Haluk Özbek

Study of phase transitions in liquid crystalline side group polymers via photon transmission method

Abstract Photon transmission measurements were applied to study the phase transitions of an acrylate monomer, 3-(4-cyanobiphenyl-4′-oxy)propyl acrylate (LC3), its homopolymer (PLC3) and its graft copolymer (GLC3) with polytetrahydrofuran grafts. In PLC3 drastic decrease in the transmitted light intensity was attributed to a first-order isotropic–nematic phase transition. The phase transitions were also confirmed by DSC and polarizing microscopy. No phase transition was observed in GLC3. This method was not able to detect the isotropic–nematic phase transition in GLC3, probably because of the small size of the phase separated domains in this graft copolymer. The isotropic–nematic transition of GLC3 was identified by polarizing microscopy only by the appearance of a weak diffuse birefringence on cooling.

Smectic A-C-A Liquid Crystal Reentrance: A Photon Transmission Study

Ultraviolet/visible photon transmission measurements were applied to study liquid crystal phase transitions in the binary BOPDOB-BOPOOB mixture. A sequence exhibiting a new reentrant, namely isotropic-nematic-smectic A-C-A as temperature is lowered, is identified. For the smectic A-C transition, the critical exponent g is seen to cross over from the mean-field 0.501 - 0.008 to the helium-like 0.336 - 0.012 as the transition is approached. The reentrant C-A transition is first-order, signalling a tricritical point in the phase diagram.

Comparative study of liquid-crystalline ordering in a monomer, linear polymer, and graft copolymer by the photon transmission technique

The photon transmission technique was used to study the phase transitions of a liquid crystalline acrylate monomer, 6-(4-cyanobiphenyl-4′oxy)hexyl acrylate (LC6), its homopolymer (PLC6) and its graft copolymer (GLC6) with polytetrahydrofuran grafts. The phase transitions were also confirmed by DSC and polarizing microscopy. We observed the phase transition sequence isotropic–nematic–smectic A–smectic C in the LC6 monomer. In PLC6 and GLC6 polymers, the nematic and smectic A phases appear dominant. The apparent nematic–smectic A transition is of first order in PLC6 and of second order in GLC6, with the transition temperature remaining the same. The effects of quenched random constraints introduced in GLC6 are consistent with the theory of quenched random interactions. The critical exponents were also evaluated.

A PHASE DIAGRAM OF SMECTOGEN-NON-SMECTOGEN BINARY MIXTURE: A PHOTON TRANSMISSION STUDY

The ultraviolet/visible photon transmission technique was applied to study the phase diagram of a mixture of 4-ethoxy-4′-(6-vinyloxyhexyloxyl) azobenzene (VE), a smectogen, and 4-hexoxy-3′-methyl-4′-(6-acryloyloxyhexyloxy) azobenzene (AH), which by itself exhibits no liquid crystalline behavior. It has been found that the N-Sm-A phase line terminates at either a tricritical point at about XVE = 0.9, where XVE is the weight percentage of VE in the binary mixture, or at least at a tricritical region 0.9 < XVE < 1 within the experimental resolution. For XVE = 1 the nematic phase region is not large enough to drive the transition second-order. It is found that the value of the critical exponent β for XVE = 0.9 is consistent with the tricritical value.

THE A-C-A PHASE DIAGRAM OF MIXED LIQUID CRYSTALS: A PHOTON TRANSMISSION STUDY

Ultraviolet/visible photon transmission technique was applied to study the phase diagram of the BOPDOB-BOPOOB liquid crystal system. It was observed that the nematic-smectic A transition becomes first order, as the weight percentage of BOPDOB in the binary mixture is increased. The smectic A-C-A reentrance disappears when the BOPDOB percentage approaches 100 (pure BOPDOB). We also present the smectic A-C transition temperatures and the critical exponents β of the all samples. The values of the exponent are consistent with mean-field theory.

THE EFFECT OF SURFACES ON THE ORIENTATIONAL PROPERTIES AND PHASE TRANSITIONS PECULIARITIES OF A NONPOLAR SMECTOGEN

The effect of thin films on the orientational and optical properties of nonpolar monolayer smectogen 4-butoxyphenyl-4′-decyloxybenzoate liquid crystal has been studied by polarizing microscopy and high-resolution photon transmission method. The uniform planar textures of on the surfaces coated with WO3, VO2, and WO3–VO2 mixture thin films have been obtained. No significant shifts in the phase transition temperatures to lower values under the influence of the surfaces have been observed. We have also presented the new experimental data on the critical behavior of the layer compression modulus B near the nematic–smectic A transition. It was observed that the data in the vicinity of the transition can be described by a power law expression after the introduction of a first-order correction-to-scaling term within the limit of experimental error. The resulting critical exponent was found to be 0.34∓0.01 which is good agreement with the previously reported values for other investigated compounds in literature.

Synthesis and liquid crystalline behavior of random copolymer of poly(ethylene oxide) macromonomer and liquid crystalline monomer by the photon transmission technique

Random copolymers of poly(ethylene oxide) macromonomer with p‐vinylbenzyl end‐functional group (PEOVB) and liquid crystalline monomer, namely 6‐(4‐cyanobiphenyl‐4′‐oxy)hexyl acrylate (COA), were prepared by conventional free radical polymerization. A living anionic polymerization technique was employed for the synthesis of PEO macromonomers bearing p‐vinylbenzyl moiety at one end. The photon transmission method was also applied to study the phase transitions of COA monomer and its random copolymer with PEO. It was found that, for both samples, the nematic‐smectic A transition is continuous, but the critical fluctuation regions do not allow to obtain 3D XY values. Instead, we have obtained the values close to mean field regime. Scaling of thermal hystersis for random copolymer sample near the nematic‐isotropic transition was studied as well. Thermal hysteresis loops were produced under linearly varying temperature. It was shown that the areas of the hysteresis loops scale with the temperature scanning rate with an exponent being equal to 0.614 which is in good agreement with the field‐theoretical value.

Convincing evidence for the Halperin-Lubensky-Ma effect at the N-SmA transition in alkyloxycyanobiphenyl binary mixtures via a high-resolution birefringence study

Abstract.We present new high-resolution experimental data for the temperature behavior of optical birefringence for a series of mixture of the liquid crystals octyloxycyanobiphenyl (8OCB) and nonyloxycyanobiphenyl (9OCB) by using a rotating analyzer technique. The birefringence data have been used to probe the temperature dependence of the nematic order parameter

Critical behavior of a nonpolar smectogen from high-resolution birefringence measurements.

S(T)

A simple method to determine high-accuracy refractive indices of liquid crystals and the temperature behavior of the related optical parameters via high-resolution birefringence data

S(T). We have then arrived at values for possible entropy discontinuities at the nematic-smectic A transition temperature