Liquid Crystalline Polymers

Mesogen-jacketed liquid crystalline polymers (MJLCPs) are rigid side-chain polymers that can form columnar and smectic liquid crystalline (LC) phases. Their liquid crystallinity is dependent on the molecular weights (MWs), and many MJLCPs exhibit reentrant phase behaviors. The size of the structure formed by an MJLCP, either columnar or smectic, is directly correlated with the size of the side chain. While the first MJLCPs have laterally attached mesogenic side chains, bulky side chains that are not mesogenic can also be used to construct MJLCPs. The “jacketing” effect, i.e., the steric hindrance caused by the densely packed side chains, forces the polymer backbone to take an extended-chain conformation. Many factors, such as MW, backbone flexibility, tail length, and architecture, influence the phase behaviors of MJLCPs. With the understanding of the chemical structure-phase behavior relationships, molecular design for MJLCPs can be executed by carefully choosing molecular parameters including the shape and rigidity of the side chain, the structure of the polymer backbone, and so on. MJLCPs can be synthesized by radical polymerizations that do not require highly pure solvents and reagents. Many controlled/living radical polymerization methods, such as nitroxide-mediated radical polymerization (NMRP) and atom transfer radical polymerization (ATRP), can also be employed. Recently, another living polymerization method, ring-opening metathesis polymerization (ROMP), has been applied in the synthesis of MJLCPs with a polynorbornene backbone. With the use of such controlled polymerization methods, the length of the rodlike or sheetlike MJLCP chain can be tuned by adjusting the MW. In addition, MJLCPs can also be synthesized by polymer reactions from prepolymers like poly(methylsiloxane)s. Furthermore, MJLCPs can also be obtained by taking advantage of supramolecular chemistry, with the application of hydrogen bonding, ionic interaction, and other non-covalent interactions. Future directions in the study of MJLCPs will be to address the main challenges by designing monomers that can be more easily synthesized and polymerized, establishing cost- and time-saving synthetic methods, applying calculation and computer simulation that can accurately predict phase behaviors and guide molecular design, and developing functional MJLCP materials.

Discotic liquid crystalline polymers have been attracting a great deal of interest recently due to their unusual combination of properties (self-assembly, mechanical and thermal stability as well as easy processability) as compared to conventional polymers. Due to their ability of self-organization and self-healing, these polymers are used in a variety of applications. The self-assembling nature of discotic mesogens (present in these polymers) into columnar phase helps in unidirectional charge or ion transfer in these polymers. So, we put together this entry as an overview on the recent advances in the chemistry of polymeric discotic liquid crystals (DLCs) with a particular focus on their self-assembly which is useful for the preparation of new functional soft materials. The properties of DLC polymers depend on the discotic core as well as on the polymer backbone. It has been observed that a particular set of properties can be tuned by changing core or backbone. We have categorized all the discotic polymers into six categories, and each of the category is further subdivided based on the type of core structure of the discotic molecules. The dependence of mesomorphism on the influence of position of discotic mesogens and connectivity in determining the supramolecular organization of these compounds has been discussed briefly. Overall, this entry will serve as general overview of polymeric DLCs and their structure-property relationship based on each core/backbone.

Anisotropic liquid crystal (LC) networks prepared from reactive mesogens (RMs) have numerous advantages in optoelectronic devices especially because of the excellent processability. To fabricate the robust thin LC film with excellent thermal, chemical, and mechanical stability, the photopolymerization of the anisotropically pre-oriented RMs should be conducted under optimized conditions. Since the final physical properties of an anisotropic LC network depend on chemical functions and physical intermolecular interactions, the hierarchical superstructures of the programmed RMs with specific chemical functions must be controlled on the different length and time scales before polymerization. This chapter describes the fundamental characteristics and recent research interests of anisotropic LC networks, elastomers, and gels fabricated using a variety of programmed RMs.

Molecular features that are essential to forming a columnar liquid crystalline phase in a polymer are discussed, along with the corresponding phase structure. The formation of internally short-range ordered and overall straight columns, the fundamental structure elements of the phase, has been emphasized.

