Polyoxyethylene-Poly(L-lactide) - ACS Publications

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O/W Emulsion Templated and Crystallization-Driven Self-Assembly Formation of Poly(L-lactide)-Polyoxyethylene-Poly(L-lactide) Fibers Chunyu Li, Rui Liu, Qingbin Xue, Yaping Huang, Yunlan Su, Qiang Shen, and Dujin Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02596 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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O/W Emulsion Templated and Crystallization-Driven Self-Assembly Formation of Poly(L-lactide)-Polyoxyethylene-Poly(L-lactide) Fibers Chunyu Li,a Rui Liu,a Qingbin Xue,a Yaping Huang,b Yunlan Su,b Qiang Shen,*,a and Dujin Wang*,b

Key Laboratory for Colloid and Interface Chemistry of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China, and Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

a

Shan Da Nan Road 27, Jinan 250100, P. R. China.

b

Zhongguancun North First Street 2, Beijing 100190, P. R. China.

* E-mail: [email protected] (Q. Shen); [email protected] (D. J. Wang).

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ABSTRACT A molecular solution of an amphiphilic block copolymer may act as an oil phase by dispersing into an aqueous micellar system of small-molecular surfactant, forming oil-in-water (O/W) emulsion droplets. In this paper, an as-synthesized triblock copolymer poly(L-lactide)-polyoxyethylene-poly(L-lactide) (PLLA-PEO-PLLA) was dissolved in tetrahydrofuran (THF) and then added to an aqueous micellar solution of nonaethylene glycol monododecyl ether (AEO-9), forming initially coalescent O/W emulsion droplets in the size range of 35 nm - 1.3 µm. Along with gradual volatilization of THF and simultaneous concentration of PLLA-PEO-PLLA molecules, the amphiphilic copolymer backbones themselves experience solution-based self-assembly forming inverted core-corona aggregates within an oil-phase domain. Anisotropic coalescence of adjacent O/W emulsion droplets occurs, accompanied by further volatilization of THF. The hydrophilic block crystallization of core-forming PEOs and the hydrophobic chain stretch of corona-forming PLLAs together induce the intermediate formation of rodlike architectures with an average diameter of 300-800 nm and this leads to a large-scale deposition of the triblock copolymer fibers with an average diameter of ~2.0 µm. Consequently, this strategy could be of general interest in the self-assembly formation of amphiphilic block copolymer fibers, and could also provide access to aqueous solution crystallization of hydrophilic segments of these copolymers.

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INTRODUCTION Research and development of tissue engineering, catalytic technology and energy storage require novel materials with complex shapes and architectures and with structural characteristics at molecular, nanoscopic, mesoscopic and/or macroscopic levels.1-4 Droplets of an emulsion have been recognized as possessing useful templates for the formation of complex structures in which the interface between oil and water phases is typically stabilized by small-molecular emulsifying agents or amphiphilic polymeric surfactants.5-7 In particular, copolymer-containing emulsion droplets could even self-assemble into a configuration through the synergistic actions among small-molecular surfactants and amphiphilic copolymers, and for instance, induce uniform formation of a macroscopic porous structure by tuning reversible hydrogen-bonding interactions of polymeric surfactants or surfactant-polymer interactions.4,8,9 In recent years, a method of emulsification/solvent evaporation has emerged as a promising route with which to process amphiphilic block copolymer assemblies such as spheres, vesicles or cylinders and other exotic structures.10-14 The acquired structure and chemical diversity of copolymer aggregates make them attractive for potential applications in drug delivery and for controlling syntheses of multifunctional materials.15,16 During the emulsification/solvent evaporation self-assembly process of amphiphilic block copolymers, two aspects are: (i) choice of organic solvents, experimental temperature, the composition and polymerization degree of chemically different subunits may jointly determine a well-defined structure of an aggregate, and (ii) the presence of small-molecular surfactants could tune the size and/or configuration of copolymer self-assemblies, depending upon a balance between the properties of the two amphiphiles.17,18 Without the emulsification of the oil phase and evaporation of the solvent, self-assembly of coil-coil, rod-coil or crystalline-coil amphiphilic block copolymers may induce the uniform formation 3


