Inorganic Polyhedral Oligomeric Silsesquioxane

Jun 12, 2014 - Beibei Lu , Lei Li , Lulu Wei , Xuhong Guo , Jun Hou , Zhiyong Liu ... Lei Li , Beibei Lu , Qikui Fan , Lulu Wei , Jianning Wu , Jun Ho...
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Synthesis of Organic/Inorganic Polyhedral Oligomeric Silsesquioxane-Containing Block Copolymers via Reversible Addition−Fragmentation Chain Transfer Polymerization and Their Self-Assembly in Aqueous Solution Lizhi Hong, Zhenghe Zhang, and Weian Zhang* Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China S Supporting Information *

ABSTRACT: Polyhedral oligomeric silsesquioxane (POSS)-containing homopolymers (PHEMAPOSS) with a high degree of polymerization (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-bPMAA (poly (methyl methacrylate)), which possessed a different length of hydrophilic chains. The self-assembly behavior of PHEMPOSS-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 typical core−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 irregular aggregates with POSS moieties dispersed in the aggregates. On the other hand, PHEMAPOSS45-b-PMAA308 with a shorter hydrophilic PMAA chain could self-assemble into a dendritic cylinder structure.

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isobutyl-POSS via ring-opening polymerization of γ-benzyl-Lglutamate N-carboxyanhydride and investigated their selfassembly behavior in toluene; they found the aggregation of nanoribbon could be prevented by incorporation of POSS. Cheng et al.32 synthesized a “giant surfactant” PS-APOSS, possessing a hydrophilic POSS headgroup and a hydrophobic PS tail. A variety of morphologies, from vesicles to wormlike cylinders and further to spheres, can be obtained in selective solutions. Our group also has contributed works on POSScontaining hybrid polymers to this field.27,33 We synthesized a series of hemitelechelic POSS-containing hybrid polymers such as poly(acrylic acid) (POSS-PAA) and poly(ethylene oxide) (POSS-PEO) and studied their self-assembly behavior in aqueous solution and found that these hybrid polymers selfassembled in aqueous solution to form the aggregates with a structure that is different from a typical core−shell micelle. Besides, POSS-containing amphiphilic star-shaped hybrid polymers have also been prepared using the living polymerization technique, and their self-assembly and application has been further explored in selective solution.30,34,35 He’s group23−25 also investigated amphiphilic di- and triblock copolymers of poly(ethylene glycol) (PEG) and poly(methacrylisobutyl-POSS) (P(MA-POSS)), and the selfassembly behaviors of amphiphilic hybrid block copolymers in aqueous solution were further investigated. However, in the past research, much effort has been tried to prepare POSS-

INTRODUCTION Inorganic/organic hybrid polymers with synergetic functions of two components have attracted a great deal of research interest.1,2 Self-assembly of block copolymers containing inorganic components such as quantum dots, metal nanoparticles, carbon nanoscaled materials, and silica-based nanoparticles is another growing technique to develop inorganic/ organic hybrid polymers with novel morphologies and novel properties.3−5 A variety of novel self-assembled morphologies could be acquired by varying the size, nanostructure, and chemical composition depending on the choice of inorganic components and polymers, as well as self-assembly conditions.6,7 A particularly noticeable example is organic/ inorganic hybrid polymers based on polyhedral oligomeric silsesquioxane (POSS).8−12 POSS, as a hybrid molecule with precisely defined nanostructure, has a rigid cubic silica core and organic corners, which could be reactive or nonreactive. These organic groups provide a POSS molecule with higher reactivity and solubility in the construction of POSS-containing hybrid polymers.13−17 Thus, POSS could be easily introduced into polymeric matrices to prepare novel polymer hybrids with advantages such as mechanical, thermal, and flammability-resistant properties.18−21 More recently, many novel POSS-containing well-defined hybrid polymers have been prepared using the living/controlled polymerization technique including hemitelechelic, telechelic, and multitelechelic block hybrid polymers.22−30 Meanwhile, much attention has also been focused on the self-assembly of POSS-containing organic/inorganic hybrid polymers. For example, Kuo et al.31 prepared the POSS-containing hemitelechelic helical polypeptide copolymers using aminopropyl © 2014 American Chemical Society

Received: Revised: Accepted: Published: 10673

April 14, 2014 June 9, 2014 June 12, 2014 June 12, 2014 dx.doi.org/10.1021/ie501517m | Ind. Eng. Chem. Res. 2014, 53, 10673−10680

