-
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
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pubs.acs.org/Macromoleculeshttp://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.macromol.8b00986http://dx.doi.org/10.1021/acs.macromol.8b00986
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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
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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.
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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.
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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.
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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.
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DOI: 10.1021/acs.macromol.8b00986Macromolecules 2018, 51,
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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.
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