Main-chain Liquid Crystalline Polymers Research Articles

Theoretical simulation of thermally induced phase separation in a main‐chain liquid‐crystalline polymer solution

AbstractPhase diagrams of main‐chain liquid‐crystalline polymer (MCLCP) solutions have been calculated self‐consistently on the basis of a simple addition of the Flory–Huggins free energy for isotropic mixing, the Maier–Saupe free energy for nematic ordering, and the Flory free energy for chain rigidity of the MCLCP backbone. The calculated phase diagram is an upper critical solution type overlapping with the nematic–isotropic transition. The phase diagram consists of liquid–liquid, liquid–nematic, and pure nematic regions. Subsequently, the dynamics of thermally induced phase separation and morphology development have been investigated by the incorporation of the combined free energy density into the coupled time‐dependent Ginzburg–Landau (model C) equations, which involve conserved compositional and nonconserved orientational order parameters. The numerical calculations reveal a variety of the morphological patterns arising from the competition between liquid–liquid phase separation and nematic ordering of the liquid‐crystalline polymer. Of particular interest is the observation of an inflection in the growth dynamic curve, which may be attributed to the nematic ordering of the MCLCP component, which leads to the breakdown of the interconnected domains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 913–926, 2003

Thermal and morphological properties of main chain liquid crystalline polymers

Temperature modulated differential scanning calorimetry (TMDSC), variable heating rate DSC, and tapping atomic force microscopy (AFM) were used to study semi-crystalline liquid crystalline polymers (LCPs). Main chain LCPs included a random copolyester (Vectra® A950) and an azomethine alternating copolymer. For the azomethine LCP the TMDSC non-reversing signal detected broad exothermic transitions associated with melting and recrystallization as the slow DSC heating scan induced surprisingly large morphological changes. Non-isothermally crystallized Vectra® and some isothermally crystallized samples at lower temperatures exhibited different levels of DSC scan induced crystal reorganization. Such crystal metastability was also studied by variable heating rate DSC and an independent technique for estimating the melting point at very rapid heating rates. The TMDSC characterization of the scan induced crystal perfection in Vectra® was substantially different than for the other polymers studied. In most cases even though crystal perfection was occurring, no clear exotherm was detected in the non-reversing signal. High temperature annealing for long times resulted in degrees of crystal perfection which could be studied by DSC with minimal scan induced reorganization. High resolution tapping AFM was used to elucidate details of crystal morphology for mechanically oriented and non-oriented Vectra® before and after annealing. Structures resembling lamellae were found to be oriented perpendicular to the chain direction in the oriented Vectra®. In the non-oriented film broad and sometimes curved ‘lamellae’ were detected. They were about 1000 nm long and between 20 and 35 nm wide, with the width increasing slightly as a function of increased annealing time at 260 °C melt crystallization conditions. Substructure of the lamellae in both oriented and non-oriented Vectra® consisted of smaller stacked crystallites which are detected by AFM studies of these surfaces.

Rheology of Nematic Side-Chain Liquid-Crystalline Polymer: Comparison with Main-Chain Liquid-Crystalline Polymer

A nematic side-chain liquid-crystalline polymer (SCLCP) was synthesized by grafting a liquid-crystalline monomer, 6-[(4-cyano-4‘-biphenyl)oxy]hexanoic acid (5CN-COOH), onto a nearly monodisperse hydroxylated polyisoprene. The linear dynamic viscoelasticity, steady shear flow, transient and intermittent shear flows, and stress relaxation of the SCLCP in the nematic state were investigated. For comparison, the rheological behavior of a main-chain liquid-crystalline polymer (MCLCP) was also investigated. Similarities and dissimilarities in the rheological behavior between the SCLCP and MCLCP are presented. Some of the important similarities observed are as follows. Upon start-up of shear flow, both SCLCP and MCLCP exhibited a very large overshoot peak of first normal stress difference (N1+ ) and shear stress (σ+ ). The values of first normal stress difference in steady shear flow were positive in both SCLCP and MCLCP over the entire range of shear rates tested. Some of the important dissimilarities observed ...

