3338
Langmuir 1997, 13, 3338-3344
Crystal Structures of Monodisperse Colloidal Silica in Poly(methyl acrylate) Films Jagdish M. Jethmalani and Warren T. Ford* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
Greg Beaucage Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221 Received September 13, 1996. In Final Form: March 25, 1997X Dispersions of 22.3 vol % of 153 nm diameter silica particles coated with 3-(trimethoxysilyl)propyl methacrylate in liquid methyl acrylate (MA) as 264-528 µm thick films in glass cells spontaneously order into colloidal crystals and Bragg diffract visible light from lattice planes parallel to the film plane. Photopolymerization of the MA gives elastomeric poly(methyl acrylate) (PMA) films with retention of the crystalline order. Swelling the films with more MA and further polymerization give colloidal crystalline composite films with larger lattice spacings. The crystal structures are modeled as face centered cubic for the monomer dispersions and rhombohedral for the polymer films. Layer spacings observed in scanning electron micrographs (SEM) of microtomed PMA films are used to calculate the (111) spacings and the lattice parameter a of the model. The layer spacings correlate well with the diffraction wavelengths from optical spectra. Ultrasmall angle X-ray diffraction also was performed to demonstrate the three-dimensional crystalline order. The SEM and optical spectra also fit hexagonal close packed (hcp) or mixed rhombohedral and hcp structures.
Introduction Monodisperse colloidal particles in liquids spontaneously form three-dimensional lattices known as colloidal crystals.1-3 The lattice dimensions usually exceed 100 nm, causing Bragg diffraction of visible light, which is given by
λ ) 2dn sin θ
(1)
where λ is the diffracted wavelength, d is the interplanar spacing, n is the refractive index of the dispersion, and θ is the Bragg angle between the incident light beam and the d planes. The crystal structures, morphologies, and orientations of colloidal particles in aqueous and nonaqueous dispersions confined between two plates have been studied under the stresses of shear, electrical fields, and elevated temperatures using light scattering,4-9 metallurgical microscopy,10-12 optical microscopy,13,14 digital-imaging,15 X
Abstract published in Advance ACS Abstracts, June 1, 1997.
(1) Pieranski, P. Contemp. Phys. 1983, 24, 25. (2) Okubo, T. Prog. Polym. Sci. 1993, 18, 481. (3) Dosho, S.; Ise, N.; Ito, K.; Iwai, S.; Kitano, H.; Matsuoka, H.; Nakamura, H.; Okumura, H.; Ono, T.; Sogami, I. S.; Ueno, Y.; Yoshida, H.; Yoshiyama, T. Langmuir 1993, 9, 394. (4) Williams, R; Crandall, R. S. Phys. Lett. 1974, 48A, 225. (5) Clark, N. A.; Hurd, A. J.; Ackerson, B. J. Nature 1979, 281, 57. (6) Carlson, R. J.; Asher, S. A. Appl. Spectrosc. 1984, 38, 297. (7) Rundquist, P. A.; Photinos, P.; Jagannathan, S.; Asher, S. A. J. Chem. Phys. 1989, 91, 4932. (8) Monovoukas, Y.; Gast, A. P. J. Colloid Interface Sci. 1989, 128, 533. (9) Kesavamoorthy, R.; Tandon, S.; Xu, S.; Jagannathan, S.; Asher, S. A. J. Colloid Interface Sci. 1992, 153, 188. (10) Kose, A.; Ozaki, M.; Takano, K.; Kobayashi, Y.; Hachisu, S. J. Colloid Interface Sci. 1973, 44, 330. (11) Ise, N.; Okubo, T.; Sugimura, M.; Ito, K.; Nolte, H. J. J. Chem. Phys. 1983, 78, 536. (12) Okubo, T. J. Chem. Phys. 1987, 86, 2394. (13) Monovoukas, Y.; Gast, A. P. Phase Transitions 1990, 21, 183. (14) Monovoukas, Y.; Gast, A. P. Langmuir 1991, 7, 460. (15) Van Winkle, D. H.; Murray, C. A. Phys. Rev. A 1986, 34, 562.
