Synthesis and characterization of heptaphenyl polyhedral oligomeric silsesquioxane-capped poly(N-isopropylacrylamide)s

pta py etal M Received 3 November 2011 Received in revised form 22 February 2012 Accepted 4 March 2012 Available online 11 March 2012 Keywords: Poly(N-isopropylacrylamide) Polyhedral oligomeric silsesquioxane Organic–inorganic amphiphiles (POSS) was synthesized via the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click of which is reactive (Scheme 1). POSS-containing hybrid nanocomposites have been becoming the focus of many studies due to their excellent thermomechanical chanical properties of POSS-containing nanocomposites. Poly(N-isopropylacrylamide) (PNIPAAm) is a kind of interesting thermoresponsive polymer. In aqueous solu- tion PNIPAAm can display a lower critical solution temper- ature (LCST) behavior at ca. 32 �C [19–23]. Below this temperature, individual PNIPAAm chains adopt a random 0014-3057/$ - see front matter � 2012 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +86 21 54743278; fax: +86 21 54741297. E-mail address: [email protected] (S. Zheng). European Polymer Journal 48 (2012) 945–955 Contents lists available at SciVerse ScienceDirect European Poly else M A CR O M O LE CU LA R N A N O TE CH N O LO G Y http://dx.doi.org.10.1016/j.eurpolymj.2012.03.007 1. Introduction Polyhedral oligomeric silsesquioxanes (POSS) are a class of important nanosized cage-like molecules, derived from hydrolysis and condensation of trifunctional organosilanes. In the past years, POSS reagents, monomers and polymers have been emerging as a new chemical technology for pre- paring the organic–inorganic hybrids [1–8]. A typical POSS molecule possesses a structure of cube-octameric frame- work represented by the formula (R8Si8O12) with an inor- ganic silica-like core (Si8O12) (�0.53 nm in diameter) surrounded by eight organic corner groups, one (or more) properties [1–8]. Recently, POSS cages have been incorpo- rated into organic polymers to optimize the functional properties of the materials via the formation of specific morphological structures [9,10]. For instance, POSS cages were incorporated into conjugated luminescent polymers such as polyfluorene, polyphenylene and polythiophene to reduce the formation of aggregates and to increase ther- mal stability [11–13]. It was reported that the incorpora- tion of POSS can enhance deswelling and reswelling rates of poly(N-isopropylacrylamide) hydrogels [14–18]. How- ever, such an investigation remains largely unexplored vis-à-vis the studies on the improvement of thermome- Self-assembly chemistry). With this initiator, the atom transfer radical polymerization (ATRP) of N-iso- propylacrylamide (NIPAAm) was carried out to afford the POSS-capped PNIPAAm. The organic–inorganic amphiphiles were characterized by means of nuclear magnetic reso- nance spectroscopy (NMR) and gel permeation chromatography (GPC). Atomic force microscopy (AFM) showed that the POSS-capped PNIPAAm amphiphiles in bulk displayed microphase-separated morphologies. In aqueous solutions, the POSS-capped PNIPAAm amphiphiles were self-assembled into micelle-like aggregates as evidenced by dynamic light scattering (DLS) and transmission election microscopy (TEM). It was found that the sizes of the self-organized nanoobjects decreased with increasing the lengths of PNIPAAm chains. By means of UV–vis spectroscopy, the lower critical solution temperature (LCST) behavior of the organic–inorganic amphiphiles in aqueous solution was investigated and the LCSTs of the organic–inorganic amphiphiles decreased with increasing the percentage of POSS termini. It is noted that the self-assembly behavior of the POSS-capped PNIPAAm in aqueous solutions exerted the significant restriction on the macromolecular conformation alteration of PNIPAAm chains while the coil-to-globule collapse occurred. � 2012 Elsevier Ltd. All rights reserved. Article history: In this work, a novel initiator bearing heptaphenyl polyhedral oligomeric silsesquioxane Macromolecular Nanotechnology Synthesis and characterization of he silsesquioxane-capped poly(N-isopro Yaochen Zheng, Lei Wang, Sixun Zheng ⇑ Department of Polymer Science and Engineering and State Key Laboratory of M a r t i c l e i n f o a b s t r a c t journal homepage: www. phenyl polyhedral oligomeric lacrylamide)s atrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China mer Journal vier .com/locate /europol j 946 Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 M A CR O M O LE CU LA R N A N O TE CH N O LO G Y OCH3 coil conformation. While the solution is heated up to 32 �C or higher, the random coils will collapse into globules. It is proposed that in the former case, the intermolecular hydrogen bonding interactions between amide groups of Si OCH3 OCH3 NaN3 Si O Si O Si O Si O Si O Si O O O O O O O Si Si N3 Si O Si O SiO Si O Si O Si O O O O O O OH + Cl Cl O Si O Si O SiO Si O Si O Si O O O O O O O Si Si N3 CuBr / PMDETA NIPAAm CuCl / Me6TREN Si O Si O SiO Si O Si O Si O O O O O O O S S Scheme 1. Synthesis of PO Si O Si O Si O O- O- O- PNIPAAm and water promote the solubility of PNIPAAm with water. In the latter cases, the intermolecular hydro- gen bonding interactions are interrupted owing to the con- formational