Phase transitions in liquid crystals

The low value of , where is the nematic-isotropic phase transition temperature and denotes the virtual transition temperature, is a long-standing puzzle in the physics of liquid crystals. The present review presents experimental and theoretical results on this long-standing problem. New experimental and theoretical results for the critical behaviour in the isotropic phase of nematogens are reviewed. We calculate in a unified approach the low value of , at both critical and tricritical points. The possibility of tricritical behaviour at the nematic-isotropic transition is also discussed by means of Landau theory. The various predictions are compared with the available experimental results.

http://iopscience.iop.org/article/10.1088/0953-8984/10/41/003/pdf

The puzzle of the nematic-isotropic phase transition

Ion irradiation offers a promising strategy to emulate the irradiation damage induced by energetic neutrons. However, the characterization of the mechanical properties of ion-irradiated materials is not easy considering the limited irradiation depth and inhomogeneous distribution of irradiation-induced defects. Over the last decades, nano-indentation has been recognized as a valid technique to investigate the localized mechanical responses at micro-scale. Therefore, the combination of ion irradiation and nano-indentation has been extensively developed in recent years to study the mechanical behaviors of nuclear structural materials with irradiation effect. In this review, current developments of experimental observations, numerical simulations and theoretical models are summarized concerning both microstructural evolution and macroscopic deformation. Corresponding analysis and discussion could help obtain a sophisticated comprehension of the fundamental deformation mechanisms resulting in mechanical degradation of ion-irradiated materials, and further assist the development of next-generation nuclear structural materials.

Nano-indentation of ion-irradiated nuclear structural materials: A review

Results of studies of the static and dynamic dielectric properties in rod-like 4-n-octyloxy-4'-cyanobiphenyl (8OCB) with isotropic (I)-nematic (N)-smectic A (SmA)-crystal (Cr) mesomorphism, combined with measurements of the low-frequency nonlinear dielectric effect and heat capacity are presented. The analysis is supported by the derivative-based and distortion-sensitive transformation of experimental data. Evidence for the I-N and N-SmA pretransitional anomalies, indicating the influence of tricritical behavior, is shown. It has also been found that neither the N phase nor the SmA phase are uniform and hallmarks of fluid-fluid crossovers can be detected. The dynamics, tested via the evolution of the primary relaxation time, is clearly non-Arrhenius and described via τ(T) = τ(c)(T-T(C))(-φ). In the immediate vicinity of the I-N transition a novel anomaly has been found: Δτ is proportional to 1/(T - T*), where T* is the temperature of the virtual continuous transition and Δτ is the excess over the 'background behavior'. Experimental results are confronted with the comprehensive Landau-de Gennes theory based modeling.

https://iopscience.iop.org/article/10.1088/0953-8984/25/24/245105/pdf

Distortion-sensitive insight into the pretransitional behavior of 4-n-octyloxy-4′-cyanobiphenyl (8OCB)

The isotropic phase of nematogenic liquid crystals has nanometer length scale domains with pseudonematic ordering. As the isotropic to nematic phase transition temperature (TNI) is approached from above, the orientational correlation length, ξ, of the pseudonematic domains grows as (T - T(*))(-1/2), where T(*) is 0.5-1 K below TNI. The orientational relaxation, which is a collective property of the pseudonematic domains, was measured with optical heterodyne detected-optical Kerr effect (OHD-OKE). The orientational relaxation obeys Landau-de Gennes theory, as has been shown previously. To examine the environmental evolution experienced by molecules in the pseudonematic domains, two-dimensional infrared (2D IR) vibrational echo experiments on the CN stretching mode of the non-perturbative vibrational probes 4-pentyl-4(')-selenocyanobiphenyl (5SeCB) and 4-pentyl-4(')-thiocyanobiphenyl (5SCB) in the nematogen 4-cyano-4(')-pentylbiphenyl (5CB) were performed. The 2D IR experiments measure spectral diffusion, which is caused by structural fluctuations that couple to the CN vibrational frequency. Temperature dependent studies were performed just above TNI, where the correlation length of pseudonematic domains is large and changing rapidly with temperature. These studies were compared to 2D IR experiments on 4-pentylbiphenyl (5B), a non-mesogenic liquid that is very similar in structure to 5CB. The time constants of spectral diffusion in 5CB and 5B are practically identical at temperatures ≥5 K above TNI. As the temperature is lowered, spectral diffusion in 5B slows gradually. However, the time constants for spectral diffusion in 5CB slow dramatically and diverge as T(*) is approached. This divergence has temperature dependence proportional to (T - T(*))(-1/2), precisely the same as seen for the correlation length of pseudonematic domains, but different from the observed orientational relaxation times, which are given by the Landau-de Gennes theory. The data and previous results show that spectral diffusion in 5CB has no contributions from orientational relaxation, and the structural dynamics responsible for the spectral diffusion are likely a result of density fluctuations. The results suggest that the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length, ξ. The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length. The results presented here show that there is another divergent dynamical process, likely associated with density fluctuations.

