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Article
Cite This: Macromolecules XXXX, XXX, XXX-XXX
pubs.acs.org/Macromolecules
Two-Dimensional Self-Assembled Structures of Highly Ordered
Bioactive Crystalline-Based Block Copolymers
Sylvia Ganda,† Martin Dulle,§ Markus Drechsler,‡ Beate Förster,‡ Stephan Förster,*,§
and Martina H. Stenzel*,†
†
Centre for Advanced Macromolecular Design, School of Chemistry, The University of New South Wales, UNSW Australia, Sydney,
NSW 2052, Australia
‡
Bavarian Polymer Institute (BPI), Keylab for Electron and Optical Microscopy, and §Physikalische Chemie I, University of Bayreuth,
D-95447 Bayreuth, Germany
S Supporting Information
*
ABSTRACT: Viewing from the material design perspective,
the sophistication of nature in generating materials with great
precision provides opportunities to learn from in order to
achieve the controlled generation of functional materials with
well-defined architectures, ordered periodicity, and stability.
Inspired by the two-dimensionality and surface chemistry of
red blood cells and blood platelets, we attempted to
implement the forces induced by crystallization and phase
separation of amphiphilic carbohydrate-based crystalline-coil
block copolymers to induce self-assembly generating two-dimensional (2D) lamellar platelet structures. With the current
generation of functional 2D platelet structures via crystallization-driven self-assembly (CDSA) of block copolymers, transitioning
the existing system into a biocompatible and bioactive system is mandatory in order to bring their functionality and applicability
to another level. In this study, we introduce the crystallization-driven self-assembly of D-fructose-functionalized crystalline-coil
block copolymers featuring poly(ε-caprolactone) as the crystallizable core-forming block. By fine-tuning the corona length and
composition, we obtained 2D platelets ranging in the scale between nanometer (183 nm, length) to micrometer size range (2−4
μm, length), with the latter featuring intrinsically highly ordered core-crystalline structure of orthorhombic single crystals as
observed by the means of electron microscopy techniques and selected-area electron diffraction (SAED) experiment. We
discovered the platelet structures to grow epitaxially through the addition of free polymer, forming supersized hexagonal 2D
platelets (ca. 19−21 μm), in a process akin to the growth of living polymers. The seeded growth of these platelets suggests a
memory effect, providing a platform for further hierarchical self-assembly and functionalization. The overall approach presents a
facile strategy in fabricating the increasingly important colloidally stable bioinspired 2D structures with characteristic features and
functional properties.
■
INTRODUCTION
Molecular self-assemblies (and self-organizations) are ubiquitous in nature, inclusive of the formation of molecular crystals
and protein folding. Driven by the inquisitiveness to understand
how nature works, countless efforts have been poured into
studying how disordered molecules interact by noncovalent
forces to form an ordered array of supramolecular architectures.1,2 One of the most unique and intriguing instances of
natural self-organization phenomena is attributed to those of
life-forming materials and living organisms. Biomolecular
components are able to undergo highly specific coordinated
miniaturization and hierarchical self-assembly forming complex
architectures,3,4 featuring intricate properties and functionalities. A remarkable feature of the self-assembled biological
particles is their nonspherical geometry as observed in most
bioparticles such as the capsid and viral geometry of viruses like
the classic tobacco mosaic virus 5,6 and bullet-shaped
Rhabdoviruses.7,8 The nonspherical shape of these viruses
and their surface chemistry give rise to a characteristic cellular
© XXXX American Chemical Society
internalization pathway into the cell membrane. The binding of
the glycoproteins, that reside on the surface of these viruses, to
oligosaccharides on cell surface accommodates for the infection
of cells.9 Furthermore, human red blood cells and platelets are
another example of biological entities exhibiting this effect.
Blood platelets play a vital role in orchestrating a complex set of
events associated with numerous life-dependent tasks including
inflammation, host defense, and hemostasis (blood clotting).
The surface chemistry of platelets in their native form dictates
the heterogeneity of specific glycoprotein expressions, where
receptor activation occurs when endothelium is damaged,
acting specifically to recruit key glycoproteins and induce
aggregation and adhesion to stop wound bleeding. The absence
of these receptors causes the platelets to fail, dictating the
inherent importance of surface chemistry. Meanwhile, the
Received: July 7, 2017
Revised: September 15, 2017
A
DOI: 10.1021/acs.macromol.7b01453
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Article
Macromolecules
Figure 1. Chemical structures of the block copolymers used in this study where A and B denote the different types of core-forming block bearing
crystalline poly(ε-caprolactone) (PCL) and amorphous poly(6-ε-methyl-caprolactone) (PMCL), respectively.
