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Molecular Assembling by the Radiation Pressure of a Focused Laser Beam: Poly(N-isopropylacrylamide) in Aqueous Solution J. Hofkens,†,‡ J. Hotta,† K. Sasaki,† H. Masuhara,*,† and K. Iwai§ Department of Applied Physics, Osaka University, Suita, Osaka 565, Japan, and Department of Chemistry, Nara Women’s University, Nara 630, Japan Received June 25, 1996. In Final Form: November 5, 1996X Phase transitions in aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) with a molecular weight (M h w) of 63 000 were achieved by irradiating the solutions (0.2-3.6 wt %) with an IR laser beam (1064 nm) through an optical microscope. First, a microparticle with the size of the focused laser beam was formed (=1.5 µm). This microparticle continuously grew and after prolonged irradiation (up to 10 min), a microparticle with a maximum size of 25 µm was obtained. Upon further irradiation, the microparticle became unstable and finally disappeared. The importance of the optical alignment of the microscope/laser system is discussed. Particle formation was also found in D2O solutions of PNIPAM. These experimental results indicate that, besides a photothermal effect (heating up of the solution due to absorption of water at 1064 nm), there is influence of the “radiation force” upon particle formation and conformation properties of the polymer. The observations mentioned above are discussed in connection with the theory of the single beam gradient force optical trap for dielectric particles.
1. Introduction Poly(N-isopropylacrylamide) (PNIPAM), an amphiphilic polymer, containing hydrophobic and hydrophilic segments in the same molecule, shows interesting solution properties, especially in water. Heskins and Guillet found a reversible phase transition at a LCST (lower critical solution temperature) of 31 °C for PNIPAM in water.1 Since then, several experimental studies of this transition have been reported.2-10 A wide variety of experimental approaches, including light scattering, microcalorimetry, fluorometry, and so on, has been used to elucidate the principles of this phase transition. In diluted solution, the transition is attended to a “coil to globule” transition. Water molecules arrange themselves around the hydrophobic and hydrophilic sections of the molecule in different configurations. Several authors11-14 reported that the water molecules near the hydrophobic segments form hydrogen binding networks among themselves. This arrangement of the water helps to keep the polymer in †
Osaka University. On leave from the Catholic University Leuven. Permanent address: Department of Organic Chemistry, KULeuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium. § Nara Women’s University. X Abstract published in Advance ACS Abstracts, January 1, 1997. ‡
(1) Heskins, M.; Guillet, J. E. J. Macromol. Sci. Chem. 1968, A2, 1441. (2) Meewes, M.; Ricka, J.; de Silva, M.; Nyffenegger, R.; Binkert, Th. Macromolecules 1991, 24, 5811-5816. (3) Ricka, J.; Meewes, M.; Nyffenegger, R.; Binkert, Th. Phys. Rev. Lett. 1991, 24, 5811-5816. (4) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7, 905-911. (5) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7, 912-917. (6) Ringsdorf, W.; Venzmer, J.; Winnik, F. M. Macromolecules 1991, 24, 1678-1686. (7) Schild, H. G.; Tirrell, D. A. Langmuir 1991, 7, 665-671. (8) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 33113313. (9) Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 51545158. (10) Inomata, H.; Yagi, Y.; Otake, K.; Konno, M. Macromolecules 1989, 22, 3494. (11) Urry, D. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 819-841. (12) Urry, D. W. Int. J. Quantum Chem.: Quantum Biol. Symp. 1994, 21, 3-15. (13) Urry, D. W. Sci. Am. 1995, 44-49. (14) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352-4356.
