Size-Tunable Nanosheets by the Crystallization-Driven 2D Self-Assembly of Hyperbranched Poly(ether amine) (hPEA)Bing Yu, Xuesong Jiang,* and Jie Yin

School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao TongUniversity Shanghai 200240, People’s Republic of China

*S Supporting Information

ABSTRACT: We reported the preparation of uniform square nanosheets withtunable size by the living crystallization-driven 2D self-assembly of hyper-branched poly(ether amine) capped with heptaisobutyl polyhedral oligomericsilsesquioxane (POSS). The nanosheets of HP1 containing both anthracene(AN) and POSS moieties in a solution of 1,4-dioxane and water can befragmented after the melting of the POSS moieties upon heating and can beregenerated after the recrystallization of POSS moieties, which was confirmedby microdifferential scanning calorimetry (μDSC) and dynamic light scattering(DLS) studies and transmission electron microscopy (TEM) images. Theobtained fragmented nanosheets (HP1-NSs) with a relatively small size wereused as seeds for the 2D epitaxial living growth of HP1 unimers to fabricateuniform square nanosheets with tunable edge lengths from ∼0.5 to ∼4.5 μm,which is dependent on the unimer-to-seed ratio. Furthermore, dual-componentnanosheets can also be obtained by random cocrystallization of HP1 with another type of hPEA capped with POSS andferrocene (HP2). This crystallization-driven 2D self-assembly behavior of POSS-capped hPEA might provide potentialsignificance in the preparation of functional nanosheets with different sizes and components, which could be further used astemplates for inorganic nanosheets and 2D-platforms for metal nanoparticles.

1. INTRODUCTION

The crystallization-driven self-assembly (CDSA) of polymers insolution is attractive for the preparation of complex micro- andnanoarchitectures as a result of the epitaxial growth of thecrystallized micelles.1−5 Because of the “living” characteristic ofthe CDSA process, the obtained nanostructures can be fine-tuned in both size and size distribution.6−10 Another advantageof CDSA is the easily tuned composition of the obtainednanostructures, and the dual-component or multicomponentnanostructures with different morphologies can be formed viarandom or block cocrystallization of the polymers with similarcrystallized moieties.11 The crystallized blocks in thesepolymers can be composed of poly(ferrocenyl dimethylsilane)(PFDMS),12 poly(3-hexylthiophene) (P3HT),13,14 poly(ε-caprolactone) (PCL),15,16 stereoregular polylactides(srPLA),17,18 poly(ethylene oxide),19 polyethylene (PE),20

and polyacrylonitrile (PAN).21 Most of these types of blockpolymers can only form one-dimensional cylindrical or worm-like crystalline-core micelles through the living CDSA method.Two-dimensional (2D) polymer nanosheets have attracted

tremendous attention due to their potential application inoptical devices,22 organic electronics,23 emulsion stabilizers,24

enzyme/catalyst arrays,25 and templates.26 As a result of theaforementioned advantages of CDSA, it is significant tofabricate size-tunable or multicomponent nanosheets withnarrow size-distributions through the living CDSA method.Although the preparation of some polymer nanosheets by self-

assembly of block polymers was reported,27−32 as well as somestudies involving CDSA with 2D platelets as buildingblocks,33−35 few of the self-assembly processes possessed theliving characteristic of CDSA, which is beneficial for obtainingsize-tunable nanosheets. As the self-assembly of polymerscontaining polyhedral oligomeric silsesquioxane (POSS) wasalready investigated,36−39 and this type of polymers couldcrystallize under a proper condition, recently, we preparedsquare hybrid nanosheets with an ultrathin thickness of ∼5 nmthrough the self-assembly of hyperbranched poly(ether amine)(hPEA) capped with anthracene (AN) and heptaisobutyl POSS(Scheme 1).40 The obtained nanosheets were multiresponsiveand could be used as a 2D-platform for metal nanoparticles,which made metal nanoparticles transfer reversibly between theoil and water phases.41 Meanwhile, square silica/titaniamesoporous nanosheets can also be prepared using thesehPEA nanosheets as templates.42 Many other functionalmoieties, such as naphthalene and pyrene, can also beintroduced into the hybrid nanosheets in the same way,which is beneficial for the preparation of functional polymernanosheets. Because POSS prefers the ordered crystallizedaggregation in the formation of these hPEA nanosheets, it is of