architectures have been prepared under similar principles by
Manners and co-workers,21−23 such as scarf-shaped comicelle24
pointed-oval-shaped25 and lenticular platelet micelles.26 This
concept has also been explored by O’Reilly’s group where
biodegradable and biocompatible spherical and cylindrical
micelles comprising semicrystalline poly(lactide) core-forming
block were generated.27−29 Fine-tuning the process by selecting
suitable solvent can furthermore lead to well-defined diamond
shaped platelets.30 Despite the exploration into the effect of
core-crystallization of these BCPs, the corona forming block
utilized in these systems accommodated no specific interaction
with biological system. The generation of biodegradable
carbohydrate-decorated crystalline-coil 2D platelets bearing
intrinsic bioactivity has not been reported. Detailed intrinsic
physical characterization and coherent morphological transition
pathway remained unclear. Consequently, we aimed to
investigate the relationship between the principles of static
self-assembly system and polymer crystallization and translate it
toward the generation of structural control over synthetic
material design that is characteristic for biological systems.
Herein, we demonstrated a facile approach to prepare twodimensional (2D) lamellar (planar) structures comprising
tailor-made crystalline-coil BCPs that are driven by the
crystallization force of poly(ε-caprolactone) (PCL) as the
core-forming block. The integration of D-fructose-functionalized glycopolymer as the hydrophilic block serves the purpose
of surface functionality, mimicking glycoproteins found on the
surface of red blood cells and platelets. Glycopolymers are well
acknowledged for their ability to induce multivalent effect,
enhancing binding affinity with cell receptors.31,32 In this case,
it also provided a stabilizing effect for the 2D structures
resulting from the crystallization of PCL core, which are
otherwise unstable for crystalline homopolymers.33 By employing these two parameters, we were able to fashion 2D platelets
with different sizes governed by the balance between the
crystallization force and thermodynamics of phase separation.
Detailed physical characterization of the materials generated
exhibited the feature of highly ordered molecular arrangement
attributed to single crystals. Further addition of more polymer
dissolved in a common good solvent to the platelet solution
depicted an extended growth of the 2D platelets. This
observation exhibited “living” crystallization characteristic of
this system, providing further potential toward the conception
of complex hierarchical supramolecular self-assembled structures. Although the pursuit of “living” CDSA experiments was
conducted only for proof-of-concept purposes, the versatility
and control of this system can be fully explored in the future.
The overall strategy presents a novel approach toward material
synthesis in the merging perspective of bioinspired structural
organization and static self-assembly system, which has come to
discoidal shape of red blood cells (discocyte) is vital for the
elastic deformation required for intravenous circulation and
passing through small capillaries to transport oxygen and
carbon dioxide.10,11
The interplay between these distinguished aspects of
geometry and surface chemistry plays a unique role in the
determination of bioparticle transportation and their cell−cell
interaction within a biological environment. Inspired by this, we
were motivated to adopt this perspective and apply it outside of
the living system into our material design in the fabrication of
bioinspired nanomaterials to mimic their biological properties.
The construction of synthetic two-dimensional (2D) materials
with surface functionality is deemed challenging due to their
thermodynamic instability. Several approaches have been
pursued in this effort.12,13 One example of bioactive surfaces
is the surface modification of adamantyl-functionalized
thermally reduced graphene oxide sheets with multivalent
cyclodextrin-based carbohydrate ligands accommodated via
host−guest interaction. These supramolecular functionalized
2D surface was demonstrated to possess specific bioactivity to
selectively bind with Escherichia coli.14 However, the synthesis
of 2D platelet structures featuring bioactive functional surfaces
from block copolymer (BCP) system has not been reported.
