Synthesis, micelle formation, and bulk properties of poly(ethylene glycol)-b-poly(pentafluorostyrene)-g-polyhedral oligomeric silsesquioxane amphiphilic hybrid copolymers

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<ul><li><p>Synthesis, Micelle Formation, and Bulk Properties of Poly(ethylene glycol)-b-Poly(pentafluorostyrene)-g-polyhedral Oligomeric SilsesquioxaneAmphiphilic Hybrid Copolymers</p><p>H. HUSSAIN,1 B. H. TAN,1 K. Y. MYA,1 Y. LIU,1 C. B. HE,1 THOMAS P. DAVIS1,2</p><p>1Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research),</p><p>3, Research Link, Singapore 117602</p><p>2Centre for Advanced Macromolecular Design (CAMD) School of Chemical Sciences and Engineering, UNSW,</p><p>Sydney, New South Wales 2052, Australia</p><p>Received 25 August 2009; accepted 3 October 2009</p><p>DOI: 10.1002/pola.23773</p><p>Published online in Wiley InterScience (www.interscience.wiley.com).</p><p>ABSTRACT: The synthesis, micelle formation, and bulk properties</p><p>of semifluorinated amphiphilic poly(ethylene glycol)-b-poly-</p><p>(pentafluorostyrene)-g-cubic polyhedral oligomeric silsesquioxane</p><p>(PEG-b-PPFS-g-POSS) hybrid copolymers is reported. The syn-</p><p>thesis of amphiphilic PEG-b-PPFS block copolymers are</p><p>achieved using atom transfer radical polymerization (ATRP) at</p><p>100 C in trifluorotoluene using modified poly(ethylene glycol)as a macroinitiator. Subsequently, a proportion of the reactive</p><p>para-F functionality on the pentafluorostyrene units was</p><p>replaced with aminopropylisobutyl POSS through aromatic</p><p>nucleophilic substitution reactions. The products were fully char-</p><p>acterized by 1H-NMR and GPC. The products, PEG-b-PPFS and</p><p>PEG-b-PPFS-g-POSS, were subsequently self-assembled in</p><p>aqueous solutions to form micellar structures. The critical mi-</p><p>celle concentrations (cmc) were estimated using two different</p><p>techniques: fluorescence spectroscopy and dynamic light scat-</p><p>tering (DLS). The cmc was found to decrease concomitantly with</p><p>the number of POSS particles grafted per copolymer chain. The</p><p>hydrodynamic particle sizes (Rh) of the micelles, calculated from</p><p>DLS data, increase as the number of POSS molecules grafted per</p><p>copolymer chain increases. For example, Rh increased from</p><p>60 nm for PEG-b-PPFS to 80 nm for PEG-b-PPFS-g-POSS25 (25is the average number of POSSparticles grafted copolymer chain).</p><p>Static light scattering (SLS) data confirm that the formation of</p><p>larger micelles by higher POSS containing copolymers results</p><p>from higher aggregation numbers (Nagg), caused by increased</p><p>hydrophobicity. The Rg/Rh values, where Rg is the radius of gyra-</p><p>tion calculated fromSLS data, are consistent with a spherical parti-</p><p>cle model having a core-shell structure. Thermal characterization</p><p>by differential scanning calorimetry (DSC) reveals that the grafted</p><p>POSS acts as a plasticizer; the glass transition temperature (Tg) of</p><p>the PPFS block in the copolymer decreases significantly with</p><p>increasing POSS content. Finally, the rhombohedral crystal</p><p>structure of POSS in PEG-b-PPFS-g-POSS was verified by wide</p><p>angle X-ray diffraction measurements. VC 2009 Wiley Periodicals,</p><p>Inc. J PolymSci Part A: PolymChem48: 152163, 2010</p><p>KEYWORDS: amphiphilic block copolymers; atom transfer radical</p><p>polymerization; fluoropolymers; micelle; polyhedral oligomeric sil-</p><p>sesquioxane; self-assembly; semifluorinated</p><p>INTRODUCTION Fluoropolymers have attracted a great dealof attention both from academia and industry as they canimpart chemical resistance, thermal stability, low surfaceenergy, low dielectric constant, and low refractive index tomaterials.1,2 Recent advances in polymerization have enabledthe synthesis of well-defined semifluorinated copolymerswith defined architectures and compositions, precisely