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Polymer 52 (2011) 1925e1931

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Polymer

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Functionalization of amphiphilic coil-rod-coil triblock copolymerpoly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine)with florescence moiety and C60M. Shahinur Rahman c, Mohammad Changez a,b, Joonkeun Min a,b, Priyank N. Shah a,b,Shashadhar Samal d, Jae-Suk Lee a,b,*

a School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of KoreabDepartment of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of KoreacDepartment of Chemistry, The University of Tennessee, Knoxville, TN 37996, USAdDepartment of Chemistry, S.B.R. Govt. (Autonomous) Women’s College, Berhampur 760001, India

a r t i c l e i n f o

Article history:Received 8 December 2010Received in revised form28 February 2011Accepted 6 March 2011Available online 16 March 2011

Keywords:Block copolymersNanocompositesSelf-assembly

* Corresponding author. School of Materials SciencInstitute of Science and Technology (GIST), 1 Oryong-doRepublic of Korea. Fax: þ82 62 970 2304.

E-mail address: jslee@gist.ac.kr (J.-S. Lee).

0032-3861/$ e see front matter Crown Copyright � 2doi:10.1016/j.polymer.2011.03.011

a b s t r a c t

We recently achieved quantitative synthesis of an amphiphilic coil-rod-coil triblock copolymer, poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine), by coupling in situ living diblockcopolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate) (P2VP-b-PHIC) using malonyl chloride inthe presence of pyridine. This led to the introduction of an active methylene group that is a site forfurther functionalization in the rod block. The Michael addition reaction of the triblock copolymer with7-(4-trifluoromethyl) coumarin acrylamide led to copolymer bearing a fluorescent pendent in the rodblock. The fluorescent labeled copolymers were isolated in w94% yields. Similarly C60 pendent wasintroduced to the rod block by the Bingel reaction. The yields of C60 functionalized copolymers werew54%. The precursor and functionalized amphiphilic coil-rod-coil copolymer show diverse morphol-ogies, such as micelles and vesicles by simply changing the solvent. For the C60 functionalized blockcopolymer, structural constraints in micelles and vesicles prevented C60 pendents to aggregate.

Crown Copyright � 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Block copolymers are regarded as essential substances toproduce well-defined nano-scale structures with many provenapplications in nanotechnology [1]. As the necessities for moreadvanced nanostructures increase, material scientist are looking forfunctionalized block copolymers, which can endow advantagescoming from both the ordered nanostructure and the chemicallyattached group of the polymer. Among the recently developedcontrolled polymerization methodologies, living anionic polymer-ization is the most suited for the synthesis of such block copoly-mers with predictable molecular weight and narrow molecularweight distribution [2e4]. By using this methodology, it is possibleto synthesize well-defined block copolymers with precise molec-ular architectures such as chain-end and in-chain-functionalizedpolymers [5]. Further the functionalized polymers obtained are

e and Engineering, Gwangjung, Buk-gu, Gwangju 500-712,

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expected to provide supramolecular assembly with a wide varietyof potential applications.

The chemical modifications or functionalizations of the blockcopolymers are generally performed by hydrogenation, hydrolysis,quaternization, and sulfonation [6e11]. The polymers containingactive methylene group can be functionalized by using the well-known organic reactions such as alkylation, Michael additionreaction, and Bingel reaction [12e17]. Dendrimer having activemethylene group in its center has been modified with C60 whicheffectively prevent the aggregation of C60 molecules. The function-alization of dendrimers with various mesomorphic groups leads toliquid-crystalline materials and showed tailor-made properties.

Amphiphilic coil-rod block copolymers are fascinating materialsfor their novel self-assembling behavior such as formation ofmicelles and hollow sphere, having unique characteristics of core-shell architecture, low critical micellar concentration, and findpotential applications as nanoreactors, nanoreservoirs, genedelivery vehicles, and reaction media for biocatalysis [18,19]. Suchcopolymers having polyisocyanate blocks could still be moreattractive, since polyisocyanates exhibit proven applications inseveral field [20e23].

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We are involved in the controlled synthesis of the poly-isocyanate homo- [24e26] and their di- and tri-block copolymers[27e29]. Recently we have reported a controlled synthesis of coil-rod-coil triblock copolymers of P2VP and PHIC by coupling livingdiblock copolymers in situ using diacid chlorides [30]. The methodled to introduction of an activemethylene group at the center of thecopolymers thus providing a facile reaction site for the furtherfunctionalization of these copolymers by simple organic reactions[31].

