From a Colloidal Crystal to an Interconnected Colloidal Array: A

University of California, Los Angeles, Los Angeles, California 90095-1569, .... Journal of Applied Polymer Science 2015 132 (10.1002/app.v132.13),...
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From a Colloidal Crystal to an Interconnected Colloidal Array: A Mechanism for a Spontaneous Rearrangement Alessandro Rugge,† Warren T. Ford,‡ and Sarah H. Tolbert*,† Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569, and Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received April 10, 2003. In Final Form: June 24, 2003 Colloidal crystals represent important templates which can be chemically modified to produce complex nanostructured materials with interesting photonic applications. The present study focuses on two potentially useful spontaneous transformations involving a thin-film colloidal crystal array made of particles with a core-shell structure. When the colloidal crystal is exposed to moderate amounts of toluene vapors it is swollen reversibly, resulting in a marked shift of its reflectance peak. This property, combined with the system’s sensitivity and the ability to repeat the process many times, makes this colloidal crystal a good candidate for the detection of toluene and similar pollutants in the atmosphere. The second transformation takes place when the colloidal crystal is exposed to higher pressures of toluene and then dried. This irreversible process yields an ordered crystal of colloidal cores embedded in a polymer matrix (an interconnected colloidal array or ICA). The simple principles driving these transformations suggest that this technique can be generalized and applied to related colloidal crystals to produce more complex and useful materials. An account of the morphology of the ICA and an explanation of the mechanism of the two transformations are presented.

Introduction Monodisperse colloidal particles of a variety of chemical compositions have the ability to crystallize into ordered arrays with a periodicity on the nanometer length scale. Colloidal crystals may also be used as templates to create inverse opal structures via backfilling with ceramic, metal, or polymeric precursors.1-8 These materials have interesting potential applications as photonic devices due to the periodic variation of dielectric and their potential to show a photonic band gap.9-13 Colloidal crystals embedded in a matrix of a polymeric material that fills the porosity between the particles represent an important class of colloidally templated nanostructures. The presence of an elastic matrix allows for reversible changes in the interlayer spacing of the colloidal cores, which results in a smaller or larger periodicity of dielectric in the direction perpendicular to the substrate. Because the wavelength of the stop band is proportional to both the periodicity of the structure and * Corresponding author. E-mail: [email protected]. † University of California, Los Angeles. ‡ Oklahoma State University. (1) Wijnhoven, J. E. G.; Vos, W. L. Science 1998, 281, 802. (2) Braun, P. V.; Wiltzius, P. Nature 1999, 402, 603. (3) Bertone, J. F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. Rev. Lett. 1999, 83, 300. (4) Jiang, P.; Bertone, J. F., Colvin, V. L. Science 2001, 291, 453. (5) Yan, H.; Blanford, C. F.; Smyrl, W. H.; Stein, A. Chem. Commun. 2000, 1477. Blanford, C. F.; Schroden, R. C.; Al-Daous, M.; Stein, A. Adv. Mater. 2001, 13, 26. (6) Lu, Y.; Yin, Y.; Xia, Y. Adv. Mater. 2001, 13, 271. (7) Subramanian, G.; Manoharan, V. M.; Thorne, J. D.; Pine, D. J. Adv. Mater. 1999, 11, 1261. (8) Manoharan, V. M.; Imhof, A.; Thorne, J. D.; Pine, D. J. Adv. Mater. 2001, 13, 447. (9) Yablonovitch, E. J. Opt. Soc. Am. B 1993, 10, 283. Yablonovitch, E.; Gmitter, T. L. Phys Rev. Lett. 1989, 63, 1950. (10) Busch, K.; Sajeev, J. Phys. Rev E 1998, 58, 3896. (11) Ho, K. M.; Chan, C. T.; Soukoulis, C. M. Phys. Rev. Lett. 1990, 65, 3152. (12) Biswas, R.; Sigalas, M. M.; Subramania, G.; Ho, K. M. Phys. Rev. B 1998, 57, 3701. (13) Moroz, A.; Sommers, C. J. Phys.: Condens. Matter 1999, 11, 997.

the average dielectric, a matrix sensitive to a stimulus can directly produce a change in optical properties.14-16 These types of arrays are readily incorporated into sensing devices because of the response of specifically designed matrixes to a variety of stimuli. Such devices have been used for the detection of heavy metals,17 pH, and temperature.18 A recent example proves that a redox-active matrix may be also used to control the photonic properties of these systems.19 In general, however, the preparation of such interconnected arrays is a fairly involved process involving several precursors and multiple steps. A promising recent approach to the preparation of colloidal arrays in a polymer matrix involves the use of core-shell colloidal particles composed of thick polymer shells with a glass transition temperature that is much lower than that of the cores.20,21 To synthesize these colloids, the core is first prepared and then the shell is grown onto the core using a variety of methods. Colloidal arrays of these particles can be annealed to generate an orderly collection of cores dispersed in an interconnected polymer matrix. The core and the matrix can both be selectively functionalized during synthesis of the coreshell colloidal building block.22 In this work, we add to the growing arsenal of techniques to manipulate and transform an array of colloidal particles. Here we use colloidal arrays of simple core-shell particles (14) Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1996, 8, 2138. (15) Foulger, S. H.; Kotha, S.; Sweryda-Krawiec, B.; Baughman, T. W.; Ballato, J. M.; Jiang, P.; Smith, D. W. Opt. Lett. 2000, 25, 1300. (16) Foulger, S. H.; Jiang, P.; Lattam, A. C.; Smith, D. W., Jr.; Ballato, J. Adv. Mater. 2001, 13, 1898. Foulger, S. H.; Jiang, P.; Ying, Y.; Lattam, A. C.; Smith, D. W., Jr.; Ballato, J. Langmuir 2001, 17, 6023. (17) Asher, S. A.; Peteu, S. F.; Reese, C. E.; Lin, M. X.; Finegold, D. Anal. Bioanal. Chem. 2002, 373, 632. (18) Reese, C. E.; Baltusavich, M. E.; Keim, J. P.; Asher, S. A. Anal. Chem. 2001, 73, 5038. (19) Arsenault, A. C.; Miguez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. Adv. Mater. 2003, 15, 503. (20) Li, H.; Kumacheva, E. Colloid Polym. Sci. 2003, 281, 1. (21) Kumacheva, E. Macromol. Symp. 2003, 192, 191. (22) Zhang, J.; Coombs, N.; Kumacheva, E. J. Am. Chem. Soc. 2002, 124, 14512.

