Amphiphilic Block Copolymers via Associative Hydrophobic Effects

Jun 10, 2010 - †Institute of Materials Research and Engineering (IMRE), A*STAR ... The University of New South Wales, Sydney, NSW, Australia 2052. ^...
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Micelle Formation and Gelation of (PEG-P(MA-POSS)) Amphiphilic Block Copolymers via Associative Hydrophobic Effects H. Hussain,*,†,^ B. H. Tan,†,^ G. L. Seah,‡ Y. Liu,† C. B. He,*,† and T. P. Davis*,†,§ †

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, ‡School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, and §Centre for Advanced Macromolecular Design (CAMD) School of Chemical Engineering, The University of New South Wales, Sydney, NSW, Australia 2052. ^ Both these authors have made an equal contribution to this work Received April 27, 2010. Revised Manuscript Received May 31, 2010 A series of well-defined amphiphilic di- and triblock copolymers have been synthesized, using atom transfer radical polymerization, with poly(ethylene glycol) (PEG) and poly(methacrylisobutyl polyhedral oligomeric silsesquioxane) P(MAPOSS) as the hydrophilic and hydrophobic blocks, respectively. The detailed self-assembly behavior of the amphiphilic macromolecules in aqueous media was studied using both static and dynamic light scattering (SLS and DLS) techniques. The evolution of block copolymer micelle formation in THF/water mixture (20/80 v/v) was monitored as the THF evaporated from the solvent mixture. Initially the block copolymer chains existed as unimers in solution, followed by the formation of smaller aggregates (Rh < 2 nm) after 30 min, eventually growing in size to reach an equilibrium size when all the THF evaporated within 24 h. The micelles formed by the block copolymers were found to be kinetically unstable (not frozen); i.e., they tended to revert to individual copolymer chains on dilution. The hydrodynamic radii, Rh, of the micelles varied with the degree of polymerization (DP) of the hydrophobic P(MA-POSS); for example, for PEG5K-b-P(MA-POSS), an increase from Rh ∼ 13.3 ( 1.1 nm to Rh ∼ 17.5 ( 1.4 nm was observed with a nominal change in the DP of P(MA-POSS) from 4 to 6. The micelles formed by the triblock copolymers (P(MA-POSS)-b-PEG10K-b-P(MA-POSS)) were comparable in size to the diblock copolymer micelles; e.g., Rh ∼ 14.0 ( 1.3 nm was found for P(MA-POSS)4-b-PEG10K-b-P(MA-POSS)4. The micellar structures created by the triblocks in aqueous media were “flowerlike”, where the PEG middle block adopted a loop conformation in the micelle corona. In addition to micelles, larger aggregates formed by P(MA-POSS)-b-PEG10K-b-P(MA-POSS) were also detected in solution. The larger aggregates may suggest a contribution from some PEG blocks adopting an extended conformation with one end dangling in solution, causing gelation at higher copolymer concentrations via intermicellar interactions. The P(MAPOSS)4-b-PEG10K-b- P(MA-POSS)4 formed a gel in water at ∼8.8 wt % copolymer concentration. No gel formation by diblock copolymers was observed; however, the addition of a small amount of triblock copolymer to an aqueous solution of diblock copolymer results in gel formation. Finally, rheological behavior of the obtained gels was also investigated.

Introduction Amphiphilic block copolymers combine characteristic properties of the constituent blocks in a unique way creating hybrid materials

that have the ability to self-assemble in selective solvents to form supramolecular aggregates1-9 of various morphologies including spherical,10-14 toroidal,15-17 wormlike,18,19 and multicompartment micelles.20-24 These self-assembled structures have found many applications25,26 in areas such as water purification,27,28 viscosity

