Crystallization-Driven Two-Dimensional Nanosheet from Hierarchical

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Crystallization-Driven Two-Dimensional Nanosheet from Hierarchical Self-Assembly of Polypeptoid-Based Diblock Copolymers Zhekun Shi,† Yuhan Wei,† Chenhui Zhu,‡ Jing Sun,*,† and Zhibo Li*,† †

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Key Laboratory of Biobased Polymer Materials, Shandong Provincial Education Department, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China ‡ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Two-dimensional (2D) nanomaterials have received increasing interest for many applications such as biomedicine and nanotechnology. Here, we report a facile strategy to prepare highly flexible 2D crystalline nanosheets with only ∼6 nm thickness from poly(ethylene glycol)-block-poly(N-octylglycine) (PEG-b-PNOG) diblock copolymer in high yield. To our best knowledge, this is the first report of free-floating, 2D extended nanosheets from polypeptoid-based block copolymers. The faceted nanostructures are achieved from hierarchical self-assembly through a sphere-to-cylinder-to-nanosheet transition pathway. The preliminary assembled spheres can behave like a fundamental packing motif to spontaneously stack into a 2D lattice via an intermediate cylinder structure, driven by crystallization of PNOG domains. The nanosheet formation process follows theoretical model for morphology development of crystalline block copolymers in selective solvents. Particularly remarkable is that we obtained the hierarchical nanostructure from synthetic block copolymers through a multiple-step strategy mimetic to protein crystallization. This is fairly distinct from the previously reported crystalline nanosheets. The ability to efficiently create 2D crystals from synthetic polymers by spontaneous assembly will enable new generations of bioinspired nanomaterials for a variety of potential applications in biomedicine and nanotechnology.

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INTRODUCTION

cocrystallization approach. Great efforts have been devoted to the achievement of supramolecular nanostructures from crystalline block copolymers.11−13 Manners and Winnik et al. demonstrated a crystallization-driven self-assembly process to prepare well-defined and functional hierarchical nanostructures from block copolymers with crystallizable core-forming metalloblock.2,13 Lately, two-dimensional (2D) nanomaterials have been receiving interest for many applications such as surface science, biomedicine, and energy storage. A few strategies have emerged to fabricate 2D nanostructures from polymers/ oligomers.14−17 The polymer crystallization has been reported

In nature, many biomolecules can fold into highly ordered structures through different pathways, particularly hierarchical self-assembly, which enable excellent functional performance.1 A variety of one-, two-, and three-dimensional complex architectures constructed via preassembled subunits of block copolymers have been prepared that offer potential applications in nanotechnology, biomedicine, environmental technology, etc.2−6 The driving forces for the formation of complex structures include inter- and intramolecular interactions such as hydrogen bonding, ionic interaction, hydrophobicity, crystallization, and external stimuli like pH, temperature, light, etc.7−10 In particular, crystallization endows self-assembly of block copolymers with a large number of unique properties, including tunable morphology and stability, living growth characteristics, and multicomponent nanostructure by a facile © XXXX American Chemical Society

