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Subscriber access provided by the Henry Madden Library | California State University, Fresno
Article
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: qshen@sdu.edu.cn (Q. Shen); djwang@iccas.ac.cn (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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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;
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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.
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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
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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
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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
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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
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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: qshen@sdu.edu.cn (Q. Shen), djwang@iccas.ac.cn (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
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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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