Telechelic Hybrid Poly(acrylic acid)s Containing Polyhedral Oligomeric Silsesquioxane (POSS) and Their Self-Assembly in Water

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<ul><li><p>Published: August 10, 2011</p><p>r 2011 American Chemical Society 6891 dx.doi.org/10.1021/ma201152t |Macromolecules 2011, 44, 68916898</p><p>ARTICLE</p><p>pubs.acs.org/Macromolecules</p><p>Telechelic Hybrid Poly(acrylic acid)s Containing PolyhedralOligomeric Silsesquioxane (POSS) and Their Self-Assembly in WaterWeian Zhang,*, Jiayin Yuan, Stephan Weiss, Xiaodong Ye, Chunliang Li, and Axel H. E. Muller*,</p><p>Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, ChinaMakromolekulare Chemie II, Universitat Bayreuth, D-95440 Bayreuth, GermanyHefei National Laboratory for Physical Sciences at the Microscale and the Department of Chemical Physics, University of Science andTechnology of China, Hefei, Anhui 230026, China</p><p> INTRODUCTION</p><p>During the past decade, the self-assembly of polymers, espe-cially amphiphilic block copolymers, has received considerableattention because these assemblies show a rich variety ofmorphologies such as spheres, vesicles, cylinders, lamellae, andgyroids in bulk or in solution.18 These morphologies can becontrolled via the polymer type, structure, external stimuli(temperature, pH, etc.), solvent, additives (nanoparticles, salts,polymers, etc.), and many other factors. However, these self-assembly morphologies have mostly been formed from pureorganic polymers, and the self-assembly behavior of hybridpolymers containing inorganic components has been rarelyreported.911</p><p>Polyhedral oligomeric silsesquioxanes (POSS), as nanoscaleenhancing agents, have been widely applied in the preparation ofthe polymer hybrids. The most common variety consists of eightRSiO1.5 units forming a cage-like inorganic core surrounded byeight organic corner groups. Thus, POSS can be easily incorpo-rated into polymeric matrices to prepare novel polymer hybridsby physical blending or chemical copolymerization.1226 Nowa-days, with the emergence of new polymerization techniques suchas living/controlled radical polymerization, some novel POSS-containing polymers with well-defined structures have beensynthesized, and the self-assembly behavior of these hybridpolymers has also attracted great attention. For example, POSS</p><p>methacrylate as a monomer was incorporated into polymermatrices by random copolymerization to prepare POSS-contain-ing random copolymers, and an ordered nanostructure wasobtained from the random copolymers.27 POSS-containing di-block copolymers were also prepared by anionic polymerization,and lamellar self-assembly structures were observed in bulk.28</p><p>POSS-containing ABA triblock copolymers were also synthe-sized via ATRP, and the self-assembly structure formed inthin films was investigated.29,30 POSS molecules were alsoused to modify polystyrene-block-polybutadiene-block-polystyr-ene (SBS) triblock copolymers, and the self-assembly morphol-ogies of the block copolymers were still preserved.31,32 POSS wasattached to the end of the polymer chain to prepare endfunc-tional (monochelic) hybrid polymers, and the self-assemblyof these hybrid polymers was also studied.3336 We preparedtadpole-shaped (monochelic) hybrid poly(N-isopropylacryl-amide) (PNIPAAm) and poly(acrylic acid) using reversibleadditionfragmentation transfer (RAFT) polymerization.37,38The tadpole-shaped POSS-PAAs self-assemble into ratherlarge aggregates where the POSS moieties are dispersed in theparticle. Recently, Cheng et al. prepared an inverse polymer,</p><p>Received: May 19, 2011Revised: July 28, 2011</p><p>ABSTRACT: Telechelic (dumbbell-shaped) POSS-contain-ing inorganic/organic hybrid poly(tert-butyl acrylate) (POSS-PtBA-POSS) was prepared by the combination of ATRP andthe copper-catalyzed