synthesis and characterization of heptaphenyl polyhedral oligomeric silsesquioxane-capped...

European Polymer Journal 48 (2012) 945–955

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European Polymer Journal

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Macromolecular Nanotechnology

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

Yaochen Zheng, Lei Wang, Sixun Zheng ⇑Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China

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Article history:Received 3 November 2011Received in revised form 22 February 2012Accepted 4 March 2012Available online 11 March 2012

Keywords:Poly(N-isopropylacrylamide)Polyhedral oligomeric silsesquioxaneOrganic–inorganic amphiphilesSelf-assembly

0014-3057/$ - see front matter � 2012 Elsevier Ltdhttp://dx.doi.org.10.1016/j.eurpolymj.2012.03.007

⇑ Corresponding author. Tel.: +86 21 54743278; faE-mail address: [email protected] (S. Zheng).

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In this work, a novel initiator bearing heptaphenyl polyhedral oligomeric silsesquioxane(POSS) was synthesized via the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., clickchemistry). With this initiator, the atom transfer radical polymerization (ATRP) of N-iso-propylacrylamide (NIPAAm) was carried out to afford the POSS-capped PNIPAAm. Theorganic–inorganic amphiphiles were characterized by means of nuclear magnetic reso-nance spectroscopy (NMR) and gel permeation chromatography (GPC). Atomic forcemicroscopy (AFM) showed that the POSS-capped PNIPAAm amphiphiles in bulk displayedmicrophase-separated morphologies. In aqueous solutions, the POSS-capped PNIPAAmamphiphiles were self-assembled into micelle-like aggregates as evidenced by dynamiclight scattering (DLS) and transmission election microscopy (TEM). It was found that thesizes of the self-organized nanoobjects decreased with increasing the lengths of PNIPAAmchains. By means of UV–vis spectroscopy, the lower critical solution temperature (LCST)behavior of the organic–inorganic amphiphiles in aqueous solution was investigated andthe LCSTs of the organic–inorganic amphiphiles decreased with increasing the percentageof POSS termini. It is noted that the self-assembly behavior of the POSS-capped PNIPAAm inaqueous solutions exerted the significant restriction on the macromolecular conformationalteration of PNIPAAm chains while the coil-to-globule collapse occurred.

� 2012 Elsevier Ltd. All rights reserved.

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1. Introduction
Polyhedral oligomeric silsesquioxanes (POSS) are a classof important nanosized cage-like molecules, derived fromhydrolysis and condensation of trifunctional organosilanes.In the past years, POSS reagents, monomers and polymershave been emerging as a new chemical technology for pre-paring the organic–inorganic hybrids [1–8]. A typical POSSmolecule 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)of which is reactive (Scheme 1). POSS-containing hybridnanocomposites have been becoming the focus of manystudies due to their excellent thermomechanical

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properties [1–8]. Recently, POSS cages have been incorpo-rated into organic polymers to optimize the functionalproperties of the materials via the formation of specificmorphological structures [9,10]. For instance, POSS cageswere incorporated into conjugated luminescent polymerssuch as polyfluorene, polyphenylene and polythiopheneto 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 ratesof poly(N-isopropylacrylamide) hydrogels [14–18]. How-ever, such an investigation remains largely unexploredvis-à-vis the studies on the improvement of thermome-chanical properties of POSS-containing nanocomposites.

Poly(N-isopropylacrylamide) (PNIPAAm) is a kind ofinteresting thermoresponsive polymer. In aqueous solu-tion PNIPAAm can display a lower critical solution temper-ature (LCST) behavior at ca. 32 �C [19–23]. Below thistemperature, individual PNIPAAm chains adopt a random

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Si OCH3

OCH3

OCH3 Si O SiO

SiO

Si O SiO

Si

O

O-

O

O-O-

OO

Si .

SiCl

ClCl Br

Si O SiO

SiO

Si O SiO

Si

O

O

O

O O

OO

Si

Si

NaN3

Br

Si O SiO

SiO

Si O SiO

Si

O

O

O

O O

OO

Si

Si

N3

3Na+

Si O SiO

SiO

Si O SiO

Si

O

O

O

O O

OO

Si

Si N

OH

+ ClCl

OO

Cl

O

Si O SiOSiO

Si O SiO

SiO

O

O

O OOO

Si

Si

N3

CuBr / PMDETA

NN

O

O

Cl

NIPAAm

CuCl / Me6TRENSi O Si

OSi

O

Si O SiO

Si

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O

O O

OO

Si

Si NN

N

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O

HNO

n

Scheme 1. Synthesis of POSS-capped PNIPAAm.

