[acs symposium series] hybrid organic-inorganic composites volume 585 || solidification of colloidal...

Chapter 14

Solidification of Colloidal Crystals of Silica

Hari Babu Sunkara, Jagdish M. Jethmalani, and Warren T. Ford1

Department of Chemistry, Oklahoma State University, Stillwater, OK 74078

Photopolymerization of methyl acrylate, methyl methacrylate, and a mixture of methyl methacrylate and 2-hydroxyethyl methacrylate, each containing ordered 3-(trimethoxysilyl)propyl-coated monodisperse 152 nm diameter silica particles, produces polymer composites in which the ordering of the particles is maintained. The silica particles formed face centered cubic colloidal crystals in both the monomers and the polymers with (111) lattice planes parallel to the film plane. Both the monomer and the polymer dispersions are selective optical filters of a narrow bandwidth of visible light by Bragg diffraction.

Organic and inorganic polymer hybrid composites are of current research interest because their mechanical and optical properties are often better than those of either component. Silica has been widely incorporated in the organic polymer matrix to reinforce the polymer, and the degree of mixing of the two phases influences the strength of the materials. Optically clear composites of silica- or glass fiber-filled poly(methyl methacrylate) (PMMA) have been prepared either by using silica spheres of diameters much smaller than the wavelength of visible light or by carefully matching the refractive index of the glass fiber with the P M M A (7,2). However, in some composites of silica-PMMA, segregation of the particles causes poor optical quality (3).

Monodisperse colloidal polymer particles in aqueous and nonaqueous dispersions form crystal-like arrays that diffract light of wavelength corresponding to the interparticle spacings and Bragg's law (4-18). The ordered arrays of particles in dispersions are known as colloidal crystals. The composites described here constitute a new class of materials in which the compatible colloidal silica particles are ordered in the polymer matrix, and the silica particles are covalently attached to the polymer. The 264-μπι thick composite films transmit more than 70 % of incident visible light normal to the film plane at wavelengths where the light is not diffracted, even though they contain 20 volume percent of 152 nm diameter silica particles. The films are iridescent because the crystalline arrays of silica particles in the polymer matrix diffract visible light. Liquid colloidal optical rejection filters having a narrower bandwidth of rejected light than commercial filters have been developed by exploiting the diffraction of colloidal crystals of polystyrene latexes in water (8-10). In these filters, the colloidal crystals of latexes orient with dhkl planes parallel to the plane of

1Corresponding author

0097-6156/95/0585-0181$12.00/0 © 1995 American Chemical Society

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In Hybrid Organic-Inorganic Composites; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

182 HYBRID ORGANIC-INORGANIC COMPOSITES

the quartz cell. A major drawback with these filters is that weak shear, gravitational, electrical, and thermal forces disturb the order, due to the low elastic modulus of the colloidal crystals, on the order of l f r 2 to 10"3 Pa (7,77,72). Sturdier colloidal crystals in a highly resistant medium may have wider use as selective filters and in other optical devices. Our approach to stable colloidal crystals is solidification of colloidal crystallites of silica in a polymer matrix. Two patents have appeared recently on colloidal crystals in polymer matrices (13,14). One reports no examples (75), and the other describes colloidal crystals of polystyrene latexes in hydrogel matrices such as polyacrylamide (14). These filters are not as rigid as the ones we have prepared with P M M A (75) because the hydrogels retain a large amount of water.

Theory

Diffraction from colloidal crystals of monodisperse charged polystyrene particles in water has been successfully explained by dynamical diffraction theory, which was originally proposed for atomic crystals, by taking into account the interaction between incident and diffracted beams in the medium (16,17):

^corr = ^

where λ = Ifigd^ sin θ and ψ0 « 30 — j (m2-\)

(rn + 2)

ψ0 is the real part of the crystal polarizability, λ is the Bragg diffracted wavelength, dhkl is the interplanar spacing, θ is the Bragg angle, m is the ratio of the refractive index of the particles to that of surrounding medium, φ is the particle volume fraction, and ns is the refractive index of the suspension calculated by

ns - ηηι(ι-φ)+ηΡΦ (2)

where nm and np are the refractive indexes of the medium and the particles (16). The d spacings of the most common structures of colloidal crystals can be calculated theoretically from the volume fraction of the particles. For fee (111)

φ ^ ( ^ ) 3 (3) Ψ 3 V a 1 d = al& (4)

where a is the lattice constant and DQ is the particle diameter. The bandwidths (Δλ 0 ) ' of the Bragg diffracted peaks from the dispersions

containing colloidal crystals can be calculated from the equation 5 given by Kosan and Spry (78).