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of nanofibers. The primary building units of a fiber-like architecture are usually spheres for coil-coil block copolymers, and disk-like or cylinder structures for rod-coil or crystalline-coil block copolymers.19-23 Compared to the flexible feature of block copolymer coil segments in a selective solvent, the rod or crystalline counterparts are relatively rigid and prefer to adopt a parallel alignment for self-assembly in solution. This is probably the main reason why rod-coil or crystalline-coil block copolymers can be predicted to form one-dimensional structures with intrinsically low interfacial curvature, such as cylinders and fibers or even two-dimensional structures including rectangular and lenticular platelets.21-25 As for the self-assembly in solution of amphiphilic block copolymers, a recent research topic concerning the crystallization of one or two blocks should be considered. This deals with the further oriented aggregation, epitaxial growth or anisotropic self-assembly of primary building units.21,22,26-38 In one of crystallization-driven self-assembly systems, the crystallized copolymer (or its segments) includes polyethylene,21,28-30 poly(ε-caprolactone),31,32 poly(ethylene oxide),33 poly(ε-caprolactone-bL-lactide),34 polylactide,26-35 polyacrylonitrile,36 poly(ferrocenyl dimethylsilane),27 poly(ferrocenyl dimethylgermane),37 or poly(perfluorooctyl ethyl methacrylate).38 Simultaneously, various methods of the fabrication of fiber-like semi-crystalline supramolecular materials of block copolymer have been explored. As an example, crystallization and epitaxial growth of the polyferrocenylsilane (PFS) blocks follows the addition of PFS block-copolymer unimers into the sonication of short-rod and stub-like seeds with active ends.39,40 Another example is the preparation of semi-crystalline PEO-b-PLLA copolymer fibers in which discoid aggregates in aqueous dispersion were initially deposited onto a mica substrate and subsequent annealing induced crystallization of PLLA blocks and facilitated the formation of fiber-like superstructures.41 In this paper, serial PLLA-PEO-PLLA triblock copolymers were first synthesized by adjusting the 4


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polymerization degrees of hydrophilic and/or hydrophobic segments. Secondly, each of the triblock copolymers was dissolved in THF, and then as an oil phase the copolymer solution was added dropwise into a vigorously stirred aqueous micellar solution of non-ionic surfactant AEO-9, homogeneously forming O/W emulsion droplets. Along with the gradual volatilization of the THF and the simultaneous anisotropic coalescence of O/W emulsion droplets, the phase separation of copolymer hydrophilic and hydrophobic segments within an oil droplet, and the crystallization of core-forming PEOs within O/W emulsion droplets, drives the large-scale formation of triblock copolymer fibers with an average diameter of ~2.0 µm. A plausible mechanism of the formation of the micrometer-sized semi-crystalline copolymer fibers was postulated and will be discussed in detail below.

EXPERIMENTAL SECTION Materials and Polymerization All organic solvents are of analytic grade and toluene was dried by sodium/potassium alloy (Na/K benzophenone) for polymerization. L-lactide (99.9%) were supplied by Jinan Daigang Biomaterials and recrystallized from ethyl acetate. Dihydroxyl polyoxyethylene (PEO) with molar masses of 10000 and 20000 were obtained from Sinopharm, recrystallized from tolubene and then further purified by reduced pressure to remove water and toluene three times. Stannous octoate Sn(Oct)2 was purchased from Sinopharm and redistilled before use. Serial copolymers of poly(L-lactide)-polyoxyethylene-poly(L-lactide) (PLLA-PEO-PLLA) were synthesized by ring-opening polymerization of L-lactide using PEO and Sn(Oct)2 as the macroinitiator and catalyst, respectively. First, PEO was dried at room temperature in vacuum for at least 12 h to constant weight and then predetermined amounts of PEO and L-lactide were introduced into a two-neck flask under purified nitrogen. Second, after the addition of toluene and stannous octoate at a 5


