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Laser-Controlled Association of Poly(N-vinylcarbazole) in Organic Solvents: Radiation Pressure Effect of a Focused Near-Infrared Laser Beam Pawel Borowicz,† Jun-ichi Hotta, Keiji Sasaki,* and Hiroshi Masuhara* Department of Applied Physics, Osaka UniVersity, Suita, Osaka 565, Japan ReceiVed: March 28, 1997; In Final Form: May 22, 1997X
Microassociation of poly(N-vinylcarbazole) by a focused near-infrared laser beam (cw-YAG, 1064 nm) was first demonstrated in organic solvents. Investigations were made in cyclohexanone and N,N-dimethylformamide, where only the radiation force is responsible for formation of the micrometer-sized particle. The following conditions of the formation process were examined for elucidating the mechanism: polymer association as a function of the concentration (range 3.5-6.8 wt %), the relation between the trapping laser power and the time of condensation, and the influences of the temperature and the solvent properties upon the formation process.
Introduction The radiation pressure-induced phenomena have received much attention in the field of optics and were first applied for trapping micrometer-sized particles by Ashkin.1 In his series of studies the phenomena have been clarified, and their application as a laser-trapping method has been conducted widely in optical measurements of microscopic systems.2 The potential of laser trapping was demonstrated particularly in investigations of biological systems and also the superiority of using near-infrared (near-IR) laser light (1064 nm) instead of visible beam (514.5 nm) was confirmed.3 The three-dimensional manipulation of microparticles with use of near-IR laser trapping was demonstrated more recently by us4 and extended to the particles with a high reflection coefficient or with a refractive index lower than that of the surrounding medium.5 Thus this technique is now recognized as a potential technique in physical chemistry. The spatial pattern formation, size selection, and assembling of microparticles in solution were achieved in noncontact mode.6 Also studies of chemical processes like laser ablation7 as well as the spectroscopic investigations of manipulated individual microparticles8 were made possible in small domains. More sophisticated output is a work on lasing of a single microparticle and its application to intracavity spectroscopy.9 The other possibility of using laser beam to trap small objects is the formation of the condensed polymer phase in small volumes. Hereafter we call it a “particle” as it looks like a microsphere in the transmission image of the optical microscopic pictures. The first reported case was on poly(N-isopropylacrylamide) (PNIPAM)10 and its derivatives.11 These polymers undergo a phase transition at lower critical solution temperature (LCST) equal to about 31 °C.12 At this temperature polymer chains start to change their shape from extended to collapsed conformations and associate with each other due to breaking intermolecular (polymer chains-water molecules) hydrogen bonds. This behavior can be observed under a microscope as a change of the PNIPAM solution from transparent to nontransparent matter. Since water (H2O) has a nonzero absorption at the trapping wavelength (overtone of O-H vibration) and the polymer itself shows no absorption in this wavelength range, irradiation resulted in a local photoinduced phase transition via † Permanent address: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. X Abstract published in AdVance ACS Abstracts, July 15, 1997.
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local heating of the solvent. During irradiation collapsed polymer chains are trapped by radiation pressure, which results in the formation of the particle within the diameter of the focal spot. Prolonged irradiation enlarges the volume where the temperature increases to the LCST value. This provides an increase of the diameter of the condensed polymer. After about 10 min the saturation (hereafter, steady state) is reached. This means the system is in dynamic equilibrium; the polymer chains move in and out of the particle, but its size is constant10 (about 8 µm). Heavy water, D2O, has no absorption at 1064 nm, so that PNIPAM chains are condensed purely by radiation pressure and not by the local heating. It results in reduction of the particle dimension to about the size of a focal spot (∼1 µm) and prolongation of the time, which is necessary to establish the steady state.11 In order to establish more firmly the molecular assembling by radiation pressure, it is necessary to demonstrate the phenomena for other polymers in organic solvents. Here we have investigated the laser-controlled behavior of poly(Nvinylcarbazole) (PVCz) in cyclohexanone (hereafter CHN) and N,N-dimethylformamide (hereafter DMF). By focusing a nearinfrared laser beam, we have clearly identified microparticle formation at the focal spot of a microscope and by varying the experimental conditions elucidated the mechanism. Experimental Section Sample. PVCz (Takasago International Corp.) was reprecipitated three times from a benzene-methanol solution. Solvents. CHN (Nacalai Tesque, Inc., GR grade > 99%) and DMF (Wako, spectroscopic grade) were checked before use in absorption with the Shimadzu UV/VIS/near-IR 3100 spectrophotometer. Experimental Setup. The experimental setup for particle formation was described extensively in previous papers.4-7 In essence, a cw-Nd3+:YAG (1064 nm) (Spectron, Sl-903U) laser beam was introduced into a Nikon (Optiphot 2) microscope where it was focused to a micrometer-sized spot in the sample solution through the microscope objective with high numerical aperture (NA ) 1.30). The He-Ne laser beam was introduced coaxially with the near-IR beam (a) to locate the focal spot at the depth of 40 µm in a sample cell and (b) to detect particles with dimensions smaller than the resolution of visible light (hereafter we call them “invisible” particles). The microscopic image was monitored by a CCD camera and recorded on the © 1997 American Chemical Society
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Figure 1. Microscope pictures of a single-particle formation of PVCz in DMF, concn of 3.5 wt %: (a) 270, (b) 600, (c) 1380, (d) 1440, (e) 1560, (f) 1570, (g) 1740, (h) 2040, and (i) 2330 s after introducing the near-infrared laser beam; (j) 2340 s after switching off the beam. Photographs a-e were obtained with backscattering of the He-Ne laser, while f-j were obtained with the optical transmission image.
