“…On the other hand, CHOL has many plastic phases 11,23,33 and, hence, many T g 's, although even in this case, controversies exist . The X-ray diffraction studies 36 indicate that phase I is a simple cubic with the unit cell parameter of 11.54(1) Å at 273 K and phase II is tetragonal with the unit cell parameters of a = 19.487(2) Å and c = 11.7805(2) Å at 248.15 K. Because of the similarity in the cell structure and lattice parameters of phase I of COOL and CHOL, the mixtures of these materials form a continuous solid solution in phase I, hereafter designated as S I , which too is simple cubic in structure, exhibiting a linear variation of the lattice parameter with concentration. , This SC-mixed crystal (S I ) is orientationally disordered and is the subject of the present study. 2 Double logarithmic plot of ε‘ ‘ versus frequency of (a) pure COOL and (b) the COOL−CHOL binary system with x m = 0.25 at different temperatures.…”
In the present communication, investigations of two interesting (two-component) solid solutions are reported where one is a hydrogen (H-)-bonded pair and the other is a non-H-bonded pair. The former is the two-component system cyclooctanol (COOL) + cycloheptanol (CHOL), which forms a simple cubic phase [Rute, M. A.; Salud, J.; Negrier, P.; López, D. O.; Tamarit, J. Ll.; Puertas, R.; Barrio, M.; Mondieig, D. J. Phys. Chem. B 2003, 107, 5914]. This solid phase has been investigated at low temperatures and for several concentrations by means of low-frequency dielectric spectroscopy and differential scanning calorimetry (DSC). Depending upon the concentration, this phase reveals a glass transition in the temperature range of 138-172 K and a pronounced relaxation process identifiable with the so-called alpha process characteristic of a single-component orientationally disordered crystal. The dielectric spectra are found to follow the Havriliak-Negami (HN) equation. The analysis of the various parameters obtained show an isomorphic relationship between the simple cubic phases of both pure components through a continuous change of parameters. In addition, a sub-T(g) process, which is Arrhenius, is found. The kinetic freezing of the various dielectric processes has been critically examined in relation to the T(g) found in the DSC experiments. The non-H-bonded pair that has been studied is cis-1,2-dimethylcyclohexane (DMCH) and cyclohexylchloride (CHC). The liquid mixture of DMCH and CHC upon lowering the temperature forms a solid solution on the DMCH-rich side, which is an orientationally disordered crystal. This phase demonstrates a pronounced alpha process in the dielectric measurements that follows the HN equation. The results are discussed in the context of the solid-liquid phase diagram of this binary system. The observed deviations from Arrhenius and Debye behaviors in the solid solutions studied in this paper are shown to follow the "strong-fragility" pattern of Angell.
“…On the other hand, CHOL has many plastic phases 11,23,33 and, hence, many T g 's, although even in this case, controversies exist . The X-ray diffraction studies 36 indicate that phase I is a simple cubic with the unit cell parameter of 11.54(1) Å at 273 K and phase II is tetragonal with the unit cell parameters of a = 19.487(2) Å and c = 11.7805(2) Å at 248.15 K. Because of the similarity in the cell structure and lattice parameters of phase I of COOL and CHOL, the mixtures of these materials form a continuous solid solution in phase I, hereafter designated as S I , which too is simple cubic in structure, exhibiting a linear variation of the lattice parameter with concentration. , This SC-mixed crystal (S I ) is orientationally disordered and is the subject of the present study. 2 Double logarithmic plot of ε‘ ‘ versus frequency of (a) pure COOL and (b) the COOL−CHOL binary system with x m = 0.25 at different temperatures.…”
In the present communication, investigations of two interesting (two-component) solid solutions are reported where one is a hydrogen (H-)-bonded pair and the other is a non-H-bonded pair. The former is the two-component system cyclooctanol (COOL) + cycloheptanol (CHOL), which forms a simple cubic phase [Rute, M. A.; Salud, J.; Negrier, P.; López, D. O.; Tamarit, J. Ll.; Puertas, R.; Barrio, M.; Mondieig, D. J. Phys. Chem. B 2003, 107, 5914]. This solid phase has been investigated at low temperatures and for several concentrations by means of low-frequency dielectric spectroscopy and differential scanning calorimetry (DSC). Depending upon the concentration, this phase reveals a glass transition in the temperature range of 138-172 K and a pronounced relaxation process identifiable with the so-called alpha process characteristic of a single-component orientationally disordered crystal. The dielectric spectra are found to follow the Havriliak-Negami (HN) equation. The analysis of the various parameters obtained show an isomorphic relationship between the simple cubic phases of both pure components through a continuous change of parameters. In addition, a sub-T(g) process, which is Arrhenius, is found. The kinetic freezing of the various dielectric processes has been critically examined in relation to the T(g) found in the DSC experiments. The non-H-bonded pair that has been studied is cis-1,2-dimethylcyclohexane (DMCH) and cyclohexylchloride (CHC). The liquid mixture of DMCH and CHC upon lowering the temperature forms a solid solution on the DMCH-rich side, which is an orientationally disordered crystal. This phase demonstrates a pronounced alpha process in the dielectric measurements that follows the HN equation. The results are discussed in the context of the solid-liquid phase diagram of this binary system. The observed deviations from Arrhenius and Debye behaviors in the solid solutions studied in this paper are shown to follow the "strong-fragility" pattern of Angell.