Liquid Crystalline Phases, Microtwinning in Crystals and Helical Chirality Transformations in a Main-Chain Chiral Liquid Crystalline Polyester

A main-chain nonracemic chiral liquid crystalline polymer was synthesized from (R)-(−)-4‘-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-decyloxy}-4-biphenylcarboxylic acid. This polymer contained 10 methylene units in each chemical repeating unit and was abbreviated PET(R*-10). On the basis of differential scanning calorimetry, wide-angle X-ray diffraction, and polarized light microscopy experiments, chiral smectic C (SC*) and chiral smectic A (SA*) phases were identified. Both flat−elongated and helical lamellar crystal morphologies were observed in transmission electron microscopy. Of particular interest was the flat−elongated lamellar crystals were constructed via microtwinning of an orthorhombic cell with dimensions of a = 1.42 nm, b = 1.28 nm, and c = 3.04 nm. On the other hand, the helical lamellar crystals were exclusively left-handed, which was opposite to the right-handed helical crystals grown in PET(R*-9) and PET(R*-11) (having 9 and 11 methylene units, respectively). Note that these three polymers had identical right-handed chiral centers (R*-). Therefore, a single methylene unit difference on the polymer backbones on an atomic length scale substantially changed the chirality of the crystals in the micrometer length scale. Furthermore, aggregates of these helical crystals in PET(R*-10) did not generate banded spherulites in polarized light microscopy. Possible reasons for this change and loss of helical senses (handedness) on different length scales in chirality transferring processes were discussed.

A tensor model of liquid crystalline polymers: application to basic disclination processes

One of the features of liquid crystalline polymers (LCPs) is their strong elastic anisotropy, which means they have unequal elastic constants. Elastic anisotropy plays a crucial role in the microstructure and macroscopic properties of LCPs. In this paper, the effect of unequal elastic constants on microstructure is investigated without an external field by using a deterministic tensorial approach. In this model, the evolution of the director field can be viewed as a process driven towards the state of zero elastic torque. A tensor expression of the elastic torque is used so that the nematic symmetry is automatically conserved. In simulations of bulk samples, disclination lines of strength half and escaped integer disclinations are observed. The distortion fields around the disclinations are found to depend on elastic anisotropy. If the twist constant is the lowest, as is the case for main chain liquid crystalline polymers, the disclination lines are predominantly of the twist type.

Bimodal Orientation Defects in Main-Chain Thermotropic Liquid Crystalline Polymer Fibers

The molecular orientation in as-spun fibers of main-chain thermotropic liquid crystalline polymers (TLCP) has been investigated using electron diffraction at high spatial resolution. Although a lar...

Layer Morphology of a Poly(ester imide) LCP in Different Solid States

Full Paper: A poly(ester imide) (PEI) in the smectic crystalline (SE) state exhibits both a medium-angle X-ray scattering (MAXS) peak and discrete small-angle X-ray scattering (SAXS). Based on quantitative analysis of absolute scattering data the observed SAXS is attributed to the formation of a sandwich structure in which a central, wellordered layer-shaped zone is enclosed in disordered buffer zones. Additionally the MAXS of material in the frozen liquid-crystalline SA and SB states is analysed. The results show that the SA state is characterised by long ranging correlation in the direction of the chain, whereas the lateral correlation range is short. Thus the smectic layer is considerably warped. The lateral range of order is wider in the SB state and approaches infinity in the SE state. This increase of lateral order and layer flatness goes along with considerable loss of longitudinal order. Nevertheless the range of longitudinal order remains high enough to assure correlation among the mesogens from the ordered zone with those from the disordered zones. Thus chain-folding cannot be the primary reason for the formation of ordered and disordered zones. Results indicate that asymmetry, orientation and internal twist of mesogenic groups in a liquid-crystalline main-chain polymer are important parameters controlling structure formation of the smectic morphology.

A Rouse-like model of liquid crystalline polymer melts: Director dynamics and linear viscoelasticity

We describe the linear viscoelastic response and dynamics of director orientation in monodomains of unentangled nematic liquid-crystalline polymers (LCPs), using a Rouse-like model appropriate to polymers with a significant degree of backbone flexibility, such as segmented main-chain thermotropic LCPs and side-chain LCPs. We consider the limits of: (i) main-chain directed polymers, for which the polymer backbone is extended along the director, with no hairpins, but executes a random walk in the plane perpendicular to the director, and (ii) anisotropic Gaussian polymers, for which the backbone executes an anisotropic three-dimensional random walk, as for main-chain LCPs with many hairpins and many side-chain LCPs. The analysis is based upon the assumption that, in fluids of such polymers, the distribution of nematogen orientations relaxes rapidly to a state of local equilibrium, via relatively rapid local rearrangements, which follows the slower evolution of the backbone conformation. We describe the linear response in terms of five independent dynamic moduli, which are time- or frequency-dependent generalizations of the Leslie viscosities. We find that directed chains generally tumble in steady shear flow, and that anistropic Gaussian chains flow align. In transient experiments, the coupling between director rotation and the relaxation of backbone conformations is found to lead to an overshoot of the director orientation following a step shear, and a phenomenon of “director recoil” in response to the temporary application of a magnetic aligning field.

