Programmable Phase Transitions in a Photonic Microgel System

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Programmable Phase Transitions in a Photonic Microgel System – Linking Soft Interactions to a Temporal pH Gradient Dennis Go, Dirk Rommel, Lisa Chen, Feng Shi, Joris Sprakel, and Alexander J. C. Kuehne Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04433 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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Programmable Phase Transitions in a Photonic Microgel System – Linking Soft Interactions to a Temporal pH Gradient a

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Dennis Go, Dirk Rommel, Lisa Chen, Feng Shi, Joris Sprakel and Alexander. J. C. Kuehne* a

DWI – Leibniz Institute for Interactive Materials, RWTH Aachen University, 52074 Aachen, Germany. E-mail: [email protected] State Key Laboratory of Chemical Resource Engineering & Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, China. c Laboratory of Physical Chemistry and Colloid Science, Wageningen University, 6703 HB Wageningen, The Netherlands. b

Soft amphoteric microgel systems exhibit a rich phase behavior. Crystalline phases of these material systems are of interest because they exhibit photonic stop-gaps, giving rise to iridescent color. Such microgel systems are promising for applications in soft, switchable and programmable photonic filters and devices. We here report a composite microgel system consisting of a hard and fluorescently labeled core and a soft, amphoteric microgel shell. At pH above the isoelectric point (IEP), these colloids easily crystallize into three-dimensional colloidal assemblies. By adding a cyclic lactone to the system the temporal pH profile can be controlled and the microgels can be programmed to melt, while they lose charge. When the microgels gain the opposite charge they recrystallize into assemblies of even higher order. We provide a model system to study the dynamic phase behavior of soft particles and their switchable and programmable photonic effects.

Introduction The potential of colloidal building blocks for applications in the optical sciences has been recognized since the description of opaline geometries produced via spontaneous self-assembly of 1,2,3 monodisperse colloids . Glassy and crystallized colloidal systems can be used, when the particles have diameters on the order of the wavelength of visible 4 light. Such systems are termed photonic glasses and 5,6 photonic crystals and allow the generation of colour 7,8 by isotropic diffraction or angle-dependent iridescence, respectively. In principle, this colloidal 9 self-assembly approach is compatible with roll-to-roll 10,11 and ink-jet printing technology; however, problems with defects in the assembled structure have prevented the translation of this technology into real world applications to date. The systems applied usually comprise hard-sphere-like particles with shortranged interactions. However, using hard-sphere-like building blocks for the generation of particle derived self-assembled geometries entails several restrictions: (i) The formation of crystalline structures requires a very narrow size distribution with sufficiently high uniformity to warrant nucleation. (ii) Hard-sphere-like particle interactions are purely entropic, which limits their phase behavior and often drives self-assembled structures into meta-stable glassy trapped states, 12 resulting in defects. (iii) Grain boundaries, dislocations and cracks in these self-assemblies often corrupt the optical performance of crystallized particle

systems. These defects are caused by intrinsic 13 effects , such as the irreversible nature of the 14,15 colloidal self-assembly process. Effectively, the magnitude of the attractive forces is so large, that non-equilibrium and kinetically trapped structures can form. To generate high-fidelity self-organized structures, nature employs weak interactions, while maintaining its systems close to equilibrium to remedy 16 faults and defects in the generated architectures. Therefore, it is desirable to create a more dynamic assembly process to be able to minimize or eradicate defects. We and others have previously reported hardcore soft-shell microgels, which carry charges and can be programmed in their aggregation behaviour via 17,18 pH. Microgels represent a promising alternative to hard-spheres as soft matter systems for photonic applications. Microgel networks composed of poly(Nisopropylacrylamide) (PNIPAM) reveal a rich phase behavior, which is mostly due to their critical solution behavior entailing a specific swelling and shrinking 19 capability. Swollen PNIPAM microgels collapse when heated above the lower critical solution temperature (LCST). At this point the interactions between the hydrophobic moieties become dominant releasing entrapped water molecules and leading to collapse 20–23 and shrinkage of the swollen polymer network. The crystallization and melting of PNIPAM microgels based on traversing of the LCST has been studied and the kinetics of these phase transitions are well 19,24,25 understood. Here, it is possible to anneal defects in the colloidal microgel assembly via thermal 26 treatment to improve the quality of crystal lattices.

