synthesis of organic/inorganic polyhedral oligomeric silsesquioxane-containing block copolymers via...

Synthesis of Organic/Inorganic Polyhedral OligomericSilsesquioxane-Containing Block Copolymers via ReversibleAddition−Fragmentation Chain Transfer Polymerization and TheirSelf-Assembly in Aqueous SolutionLizhi Hong, Zhenghe Zhang, and Weian Zhang*

Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China Universityof Science and Technology, 130 Meilong Road, Shanghai 200237, China

*S Supporting Information

ABSTRACT: Polyhedral oligomeric silsesquioxane (POSS)-containing homopolymers (PHEMAPOSS) with a high degree ofpolymerization (DP) were synthesized via reversible addition−fragmentation chain transfer (RAFT) polymerization.PHEMAPOSS was used as a macro-RAFT agent to construct a series of amphiphilic diblock copolymers, PHEMPOSS-b-PMAA (poly (methyl methacrylate)), which possessed a different length of hydrophilic chains. The self-assembly behavior ofPHEMPOSS-b-PMAA in aqueous solution was studied by transmission electron microscopy (TEM), atomic force microscopy(AFM), and dynamic light scattering (DLS), respectively. The results showed that PHEMAPOSS45-b-PMAA523 forms typicalcore−shell spherical micelles where the hydrophobic PHEMAPOSS blocks as the core and hydrophilic PMAA blocks as the shell.With increasing PMAA chain length, PHEMAPOSS45-b-PMAA1173 with a longer PMAA chain self-assembled into irregularaggregates with POSS moieties dispersed in the aggregates. On the other hand, PHEMAPOSS45-b-PMAA308 with a shorterhydrophilic PMAA chain could self-assemble into a dendritic cylinder structure.

■ INTRODUCTION

Inorganic/organic hybrid polymers with synergetic functions oftwo components have attracted a great deal of researchinterest.1,2 Self-assembly of block copolymers containinginorganic components such as quantum dots, metal nano-particles, carbon nanoscaled materials, and silica-based nano-particles is another growing technique to develop inorganic/organic hybrid polymers with novel morphologies and novelproperties.3−5 A variety of novel self-assembled morphologiescould be acquired by varying the size, nanostructure, andchemical composition depending on the choice of inorganiccomponents and polymers, as well as self-assembly con-ditions.6,7 A particularly noticeable example is organic/inorganic hybrid polymers based on polyhedral oligomericsilsesquioxane (POSS).8−12

POSS, as a hybrid molecule with precisely definednanostructure, has a rigid cubic silica core and organic corners,which could be reactive or nonreactive. These organic groupsprovide a POSS molecule with higher reactivity and solubility inthe construction of POSS-containing hybrid polymers.13−17

Thus, POSS could be easily introduced into polymeric matricesto prepare novel polymer hybrids with advantages such asmechanical, thermal, and flammability-resistant properties.18−21

More recently, many novel POSS-containing well-definedhybrid polymers have been prepared using the living/controlledpolymerization technique including hemitelechelic, telechelic,and multitelechelic block hybrid polymers.22−30 Meanwhile,much attention has also been focused on the self-assembly ofPOSS-containing organic/inorganic hybrid polymers. Forexample, Kuo et al.31 prepared the POSS-containing hemi-telechelic helical polypeptide copolymers using aminopropyl