10.1021/la034617g CCC: $25.00 © 2003 American Chemical Society Published on Web 08/15/2003

From a Colloidal Crystal to a Colloidal Array

and report two different transformations taking place upon exposing these samples to solvent vapors. The particles used in our study were prepared by a simple batch emulsion polymerization using 90 mol % styrene and 10 mol % 2-hydroxyethyl methacrylate (HEMA). This produces a polystyrene core and a thin shell rich in polyHEMA. The transformations were observed when the colloidal crystal was exposed to vapors of a solvent with good affinity for polystyrene (such as toluene or styrene) but poor affinity for the more hydrophilic poly-HEMA shell. The first transformation was completely reversible and took place when the sample was exposed to small amounts of toluene vapors. The particles were swollen and the primary absorbance peak measured at normal incidence to the film shifted to longer wavelengths by an extent that was proportional to the amount of toluene vapors used. The original peak position and colloidal crystal were recovered when the film was dried in air, indicating that toluene absorption is a reversible, equilibrium process. When larger vapor pressures of solvent were used, an irreversible and spontaneous rearrangement took place. By means of direct imaging and other macroscopic characterizations, it was possible to describe the morphology of the product of the irreversible transformation. The final structure of the film was nonporous and consisted of polystyrene ellipsoidal cores embedded in a matrix containing both polystyrene and poly-HEMA. In addition, this change in film morphology was associated with a very different absorbance spectrum for the sample: the primary peak for the colloidal crystal was replaced by a much less intense peak occurring at shorter wavelength. We refer to this new morphology as an interconnected colloidal array (ICA). The goal of the present study is to characterize the new sample morphology and develop an understanding of the mechanism of this spontaneous rearrangement. The system generates the final nanostructured material spontaneously without the need for multistep transformations carried out by the experimenter. Explaining the process that leads to this new nanostructure will potentially help in the design of related systems with useful properties. Because this transformation is explained by simple principles and happens spontaneously, the phenomenon is likely to be quite general and easy to extend to other polymer systems. As new layer-by-layer techniques are developed to selectively modify the core-shell structure of latex particles,23 the process presented here has the potential to be a convenient synthetic route applicable to an increasingly large class of colloidal crystalline arrays. This work represents a follow-up to a study by Chen et al.24 which reported the spontaneous rearrangement to a material with a new morphology. While several different techniques were utilized in the original characterization of the transformed film morphology,24,25 a full account of the mechanism of transformation and of the structure of the new film was not possible. As a result, the conclusions presented here differ from the ones offered initially (ref 25). Mechanisms for both the new reversible transformation and the already known irreversible one are proposed. (23) Goldenberg, L. M.; Jung, B. D.; Wagner, J.; Stumpe, J.; Paulke, B. R.; Gornitz, E. Langmuir 2003, 19, 205 and references therein. (24) Chen, Y.; Ford, W. T.; Materer, N. F.; Teeters, D. J. Am. Chem. Soc. 2000, 122, 10472. (25) Chen, Y.; Ford, W. T.; Materer, N. F.; Teeters, D. Chem. Mater. 2001, 12, 2697.

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Figure 1. Core-shell colloidal particles used in this study. The particles have a rough surface because the poly-HEMA shell is not uniform and has a thickness of 5-10 nm. The polyHEMA accounts for about 10% of the volume fraction of each colloid; the polystyrene core accounts for the rest. The scale bar measures 500 nm.

Experimental Section Particle Synthesis. The colloidal particles used in this study were synthesized and purified as reported elsewhere26 using a 10% mol ratio of HEMA. The size and size distribution of the particles were estimated by transmission electron microscopy (TEM) by averaging over ∼75 particles. Two different monodisperse samples of different size were used in this study. The reversible swelling experiments used particles with an average size of 392 nm ( 2%. A second sample was used for the irreversible transformation experiments and had size of 301 nm ( 2% as shown by the TEM micrograph of Figure 1. Colloidal Crystal Preparation. The colloidal crystals were prepared by an established vertical deposition technique.27 A thin glass slide was held vertically in a 20 mL vial containing an aqueous suspension of 0.25-1.0 vol % particles. The vial was placed in a Plexiglas box connected to a compressed air inlet and equipped with a venting outlet. A moderate air flow through the box and over this setup was used to enhance water evaporation. As the meniscus sweeps down the substrate, capillary forces induce ordering of the particles into a close-packed arrangement. Films were typically grown at a rate of 5-8 mm/day. Most of the experiments in this study were performed on films of thickness between 8 and 10 µm. Reversible Transformation Absorbance Data. The realtime absorbance data in the presence of toluene vapors were collected using an Ocean Optics SD2000 spectrophotometer coupled to a gas-flow system designed to deliver toluene vapors at a desired pressure. A colloidal crystal film was held inside a cuvette perpendicular to the light beam using a Teflon support. A gentle flow of compressed air was passed through the cuvette whose seal was equipped with an inlet and a venting outlet. Before reaching the sample, the compressed air was finely dispersed through two toluene reservoirs in order to saturate it with solvent vapors. The first reservoir was gently heated above room temperature in order to supersaturate the gas with solvent vapors. The second reservoir was placed in a controlledtemperature bath. The residual vapor pressure of toluene leaving the second reservoir was determined by its temperature. This bath was typically cooled to temperatures ranging from -5 °C to room temperature. The cuvette and sample holder were maintained at room temperature. When the second reservoir was at room temperature, the temperature at the sample was (26) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447. (27) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303.

