ORIGINAL CONTRIBUTION

Hierarchical mesostructures of biodegradabletriblock copolymers via evaporation-induced self-assemblydirected by alkali metal ions

Jun-Bing Fan & Feng Long & Zhi-Wu Liang &

Matthew P. Aldred & Ming-Qiang Zhu

Received: 8 March 2012 /Revised: 3 May 2012 /Accepted: 8 May 2012 /Published online: 9 June 2012# Springer-Verlag 2012

Abstract Hierarchical mesostructures of poly(ε-caprolac-tone)-b-poly(ethylene oxide)-b-poly(ε-caprolactone) (PCL-PEO-PCL) triblock copolymers have been grown fromevaporation-induced self-assembly directed by alkali metalions. The self-assembly process began with a dilute homo-geneous solution of the triblock copolymers in a mixture oftetrahydrofuran (THF) and water. THF preferentially evap-orated under reduced pressure and induced the formation ofamphiphilic polymer micelles. The spherical polymermicelles formed both in deionized water and NaOH aqueoussolution. However, different mesostructures were discov-ered during the film depositing process for scanning electronmicroscopy observation. The polymer micelles were ob-served for the deposition sample in deionized water whilesisal-like hierarchical mesostructures resulted from the filmdeposition of polymer micelles in NaOH aqueous solution.The sisal-like mesostructures and their formation processwere observed through scanning electron microscopy, trans-mission electron microscopy, fluorescent microscopy, X-raydiffraction, and Fourier transform infrared spectroscopy.Detailed study revealed that during evaporation-induced

self-assembly of PCL-PEO-PCL amphiphilic triblock copol-ymer directed by alkali metal ions, the sodium ions andpolymer micelles increasingly concentrated in NaOH aque-ous solution and the solvent quality for the diblock progres-sively decreased, which resulted in the stronger coordinationbetween alkali metal ions and PEO ligands in the blockcopolymer and PEO segment crystallization.

Keywords PCL-PEO-PCL . Evaporation-induced self-assembly . Alkali metal ion . Biodegradable·blockcopolymers

Introduction

Amphiphilic block copolymers containing both hydrophilicand hydrophobic blocks in the polymer chain have attractedmuch attention for their self-assembly behavior in selectivesolvents, which allows the formation of many aggregates,such as spherical micelles, vesicles, cylinders, worm-likemicelles, tubules, lamellae, rods, and rings with sizes rang-ing from a few nanometers to microns [1–12]. Importantly,the morphologies of aggregates are controlled at all dimen-sions from the nanoscale to macroscopic scale with potentialapplications in material science and biomedical engineering[13–16]. There is increasing evidence to suggest that theoverall properties of block copolymer aggregates are depen-dent not only on their size and stability but also on theirmorphology [17, 18]. It has been found that the drivingforce for the formation of aggregate involves a force balancebetween molecular interactions, which is determined bythree factors: stretching (deformation) of the core-formingblocks in the core, the repulsive interaction among thecorona chains, and the interfacial tension between the mi-celle core and the solvent [19, 20]. Based on these three

Electronic supplementary material The online version of this article(doi:10.1007/s00396-012-2681-3) contains supplementary material,which is available to authorized users.

J.-B. Fan : F. Long : Z.-W. Liang (*) :M.-Q. Zhu (*)College of Chemistry and Chemical Engineering,State Key Laboratory of Chemo/Biosensing and Chemometrics,Hunan University,Changsha, Hunan 410082, Chinae-mail: zwliang@hnu.edu.cne-mail: mqzhu@hust.edu.cn

M. P. Aldred :M.-Q. ZhuWuhan National Laboratory for Optoelectronics,Huazhong University of Science and Technology,Wuhan, Hubei 430074, China

