INTRODUCTION
Over the past few years, combshaped liquidcrystal line polymers containing mesogenic and photochromic groups have attracted the attention of researchers engaged in various fields of science and engineering as promising photocontrollable smart materials [1, 2].
Among three main structural modifications of LC polymers, namely, nematic, smectic, and cholesteric, cholesteric LC polymers having helical supramolecu lar structure and possessing unique optical properties (selective reflection of light, high optical activity, etc.) are of particular interest. Precisely these optical prop erties determine application areas of these polymers: selective filters and reflectors of circularly polarized light, color photocontrollable films and coatings, sys tems for data recording, etc. [3–5].
Cholesteric LC polymers (Fig. 1a) are mostly synthe sized via the freeradical copolymerization of mesogenic (nematogenic) and chiral monomers; in addition, pho tochromic LC copolymers containing mesogenic and photochromic groups can be prepared (Fig. 1b). Photo
chromic cholesteric LC polymers (Fig. 1c) are synthe sized through the ternary copolymerization of mesogenic, photochromic, and chiral monomers. Another method, which is based on the incorporation of chiral lowmolecularmass dopants into nematic photo chromic LC copolymers (Fig. 1d), has received wide acceptance, since in the case of many cholesteric LC polymers, optical properties can be easily controlled in a wide spectral range through a change in the concentra tion of chiral additives added to mixture formulations while preserving their high stability.
The main task of this study is to compare the opti cal and photooptical properties of photochromic polymer LC systems of two types: the simplest nematic photochromic LC polymers (Fig. 1b) and more com plex photochromic cholesteric LC systems character ized by the helical supramolecular structure (Fig. 1d). As a nematic photochromic polymer, we synthe sized the acrylic LC copolymer containing equimolar amounts of nematic phenyl benzoate and photochro mic azobenzene groups
COO O O O O O CH CH2 COO O N N CN CH CH2 0.50 0.50 PAAzo
Photoinduced Reorientation Processes in Thin Films
of Photochromic LC Polymers on Substrates
with a Photocontrollable Command Surface
1A. V. Ryabchun, A. Yu. Bobrovsky, and V. P. Shibaev
Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia email: alexmsu@bk.ru
Received October 7, 2009;
Revised Manuscript Received February 11, 2010
Abstract—Orientation and reorientation processes that occur in nematic and cholesteric LC polymer sys tems under irradiation with planepolarized light are studied. A copolyacrylate containing phenyl benzoate and azobenzene side groups is synthesized as a nematic polymer; the cholesteric mixture is prepared via dop ing of the nematic copolymer with the chiral dopant, the derivative of Disosorbide. Thin layers of the azobenzenecontaining photoorientant SD1 are first used as orienting substrates for polymer liquid crystals. Thin layers of the copolymer and of the mixture are spincoated on the substrate after irradiation of the pho toorientant layer with polarized light. It is shown that after annealing phenyl benzoate and azobenzene side groups of the nematic copolymer orient strictly along the direction of orientation of surface molecules, whereas in the case of the cholesteric mixture, a partial formation of the helical structure is observed. It is demonstrated that all the systems under examination can experience the repeated cyclic reorientation of the cooperative type under irradiation and subsequent annealing of the films.
DOI: 10.1134/S0965545X10080079
LIQUIDCRYSTALLINE POLYMERS
The cholesteric photochromic mixture (Fig. 1d) was prepared via mixing of the photochromic LC copolymer PAAzo with a small amount of the chiral dopant based in Disosorbide characterized by a high twisting power.
Copolymer PAAzo forms the nematic phase with isotropization temperature Тi = 120°С. The mixture
of this copolymer with 4 wt % dopant HexSorb is char acterized by the cholesteric type of mesophase with
Тi= 108°С. Planarilyoriented films based on this
mixture selectively reflect light with maximum at λmax= 730 nm. The glasstransition temperatures of
the copolymer and the mixture are 24°С.
Our study of the photooptical behavior of both types of polymer systems with different structural organization pursued several goals.
