Synthesis and self-assembly in water of coil-rod-coil amphiphilic block copolymers with central...

Synthesis and Self-Assembly in Water of Coil-Rod-CoilAmphiphilic Block Copolymers with Centralp-Conjugated Sequence

ANNE de CUENDIAS, EMMANUEL IBARBOURE, SEBASTIEN LECOMMANDOUX, ERIC CLOUTET, HENRI CRAMAIL

Universite Bordeaux 1, Laboratoire de Chimie des Polymeres Organiques, UMR 5629, CNRS, ENSCPB,16 av. Pey Berland, 33607 Pessac, France

Received 31 January 2008; accepted 1 April 2008DOI: 10.1002/pola.22793Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The purpose of this study is to correlate the nano-organization in waterof coil-rod-coil amphiphilic block copolymers constituted of a conjugated segment totheir optoelectronic properties. The ABA block copolymer structures, easily achievedvia coupling reactions, are based on conjugated rod of dihexylfluorene and 3,4-ethyle-nedioxythiophene units linked to two flexible poly(ethylene oxide) or poly[(ethyleneoxide)-ran-(propylene oxide)] chains. These well-defined copolymers exhibited a rangeof specific morphologies in water, a good solvent of coil blocks and a bad solvent ofthe conjugated rod. Particularly, vesicles and micelles with spherical, cylindrical, orelongated shape were noticed. Correlations were attempted to be established betweenthe weight percent of the conjugated sequence contained in the copolymers, the mor-phology of the nanostructures obtained by self-assembly in solution and the resultingoptical properties. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4602–

4616, 2008

Keywords: amphiphiles; amphiphilic; block copolymers; coil-rod-coil copolymers;conjugated polymers; p-conjugated segment; optoelectronics; self-assembly; self-orga-nization; structure-property correlation; structure-property relations

INTRODUCTION

The synthesis of well-defined p-conjugated seg-ments with tunable electronic properties foroptoelectronic devices is a field that has un-dergone considerable development.1,2 However,these rod-like semiconducting segments are gen-erally insoluble in most common solvents due tointer- and/or intra-chain interactions that leadto the formation of crystalline aggregates. The

incorporation of conjugated sequence withinblock copolymers is a route to improve theirprocessability. This route also permits formationof self-assembled electroactive materials thatcan exploit nanodimension effects.3–5 In particu-lar, rod-coil block copolymers have attractedincreasing interest in the two last decadesbecause of their high propensity to self-organiza-tion. This is a result of the inherent immiscibil-ity of the different blocks and the formation ofphase segregated nanostructures.6 Several origi-nal morphologies have already been reportedsuch as nanostructures with mushroom shape,7

porous membrane or spherical,8–11 vesicular,12,13

cylindrical and lamellar aggregates.14–19

Among the numerous synthesized blockcopolymers, coil-rod-coil systems have been

This article includes Supplementary Material availablefrom the authors upon request or via the Internet at www.interscience.wiley.com/jpages/0887-624X/suppmat.

Correspondence to: E. Cloutet (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 4602–4616 (2008)VVC 2008 Wiley Periodicals, Inc.

4602

largely studied. They can be prepared from vari-ous synthetic pathways. Probably the most facileroute is the end-capping of stiff conjugated seg-ments with flexible reactive chains by condensa-tion.20,21 Another elegant investigated method isto use end-functionalized rigid conjugated seg-ment as macroinitiator for ring opening,22 con-trolled radical23,24 or living anionic25 polymer-izations of monomers, leading to the flexiblecounterparts. Moreover, water-soluble conju-gated polymers have drawn considerable atten-tion because they not only allow environment-friendly processing into electronic devices butalso present a novel platform for chemical orbiological sensors applications. Many workershave already prepared amphiphilic block copoly-mers based on oligo- or poly(ethylene oxide)(PEO) chains, and studied the influence of theirself-assembly in solution or in the solid stateon their optoelectronic properties.5,20,26–28 Theformation of liquid crystalline mesophaseshas an effect on exciton formation, the chargecarrier mobility, and the fluorescence, allowinga potential improvement of solar cells efficiencyor organic light-emitting diodes (OLEDs)performances.

We recently reported a series of functional-ized rigid ‘‘bricks’’ combining a central dihexyl-fluorene enclosed on each side by one functional-ized 3,4-ethylenedioxythiophene (EDOT) unitthat revealed interesting optical properties.29

In previous studies, this type of associationwas carried out for the design of materials withelectroluminescence properties.30–35 Indeed,poly(3,4-ethylenedioxythiophene) (PEDOT) andits derivatives exhibit weak gaps36,37 and pres-ent excellent electrochromism38,39 and photolu-minescence properties.40 In addition, polyalkyl-fluorenes, known to emit in the blue wave-length, can allow a better processability of thesep-conjugated segments and offer a higher holemobility.