Received: April 22, 2014Revised: June 19, 2014Published: June 27, 2014

Article

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great interest to know whether these nanosheets were formedthrough the living two-dimensional (2D) CDSA approach.In this research, we showed that uniform hPEA nanosheets

with a controlled size can be obtained by living crystallization-driven 2D self-assembly. Upon heating to a higher temperaturethan the melting point of POSS crystallites, hPEA nanosheetswere fragmented in the mixture of water/dioxane and thenregenerated after crystallization at a lower temperature.Meanwhile, small fragments of the hPEA nanosheets could besubsequently used as seed for the 2D epitaxial growth of thehPEA unimers to obtain uniform nanosheets with tunable size.Moreover, dual-component nanosheets could be preparedthrough the random cocrystallization of the fragments ofanthracene-containing POSS-capped hPEA (HP1) and ferro-cene-containing POSS-capped hPEA (HP2). To the best of ourknowledge, this is the first example of the preparation ofuniform nanosheets with tunable size through the livingcrystallization-driven 2D self-assembly of hyperbranchedpolymers.

2. RESULTS AND DISCUSSION

Amphiphilic HP1 comprises hydrophilic hPEA as the backboneand hydrophobic POSS and AN moieties in the periphery. The2D self-assembly process of HP1 was investigated in ourprevious research,40 upon the addition of water to the dioxanesolution of HP1, the solvent became progressively worse forPOSS and AN moieties. When the water content reached acritical value, two HP1 molecules can assemble together by thecrystallized aggregation of hydrophobic POSS, while hydro-philic hPEA chains were extended to the opposite side in the z-axis direction, and then a “dual-pyramid”-like structure formed,which may further assemble along the x-axis and y-axisdirections through the crystallized aggregation of POSS andπ−π stacking aggregation of AN, resulting in the formation ofnanosheets (HP1-NS). The HP1-NS comprised three layers,like a sandwich: the hydrophobic POSS and AN moietiesformed the hydrophobic inner layer of the nanosheet, whereasthe hydrophilic hPEA formed the outer layer in contact withwater (Scheme 1). The crystallized structure could bedestroyed when heated above the melting temperature. Becausethe melting temperature of crystallized POSS in HP1-NS was∼124 °C, which could not be reached for aqueous solutions, acertain amount of dioxane was added to the aqueous solution ofHP1-NS as plasticizer, which was also a good solvent for POSSand could lower the crystallization melting temperature.20 Theaddition amount of dioxane should be controlled in order toensure that HP1-NS can keep the nanosheet structure at roomtemperature. To optimize the addition amount of dioxane,water was progressively dropped into the dioxane solution ofHP1. This process was monitored by UV−vis transmittanceand DLS experiments (Figure S2), which exhibited that theassemblies were completely formed when the water content ofthe solution was 50%. As a result, the water content of thesolution was 50% in the following investigation.The melting behavior of crystallized HP1-NSs in water/

dioxane solution was then investigated with microdifferentialscanning calorimetry (μDSC). As shown in Figure 1a, anendothermic peak was observed at ∼68 °C, which is attributedto the melting of the crystallized POSS in HP1-NS. Meanwhile,with increasing temperature, the apparent hydrodynamic radiusof HP1-NS decreased, whereas the transmittance of thesolution increased during the heating process from 55 to 80°C, indicating that these HP1-NSs were also progressivelyfragmented with the melting of the crystallized POSS (Figure1b). To ensure to obtain fragmented HP1-NS with the smallest

