U. Pedretti

Synthesis and structure of a π-arene complex of uranium(III) - aluminum chloride

Abstract The synthesis of a π-arene uranium(III) complex with aluminium chloride obtained from UCl 4 , AlCl 3 and Al in benzene, is reported. The isolated big black crystals are thermally stable up to 100°C and are immediately decomposed by moisture. The crystal and molecular structure of the complex is reported as determined by single crystal X-ray method.

The synthesis and nuclear magnetic resonance investigation of the structure and chemical dynamics of new anionic tetra-allyl complexes of lanthanide ions

Abstract The synthesis and spectroscopic characterization of LiLn(allyl) 4 dioxane (Ln = Ce, Nd, Sm, Gd, Dy) from LnCl 3 and Sn(allyl) 4 are described. The dynamic behaviour of LiSm(allyl) 4 at high and low temperatures was studied and activation parameters were determined. An unusually pronounced broadening of line-width was observed at low temperature for LINd(allyl) 4 and LiCe(allyl) 4 complexes.

Extrusion, spinning and injection moulding of blends of poly(ethylene terephthalate) with liquid crystalline polymers

Abstract Blends of poly(ethylene terephthalate) (PET) and two different types of liquid crystalline polymers (LCP) have been prepared with different processing operations and characterized. The processability of PET improves in all processing operations by adding small amounts of LCP. Both melt and solid properties of the blends seem mainly to depend on the ratio between the viscosity of the matrix and that of the LCP. Indeed, fibrillation of the LCP particles, and then the improvement of the mechanical properties, is possible only when the viscosity of the LCP is lower than that of the flexible matrix, also in elongational flow. The adhesion between matrix and LCP particles depends on the structure of the LCP. The presence of flexible spacers in the chain of the LCP improves the adhesion with the thermoplastic matrix.

CHARACTERIZATION OF BLENDS OF POLYPROPYLENE WITH A SEMIRIGID LIQUID-CRYSTAL COPOLYESTER

Abstract Blends with a liquid-crystal polymers (LCP) as one component show, in general, very interesting properties. Reduction of shear viscosity and improvement of mechanical properties are very remarkable. High melting temperatures and high costs of the LCP limit the use of these blends. A new class of thermotropic LCPs with flexible spaces, with relatively low melting temperatures, can overcome the first problem. In this work, rheological and mechanical properties of blends of polypropylene with low contents of this LCP are presented. Torque during extrusion and viscosity decrease with LCP content. Elastic modulus is remarkably increased when the LCP phase is oriented.

Mechanical and Electrical Properties of Highly Oriented Ribbons of Polyacetylene

Abstract Films of polyacetylene have been prepared which can be converted into highly oriented ribbons (HOPA) with high tensile strength (up to 150 MPa). Iodine-doped HOPA shows an electrical conductivity one order of magnitude higher than that of unstretched films.

Effects of filler type and mixing method on the physical properties of a reinforced semirigid liquid crystal polymer

Abstract Semirigid liquid crystalline polymers (LCPs) show some advantages with respect to rigid LCP: in particular lower processing temperatures, better compatibility with flexible thermoplastics, but also some disadvantages. Thermal resistance of rigid LCPs is much better and the maximum working temperature is, in general, very high. The thermomechanical properties can be improved by adding inorganic fillers. In this work processing, mechanical and thermomechanical properties of a filled semirigid liquid crystal copolyester are reported. Several inorganic fillers have been used in order to put in evidence the influence of shape and dimensions of the particles on the properties of the filled materials. Both elastic modulus and tensile strength improve by increasing the filler content while the elongation at break decreases. The more eclatant improvement, however, arises from the creep and from thermomechanical behaviour. The creep rate is drastically reduced and the maximum working temperature is remarkably increased. The glass fibre filled polymer displays the best results. The only negative feature is the rise of the power necessary to process the material.

