Proceedings Volume 4017

Polymers and Liquid Crystals

Andrzej Wlochowicz
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Proceedings Volume 4017

Polymers and Liquid Crystals

Andrzej Wlochowicz
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 14 December 1999
Contents: 5 Sessions, 37 Papers, 0 Presentations
Conference: International Conference on Dielectric and Related Phenomena '98 1998
Volume Number: 4017

Table of Contents

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Table of Contents

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  • Investigations of Polymers by Using Microwave Heating, Electrothermal Techniques, and Thermostimulated Current
  • Dielectric, Electrical, and Optical Investigations of Polymers
  • Polymer-like Materials, Epoxies, and Composites: Dielectric, Optical, and Electrical Studies of their Properties
  • Generalities About Liquid Crystals and Related Materials
  • Investigations of Dielectric, Electrical, and Electro-optic Properties of Liquid Crystals
Investigations of Polymers by Using Microwave Heating, Electrothermal Techniques, and Thermostimulated Current
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Microwave heating and the acceleration of polymerization processes
Fabrizio Parodi
Microwave power irradiation of dielectrics is nowadays well recognized and extensively used as an exceptionally efficient and versatile heating technique. Besides this, it revealed since the early 1980s an unexpected, and still far from being elucidated, capacity of causing reaction and yield enhancements in a great variety of chemical processes. These phenomena are currently referred to as specific or nonthermal effects of microwaves. An overview of them and their interpretations given to date in achievements in the microwave processing of slow-curing thermosetting resins is also given. Tailored, quaternary cyanoalkoxyalkyl ammonium halide catalysts, further emphasizing the microwave enhancements of curing kinetics of isocyanate/epoxy and epoxy/anhydride resin systems, are here presented. Their catalytic efficiency under microwave irradiation, microwave heatability, and dielectric properties are discussed and interpreted by the aid of the result of semi-empirical quantum mechanics calculations and molecule dynamics simulations in vacuo. An ion-hopping conduction mechanism has been recognized as the dominant source of the microwave absorption capacities of these catalysts. Dipolar relaxation losses by their strongly dipolar cations, viceversa, would preferably be responsible for the peculiar catalytic effects displayed under microwave heating. This would occur through a well-focused, molecular microwave overheating of intermediate reactive anionic groupings, they could indirectly cause as the nearest neighbors of such negatively-charged molecular sites.
Pulsed electrothermal technique for the characterization of dielectric films
Simona Bauer-Gogonea, Siegfried Bauer, Werner Wirges
Several adaptations of the well known thermal-pulse technique for the determination of the spatial distribution of polarization across the film thickness will be discussed, which enable the in-situ investigation of piezo-, pyroelectric, elastic and thermal material properties during i.e. thermal cycles and chemical reactions within one easy- to-use experimental setup. Illustrations of the techniques are given by employing ferroelectric polymers, as well as amorphous, photonics polymers. Pulsed electrothermal techniques are easy-to-use, fast, highly sensitive and reliable, and therefore suitable for opening broad new fields of research in thin film materials characterization.
Different molecular mechanisms for the dielectric hysteresis and pyroelectricity in a poly(vinyl alcohol) with azobenzene-alkoxy side chains
P. Fruebing, M. Wegener, Reimund Gerhard-Multhaupt, et al.
By means of pyroelectrical measurements and dielectric spectroscopy as well as structural information from differential scanning calorimetry, it is shown that, in a poly(vinyl alcohol) with azobenzene-alkoxy side chains, pyroelectricity and dielectric hysteresis which are usually related to each other have different origins. The pyroelectric effect is explained with reversible dipole- density changes upon thermal expansion, whereas the dielectric hysteresis is proposed to result from a charge- carrier polarization.
Thermal aging of pyroelectricity in PVDF and P(VDF-TrFE) copolymers
Laurent Ibos, G. Teyssedre, A. Bernes, et al.