946 Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955

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coil conformation. While the solution is heated up to 32 �Cor higher, the random coils will collapse into globules. It isproposed that in the former case, the intermolecularhydrogen bonding interactions between amide groups of

PNIPAAm and water promote the solubility of PNIPAAmwith 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,

Y. Zheng et al. / European Polymer Journal 48 (2012) 945–955 947

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the hydrophobic association among the collapsed PNI-PAAm chains takes place. During the past decades, theLCST behaviors of PNIPAAm in aqueous solutions havebeen extensively investigated [19–30]. It is identified thatsome structural factors such as co-monomer, tacticity,crosslinking, grafting, topology of macromolecules, molec-ular weights and end groups all affect the LCST behavior ofPNIPAAm in aqueous solutions [19,31–37]. Recently, thePNIPAAm amphiphiles containing hydrophobic end groupsor 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 ofmesoglobules may be the prevalent cause of their stabilityagainst aggregations [20]. Zhang et al. [40] found that theflower-like aggregates resulting from PS-b-PNIPAAm-b-PStriblock copolymers are much less collapsed than thestar-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 topologiesand self-assembly behavior [16,42–46]. Depending on thechemical 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(hydroxyetherof bisphenol A) [50] have been reported by various invesit-gators; these POSS-capped telechelic polymers can displaysome interesting morphologies and properties. Wang et al.[17] reported synthesis of POSS-capped PNIPAAm teleche-lics with a ‘‘POSS-spreading’’ strategy. In this approach, aPOSS-capped trithiocarbonate was synthesized and usedas a chain transfer agent, with which the reversible addi-tion-fragmentation chain transfer (RAFT) polymerizationof NIPAAm was carried out and thus two ends of PNIPAAmchain 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 physicalhydrogels in aqueous solutions. It was found that suchphysical 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 PNIPAAmvia atom transfer radical polymerization (ATRP). Firstly, aninitiator bearing heptaphenyl POSS was synthesized via thecopper-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 affordthe POSS-capped semi-telechelic PNIPAAm. It should bepointed 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 RAFTagent was prepared via the direct reaction between3-aminopropylheptaisobutyl POSS and 3-benzylsulfanyl-thiocarbonylsufanylpropionic chloride. In this work, theself-assembly behavior of the organic–inorganic amphi-

philes was investigated by means of atomic force micros-copy (AFM), transmission electron microscopy (TEM) anddynamic laser scattering (DLS). The lower critical solutiontemperature behavior in aqueous solutions was addressedaccording 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 wasdried 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 thislab via the reaction between isopropylamine and acryloylchloride. Both copper (I) bromide (CuBr) and copper (I)chloride (CuCl) were of chemically pure grade, suppliedby Shanghai Reagent Co., China. Tris(2-(dimethylamino)ethyl)amine (Me6TREN) was prepared by following themethods of literature by Matyjaszewski et al. [52]. Allthe 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 thendistilled; N,N-dimethylformamide (DMF) was dried overanhydrous magnesium sulfate and distilled under reducedpressure.

2.2. Synthesis of 3-bromopropylheptaphenyl POSS

Heptaphenyltricycloheptasiloxane trisodium silanolate[Na3O12Si7(C6H5)7] was synthesized by following themethod 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 amagnetic stirrer. After refluxed for 5 h, the reactive systemwas cooled down to room temperature and held at thistemperature with vigorous stirring for additional 15 h. Allthe solvent and other volatile compounds were removedvia 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-ylamine (1.3 ml, 8.8 mmol) were charged to a flaskequipped with a magnetic stirrer and then anhydrousTHF (250 ml) were added with vigorous stirring. The flaskwas immersed into an ice-water bath and purged withhighly 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


