Jul 25, 1989 - were found to be 0.002 for films with homeotropic and 0.03 for films ... and the maximum diffraction efficiency of 50% for thick phase gratings.
1478
V.P. SHIBAEVet al.
non-plasticized PVC samples is the reason for the appearance of compositional inhomogeneity in plasticized composites. Translated by M. KUBfN REFERENCES 1. Yu. K. GODOVSKII, Teplofizicheskie metody issledovania polimerov (Thermophysical Methods of Polymer Investigation), Moscow, 216 pp., 1976. 2. V. P. PRIVALKO, Svoistva polimerov v blochnom sostoianii: Spravochnik po fizicheskoi khimii polimerov (Properties of Solid Polymers, in Handbook of Polymer Physical Chemistry), Voi. 2, Kiev, 330 pp., 1984. 3. Khimicheskii entsyklopedicheskii slovar (Chemical Encyclopedia), Moscow, 791 pp., 1983. 4. D. M. GEZOVICH, and P. H. GEIL, Int. J. Polym. Mater. 1: 3, 1971. 5. D . J . BLUNDELL, Polymer 20: 934, 1979. 6. P. R. COUCHMAN, Polym. Eng. Sci. 27: 618, 1987. 7. L. A. FAMINSKAYA, PhD Thesis. Gor'kii: Gor'kii State University, 235 pp., 1983.
PolymerScience U.S.S.R. Vol. 32, No. 7, pp. 1478-1486, 1990 Printed in Great Britain.
0032-3950/90 $10.00+ .00 © 1991PergamonPresspic
SPECIFIC FEATURES OF OPTICAL INFORMATION STORAGE IN ORIENTED FILMS OF LIQUID CRYSTALLINE COMB-LIKE POLYMERS BY MEANS OF SELECTIVE OPTICAL EXCITATION* V. P.
SHIBAEV,I. V. YAKOVLEV,S. G. KOSTROMIN,S. A. IVANOV and T. I. ZVERKOVA M. V. Lomonosov Moscow State University (Received 25 July 1989)
The effect of laser irradiation on optical properties of a comb-like liquid crystal acrylic copolymer containing cyano-biphenyl groups and groups of an azo-dye was investigated. It has been demonstrated that films with homeotropic or planar orientation made from the LC copolymer can be employed as materials for phase recording. Dependences of the induced birefringence on the intensity and polarization of the incident beam at various wavelengths have been established. The maximum attainable magnitudes of birefringence were found to be 0.002 for films with homeotropic and 0.03 for films with planar orientation.
THE RAPID progress of investigations in the field of synthesis and elucidation of properties of linear and comb-like liquid crystal (LC) polymers during the last ten to twenty years resulted in the foundation of a new branch of science aimed at producing novel LC polymeric materials [1]. The potential of comb-like LC polymers, due to their special structure, has been recently analysed in a number of monographs [2-5], where it is shown that these polymers open interesting routes to *Vysokomol. soyed. A32: No. 7, 1552-1559, 1990.