con-trolled chain lengths and narrow molecular weight distribu-tions. These predesigned polymers have been exploited in arange of applications including low surface energy coat-ings,35 stimuli responsive brushes,68 surface modifiedmembranes for water treatment,9,10 and fouling release coat-ings.11 Well-defined semifluorinated copolymers have beensynthesized using living polymerization techniques, such as</p><p>anionic,12 cationic,13 and living radical.1421 Hansen et al.22</p><p>and Hirao et al.23 have recently published excellent reviewson the design of well-defined fluorinated homo- and copoly-mers by living radical polymerization techniques, such asnitroxide mediated radical polymerization (NMRP),24,25 tran-sition-metal-catalyzed atom transfer radical polymerization(ATRP)26,27 and reversible addition fragmentation chaintransfer (RAFT)/macromolecular design through the inter-change of xanthates (MADIX).2832 ATRP, in particular, hasbeen widely exploited for the synthesis of well-defined semi-fluorinated amphiphilic block copolymers.4,17,19,3335 Livingradical polymerization works by establishing a dynamic equi-librium between a low concentration of active propagatingspecies and a dormant species unable to propagate or</p><p>Additional Supporting Information may be found in the online version of this article. Correspondence to: Thomas P. Davis (E-mail: t.davis@unsw.edu.au), H. Hussain (E-mail: h-hussain@imre.a-star.edu.sg) or C. B. He (E-mail: cb-he@imre.a-star.edu.sg)</p><p>Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 152163 (2010) VC 2009 Wiley Periodicals, Inc.</p><p>152 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA</p></li><li><p>terminate.36 The equilibrium can be established by eitherthe persistent radical effect37,38 or by addition-fragmentationchemistry.3942</p><p>Polyhedral oligomeric silsesquioxane (POSS) has a well-defined, nanometer scale, cage-like cubic structure.43 POSSnanocages can be functionalized easily with organic func-tional groups (reactive or inert) that facilitate miscibilityand/or covalent incorporation into organic polymers, leadingto nanohybrids with improved bulk properties such as glasstransition temperature, mechanical strength, thermal andchemical resistance, and low dielectric constant.4452 Severalrecent studies have reported a decrease in glass transitiontemperature on the incorporation of POSS into polymers toform nanocomposites.53,54 We recently reported fluorinatedhybrid star polymers having POSS nanocages as the coreswith fluorinated arms synthesized by ATRP.55 In this work,we report the synthesis, self-assembly, and bulk propertiesof POSS containing amphiphilic semifluorinated poly(ethyl-ene glycol)-b-poly(pentafluorostyrene)-g-polyhedral oligo-meric silsesquioxane (PEG-b-PPFS-g-POSS) hybrid blockcopolymers, where the pendant POSS molecules are statisti-cally distributed along the PPFS block. The influence of thePOSS functionality on the material and solution properties ofthe block copolymer is investigated.</p><p>EXPERIMENTAL</p><p>MaterialsTrisilanolisobutyl POSS was purchased from Hybrid Plastics(product no.: SO1450). Aminopropyltriethoxysilane (APTEOS)and triethylamine (TEA) were purchased from Aldrich. Tri-ethylamine was distilled over calcium hydride (CaH2) undernitrogen before use. Tetrahydrofuran (THF) was purified bydistillation over Na/benzophenone under a nitrogen atmos-phere immediately before use. 2,3,4,5,6-pentafluorostyrene(Aldrich, 99%) was purified by an alumina column. Anhy-drous trifluorotoluene (Aldrich, 99%), 2-bromoisobutyrylbromide (Aldrich, 98%) CuBr (Aldrich, 99.99%,), 2,2-bipyri-dine (Strem Chemicals, 99%), and n-ethyldiisopropylamine(Fluka, 98%) were used as received. PEG (Mn 5000 g/mol) was purchased from Fluka and dried under vacuumbefore use.