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2D Spatially Periodic Architectures via the Drying of 1D Holographically Photopatterned Polymer Solutions Pavel A. Kossyrev,† Michael R. Bockstaller,‡ and Edwin L. Thomas* Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, 77 Massachusetts Avenue, NE47-4th floor, Cambridge, Massachusetts 02139 Received July 27, 2004. In Final Form: December 7, 2004 We demonstrate an efficient and versatile method for selectively generating 1D and 2D periodic polymer structures in the submicron to tens of micron range by the directed drying of 1D photopatterned polymer solutions. Ultraviolet (UV) holographic lithography (Xia et al. Chem. Rev. 1999, 99, 1823-1848) is initially used to create 1D periodic cross-link and density variations in the polymer/volatile solvent solutions. These variations act as anisotropic barriers (walls) that direct the subsequent solvent evaporation process. Somewhat akin to directional drying (Allain and Limat Phys. Rev. Lett. 1995, 74, 2981-2984) and directional solidification (Pelc, P. Dynamics of Curved Fronts; Academic Press: San Diego, CA, 1988) experiments, the drying exhibits channel-like interface propagation behavior. The combination of the instabilities and minimization of the interface area during drying can be effectively used to produce larger scale 2D pseudohexagonal polymer-strut structures, or by the addition of a monomeric component to the polymer solution, the instabilities can be suppressed, resulting in the formation of 1D linear gratings.
Among various techniques for producing spatially periodic structures in 1D, 2D, or 3D, holographic lithography has recently received much attention due to its technological simplicity. Visible and ultraviolet (UV) holographic lithography, where multiple-laser-beam interference is used to create a spatially periodic distribution of light, require the use of photocurable materials such as photoresists or mixtures of photopolymerizable precursor and liquid crystals. Due to the dependence of material photoreactivities on the light distribution, the desired periodic structures can be created by controlling the intensities and polarizations of interfering beams, the angles between them, and the laser wavelength. In case of photoresists, exposed samples have to be additionally processed by solvents to develop the structure, in contrast to mixtures of precursor/liquid crystals, where the structure is created right after exposure due to photoinduced phase separation.4,5 Using holographic lithography, linear gratings and 2D structures1,6 as well as 3D structures7-9 were produced by single or multiple exposures of two-, three-, and four-beam interferences. Recently discovered bicontinuous 3D photonic crystal structures7,10,11 can also be fabricated by holographic lithography and are currently being developed. † Present address: Brown University, Division of Engineering, 182 Hope St., Box D, Providence, RI 02912. ‡ Present address: Institute for Technical and Macromolecular Chemistry, RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany.
(1) Xia, Y.; Rogers, J.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823-1848. (2) Allain, C.; Limat, L. Phys. Rev. Lett. 1995, 74, 2981-2984. (3) Pelc, P. Dynamics of Curved Fronts; Academic Press: San Diego, CA, 1988. (4) Doane, J. W.; Vaz, N. A.; Wu, B. G.; Zumer, S. Appl. Phys. Lett. 1986, 48, 269-271. (5) Penterman, R.; Klink, S. I.; de Koning, H.; Nisato, G.; Broer, D. J. Nature 2002, 417, 55-58. (6) Krauss, T. F.; De La Rue, R. M. Prog. Quantum Electron. 1999, 23, 51-96. (7) Campbell, M.; Sharp, D. N.; Harrison, M. T.; Denning, R. G.; Turberfield, A. J. Nature 2000, 404, 53-56. (8) Sutherland, R. L.; et al. Opt. Express 2002, 10, 1074-1082. (9) Escuti, M. J.; Qi, J.; Crawford, G. P. Opt. Lett. 2003, 28, 522-524. (10) Wohlgemuth, M.; Yufa, N.; Hoffman, J.; Thomas, E. L. Macromolecules 2001, 34, 6083-6089.
