Letter pubs.acs.org/macroletters
Responsive Shape Change of Sub‑5 nm Thin, Janus Polymer Nanoplates Hao Qi, Tian Zhou, Shan Mei, Xi Chen, and Christopher Y. Li* Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Responsive shape changes in soft materials have attracted significant attention in recent years. Despite extensive studies, it is still challenging to prepare nanoscale assemblies with responsive behaviors. Herein we report on the fabrication and pH-responsive properties of sub-5 nm thin, Janus polymer nanoplates prepared via crystallization-driven self-assembly of poly(ε-caprolactone)-b-poly(acrylic acid) (PCL-b-PAA) followed by cross-linking and disassembly. The resultant Janus nanoplate is comprised of partially cross-linked PAA and tethered PCL brush layers with an overall thickness of ∼4 nm. We show that pronounced and reversible shape changes from nanoplates to nanobowls can be realized in such a thin free-standing film. This shape change is achieved by exceptionally small stressa few orders of magnitude smaller than conventional hydrogel bilayers. These three-dimensional ultrathin nanobowls are also mechanically stable, which is attributed to the tortoise-shell-like crystalline domains formed in the nanoconfined curved space. Our results pave a way to a new class of free-standing, ultrathin polymer Janus nanoplates that may find applications in nanomotors and nanoactuators.
R
Polymer single crystals (PSCs) have been extensively investigated during the past 60 years.11 Recently, we demonstrated that surface-functionalized two-dimensional (2D) PSCs can be used in applications such as nanoparticle asymmetric functionalization, nanomotors, surface-enhanced Raman spectroscopy, catalysis supports, and polymer brush synthesis.12 Curved polymer single crystals have also been obtained by using the liquid/liquid interface as the template.13 Herein we report on the PSC-directed assembly and responsive behaviors of sub-5 nm thick, 2D free-standing block copolymer Janus nanoplates that are able to undergo a pH-triggered 2D (nanoplate) to 3D (nanobowl) shape change. The crystallization behavior of the PCL on this ultrathin curved film was also investigated. Our results show a new class of free-standing, ultrathin polymer Janus nanoplates that may find applications in nanomotors and drug delivery applications. Scheme 1 outlines the detailed fabrication procedure of the Janus assembly. Poly(ε-caprolactone)-b-poly(acrylic acid) (PCL-b-PAA) (Mn = 6156−1944 Da) was synthesized according to a previous report.14 Uniform PCL-b-PAA single crystals were obtained using a self-seeding method,15 where crystalline PCL was sandwiched between two amorphous PAA layers. PAA on the single crystals was then partially cross-linked following the literature.16 Subsequent dissolution of the PCL domains of the single crystals in a tetrahydrofuran (THF) and
esponsive and dynamic structures in nature have inspired researchers to fabricate synthetic actuators that are able to change size/shape in response to external stimuli. Among different materials, polymeric actuators have become a topic of significant interest because of their potential applications in sensing, drug delivery, and molecular motors.1 Polymer chain conformation can be tuned by utilizing various types of stimuli; reported mechanisms include light-induced structural changes of polymers with photoactive groups,2 humidity or solvationinduced hydrogel swelling,3 and temperature-induced phase transformation.4 Liquid crystalline polymer actuators have also been extensively studied.5 To utilize these polymer molecular responses to fabricate micro-/macroscale shape change devices, stimuli-responsive polymers are often patterned into predesigned architectures.6 Another simple way to control actuation is to create asymmetric Janus structures.7 Because of the mechanical property contrast in their two compartments, Janus rods and sheets can exhibit stimuli-responsive shape changes.8 While extensive work has been reported on macro-/ microscale stimuli-responsive deformation of Janus structures, reports on nanoscale-responsive Janus assemblies are limited. Klinger et al. showed reversible anisotropic swelling behavior of block copolymer nanoparticles.9 Most recently, Liu et al. fabricated Janus silica nanosheets through a surface sol−gel process onto a template and surface modification. Such a flexible nanosheet can wrap desired species upon pH change.10 The obvious challenge for achieving nanoscale-responsive Janus assemblies is controlling the nanoscale size and shape of the asymmetric, stimuli-responsive objects. © XXXX American Chemical Society
Received: March 29, 2016 Accepted: May 11, 2016
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DOI: 10.1021/acsmacrolett.6b00251 ACS Macro Lett. 2016, 5, 651−655
Letter
ACS Macro Letters Scheme 1. Schematic Illustration of the Preparation of Responsive Janus Nanoplatesa
a (a) Crystallization-driven self-assembly of PCL-b-PAA; (b) cross-linking top and bottom PAA layers; (c) dissolution of the PCL layers leads to a free-standing, ultrathin Janus assembly that undergoes reversible shape change at different pH; (d) molecular structures of PCL-b-PAA and 4,7,10trioxa-1,13-tridecanediamine (TDDA).
