Thermotropic polymers optical properties

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

Liquid crystal polymers (LCPs) were introduced over the last three decades. In the liquid state, either as a solution (lyotropic) or a melt (thermotropic), they lie between the boundaries of solid crystals and isotropic liquids. This polymeric state is also referred to as a mesomorphic structure, or a mesophase, a combined term adopted from the Greek language (mesos = intermediate morphe = form). This state does not meet all the criteria of a true solid or a true liquid, but it has characteristics similar to both a solid and a liquid. For instance, the anisotropic optical properties of LC polymeric fluids are like those of crystalline solids, but their molecules are free to move as in liquids. 

In this chapter we shall only be concerned with electro-optic and thermo-optic switching effects in thermotropic side-chain polymer liquid crystals. We will consider briefly the synthesis and structure of such compounds and show how the nematic, cholesteric and smectic phases arise. Since the optical properties of each of these phases are different, and may be altered depending on alignment within the phase, each gives rise to different electro-optic effects. If these are coupled to the use of dye additives or substituents, then it will be realized that a wide range of electro-optic devices based on dichroism or fluorescence as well as birefringence or scattering power may be fabricated. These will be considered and discussed in terms of their performance and potential applications. Finally, possible uses of polymer-low molar mass liquid crystal solutions will be considered in terms of electro-optic device applications. 

Polymer/surfactant complexes may exhibit thermotropic mesomorphism as well as the lyotropic one discussed above. Among conducting polymers, PANi has been of considerable scientific interest because of its unique electrical and optical properties and environmental stability. Besides the covalent alkyl substitution di.scussed above, a significant approach has been to use functionalized acids, which usually serve as surfactants. Dodedcylbenzenesulfonic acid (DBSA) has been widely used for protonation of PANi and found to improve the solubility and processibility of the conducting polymer. 

Broer and coworkers prepared thermotropic LC monomers with acrylate-containing moieties and achieved preservation of LC order by using in situ photo-polymerization. It was observed that the resulting liquid-crystalline polymer exhibited anisotropic optical properties that could be varied by structurally tuning spacer length and substituents of monomers. Polymerization conditions were found to have an important effect in this system. 

Pequ Elena, Paslaru Elena, and Marin Luminita. Polymer-dispersed liquid crystal composites for bio-applications Thermotropic, surface and optical properties. Liq. Cryst. 42 no. 3 (2015) 370-382. 

Side-chain homopolymers based on poly(meth)acrylic derivatives (Fig. 16.14a) have been also widely studied (Andruzzief al., 1999 Rodriguez et al, 2005, 2006). When a suitable flexible spacer mediates between the main chain and the chromophore, these side chain polymers tend to exhibit thermotropic hquid crystalline properties, which favor cooperative chromophores motion during photoorientation and lead to more stable anisotropic optical properties. The azobenzene chromophore strongly influences a material s photoresponse. As a representative example, Ikeda... 

In the experimental study of the optical properties of molecules by the method of flow birefringence (FBF), the question of the molecular dispersion of the solution is always important, particularly in the analysis of the properties of mesogenic macromolecules. The complex structure of thermotropic polymers and their tendency to form supermolecular structures require special monitoring of possible association or decomposition of the macromolecules in solution. The linear dependence of An in a wide range of rate gradients (Figs. 3.7 and 3.8) and... 

Cholesteric mesophases in polymers were limited to these systems alone for a long time. However, due to the greatly increased scientific and practical interest in low-molecular-weight liquid crystals and cholesterics in particular at the end of the 1960s, studies began to be conducted on obtaining new types of thermotropic and lyotropic LC polymer cholesterics. These studies were stimulated by the unique optical properties of cholesterics, which permit widely... 

A specific property of liquid crystals is the ability to undergo structural transformadons under the effect of electric and magnetic fields. The appearance of lyo- and thermotropic mesomorphism in polymer systems also implies the possibility of controlling their structural-optical properties with external fields. This ability to alter the structure in electric and magnetic fields was one of the most convincing arguments in support of the validity of existence of polymers in the LC phase in general. 

Marin L, Zabulica A, Sava M (2013) Symmetric liquid crystal dimmers containing a luminescent mesogen synthesis, mesomorphic behavior, and optical properties. Soft Mater 11 32-39 Shukla U, Rao KV, Rakshit AK (2003) Thermotropic liquid-crystalline polymers synthesis, characterization and properties of poly(azomethine esters). J Appl Polym Sci 88 153-160 Shukla SR, Harad AM, Mahato D (2006) Depolymerization of nylon 6 waste fibers. J Appl Polym Sci 100(1) 186-190... 

The optical properties are directly related to the polymer and LC structure formation, consequently it is of paramount importance to determine the effect of side chain size and number of substituents, degree of esterification and degree of polymerization in pitch values and the optical properties of the mesophases of thermotropic liquid crystals. 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

Liquid crystalline polymers (LCPs) have gained attraction as materials with interesting optical, mechanical and rheological properties [3-7]. This review summarizes research on thermotropic liquid crystalhne polymers synthesized by metathesis routes, as this chemistry has proven to be a versatile way to build up well-defined polymer architectures [8]. Recent results promise to ejq)and the possible uses of these methods. 

The main part of the side group in these macromolecules consists of the alkoxy-benzoic acid moiety. This acid may form thermotropic liquid crystals. The investigation of the hydrodynamic properties of PPhEAA molecules in dilute solutions has revealed that the equilibrium rigidity of their main chains is relatively low (Table 12). Hence, since for all flexible-chain polymers, the shear optical coefficient An/Ar in PPhEAA solutions is independent of molecular weight the segmental anisotropy - tt2 and the anisotropy of the monomer unit Aa may be determined by use ofEq. (67). 

Thermotropic liquid crystal polymers (LCI ) are of considerable current interest, because of their theoretical and technological aspects [1-3]. Evidently, a new class of polymers has been developed, combining anisotropic physical properties of the liquid crystalline state with diaracteristic polymer features. This unique combination promises new and interesting material properties with potential ai lications, for example in the field of high modulus fibers [4], storage technology, or non-linear optics [5]. 

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