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added slowly into the above POSS solution at 0 °C under argon atmosphere, and the mixture was stirred for 24 h at room temperature. Then, a white precipitate (triethylamine hydrochloride) was removed by filtration, and the resulting solution was concentrated under vacuum at room temperature. The residue was purified by a silica gel chromatography with petroleum ether/ethyl acetate (2:1, v/v) as the eluent. A white solid was obtained (5 g, yield 80.5%). 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 6.13 (s, 1H, HCHC(CH3)−), 5.60 (s, 1H, HCHC(CH 3 )−), 4.35 (s, 4H, −OCO(CH 2) 2OCO−), 3.26−3.20 (m, 2H, −CH 2−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 typical experiment, 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) and 0.6 mL of toluene were charged in a dry glass tube equipped with a magnetic stirring bar. The mixture was degassed by at least three freeze−pump−thaw cycles. After the flask was flamesealed under vacuum, the reaction was performed at 65 °C. After 48 h, the polymerization was quenched by plunging the reaction flask into liquid nitrogen. The reaction solution was precipitated with a solvent mixture (methanol/acetic ether = 6/ 1, volume ratio) three times, and the final production was dried under vacuum at 30 °C. Mn = 16 900 g/mol, Mw/Mn = 1.13. Synthesis of PHEMAPOSS-b-PtBMA. A representative example of the synthesis of PHEMAPOSS-b-PtBMA block copolymer is as follows: PHEMAPOSS45 (0.2 g, 0.0041 mmol), tBMA (0.8 g, 5.6 mmol), and AIBN (22 μL, 10 mg/mL AIBN THF solution, 0.0013 mmol) and 1 mL of THF were placed in a dry glass tube equipped with a magnetic stirring bar. The mixture was degassed by at least three freeze−pump−thaw cycles. After the flask was flame-sealed under vacuum, the reaction was performed at 65 °C. After 10 h, the reaction was quenched by plunging the flask into liquid nitrogen. The reaction solution was precipitated with a solvent mixture (methanol/water = 4/1, volume ratio) three times, and the final product was dried under vacuum at 50 °C for 48 h. Mn = 73 900 g/mol, Mw/Mn = 1.38. Synthesis of PHEMAPOSS-b-PMAA Block Copolymers. Hydrolysis of the tert-butyl esters was accomplished by treating the block copolymers with an excess of trifluoroacetic acid (TFA) in dichloromethane. In a typical experiment, 0.2 g of block copolymer PHEMAPOSS45-b-PtBMA523 was dissolved in 10 mL of dichloromethane and stirred for 15 min to dissolve the polymer. Then, 2 mL of TFA was added, and the solution was left to stir for 24 h at room temperature. The polymer solution was concentrated on a rotary evaporator to afford a yellow solid residue and dried under vacuum at 50 °C. Self-Assembly of PHEMAPOSS-b-PMAA in Aqueous Solution. In a typical process, PHEMAPOSS45-b-PMAA523 (10 mg) was first dissolved in 10 mL of dioxane (common solvent). The solution was stirred for 12 h and gradually dialyzed against pH = 8.5 ultrapure water (selective solvent) for 3 days. The self-assembly solution was further dialyzed against ultrapure water at least three times to remove dioxane completely.

containing hybrid block copolymers, but it is difficult to obtain hybrid polymers with a high degree of polymerization (DP) of POSS-based monomer in free-radical polymerization, due to the steric hindrance of POSS unit. In this contribution, we synthesized POSS-containing hybrid polymer, PHEMAPOSS with a higher DP via reversible addition−fragmentation chain transfer (RAFT) polymerization. PHEMAPOSS homopolymer was further used as macro-RAFT agent in RAFT polymerization of tert-butyl methacrylate (tBMA) to construct a series of POSS-containing hybrid diblock copolymers, PHEMAPOSS-b-PtBMA. Then, the amphiphilic hybrid block copolymers, PHEMAPOSS-b-PMAA (poly (methyl methacrylate)), were obtained via the hydrolysis of the tert-butyl esters of PtBMA block with an excess of trifluoroacetic acid (Scheme 1). Finally, the self-assembly Scheme 1. Synthesis of PHEMAPOSS Homopolymers and PHEMAPOSS-b-PMAA Block Copolymers via RAFT Polymerization

behavior of PHEMAPOSS-b-PMAA in aqueous solution with different chain lengths of PMAA block was investigated by transmission electron microscopy (TEM) and atomic force microscopy (AFM).

