Self-assembly and optical properties of poly(acrylic acid)-based azo polyelectrolyte

Thin Solid Films 458 (2004) 143–148

Self-assembly and optical properties of poly(acrylic acid)-based azo polyelectrolyte Guojie Wanga,*, Yaning Heb, Xiaogong Wangb, Lei Jianga a

Center of Molecular Science, Institute of Chemistry, Chinese Academy of Science, No. 2, 1st North Street, Zhongguan, Beijing 100080, PR China b Department of Chemical Engineering, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 4 June 2003; received in revised form 9 September 2003; accepted 9 December 2003

Abstract A novel photodynamic azo polyelectrolyte was synthesized by incorporating azobenzene units into poly(acrylic acid) backbone and used as a polyanion. This polyelectrolyte was assembled into multilayer thin films by an electrostatic layer-by-layer deposition technique in conjunction with a polycation. Studies on the optical properties such as photoinduced surface relief gratings and dichroism of the multilayer films and spin-coated films were carried out. Good quality surface relief structures on the spin-coated film were clearly generated when exposed upon the polarized interferencing Arq laser beams, yet no surface relief structures occurred on the multilayer films of the synthesized azo polyelectrolyte investigated by atomic force microscopy. The extent of photoinduced dichroism of the films was evaluated by dichroic ratio and orientational order parameter from their polarized absorption spectra. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Multilayers; Optical properties; Polymers; Surface structure

1. Introduction Azo chromophore functionalized polymers have been paid great attention recently for their potential applications in the field of communication w1–4x. The azo polymers possess some interesting properties such as light-induced phase transition of liquid crystalline polymers w5x, ‘command surface’ w6x, light-induced birefringence and dichroism w7x, and light-induced surface relief gratings w8,9x. In 1995, Natansohn’s group w8x and Tripathy’s group w9x reported that large surface relief gratings could be recorded on azobenzene functionalized polymer films at room temperature using two interfering polarized argon ion laser beams, and then a great deal of data w10–15x has been accumulated quite rapidly due to its basic phenomenological interest and attractive technological applications such as holographic storage, optical filters, and resonant couplers. This photodynamic process has particular advantages w12x since: (1) it offers *Corresponding author. Tel.: q86-1082621396; fax: q861082627566. E-mail address: [email protected] (G. Wang).

an easy, all-optical, and single-step fabrication process; and (2) the surface topology is erasable by application of a single specific polarized laser beam or heating above the glass-transition temperature, which makes the process repeatable. However, the photodynamic behavior of azo polyelectrolyte was rarely reported w16,17x. Azo polyelectrolyte can be fabricated to self-assembled films by electrostatic layer-by-layer assembly technology. This self-assembly technology has been considered as a simple, versatile, and effective technique for fabrication of ultrathin organic multilayer films by alternately dipping substrates into dilute solutions of cationic and anionic polyelectrolytes w18–20x. The alternative approach is far more general than the Langmuir–Blodgett technique and chemisorption self-assembly technique. The Langmuir–Blodgett technique requires special equipment and has severe limitations with respect to substrate size and topology as well as film quality and stability. The chemisorption technique based on covalent or coordination chemistry is restricted to certain classes of organics, and highquality multilayer films cannot be reliably obtained for

0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.046

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under vacuum for 48 h. The synthesized polyelcetrolyte was dark brown and the yield was 45%. 2.3. Characterization 1

Scheme 1. Synthetic route to the azo polyelectrolyte PAA-AZ.

the severely limited number of reactions with exactly 100% yield. He and Tripathy et al. w21x fabricated surface relief gratings using low-molecular-weight azo dyes by the electrostatic layer-by-layer assembly technology and they found that the 80 bilayer film showed relief structures with depth modulations of approximately 81 nm after exposure to the interference pattern formed by p-polarized Arq laser beams. In this study, we present the synthesis of a new azo polyelectrolyte PAA-AZ by incorporating azobenzene units into poly(acrylic acid) backbone. The multilayer films of the azo polyelectrolyte were fabricated by electrostatic layer-by-layer self-assembly technique, which showed good photoresponsive properties. The surface relief gratings and photoinduced dichroism of the polyelectrolyte multilayer film and spin-coated film are reported here.