This translation is eminent in order to bring their functionality
and applicability further toward biological application. The
versatility of self-assembled BCP systems renders the synthesis
of 2D functional materials resembling the characteristic
biological features of discoidal red blood cells and blood
platelets. By employing the force of crystallization of BCP
system and the principle of minimization of free energy through
static self-assembly, we undertook to study how amphiphilic
carbohydrate-based crystalline-coil block copolymers behave
when exposed to an energetically unfavorable condition for the
crystalline block. Ideally, crystalline reinforced materials display
highly ordered structures in a solvent selective for the coil
block, in which the crystallizable block is insoluble, enabling
access to 2D platelet disklike structures. Alongside the
advancement in synthetic chemistry, crystallization-driven selfassembly (CDSA) has enabled the generation of metastable,
kinetically trapped, nonequilibrium structures of crystalline-coil
BCP systems.15,16 It has also been demonstrated to be able to
generate advanced materials with various interesting morphologies and inherent functionalities under good control.15,16 The
generations of one- and two-dimensional (1D and 2D)
structures with semicrystalline polyethylene17,18 and poly(εcaprolactone)18,19 as the core-forming block have been
demonstrated. Furthermore, ever since the pioneering work
of Winnik et al. that demonstrated the living growth of
polyferrocenylsilane (PFS)-based organometallic BCP cylindrical micelles,20 various complex and hierarchical micelle
B
DOI: 10.1021/acs.macromol.7b01453
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Initially, a series of preliminary experiments was carried out
where BCP 1 PCL101-b-PF78 was first dissolved in DMSO
followed by the addition of water to give different ratios of
dimethyl sulfoxide:water (DMSO:H2O v/v %) mixture. DMSO
was chosen as the good solvent for both blocks with H2O
acting as the poor solvent to PCL. Initial characterization of the
aggregate formation was probed by dynamic light scattering
(DLS). A signal with the peak maxima at approximately 100 nm
was detected at the mixed solvent ratio of 8:2 DMSO:H2O (v/v
%), indicating a structure formation (Figure S3, Supporting
Information). The effect of different cosolvent mixtures toward
the size and resulting morphology of the platelets was eluded
by mixing the BCP into varying volume ratios of DMSO:H2O
(v/v %). Transmission electron microscopy (TEM) revealed
the size of platelets to decrease with the decreasing solvent
quality (decreasing amount of DMSO as good solvent). At a
low nonsolvent content (8:2 DMSO:H2O v/v %, Figure S4A),
the platelets were observed to reach the length within the range
of 7.3−8.2 μm. Platelet size reduction was observed to reach
2−5 μm at 6:4 DMSO:H2O v/v % ratio (Figure S4C). 2D
elongated tapelike structures were also observed in this solvent
mixture, probably a result of aging without the disruption of
agitation (stirring). However, no major size difference was
visible between 60% and 50% DMSO mixtures, resulting in the
formation of 5−7 μm 2D platelet structures also with the
presence of elongated 2D tapelike structures, with the latter
depicted in the inset of Figure S4D. Interestingly, at 30%
DMSO content, only spherical particles were seen along with
increasing water content up to 10% DMSO volume ratio
(Figure S4F−H). We hypothesize this effect to be attributed to
the competition between the crystallization of the core-forming
block and the nonsolvent-induced phase separation, with the
latter acting as the predominant force governing the selfassembly, leading to a confinement effect.16,40 Nonetheless,
these results indicated the feasibility of the corresponding BCP
system to access 2D planar structure with low curvature, owing
to the low conformational entropy or internal degrees of
freedom of crystalline-coil block copolymers.15,21,25
Based on these promising results, the self-assembly behavior
of the core-crystallizable amphiphilic BCPs was closely
investigated. The self-assembly of BCP 2 PCL101-b-PF88 was
carried out by using the cosolvent approach. Water was added
slowly at the rate of 0.2 mL h−1 at room temperature to a
polymer solution of BCP 2 dissolved in DMSO (2.0 mg mL−1)
to obtain a final polymer concentration of 1.0 mg mL−1.
Dialysis against water was carried out afterward to remove the
organic phase. This kinetically controlled crystallization
induced the formation of pointed disklike micelles as the
predominant component (ca. 1.5−2.0 μm, Figure S5A, SI). A
small number of metastable small-sized spherical micelles,
cylinders, and ill-defined 2D structures were also observed,
which may be a consequence of heterogeneous nucleation as a
function of time (Figure S5B,C, SI). As polymer crystallization
is kinetically controlled, intermediate morphologies might be
trapped due to crystallization before thermodynamic equilibrium is reached, leading to the coexistence of various micellar
morphologies.41 Nonetheless, these observations provided
coherent evidence on the restraining effect afforded by the
conformational entropy of the polar glycopolymer coronaforming block toward the core-crystallization of PCL core,
leading to the formation and stabilization of 2D platelet
structures.
be of increasing importance for the emerging generation of
colloidally stable 2D structures with unique characteristic and
functional properties.