Here, new amphiphilic ‘‘triblock’’ copolymershaving a well-defined central conjugatedsegment are prepared and studied to clarifythe effect of their self-assembly properties inwater on their optical properties. The startingmolecule bis(3,4-ethylenedioxythiophene)-9,90-di-n-hexylfluorene (BEDOT-HF), shown in Scheme1, was prepared by Stille coupling of 2,7-dibromo-9,90-dihexylfluorene and 2-(trimethyl-stannyl)EDOT. This BEDOT-HF was then end-functionalized following different strategies toobtain the ‘‘triblock’’ structures with PEO or

with statistical copolymers composed of ethyleneoxide and propylene oxide units, poly[(ethyleneoxide)-ran-(propylene oxide)], denoted P(EO-r-PO), as flexible chains. The self-assembly inwater of these ‘‘triblock’’ copolymers was studiedby dynamic light scattering (DLS), small-angleneutron scattering (SANS), atomic force micros-copy (AFM), and cryogenic transmission electronmicroscopy (cryo-TEM). Correlations were alsostudied between the weight percent of the conju-gated part within the ‘‘triblock’’ structures, themorphology obtained by self-assembly in solu-tion, and the optical properties determined byUV-visible and fluorescence spectroscopies.

EXPERIMENTAL

General

1H-NMR spectra were recorded on a BrukerAC400 spectrometer operating at 400 MHz andsamples were dissolved in CDCl3. SEC analyseswere performed at room temperature in THFwith a set-up consisting of a WATERS 880-PUpump and a series of three microstyragel col-umns with pore sizes of 103, 105, and 106 A. Theelution of the filtered samples was monitoredusing simultaneous UV and RI detections. Theelution time were converted to molecularweights using a calibration curve against PSstandards. UV–visible spectra were measured inthe spectral range of 200–700 nm in chloroformsolutions and in water, and recorded on a VarianCary 3E UV–visible Spectrophotometer. Fluores-cence spectra were recorded on a SAFAS Spec-trofluorometer flx spectrometer. ATR-IR spectrawere recorded on a Bruker Tensor 27. Sampleswere prepared by drop-casting a thin film froma diluted solution THF (1 g �L�1) onto a ZnSecrystal. A total of 100 scans were averaged foreach spectrum at a spectral resolution of 4 cm�1

and in the range 600–4000 cm�1. The dynamiclight scattering measurements (DLS) were per-formed using ALV Laser Goniometer, which con-sists of 22 mW HeNe linear polarized laser with632.8 nm wavelength, an ALV-5000/EPP Multi-ple Tau Digital Correlator with 250 ns initialsample time and a temperature control to stabi-lize the temperature of the sample cell at 25 8C6 0.1 8C. The accessible scattering angle rangeis from 308 to 1508. The data acquisition wasperformed with the ALV-Correlator Control Soft-ware, and the counting time varied for eachsample from 300 to 600 s. Millipore water was

SYNTHESIS AND SELF-ASSEMBLY OF COPOLYMERS 4603

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

Scheme 1. Synthesis of coil-rod-coil ‘‘triblock’’ copolymers (ABA). Reagents and con-ditions: (i) n-BuLi, DMF, THF; (ii) p-toluene sulfonic acid, toluene, 17 h at reflux; (iii)LiAlH4, THF, 24 h at 25 8C; (iv) xylene, 7 h at 140 8C; (v) EtO� Naþ, THF, 12 h atreflux; (vi) DPTS, DIPC, DMF, 72 h at 25 8C. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

4604 DE CUENDIAS ET AL.


thoroughly filtered with 0.1 lm filters anddirectly employed for the preparation of the solu-tions which were then introduced into 10 mm di-ameter glass cells. Small angle neutron scattering(SANS) experiments were performed at the Labo-ratoire Leon Brillouin (CEA-CNRS, Saclay) on thePACE spectrometer. Three configurations wereused varying the wavelength (k) and the sample-to-detector distance (D) to cover a broad wave vec-tor range (k ¼ 5 A, D ¼ 1.07 m, k ¼ 17 A,D ¼ 4.57 m; k ¼ 6 A, D ¼ 2.87 m). Absolutevalues of the scattering intensity (I(q) in cm�1),were obtained from the direct determination ofthe incident beam flux.41 The scattering curveshave been analyzed by fitting theoretical spectra,which takes into account the polydispersity ofthe object (using a log-normal size distribution),convoluted for each configuration with the corre-sponding resolution function. AFM images wererecorded in the air with a Nanoscope IIIa micro-scope (Digital instruments, CA) operating intapping mode (TM). The probes with silicon tipswere commercially available and used with aspring constant of 42 N �m�1, a resonance fre-quency of 285 kHz, and a typical radius of curva-ture in the 10–12 nm range. The topography orphase signal images were recorded with a resolu-tion of 512 3 512 data points. Samples for atomicforce microscopy (AFM) analysis were solvent-casted at room temperature using water solutions(from 0.1 to 1 3 10�3 g �L�1), one drop was restedon muscovite mica substrate. The mica substratewas cleaved using adhesive tape and immediatelyused to avoid contamination by ambient dustparticles or volatile molecules. For cryogenictransmission electron microscopy (cryo-TEM)analyses (CERMAV-CNRS, Grenoble), a drop(�5 mL) of solution was deposited on a lacy car-bon film supported by a 300-mesh copper grid.The grid was mounted on a Leica EM CPCfast-freezing device, blotted with filter paper,and immediately plunged into liquid ethanecooled to �172 8C. The samples were mountedonto a Gatan 626 cryostage and observed underlow-dose conditions at a temperature of �180 8C.Images obtained after either negative staining orcryo-TEM were recorded on Agfa Scientia plates,with a defocus of 1–3 lm. The reactions werecarried out by the procedures described below.

