crystallization-driven two-dimensional nanosheet from ...€¦ · crystallization-driven...

Crystallization-Driven Two-Dimensional Nanosheet fromHierarchical Self-Assembly of Polypeptoid-Based DiblockCopolymersZhekun Shi,† Yuhan Wei,† Chenhui Zhu,‡ Jing Sun,*,† and Zhibo Li*,†

†Key Laboratory of Biobased Polymer Materials, Shandong Provincial Education Department, School of Polymer Science andEngineering, 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 asbiomedicine 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. Toour 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 transitionpathway. The preliminary assembled spheres can behave like a fundamental packing motif to spontaneously stack into a 2Dlattice via an intermediate cylinder structure, driven by crystallization of PNOG domains. The nanosheet formation processfollows theoretical model for morphology development of crystalline block copolymers in selective solvents. Particularlyremarkable is that we obtained the hierarchical nanostructure from synthetic block copolymers through a multiple-step strategymimetic to protein crystallization. This is fairly distinct from the previously reported crystalline nanosheets. The ability toefficiently create 2D crystals from synthetic polymers by spontaneous assembly will enable new generations of bioinspirednanomaterials for a variety of potential applications in biomedicine and nanotechnology.

■ INTRODUCTIONIn nature, many biomolecules can fold into highly orderedstructures through different pathways, particularly hierarchicalself-assembly, which enable excellent functional performance.1

A variety of one-, two-, and three-dimensional complexarchitectures constructed via preassembled subunits of blockcopolymers have been prepared that offer potential applica-tions in nanotechnology, biomedicine, environmental technol-ogy, etc.2−6 The driving forces for the formation of complexstructures include inter- and intramolecular interactions suchas hydrogen bonding, ionic interaction, hydrophobicity,crystallization, and external stimuli like pH, temperature,light, etc.7−10 In particular, crystallization endows self-assemblyof block copolymers with a large number of unique properties,including tunable morphology and stability, living growthcharacteristics, and multicomponent nanostructure by a facile

cocrystallization approach. Great efforts have been devoted tothe achievement of supramolecular nanostructures fromcrystalline block copolymers.11−13 Manners and Winnik et al.demonstrated a crystallization-driven self-assembly process toprepare well-defined and functional hierarchical nanostructuresfrom block copolymers with crystallizable core-formingmetalloblock.2,13

Lately, two-dimensional (2D) nanomaterials have beenreceiving interest for many applications such as surface science,biomedicine, and energy storage. A few strategies haveemerged to fabricate 2D nanostructures from polymers/oligomers.14−17 The polymer crystallization has been reported

Received: May 8, 2018Revised: July 23, 2018Published: August 10, 2018

Article

pubs.acs.org/MacromoleculesCite This: Macromolecules 2018, 51, 6344−6351

© 2018 American Chemical Society 6344 DOI: 10.1021/acs.macromol.8b00986Macromolecules 2018, 51, 6344−6351

Dow

nloa

ded

via

WU

HA

N U

NIV

on

Aug

ust 2

8, 2

018

at 0

9:08

:10

(UT

C).

Se

e ht

tps:

//pub

s.ac

s.or

g/sh

arin

ggui

delin

es f

or o

ptio

ns o

n ho

w to

legi

timat

ely

shar

e pu

blis

hed

artic

les.

pubs.acs.org/Macromoleculeshttp://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.macromol.8b00986http://dx.doi.org/10.1021/acs.macromol.8b00986

as an efficient method to obtain lamellar platelets.18 However,to date, only a few reports have been addressed for theachievement, particularly laterally extended 2D nanosheet-likestructures from block copolymers.19 This is due to the highlycomplicated crystallization process occurring during nano-structure formation. For example, the crystallization of thepolymer can be confined in a nanoscale environment, resultingin low or absence of crystallinity. Appropriate polymer systemswith tunable control over the crystallization process are highlydesired.Polypeptoids, or poly(N-substituted glycine)s, have emerged

as promising bioinspired polymers that offer advantageousproperties for both fundamental research and applications innanoscience and biotechnology.20−22 The polypeptoid is aclass of peptidomimetic polymer that differs from polypeptideonly in that the pendant side-chains are attached to the amidenitrogen instead of α-carbon. The difference leads to theabsence of the hydrogen-bonding sites and chirality in themain chains, which simplifies the design of polymers mainly bytuning the properties of side chains. It has been reported thatthe polypeptoids with longer alkyl side chains are semicrystal-line with tunable melting transitions, which is in sharp contrastto polypeptides with inherent hydrogen-bonding interac-tions.23−25 The recent result demonstrated that the crystallinepeptoid molecules adopt extended and planar conformationwith all cis conformation.26 A couple of morphologies includingsphere, cylinder, and vesicle have been prepared fromcrystallizable peptoid block copolymers.12,27 These polymersare mostly based on polysarcosine,28,29 synthesized by aclassical polymerization approach. Zuckermann’s group30,31