Scheme 1. Structure of POSS/Anthracene-CappedHyperbranched Poly(ether amine) (HP1) and POSS/Ferrocene-Capped Hyperbranched Poly(ether amine)(HP2), as Well as the Proposed Mechanism for theFormation of HP1 Nanosheets through 2D Self-Assembly(HP2 can Self-Assemble into Nanosheets Similarly)

Figure 1. (a) μDSC heating and cooling thermograms of 50 mg/mL HP1-NSs solution (water/dioxane volume ratio is 1:1). The scans were run at aheating and cooling rate of 1 °C/min. (b) The dependence of the transmittance and apparent hydrodynamic radius of 5 mg/mL HP1 solution(water/dioxane volume ratio is 1:1) on the temperature during the heating process. The sample was equilibrated at the desired temperature for 5min before measurement.

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size, HP1-NS water/dioxane solution was heated to 90 °C for 2h with stirring before the following recrystallization process. Asthe POSS/AN moieties in HP1 molecules were still hydro-phobic after heating to the melting temperature of POSSmoieties though the crystallines of POSS melted, HP1-NS wasfragmented into some micelle-like fragments rather than beingcompletely dissolved at 90 °C in a solvent composed of a largeamount of water, and this process was also confirmed by theDLS results in Figure 1b. The apparent hydrodynamic radius ofHP1 in solution was ∼60 nm, which was much larger than theradius of HP1 unimolecular micelles, indicating HP1-NS wasnot completely dissolved into unimolecular micelles.The recrystallization behavior of HP1-NS in water/dioxane

solution was then investigated with these fragmented smallnanosheets. An exothermic peak could be observed at ∼60 °Cin the cooling thermal analysis curve of Figure 1a, suggestingthe possible recrystallization behavior of the POSS moieties inHP1 during the cooling process when the temperature is lowerthan ∼60 °C. As a result, the crystallization temperature was setat 60 °C to obtain the crystallized POSS with relatively fewerdefects. The water/dioxane solution of the fragmented HP1-NS was cooled at a rate of ∼3 °C/min from 90 to 60 °C, thenequilibrated at 60 °C, and the recrystallization behavior of HP1was also monitored by μDSC. At a time interval, an aliquot ofthe water/dioxane solution of fragmented HP1-NSs equili-brated at 60 °C was removed, and the crystallization degrees ofHP1 in these solutions were measured immediately. As shownin Figure 2a, the melting enthalpy increased with the increasingof the recrystallization time, indicating that the recrystallizationof POSS moieties in HP1 occurred during the equilibration at60 °C. The recrystallization process of fragmented HP1-NS inwater/dioxane solution was also traced by DLS. An aliquot ofthe water/dioxane solution of fragmented HP1-NSs equili-brated at 60 °C was also removed at interval time and dilutedwith 5 times of water immediately. Because water was a poorsolvent for POSS and AN, the morphology structure of theobtained assemblies was locked. As shown in Figure 2b, theapparent hydrodynamic radius increased with the increasingequilibration time, indicating the gradual growth of the HP1-NSs.The morphology of the HP1-NS was observed by TEM. As

shown in Figure 3a and Figure S3a (Supporting Information),the number-average edge length of HP1-NS was 2.18 ± 0.47μm before heating. After heating to 90 °C and then rapidlycooling to 25 °C, small fragmented HP1-NS with edge lengthof 0.32 ± 0.09 μm was observed by TEM, indicating that HP1-