Received: May 8, 2018 Revised: July 23, 2018

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DOI: 10.1021/acs.macromol.8b00986 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules as an efficient method to obtain lamellar platelets.18 However, to date, only a few reports have been addressed for the achievement, particularly laterally extended 2D nanosheet-like structures from block copolymers.19 This is due to the highly complicated crystallization process occurring during nanostructure formation. For example, the crystallization of the polymer can be confined in a nanoscale environment, resulting in low or absence of crystallinity. Appropriate polymer systems with tunable control over the crystallization process are highly desired. Polypeptoids, or poly(N-substituted glycine)s, have emerged as promising bioinspired polymers that offer advantageous properties for both fundamental research and applications in nanoscience and biotechnology.20−22 The polypeptoid is a class of peptidomimetic polymer that differs from polypeptide only in that the pendant side-chains are attached to the amide nitrogen instead of α-carbon. The difference leads to the absence of the hydrogen-bonding sites and chirality in the main chains, which simplifies the design of polymers mainly by tuning the properties of side chains. It has been reported that the polypeptoids with longer alkyl side chains are semicrystalline with tunable melting transitions, which is in sharp contrast to polypeptides with inherent hydrogen-bonding interactions.23−25 The recent result demonstrated that the crystalline peptoid molecules adopt extended and planar conformation with all cis conformation.26 A couple of morphologies including sphere, cylinder, and vesicle have been prepared from crystallizable peptoid block copolymers.12,27 These polymers are mostly based on polysarcosine,28,29 synthesized by a classical polymerization approach. Zuckermann’s group30,31 and Chen’s group32 reported the crystalline nanosheet-like structures from peptoid oligomers, by which the solid-phase approach was involved. With this method, the sequence specificity and precise chain length can be obtained for peptoids with shorter chain lengths. In this study, we reported the first free-standing, ultrathin crystalline nanosheet from hierarchical self-assembly of poly(ethylene glycol)-block-poly(N-octylglycine) (PEG-b-PNOG) diblock copolymers in a large-scale yield. The obtained 2D nanosheets show excellent flexibility and good thermal stability over a wide temperature range. We studied the effects of copolymer molecular weight, sample concentration, solvent, and temperature on the self-assembly behaviors of PEG-bPNOG copolymers. It is observed that the PEG-b-PNOG prefers forming 2D nanosheet structures, which is driven by the crystallization of PNOG block in selective solvent. We demonstrate that PEG-b-PNOG initially forms spherical aggregates, which evolves into nanosheet structure with uniform thickness of ∼6 nm via a cylinder structure intermediate process. The facile synthetic approach combined with the excellent biocompatibility of polypeptoids offers great potential for the next generation of 2D nanomaterials for a broad range of advanced applications.

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purchased from commercial suppliers and used without further purification unless otherwise noted. Characterizations. 1H NMR spectra were recorded on a Bruker AV500 FT-NMR spectrometer. Tandem gel permeation chromatography (GPC) was performed at 25 °C on a Waters 410 equipped with a Waters 2414 RI detector and Waters Styragel HR4 and HR2 columns. Chloroform (HPLC grade) was used as the eluent at a flow rate of 1.0 mL/min. Conventional calibrations were performed using polystyrene standards (PS). DSC studies were conducted using a TA DSC Q20 calorimeter under nitrogen. Powder samples sealed into the aluminum pans were first heated from −40 to 200 at 10 °C/min for three cycles. AFM studies were conducted using tapping mode AFM (Bruker Multimode 8 AFM/SPM system) in ambient air with Nanoscope software. A volume of polymer solution (∼10 μL, 1 mg/ mL) was drop-deposited and dried on freshly cleaved mica under ambient conditions before AFM imaging. Minimal processing of the images was done using NanoScope Analysis software from Bruker. TEM experiments were conducted on a FEI TECNAI 20, with a Gatan digital camera and Gatan Digital Micrograph analysis software. The polymer solution (6 μL, 1 mg/mL) was pipetted onto on holey carbon-coated 200 mesh copper grids. The excess amount of solution was removed, and the sample was negatively stained with 0.5 wt % uranyl acetate. The solvent was evaporated for at least 12 h except anything noted. Cryo-EM experiments were conducted on the same instrument. The vitrified specimens were prepared using a Vitrobot (FEI, Inc.). A 5 μL droplet of the ethanol solution at a concentration of 1 mg/mL was deposited on the surface of glow discharged grids with lacey carbon films. The droplet was blotted by filter paper for 1.5 s, followed by 1 s draining, and then plunged into liquid ethane to obtain a vitrified thin film. The grids were then transferred to a Gatan cryo-stage at −190 °C for analysis. The grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed with the energy of 10 keV in top-off mode at beamline 7.3.3, Advanced Light Source (ALS), Lawrence Berkeley National Lab (LBNL). The scattering intensity was recorded on a 2D Pilatus 1M detector (Dectris) with a pixel size of 172 μm. A silver behenate sample was used as a standard to calibrate the beam position and the sample− detector distance. The sample (2 mg/mL) was deposited on Si wafers, dried, and stored under ambient conditions before testing. Synthetic of PEG-b-PNOG Diblock Copolymers. In a typical procedure, mPEG-NH2 (91.7 mg, Mn = 5000 g/mol) was heated at 50 °C, dried under high vacuum for 12 h, and then dissolved in anhydrous THF to obtain a solution (10%) in a reaction flask. In the glovebox, the n-octyl-N-carboxyanhydride monomer (234 mg) was dissolved in anhydrous THF (2.5 mL), followed by adding to the reaction flask with given ratio. Polymerization was allowed to proceed at 60 °C for 24 h under an N2 atmosphere, and then the solution was precipitated in an excess amount of hexane. The white precipitate was collected and washed with ample methanol and hexane. The product was dried under vacuum to yield a white solid (177 mg, 64% yield). All the other polymers were prepared in a similar way according to the designed monomer-to-initiator ratio. Self-Assembly of PEG-b-PNOG Diblock Copolymers. A representative procedure for the self-assembly, the block polymer was dispersed in ethanol at a concentration of 1 mg/mL in a clean vial. The mixture was heated to the desired temperature for 2 h with stirring to give a clear solution. The solution was slowly cooled to room temperature and aged for different time intervals. The small aliquots (ca. 10 μL) were obtained from the solution at different time intervals to study the assembled structures. The self-assembly of PEGb-PNOG block copolymers in dioxane was prepared in a similar way.

