1994 рік

The synthesis and properties of poly(3,4-ethylenedioxythiophene) (PEDOT) are reported. The polymer shows a low band-gap, 0.5 eV lower than that ofpolythiophene. This leads to very pronounced electrochromic properties since the doped polymer is almost transparent, with a sky-blue tint, and the neutral polymer is blue-black. The low band-gap also leads to the possibility of n-doping by electrochemical methods. The conductivity of the doped polymer is up to 200 S cm- 1, and the stability is remarkable, much better than observed in our studies of the polythiophene and polypyrrole families.

Детальніше...  

The effect of a graft copolymer based on cellulose (CE) and polyacrylonitrile (PAN) on the compatibility of the two homopolymers in solution was analysed. Samples of copolymers containing different proportions of CE and PAN were synthesized. In order to reduce the degradation of the cellulose chain we tested different synthetic routes. After extraction of PAN homopolymer, the reaction products were composed of pure CE and copolymer; the percentage of PAN with respect to the total weight varied between 13 and 32.2%. A significant increase in compatibility was obtained by using graft copolymers in mixtures with the homopolymers, as evident from both ternary phase diagrams and morphology of solution-cast films. Moreover, solutions containing the graft copolymer showed a liquid crystalline phase at high concentration

Детальніше...  

The phase behaviour of blends of glycidyl methacrylate/methyl methacrylate (GMA/MMA) copolymers with styrene/acrylonitrile (SAN) copolymers and of GMA/styrene copolymers with tetramethyl polycarbonate and poly(2,6-dimethyl-l,4-phenylene oxide) has been determined and analysed using two theoretical approaches. Binary interaction densities for repeat-unit pairs were evaluated from the lower critical solution temperature type phase behaviour using the lattice-fluid theory of Sanchez and Lacombe and from copolymer composition miscibility boundaries using the Flory Huggins theory. For the GMA/MMA-SAN system the interaction energies obtained by the two approaches agree quite well with each other and with independent information obtained by the critical molecular-weight method.

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Major progress has been made in preparation of high-modulus and high-strength fibres and films by uniaxial drawing of semicrystalline polyolefins. Solid-state coextrusion followed by tensile drawing on ultra-high molecular weight polyethylene (UHMWPE) and polypropylene (UHMWPP), and poly(4-methyl- I-pentene) starting from single-crystal mats, prepared from dilute solutions, produces superdrawn films with tensile moduli approaching the theoretical values reported for the crystal for each of these three polyolefins. For PE, a modulus of 220GPa and strength of 6GPa have been achieved. For PP, the corresponding maximum values are 37 and 2.2 GPa. This two-stage drawing technique has also been successfully applied for ultradrawing PE reactor powders. Films of compacted powder, prepared below the melting point of the powder, have been solid-state coextruded at 110°C to a draw ratio of 6, followed by tensile drawing at 120-135 C. The drawability and the uniformity of the resultant fibres and films are sensitively affected by the drawing temperatures and rates, and also by the choice of reactor powder. The maximum draw ratio achieved directly for reactor powder is ~ 85, with a corresponding tensile modulus of ~< 130 GPa. For special conditions, high uniaxial draw and high modulus for selected reactor powders have been achieved simply by direct compaction plus calendering. In none of these processes is the polyolefin melting point exceeded. Applications of the oriented polyolefins are cited in terms of use as tapes, yarn and fabrics.

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The behaviour of a quenched ethylene-propylene block copolymer cast film has been investigated during melting and recrystallization experiments carried out at heating and cooling rates of 10°C min-1. The experimental techniques used to follow the melting and recrystallization processes were differential scanning calorimetry (d.s.c.) and simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). In particular, the WAXS results show that the melting of the material is preceded by a solid-state transition involving transformation of the quenched structure into the monoclinic ~ modification. This transition appears as an exotherm in the range 65-120°C on the d.s.c, melting trace. SAXS results show that an increase in the amount of long-range order occurs during this transition. The sample was then found to melt at 162°C. Upon cooling from the melt, recrystallization of the monoclinic ct modification was found to occur in the range 123-105°C without any apparent prior formation of the quenched structure.