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Sn(Oct)2:PEO molar ratio of 2:1 injected into the flask, and when all the samples were dissolved completely, the polymerization was conducted at 110 °C for 24 h. Third, after the evaporation of toluene, raw products of each target copolymer were dissolved in dichloromethane and washed with 1.0 mol L-1 HCl (aq.) three times to eliminate Sn(Oct)2. Finally, when the dichloromethane solution was washed to remove extra HCl, the as-synthesized copolymer was precipitated by cold petroleum ether and then dried at room temperature under vacuum. Preparation of Copolymer Fibers Ultrapure water (18.2 MΩ·cm) was used throughout the solution preparation, and the synthesis and rinsing of the copolymer fibers. Briefly, the nonionic surfactant of nonaethylene glycol monododecyl ether AEO-9 (C12H25O(CH2CH2O)9H, 1.8806 g) was dissolved in water (100.0 mL) and an as-synthesized copolymer PLLA-PEO-PLLA (40.0 mg) was dissolved in tetrahydrofuran THF (4.0 mL). After being filtered through a 0.45 µm membrane, the copolymer solution in THF was added dropwise into the AEO-9 stock solution, which was simultaneously stirred magnetically for 30 min. Then, as an open system the resulting admixture was allowed to sit at room temperature for ~14 days. Along with the extension of incubation time, the evaporation of THF and water could induce the deposition of flocculent PLLA-PEO-PLLA fibers. Unless otherwise stated, these copolymer fibers were thoroughly rinsed by water, freeze-dried and used for characterization. Characterization Prior to scanning electron microscope (SEM, SU8010) measurements, each water-rinsed sample was placed on a silicon wafer, dried at room temperature, coated with 1 nm Pt film and then analyzed using a field emission source at an accelerating voltage of 5 kV. X-ray diffraction (XRD, Rigaku D/max-2400) tests were conducted using Cu-Kα radiation (λ = 1.5406 Å, 40 kV, 120 mA), 0.08° step (25 s) and the 2θ range of 10-50°. Prior to the transmission electron microscopy (TEM) measurements 6


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on a JEM 2200 FS microscope (200 kV), samples were deposited onto a copper grid with carbon film. Before observation by polarized optical microscopy (POM, Olympus BX51p), each sample was taken out of its mother solution, placed directly between microslide and coverslip and examined with a Linkam THMSE 600 cold-hot stage (-196 - 600 °C). Differential scanning calorimetry (DSC Q2000, TA) was conducted under a nitrogen atmosphere, and indium was used to calibrate each measurement. As-synthesized PLLA-PEO-PLLA copolymers and corresponding fibers were scanned comparatively from -50 °C to 190 °C at a rate of 10 °C·min-1. Fourier transform infrared spectroscopy (FT IR) data was obtained on a Bruker Tensor 27 spectrometer using KBr tablets in a transmission mode over a region of 4000-400 cm-1 and with a resolution of 4 cm-1.

RESULTS AND DISCUSSION Formation and Structural Characterization of PLLA-PEO-PLLA fibers In the laboratory, triblock copolymer PLLA-PEO-PLLA can be readily synthesized by ring-opening polymerization of L-lactide using polymeric PEO as a macroinitiator (Scheme S1). As-obtained targets are linear for each molecular backbone and are both hydrophilic and hydrophobic in nature. Chemical composition determination and structural characterization of each PLLA-PEO-PLLA was performed and is shown in Figures S1-S3. Gel permeation chromatography (GPC) results of serial PLLA-PEO-PLLA and their hydrophilic and hydrophobic subunit ratios of polymerization degree are summarized and shown in Table 1. As distinct from small-molecule surfactants, as-synthesized copolymers are water-insoluble and possess almost no surface activity when initially dissolved in THF, but they can interact strongly with non-ionic surfactant AEO-9 molecules when the THF-based solution is added dropwise to an AEO-9 aqueous micellar system (Figure S4).42 As shown in Figure S5, only the visually homogeneous O/W 7


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emulsion of 4.0 × 10-2 mol L-1 AEO-9 and 800.0 mg L-1 PLLA-PEO-PLLA was used for the self-assembly of each triblock copolymer. In such cases, the molecular solution of a triblock copolymer in THF could act as oil phase and thus the emulsification/solvent evaporation method is expected to result in solution self-assembly.

