Makromol. Chem., Rapid Commun. 12, 709- 715 (1991) 709

Laser-induced birefringence in homeotropic films of photochromic comb-shaped liquid-crystalline copolymers with azobenzene moieties at different temperatures a)

Sergei Ivanov, Igor Yakovlev, Sergei Kostromin, VaIery Shibaev

Moscow State University, Department of Chemistry, 11 9899 Moscow, USSR

Lutz Llisker, Joachim Stumpe: Dieter Kreysig

Humboldt University Berlin, Department of Chemistry, Institute of Organic Chemistry, Hessische Str. 1-2, 0-1040 Berlin, FRG

(Date of receipt: February 18, 1991)b)

Introduction

Comb-shaped liquid-crystalline copolymers (LCP) containing azobenzene moieties have attracted much attention in the last few This particular interest in ordered photochromic polymer films is predominantly caused by their unique combi- nation of optically anisotropic and polymer properties, which allows different types of laser-addressed, reversible optical data storage.

The use of linearly polarized light offers a fundamentally new approach to induce optical anisotropy by rotational diffusion in amorphous photochromic polymers 1 3 , 14)

as well as in LCP’s’s2). Recent publications6-12) were aimed at showing that azo moieties undergo E-Z photoisomerization and reorientation, in such a way that optically anisotropic properties such as birefringence and dichroism of homeotropically and planary oriented films of LCP’s are modified in the glassy as well as in the visco-elastic state. It had been shown that the photoinduced rotational diffusion of azo-chromophores does not disturb the other surrounding mesogenic side groups in the glassy state of LCP’s9s10). In contrast to these conclusions, our results from conoscopic studies of LCP’s, which differ concerning the mesogenic moieties, suggest that even in the glassy state the photoinduced reorientation of a minority of azo-chromophores triggers the physical reorientation of other mesogenic groups in the same direction, i.e. perpendicular to the plane of polarization of the incident light ’7 12) .

Up to now, there is no information about the influence of temperature, type of mesophase and content of azo moieties on the kinetics of this type of photorecording. The main concern of this paper is to present quantitative results of photoinduced birefringence at different temperatures, using homeotropic films of smectic as well as nematic LCP’s with different structures and different contents of azo-chromophores.

a) This paper is part 9 in Berlin series “Photoreactions in Liquid Crystals”. b, Revised manuscript of September 16, 1991.

0 1991, Hiithig & Wepf Verlag, Base1 CCC 0173-2803/91/$01.50

710 S. Ivanov, I. Yakovlev, S. Kostromin, V. Shibaev, L. Lasker, J. Stumpe, D. Kreysig

Experimental part

The studies were performed with the following statistical liquid-crystalline comb-shaped copolymers 1-4. Synthesis and characterization have been described elsewhere 15-''). The phases and transition temperatures are given in Tab. 1.

The polymer samples were placed between two glass plates separated by 10 pm polytetrafluoro- ethylene spacers. Homeotropic alignment was achieved by cooling the sample from the isotropic melt to the glassy state under the action of electric or magnetic fields.

Homeotropically oriented samples were irradiated by polarized green light from an Ar +-laser (A = 514,s nm, power density P = 20-300 mW/cm2; writing beam). A He/Ne-laser (A = 632,8 nm, P = 5 mW/cm2; reading beam) was used to indicate the changes of birefringence An. The direction of the reading beam was exactly perpendicular to the liquid-crystalline layer. The sample cell was mounted in a thermostatted block (+0,4"C).

A previous paper presented a description of the experiment including experimental set-up '). Polarized Fourier-transform IR spectra have been recorded with a Nicolet 60 SX spectrometer.

Tab. 1. Structure and phase behaviour of LCP's 1-4

1 - 3 (mole ratio x:y)

/-cH,-?(cHJ-

COO-(CH,),-0 C O O - - ( C H ~ ) , , , - O + = N ~

4 (mole ratio x : y )

Polymer I rn 2 = b / ( x + y ) ] . 100 Tg Phase transition b, in mol-vo a) in "C temp. in "C

1 5 6 20 2 5 6 40 3 4 6 20 4 6 2 20

40 SA 105 n 110 i 35 SA 106 n 110 i 42 n 106i 70 S 106 i

a) y: Azo-chromophor. b, s: Smectic; sA : smectic A; n: nematic; i: isotropic.

