synthesis and self-assembly of tadpole-shaped organic/inorganic hybrid poly(n-isopropylacrylamide)...

Synthesis and Self-Assembly of Tadpole-Shaped Organic/Inorganic Hybrid Poly(N-isopropylacrylamide) ContainingPolyhedral Oligomeric Silsesquioxane via RAFTPolymerization

WEIAN ZHANG,1 LI LIU,2 XIAODONG ZHUANG,1 XIAOHUI LI,1 JINRUI BAI,1 YU CHEN1

1Lab for Advanced Materials, Department of Chemistry, East China University of Science and Technology,130 Meilong Road, Shanghai 200237, People’s Republic of China

2School of Pharmacy, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

Received 20 May 2008; accepted 8 August 2008DOI: 10.1002/pola.23010Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS) wasused to prepare a POSS-containing reversible addition-fragmentation transfer(RAFT) agent. The POSS-containing RAFT agent was used in the RAFT polymeriza-tion of N-isopropylacrylamide (NIPAM) to produce tadpole-shaped organic/inorganichybrid Poly(N-isopropylacrylamide) (PNIPAM). The results show that the POSS-con-taining RAFT agent was an effective chain transfer agent in the RAFT polymeriza-tion of NIPAM, and the polymerization kinetics were found to be pseudo-first-orderbehavior. The thermal properties of the organic/inorganic hybrid PNIPAM were alsocharacterized by differential scanning calorimetry. The glass transition temperature(Tg) of the tadpole-shaped inorganic/organic hybrid PNIPAM was enhanced by POSSmolecule. The self-assembly behavior of the tadpole-shaped inorganic/organic hybridPNIPAM was investigated by atomic force microscopy and dynamic light scattering.The results show the core-shell nanostructured micelles with a uniform diameter.The diameter of the micelle increases with the molecular weight of the hybrid PNI-PAM. Surprisingly, the micelle of the tadpole-shaped inorganic/organic hybrid PNI-PAM with low molecular weight has a much bigger and more compact core than thatwith high molecular weight. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem

46: 7049–7061, 2008

Keywords: polyamides; polyhedral oligomeric silsesquioxane (POSS); poly(N-isopropylacrylamide); reversible addition-fragmentation chain transfer (RAFT); self-assembly; synthesis

INTRODUCTION

Polymeric-inorganic nanocomposites with well-defined architectures have attracted much atten-

tion, because of their advantageous performancein mechanical and thermal properties.1–10 As aclass of unique inorganic agents, polyhedral oligo-meric silsesquioxanes (POSS) can be incorporatedinto polymer matrices to effectively enhance themechanical and thermal properties, oxidation re-sistance, and reduce flammability of the hybridpolymers.11–25 A typical POSS molecule, repre-sented by the formula (R8Si8O12), consists of a

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 7049–7061 (2008)VVC 2008 Wiley Periodicals, Inc.

Correspondence to: W. Zhang (E-mail: [email protected]) or Y. Chen (E-mail: [email protected])

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rigid and cubic silica core with a 0.53-nm sidelength, and the core is surrounded by eightorganic corner groups, which endow the POSSmolecule with higher reactivity and solubilityin organic solvent. POSS molecules have beenextensively incorporated into polymer matricesby chemical reaction or physical blending to pre-pare many POSS-containing hybrids with goodproperties.26–32

Recently, much attention has focused on usingPOSS molecules to construct hybrid polymerswith novel architectures.33–36 Lichtenhan et al.,described the use of POSS molecules to preparePOSS-containing homopolymers or copolymers.33

Atom transfer radical polymerization (ATRP)has been applied to the preparation of POSS-containing polymer hybrids.37 The homopolymers,triblock copolymers, and star-shaped block copoly-mers based on POSS monomers have been syn-thesized using ATRP. POSS molecules were alsomodified into ATRP initiators to prepare POSS-containing hybrid polymers with novel architec-tures such as star and tadpole-shaped hybridpolymers.38–43