A study of the thermal aging of the pyroelectric properties of commercially available bioriented PVDF and P(VDF-TrFE) 75-25 mol percent films is reported. The analysis of pyrocurrent spectra has been carried out in a large temperature range of -100 degrees C up to +150 degrees C. The pyroelectric coefficient p3 shows a distinct temperature dependence below and above thermal transitions, and especially the glass transition. We consider how this dependence changes upon aging. The influence of both aging temperature and aging time was investigated. ThermoStimulated Currents spectroscopy (TSC) has been used as a complementary technique of investigation. We have paid a particular attention to the evolution of the relaxation modes associated with the glass transition and with the (alpha) c transition as a function of aging conditions. Experimental results of (beta) -PVDF show that the decay of the pyroelectric activity is linearly dependent on the annealing temperature. Identical aging conditions induce a lower decay of p3 in P(VDF-TrFE) 75-25 than in PVDF, attesting the higher stability of the macroscopic polarization in fluorinated copolymers. The evolution of the decay of p3 as a function of the aging time is more complex. Indeed, p3 decreases quickly below a short characteristic aging time, and much slower at longer times. This characteristic time is aging temperature dependent. The evolution of the relaxation mode association with the glass transition shows that irreversible crystallinity changes occur at aging temperature higher than +110 degrees C. So, crystallization phenomena do not play a major role in the decay of p3. Besides, the (alpha) c transition seems to play an important role in the aging of pyroelectricity. Indeed, any loss of pyroelectric activity is detected after aging at temperatures lower than the (alpha) c transition.
Molecular motions in a-PVDF: a combined dielectric relaxation and thermostimulated current study
J. Menegotto, P. Demont, C. Lacabanne
The dielectric relaxation processes of poly(vinylidene- fluoride) are investigated by combining broadband Dielectric Spectroscopy (DS) and thermally stimulated currents at temperature below and above the glass transition temperature. We compare the relaxation times distribution obtained from experimental dielectric data using Tikonov regularization method with the results of fractional polarization method of TSC.
Formation and relaxation of space charge in corona-poled polystyrene
S. N. Fedosov, G.F. Leal Ferreira, Jose A. Giacometti, et al.
The 20 micrometers -thick samples of amorphous atactic nonpolar polystyrene (PS) have been poled in a corona triode at a constant current of 2 nA up to 2 kV at different constant temperatures Tp, while the kinetics of the surface potential was continuously recorded. Then the sample were subjected to the thermally stimulate depolarization (TSD) in one of three modes; with short circuited electrodes, with the 3 mm-thick air gap, or with a PTFE 9 micrometers -thick spacer. It has been shown that the constant current corona method can be applied to measure the dielectric constant at infra- low frequencies. At Tp > 80 degrees C the curves became sub-linear due to injection of the charge carriers in the bulk. The best fitting was obtained with Arkipov- Rudenko-Sessler-Ferreira's model assuming charge transport modulated by deep trapping and detrapping. The product of the carrier mobility and the trapping time has been found as 4.4 X 10-13 m2V-1 at 80 degrees C. The TSD-SC current shows a very sharp positive peak at 110 degrees C, and TSD-AG gives two negative peaks at 114 and 129 degrees C, and TSD-DS current contains a broad positive peak at 117 degrees C and a negative one at 132 degrees C. The peak temperature were higher than glass transition temperature Tg equals 95 degrees C proving that all peaks have been caused by charge redistribution and not related to the a relaxation. It has been proved that there are two components of the charge, presumably the surface and the volume ones, with the surface charge being more thermally stable than the volume charge. It appeared that the positive charge from corona penetrates deeper in PS than the negative ones.