Materials

n-butyllithium (n-BuLi), N-pivaloyl-o-toluidine,(Carbethoxymethylene)-triphenylphosphorane(Ph3P ¼ CHCO2Et, 95%), LiAlH4 (1 M in THF),

p-toluene sulfonic acid (98.5%), diisopropyl-1,3-carbodiimide (DIPC, 99%), and PEO-OH (550g �mol�1) were purchased from Aldrich. n-BuLiwas filtrated and titrated, prior to use, with N-pivaloyl-o-toluidine by the method described bySuffert.42 Jeffamine 2070 was generously fur-nished by Huntsman. PEO-NH2 (550 g �mol�1)and 4-(dimethylamino)pyridinium-p-toluene sul-fonate (DPTS) were prepared according to litera-ture.43,44 The synthetic procedure of bis(3,4-eth-ylenedioxythiophene)-9,9-di-n-hexylfluorene (1)is described in details elsewhere.29 The otherproducts were used as received without furtherpurification. THF was dried and distilled overCaH2 and sodium/benzophenone, respectively.DMF were stirred with CaH2 overnight, and dis-tilled prior to use. Toluene and xylene were usedas received without further purification. Allreactions were carried out under a dry nitrogenatmosphere, using flame-dried glassware.

Synthesis Procedure

(Bis(3,4-ethylenedioxythiophene)-9,9-di-n-hexyl-fluorene)-dicarboxaldehyde (2)

Compound 1 (1 g, 1.63 mmol) was dissolved inTHF (40 mL), and the solution was cooled to�78 8C. n-butyllithium (6.7 mL, 10.7 mmol) wasslowly added drop-wise, and the mixture turningunder a red-brown color, was kept under stirringat �78 8C during 1h. Then, DMF (0.62 mL, 8.13mmol) was added at �78 8C via a burette and themixture was allowed to warm to ambient temper-ature with stirring for 24 h. At the end of thereaction, water (30 mL) was added to the mix-ture. After, the phases were separated and theyellow organic layer was extracted from the aque-ous layer with ether. All the organic layers werethen washed several times with water, subse-quently dried over Na2SO4 and concentrated. Theproduct was then purified by chromatography onsilica gel using CH2Cl2 as eluent, leading to a yel-low solid (0.87 g, yield ¼ 80%).

1H-NMR (CDCl3): d (ppm) ¼ 9.96 (s, 2H,CHO), d ¼ 7.84–7.71 (6H, Ar), d ¼ 4.46–4.43 (4H,OCH2 CH2O), d ¼ 1.99 (4H, CH2), d ¼ 1.14–1.05(12H, CH2), d ¼ 0.76 (t, 6H, CH3), d ¼ 0.69 (4H,CH2).

29

(Bis(3,4-ethylenedioxythiophene)-9,9-di-n-hexylfluorene)-diethyl acrylate (5)

To a solution of the aldehyde 2 (1 g, 1.49 mmol)dissolved in 50 mL of xylene, (carbethoxymethy-



lene)-triphenylphosphorane (1.55 g, 4.47 mmol)was added. The whole mixture was stirredunder reflux (140 8C) for 7 h. Then, the reactionmixture which became dark orange, was allowedto cool to room temperature and xylene wasremoved by cryodistillation. Purification by chro-matography on silica gel (toluene/ethyl acetate¼ 50/50 (v/v)) afforded the molecule as a yellowsolid (1.15 g, yield ¼ 95%).

1H-NMR (CDCl3): d (ppm) ¼ 7.75–7.65 (6H,Ar; d, 2H, CH), d ¼ 6.22 (d, 2H, cis-CH), d ¼ 5.63(d, 2H, trans-CH), d ¼ 4.38 (4H, OCH2 CH2O),d ¼ 4.24 (q, 4H, CH2O), d ¼ 1.99 (4H, CH2),d ¼ 1.32 (t, 6H, CH3), d ¼ 1.14–1.05 (12H, CH2),d ¼ 0.76 (t, 6H, CH3), d ¼ 0.69 (4H, CH2).

29

Copolymer 1

Compound 2 (0.3 g, 0.447 mmol) and PEO-NH2

(543 mg, 0.984 mmol) were mixed in dry toluene(40 mL) and treated with p-toluene sulfonic acid(3 mg). The mixture was kept under stirringand reflux during 17 h, using a Dean Stark ap-paratus removing the produced water. The reac-tion mixture was washed with water and the or-ganic layer was dried over Na2SO4 and concen-trated. The crude product was then precipitatedin cold diethyl ether to remove the PEO-NH2 inexcess. The product 3 was then lyophilized, dis-solved in 10 mL of THF, and allowed to reactwith LiAlH4 (1 mL, 1 mmol) at 0 8C. The mix-ture was stirred for 24 h at room temperature.At the end of the reaction, water was added.The organic layer was separated and washedwith water. The product was dried over Na2SO4

and concentrated leading to a brown-orange vis-cous liquid. The 1H-NMR spectrum (CDCl3) is inSupporting Information as Figure SI2.