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controlled and maintained about 5 °C warmer than the reservoir in order to prevent condensation of toluene vapors on the sample. This system also allowed us to easily switch the flow to dry air to evaporate toluene from the sample. Amount of Toluene Absorbed. The mass of toluene absorbed by the colloidal crystal and released by the net was measured with a quartz crystal microbalance (QCM). A colloidal crystal was grown by vertical deposition on the gold-coated surface of a 3 MHz quartz crystal. The mass deposited on the electrode is proportional to the shift in the crystal’s resonant frequency. The resonant frequency was measured as the sample was first placed in a room-temperature chamber saturated with toluene vapors and then later allowed to dry under a gentle air flow. Thin Sections of the Sample. The ICA films could be peeled off the glass substrate by dipping the sample and substrate in 24% HF for less than a minute. The films were then embedded in resin prepared with the Eponate 12 kit purchased from Ted Pella. After curing at 60 °C for 2 days, the samples were ultramicrotomed with a diamond knife to a thickness of 60-80 nm and placed on TEM grids. These grids were then suspended over a 0.5% aqueous solution of RuO4 (Polysciences, Inc.) for 10 min to allow the vapors to stain the thin sections. The thicknesses of colloidal crystals and ICA films were measured using a Dektak 8 profilometer. A step was created by scraping a small portion of the films to expose the bare substrate; the instrument’s stylus was then dragged across this step. Scanning electron microscopy (SEM) images of ICA films grown on silicon wafers and sputtered with a gold layer of about 5 nm thickness were collected using a Hitachi S4700 microscope. Atomic force microscopy (AFM) images were collected on a Park Autoprobe CP instrument.

Results Reversible Changes. The previous studies on this system reported a transformation of the colloidal crystal (CC), occurring upon exposure to toluene vapors in equilibrium with liquid toluene at room temperature in a sealed vessel.24,25 To slow this transformation process, we exposed the CC samples to lower vapor pressures by decreasing the temperature of the liquid toluene reservoir to subambient values in the flow apparatus described above. By doing this, we discovered that reversible structural changes can occur under these low vapor pressure conditions. When a CC is exposed to these constant lower vapor pressures, its primary peak position shifts to the red without any appreciable loss of intensity or any major changes in peak shape. After a limiting value is reached, no further spectral changes are observed and the film remains in equilibrium with a constant toluene vapor pressure. When the toluene vapors are removed and the CC is exposed to dry air, the peak shifts back to the original position (Figure 2). This process may be repeated many times with no changes in the observed behavior. Toluene is a good solvent for polystyrene but a poor solvent for poly-HEMA. Therefore, it is expected to permeate the thin poly-HEMA shell and mix with the polystyrene in the cores. The spectral changes shown in Figure 2 are consistent with a transformation model which involves swelling of the polystyrene cores by toluene and a reversal of that process when the CC is dried in air. The swelling results in an increase in the size of the colloids which in turn increases the interlayer spacing of the CC. Upon toluene exposure, the CC is not observed to expand in the in-plane direction off the edges of the supporting glass slide. This is predictable as the CC is electrostatically bound to a glass slide and has no ability to expand in the plane parallel to the substrate. Therefore, any swelling of the particles must occur as an elongation in the direction perpendicular to the substrate. The particles are thus expected to maintain their close-packing and elongate into

Figure 2. Time-resolved shifts of the primary peak of the CC as it is (a) exposed to toluene vapors at a pressure of 16.5 Torr and (b) allowed to dry in a flow of air. The wetting time (tw) and drying time (td) are given in minutes and correspond to the time elapsed since the onset of exposure to toluene vapors and dry air, respectively. The peak intensity and shape remain mostly unaffected, consistent with a swelling of the polystyrene cores by the toluene vapors. This swelling is reversible upon removal of toluene and may be carried out many times on the same film.

Figure 3. For the reversible transformation, the degree to which the peak position shifts is proportional to the toluene vapor pressure used to swell the CC. The percent increase in film thickness associated with the swelling is also shown.

ellipsoids whose long axis is perpendicular to the substrate. In the range between 5 and 16.5 Torr, the degree to which the peak shifts is proportional to the vapor pressure of toluene the sample is exposed to (Figure 3). This suggests an ability by the colloidal particles to absorb toluene vapors

From a Colloidal Crystal to a Colloidal Array

Figure 4. Apparent rate constants for the forward wetting process (black) and the backward drying process (white) for the reversible transformation of the colloidal crystalline array. The wetting rate constant is not affected by the toluene vapor pressure used in the experiment. By contrast, the drying rate constant increases with the total degree of swelling of the colloidal crystal films.

until the concentration of toluene in the CC has reached an equilibrium value. The shifts in peak position for this reversible transformation can be tracked with time in order to gain a better understanding for the kinetics of the process. When the CC is exposed to toluene vapor pressures lower than 16.5 Torr, the wavelength of the peak position red-shifts with time from an initial value to a limiting value. The peak wavelength then decreases back to a return value (very similar to the initial one) when the CC is dried in air. Both of these processes show an exponential dependence on time. For the wetting process, the following equation was used to fit the time-dependent shifts in the absorbance data:

λ(t) ) λlim + (λi - λlim) exp(-kwett) Similarly, for the drying process the equation

λ(t) ) λret + (λlim - λret) exp(-kdryt) was used, where the peak position as a function of time, λ(t), is expressed as a function of λi, the initial peak position for the dry CC, λlim, the limiting red-shifted value reached in the wetting process at a given toluene vapor pressure, and λret, the wavelength value at the end of the drying process. Even though simple first-order kinetics is not expected to apply to systems whose composition is continuously changing, as in our case, the fits to the above equations are satisfactory and the rate constants extracted in this manner provide insight into the behavior of the system. The analysis was carried out for five different experiments in which toluene vapor pressures ranging from 5 to 16.5 Torr were used. The apparent rate constants for the wetting and drying processes, kwet and kdry, respectively, are presented in Figure 4. The data indicate that kwet shows no dependence on the vapor pressure of toluene utilized. By contrast, kdry increases with the vapor pressure utilized to swell the CC. These results suggest that the rate at which toluene permeates the poly-HEMA shells before reaching the polystyrene particle cores may be ratedetermining in all cases. For the wetting experiments, the sample always starts in the same dry configuration and so the same swelling rate is observed. For the drying experiments, the CC films swollen with large amounts of