Jb sin θ

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14. SUNKARA ET AL. Solidification of Colloidal Crystals of Silica 183

where Ψ Η = 0

Δ ^ (m2 +1) ( —z-—) (sin u - u cos u) πζ(3Γζ Vwr+2/

η=πββ{¥ζ\ and D =

wy is a numerical factor = 1.155 for Ewald theory, Κ is the polarization factor, equal to unity for σ polarization and Icos 201 for π polarization, b is the ratio of direction cosines of the light rays, which is unity for Bragg diffraction, m is the ratio of the refractive index of the particles to that of the medium, and D is the nearest neighbor spacing.

Experimental

Materials. Water was deionized, treated with active carbon, deionized, and distilled in glass. Tetraethyl orthosilicate (TEOS, Aldrich), absolute ethanol (Aaper), and 3-(trimethoxysilyl)propyl methacrylate (TPM, 97%, Aldrich) were distilled prior to use. Ammonium hydroxide (Baker), methyl methacrylate (MMA), methyl acrylate (MA), 2-hydroxyethyl methacrylate (HEMA, Aldrich), 2,2,2-trifluoroethyl acrylate (TFEA, Aldrich) and 2,2-dimethoxy-2-phenylacetophenone (DMPA, Aldrich) were used as received.

Colloidal silica dispersions in aqueous ammoniacal ethanol were prepared by the Stôber method (19,20). 3-(Trimethoxysilyl)propyl methacrylate coated silica particles were prepared by the procedure of Philipse and Vrij (21). The density of T P M silica particles was measured using a specific gravity bottle at 25 °C. The dispersions of T P M silica in ethanol were dialyzed against methanol using regenerated cellulose dialysis tubing with molecular weight cutoff of 50,000 (Spectra/Por 7, Spectrum) to remove any unreacted silane, water and ammonia. The particle concentration in methanol was increased to 56 wt % by distilling methanol under reduced pressure. Small portions of about 5-7 mL of a concentrated dispersion of T P M silica particles in methanol were dialyzed against M M A and M A . After replacing the monomer 4-5 times during a period of 24 h, the *H-NMR spectra of the dispersions in monomers showed no methanol peaks. We prepared initially a high particle concentration about 50 wt % in monomers, and subsequently diluted to 35-40 wt%.

Dispersions containing 1 wt % DMPA initiator were transferred by syringe into glass sandwich cells made from 1" χ 3" χ 1 mm microscope slides and 264-μπι Teflon spacers. After colloidal crystals formed, the dispersions were polymerized at ambient temperature for 4-5 h using a medium pressure 450 W mercury vapor lamp 2 cm from the cell with the film plane horizontal. The lamp was cooled by circulating water in a quartz immersion jacket.

The particle diameters were measured using transmission electron microscopy (TEM) and dynamic light scattering (DLS) as described before (22). The structural and orientational details of the colloidal crystallites of T P M silica particles both in colloidal dispersion and in thin polymeric films were observed by the orthoscopic images of the crystals between crossed polarizers. The transmission spectra of the samples were recorded using a single beam Hewlett Packard 8452A UV-vis diode-array spectrophotometer.

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Results

Colloidal TPM-Silica Particles. We chose methacryloxypropyl functionalized silica (3) because grafting of the polymer to the surface of the particles may enhance the tensile strength of the polymer film. As shown in the Scheme, monodisperse silica particles (2) were produced from TEOS (1) by the seed growth technique. Table I lists the particle diameters from T E M and DLS, and their polydispersity indexes (standard deviation of D r t/mean). The number average diameters of silica were the same from the two different batches (3HB-18 and 3HB-76), which differed slightly in the polydispersity indexes. We did not notice any increase in the particle diameter from T E M after coating the particles with silane coupling agent (3HB-76 and 3HB-77). The particle diameters from DLS measurements of diluted dispersions of particles in ethanol were always greater than those of the dried particles. We measured the density of the TPM silica at 25 °C to be 1.79 g/mL.

Table I. Particle Diameters (nm) of TPM Silica

Sample Αι

T E M

A v Dz

Polydispersity Index

DLS

Dz

3HB-18 a 153 157 159 0.08 166 3HB-76 a 152 152 153 0.04 159 3HB-77 b 151 153 155 0.04 —

aParent silica. kTPM coated silica.

