VOLUME 104, NUMBER 27, JULY 13, 2000
© Copyright 2000 by the American Chemical Society
LETTERS Thermoresponsive Photonic Crystals Justin D. Debord and L. Andrew Lyon* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: March 31, 2000; In Final Form: May 5, 2000
Hydrogel nanoparticles that undergo volume phase transitions in response to changes in temperature and pH have been used to assemble colloidal crystal gels with environmentally tunable optical properties. When monodisperse, ∼210-nm-diameter hydrogel particles are close-packed via centrifugation, the resultant viscous polymer pellet displays a bright iridescence in the visible region of the spectrum. This iridescence can then be modulated via temperature changes, which induce the component nanoparticles to undergo thermo-initiated volume phase transitions. More importantly, these crystals undergo a completely reversible order-disorder transition in response to larger temperature fluctuations; the crystal can be processed in its disordered (solution) state and then reformed to the iridescent crystal spontaneously upon cooling. The preparation and initial characterization of these materials are presented.
The assembly of nano-, micro-, and meso-scale objects into ordered superstructures continues to gain widespread interest in numerous scientific areas. By designing specific chemical, optical, or mechanical properties into the particulate building block, control over the properties of the resultant macroscopic assembly can be obtained. Particulate materials also offer numerous advantages in the preparation of composites and laminar structures. Current investigations into colloidal assemblies range from the fundamental to the applied. For example, fundamental studies have helped to elucidate the dominant factors involved in self-assembly as a function of particle shape and surface chemistry.1-6 These assemblies have found applications in a wide range of disciplines, including optical devices (especially photonic band-gap materials),7-10 biosensors,11,12 genomics,13 and bio-separations.14 As with the potential applications, the methods by which colloidal materials can be assembled are extremely diverse. Highly complex ordered structures can be created through asymmetric repulsion or attraction. For example, the Whitesides group has created large-scale assemblies of polymer objects through the spatial manipulation * Author to whom correspondence should be addressed. E-mail:
[email protected].
of the object’s solvophobicity or charge.1,2,15 Template-directed synthesis of metal nanostructures with complex shapes has been accomplished through the careful choice of multivalent organic templates bearing specific chemical affinity for the component nanoparticles.16,17 Finally, the reversibility of biomolecular interactions has been exploited to create large nanoparticle structures; metal and semiconductor nanoparticles have been assembled through directed oligonucleotide hybridization.13,18 A different approach involves the assembly of essentially noninteracting nanoparticles into close-packed arrays. Traditional colloidal crystallization methods based on sedimentation or particle repulsion have been exploited extensively for the creation of photonic materials. These techniques typically rely on nonspecific particle-particle repulsion to induce order. For very short-range repulsive forces (hard-sphere interactions), close-packed crystalline arrays are typically formed. In these cases, order arises from the propensity of the repulsive spheres to maximize the particle-particle distance (or minimize the particle-particle contact area); maximization of this spacing results in the formation of a crystalline lattice. Particle assembly via this method is attractive in terms of both simplicity and versatility. For example, close packing can be accomplished
10.1021/jp001238c CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000
6328 J. Phys. Chem. B, Vol. 104, No. 27, 2000 through slow particle sedimentation,19 centrifugation,5,20-22 spin coating,23 electrophoretic deposition,24,25 sonication,26 and lateral compression of interface-bound monolayers.27 Because closepacked order typically involves the concentration of particles into a confined space, these studies often involve hard (oxide, metal, dense polymer) spheres that undergo little deformation upon packing. If particle deformation (asphericity) does occur, the packing efficiency is significantly lowered, thereby resulting in a less crystalline sample. Longer-range interactions can also be exploited to create ordered assemblies. Specifically, suspended colloidal spheres bearing a high surface charge will spontaneously assemble into solvated crystalline arrays when the solution is rigorously desalted (thereby increasing long-range particle-particle repulsion).5,20,28-31 This method, followed by photopolymerization of a monomer solution surrounding the crystal, has been exploited by Asher and co-workers to create polymerized colloidal crystal arrays for optical filtering and sensing applications.32,33 In our research, we have taken an approach whereby hydrogel spheres (∼210-nm-diameter) are assembled into crystalline close-packed arrays via centrifugation. A distinct advantage of this method (as with other sedimentation techniques) is that successful assembly requires only that the particles be monodisperse with respect to size and that particle-particle attraction be minimized. This is in contrast to previously reported methods of hydrogel particle crystallization, in which particles possessing a high surface charge were ordered by strong charge-repulsion effects via rigorous desalting of the medium.5,20,28-31 A similar report has also demonstrated that thermoresponsiVe hydrogel spheres ordered via charge deshielding display small diffraction wavelength shifts and large intensity changes in response to a thermally induced volume phase transition.33 In this work, we have discovered that, if a colloidal crystal is composed of thermoresponsive hydrogel nanoparticles arranged in a closepacked fashion, the resulting assembly will undergo a completely reversible order-disorder phase transition upon crossing the volume phase transition