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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 withother volatile compounds were removed via rotary evapo-ration to obtain the white solids. The solids were washedwith 50 ml methanol for three times and dried in vacuoat 30 �C for 24 h and the product (6.997 g) was obtainedwith 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 viathe reaction between 3-bromopropylheptaphenyl POSSand 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 roomtemperature for 24 h. After that, the solution was concen-trated and dropped a great amount of deionized water toafford the precipitates. The precipitates were further driedat 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 ofanhydrous 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 dichloromethanewas 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 for24 h. After the solids were removed via filtration, the fil-trate was diluted by 50 ml of dichloromethane and washedwith 40 ml of distilled water (40 ml � 3). The organic layerwas dried over anhydrous magnesium sulfate. The filtratewas concentrated via rotary evaporator and then passedthrough a neutral aluminum oxide column with petroleumether as an eluent. All the solvents were removed by rotaryevaporation, a colorless liquid was obtained with the yieldof (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 cycloadditionreaction between 3-azidopropylheptaphenyl POSS andpropargyl 2-chloropropionate was used to prepare the ini-tiator bearing POSS. Typically, 3-azidopropylheptaphenylPOSS (1.0740 g, 1.00 mmol), propargyl 2-chloropropionate(0.1378 g, 0.95 mmol) and 10 ml tetrahydrofuran werecharged to a 25 ml round-bottom flask equipped with

magnetic stirrer. The reactive system was purged withhighly pure nitrogen for 40 min and then Cu(I)Br(14.3 mg, 0.1 mmol) and PMDETA (20.8 ll, 0.1 mmol) wereadded. 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 agreat 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 removePMDETA, the precipitates were dissolved with 10 mldichloromethane and the solution was washed with1 wt.% dilute hydrochloric acid for three times. The organicphase was dried with anhydrous magnesium and the sol-vent was removed via rotary evaporation. The solids weredried 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 bearingPOSS was carried out in the mixtures of THF and isopropylalcohol at room temperature. Typically, NIPAAm (2.767 g,26.9 mmol), the initiator (0.2440 g, 0.024 mmol), and4.0 ml isopropyl alcohol were added into a flask equippedwith a magnetic stirrer, and the trace of oxygen in the sys-tem was removed via bubbling with highly pure nitrogenfor 30 min. CuCl (2.4 mg, 0.024 mmol) and Me6TREN(6 ll, 0.024 mmol) were added. The system was degassedvia three pump–freeze–thaw cycles. The polymerizationwas carried out at room temperature for 14 h. The viscousreacted mixture was diluted with 10 ml THF and thenpassed through a column of neutral aluminum with THFas the eluent to remove the catalyst. After that, the solutionwas concentrated and dropped into a great amount of themixture of diethyl ether with petroleum ether (50/50, v/v)to afford the precipitates. The precipitates were dissolvedwith 10 ml THF and re-precipitated. This procedure was re-peated for three times. After drying in a vacuum oven atroom temperature for 24 h, the polymer was obtained withthe 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 PNIPAAmhomopolymer via ATRP was synthesized via the reactionbetwen 2-chloropropinoyl chloride and benzyl alcohol.Typically, to 100 ml flask equipped with a magnetic stirrerbenzyl 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 vigorousstirring. The reaction was carried out in an ice-water bath


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for 2 h and at room temperature for 24 h. The insolublecompounds were removed via filtration and then 200 mldichloromethane was added. The organic phase waswashed with 5% sodium bicarbonate aqueous solution(100 ml � 2) and with deionized water until neutrality.The organic phase was dried over anhydrous magnesiumsulfate. After concentrated via rotary evaporation, the solu-tion was passed through a neutral aluminum oxide columnwith petroleum ether as an eluent. After the solvents wereeliminated by rotary evaporator, a colorless liquid (i.e.,benzyl 2-choropropionate) (8.240 g) was obtained withthe 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 benzyl2-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 toa flask equipped with a magnetic stirrer and the trace ofoxygen 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) wereadded and the system was degassed via three pump–freeze–thaw cycles. The polymerization was carried outat room temperature for 48 h. The viscous mixture was di-luted with THF and passed through a column of neutralaluminum with THF as the eluent to remove the catalyst.The polymer solution was concentrated and precipitatedinto a great amount of the mixture of diethyl ether withpetroleum ether (50/50, v/v) and the procedure was re-peated three times. After drying in a vacuum oven at roomtemperature 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 inwater