In contrast to parent silica, which is hydrophilic and charge stabilized, the T P M silica particles are electrosterically stabilized and are colloidally stable in ethanol and in ethanol-toluene mixtures. We transferred the T P M silica particles from ethanol to the desired monomer by dialysis (23). The T P M silica particles were stable in a variety of acrylic and methacrylic ester monomers, and the colloidal dispersions were iridescent.

Morphology of the Crystallites. The transmission polarizing microscope was used to visualize directly the colloidal crystallites of silica dispersed in various monomers. The dispersions between crossed polarizers showed a mosaic of crystalline domains of various colors ranging 100-500-μπι in size. Crystal growth was slower in the center than at the edges where the dispersion was in contact with the cured epoxy resin used to seal the cells. The crystallites at the edges appeared blue and green, pillar shaped, and larger than the crystallites in the center. Over 4-7 days at 25 °C with the film plane horizontal the entire cell filled with crystallites, and the sample turned from cloudy to iridescent. The crystals in the bulk appeared black, and some of the colored crystallites at the edges turned black over this period. Upon rotation of the cell 40-55° to the incident plane polarized light, the crystals in bulk appeared colored, and the color changed with the angle of incident light.

Bragg Diffraction. We analyzed the transmitted light from the colloidal dispersions using a UV-vis spectrophotometer. Figure 1 shows the visible spectra of 264-μπι

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thick dispersions containing 152 nm T P M silica particles in M M A and in M M A / H E M A (65/35 vol %). The dispersion in the cell appeared cloudy initially and then turned to iridescent with time. The diffracted wavelength decreased as the particle concentration was increased from 35 to 40 wt % of silica in M M A . The visible spectra of M M A / T F E A (65/35 vol %) and M A dispersions are shown in Figure 2. The spectrophotometer detects the light transmitted through an ~8 mm circular cross-section of the cell, which contains approximately 102 crystallites. The spectra were reproducible as long as the light focused on the center of the cell containing crystals with lattice planes parallel to the film plane. The crystals at the edges which have different orientation gave irreproducible results.

Table II reports the volume fraction (φ) of particles, the approximate refractive indexes (nD) of the monomers and monomer mixtures, the time at which the spectral data was collected after filling the cells, the diffracted wavelengths ( A m a x ) from the dispersions, and the lattice spacings (dm) of the particles in 264-μπι thick cells. The refractive index of the T P M silica particles is 1.449 (6). The lattice spacings of the crystallites calculated from equation 1 and the bandwidths calculated from equation 5 are also reported in Table II. The spectral bandwidths from all the dispersions were in the range of 3.4 to 6.0 nm.

Table II. Properties of T P M Silica-Monomer Dispersions8

Φ Monomer time ^nax dm bandwidth (nm) (h) (nm) (nm) obsd calcd

0.227b M M A 1.4142 8 532 188.1 3.6 3.1 0.195 M M A 1.4142 72 552 195.2 3.4 3.1 0.195 M M A / H E M A 1.4267 3 562 197.0 5.0 2.0 0.195 M M A / T F E A 1.3917 43 548 196.9 5.3 5.1 0.193 M A 1.4040 18 560 199.4 6.0 4.1

aParticle diameter 152 nm. bParticle diameter 153 nm.

When the dispersions turned from cloudy to iridescent, indicating formation of colloidal crystals throughout the sample, they were photopolymerized. The maximum extinction coefficient of the initiator (at 336 nm) in ethanol was 274 c m 1

mole 1 L . The T P M groups on the silica copolymerize with monomer or comonomers as shown in the Scheme. Figures 3-5 show the visible spectra from the colloidal dispersions of 152 nm T P M silica particles in M M A / H E M A , M M A and in M A before and after polymerization. The polymer films gave broader peaks at shorter wavelengths than the corresponding monomer dispersions as reported in Table III.

The polymer films sandwiched between the two glass slides were removed to study the morphology, orientation, and crystal structures by optical and electron microscopy. No crystallites were visible when the film was viewed between two crossed polarizers with film plane normal to incident light. However, crystallites appeared in color upon tilting the film.

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1.49

2 0.73 H

<

-0.03 450 500

Wavelength (nm)

Figure 1. Visible spectra of 152 nm diameter T P M silica dispersed in M M A and M M A / H E M A (65/35 vol %). Data are in Table II. (Adapted from ref 25).

1.8

1 0.9 -Ο <

0.0

MMA/TFEA

450 500 550 Wavelength (nm)

600

Figure 2. Visible spectra of T P M silica in MMA/TFEA (65/35 vol %) and in M A .