Compared with ATRP and other living poly-merization techniques, reversible addition-frag-mentation chain transfer (RAFT) polymerization,as a novel living radical polymerization technique,was found later. RAFT polymerization is widelyused to design the architecture of many novelpolymeric materials with well-defined structures,since it can be applied to a wide range of mono-mers containing carboxyl, amino and ionic groupsand used under a broad range of experimentalconditions including aqueous solutions. In RAFTpolymerization, the molecular weight can be effec-tively controlled by a dynamic equilibriumbetween the propagating radical and the RAFTterminated chain transfer agents (CTAs).44–47

Poly(N-isopropylacrylamide) (PNIPAM) hasalso been intensely studied. Since it possesses aphase transition at the critical solution tempera-ture (LCST) in water of 32 �C, it may offer drugdelivery.48,49 Among the several living radicalpolymerization methods, RAFT polymerizationoffers a very effective method of controlling thepolymerization of NIPAM.50–54

Here, we combined RAFT polymerization withPOSS molecules to prepare novel POSS-contain-ing hybrid polymers. The POSS molecule wasmodified into POSS-containing RAFT agent,which was further used to produce tadpole-shapedorganic/inorganic hybrid PNIPAMs. The thermalproperties of the POSS-containing organic/inor-

ganic hybrid PNIPAMs were characterized by dif-ferential scanning calorimetry (DSC). The self-as-sembly behavior of the resulting novel amphi-philic organic/inorganic hybrid polymer was alsostudied.

EXPERIMENTAL

Materials

Aminoisobutyl POSS was purchased from HybridPlastics Company. Benzyl bromide and 3-mercap-topropionic were purchased from Aldrich andused without further purification. Other reagentsand solutions in analytical grade were obtainedfrom Shanghai Chemical Reagents Company. Thi-onyl chloride and carbon disulfide were distilledbefore use. Tetrahydrofuran (THF) was distilledfrom a purple sodium ketyl solution. Azobisisobu-tyronitrile (AIBN) was recrystallized from etha-nol. Dichloromethane was dried over CaH2 anddistilled under nitrogen before use. N-Isopropyla-crylamide (NIPAM) was prepared according to theliterature method and recrystallized from hexanethree times.55

Synthesis of 3-Benzylsulfanylthiocarbonyl-sufanylpropionic Acid I (BSPA, RAFT Acid)

3-Benzylsulfanylthiocarbonylsufanylpropionic acidI (BSPA) was synthesized according to the reportof Stenzel et al.56 Mercapto propionic acid(10 mL, 0.12 mol) was slowly added into a solu-tion of potassium hydroxide (13.0 g, 0.23 mol) in125 mL of water. And carbon disulfide (15 mL)was added dropwise into the solution for morethan 30 min with vigorous stirring, and the solu-tion was further stirred for 8 h. After that, benzylbromide (18.8 g, 0.12 mol) was added into the so-lution, and then the reaction mixture was heatedat reflux for 12 h. After the reaction was cooledto room temperature, 150 mL of chloroform wasadded. The reaction mixture was acidified withconcentrated hydrochloric acid until the color ofthe organic layer became yellow. The waterphase was extracted with 100 mL of chloroformtwice. The combined organiclayers were washedwith 100 mL of saturated sodium chloride andthen with 100 mL of water twice and finally driedover anhydrous magnesium sulfate overnight.After evaporation of the solvent, the remainingproduct was crystallized in dichloromethanethree times, and 25 g of yellow solid with a yieldof 80% was obtained.

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Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

1H NMR (CDCl3, ppm): 7.34–7.25 (m, 5H,APh), 4.61 (s, 2H, ACH2Ph), 3.63 (t, 2H,ASCH2CH2COOH), 2.84 (t, 2H, ASCH2CH2

COOH).

Synthesis of 3-Benzylsulfanylthiocarbonyl-sufanylpropionic Chloride

BSPA (5.0 g, 0.18 mol) and 15 mL of anhydrousdichloromethane were introduced into a dried50-mL schlenk flask with a magnetic stirringbar. This schlenk flask was connected to astandard Schlenkline system and was degassedand then refilled with highly pure nitrogen.Freshly distilled thionyl chloride (5 mL) wasadded dropwise into the BSPA mixture solutionover 30 mins. And then the reaction mixturewas heated at reflux for about 1 h. The solventwas removed by first by distilling at atmos-pheric pressure and then under vacuo (10�3

Torr), and a yellow and highly viscous liquidwas obtained, that was further dried under ahigh vacuum (10�3 Torr) overnight at room tem-perature. The 3-benzylsulfanylthiocarbonylsufa-nylpropionic chloride (BSPCl) was not charac-terized before use in the next step.