The periodicity and dimensionality of the structures thus far created by holographic lithography are fixed after exposure and are limited by the response of materials to undergo photopolymerization at specific wavelengths. In this letter, we show that, upon the drying of 1D holographically irradiated polymer/volatile solvent solutions, the emerging periodicity and dimensionality of the solventfree polymer structures are governed by interfacial phenomena and can be controlled to result in new 1D or 2D periodic structures. In particular, we demonstrate the construction of 2D pseudohexagonal microstructures with an order of magnitude larger scale periodicity due to the propagation and coalescence of channel-like interfaces during the drying of 1D photopatterned polymer solutions. For our studies, we chose a number of materials (i.e., diene homopolymers, diene containing block copolymers, and diacrylate monomers) capable of being UV crosslinked. The dilute solutions of polymers, monomers, or their blends in toluene were placed between two glass substrates separated at d ) 5, 10, and 15 µm and exposed to the interference pattern of two collinearly polarized (along the y-axis) UV laser beams at ambient temperature (Figure 1a). The exposed area was circular. The periodicity of the resulting sinusoidal interference pattern was chosen to be within the range 1-5 µm. The orientation of the samples was such that the wave vector of the interference pattern, K (|K| ) 2π/Λ), lies in the plane of the film (along the x-axis). The cross-links between photocurable molecules form preferentially in the maximum intensity regions of the sinusoidal UV pattern. Also, due to photoinduced phase separation,4 diffusion of photocurable species into the maximum intensity regions and solvent into the minimum intensity regions occurs that produces a polymer density increase in the maximum intensity regions (analogous to other two-component systems, e.g., liquid crystal/polymer dispersions5,12). Therefore, the polymer cross-link and density variations in the solutions appear with the periodicity of the UV pattern, Λ. In the following, we assume that the maximum polymer cross(11) Ullal, C. K.; Maldovan, M.; Wohlgemuth, M.; Thomas, E. L. J. Opt. Soc. Am. A 2003, 20, 948-954. (12) Bowley, C. C.; Crawford, G. P. Appl. Phys. Lett. 2000, 76, 22352237.
10.1021/la0481086 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/29/2004
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Figure 1. Experimental setup. (a) Geometry of the UV holographic lithography setup used to produce periodic polymer walls in solution. (b) 633 nm He-Ne laser diffraction from periodically spaced walls in solution (solvent is present). The zeroth order peak is on a black background.
link-density regions can be thought about as periodically spaced walls that are oriented perpendicular to the glass substrates. The sinusoidal refractive index modulation in the solutions after irradiation due to the presence of periodically spaced walls is evident by the single pair of diffraction peaks from a He-Ne laser (Figure 1b). Exposed samples were left under ambient conditions for the solvent to evaporate through the edges of the glass cells. Upon solvent evaporation, the evolution of three distinct types of polymer microstructures was observed depending on the material composition, the nature of the cross-linked polymer regions, and the degree of attachment of the cross-linked polymer walls to the glass substrates. The first type of behavior we observed is exemplified by the drying of the photopatterned diacrylate monomer compound (RM-257). We found that the drying occurs faster in the direction parallel to the walls, characterized by an advancing air/liquid interface, which is occasionally pinned (Figure 2a). By monitoring the evolution of the initially circular exposed region, we evaluated that the speed of drying in the direction parallel to the walls (0.5 µm/s) is twice as fast as that in the perpendicular direction. The final period of solvent-free structures (Figure 2b) is identical to the interference period, as the walls shrink and collapse into spherical structures during drying. This indicates that the cross-linking of the monomeric solution is insufficient to form well-defined wall regions. Blending the RM-257 diacrylate monomer compound with homopolymers or block copolymers (1/5 to 1/6 ratios) resulted in different evaporation behavior (Figure 2c). In this case, the walls are better formed due to the presence of preformed polymers in the solution. The irradiated solution exhibits anisotropic transport behavior, leading to a set of air/solvent channels propagating parallel to the walls. In prior work, an emergence of periodic channels was observed in directional drying2 and solidification3 experiments, and its origin was discussed.2,3,13 Here, the directional evaporation behavior and the resultant regularly spaced cellular air/liquid interfaces are due to the presence and the motion of the walls, which are not firmly attached to the substrates. Due to axial and radial growth of the air channels, portions of the walls relocate to the regions between air channels, resulting in regularly spaced polymer density buildup in these regions. The solventfree structures produced in this way (Figure 2d) consist of the periodic remnants of attached walls (original period, Λ) forming a surface relief grating on the glass substrates (13) Jagla, E. A. Phys. Rev. E 2002, 65, art. no. 046147.
