Thermotropic Liquid Crystal Polymers Research Articles

Self‐Reinforced melt processible polymer composites: Extrusion, compression, and injection molding

AbstractRheological properties, extrusion, fiber spinning, compression, and injection molding of blends of polycarbonate and two thermotropic liquid crystal polymers based on wholly aromatic copolyesters have been studied. Blends were prepared using an internal Banbury mixer and static Koch mixer. Based upon differential scanning calorimetry and dynamic mechanical measurements, these blends have been shown to be incompatible in the entire range of concentrations. During extrusion and injection molding at high strain rates, it has been observed that thermotropic liquid crystal polymer at concentrations 2.5, 5, and 10 percent by weight in situ forms high modulus and high strength fibers within the polycarbonate matrix leading to self‐reinforced polymer composites. The tensile strength of the composite containing 10 percent of liquid crystal polymer exceeds that of the pure components. In addition, anisotropy of properties of the injection molded parts has been found to substantially reduce in a comparison with that of liquid crystal polymer. The processing conditions and technique for the production of self‐reinforced polymer composite during processing of the blends have been identified. This has been done by measurements of mechanical properties, direct observation of morphology, and by theoretical calculation using simplified composite theory for the unidirectional continuous fiber‐reinforced composites. At the high concentrations, 25 and 50 percent by weight, thermotropic liquid crystal polymer forms large spherical droplets inside polycarbonate leading to highly brittle material. This is in distinction from the fibrous, high modulus tough composites formed at the lower concentrations.

Viscoelastic properties of thermotropic liquid crystal polymers from analysis of dynamical characteristics

The rheological properties of liquid-crystal polymers with side-chain mesogenic groups are studied by means of small-amplitude periodic deformations in the mesophase and in the isotropic melt.

Novel approach for the processing of thermotropic liquid-crystal polymers. 1. Thermodynamics and solid-state transesterification

Diagramme de phases de poly(bisphenol E isophtalate-co-naphtalate) dans un ester phenyle p-substitue de l'acide terephtalique, bis(p-methoxycarbonyl) phenyl) terephtalate

Molecular correlations in sheared thermotropic copolyesters showing banded textures

Banded textures produced in a thermotropic liquid crystal polymer by shearing between glass slides are examined by using both transmission electron and polarized light microscopy. The periodic variation in director orientation about the shear axis, as measured by light microscopy, is shown to be distinctly different from that indicated by electron diffraction. Measurements of birefringence and observation of Zernicke phase contrast indicate periodic variations in optical properties of the polymer, in step with the bands. Such effects are accounted for in terms of a synchronous rotation of the planar aromatic groups about the molecular chain axes. Evidence for an out-of-plane component of molecular orientation is also presented.

Optical textures observed during the shearing of thermotropic liquid-crystal polymers

We report optical observations of a number of main-chain thermotropic liquid-crystal polymers. In situ measurements were carried out at elevated temperatures using an apparatus that is capable of providing a controlled translational oscillatory shearing motion to samples of typically 1–10 µm thickness. In static samples we observe either a general birefringence with local scattering from line defects which we believe to be disclinations, or in thicker samples a texture dominated by light scattering due to the presence of dence disclinations. Superimposed shearing appears to result in both the multiplication of disclinations and the progressive decrease in the distance between individual disclinations. At high shear rates pure birefringence in the direction of flow is observed. Finally we report on the optical relaxation behaviour of oriented samples together with an explanation for the skin core effect observed when thermotropic liquid-crystal polymers are extruded through dies.

Structure–property relationships in main chain liquid crystal polyesters with flexible spacers

AbstractInvestigations have been carried out in this laboratory over the past four years on the structure—property relationship in main chain, thermotropic liquid crystal polymers containing a variety of different types of mesogenic groups and flexible spacers. Our goal has been to relate the effect of polymer structure to the type of mesophase formed in the melt and to the melting and clearing temperatures of the mesophases. Many interesting, unusual, and unexpected results have been obtained in our survey, and this review is concerned with our observations on the molecular factors which determine the types of mesophases formed, nematic or smectic, and their transition temperatures and behaviors.

Synthesis and Properties of Thermotropic Liquid Crystal Polymers with Main Chain Mesogenic Units

The molecular variables which control the structure-property relationships in thermotropic liquid crystalline polyesters are under investigation in this laboratory. A wide variety of polymers based on rigid, linear aromatic ester mesogenic units, with and without flexible or rigid spacers, have been prepared and characterized for their ability to form a liquid crystalline melt, the type of phase formed, their transition temperatures, and the morphology of the mesophase. Flexible spacers reduce both the melting and clearing temperatures, and the type and length of the spacer can determine whether either a nematic, cholesteric or smectic phase is formed. Variations in the structure of the rigid mesogenic group, both in the specific type and arrangement of the aromatic ester groups and the presence of pendant substituent, also cause profound changes in the properties of the mesophase melt formed.

