Thermotropic Main-chain Liquid Crystalline Polymers Research Articles

Modification of rheological properties of a thermotropic liquid crystalline polymer by melt-state reactive processing

Thermotropic main-chain liquid crystalline polymers typically have very low melt viscosity with strong temperature dependence compared to other common thermoplastics. While this is beneficial in some processing applications, such as injection molding, it presents challenges for others, such as coextrusion. In this study, the rheological properties of a thermotropic main-chain liquid crystalline polymer (Vectra A950) were enhanced by melt-state reactive processing with triphenyl phosphite (TPP), which can react with up to three polymer chain-ends through their chain-end functionalities. The influence of processing time and TPP content on the shear viscosity and other important material properties were investigated. Optimal conditions, which increased the shear viscosity by nearly a factor of 20 over the neat polymer, were found to be 4 wt% TPP and 30 min of reaction time at 290 °C. Further results from differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy confirmed that coupling with TPP did not affect the microstructure, melting/crystallization behavior or liquid crystallinity. The stability of TPP-modified samples was also studied at 80 °C in air and following melt reprocessing at 290–300 °C under N2 or air. Samples were stable (as measured by shear viscosity) for more than one month at 80 °C in air or when reprocessed in N2 at 290 °C for up to 10 min. However, when reprocessed at 300 °C in air, the viscosity enhancement was partially reversed due to scission of P–O bonds that were formed during the initial reaction between the polymer chain-ends and TPP.

Current Advances in the Carbon Nanotube/Thermotropic Main-Chain Liquid Crystalline Polymer Nanocomposites and Their Blends

Because of their extraordinary properties, such as high thermal stability, flame retardant, high chemical resistance and high mechanical strength, thermotropic liquid crystalline polymers (TLCPs) have recently gained more attention while being useful for many applications which require chemical inertness and high strength. Due to the recent advance in nanotechnology, TLCPs are usually compounded with nanoparticles to form particulate composites to enhance their properties, such as barrier properties, electrical properties, mechanical properties and thermal properties. Carbon-based nanofillers such as carbon nanotube (CNT), graphene and graphene oxide are the most common fillers used for the TLCP matrices. In this review, we focus on recent advances in thermotropic main-chain liquid crystalline polymer nanocomposites incorporated with CNTs. However, the biggest challenges in the preparation of CNT/TLCP nanocomposites have been shown to be inherent in the dispersion of CNTs into the TLCP matrix, the alignment and control of CNTs in the TLCP matrix and the load-transfer between the TLCP matrix and CNTs. As a result, this paper reviews recent advances in CNT/TLCP nanocomposites through enhanced dispersion of CNTs in TLCPs as well as their improved interfacial adhesion with the TLCP matrices. Case studies on the important role of chemically modified CNTs in the TLCP/thermoplastic polymer blends are also included.

Smectic polyester/layered silicate nanostructured hybrids: Effect of modified nanoclay in the phase transitions

The effect of incorporating different proportions of a layered silicate nanofiller into poly(diethylene glycol p, p’-bibenzoate) has been analysed. On cooling from the isotropic melt, this thermotropic main-chain liquid-crystalline polymer exhibits a smectic mesophase which is able to crystallise rather slowly by long time annealing. The results show that the nanocomposites exhibit a higher stability against thermal decomposition in oxidative atmosphere. The intercalation of polymeric chains into the clay layers is deduced from X-ray diffraction, although part of the initial silicate structure is preserved. The DSC results indicate a clear nucleation ability of the clay for both mesophase formation and crystallisation from the mesophase. The effect in this crystallisation is of major importance, causing a reduction of the time necessary to attain 50% of the total transformation by a factor of more than 4. Additional information was obtained from microhardness measurements.

Nematic Order Drives Phase Separation in Polydisperse Liquid Crystalline Polymers

In a polydisperse thermotropic main-chain liquid crystalline polymer, we observe the process of thermally induced nematic−nematic phase separation between short and long polymer chains. We study the dynamical features of this system, in particular the evolution of Schlieren textures formed by disclinations surrounding areas of relatively uniform director. We analyze the dependence of domain size ξ on temperature and the nematic order parameter, and the evolution of textures with the waiting time at a temperature well below the nematic transition Tni. We find that before phase separation the coarsening proceeds toward the uniform state, with characteristic size of the texture increasing as ξ ∼ t1/4. When the system is phase-separated, the texture in the regions with long chains is frozen at an equilibrium value ξ, a reversible function of temperature, while in the short-chain regions the coarsening accelerates. This behavior is interpreted in terms of a miscibility gap that is proportional to the degree of nematic order, which is different for the different lengths of the nematic polymers.

Chain Folding in Liquid-Crystalline Main-Chain Polymers with a Smectic Phase

Thermotropic main-chain liquid-crystalline polymers (LCPs) composed of azobenzene or azoxybenzene mesogens and flexible spacers with and without a phenyl side group have been characterized by gel permeation chromatography, differential scanning calorimetry, and X-ray diffraction. The polymers exhibit smectic phases in the well-accessible temperature range from room temperature up to 160 °C. The morphology of thin films on a solid substrate was studied by X-ray reflectometry and scanning force microscopy (SFM). The smectic layers are mainly oriented parallel to the film surface. On the basis of X-ray reflectometry and SFM investigations, it is concluded that, at least in thin films, the polymer main chains are not extended like in the common model for main-chain LCPs, but almost regularly folded.