NS was indeed fragmented upon heating (Figure 3b and FigureS3b, as the growth of the fragmented HP1-NS may occurduring the rapidly cooling process, the size of the obtainedfragmented HP1-NS in the figure was larger than the actualvalue). In contrast, large HP1-NS with the edge length of 3.32± 0.94 μm was revealed by TEM after heating to 90 °C andthen crystallization at 60 °C for 24 h, suggesting that thefragmented HP1-NSs can be recombined into the large regularsquare nanosheet (Figure 3c and Figure S3c). The TEMobservation is in good agreement with the results from DLSand μDSC. Generally, the size of assemblies from CDSAdecreased upon lowering the crystallization temperature of thenucleation and growth processes.20 As a result, the size of thefragmented HP1-NSs caused by heating was still relativelysmall after rapid cooling to 25 °C. However, when the solutionof fragmented HP1-NSs was kept at 60 °C, the recrystallizationprocess was slow and the regenerated HP1-NS became large.As proposed in Figure 3d, HP1-NS could be fragmented withthe melting of POSS moieties upon heating and regeneratedthrough the crystallization-driven 2D self-assembly at a lowertemperature, which was supported by the results from μDSC,DLS, and TEM.By using the small sterically stabilized crystalline-core

micelles as seeds for the one-dimensional epitaxial growth ofthe block polymer unimers, Winnik and Manners et al.prepared monodisperse cylindrical micelles with a controlled

Figure 2. (a) μDSC heating thermograms of 50 mg/mL HP1-NS solution (water/dioxane volume ratio is 1:1) after heating to 90 °C and thencrystallization at 60 °C for different times. The scans were run at a heating rate of 1 °C/min. (b) Dependence of the apparent hydrodynamic radiuson the crystallization time at 60 °C determined by DLS.

Figure 3. TEM images of the nanosheets formed by HP1. (a) HP1-NS obtained by 2D epitaxial self-assembly through dropping waterinto HP1 dioxane solution (water/dioxane volume ratio is 1:1). (b)Fragmented HP1-NS obtained by equilibrating the HP1 water/dioxane solution at 90 °C for 2 h and then rapid cooling to 25 °C. (c)HP1-NS obtained by equilibrating the HP1 water/dioxane solution at90 °C for 2 h and then recrystallizing at 60 °C for 24 h. (d) Theproposed mechanism for the fragmentation and regeneration of theHP1 nanosheets. The scale bar in all TEM images is 2 μm.

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length through the living crystallization-driven self-assem-bly.6,7,43,44 It is of great interest to expand this living one-dimensional self-assembly to two-dimensional self-assembly tofabricate nanosheets with a controlled size. As a result, we usedthe aforementioned fragmented HP1-NSs prepared by heatingthen rapid cooling to 25 °C as seed, and the average edgelength of these seeds was ∼0.32 μm. The 2D epitaxial growthto prepare HP1-NS was performed at 60 °C, and the 150 mg/mL HP1 dioxane solution was added in proportion as unimerat 90 °C, whereas the 5 mg/mL HP1 seed solution was addedduring the cooling process at ∼70 °C. The unimer-to-seedratios (w/w) were set at 25, 50, 75, 100, and 150. Themorphology of the obtained nanosheets, as well as the edgelength distribution of these nanosheets, was revealed in Figures4a−f and Figure S4. According to statistical analysis of thecorresponding TEM images of the obtained nanosheets, thenumber-average edge length (Ln), number-average surface area(Sn), weight-average edge length (Lw), and edge lengthdispersity (Lw/Ln) of the obtained nanosheets were summar-ized in Table 1. With an increasing unimer-to-seed ratio, thenumber-average edge length of the square nanosheets increasedfrom 0.56 to 4.41 μm, suggesting that the size of the nanosheetcan be controlled by the unimer-to-seed ratio. Meanwhile, thereis approximately a linear dependence between the number-average surface area (Sn) of the obtained nanosheets and theunimer-to-seed ratio (Figure S5), suggesting that the growthprocess of HP1-NS is indeed a living 2D CDSA. The Lw/Ln forall five HP1-NS are less than 1.024, which is lower than that ofthe nanosheet formed by CDSA without seeds (1.141).Moreover, the formation process of HP1-NS with HP1 seedswas faster than without seeds, which might be ascribed to the

epitaxial 2D growth initiated by HP1 seeds being more efficientand faster than the formation of free nanosheets occurring inthe absence of seeds. This result is similar to the 1D growth ofcylindrical crystalline-core micelles.1

