Synthesis and Characterization of Photoprocessable Lignin-Based

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Synthesis and Characterization of Photoprocessible Lignin Based Azo Polymer Jilei Wang, Bing Wu, Shang Li, Garry Sinawang, Xiaogong Wang, and Yaning He ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00975 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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ACS Sustainable Chemistry & Engineering

Synthesis and Characterization of Photoprocessible Lignin Based Azo Polymer Jilei Wang, Bing Wu, Shang Li, Garry Sinawang, Xiaogong Wang, and Yaning He* Department of Chemical Engineering, Key Laboratory for Advanced Materials (MOE), Tsinghua University, Beijing, 100084, China. *Corresponding Author: Yaning He Department of Chemical Engineering, Key Laboratory for Advanced Materials (MOE), Tsinghua University, Beijing, 100084, China. Tel: 86-10-62784561 Fax: 86-10-62781003 E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: Lignin based azo polymer bearing pseudo-stilbene type azo chromophores has been synthesized by a post azo coupling reaction between the modified alkali lignin and diazonium salts of 4-aminobenzoic acid in DMF with high yield. The polymer synthesized was characterized by using spectroscopic methods. Significant dichroism and surface relief patterns could be photoinscribed on the prepared azo polymer films by using appropriate laser beams. Self-assembly of the lignin based polymer in selective solvents (THF/H2O) can give the uniform colloidal spheres, which can be elongated along the polarization direction of the irradiation light. This novel synthesized lignin based azo polymer could potentially be used for applications such as reversible optical data storage, photoswitching, sensors, and other photo-driven devices, which provided a simple strategy for value-added utilization of lignin biomass resource.

KEYWORDS: Lignin based; Azo polymer; Self-assembly; Photo-induced deformation; Surface relief patterns INTRODUCTION Due to the imminent shortage of fossil fuels, biomass resource has attracted a lot of attention in recent years.1-4 Besides cellulose, lignin is the most naturally abundant biopolymeric material, which has been considered as the main renewable source of aromatic structures on Earth. Large quantities of industrial lignin, as the byproducts of the spent liquor, have been yearly produced from numerous pulping industries.5-7 Many methods have been used to treat the spent liquor to obtain the lignin, which has been used as filler, surfactant, and additive in materials applications. Anyway, most of the recycled lignin has been burned or treated in low-value methods. Thus, methods to convert lignin into high valued product have attracted worldwide attention.8-20


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In recent years, due to the photo induced isomerization of the azo chromophores, azo polymers have shown many interesting properties, which can be potential used in various photonic devices.21-28 According to Rau,29 the azo chromophores have been classified into azobenzene, aminoazobenzene, and pseudo-stilbene types. For the pseudo-stilbene type azobenzene with pull/push substituents, the cis isomer is thermal unstable, which relaxes back to the trans isomer very quickly. Upon light irradiation, the pseudo-stilbene type azobenzene can rapidly undergo trans-cis-trans isomerization cycles, which can result in photoprocessible properties such as surface relief gratings, photoinduced deformation and others.30-37 Thus, the pseudo-stilbene type azo polymers can be potential used in optical data storage, sensors, and actuators. Very recently, azobenzene and aminoazobenzene type azo chromophores have been just introduced to the lignin system, which have shown interesting photochromic properties.38,39 Introduction of pseudostilbene type azo chromophores into lignin backbone will bring the novel photoprocessible properties to the lignin, which is meaningful in terms of biomass utilization. Anyway, to our knowledge, lignin based polymer containing pseudo-stilbene type azo moieties has not yet been reported in the literature. In this paper, lignin based azo polymer with pseudo-stilbene type azo chromophores has been synthesized by a post azo coupling reaction with high degrees of functionalization. First, reactive anilino moieties were introduced into the alkali lignin by modification of the phenolic hydroxyl groups in lignin. The modified alkali lignin was then reacted with diazonium salts of 4aminobenzoic acid to introduce the azo chromophores with pull/push substituents. By irradiation with appropriate laser beams, significant dichroism and surface relief gratings (SRGs) were inscribed on the polymer film. The prepared lignin based azo polymer can self assemble into colloidal spheres in selective solvents (THF/H2O). By irradiation with linearly polarized light,


