Star-shaped inorganic–organic hybrid polymers with polyhedraloligomeric silsesquioxane core: Synthesis, self-assemblyand tunable thermoresponse

Weizhong Yuan a,b,n, Tianxiang Shen a, Xu Liu a, Jie Ren a,b

a Institute of Nano and Bio-polymeric Materials, School of Materials Science and Engineering, Tongji University, Shanghai 201804, People's Republic of Chinab Key Laboratory of Advanced Civil Materials, Ministry of Education, Shanghai 201804, People's Republic of China

a r t i c l e i n f o

Article history:Received 27 June 2013Accepted 13 August 2013Available online 22 August 2013

Keywords:Inorganic–organic hybridPolymersFunctionalTunable thermoresponseNanocomposites

a b s t r a c t

A series of novel star-shaped hybrid P(2-(2-methoxyethoxy)ethylmethacrylate)-co-oligo(ethylene glycol)methacrylate (P(MEO2MA-co-OEGMA)) polymers with polyhedral oligomeric silsesquioxane (POSS) core(POSS-(P(MEO2MA-co-OEGMA))8) were synthesized via atom transfer radical polymerization (ATRP).The obtained inorganic–organic hybrid polymers can self-assemble into micelles in aqueous solutionowing to the amphiphilic property resulting from the hydrophobic inorganic POSS core and thehydrophilic P(MEO2MA-co-OEGMA) segments. The hybrid polymeric micelles show a significant tunabletemperature responsive property which can be adjusted from 29.7 to 39.1 1C through altering the ratio ofMEO2MA and OEGMA in P(MEO2MA-co-OEGMA) polymers. These amphiphilic hybrid polymers havepotential applications in nano-carrier, nano-reactor, smart materials and biomedical fields.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

In the past decades, hybrid materials have drawn considerableinterest from chemists and engineers all over the world for thefantastic properties related to the combination of both inorganicand organic building blocks via blending or covalent bonding [1].Polyhedral oligomeric silsesquioxane (POSS) as a important classof cage-like nanostructured inorganic material with a formula of(RSiO1.5)n (n¼6, 8, 12, etc.) has been intensively investigatedbecause of their remarkable properties, such as well-defined threedimensional structure in nanoscale, biocompativity, oxidationresistance, and reduction in flammability [2]. Especially, the Rgroups chemically linked to every corner of a cubic POSS moleculecould be modified to gain numerous functions, which play a keyrole for the hybrid materials to gain excellent properties [3].

“Living”/controlled polymerization methods have been intro-duced as high efficiency ways for the modification of POSS to afforda wide range of amphiphilic hybrid polymers containing POSS. Forexample, reversible addition-fragmentation chain-transfer polymer-ization (RFAT) was used to synthesize the tadpole-like POSS-poly(acrylic acid) (POSS-PAA) [4] and POSS-poly(N-isopropylacrylamide)(POSS-PNIPAM) [5]. Moreover, atom transfer radical polymerization

(ATRP) was also used to synthesize POSS-containing amphiphiliccopolymer, such as star-shaped POSS-poly(2-(N,N-dimethylamino)ethyl methacrylate) (POSS-PDMAEMA) hybrid polymer [6].

Nowadays, considerable interest has been attracted to thermo-responsive polymers for their potential application in bioactivesurfaces, nano-reactor, phase separation immunoassays or drugcarrier [7]. The poly(ethylene glycol)-based random copolymers of2-(2-methoxyethoxy)ethylmethacrylate (MEO2MA) and oligo(ethy-lene glycol) (OEGMA) (P(MEO2MA-co-OEGMA)) show a tunablethermoresponse via varying the molar ratio of MEO2MA and OEGMA[8]. The critical phase transition temperature of P(MEO2MA-co-OEGMA) polymers could be adjusted in a range from 26 to 90 1C[9]. In addition, P(MEO2MA-co-OEGMA) is regarded as nontoxic,nonimmunogenic and biocompatible materials, which indicates thatthey have the potential application in biomedical fields [10].

The work presented here demonstrates the facile synthesisof star-shaped inorganic–organic hybrid polymers POSS-(P(MEO2MA-co-OEGMA))8 by ATRP of MEO2MA and OEGMA withoctafunctional POSS-(Cl)8 as the initiator (Fig. 1). The self-assemblybehavior and tunable thermoresponse of these hybrid polymerswere investigated by transmittance measurement, dynamic lightscattering (DLS) and transmission electron microscopy (TEM).

2. Experimental

Materials: MEO2MA and OEGMA (Mn¼475) (Aldrich) werepassed through a column of activated basic alumina to remove

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n Corresponding author at: Institute of Nano and Bio-polymeric Materials, Schoolof Materials Science and Engineering, Tongji University, Shanghai 201804, People'sRepublic of China. Tel.: þ86 21 69580234; fax: þ86 21 69584723.

E-mail address: yuanwz@tongji.edu.cn (W. Yuan).

