar solvents. This is consistent with close lying mixed triplet states of 3p p * and 3np * character. Both the least active 4-hydroxy and 2-methyl-4-propoxy derivatives exhibit lower triplet energies suggesting the presence of a less active lower triplet 3p p * state. Triplet–triplet absorption spectra are obtained with all the thioxanthones in the range 600–680 nm with a marked blue shift from non-polar to polar solvents owing to stabilisation of the lowest triplet state by solvent reorganisation. In photoreductive solvents such as methyl alcohol and 2-propanol a longer lived species is observed absorbing in the region 400–500 nm associated with the formation of the ketyl radical. This observation is highly solvent dependent and further supported by microsecond flash spectroscopy in 2-propanol. The 4-hydroxy derivative gave only weak transient absorption and is consistent with its much lower initiation activity. In the presence of a tertiary amine no ketyl radical is observed. Triplet lifetimes increase with solvent polarity confirming the presence of mixed 3 p p * and 3np * states where vibronic coupling influences the rate of intersystem crossing to the ground S0 state. Bimolecular triplet quenching rate constants indicate all the thioxanthones, except the 2-methyl-4-n-propoxy and 4-hydroxy, interact strongly with a range of tertiary amines, DABCO, triethylamine and methyldiethanolamine. The lower triplet quenching constants for the 2-methyl-4-n-propoxy and 2hydroxy derivatives (an order of magnitude) indicates weaker interaction by the amine and is consistent with their lower photoinitiation activities. Low triplet quenching rates are also observed in the presence of monomer (methyl methacrylate). Bimolecular triplet quenching rates are also measured with naphthalene and are similar to those for benzophenone except the 2-methyl-4-n-propoxy and 4-hydroxy, derivatives which are an order of magnitude less. From this data triplet molar extinction coefficients are determined and found to be higher than that for benzophenone owing to the presence of mixed states. Again, the 2-methyl-4-n-propoxy and 4-hydroxy derivatives exhibited lower values as did the 4-benzoyloxy derivative. Relatively high quantum yields of intersystem crossing are observed ( < 0.6) but are all lower than that of benzophenone with the side chain substituents having no significant effect on the rate. The growth rate of the triplet state could be measured and triplet maxima accurately determined via picosecond flash photolysis. Within the growth time of the triplet state (6–18 ps) there is a concurrent formation of the ketyl radical except for the 4-acetyloxy and 4-acryloxy derivatives. The importance of this technique in gaining valuable information on the relative rates of concurrent excited state reactions for thioxanthones is discussed.

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4-Trimethylsilyloxy-4 0 -vinylbiphenyl was treated with perfluorobenzene, substituted polyfluorobenzenes, and perfluoronaphthalene to give 4-[(per/polyfluoro)phenyloxy]-4 0 -vinylbiphenyls in the presence of catalytic amounts of CsF. The new monomers that resulted were characterized by mass spectra, 1H and 19F NMR, and IR spectroscopy. The new polymers obtained by polymerizing the monomers using AIBN-initiated free radical polymerization were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), 1H and 19F NMR and IR spectroscopies. The polymers were obtained as white powders and were soluble in all common organic solvents. They are thermally very stable and have high glass transition temperatures, Tg, i.e. .1008C. Copolymerization of one of the monomers, 4-pentafluorophenyloxy-4 0 -vinylbiphenyl, 3, with different molar ratios of methylacrylate gave copolymers that have low glass transition temperatures compared with the homopolymer of the monomer, 3, along with very high thermal decomposition temperatures, e.g. incorporation of even a small amount (2.1%) of 3 into the methylacrylate copolymer gave rise to a 50% char yield at 4108C and a glass transition temperature of 288C.