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the colloidal spheres can be photo stretched along the polarization direction. Thus, this novel prepared lignin based azo polymers could potentially be used for applications such as reversible optical data storage, sensors, and other photo-driven devices. This provided a simple strategy for value-added utilization of lignin biomass resource. EXPERIMENTAL SECTION Materials N-Ethyl-N-hydroxyethylaniline ( > 97.0 %) and 4-Aminobenzoic acid (99 %) were purchased from TCI and Alfa Aesar, respectively. Alkali lignin (AL) (Mw(GPC) = 5700, PDI = 2.1) was provided by Shixian Papermaking Corp. Ltd. (Yanbian, Jilin province, China), which was separated from pulping black liquor by acid precipitation with H2SO4. Tetrahydrofuran (AR, 99 %) was dried over 4 Å molecular sieves. Other chemicals and solvents were purchased from the commercial sources and if it was not mentioned specifically, the reactants and solvents were used as received without further purification. Ultrapure water (resistivity > 18.0 MΩ.cm) was supplied by a Milli-Q water purification system and used for all experiments. Characterization 1

H NMR spectra were recorded on a JEOL JNM-ECA 600 NMR spectrometer using DMSO-d6

as the solvent and tetramethylsilane (TMS) as the internal standard. Elemental analysis (C, H and N) was carried out by Elementar vario EL III analyzer. The Fourier transform infrared (IR) spectra were obtained using a Thermo-Nicolet 6700 FTIR spectrometer. The UV-Vis spectra of samples were measured using an Agilent Cary 300 UV-Vis spectrophotometer. The molecular weights and molecular weight distributions were measured by using gel permeation chromatography (GPC). The GPC instrument was equipped a PLgel 5 mm mixed-D column and


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the measurements were carried out at 25 °C with THF as the eluent (1.0 mL/min) and the molecular weights were calibrated with polystyrene standards. The colloid morphologies were examined by TEM (H-7650B, HITACHI) with an accelerating voltage of 80-120 kV. For TEM sample preparation, drops of diluted colloid dispersions were added onto a copper grid coated with carbon film and then dried under vacuum at 30 oC for 24 h. The surface images of the surface relief gratings were monitored using an atomic force microscope (AFM, Bruker Dimension Icon) in the tapping mode. Synthesis of N-(4-methyl ethyl benzenesulfonate)-N-ethylaniline (NBNE) 5 M aqueous NaOH (3 mL) was added dropwise into a mixture of N-Ethyl-Nhydroxyethylaniline (1.65 g, 10 mmol) and toluenesulfonyl chloride (2.00 g, 11 mmol) in tetrahydrofuran (THF, 25 mL) with violent stirring in ice water bath. After reaction for 5 h at room temperature, the mixture was poured into ice water (30 mL) and then was extracted from water with dichloromethane (50 mL). Organic layer was washed with saturated aqueous NaCl before dry over MgSO4. Purification by column chromatography (silica gel, petroleum ether/dichloromethane = 5/1) yielded the final product as white solid in 60% yield. 1H NMR (600 MHz, CDCl3): δ (ppm) 1.09 (t, 3H), 2.41 (s, 3H), 3.29 (q, 2H), 3.56 (t, 2H), 4.13 (t, 2H), 6.54 (d, 2H), 6.66 (t, 1H), 7.16 (q, 2H,), 7.27 (d, 2H), 7.73 (d, 2H). 13C NMR (150 MHz, CDCl3): δ (ppm) 146.9, 145.0, 132.8, 130.0, 129.5, 128.0, 116.5, 111.9, 67.0, 49.0, 45.6, 21.8, 12.2. Synthesis of Lignin-based Polymers (AL-NEHA) The mixture of AL (0.9 g, about 3 mmol of benzene unit), NBNE (1.28 g, 4 mmol) and KOH (1.25 g, 9 mmol) in DMF (50 mL) was heated at reflux for 24 h. The mixture was poured into an excess of water and the precipitate was collected and dried. The raw product was dissolved in