extended shape (coil). With increasing temperature the random thermal energy of the water molecules suffices to disrupt the hydrogen bonds. The polymer molecule can assume its folded shape (globule) due to interactions between the hydrophobic segments. Recently, some papers were published dealing with the laser-induced phase transition in aqueous PNIPAM solutions.15,16 Normally, heating of polyacrylamide solutions results in phase separation; submicrometer aggregates are formed. By exposing PNIPAM solutions to an IR laser beam, Ishikawa et al.15 observed fast (>1 s) formation of a single irregular particle of 4 µm which became more regular of shape and grew up to 8 µm after prolonged irradiation (50 s). Besides influence of the photothermal processes, some evidence was found for the influence of “radiation pressure” upon this phase transition. In this paper, we show laser-induced assembling of PNIPAM, whose M h w was 63 000, in aqueous solution and systematically investigate the effect of several experimental parameters such as the initial temperature (Tini), the concentration of polymer, the laser power as well as some optical conditions (such as the alignment of the microscope/laser system) on the novel and interesting phenomenon. Furthermore, we want to emphasize the role of the radiation pressure, caused by the photon momentum change of the laser beam, upon the formation of the microstructures. 2. Theoretical Section Electromagnetic momentum will manifest itself as a radiation force or pressure whenever the momentum of an incident field is changed by deflection or absorption. The use of the radiation pressure as an optical trap for microparticles was first demonstrated by Ashkin.17-20 (15) Ishikawa, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Chem. Lett. 1993, 481-484. Ishikawa, M.; Misawa, H.; Kitamura, N.; Fujisawa, R.; Masuhara, H. Bull. Chem. Soc. Jpn. 1996, 69, 59-66. (16) Kitamura, N.; Ishikawa, M.; Misawa, H.; Fujisawa, R. Microchemistry: Spectroscopy and Chemistry in Small Domains; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 79-91. (17) Ashkin, A. Phys. Rev. Lett. 1970, 24, 156-159. (18) Ashkin, A.; Dziedzic, J. M. Appl. Phys. Lett. 1971, 19, 283-285. (19) Ashkin, A. Phys. Rev. Lett. 1978, 40, 729-732.
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More recently, Masuhara et al.21-25 proposed threedimensional manipulation of microparticles by the radiation force of a single focused IR laser beam. The physical origin of radiation force in single beam traps is most obvious for particles in the Mie size regime, where the diameter is large compared with the wavelength (λ) of the used light. Ray optics can be used to describe the scattering and optical momentum transfer to the particle.17,20,21 If the refractive index of the particle (np) has no imaginary part (no absorption) and a larger real part than that of the surrounding medium (nm), then the particle will be attracted to the focused beam and will be three-dimensionally trapped in the vicinity of the focal point, against thermal Brownian motion, gravity, and convection. Ray optics cannot be applied to particles whose diameter is much less than λ (Rayleigh particles). Wave optics are necessary for understanding the radiation pressure exerted on the small particles. According to the Rayleigh scattering theory, a particle whose diameter is much smaller than λ works as a single electric dipole. The dipole experiences the Lorenz force20,21 exerted by the optical electromagnetic field. This force corresponds to the radiation pressure and can be written as26
F ) nmR
∂ 1 (E × B) + nmR∇E2 ∂t 2
(1)
where E and B are electric field and magnetic flux density, respectively, and ∇ represents a gradient with respect to the spatial coordinates. The polarizability, R, of the particle is given by
R ) r3
(np/nm)2 - 1 (np/nm)2 + 2
(2)
where r is the radius of the particle. The first term in eq 1 is derived from the change in direction of a pointing vector and is called the scattering force (Fscat) and points in the direction of the incident light. The second term is an electrostatic force acting on the dipole in the inhomogeneous electric field and is called the gradient force (Fgrad). When np > nm, the gradient force is directed to the high electric field intensity region. If Fgrad exceeds Fscat, the particle is attracted toward the focal point and there arises a barrier for the particles to escape from the focal region. For a focused IR beam the value of R ) Fgrad/Fscat is usually larger than 1 at the position of maximum gradient, as the gradient force is usually stronger than the scatter force, leading to effective trapping (single gradient force optical trap). As calculated by Ashkin,17 particles as small as 9 nm can be trapped (depending on refractive index of the particle and medium, focused spot size, and laser power). For PNIPAM polymer chains with a molecular weight of (20) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288-290. (21) Kitamura, N.; Sasaki, K.; Misawa, H.; Masuhara, H. Microchemistry: Spectroscopy and Chemistry in Small Domains; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 35-48. (22) Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Opt. Lett. 1991, 16, 1463-1465. (23) Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Jpn. J. Appl. Phys. 1991, 30, 907-909. (24) Misawa, H.; Sasaki, K.; Koshioka, M.; Kitamura, N.; Masuhara, H. Appl. Phys. Lett. 1992, 60, 310-312. (25) Sasaki, K.; Misawa, H. Microchemistry: Spectroscopy and Chemistry in Small Domains; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B.V.: Amsterdam, 1994; pp 23-34. (26) Shen, Y. R. The principles of nonlinear optics; John Wiley & Sons: New York, 1984; pp 366-379.