Copolymer 2

The same procedure as above was followed withP(EO31-r-PO10) (Jeffamine 2070) (2.04 g, 0.984mmol) for the preparation of the product 4which was then reduced from LiAlH4 to give thefinal copolymer. The 1H-NMR spectra (CDCl3)are showed in Supporting Information as FigureSI3.

Copolymer 3

The compound 5 (0.48 g, 0.593 mmol) was dis-solved in dried THF (25 mL) and treated with asolution of NaOH in ethanol (0.94 g in 25 mL).

The mixture was kept under stirring at refluxduring 15 h. The completion of the reaction wasfollowed by thin layer chromatography (TLC)until the disappearance of the ester bonds. Thereaction mixture was concentrated and dissolvedin CH2Cl2. The organic layer was treated with asolution of HCl until pH < 7 and then washedwith water. The organic layer was concentratedand the product 6 was lyophilized (1H-NMR(CDCl3) in Supporting Information Fig. SI4).The final compound and DPTS (0.07 g, 0.237mmol) were dissolved in dried DMF (30 mL). A so-lution of lyophilized PEO-OH in DMF (4.04 mL,1.20 mmol) was added to the solution with DIPC(0.24 mL, 1.54 mmol). The mixture was stirredat room temperature for 72 h. DMF was removedby cryodistillation and the reaction product wasdissolved in CH2Cl2. The obtained mixture wasfiltrated and concentrated. Then, the crude prod-uct was precipitated in cold diethyl ether. The1H-NMR spectrum (CDCl3) is showed in Support-ing Information as Figure SI5.

RESULTS AND DISCUSSION

Synthesis and Molecular Characterization

The ‘‘triblock’’ copolymer structures were builtfrom end-functionalized rigid segment BEDOT-HFs based on EDOT and dihexylfluorene andindicated as molecules 2 and 6 in Scheme 1. Thesynthesis of these coil-rod-coil structures (ABA)was undertaken following a similar strategy,involving the end-groups reactivity of the rigidsegment which was used for the coupling reac-tion with end-functionalized PEO or P(EO-r-PO)chains. These various pathways are described inScheme 1.

First, the formylation of the compound 1 wasreadily realized in presence of n-BuLi and DMF,leading to the dicarboxaldehyde derivative 2which required a purification on chromato-graphic column. Subsequently, the aldehydefunctions of 2 were able to react with aminoter-minated PEO (DPn ¼ 12) or P(EO31-r-PO10)(Jeffamine 2070), used in slight excess (2.2equiv), to form azomethine linkages (3 and 4,respectively) in acidic medium. The excess of re-active amino-chains was then removed by pre-cipitation in cold diethyl ether. Then by takinginto account that the azomethine group is read-ily hydrolyzable under basic conditions,45 theazomethine groups were reduced to secondary



amine with LiAlH4 to give the Copolymer 1 andCopolymer 2, respectively. For the synthesis ofCopolymer 1, it was necessary to preliminaryprepare the aminoterminated PEO from com-mercial poly(ethylene glycol) methyl ether. Wefollowed a similar procedure than the onedescribed by Wang et al.,43 whereby PEO-Cl isfirst prepared by reaction with thionyl chloride.Subsequently, a PEO-phthalimide derivativewas obtained by nucleophilic displacement ofchlorine groups with phthalimide and finally,hydrazinolysis of the phthalimide end-groupswas performed to prepare PEO-NH2. Theachievement of each derivatization was con-firmed by 1H-NMR and ATR-IR spectroscopies(Supporting Information Figs. SI1 and SI6). Theefficiency of the coupling reactions yieldingCopolymers 1 and 2 was confirmed by SEC inTHF [Fig. 1(a,b)]. The chromatograms of Copoly-mer 1 reveal one monomodal population and anarrow distribution (Mw/Mn ¼ 1.1) with molarmass of 1800 g �mol�1, close to the expectedvalue (1730 g �mol�1). In the case of Copolymer2, one monomodal population is also obtainedwith molar mass of 3800 g �mol�1 (Mw/Mn ¼ 1.3),a value slightly lower than the calculated one(4480 g �mol�1). It is important to recall thatSEC with polystyrene calibration does not allowto determine true molar masses and also thatP(EO31-r-PO10) tends to aggregate in THF(eluent used for SEC) due to its amphiphilic na-ture and its high EO content. The chemicalstructure of the copolymers obtained before and

after the reduction of the azomethine bonds wasconfirmed by 1H-NMR (Supporting InformationFigs. SI2 and SI3).

In another approach, compound 5 (diethyl ac-rylate derivative) was synthesized from 2 byWittig reaction with the aim of increasing theconjugation length of the rigid block. Cleavageof the ester bonds led to the formation of acrylicacid end groups at each side of the rod-like seg-ment, yielding molecule 6. The extent of thisreaction, that is, saponification with a largeexcess of sodium alcoholate, was followed bythin layer chromatography. 6 was then end-capped with hydroxyl-terminated PEO chains(DPn ¼ 12) via esterification to give Copolymer3 with a higher weight ratio (rr/c) of the rod con-jugated part with respect to the coil ones.Indeed, a weak excess of PEO-OH chains (2.05equiv) was allowed to react with the rigid seg-ment in presence of diisopropylcarbodiimide(DIPC) and N,N-dimethyl-aminopyridine-p-tolu-ene sulfonate (DPTS) in DMF.44 After purifica-tion by precipitation in cold diethyl ether, the co-polymer was finally obtained in quantitativeyield. The efficiency of the coupling reaction waschecked by SEC with RI detection in THF [Fig.1(c)]. The molar mass of Copolymer 3 was foundequal to 1900 g �mol�1 (Mw/Mn ¼ 1.3) in agree-ment with the expected value (1820 g �mol�1).However, the SEC curve indicated a slightshoulder toward lower molecular weights,explained by the presence either of free PEO-OH chains or diblock copolymers in the medium.