Figure 2. Drying behavior and solvent-free structures produced in monomer or monomer/polymer solutions. (a) Optical microscope image in transmission of the drying of a 3 wt % RM-257/toluene solution photo-cross-linked with a period of Λ ) 5 µm. The solvent-free polymer structure is at the top. The scale bar is 25 µm. (b) SEM image of the solvent-free RM-257 structure. The scale bar is 5 µm. (c) Optical microscope image in transmission of a partially dried 0.6 wt % RM-257/3.4 wt % PB(P2MP)3/toluene blend solution photo-cross-linked with a period of Λ ) 2 µm. The scale bar is 20 µm. (d) SEM image of the solvent-free RM-257/PB(P2MP)3 blend structure. The scale bar is 20 µm. (e) 633 nm He-Ne laser diffraction from 1D solvent-free blend structures displaying two periods. Peaks corresponding to the original period in the solution, Λ, are separated at 2 K. The zeroth order peak is on a black background. Low order peaks are due to the larger (∼20 µm) period created by wall motion during drying.
and a new, larger 20 µm period structure formed from the portions of walls relocated by propagation of the air/liquid interface. Our studies show that varying the photopatterned wall period, Λ, from 1 to 5 µm in a 5 µm thick cell does not significantly alter the period of larger structures. The presence of two periods along the common x-axis is confirmed by He-Ne laser diffraction (Figure 2e). In general, an application of two two-beam-interference exposures is required to produce two periods along a common axis in a material by conventional holographic lithography. Additionally, it is difficult to directly produce structures with large periodicity (>50 µm) via photopolymerization. Finally, detailed studies of the drying of photopatterned pure homopolymer or pure block copolymer solutions revealed a third type of drying behavior. We found that this type of drying behavior corresponds to the situation when the cross-link-density walls are not attached to the glass substrates and can be easily relocated into regions between neighboring air channels. An example from the set of air/liquid interfaces developed during the drying of irradiated homopolymer or block copolymer solutions is shown in Figure 3a. In this case, the air channel interface
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Figure 3. Pseudohexagonal solvent-free structures produced via the drying of irradiated polymer solutions where the walls do not firmly attach to the glass substrates. (a) Optical microscope image in transmission of the drying of a 3 wt % 1,2-PB/toluene solution. The propagating channel is indicated by the abbreviation “Ch”. The scale bar is 50 µm. (b and c) Optical microscope images in transmission of solvent-free polymer structures. The scale bars are 50 µm. (d) SEM image of polymer structures. The scale bar is 10 µm. (e) 633 nm He-Ne laser diffraction from 2D solvent-free structures. The scale bar is K ) 1.57 µm-1. The zeroth order peak is on a black background. (f) 1640 nm IR laser diffraction from 2D solventfree structures corresponding to the enlarged origin of part e. The scale bar is K ) 0.26 µm-1.
(see tip of channel labeled Ch) results in transversely positioned lateral wall branches. The interacting stress fields due to the presence of the set of cross-link-density walls and neighboring air channels determine the detailed motion of a particular air channel interface. In cross-linked homopolymer or block copolymer systems, the observed shape of the air channel interface (Figure 3a) may be a consequence of the relief of stress that existed in the material situated in the regions between neighboring air channels. By clumping the material in the direction normal to the air channel propagation, the system reduces stress in that direction. This is a type of buckling instability sometimes referred to as the telephone cord instability, which has also been reported for other materials.14,15 Formation of the branches in this case is a generic stress relief mode of the film of material, which does not significantly depend on the stress anisotropy, viscoelastic effects, and film thickness gradients.16 Indeed, (14) Gioia, G.; Ortiz, M. Adv. Appl. Mech. 1997, 33, 119. (15) Rogojevic, S.; et al. J. Vac. Sci. Technol., B 2001, 19 (2), 354360.