EXPERIMENTAL SECTION

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Materials and Methods. n-Octyl-N-carboxyanhydride (OctNCA) was synthesized according to a reported method.33 Tetrahydrofuran (THF) and hexane were first purified by purging with dry N2, followed by passing through a column of activated alumina. Dichloromethane (DCM) was stored over calcium hydride (CaH2) and purified by vacuum distillation with CaH2. α-Methoxy-ωaminopoly(ethylene glycol) (PEG-NH2, Mn = 2000 g/mol, PDI = 1.05; Mn = 5000 g/mol, PDI = 1.07) was purchased from JenKem Technology Co, Ltd. (Beijing, China). All other chemicals were

RESULTS AND DISCUSSION The PEG-b-PNOG diblock copolymers were synthesized by ring-opening polymerization (ROP) of Oct-NCA using mPEG-NH2 (Mn = 2000 and 5000) as the macroinitiator (Scheme S1 and Figure S1).33 The polymerization was monitored by FTIR to confirm the consumption of OctNCA monomers. All peaks of the synthesized copolymers are B


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Figure 1. (a) TEM, (b) AFM, and (c) cryo-EM images of PEG112-b-PNOG54 diblock copolymer in ethanol at a concentration of 1 mg/mL. The solution was aged for 20 days after annealing at 70 °C.

well assigned in the 1H NMR spectra, confirming the chemical structures (Figure S2). A series of PEG-b-PNOG diblock copolymers with different degrees of polymerization (DP) were synthesized by varying the ratio of Oct-NCA to the initiator. The average DPs of PNOG are in the range of 25− 97. The GPC trace shows a monomodal molecular weight distribution with dispersity (Đ) ≤ 1.57 (Figure S3). 1H NMR spectroscopy was used to determine molecular weight and composition of the copolymers. The DPs were obtained from the proton integral ratios of alkyl group to the ethylene group of PEG block. Table S1 summarizes the molecular characteristics of the diblock copolymers PEGm-b-PNOGn, where the subscripts m and n represent the average DP of PEG and PNOG, respectively. The thermal properties of PEG-b-PNOG were first investigated by DSC (Figure S4). The DSC endotherms of PEG112-b-PNOG24 contain two peaks: one peak in the vicinity of 51.5 °C and another in the vicinity of 161.9 °C. The lower melting peak is associated with the PEG block. Note that the PNOG homopolymer exhibits two Tms arising from the crystallization of backbone (∼180 °C) and noctyl side-chain packing (∼52 °C).23 We thus attribute the peak at high temperature to the melting of PNOG crystals and the one at low temperature to the melting transition of PEG overlapped with PNOG. It is observed that the crystallization of PEG is suppressed by incorporating PNOG block, as indicated by decreased melting temperature (Tm) and enthalpy (ΔH) in the block copolymer (Table S2). As the DP of PNOG increases, its Tm and ΔH significantly increase. Decreasing DP of PEG also leads to increased ΔH of PNOG at a constant DP, suggesting considerable influence of PEG on PNOG crystallization. This is expected as we previously showed that the crystallization of both PEG and PNOG can be inhibited by incorporating additional polypeptide segment.33 The sample PEG44-b-PNOG25 with low DP of PEG shows two melting peaks. Considering that the higher peak at 51.2 °C is close to the Tm of PNOG, we attribute the peak at 35.7 °C to the melting of PEG crystals. The block copolymers PEG-b-PNOG were first dispersed in ethanol, which can dissolve PEG but is a poor solvent for PNOG block. After annealing at 70 °C for 2 h, the solution

