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Convection Patterns Trapped in the Solid State by UV-Induced Polymerization Minqin Li, Shengqing Xu, and Eugenia Kumacheva* Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario, Canada M5S 3H6 Received April 15, 2000. In Final Form: June 15, 2000 A novel approach to producing ordered patterns in polymeric films was developed, which employs (i) the generation of nonequilibrium periodic convection patterns in thin liquid layers of monomers and (ii) UV-induced polymerization of convecting fluids under controlled conditions that preserve the periodicity of the pattern. We established the conditions under which an ordered structure induced by buoyancydriven convection in a layer of methacryloxypropyl-terminated dimethylsiloxane can be trapped in the solid state and analyzed the characteristic length scales of the solidified film.
1. Introduction When a fluid layer is subjected to a vertical temperature gradient and the threshold of instability is exceeded, surface-tension-driven and/or buoyancy-driven convection occurs in the film, producing patterns with a high degree of order and symmetry. Convection patterns in thin films of simple fluids have been extensively studied both theoretically and experimentally,1 whereas convection in complex systems such as polymeric or polymerizable liquids has received scant attention.2-4 Convection in polymeric fluids is important for several reasons. First, in such systems nonequilibrium convection patterns can be trapped in the solid state and then studied using techniques not applicable to liquid systems. Second, the possibility of forming well-organized patterns on a macroscopic scale leads to new approaches to producing polymer-based composite materials with periodic modulations in properties. In addition, the study of convection in polymeric liquid films has an entirely practical aspect. Convection-induced surface roughness is a frequently encountered effect in polymeric coatings cast from solutions. In typical convection experiments, surface deflections originating from gravity waves and/or capillary ripples vary from hundreds of nanometers to several microns. These length scales are much smaller than the characteristic lateral length scales of convection patterns, which are determined by the thickness of the film and generally vary from fractions of millimeters to millimeters. For this reason, in simple fluids vertical deflection of fluid surfaces is generally neglected. In contrast, convectioncontrolled roughness induced in polymeric films is a serious problem, as it lowers the gloss and the decorative properties of coatings. In our earlier work,5 we demonstrated buoyancy-driven convection (Rayleigh-Be´nard convection) in fluid films of methacryloxypropyl-terminated dimethylsiloxane (1) (a) Godre`che, C.; Manneville, P. Hydrodynamics and Nonlinear Instabilities; Cambridge University Press: Cambridge, U.K., 1998. (b) Bragard, J.; Velarde, M. G. J. Nonequilibr. Thermodyn. 1997, 22, 1-19. (2) (a) Sakurai, S.; Tanaka, K.; Nomura, S. Polymer 1993, 34, 10891092. (b) Sakurai, S.; Tanaka, K.; Nomura, S. Macromolecules 1992, 25, 7066-7068. (3) Mitov, Z.; Kumacheva, E. Phys. Rev. Lett. 1998, 81, 3427-3430. (4) Widawski, G.; Rawiso, M.; Franc¸ ois, B. Nature 1994, 369, 387389. (5) Li, M.; Xu, S.; Kumacheva, E. Macromolecules 2000, 33, 49724979.
(MAOP-DMS) subjected to vertical temperature gradients. Convection occurred when the buoyancy-driven forces exceeded the viscous drag forces and dissipation due to thermal diffusion in the liquid films. The onset of instability was characterized by a dimensionless Rayleigh number.6
Rac ) gβ∆Td3/νκ
(1)
where β is the fluid expansion coefficient, g is the gravitational acceleration, ∆T is the temperature difference between the bottom and the top surfaces of the fluid film, d is the layer thickness, and ν and κ are the fluid kinematic viscosity and the thermal diffusivity, respectively. Buoyancy-driven convection in the MAOP-DMS layers with d ) 0.3096 cm occurred when ∆T exceeded 5.14 °C, and produced symmetric roll-like patterns.5 In this work, we aimed at preserving symmetric nonequilibrium convection patterns in solid polymeric films. Trapping of the ordered convection planforms in the solid state is not a straightforward process, since viscous dissipation accompanying vitrification may lead to the distortion or dissipation of the nonequilibrium structures (see eq 1). Clearly, the conditions of film solidification should strongly affect the appearance of the patterns. For example, Sakurai et al.2 showed that convection patterns in thin layers of polymeric solutions disappear when solvent evaporates and the viscosity of the system increases. In contrast, Kumacheva et al.3 and Franc¸ ois et al.4 demonstrated that ordered structures induced by convection in phase-separating polymer solutions can be effectively “frozen” when the system undergoes the glass transition. In this study, we found conditions under which periodic patterns generated by buoyancy-driven convection in the monomeric fluid methacryloxypropyl-terminated dimethylsiloxane can be preserved by controlled photopolymerization. Second, we examined the characteristic lateral and vertical length scales of the solidified films. 2. Experimental Section 2.1. Materials. Methacryloxypropyl-terminated dimethylsiloxane (MAOP-DMS) with molecular weight 386.6 was purchased from Gelest Inc. (PA). The amount of the end-capped acrylate group was 5.18 mmol/g. Prior to the convection experiments, (6) Nield, D. A. J. Fluid Mech. 1964, 19, 341-347.