Table 1. GPC results of serial PLLAx-PEOy-PLLAx copolymers. Copolymer

EO/LAa

Mn PEO

DPPEOb

DPPLLAc

Mnd

Mne

Mw/Mnf

PLLA20PEO227PLLA20

11.35(7.57)

10000

227

20

12880

18926

1.03

PLLA53PEO227PLLA53

4.28(3.34)

10000

227

53

17632

25340

1.04

PLLA109PEO227PLLA109

2.08(1.64)

10000

227

109

25696

31495

1.34

PLLA53PEO454PLLA53

8.57(6.68)

20000

454

53

27632

30622

1.16

Note:

a

calculated from the integrations of NMR resonances belonging to the methylene protons of

ethylene oxide units of PEO at ~3.6 ppm and to the methine proton of lactyl units of PLLA at ~5.2 ppm, numbers in parentheses represent the EO/LA feed ratios; b DPPEO = Mn

PEO/44.

c

DPPLLA =

DPPEO/(EO/LA); d the calculated number-average molecular weight of copolymer Mn (Mn = Mn PEO + 72DPPLLA); e the Mn of copolymer determined by GPC; f the index of polydispersity.43,44

Scheme 1. A schematic drawing of the experimental procedure adopted for the formation of inverted core-corona structures within an O/W emulsion droplet.

As shown in Scheme 1, three steps describe the formation of inverted core-corona structures 8


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within an O/W emulsion droplet. In the first step, a PLLA-PEO-PLLA solution in THF was added into the aqueous micellar system of non-ionic surfactant AEO-9, forming the nearly transparent O/W emulsion and acquiring the O/W emulsion droplets with a diameter of 4 - 18 nm under vigorous stirring. In the second step, collision and coalescence of these primary droplets occurs, resulting in the larger droplets with a diameter of 35 nm - 1.3 µm (see also Figure S5). Along with volatilization of THF for the open emulsion system, the phase separation of PLLA and PEO segments within an O/W droplet might spontaneously emerge owing to the simultaneously concentration of PLLA-PEO-PLLA molecules (i.e., the third step). Probably, here the rigid chain stretching of corona-forming PLLAs could further contribute to the substantial increase in the diameter of the O/W emulsion droplets. After incubation for 14 days, the sedimentary fibers of each triblock copolymer were taken out of the aqueous emulsion of 4.0 × 10-2 mol L-1 AEO-9 and 800.0 mg L-1 PLLA-PEO-PLLA, and SEM images of thoroughly water-rinsed PLLA53PEO227PLLA53 fibers are shown in Figure 1a and b. These fibers may link with each other, forming a collection of strings in the incubation system. Prior to the water rinse, it is extremely difficult to distinguish an individual string (i.e., a micrometer-sized fiber) from the others under SEM observation. As shown in Figure 1a or 1b, there exist scattered nanoparticles of triblock copolymers. This could be explained by the fact that, aside from the removal of adsorbed surfactants, the thorough water rinsing may induce the partial dissolution of crystalline PEO blocks and then lead to the partial exfoliation of self-assembled copolymer molecules from a fiber. Statistical analysis suggests that these fibers are several centimeters in length and ~ 2 µm in diameter. XRD patterns of as-synthesized triblock copolymer PLLA53PEO227PLLA53 and its self-assembled fibers are revealed in Figure 1c. Because of the crystallization occurring during sample drying, as-synthesized PLLA53PEO227PLLA53 mainly presents three diffraction peaks at the 2Theta positions of 16.82°, 19.20° and 23.43°, and the three reflections could be sequentially assigned to the crystal 9


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planes of (110)PLLA/(200)PLLA, (113)PLLA/(203)PLLA/(120)PEO and (032)PEO according to literature results.45-55 With or without water rinse, the freeze-drying operation probably causes these fibers to exhibit crystallinity, and polarized optical microscopy can clearly demonstrate the visually semi-crystalline feature of fibers formed in situ (Figures 1d, S6). Only reflections of the monoclinic crystal phase of PEO blocks could be detected (Figure 1c),52-55 implying a plausible hydrophilic block crystallization-driven mechanism for the anisotropic self-assembly. Furthermore, a high-resolution optical microscopy image of unrinsed fibers clearly exhibits a linear array of “transparent” crystalline PEO beads within the center of a fiber (Figures 1d, S7).