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THF (20 mL) and precipitated with petroleum ether (300 mL). The final product was vacuum dried at 60 oC for 24 h. Yield: 85%. Synthesis of Lignin-based Azo Polymers (AL-Azo-COOH) 4-Aminobenzoic acid (0.11 g, 0.8 mmol) was dissolved in sodium hydroxide solution (0.8 M, 1.0 mL), and then hydrochloric acid (37%, 0.22 mL) was dropped into above solution. The diazonium salt of 4-aminobenzoic acid was obtained by adding an aqueous solution of sodium nitrite (0.067 g, 0.97 mmol in 0.2 mL of water) into above mixture solution. The mixture was stirred in an ice bath for 5 min to obtain a clear solution. AL-NEHA (0.2 g, about 0.5 mmol of benzene unit) was dissolved in 10 mL DMF and the solution was cooled down to 0 oC. The diazonium salt solution was added dropwise into the DMF solution. After reaction for 12 h, the solution was poured into plenty of water and the precipitate was collected and dried. The raw product was dissolved in THF (5 mL) and precipitated with petroleum ether (100 mL). The final product was vacuum dried at 60 oC for 24 h. Yield: 96%. Colloidal spheres preparation The lignin based azo polymer (AL-Azo-COOH) was dissolved in THF to yield homogeneous solutions with initial concentrations about 0.5 mg/mL. The solution was stirred at room temperature for 24 h and then filtrated with 0.22 µm membrane. For obtaining the stable colloidal suspensions, an appropriate volume of deionized water (5.0 mL) was added dropwise into the THF solution (1.0 mL) with a proper rate (7.2 mL/h). After that, an excess of water (10 fold with respect to the solution volume) was added into the suspensions to “quench” the structures formed. The suspensions were dialyzed against water for 48 h to remove THF before further measurements. Photoinduced shape deformation


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The samples used for the light irradiation experiments were obtained by casting the water suspensions of the colloidal spheres on copper transmission electron microscopy (TEM) grids. The colloidal spheres were carefully dried under vacuum at 30 oC for 24 h before light irradiation. A linearly polarized laser beam (488 nm) as the light source with the intensity about 20 mW/cm2. The laser beam was incident perpendicularly to the TEM grid surfaces containing the colloids. The light irradiation was performed for about 15 min at room temperature under the ambient conditions. Photoinduced anisotropy Photoinduced anisotropy of the lignin based azo polymer spin-coated film was investigated by using a linearly polarized Ar+ laser beam at 488 nm with an intensity of 20 mW/cm2 as the writing beam. The laser beam was incident perpendicularly to the polymer film. The optically induced dichroism was measured by using polarized UV-vis spectroscopy. Photofabrication of surface relief gratings The fabrication condition was similar with that reported before.40,41 A linearly polarized Ar+ laser beam (488 nm) was used as the light source. The p-polarized Ar+ laser beam was expanded and collimated. Half of the collimated beam was incident on the film directly and the other half of the beam was reflected onto the film from a mirror. The intensity of the beam was about 100 mW/cm2.

RESULTS AND DISCUSSION Preparation and Characterization Alkali lignin (AL) has many phenolic hydroxyl groups (Ph-OH) that can be functionalized through appropriate modifications for creating new chemical active sites, which aid in the


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synthesis of lignin based azo polymers by post azo coupling reaction. The post azo coupling reaction has been proved to be a very efficient method to prepare azobenzene containing polymers.40-44 Thus, the target product was obtained across two simple modification processes. Firstly, the AL was treated with N-(4-methyl ethyl benzenesulfonate)-N-ethylaniline (NBNE), which was modified preferentially from N-Ethyl-N-hydroxyethylaniline, affording the functional group suitable for azo coupling reaction. The final product AL-Azo-COOH was prepared by the post azo coupling reaction between diazonium salt of 4-aminobenzoic acid and AL-NEHA in DMF. The possible structure of the lignin-based azo polymers (AL-Azo-COOH) is shown in Scheme 1. For quantifying the degree of functionalization (DF) of the AL, elemental analyses (EA) were performed and the results are listed in Table 1. Based on the results of the elemental and functional group analysis, the average phenyl-propanoid unit (C9 unit) of AL was calculated to be C9H9.50O5.38(OCH3)0.75, with a monomer molecular weight of 227. From the Table 1, it also can be seen that the nitrogen content have significantly increased following the successive introduction of the NBNE and AZO group. Correspondingly, the DF of AL-NEHA and AL-AzoCOOH are respectively estimated to be about 88% (from Al to AL-NEHA) and 96% (from ALNEHA to AL-Azo-COOH) according to equation (1). Where N(%) is the mole content of nitrogen in lignin based polymers; Mw,unit is the molecular weight of AL unit, which is 227; Mw,n is the molecular weight of the end-group, which is 149.24 for -NEHA, 149.13 for -N=N-phCOOH; nazo is the number of nitrogen in end-group, which is 1 for -NEHA, 2 for -N=N-phCOOH.