</p><p>Synthesis of Aminopropylisobutyl POSSTrisilanolisobutyl POSS (5.0 g, 6.32 mmol) was dissolved in an-hydrous THF (50 mL) under an argon atmosphere. Triethyl-amine (2.6 mL, 18.96 mmol) was then added, and the mixturewas stirred at room temperature for 10 min. Aminopropyltrie-thoxysilane (1.5 mL, 6.32 mmol) was added dropwise underargon and stirring continued at room temperature for 1 day.The viscous liquid was obtained after completion of the reac-tion. The crude product was dissolved in a small amount of dryTHF and purified by precipitation in an excess amount ofmethanol. The final product was obtained after filtrationand drying in a vacuum oven at 40 C for 2 days (yield:&gt;92%). 1H-NMR (d-chloroform): 0.580.60 (SiCH2); 0.940.96 (SiCH2CH(CH3)2); 1.821.88 (SiCH2CH(CH3)2); and2.682.70 (Si CH2CH2CH2NH2). The structure of the prod-uct was further verified by Matrix-Assisted Laser Desorp-</p><p>tion/Ionization-Time of-Flight Mass Spectrometry (MALDI-TOF-MS). In addition, a monomodal GPC profile (Mn 1122, Mw 1128 g/mol, and PDI 1.01) confirmed thatno higher molecular weight byproducts were formed dur-ing the reaction, confirming the successful synthesis ofaminopropylisobutyl POSS.</p><p>Synthesis of PEG-b-PPFS Diblock CopolymerMPEG (Mn 5000 g/mol) was transformed into an ATRPmacroinitiator by reaction with 2-bromoisobutyryl bromidein THF at room temperature.56 MPEG macroinitiator (0.14mmol), 2,3,4,5,6-pentafluorostyrene (22.1 mmol), and anhy-drous trifluorotoluene (1.4 mL) were mixed under argon.The mixture was deoxygenated by four freeze-pump-thawcycles, followed by the addition of CuBr (0.14 mmol) and2,2-bipyridine (0.42 mmol) under argon. The contents weresubjected to two more freeze-pump-thaw cycles to ensurecomplete oxygen removal. The mixture was maintained at100 C for 3 h before air exposure and dilution with THF.The product solution was purified through an aluminacolumn, precipitated in hexane, and dried under vacuum at50 C overnight.</p><p>Grafting of Aminopropylisobutyl POSS to PEG-b-PPFSA typical reaction is described as follows: PEG-b-PPFS(0.3 g), aminopropylisobutyl POSS (0.19 g), and n-ethyldiiso-propylamine (80 lL) were dissolved in dried THF (2.5 mL)and refluxed at 80 C for 21 h. Aminopropylisobutyl POSSis not soluble in methanol and ethylacetate. Therefore, toremove the unreacted POSS materials, the product was dis-persed in methanol or ethylacetate and centrifuged repeat-edly. The upper transparent layer of solution containing thedesired product was recovered. The final product was recov-ered by solvent evaporation using a rotary evaporator andsubsequent drying under in vacuum at 40 C overnight. Thepurity of the product was verified by GPC, where traces dueto unreacted POSS species disappeared after purification.The synthesis of block copolymers and subsequent graftingof POSS is outlined in Scheme 1.</p><p>Characterization1H-NMR spectra were recorded on a Bruker 400 MHz spec-trometer in d-chloroform. GPC analyses were carried out inTHF at 35 C with a flow rate of 1 mL/min using a Waters2690 system fitted with an evaporative light scattering de-tector (Waters 2420) and a UV detector (Waters 996, photo-diode array). The GPC was calibrated with both PS andPMMA standards. TEM images were obtained using a JEOL2100 transmission electron microscope operating at anacceleration voltage of 200 kV. TEM samples were preparedby dip coating carbon coated copper grids with block copoly-mer solutions. Samples for surface characterization were pre-pared by spin coating the copolymer solutions in chloroform(10 mg/mL) on cleaned silicon chips. The static and advanc-ing and receding contact angles of water on copolymer surfa-ces were measured using a Rame-Hart goniometer. DSC char-acterization was performed using a TA Instruments DSCQ100 at a heating rate of 10 C min1. Wide angle X-ray</p><p>ARTICLE</p><p>PROPERTIES OF PEG-b-PPFS-g-POSS HYBRID COPOLYMERS, HUSSAIN ET AL. 153</p></li><li><p>diffraction (WAXD) studies were conducted on a BrukerGADDS XRD system.