The description of the smectic A and C phases and the smectic A–C phase transition of TCOOB with a diffuse‐cone model
01 Jul 1979-Journal of Chemical Physics (American Institute of PhysicsAIP)-Vol. 71, Iss: 1, pp 25-31
TL;DR: In this paper, it was shown that even the preferred directions of the molecules in a smectic A phase will make a nonzero angle with the layer normal, and from the data examined, this angle, ϑm, appears to be about 10-15° for the compound TCOOB (trans‐1,4−cyclohexane−di−n−octyloxybenzoate).
Abstract: A previous paper demonstrated that the orientational disorder present in all smectic A phases causes most molecules to make fairly large angles with the normal to the smectic plane. Thus, the molecular directions form a diffuse cone around the layer normal. It is now argued that even the preferred directions of the molecules in a smectic A phase will make a nonzero angle with the layer normal, and from the data examined, this angle, ϑm, appears to be about 10–15 ° for the compound TCOOB (trans‐1,4‐cyclohexane‐di‐n‐octyloxybenzoate). The smectic A‐to‐C phase transition is found to have two aspects in the diffuse‐cone model: the distribution of the molecular directions around the layer normal loses its rotational symmetry at the phase transition temperature TA–C, and the angle ϑm starts increasing sharply at a temperature Tt. In most cases these two temperatures are probably equal. The increase of ϑm with decreasing temperature in the smectic C phase of TCOOB can be described by a (Tt−T)0.35 dependency superimposed on a much smaller linear temperature dependency already present in the smectic A phase.
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References
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TL;DR: In this paper, the authors describe a mittlere Innere Feld einer nematischen Molekülanordnung abgeleitet, and gezeigt, daß this Innere Field in einem gewissen Temperaturbereich einen nematicalhen Ordnungszustand bewirken muß and daß dieser ordnung sustand was with einer bestimmten Temperatur diskontinuierlich und unter Energieaufnahme in die isot
Abstract: Mit Beschränkung auf den Dipol-Dipol-Anteil der reinen Dispersionswechselwirkungen als dem für die Existenz einer kristallinflüssigen Phase vermutlich maßgebenden Teil der zwischenmolekularen Kräfte wird durch geeignete Mittelungen ein Ausdruck für das auf das Einzelmolekül wirkende mittlere Innere Feld einer nematischen Molekülanordnung abgeleitet. Es wird gezeigt, daß dieses Innere Feld in einem gewissen Temperaturbereich einen nematischen Ordnungszustand bewirken muß und daß dieser Ordnungszustand bei einer bestimmten Temperatur diskontinuierlich und unter Energieaufnahme in die isotrope Molekülanordnung übergehen muß. Die Theorie liefert also einen Umwandlungspunkt 1. Ordnung, wie er experimentell auch beobachtet wird. Die Umwandlungstemperatur wird im wesentlichen durch die Anisotropie der optischen Übergangsmomente der Moleküle bestimmt.
1,328 citations
TL;DR: In this article, the authors discuss the nature of the order in the main smectic phases (A, C, and B) together with the related phase transitions, and suggest that the B phase may correspond to a plastic crystal, or to a system of weakly coupled solid layers: to discriminate between these two models, acoustical studies using shear waves appear to be most promising.
Abstract: We discuss the nature of the order in the main smectic phases (A, C, and B) together with the related phase transitions. (1) The B phase may correspond to a plastic crystal, or to a system of weakly coupled solid layers: to discriminate between these two models, acoustical studies using shear waves appear to be most promising, (2) the transitions A N, C N, C A (where N stands for nematic) may be of second order in certain cases, which will allow for a number of interesting experiments. The C A transition should be very similar to the λ transition of helium, and some critical exponents which are not accessible in helium could be measured here. The N A transition is similar to the onset of superconductivity in a metal. A bend (or twist) deformation tends to suppress the A phase just as a magnetic field destroys superconductivity. In the N phase the twist and bend elastic constants should show pretransitional anomalies. The C N transition is more complex. A tentative calculation suggests...
244 citations
TL;DR: The concept of inherent tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains.
Abstract: Even though the average molecular direction in smectic A phases is perpendicular to the smectic planes, most molecules in these phases have rather large tilt angles because the orientational order parameter S in all smectic A phases is significantly less than unity. This molecular tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains. From the differences between d and l , S values from 0.78 to 0.84 are obtained, which yield average molecular tilt angles from 20° to 17°. It is predicted that all smectic A phases with monomolecular layers will follow the same pattern. The concept of “inherent tilt” should apply to all orthogonal smectic phases, thermotropic as well as lytropic.
128 citations
TL;DR: In this article, a general theory was developed for the relation between the molecular length and the position of a diffraction maximum found at diffraction angles of the order of 4°, for nematic and isotropic phases containing elongated molecules.
Abstract: A general theory is developed for the relation between the molecular length and the position of a diffraction maximum found at diffraction angles of the order of 4°, for nematic and isotropic phases containing elongated molecules. Using this theory and a previously developed one for the relation between the intermolecular distance and the position of the major diffraction maximum at larger diffraction angles, molecular parameters are determined for three phases of ethyl-p-sthoxybenzal-p-aminobenzoate. These parameters are: the thickness of the smectic layers (19.941 A), the length of the molecules in the isotropic phase (21.4 A), and the intermolecular distances in the smectic (4.894 A), nematic (4.932–4.950 A), and isotropic phases (4.950–5.182 A), all as a function of temperature. From these data, densities and volume expansion coefficients are calculated which agree very well with literature data on similar compounds. There appears to be no room for rotation of the molecules around their long ...
98 citations
43 citations