and Chen’s group32 reported the crystalline nanosheet-likestructures from peptoid oligomers, by which the solid-phaseapproach was involved. With this method, the sequencespecificity and precise chain length can be obtained forpeptoids 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 2Dnanosheets show excellent flexibility and good thermal stabilityover a wide temperature range. We studied the effects ofcopolymer molecular weight, sample concentration, solvent,and temperature on the self-assembly behaviors of PEG-b-PNOG copolymers. It is observed that the PEG-b-PNOGprefers forming 2D nanosheet structures, which is driven bythe crystallization of PNOG block in selective solvent. Wedemonstrate that PEG-b-PNOG initially forms sphericalaggregates, which evolves into nanosheet structure withuniform thickness of ∼6 nm via a cylinder structureintermediate process. The facile synthetic approach combinedwith the excellent biocompatibility of polypeptoids offers greatpotential for the next generation of 2D nanomaterials for abroad range of advanced applications.

■ EXPERIMENTAL SECTIONMaterials and Methods. n-Octyl-N-carboxyanhydride (Oct-

NCA) was synthesized according to a reported method.33

Tetrahydrofuran (THF) and hexane were first purified by purgingwith dry N2, followed by passing through a column of activatedalumina. Dichloromethane (DCM) was stored over calcium hydride(CaH2) and purified by vacuum distillation with CaH2. α-Methoxy-ω-aminopoly(ethylene glycol) (PEG-NH2, Mn = 2000 g/mol, PDI =1.05; Mn = 5000 g/mol, PDI = 1.07) was purchased from JenKemTechnology Co, Ltd. (Beijing, China). All other chemicals were

purchased from commercial suppliers and used without furtherpurification unless otherwise noted.

Characterizations. 1H NMR spectra were recorded on a BrukerAV500 FT-NMR spectrometer. Tandem gel permeation chromatog-raphy (GPC) was performed at 25 °C on a Waters 410 equipped witha Waters 2414 RI detector and Waters Styragel HR4 and HR2columns. Chloroform (HPLC grade) was used as the eluent at a flowrate of 1.0 mL/min. Conventional calibrations were performed usingpolystyrene standards (PS). DSC studies were conducted using a TADSC Q20 calorimeter under nitrogen. Powder samples sealed into thealuminum pans were first heated from −40 to 200 at 10 °C/min forthree cycles. AFM studies were conducted using tapping mode AFM(Bruker Multimode 8 AFM/SPM system) in ambient air withNanoscope software. A volume of polymer solution (∼10 μL, 1 mg/mL) was drop-deposited and dried on freshly cleaved mica underambient conditions before AFM imaging. Minimal processing of theimages was done using NanoScope Analysis software from Bruker.TEM experiments were conducted on a FEI TECNAI 20, with aGatan digital camera and Gatan Digital Micrograph analysis software.The polymer solution (6 μL, 1 mg/mL) was pipetted onto on holeycarbon-coated 200 mesh copper grids. The excess amount of solutionwas removed, and the sample was negatively stained with 0.5 wt %uranyl acetate. The solvent was evaporated for at least 12 h exceptanything noted. Cryo-EM experiments were conducted on the sameinstrument. The vitrified specimens were prepared using a Vitrobot(FEI, Inc.). A 5 μL droplet of the ethanol solution at a concentrationof 1 mg/mL was deposited on the surface of glow discharged gridswith lacey carbon films. The droplet was blotted by filter paper for 1.5s, followed by 1 s draining, and then plunged into liquid ethane toobtain a vitrified thin film. The grids were then transferred to a Gatancryo-stage at −190 °C for analysis. The grazing incidence wide-angleX-ray scattering (GIWAXS) measurements were performed with theenergy of 10 keV in top-off mode at beamline 7.3.3, Advanced LightSource (ALS), Lawrence Berkeley National Lab (LBNL). Thescattering intensity was recorded on a 2D Pilatus 1M detector(Dectris) with a pixel size of 172 μm. A silver behenate sample wasused 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 typicalprocedure, mPEG-NH2 (91.7 mg,Mn = 5000 g/mol) was heated at 50°C, dried under high vacuum for 12 h, and then dissolved inanhydrous THF to obtain a solution (10%) in a reaction flask. In theglovebox, the n-octyl-N-carboxyanhydride monomer (234 mg) wasdissolved in anhydrous THF (2.5 mL), followed by adding to thereaction flask with given ratio. Polymerization was allowed to proceedat 60 °C for 24 h under an N2 atmosphere, and then the solution wasprecipitated in an excess amount of hexane. The white precipitate wascollected and washed with ample methanol and hexane. The productwas dried under vacuum to yield a white solid (177 mg, 64% yield).All the other polymers were prepared in a similar way according to thedesigned monomer-to-initiator ratio.