Colloid Polym Sci (2012) 290:1637–1646DOI 10.1007/s00396-012-2681-3

factors, the final morphologies of aggregates are determinedby the composition of the block copolymer [21–23], sol-vents [24–26], initial polymer concentration [27], tempera-ture [7, 28, 29], and pH [19, 30–32]. Among these, thecomposition of block copolymer affects the aggregate mor-phology of the corona-forming block mainly through therepulsive interaction among the corona chains. With theincreasing length of the corona block, the morphology ofaggregates would be changed from vesicles to rods or evenspheres [4]. The common solvents affect the aggregates'morphology mainly through the change of solvent contentin the core and the aggregation number to modify the degreeof stretching of the core-forming block in the core [19]. Ithas been found that when the solvent changes from DMF todioxane and then to tetrahydrofuran (THF), the aggregates'morphologies of polystyrene-b-poly(acrylic acid) (PS-b-PAA) in water change from spheres to vesicles and then torods due to the increase of stretching of PS chains [24]. InpH-responding self-assembly, the aggregate structures areaffected mainly by the electrostatic repulsion interactionassociated with the degree of ionization of the coronachains. For example, on addition of NaOH, the aggregates'morphologies of the polystyrene-b-poly(acrylic acid) (PS-PAA) in aqueous solutions change from vesicles to spheresdue to the neutralization of AA in polymer chain, thusincreasing the electrostatic repulsion while the reverse pro-cess is observed on addition of HCl because of the proton-ation of the PAA [2, 20].

So far, a variety of morphologies can be obtained withcrew-cut micelle-like aggregates based on the polystyrene-b-poly(acrylic acid) and polystyrene-b-poly(ethylene oxide)copolymers, representatively by Eisenberg's group [33–35].However, the self-assembly behaviors of most amphiphilicblock copolymers in aqueous media have been performedon nonbiocompatible and nonbiodegradable systems. Only afew researches have been reported on the multiple morpho-logical aggregates of biodegradable amphiphilic copolymer.For biomedical application, indeed, transportation of aggre-gating nanocarriers in the body, regardless of the mode ofadministration, will be affected by aggregating nanostruc-tures [36]. More recently, Bates et al. reported spherical,cylindrical, and bilayered vesicle aggregates from poly(eth-ylene oxide)-b-poly(γ-methyl-ε-caprolactone) in water [37].Discher's group reported the formation of worm-likemicelles from poly-(ethylene oxide)-b-poly(caprolactone)(PEO-b-PCL) [6, 38]. However, previous studies for multi-ple morphological aggregates of biodegradable amphiphiliccopolymer mainly focus on the amphiphilic diblock copol-ymer. Wang's group demonstrated spheres, elliptic spheres,short rods, and thread aggregates from amphiphilic tri-block copolymers based on poly(lactide-co-glycolide)-b-poly(ethylene oxide)-b-poly(lactide-co-glycolide) [22]. Infact, the self-assembly behavior of the block copolymer

is intensively controlled by the chain architecture [39]. Novelmorphologies and properties of aggregates prepared from thetriblock copolymer in a dilute solution can be expected toimprove our thoughts about nanoscale self-assembly of blockcopolymers because their architecture is more complicatedthan that of the diblock copolymer [40].

However, self-assembly in the solid state is often differ-ent from that in dilute solution, particularly during thedeposition film process from dilute solution, which is apopular preparation condition for transmission electron mi-croscopy (TEM) or scanning electron microscopy (SEM)characterization. Brinker et al. reported that evaporation-induced self-assembly can organize hydrophilic, inorganicand hydrophobic, organic precursor solution into orderedlayered nanostructures and also mesoporous organosilicatematerials [41–44]. It is apparent that there is a morpholog-ical transition from the nanostructures in solution to solid-state mesostructures during evaporation-induced self-assembly of biodegradable amphiphilic triblock copoly-mers, which will improve our understanding and insight ofthe factors to control the self-assembly. Few investigationson the morphological transformation from solution tocondensed phase are reported although a lot of exam-ples on self-assembly in the solution and condensedphase have been reported, respectively. Here, we reportnovel morphologies of aggregates prepared from biode-gradable poly(ε-caprolactone)-b-poly(ethylene oxide)-b-poly(ε-caprolactone) (PCL-PEO-PCL) triblock copolymers.It has been observed that the sisal-like mesostructures insolid-state deposition sample for SEM characterization weredifferent in solution. The final sisal-like morphology of con-densed aggregates in evaporation-induced self-assembly oftriblock copolymers depends strongly on the addition of alkalimetal ions.