■
RESULTS AND DISCUSSION
The self-assembly driven by the core-crystallization of
crystalline-coil block copolymers (BCPs) is generally determined by three main factors, namely the enthalpy of fusion of
the crystallizable block, the interfacial energy between the
crystalline core and the solvent, the loss of free energy due to
the conformational entropy of the amorphous block (chain
stretching and corona−corona repulsion), and the degree of
supercooling of the crystallizable block. The interplay between
these contributing factors leads to the minimization of the total
free energy that yields the final morphology. Therefore, in order
to explore the feasibility of glycopolymer-based crystalline-coil
BCP to generate 2D lamellar structure, we synthesized a series
of BCPs bearing PCL as the crystalline block, namely poly(εcaprolactone)-b-poly(1-O-acryloyl-D-fructopyranose) (PCL-bPF) and poly(ε-caprolactone)-b-poly[(1-O-acryloyl-D-fructopyranose)-co-(5,6-benzo-2-methylene-1,3-dioxepane)] (PCL-bP[F-co-BMDO]) block copolymers as model systems (Figure 1
and Table 1). PCL was chosen as the crystallizable coreTable 1. Synthesis of Block Copolymers with Crystalline
Hydrophobic Core and Their Amorphous Analogue with
Various Chain Lengths Prepared via Sequential RingOpening Polymerization of ε-Caprolactone and 6-Methyl-εcaprolactone Followed by Radical Ring-Opening RAFT
Polymerizationa
sample
1
2
3
4
5
6
block copolymer
PCL101-b-PF78
PCL101-b-PF88
PCL101-b-PF178
PCL106-b-P(F69-coBMDO9)
PCL106-b-P(F150-coBMDO24)
PMCL121-b-PF81
NMR
MnTheor (Da)
SEC
Mn (Da)
Đ
38 300
41 500
69 700
32 400
12 300
13 900
13 900
16 000
1.21
1.21
1.20
1.29
60 500
14 000
1.38
41 200
10 800
1.40
a
Theoretical molecular weight calculated based on monomer
conversion (1H NMR, CDCl3). For samples 1−3, SEC analyses
were obtained by using DMF as the mobile phase, whereas samples 4−
6 were analyzed by using THF GPC (calibrated to PMMA standards).
Note that all the polymer characterizations were conducted before the
removal of isopropylidene protecting groups.
forming block to our BCPs due to its widely known features of
biodegradability and biocompatibility,34,35 followed by its ability
to undergo crystallization and self-assemble forming aggregates
with low curvature when attached to a hydrophilic segment.36−38 The BCPs were synthesized via sequential ringopening polymerization of ε-caprolactone monomer and
reversible addition−fragmentation chain transfer (RAFT)
polymerization of the second (di)block (co)polymer.39 In the
former, a RAFT agent with an alcohol terminal group was used
to initiate the ring-opening polymerization of ε-caprolactone. DFructose-functionalized crystalline-coil BCPs were chosen as
model polymers due to their high biocompatibility and
bioactivity. The summary of polymer characterization and 1H
NMR spectra are also provided in Table 1 and Figures S1 and
S2 in the Supporting Information.
C
DOI: 10.1021/acs.macromol.7b01453
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To confirm this, a control experiment was carried out by
employing an amorphous analogue to the crystalline PCL
BCPs. Herein, sample 6 poly(6-methyl-2-ε-caprolactone)121-bpoly(1-O-acryloyl-β-D-fructopyranose)81 (PMCL121-b-PF81)
BCP (Figure 1B) was synthesized by following the same
protocol described previously for PCL BCPs. PMCL is an
amorphous polymer with a low glass transition temperature (Tg
= −45 °C), obtained by organo-catalyzed ring-opening
polymerization of 6-methyl-ε-caprolactone. The monomer
was synthesized following a literature procedure with slight
modifications and is provided in the Supporting Information.42
The glycopolymer chain lengths of the BCPs in these
experiments were maintained within a small discrepancy to
minimize the differential effect of chain stretching. Interestingly,
TEM revealed the self-assembly of this polymer to yield small
spherical micelles under identical condition with a narrow size
distribution and diameter of ca. 14 nm (Figure S5D, SI). The
TEM-selected area electron diffraction (TEM-SAED) experiment showed no diffraction pattern, indicating the amorphous
state of these micelles (Figure S5D inset, SI). These
observations further suggest that the free energy of
crystallization accounts for the majority of the total free energy
for the PCL-based crystalline-coil BCPs investigated, giving rise
to a high bending modulus of these platelets balanced by the
thermodynamics of phase separation and corona chainstretching,43 favoring the formation of 2D lamellar structures.