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 stirringfor 2 h and then all the dispersions were dialyzed withultrapure water for 72 h, during which fresh ultrapurewater was replaced every 6 h. The micellar solution exhib-ited no macroscopic phase separation upon standing atroom 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 BruckerAvance 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 gel300 mm � 7.5 mm mixed B columns) equipped with arefractive index detector. DMF (0.01 M LiBr) was used asan eluent at the flow rate of 1.0 ml/min. The molecularweights 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 PNIPAAmon cleaned glass slides. The POSS-capped PNIPAAm sam-ples were dissolved in THF and the concentration of thesolutions were controlled to be �10 wt.%. The solventwas removed via evaporation at 40 �C for 24 h and dryingin a vacuum oven at 60 �C for 12 h. The AFM observationswere carried out on a Nanoscope IIIa scanning probemicroscope with tapping mode. Standard MikroMaschNSC11 silicon cantilevers with a resonance frequency of�330 kHz, a rounding radius of 610 nm, and a stiffnessconstant k � 48 N/m were used. Typical scan speeds duringrecording were 0.3–1.0 lines/s using scan heads with amaximum 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 thespecimens for TEM observation, a dispersion of POSS-capped PNIPAAm in water (about 10 ll) was dropped ontocarbon-coated copper grids and the water was eliminatedvia 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–visiblespectrophotometer. A thermostatically controlled cell wasemployed for variable temperature measurements at theheating rate of 0.2 �C/min. The optical transmittance ofthe 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 wasdepicted in Scheme 1. The initiator bearing POSS for theatom transfer radical polymerization of N-isopropylacryl-amide (NIPAAm) was synthesized. The starting compound

10 8 6 4 2 0

d

f

e

Si O SiO

SiO

Si O SiO

Si

O

O

O

O O

OO

Si

SiN3

a

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cd

ef

a

ab

b

TMS

Si O SiO

SiO

Si O SiO

Si

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cd

ef

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c

Chemical shift (ppm)

e

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dCDCl3

Fig. 2. 1H NMR spectra of 3-bromopropylheptaphenyl POSS and 3-azidopropylheptaphenyl POSS.

Si O SiO

SiO

Si O SiO

Si

O

O

O

O O

OO

Si

Si N NN

Ο

OCl

a

b

c

d

ef

g

h

a+e

f

hd

g

b

*

CDCl3 TMS

}


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for the POSS macromer is heptaphenyltricycloheptasilox-ane trisodium silanolate [Na3O12Si7(C6H5)7], which wasprepared via the hydrolysis, condensation and rearrange-ment of phenyltrimethoxysilane in the presence of sodiumhydroxide [53]. The corner-capping reaction betweenheptaphenyl tricycloheptasiloxane trisodium silanolateand 3-bromopropyltrichlorosilane was performed to ob-tain 3-bromopropylheptaphenyl POSS. The 3-bromopro-pylheptaphenyl POSS was used to react with sodiumazide (NaN3) to afford 3-azidopropylheptaphenyl POSS.The copper-catalyzed Huisgen 1,3-cycloaddition reactionbetween 3-azidopropylheptaphenyl POSS and propargyl2-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 resonanceat �65.37, �77.76 and �78.15 ppm are assignable to thesilicon nucleus of silsesquioxane cage as indicated inFig. 1. In terms of the ratio of integral intensity for thesesilicon resonance peaks, it is judged that the octameric sils-esquioxane was obtained. The complete substitution ofbromine atoms with azido groups was confirmed by 1HNMR spectroscopy. Shown in Fig. 2 are the 1H NMR spectraof 3-bromopropylheptaphenyl and 3-azidopropylhepta-phenyl POSS. It is seen that with the occurrence of the sub-stitution reaction, the resonance of methylene protonsconnected to bromine atom at 3.41 ppm fully shifted to3.25 ppm, suggesting that this reaction occurred to com-pletion. The 1H and 29Si NMR spectroscopy indicates thatthe 3-azidopropylheptaphenyl POSS was successfully ob-tained. With the above 3-azidopropylheptaphenyl POSS,the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., clickchemistry) was carried out between 3-azidopropylhepta-phenyl POSS and propargyl 2-chloropropionate to obtainthe initiator bearing POSS. The 1H NMR spectrum of theproduct is shown in Fig. 3. Apart from the signals of protonresonance assignable to propyl group connected to POSScage and aromatic rings, the peaks of resonance at 1.65,

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c+d

b

Fig. 1. 29Si NMR spectrum of 3-bromopropylheptaphenyl POSS.