Self-Assembly of PEG-b-PNOG Diblock Copolymers. Arepresentative procedure for the self-assembly, the block polymerwas dispersed in ethanol at a concentration of 1 mg/mL in a cleanvial. The mixture was heated to the desired temperature for 2 h withstirring to give a clear solution. The solution was slowly cooled toroom temperature and aged for different time intervals. The smallaliquots (ca. 10 μL) were obtained from the solution at different timeintervals to study the assembled structures. The self-assembly of PEG-b-PNOG block copolymers in dioxane was prepared in a similar way.

■ RESULTS AND DISCUSSIONThe PEG-b-PNOG diblock copolymers were synthesized byring-opening polymerization (ROP) of Oct-NCA usingmPEG-NH2 (Mn = 2000 and 5000) as the macroinitiator(Scheme S1 and Figure S1).33 The polymerization wasmonitored by FTIR to confirm the consumption of Oct-NCA monomers. All peaks of the synthesized copolymers are

Macromolecules Article

DOI: 10.1021/acs.macromol.8b00986Macromolecules 2018, 51, 6344−6351

6345

http://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://dx.doi.org/10.1021/acs.macromol.8b00986

well assigned in the 1H NMR spectra, confirming the chemicalstructures (Figure S2). A series of PEG-b-PNOG diblockcopolymers with different degrees of polymerization (DP)were synthesized by varying the ratio of Oct-NCA to theinitiator. The average DPs of PNOG are in the range of 25−97. The GPC trace shows a monomodal molecular weightdistribution with dispersity (Đ) ≤ 1.57 (Figure S3). 1H NMRspectroscopy was used to determine molecular weight andcomposition of the copolymers. The DPs were obtained fromthe proton integral ratios of alkyl group to the ethylene groupof PEG block. Table S1 summarizes the molecular character-istics of the diblock copolymers PEGm-b-PNOGn, where thesubscripts m and n represent the average DP of PEG andPNOG, respectively. The thermal properties of PEG-b-PNOGwere first investigated by DSC (Figure S4). The DSCendotherms of PEG112-b-PNOG24 contain two peaks: onepeak in the vicinity of 51.5 °C and another in the vicinity of161.9 °C. The lower melting peak is associated with the PEGblock. Note that the PNOG homopolymer exhibits two Tmsarising from the crystallization of backbone (∼180 °C) and n-octyl side-chain packing (∼52 °C).23 We thus attribute thepeak at high temperature to the melting of PNOG crystals andthe one at low temperature to the melting transition of PEGoverlapped with PNOG. It is observed that the crystallizationof PEG is suppressed by incorporating PNOG block, asindicated by decreased melting temperature (Tm) and enthalpy(ΔH) in the block copolymer (Table S2). As the DP of PNOGincreases, its Tm and ΔH significantly increase. Decreasing DPof PEG also leads to increased ΔH of PNOG at a constant DP,suggesting considerable influence of PEG on PNOGcrystallization. This is expected as we previously showed thatthe crystallization of both PEG and PNOG can be inhibited byincorporating additional polypeptide segment.33 The samplePEG44-b-PNOG25 with low DP of PEG shows two meltingpeaks. Considering that the higher peak at 51.2 °C is close tothe Tm of PNOG, we attribute the peak at 35.7 °C to themelting of PEG crystals.The block copolymers PEG-b-PNOG were first dispersed in

ethanol, which can dissolve PEG but is a poor solvent forPNOG 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 struc-ture in high yield is produced exclusively as observed bynegative stained TEM of PEG112-b-PNOG54 (Figure 1a). Atypical length along the long axis of the nanosheet can reach upto 10 μm, and the width long the short axis is in the range ofhundreds of nanometers. The nanosheets along the long axisdisplay an apparently straight edge, while the short edge isrelatively rough (Figure S5). This indicates that the polymersare aligned in one direction along the long edge. We willaddress this later. The AFM image shows the 2D nanosheetsare very flat with a uniform thickness of 6.3 ± 0.6 nm (Figure1b). To preclude sample preparation effects during dryingprocess, we studied the nanostructures in their solution stateby cryogenic electron microscopy (cryo-EM). An unstainedvitreous PEG112-b-PNOG54 thin film was prepared andexamined by cryo-EM. Figure 1c shows the extendednanosheet assemblies that are extremely flexible and robustin solution form in a very high yield.Insight into the local structure and molecular packing of the

2D extended nanosheet was provided by grazing incidencewide-angle X-ray scattering (GIWAXS). Figure 2 shows the in-plane line profiles of the membrane-like assemblies. Thescattering peak at q = q* = 2.9 nm−1 is associated with Braggreflections of PNOG crystals. It corresponds to the side-chainpacking, denoted as the (001) plane. The distance betweenadjacent backbones is given by d = 2π/q* = 2.2 nm. Higher-order peaks at 2q* and 3q* indicate the presence of a lamellae.Note that the spacing is calculated to be twice the length of afully extended chain of n-octyl groups, indicative of an end-to-end packing of the side chains (Scheme 1).33 We furtheridentified four additional higher order peaks as the reflectionsfrom the (100), (101), (102), and (103) planes, which give thecharacteristic domain spacing of 4.7, 4.5, 4.2, and 3.8 Å,respectively. The diffraction pattern is consistent with a recentstudy that demonstrates that the polypeptoid crystals intend toadopt an extended, all-cis conformation.26 Note that merelybroad peaks are shown in the out-of-plane line profiles,indicating the lack of ordered domains (Figure S6). A broadpeak centered at q = 4.5 nm−1 is likely related to the higher

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. Thesolution was aged for 20 days after annealing at 70 °C.