S0743-7463(96)00890-6 CCC: $14.00
freeze fracture microscopy,16 scanning and transmission electron microscopy,17-19 and ultrasmall angle X-ray diffraction (USAXRD).3,20,21 During crystallization of polystyrene latexes in water, confocal laser scanning microscopy experiments have shown that the particle order begins as hexagonal layers at the bottom surface and extends upward in stacks.3 Analyses of Kossel rings from microscopy and Bragg spots from laser diffraction indicate that the (111) planes of face-centered cubic (fcc) crystals, (100) planes of hexagonal close packed (hcp) crystals, or (110) planes of the body-centered cubic (bcc) crystals are parallel to the surface.5,6,13,14 Hexagonal arrays of colloidal particles in optical and metallurgical microscopic images suggest that (111) planes of fcc and (110) planes of bcc lattices lie parallel to the plane of the glass observation cells. Coexistence of fcc and bcc, or fcc and hcp, lattices has been observed at certain particle concentrations in dispersions of both polystyrene latexes in water and colloidal silica in ethanol-toluene mixtures.22 In general, particles with low surface charge require high concentrations to form lattices, which are fcc, whereas particles with high surface charge form bcc lattices at low concentration and fcc lattices at higher concentration.7,22 Although most investigations of colloidal crystals have been carried out with monodisperse polymer latexes in water, monodisperse silica spheres also form colloidal crystals. Modification of the surface with nonpolar organic coupling agents such as TPM ((3-trimethoxysilyl)propyl methacrylate) enables the colloidal silica to disperse in low dielectric organic solvents, such as ethanol-toluene (16) Cohen, J. A.; Scales, D. J.; Ou-Yang, H. D.; Chaikin, P. M. J. Colloid Interface Sci. 1993, 156, 137. (17) Goodwin, J. W.; Ottewill, R. H.; Parentich, A. J. Phys. Chem. 1980, 84, 1580. (18) Kamenetzky, E. A.; Magliocco, L. G.; Panzer, H. P. Science 1994, 264, 207. (19) Ishizu, K.; Sugita, M.; Kotsubo, H.; Saito, R. J. Colloid Interface Sci. 1995, 169, 456. (20) Matsuoka, H.; Ise, N. Chemtracts Macromol. Chem. 1993, 4, 59. (21) Konishi, T.; Ise, N. J. Am. Chem. Soc. 1995, 117, 8422. (22) Dhont, J. K. G.; Smits, C.; Lekkerkerker, H. N. W. J. Colloid Interface Sci. 1992, 152, 386.
© 1997 American Chemical Society
Crystal Structures of Monodisperse Colloidal Silica
mixtures, and still form colloidal crystals by coulombic repulsion of the slightly charged particles.22-27 Bragg diffraction makes colloidal crystals potentially useful as optical rejection filters, limiters, and switches.28-34 Since the crystal structures in liquid dispersions are easily disrupted mechanically and thermally, two groups have devised methods to make more robust colloidal crystals. Asher and co-workers prepared colloidal crystals of polystyrene latexes in highly cross-linked, aqueous polyacrylamide gels.35-37 We have formed colloidal crystals of TPM-silica in methyl acrylate (MA) and in methyl methacrylate (MMA) dipsersions in glass cells and photopolymerized the monomers to give 100-400 µm thick polymer composite films with retention of the crystalline order.24-26 The PMMA films can be heated to 150 °C and returned to room temperature without loss of colloidal crystalline order, and the elastomeric poly(methyl acrylate) (PMA) films can be stretched 30% uniaxially and still return to the original colloidal crystalline order.26 In this paper we propose crystal structures for the TPMsilica-PMA composites based on macroscopic changes of film dimensions during polymerization, scanning electron micrographs (SEM) of microtomed sections of the films, and optical Bragg diffraction wavelengths. Experimental Methods Composite Films. The synthesis of colloidal silica from TEOS, grafting of TPM on particle surfaces, and transfer by dialysis to MA were carried out as described before.23,24,26,38-40 The mean particle diameter was 171 nm by dynamic light scattering41-43 and 153 nm by TEM, with a polydispersity (standard deviation/mean) of 0.07. A dispersion of 40 wt % particles in MA