Experimental section

Materials

ε-Caprolactone (ε-CL) (Sigma-Aldrich, USA) is purified bydrying over calcium hydride (CaH2) and distilling underreduced pressure. Dihydroxyl-terminated poly(ethylene ox-ide) (PEO), with molecular weight of 1,000, 2,000, 4,000,and 6,000 is purchased from Sigma-Aldrich, USA. Stannousoctoate (from Sigma-Aldrich, USA) is used as received. Allother chemicals are of analytical grade and used withoutfurther purification.

Synthesis and characterization of block copolymers

PCL-PEO-PCL triblock copolymers are synthesized by aring-opening polymerization of ε-caprolactone in the

1638 Colloid Polym Sci (2012) 290:1637–1646

presence of hydroxyl-terminated poly(ethylene oxide) withstannous octoate catalyst [45]. The molecular weights ofPCL-PEO-PCL copolymers are determined by 1H NMR(Varian INOVA400 spectrometer) with CDCl3 as solventwith 0.03v/v% tetramethylsilane as an internal standard.

Preparation of the spherical aggregates

The spontaneous self-assembly of amphiphilic blockcopolymers is prepared by a modified solvent extractionmethod. Briefly, the amphiphilic block copolymer is firstdissolved in THF to obtain a homogeneous solution at aninitial concentration of 1 mg/mL. Subsequently, deionizedwater is added to the polymer solutions slowly to produce anaqueous suspension under moderate agitation. After 30 minof additional agitation at room temperature, THF in thesuspension is then removed under reduced pressure.

Typical preparation of sisal-like aggregates

In a typical synthesis, 5 mg of amphiphilic block copolymeris dissolved in 5 mLTHF, which is followed by the additionof 10 mL 0.02 M NaOH solution under moderate agitation.After 30 min of additional agitation at room temperature, theTHF in the suspension is removed under reduced pressure,and the final volume of the aqueous suspension is concen-trated to 10 mL. The obtained aggregates in the aqueoussolutions are subsequently deposited onto support films andevaporated spontaneously at 25 °C. Finally, after waterevaporated from the block copolymer aggregates, theobtained solid condense phase aggregates were character-ized by SEM, TEM, and fluorescent microscopy.

Characterization

TEM is conducted on a JEM JOEL 3010 TEM operating at100 kV. The sample for TEM is prepared by dropping a3-μL solution onto a carbon-coated formvar copper grid(300 mesh) followed by solvent evaporation at room tem-perature. The sample for SEM (S4800, JEOL, Japan) isprepared by dropping a 30-μL solution on silicon substratesat room temperature. All of the samples are coated withplatinum using a vacuum evaporator before SEM. Sizedistributions of samples are measured at 25 °C using aZetasizer Nano-Zs (Malvern Instruments, UK). The concen-tration of samples in deionized water is kept constant at0.2 mg/mL. X-ray diffraction (XRD) measurements arecarried out on a Bruker D8-Advance powder X-ray diffrac-tion with Cu Kα radiation (λ01.5418 Å) from 0° to 80° at ascanning rate of 2.4°/min. X-ray tube voltage and currentare set at 40 KV and 40 mA, respectively. Fluorescencemicroscopy is conducted on an Olympus BX51, Japan.The sample in solutions was dyed using Rhodamine B.

Fourier transform infrared (FT-IR) spectra in pressed KBrpellets were recorded on a Fourier transformation infraredspectrometer (WQF-410).

Results and discussion

A series of amphiphilic triblock copolymers based on PCL-PEO-PCL with various segment lengths of PEO and PEO/PCL molar fraction are synthesized using PEO with twoterminal hydroxyl groups as the initiator. The molecularweights of the synthesized polymers calculated from 1HNMR are shown in Table 1. Typical 1H NMR spectra ofthe PCL-PEO-PCL triblock copolymer is shown in Fig. S1.