6346

http://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://dx.doi.org/10.1021/acs.macromol.8b00986

order diffraction of nanosheet thickness (∼7 nm). This ispossibly because the 2D nanosheets stack into a lamellarstructure in the direction perpendicular to the substrate as aresult of GIWAXS sample preparation approach. It is thuscompletely absent in the in-plane line profiles (Figure 2). Thesignificant difference in both scattering patterns confirms thatthe 2D membrane-like nanostructure. It is generally acceptedthat the PEG dissolves in ethanol, confirmed by the absence oftypical crystalline peak. We thus propose a model forcrystalline polypeptoid nanosheet structure, shown in Scheme1. To diminish the exposure of PNOG blocks to ethanol, theblock copolymers are stacked into a bilayer lipid membrane-like nanostructure with a crystalline PNOG interior and twosolvated faces on both outer layers. Note that the thickness ofthe nanosheet (∼6.3 nm) is much smaller than the chainlength of the block copolymers. The fully stretched end-to-endlength of PNOG can be determined from the distance between

adjacent monomer residues and DP of PNOG block,considering the end-group contributions are trivial.34 Basedon the GIWAXS results, the value of extended chain length ofPNOG is ∼20.1 nm. It is believable that the PNOG chain foldsto fit the packing geometry, similar to traditional crystallinepolymers. The folded PNOG chains are aligned along the x-axis, resulting in straight long edge. In contrast, the lack ofalignment of polymer chains in the opposite direction leads tothe rough short edge in the y-axis. The PEG chains aredistributed randomly as isolated islands on the outer layers.The crystallization is thus confined in such nanoscaleenvironment, consistent with the lack of crystalline peak inGIWAXS results. We heated the dried nanosheets of PEG112-b-PNOG54 on a silica substrate to 60 °C (>Tm,PEG) for 2 h,followed by slowly cooling to the temperature or quenching inliquid nitrogen. In both cases, the thickness of nanosheets isnearly equivalent to that prior to heating, suggesting theabsence of crystalline domains (Figure S7). The formation ofisolated islands further enables protruding sticky ends ofPNOG to fuse with the adjacent one, which facilitates thegrowth of the crystalline PNOG core along the x- and y-axis. Inparticular, the growth along the y-axis perpendicular to thedirection of polymer chain leads to the formation of laterallyextended nanosheet. This model explains why the nanosheet isdispersed in ethanol and can propagate their 2D nanosheetstructure with one straight edge in two dimensions.To further understand the mechanism of nanosheet

formation, we examined the structural evolution at differenttime intervals. The self-assemblies from the solution shortlyafter sample preparation were first studied. TEM images showexclusively spherical micelles with a diameter of 20.5 ± 1.6 nmof assemblies with the aging period of 3 h (Figure 3). Thedetailed structure of the diblock copolymer was studied byGIWAXS. The in-plane line profile shows the scattering peakat q = 3.0 nm−1, associated with the spatial dimension of 2.1nm (Figure 2). The related lamellar structure is revealed by thecharacteristic diffractions. A broad peak centered at q = q* =13.9 nm−1 suggests the lack of ordered structures. The nearlyidentical diffraction pattern in the out-of-plane line profilesconfirms the spherical structures (Figure S6). It is conceivablethat the spheres consist of soluble PEG corona layers and

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

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

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



6347

http://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://dx.doi.org/10.1021/acs.macromol.8b00986

PNOG cores with less ordered structure. Note that the domainspacing of 2.1 nm is slightly smaller than that in 2Dnanosheets, possibly due to the less ordered structure of thePNOG block. As the solution was aged for 2 days at roomtemperature, the particles are starting to attach with each otherin appearance of necklace morphology, as indicated by the redarrows in Figure 3b. More sphere-like subunits are connectedinto cylinders after aging for 4 days. The AFM image showsthat the height of spheres and cylinders is comparable,confirming the sphere-to-cylinder transition with similarpacking geometry of polymer chains (Figure S8). Withincreasing aging time to 7 days, coexistence of long fibersand narrow 2D structures with short cylinders and spheresprotruding on the edge are visible. It is thus conceivable thatthese short cylinders emanating from the platelets are in theprocess of providing material into the naonsheets. Morespecifically, the nanosheet-like structures grow on theconsumption of cylinder-like or spherical micelles bycoalescence. Note that the thickness of the nanosheet ismuch less than the radius of spheres and rods due to thedistinct molecular packing geometry. This suggests the processis accompanied by crystallization of PNOG and thecorresponding rearrangement of PEG chains. Interestinglythe mechanism of the growth process is distinct from all thereported crystalline nanosheets. Further increasing the agingtime to 10 days results in more 2D nanosheets with largerwidth of ∼400 nm. Simultaneously, the population of shortcylinders is observed to dramatically decrease. These resultsconfirm the proposed mechanism. Interestingly, a few fringe-like aggregates on edge of the platelets are observed, asindicated by the red arrows in Figure 3e. Note that there is a

