The Mesomorphic State, Liquid Crystal Polymers

The state of mesomorphism is a spontaneously ordered liquid–fluid crystalline state. Liquid crystals were discovered as early as 1888. They are materials that exhibit order in one or two dimensions but not in all three. By comparison, the amorphous materials lack any order, while the crystalline ones exhibit order in three dimensions. All liquid crystalline polymers exhibit some degree of fluidity. They were investigated extensively in the 1900s and became commercially important in 1960s. These are macromolecules that can align into crystalline arrays while they are in solution (lyotropic) or while in a molten state(thermotropic). Such liquids exhibit anisotropic behavior [51,52]. The regions of orderliness in such liquids are called mesophases. Molecular rigidity found in rigid rod-shaped polymers, for instance, is the chief cause of their liquid crystalline behavior. It excludes more than one molecule occupying a specific volume and it is not a result of intermolecular attractive forces. Some aromatic polyesters or polyamides are good examples, like polyphenylene terephthalate:

Because the molecules posses anisotropy, they are aligned while still in a fluid form. This differs from ordinary liquids, that are isotropic, where the molecules lack any kind of arrangement.

Fig. 2.14 Illustration of the arrangement of liquid crystal into nematic and smectic orders Anisotropy is not affected by conformational changes. Generally, molecules that are rigid rod like and elongated or disc like in shape are the type that can form liquid crystal arrangements. Some biological polymers exhibit liquid crystalline behavior due to their rigid helical conformations. Among synthetic polymers, on the other hand, rigid rod structures, mentioned above are the ones that exhibit most of the liquid crystalline behavior. Polymers that form liquid crystals may exhibit multiple mesophases at different temperatures. Based on the arrangement of the liquid crystals in the mesophases, they are further classified as nematic, smectic, and cholesteric [51, 52].

Both, smectic and nematic are parallel arrangements along molecular axes. The smectic liquid crystals are  more ordered ,however ,than then maticones. This is a result of differences in the orientations of the chain ends. In smectic liquid crystals the chain ends are lined up next to each other. In nematic ones, however, they lack any particular orientation. Also, the smectic liquid crystals are layered while the nematic ones are not. Microscopic observations [51] can help distinguish between the two forms.

Smectic elastomers, due to their layered structure ,exhibit distinct anisotropic mechanical properties and mechanical deformation processes that are parallel or perpendicular to the normal orientation of the smecticlayer. Suchelastomersareimportant duetotheiroptical andferroelectric properties.Networks with a macroscopic uniformly ordered direction and a conical distribution of the smectic layer normal with respect to the normal smetic direction are mechanically deformed by uniaxial and shear deformations. Under uniaxial deformations two processes were observed [53]: parallel to the direction ofthemechanicalfielddirectlycouplestothesmectictiltangleandperpendiculartothedirectorwhilea reorientation process takes place. This process is reversible for shear deformation perpendicular and irreversible by applying the shear force parallel to the smectic direction. This is illustrated in Fig. 2.14. If the mesogens are chiral, a twisted nematic, supramolecular, cholesteric (twisted) phase can form [51, 52]. The achiral nonlinear mesogens can also form chiral supramolecular arrangements in tilted smectic phases. Recently, Tokita and coworkers [54] reported a direct transition from isotropic to smectic arrangement in a liquid crystalline polymer and determined experimentally the existence of metastable nematic orientational ordering that preceded the formation of translational smectic ordering. A polymeric material was used that exhibits very slow liquid crystalline transition dynamics [55]. This enabled use of conventional methods to study the transitions, such as of polarized light scattering and synchrotron wide-angle X-ray diffraction analyses. It was observed that at high quench rates or super cooling, metastable nematic (orientational) ordering occurs preceding full smectic (orientational and translational) order. Also, the occurrence of nematic preordering (high super cooling) resulted in morphological changes of growing liquid crystalline domains compared to solely smectic growth. Specifically, samples cooled at rates high enough to exhibit nematic preordering formed well oriented or “neat” tactoidal smectic domains. Samples cooled at lower rates, where only smectic ordering was observed, formed radially oriented or textured spherulitic domains [55]. In commenting on this observation, Abukhdeir and Rey [56] point out that through a simulation model the isotropic to smectic liquid crystalline transition experimental observations of preordering of smectic liquid crystalline transitions can be studied. Phase transition kinetics results presented by them show that nematic preordering results from both thermodynamic potential and dynamic differences in phase ordering time scales.