H nuclear magnetic resonance (NMR) spectra of the products in dimethyl sulfoxide-d6 were recorded with a Bruker AM-200 spectrometer. Elemental analysis was measured through the Heratus CHN-Rapid method. The thermal analysis was determined with a TA Instrument DSC 2910 at a heating rate of 10 8Cymin. The molecular weight of the polymer was measured by gel permeation chromatography (PL-GPC210) with styragel columns relative to polystyrene standards using tetrahydrofuran as eluent. The UV–Vis spectra were recorded on a Perkin–Elmer Lambda Bio-40 spectrometer. The surface image of the films was investigated by atomic force microscope (USA digital instruments Nanoscope III a Dimension 3100 AFM, Si3N4 tip) operating in the tapping mode with resonant frequency of 212 KHz. 2.4. Multilayer fabrication

The reagents and solvents used in this work are commercial products of high grade employed as received. Poly(acryloyl chloride) and the azo chromophore ethyl 4-w49-(N, N-ethyl-hydroxyethyl)phenylazox benzoate were prepared according to the literature w22,23x. The synthetic route to the photodynamic azo polyelectrolyte PAA-AZ is shown in Scheme 1.

The multilayers were prepared by the electrostatic layer-by-layer self-assembly technique. The synthesized azo polyelectrolyte PAA-AZ and poly(diallyl-dimethylammonium chloride) (PDAC) purchased from Aldrich were used as polyanion and polycation, respectively. PAA-AZ (4 mg) was dissolved in Milli-Q water (resistance)18 MV cm, 100 ml) and adjusted to pH 6. The concentration of PDAC was adjusted to 0.1 mmolyl with pH 5.5. Before multilayer fabrication, the slides were treated with sulfuric acid, H2O2, ammonia and Milli-Q water according to literature w24x. For multilayer fabrications, a freshly treated quartz slide was dipped in PDAC solution and then in PAA-AZ solution each for 10 min. After each dipping, the slide was washed with excess Milli-Q water for 2 min. The multilayers growth was monitored with an UV–Vis spectrometer. The whole process was performed under room temperature.

2.2. Synthesis of the azo polyelectrolyte PAA-AZ

2.5. Spin-coated film fabrication

Poly(acryloyl chloride) (0.3 g) and the azo chromophore ethyl 4-w49-(N, N-ethyl-hydroxyethyl)phenylazox benzoate (0.2 g) were dissolved in anhydrous N,N9dimethylformamide (40 ml). A little amount of triethylamine was added into the solution as HCl absorbent. The mixture was stirred at 60 8C for 12 h under N2 protection. Then some water was added into the mixture to hydrolysis the unreacted acyl chloride groups. The product was precipitated in 250 ml HCl solution (0.01 molyl), filtered off, washed with water till the pH of the filtrate was approximately 7 and dried. The polymer was purified by dissolving in tetrahydrofuran and precipitating by petroleum ether twice and dried at 70 8C

0.04 g PAA-AZ was dissolved in 0.3 ml N,N9dimethylformamide (DMF). The solution was spincoated on a clean quartz slide, and then dried in a vacuum at 70 8C overnight. The thickness of the spincoated film was approximately 340 nm.

2. Experimental details 2.1. Materials

2.6. Photoinduced surface relief gratings and dichroism The experiment setup for the surface relief structures by laser is shown in Scheme 2 w8,9x. A linearly ppolarized argon ion laser at 488 nm was used as recording beam (intensity of 140 mWycm2). The other portion of the beam was reflected onto the sample from

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Scheme 2. Experimental setup for surface relief grating fabrication.

an orthogonal mirror. The diffraction efficiency of the first order diffracted beam from the gratings in transmission mode was probed with an unpolarized low power He–Ne laser beam at 633 nm, which was used to monitor the grating formation process. The incident angle u of the recording beams was selected to be approximately 218. Using the identical experiment setup, the dichroism of the samples was induced with single p-polarized argon ion laser beam after the mirror was removed.