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Figure 5. When the CC is exposed to higher pressures of toluene vapors, an irreversible transformation occurs. The peak position shifts to the red and then its intensity decreases as the sample undergoes the transformation. The spectra shown were collected at 1-min intervals for 24 min after the sample was exposed to toluene at a pressure of 20 Torr.

toluene appear to desorb toluene more quickly than slightly swollen ones. The increased drying rate constants for the most swollen films are likely due to a thinning of the poly-HEMA shell around a swollen polystyrene core and possibly its temporary partial fragmentation with swelling. This stretching of the poly-HEMA shell should enhance its permeability to toluene vapors, especially in the early stages of toluene desorption when the system is far from equilibrium and the drying rates are faster. Conversion to an Interconnected Colloidal Array. When the CC is exposed to toluene vapor pressure of approximately 20 Torr or higher (corresponding to the vapor pressure of toluene at a temperature of 18 °C or higher), a different behavior is observed, as shown in Figure 5. Initially, the primary peak shifts to longer wavelengths as observed for the reversible change, but after a certain point, the peak intensity gradually decreases. After these spectral changes are observed, the sample undergoes an irreversible transformation and changes from an iridescent but somewhat scattering film to a virtually clear one. When the toluene vapors are removed and the sample is allowed to dry, a new and much less intense spectral feature appears at shorter wavelengths. We assign this new peak to a new nanostructure in the film which we will show can be described as an ICA. The new peak gradually shifts to shorter wavelengths as the film dries over the course of 1 day as seen in Figure 6. In a typical preparative transformation experiment, a CC is allowed to equilibrate with toluene vapors for 3 h. Even though no further spectral changes are observed after the first half hour, it is necessary to allow the sample to continue absorbing toluene for at least 2 h in order for the ICA spectroscopic signature to appear upon drying. When the amount of toluene absorbed by the CC and released by the ICA is measured using a quartz crystal microbalance (Figure 7), the mass of the CC continues to increase past the time required for the primary peak to disappear. Indeed, the sample was exposed to toluene for 3 h and the absorption does not appear to saturate during that period of time. When the sample is allowed to dry in air, all of the toluene absorbed is released, generating an ICA that (within experimental error) has the same mass as the original CC. It is clear that during both wetting and drying most of the change in mass occurs at very short times. In fact, in the drying case the first data point collected after switching the sample from the toluene

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Figure 6. Spectra of a drying ICA collected over the course of 24 h. The sloping background has been subtracted. As the film dries and the toluene escapes, the peak shifts to a shorter wavelength due to the thinning of the film and the accompanying decrease in interlayer spacing.

Figure 7. Mass changes of the sample measured with a quartz crystal microbalance during the irreversible CC to ICA transformation. The film is allowed to absorb toluene vapors for 3 h and is then dried in air. Most of the drying occurs within less than 2 min of removing the sample from the toluene vapors. The mass of the original CC is the same as the mass of the dried ICA.

atmosphere to pure air shows a mass that is 23% less than the mass at the last wetting point. In the wetting process, as mentioned earlier, the amount of toluene absorbed by the sample in the first 5 min is also close to a 23% mass change. The rate of toluene absorption after the first 5 min is much slower, probably because of reduced access by the vapors to the inner portions of a film that is no longer porous. The rate of drying, after the rapid initial drop just mentioned, proceeds with a slower exponential decay until the ICA has the same mass as the original CC and all toluene has evaporated. Unlike the CC, the ICA is not easily scraped from the glass substrate or fragmented into small pieces or individual particles. This fully interconnected material may instead be easily removed from the glass substrate by dipping the sample and substrate in hydrofluoric acid. The film resilience allows it to be peeled off as a flexible, free-standing ICA (Figure 8). The robustness of the ICA and its ability to be cut into pieces of different size and shape are expected to facilitate the incorporation of the film into devices designed to utilize its optical properties.

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Figure 8. The ICA film may be removed from the substrate and handled as a fully interconnected and free-standing colloidal array.

To explore the internal structure of the ICA films, freestanding film samples were embedded in resin and ultramicrotomed for TEM analysis. The thin sections were stained with RuO4, which has an affinity for the doublebond-rich polystyrene portions of the sample. The micrographs reveal an orderly collection of ellipsoidal wellseparated polystyrene cores encased by a solid matrix which appears lighter in color because it is not stained as effectively by RuO4 (Figure 9a). This matrix must thus contain most of the poly-HEMA which made up the shell of the particles in the original CC. By soaking the ICA in methanol, a good solvent for polyHEMA, it is also possible to partially dissolve the polyHEMA and image thinner portions of the ICA by TEM with no need for thin sectioning. Micrographs of methanolsoaked ICAs reveal the presence of colloidal cores whose diameter is significantly smaller than the diameter of the particles that made up the original CC (Figure 9b). Occasionally, treatment of the ICA film with methanol loosens the cores from the matrix. By direct imaging, these free colloids appear ellipsoidal in shape (Figure 9c). These images of thin sections of ICA samples strongly suggest that the film morphology involves polystyrene cores that are ellipsoidal, well separated, and encased in a polymer matrix. As mentioned earlier, this matrix is not stained as effectively as polystyrene and is partially solubilized by methanol, both of which are consistent with a polystyrene/poly-HEMA blend. The ICA was further characterized by measuring the film thickness relative to the thickness of the CC that generated it. The presence of ellipsoidal cores rather than spherical ones suggests that upon transforming from CC to ICA the films may flatten. The film thickness was measured for a range of ICA films and the CC films that generated them. Indeed, a decrease in thickness for the ICA was observed and the films were found to thin to 72% ( 2% of the thickness of the CC. Discussion Reversible Transformation. Normal incidence absorbance measurements represent a powerful tool for characterization of thin films of colloidal crystals attached to a transparent substrate. This technique allows the periodicity in index of refraction due to the (111) layers of close-packed particles in the colloidal crystal parallel to the substrate to be examined. The transmission dip is associated with diffraction of visible light with wavelengths satisfying the Bragg condition along the axis of