10.1021/la0005715 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/09/2000
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Figure 1. Schematics of the cross section of the convection cell. The temperature difference across the layer is calculated as ∆T ) (Tb - Tw)/[1 + (dakl)/(dka)],11 where Tb is the temperature of the aluminum plate (assumed to be equal to the temperature of the bottom surface of the liquid film); Tw is the temperature of the water cooling the top window, da and d are the thicknesses of the air gap and the fluid film, respectively; and ka and kl are the thermal conductivities of air and the fluid, respectively. ka ) 2.55 × 103 erg/(cm s °C), kl ) 1.25 × 104 erg/(cm s °C).12 MAOP-DMS was maintained under reduced pressure for 1 week to eliminate any traces of solvents. The initiator 1-hydroxycyclohexyl phenyl ketone (HPK) (Aldrich) was used as received. HPK has a quantum yield of photolysis of Φ ) 1, and it yields clear cured films of poly(dimethyl acrylates) and polyesters.7 The concentration of the initiator, cHPK, in the MAOP-DMS films varied from 0.25 to 5.0 wt %. 2.2. Methods. The experimental apparatus for the convection studies has been described elsewhere.5 The schematics of the experiment are shown in Figure 1. A thin layer of MAOP-DMS with a thickness of 0.3096 ( 0.002 cm was placed onto a polished chromium-coated aluminum plate with the diameter 5.65 ( 0.005 cm, and a vertical temperature gradient was applied to the film by heating it from the bottom and cooling it from the top. The top surface of the liquid layer was bounded by a uniform layer of air varying from 0.1 to 0.2 cm in thickness. Convection patterns generated in the fluid layer were visualized using a shadowgraph experimental setup equipped with a CCD camera (Pulnix TM 40) which was linked with a computercontrolled frame-grabber. When symmetric convection patterns were induced in the MAOP-DMS fluid layer, UV-induced photopolymerization of the fluid was carried out to trap the patterns in the solid state. A high-intensive UV cure unit (UVAPRINT 40C/CE, Dr. K. Ho¨nle GmbH UV-Technologie, Germany) was employed. The UV-unit was equipped with a parabolic reflector for uniform radiation of stationary objects. A quartz filter was used to select the wavelength region 330-380 nm from the emitted light of a 400 W mercury lamp (F-lamp). The lamp was allowed about 3 min for equilibration before polymerization was performed. The surface roughness of the solidified polymer films was measured with a Surface Profiler (WMPK Ltd., NJ) using the interferometric technique. The surface of the film was imaged over the areas of 4.7 mm × 2.6 mm with a resolution in height measurements of 3 nm. The interference patterns resulting from the interaction of the light reflected from the test surface were analyzed using Image Tool software (Health Science Center, University of Texas, San Antonio, TX).
3. Results and Discussion The method of trapping convection patterns in the solid state consists of two successive stages. At the first stage, ordered convection patterns are generated in a low-viscous fluid mixed with an initiator. At the second stage, polymerization of the fluid is carried out under controlled conditions that preserve the periodicity of the convection pattern. A very important requirement of this approach is to ensure that no polymerization, or very weak polymerization, occurs while heating the liquid layer during the first stage. (7) (a) Mu¨ller, U.; Vallejos, C. Angew. Makromol. Chem. 1993, 206, 171-191. (b) Roffey, C. G. Photopolymerization of Surface Coatings; Wiley: Chichester, 1982.
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Figure 2. Convection patterns generated in 0.3096 cm thick films of MAOP-DMS: (a) fluid film of pure MAOP-DMS, ∆T ) 6.81 °C; (b) fluid film of MAOP-DMS mixed with 1.0 wt % HPK, ∆T ) 6.39 °C; (c) polymerized film of MAOP-DMS, cHPK ) 0.25 wt %, t ) 3 min; (d) polymerized film of MAOP-DMS, cHPK ) 1 wt %, t ) 3 min; (e) polymerized film of MAOP-DMS, cHPK ) 2 wt %, t ) 3 min. The diameter of the convection pattern is 5.0 cm.