Figure 1. (a, b) SEM images of thoroughly water-rinsed PLLA53PEO227PLLA53 fibers obtained after an incubation time of 14 days. (c) Comparative XRD patterns of as-synthesized copolymers, unrinsed PLLA53PEO227PLLA53 fibers and thoroughly water-rinsed copolymer fibers. (d) An optical microscopy photograph of copolymer fibers taken rapidly out of the mother solution.

Gradually magnified SEM images of the thoroughly water-rinsed copolymer fibers show overall a network arrangement (Figure 2a, b), a ribbon-like shape with branches (Figures 2b, c) and a porous 10


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structure for the interior (Figure 2d). After the initial water rinsing and subsequent freeze-drying, the structural collapse of interior-fiber aggregates, the removal of adsorbed AEO-9 molecules and the partial dissolution of crystallized PEO blocks could simultaneously explain the ribbon-like skeleton and concave-like center of each fiber.

Figure 2. (a-d) SEM images of thoroughly water-rinsed PLLA53PEO227PLLA53 fibers, sequentially showing a ribbon-like shape in morphology and a porous interior in the structure. (e, f) TEM images of unrinsed PLLA53PEO227PLLA53 fibers observed shortly after an ultrasonic treatment in pure water.

When fresh PLLA53PEO227PLLA53 fibers were taken out of processing system and transferred into a large amount of pure water, ultrasonic processing should exert a great influence on the surface texture of each micrometer-sized fiber. This ultrasonic processing should induce almost all of the adsorbed surfactant molecules to detach, and then loosely arranged PLLA chains lose their physical 11


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protection and present a nanofiber-like surface texture (Figure 2e). This, together with the dissolution of crystallized PEO, could further promote an exfoliated O/W emulsion droplet to become a porous microsphere (Figure 2f) which acquires a coating layer of randomly arranged PLLA-based nanofibers and a random distribution of PEO-block nanoparticles near the shell.

Figure 3. SEM images and diameter-statistical histograms of thoroughly water-rinsed fibers: (a) PLLA20PEO227PLLA20;

(b)

PLLA53PEO227PLLA53;

(c)

PLLA109PEO227PLLA109.

12


PLLA53PEO454PLLA53;

(d)

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SEM images of these copolymer fibers and the statistical distributions of their diameters are shown comparatively in Figure 3. On one hand, the multiplied increase of PLLA-segment polymerization degree fails to promote the average diameter of the self-assembled copolymer fibers greatly (PLLA20PEO227PLLA20, 1.80 µm, Figure 3a; PLLA53PEO227PLLA53, 2.15 µm, Figure 3b; PLLA109PEO227PLLA109, 2.35 µm, Figure 3d), and this may be due to the possibly declining arrangement and amorphous feature of the PLLA stretching chains. On the other hand, by changing the polymerization degree of PEO segments, a nearly identical value for the average diameter of different copolymer fibers was obtained: PLLA53PEO227PLLA53, 2.15 µm (Figure 3b); PLLA53PEO454PLLA53, 2.13 µm (Figure 3c). If the number of the copolymer self-assembled core-corona aggregates is the same within an O/W emulsion droplet, the results of Figure 3 can be explained both by the confined crystallization of flexible PEO segments and by an effective volume buffering of loosely arranged PLLA segments around crystallized PEO beads.

Figure 4. DSC thermograms of as-synthesized copolymers (the second heating), unrinsed and water-rinsed fibers: (a) PLLA20PEO227PLLA20; (b) PLLA53PEO227PLLA53; (c) PLLA53PEO454PLLA53; 13


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(d) PLLA109PEO227PLLA109.

