We report on the X-ray studies of freely suspended hexatic films of three different liquid crysta... more We report on the X-ray studies of freely suspended hexatic films of three different liquid crystal compounds. By applying angular X-ray cross-correlation analysis (XCCA) to the measured diffraction patterns the parameters of the bond-orientational (BO) order in the hexatic phase were directly determined. The temperature evolution of the BO order parameters was analyzed on the basis of the multicritical scaling theory (MCST). Our results confirmed the validity of the MCST in the whole temperature range of the existence of the hexatic phase for all three compounds. The temperature dependence of the BO order parameters in the vicinity of the hexatic-smectic transition was fitted by a conventional power law with a critical exponent b E 0.1 of extremely small value. We found that the temperature dependence of higher order harmonics of the BO order scales as the powers of the first harmonic, with an exponent equal to the harmonic number. This indicates a nonlinear coupling of the BO order parameters of different order. We demonstrate that compounds of various compositions, possessing different phase sequences at low temperatures, display the same thermodynamic behavior in the hexatic phase and in the vicinity of the smectic-hexatic phase transition.
We report on a ferroelectric chiral smectic C (C*) phase obtained in a mixture of a nematic liqui... more We report on a ferroelectric chiral smectic C (C*) phase obtained in a mixture of a nematic liquid crystal (NLC) and a chiral nonmesogenic dopant. The existence of C* phase was proven by calorimetric, dielectric and optical measurements, and also by X-rays analysis. The smectic C* which is obtained in such a way can flow, allowing to restore the ferroelectric liquid crystal layer structure in the electro-optical cells after action of the mechanical stress, as it happens with the cells filled with NLC. The proposed method of obtaining smectic C* material allows us to create innovative electro-optical cell combining the advantages of NLC (mechanical resilience) and smectic C* (high switching speed).
Cold drawing of films made from a vinylidene fluoride-tetraflouroethylene copolymer
Polymer Science U.S.S.R., 1989
Structures of two cold-drawn films made of a vinylidene fluoride-tetrafluoroethylene copolymer we... more Structures of two cold-drawn films made of a vinylidene fluoride-tetrafluoroethylene copolymer were studied. The isotropic films differed in the conformation of the interlamellar layers and also in the type of supermolecular structure. A shift in the conformer equilibrium in the sense of the transition tgtg−→t3gt3g−, accompanied by the appearance of spherulitic structures, leads to a formation in the oriented films of microscopic crazes of micron size.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2001
We report on a combined x-ray reflectivity and optical ellipsometry study of freely suspended sme... more We report on a combined x-ray reflectivity and optical ellipsometry study of freely suspended smectic (Sm) films of a chiral liquid crystalline compound with the phase sequence Sm-A-Sm-C(*)(alpha)-Sm-C*-Sm-C(*)(gamma)-Sm-C(*)(A). Using tilt magnitude profiles from x-ray reflectivity as input to model the average optical properties obtained by ellipsometry, tilt direction profiles are also obtained. In this way realistic models can be elaborated for the various types of chiral Sm-C films. We find that the surface layers are more tilted than the interior layers due to surface interactions and finite size effects. For the ferrielectric Sm-C(*)(gamma) phase the tilt direction profile corresponds to a three-layer helix, in agreement with the clock model of chiral Sm-C phases. In thin films the surface interactions suppress the bulk helix structure of the Sm-C(*)(alpha) phase and a Sm-C(*)(A)-like structure is formed with an anticlinic layer-by-layer alternation of the tilt directions.
Static and dynamic x-ray scattering has been used to characterize thermal fluctuations in free-st... more Static and dynamic x-ray scattering has been used to characterize thermal fluctuations in free-standing smectic films in the crystalline-B phase. The results are remarkably similar to those in smectic-A films, in spite of the three-dimensional positional order in the crystalline-B phase. The main difference is the disappearance of the characteristic Landau-Peierls instability of the smectic-A phase. The dynamic nature of the fluctuations is explicitly demonstrated by x-ray photon correlation spectroscopy. This fluctuation behavior of crystalline-B films can be attributed to the small value of the shear elastic constant C 44 .