changes of PNIPAAm chains. As a consequence, Si O Si O Si O O O O Si . Si Cl Cl Cl Br Si O Si O Si O Si O Si O Si O O O O O O O Si Si Br 3Na+ O Si Si N O Cl O N N O O Cl i i N N N O O HN O n SS-capped PNIPAAm. ylamine (1.3 ml, 8.8 mmol) were charged to a flask equipped with a magnetic stirrer and then anhydrous Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 947 M A CR O M O LE CU LA R N A N O TE CH N O LO G Y the hydrophobic association among the collapsed PNI- PAAm chains takes place. During the past decades, the LCST behaviors of PNIPAAm in aqueous solutions have been extensively investigated [19–30]. It is identified that some structural factors such as co-monomer, tacticity, crosslinking, grafting, topology of macromolecules, molec- ular weights and end groups all affect the LCST behavior of PNIPAAm in aqueous solutions [19,31–37]. Recently, the PNIPAAm amphiphiles containing hydrophobic end groups or polymer chains are of interest since the specific archi- tectures can significantly affect the LCST behaviors of PNI- PAAm in aqueous solutions [20,38–41]. Winnik et al. investigated the effect of the hydrophobic n-octadecyl ter- mini on the merging of mesoglobules of PNIPAAm and pro- posed that the rigidity and partial vitrification of mesoglobules may be the prevalent cause of their stability against aggregations [20]. Zhang et al. [40] found that the flower-like aggregates resulting from PS-b-PNIPAAm-b-PS triblock copolymers are much less collapsed than the star-like aggregates derived from the corresponding flow- er-liked aggregates. POSS-capped telechelic polymers are a class of novel or- ganic–inorganic hybrids owing to their specific topologies and self-assembly behavior [16,42–46]. Depending on the chemical strategies used, one (or two) end of a linear or- ganic polymer chain can be bonded with POSS cage to con- stitute so-called semi-telechelics (or telechelics). Recently, several POSS-capped polymer telechelics such as poly(eht- ylene oxide) [16,45], poly(e-caprolactone) [47,48], poly (acrylic acid) [49], polystyrene [46] and poly(hydroxyether of bisphenol A) [50] have been reported by various invesit- gators; these POSS-capped telechelic polymers can display some interesting morphologies and properties. Wang et al. [17] reported synthesis of POSS-capped PNIPAAm teleche- lics with a ‘‘POSS-spreading’’ strategy. In this approach, a POSS-capped trithiocarbonate was synthesized and used as a chain transfer agent, with which the reversible addi- tion-fragmentation chain transfer (RAFT) polymerization of NIPAAm was carried out and thus two ends of PNIPAAm chain were capped with hepta(3,3,3-trifluropropyl) POSS. Owing to the highly hydrophobic of the POSS end groups, the POSS-capped PNIPAAm telechelics can form physical hydrogels in aqueous solutions. It was found that such physical hydrogels displayed fast deswelling and reswell- ing properties compared to traditional chemical hydrogels. In this contribution, we reported the synthesis of hepta- phenyl POSS-capped PNIPAAm, a semi-telechelic PNIPAAm via atom transfer radical polymerization (ATRP). Firstly, an initiator bearing heptaphenyl POSS was synthesized via the copper-catalyzed Huisgen 1,3-dipolar cycloaddition (i.e., click chemistry). Thereafter, the atom transfer radical poly- merization (ATRP) of N-isopropylacrylamide (NIPAAm) was carried out with the initiator bearing POSS to afford the POSS-capped semi-telechelic PNIPAAm. It should be pointed out that Zhang et al. [51] ever reported the synthe- sis of a semi-telechelic PNIPAAm (i.e., heptaisobutyl POSS- capped PNIPAAm) via RAFT polymerization. The RAFT agent was prepared via the direct reaction between 3-aminopropylheptaisobutyl POSS and 3-benzylsulfanyl- thiocarbonylsufanylpropionic chloride. In this work, the self-assembly behavior of the organic–inorganic amphi- THF (250 ml) were added with vigorous stirring. The flask was immersed into an ice-water bath and purged with highly pure nitrogen for 1 h. After that, 3-bromopropyltri- chlorosilane (3.11 g, 12.12 mmol) dissolved in 20 ml anhy- drous THF were slowly dropped within 30 min. The philes was investigated by means of atomic force micros- copy (AFM), transmission electron microscopy (TEM) and dynamic laser scattering (DLS). The lower critical solution temperature behavior in aqueous solutions was addressed according to the results of cloud point analysis. 2. Experimental 2.1. Materials Phenyltrimethoxysilane (98%) was supplied by Zhe- jiang Chem-Tech Ltd. Co., China and used as received. 3- Bromopropyltrichlorosilane (97%), sodium azide (NaN3) and N,N,N0,N0,N00-pentamethyldiethylenetriamine (PMDE- TA) were purchased from Aldrich Co., USA. Propargyl alco- hol was obtained from Alladin Reagent Co., China; it was dried over anhydrous magnesium sulfate and distilled un- der reduced pressure before use. 2-Chloropropinoyl chlo- ride (99%) was purchased from Alfa Aesar Co., China. N-isopropylacrylamide (NIPAAm) was prepared in this lab via the reaction between isopropylamine and acryloyl chloride. Both copper (I) bromide (CuBr) and copper (I) chloride (CuCl) were of chemically pure grade, supplied by Shanghai Reagent Co., China. Tris(2-(dimethylamino) ethyl)amine (Me6TREN) was prepared by following the methods of literature by Matyjaszewski et al. [52]. All the solvents used in this work were obtained from com- mercial sources. Before use, tetrahydrofuran (THF) was re- fluxed above sodium and distilled; triethylamine (TEA) and isopropyl alcohol (IPA) were dried over CaH2 and then distilled; N,N-dimethylformamide (DMF) was dried over anhydrous magnesium sulfate and distilled under reduced pressure. 