Optical information storage in comb-like LC polymers
1479
practical applications as optically and thermally sensitive materials in diverse devices for information storage and retrieval. Two principles that have been already realized practically, i.e., thermo-optical [5-7] and the so-called phase information storage [8, 9] underlie their application. The first application of comb-like LC polymers to thermo-optical storage of information by means of a laser, consisting essentially in homeotropic orientation of a LC polymer in an electric field, local overheating above the clearing temperature Tel (resulting in a local loss of the original orientation of mesogenic groups), followed by fixation (freezing) of information stored in this manner in a glassy matrix (assuming that Tg,~ Tel), has been demonstrated in [10, 11]. The second principle--phase recording--occurs because birefringence due to isomerization of azo-dye groups present in the polymer structure is invoked in the sample by means of polarized laser radiation [8, 9]. The "phase object" stored in this manner is a result of special modulation of the refractive index; information is retrieved by polarized light at a suitable orientation of the "writing" and "reading" rays. The first experiments concerning the application of comb-like LC polymers to phase and holographic recording were described in [8, 9]: using nematic or smectic films of LC polymers with azobenzene mesogenic groups, the authors observed birefringence An of the order of 7 x 10-31 × 10 -2 and briefly described the registered diffraction and holographix patterns. Unfortunately, no experimental details concerning these no doubt extremely interesting LC compounds are given in [8, 9], but the data presented indicate a relatively large spatial resolution (>3000 lines per mm) and the maximum diffraction efficiency of 50% for thick phase gratings. A holographic recording in cyclic siloxanes, also containing azo-dye groups and forming a mesophase of cholesteric type, is described in [12]. In this case the maximum diffraction efficiency was 38% with photosensitivity of 0.001 to 0.1 cm2/j (for A = 476 nm and 413 nm). All these results demonstrate the potential of comb-like LC polymers as highly sensitive reversible materials and prove the necessity of a detailed investigation of processes involved in the interaction of laser radiation with LC polymers. In this paper we present results obtained from studying the optical information storage in oriented films made of a comb-like LC polymer containing chemically attached groups of an azo-dye, and discuss in detail the changes of these properties induced by laser radiation. The material investigated was an acrylic copolymer containing cyanobiphenyl mesogenic groups that are easily oriented in an electric field, along with groups of an azo-dye capable of reversible conformational changes (cis-trans isomerization) when irradiated by laser light: I
CH,,
0
CI H - - CII _O_tCH2~._ ~ /*
O _ /~f - - ~
/z----~ CN
,..__..Jx
I
CH2 0
. -c-
o - ic
,l,-o
,ly
where z = y/(x+y) = 0.2. The polymer was synthesized according to the method described in [13]. Its composition was determined from absorption spectra of the azo-dye, using absorption at A = 367 nm (where the extinction coefficient of the dye is 3.4 x 106 l/(mol cm). The composition proved to be the same as that of the monomer mixture: 20% of the building blocks contained the azo-dye group.
1480
V.P. SHIBAEVet al.
The phase transition temperatures, the structural types of mesophases, and the formations produced in the polymer films were studied using a polarization microscope POLAM R-211 provided with the thermostating system Mettler FP800. The glass-transition temperatures were 40 + 5°C for the copolymer, 105°C and 110°C for the mesogenic phases SA and N, respectively. Samples for optical measurements were prepared by placing an isotropic melt of the copolymer into the slit of a sandwich-type cell consisting of two planar, transparent In203 electrodes on a glass substrate (surface area 1 to 5 cm2), separated by Teflon spacers of various thickness (6, 10, or 20 tzm). Homeotropic orientation was achieved by applying a.c. voltage (50 Hz, 30 to 100V) to the electrodes. The temperature was then gradually lowered to bring the copolymer to the LC state. The optical axis of the resulting single-domain, oriented polymer film was perpendicular to the substrate surface. To achieve the planar orientation the surface of the glass substrates was first polished unidirectionally. After prolonged (several h) annealing of the copolymer in this cell at 107°C (i.e., in the nematic phase) the film attained a planar orientation where the axis was parallel with the glass surface and coincided with the direction of the preliminary" polishing. The effect of laser radiation was studied in a specially built apparatus (Fig. 1). Vertically polarized laser beam I of 1 mm diameter was directed to a selected area of the cell surface 5. To register the ensuing structural changes the centre of the area was simultaneously irradiated with a He-Ne laser 6; it was established by preliminary experiments that the light of this laser does not affect the copolymer properties. The angle between the polarization planes of rays I and II was 45 °. To facilitate the measurement for the homeotropic polymers, a quarter-wave phase plate 8 with optical axis oriented vertically was inserted. During the measurement the LC cell was kept at room temperature. The intensity of ray II that has passed through the cell and the analyser was registered by a photodiode FD-10G (10). The intensities measured with the analyser axis oriented perpendicularly to and in parallel with the polarization plane of ray II can be then used to evaluate the birefringence of the LC polymer, An = nl -- n2 [14]. The intensity of the argon laser was gradually increased during the experiment. After changing the intensity of the incident beam the magnitude of birefringence An at first rapidly changed, attaining a new value after about 60 s. The character of time variation of An was the same as that described [15] for birefringence induced in oriented films
Z
/ IT]
-I,
A,
I
,, ' g
FIG. 1,
8
y
7
A scheme of the experimental arrangement designed for information storage in a film of a LC
polymer: 1--argon laser, 2, 3--mirrors, 4---neutral filter, 5---LC cell, 6---He-Ne laser, 7--focusinglens (focal length 30 cm), 8--A/4 phase plate, 9---analyser, 10--photocell, ll--microampermeter. I--writing beam, II--reading beam.