The versatility of this system was further evaluated by
employing BCP 4 PCL106-b-P(F69-co-BMDO9) and 5 PCL106-bP(F150-co-BMDO24) with slight compositional alterations of the
glycopolymer as the corona-forming block. In our previous
study, we have reported the complete biodegradation of these
BCPs via enzymatic hydrolysis, demonstrating the biocompatibility of this material.39 Since the crystallization of amphiphilic
BCPs is kinetically and thermodynamically controlled, the
hydrophilic (HL) chain length and composition play a major
role in determining the overall shape of the resulting
aggregates.43 Consequently, the self-assembly of 4 led to the
formation of narrow Gaussian (area) size distribution nanoplatelets with the number-average area (An) of 1.45 × 104 nm2
(area dispersity Aw/An = 1.13; Aw and An are weight- and
number-average area, respectively), number-average length (Ln)
= 183 nm, and number-average width (Wn) = 77 nm by means
of transmission electron microscopy (TEM) (Figure 2C and
Figure S3, SI). Cryo-transmission electron microscopy (cryoTEM) further confirmed the morphology of these platelets as
depicted by the face-on and edge-on structures with the overall
thickness of 10.5 nm and core diameter (dc) of 7 ± 1 nm
(Figure 2A,B, Figures S7 and S8, SI). The edge-on orientation
with face-to-face configuration of the nanoplatelets is eluded in
Figure S8. The overall thickness was measured by taking the
cross-sectional height of the platelets as indicated by the red
line on the left-hand side (Figure S8 inset). Meanwhile, the
core diameter (dc) of these platelets was deduced and estimated
by taking the gray value across the overall thickness of the
platelets. As can be seen in the inset, the core is the lighter
region sandwiched between two darker regions marked as the
red line on the right-hand side (lower gray value), suggesting
that these are the carbohydrate corona with a higher electron
density, giving rise to a higher contrast. Therefore, the lighter
region within the particle is the core. On the basis of these
results, we hypothesized that the thickness of the corresponding platelets obtained is a result of chain-fold crystallization of
PCL as proposed by Halperin et al.43
Figure 2. Thin platelet morphology obtained by crystallization-driven
self-assembly of PCL106-b-P(F69-co-BMDO9). (A, B) Cryo-TEM
micrographs of the nanoplatelets formed by the slow addition of
water into BCP 4 solution in DMSO (5:5 v/v %, 1.0 mg mL−1) stirring
at room temperature, upon removal of the organic phase by dialysis.
The inset of (A) elucidates the edge-on orientation of the platelet with
a measured overall thickness of 10.5 nm and the core thickness (dc) of
7 nm. Scale bars are 200 and 100 nm, respectively. (C) Histogram
depicting the contour area distribution of these platelets obtained by
room temperature TEM. (D) Addition of water as precipitant to PCL
block yields 2D lamellar platelets with the crystalline core sandwiched
between the carbohydrate coronas. The chain folding of crystallizing
PCL block leads to an ordered chain periodicity with orthorhombic
symmetry. (E) A schematic diagram illustrating the chain-fold
crystalline PCL core of thickness dc and a soluble solvent-swollen
corona of thickness H. One PCL chain (DP = 106) is folded for
approximately 12 folds (13 segments) and is determined by the corona
chain spacing. For simplicity, the schematic was not drawn to scale.
In order to confirm this, we performed a unit cell calculation
based on the core diameter obtained by cryo-TEM. Based on
literature values, the unit cell of PCL was reported to be
orthorhombic with the dimensions of a = 7.496 ± 0.002 Å, b =
4.974 ± 0.001 Å, c = 17.297 ± 0.023 Å, and a P212121 space
group.44 It is important to note that in polymer crystals the caxis coincides with the chain axis. Therefore, the correlation
between the c lattice constant and the measured platelet
thickness as measured by cryo-TEM (dc = 7 ± 1 nm) connotes
the platelet to consist a coil-crystalline lamellar with a singlefold crystal layer comprising of four unit cells. The unit cell
calculation revealed that one PCL block polymer (DP = 106)
permeates four unit cells in the c projection (dc/c = 4 unit cells,
adjacent folds) and is folded approximately 12 times (nf = 12)
producing 13 segments as illustrated in Figure 2D,E. According
to the model developed by Vilgis and Halperin,43 the proximity
of chain packing in the direction normal to the fold (planar)
surface is a response to the corona chain repulsion. Therefore,
D
DOI: 10.1021/acs.macromol.7b01453
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Figure 3. Formation of highly ordered 2D single-crystal block copolymer platelets of BCP 5 PCL106-b-P(F150-co-BMDO24) with orthorhombic unit
cells as recorded by (A) TEM, (B) cryo-TEM (scale bars denote 1.0 μm), (C) cryo-TEM SAED (scale bar represents 250 nm), and (D) cryo-SEM
(scale bar 2 μm, insets 1 μm). The inset of (A) is the diffraction pattern obtained from the platelet in the direction normal to the plane. The
measured lattice planes obtained from the SAED pattern is in good agreement with the calculated pattern (upper right quadrant) and theoretical
Miller indices based on PCL single crystal with orthorhombic unit cell with a = 7.496 Å, b = 4.974 Å, c = 17.297 Å, and a P212121 space group,
leading to the absence of odd (h00) and (0k0) reflections. (E) Schematic illustration of the formation of larger-sized platelets due to the balance
between the minimization of interfacial energy and crystallization force. The scheme is an idealized representation of PCL chain folding and not to
scale.
hydrophilic (HL) chains. According to the scaling model
proposed by Vilgis and Halperin43 and further confirmed by
Winnik et al.,45 longer hydrophilic corona leads to a higher
entropic penalty due to the strong corona chain repulsion and
chain stretching of overlapping solvent-swollen corona. Therefore, in order to minimize the high free energy cost, the system
drives the formation of more chain folds (nf) in the lamellar
domain which would result in a thinner lamellar domain (dc).