3-Benzylsulfanylthiocarbonylsufanyl-N-(3-(isobutylpolyhedral oligomeric silsesquioxane)propyl)propanamide (POSS-BSAP, POSS-Containing RAFTAgent)

Aminoisobutyl-POSS (2.5 g, 2.86 mmol) was dis-solved in 15 mL of absolute THF, and 0.5 mL ofdistilled pyridine was added. A solution of freshlyprepared BSPCl (1.17 g, 4.28 mmol) in 10 mL ofabsolute THF was added slowly into the abovePOSS solution at 0 �C. After stirring for 12 h atroom temperature, the mixture was filtered, andthe filtrate was concentrated under reduced pres-sure by heating evaporation. The residue waspurified by a silica gel chromatography with ethylacetate/petroleum ether (1:4, v/v) as the eluent. Ayellow solid was obtained (2.74 g, yield 85%).

1H NMR (CDCl3, ppm): 7.27–7.34 (m, 5H,APh), 4.61(s, 2H, ACH2Ph), 3.67 (t, 2H, ASiACH2

CH2CH2NHA), 3.25 (q, 2H, AHNCOCH2CH2SA),2.60 (t, 2H, AHNCOCH2CH2SA), 1.85 (m, 7H,ASiACH2CH(CH3)2), 1.60 (m, 2H, ASiACH2CH2

CH2NHA), 0.951 (d, 42H, ASiACH2CH(CH3)2),0.599 (q, 16H, ASiACH2CH(CH3)2, ASiACH2

CH(CH3)2).

Polymerization

A typical polymerization procedure for the synthe-sis of POSS-containing inorganic/organic hybridPNIPAM as follows. A 10-mL glass flask contain-ing a stir bar was charged with NIPAM (0.5 g,4.42 mmol), POSS-containing RAFT agent(25.0 mg, 0.022 mmol), AIBN (1.2 mg, 0.0074mmol), and 1.0 mL of 1,4-dioxane. The glass flaskwas connected to a standard Schlenkline systemwith highly pure nitrogen. The solution wasdegassed by three freeze-evacuate-thaw circles,and then the glass tube was sealed under a vac-uum. The polymerization was carried out in athermostated oil bath at 65 �C for 6 h. The poly-merization was stopped by plunging the tube intoice water. The polymerization tube was openedand 3 mL of THF was added and then precipitatedinto 250 mL of diethyl ether. The precipitate wasfiltered, and the product was dried at 50 �C in avacuum oven for 24 h to give a yield of 24.6%.

1H NMR (CDCl3) ppm, 4.02 (s, ANHCH(CH3)2),1.4–2.7 (m, polymer backbone protons, ASiACH2CH(CH3)2), 1.14 (s, ANHCH(CH3)2), 0.96 (d,ASiACH2CH(CH3)2)), 0.60 (d, ASiACH2CH(CH3)2, ASiACH2CH(CH3)2). Mn(GPC) ¼ 9, 558,Mw/Mn ¼ 1.13.

Characterization

Nuclear Magnetic Resonance Spectroscopy

The 1H NMR measurements were carried out on aVarian Mercury Plus 400 MHz nuclear magneticresonance (NMR) spectrometer at 25 �C. The sam-ples were dissolved with deuterated CDCl3(10 mg/mL), and the solutions were measuredwith tetramethylsilane as an internal reference.

Fourier Transform Infrared Spectroscopy

The Fourier transform infrared spectroscopy(FTIR) measurements were conducted on a Nico-let Nagma-IR 550 Fourier transform spectrometerat room temperature (25 �C). The samples mixedwith KBr (about 0.5 wt %) were granulated intopowder. The mixture powder were put intobetween two stainless steel disks out of the desic-cator and pressed into flakes for IR measurementsby a hydraulic pump.

Gel Permeation Chromatography

The molecular weights and molecular weightdistribution were measured on Waters 150C gel

POSS-CONTAINING RAFT AGENT 7051


permeation chromatography (GPC) equipped withUltrastyragel columns of 100, 10,000 A porosities.A series of monodisperse polystyrene (PS) stand-ards were used for calibration, and THF was usedas the eluent at a flow rate of 1 mL/min.

