1999 рік

The constitutive relation for the refractive index of dilute solutions of flexible polymer molecules is obtained, and calculations of the intrinsic birefringence exhibited in uniaxial elongational flow are presented. The polymer molecules are modeled as chains of beads connected by finitely extensible nonlinear elastic springs under the Peterlin approximation. It is found that at very low elongation rates there is very little birefringence, but there is a critical elongation rate above which there is substantial birefringence. This critical elongation rate scales as the inverse square of the number of beads in the chain. At this elongation rate the individual springs in the chains are stretched and oriented with the flow, but the chains have not uncoiled. It is found that the stress-optic relation holds for elongation rates below the critical elongation rate. However, the stress-optic coefficient is not constant above the critical rate. In the instantaneous inception of the flow above the critical elongation rate, the stress optic coefficient initially takes on the value that it would have below the critical elongation rate, but it decays to a much lower value as the strain increases.

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Gravimetric and dielectric measurements of water uptake by a series of carboxy terminated butadiene acrylonitrile (CTBN) -amine cured epoxy resin systems are reported. The amount of water absorbed and rate of diffusion increases up to approximately 15 w/w% CTBN content. Anomalous behaviour is observed around a composition of approximately 20% CTBN which is coincident with the occurrence of a cocontinuous phase structure in the resin. Dynamic mechanical analysis and atomic force microscopy studies provide evidence for phase separation and inversion of phase structure at 20% CTBN. The increase in the water uptake with increasing CTBN content reflects the ability of the highly polar acrylonitrile group to bind water. The general increase in the diffusion coefficient is consistent with CTBN being initially dispersed in the epoxy matrix tending to a lowering of the effective segment density inhibiting mobility of the water molecules. In the cocontinuous region subtle effects of mutual solubility of components and generation of a long percolation path leads to the observed anomalous behaviour. Dynamic mechanical thermal analysis (DMTA) indicated that water absorption suprisingly leads to an increase in the glass transition temperature and is associated with a densification of the matrix structure. Atomic force microscopy measurements reveal that the surface roughness changes with composition with a marked increase in the surface roughness at 20% CTBN.

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1,6-Bis-3-(vinyl-3-2-pyrrolidonyl)hexane (BNVP) was synthesized for use as a crosslinker in the photopolymerization of 1-vinyl-2pyrrolidone (NVP). A model compound, 3-hexyl-1-vinyl-2-pyrrolidone (HNVP), was also synthesized for use as a model comonomer in the kinetic studies of NVP copolymerization. Copolymerizations with NVP and low feed ratios of HNVP indicate that alkyl substitution in the 3-position on the pyrrolidone ring does not affect radical reactivity of the monomer. However, the comonomer containing this hexyl side chain acted as an internal plasticizer for the NVP polymers, lowering the Tg and reducing the effect of autoacceleration on the photopolymerization process. The crosslinked polymers exhibited gel-like swelling in water. The crosslinker limited the mobility of the polymer chains early in the polymerization and induced earlier onset of the Tromsdorff effect.

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The influence of toughener concentration on the fracture behavior of two-phase, rubber-toughened epoxy is studied. To vary the concentration without altering other morphological features, samples generated from dispersions of preformed rubber (acrylic) particles in liquid epoxy monomer are used. By diluting with different amounts of epoxy prior to cure, the toughener concentration can be varied over a wide range. Thermal and microscope studies support the assertion of a constant morphology. The fracture results show that the toughness increases to a maximum and then decreases as the concentration is increased. This suggests an optimum concentration of toughening. Micrographs of the initiation zone on the fracture surface at high concentrations of rubber show less deformation than the equivalent surfaces at lower concentrations. This is consistent with a toughening mechanism based on particles initiating yielding and plastic flow in the matrix.

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The superb depth resolution of low-energy forward recoil spectrometry, (LE-FRES), is demonstrated by measuring the interfacial profile between 1,4-polybutadiene (PB), and deuterated polystyrene (dPS), at 175°C. To enhance depth resolution, the top dPS layer is 'thinned’ by ion sputtering, prior to LE-FRES analysis, to achieve a resolution of 13 nm at the interface. An interfacial width of 6.0 ^ 3.5 nm is measured. To compliment this study, neutron reflectivity (NR) is used to determine the interfacial volume fraction profile between PB and an isotopic mixture of polystyrene (dPS:PS) having a dPS volume fraction of 0.30. After annealing at 175°C, the interface broadens to 3.3 nm. However, no interfacial segregation of dPS or PS is observed suggesting that the difference between the PB–dPS and PB–PS interaction parameters (x) is insufficient to drive interfacial segregation. Using a self-consistent field model (SCF) and known values of x, the width is calculated to be 2.2 nm. By adding capillary wave broadening, this width increases to 2.8 nm, in better agreement with the NR value. In agreement with the NR measurement, the SCF model predicts no segregation of dPS or PS.