Synthesis and characterization of spherical aggregatesby spontaneous self-assembly of amphiphilic blockcopolymers

In the spontaneous self-assembly of biodegradable and bio-compatible amphiphilic block copolymer system in selectivesolvents, poly(ε-caprolactone) and poly(ethylene oxide) wereused as hydrophobic segment and hydrophilic segment, re-spectively. The spontaneous self-assembly of amphiphilicblock copolymers in selective solvents generally resulted inaggregates of a core-shell structure. In the present study, theself-assembly of amphiphilic PCL-PEO-PCL triblock copoly-mers are prepared by a modified solvent extraction method.The self-assembly of block copolymers occurred by addingwater dropwise, and the aggregate solutions are quenchedafter removing the organic solvent under reduced pressure.

Figure 1 shows a typical image of spherical aggregatesself-assembled in deionized water. Both SEM and TEMimages suggest that the micelle aggregates have a perfectspherical shape. Most of the spherical micelles have a porein the surface. (Fig. 1a, b) The pores at the surface of thenanospheres are probably the vapor exit due to the evapo-ration of THF under reduced pressure. The monoporous

Table 1 The compositions and characteristics of PCEC

Polymer Mn,PEO Mn,PCECa Mn,PCL

b ε-CL/EO (molar ratio)

PCEC-1 1,000 64,800 31,900 96/4

PCEC-2 2,000 64,000 31,000 92/8

PCEC-3 4,000 56,000 25,000 83/17

PCEC-4 6,000 38,000 16,000 67/33

PCEC-5 6,000 50,200 22,100 74/26

PCEC-6 6,000 72,000 33,000 81/19

PCEC-7 6,000 80,835 37,500 83/17

aMolecular weight of PCEC triblock copolymers calculated by 1 HNMRbMn,PCL0(Mn,PCEC−Mn,PEO)/2

Colloid Polym Sci (2012) 290:1637–1646 1639

microspheres have also been discovered by other groups[46–48]. Figure 1c shows the hydrodynamic diameter dis-tribution of PCL-PEO-PCL monoporous sphere, whichexhibits a narrow size distribution with an average size of400±70 nm.

Preparation and characterization of sisal-like aggregatesinduced by alkali metal ions and solvent evaporation

When deionized water is replaced by NaOH aqueous solu-tion, which is added dropwise into the copolymer solution,spherical micelles formed in solution. However, an interest-ing self-assembly phenomenon was observed during thewater evaporation from NaOH solution of triblock copoly-mer. Compared to the spontaneous self-assembly in deion-ized water, the sodium hydroxide-induced solution self-assembly of amphiphilic block copolymer is conductedusing similar conditions. The obtained aggregates in theNaOH aqueous solutions (30 μL) are subsequently deposit-ed onto support substrates, silicon wafer and copper grid forSEM and TEM, respectively. The evaporation-induced self-assembly directed by alkali metal ions was conductedthrough spontaneously evaporating water from the blockcopolymer at 25 °C.

The SEM image in Fig. 2 shows the typical morphologyof the ultimate dried aggregates at a different magnificationprepared from PCL-PEO-PCL (PCEC-7) amphiphilic tri-block copolymers. The most remarkable feature of theaggregates is their sisal-like structure (Fig. 2a, b), which israrely found in other micelle aggregates. Figure 2b, c is theamplificatory central part of sisal-like aggregates marked inFig. 2a–1. The sisal-like aggregates extend as long as500 μm and are much larger than single spherical PCECmicelles, which suggests the transition from sphericalmicelles to sisal-like hierarchical mesostructures. Thegrowth of the sisal-like aggregates originated from the cen-ter of the sisal bud (Fig. 2c) to the fringe (Fig. 2a). Thebranch cluster of the sisal-like mesostructures marked inFig. 2a–2 is enlarged and shown in Fig. 2d with a width of200 nm.