1 × 106, the hydrodynamic radius in water is reported to be around 100 nm if the polymer has an extended shape (coil) and around 60 nm when the polymer chains are in the globule state.27,28 The refractive index of the PNIPAM in the dehydrated or globule state is 1.508, higher than the refractive index of water (1.33), so all conditions are fulfilled to see optical trapping. 3. Experimental Procedure N-Isopropylacrylamide (NIPAM, Eastman Kodak) was recrystallized twice from hexane. Polymerization was carried out in dioxane (0.5 M NIPAM) and initiated by azobis(isobutyronitrile) (AIBN) (5 mM) at 60 °C for 8 h after degassing four freezepump-thaw cycles on a vacuum line. The PNIPAM polymer was isolated by precipitation with diethyl ether and purified by a dissolution (dioxane)-precipitation (diethyl ether) step, followed by drying under reduced pressure. The molecular weight of PNIPAM in THF was determined by GPC (calibrated against polystyrene standards). A value for M h w of 63 000 was obtained. Sample cells were constructed by placing two cover glasses with a thickness of 70 µm on a slide glass (0.5 cm between the edges of the two cover glasses). A drop of PNIPAM solution (0.2, 1.8, and 3.6 wt %) was added, and a third cover glass with a thickness of 70 µm was put on top of the two other cover glasses. In this way, a solution layer of =70 µm was obtained. After each measurement, the thickness of the sample layer was checked with a HeNe laser in order to avoid concentration fluctuations due to evaporation of water. The sample was set on the stage of an optical microscope (Nikon, Optiphoto XF). The sample solution was then irradiated by a focused (=1.5 µm spot) 1064 nm laser beam from a continuous wave (CW) Nd3+:YAG laser (Spectron, S1-903U) through a microscope objective (magnification ) 100, numerical aperture (NA) ) 1.30). All measurements were done between 15 and 20 µm from the upper cover glass (measured with a HeNe laser) in order to minimize defocus effects. The optical condition is schematically shown in Figure 1A. Morphologic changes of the sample solution were monitored by a CCD camera. Further details on the experimental setup have been published elsewhere.29 Effective laser power irradiated to the sample solution was measured by a method reported earlier.29 The efficiency of the system measured in this way was 8%. Temperature effects were examined by placing the samples on a hotstage (Kimazato) suitable for usage under a microscope. For the measurements in D2O (Aldrich, 99.9999+%), extreme care was taken to avoid contamination with water; the PNIPAM polymer was dried under vacuum for several days, and preparation of the sample solution was done under a flow of dry argon gas. Glass plates used to prepare the sample cell were rinsed with D2O just before usage.
4. Results and Discussion At room temperature (20 °C), an aqueous PNIPAM solution is homogeneous and clear. Photoirradiation of the solution leads, after a certain time of exposure, to the formation of a 1 µm particle (the size of the focused laser beam). The particle starts to grow upon prolonged irradiation, and as it grows, its form becomes rough. Submicrometer to micrometer clusters of irregular shape become visible on the surface and in the surrounding of the trapped particle (after =400 s). The particle diameter reaches a steady state or maximum value for a given laser power after a certain irradiation time (10 min is a characteristic value). After the laser beam is blocked, the particle disintegrates slowly, while it sinks to the bottom of the sample cell. A typical example of the morphological changes of the solution as function of the laser irradiation time is shown in Figure 1 (3.6 wt % PNIPAM solution, P1064 ) 250 mW, 20 °C). (27) Kubota, K.; Fujishige, S.; Ando, I. Polym. J. 1990, 22, 15-20. (28) Yamamoto, I.; Iwasaki, K.; Hirotsu, S. J. Phys. Soc. Jpn. 1989, 58, 210-215. (29) Misawa, H.; Koshioka, M.; Sasaki, K.; Kitamura, N.; Masuhara, H. J. Appl. Phys. 1991, 70, 3829-3836.
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Figure 1. (A) Schematic diagram of the optical condition in the sample cell. (B) Schematic representation of the laserinduced particle formation in a sample cell containing an aqueous PNIPAM solution.