videotape. The cell was made of a slide glass with grooves and a cover glass. The cell was prepared in such a way to get the adhesion (optical contact) between slide and cover glasses to slow down the evaporation of the solvent. The position of the focal spot in the sample solution was checked after each measurement to monitor the possible increase of concentration resulting from solvent evaporation. Most of the data presented here were obtained for fresh solution at room temperature, where the effect of the preceding irradiation was completely neglected. Results Particle Formation and Its Dependencies on Laser Power, Solvent, and Concentration. The PVCz solutions of concen-
tration 3.5 wt % were examined for both solvents: CHN and DMF. For this concentration the solution was irradiated for 40 min. After 5-10 min nothing is identified by eye; however, the clear scattering image can be observed. The particle is detectable in the optical transmission image only after 25-30 min. The steady state was reached after 30-35 min. The time necessary to reach each of mentioned stages is longer in CHN than in DMF. The examples of images recorded by the CCD camera are presented in Figure 1. At the beginning (photographs a-e) the detection was provided with the scattering image. Then the image was replaced by a clear one (frames f-i); namely, the particle can be observed by eye under
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Figure 3. Time of steady state achievement as a function of the nearinfrared laser power, PVCz in DMF, concn of 6.8 wt %. Notation is similar to Figure 2; full rhombus indicates particles observed with optical transmission images, while the open one shows the particle detected only by backscattering of the He-Ne laser.
Figure 2. Diameter of the PVC particle as a function of the nearinfrared laser power: (a) concentration of 3.5 wt % in CHN, (b) concentration of 3.5 wt % in DMF, (c) concentration of 6.8 wt % in DMF. Notation: full circles (a and b) and rhombus (c) indicate the particle observed with the optical transmission image, while open symbols show particles detected only with backscattering of the HeNe laser beam.
microscope. Photograph j is the reference image obtained after cutting off the trapping beam and shows dissolution of the particle. The relation between the particle diameter and the near-IR laser power is shown in Figure 2. It can be easily seen the diameter of the particle observed in optical transmission images (hereafter called also “visible” particle) is always smaller in CHN than in DMF. The values for the 3.5 wt % solution are placed below 1 µm in CHN and in the range 0.9-2.0 µm in DMF. The power ranges where the formation process is possible is also slightly different for both solvents. For CHN the threshold of condensation, the power of the trapping laser, below which the particle cannot be detected either by scattering or by transmission image, is placed at about 100 mW, while for DMF it is shifted down to about 80 mW. Also the threshold of power of the trapping beam where the particle becomes “visible” is different for both solvents. The values are about 170 and 110 mW for CHN and DMF, respectively. Since the solubility of PVCz in DMF is better than in CHN, other investigations were done in DMF. Because of this increase of the concentration, the laser power range where condensation of polymer chains is possible is spread and the particle diameter is a little increased for the same power. The threshold of the creation of a particle is shifted down to about 30 mW; also the laser power when a particle can be detected by transmission image is lower in comparison with the 3.5 wt % solution. It is equal to about 50 mW. The diameter of the observed particles in the 6.8 wt % solution is larger in comparison with lower concentration (3.5 wt %). It is situated between 1.7 and 2.8 µm for the main part of power range. The experimental data are incorporated in Figure 2. Also the dependence of the particle formation time on the laser power was investigated for solutions with high (6.8 wt %) concentration. For the main part of the intensity range (130-250 mW), the distinct scattering can be observed after
3-5 min, and the steady state is established after about 17 min. For laser power below 130 mW, the process of particle formation is slowed down and the time when steady state is established increases with the decrease of the near-IR laser power. The longest value was about 40 min for 40 mW. The relation between the particle formation time (steady state stabilization) and the trapping beam power is shown in Figure 3. The particle is stable after formation. This means prolonged irradiation by near-IR laser causes no further change in the diameter of the particle. This effect was confirmed by irradiation for up to 40 min of the high concentrated solution (6.8 wt %), when the laser power was above 130 mW. After the laser beam is cut, the particle disappears in a few seconds. Temperature Dependence. The temperature effect was examined in the range from 22 to 34 °C in increments of 2 °C. After the assignment temperature was adjusted, the sample was stabilized for 10 min