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0.50

-0.02 450

P(MMA/HEMA)

MMA/HEMA

500 550 Wavelength (nm)

600

Figure 3. Visible spectra of T P M silica in M M A / H E M A before and after polymerization.

0.50

·£ 0.25

<

0.00 450 500

Wavelength (nm)

600

Figure 4. Visible spectra of T P M silica in M M A before and after polymerization. (Adapted from ref 25).

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PMA

450 500 550 600 Wavelength (nm)

Figure 5. Visible spectra of T P M silica in M A before and after polymerization.

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Table III. Comparison of Polymer with Monomer Dispersions

composite before polvmerization after polvmerization decrease bandwidth ^max bandwidth in dm

(nm) (nm) (nm) (nm) (%)

P M M A 554 4.0 490 13.6 14.6 P(MMA-C0-HEMA) 564 6.0 502 19.0 14.1 P(MMA-co-TFEA) 548 5.3 no diffraction

14.3 P M A 556 6.0 496 13.5 14.3

Discussion

Colloidal Crystals in Monomer Films. The 152 nm diameter T P M silica particles in monomers spontaneously formed colloidal crystals. The repulsive forces due to the negative charges on the particle surface, arising from the dissociation of the residual surface hydroxyl groups of the parent silica and of the surface layer from T P M , are responsible for this phase transition. Higher particle number of silica in monomers than of latexes in water is required to form colloidal crystals due to lower charge of the silica and lower dielectric constant of the medium. At high particle numbers the fee crystal structure is favored.

The silica particles and the monomers do not absorb in the visible range. The sharp peaks in the spectra of the colloidal dispersions (Figures 1 and 2) are due to Bragg diffraction of light by the colloidal crystals. Two different batches of T P M -silica in M M A showed similar crystalline behavior. The dm spacings of fee crystals calculated from volume fraction, particle diameter, and lattice parameter agree well with those calculated from A m a x (= Xcon ) by equation 1. The A m a x in M M A shifts to longer wavelength as the volume percent of the particles decreases, indicating the wavelength dependence on lattice spacing. The differences of A m a x at constant volume fraction in different monomers and monomer mixtures show the dependence of diffracted wavelength on the refractive index of the dispersion. The 3.4-3.6 nm bandwidths of the diffracted peaks from the dispersions of 152 nm diameter silica in M M A agree well with the diffracted bandwidth (Δλο) of 3.1 nm calculated for the same volume percent using equation 5. This indicates that the d m planes of all the crystallites in the sample are oriented parallel to the film plane. In general, the diffracted bandwidth depends on the particle diameter and the ratio of the refractive index of the particles to that of the surrounding medium. We observed Bragg diffraction from colloidal dispersions of silica in M M A in which the difference of the refractive indexes (Δη) was only 0.035, whereas Aw for polystyrene latexes in water is 0.26. The diffracted bandwidths for other dispersions are narrow and in the range of 5.0 to 6.0 nm, in close agreement with the calculated bandwidths (Table II).

High degree of ordering of the crystallites, smaller difference between the refractive indexes of the particles and the monomer, and smaller particle diameters narrow the bandwidth of the diffracted light from the dispersions. Others have reported that the degree of ordering of polystyrene latexes in water is influenced by the cell thickness (17,24): the thinner the sample, the greater the ordering of crystallites. We observed no significant differences of the bandwidths of apparent absorption peaks between dispersions in 132 and 264 μπι cells. The intensity of the diffracted peaks varied with sample composition. The intensity was maximum for M M A / T F E A and minimum for M M A / H E M A (Figure 2), as expected for dependence of diffraction intensity on the difference between the refractive indexes of the

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particles and the medium. No colloidal crystals could be seen in a dispersion of particles in pure H E M A , either because the particles and the monomer were optically index matched or because the high viscosity of the monomer (5.9 cps at 30 °C) slowed the rate of crystallization. Most of each sample appeared black except at the edges, indicating the crystallites are at extinction. Their (111) planes parallel to the plane of the cell do not change the direction of the polarization vector of transmitted light. The crystallites at the edges were colored indicating that the lattice planes were not normal to the incident plane polarized light.