In SEM observation, an adverse effect of electron-beam irradiation on the crystallinity of these PLLA-PEO-PLLA fibers is emphasized. The higher polymerization degree of the PEO segments, or the lower polymerization degree of the PLLA segments, and the more obvious is the visually-detected melting phenomenon of the fibers (Figure S8). In view of the melting points of homopolymer PLLA (140 - 180 °C) and PEO (40 - 60 °C), the DSC behaviors of serial copolymers and their self-assembled fibers are recorded at a rate of 10 °C·min-1 in the temperature region of -50 and 190 °C. In each panel of Figure 4, the results of secondly heating these copolymers generally exhibit a PEO-segment peak between 41.2 and 57.0 °C and a PLLA-segment peak above 140 °C. By the elimination of thermal history, these depend upon the feed ratio of EO to LA primary units for copolymer syntheses (Table 1), and the absence of the PLLA-segment peak of as-synthesized PLLA20PEO227PLLA20 (Figure 4a) may be attributed to the low DSC-measurement resolution and/or to the highest EO/LA feed ratio of 11.37.56 Concerning the water-rinsed fibers of each PLLA-PEO-PLLA copolymer, only the two or three melting peaks of PEO segments appear in the corresponding DSC trace (Figures 4a-d), depending upon both the quality of the crystal and the distribution of the segment location.57,58 Also in each DSC curve, the main PEO-melting peak at about 41.4 °C indicates that: (i) the confined crystallization of PEOs occurs within water-isolated domains; (2) the effect of PLLA chain length on the crystallization of PEOs can reasonably be ignored.53 Furthermore, compared with as-synthesized copolymers and their unrinsed fibers (Figure 4d), the DSC behaviors of water-rinsed fibers could give a weak PLLA-melting signal when the corresponding EO/LA feed ratio is lower than 4.3 (Table 1). Without the assistance of adsorbed surfactants, the water rinse could simultaneously decrease the degree of crystallinity of PEOs confined in the central part of a micrometer-sized fiber. 14


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Self-Assembly Formation Mechanism of PLLA-PEO-PLLA fibers

Figure 5. (a, b) TEM images of 2-day intermediates taken after negative staining with phosphotungstic acid. (c, d) SEM images of stub-like intermediates obtained at an incubation time of 4 days. (e) FT IR spectra of various samples, highlighting the hydrophobic interactions between surfactant and copolymer hydrocarbon segments. (f) XRD patterns of rodlike intermediates and final fibers, showing the crystallinity increase of PEO blocks with increasing time.

As mentioned above, these copolymers are THF-soluble and water-insoluble, and thus the miscible property of THF and water was considered to conduct the two blank experiments without AEO-9 aqueous solution and without surfactant (Figure S9). These demonstrate the template effect of O/W emulsion droplets for the formation of micrometer-sized copolymer fibers in the processing system of 15


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4.0 × 10-2 mol L-1 AEO-9 and 800.0 mg L-1 PLLA-PEO-PLLA. As soon as O/W emulsion droplets were formed, both phase separation of chemically different segments and crystallization of the triblock polymers could proceed randomly, resulting in hollow spherical aggregates composed of nanoparticles and/or sticks (Figure S10). The rare case is currently unexplained, but this supports the O/W emulsion templated and PEO-block crystallization-driven self-assembly formation of fibers shown in Figure 1d. Time-dependent experiments were used to trace the morphological evolution as shown in Figures 5a-d, S11 and S12. After the coalescence of adjacent emulsion droplets, several of them experienced random collision and anisotropic fusion that would be expected to be governed by the minimization of total free energy.59,60 The initially self-assembled rodlike intermediates with a diameter of 300 - 800 nm can be easily detected by TEM (Figures 5a, 5b and S11). Along with the slow volatilization of THF at room temperature, core-forming PEO segments may gradually shrink and then precipitate or crystallize within a water-isolated central domain of rodlike intermediates (Figures 5c, 5d and S12). Subsequently, prolongation of incubation time could induce the precipitation of PLLA blocks owing to the disappearance of THF within the external arc region of a core-corona subunit, which should be related to the chain stretching of rigid PLLAs outside each PEO-core. During the self-assembly process of amphiphilic block copolymers, hydrophobic interactions between hydrocarbon tails of the surfactant and water-insoluble segments of PLLA-PEO-PLLA fiber should be crucially important. Several FT IR stretching vibration signals of surfactant AEO-9 or homopolymer PEO (-OH and adsorbed H2O, 3640–3284 cm-1; -(CH2)11-, 3045–2759 cm-1) and homopolymer PLLA (-OH, 3517 cm-1; asymmetric and symmetric –CH3, 3002 and 2942 cm-1; -C=O, 1767 cm-1) were selected to highlight the hydrophobic association (Figure 5e).61-65 In contrast, FT IR spectra of the unrinsed samples (i.e. the intermediate rods and the final fibers) display an unexpected 16