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atmospheric moisture has the potential to partially swell the poly-HEMA shell, which may result in an average diameter slightly larger than measured by TEM under high-vacuum conditions. A more accurate estimate of the average particle diameter may be obtained from the peak position of the resulting colloidal crystals. At normal incidence to the film, the peak position occurs at

λmax ) 2neff d111 where d111 is the spacing between (111) layers in a closepacked structure and neff is the effective refractive index of the film. In a close-packed lattice, d111 ) (2/3)1/2D where D is the particle diameter and

neff ) (74% nparticle2 + 26% nbackground2)1/2

Figure 9. TEM images of the ICA. (A) Thin section of an ICA film stained with RuO4. The sample appears as a collection of ellipsoidal cores embedded in a polymer matrix. In this case, the sample was sectioned nearly parallel to a (111) plane. (B) Thin ICA film after a methanol soak. The cores making up the structure are smaller than the original colloidal particles shown in Figure 1 and have lost the rough poly-HEMA shell. (C) When the ICA cores are freed from the matrix and are allowed to rotate, their ellipsoidal shape is more obvious. The inset is a magnification of a core outlining its elliptical shape. All scale bars are 500 nm.

propagation.28 Changes in peak position are due to changes in the interlayer spacing and/or changes in the composition (and thus average index of refraction) of the film. The size of the colloidal particles used in this study was measured by TEM with statistical averaging over a large number of particles. However, the corrugated surface of the particles caused by the presence of a nonuniform polyHEMA shell introduces an error of a few percent in the colloid size when this procedure is followed. In addition, (28) Jiang, P.; Bertone, J, F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132.

In turn, nparticle is determined by the two polymers that make up the core (polystyrene, 90% mol) and the shell (poly-HEMA, 10% mol). Therefore nparticle (90% nPS2 + 10% npoly-HEMA2)1/2. When the values of nPS ) 1.592,29 npoly-HEMA ) 1.512,30 and for the air background nbackground ) 1.000 are used, the average particle diameter can be calculated to be 396 nm from the CC peak position of λmax ) 939 nm. An analogous treatment for the CC used in the second part of this study (vide infra) gives an average diameter of 306 nm from the value of λmax ) 727 nm. The sizes of these two populations as determined by TEM were 392 and 301 nm, respectively. The difference between the average diameters calculated from TEM and the ones calculated using the above procedure is about 1%. This value is smaller than the standard deviation of the diameter distribution, which was estimated by TEM to be about 2% for both populations. As the particles swell into ellipsoids by absorbing toluene, two parameters affecting the measured peak position change. The first and dominant effect is an increase in the interlayer spacing. The other effect is an increase in the effective index of refraction of the film due to the presence of toluene with its relatively high index of refraction (ntoluene ) 1.497). These two effects may be incorporated into a calculation of the expected peak position as a function of increased interlayer spacing and of the changes in average refractive index due to the changes in volume fraction of polystyrene, poly-HEMA, toluene, and air. If the calculated peak position is matched to the experimental values, it is possible to extract a fractional increase of the CC thickness (see Figure 3, right ordinate). For example, when the CC is exposed to a toluene vapor pressure of 16.5 Torr, the shift in peak position is about 6.5%. This corresponds to an increase of film thickness of 5.8%. Figure 3 shows that there is a considerable change in peak position and film thickness when the CC is exposed to toluene vapor. These trends suggest that these samples may be used to quantitate the vapor pressure of toluene and other similar solvents in the atmosphere. The determination of benzene, toluene, and xylene (BTX) is required for the detection of leaks and chemical spills. A variety of techniques have been utilized to achieve this goal. The most recent advances include ion mobility spectrometry,31 low-pressure chemical ionization mass spectrometry,32 and microfluidic devices.33 While many of these techniques have lower detection limits, somewhat (29) Aldrich Handbook of Fine Chemicals and Laboratory Equipment; Aldrich Chemical Co.: Milwaukee, WI, 2000-2001. (30) http://cqbetawww.chemquik.com/default.asp?LinkNum)LTE. 374898. (31) Sielemann, St.; Baumbach, J. I.; Schmidt, H.; Pilzecker, P. Field Anal. Chem. Technol. 2000, 4, 157.

