Differential scanning calorimetry liquid crystals studied

Characterization. The liquid crystalline properties of the side-chain monomers (III) and polymers (I) have been studied by Differential Scanning Calorimetry (DSC), Polarized Optical Microscopy (POM) and X-ray diffraction. The thermal transition data and phase types for all monomers (III) and polymers (I) are summarized in Table HI. A representative DSC scan for the monomer (El) and polymer (p with a four-carbon tail (n=4) and six-carbon flexible spacer (m=6) are shown in Figures 1 and 2 respectively. The first peak at -24°C shown in Figure 1 is the crystal to smectic... 

A complete characterization of liquid crystalline polymers should include at least two aspects the characterization of the molecular structure and that of the condensed state structure. Since the first characterization is nothing more than what is practiced for non-liquid-crystalline polymers, we will restrict the discussion to only a short introduction of methods mostly used in the characterization of the presence and the main types of polymeric liquid crystal phases. The methods include the mostly used polarizing optical microscopy (POM, Section 4.1), differential scanning calorimetry (DSC, Section 4.2) and X-ray diffraction (Section 4.3). The less frequently used methods such as miscibility studies, infrared spectroscopy and NMR spectroscopy will also be discussed briefly (Section 4.4). 

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). 

Differential scanning calorimetry was used to study the non-isothermal crystallization behavior of blends of poly(phenylene sulfide) (PPS) with the thermotropic liquid-crystalline copoly(ester amide) Vectra-B950 (VB) [126], The PPS crystallization temperature and the crystallization rate coefficient increased significantly when 2-50% VB was added. The Ozawa equation was shown to be valid for neat PPS as well as for the blends. The values of the Avrami exponents matched well against those determined previously using isothermal analysis, and they are independent of the concentration of VB. 

Finally, the highly polar tetrazine core finds apphcation in the design of liquid-crystal materials. A new series of symmetrical 3,6-diphenyl-1,2,4,5-tetrazines 61 with four alkoxy tails was synthesized and their mesogenic properties were studied by differential scanning calorimetry and polarizing optical microscopy It was shown that they possess smectic-C phases (13MCLC34). 

Calorimetric Methods.—Differential Scanning Calorimetry (d.s.c.). The use of commercially available d.s.c. apparatusto study phase separation in polymer and copolymer solutions is described. In this context perhaps the main advantage of the method is to distinguish between liquid-liquid phase separation, where the enthalpy change is usually small, and crystallization of polymer (see Chapter 12). The method is much used to study the glass transition in polymer mixtures (see p. 320). 

Nishikawa, K., Wang, S.L., Endo, T. and Tozaki, K., Melting and crystallization behaviors of an ionic liquid, l-isopropyl-3-methylimidazolium bromide, studied by using nanowatt-stabilized differential scanning calorimetry. Bull. Chem. Soc. Jpn. 82 (7), 806-812 (2009). 

Methyl cellulose is a derivative of cellulose soluble in water and widely used as a binder or thickener in pharmaceutical products, food products, in the field of ceramics, etc. Formation of the liquid crystal phase is dependent on molecular weight, concentration and temperature, as evidenced in different experimental studies employing differential scanning calorimetry, polarized light microscopy, optical rotatory dispersion [121]. This cellulose derivative has two stages of thermoreversible gelation in aqueous solution, as temperature rises, if concentration exceeds a certain critical value [117, 122]. Several studies [123] have revealed a crystal liquid phase in dilute solutions as well. 

Nishikawa, K, Wang, S., Endo, T., Tozaki, K. (2009) Melting and Crystallization Behaviors of an Ionic Liquid, l-Isopropyl-3-methylimidazolium Bromide, Studied by Using Nanowatt-Stabilized Differential Scanning Calorimetry, Bull. Chem. Soc. pn. Vol. 82 806-812. 

Differential scanning calorimetry (DSC) is by far the most commonly used thermal technique for studying liquid crystals. It is a very useful and sensitive survey technique for discovering new phase transitions and for determining the qualitative magnitude of thermal features. However, DSC is not well suited for making detailed quantitative measurements near liquid crystal phase transitions. 

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