was slowly cooled to room temperature and aged for 3 weeks. The laterally extended two-dimensional nanosheet-like structure in high yield is produced exclusively as observed by negative stained TEM of PEG112-b-PNOG54 (Figure 1a). A typical length along the long axis of the nanosheet can reach up to 10 μm, and the width long the short axis is in the range of hundreds of nanometers. The nanosheets along the long axis display an apparently straight edge, while the short edge is relatively rough (Figure S5). This indicates that the polymers are aligned in one direction along the long edge. We will address this later. The AFM image shows the 2D nanosheets are very flat with a uniform thickness of 6.3 ± 0.6 nm (Figure 1b). To preclude sample preparation effects during drying process, we studied the nanostructures in their solution state by cryogenic electron microscopy (cryo-EM). An unstained vitreous PEG112-b-PNOG54 thin film was prepared and examined by cryo-EM. Figure 1c shows the extended nanosheet assemblies that are extremely flexible and robust in solution form in a very high yield. Insight into the local structure and molecular packing of the 2D extended nanosheet was provided by grazing incidence wide-angle X-ray scattering (GIWAXS). Figure 2 shows the inplane line profiles of the membrane-like assemblies. The scattering peak at q = q* = 2.9 nm−1 is associated with Bragg reflections of PNOG crystals. It corresponds to the side-chain packing, denoted as the (001) plane. The distance between adjacent backbones is given by d = 2π/q* = 2.2 nm. Higherorder peaks at 2q* and 3q* indicate the presence of a lamellae. Note that the spacing is calculated to be twice the length of a fully extended chain of n-octyl groups, indicative of an end-toend packing of the side chains (Scheme 1).33 We further identified four additional higher order peaks as the reflections from the (100), (101), (102), and (103) planes, which give the characteristic domain spacing of 4.7, 4.5, 4.2, and 3.8 Å, respectively. The diffraction pattern is consistent with a recent study that demonstrates that the polypeptoid crystals intend to adopt an extended, all-cis conformation.26 Note that merely broad peaks are shown in the out-of-plane line profiles, indicating the lack of ordered domains (Figure S6). A broad peak centered at q = 4.5 nm−1 is likely related to the higher C