Figure 1. SEC traces of each copolymer (performed on different columns, inTHF and calibration with standard PS): (a) Copolymer 1, (b) Copolymer 2, and(c) Copolymer 3.



The latter is the most probable since thisshoulder is also observed by UV detection andfree PEO chains do not have a chromophore at254 nm. Furthermore, the 1H-NMR analysesindicated that less than 5% of acrylic acid func-tions were not esterified (Supporting Informa-tion Fig. SI5). Therefore, it is assumed that theminor presence of residual diblock copolymerswill not be of consequence on self-assemblybehavior of Copolymer 3 in solution.

The associative properties in water of theseamphiphilic ‘‘triblock’’ copolymers having rela-tively well-defined structures constituted of acentral conjugated segment (BEDOT-HF) andhydrophilic flexible blocks differing by their na-ture (PEO or P(EO-r-PO)), were then investi-gated.

Self-Assembly in Water

The supramolecular organization of each copoly-mer was first investigated by dynamic light scat-tering (DLS). The preparation of the solutionsconsisted of addition of copolymer to distilledand filtered water. Copolymer 1 and Copolymer2 were instantly soluble. However, ultrasoundover a period of a few minutes was necessary forCopolymer 3, obtained by esterification route. Itcan also be noticed that Copolymer 3 shows astronger tendency to aggregate because of itshigher rod over coil ratio value (rr/c). The copoly-

mer solutions were prepared with concentra-tions ranging from 1 to 10 g �L�1, kept undervigorous stirring during 3 days and filtered at0.45 lm. DLS measurements were achieved atvarious angles (from 508 to 1508) and the analysesof the normalized intensity autocorrelation func-tions C(q,t) were obtained following the method ofcumulants, fitted with multiexponential decayfunctions and CONTIN analysis. All the samplespresented one or two relaxation times s. Oncethe characteristic frequency G ¼ 1/s is deter-mined, the concentration and angular depend-ence can be expressed as G/q2 ¼ DZ (1þ kDcþ: : : ) where DZ is the Z-average diffusion coeffi-cient, kD is the effective interaction parameter,and q is the scattering vector. From the diffu-sion coefficient, Z-average hydrodynamic radiusRH can be calculated from the Stockes-Einsteinrelation, RH ¼ kT/(6pgDZ), where g is the sol-vent viscosity, k the Boltzmann constant, and Tthe temperature. The normalized correlationsfunctions measured at 90 presented in Figure2(a) show different relaxation times for thethree copolymers synthesized. The CONTINanalysis of the autocorrelation functions revealsa monomodal distribution for the Copolymer 3indicating the existence of a well-defined popula-tion. The other systems present another minor-ity population with a fast relaxation mode whichcan be attributed to the presence of few freechains (RH �4 nm). For each copolymer concen-tration, the evolution of G associated to the slow

Figure 2. DLS data of each copolymer in solution in water. (a) Autocorrelationfunctions at 908 for C ¼ 2 g �L�1, (b) Dependence of G with q2 for C ¼ 2 g �L�1, (c)Dependence of RH with the concentration. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]



mode versus the square magnitude of the scat-tering vector q2 is studied [Fig. 2(b)]. The linearevolution of G with q2 and the quality of the lin-ear adjustment attest that the formed objectsare homogeneous in size. The hydrodynamic ra-dius of the corresponding aggregates obtainedfrom the slope of this linear fit were calculatedand gave extrapolations to q ¼ 0 comprisedbetween 70 and 91.7 nm [Fig. 2(c)]. These firstresults would assess that the length and the na-ture of the hydrophilic block, as those of the con-jugated segment, have an influence on the sizeof the scattering objects. Indeed, comparativeresults obtained for Copolymer 1 and Copolymer2 constituted of the same rod block, show that alonger flexible chain made of PO units leads to

morphologies with larger size. Moreover, Copoly-mer 1 which has a rigid block with a shorterconjugation length than Copolymer 3, while pre-senting similar flexible chains, shows smallerorganizations.