2.2. Synthesis of 3-bromopropylheptaphenyl POSS Heptaphenyltricycloheptasiloxane trisodium silanolate [Na3O12Si7(C6H5)7] was synthesized by following the method of literature reported by Fukuda et al. [53]. Typi- cally, phenyltrimethoxysilane [C6H5Si(OMe)3] (35.258 g, 178.1 mmol), THF (195 ml), deionized water (4.064 g, 225.8 mmol) and sodium hydroxide (3.080 g, 77.0 mmol) were charged to a flask equipped with a condenser and a magnetic stirrer. After refluxed for 5 h, the reactive system was cooled down to room temperature and held at this temperature with vigorous stirring for additional 15 h. All the solvent and other volatile compounds were removed via rotary evaporation and the white solids were obtained. After dried at 60 �C in vacuo for 24 h, the product (12.262 g) was obtained with the yield of 98.5%. The corner capping reaction between Na3O12Si7(C6H5)7 and 3-bromopropyltrichlorosilane was carried out. Typi- cally, Na3O12Si7(C6H5)7 (10.030 g, 10.04 mmol) and trieth- 948 Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 M A CR O M O LE CU LA R N A N O TE CH N O LO G Y reaction was carried out at 0 �C for 3 h and at room tem- perature for 24 h. The insoluble solids (i.e., sodium chlo- ride) were filtered out and the solvents together with other volatile compounds were removed via rotary evapo- ration to obtain the white solids. The solids were washed with 50 ml methanol for three times and dried in vacuo at 30 �C for 24 h and the product (6.997 g) was obtained with the yield of 62.3%.1H NMR (CDCl3, ppm): 7.30–7.55, 7.70–7.87 (35H, C6H5�), 3.40 (2H, Si–CH2–CH2–CH2–Br), 2.06 (2H, Si–CH2–CH2–CH2–Br), 1.00 (2H, Si–CH2–CH2 –CH2–Br). 29Si NMR (CDCl3, ppm): �65.37, �77.76, �78.15. 2.3. Synthesis of 3-azidopropylheptaphenyl POSS 3-Azidopropylheptaphenyl POSS was synthesized via the reaction between 3-bromopropylheptaphenyl POSS and sodium azide (NaN3). Typically, 3-bromopropylhepta- phenyl POSS (3.11 g, 2.78 mmol) and NaN3 (0.21 g, 3.23 mmol) were added into a flask equipped with a mag- netic stirrer and then anhydrous THF (5 ml) and DMF (6 ml) was added. The reaction was carried out at room temperature for 24 h. After that, the solution was concen- trated and dropped a great amount of deionized water to afford the precipitates. The precipitates were further dried at 40 �C in a vacuum oven for 24 h and the product (2.24 g) was obtained with the yield of 76.6%.1H NMR (CDCl3, ppm): 7.29–7.54, 7.72–7.85 (m, 35H, C6H5�), 3.24 (t, 2H, Si–CH2–CH2–CH2–N3), 1.81 (m, 2H, Si–CH2–CH2–CH2–N3), 0.92 (t, 2H, Si–CH2–CH2–CH2–N3). 2.4. Synthesis of propargyl 2-chloropropionate To a 150 ml flask equipped with a magnetic stirrer, propargyl alcohol (4.60 g, 82 mmol) dissolved in 50 ml of anhydrous dichloromethane and TEA (8.10 g, 80 mmol) were charged. Cooled to 0 �C, 2-chloropropinoyl chloride (9.52 g, 75 mmol) dissolved in 10 ml of dichloromethane was added by dropping funnel within 40 min with vigor- ous stirring in a highly pure nitrogen atmosphere. Thereaf- ter, the reaction was performed at room temperature for 24 h. After the solids were removed via filtration, the fil- trate was diluted by 50 ml of dichloromethane and washed with 40 ml of distilled water (40 ml � 3). The organic layer was dried over anhydrous magnesium sulfate. The filtrate was concentrated via rotary evaporator and then passed through a neutral aluminum oxide column with petroleum ether as an eluent. All the solvents were removed by rotary evaporation, a colorless liquid was obtained with the yield of (9.03 g, 65.7%). 1H NMR (CDCl3, ppm): 4.77(2H, CH„C– CH2), 4.41–4.45 (1H, CH3–CH–Cl), 2.52(1H, CH„C–CH2), 1.68–1.70 (3H, CH3–CH–Cl). 2.5. Synthesis of initiator bearing POSS The copper-catalyzed Huisgen 1,3-dipolar cycloaddition reaction between 3-azidopropylheptaphenyl POSS and propargyl 2-chloropropionate was used to prepare the ini- tiator bearing POSS. Typically, 3-azidopropylheptaphenyl POSS (1.0740 g, 1.00 mmol), propargyl 2-chloropropionate (0.1378 g, 0.95 mmol) and 10 ml tetrahydrofuran were charged to a 25 ml round-bottom flask equipped with magnetic stirrer. The reactive system was purged with highly pure nitrogen for 40 min and then Cu(I)Br (14.3 mg, 0.1 mmol) and PMDETA (20.8 ll, 0.1 mmol) were added. The system was degassed via three pump–freeze– thaw cycles. The reaction was performed at room temper- ature for 24 h and the reacted mixture was dropped into a great amount of the mixture of methanol with water (50/ 50, v/v) to afford the precipitates; this procedure was re- peated three times to remove the catalyst. To remove PMDETA, the precipitates were dissolved with 10 ml dichloromethane and the solution was washed with 1 wt.% dilute hydrochloric acid for three times. The organic phase was dried with anhydrous magnesium and the sol- vent was removed via rotary evaporation. The solids were dried in vacuo at room temperature and the product (1.013 g) was obtained with the yield of 82.3%. 