The self-assembly of colloidal nanoparticles into ordered superlattices typically uses dynamic interactions to govern particle crystallization, as these non-permanent bonds prevent the formation of kinetically trapped, disordered aggregates. However, while the use of reversible bonding is critical in the formation of highly ordered particle arrangements, dynamic interactions also inherently make the structures more prone to disassembly or disruption when subjected to different environmental stimuli. Thus, there is typically a trade-off between the ability to initially form an ordered colloidal material and the ability of that material to retain its order under different conditions. Here, we present a method for embedding colloidal nanoparticle superlattices into a polymer gel matrix. This encapsulation strategy physically prevents the nanoparticles from dissociating upon heating, drying, or the introduction of chemicals that would normally disrupt the lattice. However, the use of a gel as the embedding medium still permits further modification of the colloidal nanoparticle lattice by introducing stimuli that deform the gel network (as this deformation in turn alters the nanoparticle lattice structure in a predictable manner). Moreover, encapsulation of the lattice within a gel permits further stabilization into fully solid materials by removing the solvent from the gel or by replacing the solvent with a liquid monomer that can be photopolymerized. This embedding method therefore makes it possible to incorporate ordered colloidal arrays into a polymer matrix as either dynamic or static structures, expanding their potential for use in responsive materials.
Colloidal assembly of nanomaterials presents an attractive means of generating hierarchically organized structures in one-, two-, and three-dimensions.1–3 Precision, complexity, and uniformity in these bottom-up syntheses are typically achieved by using weak interparticle interactions that facilitate particle mobility and rearrangement, thereby preventing the kinetic trapping of disordered states.4–7 While dynamic interactions between nanoparticles are therefore crucial for the formation of highly ordered assemblies, they also make the final structures prone to dissociation.8–10 Even modest changes to temperature or solvent content can completely destroy the structural ordering of colloidal assemblies, and thus their utility is limited to applications amenable to the limited range of conditions that the assemblies are stable in. This limitation is typically overcome by the use of post-assembly processing methods that either crosslink the particles via less labile interactions such as covalent bonds or encapsulate them in a more robust solid matrix. These methods can produce materials that are more mechanically, chemically, and thermally stable than the initial particle arrays, but at the cost of reducing or eliminating the structural dynamicity and responsiveness that are a hallmark of colloidally assembled materials.11,12 Thus, most stabilization techniques restrict the assemblies to one of two states—dynamic but fragile or stable but structurally fixed. Here, we present a method of embedding ordered nanoparticle superlattices in a crosslinked polymer matrix that stabilizes assemblies against lattice dissociation while still allowing for dynamic lattice manipulation. The resulting nanoparticle polymer gel can then either be reversibly deformed with applied mechanical, chemical, or thermal stimuli or be further processed to obtain a fully stable polymer nanocomposite solid. Together, the solution, gel, and solid states provide a spectrum of dynamicity and stability, expanding the conditions under which ordered colloidal nanoparticle superlattices can be assembled and incorporated into functional materials.