Corona poling of a ferroelectric polymer (PVDF)
S. N. Fedosov, Anna V. Sergeeva, Jose A. Giacometti, et al.
In this work, we applied constant current corona poling in order to study potential buildup and polarization switching in uniaxially and biaxially stretched PVDF. Relaxation processes have been studied by the thermally stimulated depolarization (TSD) current method. It has been confirmed that the poling and switching processes consisted of three stages. A simple technique has been suggested to find the P(E) hysteresis curves and obtain static and dynamic dielectric constants, as well as the coercive Ec and saturated Es fields in one sequence of experiments. The method can be used for any material with the hysteresis-like field dependence of polarization. It has been found that there are three components of the residual polarization in PVDF, namely the stable ferroelectric component, the slowly relaxing one and the unstable component. Two TSD peaks have been observed immediately after poling, but two more peaks appeared in the samples stored for a long time. It follows from our results that very slow redistribution of charges and polarization takes place in PVDF until the electret and ferroelectric components of polarization are finally separated, both accompanied by the trapped charges.
Isothermal and non-isothermal relaxation processes in dye-doped polystyrene
S. N. Fedosov, Jose A. Giacometti, Mauro M. Costa, et al.
The build-up of a polarized state and its isothermal and nonisothermal relaxation behavior in a NLO guest-host system obtained by doping of polystyrene (PS) with the disperse red 1 (DR1) dye molecules have been studied by the isothermal absorption currents measurements, the thermally stimulated polarization (TSP) and depolarization (TSD), the AC dielectric spectroscopy, as well as the Hamon method in order to cover wide ranges of temperatures and frequencies. The 20 micrometers -thick samples were pulsed by the 'sandwich' method. Analyzing isothermal polarization and depolarization and depolarization currents we proved the validity of the superposition principle. The Williams-Watts model supplemented with a concept of intrinsic time has been used to interpret the TSP curves, but it appeared that the isothermal behavior does not fully determine the TSP results. The a peak has been observed at dielectric loss curves with its position changing from 10 Hz at 100 degrees C to 3 X 104 Hz at 130 degrees C and narrowing with temperature. Temperature dependence of the relaxation time above Tg agreed with the WIlliams-Landel-Ferry model, while the Arrenius dependence was more appropriate at sub- Tg temperatures. It has been shown that the PS/DR1 system is not thermoreologically simple, i.e. the temperature-time superposition is not entirely valid.
Dielectric, Electrical, and Optical Investigations of Polymers
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Investigations of biopolymers by dielectric relaxation spectroscopy
Alexander Korzhenko, Mohamed Tabellout, Jaques Emery
An application of blocking layers between sample and electrodes permits to avoid the influence of electrode polarization and opens new opportunities in DRS investigations of the conductive systems, such as polysaccharides and proteins containing water and electrolytes. The proposed methodical approach is applied for plate capacitor type of the cell and for the one in coaxial lien for the measurements in the frequency region 0,1 Hz-10 MHz and 1 MHz-1GHz respectively, using Novocontorl GmbH equipment. The possibilities to investigate an evolution of the biological system in time and temperature scale avoiding the influence of electrode polarization are demonstrated.
Conformational dynamic of fibrinogen by dielectric spectroscopy
Vladimir P. Berest, Sergiy V. Gatash
The information concerning the structural changes of fibrinogen molecule at temperatures form 4 to 52 degrees C has ben obtained by means of dielectric-spectroscopy method. Besides the known conformational transition II, under physiological conditions conformational transition at 20-22 degrees C has been observed in fibrinogen. This transition might be connected with structural transition in peripheral domain of fibrinogen. Revealed conformational transition, probably, determines the character of the temperature dependence of blood platelet aggregation.
Dielectric properities of polyethylene modified by monoesters of maleic acid
Jozef Swiatek-Prokop, Grzegorz W. Bak, L. Slusarski
Dielectric properties of low-density polyethylene (LDPE) modified by monoesters of maleic acid have been investigated. The coagent molecules can interact with molecules of LDPE giving rise to cross-linking and changes of degree of crystallinity. Depending on the modifier used, the dielectric losses of modified LDPE at the lower frequencies may change significantly. The dielectric losses of LDPE modified with dodecyl maleate increases by over one order of magnitude.