Results

Qpical curves of laser-induced birefringence Anind as a function of irradiation time under the action of a "writing" laser ( t = 0. . .60 s) are presented in Fig. 1 a at room temperature (T < T,) and in Fig. Ib above the glass temperature of the LCP's (T > Tg). Switching-on the laser, optical anisotropy is induced within the initially homeotropic films. During the first period of irradiation the increase of birefringence proceeds quickly and becomes smaller on continued irradiation.

Laser-induced birefringence in homeotropic films of

c- Q

71 1

Laser a - on -- off -

Fig. 1.a.

Fig. 1.b.

0.06

0.01

0.02

0 0 20 LO 60 80 100 120

Time in s

Fig. 1 . Time dependence of laser-induced birefringence Anind in homeotropic films of polymers 1-4 on irradiation (P = 200 mW/cm2; t = 0-60 s) followed by relaxation (a) at room tempera- ture and (b) at a temperature above Tg (polymers 1-3: Temp. = 70°C; polymer 4: Temp. = 90 "C)

A comparison between Figs. 1 a and 1 b demonstrates that the photoinduced process is much more effective in the visco-elastic state than in the glassy state. At the same period of irradiation the values of Anind are approximately one order larger in the case of T > Tg . This fact demonstrates that the limited values of Anind in the case of LCP 1 and 2 at room temperature are not caused by thermal effects with respect to the glass temperature.

Comparing the polymers with the same content of azobenzene chromophores of 20 mol-To but different glass temperatures and different types of mesophases (1, 3, 4), quite similar initial inducing efficiencies have been found. The values of induced birefringence of the smectic LCP's 1 and 2 gradually increase during the first period and become constant after 20 s in the visco-elastic state and after 40 s in the glassy state. Higher limiting values of Anind are induced in LCP 2 with 40 mol-To of azo groups in

712 S. Ivanov, I. Yakovlev, S. Kostromin, V. Shibaev, L. Lasker, J. Stumpe, D. Kreysig

comparison to LCP 1 with only 20 mol-Vo below and above glass transition tempera- ture, whereby both polymers are smectic and have similar glass temperatures. A different time dependence of Anind is observed for polymers 3 and 4 in the glassy state, as well as in the mesophase. In both cases, the induced birefringence continues to increase at 60 s and even at 600 s irradiation time to much higher values of Anind.

After switching off the laser a decrease of primary induced birefringence Anind by relaxation processes is observed (t = 60- 120 s; Fig. l a and 1 b). At room temperature the relaxation leads to stable values of the birefringence Anstab which has been frozen- in in the glassy state of LCP and remains constant for several months. The extent of relaxation at room temperature strongly depends on the glass transition temperature. Thus, the relaxation can be neglected for polymer 4 with Tg = 70"C, whereas the stable birefringence of the other samples (T, = 35 -42 "C) is considerably lower than Anind. The decrease of birefringence in the nematic LCP 3 is stronger than that for the smectic samples 1 and 2 with similar Tg values. The relaxation of polymer 4 to Anstab at T = T, - 20 "C is in the same order as those of the other polymers at room tem- perature. In the visco-elastic state at 20- 30 "C above the glass temperatures the induced birefringences fall down to zero within 60 s after switching off the laser (Ansbb = 0), i. e. the LCP's relax completely to their initial state due to the mobility of the side groups (Fig. 1 b).

Fig. 2 shows the values of induced birefringence, Anind, as a function of tempera- ture for LCP 1-4 on 60 s of irradiation. In the glassy state the values are only slightly dependent on temperature. However, above the glass temperature a considerable increase of the induced birefringence to values one order of magnitude higher occurs. Maxima of inducing efficiency were achieved for all polymers at temperatures of about T - Tg = 30- 35 "C. Thus, upon 60 s of irradiation of sample 4 a maximum value of Anind = 0,068 is observed at 94 "C. Further rising temperatures cause a lowering of order and thus a decrease of the birefringence to be induced. At clearing temperature and in the isotropic melt photo-induced birefringence cannot be observed.