Differential Scanning Calorimetry

The calorimetric measurements were performedon a Perkin–Elmer Diamond differential scanningcalorimeter in a dry nitrogen atmosphere. Theinstrument was calibrated using a standard In-dium. The samples (about 8 mg) were first heatedup to 180 �C at 20 �C/min and held at this temper-ature for 5 min to remove any thermal history, fol-lowed by quenching to 30 �C. Second heatingscans were carried out from 30 to 160 �C at 20 �C/min and the thermograms were recorded.

Atomic Force Microscopy

Atomic force microscopy (AFM) images wereobtained using Tapping Mode on a Nanoscope IVof Digital Instruments equipped with a siliconcantilever 125 lm and E-type vertical engage pie-zoelectric scanner. The AFM samples were pre-pared by spin-coating the samples on the freshlycleaved mica and then dried naturally at roomtemperature for at least 24 h.

Dynamic Light Scattering

Dynamic light scattering (DLS) was used to inves-tigate the micellar behavior using a MalvernNano_S instrument (Malvern, U.K.). In all cases1 mg/mL micellar solution were analyzed at ascattering angle of 90� and at 25 �C. All the mea-surements were repeated three times and aver-aged value were used to determine the meandiameter (standard deviation).

RESULTS AND DISCUSSION

Synthesis of POSS-Containing RAFT Agent

In RAFT polymerization, the RAFT agent is themost crucial factor, and the transfer capability ofthe CTA determines the process of the controllingpolymerization and the molecular weight disper-sion. Nevertheless, for RAFT polymerization ofNIPAM, previous reports demonstrated thatdithioester and trithioester were very effective inthe polymerization process.53,57–60 Even for thetrithioester, RAFT polymerization can still be well

controlled in aqueous solution at ambient temper-ature.51 However, a dithioester is more effectivethan trithioester in many RAFT systems.44–47,61–63

In our study, we chose trithioester BSPA as theprecursor to prepare POSS-containing CTA.BSPA has been shown to be an effective CTA inmany RAFT polymerization systems. Since itcontains a carbonyl group, it can be organicallymodified to prepare novel CTAs.64,65 For exam-ple, BSPA was used to modify hyperbranchedpolyesters to prepare a hyperbranched polymercontaining a hyperbranched polyester core.56 Inthis work, aminopropylisobutyl POSS was modi-fied to a RAFT agent by BSPA via the reactionbetween the POSS amino group and BSPCl. ThePOSS-containing RAFT agent was then used toprepare the POSS-containing inorganic/organichybrid PNIPAMs (Scheme 1). Figure 1 shows thetypical 1H NMR spectrum of the POSS-contain-ing RAFT agent. The signals at d ¼ 0.951 ppm(a), 1.85 ppm (b), 0.599 ppm (c, d), 1.60 ppm (e)and 3.67 ppm (f) are respectively, assigned to themethyl protons, methine protons, and the pro-tons of the methylene of POSS moiety. The sig-nals at d ¼ 2.60 ppm (g), 3.25 ppm (h), 4.61 ppm(i) and 7.27–7.34 ppm (j) are respectively,ascribed to the resonance of the protons of themethylene (AHNCOCH2CH2SA, AHNCOCH2

CH2SA, ASCH2C6H5) and the aromatic protons.Compared with the 1H NMR spectrum of BSPA,the methylene protons peak (g) shifts from 2.84ppm in BSPA to 2.60 ppm in the POSS-contain-ing CTA. In addition, FTIR was also used to char-acterize the formation of POSS-containing CTA(Fig. 2). Compared with the spectrum for POSS-NH2, a new band appears at 1643 cm�1 in thespectrum of POSS-containing RAFT agent, whichis assigned to the stretching vibration of C¼¼O.Nevertheless, it is unfortunate that the stretch-ing vibration of C¼¼S could not be discerned inthe spectrum of POSS-containing RAFT agent,since the band overlaps with the stretchingvibration of SiAOASi. Based on the results ofNMR and IR, we conclude the POSS-containingRAFT agent was successfully prepared.

