O Nadtoka, O Yaroshchuk - Investigation of photoinduced orientation ordering in polymethacrylates with side-chain azobenzene moieties - страница 1

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CHEMISTRY & CHEMICAL TECHNOLOGY

Vol. 6, No. 4, 2012 Chemistry

Oksana Nadtoka1 and Oleg Yaroshchuk2

INVESTIGATION OF PHOTOINDUCED ORIENTATION ORDERING IN POLYMETHACRYLATES WITH SIDE-CHAIN AZOBENZENE MOIETIES

1Kyiv National Taras Shevchenko University, 64, Volodymyrska str., 01033 Kyiv, Ukraine; oksananadtoka@ukr.net 2 Institute of Physics, National Academy of Sciences, 46, Nauky Ave, Kyiv, Ukraine

Received: April 05, 2012 / Revised: June 25, 2012 / Accepted: October 01, 2012

 

© Nadtoka O., Yaroshchuk O., 2012Abstract. The initial and the photoinduced 3D orien-tational order in the films of two methacrylic polymers containing azobenzene groups with electron-donor or electron-acceptor substituents was studied by using null ellipsometry and UV/Vis absorption spectroscopy. The 3D order was induced by monochromatic polarized light of two different wavelengths corresponding to mi and np* absorption bands of azochromophores. We found that, depending on the excitation wavelength and terminal substituents of azochromophores, recording kinetics of photoinduced anisotropy in the polymers is dominated by either angular redistribution or angularly selective trans-cis isomerization of the chromophores originally being mainly in the trans-form. Before irradiation, the azobenzene groups in the films of all polymers show preference to the out-of-plane ordering. Scenario of the order transformation under irradiation depends on the prevailing mechanism of the photoinduced anisotropy. In case of angular redistribution, initial positive uniaxial order of azochromophores is transformed into the negative uniaxial order characterized by random orientation of these units in the plane perpendicular to the polarization direction of the exciting light. In turn, in case of photoselection, the chromophores approach a zero-order (spatially isotropic) state due to exhaustion of the anisotropic trans-isomers in all directions. The transient orientational structures in these kinetic processes are biaxial.

 

Keywords: azopolymer, polymethacrylate, photoinduced orientation.

 

1. Introduction

In the past years, azo-dye-containing polymer systems have been the subject of intensive researches due to their potential applications in photonics, optoelectronics, and optical signal processing [1, 2]. One of the main interests to these polymer systems is their birefringence property when they are irradiated with a linearly polarized light [3]. This birefringence comes from the ordering of the azobenzene groups through trans-cis-trans isomerization cycles, which leads to an excess of photochromic entities oriented perpendicularly to the light polarization direction. When the light is turned off, a large part of the photoinduced orientation is preserved [4, 5].

The time dependence of the photoinduced order in azobenzene polymers has been explained by Dumont et al. using a theoretical model, which takes into account the population in both the trans- and cis-metastable states [6­8]. This model considers that the polarized light induces a selective optical pumping ("angular hole-burning (AHB)") and angular reorientations (AR) during the direct trans-cis and reverse cis-trans photoisomerizations and the cis-trans thermal back-relaxation. As a result, two mechanisms are combined, in proportions depending on molecular parameters and on irradiation conditions. The first mechanism is connected with the angular selectivity of the photoexcitation, which produces angular hole burning. This selective depletion of the ground state is common in all photochromic materials. If photoproducts are thermally and photochemically stable, the saturation of hole burning leads to a total depletion of the initial state. The second mechanism occurs when the photochromism is thermally or optically reversible (like trans-cis photoisomerization of the majority of azobenzene derivatives). In that case, molecules undergo a great number of photoisomerization cycles resulting in a kind of mechanical stirring, which induces a random-walk rotation of azochromophores. This process, known asangular redistribution, leads to the accumulation of molecules perpendicularly to the polarization of the exciting light. More generally, after a great number of photoisomerization cycles, these two processes tend to minimize the probability of optical excitation.