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faster response times, and likely better selectivity, they require bulky and expensive dedicated equipment. When the data from Figure 3 are extrapolated to lower vapor pressures, the detection limit of a simple device based on a CC film may be estimated to be below 1500 ppm, a concentration of toluene at which the human central nervous system begins to be adversely affected.34 The dramatic change in film thickness suggests that toluene absorption may also be coupled into sensing through mechanical actuation. Conversion to an Interconnected Colloidal Array. When the CC is exposed to toluene vapor pressures exceeding the threshold value of about 20 Torr, the particles continue to swell until the peak position has shifted to about 787 nm, corresponding to a 7.5% shift from the original peak position. Based on the calculations introduced for the reversible transformation, it can be estimated that at this point the volume fraction of toluene in the film is 7.1% and the polystyrene fraction has dropped from 66.6% to 61.6%. Because the primary peak has maintained or even slightly increased its intensity, it is safe to assume that the nanostructure of the film is still intact at this point and that all the toluene absorbed by the film is mixed with the polystyrene in the particle cores. At the onset of the irreversible transformation, this toluene-polystyrene blend in the cores therefore contains about 7.1/(7.1 + 61.6) ) 10.3% toluene by volume. The time required to absorb this amount of toluene (7.1 vol %) may be estimated from the QCM experiment (Figure 7), which was carried out at 26 °C (corresponding to a toluene vapor pressure of about 30 Torr). Extrapolation of the QCM data to short times indicates that the CC film has absorbed 8 mass % toluene in about 40 s. The minimum amount of toluene required for the onset of transformation is likely related to the decrease in the glass transition temperature (Tg) of the polymers when swollen by solvents. In a study on the effects of toluene on the Tg for polystyrenes, Gutierrez and Ford35 found that Tg for weakly cross-linked polystyrene falls below 25 °C (the temperature of our experiments) when as little as 14 mass % toluene is dissolved in it. If those results on cross-linked polystyrenes are extrapolated to no crosslinking, as in our case, Tg for the polystyrene-toluene blend in the cores has probably dropped to room temperature by the time the primary absorption peak begins to disappear at 10.3% toluene by volume. Therefore, the irreversible transformation appears to be initiated by the ability of the polystyrene-toluene mixture in the particles cores to flow. As this fluid escapes the poly-HEMA shell and fills the pores, the loss of dielectric contrast between the cores and the space surrounding them results in a marked decrease of the peak intensity.36 An interesting feature of the spectra presented in Figure 5 is the apparent blue-shift of the peak as its intensity decreases. Because toluene vapors have access to only one face of the CC and must then diffuse toward the substrate, a gradient in degree of swelling may be transiently established in the direction perpendicular to the film. The apparent blue-shift of the waning peak could be due to the disappearance of the most swollen particle layers, which contribute to the peak intensity at longer wavelengths. This spectroscopic result could also be the (32) Chen, Q.-F.; Milburn, R. K.; DeBrou, G. B.; Karellas, N. S. J. Hazard. Mater. 2002, B91, 271. (33) Ueno, Y.; Horiuchi, T.; Morimoto, T.; Niwa, O. Anal. Chem. 2001, 73, 4688. (34) http://risk.lsd.ornl.gov/tox/profiles/toluene_c_V1.shtml. (35) Gutierrez, M. H.; Ford, W. T. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 655.

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consequence of the decrease in size in the colloids as the swollen polystyrene cores “leak” some of their volume into the surrounding interstices. At the onset of this process, the interlayer spacing is expected to decrease concurrently with the dielectric contrast because the formation of partially filled pores results in both a less intense peak and a shift to shorter wavelength. Both possibilities lead to a loss of the peak in the final spectrum when the film is allowed to equilibrate and the porosity of the CC is lost. These explanations are corroborated by the experimental observations that when the transformation is carried out using higher concentrations of toluene vapors and thus the transformation is more rapid, the primary peak intensity merely decreases with no blue-shifting of the waning peak. One of the goals of this work was to characterize the structure of the new phase in the transformed film. The thin sections of the final product we collected (Figure 9a) show an orderly collection of solid polystyrene cores. This suggests that upon transforming these samples undergo a process of densification in which the porous nature of the CC is lost and a new, dense nanostructured film is generated. This idea is confirmed by the fact that the ICA films are virtually clear and do not scatter significant light, as would be the case for a film containing a large air-polymer interface like the original CC films. While the qualitative observation of solid polystyrene cores from TEM imaging of thin sections is unequivocal, a quantitative morphological characterization is unfortunately not possible for several reasons. The embedding resin has an excellent chemical affinity for the poly-HEMA and has therefore the potential to penetrate the nanostructure and distort the portions that contain poly-HEMA by swelling them. This chemical affinity could cause the poly-HEMA to become more diluted by mixing with the resin. While we do not know the exact composition of the matrix, we do know that it is dilute because we have been unable to selectively stain the poly-HEMA containing portions of the ICA thin sections with fixing agents designed to stain pure poly-HEMA. In addition, the morphology of the ICA thin sections is altered by the process of ultramicrotoming itself. The diamond knife stretches the sample along the cutting direction, resulting in an unspecified elongation of the morphological features. Finally, because it is impossible to choose the sectioning direction relative to the lattice planes of the sample, most thin sectioning occurs at random angles. The micrograph of Figure 9a was carefully selected because the sectioning is parallel to one of the (111) planes of the sample, resulting in evenly spaced core sections of similar sizes. It is unlikely that the cores are cut through their equators, however. As a result, even under these conditions, the factors mentioned above do not allow for a quantitative measurement of size, eccentricity, and spacing of the cores in thin sections. The images do provide qualitative proof that polystyrene cores remain in the ICA and that these cores have shrunk so that they are not close packed (i.e., they are not touching). When the ICA films are imaged directly by TEM without embedding and sectioning, the films are too thick to be penetrated by the electron beam and all resolution is lost. However, if the films are previously soaked in methanol to dissolve as much of the poly-HEMA as possible and then are laid flat on the TEM grid, the thinner portions may be imaged effectively (Figure 9b). The stacked colloidal particles have a size of about 277 nm, a value significantly smaller than the diameter of 295 nm of the polystyrene cores in the original colloidal particles. The surface of these colloids is very smooth in contrast with

From a Colloidal Crystal to a Colloidal Array

the original core-shell particles whose surface is very rough due to the rugged poly-HEMA shell (Figure 1). This suggests that the polystyrene cores present in the ICA are indeed smaller (as seen from the thin sections, Figure 9a) than the ones in the CC and that their poly-HEMA shells have been lost and absorbed into the matrix that surrounds them. Figure 9c shows that when these shrunken polystyrene cores are removed from the matrix they have an ellipsoidal shape. The cores from Figure 9b appear round because they are imaged in the direction of the short axis of the ellipsoid, that is, perpendicular to the substrate. The flattening of polystyrene cores into ellipsoids is in agreement with the profilometry data which show that upon conversion from CC to ICA, the film thins to 72% ( 2% of its original thickness. Since the number of layers of colloids must remain the same in the course of the transformation, a thinner film implies smaller and/or flattened colloids. The decrease in film thickness also implies that the sample must be a dense polymer film. The original close-packed CC contained 74% polymers and 26% air; if that film volume diminishes to 72% ( 2% of its original value, it cannot contain significant amounts of air and must be essentially nonporous. Any measurable difference between the two quantities is explained by the fact that the value of 74% volume of polymers in the CC is an upper bound applicable to infinite systems. For thin films such as the ones considered here, edge effects will lower the percent volume fraction of polymers from 74% to a number closer to the center of the 72% ( 2% range measured by profilometry. These morphological characterizations allow one to interpret the spectroscopic signature of the ICA and to develop a self-consistent model for this transformation, both of which had not been previously possible. As a result of the film thinning and densification, both the interlayer spacing and the effective index of refraction change, which affects the spectroscopic signature of the ICA. The spacing between layers of polystyrene cores should be reduced by the same amount as the total film thinning because the volume change occurs purely normal to the substrate. This fractional decrease in film thickness will be referred to as H and, as mentioned, was measured to be 0.72 by profilometry. In the original CC, the interlayer spacing is d111 ) (2/3)1/2 × 306 nm ) 250 nm. When this value is reduced to 72% in the ICA, the interlayer spacing is dICA ) 180 nm. The effective index of refraction of the ICA is determined by polystyrene and poly-HEMA and their relative volume fractions. These are the only two components of the film because all porosity is lost in the ICA and all the toluene absorbed in the course of the transformation is released upon drying (Figure 7). For the ICA, neff is given by neff ) (90% nPS2 + 10% npoly-HEMA2)1/2 ) 1.582. Therefore the ICA peak position is expected at