isobutyl-POSS via ring-opening polymerization of γ-benzyl-L-glutamate N-carboxyanhydride and investigated their self-assembly behavior in toluene; they found the aggregation ofnanoribbon could be prevented by incorporation of POSS.Cheng et al.32 synthesized a “giant surfactant” PS-APOSS,possessing a hydrophilic POSS headgroup and a hydrophobicPS tail. A variety of morphologies, from vesicles to wormlikecylinders and further to spheres, can be obtained in selectivesolutions. Our group also has contributed works on POSS-containing hybrid polymers to this field.27,33 We synthesized aseries of hemitelechelic POSS-containing hybrid polymers suchas poly(acrylic acid) (POSS-PAA) and poly(ethylene oxide)(POSS-PEO) and studied their self-assembly behavior inaqueous solution and found that these hybrid polymers self-assembled in aqueous solution to form the aggregates with astructure that is different from a typical core−shell micelle.Besides, POSS-containing amphiphilic star-shaped hybridpolymers have also been prepared using the living polymer-ization technique, and their self-assembly and application hasbeen further explored in selective solution.30,34,35 He’sgroup23−25 also investigated amphiphilic di- and triblockcopolymers of poly(ethylene glycol) (PEG) and poly-(methacrylisobutyl-POSS) (P(MA-POSS)), and the self-assembly behaviors of amphiphilic hybrid block copolymersin aqueous solution were further investigated. However, in thepast research, much effort has been tried to prepare POSS-

Received: April 14, 2014Revised: June 9, 2014Accepted: June 12, 2014Published: June 12, 2014

Article

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© 2014 American Chemical Society 10673 dx.doi.org/10.1021/ie501517m | Ind. Eng. Chem. Res. 2014, 53, 10673−10680

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containing hybrid block copolymers, but it is difficult to obtainhybrid polymers with a high degree of polymerization (DP) ofPOSS-based monomer in free-radical polymerization, due tothe steric hindrance of POSS unit.In this contribution, we synthesized POSS-containing hybrid

polymer, PHEMAPOSS with a higher DP via reversibleaddition−fragmentation chain transfer (RAFT) polymerization.PHEMAPOSS homopolymer was further used as macro-RAFTagent in RAFT polymerization of tert-butyl methacrylate(tBMA) to construct a series of POSS-containing hybriddiblock copolymers, PHEMAPOSS-b-PtBMA. Then, theamphiphilic hybrid block copolymers, PHEMAPOSS-b-PMAA(poly (methyl methacrylate)), were obtained via the hydrolysisof the tert-butyl esters of PtBMA block with an excess oftrifluoroacetic acid (Scheme 1). Finally, the self-assembly

behavior of PHEMAPOSS-b-PMAA in aqueous solution withdifferent chain lengths of PMAA block was investigated bytransmission electron microscopy (TEM) and atomic forcemicroscopy (AFM).

■ EXPERIMENTAL SECTIONMaterials. Aminoisobutyl polyhedral oligomeric silsesquiox-

ane (POSS) was purchased from Hybrid Plastics Company.tert-Butyl methacrylate (tBMA, Aladdin, 99%) was passedthrough a column of basic aluminum oxide to remove inhibitorsshortly before polymerization. Azobis(isobutyronitrile) (AIBN)was recrystallized from ethanol. Tetrahydrofuran (THF) andtoluene was distilled from a purple sodium ketyl solution.Dichloromethane (DCM) and triethylamine (TEA) were driedover calcium hydride and distilled before use. Other regentsand solvents in analytical grade were obtained from Aladdin.The RAFT agent, cumyl dithiobenzoate (CDB), was preparedaccording to the previous literature.36

Synthesis of HEMAPOSS Monomer. The synthesis ofHEMAPOSS was similar to our previous work.28,37 The typicalsynthetic procedure was as follows: aminoisobutyl-POSS (5 g,5.7 mmol) was dissolved in 35 mL of anhydrous THF, and 2.4mL of distilled triethylamine (TEA) was added. 2-(Meth-acryloyloxy) ethyl succinyl chloride (HEMA-COCl, 4.32 g, 17.4mmol) was dissolved in 30 mL of anhydrous THF and was