The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crys... more The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crystalline (LC) polymer, a polyacrylate with phenylbenzoate mesogenic side groups and a narrow polydispersity. At a high degree of polymerisation the LC polymers show a nematic, a smectic-A d , a re-entrant nematic and aC phase, for shorter chains only a nematic and aC phase. This constitutes a new example of nematic re-entrance for which the driving field is the length of the polymer chain. The smectic-A d layers consist of partially overlapped side groups while in theC phase the side chains are rearranged into chevron-like blocks of bilayers. We propose an explanation of the frustrated phase behaviour in terms of these two different competing length scales and their coupling to the backbone conformations.
We report an X-ray study of the order in a 'side-on' smectic-A elastomer in which both the polyme... more We report an X-ray study of the order in a 'side-on' smectic-A elastomer in which both the polymer backbone and the mesogenic side groups are, on average, parallel to the smectic layer normal. The present system shows in all phases locally correlated structures as evidenced by systematic diffuse scattering at small angles, attributed to the fluorinated end groups of the mesogens. The elastomer shows order of the smectic layers over a limited range in spite of the relatively low crosslink concentration (4%). Upon stretching along the layer normal we find a linear increase of the smectic layer period for small loading (elastic regime). These changes parallel the stress variation for the elastic part of the macroscopic stress-strain curve and saturate at larger strains. The attendant increase in the width of the X-ray peaks indicates a decrease of the average domain dimensions. In the plastic regime no distortion of the smectic monodomain structure or layer rotation is observed. Instead the stress is relieved by 'melting' of the layer structure giving rise to a nematic-like state with short-range layer correlations: at the final stage about four smectic layers.
Two series of carbosilane LC dendrimers with terminal protonated and deuterated butoxyphenylbenzo... more Two series of carbosilane LC dendrimers with terminal protonated and deuterated butoxyphenylbenzoate mesogenic groups linked to carbosilane dendritic matrices of the first to fifth generations via an undecylene spacer have been synthesized. The chemical structure of new dendrimers has been studied by 1 H NMR spectroscopy and gel-permeation chromatography. The dendrimers of first-fourth generations are characterized by formation of the smectic C mesophase in a wide temperature range, whereas much more complex columnar supramolecular structures are formed in dendrimers of the fifth generation. Structural studied of mesophases by X-ray diffraction and small-angle neutron scattering show that segregation takes place in mixtures of deuterated and protonated LC dendrimers; as a result, huge aggregates composed of hundreds of chemically unbound molecules develop and the sizes of these aggregates reversibly change with temperature.
We present an x-ray study of liquid crystal membranes in the vicinity of hexatic-smectic phase tr... more We present an x-ray study of liquid crystal membranes in the vicinity of hexatic-smectic phase transition by means of angular x-ray cross-correlation analysis (XCCA). By applying two-point angular intensity cross-correlation functions to the measured series of diffraction patterns the parameters of bond-orientational (BO) order in hexatic phase were directly determined. The temperature dependence of the positional correlation lengths was analyzed as well. The obtained correlation lengths show larger values for the higher-order Fourier components of BO order. These findings indicate a strong coupling between BO and positional order that has not been studied in detail up to now.
X-ray study of the nematic and smectic A phases of the cyano-substituted pyridines
Liquid Crystals, 1989
We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous... more We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous series of polar liquid crystals, the 2-(4-alkylphenyl)-5-cyanopyridines (nCP). Experiments were carried out using a diffractometer with a linear position sensitive detector. In the nematic and smectic A phases of the nCP and their mixtures with non-polar 4-n-butyl-4′-methoxyazoxybenzene two types of layering were
Ferroelectric behaviour of different classes of smectic liquid crystals
Ferroelectrics, 1980
It is shown that two new classes of smectic liquid crystals - esters and salicelidenanilines - ha... more It is shown that two new classes of smectic liquid crystals - esters and salicelidenanilines - have ferroelectric properties. The new ferroelectric smectics have a hightened hydrolitical stability. Their electrical and heat properties are investigated, the correlation between a chemical composition and spontaneous polarisation values is discussed.