Figure 2a shows a typical symmetric roll-like convection pattern produced in a 0.3096 cm thick fluid film of MAOP-DMS at ∆T ) 6.81 °C. The transition to pure buoyancy-driven convection in this film occurred at ∆T ) 5.14 °C. In the figure, narrow white rings correspond to the elevated air-liquid film interface with a higher temperature1b and a lower refractive index. The dimensionless wavelength of the convective motion, λ, defined as λ ) r/nd, where r is the radius of the polished aluminum plate (see Figure 1), n is the number of concentric circular rolls, and d is the thickness of the film, was 2.28,8 while the distance between the rolls is 7.0 mm. No noticeable hysteresis was observed in the onset of convection in the heating and cooling cycles, which indicated that no change in the properties of MAOP-DMS had occurred upon heating the liquid film. Mixing of MAOP-DMS with the photoinitiator hydroxycyclohexyl phenyl ketone to cHPK ) 1.0 wt % led to a slight change in the appearance of the convection patterns, as is shown in Figure 2b. The distance between the rolls decreased to 6.8 mm. However, in the presence of HPK the transition to instability occurred at ∆T ) 5.78 °C, which was higher than ∆T ) 5.14 °C in a pure MAOP-DMS. The difference was caused by an increase in viscosity of about 5% upon heating MAOP-DMS mixed with HPK, due to the partial polymerization of the monomer.5 Polymerization of the convection patterns was studied as a function of the initiator concentration and the time of irradiation, while keeping the intensity of UV-irradiation constant.9 It was anticipated intuitively that the polymerization rate should be sufficiently high to preserve the symmetric patterns from potential dissipation caused by viscous drag but slow enough to maintain temperature gradients in the system. The effect of the change in concentration of HPK in the reaction mixture is demonstrated in Table 1. For the low concentration of the initiator, that is, when cHPK e 0.25 wt %, polymerization of MAOP-DMS was carried out too slowly, and the convection patterns gradually disappeared when the film was illuminated with UV-light for 3 min. (8) Koschmieder, E. L. J. Fluid Mech. 1969, 35, 527-530. (9) In principle, the intensity of irradiation is another important parameter that could affect the polymerization rate and the extent of the reaction.
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Table 1. Effect of Reaction Conditions on Polymerization of Convection Patterns in the MAOP-DMS/HPK Mixtures conc of H
time of irradiation (min)
morphology of the polymerized film
0.25 0.25 1.0 1.5 2.0 5.0 1.0 1.0 1.0 1.0
3 6 3 3 3 3 1 3 6 11
no patterna no patternb perfect rolls distorted rolls distorted rolls distorted rolls dissipated patternb perfect rolls perfect rolls perfect rolls
a
Viscous fluid. b Tacky sample.
The surface of the polymerized sample examined with the shadowgraph technique looked similar to that of the liquid film below the instability threshold (Figure 2c). In addition, the surface of the solidified sample was tacky, which indicated that the extent of reaction was low. An increase in the irradiation time to 6 min eliminated the tackiness of the polymerized film, but the sample structure remained featureless. When the concentration of the initiator in MAOP-DMS was increased to 1.0 wt %, symmetric convection patterns were essentially preserved in the solid state. A typical convection pattern achieved with this concentration of HPK and trapped in the solid state by illuminating the film for 3 min is shown in Figure 2d. When the concentration of the initiator exceeded 1.5 wt %, photopolymerization occurred too rapidly; the exothermic reaction changed the temperature gradients in the fluid layer, and the convection patterns appeared distorted, as is shown in Figure 2e. The duration of the irradiation had a less significant effect on the vitrification of the convection patterns if the time of illumination of the layer exceeded the critical length of time required to solidify the film. Table 1 indicates that, with an HPK concentration of 1.0 wt %, irradiation of the sample for 1 min led to the formation of a tacky sample with an essentially dissipated pattern. When the time of UV-irradiation exceeded 3 min, perfect rolls were preserved in the solid state, and their shape did not change when the time of the sample’s exposure to UV-irradiation was increased up to 11 min. The solid film with polymerized roll-like patterns shown in Figure 2d had the same wavelengthh of 2.28, and the distance between the rolls was 6.5 mm, that is, slightly reduced in comparison with that of the layer shown in Figure 2b because of the shrinkage of the material upon polymerization. However, a very close similarity of the patterns in Figure 2b and d indicated that the lateral periodicity of the convection planform was preserved in the solid state. In addition to lateral ordering, periodic vertical deflection could be easily resolved at the air-film interface: wide rolls were depressed, whereas narrow rolls protruded into the air. The results of studies of the surface topography of the polymerized MAOP-DMS film are shown in Figure. 3. Since the maximum lateral area that can be imaged by the optical profilometer is 4.7 mm × 2.6 mm, each roll was imaged independently and the surface profiles were superimposed. In Figure 3a, a 3D profilometer image demonstrates the deflection of the free surface of the film over the first and the second rolls from the center of the pattern (see inset to Figure 3a). The surface profile corresponding to the rolls is shown in Figure 3b. The maximum well-to-hump heights for the first and the
Figure 3. Surface topography of the polymeric film shown in Figure 2d. (a) 3D image of the topography of the free surface of the polymerized MAOP-DMS film over the first roll and the second rolls (the corresponding points are shown in the inset). (b) Surface profile corresponding to the first and the second rolls of the polymerized film.