before the measurement. Investigations were provided for PVC in DMF solution with higher concentration (6.8 wt %). Since the solvent does form a binary mixture with PVCz without any specific interaction like formation of the intermolecular hydrogen bonds, one should not expect any LCST where the phase transition resulting in collapse and/or association of polymer chains is induced. This expectation is supported by the fact that PVCz takes a rather rigid structure and the conformation of its chains is not so dependent on the surrounding medium like in the case of PNIPAM in water. This hypothesis was confirmed by systematic measurements at different temperatures. The solution was transparent in the whole investigated range of the temperature. Also, no changes in steady state diameter were observed. Discussion By this investigation we have confirmed that the condensation of the polymer chains provided by a focused near-IR laser is not restricted to water soluble systems. The association is made by a “pure” radiation pressure effect, where the chains are trapped directly from a transparent solution, in contrast to former investigated PNIPAM where precipitation was supported by thermally induced phase transition caused by near-infrared absorption of water (H2O). All measurements were done below the critical concentration c*, namely, where chain entanglement can be neglected. The critical density for binary solutions can be estimated from Flory-Huggins theory. When the degree of polymerization is
Letters about 200, this concentration ought to be about 7%.13 Measurements made for different fractions of polyisobutylene in diisobutyl ketone suggest treatment of this estimation as a lower limit of critical density.13 Since the radiation force depends not only on the power of the trapping beam and the optical properties of the microscope (i.e. numerical aperture and magnification) but also on the size of the trapped object, it is difficult to describe the condensation phenomenon simply as a function of radiation force generated by laser light. Trapping of the polymer chains results in their association, which changes the size and the effective refractive index of the trapped target. This changes lead to an increase of the radiation force and assist the further association. Sequentially the associated polymer phase is expanded leading to a visible microparticle. Furthermore, the additional complication due to their conformations and their relation to the trapping process is induced. This effect was clearly demonstrated for PNIPAM, by comparison with the association of the polymer chains in the two solvents H2O and D2O.11,12 The collapsed chains formed the greater particle in a time shorter than the extended ones. However, the process of polymer association is convergent and finally provides the dynamic equilibrium where the diameter of the particle is constant and the polymer chains move toward and from the particle. In this steady state the radiation force becomes time independent. When polymer chains are associated directly from the transparent phase, the diameter of the particle in the steady state is limited to the dimension of the focal spot of the trapping beam. However, the process of association depends on the concentration of the polymer chains, which was confirmed by measurements in DMF for two concentrations of 3.5 and 6.8 wt %. For the higher concentration the particle reaches a larger diameter in the steady state at the same power of the trapping laser. Since the values obtained for different condensations are scattered due to the small dimensions of the formed particle, the relation between the diameter and the concentration must be deduced from the average rather than from separate measurements. Also, the power range where the condensation of PVCz is detected changes with the polymer concentration. The threshold of particle creation is shifted down from 80 to 30 mW when the concentration is increased. Also the limit of the laser power above which the particle is “visible” changes. It moves from about 110 mW for 3.5 wt % to 50 mW for 6.8 wt %. This type of behavior is similar to that observed in the case of previously investigated polymers,11 where the concentration increase leads to the expansion of the steady state diameter and of the power range where the condensation was possible. Since collapsed PNIPAM chains formed particles with larger diameter in H2O, the relation between the concentration and the size was clearer than in the case of PVCz. The other problem is the relation between diameter and the power of the trapping laser at a given concentration. All plots presented in Figure 2 are similar, but the behavior is most clear in (c). Between the threshold of particle creation and 130 mW, the particle diameter grows with the increase of the trapping laser power. Further increase of the intensity provides no changes in the diameter. This behavior is reproduced also in Figure 3. The time necessary to achieve the steady state decreases when the laser increases (up to 130 mW). Above this value the time reaches a constant level equal to about 17 min. As we mentioned, the radiation force is dependent on many parameters, such as the properties of optical setup, laser power, and size of the trapped target. Since the process of association is convergent, the system should reach the steady state for each value of the power of the trapping laser (in the