We report on the X-ray studies of freely suspended hexatic films of three different liquid crysta... more We report on the X-ray studies of freely suspended hexatic films of three different liquid crystal compounds. By applying angular X-ray cross-correlation analysis (XCCA) to the measured diffraction patterns the parameters of the bond-orientational (BO) order in the hexatic phase were directly determined. The temperature evolution of the BO order parameters was analyzed on the basis of the multicritical scaling theory (MCST). Our results confirmed the validity of the MCST in the whole temperature range of the existence of the hexatic phase for all three compounds. The temperature dependence of the BO order parameters in the vicinity of the hexatic-smectic transition was fitted by a conventional power law with a critical exponent b E 0.1 of extremely small value. We found that the temperature dependence of higher order harmonics of the BO order scales as the powers of the first harmonic, with an exponent equal to the harmonic number. This indicates a nonlinear coupling of the BO order parameters of different order. We demonstrate that compounds of various compositions, possessing different phase sequences at low temperatures, display the same thermodynamic behavior in the hexatic phase and in the vicinity of the smectic-hexatic phase transition.
We report on a ferroelectric chiral smectic C (C*) phase obtained in a mixture of a nematic liqui... more We report on a ferroelectric chiral smectic C (C*) phase obtained in a mixture of a nematic liquid crystal (NLC) and a chiral nonmesogenic dopant. The existence of C* phase was proven by calorimetric, dielectric and optical measurements, and also by X-rays analysis. The smectic C* which is obtained in such a way can flow, allowing to restore the ferroelectric liquid crystal layer structure in the electro-optical cells after action of the mechanical stress, as it happens with the cells filled with NLC. The proposed method of obtaining smectic C* material allows us to create innovative electro-optical cell combining the advantages of NLC (mechanical resilience) and smectic C* (high switching speed).
Cold drawing of films made from a vinylidene fluoride-tetraflouroethylene copolymer
Polymer Science U.S.S.R., 1989
Structures of two cold-drawn films made of a vinylidene fluoride-tetrafluoroethylene copolymer we... more Structures of two cold-drawn films made of a vinylidene fluoride-tetrafluoroethylene copolymer were studied. The isotropic films differed in the conformation of the interlamellar layers and also in the type of supermolecular structure. A shift in the conformer equilibrium in the sense of the transition tgtg−→t3gt3g−, accompanied by the appearance of spherulitic structures, leads to a formation in the oriented films of microscopic crazes of micron size.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2001
We report on a combined x-ray reflectivity and optical ellipsometry study of freely suspended sme... more We report on a combined x-ray reflectivity and optical ellipsometry study of freely suspended smectic (Sm) films of a chiral liquid crystalline compound with the phase sequence Sm-A-Sm-C(*)(alpha)-Sm-C*-Sm-C(*)(gamma)-Sm-C(*)(A). Using tilt magnitude profiles from x-ray reflectivity as input to model the average optical properties obtained by ellipsometry, tilt direction profiles are also obtained. In this way realistic models can be elaborated for the various types of chiral Sm-C films. We find that the surface layers are more tilted than the interior layers due to surface interactions and finite size effects. For the ferrielectric Sm-C(*)(gamma) phase the tilt direction profile corresponds to a three-layer helix, in agreement with the clock model of chiral Sm-C phases. In thin films the surface interactions suppress the bulk helix structure of the Sm-C(*)(alpha) phase and a Sm-C(*)(A)-like structure is formed with an anticlinic layer-by-layer alternation of the tilt directions.
Static and dynamic x-ray scattering has been used to characterize thermal fluctuations in free-st... more Static and dynamic x-ray scattering has been used to characterize thermal fluctuations in free-standing smectic films in the crystalline-B phase. The results are remarkably similar to those in smectic-A films, in spite of the three-dimensional positional order in the crystalline-B phase. The main difference is the disappearance of the characteristic Landau-Peierls instability of the smectic-A phase. The dynamic nature of the fluctuations is explicitly demonstrated by x-ray photon correlation spectroscopy. This fluctuation behavior of crystalline-B films can be attributed to the small value of the shear elastic constant C 44 .