Optical information storage in comb-like LC polymers
1481
of PVC with the low-molecular-mass dye Methyl Orange added. When the laser beam was switched off, An dropped somewhat (also within some 60 s) and levelled off at a value determined by the experimental conditions employed. When observed in the polarization microscope, the areas of the copolymer with induced birefringence exhibit an interference pattern depending on the magnitude of An. The sign of birefringence was determined by the compensation method using a quartz plate and wedge in the polarization microscope [14]. The character of the phase record in the homeotropically oriented copolymer film, observed in the polarization microscope with crossed polaroids, is shown in Fig. 2. To obtain this pattern a diffraction grating with 25/~m period was situated on the surface of the LC cell prior to illuminating it with laser light. The optimum visibility of irradiated regions was attained when the cell was situated so that the angle between the direction of the polarization plane of the recording ray and the polarizer axis was +45°; when the angle was 0 or +90 °, the record was no longer visible. It is thus apparent that the laser radiation brings about induced birefringence, where the refractive index n2 in the direction coincident with the plane of polarization of the incident beam I remains the same. The light of the argon laser influences the refractive index nl for vibrations orthogonal to the electric vector of the laser beam I, as shown by diffraction of ray II on the recorded structure. The diffraction is the largest when the polarization planes of the writing (I) and reading (II) beams are perpendicular; if the two polarization planes coincide there is no diffraction. Thus, the laser radiation raises the refractive index nl. Figure 3a shows the results of experiments performed with the aim of determining the magnitude of induced birefringence An = nl - n 2 and the character of its dependence on the intensity of the recording beam. It is apparent that as the intensity is raised from 0.1 to 0.15 W/cm 2, An at first increases and then saturates at A n m a x = 0.0003 at a certain limiting intensity Wli m , the latter depends on the wavelength of the incident beam, being 0.10 W/cm 2 for ~1 = 457.9 nm and 0.15 W/cm 2 for A = 514.5 nm. The plot of An vs intensity (Fig. 3b) shows that the increase of birefringence An is at first proportional to W k. The sensitivity S of the LC polymer was evaluated in accord with the formula S = An/A log W from the slope of the straight line segments of curves 1 and 2 in Fig. 3b. The difference between the values $1 = 2.3 x 10 - 3 (at hi) and $2 = 1.8 x 10 - 3 (at h2) can be apparently attributed to larger absorption of radiation with AI = 457.9 nm by the azo-dye. When the laser beam is switched off, the birefringence drops to a residual value Anres = 0.002, as shown by curves 1' and 2' in Fig. 3a. The residual birefringence then remains stationary at room temperature.
i i FIG. 2.
,
J
Phase structure r e c o r d e d in a L C p o l y m e r film as visualized in a polarization microscope.
1482
V . P . SHIBAEVet al. An X 10 3
(a) !
"r
,, 22
_....g
I
0.I
iI
-3
I
0,2
-2
- /
1ogW
IN, W l c m 2
FIG. 3. The birefringence An induced in a LC polymer film as a function of laser light intensity in normal (a) and logarithmic (b) coordinates; A1= 457.9nm (1) and 514.5 nm (2). 1' and 2' represent the variation of An after switchingoff the laser.