added slowly into the above POSS solution at 0 °C under argonatmosphere, and the mixture was stirred for 24 h at roomtemperature. Then, a white precipitate (triethylamine hydro-chloride) was removed by filtration, and the resulting solutionwas concentrated under vacuum at room temperature. Theresidue was purified by a silica gel chromatography withpetroleum ether/ethyl acetate (2:1, v/v) as the eluent. A whitesolid was obtained (5 g, yield 80.5%). 1H NMR (400 MHz,CDCl3), δ (TMS, ppm): 6.13 (s, 1H, HCHC(CH3)−), 5.60(s, 1H, HCHC(CH3)−), 4.35 (s, 4H, −OCO-(CH2)2OCO−), 3.26−3.20 (m, 2H, −CH2−NHCO−),2.72−2.69 (m, 2H, −NHCO−CH2−), 2.50−2.44 (m, 2H,−CH2CH2COO−), 1.95 (s, 3H, H3CC(COO−)CH2), 1.88−1.82 (m, 7H, −Si−H2CCH(CH3)2), 1.61 (m, 2H, −Si−CH2CH2CH2−NH−), 0.96 (d, 42H, −Si−CH2CH(CH3)2),0.61 (d, 16H, −SiCH2CH(CH3)2, −SiCH2CH2CH2−NH−).

Preparation of PHEMAPOSS Homopolymer. In a typicalexperiment, HEMAPOSS (1 g, 0.92 mmol), CDB (417.1 μL,20 mg/mL CDB toluene solution, 0.031 mmol), and AIBN(164.2 μL, 10 mg/mL AIBN toluene solution, 0.01 mmol) and0.6 mL of toluene were charged in a dry glass tube equippedwith a magnetic stirring bar. The mixture was degassed by atleast three freeze−pump−thaw cycles. After the flask was flame-sealed under vacuum, the reaction was performed at 65 °C.After 48 h, the polymerization was quenched by plunging thereaction flask into liquid nitrogen. The reaction solution wasprecipitated with a solvent mixture (methanol/acetic ether = 6/1, volume ratio) three times, and the final production was driedunder vacuum at 30 °C. Mn = 16 900 g/mol, Mw/Mn = 1.13.

Synthesis of PHEMAPOSS-b-PtBMA. A representativeexample of the synthesis of PHEMAPOSS-b-PtBMA blockcopolymer is as follows: PHEMAPOSS45 (0.2 g, 0.0041 mmol),tBMA (0.8 g, 5.6 mmol), and AIBN (22 μL, 10 mg/mL AIBNTHF solution, 0.0013 mmol) and 1 mL of THF were placed ina dry glass tube equipped with a magnetic stirring bar. Themixture was degassed by at least three freeze−pump−thawcycles. After the flask was flame-sealed under vacuum, thereaction was performed at 65 °C. After 10 h, the reaction wasquenched by plunging the flask into liquid nitrogen. Thereaction solution was precipitated with a solvent mixture(methanol/water = 4/1, volume ratio) three times, and the finalproduct was dried under vacuum at 50 °C for 48 h. Mn = 73900 g/mol, Mw/Mn = 1.38.

Synthesis of PHEMAPOSS-b-PMAA Block Copolymers.Hydrolysis of the tert-butyl esters was accomplished by treatingthe block copolymers with an excess of trifluoroacetic acid(TFA) in dichloromethane. In a typical experiment, 0.2 g ofblock copolymer PHEMAPOSS45-b-PtBMA523 was dissolved in10 mL of dichloromethane and stirred for 15 min to dissolvethe polymer. Then, 2 mL of TFA was added, and the solutionwas left to stir for 24 h at room temperature. The polymersolution was concentrated on a rotary evaporator to afford ayellow solid residue and dried under vacuum at 50 °C.

Self-Assembly of PHEMAPOSS-b-PMAA in AqueousSolution. In a typical process, PHEMAPOSS45-b-PMAA523 (10mg) was first dissolved in 10 mL of dioxane (common solvent).The solution was stirred for 12 h and gradually dialyzed againstpH = 8.5 ultrapure water (selective solvent) for 3 days. Theself-assembly solution was further dialyzed against ultrapurewater at least three times to remove dioxane completely.