Fullerene-based liquid crystalline materials contain both the excellent optical and electrical properties of fullerene and the self-organization and external-field-responsive properties of liquid crystals (LCs). They have potential applications in optical and photovoltaic devices, organic field-effect transistors, especially as active materials in polymer solar cells. Here, we have summarized the results on the design strategies for [60]fullerene LCs into two approaches, namely, the molecular LC approach and the supramolecular LC approach, respectively. The molecular LC approach is introduced first to design [60]fullerene LCs via a traditional way, where C₆₀ was linked together with large liquid crystal mesogens to give nematic, cholesteric, smectic, and columnar phases of LCs by the Bingel reaction or the 1,3-dipolar cycloaddition reaction. This strategy needs large mesogens to fulfill the molecular aspect ratio of LCs. Thus the content of C₆₀ is low, usually less than 20%, and the properties from fullerene are hindered. Another approach is via the supramolecular self-assembly, where the [60]fullerene derivatives self-assemble to form supramolecular structure which meets the aspect ratio requirements of liquid crystals. The supramolecular LC approach opens up new strategies to design liquid crystals with low molecular aspect ratio, enabling high [60]fullerene content in the supramolecular LCs. Some of the interesting properties form [60]fullerene LCs are also summarized, especially the electrochemical properties, the chiral properties, the light emitting properties, and the optoelectronic properties. Among them, high fullerene content is the key for these LCs to achieve good properties.

Liquid crystalline block copolymers have been extensively investigated in the past two decades. Liquid crystals and block copolymers possess different ordering length scales. When they are chemically or physically incorporated in one materials systems such as liquid crystalline block copolymers, hierarchically ordered structures can be achieved with the ordering scales ranging from a few to a few tens nanometers. In this chapter, we will focus on the structure and assembly of such hierarchical systems in bulk states and the competition of the two different ordering processes.

Discotic liquid crystals with well-defined columnar self-assemblies are promising for technological applications in organic electronics such as organic solar cells and field-effect transistors. In this entry, we review the self-assembly behavior of discotic supermolecules comprised of various discotic and cubic building blocks, including porphyrin, phthalocyanine, triphenylene, and polyhedral oligomeric silsesquioxane (POSS). With the help of amide hydrogen bonding, triphenylenes and porphyrins stack into the ordered “lamello-columnar” phase. Without hydrogen bonding interaction, the π-π stacking interaction leads to a hexagonal columnar phase with mixed phthalocyanine and triphenylene columns. For a POSS molecule covalently attached to eight triphenylenes, the triphenylenes and the POSS core can form a super-column when the space length is short. Self-assembly of these supercolumns can lead to a hexagonal superlattice. From this study, the spacer length, molecular geometry/size, and intermolecular interactions play an important role in the self-assembly of the discotic liquid crystalline LEGOs.

At the millimeter scale and above, liquids and viscoelastic liquids are characterized by an absence of shear elasticity at low frequency (~Hz) in contrast to solids or plastic fluids that need to exceed a stress threshold to flow. Below the millimeter scale, the dynamic response exhibits viscoelastic moduli much higher than those measured at larger scale and reveals that fluids possess finite shear elasticity at low frequency. The low-frequency shear elasticity is identified on unentangled and entangled polymers away from the glass transition, molecular glass formers, alkanes, and H-bond liquids, from several tenths to hundredths of millimeter scales. It indicates that liquid molecules are long-range elastically correlated Consequently the thermal and density fluctuations are also elastically correlated, highlighting on the liquid state new mechanisms to consider to understand the microfluidic scale. How to conciliate the low frequency shear elasticity and viscoelasticity theory when the scale goes down to the submillimeter scale? The polymer viscoelasticity theory is founded on the predominance of the molecular dynamics (major intrachain contribution). In contrast, the dynamics of simple liquids is governed by intermolecular forces. How to conciliate intra- versus intermolecular interactions when the polymer weight decreases down to simple liquids whose dynamics are governed by intramolecular interactions? What are the underlying assumptions and their limitations? This entry traces a brief history of the foundations of the viscoelasticity theory, its empirical origin, and presents new developments revealing that the conventional viscoelastic and viscous behaviors might be the asymptotic part of a much broader dynamic response of the liquid state.