First of all our attention was focused on the inves tigation of photoprocesses occurring in thin films of the polymers under study, which in terms of their thickness, were comparable or even smaller than the pitch of the cholesteric helix (<500 nm). Until recently, photooptical studies were mostly performed for thick polymer LC films on the order of several microns or above. At the same time, requirements
imposed on new photoelectronic and optoelectronic LC devices necessitate the design of multifunctional and frequently multilayer film polymer materials of the minimum thickness, thus reflecting modern ten dencies toward miniaturization of most technical and household electronic devices (display technique, mobiles, telecommunication systems, etc.) [6].
Another not less important aspect of our study is associated with the examination of photoorientation processes of the abovementioned LC polymers under the action of light. It is known that the practical use of liquid crystals and LC polymers is possible only if an LC material can be well oriented by some method; that is, when molecules or fragments of an LC com pound are arranged in a strictly defined manner rela tive to boundary surfaces. Under application of the external electric or magnetic field, the orientation of molecules of liquid crystals can be changed and thus their optical characteristics can be varied. This phe O O O O O H H O O O HexSorb (a) (b) (c) (d) + Cholesteric (chiral)
LC copolymer Photochromic LC copolymer
mesogenic photochromic chiral
Chiralphotochromic Cholesteric photochromic chiral
group
group group
dopant
LC copolymer mixture
Fig. 1. Schematic representation of macromolecules of (a) cholesteric (chiral) LC polymers, (b) photochromic LC copolymers,
nomenon underlies all electrooptical devices operat ing according to this principle [7].
Among conventional methods used for orientation of liquid crystals, such as screw metal spraying, use of surfactants, mechanical rubbing of surfaces, applica tion of organic or inorganic coatings by means of cen trifugation (spincoating), the most efficient method involves the creation of photoorienting surfaces on the basis of various polymer compounds and dyes. Mate rials capable of photoisomerization [8–10], photo degradation [8, 11], photodimerization [8, 12–14], and photocrosslinking under the action of radiation with the corresponding wavelength are used for this purpose. Such a noncontact method based on irradia
tion with polarized light, which causes the appearance of photoinduced optical anisotropy and dichroism in thin polymer films, for example, due to reversible trans–cis (E–Z) isomerization of azobenzenecon taining layers on solid substrates, has found very wide use for creation of the socalled command surfaces, which make it possible to govern the optical behavior of lowmolecularmass liquid crystals applied on pho tocontrollable surfaces.
Herein as a command photocontrollable orienting surface we selected a very efficient photoorientant 3,3'{(2,2'disulfobiphenyl4,4'diyl)bis[diazene2,1 diyl]}bis(6hydroxybenzoic acid) [8]
The orienting effect of this photoorientant can be governed under the action of light. It should be emphasized that no data is available on the effect of the SD1 layer on any LC polymer systems and such experiments were performed herein for the first time.
Another important task of this study is to compare the orienting properties of the SD1 photoorientant and the polyimide orienting layer, which has found wide use.
EXPERIMENTAL
Synthesis of Monomers and Chiral Dopant
Mesogenic methoxyphenyl benzoate monomer, which was synthesized as described in [16], was iso lated in the form of white crystals with the following phase transitions: Cr 82.5–83 (N 48.8–50) I ([16] Cr 82 (N 50) I). Rf = 0.23 (toluene : ethyl acetate= 9 : 1), Rf= 0.61 (toluene : ethyl acetate = 1 : 1).
1H NMR (CDCl
3): δ 1.44–1.86 (m, 6H, ⎯CH2(CH2)3CH2–); 2.58 (t, 2H, –CH2COOAr; 3.89 (s, 3H,
⎯OCH3); 4.18 (t, 2H, ⎯CH2OOCCH=CH2); 5.80 (dd, 1H, CH2=CH– trans); 6.11 (dd, 1H, CH2=CH– cis);
6.40 (dd, 1H, CH2=CH–); 6.97 (d, 2H arom. ortho to –OCH3); 7.10–7.25 (2d, 4H, arom); 8.14 (d, 2H. arom.
meta to –OCH3) ppm.