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EXPERIMENTAL SECTION Materials. Aminoisobutyl polyhedral oligomeric silsesquioxane (POSS) was purchased from Hybrid Plastics Company. tert-Butyl methacrylate (tBMA, Aladdin, 99%) was passed through a column of basic aluminum oxide to remove inhibitors shortly before polymerization. Azobis(isobutyronitrile) (AIBN) was recrystallized from ethanol. Tetrahydrofuran (THF) and toluene was distilled from a purple sodium ketyl solution. Dichloromethane (DCM) and triethylamine (TEA) were dried over calcium hydride and distilled before use. Other regents and solvents in analytical grade were obtained from Aladdin. The RAFT agent, cumyl dithiobenzoate (CDB), was prepared according to the previous literature.36 Synthesis of HEMAPOSS Monomer. The synthesis of HEMAPOSS was similar to our previous work.28,37 The typical synthetic procedure was as follows: aminoisobutyl-POSS (5 g, 5.7 mmol) was dissolved in 35 mL of anhydrous THF, and 2.4 mL of distilled triethylamine (TEA) was added. 2-(Methacryloyloxy) ethyl succinyl chloride (HEMA-COCl, 4.32 g, 17.4 mmol) was dissolved in 30 mL of anhydrous THF and was 10674

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CHARACTERIZATION Nuclear Magnetic Resonance Spectroscopy (NMR). 1H spectra were recorded on a Bruker AVANCE 400 spectrometer. The samples were dissolved with deuterated CDCl3 and THFd8 and measured with tetramethylsilane (TMS) as an internal reference. Fourier Transform Infrared Spectroscopy (FT-IR). Fourier transform infrared spectroscopy (FT-IR) measurements were conducted on a PerkinElmer Spectrum One FT-IR spectrophotometer equipped with an ATR sampling unit (25 °C). Gel Permeation Chromatography (GPC). The molecular weight and molecular weight distribution of PHEMAPOSS homopolymers and PHEMAPOSS-b-PtBMA diblock copolymers were determined with a gel permeation chromatography (GPC, Waters 1515). Polystyrene standards with narrow molecular weight distribution were used for the calibration of the column set, and THF was used as the eluent at a flow rate of 1 mL/min. Critical Micelle Concentration (CMC). CMC measurements were estimated by fluorescence spectroscopy on Hitachi FL-4500 using pyrene as a probe. The copolymer solutions after self-assembly with different concentrations were prepared by using ultrapure water (pH = 8.5). Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) images were taken on a JEOL JEM1400 instrument operated at an accelerating voltage of 100 kV. A 15 μL droplet of self-assembly aggregate solution (0.25 mg/mL) was directly dropped onto a copper grid (300 mesh) coated with a carbon film, and the sample was allowed to dry under an infrared lamp. Atomic Force Microscopy (AFM). Atomic force microscopy (AFM) images were obtained using Tapping Mode on a Nanoscope IV of Digital Instruments. The preparation procedure of AFM samples is as follows: the self-assembled aggregates solution (0.25 mg/mL) was directly dropped onto a freshly cleaved mica and then dried at room temperature for 24 h. Dynamic Light Scattering (DLS). DLS measurements were performed by a BECKMAN COULTER Delasa Nano C particle analyzer at a fixed angle of 165°. All measurements were repeated three times, and the average result was accepted as the final hydrodynamic diameter (Dh).

ymers with different DPs by changing the molar feed ratio of monomer to RAFT agent, cumyl dithiobenzoate (CDB). The PHEMAPOSS homopolymers were characterized by GPC against the polystyrene linear standards, and the GPC traces are shown in Figure 1. The Mn,GPC and polydispersity index (PDI)

Figure 1. Evolution of GPC chromatograms of PHEMAPOSS homopolymers and PHEMAPOSS-b-PtBMA block copolymers with different molecular weights.