To further investigate the effect of sodium hydroxide onthe ultimate dried aggregates, the evaporation-induced self-assembly experiments at various initial NaOH concentra-tions are performed. Figure 3 shows the morphologies ofsisal-like mesostructures growing from water evaporation atvarious NaOH concentrations. Interestingly, when theNaOH aqueous solution of 0.01 M is dropped into thecopolymer solutions, the systems appeared transparent.

c

Fig. 1 Typical SEM (a), TEM(b), and size distribution (c)characterizations of the PCL-PEO-PCL spherical aggregatesprepared from PCEC-7

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With the water evaporation, the sisal-like mesostructuresresulted (Fig. 3a). Because of the increase of sodium hy-droxide concentration during water evaporation, the finaldried morphologies of the sisal-like mesostructures grewlarger, in which the branch clusters become broader andthicker (Fig. 3b, c). Additionally, it is observed that thebranch leaves change size from 200 to 800 nm when theNaOH concentration increases from 0.01 to 0.1 M. Further-more, the multilayer planar structures emerge at the concen-tration of 0.1 M (Fig. 3d). These results demonstrate that theinitial concentration of sodium hydroxide plays an impor-tant role in the size of the sisal-like mesostructures in thecondensed phase.

The similar structures are also observed when the NaOHaqueous solution is replaced by the same concentration ofNa2CO3 or NaHCO3 during the self-assembly process.There are no significant differences in the morphologicalpictures of the sisal-like mesostructures (Fig. 4a, b). How-ever, when the NaOH aqueous solution is replaced by thesame concentration of KOH during the self-assembly pro-cess, only spherical aggregates are observed (Fig. 4c). ForKOH-directed self-assembly, the aggregates are basicallyspherical or adhesion between the spheres with dimensionsof about 500 nm. In contrast, when the NaOH aqueoussolution is replaced by LiOH in the self-assembly process,

planar multilayer mesostructures are observed (Fig. 4d).The remarkable feature of LiOH-directed self-assemblyis the hierarchical aggregation structure composed ofmultiple layered sheets, which is different from theNaOH-directed sisal-like structures. This is probablyattributed to the relative different diameter of alkalimetal ions, R(Li+)<R(Na+)<R(K+). The Li ions andNa ions are much easier to form complexes with PEO blocks,which probably results in the alignment and crystallization ofthe polymer chains. When the aqueous solution is acidicduring the self-assembly process, the aggregates assemble tobe large compound micelles with no well-defined shape. Withthe increasing concentration of acidic medium, there are alsolarge compound micelles formed, along with cubic micelleswith a range of yield (Fig. S2).

The different hydrophilic block length and hydrophobic/hydrophilic block ratios can also greatly affect the morphol-ogy and size of the aggregates [22]. However, these impact-ing factors did not seem to be substantial under ourexperimental conditions. We prepared seven triblockcopolymers with different hydrophilic block length andhydrophobic/hydrophilic block ratios (PCLE-1 to PCLE-7,see Table 1). Copolymers PCEC-1, PCEC-2, PCEC-3, andPCEC-5 are composed of similar molecular weight butdifferent PEO block length, and PCLE-4 to PCLE-7 are

Fig. 2 Typical SEM images ofthe sisal-like aggregates pre-pared from PCEC-7. a Fullview of aggregates; b sisal-likestructure of aggregates; c sisalbud of the sisal-like aggregates;d side elevation on the branchof the sisal-like aggregates

Colloid Polym Sci (2012) 290:1637–1646 1641

composed of the same PEO block (PEO 6000) but differentPEO/PCL block ratios. For all of the copolymers, the hy-drophilic PEG blocks are shorter than the hydrophobic PCLblocks. Under the same preparation conditions, the sodiumhydroxide concentration is fixed at 0.02 M and the

amphiphilic block copolymer at 1 mg/mL. However, byadjusting the hydrophilic block length and block ratios ofPEO/PCL in this way, no distinct differences in aggregatemorphologies are observed (Fig. S3, S4), and the aggregatestructures remain sisal-like. This demonstrates that the

Fig. 3 SEM images of thebranch of sisal-like aggregatesprepared from PCEC-7 at vari-ous NaOH concentrations. a0.01 M, b 0.02 M, c 0.03 M, d0. 1 M

Fig. 4 SEM images of themorphology of dried aggregatesprepared from PCEC-7, whichis induced by a Na2CO3, bNaHCO3, c KOH, and d LiOH

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existence of sodium hydroxide determines much more theself-assembly of the triblock copolymers than the composi-tions of the triblock copolymers.