In the past decade, several groups of scientists studied the influence of the molecular architecture on the orientational characteristics of photochromic polymers. In their study, Sekkat et al. [9] have investigated the photo-induced orientation in an azobenzene-containing polyglutamate film. An anisotropy in the cis-isomer orientation has been found when the film was irradiated with a linearly polarized UV light (360 nm), and a theoretical model has described the time dependence of this anisotropy. Furthermore, Natansohn et al. [10, 11] have studied the time dependence of the birefringence of poly[4-[2-(methacryloyloxy)ethyl] azobenzene] film (called pMEA) using polarized UV (360 nm) or visible (514.5 nm) light. A similar birefringence has been measured for the two wavelengths, but its value was lower than those observed in various polymers containing polar azobenzene groups. Pump irradiance and temperature dependences of the photoinduced anisotropy in a pMEA polymer film have also been studied and compared to simple theoretical predictions [12]. Yaroshchuk et al. [13, 14] using irradiation with the polarized light of two different wavelengths have found that the kinetics of photoinduced ordering of azochromophores in the amorphous and liquid crystalline azobenzene polyesters can be dominated by either AR or AHB mechanism depending on the time of thermodynamic and photostimulated relaxation of metastable cis-isomers determined, correspondingly, by the structure of photosensitive side chains and excitation wavelength. Moreover the ordering process was considered in a real 3D space allowing one to identify various spatial distributions of azochromophores. Theoretical consideration of these results was given in [13, 15].

Currently, using the methodology developed earlier in [13, 14, 16] we study features of the photoinduced ordering of azochromophores in methacrylic azopolymers. The case of pp excitation of azochromophores was considered in [17, 18]. In the present paper we compare orientational transformation caused by pp* and np* excitation. It is demonstrated that, just as for azopolyesters, the process of 3D order transformation depends essentially on the relaxation kinetics of the excited cis-isomers. This allows us to conclude that photoordering regularities established earlier in [13] are common for different classes of azopolymers and thus the theory of 3D ordering [13, 15] predicting these regularities is rather general.

2. Experimental

 

2.1. Materials Synthesis

and Characterization

P1: R= NO2 P2: R= OCH3



We used poly[4-(methacryloyloxy)-4'-nitroazo-benzene] (P1) and poly[4-(methacryloyloxy)-4'-metho-xyazobenzene] (P2) differing by the donor/acceptor nature of end substitutes of the side-chain azochromophore. The general formula of these compounds is presented in Fig. 1. It is evident that the polymers differ only by the end fragment R, which is NO2 and OCH3 in polymers P1 and P2, respectively.

Synthesis of the two polymers was similar, and only the general steps towards making polymers are detailed.

Monomer synthesis. The corresponding monomers were synthesized by general methods. The azocompound (0.06 mol) and triethylamine (9.0 ml) were dissolved in THF (200 ml). The solution was kept in an ice bath for 10 min. A solution of distilled methacryloyl chloride (6.0 ml, 0.06 mol) was added slowly to the above mixture. After the addition of methacryloyl chloride, the resulting mixture was stirred at room temperature overnight. Then the solution was poured into distilled water (1 l) and the obtained residue was filtered and air-dried. Recrystallization of monomers was carried out in ethanol.

4 -methacryloxy-4-nitroazobenzene (M1) Orange crystals; yield 69 %; mp 418 K (by DSC). 1H NMR (CDCb), d (ppm): 8.43 (d, 2H, Ph-H ortho to NO2), 8.07

(d, 2H, Ar), 8.03 (d, 2H, Ar), 7.40 (d, 2H, Ar), 6.34 (s, 1H, =CH2), 5.91 (s, 1H, =CH2), 2.05 (s, 3H,

-CH3). UV-vis (Ethanol) Amax: 360, 485 nm. Elem. Anal. Calc. for C16H13O4N3: C, 61.74 %, H, 4.18 %; N, 13.50 %. Found: C, 61.70 %, H, 4.16 %; N, 13.52%.

4 -methacryloxy-4-methoxyazobenzene (M2):

1Yellow crystals; yield 65 %; mp 545 K (by DSC).