Herein,wereport chemicalmodificationof thenovel amphiphiliccoil-rod-coil triblock copolymers of poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine) (P2VP-b-PHIC-b-P2VP)by 7-(4-trifluoromethyl) coumarin acrylamide (TFCA) and C60.Furthermore, we also studied morphologies of the resulting copol-ymers in selective solvents by using scanning electron microscope(SEM), confocal laser scanningmicroscope (CLSM), and transmissionelectron microscope (TEM).

2. Experimental section

2.1. Chemicals

The monomers, 2-vinylpyridine (VP) (Aldrich, 97%) andn-hexyl isocyanate (HIC) (Aldrich, 97%), and the linking molecule,malonyl chlorides (Aldrich, 97%), were dried over CaH2 andvacuum distilled. Tetrahydrofuran (THF) (Fisher Scientific, GRgrade) was distilled under nitrogen after refluxing with sodiumfor 5 h and distilled again under vacuum from sodium naph-thalenide. Calcium hydride (Junsei, 95%), naphthalene (Aldrich,99%), potassium metal (Aldrich, 98%), sodium tetraphenylborate(Aldrich, 99.5%), triethylamine (Fischer Scientific, 99%) anddiphenylmethane (DPM) (Aldrich, 99%), 7-(4-trifluoromethyl)coumarin acrylamide (TFCA) (Aldrich, 98%) and C60 (Aldrich,99.9%) were used without further purification.

2.2. Physical characterization

The 1H NMR (300 MHz) spectra were measured using a JEOLJNMLA300WB using CDCl3 as the solvent. Chemical shifts werereferenced to tetramethylsilane (TMS) at 0 ppm. Molecular weightswere determined from the response of a multi-angle laser lightscattering detector system (MALLS, l ¼ 690 nm), combined withsize-exclusion chromatography (SEC) and OPTILAB-DSP Interfero-metric Refractometry 478-009-690 and DAWN EOS Laser Photom-eter (l ¼ 690 nm) 113-E, (Wyatt Technology) with 4 columns(HR 0.5, HR 1, HR 3 and HR 4, Waters Styragel columns run in serieswith column pore sizes 50, 100, 500 and 1000 Å, respectively). THFwith triethylamine (2% v/v) (for the prevention of adsorption ofhydrophilic polymer in the column)was used as themobile phase ata flow rate of 1.0 mL/min. The dn/dc values for block copolymers inTHF at 40 �C were measured with an LED (OPTI-DSP, l ¼ 690 nm)source. After dn/dc was measured for 5 different concentrations foreach polymer sample, SEC-LS data were gained with refractiveindex detection at 40 �C. Polymer aggregates were characterized bydynamic light scattering (DLS) at 25�, field emission scanningelectronmicroscope (FE-SEM,Model s-4700, Hitachi), confocal laserscanning microscope (CLSM), and energy filtering transmissionelectron microscope (EF-TEM, EM 912 OMEGA).

2.3. Synthesis

2.3.1. Synthesis of P2VP-b-PHICeCOeCH2eCOePHIC-b-P2VP(P2VP-b-PHIC-b-P2VP)

The coil-rod-coil triblock copolymer P2VP-b-PHICeCOeCH2eCOePHIC-b-P2VP here as having an active