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The open network structure of the microgel shell allows incorporation of large amounts of functional moieties, outperforming hard spheres in functionality density, where only the surface can be covered. When equipped with charges, microgels exhibit long-ranged electrostatic interactions, short-ranged steric repulsion as well as responsiveness to externally applied 18,27,28 fields. Soft microgels easily self-assemble into ordered crystalline arrays as a reaction to external stimuli, such as temperature, electric field, light or pH, 29,30 depending on the incorporated active moiety. The dynamic transition between the crystalline and liquid state of such soft particles by means of charge control is largely undeveloped. Control over soft inter-particle interactions permits morphological switching, allowing detailed study of the phase transition kinetics and 31 enables novel photonic applications. Dynamic near-equilibrium assembly can be achieved using a slowly degrading lactone gradually acidifying 18 the medium. This gradual release of acid entails a slow assembly process and the ability of the microgel assembly to heal out defects. Furthermore, due to their soft character, microgel systems display higher 32,33 tolerance towards external perturbations and are 34 more forgiving towards structural defects. Here, we report a programmable amphoteric system, which - driven by electrostatic repulsions - selfassembles into a soft colloidal crystal. Using a temporal pH gradient provided by spontaneous hydrolysis of a lactone, the crystal first melts and then reassembled via kinetically controlled assembly into highly ordered colloidal crystals with a photonic stopgap or reflection band in the visible spectrum (see Figure 1a). This system provides highly crystalline microgel structures with a much higher quality than those assembled without the intermediate liquid state, while circumventing the drawbacks of conventional photonic crystals assembled from colloidal hard spheres.

Results and discussion To produce a soft programmable photonic system we first synthesize amphoteric core-shell particles as 18 described before (see SI for synthesis). This coreshell structure contains a fluorescently labelled poly(methyl methacrylate) (PMMA) core to enable imaging via fluorescence microscopy and a nonfluorescent charged, swollen poly(Nisopropylacrylamide) (PNIPAM) microgel shell (see Figure 1b and c). This geometry generates sufficient refractive index contrast between the hard core particles and the surrounding matrix, to enable application in photonics. The charges are incorporated by co-polymerizing acrylic acid for negative charges in

Figure 1: Particle characterization: a) Schematic of the negative charge stabilized crystalline microgel assembly. The decaying acidification compound decreases the pH, which results in protonation of the microgel and reduction in negative charge. At the isoelectric point the microgel particles are amorphous as there is no electrostatic repulsion forcing the microgels into a hexagonal array. Further protonation results in positively charged microgels and reassembly in to crystalline assemblies. b) Confocal microscopy image of periodically arranged microgels consisting of a fluorescently labelled core particle surrounded by an amphoteric microgel shell. The scale bar represents 2 µm. c) TEM images of the same particles in dried state and liquid state (imaged using cryo-TEM in the inset). The contrast between the dense core particle (grey solid circle) and the collapsed microgel network (indicated by white dashed lines) displays their core-shell structure. Scale bars represent 1 µm. d) pH decay kinetics induced by the hydrolysis of 25 mM GDL (green open squares), 25 mM GGL (grey open triangles) and 25 mM MF (grey open trapezoids) in water. Dynamic electrophoretic mobility as a function of time upon gradual pH change via GDL hydrolysis (green squares). The Inset shows the electrophoretic mobility (blue diamond) and size change (blue cirles) of amphoteric core-shell particles as a function of static pH with an IEP at around 3.7.

the microgel shell. Positive charges are introduced by sub-stoichiometric post-synthetic EDC/NHS-coupling of N,N-dimethylethylendiamine. This postfunctionalization provides the particles with amphoteric charge characteristics as the tertiary amine can carry a positive charge, while sufficient acrylic acid groups remain non-functionalized to

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Figure 2: Confocal microscopy images of the microgels and their melting and crystallization transition at different points in time. At t = 2 min the first liquid nuclei become apparent. The sample is fluid for 3 min < t < 44 min. Scale bars indicate 2 µm. 18