Where N is the atomic weight of nitrogen, A is the molar mass of the repeat unit of acrylic acid, B is the molar mass of the repeat unit of acrylate with azo chromophore, and E (5.27%) is the content of nitrogen in PAA-AZ measured by elemental analysis, respectively. According to the relation, the degree of functionalization of the PAA-AZ is 15%.

3. Results and discussion

3.2. Self-assembly of PAA-AZ

3.1. Chemical structure of PAA-AZ

The synthesized azo polyelectrolyte could be easily assembled into multilayer films through an electrostatic layer-by-layer deposition process and possessed good self-assembly property. The UV–Vis spectra of PAAAZyPDAC multilayers (from one to thirty bilayers) are shown in Fig. 2a. The intensity of the absorbance of the multilayers at lmax increase linearly with the number of diping after ;5 dipings (shown in Fig. 2b), which indicates the build-up of multilayers in a layer-by-layer manner w24x. In the beginning, the maximum absorbance intensity of the multilayers increased slowly for the influence of substrate, and after five multilayers fabricated the maximum intensity increased linearly fast, which indicated that the increments in the adsorbed amount of PAA-AZ beyond the fifth layer were greater than that in the first few layers. The influence of the substrate is restricted to the first few layers, and the

PAA-AZ was synthesized by functionalization of a very reactive precursor poly(acryloyl chloride) with a azo chromophore (see Scheme 1). 1H NMR spectrum of PAA-AZ is shown in Fig. 1. The molecular weight ¯ of synthesized PAA-AZ was determined to be Mws ¯ ¯ 7944 (MwyMns2.6) by GPC. The glass transition temperature Tg was determined to be 134 8C. The maximum absorption wavelength lmax (related to p– p* transition of the azobenzene trans-configuration) of PAA-AZ in DMF solution and spin-coated film were determined to be 448 nm and 446 nm, respectively. The degree of functionalization X was determined according to the following relation by elemental analysis: 3NXy wA(1yX)qBXxsE

Fig. 1. 1H NMR spectrum of PAA-AZ.

Fig. 2. (a) UV–Vis spectra of PDACyPAA-AZ multilayers (from bottom to top: 1 to 30 bilayers). (b) Maximum UV–Vis absorbance of PDACyPAA-AZ multilayers varying with the number of bilayer.

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mately 667 nm. The grating spacing measured from AFM was consistent with the theoretically calculated spacing (L) according to the relation w27x: Lsl0 y2sinu

Fig. 3. Diffraction efficiency of PAA-AZ spin-coated film as a function of recording time.

interaction between polyanion and polycation governs the adsorbed amount in high layers w19x. 3.3. Surface relief gratings The recording of surface relief gratings on azo polymer films is dependent on the polarization and the energy of writing beams w25,26x. Tripathy and coworkers w25x have shown that the diffraction efficiency when writing beams are p-polarized (polarization parallel with the incidence plane) is larger than that when writing beams are s-polarized (polarization perpendicular to the incidence plane). In this study, p-polarized writing beams with the intensity of 140 mWycm2 were used to obtain a larger diffraction efficiency. Fig. 3 shows the increase of diffraction efficiency of spincoated film with the recording time. The surface morphology of the sample was investigated by AFM. Fig. 4 shows the AFM two-dimensional view and surface profile of the gratings recorded on PAA-AZ spin-coated film. Very regularly spaced sinusoidal surface relief structures with large surface modulation depths were obtained. The surface modulation depth was approximately 100 nm, and the grating spacing was approxi-