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we found that the structural periodicity after drying does not depend on the thickness between glass substrates (5, 10, and 15 µm thicknesses were used), thus confirming that periodicity is determined by the intrinsic strains in the material.16 The combination of the instability phenomenon and the drying induced motion of the walls resulted in the formation of large scale 2D pseudohexagonal polymer structures between the two glass substrates (Figure 3b,c). The structures can be envisioned as zigzag lines periodically spaced along the x-axis (period 50 µm) with attached lateral branches periodically spaced along the y-axis (period 20 µm). Our studies indicate that varying the photopatterned wall period, Λ, from 1 to 5 µm in a 5 µm thick cell does not produce significant variations in the final solvent-free structures. The branches and the zigzag segments of pseudohexagonal polymer structures form a 120° angle at their joints (Figure 3d). In some cases, two 120° zigzag-branch joints may be closely situated with respect to each other (Figure 3b,c). The variable lengths of the branches and zigzag segments of solvent-free structures are attributed to the alterations in air channel shapes during drying. The 2D quasi-periodicity of solventfree structures is confirmed by observing diffuse diffraction orders of a He-Ne laser (Figure 3e) and an infrared (IR) laser (Figure 3f). The diffraction pattern of Figure 3e can be explained assuming each zigzag-branch chain has a regular periodic structure along the x- and y-axes but the mutual levels of the chains in the y-direction are irregular. The diffraction orders broaden with an increase of this type of irregularity and ultimately become continuous layer lines. Analogous patterns involving the axial shift disorder of individual chains have been observed in a number of crystalline polymers by X-ray diffraction measurements.17 In summary, the results presented in this letter demonstrate that 1D photopatterning of solutions of monomeric and/or polymeric materials provides a tool to efficiently guide the subsequent solvent evaporation process and the associated deposition of solutes. For wellformed walls with weak attachment to the substrates, instabilities can occur along the air channels; the subsequent wall motion and area minimization allow selective fabrication of large period 1D linear or 2D pseudohexagonal structures. By using microphase separating solutes such as block copolymers as the cross-linkable medium,18 it may be possible to obtain structures that exhibit periodicities on a hierarchy of length scales from the microscopic to the nanoscopic regimes. Since the processes of photopatterning and solvent evaporation are important in a vast number of applications, we hope that our results will inspire new processing approaches in the fields of optoelectronics, photonics, and nanotechnology. Experimental Section We used dilute (3 wt %) solutions of the following homopolymers and block copolymers in toluene: poly(butadiene-1,2) [1,2PB, 48.5 kg/mol], poly(butadiene-1,4) [1,4-PB, 58 kg/mol], linear polystyrene-block-poly(butadiene-1,2) [PSsPB, 27/34 kg/mol], star polystyrenespolybutadienespolyisoprene [SBI, 56/31/28 kg/mol], and star polybutadienespoly(2-methylpyridine)3 [PB(P2MP)3, 120/(23.1)3 kg/mol], where the abbreviation and (16) Crosby, K. M.; Bradley, R. M. Phys. Rev. E 1999, 59, R2542R2545. (17) Tadokoro, H. Structure of Crystalline Polymers; John Wiley & Sons Inc.: New York, 1979; pp 150-153. (18) Hahn, H.; Eitouni, H. B.; Balsara, N. P.; Pople, J. A. Phys. Rev. Lett. 2003, 90, art. no. 155505.
Letters molecular weight of the homopolymers and respective blocks of copolymers are indicated in square brackets. Small amounts of UV photoinitiator (2 wt % relative to the polymer content) Irgacure 819 from Ciba Chemicals and cross-linking agent (1 wt % relative to the polymer content) trimethylolpropane tris(3mercaptopropionate) from Sigma-Aldrich were added. As diacrylate monomer compound, we used 1,4-phenylene bis{4-(3acryloyloxyhexyloxy) benzoate} distributed by EM Industries under the name RM-257. Irgacure 819 (2 wt % relative to the RM-257 content) was added. The chemical composition of RM-257 is shown in Figure 2a, where R represents (CH)3CO2CHdCH2. Cells were made from square 1 × 1 in. glass slides so that all four sides were open for solvent evaporation. To photopattern, the energy flux in each UV (λ ) 351 nm) curing beam was 600 mJ/cm2 and the exposed area of 1.1 cm2 was circular. Samples for scanning electron microscopy (SEM) were prepared by fracture-opening cells cooled to 77 K.
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Acknowledgment. We are grateful to Gregory Crawford and the Division of Engineering, Brown University, for the permission to use the UV and IR laser optical setups. We thank Apostolos Avgeropoulos, University of Ioannina, Greece, for synthesis of block copolymers. We also thank an anonymous referee for pointing out the possible connection with the telephone cord instability. This research was supported by the U.S. Army through the Institute for Soldier Nanotechnologies, under contract DAAD-19-02-D-0002 with the U.S. Army Research Office. The content does not necessarily reflect the position of the Government, and no official endorsement should be inferred. LA0481086