As mentioned above, it is well-known that an aqueous PNIPAM solution exhibits a phase transition at a LCST (lower critical solution temperature) of 31-32 °C.1 At this temperature, polymer chains start to associate with each other to form water-insoluble submicrometer polymeric particles (seen as clouding of the solution). Therefore, one factor that should be important for the association process under the microscope is the photothermal heating of the solution by the IR laser beam. Indeed, H2O shows an absorption at 1064 nm due to the vibration overtone band of OH. It is also worth noting that PNIPAM itself does not show absorption at 1064 nm, so that the laserinduced phase transition and microparticle formation cannot be ascribed to direct photoresponse of PNIPAM. So, the phase transition of the polymer (coil to globule) will be induced via photothermal local heating of the solution, initially in the vicinity of the focal spot of the laser beam. According to a rough calculation,16,29 at least a few K is elevated upon irradiation. As the refractive index (and hence the polarizability) of the collapsed polymer chains is significantly higher than that of the surrounding solution, the collapsed polymer chains will be attracted to the focal spot by the radiation pressure. This will result in the formation of a single microparticle with the size of the focal spot if the radiation force overcomes the Brownian motion of the collapsed polymer chains. The so-called Soret effect30,31 and a resultant nonlinear propagation of laser beam32 should be involved. However, the particle formation is considered not to be possible without the phase transition under the present PNIPAM concentration, as higher concentration (30) Giglio, M.; Vendramini, A. Appl. Phys. Lett. 1974, 25, 555-557. (31) Thyagarajan, K.; Lallemand, P. Opt. Lett. 1978, 26, 54-57. (32) Jean-Jean, B.; Freysz, E.; Ducasse, A.; Poulingny, B. Europhys. Lett. 1988, 7, 219-224.
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gives more viscous solution almost like glass and never induces precipitation. Upon prolonged irradiation, the phase transition temperature will be reached in a broader region, resulting in a growing particle. As the particle reaches the maximum diameter, a dynamic equilibrium is established. Polymer chains move in from the surrounding of the particle while others diffuse out toward the solution, while the size of the particle stays constant. A schematic visualization of the particle formation process in an aqueous PNIPAM solution is given in Figure 2. The morphological changes mentioned above are quite different from the behavior reported before,15,16 the very fast formation of an irregular microparticle of ∼4 µm (in fact it is 7 µm when one also counts the rough surface) which becomes a smooth 8 µm particle after 50 s of irradiation. One possible difference we could see between the present and the previous experiments is a variance in the optical alignment of the microscope/laser system. Therefore, we checked the effect of the system adjustment. When the beam was defocused in the sample solution (by shifting the focal point out of the sample solution), the fast formation of a rough particle of several micrometers was also observed (although we never observe the smooth particle after prolonged irradiation). The reason why particle formation is faster for the defocused beam is not completely clear at this moment. Probably it is due to different volume/surface ratios and therefore different cooling rates by the surrounding water. Another difference between the two experiments can be found when one looks at the molecular weight of the polymers used (M h w ) 63 000 and 1 000 000 for the present and the reported systems, respectively). As the radiation pressure is weaker as the particle is smaller, it is reasonable that a longer irradiation time is needed for the polymer with the lower molecular weight, assuming that all experimental conditions are identical. In order to establish the present interesting phenomena, we investigated the effect of several experimental parameters. First, microparticle formation was measured as function of the laser power (Figure 3, all diameters were measured after 10 min of irradiation). The laser power indicated in the figure is the laser power in solution (P ) Pin × 0.08). As can be seen in Figure 3, the maximum diameter of the formed particle depends on the concentration of the solution. All solutions show an increase in diameter of the particle upon increasing the laser power up to 250 mW. Higher laser power results in a faster achievement of this diameter but does not lead to larger particles. Laser power above 250 mW leads to fast particle formation but the particle is not stable; as it reaches the maximum diameter, the particle is no longer trapped and starts to sink to the bottom of the sample cell. Small fragments are formed around the particle and flow vertically to the focal point. Namely, the particle disappears and one can clearly observe a convection in the sample solution. Similar observations were made for the three different concentrations. As explained before, the particle with an appropriate size is trapped against thermal Brownian motion, gravity, and convection. As the particle grows, the gravity force becomes larger. As the laser power increases, also the temperature gradient induced in the solution will increase. Therefore we consider that the trapping force will be overcome by the other forces at a certain laser power (∼250 mW). When one sample cell is irradiated repetitively, the initial particle formation tends to be somewhat faster (Figure 4). This is caused by the overall increase in temperature in the sample cell due to the photothermal heating of water. In the case of a very short blocking of the laser beam (20 µm but with a very loose structure is formed very fast. Apparently the simultaneous collapse of the polymer chains and influence of radiation pressure are necessary to obtain a more structured microparticle. In order to demonstrate the effect of the radiation pressure clearly, we have tried to confirm morphologic changes for a 3.6 wt % solution of PNIPAM in D2O. As shown in Figure 6, no absorption was observed at 1064 nm for PNIPAM in D2O. This indicates that the photothermal effect is neglectable in D2O; thus the unique driving force for particle formation should be the radiation force of the laser beam. Compared to H2O, the microparticle formation was very slow. At 250 s after switching
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Figure 5. Diameter of the observed particle as function of time for a 3.6 wt % aqueous PNIPAM solution at different initial temperatures (P1064 ) 230 mW; a, Tinit ) 18 °C (2); b, Tinit ) 24 °C (9); c, Tinit ) 27 °C ([); d, Tinit ) 30 °C (b)).