containing 0.2 wt % (based on MA) of the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA) was filled in a 528 µm thick glass cell.26 The cell was fabricated from two 7.5 × 2.5 cm microscope slides that were pretreated with dichlorodimethylsilane to coat the surfaces with poly(dimethylsiloxane). Bragg diffraction was studied by UV-vis transmission spectrophotometry with incident light normal to the plane of the cell. After 6-8 h the MA dispersion (sample A, Table 1) showed a narrow diffraction peak, and the dispersion was irradiated for 3.5-4 h using a medium pressure 450 W Hg lamp. The TPM-silica-PMA composite film (sample B) was removed from the cell for swelling the composite and for SEM analysis. (23) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1988, 128, 121. (24) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1994, 6, 362. (25) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. ACS Symp. Ser. 1995, No. 585, 181. (26) Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1996, 8, 2138. (27) Okubo, T. Langmuir 1994, 10, 1695. (28) Asher, S. A.; Kesavamoorthy, R.; Jagannathan, S.; Rundquist, P. A. Proc. SPIE 1992, 1626, 238. (29) Asher, S. A. US Patent 4,627,689, 1986. (30) Asher, S. A.; Jagannathan S. US Patent 5,281,370, 1994. (31) Flaugh, P. L.; O'Donnell, S. E.; Asher, S. A. Appl. Spectrosc. 1984, 38 (6), 847. (32) Asher, S. A.; Flaugh, P. L.; Washinger, G. Spectroscopy 1986, 1, 26. (33) Okubo, T. Colloid Polym. Sci. 1993, 271, 873. (34) Okubo, T. J. Chem. Phys. 1988, 88, 6581. (35) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997. (36) Panzer, H. P.; Giovanni, L.; Cohen, M. L.; Yen, W. S. US Patent 5,338,492, 1994. (37) Haacke, G.; Panzer, H. P.; Magliocco, L. G.; Asher, S. A. US Patent 5,266,238, 1993. (38) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (39) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F. J. Non-Cryst. Solids 1988, 104, 95. (40) Badley, R. D.; Ford, W. T.; McEnroe, F. J.; Assink, R. A. Langmuir 1990, 6, 792. (41) Pusey, P. N.; van Megen, W. J. Chem. Phys. 1984, 80, 3513. (42) Ackerson, B. J.; Clark, N. A. J. Phys. (Paris) 1981, 42, 929. (43) Pecora, R., Ed. Dynamic Light Scattering-Applications of Photon Correlation Spectroscopy; Plenum Press: New York, 1985.
Langmuir, Vol. 13, No. 13, 1997 3339
Figure 1. Transmission spectra of 40 wt % 153 nm TPMsilica particles (a) in a 528 µm thick dispersion in MA before polymerization and (b) in PMA after polymerization. This sample was used for swelling and further polymerization. Normally the spectra of such dispersions have only single narrow peaks as shown in the inset for a 264 µm thick sample. Swelling of PMA Composite Films. A piece was cut from the 40 wt % TPM-silica-PMA composite film, placed between two glass slides, and swollen with MA for 24 h to give sample C. The MA was photopolymerized as described before to give sample D.26 Computer Modeling. ChemDraw v. 2.0 on a Macintosh computer was used to model the fcc and rhombohedral (trigonal) structures. The data were written in the form of a text document and then transformed into a fcc unit cell. The fcc unit cell was compressed normal to the (111) planes by 15% to give a rhombohedral lattice. Scanning Electron Microscopy. Composite films were microtomed at room temperature using a Sorvall RMC-MT 6000 instrument, and the surfaces were coated with Au-Pd under vacuum in a Hummer II sputter coater. Particle order was observed with a JEOL JSM-35U scanning electron microscope (SEM) at 20 kV and 20 000 magnification. There was no observable damage of the films by the beam, and photographs of the same sample area taken minutes apart were identical. Ultrasmall Angle X-ray Diffraction (USAXRD). Composite films of TPM-silica-PMA 264 or 396 µm thick were prepared as described above. The 40 wt % TPM-silica film B1 diffracted at λmin ) 514 nm, the MA-swollen, repolymerized D1 diffracted at 580 nm, and the similar composite D2 diffracted at 620 nm. The X-ray diffraction patterns were obtained using Cu KR radiation and a Bonse-Hart camera at the Sandia National Laboratories/University of New Mexico Scattering Center.