The colloidal superlattices synthesized in this work are built from nanocomposite tectons (NCTs), which are nanoscale building blocks that can assemble into ordered nanoparticle superlattices. NCTs consist of inorganic nanoparticle cores decorated with polymer brush chains that terminate in supramolecular binding groups; reversible interactions between the supramolecular groups on adjacent particles dictate their assembly (Fig. 1).13 NCTs containing complementary binding groups will rapidly assemble at room temperature, and thermal annealing (typically at temperatures of ∼30–50 °C) drives the formation of ordered nanoparticle superlattices.14–16 While multiple supramolecular binding chemistries have been used to mediate NCT assembly, ordered particle arrangements require the use of weak, reversible bonds (most commonly complementary hydrogen bonding interactions) to prevent the formation of kinetically trapped structures.17 However, the dynamicity of these reversible interactions makes the NCT assemblies susceptible to dissociation when subjected to stimuli, such as high temperatures, polar solvents, UV light, or shifts in solution pH.13,16,18,19 Additionally, NCTs are also susceptible to uncontrolled collapse upon sudden solvent removal since the solvent plays a structural role in supporting the polymer chains between NCT cores.20
We recently demonstrated that adding large quantities of free polymer to a suspension of assembled NCTs increases the thermal stability of the lattices, allowing their ordered structures to persist across a significantly wider temperature window (e.g., an NCT lattice that would typically dissociate at ∼30 °C in toluene could be stabilized to ∼50 °C in a toluene solution containing 200 mg/ml of ∼60 kDa polystyrene).21 We hypothesized that this added stability would also make the NCT assemblies less susceptible to dissociation upon the introduction of chemical stimuli and would therefore permit synthetic reactions to occur within and around the NCT lattices without disrupting particle organization. Thus, if the free polymer chains added to the NCT colloidal suspension were modified with appropriate functional groups, crosslinking of these free polymers would enable the formation of a gel matrix encapsulating the NCT superlattices. Previous work has shown that such a strategy is possible for DNA-linked nanoparticle lattices, though this prior method relied on electrostatic interactions between the gel matrix and the DNA strands to sufficiently stabilize the particle arrays, and thus more generalizable methods would be required if such an approach were to be adopted by other types of colloidal assemblies, such as the NCTs used here.22
RESULTS AND DISCUSSION
To evaluate the possibility of using organogel formation to stabilize colloidal NCT arrays, superlattices were first assembled using previously established NCT designs consisting of gold nanoparticle (AuNP) cores grafted with dense brushes of polystyrene (PS) chains. Multiple NCT samples were prepared: 26 nm diameter AuNP spheres grafted with 15 kDa PS brushes, 17 nm diameter AuNP spheres grafted with 15 kDa PS brushes, and 17 nm diameter AuNP spheres grafted with 11 kDa PS brushes; complete characterization and synthesis details are described in the supplementary material. Each polymer chain on an individual NCT’s brush terminates in either a diaminopyridine (DAP) or thymine (Thy) supramolecular binding group; DAP and Thy form complementary hydrogen bonding pairs (Fig. 1). To form the ordered arrays, toluene suspensions of DAP-NCTs and Thy-NCTs were combined and heated to 65 °C, above the temperature at which the NCTs would dissociate. These solutions were then slowly cooled to 25 °C, allowing for controlled assembly and reorganization in a manner that enables the formation of crystalline structures. The resulting colloidal arrays were examined using small-angle x-ray scattering (SAXS), which confirmed the formation of body-centered cubic (BCC) superlattices after this annealing process.13 These lattices were used for all subsequent experiments on the polymer gel encapsulation of NCT crystallites.
