1992 рік

Dibutyltin diacrylate, di-2-methylpropyltin diacrylate and dioctyltin diacrylate cure to give tack-free films at much lower doses than the commonly used diluents hexanediol diacrylate, trimethylolpropane, triacrylate etc. Their reactivity is ascribed to their greater stopping power due to the presence of the tin atom and to the homolysis of the weak tin-carbon bond which leads to the production of initiating radicals. Film formation is also aided by the tin carboxylates being able to form an intermolecularly associated structure. The tin acrylates can be used in conjunction with prepolymers, e.g. an epoxidized soya bean oil (Synocure 3111 ) and a urethane acrylate (Synocure 3134). The tin acrylates are less effective than tripropyleneglycol diacrylate (TPGDA) in reducing the viscosity of the prepolymers. Electron beam curing of mixtures of the tin acrylates and prepolymers gave films possessing greater solvent resistance than those obtained from corresponding mixtures of TPGDA and the prepolymers

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Differences have been observed between the enthalpies of crystallization and fusion of quenched 'amorphous' poly(ether ether ketone) (PEEK) which have been attributed to residual crystallinity produced during quenching. Comparison with samples of PEEK crystallized under well defined conditions reveals that these apparent differences arise from changes in the heat capacity of the crystalline solid and the liquid alone and there is no need to invoke abnormal crystalline structure, or the presence of residual crystallinity produced on quenching to explain the apparent differences. Comparison is made of the degree of crystallinity as measured by d.s.c., WAXS and density from which the enthalpy of fusion of totally crystalline PEEK at 437 K was determined to be 122.5 + 1.5 J g-1

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We have investigated the Stark effect on the spectral hole characteristics in the photochemical hole burning reaction in systems of anthraquinone derivatives, such as 1,4-dihydroxyanthraquinone (DAQ) and 4-amino-2,6-bis(4-butylphenoxy)l,5-dihydroxyanthraquinone (ABDAQ) as guest molecules embedded in host polymers such as 2-hydroxyethyl methacrylate (PHEMA) or poly(vinyl butyral) (PVB). The hole broadening and refilling were observed by applying an external electric field to the samples. Thus, the dipole moment differences [A/~] of the guest molecules between the ground state and the excited state were estimated as follows: 0.38, 0.51 and 0.26D in the DAQ/PHEMA, ABDAQ/PHEMA and ABDAQ/PVB systems, respectively. These material dependent characteristics of [Aft[ have been able to be explained qualitatively by Hammett's substituent constant and the Taft's polar substituent constant. Furthermore, from the application view point, 5 x 5 multiple spectral holes could be formed in both the wavelength and the electric field dimensions at 4.2 K in the ABDAQ/PVB system. This result indicates that high-density data multiplexing can be achieved in both the wavelength and the electric field dimensions using the Stark effect on the spectral holes.

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The crystal orientation and the microstructure of biaxially oriented polyethylene films are examined by means of wide (WAXS) and small angle X-ray scattering (SAXS) as a function of the final applied pressure. The SAXS patterns of the starting uniaxially oriented injection moulded materials, having a shish-kebab microstructure, yield two meridional maxima with long periods of 36 and ~ 50 nm. It is shown that with increasing applied compressive stresses the intensity of the initial meridional long period decreases, while a new equatorial long period appears in the normal direction to the initial orientation. This final long period only depends on the temperature of deformation. Results are discussed in terms of a gradual transformation from the original shish-kebab structure into oriented domains of the pressure-induced structure, appearing at right angles to the initial axial direction. The coexistence of microdomains, with chain and lamellar orientation at right angles to each other, yields tensile strength values in the two principal directions of the films which are larger than 100 MPa.