Results and Discussion Silica-PMA Composite. A 40 wt % dispersion of 153 nm diameter TPM-silica particles in MA (sample A) became iridescent 1-2 min after filling the glass cells due to diffraction of visible light by the colloidal crystals. Figure 1 shows the transmission spectrum, and Table 1 reports the wavelength of minimum transmittance (λmin) and bandwidth. By the polarized microscopic observation, the sample contained 50-200 µm crystals over its entire area. Silica spheres like ours at 0.344-0.380 g mL-1 in low dielectric ethanol/toluene mixtures form fcc or mixtures of fcc and hcp colloidal crystals, and the fcc (111) or hcp (100) planes Bragg diffract visible light.22 If the hexagonal layers order ABC, the colloidal crystals are fcc with the most dense (111) planes parallel to the plane of the glass. If the layers order ABA, the crystals are hcp with (100) planes parallel to the plane of the glass. If fcc and hcp
3340 Langmuir, Vol. 13, No. 13, 1997
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Table 1. Transmission of a 153 nm TPM-Silica in MA Dispersion and a PMA Composite sample
λmin (nm)
obsd d111 (nm)a
calcd d111 (nm)b
bandwidth (nm)
A B
552 488
195 166
186 166
6 7c
a Calculated from the Bragg equation (1) with sin θ ) 1 assuming densities of 1.795 g cm-3 for TPM-silica, 0.956 g cm-3 for MA, and 1.22 g cm-3 for PMA. b Calculated from particle size and number assuming an fcc lattice for the MA dispersion, and a 15.1% decrease in the d111 spacing for the PMA composite. c The diffraction band had a broad shoulder (Figure 1b).
Scheme 1
Figure 2. Transmission spectra of a 40 wt % 153 nm TPMsilica-PMA composite (a) before and (b) after swelling with methyl acrylate and (c) after polymerization (samples B, C, and D).
lattices coexist in the dispersion, or if there are random hexagonal layers, the interplanar distances must be constant. The MA dispersion containing 0.2 wt % photoinitiator (based on monomer) was irradiated to copolymerize MA and the TPM groups on silica to yield the silica-PMA composite (sample B) as shown in Scheme 1.26 After photopolymerization, the transmission spectrum was blue shifted with a broad shoulder, and λmin varied from one position to another in the composite film as shown in Figure 1b and Table 1. The shoulder in the spectrum of the composite film may be due to regions of the sample that have different d spacings or to crystals with planes not parallel to the glass surface. The blue shift results from a decrease in the specific volume of the matrix during polymerization. The thickness of the film decreased 15%, but the length and width did not change, apparently because the PMA adhered to the epoxy resin that sealed the edges of the cells but not to the poly(dimethylsiloxane)coated glass surface. If the crystal structure of the monomer dispersion was fcc with (111) planes parallel to the film plane, and during polymerization the particles moved only in the direction normal to that plane, the crystal structure in the polymerized film is rhombohedral. If the monomer dispersion was hcp, the spacing between the (100) planes would decrease to the same extent as in an fcc lattice, and the lattice would remain hcp. The d111 spacings of an fcc lattice calculated from transmission spectra of the silica-PMA films at most regions are in good agreement with the d111 spacings calculated from particle size and number by assuming shrinkage only in the film thickness, as seen in Table 1. Uneven shrinkage in the film thickness was measured experimentally with a micrometer caliper. Varied thickness causes the λmin to vary from one region to another. The thinner regions have shorter λmin and smaller d111 spacings than the thicker regions. Swelling the Composite Film. The λmin of the TPMsilica-PMA composite shifts to the red during swelling with MA (containing 1 wt % photoinitiator) to give sample C. Irradiation for 3-4 h polymerized the imbibed MA to form a new PMA composite film (sample D). Figure 2 shows the transmission spectra. The diffraction properties
Table 2. Transmission of a PMA Film after Swelling with MA and Polymerization sample
dimensions (mm)
vol % silica
λmin (nm)
d111a (nm)
bandwidth nm)
B C D
9.0 × 8.75 × 0.470 11.0 × 10.75 × 0.622 10.6 × 10.25 × 0.600
25.2 12.7 16.7
512 678 656
174 236 222
8b 13 14
a Calculated from eq 1 with sin θ ) 1. b The diffraction band had a broad shoulder (Figure 1).