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adsorption peak at 3742 cm-1 for occluded water within the triblock copolymer self-assemblies and an 8-cm-1 red shift for the -C=O stretching vibration in the PLLA53PEO227PLLA53 backbones. As presented in Figure 5e, the -OH and -(CH2)11- stretching vibrations of AEO-9 molecules also experience large shifts to 3640-3194 cm-1 (i.e. 90 cm-1 red shift) and 3021-2759 cm-1 (i.e. 24 cm-1 red shift), which are assigned both to the hydroxyl terminated hydrogen bonds and to the hydrophobic interactions, respectively.66-68 Furthermore, the increase in the crystallinity of the PEO segments could be detected by time-dependent approaches. Concerning the XRD reflection of the monoclinic crystal phases at 19.20° or 23.43°, the peak intensity (i.e. peak area) of unrinsed rodlike intermediates is weaker than that of the unrinsed fiber products (Figure 5f).

Scheme 2. A schematic drawing showing the possible self-assembly formation mechanism of PLLA-PEO-PLLA copolymer fibers.

As shown schematically in Scheme 2, four steps are speculated to account for a possible mechanism of formation of the triblock copolymer fibers. The first step deals with anisotropic coalescence of adjacent O/W emulsion droplets, and subsequently the non-affinity between water and PLLA blocks could further dominate the linear self-assembly of PLLA-PEO-PLLA core-corona 17


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aggregates.54 The second step may be regarded as the crystallization of PEO segments within central parts of the linear self-assemblies of O/W emulsion droplets. The hydrophilic segments gradually crystallize along with the volatilization of THF, and these partly crystallized beads separately become localized along the axis of intermediately formed rods. These intermediate rods may become increasingly straight (Figure 5d, S12-c) owing to the crystallinity increase of PEOs cores within the central part of a rod, probably leaving the ends to be still active for the next anisotropic coalescence of O/W emulsion droplets. Therefore, these processes proceed continuously until floccule sediments are clearly observed in the third step, which results in the triblock copolymer semi-crystalline fibers with branches. The fourth step relates to the thorough rinsing of collected PLLA-PEO-PLLA fibers by water, which could remove as much as possible of the adsorbed AEO-9 molecules. During the fourth step, both the removal of adsorbed surfactants and the hydrophilic nature of crystallized PEOs should exert a great influence on the surface roughness and topography of these fibers whose diameter is micrometer-sized.

CONCLUSIONS When dissolved in the polar ether THF previously, each PLLA-PEO-PLLA triblock copolymers, synthesized by tuning the polymerization degrees of hydrophobic and hydrophilic segments, could self-assemble via an O/W emulsion templated and hydrophilic block crystalline-driven route. When the open systems were allowed to sit at room temperature for the anisotropic self-assembly, intermediate rods with a diameter of 300 - 800 nm and final fibers with a diameter of ~2.0 µm spontaneously form and both acquire the linearly arranged crystalline beads of the PEO segments within their central domains. This could be briefly summarized as the crystallization-driven anisotropic self-assembly for the formation of PLLA-PEO-PLLA fibers under the template effect of O/W emulsion 18


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droplets, and probably offers an effective approach in the future to the aqueous solution-based PEO-segment crystallization of the triblock copolymers.

ASSOCIATED CONTENT Supporting Information This material is available free of charge on the ACS Publications website at DOI: Structural characterizations of PLLA-PEO-PLLA (Scheme S1 and Figure S1-S3); Laser particle size and surface tension measurement of copolymer solution (Figure S4, S5); POM (Figure S6), OM (Figure S7, S9), SEM (Figure S8, S10, S12) and TEM (Figure S11, S12) images of intermediates and final fibers (PDF).

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Q. Shen), [email protected] (D. J. Wang); Tel.: +86-531-88361387; Fax: +86-531-88364464.

Author Addresses a

Shandong University.

b

Chinese Academy of Sciences.

b Chinese Academy of Sciences. Author Contributions The manuscript was written through contributions of all authors.

Notes 19


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The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial supports from Beijing National Laboratory for Molecular Science, the National Natural Science Foundation of China (21673131) and the Taishan Scholar Project of Shandong Province (ts201511004) are greatly acknowledged.

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