Scheme 1. Synthesis of PHEMAPOSS Homopolymers andPHEMAPOSS-b-PMAA Block Copolymers via RAFTPolymerization

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie501517m | Ind. Eng. Chem. Res. 2014, 53, 10673−1068010674

■ CHARACTERIZATION

Nuclear Magnetic Resonance Spectroscopy (NMR). 1Hspectra were recorded on a Bruker AVANCE 400 spectrometer.The samples were dissolved with deuterated CDCl3 and THF-d8 and measured with tetramethylsilane (TMS) as an internalreference.Fourier Transform Infrared Spectroscopy (FT-IR).

Fourier transform infrared spectroscopy (FT-IR) measure-ments were conducted on a PerkinElmer Spectrum One FT-IRspectrophotometer equipped with an ATR sampling unit (25°C).Gel Permeation Chromatography (GPC). The molecular

weight and molecular weight distribution of PHEMAPOSShomopolymers and PHEMAPOSS-b-PtBMA diblock copoly-mers were determined with a gel permeation chromatography(GPC, Waters 1515). Polystyrene standards with narrowmolecular weight distribution were used for the calibration ofthe column set, and THF was used as the eluent at a flow rateof 1 mL/min.Critical Micelle Concentration (CMC). CMC measure-

ments were estimated by fluorescence spectroscopy on HitachiFL-4500 using pyrene as a probe. The copolymer solutionsafter self-assembly with different concentrations were preparedby using ultrapure water (pH = 8.5).Transmission Electron Microscopy (TEM). Transmission

electron microscopy (TEM) images were taken on a JEOLJEM1400 instrument operated at an accelerating voltage of 100kV. A 15 μL droplet of self-assembly aggregate solution (0.25mg/mL) was directly dropped onto a copper grid (300 mesh)coated with a carbon film, and the sample was allowed to dryunder an infrared lamp.Atomic Force Microscopy (AFM). Atomic force micros-

copy (AFM) images were obtained using Tapping Mode on aNanoscope IV of Digital Instruments. The preparationprocedure of AFM samples is as follows: the self-assembledaggregates solution (0.25 mg/mL) was directly dropped onto afreshly cleaved mica and then dried at room temperature for 24h.Dynamic Light Scattering (DLS). DLS measurements

were performed by a BECKMAN COULTER Delasa Nano Cparticle analyzer at a fixed angle of 165°. All measurementswere repeated three times, and the average result was acceptedas the final hydrodynamic diameter (Dh).

■ RESULTS AND DISCUSSION

Preparation of PHEMAPOSS Homopolymer. In ourprevious work,27,37 we prepared hemitelechelic POSS-PAA andditelechelic POSS-PAA-POSS via living radical polymerizationand further studied their self-assembly behavior in aqueoussolution. We found that amphiphilic hemitelechelic POSS-PAAself-assembles in aqueous solution into aggregates with differentsize. The aggregates are not conventional core−shell micelleswith hydrophobic POSS moieties as the core and hydrophilicPAA chains as the shell, but the POSS moieties are dispersed inthe aggregates. Additionally, POSS-PAA-POSS with a shortPAA chain can self-assemble in water into ellipsoidal aggregateswith a moderately uniform size.In this work, we prepared PHEMAPOSS-b-PMAA block

copolymers. The synthetic strategy is shown in Scheme 1.PHEMAPOSS homopolymer was first prepared by RAFTpolymerization using the novel POSS-containing monomer,HEMAPOSS. We have synthesized PHEMAPOSS homopol-

ymers with different DPs by changing the molar feed ratio ofmonomer to RAFT agent, cumyl dithiobenzoate (CDB). ThePHEMAPOSS homopolymers were characterized by GPCagainst the polystyrene linear standards, and the GPC traces areshown in Figure 1. The Mn,GPC and polydispersity index (PDI)