Anisotropic ionic conduction in a magnetically aligned liquid crystalline polymer electrolyte

Sequentially ordered thermotropic main-chain liquid crystalline polymers based on modifications of terephthaloyl-bis-4-oxybenzoyl copolymerised with polyethylene glycol (TOB-PEG) have been synthesized. Materials of interest typically have a T g below ambient, a crystalline melt transition around 100°C, and exhibit a number of complex liquid crystalline phases with clearing ranges to isotropic in the region 120–180°C. The variation of ionic conductivity with temperature of complexes formed with LiCF 3SO 3 is typically in the range 10 −9 S/cm at ambient to 10 −5 S/cm at 130°C, and reveals discontinuities of slope which correlate with changes of phase in the material. Selected unsalted materials have been aligned magnetically using a 7.1 T superconducting magnet and show ordering at ambient temperatures characteristic of a smectic C type configuration. Polymer electrolytes formed with LiCF 3 SO 3 (EO/Li in the range 30 to 75) have also been magnetically aligned and bulk ionic conductivities measured parallel ( σ ∥) and perpendicular ( σ ⊥) to the alignment direction measured. σ ∥ is significantly reduced over the isotropic polymer by a factor of more than five due to the blocking smectic-type layers. When heated to the onset of the melt, σ ⊥ shows an unexpected increase attributed to the opening up of aligned conduction channels of PEO.

Structure and linear viscoelastic behavior of main-chain thermotropic liquid crystalline polymers

The influence of molecular weight on the ordering processes and linear viscoelastic properties of a series of thermotropic main-chain liquid crystalline polymers has been investigated. The polymers are wholly aromatic copolyesters based on random units of 75 mol % 1,4-hydroxybenzoic acid (B) and 25 mol% 2,6-hydroxynaphthoic acid (N). The thermal characterization, performed between −50 °C and 360 °C, showed that annealing below 290 °C gives rise to a secondary endotherm. However, only one endotherm is observed when annealing is carried out above 290 °C, showing that all transitions are completed by 310 °C. Hot-stage wide-angle x-ray scattering demonstrates that the second endotherm is associated with a solid-solid transformation from pseudohexagonal to orthorhombic. The orthorhombic phase, found on slow heating or annealing below the melting point, has a higher melting point than the pseudohexagonal phase. The increase in melting point which can result from such thermal treatments is avoided in all the experiments reported here by comparatively rapid heating of the specimens into the melt. In situ optical microscopy and x-ray scattering measurements show that the “as-molded” samples for rheological experiments exhibit preferred orientation, which is associated with their mechanical history. However, holding the sample in the molten state over periods of time leads to a relaxation of the degree of orientation, until a macroscopically unoriented (textured) state is obtained. This reduction of degree of orientation is correlated with an increase of the complex viscosity, where a plateau value is reached in the final unoriented state. The rheological characterization, on textured samples, show that B–N copolyesters exhibit a linear viscoelastic (LVE) regime similar to that observed in common flexible chain polymers. However, it is also found that this linearity extends only up to strains of about 10%, and is independent of the molecular weight. Dynamic oscillatory experiments in the LVE regime reveal a rubberlike region (minimum in delta loss angle), suggesting that the B–N thermotropic copolyesters behave like lightly cross-linked materials, adding thus support to the elastic network hypothesis.

Investigation on the formation behaviour of banded texture in thermotropic main-chain liquid crystalline polymers with linear rod-like and two-dimensional mesogenic units

The mechanism and the formation behaviour of banded texture in non-isothermal processes have been studied for two aromatic main-chain liquid cyrstalline polyesters designated as P(2,8) and PTDT-Br which contain the X-shaped and linear rod-like mesogens along the polymer backbones, respectively. For P(2,8) regular and perfect banded textures were observed within its oriented films which were prepared by shearing in mesomorphic state and subsequent cooling down to room temperature under various conditions. Bandwidth was dependent sensitively on the cooling conditions, about 8 μm for rapid and 2 μm for slow coolings. During the cooling of an oriented film the bands were first generated around 170°C, and their regularity was improved with lowering temperature. The bandwidth as well as the extinction angle of the bands were changed drastically. In such a cooling process the birefringence Δn, the difference between the refractive indices along the shear and lateral directions, of the oriented film was decreased gradually from about 0.06 to 0.03. In the case of PTDT-Br, clear and perfect banded texture could be observed only for the rapid cooling case. The banded formation behaviour was discussed on the basis of a contraction mechanism and it could be explained as the result of zigzag rearrangement of straightforward oriented fibrils under certain contraction effects. The origins of these effects were considered to be the elastic energy stored in the specimens during shearing and the thermal contraction during cooling. The latter was more evident in the rapid cooling case. The orientational relaxation of fibrils or the formation of banded texture occurs during cooling in a small temperature range after an ‘induction stage’, while the relaxation due to free thermal motion of individual molecules may proceed in the whole temperature region before the solidification of specimens.

Effect of Bulky Pendent Side Groups on the Structure of Mesophase in a Thermotropic Main-Chain Liquid-Crystalline Polymer

We investigated the effect of bulky pendent side groups on the structure of mesophase in a thermotropic main-chain liquid-crystalline polymer, poly[(phenylsulfonyl)-p-phenylene 1,10-decamethylenebi...

Influence of processing conditions on the anisotropy in injection-moulded thermotropic LCPs

The influence of injection-moulding conditions on the anisotropy in macroscopic properties of flat rectangular plates has been evaluated for a thermotropic main-chain liquid crystalline polymer. The anisotropic mechanical and thermal behaviour is correlated to the degree of molecular ordering as determined from X-ray diffraction. The geometry of the injection-moulded plates, such as their thickness and the thickness and shape of the sprue, as well as the injection speed, significantly affect the molecular ordering and the resulting anisotropy in, for example, Young's modulus and heat conduction. The tendencies observed are explained in terms of molecular alignment as a consequence of the combination of shear, elongational fountain flow and the elongational deformation in the sprue.