Dielectric dilatometry on thin Teflon-PTFE films prepared by pulsed-laser deposition
Reinhard Schwoediauer, Simona Bauer-Gogonea, Stefan Bauer, et al.
Polytetrafluoroethylene (Teflon PTFE) films were grown by pulsed-laser deposition (PLD). Films prepared by ablation from press-sintered targets are found to be highly crystalline, with spherulite sizes adjustable over more than one order in magnitude by suitable thermal annealing. As revealed by dielectric dilatometry, PLD-PTFE films show characteristics remarkably similar to those of conventional PTFE, i.e. the same structural first-order phase transitions. Dielectric losses are low and indicate no tendency to film oxidation. PLD-PTFE films additionally show an excellent charge-stability, comparable and even superior to commercially available Teflon-PTFE foils. PLD-PTFE enlarges the family of Teflon materials and may thus become interesting for potential miniaturized electret devices. Furthermore, dielectric dilatometry provides an elegant means for the determination of the coefficient of thermal expansion in thin nonpolar films.
Electrical properties of some plasma polymers obtained by remote microwave plasma chemical vapor deposition
M. Wlodarska, E. Staryga, A. Walkiewicz-Pietrzykowska, et al.
Some electrical properties of amorphous hydrogenated silicon-carbon films produced by remote microwave hydrogen plasma chemical vapor deposition technique are discussed. The DC conduction of Al/a-Si:C:H/Au and Al/a-Si:C:H/n-Si structures has been measured. Dielectric response of these structures has also been examined. The films are weakly conducting materials with the activation energy of the conductivity equal to 0.28 eV and 0.03 eV and 0.03 eV. MIS structures containing the amorphous silicon-carbon films as an insulator show the influence of trapping phenomena at the insulator-silicon interface.
In-situ electrical analysis in view of monitoring the processing of thermoplastics
J. M. Gonnet, J. Guillet, A. Ainser, et al.
In the last recent years, electrical techniques like microdielectrometry have presented an attracting and increasing interest for continuous monitoring, in a nondestructive way, of the advancement of the reaction of thermoset resins under cure. We think that the use of electrical analysis for in situ monitoring of chemical reactions can be extended to get information on thermoplastic and the physical phenomena such sa crystallization or study of residence time distribution in processing machines such as extruders.
Electrical properties of polyimide
Shigetaka Fujita, Katsuyoshi Shinyama, Makoto Baba
This study examines the dielectric characteristics and conduction current characteristics of polyimide (PI) film in a wide temperature range from the room temperature to 500 degrees C using electron beam irradiated specimens. At frequency of 100 Hz, relative dielectric constant began increasing rapidly at about 420 degrees C. The higher the dose, the smaller (epsilon) r. At 10 kHz, the temperature at which (epsilon) r began increasing rapidly was higher and the value of (epsilon) r was smaller. The 100 MGy specimens had the smallest dielectric tangent at both 100 Hz and 10 kHz. At 10 kHz, the temperature at which tan(delta) began increasing rapidly was higher than at 100 Hz. The unirradiated specimens have smaller conduction current than the irradiated specimens up to 250 degrees C, but the unirradiated specimens have larger conduction current than the 100 MGy irradiated specimens at 250 degrees C and higher. At 500 degrees, C the unirradiated specimen became equal to the 5 MGy and 50 MGy specimens. Then, the space charge distribution in the specimens was measured by the PEA method. Both the unirradiated specimens and the 100 MGy irradiated specimens showed small polarization charge due to pulse application, but no charge distribution over 0.7 C/m3 was found.