The photochromic azobenzenecontaining monomer, which was prepared as described in [17], is a lightyel low finely crystalline substance with Тm = 88°С ([17] Тm = 87.5°С).
1H NMR (CDCl 3): δ 1.4–1.9 (m, 8H, CH2); 3.9– 4.3 (m, 4H, –СН2О–); 5.6 (d, 1H, CH2=CH– trans); 6.3 (d, 1H, CH2=CH– cis); 6.1 (m, 1H, CH2=CH–); 7.0 (d, 2H, arom.); 7.7–8.0 (m, 6H, arom) ppm. HO HOOC N N SO3H HO3S N N OH COOH SD1 CH2 CH COO O O O O O AA5 CH2 CH COO O N N CN Azo
IR (KBr): ν 2212 (C≡N), 1715 (C=O in RCOOR'), 2942, 2854 (CH2), 1596 (C–C in Ar), 1250 (COC),
1627 (C=C) cm–1.
Lowmolecularmass chiral dopant HexSorb was synthesized in accordance with [18].
Synthesis of Photochromic Copolymer
Copolymer PAAzo was prepared through the free radical copolymerization of monomers Azo and AA5 in dehydrated benzene. The reaction was performed in a sealed ampoule under argon for 100 h at 60°С; AIBN (2% based on the monomer weight) was used as an initiator. Lowmolecularmass monomer and oli gomer admixtures were removed by the longterm boiling of the copolymer in methanol. The copolymer was outgassed in vacuum during heating for 2 h. The conversion of polymerization was ~90%. The compo sition of the copolymer (50 : 50) was preset by the molar ratio of initial monomers and was confirmed by UV spectroscopy. The degree of polymerization of the copolymer was ~20; polydispersity, ~1.5. Molecular mass characteristics were estimated by GPC on a Knauer chromatograph (UV detector; LC100 col umn packed with sorbent 1000 Å; THF as a solvent; 1 ml/min; 25°C; and PS standard).
Preparation of Cholesteric Mixture
Copolymer PAAzo and chiral dopant HexSorb (4 wt %) were dissolved in chloroform. The main frac tion of the solvent was evaporated at room tempera ture, and the residue was vacuum distilled at 50°С for 2 h.
Preparation of Samples
Glass plates with the applied photocontrollable layer SD1 were used as orienting substrates. For this purpose, a solution of photoorientant SD1 in DMF (1 wt %) was applied on a preliminarily purified glass surface via the spincoating method; then, the aspre pared sample was annealed for 15 min at 145°С to remove the residual solvent and irradiated for 30 min with polychlromatic planepolarized light of a mer cury lamp. This method ensures obtainment of a uni form (homogeneous) solid film of the SD1 dye with a thickness of ~10 nm [8]. Along with the SD1 layer, a polyimide layer was employed for orientation of LC polymers. This layer was prepared as follows: a solu tion of poly(amid acid) (ZLI2650, Merck) in cyclo hexanone was spincoated on the purified surface of glass; the asprepared layer was subjected to imidiza tion at 220°С for 2 h. At the final stage, the polyimide layer was rubbed with a velvet fabric. The samples of the nematic copolymer and of the cholesteric mixture were also spincoated on the surface of the irradiated photoorientant from their chloroform solutions. The
film thickness d was estimated from the isotropic
extinction coefficient of cyanoazobenzene groups ( ) and isotropic absorption ( ) through the Beer–Lambert–Bouguer law: cd; the value of d was usually in the range 200–500 nm. Then the films were annealed at a temperature of 50°C for 60 min. The maximum degree of orientation was achieved under the specified conditions.
The isotropic extinction coefficient was deter mined by measuring the absorption of copolymer solution (2.35 × 10–5 mol/l) in chloroform in a 1cm
thick quartz cuvette. The value of was found to be 1.4 × 104 l/(mol cm). The isotropic absorption
was calculated from the absorption of amorphized films. Since the concentration of azobenzene groups in the film was 1.26 mol/l, its thickness (nm) was esti mated via equation
d = × 107/(1.4 × 104 × 1.26) (1)
Physicochemical Studies
Microcalorimetric studies were carried out with a Mettler TA4000 differential scanning calorimeter at a heating rate of 10 K/min. Samples were pellets with a weight of 5–10 mg.