molecular weight and the two-chain orthorhombic unit cell of PCL with the parameters of a = 0.748 nm, b = 0.498 nm, and c = 1.726 nm,17 shown in the inset of Figure 1b. The grafting density of PAA segments on each PSC surface can therefore be calculated to be ∼0.45 PAA chain/nm2. Figure 1c shows that the overall crystal thickness increases to 13 nm upon PAA cross-linking, due to the uptaking of the cross-linking agents in the PAA domains. To disassemble the cross-linked singlecrystal sandwich, the same volume of THF to water was added to the PSC’s aqueous suspension to selectively dissolve the PCL crystalline domain. Figure 1d shows the resultant Janus nanoplates. Because the PAA layers are cross-linked, the shape of the PSC is retained after the dissolution process. The AFM height profile indicates that the overall thickness of the Janus nanoplate is ∼4 nm. From Scheme 1, the simple design hints that when dissolving the center layer of a single-crystal sandwich two Janus bilayers are formed. The measured thickness of the Janus nanoplate is however smaller than the intended half of the cross-linked PSC (6.5 nm) depicted in Scheme 1, and the lateral size shrunk compared to the original PSC. Both observations can be attributed to partial crosslinking of the PAA layers, which was purposely designed so that the un-cross-linked carboxylic acid groups can be used for actuation. The overall cross-linking density was estimated to be approximately 33% (Figure S2). Figure 2 shows the pH-responsive behavior of PCL-b-PAA Janus nanoplates. At neutral pH, the Janus nanoplates are flat (Figure 2b). When pH is lowered to 3, these nanoplates are significantly bent, as shown in Figure 2a. As TEM imaging uses a transmission mode, various shapes of the nanoplates are observed in Figure 2a: some of them are spherical/bowl like, while a few show clear folding of the original Janus nanoplates. As previously reported, preferential bending along the long edge of an anisotropic plate is a consequence of isotropic strain and aspect ratio.18 To better illustrate the folded PSCs, a series of tilted images were taken as shown in Figure 2e−g, where 30° consecutive tilting along the dotted lines was applied. These images clearly show that the nanoplate is bent along the long edge of the PSC, adopting a 3D curved structure. Similar shape change of these Janus nanoplates can be observed when they are exposed to a high pH of 11, as show in Figure 2c. Figure S3 shows nanoparticle-decorated bowl shape PSCs, and Figure S4 reveals that after two actuations where pH value was changed as 7−3−7−11−7 the Janus nanoplates return to the flat shape at
water (VH2O/VTHF = 1/1) mixed solvent led to free-standing, sub-5 nm, bilayered Janus nanoplates, which undergo shape changes in both basic (pH = 11) and acidic (pH = 3) conditions, as detailed in the following discussion. The as-fabricated PCL-b-PAA single crystals have a hexagon shape (Figure 1a), and the size can be controlled by simply
Figure 1. (a) Bright-field TEM and (b) AFM height images of PCL-bPAA single crystals. Inset in (b): side view of the corresponding chain folding; yellow chains are PCL, while blue chains are PAA. (c,d) AFM height image and the corresponding height profile of a cross-linked PCL-b-PAA PSC (c) and PCL-b-PAA Janus nanoplates.
changing the seeding temperature (Figure S1). The overall thickness of the crystal was measured to be 11 nm by atomic force microscopy (AFM), as shown in Figure 1b. In polymer solution crystallization, the thickness of a single crystal is typically determined by crystallization temperature. Therefore, both lateral and vertical dimensions of the block copolymer single crystals can be well tuned. The block copolymer PSCs mimic nanoscale “sandwiches” with the PCL lamellae being confined by two PAA layers. Based on the density and molecular weight of each segment, the thickness of the PAA and PCL layer can be calculated to be 1.3 and 8.4 nm, respectively. As the polymer chain folds back and forth perpendicular to the crystal surface in the PSC, it can be calculated that each PCL chain folds ∼5 times using the 652
DOI: 10.1021/acsmacrolett.6b00251 ACS Macro Lett. 2016, 5, 651−655
Letter
ACS Macro Letters
Figure 2. Bright-field TEM images of small Janus PCL-b-PAA nanoplates (2.0 × 0.6 μm) at (a) pH 3, (b) pH 7, and (c) pH 11. (d) Schematic drawing shows the shape changing behavior of a Janus nanoplate at various pH. (e−g) Series of bright-field TEM images of one Janus nanoplate at pH = 11. The tilting angle is −30°, 0°, and 30°, respectively. Tilting axis is indicated as the dotted white line in f. (h−j) Bright-field TEM images of relatively larger Janus PCL-b-PAA nanoplates (4.5 × 1.3 μm) at (h) pH 3, (i) pH 7, and (j) pH 11.