SANS experiments were also performed todetermine the structural parameters of thesescattering objects, by a proper analysis of theform factors. The solutions were preparedaccording to the same procedure than for DLSanalyses, but in heavy water (D2O) and athigher concentrations for two of the copolymers(20 g �L�1) in the aim to increase the signal-to-noise ratio. Copolymer 3, presenting the moresignificant mass percentage in rigid block, wasonly studied at a lower concentration (1 g �L�1),

Figure 3. SANS intensity and theoretical form factor P(q) for each copolymer insolution in D2O: (a) Copolymer 1 at 20 g �L�1, (b) Copolymer 2 at 20 g �L�1, and (c)Copolymer 3 at 1 g �L�1. (Rc: dense core radius; L: cylinder length; d: membranethickness; and r: particles polydispersity or log-normal distribution). [Color figurecan be viewed in the online issue, which is available at www.interscience.wiley.com.]



to exempt the aggregation phenomena likely todisturb the analysis. Figure 3 gathers the SANSintensity profiles resulting from the self-assem-bly of copolymers in D2O. The adjustment of theexperimental data by theoretical form factors ofvarious morphologies was also represented onthe graphs. These fits were carried out by usinga home-made program that takes into account,at the same time, the particles polydispersityand the experimental resolutions.46

Firstly, a decrease of the intensity in q�4 inthe large q range (q > 0.03 A�1) for all copoly-mers can be noted, which is characteristic of adense object and a clear interface (Porod likebehavior). Copolymers 1 and 2 show a decreaseof the intensity in q�1 in the lowest q range (q <0.03 A�1), in agreement with the presence of rodsmeaning that these two systems self-organizeunder cylindrical structure. For these two copoly-mers, the radii obtained from these adjustmentsare similar (Rc ¼ 2.3 nm) and markedly lowerthan the hydrodynamic radii determined by DLS(RH

0 ¼ 70 and 91.7 nm, respectively). This obser-vation suggests that only the dense core of thecylinder built up of conjugated blocks contributesto the SANS signal, whereas the shell made upof stretched PEO (and PPO) chains is invisible,as previously observed for comparable core-shellmicellar structures in D20.

46,47 However thehydrodynamic radii are quite large and the corre-sponding objects may not correspond to simplecore-shell cylindrical micellar structures. Forexample, more complex morphology such as‘‘hockey puck’’ was already theoretically describedfor rod-coil systems with high volume fractions ofthe coil part and mainly observed experimentallyin coil-selective solvents.22,48–50 This kind of mi-cellar organization allows the minimization ofthe stretching constraints of the solvated flexiblechains. Copolymer 3 reveals a decrease in q�2 inthe same q range, indicating a planar interfacethat is typical of vesicular self-assemblies. Thislatter shows a radius in SANS (80 nm) close to

that obtained in DLS (86.3 nm) suggesting a hol-low structure with a conjugated dense membraneof 6 nm thick.

The Guinier representation is further com-monly used to determine the characteristicdimensions of the self-assembled nanostruc-tures.51–53 Indeed, the slope p of ln I(q) accord-ing to q2 makes it possible, within the Guinierlimit (qRg < 1, i.e., in low q regions), to reachsome structural parameters of the scatteringobjects such as the radii and the lengths. Thevalues obtained by this approximation (Sup-porting Information Fig. SI7) corroborate thosegiven by the complete adjustment of spectra ofscattered intensities.

The various data obtained in these studies ofdiffusion are reported in Table 1. We can noticethat the dimensional contribution of the conju-gated core on the size is minor and it is in factthe flexible chains inflated by water that occupythe greatest space.

To confirm the morphologies suggested fromthese last studies, additional analyses were car-ried out by microscopy techniques such asatomic force microscopy and transmission elec-tronic microscopy. The systems were character-ized by AFM in tapping mode on dried thinfilms. Very dilute solutions of copolymers inpure water were prepared via the same proce-dure previously employed and one drop of thesesolutions was deposited on a freshly cleavedmica support. Then, water was let to evaporateslowly (24 h) at room temperature under atmos-pheric pressure. Figure 4 presents two AFMimages obtained from a solution of Copolymer 1(10�3 g �L�1) and Copolymer 2 (2 3 10�2 g �L�1).The image in Figure 4(a) reveals sphericalobjects of rather homogeneous radii of 40 nm,which seem to aggregate by bundles of two orthree particles. In this range of concentration,we do not obtain cylindrical morphology as couldindicate the SANS analyses performed at 20g �L�1. However, the concentration used for

Table 1. Data of DLS and SANS Analyses for the Three Copolymers in Water

Copolymer rr/c (wt %) Morphology RH0a (nm) Rc (nm) Rves/dves (nm) r L (lm)

1 23 Cylinder 70 2.3 – 0.3 0.42 13 Cylinder 91.7 2.3 – 0.3 13 37 Vesicle 86.3 – 80/6 0.3 –

a DLS: from 1 to 10 g �L�1 and SANS: Copolymer 1 and 2 at 20 g �L�1; Copolymer 3 at 1 g �L�1 in D2O (rr/c: weight ratio ofthe conjugated part in the copolymer, Rc: dense core radius, L: cylinder length, d: membrane thickness, and r: particles polydis-persity or distribution of the radii in log-normal).