“…The fragility plot ͑not shown in figure͒ for all the samples studied here more or less reflects this view with CNCH exhibiting the most fragile plastic phase and CNADM, the strongest of the plastic phases. This view is also supported by the emperical relation related to thermodynamic fragility index [48][49][50][51][52][53][54] given by Wong and Angell 47 for liquids may be modified for plastic phase I as…”
Section: Fragilitymentioning
confidence: 87%
“…We have also discussed fragility concept in the light of some new concepts emerging in this area. [48][49][50][51][52][53][54] From Table III, it is interesting to note that the value of the exponent "r" is located in a narrow range of 9-12, as found in many of the supercooled liquids 19,26,55 and plastic crystals. 13,14,[22][23][24] Recently, Colby 56 has given a physical interpretation of this high exponent value, where it is related to the size of the cooperatively rearranging region.…”
In the present communication, dielectric relaxation investigations on three interesting supercooled plastic crystalline substances, i.e., isocyanocyclohexane (ICNCH), cyanocyclohexane (CNCH), and 1-cyanoadamantane (CNADM) are reported. All of these have the main dipole moment situated in their side group- C[Triple Bond]N or- N[Triple Bond]C. Differential scanning calorimetry (DSC) was also employed as a supporting technique. Glassy crystal were easily formed in the first two samples by slowly cooling the plastic phase, but in CNADM it was formed by rapidly quenching the room temperature plastic phase. In addition to the so called alpha process that can reasonably be described by a Havriliak-Negami (HN) shape function, a secondary (or beta) relaxation process is found in all the materials. The beta process in CNADM has an activation energy (DeltaE(beta)) of about approximately 13.8+/-1 kJmol, and is present even in the corresponding ordered crystalline phase, i.e., in its monoclinic phase. On the other hand, the magnitude of DeltaE(beta) in both the isomers of cyanocyclohexane, i.e., ICNCH and CNCH, is similar and is about 21.1 and 23.4 kJmol, respectively. Unlike CNADM, the cyclohexane derivatives are capable of exhibiting additional intramolecular process due to chair-chair conversion (i.e., in addition to the rotational motion of the side group- C[Triple Bond]N or- N[Triple Bond]C). Therefore, the secondary process of these systems is compared to that occuring in the binary liquid glass formed by dispersing a small quantity of these dipolar liquids in nearly nonpolar orthoterphenyl (OTP). Measurements were also made in the supercooled binary mixures of other cyclohexyl derivatives like cyclohexylchloride and cyclohexylbromide with OTP which lack a flexible side group. The sub-T(g) relaxation process exhibited in all these cases have almost similar activation energy as in case of pure ICNCH and CNCH. These observations together with the fact that the activation energy for this process is much below that of chair-chair conversion which is about 43 kJmol leads us to the conclusion that sub-T(g) relaxation process in the binary mixtures is JG type, and perhaps beta relaxation process in phase I of ICNCH and CNCH is also similar. With the help of semiemperical calculations of the dipolemoments for the axial and equitorial confirmers, it is concluded that the process associated with the chair-chair may not be dielectrically very active and, hence, should be relatively weaker in magnitude. The beta process in CNADM has an activation energy (DeltaE(beta)) of about 13.8+/-1 kJmol, and is present even in the corresponding ordered crystalline phase indicating that it may not be characteristic of the glass formation of phase I. The molecular structure of CNADM is such that it does not possess other intramolecular degrees of freedom of the type equitorial to axial (or chair-chair) transformation. Our experimental finding that JG relaxation for CNADM dispersed in glassy OTP matrix is about 31 kJmol, indicating that the well ...
“…Many efforts were performed to get a thermodynamic parameter that can be used instead of the m -fragility. − In glass-forming polymers, Huang and McKenna introduced the ratio C p,l /C p,g , where C p,l and C p,g are the heat capacities of the liquid and the corresponding liquid frozen as glassy state at T g , respectively, as a thermodynamic fragility parameter that correlates quite well with the m -fragility ( C p,l /C p,g increases when the m -fragility decreases). In our case, liquid crystal dimers are a kind of polymer.…”
Broadband dielectric spectroscopy (10(3) Hz-1.8 × 10(9) Hz) has been performed on the odd nonsymmetric liquid crystal dimers of the series α-(4-cyanobiphenyl-4'-oxy)-ω-(1-pyreniminebenzylidene-4'-oxy) alkanes (CBOnO.Py) with n ranging from 3 to 11, as a function of temperature. A previous thermal behavior study through heat capacity measurements has been made. Dielectric measurements enable us to obtain information about the molecular dynamics in the nematic mesophase as well as in the isotropic phase. Two orientations (parallel and perpendicular) of the molecular director with regard to the probe electric field have been investigated. In the nematic mesophase, the dielectric anisotropy is revealed to be positive for all studied compounds. Measurements of the parallel component of the dielectric permittivity are well-explained by means of the molecular theory of dielectric relaxation in nematic dimers (Stocchero, M.; Ferrarini, A.; Moro, G. J.; Dunmur, D. A.; Luckhurst, G. R. J. Chem. Phys., 2004, 121(16), 8079). The dimer is seen as a mixture of cis and trans conformers, and the model allows us to estimate their relative populations at each temperature. The main molecular motions are interpreted by the model as independent end-overend rotations of each terminal semirigid unit of the dimer.
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