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and surface modification,29,30 drug-delivery systems,31-35 etc. In addition, self-assembly of block copolymers in thin films has been frequently employed as templates to achieve ordered nanostructures36 and, more recently, to create antifouling surfaces.37-39 Synthesis of well-defined block copolymers can be achieved by various living polymerization techniques including both ionic and living radical polymerizations (LRP) such as nitroxide-mediated radical polymerization (NMRP),40,41 transition-metal-catalyzed atom transfer radical polymerization (ATRP),42,43 and reversible addition-fragmentation chain transfer (RAFT)/macromolecular design via the interchange of xanthates (MADIX).44-49 Generally, in all LRPs, the living character and control over polymerization are achieved by establishing a reversible and dynamic equilibrium (achieved either by the persistent radical effect50,51 or by additionfragmentation chemistry52-55) between a low concentration of active propagating chains and a predominant concentration of dormant chains that are unable to propagate or terminate. Thus, by maintaining a low concentration of active propagating radical species in solution, side reactions leading to dead polymer chains and ill-defined polymers are kept to a minimum. Polyhedral oligomeric silsesquioxane (POSS) has a well-defined cagelike nanostructure made of silicon and oxygen atoms linked together in a cubic form, usually functionalized with organic functional groups at each corner to facilitate miscibility and/or covalent incorporation into organic polymers leading to nanohybrids with (29) Spatz, J. P.; Sheiko, S.; Moller, M. Macromolecules 1996, 29, 3220–3226. (30) Breulmann, M.; Forster, S.; Antonietti, M. Macromol. Chem. Phys. 2000, 201, 204–211. (31) Boyer, C.; Bulmus, V.; Davis, T. P.; Ladmiral, V.; Liu, J. Q.; Perrier, S. Chem. Rev. 2009, 109, 5402–5436. (32) Adams, M. L.; Lavasanifar, A.; Kwon, G. S. J. Pharm. Sci. 2003, 92, 1343– 1355. (33) Rosler, A.; Vandermeulen, G. W. M.; Klok, H. A. Adv. Drug Delivery Rev. 2001, 53, 95–108. (34) Chan, Y.; Wong, T.; Byrne, F.; Kavallaris, M.; Bulmus, V. Adv. Drug Delivery Rev. 2008, 9, 1826–1836. (35) York, A. W.; Kirkland, S. E.; McCormick, C. L. Adv. Drug Delivery Rev. 2008, 60, 1018–1036. (36) Shin, K.; Leach, K. A.; Goldbach, J. T.; Kim, D. H.; Jho, J. Y.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Nano Lett. 2002, 2, 933–936. (37) Martinelli, E.; Agostini, S.; Galli, G.; Chiellini, E.; Glisenti, A.; Pettitt, M. E.; Callow, M. E.; Callow, J. A.; Graf, K.; Bartels, F. W. Langmuir 2008, 24, 13138–13147. (38) Grozea, C. M.; Gunari, N.; Finlay, J. A.; Grozea, D.; Callow, M. E.; Callow, J. A.; Lu, Z. H.; Walker, G. C. Biomacromolecules 2009, 10, 1004–1012. (39) Tan, B. H.; Hussain, H.; Chaw, K. C.; Dickinson, G. H.; Gudipati, C. S.; Birch, W. R.; Teo, S. L. M.; He, C.; Liu, Y.; Davis, T. P. Polym. Chem. 2010, 1, 276–279. (40) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661–3688. (41) Hawker, C. J. J. Am. Chem. Soc. 1994, 116, 11185–11186. (42) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614–5615. (43) Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921–2990. (44) Hussain, H.; Tan, B. H.; Gudipati, C. S.; Liu, Y.; He, C. B.; Davis, T. P. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5604–5615. (45) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559–5562. (46) Chong, Y. K.; Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 2071–2074. (47) Mayadunne, R. T. A.; Rizzardo, E.; Chiefari, J.; Chong, Y. K.; Moad, G.; Thang, S. H. Macromolecules 1999, 32, 6977–6980. (48) Charmot, D.; Corpart, P.; Adam, H.; Zard, S. Z.; Biadatti, T.; Bouhadir, G. Macromol. Symp. 2000, 150, 23–32. (49) Destarac, M.; Bzducha, W.; Taton, D.; Gauthier-Gillaizeau, I.; Zard, S. Z. Macromol. Rapid Commun. 2002, 23, 1049–1054. (50) Davis, T. P.; Kukujl, D.; Maxwell, I. A. Macromol. Theory Simul. 1995, 4, 195–208. (51) Fischer, H. Macromolecules 1997, 30, 5666–5672. (52) Destarac, M.; Taton, D.; Zard, S. Z.; Saleh, T.; Six, Y. Advances in Controlled/Living Radical Polymerization; American Chemical Society: Washington, DC, 2003. (53) Meijs, G. F.; Rizzardo, E.; Le, T. P. T.; Chen, Y. C. Makromol. Chem., Macromol. Chem. Phys. 1992, 193, 369–378. (54) Meijs, G. F.; Rizzardo, E.; Thang, S. H. Macromolecules 1988, 21, 3122– 3124. (55) Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1996, 29, 7717–7726.