The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crys... more The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crystalline (LC) polymer, a polyacrylate with phenylbenzoate mesogenic side groups and a narrow polydispersity. At a high degree of polymerisation the LC polymers show a nematic, a smectic-A d , a re-entrant nematic and aC phase, for shorter chains only a nematic and aC phase. This constitutes a new example of nematic re-entrance for which the driving field is the length of the polymer chain. The smectic-A d layers consist of partially overlapped side groups while in theC phase the side chains are rearranged into chevron-like blocks of bilayers. We propose an explanation of the frustrated phase behaviour in terms of these two different competing length scales and their coupling to the backbone conformations.
We report an X-ray study of the order in a 'side-on' smectic-A elastomer in which both the polyme... more We report an X-ray study of the order in a 'side-on' smectic-A elastomer in which both the polymer backbone and the mesogenic side groups are, on average, parallel to the smectic layer normal. The present system shows in all phases locally correlated structures as evidenced by systematic diffuse scattering at small angles, attributed to the fluorinated end groups of the mesogens. The elastomer shows order of the smectic layers over a limited range in spite of the relatively low crosslink concentration (4%). Upon stretching along the layer normal we find a linear increase of the smectic layer period for small loading (elastic regime). These changes parallel the stress variation for the elastic part of the macroscopic stress-strain curve and saturate at larger strains. The attendant increase in the width of the X-ray peaks indicates a decrease of the average domain dimensions. In the plastic regime no distortion of the smectic monodomain structure or layer rotation is observed. Instead the stress is relieved by 'melting' of the layer structure giving rise to a nematic-like state with short-range layer correlations: at the final stage about four smectic layers.
Two series of carbosilane LC dendrimers with terminal protonated and deuterated butoxyphenylbenzo... more Two series of carbosilane LC dendrimers with terminal protonated and deuterated butoxyphenylbenzoate mesogenic groups linked to carbosilane dendritic matrices of the first to fifth generations via an undecylene spacer have been synthesized. The chemical structure of new dendrimers has been studied by 1 H NMR spectroscopy and gel-permeation chromatography. The dendrimers of first-fourth generations are characterized by formation of the smectic C mesophase in a wide temperature range, whereas much more complex columnar supramolecular structures are formed in dendrimers of the fifth generation. Structural studied of mesophases by X-ray diffraction and small-angle neutron scattering show that segregation takes place in mixtures of deuterated and protonated LC dendrimers; as a result, huge aggregates composed of hundreds of chemically unbound molecules develop and the sizes of these aggregates reversibly change with temperature.
We present an x-ray study of liquid crystal membranes in the vicinity of hexatic-smectic phase tr... more We present an x-ray study of liquid crystal membranes in the vicinity of hexatic-smectic phase transition by means of angular x-ray cross-correlation analysis (XCCA). By applying two-point angular intensity cross-correlation functions to the measured series of diffraction patterns the parameters of bond-orientational (BO) order in hexatic phase were directly determined. The temperature dependence of the positional correlation lengths was analyzed as well. The obtained correlation lengths show larger values for the higher-order Fourier components of BO order. These findings indicate a strong coupling between BO and positional order that has not been studied in detail up to now.
X-ray study of the nematic and smectic A phases of the cyano-substituted pyridines
Liquid Crystals, 1989
We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous... more We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous series of polar liquid crystals, the 2-(4-alkylphenyl)-5-cyanopyridines (nCP). Experiments were carried out using a diffractometer with a linear position sensitive detector. In the nematic and smectic A phases of the nCP and their mixtures with non-polar 4-n-butyl-4′-methoxyazoxybenzene two types of layering were
Ferroelectric behaviour of different classes of smectic liquid crystals
Ferroelectrics, 1980
It is shown that two new classes of smectic liquid crystals - esters and salicelidenanilines - ha... more It is shown that two new classes of smectic liquid crystals - esters and salicelidenanilines - have ferroelectric properties. The new ferroelectric smectics have a hightened hydrolitical stability. Their electrical and heat properties are investigated, the correlation between a chemical composition and spontaneous polarisation values is discussed.
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Papers by B. Ostrovskii