of the PHEMAPOSS homopolymers was also listed in Table 1.We find the GPC curves are very symmetrical, and the PDI isquite lower, which suggests the RAFT polymerization ofHEMAPOSS is living/well-controlled. The 1H NMR ofPHEMAPOSS26 was shown in Figure S1, SupportingInformation, and the DP and molecular weight of PHEMA-POSS can also be estimated from Figure S1, SupportingInformation. By comparing the peaks of methylene protonsnext to silicon from the POSS unit at 0.6 ppm with thearomatic protons of the terminal benzene ring at 7.81 ppm, theDP of PHEMAPOSS and the absolute molecular weights weredetermined as DP = I0.6/(8 × I7.81) and Mn,PHEMAPOSS,NMR =I0.6/(8 × I7.81) × MHMEAPOSS + MCDB, where the MHEMAPOSS andMCDB are the molecular weights of HEMAPOSS monomer andCDB, respectively. Here “I” represents the area of a peak, andwe first set I0.6 to 16.00. The DP calculated from the 1H NMRspectrum is 26. Additionally, we can find that the Mn,GPC ismuch lower than Mn,NMR in Table 1. It could be attributed tothe relatively compact cage structure of POSS units; thus,PHEMAPOSS has a very different solution behavior with PSstandard in THF.38−40 On the basis of the 1H NMR and GPCresults, PHEMAPOSS homopolymers with a high DP weresuccessfully achieved via RAFT polymerization.

Preparation of PHEMAPOSS-b-PMAA Block Copoly-mers. The PHEMAPOSS-b-PtBMA block copolymers weresynthesized using PHEMAPOSS45 as a Macro-RAFT agent. Wesynthesized three PHEMAPOSS-b-PtBMA block copolymerswith different PtBMA block lengths. The GPC traces ofPHEMAPOSS-b-PtBMA block copolymers are shown in Figure1. All the curves of PHEMAPOSS-b-PtBMA shift to lowerelution volume compared to that of PHEMAPOSS45. Thepolymerization results were also listed in Table 1. The 1HNMR spectrum of PHEMAPOSS45-b-PtBMA523 is shown inFigure 2, and the tert-butyl ester proton resonance in thePtBMA chain was detected at 1.41 ppm. The signals ofresonance at 0.61, 0.96, and 1.84 ppm were assignable to theprotons from the iso-butyl group of POSS units. Additionally,

Figure 1. Evolution of GPC chromatograms of PHEMAPOSShomopolymers and PHEMAPOSS-b-PtBMA block copolymers withdifferent molecular weights.



we can estimate the DPPtBMA and the molecule weight of thediblock copolymers (Mn,PHEMAPOSS‑b‑PtBMA,NMR) by comparingthe peaks of methylene protons next to silicon from the POSSunit at 0.61 ppm with the tert-butyl ester group protons at 1.41ppm. The DPPtBMA is determined as DPPtBMA = (I1.41 × 16)/(I0.61 × 9) × DPPHEMAPOSS, and the Mn,PHEMAPOSS‑b‑PtBMA,NMR isdetermined as Mn,PHEMAPOSS‑b‑PtBMA,NMR = Mn,PHEMAPOSS +DPPtBMA × Mn,tBMA, where the Mn,tBMA is the molecular weightof tBMA monomer. PHEMAPOSS-b-PMAA block copolymerswere obtained by hydrolysis of the tert-butyl ester groups ofPHEMAPOSS-b-PtBMA in dichloromethane using TFA. As theFT-IR spectrum shows in Figure S2, Supporting Information,the absorption bands of the tert-butyl moiety of PtBMA at 1367cm−1 completely disappeared after hydrolysis of PtBMA toPMAA. Moreover, the absorption bands at 1725 cm−1 (COstretching vibrations) are slightly shifted to lower wave-numbers, and the peaks also become broader, due to theformation of carboxylic acid from the tert-butyl ester.Additionally, the absorbance peak in range from 2292 to3714 cm−1 is quite broad in the spectrum of PHEMAPOSS-b-PMAA, which further confirms the formation of carboxylic acidgroups. The 1H NMR spectrum of PHEMAPOSS45-b-PMAA523also shows that the proton signal of the tert-butyl ester groupsat 1.44 ppm completely disappeared after the hydrolysis (FigureS3, Supporting Information).Self-Assembly Behaviors of PHEMPOSS-b-PMAA in