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improved bulk properties.56-65 More recently, POSS nanocages, with a single polymerizable functional group per nanocage, have attracted significant attention for their potential in preparing welldefined organic-inorganic hybrid (co)polymers via living polymerization techniques.66-70 However, little attention has been paid to amphiphilic block copolymers having POSS polymers as the hydrophobic block or their self-assembly. In several examples, reported in the literature, hydrophilic polymers are end-functionalized with a single POSS nanocage either at one end or at both the ends.71-77 In this article, we report a new series of amphiphilic diand triblock copolymers based on poly(ethylene glycol) (PEG) as the hydrophilic block and poly(methacrylisobutyl-POSS) P(MAPOSS) as the hydrophobic block, i.e., PEG5K-b- P(MA-POSS) and P(MA-POSS)-b-PEG10K-b-P(MA-POSS) (where the subscript to PEG; 5K and 10K represent the molar mass of the PEG block in the respective copolymers). Details of the ATRP synthesis, self-assembly, and gelation through hydrophobic effects in aqueous solution are reported.

Experimental Section Materials. Methacrylisobutyl-POSS (MA-POSS) was purchased from Hybrid Plastics (product no. MA0702) and used as received. Triethylamine (Fisher, 99.7%) was dried over molecular sieves before use. 2-Bromoisobutyryl bromide (98%) CuBr (99.99%,), N,N,N,N,N,-pentamethyldiethylenetriamine (PMDETA) (99%), and anhydrous toluene (99.8%) were purchased from Aldrich and used as received. MPEG (Mn = 5000 g/mol) and PEG (Mn = 10 000 g/mol) were purchased from Fluka and dried under vacuum before use.