azidealkyne click reaction. Telechelicbromoterminated PtBAs with different molecular weights werefirst prepared via ATRP using a dibromo-functionalized ATRPinitiator; subsequently, the bromo groups were transformed toazido groups using sodium azide in DMF via nucleophilicsubstitution. The click coupling between azido-terminatedPtBA and alkyne-functionalized POSS was successfully accom-plished to afford telechelic POSS-PtBA-POSS, which is confirmed by NMR and GPC. The telechelic POSS-PtBA-POSS was thenhydrolyzed to amphiphilic telechelic POSS-functional poly(acrylic acid) (POSS-PAA-POSS). The self-assembly behavior of POSS-PAA-POSS with three different molecular weights in aqueous solution at pH 8.5 was investigated by transmission electronmicroscopy, scanning electron microscopy, and light scattering (SLS, DLS). The results show that POSS-PAA40-POSS and POSS-PAA82-POSS can self-assemble in water into ellipsoidal aggregates with a moderately uniform size, whereas POSS-PAA140-POSSwith a long PAA chain only self-assembles into aggregates with a broad size distribution.We discuss the formation of layered crystalsof POSS within these aggregates.</p></li><li><p>6892 dx.doi.org/10.1021/ma201152t |Macromolecules 2011, 44, 68916898</p><p>Macromolecules ARTICLE</p><p>POSS-containing tadpole-shaped polystyrene with a hydrophilicPOSS as the head, and obtained interesting self-assemblymorphologies in different solutions.39</p><p>In this contribution, we continue the study of the self-assemblyof POSS-containing hybrid polyelectrolytes. Dumbbell-shaped,POSS-containing telechelic poly(tert-butyl acrylate) (POSS-PtBA-POSS) was prepared by the combination of ATRP andclick chemistry.40 The telechelic POSS-PtBA-POSS wasfurther hydrolyzed into POSS-containing telechelic poly(acrylicacid) (POSS-PAA-POSS) using trifluoroacetic acid (TFA). Theself-assembly behavior of the telechelic amphiphilic POSS-PAA-POSS was investigated by transmission electron microscopy(TEM), scanning electron microscopy (SEM), and light scatter-ing (LS).</p><p>EXPERIMENTAL SECTION</p><p>Materials. Aminoisobutyl polyhedral octameric silsesquioxane(POSS) was purchased from Hybrid Plastics Company. tert-Butylacrylate (tBA) was kindly supplied by BASF SE and was passed througha silica column to remove the inhibitor shortly before polymerization.CuBr was purified by stirring with acetic acid overnight. After filtration, itwas washed with ethanol and ether and then dried in a vacuum oven atroom temperature overnight. Triethylamine and dichloromethane were,respectively, dried over CaH2 and distilled prior to use. Tetrahydrofuran(THF) was distilled from a purple sodium ketyl solution.Preparation of 3-Propargyl carbonyl-N-[3-(isobutyl poly-</p><p>hedral oligomeric silsesquioxane) propyl]propanamide(Alkyne-POSS)z. The preparation of alkyne-POSS has been reportedin our previous paper.40 1H NMR (CDCl3, ppm): 4.71 (d, 2H,COOCH2CCH,), 3.26 (q, 2H, Si-CH2CH2CH2NH), 2.75 (t, 2H,NHCOCH2CH2COO), 2.50 (m, 3H, NHCOCH2CH2COO,COOCH2CCH), 1.87 (m, 7H, Si-CH2CH(CH3)2), 1.61 (m,2H, Si-CH2CH2CH2NH), 0.98 (d, 42H, Si-CH2CH(CH3)2),0.62 (q, 16H, Si-CH2CH(CH3)2, Si-CH2CH2CH2NH).Preparation of 1,4-Bis(2-bromoisobutylcarbonyloxy)-</p><p>butane (BiBB). A round-bottomed flask (250 mL) equipped with amagnetic stir bar was charged with 1,4-butanediol (3.0 g, 0.033 mol),triethylamine (13.92 mL, 0.10 mol), and anhydrous THF (120 mL).After stirring for 20 min at 0 C, 2-bromo-2-methylpropionylbromide(12.34 mL, 0.10 mol) in 30 mL of anhydrous THF was slowly addedunder argon. After stirring for 24 h at ambient temperature, the reactionmixture was filtered, and the solvent was removed in vacuo. Theresulting mixture was dissolved in dichloromethane and, respectively,washed with a saturated solution of NaHCO3 and NaCl, the organiclayer was dried over anhydrous MgSO4 and concentrated in vacuo.Finally, the crude product was crystallized in toluene twice, and 9.68 gof white crystal was obtained with 75% yield. 