of the PHEMAPOSS homopolymers was also listed in Table 1. We find the GPC curves are very symmetrical, and the PDI is quite lower, which suggests the RAFT polymerization of HEMAPOSS is living/well-controlled. The 1H NMR of PHEMAPOSS 26 was shown in Figure S1, Supporting Information, and the DP and molecular weight of PHEMAPOSS can also be estimated from Figure S1, Supporting Information. By comparing the peaks of methylene protons next to silicon from the POSS unit at 0.6 ppm with the aromatic protons of the terminal benzene ring at 7.81 ppm, the DP of PHEMAPOSS and the absolute molecular weights were determined as DP = I0.6/(8 × I7.81) and Mn,PHEMAPOSS,NMR = I0.6/(8 × I7.81) × MHMEAPOSS + MCDB, where the MHEMAPOSS and MCDB are the molecular weights of HEMAPOSS monomer and CDB, respectively. Here “I” represents the area of a peak, and we first set I0.6 to 16.00. The DP calculated from the 1H NMR spectrum is 26. Additionally, we can find that the Mn,GPC is much lower than Mn,NMR in Table 1. It could be attributed to the relatively compact cage structure of POSS units; thus, PHEMAPOSS has a very different solution behavior with PS standard in THF.38−40 On the basis of the 1H NMR and GPC results, PHEMAPOSS homopolymers with a high DP were successfully achieved via RAFT polymerization. Preparation of PHEMAPOSS-b-PMAA Block Copolymers. The PHEMAPOSS-b-PtBMA block copolymers were synthesized using PHEMAPOSS45 as a Macro-RAFT agent. We synthesized three PHEMAPOSS-b-PtBMA block copolymers with different PtBMA block lengths. The GPC traces of PHEMAPOSS-b-PtBMA block copolymers are shown in Figure 1. All the curves of PHEMAPOSS-b-PtBMA shift to lower elution volume compared to that of PHEMAPOSS45. The polymerization results were also listed in Table 1. The 1H NMR spectrum of PHEMAPOSS45-b-PtBMA523 is shown in Figure 2, and the tert-butyl ester proton resonance in the PtBMA chain was detected at 1.41 ppm. The signals of resonance at 0.61, 0.96, and 1.84 ppm were assignable to the protons from the iso-butyl group of POSS units. Additionally,

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RESULTS AND DISCUSSION Preparation of PHEMAPOSS Homopolymer. In our previous work,27,37 we prepared hemitelechelic POSS-PAA and ditelechelic POSS-PAA-POSS via living radical polymerization and further studied their self-assembly behavior in aqueous solution. We found that amphiphilic hemitelechelic POSS-PAA self-assembles in aqueous solution into aggregates with different size. The aggregates are not conventional core−shell micelles with hydrophobic POSS moieties as the core and hydrophilic PAA chains as the shell, but the POSS moieties are dispersed in the aggregates. Additionally, POSS-PAA-POSS with a short PAA chain can self-assemble in water into ellipsoidal aggregates with 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 RAFT polymerization using the novel POSS-containing monomer, HEMAPOSS. We have synthesized PHEMAPOSS homopol10675

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Table 1. Results of PHEMAPOSS Homopolymers and PHEMAPOSS-b-PtBMA Block Copolymers Prepared via RAFT Polymerization samplesa

[HEMAPOSS]/[CDB]b or [tBMA]/[Macro-RAFT agent]b

10−3 Mn,NMRa

10−3 Mn,GPCc

Mw/Mnc

PHEMAPOSS26 PHEMAPOSS45 PHEMAPOSS45-b-PtBMA308 PHEMAPOSS45-b-PtBMA523 PHEMAPOSS45-b-PtBMA1173

30:1 50:1 568:1 1376:1 2837:1

28.5 48.9 92.7 123.3 215.7

16.9 25.4 50.3 73.9 144.8

1.13 1.12 1.16 1.16 1.17

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

the self-assembly behavior of POSS-containing hybrid block copolymers with a high DP of POSS-based monomer has been rarely reported.27,29,30,33,34,37 This is because it is difficult to obtain the POSS-containing hybrid block copolymers with a high DP of POSS unit. Here, we continued to study the selfassembly behavior of amphiphilic POSS-containing hybrid polymers, and we prepared POSS-containing hybrid block copolymers 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) and further studied their self-assembly behaviors in aqueous solution. To study the assembled behavior of PHEMPOSS-b-PMAA in aqueous solution, we chose dioxane as common solvent and water as selective solvent in which PHEMPOSS block is insoluble. First, the critical micelle concentration (CMC) of block copolymers in water at pH = 8.5 was measured by fluorescence spectrophotometry. Figure S4, Supporting Information, showed the intensity ratio of I3/I1 as a function of PHEMPOSS-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-assembly aggregates of the hybrid block copolymers were characterized by TEM and AFM. The TEM images of PHEMAPOSS45-b-PMAA523 with the mass fraction of PHEMAPOSS/PMAA of about 1/1 are shown in Figure 3a,b. The well-dispersed spherical aggregates revealed the formation of typical core−shell micelles with a relatively uniform diameter of about 38 nm. The aggregates were also analyzed by AFM (Figure 3c,d); the relatively uniform diameter can 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 in the difference in size between AFM and TEM measurements; the micelles have a relatively long corona in the aqueous solution due to the high repeat unit of the hydrophilic chain, and the shell of PMAA block resulting from the shrinkage of the corona also should be taken into consideration in the AFM measurements, while the TEM images only exhibit the PHEMAPOSS core. However, the height of the micelles (29.3 nm) measured by AFM is in good agreement with the diameter observed by TEM (32 nm), as shown in Figure 3e. This further confirms the formation of core−shell structure with 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 mass ratio of hydrophobic to hydrophilic segment of about 1/2, we got a different assembled morphology as the TEM images show in Figure 4a,b. There are some irregular aggregates, and the density of these aggregates is clearly nonhomogeneous. We can find many dark dots in these aggregates. The size of the little