9 8 7 6 5 4 3 2 1 0


c

Fig. 3. 1H NMR spectrum of the initiator bearing POSS.

4.38 and 5.24 ppm are assignable to the protons of methyl,methine and methylene group from 2-chloropropionatemoiety. According to the ratio of integral intensity of pro-tons from propyl groups to 2-chloropropionate moiety, itis judged that the initiator bearing POSS was successfullyobtained.

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). Bycontrolling the molar ratio of the initiator to NIPAAmmonomer, the POSS-capped PNIPAAm samples with vari-able lengths of PNIPAAm were obtained; the results ofpolymerization 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

Table 1Molecular weights and polydispersity of POSS-capped PNIPAAm.

Samples LPNIPAAm

(�10�3 Da)Mn (�10�3 Da)(GPC)

Mw/Mn

PNIPAAm 7.8 7.8 1.27POSS-PNIPAAm3.8K 3.8 4.86 1.33POSS-PNIPAAm4.7K 4.7 5.71 1.35POSS-PNIPAAm6.2K 6.2 7.22 1.34POSS-PNIPAAm15.8K 15.8 16.89 1.42POSS-PNIPAAm18.4K 18.4 19.47 1.46

10 9 8 7 6 5 4 3 2 1 0

Si O SiO

SiO

Si O SiO

Si

O

O

O

O O

OO

Si

Si N NN

O

OCl

HN

On

b

c

d

e f

gh

i

j

a

k


a+e

j

h

k

CDCl3

TMS

gf+i{

b*

Fig. 4. 1H NMR spectrum of POSS-PNIPAAm4.7 K. �: the signal isassignable to the proton resonance of a trace of tris(2-(dimethyl-amino)ethyl)amine which persistently existed after three dissolution-precipitation circles.

8 10 12 14 16 18 20

POSS-PNIPAAm3.8K

POSS-PNIPAAm4.7KPOSS-PNIPAAm6.2K

POSS-PNIPAAm15.8K

Retention time (min)

DMF

POSS-PNIPAAm18.4K

Fig. 5. GPC profiles of POSS-capped PNIPAAm samples.


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range of 7.3 � 7.9 ppm are discernable, which are ascribedto the protons of phenyl group of POSS cage. The signals ofresonance at 1.14, 1.64, 2.02 and 4.01 ppm are assigned tothe protons of methyl, methylene, methine in PNIPAAmmain chain, methine of isopropyl groups of PNIPAAm,respectively; the broad signals of resonance at 6 � 7 ppmwere 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. Allthe POSS-capped PNIPAAm samples were subjected to gelpermeation chromatography (GPC) to measure the molec-ular weights and the GPC profiles curves are presented inFig. 5 and the results of the molecular weights are listedin Table 1. In all the cases the unimodal peaks were exhib-ited, suggesting that no initiator bearing POSS was left inthe samples. The polydispersity of all the POSS-cappedPNIPAAm samples were in the range of 1.33 � 1.46. It isnoted that the polydispersity of the POSS-capped PNIPAAmwas slightly broader than the PNIPAAm homopolymer.Nonetheless, the unimodal and fairly narrow distribution

of molecular weights indicates that the initiator bearingPOSS can be successfully used to obtain the POSS-cappedPNIPAAm, i.e., the polymerization could be carried out ina living fashion.

3.2. Morphology of POSS-capped PNIPAAm

The specimens of POSS-capped PNIPAAm films wereprepared via spin-coating their solutions onto surfaces ofglass slides and the surface morphology of the POSS-capped PNIPAAm in bulks was investigated by means ofatomic force microscopy (AFM). In all the cases, the heter-ogeneous morphologies were exhibited and the morpholo-gies were quite dependent on the percentage of POSS (orthe length of PNIPAAm chains). Representatively shownin Fig. 6 are the AFM micrographs of POSS-capped PNI-PAAm samples with the shorter and longer PNIPAAmchains. In terms of the volume fraction of POSS and the dif-ference in viscoelastic properties between PNIPAAm andPOSS, 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 microdomainswith 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 whereasthe matrix is attributed to PNIPAAm phase. Careful obser-vation showed that the matrix was also heterogonous, inwhich the POSS nanophases were discernible. It is pro-posed that the spherical microdomains with the larger sizeare responsible for the POSS which were enriched onto theinterface between air and the surface of the specimens.Owing to the characteristics of low surface energy oforganosilicon moeity [54–60], the POSS component at thesurface have the tendency to coarsen to the larger sizewhereas those which were embeded in the bulks of PNI-PAAm matrix would have the smaller sizes. The similar

