678
Langmuir 2008, 24, 678-682
Articles Azo Polymer Colloidal Spheres Containing Different Amounts of Functional Groups and Their Photoinduced Deformation Behavior Junpeng Liu, Yaning He, and Xiaogong Wang* Department of Chemical Engineering, Laboratory for AdVanced Materials, Tsinghua UniVersity, Beijing, P. R. China, 100084 ReceiVed September 12, 2007. In Final Form: October 26, 2007 In this work, colloidal spheres composed of azo polymers with different chromophore loading densities were prepared, and their photoinduced deformation behavior was studied. The colloids were constructed by using a series of amphiphilic epoxy-based random copolymers containing 4-carboxylazobenzene functional groups with different degrees of functionalization (DFs). The colloidal spheres were fabricated through gradual hydrophobic aggregation of the polymeric chains in tetrahydrofuran-H2O dispersion media, which was induced by gradually adding water into the systems. The colloidal spheres were characterized by using transmission electron microscopy and dynamic light scattering. The photoinduced deformation behavior was studied by irradiating the colloidal spheres with a linearly polarized Ar+ laser beam. Results showed that the critical water content (CWC) for the colloid formation is related to the DF of the polymers, and CWC increases with the increase of DF. The hydrodynamic diameter of the colloidal spheres is also related to the DF of the polymers. When the DF of the polymers increases, the average size of the colloids gradually decreases. The hydrodynamic diameter of the colloidal spheres increases as the water dropping rate decreases. When the dropping rate is below 20 µL/s, the size of the colloidal spheres increases abruptly as the dropping rate further decreases. Upon the linearly polarized Ar+ laser beam irradiation, the colloids composed of polymers with different DFs can all be elongated along the polarization direction of the laser beam. As DF increases, the deformation degree characterized by the axial ratio (l/d) almost linearly increases. These observations can give some insight into the photoinduced deformation mechanism and can be used to construct colloids with different sizes and photoresponsive ability.
1. Introduction In recent years, various photoinduced responses of azo polymers (polymers containing azobenzene-type chromophores) have been intensively investigated.1 The photoinduced responses can be variations such as phase transition,2 photoinduced chromophore orientation,3 light-driven thin-film contraction and bending,4,5 and surface-relief-grating (SRG) formation.6 Among them, photoinduced deformations observed in different forms have aroused considerable research interest because of their unique nature and potential applications. Light-induced contraction or bending has been observed for liquid-crystal elastomeric thin films of azo polymers.4 The deformation is caused by the variation of the orientation order and conformational change of the polymer backbones triggered by the photoisomerization of azobenzenes.5(b) SRG formation has been well documented for various azo polymer thin films upon irradiation with interfering Ar+ laser beams.1,6 The reversible surface deformation has been attributed to internal
pressure gradients caused by an isomerization-driven free volume expansion in the bulk,7 the force based on the dipolar interaction of the azo chromophores with the optically induced electric field gradient,8 a translational wormlike diffusion caused by the photoisomerization of the azobenzene chromophores,9 and a mean-field force caused by the molecule alignment.10 Recent studies show that a photomechanical effect occurring in thin films of azo polymers can be a new potential mechanism to explain the formation of SRGs.11 Polymers possessing photoinduced deformation properties are promising for applications in areas such as sensors, optical data-storage, and artificial muscles.1,5 Colloidal particles have attracted much attention because of their wide application in many industrial products.12 Recently, monodispersed colloidal spheres have been widely used to construct two-dimensional and three-dimensional colloidal crystals.13,14 The colloid-based materials are expected for diversified applications such as sensors, filters, optical switches,
* Corresponding author. E-mail:
[email protected]. (1) (a) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817. (b) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139. (2) Ikeda, T.; Horiuchi, S.; Karanjit, D.; Kurihara, S.; Tazuke, S. Macromolecules 1990, 23, 43. (3) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4309. (4) (a) Finkelmann, H.; Nishikawa, E.; Pereira, G. G.; Warmer, M. Phys. ReV. Lett. 2001, 87, 015501. (b) Li, M. H.; Keller, P.; Li, B.; Wang, X. G.; Brunet, M. AdV. Mater. 2003, 15, 569. (c) Yu, Y. L.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145. (5) (a) Camacho-Lopez, M.; Finkelmann, H.; Palffy-Muhoray, P.; Shelley, M. Nat. Mater. 2004, 3, 307. (b) Ikeda, T.; Mamiya, J. I.; Yu, Y. L. Angew. Chem., Int. Ed. 2007, 46, 506. (6) (a) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136. (b) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166.