While many different chemical reactions could be used to crosslink polymer networks, copper(I)-catalyzed azide cycloaddition (CuAAC) chemistry is an ideal choice [Fig. 2(a)]. The CuAAC reaction forms triazole linkages from azide and alkyne moieties in high yield, producing minimal or no by-products, depending on reaction design. It can be run at a wide range of temperatures and in the presence of many different functional groups without the occurrence of undesirable side reactions.23,24 Copper-halogen salts are typically used to catalyze the CuAAC reaction; CuCl was specifically chosen as the Cu(I) source for this work due to the chemical incompatibility of CuI with gold and the slower reaction rates of CuBr compared to CuCl. Non-polydentate triphenylphosphine (PPh3) was selected as a coordinating ligand to the Cu(I); PPh3 was used because most other nitrogen-containing polydentate ligands used in CuAAC reactions would interfere with the hydrogen bonding between DAP and Thy, potentially disrupting ordering in assembled NCTs.25 Importantly, control experiments showed that none of the relevant chemical moieties or species (azides, alkynes, CuCl, and PPh3) involved in the CuAAC reaction interfere with the hydrogen bonding between assembled NCTs at concentrations sufficient to prevent the use of CuAAC crosslinking to form a polymer network (Fig. S2).21
To embed the colloidal NCT arrays within a gel network, the NCT superlattices were transferred to a toluene solution containing azide-terminated four-arm polystyrene star polymers (60–64 kDa, final concentration 400 mg/ml) and thoroughly mixed to ensure that the star polymers were able to both surround and intercalate into the lattice.21 These solutions were mixed with toluene solutions containing 0.1M 1,7-octadiyne, 0.1M CuCl, and 0.1M PPh3, then heated at 65 °C to yield the NCT polymer gel [Fig. 2(b)] (complete protocols can be found in the supplementary material). While the NCT designs used here typically dissociate at ∼42.5 °C due to the thermal breaking of hydrogen bonds, the addition of the star polystyrene elevated the thermal stability of assembled superlattices, allowing gelation to be run at 65 °C without causing NCT dissociation. As a result, these elevated temperatures enabled gelation to occur over short time periods (3.5 h). SAXS data of the NCT lattices before and after gel formation demonstrate that NCT lattices remain highly ordered during gelation, with a slight decrease in the lattice parameter [Fig. 2(c)]. This lattice shrinkage post-gelation is attributed to solvent loss during the gelation process as well as compression from the surrounding crosslinked polymer matrix.21
Once embedded in a polymer gel, ordered NCT lattices exhibited improved stability against a variety of external environment changes since, in contrast to ordered NCT lattices in solution, the NCT ordering no longer relied exclusively on the weak supramolecular bonds between neighboring particles to maintain particle positioning within a lattice; the gel network “trapped” the particles in place and prevented them from easily diffusing away from one another. Even under conditions where the hydrogen bonds would be expected to be completely broken (based on control experiments with non-gel-embedded NCTs, Fig. S3), the particle arrays remained intact and crystalline. For example, NCTs assembled using DAP-Thy interactions must generally be kept in nonpolar solvents, such as toluene, as polar solvents weaken the hydrogen bonding between complementary DAP and Thy binding groups, causing assemblies to rapidly dissociate into free particles. However, when these DAP-Thy assembled lattices were embedded in a crosslinked polymer network, the gel could be immersed in excess volumes of acetone or dichloromethane (DCM) for 24 h with no change in the assembled structures (as determined by SAXS analysis), other than a slight increase in lattice parameters attributed to swelling of the gel matrix [Fig. 3(a)]. These data are consistent with the formation of polymer networks that fully encapsulate and interpenetrate the NCT arrays, as polymer networks that only wrapped around the outside of an NCT array would still permit the individual particles to locally diffuse and cause a loss of BCC ordering.
Once embedded in polymer gels, NCTs can also be entirely removed from solvents without completely dissociating from one another. This stability was demonstrated by drying a set of gel-embedded NCTs under vacuum for 4 h, then re-solvating the gels in toluene [Fig. 3(b)]. SAXS data showed that the NCTs remained in a BCC lattice upon solvent removal, although with a noticeably reduced lattice parameter and a slight broadening of SAXS peaks, potentially indicating a small decrease in the degree of structural organization. However, the original BCC lattice quality and unit cell size were fully recovered after re-solvation in toluene. Thus, even though unprotected colloidal suspensions of NCTs quickly collapse upon solvent removal (resulting in disordered aggregates),13 polymer gels containing embedded NCTs can be reversibly dried and re-solvated without losing superlattice ordering. We hypothesize that this improved stability is due to the added structural support of the crosslinked polymer network and the slowed rate of solvent evaporation during drying, which allows the NCT polymer brush to collapse more gradually and uniformly upon solvent removal.