This chapter presents a comprehensive review on the flow behavior of liquid crystalline polymers (LCPs). The work presented here covers the widely known synthetic LCPs and also its biological counterpart denominated as biological liquid crystalline polymers (BLCPs), which have been recently studied extensively due to their multi-functionality and their very interesting material properties. We focus on their flow behavior and the structure of these materials and its coupled dynamics. Theory, modelling, and simulation aspects are provided through two very well-known theories: (i) the Leslie-Ericksen (L-E) and (ii) the Landau – de Gennes (LdG) theory presenting the compatibility of both theories through the projection of the LdG to the L-E theory. Other aspects that are covered in this chapter are defects and textures since they are essential characteristics of these materials and they are known for affecting the flow behavior of liquid crystalline materials. An in-depth review of physical and rheo-physical defects is presented including defect nucleation and coarsening processes. A wide range of applications of the theory and simulations results are also covered which include transient shear responses, linear viscoelasticity, flow birefringence, banded patterns, and banded textures appearing after cessation of shear flow due to stress relaxation processes. Contrast and comparison with experimental data is also included in this chapter. Moreover, applications in the context new material design and development of BLCs based on reported in vivo and in vitro processes are also provided. The applied theory and simulations provide a new way to extract additional information from experimental rheological data and allow to distinguish the role of liquid crystalline properties such as viscoelasticity and anisotropy, flow-alignment, coupling between orientation, kinematics, and flow kinematics. This comprehensive chapter provides a state-of-the-art review in this field.

Main-chain-type liquid crystalline conjugated polymers and supramolecules are discussed as hairy-rod-type polymers with rigid main chain and flexible covalently bonded side chains and as hairy-rod-type supramolecules with physically bonded side chains. Thanks to the rigid main chain these materials show both thermotropic and lyotropic liquid crystalline (LC) state which allows their facile macroscopic alignment leading to anisotropic optical and electrical properties. This chapter is a brief overview on this materials class with illustrative examples including polyfluorene, polypyridine, and polythiophene.

We review the recent progress in advanced functionalities of liquid crystalline polyacetylenes (LCPAs). We focus on properties and functionalities that include electrical anisotropy and linearly polarized luminescence (LPL). First, the synthesis of mono-substituted LCPAs using Fe-based Ziegler–Natta catalyst is briefly reviewed. The mono-LCPAs exhibit enantiotropic S_A phases resulting from spontaneous orientation of the LC side chain. Iodine doping of mono-LCPA cast films followed by macroscopic alignment of the main chain accompanied by the side chain orientation using an external magnetic force of 0.7–1.0 T enhanced the electrical conductivity by two orders of magnitude to 10⁻⁶ S/cm and gave rise to a notable electrical anisotropy. Second, the synthesis of di-substituted LCPAs using metathesis catalyst that exhibit enantiotropically thermotropic or lyotropic liquid crystallinity is discussed. LC phases of di-LCPAs are assigned through observation of polarized optical microscope (POM) and differential scanning calorimeter (DSC) and measurements of X-ray diffraction (XRD). Using schematic energy levels of ground and low-lying excited states of non-, mono-, and di-PAs, the origin of the emission of substituted PAs is elucidated, and fluorescent trends including emission color are investigated. It is found that the macroscopically aligned films of the di-LCPAs emit LPL by virtue of the functionalities associated with liquid crystallinity and fluorescence. The aligned structures of the di-LCPAs are characterized in terms of main chain and side chain type alignments through XRD measurements of the macroscopically aligned polymer films. The mechanism of the LPL of the di-LCPAs with respect to the polymer structure, alignment type, and emission color is elucidated.

Collecting and amplifying the nanoscopic molecular motions into macroscopic deformation are the basic properties of crosslinked liquid crystalline polymers (CLCPs), which can even directly transfer input light energy into mechanical work when combined with photochromophores, thus fascinating many scientists. This article reviews the macroscopic and microscopic deformation of photoresponsive CLCPs based on photochemical phase transition and photothermal effect. In addition, we highlight some new methods to trigger the deformation driven by visible and infrared light instead of ultraviolet one, such as chemical modification of azobenzene and addition of upconversion materials.

Photopolymers make a distinct class of materials that have the ability to undergo physical and chemical change by the action of light. This has another important aspect in that the light stimulus can be applied from remote location without coming in contact with the substrate. At the outset of this chapter, introduction has been made on various combinations and architectures of the photoactive liquid crystalline polymers. The importance of photoresponsive LC polymers in holograms making and their important types are discussed. This is followed by discussion on photomechanical properties of photoresponsive LC polymers, composites, and elastomers. Individual mechanical change in multiple addressable segments of a system is discussed as this will enable future smart complicated functions. Liquid single crystalline elastomers with superior optomechanical properties have been discussed. Photoresponsive LC polymer composites comprising polymer-stabilized liquid crystals (PSLC) and polymer-dispersed liquid crystals (PDLCs) have been briefly presented. This is followed by deliberations on the photoactive LC anisotropic gel and photoactive noncovalent LCP, such as liquid crystalline ionomers containing azobenzene mesogens and hydrogen-bonded liquid crystalline azo polymer, respectively. Nanoparticle-LC elastomer composites that have light-responsive nanoparticles and photoresponsive main-chain oligomers useful for studying various main-chain characteristics are elaborated. Then, the macrocycles containing azobenzene main-chain oligomers, useful in characterizing the structure–property relationship, and stimulus-induced deformations in various geometries of photoresponsive LCPs have been discussed. Ferroelectric liquid crystals, useful for electronic and photonic devices, are also added to the discussion. Light-sensitive microcapsules based on LCP and photoresponsive membranes useful in the area of biotechnology, including drug delivery, biosensing, microfluidics, light-powered molecular machines, molecular shuttles, data storage, etc., have been elaborated. The chapter ends with future directions.