1H NMR (CDCl3, ppm): 7.30–7.85 (m, 5H, C6H5�), 7.07–7.79 (m, 5H, C6H5–Si), 5.28–5.37 (s, 2H, C–CH2–O–CO–), 4.47(m, 1H, CH3–CH–Cl), 4.27 (t, 2H, Si–CH2–CH2–CH2–N), 1.66– 1.73 (d, 3H, CH3–CH–Cl), 2.10 (m, 2H, Si–CH2–CH2–CH2– N), 0.82 (t, 2H, Si–CH2–CH2–CH2–N). 2.6. Synthesis of POSS-capped PNIPAAm The ATRP of NIPAAm with the above initiator bearing POSS was carried out in the mixtures of THF and isopropyl alcohol at room temperature. Typically, NIPAAm (2.767 g, 26.9 mmol), the initiator (0.2440 g, 0.024 mmol), and 4.0 ml isopropyl alcohol were added into a flask equipped with a magnetic stirrer, and the trace of oxygen in the sys- tem was removed via bubbling with highly pure nitrogen for 30 min. CuCl (2.4 mg, 0.024 mmol) and Me6TREN (6 ll, 0.024 mmol) were added. The system was degassed via three pump–freeze–thaw cycles. The polymerization was carried out at room temperature for 14 h. The viscous reacted mixture was diluted with 10 ml THF and then passed through a column of neutral aluminum with THF as the eluent to remove the catalyst. After that, the solution was concentrated and dropped into a great amount of the mixture of diethyl ether with petroleum ether (50/50, v/v) to afford the precipitates. The precipitates were dissolved with 10 ml THF and re-precipitated. This procedure was re- peated for three times. After drying in a vacuum oven at room temperature for 24 h, the polymer was obtained with the yield of 31.2%. 1H NMR (CDCl3, ppm): 7.02–7.85(m, C6H5–Si), 6.06–6.73 [m, NH–CH–(CH3)2], 4.02 [m, NH–CH–(CH3)2], 1.54–2.95 [m, CH2–CH–(CH3)], 1.13 [d, NH–CH–(CH3)2] and 0.83 [m, CH2–CH–(CH3)]. GPC: Mn = 5700 with Mw/Mn = 1.35. 2.7. Synthesis of PNIPAAm homopolymer The initiator used for the preparation of PNIPAAm homopolymer via ATRP was synthesized via the reaction betwen 2-chloropropinoyl chloride and benzyl alcohol. Typically, to 100 ml flask equipped with a magnetic stirrer benzyl alcohol (4.4331 g, 0.040 mol), TEA (4.05 g, 0.040 mol) and dichloromethane (50 ml) were charged. Cooled to 0 �C, 2-chloropropinoyl chloride (8.252 g, 0.065 mol) was slowly dropped into the flask with vigorous stirring. The reaction was carried out in an ice-water bath Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 949 M A CR O M O LE CU LA R N A N O TE CH N O LO G Y for 2 h and at room temperature for 24 h. The insoluble compounds were removed via filtration and then 200 ml dichloromethane was added. The organic phase was washed with 5% sodium bicarbonate aqueous solution (100 ml � 2) and with deionized water until neutrality. The organic phase was dried over anhydrous magnesium sulfate. After concentrated via rotary evaporation, the solu- tion was passed through a neutral aluminum oxide column with petroleum ether as an eluent. After the solvents were eliminated by rotary evaporator, a colorless liquid (i.e., benzyl 2-choropropionate) (8.240 g) was obtained with the yield of 85.5%. 1H NMR (400 MHz, CDCl3): 7.32–7.45 (5H, C6H5–CH2–O), 5.20 (2H, C6H5–CH2–O), 4.40–4.46 (1H, CH3–CH–Cl), 1.70 (3H, CH3–CH–Cl). The polymerization of NIPAAm monomer with benzyl 2-choropropionate as the initiator was performed with iso- propyl alcohol (IPA) as the solvent. Typically, NIPAAm (1.6200 g, 15.7 mmol), benzyl 2-choropropionate (29.5 mg, 0.137 mmol) and IPA (4.0 ml) were charged to a flask equipped with a magnetic stirrer and the trace of oxygen was removed via bubbling with highly pure nitro- gen for 30 min. Copper (I) chloride (CuCl) (13.7 mg, 0.137 mmol) and Me6TREN (36 ll, 0.137 mmol) were added and the system was degassed via three pump– freeze–thaw cycles. The polymerization was carried out at room temperature for 48 h. The viscous mixture was di- luted with THF and passed through a column of neutral aluminum with THF as the eluent to remove the catalyst. The polymer solution was concentrated and precipitated into a great amount of the mixture of diethyl ether with petroleum ether (50/50, v/v) and the procedure was re- peated three times. After drying in a vacuum oven at room temperature for 24 h, the white solids (1.1129 g) were ob- tained with the yield of 68.7%. 1H NMR (CDCl3,ppm): 7.32– 7.45 (5H, C6H5–CH2–O), 6.06–6.73 [1H, NH–CH–(CH3)2], 5.20 (2H, C6H5–CH2–O), 4.02 [1H, NH–CH–(CH3)2], 1.54– 2.95 [3H, CH2–CH–(CH3)],1.13 [6H, NH–CH–(CH3)2], 0.83 [3H, CH2–CH–(CH3)]. GPC: Mn = 7800 with Mw/Mn = 1.27. 2.8. Preparation of dispersions of POSS-capped PNIPAAm in water Typically, 10 mg of POSS-capped PNIPAAm was dis- solved in 1 ml of THF with vigorous stirring. Thereafter, 50 ml of deionized water was slowly added with a drop- ping funnel. Thereafter, the dispersion was left stirring for 2 h and then all the dispersions were dialyzed with ultrapure water for 72 h, during which fresh ultrapure water was replaced every 6 h. The micellar solution exhib- ited no macroscopic phase separation upon standing at room temperature for one month, suggesting the forma- tion of stable aggregates. 2.9. Characterization and measurements 2.9.1. Nuclear magnetic resonance spectroscopy (NMR) The 1H NMR measurements were performed on a Var- ian Mercury Plus 400 MHz NMR spectrometer at 25 �C. The 29Si NMR spectra were obtained using a Brucker Avance III 400 MHz NMR spectrometer. Deuterium chloroform (CDCl3) was used as the solvent and tetrameth- ylsilane (TMS) as an internal reference. 