The record can be erased by heating the polymer above the clearing temperature TCL and applying an electric field; thus, the recording is reversible. This method of erasure is not unique, however. The ability of the copolymer to change its optical characteristics under irradiation enables one to control the process by purely optical means. The fundamental parameters are the spectral distribution, intensity, and polarization of the irradiating beam. Thus, for example, the non-polarized, intensive light of the sun restores the polymer to its original homeotropic state within several hours. A number of experiments were carried out to elucidate the nature of these phenomena. Each experiment included several stages where the sample was irradiated by a laser beam of different polarization; the value of An was registered continuously. A segment of the homeotropically oriented sample was at first irradiated by vertically polarized laser light with intensity gradually increasing up to W l i m and then decreasing to zero; the resulting induced birefringence was designated as An~. In the second stage the cell was rotated by 90° around the axis of the incident laser beam and the recording process was repeated, i.e., the selected area was again irradiated, this time with light in the plane of polarization perpendicular to that previously employed. The resulting magnitude of birefringence was denoted by An2. In the third stage the process was repeated once more with the cell in the original position; the final birefringence was An3. Results of these experiments are visualized in Fig. 4: the intensity of the laser beam with polarization plane parallel with a selected direction Y on the cell surface is plotted on the positive semiaxis of the abscissa, whilst the intensity of light with the polarization plane lying in the orthogonal direction X is plotted on the negative semiaxis. The measured birefringence represents the difference between the refractive indices for vibrations along the axes X and Y, An = n x - ny. The segment OAB of the curve corresponds to the preliminary recording (OA corresponds to the irradiation, A B represents the relaxation after switching off the laser), leading to the birefringence An~ (point B in Fig. 4). During the repeated recording with the cell rotated the birefringence at first dropped from An~ to zero (segment BC---erasure), then changed to An2.ma, at the intensity W x = W l i m (segment CD) and relaxed to An2 during the lowering of beam intensity (segment DE). The segment EFAB corresponds to the third stage of the process where the plane of polarization again coincides with the Y axis. The experiment has shown that the state of the LC copolymer at the end of each stage is characterized by induced birefringences identical in absolute value, lAnai = IAn2l -- I~n3l -- 0 . 0 0 2 ; the individual states (points B and E in Fig. 4) differ only in the direction corresponding to the more rapid vibrations in the LC polymer, identical with the plane of the electric vector in the "recording"
Optical information storage in comb-like LC polymers
1483
4 1 7 "]ff J
J3 4hi=an~
W~im
C',
an~
3 FIG. 4.
Changes of birefringence induced in a homeotrnpically oriented copolymer sample by irradiation with laser light of different polarization; see explanation in text.
....7~
i.
L
1 mm I
FIG. 5. Micrograph of homeotropically oriented LC film, irradiated in turn with laser beams of different polarization, registered by means of a polarization microscope. The polarizer and analyser in the microscope were oriented at an angle of 45° with respect to the axes X and Y. a--area II irradiated with a beam polarized along the Y axis at intensity W = 0.15 W/cm 2 (area I shielded); b--area III--IV irradiated with light polarized along X, W = 0.04 W/cm 2 (area I-II shielded); c--area V irradiated with light polarized along X, W = 0.15 W/cm 2 (area I-II-III-IV shielded).
wave. Reversible transitions between these states induced by a suitable radiation are possible (segments B C D E and E F A B ). Micrographs of a LC film shown in Fig. 5 illustrate the processes proceeding during the repeated recording. They show the same area of homeotropically oriented copolymer sample, which was PS 32:7-N
1484
V . P . SHIBAEVet al.