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adjacent monomer residues and DP of PNOG block, considering the end-group contributions are trivial.34 Based on the GIWAXS results, the value of extended chain length of PNOG is ∼20.1 nm. It is believable that the PNOG chain folds to fit the packing geometry, similar to traditional crystalline polymers. The folded PNOG chains are aligned along the xaxis, resulting in straight long edge. In contrast, the lack of alignment of polymer chains in the opposite direction leads to the rough short edge in the y-axis. The PEG chains are distributed randomly as isolated islands on the outer layers. The crystallization is thus confined in such nanoscale environment, consistent with the lack of crystalline peak in GIWAXS results. We heated the dried nanosheets of PEG112-bPNOG54 on a silica substrate to 60 °C (>Tm,PEG) for 2 h, followed by slowly cooling to the temperature or quenching in liquid nitrogen. In both cases, the thickness of nanosheets is nearly equivalent to that prior to heating, suggesting the absence of crystalline domains (Figure S7). The formation of isolated islands further enables protruding sticky ends of PNOG to fuse with the adjacent one, which facilitates the growth of the crystalline PNOG core along the x- and y-axis. In particular, the growth along the y-axis perpendicular to the direction of polymer chain leads to the formation of laterally extended nanosheet. This model explains why the nanosheet is dispersed in ethanol and can propagate their 2D nanosheet structure with one straight edge in two dimensions. To further understand the mechanism of nanosheet formation, we examined the structural evolution at different time intervals. The self-assemblies from the solution shortly after sample preparation were first studied. TEM images show exclusively spherical micelles with a diameter of 20.5 ± 1.6 nm of assemblies with the aging period of 3 h (Figure 3). The detailed structure of the diblock copolymer was studied by GIWAXS. The in-plane line profile shows the scattering peak at q = 3.0 nm−1, associated with the spatial dimension of 2.1 nm (Figure 2). The related lamellar structure is revealed by the characteristic diffractions. A broad peak centered at q = q* = 13.9 nm−1 suggests the lack of ordered structures. The nearly identical diffraction pattern in the out-of-plane line profiles confirms the spherical structures (Figure S6). It is conceivable that the spheres consist of soluble PEG corona layers and

Figure 2. GIWAXS in-plane measurements for the sphere and 2D nanosheet from PEG112-b-PNOG54 diblock copolymer.

order diffraction of nanosheet thickness (∼7 nm). This is possibly because the 2D nanosheets stack into a lamellar structure in the direction perpendicular to the substrate as a result of GIWAXS sample preparation approach. It is thus completely absent in the in-plane line profiles (Figure 2). The significant difference in both scattering patterns confirms that the 2D membrane-like nanostructure. It is generally accepted that the PEG dissolves in ethanol, confirmed by the absence of typical crystalline peak. We thus propose a model for crystalline polypeptoid nanosheet structure, shown in Scheme 1. To diminish the exposure of PNOG blocks to ethanol, the block copolymers are stacked into a bilayer lipid membranelike nanostructure with a crystalline PNOG interior and two solvated faces on both outer layers. Note that the thickness of the nanosheet (∼6.3 nm) is much smaller than the chain length of the block copolymers. The fully stretched end-to-end length of PNOG can be determined from the distance between

Scheme 1. 2D Nanosheet-like Structure of the Crystalline Diblock Copolymersa

a

The characteristics of the 2D nanosheets represent the sample PEG112-b-PNOG54. D


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Figure 3. TEM images of PEG112-b-PNOG54 aged for (a) 3 h, (b) 2 days, (c) 4 days, (d) 7 days, (e) and (f) 10 days after annealing at 70 °C in ethanol at a concentration of 1 mg/mL.

huge energetic penalty for exposing the hydrophobic domains to the solvent. To minimize the exposed edge, the intact 2D nanosheet-like structures with larger width up to 550 nm and thickness of 6.3 nm are exclusively obtained after ∼3 weeks aging. This suggests the merging of long fibrils into faceted nanostructures is possible. The morphology with similar thickness and dimension persists over a year, indicative of good stability at room temperature. This also suggests that cohesion of two nanosheets is unlikely to happen. The observation of a sphere-to-cylinder-to-nanosheet transition suggests that the spherical micelles with less ordered structure, as obtained initially, are in a metastable state. It is kinetically easier to form a spherical structure rather than a faceted nanosheet with significantly long-range ordering.35 In the presence of ethanol, PNOG in the core is slightly swelled, which facilitates the micellar core rearranges and assists the onset of crystallization to minimize the total free energy contribution. The evolution of crystallization in the micelle core induces the final formation of the hierarchical 2D nanosheets. The formation of nanosheets is remarkably coincident with theoretical model for morphology development of diblock copolymers in selective solvents where the insoluble block is crystalline, established by Vilgis and Halperin.36 They hypothesized the lamellar structure is the most common morphology except for the case of very long soluble blocks. In addition, it has been reported lately that multiple steps are involved for protein crystallization, referred as crystallization by particle attachment (CPA) strategy.37,38 In contrast to monomer-by-monomer addition, the protein crystals grow from non- or less-crystalline clusters through a hierarchical pathway. Here, we demonstrated the achievement