huge energetic penalty for exposing the hydrophobic domainsto the solvent. To minimize the exposed edge, the intact 2Dnanosheet-like structures with larger width up to 550 nm andthickness of 6.3 nm are exclusively obtained after ∼3 weeksaging. This suggests the merging of long fibrils into facetednanostructures is possible. The morphology with similarthickness and dimension persists over a year, indicative ofgood stability at room temperature. This also suggests thatcohesion of two nanosheets is unlikely to happen.The observation of a sphere-to-cylinder-to-nanosheet

transition suggests that the spherical micelles with less orderedstructure, as obtained initially, are in a metastable state. It iskinetically easier to form a spherical structure rather than afaceted nanosheet with significantly long-range ordering.35 Inthe presence of ethanol, PNOG in the core is slightly swelled,which facilitates the micellar core rearranges and assists theonset of crystallization to minimize the total free energycontribution. The evolution of crystallization in the micellecore induces the final formation of the hierarchical 2Dnanosheets. The formation of nanosheets is remarkablycoincident with theoretical model for morphology develop-ment of diblock copolymers in selective solvents where theinsoluble block is crystalline, established by Vilgis andHalperin.36 They hypothesized the lamellar structure is themost common morphology except for the case of very longsoluble blocks. In addition, it has been reported lately thatmultiple steps are involved for protein crystallization, referredas crystallization by particle attachment (CPA) strategy.37,38 Incontrast to monomer-by-monomer addition, the proteincrystals grow from non- or less-crystalline clusters through ahierarchical pathway. Here, we demonstrated the achievement

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 inethanol at a concentration of 1 mg/mL.



6348

http://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://dx.doi.org/10.1021/acs.macromol.8b00986

of the hierarchical nanostructure through protein-mimeticmechanism from a class of synthetic block copolymers.A similar sphere-to-cylinder-to-nanosheet transition was

observed for the block copolymer PEG112-b-PNOG97 (FigureS9). The 2D membrane-like nanostructure was also obtainedfrom PEG112-b-PNOG97 after ∼3 weeks aging (Figure 4). AFMimages show that the uniform thickness is 6.5 nm, similar tothat of PEG112-b-PNOG54. Note that the fully stretched chainlength of PNOG block is determined to be ∼36.9 nm fromGIWAXS (Table S3), significantly larger than the thickness ofthe 2D nanosheets. This confirms the presence of chain foldingin the PNOG crystals in self-assemblies. As the DP of PNOG isdecreased to 24, only short cylinder-like morphologies areobserved (Figure S10), possibly due to the low crystallinity ofPNOG, as indicated by the DSC results. This is also true forthe block copolymer PEG44-b-PNOG25 with reduced DP ofPEG, suggesting the morphology of the system is largelydependent on the DP of PNOG, irrespective of that of PEG.These results confirm that self-assembly of block copolymersare dominated by crystallization of PNOG block. To betterunderstand the morphology transition kinetics, the solutionconcentration effect on the 2D assemblies was also studied.Generally, increasing the solution concentration results in theless population of 2D nanostructures with considerably smallerdimension and rough edge (Figure S11). This is not surprising

as the concentration dramatically influences the crystallizationproperty of the polymer.39

The influence of solvent selectivity on the solution self-assembly of block copolymers was further investigated. Twoselective solvents, e.g., dioxane and THF, were applied. Bothshow enhanced solubility for PEG.40 Similar to themorphology transition in ethanol, both PEG112-b-PNOG54and PEG112-b-PNOG97 show sphere-to-cylinder-to-nanosheetevolution in dioxane as well (Figure 5, Figures S12 and S13).Although the 2D nanosheets assembled from both solventsshow quite comparable thickness, the width of the nanosheetsin dioxane is generally narrower than that in ethanol. Inaddition to the 2D nanosheets, one-dimensional fiber-likestructures with a few micrometers were observed as well. Thethicknesses of 2D nanosheets and 1D fibers are quite similar,e.g., 6.2 and 6.4 nm for sheets and fibers, respectively, forPEG112-b-PNOG54 (Figure S12). This indicates fairly closedimensional geometry of both crystals. Qualitatively similarscattering profiles are obtained from assemblies in dioxane(Table S3), confirming that the crystallization dominates themorphology transition. The fibers remain in spite of the longperiod aging of a year. This is possibly because enhancedsolubility of PEG in dioxane reduces the exposure ofprotruding sticky ends of PNOG and further prevents lateralgrowth of nanosheets. In the case of block copolymers with

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 aconcentration of 1 mg/mL.