and dimensions of the sample before and after swelling with MA and photopolymerization are reported in Table 2. Figure 3 illustrates the dimensions at each stage of formation of the composite film. At one 470 µm thick location of the unswollen silica-PMA composite film B, λmin shifted from 512 nm to 676 nm after swelling with MA to give film C. Further polymerization of the swollen silica-PMA composite produced a 600 µm thick film D with λmin ) 656 nm. During the swelling and repolymerization process, the Bragg diffraction wavelength from the composite at various stages varied in direct proportion to the thickness of the film. During swelling with MA the in-plane dimensions of the sample were not constrained at all, and the only force normal to the plane was due to the weight of a microscope slide. The macroscopic dimensions after swelling B to C increased 1.32 times normal to the plane and 1.22 times in the plane, which on polymerization gave a final film D 1.27 times thicker and 1.17 times larger in other two dimensions compared to the original film B. Thus, if B had a rhombohedral crystal structure and/or an hcp structure, and underwent affine deformations, both C and D also had rhombohedral and/or hcp structures. Computer Modeling. Since an fcc lattice becomes rhombohedral if it shrinks only in the dimension normal to the (111) plane, and an hcp lattice is still hcp but with a different c/a ratio if it shrinks only in that one dimension, we have modeled the crystal structure in the polymerized film as rhombohedral. The lower two models in parts b and d of Figure 4 show the side view of parallel (111) planes of an fcc lattice in MA (sample A) and of a rhombohedral lattice in the PMA composite (sample B). The top two models in parts a and c of Figure 4 show the numbering of particles in the two crystal lattices. The
Crystal Structures of Monodisperse Colloidal Silica
Figure 3. Dimensional changes during photopolymerization, swelling with MA and photopolymerization of imbibed MA of a 40 wt % silica-PMA composite. Measured dimensions are in Table 2.
Figure 4. Computer-generated models of (a and b) a unit cell of an fcc lattice (sample A) and (c and d) a unit cell of a rhombohedral lattice (sample B).
Langmuir, Vol. 13, No. 13, 1997 3341
Figure 5. (a) SEM image of a 40 wt % 153 nm TPM-silicaPMA composite film (sample B) microtomed parallel to film plane. (b) Regular hexagonal model of a (111) plane. Film thickness ) 470 µm.
interparticle distances in both the fcc and rhombohedral unit cells are given in Table 3.44 The cartesian coordinates and the internal angles of an fcc unit cell are given in Tables 4 and 5, respectively.44 Since photopolymerization of MA caused a 15.1% decrease in the Bragg diffraction wavelength, the (111) interplanar spacings of the original fcc lattice were decreased by 15% and no changes were made in the plane to model the rhombohedral lattice. The positional Cartesian coordinates and the interparticular internal angles of a rhombohedral unit cell are given in Tables 6 and 7, respectively.44 The models of the fcc and rhombohedral lattices assume regular hexagonal particle order in the (111) planes and rectangles or parallelograms of particles in the (110) planes of the fcc or rhombohedral lattice. The interparticle distances a and d111 spacings of 10.00 and 5.77 units for the model unit cell of fcc transform to 9.53 and 4.91 units for the rhombohedral unit cell. SEM Analysis. Earlier reports24-26 showed SEM images of hexagonal particle orders only of the surface layers of silica-PMMA and silica-PMA composites and not the particle order in the bulk. Now we have microtomed the silica-PMA composites both parallel and perpendicular to the film surface to observe the bulk particle order. The SEM images of samples B and D microtomed parallel to the film plane show hexagonal order. The images and the model hexagonal arrays of particles which represent (111) planes of a rhombohedral lattice or (100) planes of an hcp lattice are shown in Figures 5 and 6. The interparticle distance Do (Figures 5b and (44) Supporting information.