Colloidal Crystals in Polymer Composite Films. No phase separation of the silica particles from the polymer matrix was observed in 264 μΐη thick composite films after photopolymerization. The cell containing the colloidal dispersion must be kept horizontal during crystal growth and polymerization to prevent sedimentation of the particles. No diffraction of light was seen from a silica-PMMA film which was polymerized with the cell vertical. This suggests that the orientation and the structure of the crystallites were lost. However, the silica-poly(MMA-co-HEMA) film diffracted visible light (Figure 3), though the cell was vertical during the polymerization. This suggests that the rate of sedimentation of the particles in this mixture was much slower than in M M A alone due to the higher viscosity and density of H E M A .

When the cells containing dispersions in M M A and M A were kept horizontal during polymerization, colloidal particles maintained their order in the polymer matrix, but the diffraction bandwidths increased (Figures 4 and 5 and Table III). No diffraction of light by the crystals in the silica-poly(MMA-co-TFEA) film was seen because of index matching of the particles with the polymer matrix.

No birefringence was observed from most of the film between crossed polarizers when the light beam was normal to the film plane. This suggests a single index of refraction of the polymer composite. When the composite films were rotated at angle 40-55° to the incident light, mostly purple or blue crystals appeared throughout the film. This indicates that sample is not amorphous, but crystalline, and the lattice planes are normal to the incident light.

A scanning electron micrograph of the surface layer of the silica-PMMA composite showed the fee d\\\ planes (25). However, the surface layer does not prove the presence of the same crystalline structures throughout the sample. The average center-to-center distance between neighboring particles measured from the micrograph was 234 nm, and the average surface-to-surface distance was 82 nm, indicating that the order was due to the long range repulsions rather than hard-sphere packing.

Polymerization caused a blue shift in the rejection wavelength from the composite films, indicating that the colloidal crystals were compressed. The expected 21% decrease in volume of M M A on polymerization accounts for a 6.8% decrease in ^-spacing, compared with an observed 14.7% decrease of d, which must be due to an increase of particle number within the crystalline regions. The measured particle distances from the S E M micrograph do not agree with the calculated lattice spacing using equation 1, which suggests that the particle concentration in the bulk may be higher than in the surface layer. The greater bandwidth of the diffraction peaks of the composite films could be due to lower crystalline order of the particles in the polymer films than in the dispersions.

Conclusion

Colloidal crystals of silica in monomer dispersions form solid composite films by polymerizing with UV radiation. The composites selectively filter visible light with a bandwidth less than 20 nm. The silica filled thermoplastic or elastomeric films are

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more rigid, stable and easy to handle than the previously reported arrays of polystyrene latexes in hydrogels. These composite materials potentially could replace optical rejection filters currently in use.

Acknowledgment. We thank the National Science Foundation for financial support, and Bruce J. Ackerson for helpful discussion.

Literature Cited

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10. Asher, S. Α., U. S. Patents 4 627 689 and 4 632 517 (1986). 11. Clark, N. A.; Hurd, A. J.; Ackerson, B. J. Nature 1979, 281, 57. 12. Okubo, T. Colloid Polym. Sci. 1993, 271, 873. 13. Alvarez, J. L . , U.S. Patent 5 131 736 (1992). 14. Asher, S. Α.; Jagannathan, S., U. S. Patent 5 281 370 (1994). 15. Sunkara, H. B.; Jethmalani, J. M . ; Ford, W. T. ACS Polym. Mat. Sci. Eng.

Preprints 1994, 74, 274. 16. Rundquist, P. Α.; Photinos, P.; Jagannathan, S.; Asher, S. A . J. Chem. Phys.

1989, 91, 4932. 17. Monovoukas, Y . ; Gast, A. P. Langmuir 1991, 7, 460. 18. Spry, R. J.; Kosan, D. J. Appl. Spectrosc. 1986, 40, 782. 19. Stöber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. 20. Badley, R. D.; Ford, W. T.; McEnroe, F. J.; Assink, R. A. Langmuir 1990, 6,

792. 21. Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1988, 128, 121. 22. Ford, W. T.; Yu, H.; Lee, J.-J.; El-Hamshary, H. Langmuir, 1993, 9, 1698. 23. Hiltner, P. Α.; Papir, Y. S.; Krieger, I. M. J. Phys. Chem. 1971, 75, 1881. 24. Van Winkle, D. H.; Murray, C. A. Phys. Rev. 1986, 34, 562. 25. Sunkara, Η. B.; Jethmalani, J. M . ; Ford, W. T. Chem. Mater. 1994, 6, 362.

RECEIVED October 20, 1994

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[acs symposium series] hybrid organic-inorganic composites volume 585 || solidification of colloidal...

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