(7) Barrett, C. J.; Natansohn, A. L.; Rochon, P. L. J. Phys. Chem. 1996, 100, 8836. (8) Kumar, J.; Li, L.; Jiang, X. L.; Kim, D. Y.; Lee, T. S.; Tripathy, S. Appl. Phys. Lett. 1998, 72, 2096. (9) Lefin, P.; Fiorini, C.; Nunzi, J. M. Pure Appl. Opt. 1998, 7, 71. (10) Pedersen, T. G.; Johansen, P. M. Phys. ReV. Lett. 1997, 79, 2470. (11) (a) Tanchak, O. M.; Barrett, C. J. Macromolecules 2005, 38, 10566. (b) Yager, K. G.; Tanchak, O. M.; Godbout, C.; Fritzsche, H.; Barrett, C. J. Macromolecules 2006, 39, 9311. (c) Yager, K. G.; Barrett, C. J. Macromolecules 2006, 39, 9320. (12) (a) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker, Inc.: New York, 1986. (b) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 4th ed.; Butterworth-Heinemann: Oxford, 1992. (13) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693.
10.1021/la702846p CCC: $40.75 © 2008 American Chemical Society Published on Web 01/01/2008
Azo Polymer Colloidal Spheres
photovoltaic devices, photonic band gap materials, and processing templates among others.13,15,16 Incorporating azo polymers into colloidal particles can combine interesting properties of azo polymers and colloidal particles. Recently, it has been reported by us that, through gradual hydrophobic aggregation, uniform colloidal spheres can be constructed from amphiphilic random azo copolymers or azo homopolymers with polydispersity in the molecular weight.17-19 The colloidal spheres are formed through gradual hydrophobic aggregation of the polymeric chains in tetrahydrofuran (THF)-H2O dispersion media, which is induced by continuously increasing the water content in the media.17 The colloids exhibit some interesting properties such as photoinduced dichroism18 and photoinduced elongation upon Ar+ laser beam irradiation.19 The photoinduced elongation of the azo polymer colloidal spheres has been observed to be induced by irradiation of interfering p-polarized Ar+ laser beams19(a) or a uniform linearly polarized Ar+ laser single beam.19(b) The elongated particles can be released from the substrates and redispersed in water. The deformed shape is stable, even after the releasing and redispersing process. After the colloids have been stretched in one direction, the colloids can be further stretched in the other directions (such as that orthogonal to the first one) by the polarized laser singlebeam irradiation. For close-packed colloidal spheres such as a two-dimensional array, the colloidal spheres can be uniformly stretched along the polarization direction of the laser beam. However, the deformation degree of the colloids in the array is less than that of the “isolated” colloids, because of the restraint of the neighboring colloids. Upon exposure of the colloidal array to the interfering laser beams, only the colloids in the bright regions of the interference pattern can be deformed by the irradiation.19(b) The stretching effect of the interfering Ar+ laser beams could be attributed to the optically induced electric field gradient, which has been used to explain the SRG formation.8 The mechanism causing the colloid deformation by the uniform single laser beam is still not clear up till now. It has been reported that the degree of functionalization (DF) can show a significant effect on the SRG formation.20 Both inscription rate and saturation level are found to be determined by the loading density of the azo chromophores in the polymers. For azo polymer colloidal spheres, it is still an open question whether DF can show a similar effect on the photoinduced elongation, especially when the deformation is caused by the single laser beam irradiation. In this work, a series of amphiphilic azo polymers with the same backbone and different DFs were used to construct colloidal spheres. The effects of DF on both the formation of the colloidal spheres and the photoinduced deformation behavior were investigated. The results showed that DF together with initial polymer concentration in THF and water-dropping rate could be used to control colloidal sizes. For colloidal spheres with similar size and irradiated with an Ar+ laser single beam, the deformation (14) See, for example, (a) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (b) Norri, D. J.; Arlinghaus, E. G.; Meng, L. L.; Heiny, R.; Scriven, L. E. AdV. Mater. 2004, 16, 1393. (c) Gu, Z. Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760. (15) Ozin, G. A.; Yang, S. M. AdV. Funct. Mater. 2001, 11, 95. (16) Schroden, R. C.; Al-Daous, M.; Blanford, C. F.; Stein, A. Chem. Mater. 2002, 14, 3305. (17) Li, Y. B.; Deng, Y. H.; Tong, X. L.; Wang, X. G. Macromolecules 2006, 39, 1108. (18) Li, Y. B.; Deng, Y. H.; He, Y. N.; Tong, X. L.; Wang, X. G. Langmuir 2005, 21, 6567. (19) (a) Li, Y. B.; He, Y. N.; Tong, X. L.; Wang, X. G. J. Am. Chem. Soc. 2005, 127, 2402. (b) Li, Y. B.; He, Y. N.; Tong, X. L.; Wang, X. G. Langmuir 2006, 22, 2288. (20) (a) Fukuda, T.; Matsuda, H.; Shiraga, T.; Kimura, T.; Kato, M.; Viswanathan, N.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 4220. (b) He, Y. N.; Wang, X. G.; Zhou, Q. X. Polymer 2002, 43, 7325.