Materials Letters 111 (2013) 9–12

the inhibitors. CuCl was purified by washing in acetic acid andethanol sequentially, and then dried in vacuo before use. Dichlor-omethane, N,N-dimethylformamide (DMF) were dried over CaH2.N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), metha-nol, 3-chloropropyltrimethoxysilane, and tetrahydrofuran (THF)were purchased from Acros Organic and used as received.

Characterization: ATR FT-IR spectra of the samples were recordedon an AVATAR 360 ESP FT-IR spectrometer equipped with a singlereflection ATR system. 1H NMR data were obtained by a BrukerDMX-500 NMR spectrometer with CDCl3 as solvent. The molecularweight and molecular weight distribution of copolymers weremeasured on a Viscotek TDA 302 gel permeation chromatographand THF was used as eluent. The transmittances of copolymersaqueous solutions at various temperatures were measured at awavelength of 500 nm on a UV–visible spectrophotometer. TheLCST value of the copolymer solution was defined as the tempera-ture producing a 50% decrease in transmittance. The hydrodynamicdiameters (Dh) of the copolymers micelles were determined using adynamic light scattering spectrophotometer (DLS). Samples fortransmission electron microscopy (TEM) images were taken on anH-600 transmission electron microscope (Hitachi, Japan) operatingat 120 kV.

Synthesis of POSS-(Cl)8 and POSS-(P(MEO2MA-co-OEGMA))8: Thesynthesis of octafunctional POSS-(Cl)8 was according to the litera-ture [11]. Methanol, 3-chloropropyltrimethoxysilane and concen-trated HCl were mixed and stirred for 40 days. POSS-(P(MEO2MA-co-OEGMA))8 copolymers were synthesized by ATRP of MEO2MAand OEGMA with POSS-(Cl)8 as the initiator. CuCl/PMDETA wasused as the catalyst system. (The experimental details weredescribed in Supporting Information)

Preparation of self-assembled micelles: The samples for trans-mittance, DLS and TEM were prepared as follows. POSS-(P(MEO2MA- co-OEGMA))8 (40 mg) was dissolved in THF (5 mL)and subsequently dialyzed (molecular weight cut-off: 3500 Da)against distilled water for 72 h. During this dialysis process, thehybridized copolymers self-assembled into micelles with POSScore and P(MEO2MA-co-OEGMA) corona. Micelles solutions withdifferent concentrations can be obtained by diluting with distilledwater and equilibrating at room temperature for 48 h. The result-ing micelles solution had a concentration of 2 mg/mL for trans-mittance measurement and 1 mg/mL for DLS and TEM.

3. Results and discussion

Synthesis of POSS-(P(MEO2MA-co-OEGMA))8: Amphiphilic star-shaped hybrid polymers POSS-P(MEO2MA-co-OEGMA)8 withthermosensitive arms were easily synthesized via ATRP withoctafunctional POSS-(Cl)8 as the initiator. P(MEO2MA-co-OEGMA)8polymers with different ratio of MEO2MA and OEGMA wereprepared by changing the feed ratio of MEO2MA and OEGMAMEO2MA/OEGMA (mol/mol): 95/5% (sample 1, Mn¼25,600, Mw/Mn¼1.36), 92/8% (sample 2, Mn¼28,400, Mw/Mn¼1.32), 90/10%

(sample 3, Mn¼28900, Mw/Mn¼1.41), and 88/12% (sample 4,Mn¼32100, Mw/Mn¼1.31). Fig. 2(I) shows the ATR FT-IR spectra ofPOSS-Cl8 and POSS-(P(MEO2MA88%-co-OEGMA12%))8. It can beseen that the absorption peak of C–H of POSS-P(MEO2MA-co-OEGMA) at 2878 cm�1 was much stronger than that of C–H inPOSS-(Cl)8 due to the high content of methyl and methylene groupsin P(MEO2MA-co-OEGMA) segments. Moreover, there is a newabsorption peak with strong intensity in POSS-(P(MEO2MA-co-OEGMA))8 at 1728 cm�1 which should attributed to the carbonylgroups in P(MEO2MA-co-OEGMA) chains. Fig. 2 (II) shows the 1HNMR spectra of POSS-Cl8 and POSS-(P(MEO2MA88%-co-OEGMA12%))8. The chemical shifts in Fig. 2(II) (a) at 0.82 ppm,1.89 ppm and 3.56 ppm were attributed to the proton signals ofmethylene groups in POSS-Cl8. But for hybrid polymer (Fig. 2(II)(b)), the protons in P(MEO2MA-co-OEGMA) can be observed: 0.82–1.12 ppm (CH2C(CH3)), 1.74–2.01 ppm (CH2C(CH3)), 3.42 ppm(CH2CH2OCH3), 3.53–3.76 ppm (CH2CH2O), and 4.12 ppm(CH2CH2O). As shown in Fig. 2(II) (b), the peaks of the protons inPOSS-Cl8 were overlapped by the signals of protons in P(MEO2MA-co-OEGMA). Therefore, it is difficult to calculate the molecularweight of the copolymers according to 1H NMR spectrum. The ratioof MEO2MA to OEGMA in copolymer calculated by the integral ratioof peak g to peak h (Fig. 2(II) (b)) was similar with the feed ratio ofMEO2MA to OEGMA.