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The optical properties have been studied in chlorinated polyvinyl chloride (CPVC) film in the wavelength range from 200 to 500 nm using UV-160 Shimadzu spectrophotometer. The irradiated samples of different energies (5, 9 and 13 MeV), as well as different radiation doses were studied. This investigation were carried out to determine the optical parameters, optical energy gap (DE), absorption coefficient (a) and the energy of direct and indirect transition E (opt)1 and E (opt)2. The results show that both direct and indirect transition exist in CPVC and highly sensitive to electron irradiation doses and energies. The variation of optical energy gap with electron irradiation energies and doses can be explained as the change in the degree of disorder and the optical band energy is dose and energy dependent.

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The micromorphologic features of sheared isotactic polypropylene (iPP) in the vicinity of fibre were investigated through polarized light microscopy (PLM), phase contrast optical microscopy (PCLM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results indicate that the polymorphic structures and their boundary lines can be explained from a modified model by combining the concept of metastability and the theory of shear-induced crystallization without assuming epitaxial growth.

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t-Butylstyrene-1,4-divinylbenzene (TBS-DVB) microgels were synthesized by the arm first living anionic polymerization technique using two different solvents, heptane, a good solvent for poly(TBS) arms only and tetrahydrofuran (THF), a good solvent for both the arms and the poly(t-butylstyrene-co-4-vinylstyrene) block copolymer formed initially before crosslinking. The polymerizations were carried out using tbutyllithium as an initiator. The resulting microgels were characterised by size exclusion chromatography with an on-line multi angle laser light scattering instrument. In the polar solvent THF, much better control of the molecular weight of the microgels was observed. It is suggested that the mechanism of microgel formation is influenced by the solvent. Micelle formation is favoured in the case of heptane, which is a poor solvent for 4-vinylstyrene blocks.

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The complete cohesion of two thin films composed of melts of C 99H 200 has been achieved by simulation. The simulation employs a new 'mapping/reverse mapping’ technique that is orders of magnitude more efficient than conventional molecular dynamics simulations of fully atomistic models. Three distinct processes, each with its own time scale, can be resolved. From fastest to slowest, these are: 1. the healing of the density profile between the two films, 2. the redistribution of chain ends, and 3. the complete intermixing of the segments from the original two films. This simulation has the capability of producing new insights into the mechanism of interdiffusion across an interface and the mechanism of the development of cohesive strength at the interface.

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The transesterification reaction between polyethylene terephthalate and polybutylene terephthalate (PBT) was investigated by small-angle neutron scattering. For this purpose deuterio polyethylene terephthalate (dPET) was synthesised and fractionated to produce three samples whose molecular weight ranged from circa 14 000 g mol ⫺1 to 63 400 g mol ⫺1. Mixtures of these two polymers containing 10% of the deuterio polymer were transesterified to varying extents by heating for known times at different temperatures. The apparent molecular weight of the deuterio polymer decreases as the extent of reaction increases and this is observed in the small-angle scattering data. From these data second order rate constants were obtained, the values are somewhat larger than those obtained for main-chain mesomorphic polyesters but of the same order of magnitude for transesterification in polyethylene terephthalate alone. No influence of the large difference in molecular weight of the dPET on the rate constants was observed. The activation energy obtained from these data was 61 ^ 7 kJ mol ⫺1, a value which is less than half that reported for other polyester transesterifications. The reaction appears to be dominated by the scission of the PBT component of the mixture.

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This report presents the synthesis and chemical modification of hyperbranched poly(ether ketones). The polymer was conveniently prepared by direct polycondensation of an AB2 monomer, 3,5-diphenoxybenzoic acid, using phosphorus pentoxide/methanesulfonic acid (PPMA) as the condensing agent and solvent. The hyperbranched poly(ether ketone) could be modified via the electrophilic aromatic substitution of the active phenoxy groups at the chain ends with a variety of carboxylic acids. The thermal properties of the hyperbranched poly(ether ketones) depend heavily on the nature of the chain end, with glass transition temperature ranging from 2 248C to 1808C. Moreover, the length of the terminal alkyl groups significantly influences the solubility of the hyperbranched poly(ether ketones). By varying the chain ends, hyperbranched poly(ether ketones) soluble in either a polar or nonpolar solvent could be obtained.

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