3342 Langmuir, Vol. 13, No. 13, 1997
Figure 6. (a) SEM image of sample D, a 40 wt % 153 nm TPM-silica-PMA composite film swollen with MA and polymerized, microtomed parallel to film plane. (b) Regular hexagonal model of a (111) plane. Film thickness ) 590 µm.
6b) in the (111) plane of a rhombohedral lattice is the same as the lattice constant a of an hcp lattice.26 The SEM images of samples B and D microtomed perpendicular to the film plane are shown in Figures 7 and 8. The arrangements of particles in parallelograms represent either (110) planes of rhombohedral or (110) planes of hcp crystals. As shown in Figures 7b and 8b, the interparticle distances between the ordered diagonal arrays give the constant a of the rhombohedral lattice or the constant c of hcp, which is twice the d spacing, while the interparticle distances between the horizontal planes give the d spacings of a rhombohedral lattice. Table 8 shows that the lattice constant a and the d spacings calculated from the SEM micrographs assuming a rhombohedral crystal structure, and also the lattice constants a and c and the d spacing of (100) planes of an hcp lattice, are in close agreement with calculated a , Do, and d from optical diffraction of composite films B and D. This supports the model of a rhombohedral crystal structure in the original composite film that remains rhombohedral on swelling with MA and on polymerization of imbibed MA, or an hcp structure that remains hcp throughout swelling and repolymerization. USAXRD Analysis. Spectrophotometry with light normal to the film plane detects only crystal planes parallel to the film plane. SEM provides a two-dimensional image only of the specific plane exposed with a microtome. To examine the three-dimensional crystal structures, we have analyzed the silica-PMA composites by USAXRD (ultrasmall angle X-ray diffraction). log-log plots of data for
Jethmalani et al.
Figure 7. (a) SEM image of a 40 wt % 153 nm TPM-silicaPMA composite film (sample B), microtomed perpendicular to the film plane. (b) Rectangular particle order of a (110) plane. Film thickness ) 470 µm.
sample B1 containing 40 wt % PTM-silica in PMA and samples D1 and D2 obtained by swelling B1 with MA and repolymerization were desmeared and corrected for background (Figure 9). A calculation for a Gaussian distribution of spheres gave a mean radius of 795 Å and a standard deviation of 65 Å (vs radius ) 765 Å by TEM). The sphere function was convoluted with an amorphous radial arrangement which led to the first peak in the calculation. The correlation distance was 1750 Å, and the volumetric packing ratio of eight times the occupied volume to the available volume was 5.92, which suggests a close-packed arrangment of the particles. The powerlaw tail of -4 in the sphere calculation agrees with Porod’s law, which suggests sharp, smooth interfaces for the spheres in electron density contrast. A report of similar USAXRD of noncrystalline dispersions of 100 nm silica spheres in water appeared recently.45 Plots of diffracted intensity vs a narrow range of q for composites B1, D1, and D2 (nominally the same as B and D but different preparations) are shown in Figure 10. The XRD plots show a “powder pattern”3,20,21 for the three samples, and the most intense peak corresponds to the d-spacing. The XRD data were indexed by Hull-Davey plots46 assuming a hexagonal lattice with densest planes normal to the incident light. The data also were transformed to a rhombohedral cell. The cell parameters for samples B1 and D2 are given in Table 9. The a and c (45) Konishi, T.; Yamahara, E.; Ise, N. Langmuir 1996, 12, 2608. (46) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; Addison-Wesley: Reading, MA, 1978; pp 330-337, 504-505.