1H NMR (CDCb), d (ppm): 7,85 (d, 2H, Ar), 7,65

(d, 2H, Ar), 7,74 (d, 2H, Ar), 6,91 (d, 2H, Ar), 6.30

(s, 1H, =CH2), 5.84 (s, 1H, =CH2), 2.08 (s, 3H, -CH3),4,2 (m, 2H, CH2) 1,72 (m, 3H, CH3) UV-vis (Ethanol) Amax: 378, 500 nm. Elem. Anal. Calc. for C17H16O3N2: C, 60.59 %, H, 6.82 %; N, 7.95 %. Found: C, 60.62 %, H,

6.80 %; N, 7.93%.

Polymerization. Homopolymers P1 and P2 were synthesized by free-radical polymerization in toluene from monomers Ml and M2, respectively. The polymerization was carried out in 10 wt % toluene solution of monomer with AIBN as free radical initiator (10 wt.% of monomer) at 353 K over more than 30 h. Polymers were isolated from the reaction solution by precipitation into excess of methanol followed by reprecipitation from toluene into methanol and then dried at 293 K overnight. The synthetic work is described in more details in a separate paper [18].


The synthesized polymethacrylates were characterized by elemental analysis and 1H NMR spectroscopy. The results are in agreement with the proposed structures. Molecular weights of polymers were determined by gel permeation chromatography. The data for number-average molar mass Mn are presented in Table 1. The phase transition temperatures of the polymers were studied by DSC technique. The obtained data are presented in Table 1. As can be seen, both polymers are amorphous with a rather high temperature of glass transition. Molecular weights of the polymers (Mn) were determined by gel permission chromatography. The obtained values of Mn are rather low (Table 1), which is apparently caused by the ability of azogroups in azomonomers to serve as "traps" for radicals. As a result, in the course of polymerization, the role of termination and chain-transfer reactions markedly increases.

2.2. Film Preparation and Illumination

The polymers were dissolved in chloroform at the concentration of 2 wt %. The filtered solutions were used to spin coat the polymer films onto quartz slides. The films were kept for 30 min at 353 K to remove any remaining solvent. The film thickness measured with a profilometer was 300-500 nm. These films were of good optical uniformity.

The photoordering processes were initiated by irradiation with polarized light directed normally to the films. Two different wavelengths were used to pump the samples:

(1) Aex1 = 365 nm, from a mercury lamp, selected
by an interference filter and polarized with a Glan prism.
The light intensity
I was varied in the range of
4-10 mW/cm2.

(2)   Aex2 = 480 nm from an Ar+ laser
(I
» 20 mW/cm2).

As it will be shown below, the lines Aex1 = 365 nm and Aex2 = 480 nm correspond to л®л* and n®p* excitation, respectively. In all cases the polarization of exciting light (Eex) was chosen along the x axis of the Cartesian coordinate system with x and y axis parallel to the verges of the rectangular polymer film and z axis normal to this film (Fig. 2).

2.3. Methods of Studying of 3D Orientational Order

In these studies the transmission null ellipsometry (TNE) method dealing with refractive indices has been adapted to azopolymers. The optical scheme of the method is presented in Fig. 2. The probe beam (628 nm) is linearly polarized at 450 with respect to the in-plane main axes of the sample. The elliptically polarized transmitted beam is converted into a linearly polarized beam by a quarter wave plate (axis parallel to that of the polarizer). The angle j of the output polarization, determined by rotation of a linear analyzer, gives the in-plane retardation (ny-nx)d. Then the sample is rotated around the x axis vertically aligned and the polarization angle j is measured as a function of the incidence angle в. The out-of-plane retardation (nz-nx)d is determined by fitting a theoretical expression of j(e). The TNE was used to study 3D orientation of azochromophores in both polymers, before irradiation and after successive irradiation steps with Aex1 = 365 nm and Aex2 = 480 nm exciting light.

As a complementary characterization method we also perform 2D-dichroizm measurement in the UV/visible spectral range. The UV/vis absorption measurements were carried out using a S2000 diode array spectrometer from Ocean Optics Co. The samples were set normally to the testing light from a low intensity deuterium lamp. A Glan-Thomson prism was used to polarize the probe beam.