Langmuir, Vol. 24, No. 3, 2008 679
degree almost linearly increases as the DF increases. The colloid preparation, characterization, and photoresponsive properties will be presented in the following parts in detail. 2. Experimental Section Materials. Analytical pure THF from a commercial source was refluxed with cuprous chloride and distilled for dehydration before use. Deionized water (resistivity > 18 MΩ·cm) was obtained from a Millipore water purification system and used for the experiments described below. The chemical structure of the epoxy-based azo polymers (BP-AZ-CAs) used to prepare the colloidal spheres is given as
which contains different amounts of 4-carboxylazobenzene functional groups. BP-AZ-CAs were prepared through an azo-coupling reaction between the diazonium salt of 4-aminobenzoic acid and an epoxybased precursor polymer (BP-AN), which has a number-averaged molecular weight of 35 000 and a polydispersity of 2.2. The DF of BP-AZ-CAs, defined as the average percentage of the structure units bearing azo chromophores among the total units, was controlled by adjusting the feed ratio between the diazonium salt and BP-AN. The DFs of the polymers were estimated by 1H NMR and elemental analysis. The preparation and characterization details can be seen in our previous paper.20(b) Colloidal Sphere Preparation. Suitable amounts of BP-AZCAs were dissolved in THF to obtain solutions with initial concentrations of 0.5 mg/mL. The solutions were placed under stirring for 24 h and then put aside for at least 72 h. To obtain solutions or suspensions for CWC measurements, the required amounts of Milli-Q water were added into the THF solutions at a rate of 5-6 drops/min with durative stirring. For preparation of the stable colloidal suspensions, water was added into the THF solutions with a proper rate until the water content reached 50% (vol %). After that, an excess of water was added into the suspensions to “quench” the structures formed. The suspensions were dialyzed against water for 3 days to remove THF before further measurements. Light Scattering Measurement. The average size and size distribution of the colloidal spheres were measured with a Malvern Zetasizer 3000 dynamic light scattering (DLS) instrument equipped with a multi-τ digital time correlator and a 632 nm solid-state laser light source. The scattering angle used for the measurement was 90°, and the sample temperature was controlled to be 25 °C. Laser Irradiation Setup. A linearly polarized beam of an Ar+ laser at 488 nm was used as the light source. The spatially filtered laser beam was expanded and collimated. The intensity of the laser beam was about 100 mW/cm2. The linearly polarized laser beam was incident perpendicularly to the copper grid surfaces containing the colloids. All experiments were carried out at room temperature under ambient conditions. Transmission Electron Microscopy (TEM). TEM images of the colloidal spheres were obtained by using a JEOL-JEM-1200EX microscope with an accelerating voltage of 120 kV. The TEM samples were prepared by dropping diluted sphere suspensions onto the copper grids coated with a thin polymer film and then drying them in a 30 °C vacuum oven for 24 h. The samples were observed with the electron microscope before and after the laser light irradiation. No staining treatment was performed for the measurement.
680 Langmuir, Vol. 24, No. 3, 2008
Figure 1. Scattering light intensity as a function of the water content (vol %) in the THF-H2O dispersion media; the initial polymer concentrations in THF were 0.5 mg/mL.
Liu et al.
Figure 2. A plot of the CWC versus the DF.