PNOG cores with less ordered structure. Note that the domain spacing of 2.1 nm is slightly smaller than that in 2D nanosheets, possibly due to the less ordered structure of the PNOG block. As the solution was aged for 2 days at room temperature, the particles are starting to attach with each other in appearance of necklace morphology, as indicated by the red arrows in Figure 3b. More sphere-like subunits are connected into cylinders after aging for 4 days. The AFM image shows that the height of spheres and cylinders is comparable, confirming the sphere-to-cylinder transition with similar packing geometry of polymer chains (Figure S8). With increasing aging time to 7 days, coexistence of long fibers and narrow 2D structures with short cylinders and spheres protruding on the edge are visible. It is thus conceivable that these short cylinders emanating from the platelets are in the process of providing material into the naonsheets. More specifically, the nanosheet-like structures grow on the consumption of cylinder-like or spherical micelles by coalescence. Note that the thickness of the nanosheet is much less than the radius of spheres and rods due to the distinct molecular packing geometry. This suggests the process is accompanied by crystallization of PNOG and the corresponding rearrangement of PEG chains. Interestingly the mechanism of the growth process is distinct from all the reported crystalline nanosheets. Further increasing the aging time to 10 days results in more 2D nanosheets with larger width of ∼400 nm. Simultaneously, the population of short cylinders is observed to dramatically decrease. These results confirm the proposed mechanism. Interestingly, a few fringelike aggregates on edge of the platelets are observed, as indicated by the red arrows in Figure 3e. Note that there is a E


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Figure 4. TEM (a) and AFM images (b) of PEG112-b-PNOG97 aged 20 days after annealing at 70 °C in ethanol at a concentration of 1 mg/mL.

Figure 5. TEM images of PEG112-b-PNOG54 aged for (a) 3 h, (b) 1 days, (c) 2 days, and (d) 20 days after annealing at 70 °C in dioxane at a concentration of 1 mg/mL.

of the hierarchical nanostructure through protein-mimetic mechanism from a class of synthetic block copolymers. A similar sphere-to-cylinder-to-nanosheet transition was observed for the block copolymer PEG112-b-PNOG97 (Figure S9). The 2D membrane-like nanostructure was also obtained from PEG112-b-PNOG97 after ∼3 weeks aging (Figure 4). AFM images show that the uniform thickness is 6.5 nm, similar to that of PEG112-b-PNOG54. Note that the fully stretched chain length of PNOG block is determined to be ∼36.9 nm from GIWAXS (Table S3), significantly larger than the thickness of the 2D nanosheets. This confirms the presence of chain folding in the PNOG crystals in self-assemblies. As the DP of PNOG is decreased to 24, only short cylinder-like morphologies are observed (Figure S10), possibly due to the low crystallinity of PNOG, as indicated by the DSC results. This is also true for the block copolymer PEG44-b-PNOG25 with reduced DP of PEG, suggesting the morphology of the system is largely dependent on the DP of PNOG, irrespective of that of PEG. These results confirm that self-assembly of block copolymers are dominated by crystallization of PNOG block. To better understand the morphology transition kinetics, the solution concentration effect on the 2D assemblies was also studied. Generally, increasing the solution concentration results in the less population of 2D nanostructures with considerably smaller dimension and rough edge (Figure S11). This is not surprising