Synthesis of PEG5K-b-P(MA-POSS) and P(MA-POSS)b-PEG10K-b-P(MA-POSS). As an example, first PEG (Mn = 10 000 g/mol) was transformed into an ATRP macroinitiator by reaction with 2-bromoisobutyryl bromide in THF at room temperature.78 In a second step, PEG macroinitiator (0.19 mmol (Br)), MA-POSS (1.22 mmol), and anhydrous toluene (∼4 mL) were added to a Schlenk tube. The mixture was deoxygenated by (56) Hussain, H.; Tan, B. H.; Gudipati, C. S.; Xaio, Y.; Liu, Y.; Davis, T. P.; He, C. B. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 7287–7298. (57) Liu, L.; Wang, W. P. Polym. Bull. 2009, 62, 315–325. (58) Hussain, H.; Mya, K. Y.; Xiao, Y.; He, C. B. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 766–776. (59) Amir, N.; Levina, A.; Silverstein, M. S. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 4264–4275. (60) Xu, H. Y.; Yang, B. H.; Wang, J. F.; Guang, S. Y.; Li, C. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5308–5317. (61) Yang, B. H.; Xu, H. Y.; Wang, J. F.; Gang, S. Y.; Li, C. J. Appl. Polym. Sci. 2007, 106, 320–326. (62) Liu, Y. L.; Tseng, M. C.; Fangchiang, M. H. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5157–5166. (63) Leu, C. M.; Chang, Y. T.; Wei, K. H. Chem. Mater. 2003, 15, 3721–3727. (64) Chen, Y. W.; Kang, E. T. Mater. Lett. 2004, 58, 3716–3719. (65) Wahab, M. A.; Mya, K. Y.; He, C. B. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5887–5896. (66) Chen, R. X.; Feng, W.; Zhu, S. P.; Botton, G.; Ong, B.; Wu, Y. L. Polymer 2006, 47, 1119–1123. (67) Pyun, J.; Matyjaszewski, K.; Wu, J.; Kim, G. M.; Chun, S. B.; Mather, P. T. Polymer 2003, 44, 2739–2750. (68) Pyun, J.; Matyjaszewski, K. Macromolecules 1999, 33, 217–220. (69) Escude, N. C.; Chen, E. Y. X. Chem. Mater. 2009, 21, 5743–5753. (70) Hirai, T.; Leolukman, M.; Jin, S.; Goseki, R.; Ishida, Y.; Kakimoto, M. A.; Hayakawa, T.; Ree, M.; Gopalan, P. Macromolecules 2009, 42, 8835–8843. (71) Zhang, W.; Fang, B.; Walther, A.; M€uller, A. H. E. Macromolecules 2009, 42, 2563–2569. (72) Kim, B. S.; Mather, P. T. Macromolecules 2006, 39, 9253–9260. (73) Kim, B. S.; Mather, P. T. Polymer 2006, 47, 6202–6207. (74) Zhang, W. A.; Liu, L.; Zhuang, X. D.; Li, X. H.; Bai, J. R.; Chen, Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 7049–7061. (75) Markovic, E.; Ginic-Markovic, M.; Clarke, S.; Matisons, J.; Hussain, M.; Simon, G. P. Macromolecules 2007, 40, 2694–2701. (76) Lee, W.; Ni, S. L.; Deng, J. J.; Kim, B. S.; Satija, S. K.; Mather, P. T.; Esker, A. R. Macromolecules 2007, 40, 682–688. (77) Kim, B. S.; Mather, P. T. Macromolecules 2002, 35, 8378–8384. (78) Hussain, H.; Budde, H.; Horing, S.; Kressler, J. Macromol. Chem. Phys. 2002, 203, 2103–2112.

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Scheme 1. Syntheses of PEG5K-b-P(MA-POSS) and P(MA-POSS)-b-PEG10K-b-P(MA-POSS) Block Copolymers by ATRP

four freeze-pump-thaw cycles, followed by the addition of CuBr (0.19 mmol) and PMDETA (0.19 mmol) under argon. The contents were subjected to two more freeze-pump-thaw cycles to ensure complete oxygen removal. The mixture was maintained at 80 C for ∼21/2 h prior to air exposure and dilution with THF. The product solution was passed through an alumina column to remove copper catalyst. To remove unreacted MA-POSS, the crude product was dispersed in cold ether and centrifuged at 9000 rpm. The process was repeated several times with fresh batches of ether. Finally, the product was dried under vacuum at 40 C overnight. It is worth mentioning here that although cold ether is a good precipitating solvent for PEG, significant amounts of block copolymers were lost during the extensive purification steps because the extension of the PEG with MA-POSS enhanced the overall solubility of the block copolymers in ether. A schematic of the synthesis procedure is presented in Scheme 1. Characterization. 1H NMR and 29Si NMR spectra were recorded on a Bruker 400 MHz spectrometer in d-chloroform. GPC analyses were carried out against PMMA standards in THF at 35 C with a flow rate of 1 mL/min using a Waters 2690 system fitted with an evaporative light scattering detector (Waters 2420). Three Phenomenex linear 5 mm styragel columns (500, 104, and 106 A˚) were used in the GPC system. Dynamic Light Scattering (DLS). Room-temperature light scattering measurements were made with a Brookhaven BI-200SM multiangle goniometer equipped with a BI-APD detector. The light source was a 35 mW He-Ne laser emitting vertically polarized light of 632.8 nm wavelength. In DLS measurements, the intensity correlation function was measured at a temperature of 25 C (unless otherwise stated) with a maximum number of 256 channels using a BI-9000AT digital autocorrelator. Non-negative leastsquares (NNLS) algorithm, developed by Lawson and Hansen,79 was applied to obtain best-fit values of the parameters from DLS measurements. DLS measures the intensity autocorrelation (79) Lawson, C. L.; Hanson, R. J. Solving Least Squares Problems; Prentice-Hall: Englewood Cliffs, NJ, 1974.