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New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy

The effect of irradiation damage on the microstructure evolution and mechanical properties in nanocrystalline (nc) Ni with an average grain size of � 60 nm was studied. Samples were irradiated at doses of 1.6 � 10 15 , 2.3 � 10 15 ,5 � 10 15 and 2.3 � 10 16 He 2+ cm � 2 by 12 MeV He ions at room temperature. Microstructural parameters like domain size and microstrain were studied in detail using the X-ray diffraction (XRD) technique with the simplified breadth method and Williamson–Hall analysis. The average domain size was found to decrease systematically with the increase in irradiation dose. Microscopic observations made with transmission electron microscopy showed dislocation loops and dislocation networks within the grain interior of the irradiated sample. In addition, irradiation at a higher dose showed small amounts of the sand-like black dots inside some grains, which could be due to the accumulation of point defects. Tensile tests performed on the irradiated sample were compared with unirradiated samples. The irradiated sample showed higher ultimate tensile strength and average work-hardening rate as compared to the unirradiated sample. Nanoindentation studies were performed to study the effect of irradiation on deformation parameters like strain rate sensitivity (m) and activation volume (V � ). The value

Study of the effect of a irradiation on the microstructure and mechanical properties of nanocrystalline Ni

Nematic-Isotropic (N-I) phase transition is discussed in an alternative way by using Landau–de Gennes phenomenological theory. The temperature difference of TNI-T* is obtained near the tricritical point. We also obtained the value of the ratio (Q*-QNI)/QNI, where Q* is the order parameter at the temperature T* of the absolute stability limit of the isotropic phase and QNI is the order parameter at the nematic-isotropic transition temperature TNI.

Nematic Isotropic Phase Transition: An Alternative Formulation

The Landau-de Gennes model of the nematic-isotropic phase transition with the inclusion of the density change and a higher order term (sixth-order term) in a simple way is examined. The temperature difference TNI−T* obtained is in good agreement with the experimental one. We also obtained the value of the ratio (Q*–QNI)/QNI. The value of (Q*–QNI)/QNI is not improved over the earlier attempt, but there is a definite improvement in the value of TNI–T* which is obtained as 1.045 K.

Landau-de Gennes model with the inclusion of density change for the nematic-isotropic phase transition

The phase transition from isotropic liquid to nematic liquid crystal is a weak first order one. The present paper investigate the possibility of the critical region near the isolated critical point at the first order nematic-isotropic phase transition line.

Critical Region at the Nematic-Isotropic Phase Transition

J Banerj i ··, A Chatterjeeb, M Saha", K P Dhara", S Kanrar", P Mukherjeed, A Neuman· & T Prange· IDepartment of Chemistry, bCentre of Advanced Studies on Natural Products, University College of Science, 92, Acharya Prafulla Chandra Road, Calcutta 700 009, India CDepartment of Chemistry, Ananda Mohan College, Calcutta 700 009, India dKanailal Vidyamandir, Chandannagore 7 1 2 1 36, India ·Ch1mie Structurale Biomoleculaire (URA 1 430, CNRS), UFR Biomedicale, 74 rue M. Cachin, 9301 2 Bobigny Cedex, France Received 12 March 1 999; accepted 16 July 1 999

/pdf/reactions-and-rearrangements-of-triterpenoids-3-epitaraxerol-22f9bea47k.pdf

P. Mukherjee

Papers

Study of the effect of a irradiation on the microstructure and mechanical properties of nanocrystalline Ni

Nematic Isotropic Phase Transition: An Alternative Formulation

Landau-de Gennes model with the inclusion of density change for the nematic-isotropic phase transition

Critical Region at the Nematic-Isotropic Phase Transition

Reactions and rearrangements of triterpenoids-3-Epitaraxerol and its transformation products