methylene group in its center has been synthesized as describedpreviously. Briefly, 2VP was first polymerized by using DPM-K(0.026 g, 0.130 mmol) initiator under high vacuum conditions(10�6 torr) in an all-glass apparatus. After the homopolymerizationof 2VP (1.40 g, 13.1 mmol) for 30 min in tetrahydrofuran (THF)at �78 �C, an excess of NaBPh4 (0.290 g, 0.850 mmol) was intro-duced and the reactor was cooled down to �98 �C. The secondmonomer HIC (0.680 g, 5.36 mmol) was then introduced andcopolymerization was carried out for 20 min. Addition of malonylchloride (0.010 g, 0.070 mmol) to the living P2VP-b-PHIC led tolinking two living P2VP-b-PHIC anions. The coupling reaction wascarried out in situ in the presence of pyridine (0.012 g, 0.150 mmol)at �98 �C for 10e20 min. The coil-rod-coil triblock copolymerswere isolated in 95% yield. The reaction mixture was poured intoa large amount of hexane or methanol. The choice of the solventneeded for precipitation depends on the block copolymer compo-sition. The precipitated polymer was filtered and dried undervacuum. Dissolution in a good solvent and precipitation usinga non-solvent such as hexane or methanol was repeated to obtainhighly pure copolymer samples. The copolymer samples werefinally dissolved again in benzene and freeze-dried for 12 h.Solvent-soluble parts were analyzed quantitatively byweighing theresidue after evaporation of solvent and using 1H NMR to checkwhether there were any un-reacted monomers or trimers. Themechanism for formation of coil-rod-coil triblock copolymerP2VP-b-PHICeCOeCH2eCOePHIC-b-P2VP had shown inSupporting Information Fig. 1S. Number average molecular weight(Mn) ¼ 31,400 g mol�1, fPHIC ¼ 0.29, polydispersity index(PDI) ¼ 1.07, and yield ¼ 95%. P2VP-b-PHIC-b-P2VP. 1H NMR(CDCl3, 300 MHz), d (ppm): 0.9 (3H, CH3), 1.00e2.10 (10H, (CH2)4 inhexyl group of HIC and CH2 in vinyl main chain of 2VP), 2.10e2.89(1H, CH), 3.36e4.14 (2H, eCH2eN in HIC) (Fig. 1a), 6.11e7.35 (3H,CH in pyridine), 8.02e8.55 (1H, CH in pyridine). 13C NMR (CDCl3,150MHz), d (ppm): 13.9 (CH3), 22.4e31.4 (CH2 in hexyl group of HICand CH2 in vinyl main chain of 2VP), 40.2 (CH), 43 (eCH2eN in HIC),156.6 (C]O). FT-IR (KBr, cm�1): 3432 (NH), 3076 (CH, in pyridinering), 2935 (aliphatic, CH2eCH in backbone), 1728 (C]O fromlinking agent and HIC), 1590 (C]C), 1474 (C]N) (Fig. 1b).

2.3.2. Functionalization of the triblock copolymerThe coil-rod-coil triblock copolymer was functionalized by

Michael addition reaction utilizing the known reactivity of theactive methylene group (Scheme 1). For the synthesis of copolymerwith a fluorescent moiety, the triblock copolymer sample (0.100 g,0.022 mmol) was dissolved in THF (10 mL) and then 7-(4-tri-fluoromethyl) coumarin acrylamide (TFCA, 0.008 g, 0.030 mmol)was added in the presence of triethylamine (TEA, 0.003 g,0.030 mmol). The reaction mixture was stirred for 7 h at 40 �C. Theproduct was precipitated in hexane and filtered. The crude polymersample was repeatedly dissolved in small amounts of THF andre-precipitated in hexane to remove the un-reacted TFCA. Thesample was then dried in vacuum at 40 �C. The sample analyzed by1H NMR and was found to contain w90% of the functionalizedpolymer, with the remaining 10% parent polymer as an impurity.Separation of the mixture via column and quantitative thin layerchromatography was not feasible as the polymers were entrappedin the sorbent media. We are now purifying the sample by high-pressure liquid chromatography (HPLC). Number average molec-ular weight (Mn) ¼ 4500 g mol�1, fPHIC ¼ 0.45, polydispersity index(PDI) ¼ 1.08, and yield ¼ 93% (Table 1). P2VP-b-PHIC(TFCA)-b-P2VP. 1H NMR (CDCl3, 300 MHz), d (ppm): 0.9 (3H, CH3), 1.00e2.10(10H, (CH2)4 in hexyl group in HIC and CH2 in vinyl main chain of2VP), 2.10e2.35 (5H, 1H in vinyl and 4H from TFCA), 2.9 (1H inmalonyl) 3.6e3.6 (2H, eCH2eN in HIC), (Fig. 1a) 6.11e7.35 (3H, CHin pyridine), 8.02e8.55 (2H, CH in pyridine and NH in TFCA). The 1H

Fig. 1. 1H NMR spectra of poly (2-vinylpyridine-block-hexyl isocyanate-block-2-vinylpyridine) triblock coil-rod-coil copolymer (P2VP-b-PHIC-b-P2VP) (a) without and (b) withfunctionalization of fluorescent 7-(4-trifluoromethyl) coumarin acrylamide (TFCA). (P2VP-b-PHIC(TFCA)-b-P2VP), FT-IR spectra of (c) P2VP-b-PHIC-b-P2VP, (d) TFCA, and (e) (P2VP-b-PHIC(TFCA)-b-P2VP) coil-rod-coil triblock copolymer.