</p><p>Dynamic Light ScatteringRoom-temperature light scattering measurements were madewith a Brookhaven BI-200SM goniometer equipped with aBI9000AT digital correlator. An inverse Laplace transformationof REPES in the GENDIST software package was used to obtain</p><p>decay time distribution functions with the probability to-rejectset at 0.5. The copolymer concentration in aqueous solution was0.2 mg/mL while the scattering angle was set at 90. From theexpression C Dappq2, the apparent translational diffusion coef-ficients, Dapp, were determined. C is the decay rate, which is theinverse of the relaxation time, s; q is the scattering vectordefined as q (4pn sin(h/2)/k) (where n is the refractive indexof the solution, h is the scattering angle, and k is the wavelength</p><p>SCHEME 1 Synthesis of amphiphilic PEG-b-PPFS-g-POSS hybrid copolymer.</p><p>JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA</p><p>154 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA</p></li><li><p>of the incident laser light in vacuum).5759 The particle interac-tions in dilute solution can be assessed using a virial expansion,describing the concentration; c, dependence of the apparent dif-fusion coefficient, Dapp:</p><p>58</p><p>Dapp c D0 1 kcDc </p><p>(1)</p><p>where kcD is the diffusion constant.</p><p>After extrapolation to infinite dilution, the diffusion coeffi-cient at infinite dilution, D0 and hence the hydrodynamicradius (Rh) can be determined by the Stoke-Einstein relation-ship:57,59</p><p>Rh kBT6pgD0 (2)</p><p>where kB, g, and T are the Boltzmann constant, viscosity ofsolvent, and the absolute temperature, respectively.</p><p>The dynamic light scattering (DLS) method provides a simplebut accurate method to obtain the size distribution of themicelles using the cumulants method where the field autocor-relation function, g1(t) is fitted to a second order polynomial,which yields the first cumulant, C and the second cumulant,l2.</p><p>60 The first cumulant, C describes the average decay rateof the distribution, whereas the second cumulant, l2 is ameasure of the width of the distribution w(C) and hence theeffective polydispersity of the sample. For moderately poly-disperse spherical particles in solution, the following relationhas been derived:57</p><p>l2=C2 Mw=Mn 1=4 (3)</p><p>where Mn is the number average molecular weight, Mw theweight average molecular weight, and the ratioMw/Mn is knownas the polydispersity index. The method of cumulants is verysensitive to the distribution of particle sizes, thus providing aquantitative measure of the dispersity of particle sizes in thesample. Information on the polydispersity of the micellar aggre-gates in aqueous solutions obtained using eq 3 is included inTable 2.</p><p>Static Light ScatteringStatic light scattering (SLS) was used to measure and analyzethe timeaverage scattered intensities where the weightav-erage molecular weight ( Mw) and the second virial coeffi-cient (A2) of the block copolymer unimers and micelles canbe determined.61,62 The refractive index increment of the co-polymer solution, (dn/dc) was measured using a BI-DNDCdifferential refractometer at a wavelength of 620 nm todetermine the refractive index increment (dn/dc) of each so-lution. The instrument was calibrated primarily with potas-sium chloride (KCl) in aqueous solution.</p><p>Fluorescence SpectrometryThe excitation spectra (300360 nm) of the block copoly-mers in aqueous solutions were recorded with an emissionwavelength of 395 nm and the excitation and emission band-widths being set at 3 nm. The ratios of the peak intensities</p><p>at 338 nm and 333 nm (I338/I333) of the excitation spectrawere analyzed as a function of polymer concentration. Thecmc values were taken from the intersection of the tangentto the curve at the inflection with the horizontal tangentthrough the point at the low concentrations.</p><p>Micelle Formation of PEG-b-PPFS in Aqueous SolutionThe PEG-b-PPFS and PEG-...</p></li></ul>