In addition to the size of the obtained nanosheets, thethicknesses of these nanosheets were examined by AFM. Asshown in Figure 5, the thicknesses of the obtained nanosheetswere all within 4−5 nm, regardless of the unimer-to-seed ratio,which was the same as that of the HP1-NS before beingfragmented, indicating that these nanosheets were also formedby crystallized aggregation of HP1 molecules from the x-axisand y-axis directions. As proposed in Figure 4g, the squarenanosheets with tunable size could be progressively formedwhen the HP1 unimers in the solution attached to and

Figure 4. TEM images of the square nanosheets formed through CDSA of HP1 unimers with fragmented HP1-NSs as seeds at 60 °C for 24 h, andthe unimer-to-seed ratio is 25 (a), 50 (b), 75 (c), 100 (d), and 150 (e). The scale bar is 2 μm for all TEM images. (f) Edge length distributionhistograms of the nanosheets formed through CDSA with different unimer-to-seed ratios. (g) The proposed mechanism for the formation of thesize-tunable square nanosheets by CDSA of HP1 with fragmented HP1-NSs as seed.

Table 1. Characterization of the Square Nanosheets Formedthrough CDSA of HP1 with Fragmented HP1-NSs as Seedsat 60 °C for 24 ha

unimer-to-seed ratio

number-averageedgelength

(Ln, μm)

number-averagesurfacearea

(Sn, μm2)

weight-averageedgelength

(Lw, μm)

edgelength

dispersity(Lw/Ln)

fragmented HP1-NS seed 0.32 0.11 0.38 1.15725 0.56 0.32 0.58 1.02450 1.28 1.65 1.30 1.01575 1.86 3.48 1.88 1.006100 2.59 6.73 2.61 1.009150 4.41 19.56 4.44 1.006

aThe results were obtained from the statistical analysis of the TEMimages and the detailed process was shown in the ExperimentalSection.

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epitaxially grew on the active POSS moieties in the middle-layerof the HP1 seeds. Some particles were observed on the surfaceof the HP1-NSs in Figure 5, which might be ascribed to theimpurities formed by a few HP1 molecules that did notparticipate in the 2D assembly.Another interesting and useful application of CDSA is the

preparation of multicomponent nanostructures with the desiredfunctions.34 As reported by our previous research, variousfunctional molecules, such as fluorescent naphthalene andpyrene can be incorporated into the square nanosheet of hPEAin the case that an adequate amount of POSS moieties wereattached to the end of hPEA, these hydrophobic functionalmolecules were incorporated into the inner layer of thenanosheet, and possessed no effect on crystallization of POSSmoieties.40 Motivated by this characteristic, multicomponenthPEA nanosheets formed by POSS-capped hPEA containingdifferent hydrophobic functional moieties could be expectedthrough the crystallization-driven coassembly if POSS-cappedhPEA with different functional groups were mixed in hotwater/dioxane solvent followed by cooling-induced crystalliza-tion. hPEA capped with ferrocene (FC) and POSS (HP2) waschosen as another component for the formation of dual-

component nanosheets by random cocrystallization, which wasascribed to two reasons. The first one was that ferrocene was animportant functional group with electrochemistry and redoxproperties.45−49 The second one was that FC exhibited UV−visabsorption at approximate 436 nm, which was overlapped withthe fluorescence emission of AN. This characteristic wasbeneficial for monitoring the formation of the HP1/HP2 dual-component nanosheets by the fluorescence changes.The synthesis process of HP2 was the same as HP1 through