Figure 4. AFM topography images obtained from a drop of copolymer solutions inwater deposed on a mica surface: (a) Copolymer 1 (10�3 g �L�1) and (b) Copolymer 2(2 3 10�2 g �L�1). (c) Schematic representation of the concentration effect on the mor-phology. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 5. AFM topography images obtained from a drop of a solution of Copolymer3 in water (0.1 g �L�1) deposited on a mica surface. (a) Under atmospheric pressureand (b) when the sample was placed under high vacuum (8 3 10�5 mbar) beforeanalysis. [Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]



AFM preparation is at least three orders of mag-nitude lower than that for SANS experiments,possibly indicating a transition in morphology.Moreover, this technique used for the prepara-tion of the samples with a slow evaporation ofwater have to support the high interactionsbetween PEO blocks and the absorbent surfaceof mica, likely to disrupt the organization of thesystem.54 In addition, the application of a modewith a stronger tapping (Supporting InformationFig. SI8) where the oscillation amplitude of thetip decreases, led to the appearance of whitehusks within the heart of the particles indicat-ing that the core is of harder nature than theshell. These observations confirm the presenceof ‘‘core-shell’’ structures with a core based onconjugated rigid blocks and a flexible PEO shell.Figure 4(b) represents AFM image obtainedfrom a solution of Copolymer 2 at 2 3 10�2

g �L�1. Spherical particles and objects with elon-gated rice-like forms are clearly observable. Fur-thermore, these ovoid structures would seem tocome from the fusion of aggregated sphericalparticles. We can indeed distinguish very homo-geneous spherical particles with radii of 50 nm,and also more elongated objects around 80 nmin length. It appears clearly that the initialspherical micelles observed at low concentrationhave a strong tendency to assemble into theseintermediate rice grain-like structures, probablygrowing up to worm-like micelles for more con-centrated solutions, as observed by SANS for aconcentration of 20 g �L�1. In a general way, wecan propose that the morphology of the obtainedobjects with the systems based on Copolymer 1and Copolymer 2 evolves with the concentration,as illustrated in the drawing presented on Fig-ure 4(c). AFM characterizations with concentra-tions of 20 g �L�1 were undertaken to confirmthese hypotheses, but the results were not clearas the AFM tip was probably deflected by stronginteractions with high concentrations of conju-gated aggregates.

Figure 5(a) represents AFM images from a so-lution of Copolymer 3 (0.1 g �L�1). These imagesshow slightly deformed spherical objects withvery homogeneous sizes and radii around68 nm. These values are lower than those deter-mined by DLS (86.3 nm) or SANS (80 nm), dueto the collapse of PEO chains probably duringthe slow evaporation of solvent. To confirm thehollow structure of these morphologies, theywere characterized prior to and after the appli-cation of a secondary vacuum (8 3 10�5 mbar).

It is possible that such a vacuum can deformhollow structures. Indeed, the sample showscompletely disorganized objects that had for themost part doubled in size to an average width ofabout 320 nm [Fig. 5(b)]. Moreover, the meas-ured height decreased by a factor of 4, corre-sponding now to the membrane thickness eval-uated by SANS (6 nm). This disorganization canbe interpreted as the consequence of a too im-portant pressure together with residual waterwithin the system leading to the bursting of thehollow structures. The presence of vesicularmorphologies would explain the observed phe-nomena. To confirm this, cryo-TEM analysis wasperformed on samples.

The characterization of the objects resultingfrom the self-organization of Copolymer 3 inwater appeared more clearly by cryo-TEMbecause this technique makes it possible to bet-ter preserve the morphologies by freezing thepolymer solution. Figure 6 illustrates a cryo-TEM image of Copolymer 3 in water (1 g �L�1).A rather homogeneous distribution of vesiclesradii between 26 and 55 nm can be seen, that is,lower than those expected (RH

0 ¼ 86 nm). In fact,the method of preparation by cryo-TEM requiresthe formation of a water film with a thickness ofabout 100 nm, leading to a selective formationof smaller objects. Moreover, the membranethickness is estimated at 12 nm, a value twicelarger than that obtained by SANS, for whichthe hydrophobic rigid block was only considered.Nevertheless, these last results testify clearly to

Figure 6. Cryo-TEM images obtained from a solu-tion of Copolymer 3 in water (1 g �L�1).



the organization of Copolymer 3 in water intovesicular shape.

Finally, the morphology transitions observedwhen increasing the rigid block length can becompared to theoretical models. Most of themodels developed so far have been realized forblock copolymers in the solid state, but thephase boundaries as a function of the rod vol-ume ratio generally reflect the solution proper-ties. Especially, ten Brinke and coworkers pre-dicted transitions from cubic to hexagonal andlamellar morphologies in the solid state, with avery strong stabilization of the last one.55 Ourexperimental observations are in perfect agree-ment with these theoretical predictions.

Optical Properties

UV–visible and fluorescence analyses were per-formed to evaluate the effect of the polymernano-organizations on the absorption and emis-sion properties. For comparison, the opticalbehavior of the copolymers was systematicallyanalyzed both in water and in chloroform.Indeed, chloroform is considered as a good sol-vent in which the chains of the copolymer donot interact. In water, as previously discussed,the copolymers self-organize under variouskinds of supramacromolecular assemblies. Theabsorption spectra of Copolymers 1 and 2 inchloroform (0.2 g �L�1) show an intense p-p*transition with an absorption maximum atabout 370 nm and a well-defined vibronic struc-