The mechanism for the self-assembly of PCL-PEO-PCLamphiphilic triblock copolymer

The effect of added alkali metal ions on the aggregatemorphology is remarkably strong. It is very desirable tofigure out the formation mechanism—how the aggregatingmesostructures are obtained. SEM and TEM images onlyshow the morphologies of samples in the dried state, not insitu in solution. Hence, we first need to make sure of the realmorphology of PCL-PEO-PCL in the NaOH aqueous solu-tion. Fluorescence microscopy is employed to observe theaggregates' morphology in NaOH aqueous solution after theTHF in the suspension is removed under reduced pressure.The aggregates in solution are stained by Rhodamine B. It isfound that when the concentration of NaOH is 0.02 M, theobtained aggregates in NaOH aqueous solution are stillspherical (Fig. 5a). However, when the obtained aggregates(30 μL) in NaOH aqueous solution are subsequently depos-ited onto support films and dried at 25 °C for 12 h, theremarkable feature of sisal-like mesostructures is observedunder the fluorescence microscope (Fig. 5b), which is inaccordance with the SEM images. In order to eliminate theeffect of alkali metal ions during water evaporation, theNaOH aqueous solution of copolymer micelles are centri-fuged to remove additional metal ions, and then, the

obtained centrifugal solids are redispersed into deionizedwater and the same concentration of NaOH aqueoussolution, respectively. The process is repeated threetimes, and the resulting sample is deposited onto sup-port films and dried at room temperature for SEM. Asshown in Fig. 5c and d, only spherical nanoparticleswith a size of 400±50 nm are observed when the obtainedcentrifugal solid is dispersed into deionized water. However,the sisal-like mesostructures are observed when the obtainedcentrifugal solid is dispersed into the same initial concentra-tion of NaOH (0.02 M) aqueous solution. These results sug-gest that the morphology of the aggregates at lowerconcentrations of the NaOH aqueous solution is presented inthe form of spherical nanoparticles, while spherical nanopar-ticles in solutions transformed into sisal-like mesostructuresalong with the increase of alkali metal ion concentrationsduring the evaporating process.

Since the PCL-PEO-PCL block copolymer includes bothhydrophobic PCL and hydrophilic PEO segments, duringthe self-assembly process, the hydrophilic PEO segmentwould extend towards the aqueous medium to form a shell,while hydrophobic PCL segments will entangle to the rela-tive water-free core. We measured the water contact anglesof spherical aggregates and sisal-like aggregates self-assembled on the silicon substrates. The water contact angleof spherical aggregates is measured at 69° (Fig. 6a), whilethat of the sisal-like aggregates measured under the sameconditions is 40° (Fig. 6b). It is demonstrated that duringformation of the aggregates, the hydrophilic PEO all along

Fig. 5 Fluorescencemicroscopy images of a self-assembled polymer sphericmicelles in NaOH aqueous so-lution and b naturally con-densed aggregates after waterevaporation. SEM images of cthe self-assembled aggregatesdispersed into deionized waterfollowed by drying and d for-mation of sisal-like aggregateswhen polymer sphericalmicelles are dispersed in thesame concentration of NaOHaqueous solution followed byevaporation-induced self-assembly

Colloid Polym Sci (2012) 290:1637–1646 1643

is exposed to the surface of the aggregates. The sisal-likemesostructures reported herein possess more affinity to wa-ter and surface wettability than spherical micelles due to thehigh Na+ content.