RAFT Polymerization of NIPAM UsingPOSS-Containing RAFT Agent

All polymerizations of NIPAM were carried out in1,4-dioxane at 65 �C with AIBN as the initiator.The results are listed in Table 1. A kinetic plot ofthe polymerization rate for NIPAM is shown inFigure 3. It can been seen that a linear

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relationship exists between ln(1/(1 � x)) and reac-tion time, which indicates the concentration ofchain radicals is constant in the RAFT polymer-ization of NIPAM using the POSS-containingRAFT agent. Thus, the polymerization exhibitspseudo-first-order kinetics. Nevertheless, the ki-netic straight line does not begin from the origin

of coordinate in the plot, which reveals an induc-tion period for the RAFT polymerization ofNIPAM. A similar induction period was beenobserved by other groups.60,66,67 The occurrence ofan induction period may be due to slow initiationor fragmentation of the leaving group at the earlystage of RAFT polymerization. Figure 4 shows the

Scheme 1. Synthesis of POSS-containing RAFT agent and RAFT polymerization ofisopropylacrylamide using POSS-containing RAFT agent. [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com.]



relationship of number-average molecular weightand polydispersity of PNIPAM with NIPAM con-version. It was observed that the molecularweight linearly increase of NIPAM conversion.However, the experimental molecular weightmeasured by GPC is in disagreement with thetheoretical molecular weight. The theoretical mo-lecular weight was calculated according to theequation:

MnðthÞ ¼ ½M�=½RAFT� �MNIPAM � xþMPOSS-CTA

where Mn(th) is the theoretical molecular weightof the POSS-containing hybrids; [M] and [RAFT]are respectively, the initial concentrations of theNIPAM and the POSS-containing RAFT agent;MNIPAM is the molecular weight of NIPAM;MPOSS-CTA is molecular weight of POSS-contain-ing RAFT agent; and x is the conversion ofNIPAM. Similar differences between experimen-tal molecular weight and theoretical molecularweight were reported by several research groups,where they conducted some detail experimentand highlighted some problems about the molecu-lar weight of PNIPAM measured by GPC.51,68

Figure 1. 1H NMR spectrum of POSS-containing RAFT agent.

Figure 2. FTIR spectra of aminopropylisobutylPOSS, POSS-containing RAFT agent and POSS-con-taining inorganic/organic hybrid poly(N-isopropylacry-lamide).

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Since GPC is used to evaluate the molecularweight of polymers relative to PS standards, it isassured that the hydrodynamic volume of PNI-PAM in THF is different than that of PS, whichresults in some discrepancy between the molecu-lar weight of PNIPAM measured by GPC and the-oretical molecular weights. In addition, there areoften some aggregations of PNIPAM occurringduring GPC measurement, especially for the PNI-PAM of higher molecular weight, because of traceamounts of water in the elution solvents, whichwill result in an increase of GPC measured molec-ular weights. To avoid such aggregations of PNI-PAM chains, rigorous anhydrous GPC conditionsare required.

The narrow molecular weight distributions(almost all below 1.15) are provided in Table 1

and Figure 5. The GPC traces of the POSS-con-taining hybrids are highly symmetric, and nar-rowly dispersed. The GPC and molecular weightare typical of a living/controlled as a RAFT poly-merization of NIPAM. Further these results alsoreveal that POSS-containing CTA is very effectiveas a RAFT polymerization agent for NIPAM.

The 1H NMR spectrum of POSSPNIPAM6 isshown in Figure 6. The characteristic signals at d¼ 4.00 ppm (a) and 1.14 pm (c), ascribed to themethane (ACONHCH(CH3)2) and methyl protons(ACONHCH(CH3)2) of PNIPAM, respectively, arereadily identified in addition to the proton signalsof POSS-containing CTA (The signals at d ¼ 0.95ppm (a) and 0.60 ppm (c) are respectively,assigned to the protons of methyl protons(ASiACH2CH(CH3)2) and methylene protons

Table 1. Results and Conditions of the Reversible Addition-Fragmentation Chain Transfer (RAFT)Polymerization of Isopropylacrylamide Using POSS-Containing RAFT Agenta

Sample Polymerization Time (h) Mn(GPC)b Mn(th)c Mw/Mn

d Conversion (%)