Effect of polymerization conditions on the microstructure of a liquid crystalline copolyester

The melt polycondensation of mixtures of sebacic acid (S), 4,4′-diacetoxybiphenyl (B), and 4-acetoxybenzoic acid (H), carried out for the synthesis of semiflexible liquid–crystalline copolyesters referred to as SBH 1 : 1 : x, has been studied with the aim of clarifying the effect of the reaction conditions on the microstructure and the thermal properties of the products. It has been shown that the segregation of a liquid–crystalline phase within the polymerizing mixture, coupled with the thermodynamic tendency of the two phases to undergo compositional differentiation as polymerization proceeds, is responsible for the formation of blocky, rather than ideally random, copolyesters with poor processibility, when the mole ratio of H to the other two monomers is higher than x ≈ 1.90. The results of this study have shown that this unwanted effect can be considerably limited by carrying out the polycondensation at a relatively high temperature from the very beginning, rather than by the standard technique involving progressive heating of the reaction mixture, thus allowing the production of SBH copolyesters with a higher degree of aromaticity. The results are discussed in terms of the relative rates of the condensation reactions, which are responsible for chain growth, and of the concurrent acidolysis and esterolysis reactions leading to copolyester sequence reorganization.

Characterization of a Fiber-Reinforced New Fully Aromatic Liquid Crystalline Polymer

A new rigid liquid crystalline polymer that demonstrates good mechanical and thermomechanical properties and acceptable processing temperature has been recently synthesized and characterized. The melting temperature and some thermomechanical properties do not seem, however, suitable for use in some applications at high temperature or in the presence of aggressive fluids. The use of appropriate inorganic fillers can overcome these shortcomings. In this work the new LCP, polymerized in a bench-scale apparatus, and the same sample filled with carbon fibers are characterized. The properties of the carbon fiber filled LCP are compared with those of another filled wholly aromatic commercial LCP, a carbon filled semirigid LCP, and the same semirigid LCP reinforced with glass fibers. In general, the mechanical properties of the filled LCP sample increase remarkably when the filler concentration is about 20%; however, with further increases in filler content, a decrease is observed. The thermal and chemical resistance of this new rigid LCP seems slightly influenced by the addition of fibers, while the thermal resistance of the other two LCP samples is remarkably improved.

Processing and characterization of blends of fluoroelastomers with semirigid liquid crystal polymers

Fluoroelastomers (FEs) usually have working temperatures above 150°C and a great resistance to aggressive agents such as oils, fuels, aliphatic and aromatic solvents, steam, moderate acid, and basic environments. Liquid crystal polymers (LCPs) can be effective processing aids and reinforcing agents for elastomers. These characteristics are very attractive to lower melt viscosity and to stiffen and strengthen the final product through a simple blending. Among the LCPs, the semirigid LCPs seem the most appealing for blending with flexible thermoplastics (FTs) because their processing temperatures can be arranged to be in the same processing temperature range of FTs and because the presence of flexible segments can improve the compatibility with the flexible matrix. This is very important especially for FEs that are subjected not only to mechanical degradation, but also to thermal degradation. Blends of FEs with two types of semirigid LCPs show easy processability and enhanced mechanical and thermomechanical properties. These improvements were observed both for pure FEs and for filled vulcanized systems. The improvement of the mechanical properties is below that expected on the basis of the additive rule due to the incompatibility between the components; nevertheless, an impressive increase of the elastic modulus, up to 40 times, was noticed by adding 20% of LCP. In vulcanized systems, the tensile strength is also increased, whereas the elongation at break is slightly reduced. The working temperature is also drastically increased both in vulcanized and in unvulcanized systems.

Synthesis and Characterization of a New Fully Aromatic LCP

Abstract Liquid crystal polymers (LCP)are a relatively new class of polymers with very interesting and unusual properties. In particular, easy processability and outstanding mechanical and barrier properties make them very attractive. Unfortunately the high stiffness of their chains often leads to compounds having very high melting points and consequently to unprocessable materials. By adding a flexible segments in the chain backbone the LCPs become processable at lower temperatures but lower properties are displayed. A new rigid liquid crystalline polymer, having excellent physical properties, good processing temperature and a good cost/performance compromise, is described in this work. The properties of this liquid crystalline polymer are compared with those of a commercial rigid LCP and with those of a semirigid one.