1H NMR (CDCl3-d ppm):4.24 (m, 4H, OCH2(CH2)2CH2O), 1.94 (s, 12H, Br(CH3)2-CO,OC(CH3)2Br), 1.83 (m, 4H,OCH2(CH2)2CH2O).Preparation of Dibromo-Terminated PtBA (Br-PtBA-Br)</p><p>Using ATRP. A round-bottomed glass flask (25 mL) charged with amagnetic stir bar, tBA (6.80 mL, 0.047 mol), BiBB (1.74 g, 0.45 mmol),CuBr (0.032 g, 0.22 mmol), acetone (1.5 mL), and toluene (0.50 mL)was purged with argon. After 20 min, PMDETA (0.047 mL, 0.22 mmol)was added under argon, and the flask was purged with argon for another5 min. The polymerization was carried out at 60 C. Small samples(0.05 mL) were taken out from the reaction flask at intervals to checkthe conversion, which was measured by 1H NMR by comparing thepeaks of the double-bond protons at 5.78 ppm and those of the toluenearomatic protons at 7.29 ppm. At the end of the polymerization reaction,the reaction solution obviously became viscous. Final conversiondetermined by 1H NMR reached 84%. The reaction was stopped by</p><p>cooling to room temperature and opening to the air, and THF wasadded. After passing through a basic alumina column, the solution wasconcentrated by a rotary evaporator. Afterward, it was precipitated into amixed solution of water/methanol (v/v, 1/4) to remove the residualmonomer and other impurities. The polymer was dried under vacuum at50 C for 24 h (Mn = 10 880 g/mol, Mw/Mn = 1.30).Preparation of Diazido-Terminated PtBA (N3-PtBA-N3). A</p><p>round-bottomed glass flask (20 mL) with a magnetic stirring bar wascharged with Br-PtBA-Br (1.50 g, 0.14 mmol), sodium azide (0.18 g,2.8 mmol), and DMF (10 mL). The solution was allowed to stirovernight at room temperature. At the end of the reaction, the solutionwas precipitated into a mixed solution of water/methanol (v/v 1/4).The resulting product was filtered and dried at 50 C for 24 h to give 1.28g of N3-PtBA-N3, 85.0% yield.Preparation of Telechelic POSS-Containing PtBA Hybrids</p><p>(POSS-PtBA-POSS) via CuBr-Catalyzed Click Coupling. Around-bottomed glass flask (20 mL) charged with a magnetic stirringbar, N3-PtBA82-N3 (0.60 g, 0.056 mmol), alkyne-POSS (0.33 g, 0.34mmol), PMDETA (0.23 mL, 1.1 mmol), and DMF (10 mL) was purgedwith argon. After 20 min, CuBr (0.16 g, 1.1 mmol) was added underargon, and the flask was purged with argon for another 5 min. Thereaction was carried out at 80 C. After 24 h, the flask was opened,and THF was added to quench the reaction. The solution waspassed through a basic alumina column. The solution was diluted into50 mL and dialyzed in THF using a dialysis membrane (MWCO =12 00014 000). After 7 days, the unreactive alkyne-POSS was com-pletely removed. The resulting product was dried under vacuum at 50 Cfor 24 h (0.54 g, yield 82%). Mn = 14 400, Mw/Mn = 1.21.Preparation of POSS-PAA-POSS. A round-bottomed flask</p><p>(50 mL) with a magnetic stir bar was charged with POSS-PtBA82-POSS(0.4 g, 2.7 mmol of tBA unit), followed by dichloromethane (40 mL).The mixture was allowed to stir for 15 min to dissolve the polymer.Then, TFA (3.0 mL, 40.0 mmol) was added. Themixture was allowed tostir at room temperature. After 24 h, dichloromethane and TFA wereremoved by a rotary evaporation. The product was washed three timeswith dichloromethane, redissolved, and freeze-dried from dioxane.Mn =6217, Mw/Mn = 1.11 (MALDI-TOF).Self-Assembly of POSS-PAA-POSS in Aqueous Solution. A</p><p>typical self-assembly aggregate solution of telechelic POSS-PAA-POSSwas prepared as that of monochelic POSS-PAA.38 POSS-PAA82-POSS(20 mg) was first dissolved in dioxane (5 mL). The solution was stirredovernight and gradually dialyzed against pH 8.5 Millipore water(resistance = 18M) for 3 days. After gradual dialysis, the self-assemblysolution was dialyzed at least three times using pH 8.5Millipore water tomake sure to remove dioxane completely. The critical micelle concen-tration (CMC) of POSS-PAA82-POSS in water is 2.4 mg/L, as deter-mined by the fluorescent probe technique with pyrene as a probe.41</p><p>CHARACTERIZATION</p><p>Nuclear Magnetic Resonance (NMR) Spectroscopy. The1H NMR and 13C NMR measurements were carried out on aBruker AC-250 instrument. The samples were dissolved withdeuterated CDCl3 or dioxane-d8, and measured with tetra-methylsilane (TMS) as an internal reference.