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Thermodynamic transition properties of highly ordered smectic phases

Series of main-chain liquid crystalline polyethers

Journal of Thermal Analysis and Calorimetry
Authors: Y Yoon, R. -M. Ho, F. Li, B. Moon, D. Kim, J. -Y. Park, F. W. Harris, S. Z. D. Cheng, V. Percec, and P. Chu

A series of polyethers have been synthesized from 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)propane and α, Ω-dibromoalkanes having different numbers of methylene units [TPPs]. Both odd- and even-numbered TPPs [TPP(n=odd)s and TPP(n=even)s) exhibit multiple transitions during cooling and heating and they show little supercooling dependence, indicating close-to-equilibrium nature of these transitions. Combining the structural characterization obtainedvia wide angle X-ray diffraction powder and fiber patterns at different temperatures and the morphological observations from microscopy techniques, not only the nematic liquid crystalline phase but also highly ordered smecticF, smectic crystalG andH phases have been identified. The phase diagrams for both TPP(n=odd)s and TPP(n=even)s have been constructed [1–3]. Thermodynamic properties (enthalpy and entropy changes) during these transitions are studied based on differential scanning calorimetry experiments. The contributions of the mesogenic groups and methylene units to each ordering process can be separated and they indicate the characteristics of these processes thereby providing estimations of the transition types.

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Phase transition process of polyester from phenanthrene, poly(oxyheptamethyleneoxy-2,7-phenantrenedicarbonyl), a main chain type liquid crystalline polymer, was investigated by simultaneous DSC-XRD measurements using the synchrotron radiation facility (PF). Three exothermic DSC peaks were observed during cooling from the isotropic liquid state. These DSC peaks were assigned to the transition from the isotropic liquid to the smectic A, that from the smectic A to C, and that from the smectic C to the crystalline state, respectively, as determined by XRD profiles. The rate of transition from the smectic A to C was very slow compared with the liquid crystalline transition and the crystallization. From the DSC-XRD results, the thermal expansion along c-axis in the crystal and smectic phases are 4.110−4 and 1.410−3 nm K−1 , respectively.

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This study explores the calorimetric analysis of an aligned nano-colloidal aerosil dispersed octyl-cyanobiphenyl gel. This system was prepared by solvent dispersion method (SDM). Heating scans were performed at different heating rates from 20 to 1 K min−1 using DSC. Aligned samples follow Arrhenius behavior and showed a temperature shift in SmA-N and N-I transitions towards lower temperature. These samples show a decreased activated kinetics and an interesting relationship with their enthalpy. This behavior can be explained in terms of surface and molecular interaction between aerosil nano-particles and 8CB molecules and produced strain in the system.

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Phase transition process of PEOm-b-PMA(Az)n was investigated by the simultaneous DSC-XRD measurement using the synchrotron radiation facility. Four endothermic DSC peaks were observed during heating process. These DSC peaks were assigned to the melting of PEO, the transition from SmX, which is a mixture of super-cooled SmC and crystal, to SmC, from SmC to SmA, and from SmA to isotropic liquid state as determined by XRD profiles. In SmC phase, the liner expansion coefficient calculated from the spacing variation of the smectic layer distance was larger than that of the other phases. This result reflected the fact azobenzene moieties in the long-side chains of PMA(Az)n forming the smectic layers and then they were tilted and stood up during the heating process.

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To understand better the antagonistic behaviour of the backbone in comparison with the mesogenisity of the pendant side groups in side-chain liquid crystal polymers series of measurements using mechanical spectroscopy as well as DSC were carried out on two disubstituted polynorbornene derivatives. In order to clarify the specific arrangement of chains, the influence of thermal history on the smectic phase stabilisation were studied. By quenching samples from different temperatures, it was possible to obtain the systems which were either in the glassy liquid-crystalline (LC) state or in the glassy isotropic amorphous state. The samples prepared in such ways were used to study the influence of the LC structure on the mobility of chains in amorphous regions. The quality model of the supermolecular structure of the LC polymers is proposed.

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This article described the synthesis and mesomorphic behavior transition of a novel liquid crystalline (LC) epoxy resin 4-(2,3-epoxypropoxy)biphenyl,4″-(2,3-epoxypropoxy)phenyl-4′carboxylate (EBEPC), which combined a hydroxyl benzoic aromatic ester and biphenol rigid-rod group. EBEPC showed a clear nematic schlieren texture under curtain conditions. The reaction kinetics of EBEPC cured by 4,4′-diaminodiphenyl-methane (DDM) was studied by using an isoconversional method under isothermal conditions with differential scanning calorimetry (DSC). The isothermal DSC data can be fitted reasonably by an autocatalytic curing model. Smectic phases had been observed in the EBEPC/DDM curing system. The results of DSC showed that the formation of the LC phase had pronounced influence on the curing reaction.

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Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4′-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases

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The binary systems of cholesteryl nonanoate with 4,4′-bis(hexyloxy)azoxybenzene (1) and with 4,4′-bis(heptyloxy)azoxybenzene (2) show induced smecticA phases with transition point maximum. Whereas in (1) the smecticA phase is transformed into the cholesteric phase on heating within the whole range of its existence, in (2) the maximum of the smectic phase is stabilized compared with the cholesteric phase and clears into the isotropic melt directly. The molecular ratio of the maximum is 1∶1 in both cases. Moreover in system (2) a smecticC phase was observed.

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Bis [(ω-(4′-cyanobiphenyl)-4-yl)oxy-n-alkyl]norborn-5-ene-2,3-dicarboxylate was polymerised via ring opening metathesis polymerisation (ROMP). Two disubstituted polynorbornene derivatives both of cis configuration with different length of the side-chain were studied. Differential scanning calorimetry (DSC) was used to study the effect of thermal history on the assignment of the glass transition event associated with the biaxial orientation of a smectic phase. Glass transition temperatures, the change of isobaric specific heats at Tg and the enthalpies of isotropisation were calculated. The DSC traces only show the classic step-wise change in Tg in some cases, giving the evidence that the amorphous domains are constrained and highly restricted in movement due to the morphology developed as a result of the biaxial stretching. Based on the literature data of mono- and disubstituted polynorbornene derivatives and our calorimetric experiments, the shape of Tg dependence on number of (CH2) units is interpreted. The origin of this shape is discussed.

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Data are presented on the heats of phase changes and heat capacities for the even chain number mercury(II) carboxylates from octanoate to octadecanoate. The octanoate and dodecanoate melt directly to the liquid, while the decanoate and octadecanoate pass through a solid → solid transition before melting to the liquid. The tetradecanoate and hexadecanoate pass through a mesophase before finally melting. It is proposed that this mesophase is aG (smectic) phase. Addition of mercuric oxide to the tetradecanoate and hexadecanoate causes the appearance of an additional mesophase. The results are explained in terms of theR theory of fused micellar phases.

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