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid-crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra-thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid-crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole-conducting alignment layers in polymer light-emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole-transporting molecule (HTM) have been mixed into the PAP layer. These hole-conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.-%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m2 and an efficiency of 0.3 cd/A could be realized.

LC Ionomers with Phosphonate Groups and Their Multilayer Build-Up

This paper describes the synthesis of new liquid-crystalline (LC) ionomers (semiflexible LC main-chain polymers with ionic groups as side groups) by polycondensation of substituted malonates with mesogenic diols. Phosphonic acids were used as negative groups, and phosphonium and ammonium ions as positive groups. The presence of these ionic groups leads to gelation (physical crosslinking) of the polymers, which display a rubbery consistency. As expected from the reference polymers without an ionic group, all polymers show smectic phases. These phases are destabilized by bulky side groups (e. g., phosphonium ions or phosphonic acid esters). They are, however, stabilized by phosphonic acid groups and the corresponding salts. Together with X-ray measurements this supports the assumption that different smectic sublayers are formed for the mesogens and the ionic groups. Dielectric relaxation measurements show four different relaxation processes for these polymers. They are, with rising temperature, the χ- and the β-relaxation (local relaxations, active below Tg) and an α- and a δ-relaxation above Tg. The α-relaxation corresponds to the dynamic glass process and the δ-relaxation is due to a reorientation of the mesogenic groups around their short axis. Despite the fact that these LC ionomers have only one ionic group per repeating unit (one group per 3.3 nm), they could be successfully used for a multilayer built-up by electrostatic deposition. The resulting multilayers are rather hydrophobic, if the LC ionomer is the outermost layer.

Liquid crystalline elastomers as artificial muscles

AbstractNew nematic co-elastomers were synthesized which consist of a combination of nematic side- and main-chain polymers and where the liquid crystalline main-chain polymer can be considered as a macromolecular crosslinking component. By applying a mechanical field during the crosslinking process, the director of the nematic phase becomes macroscopically uniformly aligned and liquid-single-crystal elastomers were obtained. Within a narrow temperature regime, these networks can change their shape in one dimension by a factor > 3 and considerably improve the ability of being used as artificial muscles or mechanical actuators compared to the well known nematic side-chain elastomers. Networks having a non-ordered polydomain structure exhibit “soft (or semi-soft) elasticity” which is not only determined by a director reorientation process. Obviously, hairpin conformations within the main-chain segments contribute to this effect.

Molecular orientation in quenched channel flow of a flow aligning main chain thermotropic liquid crystalline polymer

We report measurements of molecular orientation in solid specimens of a main-chain thermotropic liquid crystalline polymer (LCP) that were quenched from mixed shear-extensional channel flows. The polymer under investigation is a random copolyether with mesogens separated by flexible hydrocarbon spacers. This polymer is known to exhibit ‘flow aligning’ dynamics under slow shear flow. Experiments were designed to preserve the molecular orientation state, representative of steady, isothermal channel flow in the solid samples, so that comparisons could be made against in situ channel flow measurements on other main chain thermotropes without flexible spacers, including a commercial fully aromatic copolyster. In the flow aligning material, little change in orientation was found in slit-contraction flows, and only modest drops in orientation were found in slit-expansion flow. This contrasts strongly with data on commercial LCPs, suggesting that these materials may be of the ‘tumbling’ type.

Conducting blends of polyaniline and aromatic main-chain liquid crystalline polymer, XYDAR SRT-900

Conductive polyaniline/fully aromatic main-chain liquid crystalline polymer, XYDAR SRT-900 films have been prepared by solution blend of polyaniline (PANI) emeraldine salt and the liquid crystalline polymer. The polymer blends exhibit excellent mechanical properties and good conductivity at moderate PANI concentration. Moreover, polymer films have optical transparent property at concentration lower than 10 wt.%. The morphology of the blends has been investigated using scanning electronic microscopy and X-ray scattering.

PHASE SEPARATION KINETICS OF MAIN-CHAIN LIQUID CRYSTALLINE POLYMERS

We investigate phase separation kinetics of main-chain liquid crystalline polymers (LCP's) by numerical calculations based on a continuum description. We introduce two order parameters, the density and the orientational order of LCP's and construct a set of time-dependent Ginzburg-Landau equations to describe the dynamics of these two order parameters. We show that the morphology of the phase-separated system is crucially influenced by the coupling between the density and the orientational order which appears in the kinetics as well as in the free energy.