ture [Fig. 7(a,b)]. Copolymer 3 which differsfrom the two others by a higher intrinsic conju-gation length, exhibits in chloroform an absorp-tion maximum at lowest energies (i.e., 433 nm)[see Fig. 7(c)]. In water, all the systems adoptquite distinct behaviors as compared to thoseobserved in chloroform. The intensity of eachabsorption band decreases, the phenomenoncharacteristic of supramacromolecular organiza-tions.15,56,57 In addition, Copolymers 1 and 2,obtained from the same synthetic route butbased on flexible blocks of different nature andsize (PEO12 or P(EO31-r-PO10)), show a similarbehavior with a red-shift of the absorption maxi-mum up to 10 nm [Fig. 7(a,b)]. This variation ofabsorption range can be explained by the forma-tion of J-aggregates (Scheme 2). This kind ofaggregation related to the presence of aromaticcycles with a more planar configuration leads toan increase of the conjugation effect within thesystem.58,59 Copolymer 3 reveals a significantblue shift of its absorption maximum of �105 nm[Fig. 7(c)], evidence of the existence of H-aggregates, for which the rigid blocks arealigned in parallel (Scheme 2). The H or J-stack-ing observed by UV–visible spectroscopy shouldbe comparable to the characteristic sizesobtained by SANS and to the theoretical lengthsof the various molecules. It is worth noting thatthe values of the hard core radii, evaluated bySANS for Copolymers 1 and 2 are not so inco-herent. Indeed, the length of the conjugatedblock, which was evaluated from MM2 calcula-tion, represents only �1.8 nm (Supporting Infor-

Figure 7. Absorption spectra of the copolymers in chloroform (solid line) and inwater (dashed line) for a same concentration. (a) Copolymer 1, (b) Copolymer 2, and(c) Copolymer 3.



mation Fig. SI9.a) and a slight shift in thestacking of these blocks could easily lead to av-erage distances of 2.3 nm. For Copolymer 3, thelength of the rigid block is about 3 nm (Sup-porting Information Fig. SI9.b) but the width ofthe vesicular membrane was estimated to 6 nmaccording to the SANS analyses. It is difficult tofigure out that only the formation of H-aggre-gates can lead to a doubling of the size. More-over, the formation of bilayer stacking withinthe vesicle membrane seems to be hardly con-ceivable from this kind of coil-rod-coil structure.Nevertheless, we can postulate that the sterichindrance induced by the hexyl groups attachedto the fluorene moiety is also responsible for thisdifference in size.

Knowing the absorption maximum of eachsystem in water, the fluorescence emission wasthen determined and correlated to the dataobtained in chloroform (Supporting informationFig. SI10). Except the case of Copolymer 3, thesystems exhibit in water a significant shift ofthe emission peak maxima, from 60 to 106 nmtoward the higher wavelengths. These emissionsin the green are characterized by broader spec-tral bands than those obtained in chloroform.Besides, the optical-electronic properties of theconjugated blocks studied beforehand revealedthat increase in the conjugation by the additionof two vinyl ester groups on both sides of theBEDOT-HF segment clearly lead to an improve-ment of the electroluminescence performances.29

Here also, the length of the conjugated part in

the copolymers shows its important role on theoptical properties closely dependant on thesupramacromolecular organization adopted bythe system, as illustrated in the generalScheme 2.

CONCLUSIONS

In this article, the synthesis of well-defined coil-rod-coil block copolymers, constituted of a cen-tral conjugated segment based on EDOT anddihexylfluorene units and lateral flexible PEO orP(EO-r-PO) chains is reported. Similar conven-ient synthetic routes based on coupling reactionwere used to prepare three different ‘‘triblock’’copolymers varying from the nature and the sizeof the hydrophilic flexible blocks as well as thelength of the hydrophobic conjugated segment.As expected, the synthesized copolymers showeda strong propensity to self-assemble in water.The nature and the volume fraction of eachblock proved to be key factors on the resultantself-assembled morphologies. The copolymerscontaining the smallest conjugated block(BEDOT-HF) exhibit self-assemblies under theshape of spherical micelles and cylinders accord-ing to the concentration of the solution, whereasthe system having longer conjugated block hastendency to generate vesicular organizations. Itmeans that the presence of additional doublebonds is sufficient to modify in a significant

Scheme 2. Correlation between the weight percent of the conjugated part in theamphiphilic copolymers (rr/c), the self-assembly structures, and the optical properties.The vials of copolymers in water being placed under a UV lamp (365 nm).



manner the repulsive interactions betweenwater and the rigid segment. Finally, UV–visibleand fluorescence analysis of these copolymersboth in chloroform, good solvent of the twoblocks, and in water, selective solvent for theflexible counterpart, enabled to stress modifica-tions of the optical properties, related to the dif-ferent possibilities of layout of such organizedcompact structures (H or J-aggregates).

The self-assembly of the copolymers at thesolid state for application as diode devices is cur-rently under investigation and should allow usto evaluate the effect of the exciton containmenton the electroluminescence properties. In addi-tion, the presence of PEO spacers known tohave good ionic conductivities can also be advan-tageously used for applications of these copoly-mers in electrochemical diodes.60,61 The covalentlinkage between the electroactive blocks and thePEO chains can minimize the phase separationproblem often associated with conventionalLECs.62–64

Financial support has been provided by the Ministerede la Recherche et de la Technologie for a thesis grantto A. de C. The authors are grateful to Jean-LucPutaux (CERMAV-CNRS, Grenoble) for his help inthe cryo-TEM analysis, to Annie Brulet (LLB, CEA-Saclay) for her contribution in the SANS experiments,to the CNRS and to the University of Bordeaux 1.The authors are indebted to Roger C. Hiorns for helpin the preparation of this manuscript.

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