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function, g(2)(τ), which is related to the electric field autocorrelation function, g(1)(τ), by means of the Siegert relation.80 From the expression Γ = Dappq2, the apparent translational diffusion coefficients, Dapp, were determined. Γ is the decay rate, which is the inverse of the relaxation time, τ; q is the scattering vector defined as q = (4πn sin(θ/2)/λ) (where n is the refractive index of the solution, θ is the scattering angle, and λ is the wavelength of the incident laser light in vacuum).80,81 The apparent hydrodynamic radius (Rh) can be determined by the Stokes-Einstein relationship80,81 Rh ¼

kT 6πηDapp

ð1Þ

where kB, η, and T are the Boltzmann constant, viscosity of solvent, and the absolute temperature, respectively. The critical micelle concentration (cmc) of the diblock and triblock copolymers in aqueous solution was determined using DLS. The scattering intensity of each concentration of block copolymer in aqueous solution was measured and plotted against the copolymer concentration. The concentration at which the scattering intensity increases sharply was defined as the cmc. Fluorescence Spectrometry. A series of block copolymer solutions with concentrations ranging from 0.005 to 1.5 mg/mL were prepared by diluting a stock solution of block copolymer having a concentration way above the cmc with water. The pyrene solution in acetone was transferred to separate vials; acetone was then evaporated under nitrogen before adding the block copolymer solutions to give a final pyrene concentration of 6.0  10-7 M in each vial. After equilibration overnight at ambient temperature, the excitation spectra (300-360 nm) of the solutions were recorded at an emission wavelength of 395 nm with the excitation  ipanek, P. Data analysis in dynamic light scattering. In Dynamic Light (80) St Scattering - The Method and Some Applications; Brown, D., Ed.; Clarendon Press: Oxford. 1993; pp 177-241. (81) Chu, B. Laser Light Scattering: Basic Principles and Practice; 2nd ed.; Academic Press: New York. 1991.