The optical densities (Dx and Dy) corresponding to x and y in-plane polarizations, are measured with a probe beam propagating perpendicularly to the sample. The third component (Dz) is estimated by the total absorption (TA) method, which presumes the conservation of the total absorption Dtot = Dx + Dy + Dz. The Dtot can be easily obtained, if at some instant of time t0 the sample is uniaxial, with an in-plane orientation of the axis of anisotropy, say x. Then:

Dot ° Dx(t0) + Dy(t0) + Dz(t0) = Dx(t0) + Wyik) (1)If the number of azobenzene units in trans configuration remains constant at each instant of time t, Dz can be estimated as:

Dz(t) = Do - Dx(t)-Dy(t) (2) where Dx(t) and Dy(t) are experimentally measured. Then, the diagonal terms of the tensor of orientational order Si} can be estimated. For example:

(3)

1

s

xx

Dx + Dy + Dz

The components Syy and Szz can be obtained by cyclic permutation in expression (3).

The total absorption method can be applied in some experimental situations described below.

 

3. Results and Discussion

 

3.1. Electronic Spectra

In our study we perform polymers containing azobenzene groups with electron-donor and electron-acceptor substituents, which strongly absorb in the visible region of the electronic spectrum. Fig. 3 shows the absorption spectrum in the 300-600 nm regions of a P1 and P2 polymer films before irradiation. These UV-vis spectra display high-intensity pp* bands in the UV (at about 350 nm) and low-intensity np* bands in the visible region (at about 450 nm) [19]. Considering the absorption spectra of trans- and cz's-azobenzene [18], the band at nearly 350 nm corresponds essentially to the absorption of the trans-isomers while the band at about 450 nm is mainly due to the absorption of the czs-isomers. The exact positions of the maximum of the pp* absorption bands, 1max, are presented in Table 1. A red shift of P1 spectrum is explained by stretching of p electronic system of azochromophore due to the presence of electron-donor NO2 group.

ro

<D

о с го

_Q

О (Л _Q ГО

= 365 nm

1,5 1,0 0,5

 

0,0

1 = 480 nm

ex

250 300 350 400 450 500 550 600 wavelength, nm

Fig. 3. The absorption spectra of P1 and P2 polymer films

 

3.2. Photoinduced Ordering: Case of 1ex = 365 nm Irradiation

At first we consider polymer P1 containing push-pull chromophore. The values of the in-plane, (ny-nx)d, and the out-of-plane (nz-nx)d retardation measured for the film of this polymer after successive exposure steps are presented in Fig. 4a. It is evident that before irradiation azochromophores demonstrate slight preference to the out-of-plane orientation (nz > nx = ny, Fig. 4a). This prolate uniaxial order is caused by a selforganization in the P1 film. In the stationary state of irradiation, nx < ny = nz. This implies that the UV exposure results in transformation of the initial prolate order with z axis into the oblate uniaxial order with x axis determined by the symmetry of light field. The ratio nx < ny = nz implies that the azochromophores are randomly distributed in the plane perpendicular to the light polarization direction. The transient orientational structures are biaxial (nx Ф ny Ф nz).

The in-plane components of optical density, Dx and Dy, of the same P1 film are presented in Fig. 4b.Initially, Dx=Dy, which reflects in-plane isotropy of azochromophores. In a course of irradiation, Dy increases while Dx decreases. This suggests that azochromophores are partially redistributed from x to y direction, i.e., perpendicularly to Eex. These changes correspond to reorientation mechanism [6-8] implying that concentration of cis-isomers after irradiation steps is negligible. In this case, total absorption approach can be applied. Dtot can be easily obtained, because, according to Fig. 4a, Dz= Dy in the photostationary state. Then Dz after each step of exposure can be estimated by using Eq. (2). These data are presented in Fig. 4a too.

The values of Dx, Dy, and Dz are used to calculate order parameters as described above in the Experimental section. The oblate initial order is described with the parameters S ° Szz = 0.08 and Sxx= Syy =-Szz/2 = = -0.04 (positive uniaxial order along z axis). At saturation, S ° Sxx = -0.145 and Syy= Szz = -Syy/2 = = 0.073 (uniaxial negative order along x axis). The transient order is biaxial, i.e., SxxФ Syy Ф Szz.

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O Nadtoka, O Yaroshchuk - Investigation of photoinduced orientation ordering in polymethacrylates with side-chain azobenzene moieties