Microscopic measurements were performed on a POLAMR112 polarization microscope equipped with a Mettler FP86 hot stage.
1H NMR spectra were recorded with a Bruker
Avance400 instrument operating at a frequency of 400 MHz. Chemical shifts were measured relative to TMS.
The photochemical properties of the films were investigated on a specially designed setup equipped with a DRSh250 ultrahighpressure mercury lamp (Fig. 2). We used the polychromatic light of the mercury lamp with wavelengths initiating both E–Z and Z–E photoi somerization processes (365, 405, and 436 nm). As a result, high rates of a gain in the induced anisotropy in films of azobenzenecontaining compounds were attained. The intensity of light, as measured with a LaserMateQ intensity meter, was ~7.0 mW/cm2 (for
polychromatic light of the lamp). Spectral measure ments were performed with a UNICAM UV500 UV visible spectrometer (United States).
The degree of orientation was estimated by polar ized UVvisible spectroscopy with allowance for the fact that the moment of transition of the Eform of azobenzene groups is directed along their long axes. For this purpose, the angular dependence of polariza tion absorption was measured with a step of 10° using a J&M photodiode UVvisible spectrophotometer (Germany). 365 i ε 365 i A 365 365 i i A = ε 365 i ε 365 i A 365 i A
The value of dichroism was calculated from the spectral data
D = (A|| – A⊥)/(A|| + A⊥), (2) where A|| is polarized absorption along orientation of chromophore groups and A⊥ is absorption in the per pendicular direction (Fig. 2).
RESULTS AND DISCUSSION
Photoorientation Processes in Films of Photocontrollable Command Surface SD1
Let us consider the photooptical behavior of the photoorienting layer. In the spincoated film of SD1, dye molecules are arranged in a random manner (Fig. 3a); therefore, the absorption of such a film is isotropic. This implies that the parallel (||) and perpen dicular (⊥) components of polarized absorption coin cide, as confirmed by the solid line in Fig. 4. Irradia tion of the SD1 film by planepolarized polychro matic light results in the uniaxial orientation of dye molecules (Fig. 3b), as evidenced by a marked differ ence in the polarized absorption components (dashed and dotted lines || and ⊥ in Fig. 4).
In this case, surface molecules orient perpendicu lar to the plane of polarization of irradiating light. It should be noted that the photoorientation model developed in detail for SD1 films is in good agree ment with the experimental data [19]: no E–Z isomerization takes place during excitation of SD1 molecules by polarized UV light; however, the rota tional diffusion of molecules in the excited state leads to their orientation in direction orthogonal to the plane of polarization. As opposed to molecules of organic orientants containing C=C double bonds, for which irreversible photoprocesses are observed [20, 21], the repeated reorientation of molecules may occur in SD1 films due to a change in the direction of
light polarization. This effect is of significant interest for creation of reverse command surfaces. A signifi cant advantage of such surfaces is their high thermal stability (~200°С), while the orientation of molecules is preserved.
Photoreorientation Processes in Nematic Copolymer Films on Substrates with Photocontrollable Command
Surface SD1
Let us begin examination of photoreorientation processes from the simplest photochromic system, the nematic copolymer. The spincoated copolymer film is amorphized, as schematically shown in Fig. 3c. Fur ther annealing at a temperature above the glass transi tion temperature of the copolymer causes orientation of side groups of the copolymer in direction coinciding with the predominant orientation of SD1 surface molecules (Fig. 3d).
Figure 5 shows the polarized absorption spectrum for the thin annealed copolymer film on the SD1 sur face. Note that the thickness of the copolymer film is several times greater than the thickness of the film of the SD1 orientant; therefore, the absorption of the latter compound may be neglected. It is seen that the absorption of azobenzene groups depends on the direction of polarization of the detecting beam. This value attains a maximum when the polarizer direction coincides with the orientation of photoorientant mol ecules (i.e., perpendicularly to the plane of polariza tion of light used to set the direction of orientation of SD1 molecules; Fig. 2). The value of dichroism, as calculated through formula (2), was 0.76. It is worth noting that, if the layer of the uniaxially rubbed poly imide is used an orienting coating, then copolymer PAAzo films are characterized by almost the same dichroism value (D = 0.77) [22].