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Article and emission bandwidths being set at 3 nm. The ratios of the peak intensities at 338 and 333 nm (I338/I333) of the excitation spectra were analyzed as a function of polymer concentration. Static Light Scattering (SLS). Static light scattering was used to measure and analyze the time-average scattered intensities where the weight-average molecular weight (Mw) and the second virial coefficient (A2) of the block copolymer unimers and micelles can be determined.82,83 The refractive index increment of the copolymer solution (dn/dc) was measured using a BI-DNDC differential refractometer at a wavelength of 620 nm to determine the refractive index increment (dn/dc) of each solution. The instrument was calibrated primarily with potassium chloride (KCl) in aqueous solution. Rheology Measurement. Rheological studies were carried out using the TA Instruments AR-G2 controlled-stress rheometer with cone and plate (60 mm, 1). All the geometry setups were equipped with a water-circulating bath and Peltier plate to ensure a uniform temperature of 25 ( 0.1 C in the samples. The rheometers are calibrated once a month using silicon calibration oils at three different rotational speeds to verify the accuracy and response of the rheometers when sensing low, medium, and high torque values. Polymer samples were equilibrated for 24 h prior to measurements, and each measurement of the same sample was repeated at least twice in order to ensure that the viscosity data were reproducible. An amplitude sweep experiment at a fixed frequency was performed to determine a suitable amplitude value, where the material exhibits the linear viscoelastic behavior. This amplitude value was later used in the frequency sweep experiments to determine the frequency dependence of storage and loss modulus, G0 and G00 , respectively. This procedure ensured that the dynamic property measurements were carried out in the linear viscoelastic region. Preparation of Block Copolymer Aqueous Solutions. The block copolymers were first dissolved in a small volume of THF, followed by the slow addition (∼1 mL/min) of a known volume of water. An example is given for the preparation of 1.0 mg/mL of block copolymer in aqueous solution. Polymer (2.0 mg) was dissolved in THF (0.5 mL), and the solution was diluted with deionized water (2.0 mL). To follow the THF evaporation from solution, the 2.5 and 2.0 mL volumes representing the volume of solution with and without THF, respectively, was marked on the sample vial. The mass of the initial polymer solution (2.5 mL) was also measured. Next, the solution was left to evaporate at room temperature until the volume dropped to the 2.0 mL level, and the mass was observed to decrease until it remained constant (∼12 h). The drop in the volume and mass of the solution could be mainly due to the evaporation of THF (0.5 mL of THF was added to dissolve the polymer) as the rate of evaporation of THF is much faster as compared with water at room temperature. Gel Formation by the Block Copolymers. For rheology measurements, three block copolymer samples in aqueous solution were prepared. The first sample (gel) was the triblock copolymer, where a concentrated solution of P(MA-POSS)4-bPEG10K-b-P(MA-POSS)4 in THF (∼0.2 g/mL) was added dropwise (0.1 mL solution at every 2 min interval) into deionized water (3 mL) with continuous stirring until gel formation was observed and the final copolymer concentration in water was noted. The second sample (concentrated solution) of a similar concentration was PEG5K-b-P(MA-POSS)6 diblock copolymer in aqueous solution, which was prepared as a control. The third sample (gel), from a mixture of di- and triblock copolymers, and of overall concentration (di- þ triblock) similar to the first sample, was prepared as follows: a concentrated solution of P(MA-POSS)4-bPEG10K-b-P(MA-POSS)4 in THF (∼0.2 g/mL) was added dropwise into an aqueous solution of PEG5K-b-P(MA-POSS)6 diblock copolymer (3 mL) (prepared 24 h earlier) with continuous stirring, (82) Zimm, B. H. J. Chem. Phys. 1948, 16, 1099–1116. (83) Zimm, B. H. J. Chem. Phys. 1948, 16, 1093–1099.

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Hussain et al. so that the final composition of the mixture was triblock (20 wt %) and diblock (80 wt %).