Fig. 6. Tapping mode atomic force microscopy images: (A) POSS-PNIPAAm4.7K and (B) POSS-PNIPAAm15.8K.

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 theaqueous solutions (0.2 g/l) at 24 �C.


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result has been found in other POSS-containing PNIPAAmcopolymers [15]. While the percentage of POSS in the or-ganic–inorganic amphiphiles was sufficiently low for thePOSS-capped PNIPAAm with logner PNIPAAm chains, thetendency for POSS microdomain to coarsen to the largersize at the surface of specimens would be significantly sup-pressed as shown in Fig. 6B for the AFM micrograph ofPOSS-PNIPAAm15.8K. It is seen that a nanophase-separated morphology was displayed. The heterogeneousmorphology 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 behaviorof POSS-capped PNIPAAm in aqueous solutions. In thiswork, dynamic light scattering (DLS) and transmissionelectron microscopy (TEM) were employed toward thisend. Shown in Fig. 7 are the plots of hydrodynamic radiusdistribution as functions of hydrodynamic radius (Rh) forthe dispersions of POSS-capped PNIPAAm in water withvariable lengths of PNIPAAm at 24 �C. In all the cases, the

intensity-averaged hydrodynamic radius (Rh) displayedunimodal distribution with the values of Rh ranged from50 to 80 nm. The values of Rh decreased with increasingthe length of PNIPAAm chains. The results of DLS indicatethat the organic–inorganic amphiphiles were self-assme-bled into nanoobjects in the aqueous solutions. The self-organzied nanoobjects could be micelle-like aggregates,in which the hydrophobic POSS cages aggregated into thecore whereas PNIPAAm chains consititue the coronas. Interms 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 valuesof Rh decreased with the length of PNIPAAm chains, indi-cating that hydrophobic/hydropholic balance can be estab-lished and modulated by controlling the lengths ofhydrophilic PNIPAAm. The morphology of the self-assem-bled nanoobjects was further investigated by transmissionelectron microscopy (TEM). The specimens for TEM mea-surement were prepared via freeze-drying approach withthe dispersion of POSS-capped PNIPAAm in water at theconcentration of 0.2 g/l at 24 �C. The spherical nanoobjectswere detected in all the cases. Representatively shown inFig. 8 is the TEM micrograph for the specimen preparedfrom POSS-PNIPAAm4.7K. It is seen that the self-assemblyof this sample in aqueous solution generated the sphericalnanoparticles with the size of ca. 50 nm. It is proposed thatthe organic–inorganic aggregates are composed of POSScores and PNIPAAm coronas. It is should be pointed outthat the sizes of aggregates measured with TEM (Fig. 8)are not necesarrily indentical with the Rh values by meansof DLS. The former were obtained in the dry state and aremuch lower than the latter obtained in solution, whichcontains the dominant contribution of the solvated coro-nas (viz. PNIPAAm).

Scheme 2. Self-assembly behavior of POSS-capped PNIPAAm in aqueoussolution.

Fig. 8. TEM micrograph of the specimen prepared from the aqueous ofsolution (0.2 g/l) for POSS-PNIPAAm4.7K via freeze-drying technique.

20 24 28 32 36 40 440

20

40

60

80

100

POSS-PNIPAAm3.8K

Tran

smitt

ance

( %

)

Temperature ( oC)

PNIPAAm



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.