POSSPNIPAM6 6 9,558 6,683 1.13 24.6POSSPNIPAM12 12 16,100 16,754 1.07 69.2POSSPNIPAM18 18 18,290 20,276 1.13 84.8POSSPNIPAM24 24 18,484 22,625 1.15 95.2BSPAPNIPAM 12 17,752 18,900 1.16 82.5

aPolymerization conditions: [NIPAM]/[RFAT agent] ¼ 200, [RFAT agent]/[AIBN] ¼ 3, in 1,4-dioxane at 65 �C.bCalculated based on the GPC method, in which narrow PDI polystyrene standards were used in the calculation.c Calculated from the following equation: Mn(th) ¼ [M]/[RAFT] � MNIPAM � x þ MCTA, [M] and [RAFT] are respectively the

initial concentrations of the NIPAM and RAFT agent; MNIPAM is the molecular weight of NIPAM; MCTA is molecular weight ofRAFT agent; and x is the conversion.

dEvaluated from GPC in THF with polystyrene standards.

Figure 3. Pesudofirst-order kinetic plot of the prep-aration of POSS-containing inorganic/organic hybridpoly(N-isopropylacrylamide) at 65 �C in 1,4-dioxanein the presence of POSS-containing RAFT agent.

Figure 4. Evolution of the number-average molecu-lar weight and polydispersity with conversion forthe RAFT polymerization of N-isopropylacrylamide at65 �C in 1,4-dioxane in the presence of POSS-containing RAFT Agent.



(ASiACH2CH(CH3)2)). To more accurately calcu-late the molecular weight measured by GPC, wehave estimated the molecular weight using 1HNMR spectrum. The number average molecularweight of POSSPNIPAM was calculated accordingto the following equation:

MnðNMRÞ ¼ ½I4:00=ðI0:60=16Þ� �MNIPAM

þMPOSS-CTA

where I4.00 and I0.60 are respectively, therelative integrations of the methane protons(ACONHCH(CH3)2) of PNIPAM and methyleneprotons (ASiACH2CH(CH3)2) of POSS-contain-ing CTA. The calculated results show Mn(NMR)of POSSPNIPAM6 is 7246, which is very close tothe theoretical molecular weight. Unfortunately,NMR could not be used to calculate the molecu-lar weight of POSS-containing hybrid PNIPAMwith much higher molecular weight, since themethylene protons signal (ASiACH2CH(CH3)2)of POSS-containing CTA is difficult to be identi-fied because of overlapping resonances. Not

Figure 5. Evolution of GPC chromatograms forthe RAFT polymerization of N-isopropylacrylamide at65 �C in 1,4-dioxane in the presence of POSS-contain-ing RAFT agent.

Figure 6. 1H NMR spectrum of the POSS-containing inorganic/organic hybridpoly(N-isopropylacrylamide) (POSSPNIPAM6) (Table 1, entry 1).

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surprisingly the stretching vibration of SiAOASiband at 1114 cm�1 becomes lower in the IR spec-trum of POSS-containing PNIPAM, because ofthe low content of the SiAOASi group in POSS-containing PNIPAM (Fig. 2). The IR findings areconsistent with an increase of the molecularweight relative to POSS content.

Thermal Properties of POSS-ContainingOrganic/Inorganic Hybrid PNIPAM

The POSS-containing hybrids were subject toDSC. To evaluate the thermal properties of thePOSS-containing organic/inorganic hybrid PNI-PAM, the control sample without POSS, BSPAP-NIPAM, was synthesized by using the BSPA asthe RAFT agent. Figure 7 shows the DSC curvesof the BSPAPNIPAM and POSS-containing or-ganic/inorganic hybrid PNIPAM. The BSPAPNI-PAM has a glass transition temperature (Tg) atabout 114 �C. However, the POSSPNIPAM6 (withabout 5 wt % POSS) displays its Tg at 107 �C,which is obviously lower than that of BSPAPNI-PAM. With the increase of molecular weight ofhybrid PNIPAM, corresponding to the decrease ofthe amount of the POSS in the hybrid PNIPAM,POSSPNIPAM12 and POSSPNIPAM18 show ahigher Tg than POSSPNIPAM6. As the POSS con-tent decreases below that of POSSPNIPAM18, theTg of POSSPNIPAM24 does not further increase,but slightly decreases. Comparing BSPAPNIPAM

with POSSPNIPAM12, we find that the Tg ofPOSSPNIPAM12 is slightly higher than that ofBSPAPNIPAM. This indicates Tg of PNIPAM isenhanced by POSS molecule, because the POSSmolecule has organic core and connects with PNI-PAM chain by chemical bond, which can preventthe movement of PNIPAM chain. For POSSPNI-PAM24, the molecular weight is sufficient todilute. The effect of POSS and a Tg value and en-thalpy nearly equal to that of BSPAPNIPAM areobserved.