Crystal Structures of Monodisperse Colloidal Silica
Langmuir, Vol. 13, No. 13, 1997 3343
Table 8. Crystal Parameters of Ordered TPM-Silica in PMA Composites SEMa
Bragg diffractionb
fcc and hcp plane
2D order
a (nm)
d (nm)
110c
parallelogram hexagon rectangle parallelogram hexagon rectangle
325 218
167
100c 110c 110d 100d 110d
fcc c (nm)
hcp
a (nm)
d111 (nm)
322
166
a (nm)
d100 (nm)
c (nm)
166
332
222
445
221
413 304
165 225
330
206
413
393
222 310
a
Crystal parameters a and d were calculated from the interparticular distances of the SEM images, assuming a rhombohedral structure. Calculated from the Bragg equation (1) assuming a rhombohedral structure. c Observed from sample B. d Observed from sample D. The blank spaces indicate that the crystal lattice spacings could not be calculated from the SEM images.
b
Table 9. Crystal Parameters of Ordered TPM-Silica in PMA Composites SEM and spectrophotometryb
XRD samplea
structure
a (nm)
dc (nm)
B1 D2 B1 D2
rhombohedral rhombohedral hexagonal hexagonal
242 240 358 299
126 167 189 250
c (nm)
378 500
c/a
a (nm)
dc (nm)
c (nm)
c/a
1.06 1.67
329 386 241 280
175 210 175 210
350 420
1.45 1.50
a The samples B and D diffract in a spectrophotometer at λ 1 2 min ) 514 and 620 nm, respectively, and are slightly different from B and D used in SEM analysis. b Calculated from the Bragg equation 1. c Rhombohedral (111); hexagonal (110).
Figure 9. log-log plots of USAXS data for the sphere function analysis.
Figure 8. (a) SEM image of sample D, a 40 wt % 153 nm TPM-silica-PMA composite film swollen with MA and polymerized, microtomed perpendicular to the film plane. (b) Parallelogram-like particle order of a (110) plane. Film thickness ) 590 µm.
distances and the d-spacings that fit the Hull-Davey plots and the a and d assuming either a hexagonal or a rhombohedral lattice with (111) planes parallel to the glass cell do not agree with the optical diffraction wavelength and the SEM analysis. The data in Figure 9 show four
Figure 10. XRD plot of intensity versus q for 40 wt % TPMsilica-PMA composite (sample B1) and swollen polymerized composites (samples D1 and D2).
peaks that can be indexed to three-dimensional structures, but they are insufficient to identify a specific structure. The data fit a simple cubic structure as well as they fit an hcp or a rhombohedral structure, which we attribute
3344 Langmuir, Vol. 13, No. 13, 1997
to a very low level of order of the colloidal crystals. The XRD results indicate how little long range crystallographic order is needed to produce a colloidal crystalline material that is a selective filter of visible light. The diffraction application forgives a great deal of disorder in the sample.
Jethmalani et al.
observed spectrophotometrically at each stage of the process are in close agreement with the proposed lattice transformations. The composite films diffract narrow bandwidths of visible light despite having little long range order detected by ultrasmall angle X-ray diffraction measurements.
Conclusions Monodisperse TPM-silica in an MA dispersion selfassembles into hexagonally ordered planes which form a fcc lattice, an hcp lattice, or a mixture of fcc and hcp lattices. Photopolymerization of the MA transforms the fcc lattice to a rhombohedral lattice due to shrinkage in only the thickness of the film. Polarizing microscopic images of the composites show 50-200 µm crystallites over most of the sample. The TPM-silica-PMA composite swells in three dimensions with MA, and then shrinks in three dimensions during further polymerization of the imbibed MA. SEM images of planes both parallel and normal to the film plane show some local order and considerable disorder in the samples. The interparticle spacings measured from the SEM images support the proposed lattice transformations. Bragg diffraction wavelengths
Acknowledgment. This research was supported by National Science Foundation Grant DMR-9503626. We thank Bruce J. Ackerson and Hari Babu Sunkara for helpful discussions and Ginger Baker for help with SEM analyses. Supporting Information Available: Table 3, interparticle distances of TPM-silica in fcc and rhombohedral lattices, Table 4, Cartesian coordinates of TPM-silica particles in an fcc lattice, Table 5, internal angles of TPM-silica in an fcc lattice, Table 6, Cartesian coordinates of TPM-silica particles in a rhombohedral lattice, Table 7, internal angles of TPM-silica in a rhombohedral lattice (5 pages). Ordering information is given on any current masthead page. LA960890P