The balance between the minimization of interfacial energy
(thick core) and entropic penalty attributed to chain stretching
(thinner core) dictates the morphology of the equilibrium
structure, favoring the formation of larger-sized lamellar
aggregates. This phenomenon is illustrated in Figure 3E.
Selected-area electron diffraction (SAED) pattern obtained
from the platelets in the direction normal to the disk plane
revealed an identical electron diffraction pattern with the
calculated pattern (Figure 3A and inset, upper right quadrant)
and literature values,44 with the absence of odd (h00) and
(k00) reflections. The lattice plane spacings (d-spacings)
obtained from SAED were able to be indexed according to
the Miller indices reported for PCL single crystal44 with
orthorhombic symmetry as provided in Table 2 and Figure 3A.
The calculated pattern was acquired based on the PCL
orthorhombic unit cell with the dimensions of a = 7.496 ±
the number of chain folds per core block (nf) determines the
space occupied by the solvent-swollen corona (H) (Figure 2E).
Unlike the chain packing of coil−coil BCP that has a circular
interfacial cross section, the folding of PCL chains leads to the
end-to-end packing of rectangular unit cells as depicted in
Figure 2D. Each of these unit cells accommodates two extended
PCL unimers. Note that the inherent errors of cryo-TEM need
to be accounted as a part of the standard deviation of the
measured core diameter by this technique, affecting also the
estimated number of chain folds. The corona structure was
unable to be resolved in cryo-TEM due to the insufficient
contrast between the corona with the respecting solvent. The
crystallinity of these platelets was unable to be characterized by
electron diffraction due to the small size of the crystalline PCL
core.
Subsequently, in order to validate this hypothesis, we
investigated the self-assembly behavior of BCP 5 to elucidate
more detailed information about the chain packing of the core.
BCP 5 PCL106-b-P(F150-co-BMDO24) consisted of a longer
hydrophilic chain. Under similar conditions, thin platelets with
greater lateral dimensions of ca. 4 μm in length and 1 μm in
width were prepared as depicted by means of TEM (Figure
3A). We observed the increase of the lateral dimension of the
platelets compared to sample 4 to be an effect of the longer
E
DOI: 10.1021/acs.macromol.7b01453
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of PCL. This approach facilitates a unique opportunity to study
and confirm the crystal structure locally via TEM and SAED to
the respective micrometer-sized platelets. Cryo-TEM provided
a further evidence of the thin disklike structure and electron
diffraction of these platelets in their native state (Figure 3B,C).
Cryo-scanning electron microscopy (cryo-SEM) exhibited the
isolated thin disklike structures after deep etching (Figure 3D).
These single crystals were found to be stable over the course of
one year, suggesting the colloidal stability of the 2D lamellar
structures.
Living crystallization-driven self-assembly (CDSA) of semicrystalline micelles has been demonstrated to generate growth
in epitaxial direction.46−48 Here, the edges of preconceived
crystalline micelles used as seed precursors remain active to the
addition of polymeric unimers. Careful selection of the unimers
with matching crystal lattice to the seed platelet micelles allows
for the crystallization force to extend the growth of these seeds
upon unimer addition, exhibiting the livingness of this
system.23,24,49 Seeded growth of 2D platelet structures as
precursors forming hollow rectangular morphologies22 and
branched hybrid structures50 has also been demonstrated. For
this reason, we investigated the potential of our PCL-based
block glycopolymer as a model system to undergo living CDSA
as a potent platform for further functionalization. In this case,
the slow kinetics of self-assembly allows us to probe various
Table 2. Reported Literature Values of the d-Spacings (Å) of
PCL Homopolymer Single Crystalsa Compared to the
Measured Lattice Plane Spacingsb of PCL106-b-P(F150-coBMDO24) BCP Platelet Single Crystals by SAED
d-spacings (Å)
layer
line
Miller
indices
PCL single
crystalsa
PCL106-b-P(F150-coBMDO24)b
0
110
200
210
020
120
310
220
400
410
4.122
3.710
2.984
2.498
2.365
2.214
2.080
1.859
1.754
4.097
3.723
2.963
2.455
2.212
2.049
1.861
a
Reported literature values denoting the lattice plane spacings (dspacings) of PCL homopolymer single crystals.44 bMeasured lattice
spacings obtained from the diffraction pattern of PCL106-b-P(F150-coBMDO24) BCP platelets by means of TEM-SAED (Figure 3A, inset).