Detection of structural inhomogeneity in graded-index polymer materials by space charge measurements: numerical solutions
Marian Kryszewski, Grzegorz W. Bak
Introducing by diffusion guest molecules into a host polymer we can obtain very interesting materials with gradually changing refractive index. Such graded index materials may be used for many optical purposes. It has been shown previously that the gradient profile of guest molecules may be not consistent with the Fick's law. This means that it might be useful to find an independent non-destructive method of determination of gradient profile of guest molecules. If the guest polar molecules are diffused into a host non-polar matrix then the guest molecules may form charge traps. Detailed numerical analysis of space charge limited currents shows that it is possible to estimate the gradient profile of the charge traps at the injecting electrode if the concentration of the traps at the very surface is known.
Tunnelling-mediated recombination-induced luminescence in polyethylene naphthalate
G. Teyssedre, C. Laurent
Electroluminescence-EL- is excited in Poly(ethylene 2,6- naphthalate)-PEN-films submitted to a DC field of the order of 3 MV/cm. The decay of the light emitted after field removal, so-called delayed luminescence, is analyzed and it is shown that light emission can be interpreted in terms of tunnelling recombination of deeply trapped electrons to the luminescent centers forme d by the ionized molecules. The EL decay obeys an equation of the form I varies direct as (1 + (alpha) .t)-m and appears temperature independent. A similar behavior was obtained after ambipolar charging of the film surface by a cold plasma discharge. Wavelength resolved emission spectra obtained in the different situations are compared to the photoluminescence spectrum of the polymer. The predominance of phosphorescence in the electroluminescence spectrum of PEN is explained by a direct coupling between the trapping center and the triplet state of the molecules.
Polymer-like Materials, Epoxies, and Composites: Dielectric, Optical, and Electrical Studies of their Properties
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Dielectric and light scattering analysis of the -relaxation of an epoxy system
L. Comez, Daniele Fioretto, L. Palmieri, et al.
The dynamics of an epoxy compound has been studied by wideband dielectric spectroscopy and light scattering above the glass transition temperature, Tg. A change of diffusion mechanism in the supercooled liquids has been recognized some tens of degrees above the glass transition, where a change in the temperature behavior of the main relaxation time occurs, together with the splitting of the structural and secondary relaxations and deviation from the Stokes-Einstein-Debye diffusion law. The light scattering results can be interpreted in terms of a correlation length for the structural relaxation which increases for decreasing temperature.
Epoxide crosslinking studies by means of recording electrode current
Aleksander B. Szymanski, A. Bukowska, W. Bukowski
Monitoring of adhesive cure can be effected by means of recording of different physical values. In this paper, the results are reported about recording of samples electrical conductivity. In present contribution a modification of old proposal due to crosslink monitoring by DC technique is proposed. To avoid distortion measurement by electrode polarization process pseudo-non polarizable electrodes were applied.
Low-frequency electrical properties of non-crosslinked epoxide materials
A. Blahut, Wiktor L. Szymanski, Aleksander B. Szymanski
Our investigations are mostly focused on sample electrical properties in isothermal conditions. Temperature of these test was varied from 298 K up to 353 K. There was possible to observe different regimes of electrical conduction: bulk transport of impurity ions, electrode limited conduction and AC contribution of trapped carriers.
Dielectric properties of different composite structures
Enis Tuncer, Stanislaw M. Gubanski
Finite element calculations of frequency dependent dielectric permittivity of different binary system composed of conductive cylindrical inclusions in a less conductive matrix are presented. The inclusions were distributed regularly in three different geometrical structures. The smallest repeatable units of the structures were used in the computations. The relative permittivities of the inclusions and matrix were taken to be the same and equal to 4, whereas their conductivities were 1 X 10-7 and 1 X 10-20 ((Omega) X m)-1, respectively. The systems showed classical Debye behavior. An increase in the dielectric constant at the low frequency side as observed with did not show any differences when q <EQ 0.30. However, the relative permittivities were not the same for different lattices at higher q values. This effect was due to the interaction of the inclusions with each other.