2.9.2. Gel permeation chromatography (GPC) The molecular weights and the molecular distribution (Mw/Mn) were measured on a Perkin-Elmer Series 200 sys- tem (100 ll injection column, 10 lm PL gel 300 mm � 7.5 mm mixed B columns) equipped with a refractive index detector. DMF (0.01 M LiBr) was used as an eluent at the flow rate of 1.0 ml/min. The molecular weights were expressed with polystyrene standard. 2.9.3. Atomic force microscopy (AFM) The specimens for AFM measurements were prepared via spin coating the solutions of the POSS-capped PNIPAAm on cleaned glass slides. The POSS-capped PNIPAAm sam- ples were dissolved in THF and the concentration of the solutions were controlled to be �10 wt.%. The solvent was removed via evaporation at 40 �C for 24 h and drying in a vacuum oven at 60 �C for 12 h. The AFM observations were carried out on a Nanoscope IIIa scanning probe microscope with tapping mode. Standard MikroMasch NSC11 silicon cantilevers with a resonance frequency of �330 kHz, a rounding radius of 610 nm, and a stiffness constant k � 48 N/mwere used. Typical scan speeds during recording were 0.3–1.0 lines/s using scan heads with a maximum range of 16 lm � 16 lm. 2.9.4. Dynamic light scattering (DLS) Laser light scattering experiments were conducted on a Nano ZS90 (Malvern Instrument, Malvern, Worcestershire, UK) equipped with a He-Ne laser operating at the wave- length of 633 nm. The dispersions of POSS-capped PNI- PAAm in water were measured at room temperature. 2.9.5. Transmission electron microscopy (TEM) TEM observations were conducted on a JEOL JEM 2100F electron microscope at a voltage of 200 kV. To prepare the specimens for TEM observation, a dispersion of POSS- capped PNIPAAm in water (about 10 ll) was dropped onto carbon-coated copper grids and the water was eliminated via freeze-drying approach. 2.9.6. Ultraviolet–visible spectroscopy (UV–vis) The lower critical solution temperature (LCST) was measured on a Perkin-Elmer Lambda 20 Ultraviolet–visible spectrophotometer. A thermostatically controlled cell was employed for variable temperature measurements at the heating rate of 0.2 �C/min. The optical transmittance of the aqueous solution at the concentration of 0.2 g/l was re- corded at a fixed wavelength of k = 550 nm. 3. Results and discussions 3.1. Synthesis of POSS-capped PNIPAAm The synthesis route of the POSS-capped PNIPAAm was depicted in Scheme 1. The initiator bearing POSS for the atom transfer radical polymerization of N-isopropylacryl- amide (NIPAAm) was synthesized. The starting compound for the POSS macromer is heptaphenyltricycloheptasilox- ane trisodium silanolate [Na3O12Si7(C6H5)7], which was prepared via the hydrolysis, condensation and rearrange- ment of phenyltrimethoxysilane in the presence of sodium hydroxide [53]. The corner-capping reaction between heptaphenyl tricycloheptasiloxane trisodium silanolate and 3-bromopropyltrichlorosilane was performed to ob- tain 3-bromopropylheptaphenyl POSS. The 3-bromopro- pylheptaphenyl POSS was used to react with sodium azide (NaN3) to afford 3-azidopropylheptaphenyl POSS. The copper-catalyzed Huisgen 1,3-cycloaddition reaction between 3-azidopropylheptaphenyl POSS and propargyl 2-chloropropionate was utilized to obtain the targeting ini- tiator. Shown in Fig. 1 is the 29Si NMR spectrum of the 3- bromopropylheptaphenyl POSS. The signals of resonance at �65.37, �77.76 and �78.15 ppm are assignable to the silicon nucleus of silsesquioxane cage as indicated in Fig. 1. In terms of the ratio of integral intensity for these silicon resonance peaks, it is judged that the octameric sils- esquioxane was obtained. The complete substitution of bromine atoms with azido groups was confirmed by 1H NMR spectroscopy. Shown in Fig. 2 are the 1H NMR spectra of 3-bromopropylheptaphenyl and 3-azidopropylhepta- 4.38 and 5.24 ppm are assignable to the protons of methyl, methine and methylene group from 2-chloropropionate moiety. According to the ratio of integral intensity of pro- tons from propyl groups to 2-chloropropionate moiety, it is judged that the initiator bearing POSS was successfully obtained. The atom transfer radical polymerization (ATRP) of NIP- AAm with the initiator bearing POSS was carried out to af- ford the POSS-capped PNIPAAm (See Scheme 1). By controlling the molar ratio of the initiator to NIPAAm monomer, the POSS-capped PNIPAAm samples with vari- able lengths of PNIPAAm were obtained; the results of polymerization are summarized in Table 1. Representa- tively shown in Fig. 4 is the 1H NMR spectrum of POSS- capped PNIPAAm4.7 K. The signals of resonance in the 10 8 6 4 2 0 Si O Si ab c Chemical shift (ppm) Fig. 2. 1H NMR spectra of 3-bromopropylheptaphenyl POSS and 3- azidopropylheptaphenyl POSS. 9 8 7 6 5 4 3 2 1 0 Si O Si O Si O Si O Si O Si O O O O O O O Si Si N N N Ο O Cl a b c d e f g h Chemical shift (ppm) a+e f h d c g b * CDCl3 TMS } Fig. 3. 