The Preparation and Properties of the α,ω-bis(4,4′-Cyanobiphenyloxy)Alkanes: Nematogenic Molecules with a Flexible Core

Abstract The first twelve members of the homologous series of α,ω-bis(4,4′-cyanobiphenyloxy) alkanes have been synthesised. The compounds are nematogenic, although the mesophases for the first and third members of the series are monotropic. Both the nematic-isotropic transition temperature and the entropy of transition exhibit a pronounced dependence on the length of the flexible core; this is analogous to that found for main chain thermotropic liquid crystal polymers.

Macromolecular composites of extruded thermotropic polymer sheets

Abstract Using two types of thermotropic liquid-crystal polymers, experimental results reconfirmed previous reports that the relaxation time of extruded liquid-crystal polymers is much longer than that of conventional polymers. The initial modulus and elongation of sheets extruded with liquidcrystal polymers were found to be strongly dependent on the gauge length. In addition, it was observed, for the first time, that the mechanical properties of the extruded thermotropic copolyesters follow the general equations for fiber-reinforced composites such that the angular dependence of the tensile strength of the extruded liquid-crystal polymeric sheets obeys the Tsai-Hill theory, while the angular dependence of the initial modulus follows the Lees equation. These results suggest that the highly oriented liquid-crystal domains may be considered as reinforcing fibers in the extruded articles, and their mechanical properties can be predicted using existing composite theories.

Shear Induced Optical Textures and Their Relaxation Behaviour in Thermotropic Liquid Crystalline Polymers

Abstract We describe and classify optical textures observed in thermotropic liquid crystal polymers at elevated temperatures. The evolution from the static state to different textures observed as a consequence of controlled shearing experiments is reported together with the subsequent relaxation behaviour at the cessation of flow. Some of the observations show similarities with the behaviour of the phases of small molecule liquid crystals whilst others appear unique to liquid crystal polymers.

Transmission loss of thermotropic liquid-crystal polyester-jacketed optical fibre at low temperature

The transmission loss at low temperature of an optical fibre tightly jacketed with a new low-linear-expansion-coefficient plastic, thermotropic liquid-crystal polymer, is reported. The new type of tight-jacket fibre is free from microbending losses due to jacket contraction when exposed to heat cycles from ?70 to 80°C.

Orientation development in thermotropic liquid crystal polymers

AbstractThermotropic liquid crystal polymers are a new class of polymeric materials that consist of rigid backbone molecules and thus, even in the quiescent condition, take extended chain conformation to form optically anisotropic melts. A systematic investigation was carried out on how this type of material responds to two basic flow fields: shear and elongation. Rheological properties of the polymer in these flow fields have also been measured. It was found that a high level of molecular orientation was readily obtained by elongational flow but not with shear flow. Specifically, extraordinarily high orientation was obtained when the melt was subjected to small elongational strains, whiel shear strain or shear rate had little effect. A possible mechanism to explain these behaviors is illustrated based on the existing observations or theories of rodlike molecules. This finding was used to interpret the orientation distribution in the extruded and injection‐molded articles.

Thermal Analysis, Mechanical and Rheological Behaviour of Melt Manufactured Polyethylene/Liquid Crystal Polymer Blends

Modification of properties of conventional thermoplastics with thermotropic liquid crystal polymers, from one hand, allows decrease their viscosities, substantially facilitating processing conditions, and, from another hand, allows increase their exploitation properties. Orientation of the labile structure of liquid crystal polymer in extrusion or injection moulding causes specific reinforcement (so-called self-reinforcement) to occur in the blends containing liquid crystal polymer. Up to now the effect of self-reinforcement is mostly investigated in the blends, containing considerable amount of liquid crystal polymer. In this research the effect of minor amounts of liquid crystalline co-polyester modifier on the properties of polyethylene is investigated. Various compositions of laboratory synthesized hydroxybenzoic acid /polyethylene terephtalate copolymer containing polyethylene composites have been manufactured by thermoplastic blending. It has been observed that 1) modulus of elasticity, yield strength and ultimate strength increase with raising the content of liquid crystalline modifier; 2) void content in the investigated polyethylene/liquid crystal copolymer composites is not greater that 1 %; 3) addition of liquid crystalline co-polyester modifier improves arrangement of PE crystalline phase.http://dx.doi.org/10.5755/j01.ms.17.2.483