gradually irradiated with individual parts shielded by the opaque grating. The orientation of the vector E of the incident laser beam during each stage is shown in the figure. The segments nx and ny are proportional to the refractive indices measured along the respective axes X and Y. The micrograph in Fig. 5a was obtained after the first stage of irradiation of film area II; the shielded area I preserved the original homeotropic orientation and appears dark between the crossed polarizers of the microscope. This state of the copolymer is represented in Fig. 4 by point O where An = 0, i.e., the refractive indices ny and ny are identical. Area II was irradiated by a laser beam of intensity Wli m and with the plane of polarization parallel with the Y axis, to achieve in this area a residual birefringence An1 = nx - ny > 0; in other words, the refractive index nx for vibrations orthogonal with the direction of vector E of the incident beam increased, nx >ny. This area, when observed in the microscope in an orientation where the X axis of the sample lay diagonally to the axes "of the crossed polarizers, assumed an interference colouring of first order (light-grey). The state of area II at the end of the first stage corresponds to point B on the segment d A B in Fig. 4. This part of the experiment is a reproduction of experiments described previously (Fig. 3a). Fig. 5b shows the sample after irradiation of its area III-IV by laser light polarized in parallel with the X axis (areas I-II were shielded); the intensity was first increased from zero to W' (Fig. 4) and the laser was thereafter switched off. Area III of the micrograph again became dark: the birefringence changed from Anl to zero, nx = fly. Thus, after repeated irradiation with another polarization (along the X axis), area III of the LC film returned to the original state of homeotropic orientation (point O at the end of the curve B C ' O in Fig. 4); in other words, the stored information was erased by means of laser light. Irradiation increased the refractive index ny in area IV; accordingly, the difference An changed from zero (point O in Fig. 4) to An2>0 (point G in the segment O C " G ) . It is apparent from Fig. 5b that area IV became light-grey, although less bright than area II. This is understandable since irradiation with an intensity W' smaller than the limiting value Wli m led to a smaller change of the difference An (JAn2' [< [Anl[). In the third stage (Fig. 5c) area V was irradiated with light of intensity Wjim without changing the beam polarization (still along the x axis). The birefringence attained the maximum value An2.max (curves O D and G D in Fig. 4 for segments corresponding to areas III and IV) and after switching the laser off relaxed to the residual value An2 (point E), where JAn2[ = [anll. It follows from the above discussion that the structure of the LC copolymer can be reversibly changed by irradiation with a suitably polarized light. Changes proceeding in each area of the sample are characterized by two parameters: (1) the induced birefringence An = n 1- - n 2 and (2) the orientation of the plane in which the vibrations of the slow wave propagate through the LC polymer (refractive index nl ). The possibility to create structures where parameters such as the magnitude of birefringence and orientation of the polarization plane of light vary across the sample volume enables one to use the material for either phase or polarization recording [15, 16]. The differences in An play the decisive role in the former case; depending on the state of polarization of the incident light, the different directions of optical axes in individual microvolumes of the LC polymer are the controlling factor in the latter case. The dependence of the induced birefringence on polarization of the incident light can be explained, in analogy to [17], by photoinduced cis-trans isomerization of the aromatic azo-groups in the branches of the LC polymer. Figure 3 demonstrates that the maximum attainable value of birefringence in homeotropically oriented mesogenic groups is 0.002. However, it has been shown by similar experiments carried but with films having planar orientation that in this case the maximum value of An can be raised by an order of magnitude.
Optical information storage in comb-like LC polymers
1485
An x 10 2
I 2
l - / z I
da
0..I
0,2 W, Wlcm 2
FIG. 6. Birefringence induced in films of the LC polymer with a planar orientation. Curves 112, and 3, 4 were registered for two samples, each irradiated with a laser beam at two areas presumably differing in the quality of planar texture. 1' to 4' are the relaxation curves.
Figure 6 provides evidence that the character of the dependences of rise and subsequent relaxation of birefringence on the light intensity is fully analogous to that described previously for the homeotropically oriented LC polymer. The somewhat larger scatter of experimental points in the "planar" samples is probably due to variations in the quality of the planar texture across the sample. Nevertheless, the maximum observed values of An (between 0.02 and 0.03) subsequently exceed those obtained in [8, 12]. The authors wish to thank A. V. Parfenov from the Physical Institute of the U.S.S.R. Academy of Sciences for advice and help with the design of the experimental setup.
Translated by M. KuafN
REFERENCES 1. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13.