</p><p>29Si NMR spectra were measured on a Varian Inova 300 MHzspectrometer at 23 C. The samples were dissolved in dioxane-d8;chemical shifts are given relative to TMS [29Si = 0 ppm for(29Si) = 19.867184 MHz]; Chemical shifts are given to (0.1for 29Si; coupling constants are given (0.4 Hz for J(29Si). Thespectra were measured by using the refocused INEPT pulsesequence based on 2/3J(29Si) = 10 Hz after optimizing the delaytimes in the pulse sequence.</p></li><li><p>6893 dx.doi.org/10.1021/ma201152t |Macromolecules 2011, 44, 68916898</p><p>Macromolecules ARTICLE</p><p>Fourier Transform Infrared Spectroscopy. Fourier Trans-form Infrared Spectroscopy (FTIR) measurements were con-ducted on a Perkin-Elmer Spectrum One FT-IR spectro-photometer equipped with an ATR sampling unit (25 C).Gel Permeation Chromatography. The molecular weights</p><p>and molecular weight distribution were measured by conven-tional gel permeation chromatography (GPC). Column set:5 m SDV gel, 102, 103, 104, and 105 , 30 cm each (PSS,Mainz). Detectors used are RI and UV operated at 254 nm.Polystyrene standards (PSS, Mainz) with narrow molecularweight distribution were used for the calibration of thecolumn set, and THF was used as the eluent at a flow rate of1 mL/min.MALDI-TOF Mass Spectrometry. MALDI-TOF mass spec-</p><p>trometry was performed on a Bruker Reflex III instrumentequipped with a 337 nm N2 laser in the reflector mode and20 kV acceleration voltage. 2,5-Dihydroxybenzoic acid (Aldrich,97%) was used as a matrix. Samples were prepared from dioxanesolution bymixingmatrix (20mg/mL) and polymer (10mg/mL)in a ratio of 4:1. The number-average molecular weights of thepolymers were determined in the linear mode.Transmission ElectronMicroscopy.TEM images were taken</p><p>on a LEO 922 OMEGA electron microscopes operated at200 kV. A 5 L droplet of self-assembly aggregate solution(1 mg/mL) was directly dropped onto a copper grid (300 mesh)coatedwith a carbon film, followed by drying at room temperature.Scanning Electron Microscopy. SEM was performed using a</p><p>LEO 1530 Gemini instrument equipped with a field emissioncathode with a lateral resolution of 2 nm. The accelerationvoltage was chosen between 0.5 and 3 kV.Wide-Angle X-ray Diffraction. Wide-angle X-ray diffraction</p><p>(WAXD) patterns were recorded in transmission with a BrukerD8 Advance X-ray diffractometer. The wavelength used was CuKR ( = 0.154 nm), and spectra were recorded in the 2 range of440 (step size 0.025).Static and Dynamic Light Scattering. Static and dynamic</p><p>light scattering (SLS, DLS) were measured on a DLS/SLS-5022F instrument (ALV Germany) equipped with a multi-digital time correlation (ALV5000) and a cylindrical 22 mWUNIPHASE HeNe laser (0 = 632 nm) as the light source. InSLS,42,43 the angular dependence of the absolute excess time-average scattering intensity, known as the Rayleigh ratio, Rvv(q),is found, and we can obtain the weight-average molar mass,Mw,the root-mean-square radius of gyration, Rg2z1/2 (or written asRg), and the second virial coefficient, A2, by using the Zimmequation. The refractive index increment (dn/dC = 0.044 mLg1) of POSS-PAA82-POSS aggregates was determined with aprecise differential refractometer at 25 C and 633 nm.44In DLS,45 the intensityintensity time correlati...</p></li></ul>