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NMR spectra of P2VP-b-PHIC-b-P2VP and P2VP-b-PHIC(TFCA)-b-P2VP shown in Fig. 1a and b, respectively. A FT-IR spectrums ofP2VP-b-PHIC-b-P2VP, pure TFCA, and P2VP-b-PHIC(TFCA)-b-P2VP shown in Fig. 1cee, respectively. Comparing the FT-IRspectra of TFCA, P2VP-b-PHIC-b-P2VP, and P2VP-b-PHIC(TFCA)-b-P2VP, the presence of peaks marked as x and y (Fig. 1e) in func-tionalized polymer confirmed the substitution of active methylenehydrogen by THCA molecules which was consistent with 1H NMRdata (Fig. 1b).

Scheme 1. (a) Functionalization of coil-rod-coil triblock copolymers poly (2-vinylpyridinfluoromethyl) coumarin acrylamide (TFCA) using Michael addition reaction in presence ofPHIC-b-P2VP with C60 using Bingel reaction.

Following the same procedure, introduction of C60 to the coil-rod-coil triblock copolymers was not possible since TEA getsattached to the C60. By using the Bingel reaction, C60 has beenincorporated to the coil-rod-coil triblock copolymers (Scheme 1). Ina typical procedure, the polymer sample (0.050 g, 0.011 mmol),solid C60 (0.008 g, 0.011 mmol) and solid carbon tetrabromide(0.005 g, 0.013 mmol) were taken in a round bottom flask anddissolved in toluene (40 mL). The 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (0.004 g, 0.026 mmol) was then added to the reactor and

e-block-hexyl isocyanate-block-2-vinylpyridine) (P2VP-b-PHIC-b-P2VP) with 7-(4-tri-triethylamine (TEA). (b) Functionalization of coil-rod-coil triblock copolymer P2VP-b-

Table 1Synthesis results of coil-rod-coil triblock copolymers of poly(2-vinylpyridine) (P2VP) and poly(n-hexyl isocyanate) (PHIC).

Sample ID Precursor copolymera Mn � 10�3 (g mol�1)b Mw/Mnb fPHIC Sample ID with TFCA Sample ID with C60

1 P2VP11-b-PHIC8-L-PHIC8-b-P2VP11 4.5 1.08 0.45 1TFCA 1C602 P2VP101-b-PHIC41-L-PHIC41-b-P2VP101 31.4 1.13 0.29 2TFCA 2C603 P2VP43-b-PHIC99-L-PHIC99-b-P2VP43 33.5 1.08 0.70 3TFCA 3C60a L is a linking agent eCOCHRCOe, where; R ¼ H for un functionalized polymer and fluorescent molecules substitution R ¼ TFCA. For C60 pendent the linking agent L is

eCOCR0COe, and R0 is C60.b Number average molecular weight (Mn) and polydispersity index (Mw/Mn) were measured by size-exclusion chromatography, multi-angle laser light scattering

(SEC-MALLS) at 90� in THF/Et3N at 40 �C.

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the reactionmixturewas stirred for 18 h at room temperature. Afterthe reaction toluene was removed by using vacuum evaporatorand the crude product was dissolved in THF. The un-reacted C60,which was not soluble in THF, was removed by filtration. Theproduct containing THFwas then evaporated and re-precipitated inhexane. The sample was then dried in vacuum at 40 �C. Numberaverage molecular weight (Mn) ¼ 5200 g mol�1, fPHIC ¼ 0.45,polydispersity index (PDI) ¼ 1.08, and yield ¼ 54% (Table 1). P2VP-b-PHIC(C60)-b-P2VP. 1H NMR (CDCl3, 300 MHz), d (ppm): 0.9 (3H,CH3), 1.00e2.10 (10H, (CH2)4 in hexyl group in HIC and CH2 in vinylmain chain of (2VP)), 2.10e2.89 (1H, CH, 2H in eCH2 of remainingmethylene), 3.36e4.14 (2H, eCH2eN in HIC), 6.11e7.35 (3H, CH inpyridine), 8.02e8.55 (1H, CH in pyridine).