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

we can estimate the DPPtBMA and the molecule weight of the diblock copolymers (Mn,PHEMAPOSS‑b‑PtBMA,NMR) by comparing the peaks of methylene protons next to silicon from the POSS unit at 0.61 ppm with the tert-butyl ester group protons at 1.41 ppm. The DPPtBMA is determined as DPPtBMA = (I1.41 × 16)/ (I0.61 × 9) × DPPHEMAPOSS, and the Mn,PHEMAPOSS‑b‑PtBMA,NMR is determined as Mn,PHEMAPOSS‑b‑PtBMA,NMR = Mn,PHEMAPOSS + DPPtBMA × Mn,tBMA, where the Mn,tBMA is the molecular weight of tBMA monomer. PHEMAPOSS-b-PMAA block copolymers were obtained by hydrolysis of the tert-butyl ester groups of PHEMAPOSS-b-PtBMA in dichloromethane using TFA. As the FT-IR spectrum shows in Figure S2, Supporting Information, the absorption bands of the tert-butyl moiety of PtBMA at 1367 cm−1 completely disappeared after hydrolysis of PtBMA to PMAA. Moreover, the absorption bands at 1725 cm−1 (CO stretching vibrations) are slightly shifted to lower wavenumbers, and the peaks also become broader, due to the formation of carboxylic acid from the tert-butyl ester. Additionally, the absorbance peak in range from 2292 to 3714 cm−1 is quite broad in the spectrum of PHEMAPOSS-bPMAA, which further confirms the formation of carboxylic acid groups. The 1H NMR spectrum of PHEMAPOSS45-b-PMAA523 also shows that the proton signal of the tert-butyl ester groups at 1.44 ppm completely disappeared after the hydrolysis (Figure S3, Supporting Information). Self-Assembly Behaviors of PHEMPOSS-b-PMAA in Aqueous Solution. Although the self-assembly of POSScontaining hybrid polymers in selective solutions has been studied by some groups and some interesting assembled morphologies have been obtained using hemitelechelic, ditelechelic, and star-shaped POSS-containing hybrid polymers, 10676

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

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

hydrophilic PMAA chain. We decreased the length of PMAA chain to prepare PHEMAPOSS45-b-PMAA308 and rendered the mass ratio of hydrophobic/hydrophilic segment to around 2/1. As shown in Figure 5a,b, the morphology of the aggregates was tuned from spherical micelles to dendritic cylinders with decreasing chain length of hydrophilic PMAA block, and typical crew-cut micelles formed; there are some irregular burls in size and shape in the dendritic PHEMAPOSS core. This morphology was also characterized by AFM, as shown in Figure 5c,d. The network cylinders interconnected by Y-shaped junctions, which were also irregular in shape. The self-assembled aggregates of PHEMAPOSS-b-PMAA ABCPs in aqueous solution (0.25 mg/mL) were further characterized by dynamic light scattering (DLS). Figure S5, Supporting Information, shows the intensity-weighted hydrodynamic diameter distributions and hydrodynamic diameter of PHEMAPOSS-b-PMAA ABCPs self-assembled aggregates in water at pH = 8.5. It can be seen that the uniform spherical structure formed from PHEMAPOSS45-b-PMAA523 has a very narrow distribution, but the diameter of the spheres measured by DLS is larger than that in AFM or TEM images. This is because the DLS result directly reflects the dimension of micelles in solution, where the PMAA chains as the corona are