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Semicrystalline segmented polyurethanes based on hydrogenated polybutadiene soft segments (H-PBD) and butanediol (BDO) as chain extender were synthesized in bulk. Because of the initial immiscibility of the reactants, macroscopic phase separation occurs during a two-step polymerization in bulk. As a previous work on the same materials prepared in polar solvent has shown the high level of microphase separation with such polymers, the challenge of the present work is also to obtain a high level of microphase separation for bulk-prepared polyurethanes. For this purpose the synthesis of polyurethanes in bulk is carried out in the presence of different emulsifiers to minimize the effects of the premature macrophase phenomenon. The morphology of such polyurethanes with non-polar soft segments can be well characterized by dielectric measurements. The dielectric relaxations confirm the thermal behaviour and also the presence of a mixed interfacial region. The addition of emulsifiers to bulk samples decreases the number of short isolated or amorphous hard segments, as shown by size exclusion chromatography results, and suppresses the related conduction process appearing in the dielectric measurements. The results show the control of the structures by appropriate synthesis and the monitoring of phase separation using dielectric spectroscopy.

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The new iterative linear least-squares method for calculation of monomer reactivity ratios for the terminal model, proposed by Mao and Huglin, is applied to the penultimate model system with r2 =0. The principle and the calculation procedure are similar for the two different models. All advantages of the new method when applied to the terminal model, i.e. fast convergence, exact calculation and the fact that any initial estimation of reactivity ratios always leads to the same correct results, remain valid when applied to the penultimate model. The superiority of the new method over the Kelen-TiidSs and extended Kelen-Tiid6s procedures is even more evident for the penultimate model than for the terminal model.

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A series of polyphosphazenes with ester side groups was prepared from a derivative of poly(methylphenylphosphazene) that contained alcohol functional groups. The synthesis involved treatment of {[Ph(Me)P=N]z(Ph[MezC(OH)CHz]P=N)} . with acid chlorides, RC(=O)C1, where R=(CHz),CH 3 (n=2, 4, 6, 8, 10, 12 and 14) in the presence of triethylamine. These new polymers were characterized by 1H, 13C and 3~p n.m.r, spectroscopy, i.r. spectroscopy, size exclusion chromatography and thermal analysis (d.s.c. and t.g.a.). The systematic dependence of the glass transition temperatures of these new polymers on the number of methylene groups is discussed.

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Wide-angle X-ray scattering studies on equibiaxially deformed isotactic polypropylene (i-PP) have been carried out. The equibiaxial deformation is achieved by uniaxial compression below the melting point at 60 and 140°C. The study proves the equibiaxiality of the deformation with the observation of two types of crystal textures referred to as planar and fibrillar. These textures are quantified by relating the intensity distribution for a (110) reflection to the orientation distribution of the crystals. The signature of the fibre-like texture in the intensity distribution (of a given (hkl) reflection) is obvious; the manifestation of the planar texture requires a closer analysis of the pole figure. An 'exact method' of formulation is used to estimate the relative fractions of the two textures. The quantitative change in the relative fraction as a function of compression ratio is explained in terms of a qualitative model based on slip systems. The mechanism of the texture developed in i-PP as a function of compression ratio is discussed in the light of crystal slip systems and the order~lisorder transition during the deformation process. The latter phenomenon is shown to be a favourable route to obtaining a high chain orientation and a larger amount of the planar texture relative to the fibrillar texture

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The thermal expansion behaviour of unmodified and siloxane-modified epoxy resins has been investigated to clarify the mechanism by which siloxane modifiers lower the thermal expansion coefficient of epoxy moulding compounds containing inorganic fillers. A mechanism that accounts for all the observed characteristics is proposed. The thermal expansion coefficient of epoxy resins is increased by siloxane modifiers with carboxyl end groups, and the degree of the increase is dependent on the molecular weight of the modifiers. This increase is considered to be due to two effects: an increase in the local free volume in the epoxy matrix caused by the development of thermal stresses, and the building of siloxane molecules into the network structure. For sufficiently low molecular weight siloxane modifiers the latter effect is dominant, whereas with increasing molecular weight the former effect dominates. The increased coefficient of epoxy resins due to siloxane modifiers is likely to result in thermal stress near the filler-polymer interface because of an increased difference in the thermal expansion coefficient between the materials. This is one factor causing siloxane-modified epoxy moulding compounds to have a lower thermal expansion coefficient than unmodified ones.

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