The thermal stability of NCTs is also greatly improved upon being embedded into a polymer matrix, even beyond the melting temperature elevation previously seen in NCT lattices suspended in high-polymer-content solutions21 [Fig. 3(c)]. Even after heating the embedded NCT arrays to 85 °C and holding them at this temperature for 40 min, the NCT lattice quality was completely unaffected. For comparison, two different control samples (colloidal suspensions of NCTs without added polymer; suspensions of NCTs in toluene with un-crosslinked star PS at equal concentrations to the sample above) were also heated to 85 °C (Fig. S3). As expected, the NCT control containing no excess free polymer rapidly disassembled as the solution was heated to 85 °C, with no observable structure factor remaining. While NCTs in the solution containing large quantities of uncrosslinked polymer remained assembled at 85 °C (due to the aforementioned added thermal stability21), ordering quality was severely diminished, as indicated by the weakened intensity and loss of higher order peaks. The improved retention of NCT ordering of samples in the crosslinked gel vs samples in the solution containing equal amounts of uncrosslinked polymer shows that the observed stability at high temperatures is not merely due to the previously documented melting temperature elevation from the presence of high weight percentages of polymer.21
A key advantage of the polymer gel matrix as a stabilizing medium lies not only in the way it improves NCT stability against multiple kinds of external environment changes, but also in its ability to do so without locking the lattices into a rigid solid. As a result, the NCT lattices can be easily and reversibly deformed by mechanically compressing the surrounding gel matrix (Fig. 4). To demonstrate this ability to dynamically alter structure, NCT gels were placed on a linear compression stage and monitored with SAXS during the application of mechanical force. Before compression, gels displayed an isotropic, circular 2-D SAXS pattern [Fig. 4(b)], indicative of a random distribution of BCC crystallites with no preferential orientation. As the gel was compressed, NCTs experienced anisotropic deformation, resulting in a shift of the 2-D SAXS from a circular to an oval pattern that indicated lattice compression and expansion along orthogonal axes. Converting different radial sectors of these 2-D patterns to 1-D line scans of intensity as a function of scattering angle revealed significant differences in lattice structure at 0° (perpendicular to the direction of compression) and 90° (parallel to the direction of compression) [Fig. 4(a)]. At 0% compression, 1-D SAXS patterns for the 0° and 90° sectors were identical, confirming no preferential orientation or anisotropy in NCT lattice structure. As the gel was compressed, line scans along the 90° sector showed qualitatively the same lattice structure but with a shift to larger scattering angles, corresponding to lattice compression parallel to the applied force. Conversely, line scans along the 0° sector showed the opposite effect (peak shifts to smaller angles), corresponding to lattice expansion. It should be noted that the ordering was better retained in the direction perpendicular to lattice compression (though significant structure factors are still observed even at 70% compression). Upon removal of the applied compressive force, however, NCTs within the gel recovered their initial ordering and spacing, returning from an oval-shaped pattern to the original circular 2-D scattering pattern (Fig. S4). NCT gels were also robust enough to undergo five complete cycles of mechanical compression and release without permanent deformation and always returned to their initial BCC lattice structures without significant changes in lattice quality [Fig. 4(c)]. These data demonstrate that not only does the gel embedding process stabilize NCTs against polar solvents, uncontrolled solvent loss, and high temperatures, it also enables reversible mechanical deformation to break lattice symmetry.
Once NCTs are embedded within a polymer gel, they can be further processed into a solvent-free form to obtain a fully stable, ordered nanocomposite solid by replacing the solvent with a photopolymerizable monomer.26–28 To achieve these solids, gels were immersed in styrene solutions containing 5 wt. % AIBN for 24 h to allow the monomer to fully permeate the gel, then exposed to UV light (254 nm, 8 W) for 12 h. Upon completion of the photopolymerization of styrene into polystyrene, the resulting composites were fully solid, with SAXS indicating that the ordering of the lattice had been completely retained [Fig. 5(a)]. The solid form also enabled the use of characterization methods that require a solvent-free state, specifically scanning electron microscopy (SEM), making it possible to visually confirm the nanostructure of NCTs embedded within the polymer solid [Fig. 5(b)]. SEM of microtomed and ion-beam milled cross-sections of the photopolymerized solids clearly show that the BCC ordering was retained. Additionally, the shape of this cross-section is consistent with the silhouette of a bisected rhombic dodecahedron, the microscale crystal habit that has previously been demonstrated as the thermodynamically preferred shape for these NCT BCC lattices.20 Thus, both the nanoscale ordering and the microscale crystal habit are preserved upon embedding within the photopolymerized solid.