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Polydimethylsiloxane (PDMS) bimodal networks were prepared by tetrafunctionally end-linking very short and relatively long hydroxyl-terminated chains, both in solution and in the bulk state. A constant weight fraction of short chains, 30 wt%, was employed. The networks were studied with regard to their stress-strain isotherms, covering the entire range from high elongations to high compressions. In the first type of experiment, the bimodal networks crosslinked in solution were studied in the unswollen state. The dependence of the elastic properties of the networks on the volume fraction uzs of polymer chains in the solution being crosslinked was determined. In particular, as tlzs increases, the modulus was found to go through a pronounced maximum with increase in deformation. In the second type, the bimodal networks crosslinked in the undiluted state were studied swollen. Decrease in the volume fraction u2 of polymer network present during the stress-strain measurements was found to increase the modulus, which is the opposite of that invariably found for unimodal networks. This effect of swelling on the elastic properties of bimodal networks confirmed that the self-reinforcement they exhibit is due to the limited extensibility of short chains.

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A reliable method for the anionic synthesis of block copolymers from polystyrene and polymethylphenylsiloxane is reported. Analysis of the block copolymers by h.p.l.c, proved the absence of homopolymer. The evaluation of g.p.c, measurements demonstrated that the copolymers exhibit a narrow molecular weight distribution (M,/M, <~ 1.1). Depending on molecular weight and composition the copolymers are homogeneous or undergo microphase separation.

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Neutron scattering on polymer blends of deutero-poly(methyl methacrylate) (D-PMMA) and poly(styrene-co-acrylonitrile) with 19 wt% acrylonitrile (PSAN-19) exhibits excess scattering for scattering vectors Q ~<0.2 nm-~ if the volume fraction of D-PMMA is less than 0.5 and greater than 0.05. The range of occurrence and the amount of excess scattering depends to a certain degree on the procedure of sample preparation. If the excess scattering is eliminated by linear extrapolation from large to small Q in the Zimm representation, the residual scattering intensity in the small and intermediate Q-range can be interpreted by the mean field approach of de Gennes with a concentration-dependent effective interaction parameter. The scattering intensity in the intermediate Q-range can be described with the coil scattering function of Debye using effective radii of gyration and interaction parameters obtained from Q~0 extrapolated scattering data. In the intermediate Q-range the scattering curves of D-PMMA in the bulk and in the investigated blends show small differences outside the limits of experimental error. These differences are interpreted as an environmental influence of the conformation of the PMMA chain.

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Diesters with the structure C6Hs--COO--(CH2)m--OOC--C6Hs, m = 2-6, have been synthesized, and their fluorescence has been measured in dilute solution in viscous solvents. These diesters are model compounds for polyesters prepared from isophthalate or terephthalate and HO(CH2)mOH. Comparison of the fluorescence of the diesters with the fluorescence of the polyesters, reported earlier, shows that energy migration is not an important mechanism for the population of excimers in the polymers. The agreement between the experimentally determined dependence of excimer emission on m and the theoretical dependence of the population of excimer forming conformations on m is improved by consideration of interactions higher than second order, especially for the largest member of the series.

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The tensile deformation and failure of a wide range of poly(p-phenylene terephthalamide) fibres is investigated. Fibre tensile modulus and tensile strength can be varied widely through simple processing variations, yet the fibres are found to exhibit similar non-linear elastic deformation responses as elucidated in stress perturbation experiments. An asymptotic modulus characteristic of the local orientation in the fibres is determined and is found to correlate well with the tensile strength of the fibres in accordance with straightforward shear failure criteria. The tensile failure of these fibres is thus dominated by the inherent strength anisotropy of linear chain polymers and the imperfect orientation in the fibres.

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Amorphous polycarbonate with various mechanical and/or thermal histories was investigated via macroradical decay in the temperature interval between -30°C and 100°C by e.p.r, spectroscopy. The decay of end macroradicals was found to take place at low temperatures more rapidly in anisotropic drawn samples than in isotropic materials. The rate constants of both samples show breaks at ~20°C and 70°C and coincide at higher temperatures. After heating in the region below the glass transition temperature, the order of the rate constants is maintained but the decay is more rapid in aged samples compared to unaged materials. The concentration and kinetic characteristics are correlated with previous small-angle X-ray scattering results and the dynamic-mechanical relaxation results presented here and interpreted in terms of changes in free volume characteristics after cold deformation and/or annealing of polycarbonate.

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