6349

http://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://dx.doi.org/10.1021/acs.macromol.8b00986

decreased DP of PNOG, only short cylinder-like morphologiesare observed in dioxane solution, which is in a good agreementwith that in ethanol (Figure S10). Note that a few narrownanosheets and spheres are occasionally present in PEG112-b-PNOG24.In the case of THF solution, PEG112-b-PNOG54 was heated

to 50 °C and slowly cooled back to the room temperature dueto the low boiling point of THF. However, only short cylindersand spheres were observed (Figure S14). Whether it is aneffect of temperature is not clear. A detailed study of thetemperature effect on the self-assemblies of PEG-b-PNOG wassubsequently performed. As the samples were heated to alower temperature of 30 °C, the block copolymers in bothethanol and dioxane show irregular morphologies (Figures S15and S16). Obviously the PEG block dissolves well in 70 °C,which is higher than its melting temperature. Meanwhile, themelting transition of side-chain packing of PNOG coincideswith this temperature range as well. Both factors can facilitatethe alignment of polymer chains that promotes the formationof hierarchical structures. Because of the volatility of ethanol,only the dioxane solution was heated to 90 °C. No significantvariation was observed in morphology of PEG112-b-PNOG54and PEG112-b-PNOG97 as compared to 70 °C (Figure S16).

■ CONCLUSIONSIn conclusion, we have shown that the diblock copolymerbased on polypeptoid can assemble into sphere-like structuresin ethanol, which serves as fundamental packing motifs to form2D ultrathin nanosheets with uniform thickness. This growthprocess is very different from the reported crystallinenanosheet. We demonstrated that the evolution of crystal-lization the micelle core induces the formation of thehierarchical 2D nanosheets. The sphere-to-cylinder-to-nano-sheet transition mimics the multiple pathways of proteincrystallization and coincides with theoretical model formorphology development of diblock copolymers in selectivesolvents where the insoluble block is crystalline. The flexibilityof the peptoid backbone allows the dynamic chain to rearrangetheir interactions for the thermodynamically favorabletransition from the initial assemblies to crystalline nanosheets.The traditional ring-opening polymerization (ROP) syntheticmethod allows access to higher molecular weights and largerscale yields. Furthermore, the great biocompatibility andpotential bioactivities of polypeptoids offer great potential forthe biomedical application.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.8b00986.

Detailed thickness of the assemblies, additional 1H NMRdata, DSC results, GPC data, GIWAXS results, TEMimages, and AFM images (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*(Z.L.) E-mail [email protected]; Tel +86 053284022927.*(J.S.) E-mail [email protected]; Tel +86 053284022950.ORCIDJing Sun: 0000-0003-1267-0215Zhibo Li: 0000-0001-9512-1507

Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript. J.S. and Z.B.L. designed research; Z.K.S. andJ.S. performed research; Y.H.W. and C.H.Z. contributed newanalytic 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.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Natural ScienceFoundation of China (51722302, 21674054, 51503115, and21434008), Qingdao Innovation leader talent Program (third),and the Taishan Scholars Program. The beamline 7.3.3 at theAdvanced Light Source is supported by the Director of theOffice of Science, Office of Basic Energy Sciences, of the U.S.Department of Energy under Contract DE-AC02-05CH11231.