J. Phys. Chem. B, Vol. 101, No. 31, 1997 5903 range where the association occurs). However for different powers the “level” of the steady state should be different, which is reflected in an increase of the particle diameter with the increase of the laser intensity. The steady state diameter cannot increase without limit, due to other parameters like properties of the optical setup, which can play the limiting role. In such a case, the diameter of the particle becomes constant and further increase of the laser power does not cause any changes in the diameter. In the range where the diameter is constant, the laser power is not effectively used for polymer association. The next point is the solvent dependence of particle formation. It is shown as a difference in particle diameter and power range: both smaller for CHN than DMF solution (with equal concentration). The simplest explanation as a difference in radiation force generated by a near-IR beam is excluded due to almost equal refractive index of both solvents (1.45 for CHN and 1.43 for DMF). The other possibility is the solubility of PVCz in both media. Different solubility may result in slightly different conformation of polymer chains in both solvents, which may have different effective size from the point of view of radiation pressure. It may lead to different trapping force in both solvents even under the same conditions (laser power and concentration). This difference should be detected as a difference of the particle diameter in the steady state. The detailed study of these problems will be the subject of separate investigations. Acknowledgment. One of the authors (P.B.) gratefully acknowledges New Energy and Industrial Technology Development Organization (NEDO) for financial support of his stay in Japan. J.H. is a Research Fellow of the Japan Society for the Promotion of Science. The present work was partly supported by NEDO, Proposal-Based Advanced Industrial Technology R&D Program “Control of Molecular Assembling by Radiation Pressure of a Laser Beam”, and by Grant-in-Aid from Ministry of Education, Science, Sport and Culture (94170, 07241243, 08231247, 3074). PVCz was kindly provided by Takasago International Inc. References and Notes (1) Ashkin, A. Phys. ReV. Lett. 1970, 24, 156. (2) Ashkin, A.; Dziedzic, J. M. Appl. Phys. Lett. 1971, 19, 283. Ashkin, A. Science 1980, 210, 1081. Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288. (3) Ashkin, A.; Dziedzic, J. M.; Yamane, T. Nature 1987, 330, 769. (4) Misawa, H.; Koshioka, M.; Sasaki, K.; Kitamura, N.; Masuhara, H. Chem. Lett. 1990, 1479. (5) Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Appl. Phys. Lett. 1992, 60, 807. (6) Misawa, H.; Koshioka, M.; Sasaki, K.; Kitamura, N.; Masuhara, H. Chem. Lett. 1991, 469. Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Jpn. J. Appl. Phys. 1991, 30, 907. Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Opt. Lett. 1991, 16, 1463. Misawa, H.; Sasaki, K.; Koshioka, M.; Kitamura, N.; Masuhara, H. Macromolecules 1993, 26, 282. Sasaki, K.; Misawa, H. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 23-34. Kitamura, N.; Sasaki, K.; Misawa, H.; Masuhara, H. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 35-48. (7) Misawa, H.; Koshioka, M.; Sasaki, K.; Kitamura, N.; Masuhara, H. J. Appl. Phys. 1991, 70, 3829. (8) Sasaki, K.; Koshioka, M.; Masuhara, H. Appl. Spectrosc. 1991, 45, 1041. Koshioka, M.; Misawa, H.; Sasaki, K.; Kitamura, N.; Masuhara, H. Chem. Lett. 1991, 469. Masuhara, H. J. Photochem. Photobiol. A 1992, 62, 397. Masuhara, H.; Kitamura, N.; Misawa, H.; Sasaki, K.; Koshioka, M. J. Photochem. Photobiol. A 1992, 65, 235. Sasaki, K.; Kashioka, M. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 185-196. Sasaki, K.; Kamada, K.; Masuhara, H. Jpn. J. Appl. Phys. 1994, 33, 1413. Masuhara, H.; Sasaki, K. Anal. Chim. Acta 1995, 299, 309.
5904 J. Phys. Chem. B, Vol. 101, No. 31, 1997 (9) Kamada, K.; Sasaki, K.; Fujisawa, R.; Misawa, H. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 287-300. (10) Ishikawa, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Chem. Lett. 1993, 481. Kitamura, N.; Ishikawa, M.; Misawa, H.; Fujisawa, R. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 79-92. Ishikawa, M.; Misawa, H.; Kitamura, N.; Fujisawa, R.; Masuhara, H. Bull. Chem. Soc. Jpn. 1996, 69, 59.
Letters (11) Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Faes, H.; De Schryver, F. Mol. Cryst. Liq. Cryst. 1996, 283, 165. Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Iwai, K. Langmuir 1997, 13, 414. (12) Heskins, M.; Guilliet, J. E. J. Macromol. Sci. Chem. 1968, A2, 1441. Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 5154. Irie, M. AdV. Polym. Sci. 1993, 110, 50. (13) Kwei, T. K. Macromolecules in solution. In Macromolecules. An Introduction to Polymer Science; Bovey, F. A., Winslow, F. H., Eds.; Academic Press: New York, San Francisco, London, 1979; pp 273-289.