1H NMR spectrum of the initiator bearing POSS. 950 Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 M A CR O M O LE CU LA R N A N O TE CH N O LO G Y phenyl POSS. It is seen that with the occurrence of the sub- stitution reaction, the resonance of methylene protons connected to bromine atom at 3.41 ppm fully shifted to 3.25 ppm, suggesting that this reaction occurred to com- pletion. The 1H and 29Si NMR spectroscopy indicates that the 3-azidopropylheptaphenyl POSS was successfully ob- tained. With the above 3-azidopropylheptaphenyl POSS, the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click chemistry) was carried out between 3-azidopropylhepta- phenyl POSS and propargyl 2-chloropropionate to obtain the initiator bearing POSS. The 1H NMR spectrum of the product is shown in Fig. 3. Apart from the signals of proton resonance assignable to propyl group connected to POSS cage and aromatic rings, the peaks of resonance at 1.65, 0 -20 -40 -60 -80 -100 -64 -66 -68 -70 -72 -74 -76 -78 -80 Si O Si O Si O Si O Si O Si O O O O O O O Si Si Brab c d a c+d b Chemical shift (ppm) a c+d b Fig. 1. 29Si NMR spectrum of 3-bromopropylheptaphenyl POSS. d f e O Si O Si O Si O Si O O O O O O O Si Si N3 a b c d ef a b TMS Si O Si O Si O Si O Si O Si O O O O O O O Si Si Bra b c d ef c e f d CDCl3 Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 951 Table 1 Molecular weights and polydispersity of POSS-capped PNIPAAm. Samples LPNIPAAm (�10�3 Da) Mn (�10�3 Da) (GPC) Mw/Mn PNIPAAm 7.8 7.8 1.27 POSS-PNIPAAm3.8K 3.8 4.86 1.33 POSS-PNIPAAm4.7K 4.7 5.71 1.35 POSS-PNIPAAm6.2K 6.2 7.22 1.34 POSS-PNIPAAm15.8K 15.8 16.89 1.42 POSS-PNIPAAm18.4K 18.4 19.47 1.46 Si O Si O Si O Si O Si O Si O O O O O O O Si Si N N N O O Cl HN O n b c d e f g h i j a range of 7.3 � 7.9 ppm are discernable, which are ascribed to the protons of phenyl group of POSS cage. The signals of resonance at 1.14, 1.64, 2.02 and 4.01 ppm are assigned to the protons of methyl, methylene, methine in PNIPAAm main chain, methine of isopropyl groups of PNIPAAm, respectively; the broad signals of resonance at 6 � 7 ppm were assignable to the protons of N–H moiety of PNIPAAm. The 1H NMR spectroscopy indicates that the polymer com- bined the structural features from POSS and PNIPAAm. All the POSS-capped PNIPAAm samples were subjected to gel permeation chromatography (GPC) to measure the molec- ular weights and the GPC profiles curves are presented in Fig. 5 and the results of the molecular weights are listed in Table 1. In all the cases the unimodal peaks were exhib- ited, suggesting that no initiator bearing POSS was left in the samples. The polydispersity of all the POSS-capped PNIPAAm samples were in the range of 1.33 � 1.46. It is noted that the polydispersity of the POSS-capped PNIPAAm was slightly broader than the PNIPAAm homopolymer. Nonetheless, the unimodal and fairly narrow distribution 10 9 8 7 6 5 4 3 2 1 0 k Chemical shift (ppm) a+e j h k CDCl3 TMS g f+i{ b * Fig. 4. 1H NMR spectrum of POSS-PNIPAAm4.7 K. �: the signal is assignable to the proton resonance of a trace of tris(2-(dimethyl- amino)ethyl)amine which persistently existed after three dissolution- precipitation circles. LE CU LA R N A N O TE CH N O LO G Y of molecular weights indicates that the initiator bearing POSS can be successfully used to obtain the POSS-capped PNIPAAm, i.e., the polymerization could be carried out in a living fashion. 3.2. Morphology of POSS-capped PNIPAAm The specimens of POSS-capped PNIPAAm films were prepared via spin-coating their solutions onto surfaces of glass slides and the surface morphology of the POSS- capped PNIPAAm in bulks was investigated by means of atomic force microscopy (AFM). In all the cases, the heter- ogeneous morphologies were exhibited and the morpholo- 8 10 12 14 16 18 20 POSS-PNIPAAm3.8K POSS-PNIPAAm4.7K POSS-PNIPAAm6.2K POSS-PNIPAAm15.8K Retention time (min) DMF POSS-PNIPAAm18.4K Fig. 5. GPC profiles of POSS-capped PNIPAAm samples. gies were quite dependent on the percentage of POSS (or the length of PNIPAAm chains). Representatively shown in Fig. 6 are the AFM micrographs of POSS-capped PNI- PAAm samples with the shorter and longer PNIPAAm chains. In terms of the volume fraction of POSS and the dif- ference in viscoelastic properties between PNIPAAm and POSS, the light continuous regions are attributable to PNI- PAAm matrix whilst the spherical dark regions are assign- able to POSS domains. While PNIPAAm chains were shorter (e.g., POSS-PNIPAAm4.7 K), the spherical microdomains with the size of ca. 200 nm in diameter were detected, which were dispersed in continuous matrix (see Fig. 6A). The spherical microdomains are ascribed to POSS whereas the matrix is attributed to PNIPAAm phase. Careful obser- vation showed that the matrix was also heterogonous, in which the POSS nanophases were discernible. It is pro- posed that the spherical microdomains with the larger size are responsible for the POSS which were enriched onto the interface between air and the surface of the specimens. Owing to the characteristics of low surface energy of organosilicon moeity [54–60], the POSS component at the surface have the tendency to coarsen to the larger size whereas those which were embeded in the bulks of PNI- PAAm matrix would have the smaller sizes. The similar M A CR O M O 952 Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 M A CR O M O LE CU LA R N A N O TE CH N O LO G Y result has been found in other POSS-containing PNIPAAm