Results and Discussion Synthesis of Block Copolymers. Amphiphilic di- and triblock copolymers of PEG and P(MA-POSS), i.e., PEG5K-bP(MA-POSS) and P(MA-POSS)-b-PEG10K-P(MA-POSS), were synthesized by ATRP using modified PEG as the macroinitiator. For the synthesis of diblock copolymers, a monofunctional PEG (Mn =5000 g/mol) macroinitiator was used, while for the triblock copolymers a bifunctional PEG (Mn = 10 000 g/mol) macroinitiator was employed. Typically, the ATRP of MA-POSS was carried out at 80 C in anhydrous toluene as the solvent using CuBr/ PMDETA as the catalyst system. A few experiments were also carried out in anhydrous xylene as solvent and at 70 C. The structure of the obtained block copolymers was verified by 1H NMR spectroscopy and GPC. A representative 1H NMR spectrum of the purified amphiphilic PEG-b-P(MA-POSS) copolymer in d-chloroform is shown in Figure 1, clearly confirming the presence of protons from both the PEG and P(MA-POSS) blocks. Signals f and g at ∼3.6 and ∼3.38 ppm can be assigned to (-CH2-CH2-O-)- and terminal CH3O- of MPEG, respectively. Resonances observed at δ ∼ 0.58-0.60 ppm could be assigned to the methylene protons (c and d in Figure 1) of P(MA-POSS) block. Methyl protons (a in Figure 1) and methine protons (b in Figure 1) of isobutyl groups MA-POSS could be identified at δ ∼ 0.95 ppm and δ ∼ 1.9 ppm, respectively. Additionally, methylene protons (e in Figure 1) from P(MA-POSS) block could be seen at δ ∼ 3.9 ppm, indicating that successful chain extension has occurred. The small shoulders at δ ∼ 0.84 ppm and δ ∼ 1.7 ppm indicated by an asterisk in Figure 1 could be due to the methyl and methylene protons of poly(methacrylate) backbone in P(MA-POSS), respectively; however, due to low intensity, probably because of small block lengths, these signals could not be clearly verified. The composition and hence the degree of polymerization of P(MA-POSS) block were estimated by comparing the integral values of signals representing -(CH2-CH2-O-)- ( f in Figure 1) of PEG and protons c and d (Figure 1) of the P(MAPOSS) block, and the data are included in Table 1. Additionally, a representative 29Si NMR spectrum of the block copolymers is also given in the Supporting Information, revealing clear signals for Si at -59.2 ppm. Further verification of the successful synthesis was obtained from GPC analyses. Figure 2 shows representative GPC profiles of the macroinitiaors: PEG5K (Figure 2a) and PEG10K (Figure 2b) and the subsequent di- and triblock copolymers. A significant shift in the GPC traces (to a higher molecular weight) of the PEG and monomodal molecular weight distributions of the block copolymers indicates a successful chain extension producing well-defined PEG5K-b-P(MA-POSS) and P(MA-POSS)-bPEG10K-b-P(MA-POSS) block copolymers. More importantly, low values for Mw/Mn, where Mw is the weight-average molecular weight and Mn is number-average molar mass of the block copolymer, were observed after the chain extension of PEG macroinitiator with MA-POSS for both the di- and triblock copolymers. The self-assembly behavior of the block copolymers in aqueous solution as described below also confirms that block copolymers were obtained. The GPC data are summarized in Table 1. Micelle Formation in Aqueous Solution. Micelles with P(MA-POSS) blocks as the core and PEG as the corona are expected to form in aqueous solution. DLS was used to determine the critical micelle concentration (cmc) of the PEG5K-b-P(MAPOSS) and POSSM-b-PEG10K-b-P(MA-POSS) block copolymers in aqueous solution at 25 C as shown by the data in Figure 3a (square symbols) and Figure 3b for PEG5K-b-P(MA-POSS)4 and P(MA-POSS)4-b-PEG10K-b-P(MA-POSS)4, respectively, where Langmuir 2010, 26(14), 11763–11773

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Figure 1. Representative 1H NMR spectrum and assignment of the respective peaks of the PEG5K-b-P(POSS-MA) diblock copolymer in d-chloroform. The structure of the diblock copolymer is given at the top. Table 1. Synthesis Parameters, GPC, 1H NMR, Cmc, and Particle Size Analyses of the Synthesized PEG5K-b-P(MA-POSS) and P(MA-POSS)-b-PEG10K-b-P(MA-POSS) Block Copolymers

samples

[M0]/[I]a

reaction solvent/temp (C)

Mnb (1H NMR) (g/mol)

Mnc (GPC) (g/mol)

PDId

cmce (mg/mL)  102

Rhf(micelle) (nm)

Rhf (large aggregates) (nm)

6.4 toluene/80 17 000 15 000 1.2 1.5 13.3 ( 1.2 89.5 ( 9.5 P(MA-POSS)3.4-b-PEG10Kb-P(MA-POSS)3.4 3 xylene/70 18 000 13 000 1.19 1.0 13.8 ( 1.1 105.0 ( 11.0 P(MA-POSS)4-b-PEG10Kb-P(MA-POSS)4 6 xylene/70 21 000 15 000 1.14 0.6 16.3 ( 1.5 124.5 ( 11.6 P(MA-POSS)5.5-b-PEG10Kb-P(MA-POSS)5.5 PEG5K-b-P(MA-POSS)4 4.3 toluene/80 9000 8000 1.1 10.0 13.6 ( 1.3 4.7 toluene/80 9000 9500 1.1 8.0 14.7 ( 1.2 PEG5K-b-P(MA-POSS)4.6 6 toluene/80 10 000 9800 1.1 7.0 14.8 ( 1.1 PEG5K-b-P(MA-POSS)4.7 3 xylene/70 11 000 9700 1.1 4.5 18.2 ( 1.5 PEG5K-b-P(MA-POSS)∼6 a [M0] and [I] is the relative number of moles of the monomer and the initiator (with respect to -Br end groups). b Rounded to the nearest thousand; however, the DPs of P(MA-POSS), given in column 1, were calculated from the Mn (1H NMR) values before rounding to the nearest thousand). Calculated as Mn(block copolymer) = Mn of PEG macroinitiator þ MW of MA-POSS  degree of polymerization of P(MA-POSS) determined by 1H NMR spectroscopy. c Measured against PMMA standards using THF as eluent. d Mw/Mn. e Estimated fom scattering light intensity as a function of block copolymer concentration. f Determined by DLS measurements at copolymer concentration of 0.5 mg/mL in aqueous solutions and at room temperature.