Self-Assembly Behavior of POSS-ContainingOrganic/Inorganic Hybrid PNIPAM

PNIPAM is a typical smart polymer, which has aphase transition in water at the LCST of 32 �C.PNIPAM can be dissolved in water because of theintramolecular hydrogen bonding. At tempera-tures below the LCST, the intramolecular hydro-gen bonding is predominant, and the PNIPAMchains are well soluble in water, but as the tem-perature increases beyond its LCST, the intramo-lecular hydrogen bonding become sharply weaker,which leads to partial solubility of PNIPAMchains and eventual insolubility. Based on thisunique property of PNIPAM, it has been widelyused to study the physical nature of the polymerin solution, and can potentially be applied as afunctional material for drug delivery. Recently,PNIPAM block copolymers were widely investi-gated, especially in the influence of temperatureon their self-assembly behavior. For example,poly(N-isopropylacrylamide)-b-poly(ethylene ox-ide) (PNIPAM-b-PEO) can form a core-shell nano-structured micelle with a PNIPAM core and PEOshell at temperatures above the LCST.69 We havesynthesized polystyrene-b-poly(N-isopropylacryla-mide) (PS-b-PNIPAM) diblock copolymers, andinvestigated the conformational changes of PNI-PAM blocks in the coronas of micelles and vesiclesby a combination of static and dynamic laser lightscattering.70 In the current work, the tadpole-shaped POSSPNIPAM can be regarded as amphi-philic block copolymers with an inorganic/organichybrid structure, since the POSS molecule ishydrophobic and PNIPAM chains is hydrophilicat the temperature below LCST. Therefore, thetadpole-shaped inorganic/organic hybrid copoly-mers should also self-assemble into a core-shellnanostructure (Scheme 2).

To investigate this prospect, the tadpole-shapedinorganic/organic hybrid copolymers (POSSPNI-PAM) were investigated by AFM, which can

Figure 7. The heating DSC curves of the controlPNIPAM (a: BSPAPNIPAM) and POSS-containinginorganic/organic hybrid poly(N-isopropylacrylamide)s(b: POSSPNIPAM6, c: POSSPNIPAM12, d: POSSPNI-PAM18, e: POSSPNIPAM24).



Scheme 2. The self-assembly process of POSS-containing inorganic/organic hybridpoly(isopropylacrylamide).

Figure 8. Tapping-mode AFM images of POSS-containing inorganic/organic hybridpoly(N-isopropylacrylamide)s (a: POSSPNIPAM6 chloroform solution (1 lm � 1 lm);b: POSSPNIPAM6 aqueous solution (1 lm � 1 lm); c: POSSPNIPAM24 aqueous solu-tion (1 lm � 1 lm); d: POSSPNIPAM6 aqueous solution (500 nm � 500 nm)). Thesamples were prepared by spin-coating the 1 mg/1 mL dilute solution onto freshlycleaved mica and allowed to dry in air.

directly revealed the self-assembly morphology.Samples for AFM were respectively, prepared inchloroform and water diluted solutions (1 mg/mL), and then spin-coated on mica. We had hopedto obtain a unique morphology for the tadpole-shaped inorganic/organic hybrid PNIPAM usingchloroform, since the POSS molecule and PNI-PAM chains are all soluble in chloroform. As seenfrom Figure 8(a), the AFM image of the sampleprepared from POSSPNIPAM6 in chloroform so-lution is flat, and not revealing of any obviousphase discrepancy. However, in water the tadpole-shaped inorganic/organic hybrid PNIPAM inwater can self-assemble into micelle [Fig. 8(b–d)].Figure 8(b) shows an AFM image of the core-shellnanostructured micelle of the POSSPNIPAM6with POSS molecule as core and PNIPAM asshell. Since POSSPNIPAM6 has the shortest tail(the length of PNIPAM chain) in the four samplesmentioned above, we hope it can be assemble intothe micelle with much bigger core. It can be seenthat some micelles connect each other, which isdue to the higher micelle concentration, but themicelles are relatively uniform in diameter, about15 nm. In addition, the height of the micelles isonly about 4 nm, which is significantly less thanthe micellar diameter.