provide negative charges. The incorporation of both types of chargeable moieties leads to a specific response to the pH of the surrounding medium, which can be followed via electrophoretic mobility (EPM) measurements. Static EPM measurements at fixed pH reveal an equi-Coulombic charge profile, where the particles have a EPM magnitude of 2 µmcm/Vs between a pH range of 2.5 - 5.5 (see Figure 1d inset, blue diamonds). The isoelectric point, where the amount of positive and negative charges is equal and the particle has a zero net charge, is at pH ̴ 3.7 (see inset of Figure 1d, blue diamonds). Associated with the change in charge, the microgels exhibit variation in the degree of swelling. When the microgel shell is carrying charge the electrostatic repulsions leads to an extended network. At the IEP, when effectively the microgel is uncharged, the network relaxes and is less swollen (see inset Figure 1d, blue circles). The sequential synthesis allows independent tuning of core particle size, degree of functionalization in the shell and the scattering profile of the core-shell system by tailoring the core-shell size and refractive indices. The resulting core-shell particles are monodisperse and easily crystallize at a sufficiently high concentration (Figure 1b). Their core-shell structure is evidenced by the fact that the fluorescent cores are separated by a well-defined distance, which is occupied by the unlabeled microgel shell, as can be imaged via confocal microscopy. Transmission electron microscopy (TEM) further substantiates this observation. A clear difference in contrast between core particles and the collapsed and less dense microgel corona is apparent (see white dashed circles in Figure 1c). Cryo-TEM measurements also show a clear contrast between the water-insoluble PMMA cores (see grey circle in Figure 1c inset) and the swollen microgel shell (see white dashed line in Figure 1c inset). Dynamic light scattering (DLS) of only the core particles gives a hydrodynamic diameter of d = 215 nm, while the particles functionalized with a microgel shell have d = 1.1 µm when dispersed in

Figure 3: Calculation based on confocal microscopy imaging data displays bond order parameter Ψ6 versus time (top) and spatial correlations of refractive indices (bottom) for crystalline (I & III) and liquid states (II).

deionized water at pH 5.5. These sizes correspond well with the values obtained from confocal microscopy and TEM. To gain insight into the phase transition behaviour of our microgel system at higher concentrations, we turn to dynamic systems based on esters. Such esters hydrolyze with different decay kinetics into acids to 18 decrease the pH of the system. We choose methyl formate (MF), D-(+)-glucuronic acid γ-lactone (GGL) and glucono-δ-lactone (GDL) as acidifying compounds with different decay kinetics. All compounds show biexponential decay kinetics with fast decay at the beginning of the experiment and a decay rate constant of τ1 = 0.7 min. MF and GGL decompose within a few minutes and lead to static pHs of 4.1 and 3.5 respectively (τ2 = 0.0007 min for MF, τ2 = 0.0138 min for GGL). This behaviour is due to the different pKa 35 acid dissociation constants of the compounds. The best decay kinetics delivers GDL with a significantly higher τ2 of 35.8 min. GDL gives us temporal control over the phase behaviour of the composite microgel assemblies and we reach a sufficiently low pH for gaining positive charge in our microgel system. Moreover, the GDL decomposes slow enough so that we can follow the melting and rearranging processes using confocal microscopy. This is not possible with the MF and GGL, which is why we continue our study with GDL as am acidifying compound (see Figure 1d). While the GDL hydrolyses we record the EPM of the system, which reflects the dynamic change of the microgel charge over time (see Figure 1d, green full squares). The microgels change their charge from negative to neutral after about 10 min, and then gain positive charge over 30 min until a stable EPM is obtained. In this dynamic measurement the isoelectric point, where the microgels are neutral, is at pH = 3.6,

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Figure 4: Macroscopic experiment to follow the photonic response after melting and recrystallization. The sample is loaded into a well plate and irradiated with a white light source to create Bragg diffraction at a fixed angle. The macroscopic behaviour corresponds with the processes on the microscopic scale (cf. Figure 2). i) Angle dependent reflection spectra of the assembled microgel photonic crystal in (h). The inset shows the relation between the reflectance maximum and the reflection angle θ in accordance with Bragg’s law for fcc crystals in the [1 1 1] direction. The data points are colour coded for facile comparison with figure i.

which corresponds well with the value obtained in the static measurement; this indicates that the temporal response of the microgel charges is governed only by the GDL decomposition kinetics and that charging or discharging of the ionic groups of the microgel is instantaneous. When we follow the particle system on the confocal microscope, we observe a crystalline microgel structure with a face-centered cubic threedimensional symmetry, characterized by twodimensional crystal planes, in which the particles are clearly ordered in a hexagonal pattern (Figure 2 and Movie 1 in ESI). In this state, all particles have like charges resulting in electrostatic repulsion (of intramolecular charged groups as well as between 36 microgels) and a colloidal crystal is formed. When the particles lose their charge due to the decreasing pH (induced by the hydrolysis of GDL), we observe fast crystal melting, with the first isolated amorphous domains appearing already after 2 min. After 10 min the microgels have completely lost their charge, which reduces Coulombic repulsion and entails a decreased volume fraction. The microgels are mobile and exhibit Brownian and concerted fluid motion (see (Figure 2, t = 12 min and Movie 2 in the ESI). When the microgels gradually acquire positive charge, they slowly reassemble into a crystalline array. To quantify the quality of the crystalline arrangement and to probe temporal changes in the degree of order we find the positions of all particles in the field-of-view in our confocal microscopy images using established particle 37 tracking algorithms. We first compute the 2D bond order parameter Ψ6, which probes the degree of local hexagonal order, as a function of time; Ψ6 = 1 corresponds to a perfect hexagonal plane, while lower values indicated increasing degrees of disorder. Melting of the microgel assembly is signified by a sharp transition to low Ψ6 values (Figure 3 top). After some time, recrystallization occurs from the fluid