the copolymer concentration at which the scattering intensity increases sharply was defined as the cmc. Note that each concentration in these experiments was prepared separately from a stock solution of the copolymer in THF. The cmc values for all the investigated PEG5K-b-P(MA-POSS) diblock and P(MA-POSS)-b-PEG10K-b-P(MA-POSS) copolymers are tabulated in Table 1. A gradual decrease of the cmc with increasing MA-POSS content in the copolymer (see Table 1) was observed, suggesting that the higher Langmuir 2010, 26(14), 11763–11773

MA-POSS content in the block copolymers favors micellization promoted by an increased hydrophobicity. It is also interesting to note that the cmc of the triblock copolymers is significantly lower when compared against the diblock copolymers with similar relative MA-POSS content (see Table 1). For example, the cmc of PEG5Kb-P(MA-POSS)4 and P(MA-POSS)4-b-PEG10K-b-P(MA-POSS)4 was estimated to be 0.1 ( 0.012 and 0.01 ( 0.008 mg/mL, respectively. DOI: 10.1021/la101686q

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Figure 2. GPC profiles of (a) PEG5K and the obtained PEG-b-P(MA-POSS)4.7 and (b) PEG10K and the respective P(MA-POSS)4-bPEG10K-b-P(MA-POSS)4 triblock copolymer.

Figure 3. Scattering intensity from DLS measurements as a function of block copolymer concentration (mg/mL) in aqueous solution: (a) PEG5K-b-P(MA-POSS)4 (square symbols and added solid trend line) and (b) P(MA-POSS)4-b-PEG10K-b-P(MA-POSS)4. Also added to (a), the data (circle symbols and an added dotted trend line) obtained from the stepwise dilution of the block copolymer solution, where the scattering intensity decreases as the concentration of the block copolymer decreases. The arrows are included only to guide the eye. (c) Intensity ratio (I338/I333), obtained from fluorescence measurements, as a function of decreasing PEG5K-b-P(MA-POSS)4 concentration in aqueous solution, the direction of the arrow shows that the intensity ratio decreases with dilution.

Time Evolution of Micelles in THF/Water Mixture. As described in the Experimental Section, the block copolymer was dissolved in small known volumes of THF followed by the addition of a designated amount of water. Evaporation of THF from the solution drove the formation of micelles in aqueous media. The time evolution of micelle formation with THF evaporation was followed by measuring the light scattering intensity of the solution at regular time intervals for 24 h using DLS. As an example, the data from two different concentrations, 0.5 and 1.0 mg/mL, of PEG5K-b-P(MA-POSS)4 dissolved in THF/water mixture (20:80 v/v) are depicted in Figure 4a. The scattering intensities of both the concentrations increased rapidly within the first hour and reached a plateau within 5 h with no further change 11768 DOI: 10.1021/la101686q

up to 24 h. The respective time evolution of the distribution functions of relaxation times, t (extracted by analysis of the dynamic correlation functions g(1)(t), using NNLS79), for PEG5K-bP(MA-POSS)4 (concentration of 0.5 mg/mL) is shown in Figure 4b, where the data at time zero (immediately after adding water to the THF solution of block copolymer) shows no significant scattering, meaning that no micelles in solution at this point; however, measurement carried out after 30 min clearly shows the appearance of scatterers in solution as represented by a broad distribution function of time relaxation (Figure 4b), where the peak maximum corresponds to particles of equivalent hydrodynamic radius Rh