1994 рік

Since the early 1950s a number of theoretical studies have dealt with the prediction of the dimensions and behaviour of macrocyclic polymers. However, owing to the difficulty of synthesis, the comparison of theories with experimental data still remains restricted to a small number of macromolecules. The first cyclic polymers studied were those produced from systems in which ring-chain equilibrium exists, but it is only in the last decade that a method for the preparation of ring-shaped vinylic polymers of controlled dimensions has become available. The method involves the bimolecular end-to-end ring closure of an anionically prepared ~,~o-difunctional linear polymer using a coupling agent. Low cyclization yields are generally observed. Besides theoretical considerations the experimental conditions which may lead to these results have been examined. In this paper a new approach to the synthesis of macrocyclic polymers with controlled dimensions is examined. In contrast to previous procedures, it involves a single unimolecular ring-closure process which is much less sensitive to the extremely low concentrations required for cyclizations. It is achieved by the direct end-coupling of an :~,og-heterodifunctional linear polymer precursor after the appropriate activation under high dilute conditions of one of its two end-functions. The advantages of this new approach are discussed both on theoretical and experimental grounds. Its application to the preparation of poly(vinyl ether)s, polystyrenes and poly(vinyl ether-b-styrene) diblock copolymers of controlled dimensions with a macrocyclic architecture is described.

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The photo-fluorescence of a solution-cast film containing a 30/70 blend ofpoly(p-phenylene benzobisoxazole) (PBO)/amorphous nylon shows a substantial decrease in fluorescence yield, and a blue-shifted spectrum upon annealing at around 220°C, corresponding to the transition from an entrapped single phase to a multiphase system. This indicates a substantial dependence of PBO fluorescence on its domain structure in this blend

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New polysilsesquioxanes are prepared by incorporating tetrahydropyranyl moieties into silsesquioxanes. These polymers, which are soluble in organic solvents, are synthesized by means of HCl-catalysed reactions.

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Highly oriented films of poly(p-phenylene vinylene) (PPV) were prepared by uniaxial stretching during thermal conversion of a water-soluble sulfonium polyelectrolyte precursor polymer. The precursor films were stretched up to seven times their original length. Doping of the stretched PPV films was achieved by dipping into a 0.12 M solution of FeC13 in nitromethane. The d.c. electrical conductivity of these doped films was measured as a function of the doping time and as a function of the temperature between 80 and 300 K. Compared with the maximum conductivity obtained in unstretched PPV films doped with FeCI a (ad? = 35 f~- 1 cm 1), the stretched films showed a higher conductivity (ad? = 230 ~- 1 cm 1) when measured along the stretching orientation. The molecular structure and the changes caused by doping of stretched PPV films were studied by infra-red dichroism, transmission electron microscopy and electron diffraction.

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Poly(ether ether ketone) (PEEK) was used to prepare a sulfonated terbium salt (Tb-SPEEK). This salt emits green fluorescent light under u.v. excitation. The fluorescence originates from the transition of the 5D 4 level of Tb 3 ? to the levels of 7Fj (J= 6, 5, 4, 3). The fluorescence intensity increases with increasing Tb content, reaching a maximum value at around 9 wt% Tb, and then decreases due to the formation of ionic aggregates in the salt. The excitation of the salt is related to its Tb content and corresponding solid state, and changes after experiencing thermal treatment

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We present a new methodology that couples neutron diffraction experiments over a wide Q range with single chain modelling in order to explore, in a quantitative manner, the intrachain organization of non-crystalline polymers. The technique is based on the assignment of parameters describing the chemical, geometric and conformational characteristics of the polymeric chain, and on the variation of these parameters to minimize the difference between the predicted and experimental diffraction patterns. The method is successfully applied to the study of molten poly(tetrafluoroethylene) at two different temperatures, and provides unambiguous information on the configuration of the chain and its degree of flexibility. From analysis of the experimental data a model is derived with C-C and C-F bond lengths of 1.58 and 1.36 A, respectively, a backbone valence angle of 110 ° and a torsional angle distribution which is characterized by four isometric states, namely a split trans state at + 18 °, giving rise to a helical chain conformation, and two #auche states at + 112 °. The probability of trans conformers is 0.86 at T= 350°C, which decreases slightly to 0.84 at T=400°C. Correspondingly, the chain segments are characterized by long all-trans sequences with random changes in sign, rather anisotropic in nature, which give rise to a rather stiff chain. We compare the results of this quantitative analysis of the experimental scattering data with the theoretical predictions of both force fields and molecular orbital conformation energy calculations.