N% ≈

DF ×14 × nazo M w,unit + DF × M w,azo

(1)


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For the in-depth elucidation of structural features of lignin-based polymers, the 1H NMR spectra of AL, AL-NEHA and AL-Azo-COOH are presented in Figure 1. Compared with AL, the dramatic increase of aliphatic proton signals at about 0.7-1.2 ppm is related to the protons from the methyl groups carried by NBNE. Two distinct doublets are discernible in the aromatic protons regions, at respectively 7.06 and 6.63 ppm. They are assigned to the protons of the newly attached aromatic ring (AL-NEHA). The more powerful evidences can be found in the aromatic protons regions of AL-Azo-COOH. When a slightly excessive amount of diazonium salts was used, the electrophilic substitution reaction could exclusively occur at the p-position of the aniline moiety with high yield, which is about 96% according above elemental analyses. This also can be proved by the changes in the 1H NMR spectra observed for aniline moieties of ALNEHA after azo coupling reaction. The resonance (7.06 ppm) corresponding to the protons at pposition of amino group totally disappears. The chemical shifts of protons ortho and meta to the amino groups shift to lower magnetic field, due to the presence of electron withdrawing groups introduced by the azo coupling reaction and the increase of the conjugation length. Therefore, another two new characteristic peaks can be found at 8.0 and 7.76 ppm. The FT-IR spectra of lignin based polymers before and after modification are presented in Figure 2. It is clearly revealed the differences in the frequency range and fingerprint region which came from the successful introduction of NBNE and 4-Aminobenzoic acid groups into AL. Compared with AL, the absorption band intensity weakened at 3440 cm−1 due to the phenolic hydroxyl groups reacted with NBNE. The peak at 2945 cm−1 attributed to the C-H stretching vibration of -CH2- groups is enhanced simultaneously the characteristic bands for the aromatic skeletal vibration at 1600 cm−1, 1520 cm−1 are also increased. Experiments have indicated that the first step modification is very successful. Compared with AL-NEHA, the more powerful


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evidence is the strong absorption peaks appearing at 1720 cm−1and 1395 cm−1, which are absent in spectrum of AL and AL-NEHA. These two peaks belong to carboxylic acids (symmetrical stretching vibrations of carboxyl anions-COO-), illustrating the efficient post azo coupling reaction. Furthermore, the formation of the azo group was also confirmed by the UV-Vis absorption spectra in DMF (Figure 3). Compared with that of AL, the spectra of AL-NEHA did not seem to change much, however the target product showed obvious characteristic absorption peak in the visible region (λmax = 445 nm), which was the typical absorption behavior of the pseudo-stilbene type of azo chromophores. From above experimental results, it can be seen that the degree of the post functionalization of AL is very high. As it was difficult to measure the molecular weight of the prepared lignin based azo polymer directly by GPC due to the strong polarity of the azobenzene and carboxyl groups. The inherent viscosity of the prepared lignin based azo polymer was measured by using Ubbelodhe viscosimeter at 30 oC (Table S1). The [η] of the prepared polymer is calculated to be about 0.32 dL/g. Photoinduced Anisotropy and Surface Relief Gratings. It has been well known that azobenzene and their derivatives will undergo the trans-cis isomerization upon light irradiation. When the light resource is linearly polarized light, the azobenzene groups that are not perpendicular to the light polarization direction will undergo repeated trans-cis-trans isomerization. As a result, the azobenzene moieties will be forced to orientate in the direction perpendicular to the polarization of the incident light. Thus, significant dichroism can be produced by the photoinduced orientation in this lignin based azo polymer thin