λmax ) 2neff d111 ) 2(1.582)(180 nm) ) 570 nm in excellent agreement with the experimental value of 568 nm from the spectrum of dry ICA films (Figure 6). While the general structure is now clear, direct imaging of the ICA samples (Figure 9) may be utilized to develop both a qualitative understanding of the transformation mechanism and a quantitative description of the final ICA morphology. TEM data suggest that, contrary to the CC which is a closed-packed structure, the polystyrene cores in the ICA are smaller, well separated, and encased in a polymer matrix. This must mean that some of the polystyrene originally in the cores of the CC has relocated to become part of the matrix surrounding the smaller cores

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of the ICA. We believe this process happens by means of the fluidification of polystyrene by toluene. The fluid polystyrene can fragment and permeate the poly-HEMA shell eventually filling the film porosity and joining with other fluid polystyrene to form a continuous matrix. The thinning of the film is due to the loss of the air-filled pores present in the CC. When the toluene escapes the film during drying, it leaves behind a dense film composed of smaller polystyrene cores encased in a matrix containing both polystyrene and poly-HEMA. The shift to shorter wavelength associated with the drying of the ICA (Figure 6) is most likely due to the thinning of the film which can reach the final value only when all toluene has left the sample. The first part of this mechanism is very similar to the ones postulated previously.16 Detailed characterization of the ICA morphology, however, allowed us to present a deeper understanding of the full transformation process. We can use a variety of methods to quantify the redistribution of materials within the film over the course of the transformation. We begin by estimating the size of the polystyrene ellipsoidal cores present in the ICA. The image in Figure 9c suggests that the polystyrene cores have lost their spherical shape due to the thinning of the film during the transformation. Because the film is attached to the substrate, it is not expected (or observed) to swell or shrink in the plane of the substrate. However, the thinning of the film measured by profilometry is a result of the loss of porosity and is expected to introduce some perpendicular anisotropy in the ICA. This translates into ellipsoidal cores with equivalent long axes parallel to the plane of the film and with a short axis perpendicular to the film that is compressed to 72% of the original value. As a result, when films are imaged from the top (Figure 9b), both ellipsoidal long axes have the same length and the colloids appear circular. When the cores are loosened from the surrounding material and are allowed to rotate freely, as shown in Figure 9c, the eccentricity is more obvious. In more quantitative terms, the long axes of the ellipsoidal cores (Figure 9b) measure 277 nm. The size of the short axis for these ellipsoidal cores could range between 72% of the original polystyrene core diameter (212 nm) and 72% of the length of the long axes of the ellipsoids imaged in Figure 9b (199 nm). The average eccentricity measured over several particles was 0.91, a value that is nicely between the maximum possible eccentricity of 0.72 or 0.77 (using a short-axis length of 199 or 212 nm, respectively) and the minimum possible eccentricity of 1. The specific value measured for each colloid depends on the exact orientation of each individual core. This combination of thickness measurements and direct imaging can now be used to calculate the distribution of polymers in the final structure. The fractional volume change γ for each colloid as it transforms from a spherical polystyrene core in a core-shell particle to a polystyrene ellipsoid is given by

γ)

ICA V ellipsoid CC V sphere

The volume of the ellipsoidal polystyrene core in the ICA is given by

π π ICA V ellipsoid ) a2b ) a3H 6 6 where a is the long axis of the ellipsoid, b is the short axis,

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and H is the fractional thinning measured by profilometry, that is, H ) 0.72. For this calculation, we will assume b ) aH. For the spherical core in the CC CC V sphere )

4π D 3 CC φ 3 2 PS

()

Here D is the particle diameter and φCC PS ) 0.90 represents the volume fraction of PS in the core-shell particles. The fractional core shrinkage γ is 0.593, which is a clear indication that indeed the cores have lost a significant portion of polystyrene to the matrix. We can now calculate the volume fraction of the total ICA : film volume occupied by the polystyrene cores φellipsoid ICA φellipsoid )

74% φCC PS γ ) 0.55 H

where 74% is the filling ratio of the close-packed CC, and the factor of H in the denominator represents the shrinkage in film volume due to film thinning. Clearly, the ellipsoidal polystyrene cores are far from close packed and occupy only about 55% of the ICA volume. To estimate the composition of the matrix, which contains both polystyrene and poly-HEMA, we can carry out a similar calculation for the amount of polystyrene transferred to the matrix as a fraction of total film volume.