Where L is the grating spacing, l0 is the wavelength of the writing beam, and u is the incident angle of the recording beams. The gratings could be erased by heating the PAA-AZ spin-coated film above its Tg and were stable when kept below the Tg. Surface relief gratings of azo polymers have been comprehensively reviewed by Tripathy’s group w28x, Delaire and Nakatani w29x, Natansohn and Rochon w30x. Until now, the mechanism leading to the formation of the grating (the light-driven mass transport process) is not well understood. Four models have been proposed to explain the origin of this light-driven force. Barrett, Rochon and Natansohn w31x thought that the laser-driven mass transport was related to the pressure gradient effects created by the isomerizaiton reaction. Lefin and Nunzi et al. w32x built a molecualr nodel based on an anisotropic translational diffusion of photochromes in a direction, which is parrallel to the polarization. Pedersen et al. w33x proposed a mean-field model to explain the surface relief grating formation in side chain liquid crystalline azo polymers. Kumar, Tripathy and co-workers w34,35x proposed a gradient electric field model (Forces leading to migration of polymer chains are attributed to molecular dipoles interacting with the gradient of the electric field present in the polymer material), which stresses the importance of the film surface in the mechanism. They fabricated multilayer films with azo chromophore on the surface by post azo functionalization and found that surface relief gratings in the films could not be generated in most conditions w16,17x. In our experiments, surface relief structure of the fabricated multilayers PDACyPAA-AZ was investigated with the same recording process as that of the spin coated film and formation of surface relief

Fig. 4. AFM two-dimensional view (a) and surface profile (b) of the grating on PAA-AZ spin-coated film.

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Fig. 5. Polarized UV–Vis absorption spectra of PAA-AZ self-assembled film (a) and spin-coated film (b) with polarizer oriented perpendicular (solid line) and parallel (dot line) to the recording Arq beam.

grating was not observed. The low degree of functionalization of the PAA-AZ (only 15%) and the restriction resulted from strong ionic bonds between the layers could be responsible for working against the light-driven mass transport process.

PDACyCR multilayers and found that the order parameter S was 0.09 according to the above equation. The small orientaiton factor indicated that the transition moment of the azobenzene in the multilayers tended to be dominantly parallel to the plane of the multilayer film w39x.

3.4. Photoinduced dichroism Although the p-polarized Arq writing beams could not induce the surface relief gratings on the fabricated PDACyPAA-AZ multilayers, the writing beam could induce PAA-AZ in the assembled film and spin-coated film to align in the direction perpendicular to the writing beam. To investigate the alignment behavior of PAAAZ films polarized optical absorption measurements (UV-dichroism) were carried out. The results presented in Fig. 5a,b show that dichroism has been optically induced by the writing beam. The absorbance of the films is higher for light polarized in the direction perpendicular to the writing beam. This indicated that a significant number of azo molecules have been aligned in the direction perpendicular to the writing beam. The origin of this photoinduced dichroism is attributed to trans–cis photoisomerization of the azo group followed by a cis–trans thermal isomerization w36,37x. By repetition of these trans–cis– trans isomerization cycles and motion of their molecular long axis, the optic axis of azo groups becomes aligned perpendicular to the electric vector of polarized actinic light. The dichroic ratio R and orientational order parameter S are determined by the following relation w38x:

4. Conclusion A novel photodynamic azo polyelectrolyte PAA-AZ has been synthesized by incorporating azobenzene units into poly(acrylic acid) backbone. Two kinds of PAAAZ thin films were fabricated: electrostatic deposition self-assembled film and spin-coated film. The selfassembled multilayers on the substrate grew in a layerby layer manner. The surface relief gratings on the films recorded by a linearly p-polarized argon ion laser at 488 nm were investigated. Good quality surface relief structures on the spin-coated film were clearly generated (the modulation depth was 100 nm), yet no surface relief gratings occurred on the self-assembled film. The low degree of functionalization of the azo polyelectrolyte and the strong ionic bonds between the layers might restricte the light-driven mass transport of PAA-AZ. Both the self-assembled film and the spin-coated film showed ultraviolet dichroism after irradiating with the linearly p-polarized argon ion laser. The PAA-AZ in the films oriented along the direction perpendicular to the electric vector of writing beam. Acknowledgments

RsA≤yAH, Ss(A≤yAH)y(A≤q2AH) A≤ and AH are the values of the maximum absorption parallel and perpendicular to the alignment of azo chromophore, respectively. The dichroic ratio R and orientational order parameter S of the self-assembled film (30 bilayers) were determined to be 0.88 and 0.043, and those of the spincoated film were determined to be 0.78 and 0.088, respectively. He and Tripathy et al. w21x also investigated the orientation of azo molecules (Congo Red) in the

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