The Anind values of samples 2 and 4 are larger than those of the smectic LCP 1. Whereas the maxima of LCP's 1 and 2 are constant (see limiting values in Fig. 1 b) the maxima of LCP's 3 and 4 increase upon continued irradiation (see the corresponding curves in Figs. 1 b and 2).

Fig. 2. Temperature de- pendence of the induced birefringence Anind of the polymers 1-4 (ti I = 60 s; P = 200 mw/cm5j

20 LO 60 80 100 120 Temp. in O C

Laser-induced birefringence in homeotropic films of . . . 713

Fig. 3 shows the temperature dependence of stable values of birefringence, Anstab, which have been achieved upon 60 s of irradiation and relaxation at an operating tem- perature of 20°C. The Anstab values are nearly constant in the glassy state, however, a back relaxation process occurs above glass temperature. The loss of any stable birefringence is observed at T - Tg = 20-30°C for all samples.

20 LO 60 80 100 Temp. in O C

Fig. 3. Temperature dependence of the stable birefringence Anstab of polymers 1-4. The values of Anstab at temp. = 20 "C were attained by the procedure of irradiation and relaxation as shown in Fig. 1 a

Discussion

Birefringence was generated by irradiating homeotropically oriented films of LCP's containing azo-chromophores and different mesogenic moieties with linearly polarized laser light at different temperatures. Based on results obtained by polarizing micro- scopy and conoscopic studies we have recently characterized this type of angular- dependent photoselection in the glassy state of LCP's as a physical reorientation of the optical axis of the whole system7).

Irradiating the photochromic LCP's 1-4 the azo moieties undergo E-Z photoiso- merization, and a photostationary equilibrium between the isomers is easily established, whereas the induction of optical anisotropy proceeds in a much longer time of irradiation. Thus, observation of UV/VIS spectra during irradiation with unpolarized light of low intensity (P < 1 mW/cm2) have demonstrated that a constant ratio of the isomers is generated after 2 min. However, irradiating this film with linearly polarized light of the same intensity, the process of inducing birefringence occurs in a period of more than 3 hlS). Consequently, successive E + Z isomerization steps within the photochemical equilibrium cause a rotational diffusion of azobenzene chains in such a way that the angle between the axis of the azo moiety and the plane of polarization of incident light is continuously enlarged to a maximum limit of perpendicular orientation lo).

The different values of the induced birefringences as well as the different dynamics of the inducing process in polymers with the same content of azo-chromophores show that this process is affected by the molecular environment of the photochromic groups. Thus, the high values of Anind show that the reorientation process is especially efficient in polymer 4. Irradiating a glassy sample of this LCP with linearly polarized

714 S. Ivanov, I. Yakovlev, S. Kostromin, V. Shibaev, L. Llsker, J. Stumpe, D. Kreysig

light, IR dichroism of both, phenyl as well as amide bands (1 308, 1535 and 1661 cm-I), was generated 19). This finding demonstrates that the orientation of the meso- genic groups is affected by the photoselection of the azo moieties.

Regarding polymers 1- 3 with similar chemical structures, the behaviour differs in dependence on the type of mesophase as well as on the content of photochromic groups. A comparison of LCP 4 with the other smectic polymers 1 and 2 demonstrates that not only the content of photochromic moieties and the type of mesophase but also the chemical structure influences the efficiency of the process.

Moreover, the induced birefringence as well as the orientational relaxation processes strongly depend on the operating temperature with respect to the glass transition. Below Tg only small-scale rotations such as functional-group rotation and isomeriza- tion may occur. However, above Tg the increased free volume and thermal energy permit segmental motion and in this way much more efficient light-induced cooperative disturbance and reorientation processes. We suggest that also in the glassy state the photoselection of a minority of photochromic groups can cause a more or less efficient physical reorientation of the other mesogenic moieties in a multistep process of adjacent side groups.

Our results demonstrate that the stable induced birefringence for each LCP depends on temperature with respect to its photochromic, liquid-crystalline as well as polymer properties.

We would like to thank L Riibner and R. Ruhmann, Institute of Organic Chemistry, Berlin- Adlershof, for synthesis of copolymer 4, and W Scholdei, MPI-P Mainz, for the FTIR measure- ments.

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