state; interestingly, this is significantly slower than melting, in agreement with nucleation theory as the energy barrier for crystal nucleation is higher than that of liquid nucleation from a superheated state. This is exacerbated by the fact that the slope for the decrease in pH is much smaller for recrystallization, as compared to the melting regime (see Figure 1d). The transition from crystal-fluid-crystal allows annealing and healing of defects and the complete removal of sample-loading history. As a result, the reformed crystals are virtually defect free and exhibit a much higher degree of order as evidenced from the higher value of the bond-order parameter in stage III (see Figure 3, top). To extend this analysis, we use the particle coordinates to compute the real-space refractive correlation function. To do so, we first use the particle coordinates and our knowledge of the particle core-shell architecture to create a refractive index map of the sample; for the PMMA core we use a refractive index of 1.48, for the highly-swollen hydrogel shell a refractive index of 1.40 and 1.33 for the aqueous medium. Spatial autocorrelation, within the imaging plane, of the refractive indices, gives a real-space representation of the scattering contrast correlations that give rise to Bragg reflections. For the initial crystalline state we observe clear correlation peaks, indicative of an ordered arrangement. We note that the peaks are blurred in the angular direction, indicative of defects and a non-perfect hexagonal structure, possibly due to lattice deformations during sample loading (Figure 3-I). In the liquid state, no peaked correlations are found, but only an isotropic liquid structure, reminiscent of Scherrer rings, in which no Bragg scattering or photonic bandgap may be expected (Figure 3-II). As the crystal reforms gradually from the liquid state, we again observe clear Bragg-like peaks in the scattering contrast. Interestingly, these peaks have an isotropic Gaussian shape, the width of which is determined by the particle scattering cross-

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section, indicative of near perfect crystalline order with virtually no extended defects or lattice deformations (Figure 3-III). The scattering contrast patterns indicate that the microgel system can be programmed for photonic applications, where the system is initially iridescent and colored, becomes opaque and colorless and then regains its color and reflective quality. We observe this melting and recrystallization transition on the macroscale. The microgels are loaded into a well-plate compartment and are irradiated using a broadband white light source (Figure 4). The iridescent photonic effect is evoked by the refractive index contrast between the PMMA core particles as the swollen PNIPAM microgel shell, while the microgel refractive index is almost identical to that of the surrounding 18,24 water. Due to the spatial periodic distribution of the colloids and the resulting Bragg diffraction we observe colored reflection from deposited droplets carrying our programmable microgel system. (The charge in color across the images in Figure 4 a-h is caused by the curved surface of the liquid droplet we image). Depending on the angle of irradiation the reflection wavelength maximum changes (see Figure 4i). For smaller angles θ, the reflection peak is shifted towards longer wavelengths. We plot the data and obtain the theoretically predicted linear dependency for a face-centered cubic geometry and reflectance of the (111) direction: ߣଶሺଵଵଵሻ = 2 2ൗ3 ݀ଶ ሺ݊ଶ − sinଶ ߠሻ (see inset Figure 4i).

Technology CPT, which was supported by the EU and the federal state of North Rhine-Westphalia (Grant EFRE 30 00 883 02).

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Conclusions The development of a composite microgel system with a programmable photonic response allows detailed insight into phase transitions, and presents potential for new photonic materials. Their pH-dependent charge behaviour allows fine-tuning of inter-particle interactions and control of the corresponding photonic effect. Upon changing the pH of the system we are able to melt and grow the colloidal crystal. With such smart colloidal building blocks at hand it will in the future be possible to couple soft photonic systems to oscillating pH or feedback system. Incorporation of photo-responsive constituents to the microgel system could lead to self-organizing soft photonic materials.

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Acknowledgements The authors acknowledge funding by the Deutsche Forschungsgemeinschaft DFG through the collaborative research center SFB 985 ”Functional microgels and microgel systems”. This work was performed in part at the Center for Chemical Polymer

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