Light is external stimuli which is clean and highly accessible and can be precisely manipulated. Due to the fascinating photoinduced changes from molecular to nano- and macroscopic scales, photoresponsive liquid crystalline polymers (LCPs) have attracted wide interests in the fields of stimuli-responsive materials, soft robotics, biomaterials, nanotechnology and photonic devices, and so on. This entry mainly focuses on the booming researches related to photoresponsive LCPs, especially azobenzene-containing LCPs. First, the definition and photochemical properties are clarified. Then, recent advances on intriguing light-responsive behaviors of azobenzene-containing LCPs are introduced in terms of homopolymers, block copolymers, crosslinked LC systems, composite LC systems, and supramolecular liquid crystalline polymers. These photoinduced behaviors include molecular cooperative motion, nanoscale self-assembled photocontrollable structures, and macroscale photo-driven 2D and 3D mechanical movements. Finally, the researches are summarized and the possible applications are proposed.

Photoresponsive Liquid Crystalline Polymers bear light sensitive units that reversibly undergo transformations between their isomer forms when exposed to light in their absorption bands. These transformations, taking place at the molecular level, influence the electronic molecular structure and as a result the optical absorption of the molecule, a phenomenon that is known as photochromism. This light-induced change of the color of the material is of great interest in the preparation of systems such as optical memories and switches. Besides, the isomerization of the chromophores can lead to motions at the molecular level that can be amplified through cooperative effects typical in liquid crystalline materials resulting in photoinduced effects and properties.

Besides fulgides, diarylethenes, spiropyrans, and spirooxazines, azobenzene molecules have drawn a great deal of attention in combination with liquid crystalline polymers since their synthetic versatility and the promesogenic shape of the thermodinamically stable trans isomer that allows to keep the liquid crystalline phase of the material, in which they are incorporated. The bent shape shown by the light-induced cis isomer strongly distorts the liquid crystalline character of the polymer in which the chromophore is incorporated and can lead to important changes of the material morphology and macroscopic properties.

Rational design of the liquid crystalline polymeric photoresponsive materials has been carried out to exploit their potential in very diverse fields of application. Photoresponsive systems have been optimized due to their potential in (holographic) optical storage. Photoinduced generated surface relief structures with interest in photonic applications have been explored. Photoalignment is being studied as a tool to generate periodic patterns at the nanoscale with interest in nanolithography. Finally, macroscopic motions of film of these materials have shown their potential to generate new robotic functions for soft actuators.

We put together this entry to present a novel liquid crystalline brushlike block copolymer (LCBBC) architecture consisting of norbornenyl functionalized magnetically responsive liquid crystalline (LC) side chains in one block and brushlike semicrystalline PEG side chains in another block. At asymmetric LC block composition (73 wt%), LCBBCs show cylindrical PEG domains embedded in the major matrix having LC order. Molecular, thermal and microstructural characterizations are carried out to gain understanding of polymer structure, hierarchical self-assembly, and PEG crystallization. In order to get more insights on the role of PEG topology in different architectures, LCBBCs are compared with previously reported linear counterparts. LCBBC architecture presents a strong case to obtain magnetically aligned amorphous PEG domains in the LC major matrix for anisotropic ion transport (Li⁺) to develop electrochemical devices and batteries. Furthermore, norbornene backbones can be UV cross-linked in aligned states by using thiol-ene chemistry to attain robust platform with amorphous PEG domains for the ion transport to envision solid-state electrolyte membranes.