second rolls, as well as for the third roll not shown in the figure, were very close. The average height between the humps and the wells at the interface is 19.4 µm. This parameter compares reasonably well with the surface deflection measured in liquid films of silicon oil with a comparable thickness, in which buoyancy forces dominated convection.10 It should be noted, however, that polymerization of MAOP-DMS does not occur instantaneously; thus, heat fusion occurs in the film. Another noticeable feature of the polymerized film is that the slope over the second roll appears as more shallow than the slope over the first roll, which indicates that the wells in the polymerized film are not symmetric. This observation has not been reported for convection patterns in silicon fluids, and it needs further investigation. Finally, an interesting phenomenon was observed at the surface of the chromium-coated aluminum plate, that was maintained in contact with the convecting fluid. When (10) Kayser, W. V.; Berg, J. C. J. Fluid Mech. 1973, 57, 739-752. (11) Koschmieder, E. L.; Biggersraff, M. I. J. Fluid Mech. 1986, 167, 49-64. (12) Schatz, M. F.; VanHook, S. J.; McCormick, W. D.; Swift, J. B.; Swinney, H. L. Phys. Rev. Lett. 1995, 75, 1938-1941.
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associated with the stronger adhesion of the downflowing fluid to the metal surface than that of the uprising fluid. 4. Conclusions
Figure 4. Photograph of the surface of the chromium-coated aluminum plate after the polymerized MAOP-DMS film shown in Figure 2d was removed.
the solidified films were removed from the polished chromium-coated aluminum plate, the two-dimensional convection planform induced in the liquid state was replicated at the metal surface. As an example, Figure 4 shows a symmetric roll-like pattern “drawn” on the metal substrate by the convecting fluid. Microscopic imaging of the metal surface and Raman spectroscopy showed that the surface was coated with a strongly adhering thin layer of polymerized MAOP-DMS. The surface coverage was not uniform: in wide rolls a dense polymer layer was formed, whereas in narrow rings the surface coverage was loose. The periodicity of the pattern on the metal plate can be explained as follows. The narrow rings correspond to the fluid stream with a somewhat lower density, which moves upward, whereas the wide rings are formed by the denser liquid moving downward. Since the surface of the aluminum plate has a uniform temperature and the flow of liquid is continuous, polymerization of MAOP-DMS in wide rolls at the surface of the plate is presumably
We have developed a new procedure for producing polymeric films with periodic structures on millimeter scales by generating convection patterns in thin films of polymerizable fluids and then trapping them in a solid state by UV-polymerization. The periodicity of the solid films can be reduced by about 30% by reducing the thickness of the liquid film. In the current work, roll-like convection patterns were induced by buoyancy-driven convection. Alternatively, other types of nonequilibrium structures can be trapped in a solid state. For example, hexagonal patterns with a lateral periodicity as low as fractions of a millimeter can be generated in polymeric films by inducing surface-tension-driven convection in thin fluid layers of monomers or low-viscous oligomers, whereas parallel stripes can be produced by buoyancy-driven convection in polymerizable fluids placed in cells with a square shape. The control parameters for the polymerization of convecting fluids are the concentration of the initiator and the duration of the illumination of the sample. Acknowledgment. This work is supported by the National Science and Engineering Research Centre of Canada (NSERC) and DuPont Canada Corp. E.K. thanks the Ontario Government for a PREA award. We thank Prof. S. Morris and W. Tokaruk for their assistance in assembling the experimental setup for convection studies and Dr. R. Tam for his help in measurements of the surface profiles of polymerized films. LA0005715