PAA hydrogel layer is elastic, having a biaxial Young’s modulus of E′. The predicted surface stress and radius of curvature are20
pH 7, suggesting that the pH-dependent shape changing is reversible. The above observation can be correlated with the pHresponsive behavior of PAA chains. PAA is known to undergo a different degree of ionization as pH changes.19 The pKa of PAA is approximately 5.7−6.5, and its degree of ionization is ∼0, 70, and 100% at pH = 3, 7, and 11, respectively.19 Since PAA chemical cross-linking was conducted at neutral pH, the PAA and PCL layers are balanced in lateral expansion force, and the Janus plate is flat at pH = 7. In an acidic environment, the uncross-linked carboxylic acid groups of PAA are protonated, inducing the PAA layer to contract; the Janus assembly therefore bends toward the PAA layer. On the other hand, the cross-linked PAA layer swells at pH ∼ 11 due to deprotonation of unreacted carboxylic acid groups, and the Janus assembly bends toward the PCL layer, as shown in Figure 2d. Therefore, out-of-plane bending of the Janus nanoplates takes place in both acidic and basic environments. Shape changes of larger PCL-b-PAA Janus nanoplates (4.5 × 1.3 μm) are shown in Figure 2h−j, where the 2D flat Janus sheets transformed to 3D bowl-like shapes and the bending occurred along the long edge of the nanoplate. The shape changes of large sized Janus nanoplates led to nanobowls with a much greater radius of curvature RL ∼ 1000−1500 nm, compared with the radius of curvature Rs of ∼250−500 nm for smaller nanobowls shown in Figure 2a,c. This can be explained by the predicted surface stress and radius of curvature based on a bilayer assembly comprised of one elastic layer and one layer of polymer brush.20 In the present case, we can assume that the
σ=
R=
⎞ E′h ⎛ S − 1⎟ ⎜ 3 ⎝ A0 ⎠
(1)
E′h2 6σ
(2)
where σ is the surface stress; A0 is the original interfacial area; S is the PAA surface area after actuation; h is the thickness of the PCL layer; and E′ is the biaxial Young’s modulus of PAA, E′ = E/(1 − ν) (where E is the linear Young’s modulus and ν is the Poisson’s ratio). Since polymer chains are tethered at the interface and polymer brushes at the edge of the nanoplates are less constrained by the tethering effect, the relative strain S/A0 (hence the stress σ based on eq 1) at a given pH condition after actuation decreases with PSC size. Because R is inversely proportional to σ (eq 2), the radius of curvature therefore increases with the PSC size as observed in Figure 2. Furthermore, based on our experimental results and previously reported data, assuming h ∼ 2 nm, R ∼ 500 nm, ν ∼ 0.5,21 and E ∼ 50 kPa,22 the surface stress can then be estimated to be in the order of 10−7 N/m according to eqs 1 and 2. This number certainly is an approximation due to various assumptions we made and the experimental challenges to precisely measure h and R in the present case. Nevertheless, it provides an important guideline to understand the present system. The estimated stress is much smaller than traditional bilayer hydrogels or polymer brushed coated bilayers. For instance, polymer brush coated cantilevers have a typical surface stress of 653
DOI: 10.1021/acsmacrolett.6b00251 ACS Macro Lett. 2016, 5, 651−655
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ACS Macro Letters
might be because the crystallization occurred on a curved nanosurface.24 Because the curved surface is incommensurate with the 3D translational symmetry required for crystallization, one would anticipate distorted lattice packing in the crystallized nanobowl.13 It has been proposed in polymer crystallization that continuous and discrete lattice distortion could occur when forming polymer crystals in a helical/curved environment. Continuous lattice distortion typically leads to smeared archshaped diffraction patterns.25 Observing the multiple sets of (hk0) patterns with similar intensity suggests that the distortion of the crystalline lattice due to curved space is discrete. Since the PCL layer is templated by the curved surface, while multiple nucleation events could occur in one nanobowl, each crystallite can only develop to a finite size until the curvature retards the crystal growth. The resultant lamella is polycrystalline with multiple domains that are separated by grain boundaries, and each domain has a similar size. The lattice orientation of each domain is slightly shifted (∼3°) from that of its neighboring domain, also due to the curved