Liquid crystal (LC) alignment methods are very important for manufacturing liquid crystal displays (LCDs). The photoalignment method in which polyimide films are radiated with linearly polarized ultraviolet (LPUV) light is one of the most effective non-rubbing processes to solve problems such as electrostatic charge and dust accumulation. This entry presents a review on photoalignment film of LC using new fluorine-containing polyimides. We have provided detailed spectroscopy study to understand the possible mechanism behind the photoalignment effect for liquid crystal molecules.

This chapter reviews gas permeation and barrier properties of crystalline and liquid crystalline polymers from both theoretical mechanism and experimental measurement. The permeability is closely related to both solubility and diffusivity in polymers. Various factors such as crystallinity and crystalline-amorphous morphology play important roles in the solubility and diffusivity, and thus the permeability of crystalline and liquid crystalline polymers.

The flame retardation for polymer materials can be easily achieved by blending small-molecular flame retardants. However, traditional small molecule flame retardants exhibit potential drawbacks during application, including migration and blooming of the additives; deterioration of the polymer performance; and potential persistence, bioaccumulation, and toxicity (PBT); etc. High molecular weight polymers have been found to be less accessible by living organisms, thus have an automatically lower PBT profile than small molecules. As a typical kind of highly flame-retardant liquid crystalline polymers (LCP), phosphorus-containing LCPs have been proved to be a class of efficient high molecular weight flame retardants, which can overcome the aforementioned drawbacks, and have potential industrial applications to replace some existing small molecular flame retardants. The recent relevant developments of phosphorus-containing LCPs with high flame retardance and the corresponding in situ flame-retardant composites are reviewed in this chapter.

Nanocomposites of three thermotropic liquid crystalline polymers (TLCPs) with organoclay were prepared. The first TLCP, poly(2-ethoxyhydroquinone-2-bromoterephthaloyl), EHBT, consists of wholly aromatic ester type mesogenic units containing an ethoxy side group, and the second poly(oxybiphenyleneoxy-2,5-dihexyloxyterephthaloyl) (OBDT) is an aromatic polyester TLCP having alkoxy side groups on the terephthaloyl moiety. The last TLCP polyazomethine (PAM) consists of diad aromatic azomethine type mesogenic units. An EHBT with an alkoxy side-group was synthesized from 2-ethoxyhydroquinone and 2-bromoterephthalic acid. Nanocomposites of EHBT with Cloisite 25A (C25A) as an organoclay were prepared by the melting intercalation method above the melt temperature (T_m) of the TLCP. Liquid crystallinity, morphology, and thermo-mechanical behaviors were examined with increasing organoclay content from 0 to 6 wt%. Liquid crystallinity of the C25A/EHBT hybrids was observed when organoclay content was up to 6 wt%. Regardless of the clay content in the hybrids, the C25A in EHBT was highly dispersed in a nanometer scale. The hybrids (0–6 wt% C25A/EHBT) were processed for fiber spinning to examine their tensile properties. Ultimate strength and initial modulus of the EHBT hybrids increased with increasing clay content and the maximum values of the mechanical properties were obtained from the hybrid containing 6 wt% of the organoclay. A TLCP (OBDT)/organoclay nanocomposite was synthesized via in-situ intercalation polycondensation of diethyl-2,5-dihexyloxyterephthalic acid and 4,4′-biphenol in the presence of organically modified montmorillonite (MMT). The organoclay, C18–MMT, was prepared by the ion exchange of Na⁺–MMT with octadecylamine chloride (C18–Cl⁻). OBDT/C18–MMT nanocomposites were prepared to examine the variations of the thermal properties, morphology, and liquid crystalline phases of the nanocomposites with clay content in the range 0–7 wt%. It was found that the addition of only a small amount of organoclay was sufficient to improve the thermal behavior of the OBDT hybrids, with maximum enhancement being observed at 1 wt% C18–MMT. Nanocomposites of PAM with the organoclay C12-MMT were also synthesized by using the in-situ interlayer polymerization method. The variations with organoclay content of the thermal properties, morphology, and liquid crystalline mesophases of the hybrids were determined for concentrations from 0 to 9 wt% C12-MMT. The wide-angle X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) micrographs show that the levels of nanosize dispersion can be controlled by varying the C12-MMT content. The clay particles are better dispersed in the matrix polymer at low clay contents than at high clay contents. With the exception of the glass transition temperature (T_g), the maximum enhancement in the thermal properties was found to arise at an organoclay content of 1 wt%. Further, the PAM hybrids were shown to exhibit a nematic liquid crystalline phase for organoclay contents in the range 0–9 wt%.