Aqueous Solution. Although the self-assembly of POSS-containing hybrid polymers in selective solutions has beenstudied by some groups and some interesting assembledmorphologies have been obtained using hemitelechelic,ditelechelic, and star-shaped POSS-containing hybrid polymers,

the self-assembly behavior of POSS-containing hybrid blockcopolymers with a high DP of POSS-based monomer has beenrarely reported.27,29,30,33,34,37 This is because it is difficult toobtain the POSS-containing hybrid block copolymers with ahigh DP of POSS unit. Here, we continued to study the self-assembly behavior of amphiphilic POSS-containing hybridpolymers, and we prepared POSS-containing hybrid blockcopolymers with three different mass ratios of hydrophobic/hydrophilic block (PHEMAPOSS/PMAA) of around 2/1, 1/1,and 1/2 (the exact ratios are 2/1.04, 1/0.92, and 1/2.06) andfurther studied their self-assembly behaviors in aqueoussolution.To study the assembled behavior of PHEMPOSS-b-PMAA in

aqueous solution, we chose dioxane as common solvent andwater as selective solvent in which PHEMPOSS block isinsoluble. First, the critical micelle concentration (CMC) ofblock copolymers in water at pH = 8.5 was measured byfluorescence spectrophotometry. Figure S4, Supporting In-formation, showed the intensity ratio of I3/I1 as a function ofPHEMPOSS-b-PMAA concentration in aqueous solution.Apparently, with the increase of the length of PMAA segment,the CMC value increased remarkably, and all the self-assemblyaggregates of the hybrid block copolymers were characterizedby TEM and AFM.The TEM images of PHEMAPOSS45-b-PMAA523 with the

mass fraction of PHEMAPOSS/PMAA of about 1/1 are shownin Figure 3a,b. The well-dispersed spherical aggregates revealedthe formation of typical core−shell micelles with a relativelyuniform diameter of about 38 nm. The aggregates were alsoanalyzed by AFM (Figure 3c,d); the relatively uniform diametercan be estimated from the AFM images and is about 95 nm,which is larger than that observed in the TEM image. Here,beside the tip, convolution effects should be accounted for inthe difference in size between AFM and TEM measurements;the micelles have a relatively long corona in the aqueoussolution due to the high repeat unit of the hydrophilic chain,and the shell of PMAA block resulting from the shrinkage ofthe corona also should be taken into consideration in the AFMmeasurements, while the TEM images only exhibit thePHEMAPOSS core. However, the height of the micelles(29.3 nm) measured by AFM is in good agreement with thediameter observed by TEM (32 nm), as shown in Figure 3e.This further confirms the formation of core−shell structurewith PHEMAPOSS as the core and PMAA block as the shell.When we extended the length of PMAA chain to synthesize

PHEMAPOSS45-b-PMAA1173 block copolymer with the massratio of hydrophobic to hydrophilic segment of about 1/2, wegot a different assembled morphology as the TEM images showin Figure 4a,b. There are some irregular aggregates, and thedensity of these aggregates is clearly nonhomogeneous. We canfind many dark dots in these aggregates. The size of the little

Table 1. Results of PHEMAPOSS Homopolymers and PHEMAPOSS-b-PtBMA Block Copolymers Prepared via RAFTPolymerization

samplesa [HEMAPOSS]/[CDB]b or [tBMA]/[Macro-RAFT agent]b 10−3 Mn,NMRa 10−3 Mn,GPC

c Mw/Mnc

PHEMAPOSS26 30:1 28.5 16.9 1.13PHEMAPOSS45 50:1 48.9 25.4 1.12PHEMAPOSS45-b-PtBMA308 568:1 92.7 50.3 1.16PHEMAPOSS45-b-PtBMA523 1376:1 123.3 73.9 1.16PHEMAPOSS45-b-PtBMA1173 2837:1 215.7 144.8 1.17

aThe degree of polymerization and Mn,NMR were determined from the integration of 1H NMR spectra. bThe feed molar ratio between the RAFTagent and the initiator AIBN was maintained at 1:0.33. cMeasured by GPC against polystyrene standards.