as the concentration dramatically influences the crystallization property of the polymer.39 The influence of solvent selectivity on the solution selfassembly of block copolymers was further investigated. Two selective solvents, e.g., dioxane and THF, were applied. Both show enhanced solubility for PEG. 40 Similar to the morphology transition in ethanol, both PEG112-b-PNOG54 and PEG112-b-PNOG97 show sphere-to-cylinder-to-nanosheet evolution in dioxane as well (Figure 5, Figures S12 and S13). Although the 2D nanosheets assembled from both solvents show quite comparable thickness, the width of the nanosheets in dioxane is generally narrower than that in ethanol. In addition to the 2D nanosheets, one-dimensional fiber-like structures with a few micrometers were observed as well. The thicknesses of 2D nanosheets and 1D fibers are quite similar, e.g., 6.2 and 6.4 nm for sheets and fibers, respectively, for PEG112-b-PNOG54 (Figure S12). This indicates fairly close dimensional geometry of both crystals. Qualitatively similar scattering profiles are obtained from assemblies in dioxane (Table S3), confirming that the crystallization dominates the morphology transition. The fibers remain in spite of the long period aging of a year. This is possibly because enhanced solubility of PEG in dioxane reduces the exposure of protruding sticky ends of PNOG and further prevents lateral growth of nanosheets. In the case of block copolymers with F


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decreased DP of PNOG, only short cylinder-like morphologies are observed in dioxane solution, which is in a good agreement with that in ethanol (Figure S10). Note that a few narrow nanosheets and spheres are occasionally present in PEG112-bPNOG24. In the case of THF solution, PEG112-b-PNOG54 was heated to 50 °C and slowly cooled back to the room temperature due to the low boiling point of THF. However, only short cylinders and spheres were observed (Figure S14). Whether it is an effect of temperature is not clear. A detailed study of the temperature effect on the self-assemblies of PEG-b-PNOG was subsequently performed. As the samples were heated to a lower temperature of 30 °C, the block copolymers in both ethanol and dioxane show irregular morphologies (Figures S15 and S16). Obviously the PEG block dissolves well in 70 °C, which is higher than its melting temperature. Meanwhile, the melting transition of side-chain packing of PNOG coincides with this temperature range as well. Both factors can facilitate the alignment of polymer chains that promotes the formation of hierarchical structures. Because of the volatility of ethanol, only the dioxane solution was heated to 90 °C. No significant variation was observed in morphology of PEG112-b-PNOG54 and PEG112-b-PNOG97 as compared to 70 °C (Figure S16).

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.S. and Z.B.L. designed research; Z.K.S. and J.S. performed research; Y.H.W. and C.H.Z. contributed new analytic tools; Z.K.S., J.S., C.H.Z., and Z.B.L. analyzed data; and J.S. and Z.B.L. wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51722302, 21674054, 51503115, and 21434008), Qingdao Innovation leader talent Program (third), and the Taishan Scholars Program. The beamline 7.3.3 at the Advanced Light Source is supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231.

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CONCLUSIONS In conclusion, we have shown that the diblock copolymer based on polypeptoid can assemble into sphere-like structures in ethanol, which serves as fundamental packing motifs to form 2D ultrathin nanosheets with uniform thickness. This growth process is very different from the reported crystalline nanosheet. We demonstrated that the evolution of crystallization the micelle core induces the formation of the hierarchical 2D nanosheets. The sphere-to-cylinder-to-nanosheet transition mimics the multiple pathways of protein crystallization and coincides with theoretical model for morphology development of diblock copolymers in selective solvents where the insoluble block is crystalline. The flexibility of the peptoid backbone allows the dynamic chain to rearrange their interactions for the thermodynamically favorable transition from the initial assemblies to crystalline nanosheets. The traditional ring-opening polymerization (ROP) synthetic method allows access to higher molecular weights and larger scale yields. Furthermore, the great biocompatibility and potential bioactivities of polypeptoids offer great potential for the biomedical application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00986. Detailed thickness of the assemblies, additional 1H NMR data, DSC results, GPC data, GIWAXS results, TEM images, and AFM images (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*(Z.L.) E-mail [email protected]; Tel +86 053284022927. *(J.S.) E-mail [email protected]; Tel +86 053284022950. ORCID

Jing Sun: 0000-0003-1267-0215 Zhibo Li: 0000-0001-9512-1507 G


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