) φICA,matrix PS

74% φCC PS (1 - γ) H

And similarly for poly-HEMA, all of which ends up in the matrix, ICA,matrix ) φp-HEMA

CC 74% φp-HEMA H

CC ) 0.10 is the volume fraction of polywhere φp-HEMA HEMA in the original core-shell particles. From these relationships, it is easy to calculate that the matrix composition is roughly 79% polystyrene and 21% polyHEMA. When the larger value for the short axis of the ellipsoidal cores is used, a similar result is obtained and the matrix composition is 77% polystyrene and 23% polyHEMA. The high concentration of polystyrene in the matrix is explained by the blocklike nature of the polymer of the original colloidal particles. In the emulsion polymerization reaction, styrene polymerizes before HEMA, resulting in polystyrene chains covalently linked to the poly-HEMA-rich blocks that make up the shell. When the CC is exposed to toluene and the polymer chains are allowed to redistribute, the poly-HEMA-rich blocks drag some of the covalently bound polystyrene away from the toluene-rich cores, which produces a matrix that is moderately dilute in poly-HEMA. This result confirms several experimental observations. First, because the ICA is composed of a continuous matrix of polystyrene and poly-HEMA, the film is fully interconnected, flexible, resilient, and removable from the glass substrate as a continuous film. Second, the chemical resistance of the ICA to a methanol soak (which destroys the CC) should not be surprising since poly-HEMA is only a minority component in a matrix containing mostly polystyrene. Vigorous ultrasonication in methanol was required to be able to image the thinnest portions of the ICA as shown in Figure 9b,c. Even under these extreme conditions, the bulk of the film appeared unaltered.

Figure 10. Schematic of the mechanism for the transformations. The reversible transformation involves an equilibrium between step 1 and step 2. When the amount of toluene exceeds a threshold value, the transformation is irreversible and proceeds to the following steps. The morphological features are not drawn to scale; the poly-HEMA shell thickness has been magnified for clarity.

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thus to quantitatively compare the intensity of its reflectance peak with the most recent theoretical models for the optical properties of colloidal crystal arrays.36 These models predict that the ICA peak intensity should be 80120 times less intense than the CC peak, depending on film thickness, in good agreement with the experimental observation of an 85× decrease in peak intensity in going from CC to ICA. The fact that these models are in very good agreement with our findings as well as with the results originally reported by Chen et al.24 corroborates the proposed morphology for the ICA. Conclusions

Figure 11. SEM image of the top surface of the ICA. On the ICA film surface, a collection of hexagonally packed dimples are visible. This ICA sample was prepared on a silicon wafer, and the image was collected at a 40° angle to enhance the depth of the dimples. This results in the horizontal distortion of the hexagonal pattern visible in the SEM image. The ICA surface was also characterized by AFM as shown in the inset. The dimples have a depth of approximately 25 nm. The scale bar is the same for both images.

The proposed mechanism for the transformations described in this work is summarized by the scheme of Figure 10. When a CC is exposed to small vapor pressures of toluene, the particle cores are reversibly swollen in the direction perpendicular to the substrate. If higher vapor pressures of toluene are used, the swollen colloids can rupture and the transformation progresses irreversibly from this point. Fluid polystyrene fills the film porosity and is partially transferred from the cores to the surrounding continuous matrix. With sufficient swelling, polyHEMA containing chains and toluene-swollen pure polystyrene can completely separate. As the toluene evaporates from the pure polystyrene domains, this process results in a thinner ICA film composed of smaller polystyrene cores embedded in a continuous matrix containing both polystyrene and poly-HEMA. The model for the transformation presented here and the structure proposed for the ICA are consistent with SEM imaging of the ICA surface published by Chen et al.24 and confirmed here (Figure 11). The surface of the ICA shows hexagonally packed holes or dimples arranged on the same lattice as the top layer of particles in the original CC. These holes were measured to be about 25 nm deep by atomic force microscopy (Figure 11, inset). As the toluene evaporates from the swollen PS/toluene core, the top half of the spheres can “deflate” into a convex dimple (see Figure 10, step 4). This would leave a collection of round holes on the top surface as shown in Figure 10. The preponderance of surface polystyrene measured on the ICA relative to the CC by X-ray photoelectron spectroscopy in previous studies25 is unsurprising since the poly-HEMA originally present on the surface of the CC particles is mixed with polystyrene in the ICA matrix. The above estimate of the composition of the matrix allows one to calculate its average index of refraction and

We have studied the morphological transformations occurring in a thin-film colloidal crystal of core-shell particles upon exposure to toluene vapors. A reversible transformation was observed when the toluene vapor pressure was 16.5 Torr or less. When the vapor pressure of solvent exceeds that threshold value, the colloidal crystal undergoes an irreversible transformation. Upon evaporation of the toluene, a new nanostructured material, an interconnected colloidal array, is generated. The mechanism for the transformation involves solubilization of the polystyrene cores by toluene and subsequent filling of the interstitial voids with the fluidized polymer. The CC presented in this study is reversibly swollen by toluene vapors, and its absorption peak is markedly shifted to a longer wavelength. The ability to undergo this process many times makes this system a good candidate for the detection and quantitation of moderate amounts of toluene and similar pollutants in the atmosphere. The irreversible transformation from CC to ICA generates a nanostructured material with interesting optical and mechanical properties with minimal intervention from the experimenter. Our work expands the range of methods that can be used to generate photonic materials from core-shell colloids using spontaneous processes. Indeed, our results and those of others21,22 suggest that such transformations are quite general and should be extendable to many related systems. Because many materials with complex nanoscale architectures can only be prepared by involved multistep processes, this type of spontaneous transformation represents an important potential route to the facile synthesis of new robust photonic materials. Acknowledgment. We heartily thank Birgitta Sjostrand of the EM Core Facility of UCLA for her help with thin sectioning and staining of the ICA samples. We also thank Dr. Jeong-Yeol Yoon and Professor Robin Garrell for their assistance with the QCM experiments and Erik Richman for collecting the AFM images. Dr. Yiyan Chen is acknowledged for helpful discussions in the course of the experimental work. This work was supported by the Office of Naval Research under Grant N00014-99-1-0568. S.H.T. is an Alfred P. Sloan Foundation Research Fellow. LA034617G (36) Mittleman, D. M.; Bertone, J. F.; Jiang, P.; Hwang, K. S.; Colvin, V. L. J. Chem. Phys. 1999, 111, 345.