■ REFERENCES(1) Chung, S.; Shin, S.; Bertozzi, C.; De Yoreo, J. Self-catalyzedgrowth of S layers via an amorphousto-crystalline transition limited byfolding kinetics. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 16536−16541.(2) Hudson, Z. M.; Boott, C. E.; Robinson, M. E.; Rupar, P. A.;Winnik, M. A.; Manners, I. Tailored hierarchical micelle architecturesusing living crystallization-driven self-assembly in two dimensions.Nat. Chem. 2014, 6, 893.(3) Murnen, H. K.; Rosales, A. M.; Jaworski, J. N.; Segalman, R. A.;Zuckermann, R. N. Hierarchical self-assembly of a biomimetic diblockcopolypeptoid into homochiral superhelices. J. Am. Chem. Soc. 2010,132, 16112−16119.(4) Mai, Y.; Eisenberg, A. Self-assembly of block copolymers. Chem.Soc. Rev. 2012, 41, 5969−5985.(5) Cademartiri, L.; Bishop, K. J. M. Programmable self-assembly.Nat. Mater. 2015, 14, 2.(6) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P.Multicompartment Micelles from ABC Miktoarm Stars in Water.Science 2004, 306, 98−101.(7) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L.Toroidal Triblock Copolymer Assemblies. Science 2004, 306, 94.(8) Qiu, H.; Gao, Y.; Boott, C. E.; Gould, O. E.; Harniman, R. L.;Miles, M. J.; Webb, S. E.; Winnik, M. A.; Manners, I. Uniform patchyand hollow rectangular platelet micelles from crystallizable polymerblends. Science 2016, 352, 697.(9) Löbling, T. I.; Borisov, O.; Haataja, J. S.; Ikkala, O.; Gröschel, A.H.; Müller, A. H. E. Rational design of ABC triblock terpolymersolution nanostructures with controlled patch morphology. Nat.Commun. 2016, 7, 12097.(10) Zhuang, Z.; Jiang, T.; Lin, J.; Gao, L.; Yang, C.; Wang, L.; Cai,C. Hierarchical Nanowires Synthesized by Supramolecular StepwisePolymerization. Angew. Chem., Int. Ed. 2016, 55, 12522−12527.(11) Lee, I. H.; Amaladass, P.; Yoon, K. Y.; Shin, S.; Kim, Y. J.; Kim,I.; Lee, E.; Choi, T. L. Nanostar and nanonetwork crystals fabricatedby in situ nanoparticlization of fully conjugated polythiophene diblockcopolymers. J. Am. Chem. Soc. 2013, 135, 17695−17698.(12) Lee, C. U.; Smart, T. P.; Guo, L.; Epps, T. H., III; Zhang, D.Synthesis and Characterization of Amphiphilic Cyclic DiblockCopolypeptoids from N-Heterocyclic Carbene-Mediated ZwitterionicPolymerization of N-Substituted N-Carboxyanhydride. Macromole-cules 2011, 44, 9574−9585.(13) Li, X.; Gao, Y.; Harniman, R.; Winnik, M. A.; Manners, I.Hierarchical Assembly of Cylindrical Block Comicelles Mediated bySpatially Confined Hydrogen-Bonding Interactions. J. Am. Chem. Soc.2016, 138, 12902.(14) Ballauff, M. Self-assembly creates 2D materials. Science 2016,352, 656−657.



6350

http://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfhttp://pubs.acs.orghttp://pubs.acs.org/doi/abs/10.1021/acs.macromol.8b00986http://pubs.acs.org/doi/abs/10.1021/acs.macromol.8b00986http://pubs.acs.org/doi/suppl/10.1021/acs.macromol.8b00986/suppl_file/ma8b00986_si_001.pdfmailto:[email protected]:[email protected]://orcid.org/0000-0003-1267-0215http://orcid.org/0000-0001-9512-1507http://dx.doi.org/10.1021/acs.macromol.8b00986