2.3.3. Sample preparation for morphologies characterizationsTEM samples were prepared by spreading a drop of copolymer

solutions on carbon coated copper grid and characterized afterstaining with I2 vapor for 8 h. The TEM images of the copolymerscontaining C60 were gained without staining. The aggregationbehaviors of the block copolymers were studied in CHCl3,CH3OH, THF:CH3OH (1:9 v/v) and THF:CH3OH:H2O (1:2:2 v/v/v)(5 mg/mL).Hydrodynamic size of aggregates were measured bydynamic light scattering (DLS). The sample solutions were spin caston silicon wafer and dried at room temperature for SEM.

3. Results and discussion

3.1. Funtionalization of coil-rod-coil triblock copolymerswith fluorescent molecules

The functionalized block copolymers are triblock copolymers ofpoly(2-vinylpyridine) and poly(n-hexyl isocyanate) (Table 1).Fluorescent labeling of the copolymers has been performed byMichael addition reaction using known reactivity of active methy-lene group. For quantitative analysis of crude sample by 1H NMR,a low molecular weight copolymer (Mn ¼ 4500 g mol�1, Table 1,entry 1) was used since the appearance of functional group in highMW copolymers is not prominent from NMR spectra.

3.2. Funtionalization of coil-rod-coil triblock copolymerswith C60 molecules

Following the same procedure introduction of C60 to thecopolymers was not possible since TEA gets attached to the C60. Byusing the Bingel reaction C60 has been incorporated to the coil-rod-coil triblock copolymers in presence of CBr4 and DBU in toluene(Scheme 1). The reaction mechanism is the initial formation ofa carbanion, which attacks C60, resulting in bridging of the carbonatom of active methylene group. The controlled experiment witha low MW polymer and quantitative analysis by NMR shows theincorporation of C60 was w54%. Attempt toward separation orfractionation was not successful and hence the product has beenused as such for morphological studies.

3.3. Aggregation behaviors

3.3.1. Aggregation behavior of coil-rod-coil triblock copolymersThe self-assembly of the block copolymers composed of two

distinct blocks having different solubility in selective solventsleading to the formation of various nano aggregates such micelles,vesicles, nanotubes etc. Two seed copolymers P2VP101-b-PHIC41-L-PHIC41-b-P2VP101,Mn ¼ 31,400 g mol�1, (fPHIC ¼ 0.29, sample 2) andP2VP43-b-PHIC99-L-PHIC99-b-P2VP43, Mn ¼ 33,500 g mol�1,(fPHIC¼ 0.70, sample 3) of Table 1 were used for aggregation studies.The hydrodynamic size of the aggregates of copolymer2 (fPHIC ¼ 0.29) measured by DLS was 32 � 4 nm in CHCl3, which issolvent selective for the rod block. Even though PHIC is the midblock of the triblock copolymer, the linking methylene group andthe kinks in the polyisocyanate chain impart enough flexibility toallow the terminal P2VP chains to assemble at the core of themicelles and formed the solid spherical micelles with20 nm � 2 nm size (measured by SEM, Fig. 2a) with 6 � 1 nm P2VPcore (measured by TEM, Fig. 2b).

In CH3OH, the solvent selective for the P2VP block, the PHIC(fPHIC ¼ 0.29) blocks remain associated by hydrophobic interactionformed a larger aggregates of hydrodynamic size of 160 � 10 nm.After solvent evaporation the hydrophilic P2VP blocks diffused ontothe hydrophobic PHIC generating the vesicles with average diam-eter varying in the range 80e100 nm, as observed in the SEM andTEM micrographs (Fig. 2cand 2d, respectively). When the PHICfractionwas increased to 0.70 mol%, polymer was no longer solublein CH3OH. However, THF dissolves both PHIC and P2VP irrespectiveof the rod or coil fractions. Since CH3OH is a good solvent only forP2VP, the copolymer was first dissolved in THF and the effect ofadded CH3OH was studied. Interestingly, in THF:CH3OH (1:9 v/v),the copolymer formed well-defined bilayer vesicles typemorphologies (Fig. 2e and 2f) with PHIC rod at the interior of thevesicles membrane and P2VP segment exposed the solvent andformed outer layer (inset TEM micrograph of Fig. 2f). Severalsolvent ratios were tried but this composition was ideal for theformation of such vesicles. Thus, by suitably choosing the solventand the block composition, it was possible to generate differentmorphologies as schematically shown in Fig. 2e (Fig. 2).