0.002 Å, b = 4.974 ± 0.001 Å, c = 17.297 ± 0.023 Å, and a
P212121 space group. This proves that the disklike PCL core is a
thin, planar single crystal driven by the chain-fold crystallization
Figure 4. (A) Schematic illustration of the seeded growth of 2D platelets used as precursors upon the addition of BCP unimers resulting in the
significant increase in the lateral dimension of preformed platelets. TEM micrographs of a representative sample prepared by the seeded growth of
(B) PCL101-b-PF178 platelet seed precursors (2−4 μm) in aqueous solution by the addition of the corresponding BCP 3 PCL101-b-PF178 unimers (10
μL, 10.0 mg mL−1 in DMSO) aged at 3−4 °C led to the formation of (C, D) supersized 2D hexagonal platelets (19−21 μm). Scale bars represent 2
μm. The grayscale analysis (D) depicts the periodic lamellar multilayered structure. (E) Schematic illustration showing the helicoidal growth of
platelets with screw-dislocation effect due to defect of crystal growth on the face of (001) lattice plane in the direction normal to the fold symmetry.
F
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amorphous core. Subsequently, crystallization of the core starts
to occur giving rise to the final aggregated structure. We
observed this transition in the control platelet solution of BCP
3 aged at 3−4 °C where no additional unimers were added,
with the fusion (coalescence) of small spherical aggregates (ca.
20−40 nm, Figure S10, SI) to the growth front and surface of
the platelets (Figure 5). This observation further confirms the
intermediate structures before reaching colloidally stable,
energetically favorable structures. As discussed previously, we
utilized a longer hydrophilic block length to induce the
formation of larger sized platelets as an effect of entropic
compensation of PCL block, resulting in an increase of the
number of chain foldings.45 The seed platelets used as
precursors were obtained by dissolving BCP 3 PCL101-b-PF178
into DMSO followed by the slow addition of water (5:5 v/v %,
1.0 mg mL−1) and dialysis to remove the organic phase. The
solvent exchange during dialysis facilitated kinetic trapping of
the platelets with the size range between 2 and 4 μm (Figure
4B). The number-average length (Ln) and width (Wn) of these
seed platelets were measured to be Ln = 2.77 × 106 nm and Wn
= 8.96 × 105 nm with the dispersity of Lw/Ln = 1.37 and Ww/
Wn = 1.16, respectively. The addition of 10.0 μL of PCL101-bPF178 unimers (10 mg mL−1) in DMSO to PCL101-b-PF178
seeds in water (An = 2.51 × 106 nm2, Aw/An = 1.67) at the
unimer-to-seed mass ratio (munimer/mseed) of 10:1 led to the
formation of supersized hexagonal 2D platelet structures with
An = 9.79 × 107 nm2 and the aspect ratio of 5.3, indicating the
“living” growth of the 2D seed precursors. The lateral
dimension of the 2D platelets reached Ln = 1.78 × 107 nm
(17.8 μm) and Wn = 5.37 × 106 nm (5.37 μm) with the length,
width, and area dispersity of Lw/Ln = 1.02, Ww/Wn = 1.06, and
Aw/An = 1.14. From these results, we observed the resulting
platelets to be an order of magnitude greater than the parent
seeds, with a decrease in dispersity. It is important to note here
that the statistical analysis carried out over the resulting
platelets was done by considering only the population of the
platelets that underwent continuous growth. This was
conducted merely to observe the size increase of the platelets
following seeded growth. The structures formed resemble the
structure of the parent seeds, reminiscent of PCL homopolymer single crystals.44 Because of the slow kinetics of this
process, only a small number of platelets were observed to have
increased in size after aging at 3−4 °C for 1 day (Figure
S9A,B). However, prolonged aging of 30 days depicted the
presence of mostly well-defined hexagonal platelets in solution,
reaching the length of ca. 19−21 μm, further suggesting the
slow kinetics of the seeded growth (Figure 4C,D). This
observation indicated the susceptibility of the edges of the 2D
platelets toward unimeric addition as illustrated in Figure 4A.