A⊥
A|| Detector
PC Rotation of light polarization plane Side groups of copolymer
Control over Polarizer S am ple Light Hg lamp or polarizer rotation source laser
Fig. 2. Schematic representation of the experimental setup for photooptical studies of thin polymer films (see text for explana
Thus, the orienting ability of the SD1 photoorien tant is similar to that of the uniaxially rubbed polyim ide coating widely used in the display technique. The main difference is that the nature of the orienting effect of the polyimide coating is associated with anisotropy of the surface energy arising due to surface microrelief that is set during rubbing; note that a change in the direction of orientation after substrate coating by the LC polymer is impossible. In the case of the SD1 photoorientant, orientation likewise results from anisotropy of the surface energy; it appears due to the uniaxial orientation of surface molecules photo induced by polarized light; however, the direction of orientation can be repeatedly changed.
Naturally, this raises the question of whether the reorientation of side groups of the copolymer applied on the substrate of the photoorienting coating can take place. To answer this question, let us analyze absorp tion diagrams obtained for copolymer films applied on the photoorienting coating and subjected to photoir radiation and thermal treatment.
The initial copolymer film spincoated on the sub strate is amorphized, as evidenced by the isotropic dis tribution of polarized absorption (Fig. 6a). Subse quent annealing of this film, as was shown above, brings about orientation of the side groups of the copolymer which arrange along the direction of orien
tation of SD1 suraface molecules (Fig. 3d, Fig. 6b). Upon further irradiation of the annealed film by poly chromatic light with polarization plane parallel to the direction of the orienting effect of the SD1 layer, the reorientation of side groups of the copolymer occurs in perpendicular direction (cf. diagrams in Figs. 6b and 6c). However, the value of photoinduced dichroism turns out to be somewhat smaller than that in the ini tial film (~–0.56) (Fig. 7). The negative dichroism value corresponds to the orientation of side groups in direction perpendicular to their initial position. A lower dichroism value is apparently associated with the fact that in the photostationary state the system contains lowanisometric Z isomers of photochromic fragments of side groups which somewhat disturb their orientation [23].
During subsequent annealing (the transition from diagram in Fig. 6c to diagram in Fig. 6d), the dichro ism of the system reaches its initial value; in this case, the direction of the predominant orientation of side groups in the copolymer remains the same as after irradiation. Diagrams d–f (Fig. 6) illustrate the second cycle of reorientation. Three irradiation–annealing cycles were performed, and it was shown that the ori enting effect of the coating remains nearly unchanged (Fig. 7) and only the direction of orientation changes. Thus, irradiation by planepolarized light of the nem
(a) (b) (c) (d) PAAzo copolymer mesogenic photochromic glass SD1 molecules Polarized E light groups groups
Fig. 3. Schematic representation of the orientation of SD1 surface molecules (a) before and (b) after irradiation with plane
polarized light and the orientation of side mesogenic and photochromic groups in the PAAzo copolymer film (c) before and (d) after annealing. Е is the electric vector of light wave.
atic copolymer film applied on the orienting coating causes the cooperative reorientation of side groups of the copolymer and SD1 photoorientant molecules. Our experimental data demonstrate the active nature of the surface (in contrast to the passive role of the polyimide coating) which appears as the feasible con trol over the direction of its orienting effect on the polymer film. When the rubbed polyimide layer is used as an orienting coating, the photoreorientation of PAAzo polymer films also occurs due to photoorienta tion of its azobenzene side groups. However, the sub stantial difference in this case is that upon subsequent annealing side groups recover their previous orienta tion (along initially preset rubbing direction of the polyimide coating).