The Effect of -(C6F4)- on the Surface Free Energy of Main-Chain Liquid Crystalline and Crystalline Polymers

Novel fluorinated main-chain liquid-crystalline/crystalline polymers were prepared through thin film polymerization to investigate the effect of –(C6F4)– on the surface free energy. The fluorine in the phenyl rings does not lower the total surface free energy of the thin copolymer films, compared to those without fluorine. Interestingly, the Lewis acid components (γ+) of the surface free energy of the fluorine-containing polymers increase with an increase in the –(C6F4)– content, indicating the increasing electron accepting character of the surface.

Miscibility of a nematic liquid crystalline polymer pair

This work investigates the miscibility of two commercially important thermotropic main chain liquid crystalline polymers (LCPs), Vectra A950 and Vectra B950, using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and positron annihilation lifetime spectroscopy (PALS). Although previously reported to be either miscible or show a compositional-dependent miscibility, they are shown here to be immiscible based on DMA, DSC and PALS results. The latter technique is somewhat more novel in its use to assess miscibility, by probing the free volume of the blends and comparing this to rule-of-mixtures values of the two components.

Main chain liquid crystalline polymers with laterally linked organometallic mesogens

The synthesis and characterization of the phase behaviour of a homologous set of polymers of 1,m-diamino-n-alkanes with bis[4-(4-alkoxybenzoyloxy)-2-hydroxybenzaldheyde]copper(II) is reported. The structural peculiarity of these polymers, which is indicated in the title, stems from the presence of metal-complexed salicylideneaminato segments connected via the nitrogen atoms. Calorimetric analysis, polarizing optical microscopy, X-ray diffraction and dynamic viscosity measurements indicate the occurrence of nematic mesomorphism for all the polymers-including two structural analogues containing Ni(II) or Pd(II) in place of Cu(II).

Shear Aligning Properties of a Main-Chain Thermotropic Liquid Crystalline Polymer

We report the first direct, quantitative measurements of the shear aligning properties of a main-chain thermotropic liquid crystalline polymer (LCP). We find that a model thermotrope with alternating mesogen and spacer structure is of the shear aligning type throughout its nematic range. The director rotates uniformly in the shear flow toward the Leslie alignment angle as probed by in situ flow conoscopy. The Leslie alignment angle becomes progressively closer to the flow direction as temperature decreases, corresponding to a decrease of the tumbling parameter λ with increasing order parameter S. Our measurements of λ(S) enable direct comparison with predictions from molecular models, which predict that shear alignment prevails in the limit of flexible nematic chains. This is in direct contrast to rodlike lyotropic LCPs for which director tumbling is the rule.

Shear Stress Overshoots in Flow Inception of Semiflexible Thermotropic Liquid Crystalline Polymers: Experimental Test of a Parameter-Free Model Prediction

We explore the origins of shear stress overshoots upon flow inception of main-chain thermotropic liquid crystalline polymers from a random polydomain initial condition. A simple polydomain simulation strategy, based on Ericksen's transversely isotropic fluid model, is capable of predicting a shear stress overshoot arising from the reorientation dynamics of domains toward a steady state flow aligned state. When model parameters are estimated using elementary molecular theories for semiflexible main-chain thermotropes, both the relative magnitude of the shear stress overshoot and the steady state ratio of first normal stress difference to shear stress are determined solely by the “tumbling” parameter, λ. This implies a universal correlation between these two variables. We test this prediction against an extensive data set on semiflexible polyesters and find good qualitative agreement between model and experiment.

Shear-induced band texture of side group liquid crystalline polymers

The shear-induced band texture of conventional end-on fixed side group liquid crystalline polymers (LCPs) has been investigated by using polarizing optical microscopy (POM), small angle light scattering (SALS) and infra-red dichroism techniques. The band spacing is about 1 μm, which increases very slightly on increasing the temperature of shearing and is independent of shearing rate within the range studied. The band texture is not seen to exhibit an interchange of dark and bright bands on rotation of the sample with respect to the polarizer/analyser, but a typical periodical structure is reflected by the SALS patterns of the band texture. The relaxation behaviour of the bands indicates that the band texture formed here is the result of the orderly aligning of domains exhibiting the focal-conic texture, and this is totally different from the case of main chain LCPs where the band texture is substantially an optical effect of the periodic zigzag or sinusoidal structure of parallel aligned microfibrils. Infra-red dichroism and rotating parallel-plate shearing measurements show that the axes of the backbone of the polymer tend to orient in the shearing direction and the end-on fixed mesogenic side groups tend to align perpendicular to the shearing direction.