temperature of the particle. In the ordered state, the material is highly iridescent and viscous; a sharp Bragg diffraction peak is observed. Upon being heated to the phase transition temperature (∼32 °C), the crystal becomes a disordered, turbid, colorless fluid. However, when the solution is cooled back to room temperature, the crystal spontaneously reorders to a degree of order that is equal to or greater than that of the original crystal (as indicated by the width of the Bragg peak). Because of the remarkable tendency of the crystal to reorder, the material can survive extensive physical and chemical manipulation without degradation of the photonic properties. This description of a simple and rapid self-assembly method for a processable photonic material represents a new tool for the construction of functional superstructures from polymeric nanomaterials. Poly-N-isopropylacrylamide (NIPA)/acrylic acid (AA) (90/ 10%) copolymer hydrogel nanoparticles were prepared via aqueous, free-radical, precipitation polymerization.34,35 Relatively low cross-linker (N,N′-methylenebisacrylamide, BIS) concentrations (∼2%) were used, thereby producing low-density network polymers that are solvent-swollen at room temperature (>90% water by volume). The NIPA portion of the polymer is responsible for the thermoresponsive characteristics of the material. Below the phase transition temperature [lower critical solution temperature (LCST)]), the material is highly solventswollen, but becomes partially or wholly desolvated (depending on the details of the polymer structure) when heated above its characteristic phase transition temperature. For macroscopic
Letters monoliths of the hydrogel, the transition is visibly evident as a dramatic shrinkage of the hydrogel. Similarly, the AA groups introduce a pH dependence to solvation of the copolymer hydrogel. When the AA groups are deprotonated (pH > 5), increased polymer hydrophilicity and Coulombic repulsion between polymer chains causes further swelling of the hydrogel and suppression of the volume phase transition. Conversely, upon neutralization of the acid groups (pH < 5), hydrogel thermoresponsivity is maximized. Because our materials have submicron dimensions, it is necessary to use indirect methods to monitor the phase transitions. Accordingly, characterization of hydrogel particle diameter and polydispersity was accomplished via temperature-programmed photon correlation spectroscopy (TP-PCS, Protein Solutions Inc) of dilute particle suspensions. This technique (when combined with high-quality fitting algorithms, as employed herein) has been proven to be quite accurate in the determination of hydrogel particle diameters.36 For solvated polymer particles such as those described in this paper, this technique is ideal for size determination, as it can be performed on dilute aqueous solutions with only minor sample preparation. This is in contrast to electron or probe microscopy techniques for which samples can be difficult to prepare reproducibly and the measured particle size is perturbed by polymer dehydration and substrate-induced deformation. Because we are interested in the behavior of solvated spheres in aqueous media, those perturbations prohibit an accurate size determination of these materials as a function of solution conditions. Furthermore, the temperature-programmed aspect of the technique allows for accurate analysis of the temperaturecontrolled solvation of p-NIPA-co-AA particles.37 Because these materials are thermoresponsive at acidic pH, it is necessary to obtain a precise measure of the particle diameter as a function of temperature. Specifically, at the LCST, a p-NIPA hydrogel will undergo a volume phase transition that involves hydrophobic chain-chain association and the concomitant expulsion of water from the network. TP-PCS measurements show that 210-nm-diameter p-NIPA-co-AA particles prepared with a 2% cross-linker concentration collapse to ∼140-nm in diameter upon crossing the transition temperature. Figure 1a shows the variation of the mean hydrodynamic radius as a function of solution temperature for a dilute particle suspension following adjustment of the pH to 3.4 (protonated AA groups). Particle sizes were obtained by fitting collected correlelograms to a distribution of diffusion coefficients. These values were then converted to a range of particle sizes via the Stokes-Einstein relationship. Data fitting was accomplished with a proprietary regularization algorithm that accounts for multimodal populations and nonGaussian distributions (Protein Solutions). From the TP-PCS data, it is evident that the phase transition is broad (occurring over a ∼3° span). Because macroscale monoliths of these polymers are typically observed to undergo first-order, discontinuous phase transitions, it is notable that the colloidal suspension exhibits only pseudo-first-order behavior (continuous change in size with temperature). We tentatively ascribe this observation to a degree of size and structure polydispersity in our preparations. Another measure of the size polydispersity is offered by PCS size distribution histograms (Figure 1b). As can be seen from the peak width of the histogram, our preparations yield polydispersities that are less than 15%. Having established the size, size distribution, and responsivity of the nanoparticles, we set out to investigate the creation of ordered (crystalline) assemblies of the hydrogels. As described above, the literature is rich with examples of polymeric colloidal crystals formed by chemical or mechanical means. Indeed,
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Figure 1. Temperature-programmed photon correlation spectroscopy analysis of p-NIPA-co-AA hydrogel nanoparticles at a solution pH of 3.4. (a) Particle diameter variation as a function of temperature is shown. A continuous change in size is observed, with a sharper decrease in particle size occurring between 30 and 34 °C. The apparent size increase at higher temperatures reflects a small amount of aggregation following polymer phase separation. The line is drawn to guide the eye. (b) Particle size histogram for the same particles at 25 °C. The apparent polydispersity is ∼14%.