copolymers [15]. While the percentage of POSS in the or- ganic–inorganic amphiphiles was sufficiently low for the POSS-capped PNIPAAm with logner PNIPAAm chains, the tendency for POSS microdomain to coarsen to the larger size at the surface of specimens would be significantly sup- pressed as shown in Fig. 6B for the AFM micrograph of POSS-PNIPAAm15.8K. It is seen that a nanophase- separated morphology was displayed. The heterogeneous morphology indicates that the organic–inorganic amphi- philes were microphase-separated owing to the immisci- bility of PNIPAAm with the POSS. 3.3. Self-assembly behavior in aqueous solutions It is of interest to investigate the self-assembly behavior of POSS-capped PNIPAAm in aqueous solutions. In this work, dynamic light scattering (DLS) and transmission electron microscopy (TEM) were employed toward this end. Shown in Fig. 7 are the plots of hydrodynamic radius distribution as functions of hydrodynamic radius (Rh) for the dispersions of POSS-capped PNIPAAm in water with variable lengths of PNIPAAm at 24 �C. In all the cases, the Fig. 6. Tapping mode atomic force microscopy images: (A) POSS- PNIPAAm4.7K and (B) POSS-PNIPAAm15.8K. intensity-averaged hydrodynamic radius (Rh) displayed unimodal distribution with the values of Rh ranged from 50 to 80 nm. The values of Rh decreased with increasing the length of PNIPAAm chains. The results of DLS indicate that the organic–inorganic amphiphiles were self-assme- bled into nanoobjects in the aqueous solutions. The self- organzied nanoobjects could be micelle-like aggregates, 100 101 102 103 POSS-PNIPAAm15.8K POSS-PNIPAAm6.2K POSS-PNIPAAm4.7K Rh (nm) POSS-PNIPAAm3.8K POSS-PNIPAAm18.4K Fig. 7. Hydrodynamic radius (Rh) of POSS-capped PNIPAAm in the aqueous solutions (0.2 g/l) at 24 �C. in which the hydrophobic POSS cages aggregated into the core whereas PNIPAAm chains consititue the coronas. In terms of the values of hydrodynamic radius (Rh), it is pro- posed that the cores of micelle-like aggrgates were com- posed of tens of POSS as dipicted in Scheme 2. The values of Rh decreased with the length of PNIPAAm chains, indi- cating that hydrophobic/hydropholic balance can be estab- lished and modulated by controlling the lengths of hydrophilic PNIPAAm. The morphology of the self-assem- bled nanoobjects was further investigated by transmission electron microscopy (TEM). The specimens for TEM mea- surement were prepared via freeze-drying approach with the dispersion of POSS-capped PNIPAAm in water at the concentration of 0.2 g/l at 24 �C. The spherical nanoobjects were detected in all the cases. Representatively shown in Fig. 8 is the TEM micrograph for the specimen prepared from POSS-PNIPAAm4.7K. It is seen that the self-assembly of this sample in aqueous solution generated the spherical nanoparticles with the size of ca. 50 nm. It is proposed that the organic–inorganic aggregates are composed of POSS cores and PNIPAAm coronas. It is should be pointed out that the sizes of aggregates measured with TEM (Fig. 8) are not necesarrily indentical with the Rh values by means of DLS. The former were obtained in the dry state and are much lower than the latter obtained in solution, which contains the dominant contribution of the solvated coro- nas (viz. PNIPAAm). PNIPAAm chains adopt a random coil conformation. Above the LCST, the PNIPAAm random coil would collapse into a globule. With the occurrence of the coil-to-globule transi- tion, PNIPAAm chains would become dehydrated. In the present case, one bulky and hydrophobic POSS cage was bonded to one end of a single PNIPAAm chain and thus it Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 953 3.4. LCST behavior It is known that PNIPAAm homopolymer is capable of undergoing a coil-to-globule transition in aqueous solu- tions in the vicinity of its lower critical solution tempera- ture (LCST) at �32 �C [19–30]. Below the LCST, individual is of interest to examine the LCST behavior of the organ- ic–inorganic amphiphiles. The LCST behavior was investi- gated with the cloud-point analysis by measn of UV–vis spectroscopy at the wavelength of k = 550 nm at the heat- ing rate of 0.2 �C min�1. Shown in Fig. 9 are the plots of vis- ible light transmittance for POSS-capped PNIPAAm samples with various lengths of PNIPAAm as functions of temperature. At the temperature below the LCSTs, the opti- cal transmittance of all the aqueous solutions was as high as 98%. At elevated temperatures, the optical transmittance of the POSS-capped PNIPAAm dispersion in water was sig- nificantly decreased as that of PNIPAAm homopolymer, indicating the occurrence of the LCST phenomena. For pure PNIPAAm homopolymer (Mn = 7800 with Mw/Mn = 1.27), the coil-to-globule transition occurred at 32 �C and the transition range of temperature was as narrow as 2 � 3 �C. After the coil-to- globule transition of PNIPAAm chains, the optical transmittance was decreased as low as 4%. Nonetheless, the POSS-capped PNIPAAm displayed the features quite different from PNIPAAm homopolymer. Scheme 2. Self-assembly behavior of POSS-capped PNIPAAm in aqueous solution. Fig. 8. TEM micrograph of the specimen prepared from the aqueous of solution (0.2 g/l) for POSS-PNIPAAm4.7K via freeze-drying technique. M A CR O M O LE