3.3.2. Aggregation behavior of fluorescent labeled coil-rod-coiltriblock copolymers

The parent coil-rod-coil triblock copolymers show diversemorphology in selective solvent for the specific block as observed inprevious section. The self-assembly of the functionalized blockcopolymers was expected to be nearly the same or significantlydifferent due to the effect of the functional groups. Samples of theP2VP-b-PHIC-b-P2VP block copolymers functionalized with TFCA(3TFCA, 33,500 g mol�1, Table 1) having PHIC volume fractions(fPHIC) 0.70, was chosen to observe aggregation behavior. Fig. 3a(using UV filter) and Fig. 3b (without UV filter) shows the CLSMmicrograph of 3TFCA, respectively with fluorescent pendent at thePHIC rod block. The sample 3TFCA (Table 1) in THF:CH3OH:H2O(1:2:2 v/v/v), the solvent selective for the P2VP block, the PHIC

Fig. 2. Scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs of micelles and vesicles of poly (2-vinylpyridine-block-hexyl isocyanate-block-2-vinylpyridine) triblock coil-rod-coil copolymer (P2VP-b-PHIC-b-P2VP), (a) SEM and (b) TEM micrograph of solid micelles of the P2VP-b-PHIC-b-P2VP in CHCl3 (fPHIC ¼ 0.29,Mn ¼ 31,400 g mol�1), (c) SEM, and (d) TEM micrograph of vesicles of the P2VP-b-PHIC-b-P2VP in CH3OH (fPHIC ¼ 0.29, Mn ¼ 31,400 g mol�1), (e) SEM and (f) TEM micrograph ofvesicles of the P2VP-b-PHIC-b-P2VP vesicles in THF:CH3OH (1:9 v/v) (fPHIC ¼ 0.70, 33,500 g mol�1), and (g) schematic representations of the aggregates formation of P2VP-b-PHIC-b-P2VP in different solvent. A sample concentration was 5 mg mL�1.

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blocks remain associated by hydrophobic interaction at the interiorof bilayer vesicles membrane and P2VP chains of the P2VP-b-PHIC(TFCA)-b-P2VP exposed to the solvent which can clearly distin-guished in CLSM image Fig. 3b. Comparing the Fig. 3a with b weconclude that the gray area represents P2VP segment and drankzone represents PHIC rod containing high electron density TFCA in

Fig. 3. Confocal laser scanning microscope (CLSM) image of vesicles of fluorescent 7-(4-triymers P2VP-b-PHIC(TFCA)-b-P2VP, (a) with UV filter, deformed fluorescent vesicles in THF:filter, shows empty vesicles morphologies with dark (high electron density TFCA containingfunctionalized P2VP-b-PHIC-b-P2VP (fPHIC ¼ 0.70, 33,500 g mol�1) in THF:CH3OH:H2O (1:2:

Fig. 3b of bilayer vesicles. The hydrophilic P2VP blocks diffused[22e24] on the hydrophobic PHIC generating vesicles as observedin the CLSM image (Fig. 3a and b). The addition of water to thesystem increases the hydrophobic interaction; rod like PHICaggregated strongly and formed the large size of vesicles (hydroy-namic size w1.8 mm). Schematic representation for the vesicle

fluoromethyl) coumarin acrylamide (TFCA) functionalized coil-rod-coil triblock copol-CH3OH:H2O (1:2:2 v/v/v) from fPHIC ¼ 0.70 and Mn ¼ 33,500 g mol�1, (b) without UVPHIC at the interior), and (c) schematic representation for the vesicle formation of TFCA2 v/v/v). A sample concentration was 5 mg mL�1.

Fig. 4. TEM image of micelles and vesicles of C60 functionalized coil-rod-coil triblock copolymers P2VP-b-PHIC-b-P2VP, (a) C60 nano particles of solid micelles in CHCl3, (fPHIC ¼ 0.29,Mn ¼ 31,400 g mol�1), (b) the high resolution image of a shows crystalline order of C60, (c) C60 containing vesicles in THF:CH3OH (1:9 v/v) (fPHIC ¼ 0.70, Mn ¼ 33,500 g mol�1)showing the nano-ring, and (d) schematic representation for the micelles and vesicle of C60 functionalized P2VP-b-PHIC-b-P2VP in CHCl3 (fPHIC ¼ 0.29, Mn ¼ 31,400 g mol�1), andTHF:CH3OH (1:9 v/v) (fPHIC ¼ 0.70, Mn ¼ 33,500 g mol�1). The sample concentrations in all cases were 5 mg mL�1.