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New main-chain type poly(5,8-dialkoxyanthraquinone-1,4-diyl)s have been prepared by using a Ni(0) complex. They take an ordered structure assisted by side-chain crystallinity. The ordered structure has been elucidated by X-ray diffraction pattern. Their d spaces and the relationship between the d space and the number of carbon atoms in the alkoxyl side chain indicate that these polymers take an end-to-end packing mode and their main chain has an s-trans conformation. These polymers exhibit liquid crystalline behaviour at a wide range of temperature.

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The effect of a number of dyes on the glass transition temperatures of a variety of polymers has been investigated. The glass transition temperatures of each dye–polymer blend were compared to the Kwei equation to obtain Kwei q parameters. It was demonstrated that the q parameter correlated to dye–polymer interaction independent of the variation in dye structure and polymer composition. The highest q parameters were obtained as the solubility parameters of dye and polymer approached equality corresponding to the maximum dye–polymer affinity.

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Molecular dynamics (MD) simulations have been used to study diffusion of methane in three highly impermeable aromatic polyesters that are good barrier materials. These are amorphous poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalene dicarboxylate) (PEN), and the nematic mesophase of the thermotropic liquid crystalline copolyester (LCP) of p-hydroxy benzoic acid (HBA) and 2,6 hydroxy naphthoic acid (HNA). Diffusion coefficients were determined in the temperature ranges of 450–625 K for PET, 500–625 for PEN, 425–530 K for the LCP, where values are large enough to be accessible to MD in practical computation times. Extrapolation, via Arrhenius plots, of the coefficients to near room temperature gave good agreement with experimental data in that region. This was found even though the glass transition temperatures (Tg) of PET (350 K) and PEN (390 K) lie in the intervening temperature range. This finding confirms previous observations that a low temperature hopping regime for small penetrants sets in on cooling well before the Tg of the host. Analysis of diffusant trajectories in terms of diffusive jump size distribution also shows that the low temperature hopping regime remains in place over the temperature range studied in these low diffusion coefficient polymers. Correlation of diffusion coefficients with free volume was examined. The LCP, even though diffusion there is highly anisotropic, is found to lie on a correlation found previously for five other polymers studied via MD. However, PET and PEN fail badly and are found to diffuse much more slowly than inferred from free volume vs diffusion coefficient behavior in the other polymers.

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Reasonable models for the crystalline structures of unsubstituted poly(azomethine) and methoxy substituted poly(azomethine) are proposed on the basis of X-ray diffraction and computer modeling. Both compounds exhibit a relatively high degree of crystallinity (,30%). The calculated diffraction patterns (almost identical to the experimental ones) show that the polymers investigated crystallize in a pseudomonoclinic or a triclinic lattice. The structural properties of the polymers studied, based on their crystalline structure models, are discussed.

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New, wholly aromatic polyesters carrying 9,10-diphenylanthracene fluorophores along the main chain or both in the pendent and main chain were prepared and their properties were studied by differential scanning calorimetry, wide-angle X-ray diffractometry and fluorometry in solution as well as in thin solid films. The diphenylanthracene pendent groups are attached to the terephthaloyl unit of the aromatic polyester main chains through the oxyhexamethyleneoxy spacer. Two of the polymers were found to be thermotropic and all of the polymers exhibited relatively strong photoluminescence in solution as well as in thin films. Their fluorescence behavior is discussed in relation to their structures. When the diphenylanthracene fluorophores exist either both in the main chain and the side chain or only in the side chain, the polyesters reveal photoluminescence over a broad range of wavelength from 400 to about 700 nm with the maximum emitting intensity at about 450 nm. The polymer that contains the same fluorophore only in the main chain, however, shows a rather sharp luminescence from about 420 to 550 nm, although its maximum emitting intensity is observed at the same wavelength (ca. 450 nm) as for other polyesters.

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