Photoreorientation Processes in Cholesteric Mixture Films on Substrates with Photocontrollable Command
Surface SD1
Let us turn to examination of photoorientation and photoreorientation processes in cholesteric thin films. This mesophase type is characterized by the supramo lecular structure with helix pitch Р, which corresponds to the turn of director n by angle 2π (Fig. 8). Note that the thickness of the used films d for given polymer sys tems does not exceed 500 nm, that is, the value com parable with pitch P of the cholesteric helix: d ≤ P.
As was shown in [22], such thin films of cholester ics are distinguished by a number of typical spectral features. Thus, the maximum of polarized absorption corresponding to direction of the predominant orien tation of side groups of the polymer in the film is
turned at a certain angle α relative to direction of the orienting effect of the coating (in [22], the uniaxially rubbed polyimide layer was used); dichroism in such films is much smaller than that in films of the nematic copolymer with the same thickness. Moreover, with the increasing thickness of cholesteric films, the linear dichroism decreases, while angle α increases. On the basis of the above data, the model of arrangement of side groups in thin cholesteric films was advanced (Fig. 8a). In terms of this model, the film is comprised of three arbitrary layers: a lower layer, which is imme diately linked to the substrate, the middle layer with the deformed helical structure corresponding to the uniaxial planar orientation, and the upper layer with the homeotropic orientation at the polymer/air inter face (Fig. 8a). It is the layer with the helical structure that is responsible for spectral features described below that explain peculiar optical properties of these films.
Figures 9 and 10 display the photooptical data illus trating the effect of polarized light on thin films based on the cholesteric mixture. If the initial annealed thin film based on the cholesteric mixture is irradiated with light with the plane of polarization parallel to the direction of the orienting effect of the SD1 surface, then, unwinding of elements of the helical supramo lecular structure takes place in perpendicular direction relative to the plane of polarization of incident light and the uniaxial orientation forms (in Fig. 9, diagrams a and b); i.e., the transition to the oriented nematic phase occurs (α = 0°). In this case, the value of dichroism after irradiation (D ≈ 0.70) is comparable
Absorption 0 0.04 0.08 400 ⊥ 500 λ, nm 300 ||, ⊥ ||
Fig. 4. Polarized absorption spectrum of the SD1 surface
before (solid line) and after irradiation (dashed and dotted lines) with polarized polychromatic light for 30 min. || and
⊥ correspond to the parallel and perpendicular compo nents of polarized absorption.
Absorption 0 0.4 0.8 400 500 λ, nm 300 0° 350 450 550 30° 60° 90° 365 nm 0.2 0.6
Fig. 5. Polarized absorption spectrum of the film
(~310nm thick) of the nematic copolymer on photocon trollable surface SD1. Numbers at curves correspond to angles between the polarization plane of the detecting light of spectrophotometer and the direction of predominant orientation of chromophores.
0.4 0.2 0 0.2 0.4 Absorption at 365 nm (a) 90 270 120 150 180 0 210 240 300 330 30 60 T 0.4 0.8 0 0.4 0.8 (b) 90 270 120 150 180 0 210 240 300 330 30 60 E 0.4 0.2 0 0.2 0.4 (c) 90 270 120 150 180 0 210 240 300 330 30 60 T 0.4 0.8 0 0.4 0.8 (d) 90 270 120 150 180 0 210 240 300 330 30 60 E 0.4 0.2 0 0.2 0.4 (e) 90 270 120 150 180 0 210 240 300 330 30 60 T 0.4 0.8 0 0.4 0.8 (f) 90 270 120 150 180 0 210 240 300 330 30 60
Fig. 6. Polar diagrams of the 365nm absorption band for the copolymer film (~310nm thick) illustrating changes during irradi
with that measured for the nematic copolymer PAAzo (Fig. 7).
After subsequent annealing owing to the twisting effect of the chiral dopant, the reverse nematic–cho lesteric transition occurs and the helical structure par tially recovers (Fig. 9, diagrams b, c) but now along the new direction of the orienting effect of the coating (90°). This is another evidence for the cooperative photoorientation of both side groups of the polymer and molecules of the SD1 photoorientant. It is important that angle α1 does not attain its initial values (α1< α0), thereby demonstrating the dependence of
angle α on the irradiation–annealing cycle number. For the sake of simplicity, projections of helical order elements are shown in Fig. 10 for every state of the sys tem.