Figure 2. 1H NMR spectrum of PHEMAPOSS45-b-PtBMA523.



dots in the aggregates is quite uniform at about 4.5 nm,although the aggregates are relatively polydisperse in size (from32 to 51 nm). The dark dots should be formed from a singlePHEMAPOSS block, which was embedded in the PMAAmatrix. This is obviously different from the above core−shellmicelles. In our previous study27 of self-assembly of hemi-telechelic POSS-containing poly(acrylic acid) (POSS-PAA)where the mass ratio of hydrophobic POSS/hydrophilic PAAportion is much more than 1/2, their self-assemblymorphologies are also not the typical core−shell micelles, andthe density observed of the aggregate is not uniform in a singleaggregate, which is consistent with this case. The aggregates canbe characterized by an AFM image (shown in Figure 4c,d). Thesize of the aggregates observed in the AFM images is about 54−70 nm, which is slightly larger than that in the TEM images.From the AFM phase image (Figure 4d), the aggregates areclearly nonhomogeneous, which is agreeable with TEM results.The height of the aggregates in Figure 4e is about 25 nm, whichis smaller than the dimension shown in the TEM images. Thisfurther confirms this is not typical core−shell assembledmorphology.On the other hand, we also studied self-assembly behavior of

PHEMAPOSS-b-PMAA block copolymer with a shorter

hydrophilic PMAA chain. We decreased the length of PMAAchain to prepare PHEMAPOSS45-b-PMAA308 and rendered themass ratio of hydrophobic/hydrophilic segment to around 2/1.As shown in Figure 5a,b, the morphology of the aggregates wastuned from spherical micelles to dendritic cylinders withdecreasing chain length of hydrophilic PMAA block, and typicalcrew-cut micelles formed; there are some irregular burls in sizeand shape in the dendritic PHEMAPOSS core. Thismorphology was also characterized by AFM, as shown inFigure 5c,d. The network cylinders interconnected by Y-shapedjunctions, which were also irregular in shape.The self-assembled aggregates of PHEMAPOSS-b-PMAA

ABCPs in aqueous solution (0.25 mg/mL) were furthercharacterized by dynamic light scattering (DLS). Figure S5,Supporting Information, shows the intensity-weighted hydro-dynamic diameter distributions and hydrodynamic diameter ofPHEMAPOSS-b-PMAA ABCPs self-assembled aggregates inwater at pH = 8.5. It can be seen that the uniform sphericalstructure formed from PHEMAPOSS45-b-PMAA523 has a verynarrow distribution, but the diameter of the spheres measuredby DLS is larger than that in AFM or TEM images. This isbecause the DLS result directly reflects the dimension ofmicelles in solution, where the PMAA chains as the corona are

Figure 3. TEM images (scale bar, 100 nm (a), 50 nm (b)), AFMimages (height (c), phase (d)), and the height profile along the line inthe height image (e) of PHEMAPOSS45-b-PMAA523 self-assembledaggregates in aqueous solution.