Pyroelectricity in PVDF/PZT ceramic composites of 0-3 connectivity
Marian Olszowy
The pyroelectric properties of PVDF/PZT composites with 0-3 connectivity pattern are studied experimentally. PVDF/PZT composite films have been produced by introducing fine grain PZT powder in the matrix of PVDF using a well-known hot- pressure method. These films were polarized in a strong DC electric field at elevated temperatures. The pyroelectric coefficient of PVDF/PZT depends on the volume fraction PZT in composite, on the electric field strength and temperature. The results of measurements of the pyroelectric properties of the PVDF/PZT mixture with the various volume fraction of the components are given in this report.
Generalities About Liquid Crystals and Related Materials
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Time-domain investigation of liquid crystals and epoxide materials
Aleksander B. Szymanski
The contribution is related to experimental studies on electrical properties of organic materials carried in time domain, molecular liquids, particularly.
Correlations in a system with complex dynamics
Maria Massalska-Arodz, Jan Krawczyk
The (alpha) -relaxation connected with reorientations of elongated molecules around the short axes was extracted using Cole-Davidson formula from the process observed in the isotropic and cholesteric phase of isopentycyanobiphenyl at the frequency range from 0.1 Hz to 100 kHz. Description gives the following parameters pointing to non-Debye and non-Arrhenius features of the process: the Jonscher parameters n1 approximately equals 0.5 and n2 equals 1, fragility m equals 50, exponent of the Kohlrausch-Williams-Watts relaxation function (beta) equals 0.7-0.87 and T0 equals 172 K from the Vogel-Fulcher-Tamman equation. The data obeys the scaling relation proposed in 1990 by group of Nagel.
Investigations of Dielectric, Electrical, and Electro-optic Properties of Liquid Crystals
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Dielectric properties of liquid-crystalline S-MHPOBC in the low-frequency range
Krzysztof Wojciechowski, Agnieszka L. Gromiec, Grzegorz W. Bak
Collective motions of molecules in a ferroelectric liquid crystal have been investigated using broadband dielectric spectroscopy. S-MHPOBC is a smectic liquid crystal with antiferroelectric and ferroelectric phases. In those phases (epsilon) ' and (epsilon) ' were measured in the frequency range 10-2 divided by 5*104 Hz. Two dielectric loss peaks were detected. The low-frequency peak has its maximum at about 0.1 Hz and is caused by Maxwell-Wagner relaxations. Goldstone mode is responsible for the second high-frequency loss peak. Both peaks may be well described by Havriliak- Negami equation. The changes of Havriliak-Nagami parameters (alpha) and (alpha) (beta) well correspond to the phase transitions detected in S-MHPOBC.
Dielectric properties of antiferroelectric liquid-crystalline mixtures
F. Goc, Wojciech Kuczynski, J. Malecki, et al.
Antiferroelectric liquid crystalline mixtures having smectic CA* phase in very broad temperature range have been investigated. Measurements of dielectric relaxation and spontaneous polarization were performed. Two absorption peaks in the existence range of the antiferroelectric CA* phase in the kHz-MHz range of frequency were observed. Both peaks are quite weak and of Debye type. Their characteristic dielectric strengths are almost temperature independent. The peaks are probably related to the motions of molecules on the side of a cone, defined by the molecule tilt angle, the slower process is related to the motions of molecules, tilted in opposite directions in subsequent layers and moving in the same direction. The process with shorter relaxation time is attributed to the movement of molecules in opposite directions. Both modes are active in dielectric measurements due to small residual polarization being a consequence of the helical superstructure.
Switching behavior of antiferroelectric liquid-crystalline mixtures
Results of investigations of the switching phenomenon in antiferroelectric liquid crystals are presented. Mixtures possessing the smectic C*A phase in very broad temperature range, including the room temperature, were investigated. The influence of several experimental conditions on switching phenomenon was studied. The switching process was detected using two independent methods. In the electrooptic method, the intensity of light passing the sample placed between crossed polarizers was registered. In the second method the current flowing across the cell during the change of the polarization direction was measured. Both methods have given consistent results. The critical electric field strength needed for changing the polarization direction by 180 degrees has been determined as function of temperature, pulse shape and frequency. Two kinds of switching were observed. At low frequency of triangle-shaped of temperature, pulse shape and frequency. Two kinds of switching were observed. At low frequency of triangle-shaped voltage the switching between three stable states took place. At higher frequencies, or when the voltage has rectangular shape, we observed the switching between two ferroelectric states. A simple and accurate method of measurement of the threshold field for bistable switching is proposed.