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films, which are promising for applications in optical data storage, diffraction gratings, and other photo-driven devices. The spin-coated AL-Azo-COOH films on quartz glass slides reveal perceptible photoinduced anisotropy feature after being irradiated with linearly polarized 488nm (20 mW/cm2) Ar+ laser beam. The polarized UV-vis spectra were collected in two directions, which were perpendicular (A⊥) and parallel (A||) to the exciting laser polarization, respectively. In-plane dichroism was induced in the film as indicated by the polarized UV-vis spectroscopy. It can be seen that the absorbance in the direction of the polarization of the laser beam is smaller than that in the direction perpendicular to the polarization of the laser beam (Figure 4). An orientation order parameter of ca. 0.15 can be estimated from the dichroic ratio. For giving a more intuitive understanding, the optical image was showed in Figure 4 as an insert. The round spot appearance in the middle of the film was the irradiation area. The color of this area was relative weak due to the partial orientation of the azobenzene group perpendicular to the film. Due to the repeated trans-cis-trans isomerization, large surface modulations can also be reversibly photoinduced on surfaces of azo polymer films, which have been reported in the past years.45-48 The photoinduced surface-relief-gratings (SRGs) can be formed upon exposure to an interference pattern of Ar+ laser beams at modest intensities. The easy photofabrication of various micro/nano surface structures can be very useful in many future applications. In this work, photofabrication of SRGs was performed on the spin-coated lignin based azo polymer thin solid films. The interference pattern of Ar+ laser beams (488 nm) was produced by two ppolarized beams with modest intensity of 100 mW/cm2. Figure 5 (left) shows a typical AFM three-dimensional (3D) image of the SRGs appearing as regularly spaced sinusoidal surface relief structures and corresponding 2D-view AFM image (right). The spatial period can be


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adjusted by changing the wavelength of the writing beam and the angle between the two interfering beams. Self-assembly and Photoinduced Shape Deformation. Nano/micro colloidal particles can be widely applied in many industrial products. Particularly, the preparation of nonspherical colloids has aroused significant interest, which can be potentially used as photonic band gap (PBG) crystals, optical data storage and many others. A convenient method to prepare colloidal particle is by the self-assembly of amphiphilic polymers in selected solvents. For the photoinduced deformation properties of azo polymers,49-54 the colloidal spheres from the self-assembly of amphiphilic polymers bearing push-pull azo chromophores can be easily elongated along the polarization direction of the laser beam. In this work, the prepared lignin based azo polymer contained both hydrophobic and hydrophilic segments, colloidal spheres could be simply prepared by self-assembly in select solvents (THF/H2O). When deionized water was gradually added into the azo polymer THF solution with a proper rate, the solubility of the azo polymer in the mixed solvents gradually declined, which caused the aggregation of the molecular chains to form colloidal spheres. To obtain well-developed aggregates, more water was continually added to the solutions to ensure that the aggregation process could be completed. Figure 6a gives the typical TEM images of the AL-Azo-COOH colloidal spheres with average grain size 200 nm. After irradiation with linearly polarized 488 nm Ar+ laser beam (20 mW/cm2) for 15 min, the colloidal spheres were obviously elongated along the polarization direction of the light with an average axial ratios (l/d) 1.52 (Figure 6b). CONCLUSIONS


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Alkali lignin was chemically modified to incorporate pseudo-stilbene type azo chromophores. Initially, the phenolic hydroxyl groups in lignin were reacted with N-(4-methyl ethyl benzenesulfonate)-N-ethylaniline to give the anilino moieties, which was suitable for the azo coupling reaction. Then, the modified alkali lignin was reacted with diazonium salts of 4aminobenzoic acid to introduce the pseudo-stilbene type azo chromophores. High degree of azobenzene functionalization can be achieved by this approach. Significant dichroism was produced by the photoinduced orientation in the lignin based azo polymer thin films. The orientation order parameter can be reached about 0.15. SRGs have been well inscribed on the polymer film by irradiation of interfering laser beams. Uniform colloidal spheres have also been easily prepared by self-assembly of the prepared lignin based azo polymer in selective solvents. By irradiation with linearly polarized light, obvious photoinduced deformation of the colloidal spheres along the polarization direction was observed. This work has provided a convenient way to prepare lignin based azo polymers, which could potentially be used for reversible optical data storage, photoswitching, and other photo-driven devices. This work provided a simple strategy for value-added utilization of lignin biomass resource. ASSOCIATED CONTENT Supporting Information. 1