dots in the aggregates is quite uniform at about 4.5 nm, although the aggregates are relatively polydisperse in size (from 32 to 51 nm). The dark dots should be formed from a single PHEMAPOSS block, which was embedded in the PMAA matrix. This is obviously different from the above core−shell micelles. In our previous study27 of self-assembly of hemitelechelic POSS-containing poly(acrylic acid) (POSS-PAA) where the mass ratio of hydrophobic POSS/hydrophilic PAA portion is much more than 1/2, their self-assembly morphologies are also not the typical core−shell micelles, and the density observed of the aggregate is not uniform in a single aggregate, which is consistent with this case. The aggregates can be characterized by an AFM image (shown in Figure 4c,d). The size 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 are clearly nonhomogeneous, which is agreeable with TEM results. The height of the aggregates in Figure 4e is about 25 nm, which is smaller than the dimension shown in the TEM images. This further confirms this is not typical core−shell assembled morphology. On the other hand, we also studied self-assembly behavior of PHEMAPOSS-b-PMAA block copolymer with a shorter 10677

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

by changing the mass ratio of hydrophobic/hydrophilic block. In previous research, the self-assembly of amphiphilic copolymers has presented promising applications in the fields of medicine, microelectronics, optics, etc.1,7,29 Here, the micelles based on PHEMAPOSS-b-PMAA block copolymers would also have many potential applications such as drug and gene delivery systems.

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

CONCLUSIONS POSS-containing homopolymer (PHEMAPOSS) with a high DP was synthesized via RAFT polymerization, which was used as the macro-RAFT agent to prepare amphiphilic PHEMPOSSb-PMAA diblock copolymers with different lengths of hydrophilic chain. TEM, AFM, and DLS results showed that the selfassembled morphology of PHEMPOSS-b-PMAA is dependent on the mass ratio of hydrophilic/hydrophobic block. PHEMAPOSS45-b-PMAA523 (f PHEMAPOSS = 52.1%) can selfassemble in aqueous solution into the typical core−shell spherical micelles with the hydrophobic PHEMAPOSS block as the core and hydrophiphilic PMAA block as the shell. PHEMAPOSS45-b-PMAA1173 with a longer PMAA chain ( f PHEMAPOSS = 32.7%) self-assembled into irregular aggregates with POSS moieties dispersed in these aggregates, whereas PHEMAPOSS45-b-PMAA308 with a shorter hydrophilic PMAA chain (f PHEMAPOSS = 65.8%) could self-assemble into dendritic cylinder structure. Thus, the self-assembled morphology of PHEMPOSS-b-PMAA can be tuned by changing the mass ratio of hydrophilic/hydrophobic block. This formation of micelles might have potential applications such as drug and gene delivery systems.

highly stretched in aqueous solution at pH = 8.5. PHEMAPOSS45-b-PMAA1173 possesses the largest average hydrodynamic diameter due to the longest PMAA chain as the corona. Additionally, PHEMAPOSS45-b-PMAA308 has a very broad distribution, which is attributed to the assembled dendritic cylinders. We summarized the self-assembly process of PHEMAPOSSb-PMAA in Scheme 2. PHEMAPOSS 4 5 -b-PMAA 5 2 3 (f PHEMAPOSS = 52.1%) can form the typical core−shell spherical micelles where the hydrophobic PHEMAPOSS block as the core and hydrophiphilic PMAA block as the shell. This is a classical assembled morphology, which could also be obtained from pure organic amphiphilic block copolymers in selective solution. With an increase in PMAA chain length, PHEMAPOSS45-b-PMAA1173 ( f PHEMAPOSS = 32.7%) self-assembled into irregular aggregates with some dark dots, which is similar to that of our previous study in hemitelechelic POSS-PAA. This suggests that only irregular aggregates formed with a low content of POSS. This is rarely reported from the self-assembly of pure organic amphiphilic block copolymer. On the other hand, PHEMAPOSS45-b-PMAA308 with a shorter hydrophilic PMAA chain (f PHEMAPOSS = 65.8%) self-assembled into a dendritic cylinder structure. Thus, the assembled morphologies of PHEMAPOSS-b-PMAA block copolymers can be mediated

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectrum of PHEMAPOSS26 and PHEMAPOSS45-bPMAA523, FT-IR spectra of PHEMAPOSS-b-PMAA and PHEMAPOSS-b-PtBMA, and CMC measurement of PHEMA10678

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

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86(21) 64253033. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21074035 and 51173044), Research Innovation Program of SMEC (No. 14ZZ065), and the Project-sponsored by SRF for ROCS, SEM. W.Z. also acknowledges the support from the Fundamental Research Funds for the Central Universities.

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