Figure 4. TEM images (scale bar, 100 nm (a), 50 nm (b)), AFMimages (height (c), phase (d)), and the height profile along the line inheight image (e) of PHEMAPOSS45-b-PMAA1173 self-assembledaggregates in aqueous solution.



highly stretched in aqueous solution at pH = 8.5.PHEMAPOSS45-b-PMAA1173 possesses the largest averagehydrodynamic diameter due to the longest PMAA chain asthe corona. Additionally, PHEMAPOSS45-b-PMAA308 has avery broad distribution, which is attributed to the assembleddendritic cylinders.We summarized the self-assembly process of PHEMAPOSS-

b -PMAA in Scheme 2. PHEMAPOSS45-b -PMAA523( f PHEMAPOSS = 52.1%) can form the typical core−shell sphericalmicelles where the hydrophobic PHEMAPOSS block as thecore and hydrophiphilic PMAA block as the shell. This is aclassical assembled morphology, which could also be obtainedfrom pure organic amphiphilic block copolymers in selectivesolution. With an increase in PMAA chain length, PHEMA-POSS45-b-PMAA1173 ( f PHEMAPOSS = 32.7%) self-assembled intoirregular aggregates with some dark dots, which is similar tothat of our previous study in hemitelechelic POSS-PAA. Thissuggests that only irregular aggregates formed with a lowcontent of POSS. This is rarely reported from the self-assemblyof pure organic amphiphilic block copolymer. On the otherhand, PHEMAPOSS45-b-PMAA308 with a shorter hydrophilicPMAA chain ( f PHEMAPOSS = 65.8%) self-assembled into adendritic cylinder structure. Thus, the assembled morphologiesof PHEMAPOSS-b-PMAA block copolymers can be mediated

by changing the mass ratio of hydrophobic/hydrophilic block.In previous research, the self-assembly of amphiphiliccopolymers has presented promising applications in the fieldsof medicine, microelectronics, optics, etc.1,7,29 Here, themicelles based on PHEMAPOSS-b-PMAA block copolymerswould also have many potential applications such as drug andgene delivery systems.

■ CONCLUSIONSPOSS-containing homopolymer (PHEMAPOSS) with a highDP was synthesized via RAFT polymerization, which was usedas the macro-RAFT agent to prepare amphiphilic PHEMPOSS-b-PMAA diblock copolymers with different lengths of hydro-philic chain. TEM, AFM, and DLS results showed that the self-assembled morphology of PHEMPOSS-b-PMAA is dependenton the mass ratio of hydrophilic/hydrophobic block.PHEMAPOSS45-b-PMAA523 ( f PHEMAPOSS = 52.1%) can self-assemble in aqueous solution into the typical core−shellspherical micelles with the hydrophobic PHEMAPOSS block asthe core and hydrophiphilic PMAA block as the shell.PHEMAPOSS45-b-PMAA1173 with a longer PMAA chain( f PHEMAPOSS = 32.7%) self-assembled into irregular aggregateswith POSS moieties dispersed in these aggregates, whereasPHEMAPOSS45-b-PMAA308 with a shorter hydrophilic PMAAchain ( f PHEMAPOSS = 65.8%) could self-assemble into dendriticcylinder structure. Thus, the self-assembled morphology ofPHEMPOSS-b-PMAA can be tuned by changing the mass ratioof hydrophilic/hydrophobic block. This formation of micellesmight have potential applications such as drug and genedelivery systems.

■ ASSOCIATED CONTENT*S Supporting Information1H NMR spectrum of PHEMAPOSS26 and PHEMAPOSS45-b-PMAA523, FT-IR spectra of PHEMAPOSS-b-PMAA andPHEMAPOSS-b-PtBMA, and CMC measurement of PHEMA-

Figure 5. TEM images (scale bar, 200 nm (a), 50 nm (b)), AFMimages (height (c), phase (d)), and the height profile along the line inheight image (e) of PHEMAPOSS45-b-PMAA308 self-assembledaggregates in aqueous solution.

Scheme 2. Self-Assembly Behaviors of PHEMAPOSS-b-PMAA in Water with Different Lengths of HydrophilicPMAA Moiety



POSS-b-PMAA. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Tel.: +86(21) 64253033.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the National NaturalScience Foundation of China (Nos. 21074035 and 51173044),Research Innovation Program of SMEC (No. 14ZZ065), andthe Project-sponsored by SRF for ROCS, SEM. W.Z. alsoacknowledges the support from the Fundamental ResearchFunds for the Central Universities.

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