Self-assembly behavior and thermoresponse of POSS-(P(MEO2MA-co-OEGMA))8: As a hybridized copolymer with an inorganic POSScore and eight organic P(MEO2MA-co-OEGMA) arms, POSS-(P(MEO2MA-co-OEGMA))8 can assemble into micelles in selectivesolvent. The hydrophilic P(MEO2MA-co-OEGMA) arms are mainlyin the corona of the micelles, whereas the hydrophobic POSS coresin the star-shaped copolymer are mainly in the core of themicelles. The obtained four samples of POSS-(P(MEO2MA-co-OEGMA))8 were used to self-assemble into micelles in aqueoussolution and the tunable thermo responsive property was sur-veyed. Fig. 3(I) shows the transmittance curves of POSS-(P(MEO2MA-co-OEGMA))8 micelles in water with different molarratio of MEO2MA and OEGMA. It can be seen that the transmit-tance curves show sharp transition during heating process. TheLCST values increase from 29.7, 33.5 and 36.8 to 39.1 1C with theincrease of molar ratio of OEGMA in polymers from 5%, 8% and 10%to 12%, which indicated that the LCST values of micelle solutionscan be easily adjusted through altering the ratios of MEO2MA andOEGMA in the eight hydrophilic arms. Fig. 3(II) shows the plot ofthe hydrodynamic diameters (Dh) of samples 1–4 in water as afunction of temperature. When the temperature is relatively lower,the Dh values are small the change slightly. In contrast, the valuesincrease significantly in the higher temperature ranges due to theaggregation among micelles. For example, the Dh value of POSS-(P(MEO2MA95%-co-OEGMA5%))8 (sample 1) is about 20 nm at 28 1C,and the Dh value increases significantly to 550 nm when tempera-ture is raised to 36 1C. The schematic process of micelles aggrega-tion with the increase of temperature was shown in Fig. 3(III).At low temperature range, the P(MEO2MA-co-OEGMA) chains exist

Fig. 1. Synthesis of star-shaped inorganic–organic hybrid polymer POSS-(P(MEO2MA-co-OEGMA))8.

W. Yuan et al. / Materials Letters 111 (2013) 9–1210

in random coil conformation due to the hydrogen-bonding reac-tion between the ether oxygen and water molecules. When thetemperature increases to a critical value, P(MEO2MA-co-OEGMA)chains will shrink into a globular structure because the hydrogenbonds between the ether oxygen of P(MEO2MA-co-OEGMA) andwater molecules collapse and become hydrophobic. As a result,the intermolecular hydrophobic attractions are thermodyna-mically favored and the micelles aggregation will occur. Fig. 3(IV)shows the typical TEM image of POSS-(P(MEO2MA92%-co-OEGMA8%))8. The result confirms the spherical nano-micelles

can form by self-assembly of the obtained hybridized copolymer.In addition, the diameter of spherical micelles shown in this imageis consistent with the results obtained from the DLS analysis.

4. Conclusions

The star-shaped inorganic–organic hybrid polymer POSS-(P(MEO2MA-co-OEGMA))8 was synthesized with octafunctionalPOSS-(Cl)8 initiator via ATRP. The hybrid copolymers can be

Fig. 3. (I) Transmittance curves and (II) Temperature dependence of hydrodynamic diameters (Dh) of (a) POSS-(P(MEO2MA95%-co-OEGMA5%))8, (b) POSS-(P(MEO2MA92%-co-OEGMA8%))8, (c) POSS-(P(MEO2MA90%-co-OEGMA10%))8, and (d) POSS-(P(MEO2MA88%-co-OEGMA12%))8 micelle solutions; (III) The schematic process of micelleaggregation with the increase of temperature; (IV)TEM image of POSS-(P(MEO2MA92%-co-OEGMA8%))8 micelles at 25 1C.

Fig. 2. IR spectra(I) and 1H NMR spectra (II) of (a) POSS-Cl8 and (b)POSS-(P(MEO2MA88%-co-OEGMA12%))8.

W. Yuan et al. / Materials Letters 111 (2013) 9–12 11

self-assemble into micelles with POSS core and P(MEO2MA-co-OEGMA) shell. The tunable thermoresponse of the micelle solu-tions can be achieved by varying the molar ratio of MEO2MA andOEGMA. The transmittance and DLS measurements were used toinvestigate and determine the self-assembly behavior and thetunable thermoresponsive properties of these hybrid polymers.

Acknowledgment

The authors gratefully acknowledge the financial support of theNational Key Technology R&D Program (no. 2012BAI15B06) andthe Fundamental Research Funds for the Central Universities.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2013.08.062.

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