Finally, the methods presented here for star polymer synthesis, CuAAC-driven gel formation, and photopolymerization are compatible with other vinyl monomers in addition to styrene, allowing for photopolymerization encapsulation of the NCT arrays in different polymer solids like PnBA (Fig. S5). Thus, this method of enhancing the stability of colloidal assemblies via first forming a gel network allows NCT arrays to be incorporated into a wider range of polymer matrices, potentially permitting future investigations of how the mechanical, chemical, optical, or other properties of these possible polymer matrices are affected by the presence of the NCT assemblies.
While the use of dynamic, reversible bonding interactions is a useful method to facilitate nanoparticle assembly into ordered structures, these weak interactions become detrimental in the subsequent processing of the assembled nanostructures. Incorporating nanoparticle assemblies into a polymer gel matrix provides a simple method for greatly improving their thermal and chemical stability while still preserving a degree of dynamicity in the embedded nanocrystals. Moreover, replacing the solvent with a gel allows for macroscopic manipulation of nanoparticle organization simply by deforming the surrounding polymer matrix. The formation of a stabilizing gel network also permits further post-assembly processing to produce free-standing solids with embedded crystallites, thereby providing a path to produce polymer nanocomposites containing nanoparticle superlattices that would normally deform or dissociate under the harsh chemical, thermal, or physical stimuli needed to process the polymer matrix of a composite. By enabling NCT lattices to access these three environments (solution, gel, and solid) without disruption of particle organization, NCT assemblies can access a spectrum of dynamicity and stability, making it possible to expand the conditions under which NCT crystals can be handled and applied. Furthermore, it permits the incorporation of NCTs into a wider array of polymer matrices, enabling the investigation of structure-property relationships in NCT-based composites. The ability to incorporate ordered colloidal arrays as either dynamic or static structures within a polymer matrix is an important step toward the use of these nanomaterials in advanced composites and also introduces their potential as smart, responsive materials.
See supplementary material for complete synthesis protocols and full characterization of polymers, nanoparticles, and assemblies, including additional UV–Vis spectroscopy and x-ray scattering data.
This material is based upon work supported in part by the U.S. Army Research Office under Grant No. W911NF-18-1-0197. This work was primarily supported by an NSF CAREER Grant (Award No. CHE-1653289) and made use of the MRSEC Shared Experimental Facilities at MIT, supported by the NSF under Award No. DMR 14-19807. It was also supported by funding from the Department of the Navy, Office of Naval Research under ONR Award No. N00014-22-1-2148. SAXS experiments at beamline 12-ID-B at the Advanced Photon Source at Argonne National Laboratory were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
Conflict of Interest
The authors have no conflicts to disclose.
M. S. Lee, D. W. Yee, J. M. Kubiak, and P. J. Santos performed experiments. All authors analyzed the data and wrote the manuscript.
Margaret S. Lee: Conceptualization (supporting); Formal analysis (equal); Methodology (equal); Writing – original draft (lead); Writing – review & editing (supporting). Daryl W. Yee: Data curation (equal); Formal analysis (equal); Visualization (equal); Writing – review & editing (supporting). Joshua M. Kubiak: Data curation (equal); Formal analysis (equal). Peter J. Santos: Data curation (equal); Formal analysis (equal). Robert J. Macfarlane: Conceptualization (lead); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Project administration (lead); Writing – original draft (equal); Writing – review & editing (lead).
The data that support the findings of this study are available within the article and its supplementary material.