3. Results and Discussion BP-AZ-CAs are a series of epoxy-based polymers containing different amounts of pseudo-stilbene-type azo chromophores.21 The polymers were synthesized by azo-coupling reactions between the diazonium salt of 4-aminobenzoic acid and an epoxybased precursor polymer (BP-AN), which contains an anilino moiety in each structure unit. Depending on the molar ratio between the diazonium salt molecules and anilino moieties, BPAZ-CAs can be a series of copolymers with different DFs. The DF is defined as the average percentage of the azobenzenecontaining units among the total structure units. Because of the synthetic method, the polymer possesses polydispersity both in the molecular weight and in the loading density of the pseudostilbene-type azo chromophores. The DF of the sample used for this study was in the range of 18-91%, which should be understood as a statistically averaged value. The details of the synthesis and characterization can be seen in our previous paper.20(b) Influence of DF on the Critical Water Content (CWC). The colloidal spheres were prepared by hydrophobic aggregation of the polymer chains in the selective solvent.19 This method has been developed by Eisenberg et al. to prepare micellar aggregates from amphiphilic block copolymers.22 In the process, Milli-Q water was gradually added into the homogeneous solutions of BP-AZ-CAs in THF. When the water content reached a critical value, the scattered light intensity was observed to increase suddenly. As BP-AZ-CAs possessed polydispersity, this transition indicated that the most hydrophobic segments or chains started to aggregate in the solutions. When the water content further increased, more polymeric chains were involved in the colloid growth process according to their hydrophobicity and molecular weight. The stable colloidal suspensions were obtained by adding an excess of water into the suspensions to “quench” the structures formed after the water content reached 50% (vol %). The CWC is defined as the water content at which polymer chains start to aggregate.23 CWC is an important parameter to indicate the relative hydrophobicity of the polymeric chains. CWC can be obtained by the light scattering measurement mentioned above. Figure 1 shows the plot of the scattered light intensity versus the water content for the series of BP-AZ-CAs. When the water content is low, the scattered light intensity is almost zero and remains unchanged as the water content increases for all the samples. When the water content reaches the critical values, the scattered light intensity increases sharply. CWCs were evaluated from the turning-up points of the curves. Figure (21) Rau, H. Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. II, Chapter 4. (22) See, for example, (a) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (b) Moffit, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95. (23) Zhang, L. F.; Shen, H. W.; Eisenberg, A. Macromolecules 1997, 30, 1001.
Figure 3. Hydrodynamic diameter of the colloidal spheres as a function of the water-dropping rate. DFs of BP-AZ-CAs were in the range of 18-91%, and the initial polymer concentrations in THF were 0.5 mg/mL
Figure 4. The relationships between the hydrodynamic diameter of the colloidal spheres and DF for different water-dropping rates. Table 1. The DLS Experimental Results of the Colloidal Spheres DF (%) 18 38 48 58 69 79 91 diameter (nm) 283.1 235.2 258.9 268.3 191.1 230.6 259.4 PIa 0.005 0.004 0.056 0.003 0.021 0.031 0.041 a
The polydispersity index.
2 shows the plot of CWC versus the DF of BP-AZ-CAs. The CWC increases almost linearly as the DF increases. When the DF changes from 18% to 91%, CWC increases from 22.5% (vol %) to 26.8% (vol %). CWC depends on both the initial concentration of polymers in solutions and the hydrophobicity of the polymers. As the initial concentration is the same for the samples (0.5 mg/mL), the result shows that the hydrophobicity decreases as the DF increases. This is an expectable result because the density of the carboxylic groups increases as the DF increases. Relationship between DF and Colloid Size. The colloids formed in the stable suspensions were characterized by DLS and TEM after separation from the suspensions. Both results indicated that BP-AZ-CAs with different DFs all form uniform colloidal spheres. When the initial concentrations of the polymers in THF are fixed, the average sizes of the colloidal spheres are related to the water addition rate and the DF of the polymers. Figure 3 shows the relationship between the average colloid sizes and water-dropping rate for BP-AZ-CAs with different DFs. When the dropping rate is higher than 50 µL/s, the colloid sizes of all
Azo Polymer Colloidal Spheres
Langmuir, Vol. 24, No. 3, 2008 681
Figure 5. Typical TEM images of the colloidal spheres composed of BP-AZ-CAs with different DFs: (A) 18%, (B) 38%, (C) 48%, (D) 58%, (E) 69%, (F) 79%, and (G) 91%. The scale-bars in panels A-F are 200 nm, and the scale-bar in panel G is 500 nm.
Figure 6. Typical TEM images of the colloidal spheres after being irradiated by the linearly polarized Ar+ laser beam (488 nm, 100 mW/cm2) for 15 min. The colloidal spheres are composed of BP-AZ-CAs with different DFs: (A) 18%, (B) 38%, (C) 48%, (D) 58%, (E) 69%, (F) 79%, and (G) 91%. The scale-bars in the all figures are 500 nm.