Fig. 2.15 Illustration of arrangements of liquid crystal structure

The chemical structure of the polymers determines whether the molecules can form rigid rods. If the backbone of the polymer is composed of rigid structures then it tends to form main chain liquid crystals. If, however, the side chains are rigid, then the polymer will tend to form side chain liquid crystals. From practical considerations, these two properties are of prime interest. The structures are illustrated in Fig. 2.15. Liquid crystalline behavior affects the melt viscosity of the polymer and the ability of the polymer to retain the ordered arrangement in the solid state. Thus, liquid crystalline behavior during the melt results in lower viscosity because the rigid polymeric mesophases align themselves in the direction of the flow. As a result, the polymer is easier to process. Also, retention of the arrangement upon cooling yields a material with greatly improved mechanical properties. Several thermotropic liquid crystal line co polyesters and polyamides are available commercially. Samulski [57] gives examples of molecules that can typically form liquid crystals These are 1. A discotic liquid crystal

The above shown structure has a mesogenic core, (hard central segment) correlated with dynamic packing of anisometric shapes. The flexible tales, often hydrocarbon chains, extend from the mesogenic core and facilitate the transformation from the solid state to the liquid crystalline phase. 2. A calamitic liquid crystal

In this case there is a prolate mesogen axis and flexible hydrocarbon chains that extend from it.

3. A nonlinear liquid crystal

Many liquid crystal polymers tend to exhibit multiple mesophases at different pressures and temperatures [57]. When heated, these polymers will go through multiple first-order transitions. Such transitions are from more ordered to less ordered arrangements. This is referred to as the clearing temperature with the last one resulting in isotropic melts. A lyotropic liquid crystalline aromatic polyamide, sold under the trade name of Kevlar, is an example of such a polymer that is available commercially:

The polyamide forms liquid crystals in sulfuric acid solution from which it is extruded as a fiber. After the solvent is removed, the remaining fiber possesses greater uniform alignment than would be obtained by mere drawing. This results in superior mechanical properties. There are polymers, however, that exhibit liquid crystalline behavior, but are very high melting and in soluble in most common organic solvents. This is a drawback, because such material sare hard to process. Apreparation of new liquid crystal polymers with bilaterally linked mesogens in main-chain was reported [57]. Such materials exhibit biaxial fluctuation in the nematic phase. This is interesting because most commonly encountered polymeric liquid crystals have mesogens linked at their ends to the polymer backbone by flexible spacers. On the other hand, liquid crystal polymers with mesogens linked bilaterally by flexible spacers are not common and only a few examples were reported [58]. One such material can be illustrated as follows:

It was also shown that it is possible to synthesize polymethacrylate liquid crystal polymers with mesomorphic properties that contain ferrocenes with two flexible chains at the l,l0-positions [59]. Based on dilatometric measurements, a head-to-tail molecular arrangement of the monomeric units occurs within the smectic A phase. Because of special electrochemical properties of ferrocene, these materials are of interest for developing electroactive mesomorphic polymers. The structure of the polymer can be shown as follows [59]:

Finkelman reported synthesis of a novel cross-linked smectic-C main-chain liquid-crystalline elastomer that was formed by polycondensation of vinylogy-terminated mesogens, tetramethyl Di siloxane, and pentamethyl-pentaoxapentasilicane. The introduction of the functional vinyloxy group allows the synthesis of well-defined networks with good mechanical properties due to elimination of side reactions as in the case of vinyl groups [60]. Large amplitude oscillatory shear is frequently capable of generating macroscopic alignment from an initially random orientation distribution in ordered polymer fluids. Burghardt and coworkers [61] reported that by combined rheological and in situ synchrotron X-ray scattering to investigate of such induced alignment in smectic side-groups pf liquid crystalline polymers. In all cases, they found that shear promotes anisotropic orientation states in which the lamellar normal tends to align along the vorticity direction of the shear flow (“perpendicular” alignment). Rheological measurements of the dynamic moduli by them revealed that large amplitude shearing in the smectic phase causes a notable decrease in the modulus. They also observed that increasing strain promotes higher degrees of orientation, while increasing molecular weight impedes development of smectic alignment. Ahn et al. reported [62] preparation of a smetic liquid crystalline elastomer with shape memory properties. Shape memory polymeric materials can recover their equilibrium, permanent shapes from nonequilibrium, temporary shapes as a result of external stimuli, like heat or light. Such materials have application in medical practice. Main-chain polynorbornenes were linked with three different side-chains, cholesterol, poly(ethylene glycol), and butylacrylate.

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مواضيع ذات صلة

Steric Control in Free-Radical Polymerization

The rmal Polymerization

Inhibition and Retardation

The Termination Reaction

Ring Forming Polymerization

Polymerization of Monomers with Multiple Double Bonds

Methods for Measuring Molecular Weights of Polymers

Radius of Gyration

Solutions of Polymers

Orientation of Polymers

Kinetics of Crystallization

Thermodynamics of Crystallization