The AFM image of the core-shell nanostruc-tured micelle of the POSSPNIPAM24 was shownin Figure 8(c). Compared with the AFM image ofPOSSPNIPAM6, it can be found the core of themicelle becomes small and dim, which means thenumber of POSS molecules in core is less thanthat of POSSPNIPAM6. Nevertheless, the size ofthe micelle (about 17 nm) is not smaller than thatof POSSPNIPAM6, since the length of PNIPAM inPOSSPNIPAM24 is much longer than that ofPOSSPNIPAM6. From its magnified image [Fig.8(d)], the height of the micelles is about 2 nm,which is much less than that of POSSPNIPAM6.

The DLS result was shown in Figure 9. It canbe seen that the hydrodynamic diameter (Dh) ofmicelle in aqueous solution at 25 �C increaseswith the molecular weight of hybrid PNIPAMincreased, which is attributed to the contributionof PNIPAM coronas. The Dh measured by DLS inaqueous solution at 25 �C is much bigger thanthat measured by AFM, since DLS data directlyreflects the dimension of micelles in solution,where the PNIPAM chains are well dispersed inwater, although one side of the PNIPAM chain isattached to the core of micelle. However, for AFMmeasurement, the micellar solution was spin-coated on the mica surface, where PNIPAM

sharply shrinks at the evaporation of water,which results in the smaller Dh measured byAFM. In addition, the theoretical diameter of themicelle could be calculated according to the molec-ular weight of POSSPNIPAM. The calculatedresult shows the theoretical diameter of POS-SPNIPAM6 micelle is 40 nm, (per repeat unit ofPNIPAM is about 0.25 nm,71 and the diameter ofPOSS molecule is 1.5 nm), which is much largerthan that measured by DLS and AFM. We alsofound the micellar behavior was influenced bytemperature, which is similar with that of amphi-philic block polymers containing PNIPAM. Thedetailed work of the conformational changes ofPNIPAM blocks in the coronas of micelles studiedby a combination of static and dynamic laser lightscattering will be discussed in the next article.

CONCLUSIONS

A POSS-containing RAFT CTA was successfullyprepared using aminopropylisobutyl POSS. ThePOSS-containing RAFT agent was further appliedin the RAFT polymerization of NIPAM to producetadpole-shaped organic/inorganic hybrid PNI-PAM. POSS-containing RAFT agent was con-firmed to be an effective CTA in the polymeriza-tion of NIPAM, and the polymerization kinetics ispseudo-first-order behavior. The thermal proper-ties of the organic/inorganic hybrid PNIPAM werecharacterized by DSC. The DSC result showedthe glass transition temperature of the tadpole-shaped inorganic/organic hybrid PNIPAM isenhanced by the POSS molecule, until sufficientmolecular weight is achieved to dilute the effect ofPOSS. The self-assembly behavior of the tadpole-shaped inorganic/organic hybrid PNIPAM wasinvestigated by AFM and DLS. The result showedthe core-shell nanostructured micelle is well-

Figure 9. Typical hydrodynamic diameter distribu-tions for the micelles prepared from POSS-containinginorganic/organic hybrid poly(N-isopropylacrylamide)in aqueous solution at 25 �C.



uniform in their diameter, and the diameter of mi-celle of tadpole-shaped inorganic/organic hybridPNIPAM increases with molecular weight. Never-theless, the micelle of tadpole-shaped inorganic/organic hybrid PNIPAM with low molecularweight has a much bigger and more compact corethan those of higher molecular weight.

This work was supported by the National Natural Sci-ence Foundation of China (No. 50503013) and YouthFoundation of East China University of Science andTechnology. W. Zhang also acknowledges the AlexandervonHumboldt Foundation for the support.

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