M. Shahinur Rahman et al. / Polymer 52 (2011) 1925e19311930

formation shown in Fig. 3c. These vesicles, due to their large size ofw1.8 mm are deformed. Apparently, increasing the polarity of thesolvent led the P2VP chain relaxation that affected the size and theshape of the vesicles.

3.3.3. Aggregation of C60 functionalized coil-rod-coil triblockcopolymers

Fullerene containing polymers, dendrimers or functionalizedsingle molecules is interesting due to their potential application inelectron transporting semiconductors, field-effect transistors, aswell as building blocks for the development of functional nano-structures [32]. Fullerenes are known to undergo strong aggrega-tion that hinders ordered arrangement, an essential requirementfor the construction of functional devices. To solve this problemisolated aggregates of fullerene particles from amphiphilic roderoddiblock copolymers have been reported recently [33]. However, thesizes of these aggregated fullerenes are over micrometer rangewhich limits their applications.

Our target was to controlling the fullerene arrays in nanometerregimewith defined structure by utilizing the amphiphilic coil-rod-coil triblock copolymer self-assembly. The coil-rod-coil triblockcopolymers used for the purpose show diverse morphology, suchas, lamellar, solid, and hollow spheres [27e30] of greatly varyingsizes by simply changing the solvent and block composition. TheP2VP-b-PHIC-b-P2VP block copolymers containing C60 as a func-tional group at the PHIC segment (2C60, Mn ¼ 31,400 g mol�1 and3C60, Mn ¼ 33,500 g mol�1, Table 1) with PHIC volume fractions(fPHIC) 0.29 and 0.70, respectively, were chosen to study theaggregation behavior. The copolymer 2C60 in CHCl3, the solventselective for the rod block, formed spherical micelles with P2VPcore and PHIC shell structure. The C60 which is attached to the PHICblock eventually remain in the solvated media. The TEM samplecast on a copper grid by drop cast method was analyzed afterthe solvent evaporation without staining. The Fig. 4a shows the

well-defined fullerene nano particle of average sizes w4 nm. Thehigh resolution TEM image and FFT shows the fullerene nanoparticles are crystalline (Fig. 4b).

TEM micrograph of C60 containing vesicles from 3C60 inTHF:CH3OH (1:9 v/v) (Fig. 4c) shows a fullerene nano-ring. Themicrograph was obtained without staining the vesicles. The darkring represents the fullerene rich region. In THF:CH3OH (1:9 v/v)the PHIC containing C60 blocks associate by hydrophobic interac-tion and produce the interior of the vesicles (Fig. 4d). After solventevaporation, the hydrophilic P2VP blocks collapsed on the hydro-phobic PHIC generating well-defined vesicles of w400 nm sizes.Furthermore, C60 is known to aggregate readily. In the present caseit is covalently bonded to the block copolymer. The structuralconstraints of the copolymer prevented aggregation of C60 toa significant extent indicated by the formation of the nano-ringof C60.

4. Conclusion

The amphiphilic coil-rod-coil copolymers of PHIC and P2VPwithan active methylene group in its center have been used for func-tionalization and aggregation studies. The more than 90% yield wasobtained for substitution of active hydrogen with 7-(4-tri-fluoromethyl) coumarin acrylamide by Michael addition reaction.However, only 54% yield was observed for C60 substitution by usingthe Bingel reaction. The amphiphilic copolymers formed solidmicelles (w20 nm) and vesicles (100e500 nm) in solvent selectivefor the specific block. The sizes of the vesicles greatly varied withthe volume fraction of the rod components and the solventselected. The fluorescent labeled copolymers also furnished vesi-cles. Furthermore, the strong aggregation of C60 has been signifi-cantly suppressed by the copolymer self-assembly. Thus with theease of functionalization and its self-assembly, these amphiphilic

M. Shahinur Rahman et al. / Polymer 52 (2011) 1925e1931 1931

copolymers can be suitably tailored for potential applications inarea of biomedical and molecular electronics.

Acknowledgments

This workwas partially supported by the Program for IntegratedMolecular System (PIMS) and World Class University (WCU)program (Project Number: R31-20008-000-10026-0). Authors alsothank the Korea Basic Science Institute (KBSI), Daejon, for TEMmeasurements.

Appendix. Supplementary information

Supplementary information related to this article can be foundonline at doi:10.1016/j.polymer.2011.03.011.

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