Therefore, we deduce that the formation of sisal-likemesostructures is relative to the increase of alkali metalion concentration during the evaporation-induced self-assembly process. It is well known that PEO is an excep-tional polymer for the development of high-performancesolid-state ion batteries and fuel cells, as well as sensorsand electrochromic devices, because PEOs are great ligandsfor alkali metal ions. The ion conduction mechanism isassumed to be cationic mobility, in which polymer chainmotion plays a significant role [49, 50]. In the past decades,the coordination of PEO with alkali metal ions has beenextensively studied using XRD and FT-IR. When thePEO is coordinated with alkali metal ions, a new crys-tallizing point of the metal ion–polymer system proba-bly appears. We deduce that the formation of sisal-likemesostructures is attributed to the interaction of alkalimetal ions with PEO. In the present study, we take theexample for the coordination of Na+ with PEO to fur-ther confirm our deduction by XRD. The XRD patternsof the original polymer of PCL-PEO-PCL and coordi-nated polymer aggregates PCL-(PEO-Na)-PCL areshown in Fig. 7. A comparison of X-ray diffractionpatterns shows that there are obvious differences indiffraction patterns between the original polymer ofPCL-PEO-PCL and that of the PCL-(PEO-Na)-PCL

polymer complex. Obviously, for the original polymer ofPCL-PEO-PCL, there are only two major peaks at about21.3° and 23.8° of 2θ, which corresponded to the typicalPCL crystalline peaks [51]. Both PCL and PEO-Na blocks inthe PCL-(PEO-Na)-PCL complex could crystallize and formseparate crystal phases because their XRD curves are just asummation of the PEO and PCL crystal patterns, where thepeaks corresponding to PEO-Na crystalline are at about 19.3°and 23.4° of 2θ. These results suggest that the crystallinity ofthe middle PEO chain is restricted extensively by both sides ofthe PCL when the PCL chains are long enough; thus, the PEOcrystalline peak disappears, and only the crystal patterns typ-ical for the PCL crystal phase are observed in the originalpolymer of PCL-PEO-PCL, which is accorded with the previ-ous report [51]. However, the coordination of PEO with Na+

would change the steric configuration of the triblock copoly-mer, which might decrease the restriction of PCL to PEO sothat the crystalline peaks of PEO-Na become stronger thanPCL. The diffraction profiles of PEO-Na and PCL-(PEO-Na)-PCL are very analogous (Fig. 7c, e), indicating the formationof a stable crystalline PEO-Na, both in PEO-Na and the PCL-(PEO-Na)-PCL complex, and the limitation of crystalline PCL.

The FT-IR spectra of the PCL-PEO-PCL original poly-mer and PCL-(PEO-Na)-PCL polymer complex also con-firm our deduction. As shown in Fig. 8, for the originalPCL-PEO-PCL polymer, the peak at 2,949 cm−1

Fig. 6 The water contactangles of a spherical micellesand b sisal-like mesostructures

10 20 30 40

D

E

C

B

Inte

nsity

(a.

u.)

2 theta (degree)

A

Fig. 7 XRD spectra of a NaOH, b PEO, c PEO-Na complex, doriginal polymer of PCL-PEO-PCL (PCEC-7), and e coordinated poly-mer aggregates of PCL-(PEO-Na)-PCL

4000 3000 2000 1000

1590

11861244

1450

1470

1450

1103

1186

34372889

11071242

11051244

1570

1730

1728

Tra

nsm

ittan

ce (

%)

Wavenumbers (cm-1)

3440

3330

2949

2939

2868

2864

A

B

C

D

34352887 1242

1107

1470

Fig. 8 FT-IR spectra of a PEO, b PEO-Na complex, c original poly-mer of PCL-PEO-PCL, and d coordinated polymer aggregates of PCL-(PEO-Na)-PCL

1644 Colloid Polym Sci (2012) 290:1637–1646

corresponds to the absorption of C–H stretch of CH2 fromthe PCL blocks, while the peak at 2,868 and 1,107 cm−1

belongs to the C–H stretching band and C–O–C stretchingvibration band of PEO, respectively. The strong absorptionat 1,728 cm−1 corresponds to the stretching vibration bandof C0O from PCL segment, and the peaks at 1,186 and1,244 cm−1 are due to the stretch vibration band of C–O–Cand C–O. These three peaks demonstrate that the estergroups exist in the triblock copolymer (Fig. 8c). The fol-lowing change in the spectral feature has been observed oncomparing the spectrum of PCL-(PEO-Na)-PCL polymercomplex. If Na+ ions get coordinated with the oxygen ofPEO, the spectral changes are expected to be in the C–O–Cstretching and deformation ranges [52]. As shown inFig. 8d, in the PCL-(PEO-Na)-PCL polymer complex, theintensity of C–O–C stretching vibration at 1,103 cm−1