a “click chemistry” reaction between an amino and epoxy group(Scheme S1), and the structure of HP2 was confirmed by 1HNMR spectra (Figure S1). HP2 could also self-assemble intonanosheets (Figure S6). However, the nanosheets formed byHP2 (HP2-NS) were not as regular as those formed by HP1,which might be due to the strong π−π stacking between theAN groups in the HP1 molecules.40 The water/dioxanesolution of HP1-NS and HP2-NS was mixed in proportionand heated to 90 °C while stirring for 2 h. Then, the solutionwas equilibrated at 60 °C for 24 h, and the cocrystallization ofthe POSS moieties in HP1 and HP2 was expected to occur(Figure 6a). As shown in Figure 6, parts b and c, the obtainedassemblies exhibited morphologies similar to square nano-sheets. With the increasing amount of HP1, the obtainednanosheets were closer to square shape, which might be due tothe stronger π−π stacking between the AN groups in HP1molecules than the FC groups in HP2 molecules.40

To confirm that the nanosheets were composed of HP1 andHP2 instead of only one component, the fluorescence emissionspectra of the obtained nanosheets with different mass ratios ofHP1 to HP2 were measured and are shown in Figure S7.Meanwhile, the fluorescence emission of the nanosheetsmixture with the same ratio of HP1-NS to HP2-NS was alsomeasured for comparison. As shown in Figure 6d, thefluorescence of the nanosheets decreased with the increasingamount of HP2, whereas the concentration of HP1 wasconstant at 0.1 mg/mL, which can be ascribed to the quenchingof the fluorescence emission of the AN moieties in HP1 by the

Figure 5. AFM images and the corresponding height profiles along theAFM image of the nanosheets formed through CDSA of HP1 withfragmented HP1-NSs as seeds at 60 °C for 24 h. The unimer-to-seedratio is 25 (a), 75 (b), and 150 (c).

Figure 6. Proposed mechanism for the formation of dual-component nanosheets with both AN and FC moieties by CDSA of fragmented HP1 andHP2 nanosheets (a) and TEM images of the nanosheets fabricated by the random cocrystallization of fragmented nanosheets formed by HP1 andHP2, respectively, in their solution (water/dioxane volume ratio is 1:1) at 60 °C for 24 h. The mass ratio of HP1 and HP2 is 2:3 (b) and 4:1 (c).The scale bar for these TEM images is 2 μm. (d) The dependence of the fluorescence emission intensity of dual-component nanosheets at 414 nmand the fluorescence emission intensity ratio (414 nm/464 nm) on the mass content of HP1, the mixture of nanosheets with the same ratio of HP1and HP2 formed by the respective 2D epitaxial self-assembly were also prepared for comparison. The excitation wavelength was set at 381 nm.

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FC moieties in HP2. The monomer fluorescence emission ofAN was at ∼414 nm, whereas the fluorescence of the face-to-face overlapped AN excimers was at ∼464 nm.50 Thefluorescence emission ratio of the dual-component nanosheetsat 414 to 464 nm was larger than that of the nanosheet mixtureswith the same ratio of HP1 to HP2. This might be ascribed tooverlapping AN moieties in HP1 being isolated by the FCmoieties in HP2, suggesting that HP2 molecules were closer tothe HP1 molecules than the mixtures, and these nanosheetswere composed of random cocrystallization of HP1 and HP2molecules. This result was further confirmed by the normalizedUV−vis spectra of the dual-component nanosheets in THF(Figure S8), which indicated that the dual-componentnanosheets were almost with the same composition as themass ratio of HP1 to HP2 in the feed.

3. CONCLUSIONS

In summary, we presented an approach to prepare uniformnanosheets with controlled size through the living crystal-lization-driven 2D self-assembly of POSS-capped hPEA. Thenanosheets of hPEA could be fragmented upon heating abovethe melting point of the POSS moieties and regeneratedthrough the recrystallization of the POSS moieties at a lowtemperature. The crystallization-driven 2D self-assemblyprocess exhibited living characteristics. Uniform HP1 nano-sheets with tunable size can be obtained using fragmented HP1nanosheets as seeds and HP1 in a dioxane solution as unimers.The edge length of the obtained nanosheets is tunable from∼0.5 to ∼4.5 μm by increasing the unimer-to-seed ratio.Moreover, dual-component nanosheets can be obtained byrandom cocrystallization of HP1 and HP2, which comprise ANmoieties and FC moieties, respectively. This approach based onthe crystallization-driven 2D self-assembly behaviors of POSS-capped hPEA might allow significant advancement in 2Dnanostructures with the controlled sizes and desired functions.