space. Defects are expected in the grain boundaries. This discrete packing of polymer crystalline domains mimics tortoise shell structure (Figure 3d) and explains why such a thin film can maintain its shape during TEM sample preparation. Note that although both the diffraction patterns in Figure 3b and 3c represent polycrystalline diffraction pattern Figure 3c is much more uniform in both relative lattice orientation and diffraction intensity, indicating that individual crystalline domains have similar sizes and similar shifting angle upon packing. In summary, we report sub-5 nm, free-standing block copolymer Janus assemblies that undergo a pH-triggered 2D to 3D shape transformation. These free-standing nanoplates are ultrathin, with an average thickness of ∼4 nm. Pronounced, reversible shape change was realized by varying solution pH from neutral to either acidic or basic, which was attributed to the controlled degree of PAA ionization. Because of the ultrathin nature of the Janus structure, this shape change was driven by exceptionally small stress: the estimated stress for the nanoplate is on the order of 10−7 N/m, which is a few orders of magnitude smaller than typical polymer brush-coated cantilevers and hydrogel bilayers. Despite the thinness of the 3D nanobowl, they sustained the 3D shape upon solvent drying, thanks to the fast PCL crystallization process and the tortoise shell-like domain packing of the crystals, as evidenced by SAED experiments. We anticipate our approach to be a new method to fabricate uniform ultrathin Janus nanostructures for novel polymer actuators.
0.1−1 N/m, while the stress is 10−100 N/m for bilayer hydrogels.23 This ultralow stress observed in our Janus nanoplates is because of the extremely thin PAA layer, which provides the driving force for actuation. Typical polymer brushes in the cantilever study are around a few tens to hundreds nanometer thick, while in bilayer hydrogels, hundreds of micron thick gel layers have been utilized.23 The PAA layer in our system is only 2 nm, leading to a few orders of magnitude smaller stress. This also interestingly implies that our Janus nanoplates are a new class of actuator: significant shape changes of several nanometer thin polymer actuators could be achieved with ∼5 orders of magnitude lower stress compared with reported systems. Also of interest in our system is that, although the Janus nanoplate is ultrathin, the 3D nanobowl remarkably maintained its shape during TEM sample preparation: in most similar nanostructures such as polymersomes, polymer capsules collapse during TEM sample preparation. We attribute this mechanical stability to PCL recrystallization: when dropcasting the curved, bowl-shaped Janus nanobowl onto a solid substrate, PCL crystallizes upon solvent evaporation. Figure 3a shows a
Figure 3. TEM electron diffraction pattern of (a) a flat PCL single crystal; (b) a flat PCL-b-PAA Janus nanoplate; (c) a curved PCL-bPAA nanobowl. Insets in (b) and (c) show the corresponding morphologies. (d) Schematic drawing shows polymer crystallizes on (i) a flat nanoplate and a curved nanobowl (ii). The crystallites’ packing mimics the morphology of a tortoise shell (iii).
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selected area electron diffraction (SAED) pattern of a flat PCL single crystal, which is consistent with the reported PCL orthorhombic unit cell structure.17 Only (hk0) diffraction planes are observed, indicating that the c-axis and the polymer chains are normal to the crystal surface. Figure 3b shows an SAED pattern from a flat Janus nanoplate (the inset of Figure 3b); multiple sets of (hk0) diffractions are observed. This can be attributed to the evaporation-induced crystallization process: the fast evaporation and the thin nanoplate geometry led to relatively fast/multiple nucleation events in the nanoplate and consequently a polycrystalline structure and diffraction pattern as seen in Figure 3b. Figure 3c reveals an SAED pattern from one Janus PCL-bPAA nanobowl (the inset of Figure 3c). Interestingly, five sets of (hk0) diffractions can be observed, and the adjacent patterns are offset by ∼3°. The unique SAED pattern observed herein
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00251. Detailed experimental procedures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 654
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Grant DMR-1308958 and CBET 1438240.
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