An aliquot taken at 60 days after seeded growth showed no
further growth observable (Figure S9C), suggesting the kinetic
stability of these structures once the unimers were consumed. It
is worth mentioning that this experiment was carried out for
proof-of-concept purposes only; therefore, further investigation
into the living CDSA behavior was not pursued, in which case
the area of platelet is linearly dependent on the unimer:seed
ratio as demonstrated previously by Manners et al.20,23
Furthermore, the structure observed in Figure 4C with the
orientation of terrace topography feature suggested the
presence of defects during crystallization. These continuous
spiral overgrowths are observed to be an effect of an emergent
lattice displacement with a Burgers vector b on the face of
(001) lattice plane as illustrated in Figure 4D. The displacement of each lattice plane leads to the formation of winding,
spiral concentric loops known as the screw-dislocation
effect.51−54
The self-assembly of crystalline BCPs via the cosolvent
approach is suggested to undergo two stages. When a
nonsolvent for the core-forming block is introduced, phase
separation starts to take place forming spherical micelles with
Figure 5. Coalescence of spherical micelles to platelet micelles driven
by the crystallization force of PCL as observed on TEM.
assumption that the 2D platelets were formed via the
crystallization-induced aggregation of spherical micelles. It is
noteworthy that, apart from fusion of micelles onto platelet
rims, the growth of 2D platelets also takes place by unimeric
adsorption onto the crystal growth front. In the case of BCP
with crystalline core-forming block, dynamic unimeric exchange
between particles is suppressed due to the rigidity, slow lateral
mobility, and low internal degrees of freedom of the polymer
chain at ambient temperature.
We mentioned previously that the seed platelet solution was
dialyzed against water to remove DMSO and induce kinetic
trapping of the precursor platelets. Without dialysis in priori, the
presence of DMSO in solution causes the crystalline core to
remain softer and more dynamic in solution, more susceptible
to structural rearrangement thus reduces the chance of kinetic
trapping. During the seeded growth process, we observed the
occurrence of a number of intermediate structures. The slow
self-assembly kinetics of this system allowed the visualization of
the transformation from spherical micelles to rods to 2D
lamellae structures like platelets, tubes, and sheets. In the
presence of DMSO as cosolvent in the solution mixture,
immediately after the addition of 10.0 μL of PCL101-b-PF178
unimers (10 mg mL−1) into the seed solution of BCP 3
PCL101-b-PF178 in DMSO:H2O 5:5 v/v % mixture (1.0 mg
mL−1), it led to the formation of tubes, ribbons, and sheets with
protrusion of short rods from the sheet edges (Figures S11 and
S12, SI). We observed the edge of the ribbons to curl to
minimize contact with the solvent (Figure S11C). The fusion of
spherical micelles to rods then sheets is clearly observed during
this transition (Figure S12). The morphologies arose may be
attributed to the DMSO plasticized core that accommodates
G
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Notes
structural rearrangement through these intermediate structures.
It has been reported in the literature before that the presence of
good solvent toward the hydrophobic core in the self-assembly
solution promotes a plasticizing effect on the core, facilitating a
more dynamic environment and enhanced chain mobility, thus
affecting the particle morphological transition pathway and the
resulting morphology.40,56−58 Interestingly, after aging for 30
days at room temperature, we observed the disappearance of
the intermediate structures and the presence of sheet/platelet
structures with the size between 2 and 5 μm (Figure S13, SI),
which suggests the unsuccessful seeded growth. We hypothesize that upon unimeric addition, new 2D platelet structures
along with several metastable intermediates were formed
preferentially against the growth of the 2D seed precursors.
This indicates that the core-crystallization force of the seed
platelet precursors was weak in the presence of DMSO, leading
to a faster new aggregate formation compared to the initiation
of 2D seeded growth driven by core-crystallization. These
observations further suggest the metastability of the intermediate structures to be kinetically trapped, which evolve over
time to form 2D platelet structures as the thermodynamically
preferred morphology.26 The long tubes and ribbons were
probably energetically unfavorable; therefore, facilitated by
DMSO that aids the rearrangement of the polymer chains, the
transition to micrometer-sized 2D platelets occurs as a
thermodynamically preferred morphology. This observation
is, however, open to experimental verification.
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors thank Janina-Miriam Noy for the synthesis of 6methyl-ε-caprolactone monomer. S.G. gratefully acknowledges
the financial support by an Australian Government Research
Training Program (RTP) Scholarship. The authors also thank
the Australian Research Council for funding (ARC
DP160101172).
■
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■
CONCLUSIONS AND OUTLOOK
In summary, we demonstrated a simplistic approach toward the
generation of well-defined, highly ordered, colloidally stable
two-dimensional platelets featuring biocompatibility and
bioactive component based on tailor-made crystalline-coil
block glycopolymers. The fine-tuning of hydrophilic block
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Seeded growth of 2D platelets fabricated led to the formation
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means of the use of other functional polymers, supramolecular
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■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macromol.7b01453.
Experimental details; Figures S1−S13 (PDF)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail M.Stenzel@unsw.edu.au (M.H.S.).
*E-mail stephan.foerster@uni-bayreuth.de (S.F.).
ORCID
Martina H. Stenzel: 0000-0002-6433-4419
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