H NMR, optical setup and viscosity measurement (Figure S1, S2, Table S1). This material is

available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the Natural Science Foundation of China (Grant No. 21474056). REFERENCES (1) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110, 35523599. (2) Luque, R.; Herrero-Davila, L.; Campelo, J. M.; Clark, J. H.; Hidalgo, J. M.; Luna, D.; Marinas, J. M.; Romero, A.A. Biofuels: a technological perspective. Energy Environ. Sci., 2008, 1, 542-564. (3) Fernandes, E. M.; Pires, R. A.; Mano, J. F.; Reis, R. L. Bionanocomposites from lignocellulosic resources: Properties, applications and future trends for their use in the biomedical field. Prog. Polym. Sci. 2013, 38, 1415-1441. (4) Faruk, O.; Bledzki, A. K.; Fink, H. -P.; Sain, M. Biocomposites reinforced with natural fibers: 2000-2010. Prog. Polym. Sci. 2012, 37, 1552-1596. (5) Gosselink, R. J. A.; De Jong, E.; Guran, B.; Abacherli, A. Co-ordination network for lignin: Standardisation, production and applications adapted to market requirements. Ind. Crops Prod. 2004, 20 (2), 121-129. (6) Sixta, H.; Ed. Handbook of Pulp; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2008. (7) Lora, J. In Monomers, Polymers and Composites from Renewable Resources; Belgacem, M. N., Gandini, A., Eds.; Elsevier: Amsterdam, 2008; pp 225-241.


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Captions of Figures and Tables Scheme 1. Possible structural formula of AL, AL-NEHA and AL-Azo-COOH. Table 1. Elemental analyses (EA) of AL, AL-NEHA and AL-Azo-COOH. Figure 1. 1H NMR spectra of AL, AL-NEHA and AL-Azo-COOH in DMSO-d6. Figure 2. FT-IR spectra of AL, AL-NEHA and AL-Azo-COOH. Figure 3. UV/Vis absorption spectrum of AL, AL-NEHA and AL-Azo-COOH in DMF solution (0.03 mg/mL). Figure 4. UV-vis spectra of AL-Azo-COOH solid film: curve a, perpendicular (A⊥) to the polarization direction of laser beam; curve b, parallel (A||) to the polarization direction of laser beam. The samples were prepared by casting an anhydrous DMF solution on quartz and measured after irradiation with linearly polarized Ar+ laser beam (488 nm, 20 mW/cm2) for 30 min. The inset was the photograph of partially photo irradiated film. Figure 5. Topographical AFM images of AL-Azo-COOH (a) (left) and the corresponding 2Dview AFM image (right). Figure 6. Typical TEM images of the AL-Azo-COOH colloidal spheres with average grain size 200 nm (a) and after being irradiated with a linearly polarized laser beam (488 nm, 20 mW/cm2) 15 min (b).


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Scheme 1.


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Table 1.

Sample

C%

AL

51.50±0.36

AL-NEHA

AL-AZO-COOH

H%

O%

N%

DF

5.20±0.02 43.30±0.38

---

---

69.07±0.28

6.18±0.05 21.30±0.48

3.45±0.15

88%

62.24±0.34

5.14±0.04 24.80±0.50

7.82±0.12

96%


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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For Table of Contents Use Only

Synthesis and Characterization of Photoprocessible Lignin Based Azo Polymer Jilei Wang, Bing Wu, Shang Li, Garry Sinawang, Xiaogong Wang, and Yaning He* Synopsis: Photoprocessible lignin based azo polymer bearing pseudo-stilbene type azo chromophores has been efficiently synthesized by post azo coupling reaction.


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Synthesis and Characterization of Photoprocessable Lignin-Based

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