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Films of poly(2,5-thienylene vinylene) (PTV) were prepared via thermal elimination under vacuum of an HCl-treated methoxy precursor polymer. Optimum reaction conditions were obtained from on-line Fourier transform infra-red experiments. Doping was achieved either by immersing the PTV films in a solution of 129I 2 in pentane or by exposing them to I 2 vapour. The doping level of the samples was measured by electron probe X-ray micro-analysis; values of the iodine/sulfur atomic ratio ranged from 0.4 to 0.57. Dopant profiles were determined on sections of the doped films by scanning electron microscopy-energy dispersive spectral analysis. Electron microscopy also revealed the porous structure of the PTV films, rationalizing the ease with which the dopant penetrates. Despite the porosity, the films could be doped homogeneously and reproducibly. M6ssbauer spectroscopy showed that predominantly I~- is present as the dopant species, which leads to one dopant ion per 10-13 monomeric units. The temperature dependence of the d.c. conductivity (measured between 90 and 300K) is best described by variable-range hopping (VRH) in three dimensions. Using Mott's formulae the physical parameters involved in VRH, i.e. the density of states at the Fermi level, the hopping distance and hopping energy, are calculated.

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Aqueous polyurethane anionomer dispersions were prepared from isophorone diisocyanate, poly(tetramethylene adipate glycol) (PTAd) and dimethylol propionic acid as potential anionic centres. The effects of polyol molecular weight (Mn) on the state of dispersion, thermal, mechanical and viscoelastic properties and swelling of emulsion-cast film were determined. With increasing Mn of PTAd, the particle size of the emulsion and soft segment glass transition temperature, Tg, decreased, and solvent swelling, emulsion viscosity and hard segment Tg increased. Tensile strength showed a minimum with PTAd 1000, and elongation at break generally increased with the M n of PTAd. These results could be interpreted in terms of soft segment-hard segment phase separation and crystallization of high molecular weight PTAd

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Charge-transfer (CT) fluorescence of poly(N-vinylcarbazole) (PVCz) films doped with 1,2,4,5-tetracyanobenzene was investigated in comparison with exciplex (and/or exterplex) fluorescence of PVCz films doped with 1,4-dicyanobenzene by using the nanosecond time-correlated single-photon-counting method. For excitation of PVCz absorption band and CT absorption band, the fluorescence peak in the time-resolved spectra shifted continuously to short wavelength with time. The direction of the shift was opposite to that of the exterplex fluorescence. The dependence of the steady-state CT fluorescence intensity on concentration of the acceptor was also different from the exciplex system. These different behaviours of fluorescence dynamics between the CT and the exciplex systems were discussed from the viewpoint of the overlap of CT and exterplex fluorescence for the CT system and of a morphological change of PVCz films caused by formation of ground-state CT complexes

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The mixed crystal i.r. technique has been applied to polyethylene single crystal mats after rolling at room temperature to roll ratios up to 6.0. Reduction in the absorbance of the outer CD2 bending components provides evidence of lamellar breakup. Model calculations indicate that the size of blocks which break away from the crystals is reduced to ~ 36 A for a roll ratio of 6, while X-ray diffraction data show that crystallite reorientation occurs progressively with increasing roll ratio. Changes in small-angle X-ray scattering and the Raman longitudinal acoustic mode with rolling demonstrate that crystallite stacking periodicity is gradually lost and that crystal thickness remains unchanged.

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