increases, which is very analogous with the pure PEO-Nacomplex in Fig. 8b. New peaks around 1,450 and1,570 cm−1 have been observed in the PCL-(PEO-Na)-PCL polymer complex, where the appearance of new peaksalong with changes in existent peaks in the FT-IR spectradirectly indicates the coordination of PEO with Na+. Thus,the results of XRD and FT-IR data clearly indicate thecoordination of PEO with Na+.

Therefore, we can conclude that the formation of sisal-like mesostructures is attributed to the strong coordinationbetween alkali metal ions and PEO in the block copolymer,which act as a structure-directing agent and induce thereorganization of spherical micelles into the sisal mesostruc-tures during water evaporation from triblock copolymersolutions. As described in the introduction, the aggregatemorphology is determined by three factors: stretching (de-formation) of the core-forming blocks in the core, the repul-sive interaction among the corona chains, and the interfacialtension between the micelle core and the solvent. Althoughthe PCL-PEO-PCL block copolymers differ from the PS-based amphiphilic block copolymer investigated by Eisen-berg's group, the major factors that control the morphologycan be described employing the same terms. The equilibri-um structure and stability of aggregates can be attributed tothe free energy of the system [53]. In summary, if kineticallypossible, the free energies in the system influence greatly theforce balance and control the absorption of the lowest ener-gy morphology [54, 55]. In this case, the morphology ofsisal-like mesostructures observed may be explained by theforce balance factors discussed above, because PEO in thePCL-PEO-PCL block copolymer all along is exposed to thesurface of the spherical nanoparticles, which extend towardsthe alkali metal ion aqueous medium. As the concentrationof alkali metal ions increases up to a certain range duringwater evaporation, it exhibits great affinity and coordinatesstrongly to the PEO block, and induces a new crystallizingpoint in the polymer complex. The strong binding of PEO

with alkali metal ions leads to the decrease of free energy inthe system. It is well recognized that the major driving forceunderlying micelle formation is the decrease of the freeenergy of the system [56]. In this case, with the support ofthe driving force from the strong coordination between PEOand alkali metal ions, the stable system of spherical micellesin solution is destroyed, a large number of spherical micellesmove and merge, and some of the micelles possibly fusetogether, resulting in the formation of sisal-like mesostruc-tures. The alkali metal ion concentration increases duringthe water evaporation process, and larger sisal-like meso-structures are obtained.

Conclusions

We have described novel sisal-like aggregating morpholo-gies of PCL-PEO-PCL copolymers, which have been grownfrom evaporation-induced self-assembly directed by alkalimetal ions. When adding NaOH aqueous solution dropwiseinto the copolymer solutions, spherical micelles are formedin aqueous solution. When the solutions are subsequentlydeposited onto supporting substrates, the large sisal-likehierarchical mesostructures at the condensed state are ob-served by SEM and fluorescence microscopy. We find thatthe initial concentration of NaOH plays an important role inthe growth of the final dried mesostructures, while thecompositions of triblock copolymer have no obvious influ-ence on the morphologies of the dried mesostructures. Sim-ilar structures are also found during evaporation-inducedself-assembly when the NaOH aqueous solution is replacedby the same concentration of Na2CO3 or NaHCO3 duringthe self-assembly process. The mechanism for the self-assembly of the PCL-PEO-PCL amphiphilic triblock copol-ymer is attributed to the continuous increase of the alkalimetal salt concentration and thus the stronger coordinationbetween alkali metal ions and PEO ligands in the blockcopolymer during water evaporation process.

Acknowledgments We are thankful for the financial support fromNSFC (20874025, 21174045), the Fundamental Research Funds forthe Central Universities (HUST2010MS101), and the Open Founda-tion of Beijing National Laboratory of Molecular Science.

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