highly charged (sulfonated) versions of similar particles (pNIPA-co-2-acrylamido-2-methyl-1-propanesulfonic acid) have been used to assemble aqueous suspensions of thermoresponsive colloidal crystals by charge deshielding.33 In our method, an aqueous polymer nanoparticle solution (pH ) 3.4, adjusted with HCl) is placed in a 1.5-mL centrifuge tube and spun in a microcentrifuge (Eppendorf) at 16 000g for 30-60 min at room temperature. Following centrifugation, a viscous, blue-green polymer layer is observed at the bottom of the tube. This polymer pellet is then stabilized against redispersion by simply decanting off the supernatant. To interrogate the optical properties of this material, the viscous, iridescent liquid was spread between two glass microscope slides that comprised the faces of a 1-mm-path-length thin-layer cell. Upon standing for 1 to 2 h at room temperature, the polymer developed an even more intense green-red iridescence. A representative visible spectrum of this material is shown in Figure 2a, in which a sharp peak in
Figure 2. Visible spectra of the photonic crystals formed by centrifugation from pH 3.4 particle suspensions. (a) Spectrum of the crystal in a 1-mm-path-length optical cell at 20 °C. The λmax for the Bragg diffraction peak is 604 nm with a full width at half-maximum intensity (fwhm) of ∼9 nm. (b) Variation of the Bragg peak at a series of temperatures from 8 to 34 °C. Small changes in the peak position, breadth and intensity are observed below the LCST, followed by disappearance of the peak above the LCST because of crystal disordering. (c) Comparisons of the crystal diffraction before and after a temperature scan. An increase in order is observed following the crystal annealing, as evidenced by the decrease in the peak fwhm.
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the spectrum is observed at ∼605 nm. These optical properties are reminiscent of those observed for colloidal suspensions assembled by charge deshielding of anionic particles. In those cases, the observed color was interpreted as diffraction from ordered colloidal arrays that possessed lattice spacing on the order of visible wavelengths. It has been shown that the optical properties of these colloidal crystals closely follow behavior predicted by Bragg’s law.