V. P. SHIBAEV, Khim. volokna 1987, No. 3, p. 4. N. A. PLATE and V. P. SHIBAEV, Comb-shaped Polymers and Liquid Crystals, New York, p. 415, 1987. Zhidkokristallicheskie polimery (Polymer Liquid Crystals), N. A. Platr, Ed. Moscow, p. 415, 1988. Side-Chain Liquid Crystal Polymers, C. B. McArdle, Ed. Glasgow, 450 pp., 1989. S. A. IVANOV, I. A. YAKOVLEV, V. Yu. VETROV, S. G. KOSTROMIN and V. P. SHIBAEV, Pis'ma v Zh. teor. fiz 9, 448, 1983. V. P. SItIBAEV, S. G. KOSTROMIN, N. A. PLATI~, S. A. IVANOV, V. Yu. VETROV and I.A. YAKOVLEV, Polym. Commun. 24: 364, 1983; Polymeric Liquid Crystals, A. Blumstein, Ed. New York, p. 345, 1985. H . J . COLES, Faraday Disc. Chem. Soc. 1985. No. 79, p. 1, 1985. M. EIClt, J. WENDORFF, B. RECK and J. RINGSDORF, Makromol. Chem. Rapid Commun. 8: 59, 1987. M. EICH and J. WENDORFF, Makromol. Chem. Rapid Commun. 8: 467, 1987. C. McARDLE, m. CLARK, C. HAWS, M. WILTSHIRE, A. PARKER, G. NESTOR, G. GRAY) D. LACEY and K. TOYNE, Liquid Crystals 2: 573, 1987. C. B. MeARDLE, Side-Chain Liquid Crystal Polymers, C. B. McArdle, Ed. Glasgow, p. 357, 1989. R. ORTLER, Ch. BRAUCHLE, A. MILLER and G. RIEPL, Makromol. Chem. Rapid Commun. 10: 189, 1989. S. V. BELYAEV, T. I. ZVERKOVA, Yu. P. PANARIN, S.G. KOSTROMIN and V.P. SHIBAEV, Vysokomoi. soyed. B28: 789, 1986 (not translated in Polymer Sci. U.S,S.R.).
1486
Yu. V. ZELENEVand V. A. IVANOVSKII
14. N. M. MELANKHOLIN, Metody issledovania opticheskikh svoistv kristallov (Methods for Studying Optical Properties of Crystals). Moscow, p. 42, 1970. 15. T. TODOROV, L. NIKOLOVA and N. TOMOVA, Appl. Optics 23: 4309; 4588, 1984. 16. Sh. D. KAKICHASHVILI, Optika i spektroskopia 32: 324, 1972. 17. A. M. MAKUSHENKO, B. S. NEPORENT and O. V. STOLBOVA, Optika i spektroskopia 21" 557, 1971.
PolymerScienceU.S.S.R.Vol. 32, No. 7, pp. 1486-1490, 1990
0032-3950/90 $10.00+ .00 © 1991 Pergamon Press plc
Printed in Great Britain.
THERMO-ELECTROFLUCTUATION METHOD POLYMER PROPERTIES*
FOR
STUDYING
Yu. V. ZELENEV and V. A. IVANOVSKII F. E. Dzerzhinskii Military Technical Institute of Aviation Engineering, Tambov; A. N. Kosygin Moscow Textile Institute
(Received23 June 1989) The potential of thermo-electrofluctuation measurements for studying polymer properties has been evaluated. The possibility was tested as to how to measure the information-carrying signal--the meansquared voltage of electrical fluctuations on the clamps of a primary capacitor-type transducer with the investigated polymer as the dielectric--and to aeduce the internal pressure, the cohesion energy, and the moduli of elasticity in shear and elongation from the experimental data. The internal pressure was evaluated from the measured intensity of electromagnetic fluctuations for poly(vinyl chloride), polystyrene, poly(methyl methacrylate), and natural rubber.
APPLICATION of various polymers and polymeric composites in industrial practice often requires very fast and reliable estimation of their physical characteristics. Of considerable promise in this respect is the non-destructive thermo-electrofluctuation method [1], which uses as the informationcarrying signal the intensity of electromagnetic fluctuations due to thermally-induced oscillations of structural elements present in the material analysed: these include the individual atoms and their groups such as the side groups, macromolecular segments and various supermolecular formations [2]. As a result of the chaotic motion of all these elements (relaxators) characterized by effective mass m and intrinsic or induced charge q, continuous fluctuations of charge density proceed in each volume element. The ensuing electromagnetic waves and electromagnetic field extends to a distance of several thousand/~ [3] and forms the basis of intramolecular covalent and intermolecular van der Waals interactions. The overall fluctuational electromagnetic field represents a sum of individual microfields due to fluctuating charges and microcurrents in various volume elements in the medium; their voltage is
Ei (o~, r) = Ei ° (oo, r)
4~d
drldr2Gik (~o, r, rt ) [ekm (oo, ri, r2) -- 8,,,l Em° (co, r2)
i° I drlGk(ta, r, rl)jkSP(W, rl),
~C2
*Vysokomol. soyed. A32: No. 7, 1560-1563, 1990.
(1)