4. EXPERIMENTAL SECTIONPreparation of HP1 Nanosheets by 2D Self-Assembly. HP1

was first dissolved in 1,4-dioxane, which is a good solvent for hPEA,POSS, and anthracene moieties, at a concentration of 10 mg/mL. Thesolution was left to equilibrate at 30 °C, and then ultrapure water wasadded very slowly (83.3 μL/min) to the solution while stirring untilthe volume ratio of water and 1,4-dioxane in the solution was 1:1.After overnight stirring, a 0.5 mL aliquot of solution was diluted into 1mg/mL with ultrapure water, and 1 mg/mL aqueous solutions of HP1nanosheets were obtained.Preparation of HP1 Nanosheets with Fragmented HP1-NSs

as Seeds. A 5 mg/mL solution of HP1 nanosheets was prepared bydropping water into a dioxane solution of HP1 (water/dioxane volumeratio is 1:1) heated to 90 °C while stirring for 2 h and rapidly coolingto 25 °C. Then, small and fragmented nanosheets (Ln = 0.32 μm, Lw =0.38 μm, and Lw/Ln = 1.157) were obtained, which were used as seedsfor the growth experiments. Growth from the fragmented micelles wasdemonstrated by the addition of HP1 unimer. For example, 2.79, 2.84,2.86, 2.87, and 2.88 mL of mixed solvent (water/dioxane volume ratiois 1:1) were heated to 90 °C, and 96.0, 98.0, 98.5, 99.0, and 99.5 μL of150 mg/mL dioxane solution of HP1 were added, respectively, duringstirring. The obtained unimer solutions were rapidly cooled from 90 to60 °C. During this process, and at ∼70 °C, 0.12, 0.06, 0.04, 0.03, and0.02 mL of the prepared seed solution were added, and the unimer-to-seed ratios (w/w) were 25, 50, 75, 100, and 150, respectively. Thesolutions were equilibrated at 60 °C for 24 h without stirring, and a 0.5mL aliquot of solution was diluted into 1 mg/mL with ultrapure water.Solutions of square HP1 nanosheets with tunable edge lengths wereobtained.

Preparation of Dual-Component Nanosheets by Cocrystal-lization of Fragmented HP1 and HP2 Nanosheets. HP2nanosheets were prepared similarly to HP1 by 2D assembly. HP2was first dissolved into 1,4-dioxane, which is a good solvent for thehPEA, POSS, and ferrocene moieties, at a concentration of 10 mg/mL.The solution was left to equilibrate at 30 °C, and then ultrapure waterwas added very slowly (83.3 μL/min) to the solution while stirringuntil the volume ratio of water and 1,4-dioxane in the solution is 1:1.The water/dioxane solution of HP1 nanosheets and HP2 nanosheetswas mixed in proportions with mass ratios of 1:4, 2:3, 3:2, and 4:1.The mixed solutions (water/dioxane volume ratio is 1:1) of HP1 andHP2 were heated to 90 °C while stirring for 2 h, then equilibrated at60 °C for 24 h without stirring. A 0.5 mL aliquot of solution wasdiluted to 1 mg/mL with ultrapure water, and solutions of dual-component nanosheets formed by both HP1 and HP2 were obtained.

Nuclear Magnetic Resonance (NMR). 1H NMR and 13C NMRspectra were acquired with a Mercury Plus spectrometer (Varian, Inc.)operating at 400 MHz using CDCl3 as a solvent and TMS as aninternal standard at room temperature.