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3.4. LCST behavior

It is known that PNIPAAm homopolymer is capable ofundergoing 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

PNIPAAm chains adopt a random coil conformation. Abovethe LCST, the PNIPAAm random coil would collapse into aglobule. With the occurrence of the coil-to-globule transi-tion, PNIPAAm chains would become dehydrated. In thepresent case, one bulky and hydrophobic POSS cage wasbonded to one end of a single PNIPAAm chain and thus itis 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–visspectroscopy 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 PNIPAAmsamples with various lengths of PNIPAAm as functions oftemperature. At the temperature below the LCSTs, the opti-cal transmittance of all the aqueous solutions was as highas 98%. At elevated temperatures, the optical transmittanceof the POSS-capped PNIPAAm dispersion in water was sig-nificantly decreased as that of PNIPAAm homopolymer,indicating the occurrence of the LCST phenomena. For purePNIPAAm homopolymer (Mn = 7800 with Mw/Mn = 1.27),the coil-to-globule transition occurred at 32 �C and thetransition range of temperature was as narrow as2 � 3 �C. After the coil-to- globule transition of PNIPAAmchains, the optical transmittance was decreased as low as4%. Nonetheless, the POSS-capped PNIPAAm displayedthe features quite different from PNIPAAm homopolymer.On the one hand, the LCSTs of the POSS-capped PNIPAAmwere much lower than PNIPAAm homopolymer; the LCSTsdecreased with increasing the percentage of POSS (or withdecreasing the length of PNIPAAm chains). The decreasedLCSTs for POSS-capped PNIPAAm are attributable to thepresence of the bulky and hydrophobic POSS termini. It isproposed that the introduction of hydrophobic POSS cageat the one end of PNIPAAm chain could cause the reduced


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solvation of PNIPAAm by water and thus the LCSTs weredecreased. On the other hand, the coil-to-globule transi-tional ranges for the POSS-capped PNIPAAm were signifi-cantly broadened especially for the organic–inorganicamphiphiles with the shorter PNIPAAm chains (e.g.,POSS-PNIPAAm3.8 K, 4.7 K and 6.2 K), which is in markedcontrast to PNIPAAm homopolymer. The broadened coil-to-globule collapse ranges for the POSS-capped PNIPAAmcould be associated with the self-assembly behavior ofthe organic–inorganic amphiphiles in aqueous solutions.By means of DLS and TEM, it was found that in aqueoussolutions, the organic–inorganic amphiphiles were self-assembled into the spherical nanoobjects with the size of50 � 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 ofthe hydrophobic cores composed of tens of POSS cages.The hydrophobic cores could exert significant restrictionon the conformational alteration of PNIPAAm chains atthe intimate surface of the cores due to steric hindrancewhile 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 fairlybroad 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 ofPOSS cores could remain hydrated at the temperatureabove the LCSTs. In fact, we indeed observed that the opti-cal transmittance of the aqueous solutions of POSS-cappedPNIPAAm samples were significantly higher than PNIPAAmhomopolymer after the coil-to-globule transition occurred.

4. Conclusions

The novel initiator bearing heptaphenyl POSS and 2-chloropropionate moiety was successfully synthesized viathe copper-catalyzed Huisgen 1,3-cycloaddition (i.e., clickchemistry) between 3-azidopropylheptaphenyl POSS andpropargyl 2-chloropropionate. The initiator was success-fully employed to obtain the POSS-capped PNIPAAm withvariable length of PNIPAAm via ATRP approach. ThePOSS-capped PNIPAAm amphiphiles displayed micro-phase-separated morphologies in bulk. In aqueoussolutions the POSS-capped PNIPAAm amphiphiles wereself-assembled into spherical micelle-like nanoobjects asevidenced by dynamic light scattering (DLS) and transmis-sion election microscopy (TEM). The organic–inorganic mi-celle-like nanoobjects are composed of PNIPAAm coronasand hydrophobic POSS cores in which tens of POSS cageswere aggregated. The sizes of the micelle-like nanoobjectsdecreased 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. Itis found that the LCSTs of the POSS-capped PNIPAAm sam-ples decreased with increasing the percentage of POSStermini. It is found that the hydrophobic cores of themicelle-like nanoobjects exerted the restriction on the

macromolecular conformation alteration of the coronas(viz. PNIPAAm chains) while the coil-to-globule collapseoccurred especially for the POSS-capped PNIPAAm withthe shorter PNIPAAm chains.

Acknowledgments

The financial supports from Natural Science Foundationof China (Nos. 20474038, 50873059 and 51133003) andNational Basic Research Program of China (No.2009CB930400) were gratefully acknowledged.

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