CU LA R N A N O TE CH N O LO G Y On the one hand, the LCSTs of the POSS-capped PNIPAAm were much lower than PNIPAAm homopolymer; the LCSTs decreased with increasing the percentage of POSS (or with decreasing the length of PNIPAAm chains). The decreased LCSTs for POSS-capped PNIPAAm are attributable to the presence of the bulky and hydrophobic POSS termini. It is proposed that the introduction of hydrophobic POSS cage at the one end of PNIPAAm chain could cause the reduced 20 24 28 32 36 40 44 0 20 40 60 80 100 POSS-PNIPAAm3.8K Tr an sm itt an ce ( % ) Temperature ( oC) PNIPAAm POSS-PNIPAAm4.7K POSS-PNIPAAm6.2K POSS-PNIPAAm15.8K POSS-PNIPAAm18.4K Fig. 9. Plots of light transmittance (k = 550 nm) as functions of temper- ature for the aqueous solution (0.2 g/l) of plain PNIPAAm and POSS- capped PNIPAAm samples at the heating rate of 0.2 �C/min. telechelics. J Phys Chem B 2009;113(35):11831–40. 954 Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 M A CR O M O LE CU LA R N A N O TE CH N O LO G Y solvation of PNIPAAm by water and thus the LCSTs were decreased. On the other hand, the coil-to-globule transi- tional ranges for the POSS-capped PNIPAAm were signifi- cantly broadened especially for the organic–inorganic amphiphiles with the shorter PNIPAAm chains (e.g., POSS-PNIPAAm3.8 K, 4.7 K and 6.2 K), which is in marked contrast to PNIPAAm homopolymer. The broadened coil- to-globule collapse ranges for the POSS-capped PNIPAAm could be associated with the self-assembly behavior of the organic–inorganic amphiphiles in aqueous solutions. By means of DLS and TEM, it was found that in aqueous solutions, the organic–inorganic amphiphiles were self- assembled into the spherical nanoobjects with the size of 50 � 80 nm. It is plausible to propose that in the micelle- like aggregates, thermoresponsive PNIPAAm chains (viz. coronas) have to be densely attached onto the surface of the hydrophobic cores composed of tens of POSS cages. The hydrophobic cores could exert significant restriction on the conformational alteration of PNIPAAm chains at the intimate surface of the cores due to steric hindrance while the coil-to-globule transition of PNIPAAm chains oc- curred. The shorter the PNIPAAm chains the higher the re- stricted portion of PNIPAAm chains. Therefore, the fairly broad ranges of transition were observed for the POSS- capped PNIPAAm with the shorter PNIPAAm chains (e.g., POSS-PNIPAAm3.8 K, 4.7 K and 6.2 K). Owing to the restric- tion of POSS cores on PNIPAAm coronas considerable por- tion of the PNIPAAm chain at the intimate surface of POSS cores could remain hydrated at the temperature above the LCSTs. In fact, we indeed observed that the opti- cal transmittance of the aqueous solutions of POSS-capped PNIPAAm samples were significantly higher than PNIPAAm homopolymer after the coil-to-globule transition occurred. 4. Conclusions The novel initiator bearing heptaphenyl POSS and 2- chloropropionate moiety was successfully synthesized via the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click chemistry) between 3-azidopropylheptaphenyl POSS and propargyl 2-chloropropionate. The initiator was success- fully employed to obtain the POSS-capped PNIPAAm with variable length of PNIPAAm via ATRP approach. The POSS-capped PNIPAAm amphiphiles displayed micro- phase-separated morphologies in bulk. In aqueous solutions the POSS-capped PNIPAAm amphiphiles were self-assembled into spherical micelle-like nanoobjects as evidenced by dynamic light scattering (DLS) and transmis- sion election microscopy (TEM). The organic–inorganic mi- celle-like nanoobjects are composed of PNIPAAm coronas and hydrophobic POSS cores in which tens of POSS cages were aggregated. The sizes of the micelle-like nanoobjects decreased with increasing the lengths of PNIPAAm chains. By means of UV–Vis spectroscopy, the lower critical solu- tion temperature (LCST) behavior of the POSS-capped PNI- PAAm amphiphiles in aqueous solution was investigated. It is found that the LCSTs of the POSS-capped PNIPAAm sam- ples decreased with increasing the percentage of POSS termini. It is found that the hydrophobic cores of the micelle-like nanoobjects exerted the restriction on the [17] Wang L, Zeng K, Zheng S. 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Synthesis and characterization of heptaphenyl polyhedral oligomeric silsesquioxane-capped poly(N-isopropylacrylamide)s 1 Introduction 2 Experimental 2.1 Materials 2.2 Synthesis of 3-bromopropylheptaphenyl POSS 2.3 Synthesis of 3-azidopropylheptaphenyl POSS 2.4 Synthesis of propargyl 2-chloropropionate 2.5 Synthesis of initiator bearing POSS 2.6 Synthesis of POSS-capped PNIPAAm 2.7 Synthesis of PNIPAAm homopolymer 2.8 Preparation of dispersions of POSS-capped PNIPAAm in water 2.9 Characterization and measurements 2.9.1 Nuclear magnetic resonance spectroscopy (NMR) 2.9.2 Gel permeation chromatography (GPC) 2.9.3 Atomic force microscopy (AFM) 2.9.4 Dynamic light scattering (DLS) 2.9.5 Transmission electron microscopy (TEM) 2.9.6 Ultraviolet–visible spectroscopy (UV–vis) 3 Results and discussions 3.1 Synthesis of POSS-capped PNIPAAm 3.2 Morphology of POSS-capped PNIPAAm 3.3 Self-assembly behavior in aqueous solutions 3.4 LCST behavior 4 Conclusions Acknowledgments References