(15) Crassous, J. J.; Schurtenberger, P.; Ballauff, M.; Mihut, A. M.Design of block copolymer micelles via crystallization. Polymer 2015,62, A1−A13.(16) Ni, B.; Huang, M.; Chen, Z.; Chen, Y.; Hsu, C. H.; Li, Y.;Pochan, D.; Zhang, W. B.; Cheng, S. Z. D.; Dong, X. H. Pathwaytoward Large Two-Dimensional Hexagonally Patterned ColloidalNanosheets in Solution. J. Am. Chem. Soc. 2015, 137, 1392−1395.(17) Sun, Y.; Li, Z.; Wang, Z. Self-assembled monolayer andmultilayer films based on L-lysine functionalized perylene bisimide. J.Mater. Chem. 2012, 22, 4312−4318.(18) He, X.; Hsiao, M. S.; Boott, C. E.; Harniman, R. L.; Nazemi, A.;Li, X.; Winnik, M. A.; Manners, I. Two-dimensional assemblies fromcrystallizable homopolymers with charged termini. Nat. Mater. 2017,16, 481.(19) Gad̈t, T.; Schacher, F. H.; Mcgrath, N.; Winnik, M. A.;Manners, I. Probing the Scope of Crystallization-Driven Living Self-Assembly: Studies of Diblock Copolymer Micelles with a Poly-isoprene Corona and a Crystalline Poly(ferrocenyldiethylsilane)Core-Forming Metalloblock. Macromolecules 2011, 44, 3777−3786.(20) Sun, J.; Zuckermann, R. N. Peptoid polymers: a highlydesignable bioinspired material. ACS Nano 2013, 7, 4715−4732.(21) Zhang, D.; Lahasky, S. H.; Guo, L.; Lee, C. U.; Lavan, M.Polypeptoid Materials: Current Status and Future Perspectives.Macromolecules 2012, 45, 5833−5841.(22) Gangloff, N.; Ulbricht, J.; Lorson, T.; Schlaad, H.; Luxenhofer,R. Peptoids and Polypeptoids at the Frontier of Supra- andMacromolecular Engineering. Chem. Rev. 2016, 116, 1753.(23) Lee, C. U.; Li, A.; Ghale, K.; Zhang, D. Crystallization andMelting Behaviors of Cyclic and Linear Polypeptoids with Alkyl SideChains. Macromolecules 2013, 46, 8213−8223.(24) Rosales, A. M.; Murnen, H. K.; Zuckermann, R. N.; Segalman,R. A. Control of Crystallization and Melting Behavior in SequenceSpecific Polypeptoids. Macromolecules 2010, 43, 5627−5636.(25) Secker, C.; Völkel, A.; Tiersch, B.; Koetz, J.; Schlaad, H.Thermo-Induced Aggregation and Crystallization of Block Copoly-peptoids in Water. Macromolecules 2016, 49, 979.(26) Greer, D. R.; Stolberg, M. A.; Kundu, J.; Spencer, R. K.; Pascal,T.; Prendergast, D.; Balsara, N. P.; Zuckermann, R. N. UniversalRelationship between Molecular Structure and Crystal Structure inPeptoid Polymers and Prevalence of the cis Backbone Conformation.J. Am. Chem. Soc. 2018, 140, 827.(27) Fetsch, C.; Gaitzsch, J.; Messager, L.; Battaglia, G.; Luxenhofer,R. Corrigendum: Self-Assembly of Amphiphilic Block Copolypeptoids− Micelles, Worms and Polymersomes. Sci. Rep. 2016, 6, 33491.(28) Hörtz, C.; Birke, A.; Kaps, L.; Decker, S.; Wac̈htersbach, E.;Fischer, K.; Schuppan, D.; Barz, M.; Schmidt, M. Cylindrical BrushPolymers with Polysarcosine Side Chains: A Novel BiocompatibleCarrier for Biomedical Applications. Macromolecules 2015, 48, 2074−2086.(29) Klinker, K.; Schaf̈er, O.; Huesmann, D.; Bauer, T.; Capelôa, L.;Braun, L.; Stergiou, N.; Schinnerer, M.; Dirisala, A.; Miyata, K.; et al.Secondary Structure-Driven Self-Assembly of Reactive Polypept(o)-ides: Controlling Size, Shape and Function of Core Cross-LinkedNanostructures. Angew. Chem., Int. Ed. 2017, 56, 9608.(30) Nam, K. T.; Shelby, S. A.; Choi, P. H.; Marciel, A. B.; Chen, R.;Tan, L.; Chu, T. K.; Mesch, R. A.; Lee, B. C.; Connolly, M. D.; et al.Free-floating ultrathin two-dimensional crystals from sequence-specific peptoid polymers. Nat. Mater. 2010, 9, 454−460.(31) Mannige, R. V.; Haxton, T. K.; Proulx, C.; Robertson, E. J.;Battigelli, A.; Butterfoss, G. L.; Zuckermann, R. N.; Whitelam, S.Peptoid nanosheets exhibit a new secondary-structure motif. Nature2015, 526, 415−420.(32) Jin, H.; Jiao, F.; Daily, M. D.; Chen, Y.; Yan, F.; Ding, Y. H.;Zhang, X.; Robertson, E. J.; Baer, M. D.; Chen, C. L. Highly stableand self-repairing membrane-mimetic 2D nanomaterials assembledfrom lipid-like peptoids. Nat. Commun. 2016, 7, 12252.(33) Ni, Y.; Sun, J.; Wei, Y.; Fu, X.; Zhu, C.; Li, Z. Two-DimensionalSupramolecular Assemblies from pH-Responsive Poly(ethyl glycol)-b-

poly(l-glutamic acid)-b-poly(N-octylglycine) Triblock Copolymer.Biomacromolecules 2017, 18, 3367−3374.(34) Sun, J.; Teran, A. A.; Liao, X.; Balsara, N. P.; Zuckermann, R.N. Crystallization in sequence-defined peptoid diblock copolymersinduced by microphase separation. J. Am. Chem. Soc. 2014, 136,2070−2077.(35) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Laterally NanostructuredVesicles, Polygonal Bilayer Sheets, and Segmented WormlikeMicelles. Nano Lett. 2006, 6, 1245−1249.(36) Vilgis, T.; Halperin, A. Aggregation of coil-crystalline blockcopolymers: equilibrium crystallization. Macromolecules 1991, 24,2090−2095.(37) De Yoreo, J. J.; Gilbert, P. U.; Sommerdijk, N. A.; Penn, R. L.;Whitelam, S.; Joester, D.; Zhang, H.; Rimer, J. D.; Navrotsky, A.;Banfield, J. F.; et al. Crystallization by particle attachment in synthetic,biogenic, and geologic environments. Science 2015, 349, aaa6760.(38) Erdemir, D.; Lee, A. Y.; Myerson, A. S. Nucleation of Crystalsfrom Solution: Classical and Two-Step Models. Acc. Chem. Res. 2009,42, 621−629.(39) Zhang, J.; Muthukumar, M. Monte Carlo simulations of singlecrystals from polymer solutions. J. Chem. Phys. 2007, 126, 234904.(40) Jia, L.; Albouy, P. A.; Di Cicco, A.; Cao, A.; Li, M. H. Self-assembly of amphiphilic liquid crystal block copolymers containing acholesteryl mesogen: Effects of block ratio and solvent. Polymer 2011,52, 2565−2575.



6351
http://dx.doi.org/10.1021/acs.macromol.8b00986

crystallization-driven two-dimensional nanosheet from ...€¦ · crystallization-driven...

Documents