BP-AZ-CAs samples change slightly with the dropping rate change. When it is below 20 µL/s, the sizes increase abruptly with the further decrease of the dropping rate. This result indicates that the colloidal nucleation and growth are related to the dynamic process occurring in the system. The colloid size is determined by the relative rates of these two stages. The above observation could be attributed to the strong tendency for smaller particles to redissolve and aggregate on the larger particles when the dropping rate is low. Similar processes occur in the aging of colloidal dispersions (often referred to as Ostwald ripening).12(b) When the water-dropping rate is in a proper range (such as larger than 2 µL/s), the colloid size decreases as the DF increases. The relationships between the average colloid size and DF for different dropping rates are shown in Figure 4. It can be seen that, for the different dropping rates, the average colloid size decreases almost linearly as the DF increases, and the decrease is more significant for the lower dropping rate. As mentioned in the above section, the hydrophilicity of BP-AZ-CAs increases with the DF increase. These more hydrophilic chains can decrease the interfacial tension of the colloids, which leads to the reduction of the colloid sizes. Effect of DF on the Photoinduced Deformation Behavior. The photoinduced deformation behavior was studied for the colloids composed of BP-AZ-CAs with different DFs. From the
above understanding, colloidal spheres of BP-AZ-CAs with required size can be obtained. In order to minimize the possible influence of colloid size, the average colloid sizes were controlled to be in the same range by adjusting the water-dropping rate (in the range of 2-15 µL/s). The average sizes of the spheres in the stable suspensions were characterized by using DLS. The average sizes and the polydispersity index of the colloidal spheres are given in Table 1. Figure 5A-G shows typical TEM images of the colloidal spheres, which were separated from the suspensions and then dried under vacuum. The samples for the opticalstretching experiment were prepared by casting the water suspensions of the colloidal spheres (0.05 mg/mL) on copper grids, which left the “isolated” colloids on the surfaces. The colloidal spheres were exposed to the collimated laser beam that was incident perpendicularly to the copper grids. Figure 6 gives typical TEM images of the “isolated” colloidal spheres after being irradiated by the linearly polarized Ar+ laser beam (488 nm, 100 mW/cm2) for 15 min. From panel A to panel G in Figure 6, DF increases in a range from 18% to 91%. It can be seen that the colloids are significantly elongated, especially for those with the higher DF. The elongated direction of the colloids is parallel to the polarization direction of the laser beam. The average axial ratios (l/d) of the colloids (estimated statistically from TEM images of 50 colloidal particles) are used to indicate
682 Langmuir, Vol. 24, No. 3, 2008
Liu et al.
is consistent with those reported for SRG formation of the same series of azo polymers.20(b) An important difference existing in the above comparison should be mentioned. In contrast to the interfering beams used for SRG inscription, the photoinduced deformation reported in this work is induced by a uniform Ar+ laser single beam. The exact mechanism of the colloid deformation induced by the azo chromophores needs further exploration.
Summary Figure 7. Relationship between the average axial ratio (l/d) of the colloids and DF. The colloids were irradiated by a linearly polarized Ar+ laser beam (488 nm, 100 mW/cm2) for 15 min.
the deformation degree of the colloids. Figure 7 gives the relationship between the average axial ratio and DF. The average axial ratio (l/d) increases almost linearly as DF increases. The above result clearly indicates that the azo chromophores play a key role to induce the colloid deformation. A study on photoinduced SRG formation has shown that the surface deformation rate increases with the increase of DF of azo polymers.20 Fukuda et al. reported that the SRG formation rate exhibits an S-shaped dependence on the azo functionalization degree.20(a) When the chromophore density is above 40-50 wt %, the inscription rate is independent of the DF. Because of the difference of polymer backbone, DFs (given in terms of the mole percentage) of BP-AZ-CAs used in the current study are lower than the saturation point reported by Fukuda. The above result
Uniform colloidal spheres were constructed from a series of BP-AZ-CAs containing different amounts of the azo functional group through a gradual hydrophobic aggregation scheme. Both the colloid sizes and photoresponsive property were found to be closely related to the DF of the polymers. When DF increases, the hydrophilicity of the polymers increases, and the average size of the colloidal spheres decreases accordingly. Upon the irradiation of the linearly polarized Ar+ laser beam, the colloids can be elongated along the polarization direction of the laser beam. The deformation degree increases almost linearly as DF increases for the same irradiation condition. Acknowledgment. The financial support from the NSFC under Projects 20374033, 50533040, and 20504017 is gratefully acknowledged. LA702846P