Dynamic Light Scattering (DLS). The DLS measurements wereperformed using a ZS90 Zetasizer Nano ZS instrument (MalvernInstruments Ltd., U.K.) equipped with a multi-τ digital timecorrelation and a 4 mW He−Ne laser (λ = 633 nm) at an angle of90°. Regularized Laplace inversion (CONTIN algorithm) was appliedto analyze the obtained autocorrelation functions. For the purpose ofthe light scattering studies, the refractive index of HP1 and HP2involved were all assumed to be 1.46. The results were expressed asapparent hydrodynamic radius (RH,app), which was determined using astandard spherical particle model. The 2D epitaxial growth of thePOSS-capped hPEA nanosheets was monitored by the changes of theRH,app obtained by DLS.51−53

UV−Vis Spectra. The UV−vis spectra of the nanosheets solutionwas analyzed with a UV-2550 spectrophotometer (Shimadzu, Japan).

Fluorescence Spectra. The fluorescence emission spectra of thenanosheets solutions were recorded under a QM/TM/IM steady-stateand time-resolved fluorescence spectrofluorometer (PTI Company).

Micro-Differential Scanning Calorimetry (μDSC). The crystalliza-tion behaviors of the HP1 nanosheets in water/dioxane mixtures weredetermined by a microdifferential scanning calorimeter (MicroDSC3evo, Setaram Instruments Ltd., France). The concentration of the HP1nanosheets was 50 mg/mL, and the heating and cooling rate for eachsample was 1 °C/min.

Transmission Electron Microscopy (TEM). The TEM images of theHP1 nanosheets were obtained using a JEM-2100 (JEOL Ltd., Japan)transmission electron microscope operated at an acceleration voltageof 200 kV. The sample was prepared by dropping the HP1 nanosheetssolution onto copper grids coated with a thin polymer film, and thendried at 30 °C for 24 h. No staining treatment was performed for themeasurement. For the statistical edge length analysis, 25 squarenanosheets from a single TEM image (small unimer-to-seed ratio) orfrom several random TEM images (large unimer-to-seed ratio) werecarefully traced by hand to determine the edge length, and for eachsquare nanosheet, two orthogonal edge lengths along the surface of thenanosheet were measured. The number-average edge length (Ln),number-average surface area (Sn), weight-average edge length (Lw),and edge length dispersity (Lw/Ln) of each sample were calculatedfrom eqs 1−4, in which L = edge length of the square nanosheets, S =surface area of the square nanosheets, N = number, and W = weight.The density (ρ) and thickness (t) of the nanosheets were assumed tobe constant in eq 3.

=∑∑

=

=L

NL

Nin

i i

in

in

1

1 (1)

=∑∑

=∑

∑=

=

=

=S

NS

N

NL

Nin

i i

in

i

in

i i

in

in

1

1

12

1 (2)

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ρρ

=∑∑

=∑∑

=∑∑

=

=

=

=

=

=

LWL

W

N tL L

N tL

NL

NLin

i i

in

i

in

i i i

in

i i

in

i i

in

i iw

1

1

12

12

13

12

(3)

=L LLL

/w nw

n (4)

Atomic Force Microscopy (AFM). The AFM images of the HP1nanosheets were obtained by using a scanning probe microscope(Nanonavi E-Sweep, SII. Japan) operated in the tapping mode. Thesamples were prepared by dropping dilute HP1 nanosheets solutionon a mica sheet and then drying at 30 °C for 24 h.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental section, 1H NMR of HP2, additional DLS resultsand TEM image, fluorescence spectra, and UV−vis spectra.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*(X.J.) Telephone: +86-21-54743268. Fax: +86-21-54747445.E-mail: ponygle@sjtu.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the National Nature Science Foundation ofChina (21174085, 21274088, 51373098), Education Commis-sion of Shanghai Municipal Government (12ZZ020), and theShanghai Key Lab of Polymer and Electrical Insulation for theirfinancial support. X.J. is supported by the NCET-12-3050Project.

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