Dielectric relaxation phenomena in side-chain liquid-crystalline polycarbosilane of various spacer lengths
Maria Mucha, Joseph Grzyb, Andreas Schoenhals
Dielectric loss has been measured over the frequency 10-2-106 Hz between -100 degrees C to 80 degrees C for side-chain liquid crystalline carbosilanes, whose different mesophase structure was reached by the variation of the number of alkyl spacer groups. Three relaxation processes known as (beta) , (alpha) and (delta) were found and discussed. The characteristic (beta) relaxation corresponds to fluctuational dynamics of a transverse component of dipole moment which has Arrhenius-like character. Its activation energy Ea(beta ) changes with an increasing spacer length. The non Arrhenius-like (alpha) process is related to a dynamic glass transition of the polycarbosilanes. The (delta) process connected with rotational fluctuation of mesogenic longitudinal dipoles around short axis is observed over the whole range of frequency. The activation energy Ea(delta ) of (delta) relaxation depends on the polymer order and the phase structure such as: glassy amorphous, viscoelastic amorphous, mesomorphic or isotropic reached at various temperatures. Ea(delta ) value ranges from 32 kJ/mol to 120 kJ/mol. Electro-optical effect for the sample with x equals 4 is observed at various temperatures with varying intensity. In the vicinity of isotropization the reorientational effect due to electric field is suppressed by heat disorder.
Dielectric relaxation of 6-CB dispersed in polymer matrix
Jan Jadzyn, Grzegorz Czechowski, Maria Mucha, et al.
Dielectric relaxation studies for n-hexylcyanobiphenyl in pure state and dispersed in polymer matrix, have been performed. Polymer dispersed liquid crystal (PDLC) films have been prepared with different 6-CB contents. The measurements have been carried out in the frequency range 10 kHz divided by 100 MHz for pure 6-CB and 10 kHz divided by 10 MHz for PDLC films. A method for evaluation of the fraction of the 6-CB molecules which are not affected by the polymer surface anchoring forces in the nematic droplets, has been presented.
Static and dynamic dielectric properties of 6-OCB and 8-OCB
Susan Czerkas, A. Burczyk, Grzegorz Czechowski, et al.
Static electric permittivity and dielectric relaxation have been measured in isotropic and nematic phase of 6-OCB and 8- OCB. On the basis of the temperature dependence of the principal static permittivities of nematics, (epsilon) (parallel)(T) and (epsilon) (perpendicular)(T), using the Maier-Meier equations, the following quantities have been obtained: i) the angle between the dipole moment vector and the long axis of mesogenic molecules, ii) the molecule apparent dipole moment square (mu) 2app(T) and iii) the nematic order parameter S(T). From the temperature dependence of the dielectric relaxation time, the activation energy for the molecular rotation around the short axis has been calculated and, taking into account the viscosity data, the effective dimensions of the nematogen molecules have been evaluated.
Nonlinear dielectric relaxation in dipolar systems
Przemyslaw Kedziora, Jan Jadzyn, Louis Hellemans
Studies of the nonlinear dipolar relaxation in the frequency range from 1 MHz to 3 GHz have been carried out for diluted solutions of 4,4'-n-hexylcyanobiphenyl and 4-(trans-4'-n- hexylcyclohexyl)isothiocyanatobenzene in benzene at 25 degrees C. The nonlinear dielectric effect has been measured with a weak alternating field superimposed on a strong quasi-static bias field of the amplitude of 1.1 X 107 V/m. The nonlinear relaxation spectrum has been interpreted in terms of the theory proposed by Coffey et al.