Figure 6. Absorption spectra of 3.6 wt % solutions of PNIPAM in H2O (1) and D2O (2) in a cell with 1 cm path length.
on the laser, we could not identify a particle, although visible scattering of a HeNe beam by a submicrometer cluster or particle at the focal point was already observed. After prolonged irradiation (400 s), a micrometer-size particle could be seen in solution. The maximum size
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that could be obtained for the PNIPAM particle in D2O was between 3 and 4 µm (observed after more than 15 min of irradiation). As extreme care was taken to avoid contamination with water, we think that the conformational change of the polymer in this case is induced by the radiation pressure. The different behavior compared with aqueous solutions is of course ascribed to the absence of the photothermal temperature elevation. After the IR laser beam was blocked, the particle disintegrated very slowly (more than 120 s before complete dissolution), while a microparticle of the similar size in water dissolved in less than a second. The very slow particle dissolution in D2O compared with the very fast dissolution of a particle with similar size in H2O can indicate a different association or organization of the polymer chains in the particle. Also the temperature gradient in the focal point should be mentioned. After the laser cutoff, the temperature becomes lower than the LCST, accelerating the dissolution. In D2O, the temperature effect is completely absent. Finally, we mention the overall structure of the formed particle. As the particle moved due to Brownian motion, one could clearly see that the shape of the microparticle was not spherical but ellipsoidal (Figure 7). The ellipsoidal shape of the particle is again consistent with the theoretical background outlined before. As the focused laser beam is conical, the formed particle should not be spherical but more or less ellipsoidal. It is observed as a sphere (circle) when one sees the cross section at the focal plane through the microscope. The present observation has been made possible for the first time here, since the disintegration is so slow in D2O. 5. Conclusions Reversible microparticle formation was attained by irradiation of PNIPAM solutions with an IR laser beam through a microscope. The creation of a particle with a diameter between 1 and 25 µm and the disintegration of that particle could be controlled. It is well demonstrated that both the photothermal heating and radiation pressure cause the unusual particle formation in H2O. It was also clearly demonstrated that associations of the polymer in D2O can be controlled with the radiation force of the focused laser beam only. The M h w of the present polymer
Figure 7. Photographs of the ellipsoidal structure of the PNIPAM particle in D2O, obtained after 1000 s of irradiation (P1064 ) 250 mW). The pictures were obtained 20 s (a) and 50 s (b) after blocking the laser beam. Tinit ) 20 °C.
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is 63 000, which means hydrodynamic radii of ∼20 nm and ∼10 nm for coil and globule conformation, respectively. Apparently, the size of this molecule is large enough to receive the radiation force of the laser light. We consider the molecular assembling by the radiation pressure of laser light to be quite general. Actually, Hotta et al.33 reported attraction and fusion of micelles with a diameter of 100 nm in D2O, Hofkens et al.34 observed attraction and particle formation of a hydrophobically modified poly(N(33) Hotta, J.; Sasaki, K.; Masuhara, H. Abstracts of the 7 UPS symposium 1995. (34) Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Faes, H.; De Schryver, F. C. Mol. Cryst. Liq. Cryst. 1996, 283, 165.
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isopropylacrylamide) which is known to form a kind of polymeric micelles of around 30 nm).4,5 Acknowledgment. The authors thank Professor H. Fukumura (Osaka University) for his stimulating discussion. Financial support in the form of a scholarship from the Japan Society for the Promotion of Science (JSPS) to J. Hofkens is gratefully acknowledged. The present work is partly defrayed by Grands-in aid from the Ministry of Education, Science, Sports and Culture of Japan (94170, 07241243, 08231247). LA9606308