After subsequent irradiation and annealing (Fig. 9, diagrams c, e), the system returns to the initial state: angle α precisely corresponds to the initial α0 value
(Fig. 10). As is seen, this phenomenon is observed after each even cycle and probably may be associated with the memory effect; i.e., the system remembers its ini tial state.
Let us discuss in more detail the cause of such a memory effect. A similar phenomenon is observed in various forms for polymers and is characterized by the D 0 0.8 2 N 1 3 4 −0.4 0.4 0 −0.8 hν T Direction of polarization plane of incident light
Fig. 7. Dichroism of the copolymer film (applied on the
orienting coating SD1) subjected to cyclic photoirradia tion hν and annealing Т processes vs. the number of treat ment cycles N. Each time, irradiation was performed for 30 min with polychromatic light with the polarization plane parallel to the previous direction of orientation of side groups. Annealing was performed at 50оС for 30 min.
(a) (b) air d substrate α top direction of (c) P n projection substrate rubbing
Fig. 8. Model illustrating (a) arrangement of side groups of cholesteric LC polymer in thin film on the orienting substrate; and
(b) projection of a partially unwound helical structure along normal to the film surface (for the sake of simplicity, main chains and spacers are omitted) [22]. (c) Schematic representation of director n distribution in the helical structure of cholesteric; Р is the cholesteric helix pitch.
0.4 0.2 0 0.2 0.4 Absorption at 365 nm (a) 90 270 120 150 180 0 210 240 300 330 30 60 T (b) 90 270 120 150 180 0 210 240 300 330 30 60 E 0.4 0.2 0 0.2 0.4 0.4 0.2 0 0.2 0.4 (c) 90 270 120 150 180 0 210 240 300 330 30 60 T (d) 90 270 120 150 180 0 210 240 300 330 30 60 E 0.4 0.2 0 0.2 0.4 0.4 0.2 0 0.2 0.4 (e) 90 270 120 150 180 0 210 240 300 330 30 60 α0 α0 α1 < α0
Fig. 9. Polar diagrams of the 365nm absorption band for the cholesteric mixture film (~280nm thick) illustrating changes during
α , deg 20 40 2 N 1 3 4 10 30 0 0 hν T polarization plane 5 6 α = 0 α = 0 α1 < α0 α0 of incident light
Fig. 10. Change in angle α during several irradiation–annealing cycles N for the cholesteric mixture film.
entropy nature [24, 25]. In our case, this effect may be explained by the fact that during annealing of the as prepared (amorphized) film, polymer chains assume the most energetically favorable conformation. A strictly defined arrangement of side mesogenic groups and fixed angle α0 correspond to this set of conforma
tions. After reorientation of side groups, polymer chains assume another less favorable conformation characterized by another angle α1. Under the action of polarized light that reorients side groups back, poly mer chains reassume the most favorable initial confor mation and force chromophores and mesogens to recall the initial arrangement.
CONCLUSIONS
Processes of orientation and photoinduced reori entation of side groups in thin films of photochromic combshaped LC polymers of nematic and cholesteric types on the photocontrollable command surface have been first studied. With the use of these systems, we have demonstrated that there is the key principal dif ference between the SD1 photocontrollable surface and the polyimide orienting coating, namely, the active nature of the photoorientant which consists in ability to switch the direction of the orienting effect with a change in the direction of polarization plane of incident light. During irradiation of thin films of pho tochromic polymer systems spincoated on the SD1 photoorientant, the cooperative reorientation of side groups of the LC polymer and surface molecules takes place. It was been shown that both the nematic poly mer and the cholesteric mixture are capable of the
repeated cyclic photoreorientation. In addition, the study of cyclic photoreorientation processes in choles teric mixture films has revealed that the initial (ini tially preset) orientation in the film is the most ener getically favorable and that after each even irradia tion–annealing cycle, the system returns to its initial state, that is, is characterized by the memory effect.
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