Dynamics of n-p-(ethoxybenzylidene) p'-propylaniline (EBPA) in solid phases
Andrzej Bak, Antoni Kocot, Maria Massalska-Arodz
In both solid phases of EBPA the dielectric relaxation phenomenon has been observed in the frequency range from 1 Hz to 10 MHz. The process observed in a metastable phase K1 is stronger than this observed in a stable phase K2. Analysis of the temperature dependence of complex dielectric permittivity points to a non-Debye relaxation in each solid phase. THe composition of two Cole-Cole functions has been fitted successfully to the observed results. Each of two molecular motions extracted is of the Arrhenius type. Processes have the same activation energy barriers in a stable phase K2 and in a metastable phase K1.
Polarizability anisotropy of CBn molecules
Przemyslaw Adamski
The temperature dependence of the polarizability anisotropy and polarizability tensor components.
Interfacial effects in nematic liquid crystals
Wiktor L. Szymanski, Aleksander B. Szymanski
Long-time isothermal currents in liquid crystals are in test. It is shown, that in nematic as well as isotropic pentacyanobiphenyl/5CB/quasi DC conductivity is observed for elevated time of measurements. The investigation of these current are presented. It is suggested that these current may originate from carrier exchange between solute molecules and electrode due to strong interface field.
Electrical conductivity of liquid-crystalline materials; old results in light of new findings
Aleksander B. Szymanski, Elzbieta Szwajczak, J. Michalski, et al.
Here is considerable literature related to the electrical conductivity of liquid-crystalline materials. However, it seems, that the electrode interfacial phenomena were not taken into account consequently. This provoked some criticism. This criticism is supported by frequency domain data. These data are indicating electrode polarization as dominant mechanics, in long period time, particularly. The data related to frequency domain and time domain were analyzed. Now, it seems, that at least in liquid-crystalline cyanobiphenyls materials, due to electrode effects, DC conductivity can be observed. However, in this case one can expect non-uniformity of electrical field in the plane parallel/sandwich/sample. The aim of this contribution is an analysis of these effects and their relations with the result obtained in the past and the critical evaluation of a previously assumed material characterization.
Electro-optical investigation of nematic liquid crystals using dielectrometry
Aleksandar Andonovski, Margarita Ginovaska
An investigation is carried on two samples of nematic liquid crystals (NLC), products of 'Merck' - Germany ZLI-1800-000 with (Delta) (epsilon) > 0 and ZLI-4788-000 with (Delta) (epsilon) < 0. Method of dielectrometry is used for measurements in nematic phase of these liquid crystals. Parallel and perpendicular components of dielectric constants (epsilon) (parallel) and (epsilon) (perpendicular) are determined for different temperatures and frequencies from 5 Hz to 10 MHz of applied field. Changes of dielectric anisotropy versus frequency are presented, according to frequency-dispersion of dielectric constants and their imaginary and real components are obtained. Debye's and Coul-Coul diagrams are constructed for given frequencies only for the first LC caused by the low frequency dispersion for (epsilon) (parallel). Physical properties and accurate values of clearing point are obtained from measurements of temperature dependence of dielectric constants.
Electro-optical effect in PDLC complex system prepared by solution cast method
Maria Mucha, E. Nastal-Grosicka
THe present work has involved a fragment of our studies on phase separation processes in polymer-liquid crystal systems. We have studied the relationship between morphology and thermo- and electro-optical properties of liquid crystal - polymer composite materials achieved by SIPS method. The PDLC materials consists of the nematic liquid crystal of 20 to 60 percent weight fraction dispersed in amorphous polycarbonate. The PDLC films were prepared by casting from solution in methylene chloride. The result show that about 35 percent of the initial content of LC stays dissolved in the polymer matrix affecting its glass transition temperature. The PDLC samples exhibit good electrooptical properties in the case when the initial LC content is higher than 20 percent by weight. The threshold voltage is ranges from 10 to 20 V.