mλ ) 2nd sin θ
(1)
In eq 1, m is the diffraction order, λ is the diffracted wavelength, n is the refractive index of the particles, d is the lattice spacing, and θ is the angle between the incident light and the crystal planes. Judging from the similarity of our results to those reported by others, it is apparent that a significant degree of crystalline order must exist for these hydrogel assemblies. Crystallinity is also suggested by the decrease in λmax as the angle of the sample with respect to the beam is changed (data not shown). Such an angle dependence is typically taken as evidence that the color is due to crystalline order at optical length scales. The thermoresponsivity of the component nanoparticles renders the assembly similarly responsive. This behavior is shown in Figure 2b, where, below the LCST, the assembly displays strong Bragg diffraction. As the temperature is raised from 8 to 26 °C, small changes in the particle size and refractive index result in small shifts in the Bragg peak intensity and wavelength. As the temperature approaches and crosses the LCST (26-34 °C), the particles begin to desolvate and collapse, resulting in a large decrease in the Bragg intensity until the assembly becomes colorless. It is important to note that this change in the optical properties reflects a loss of crystalline order due to polymer phase separation, rather than a simple change in lattice spacing. If the assembly retained its crystallinity upon particle collapse, we would expect a shift in the position of the Bragg peak and/or a large increase in its intensity (because of a higher particle refractive index).33 Instead, the peak disappears, indicating that the particles restructure from a crystalline array into a disordered suspension. When the solution is cooled below the LCST, however, the particles rehydrate and spontaneously assemble into a crystal with nearly identical spacing and order to that of the sample prior to heating. This is evident from the complete recovery of the original Bragg peak, as shown in Figure 2c. In contrast to the original color evolution, however, this restructuring takes place on the time scale of minutes, where thermal equilibration of the optical cell appears to be the ratelimiting step. When care is taken to eliminate water evaporation, the order-disorder transition is extremely reversible; we do not observe any large decreases in intensity, any significant peak broadening, or any peak energy shifts that would indicate disordering, even after repeated thermal cycling (>10 cycles). Conversely, the first 4-5 thermal cycles following initial sample preparation result in decreases in peak width (1-4 nm per cycle), increases in intensity (0.02-0.05 AU per cycle), and shifts in λmax (1-5 nm per cycle). The shifts in intensity and peak position are, to a first approximation, indicative of very small changes in particle refractive index and size. However, the observed decrease in the peak width can only be due to an increase in the crystalline order as this value reports directly on the polydispersity in the crystal lattice constant. The observed peak sharpening therefore indicates that thermal cycling of the crystal across the order-disorder transition results in enhanced ordering of the assembly. The ability of these materials to order spontaneously upon rehydration survives extensive manipulation of the sample in
Figure 3. Photographs of a thin (∼1-mm) film of the photonic material between two microscope slides. This film was prepared by filling the space between the slides with the polymer suspension in its fluid disordered state. The sample was then allowed to cool at 4 °C for ∼2 h. Photographs were taken with sample illumination from (a) diffuse room light, (b) white light from a bulb at a 90° angle with respect to the camera, and (c) white light at grazing angle incidence. The variation in color with illumination angle suggests that the color is the result of diffraction from an ordered material.
Letters its disordered (phase-separated) state. We have subjected disordered samples held at temperatures above the LCST to vigorous shaking and sonication with no visually detectable decrease in order following cooling. This remarkable stability, combined with the enhanced fluidity of the disordered material (because of the polymer phase separation), allows us to cast the material into any desired form. Figure 3 shows a film of the material sandwiched between two microscope slides (with ∼1-mm spacers) under different illumination conditions. Changing the angle between the light source and the camera allows for the manipulation of the observed color just as changing the sample angle in the spectrophotometer shifts the Bragg peak. This sample was prepared by heating a centrifuged sample above the transition temperature and then pipetting (with a pre-warmed pipet) the milky suspension between the slides. Upon cooling, the sample assumes the original crystallinity and high viscosity. The thermal processability of the crystal into other geometries (especially thin films) should expand significantly the utility of this assembly method to a wider range of photonic materials applications. We have demonstrated the assembly of water-swollen polymer spheres (hydrogels) into ordered crystalline arrays via centrifugation. In theory, these materials can be constructed with particles of any composition, provided that the polydispersity is low and the particle interaction potential is small. In this case, we have utilized 210-nm-diameter particles of a pHand temperature-responsive hydrogel (p-NIPA-co-AA) to create a photonic crystal that undergoes a thermo-initiated orderdisorder transition. In the ordered state, the material is a viscous solution that is brightly iridescent because of strong Bragg diffraction. When the temperature is raised above the LCST for the component particles, the array becomes a milky-white, disordered, and free-flowing solution. This solution can then be manipulated extensively without destroying the ability of the material to recrystallize upon cooling. Indeed, thermal cycling has been observed to increase the crystalline order. This remarkably simple technique offers a new route to the creation of ordered monolithic or thin-film arrays from soft, environmentally responsive nanomaterials. Acknowledgment. L.A.L. gratefully acknowledges financial support from Research Corporation in the form of a Research Innovation Award and from the National Science Foundation for a CAREER Award. The authors would also like to thank Prof. Robert M. Dickson (School of Chemistry and Biochemistry) and Prof. Mohan Srinivasarao (School of Textile and Fiber Engineering) for many helpful discussions regarding this manuscript. References and Notes (1) Bowden, N.; Choi, I. S.; Grzybowski, B. A.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 5373-5391. (2) Breen, T. L.; Tien, J.; Oliver, S. R. J.; Hadzic, T.; Whitesides, G. M. Science 1999, 284, 948-951.
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