Tetrapod Polymersomes

Article abstract of DOI:10.1021/jacs.9b12925

Hollow nanoparticles such as polymersomes have promising potentials in many fields. However, the design and construction of higher-order polymersomes with precisely controlled spatial compartments is still very challenging. Herein, we report a unique tetr

Full text of DOI:10.1021/jacs.9b12925

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Article
Tetrapod Polymersomes
Jiangang Xiao, and Jianzhong Du
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 09 Mar 2020
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Tetrapod Polymersomes
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Jiangang Xiao†,‡ and Jianzhong Du†,‡,*
†Department of Orthopedics, Shanghai Tenth People’s Hospital, Tongji University School of Medicine, 301 Middle Yan-
chang Road, Shanghai 200072, China.
‡Department of Polymeric Materials, School of Materials Science and Engineering, Key Laboratory of Advanced Civil Engi-
neering Materials of Ministry of Education, Tongji University, 4800 Caoan Road, Shanghai 201804, China.
ABSTRACT: Hollow nanoparticles such as polymersomes have promising potentials in many fields. However, the design and con-
struction of higher-order polymersomes with precisely controlled spatial compartments is still very challenging. Herein, we report a
unique tetrapod polymersome that is evolved via precisely controlled fusion of four traditional spherical polymersomes that are self-
assembled from poly(ethylene oxide)113-block-poly[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl methacrylate61-statistic-2-
(diethylamino)ethyl methacrylate23] [PEO113-b-P(TBA61-stat-DEA23)] in DMF/water at lower water content (Cw). To unravel the se-
cret behind the tetrapod polymersomes, a series of block copolymers with various comonomer types and degrees of polymerization
were synthesized. PEO113-b-PTBA80 self-assembles into spherical micelles in DMF/water, and the subsequent evolution into tripod
and multipod micelles, and finally micelle clusters upon increasing Cw suggests that the TBA is a ‘pro-fusion’ component that facil-
itates the fusion due to its providential hydrophobicity and chain mobility. When one fourth of TBA of PEO113-b-PTBA80 is substi-
tuted by DEA, spherical polymersomes of PEO113-b-P(TBA61-stat-DEA23) are born in DMF/water, and then fused into dipod, tripod
(Cw = 95%), and finally tetrapod polymersomes (Cw = 100%) upon increasing Cw, suggesting that the DEA is not only a promotor of
hollow pods, but also an ‘anti-fusion’ component that can compromise with ‘pro-fusion’ force for its high chain mobility. Either
tetrapod polymersome or micelle cluster is a matter of balance between ‘pro-fusion’ and ‘anti-fusion’ forces. Overall, we provide a
fresh insight for creating tetrapod polymersomes as well as other higher-order structures with precisely defined spatial compartments.
1. INTRODUCTION
cancer therapeutics.35-37 Tetrapod structures showed enhanced
bacterial adhesion property.13 Spiky nanoparticles exhibited
physical activation of innate immunity.38 Combining the ad-
vantages of polymersomes and geometry effect, tetrapod poly-
mersomes may have promising potential applications.
Nature fascinates with its rich functionality and geometries
from complex structures, and it is an ultimate goal to design
nature-inspired materials with beautifully defined subtle struc-
tures.1-5 Among them, highly symmetrical tetrapod nanostruc-
tures are promising in diverse fields such as biolabeling, sensing,
spintronics, optoelectronics, etc.6-8 Recently, inorganic tetrapod
nanocrystals have shown excellent performance and can be syn-
thesized from a variety of inorganic materials through certain
crystal growth.9,10 ‘Colloidal molecules’ of tetrapod type struc-
tures have been made from controlled phase separation of im-
miscible macromolecules.11,12 However, most of multipod na-
nomaterials are based on dense crystals or polymers without
enough storage space for functional guest molecule loading, re-
stricting their further practical applications.13 At present, the de-
sign and preparation of tetrapod polymersomes (also known as
polymer vesicles) still remains an important challenge.
Fusion is an important spontaneous process to form higher-
order structures. For example, micelles can be fused to chains
and rings,39,40 cylinders to cylindrical supermicelles and multi-
dimensional superstructures,2,41-43 and patchy nanoparticles to
viral capsid structure, monolayer microcapsules and colloidal
kagome lattices.44,45 So we are considering the construction of
precisely defined higher-order structures such as tetrapod poly-
mersomes by controlled fusion of polymersomes.
Herein, we report the preparation and the formation mecha-
nism of an unusual tetrapod polymersome (Scheme 1). Upon
simply changing Cw, it can be made from precisely controlled
fusion of four traditional spherical polymersomes that are self-
assembled from PEO113-b-P(TBA61-stat-DEA23) block copoly-
mer (P3 in Table 1), which was synthesized by one-step reversi-
ble addition–fragmentation chain transfer (RAFT) polymeriza-
tion. The hydrophobic TBA and DEA moieties form the mem-
brane, whereas the hydrophilic PEO chains form the coronas.
The TBA is designed as a ‘pro-fusion’ component due to its
providential hydrophobicity and chain mobility at room temper-
ature, while the DEA is not only an ‘anti-fusion’ component
during the self-assembly process, but also a promotor of hollow
pods for its high chain mobility.
Polymersomes have diverse promising potential applications
such as drug delivery,14 gene delivery,15 theranostics,14,16 regu-
lation of blood glucose levels,17,18 nanoreactors,19,20 antibiosis,21
etc. Recently, non-spherical polymersomes22,23 such as stomato-
cytes,24 tubes,25,26 ellipsoids26-28 and polyhedrons29 have been
prepared, showing potential applications as nanomotors/nano-
reactors,30,31 drug delivery vehicles,32,33 and providing space for
confined crystallization.34 Recently, the geometry effect of na-
nomaterials on their biomedical applications has been exten-
sively studied. For example, nanodisks and patchy structures
displayed faster cellular uptake rate and enhanced delivery of
2. RESULTS AND DISCUSSION
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Scheme 1. Evolution of Tetrapod Polymersomes a
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a The PEO113-b-P(TBA61-stat-DEA23) block copolymer (P3) can self-assemble into traditional spherical polymersomes in DMF/water at Cw
of 10%, which can be fused into dipod, tripod at Cw of 95% and finally tetrapod polymersomes at Cw of 100%.
Table 1. Properties of Block Copolymers and Corresponding Morphologies
a
b
copolymer
P1
composition
Mn, NMR
Mn, SEC
Ðb
ζc
morphology
PEO113-b-PTBA45
18.6
14.2
1.25
-27.9
micelles
PEO113-b-P(TBA41-stat-
P2
P3
21.6
27.7
18.4
24.2
1.24
1.27
-11.3
-11.2
cylinders
DEA23)
PEO113-b-P(TBA61-stat-
tetrapod polymersomes
DEA23)
P4
P5
P6
PEO113-b-PTBA65
PEO113-b-PTBA80
PEO113-b-PTBEA69
24.6
29.2
32.0
20.7
26.7
28.5
1.28
1.27
1.12
-22.2
-24.3
-18.7
micelle clusters
micelle clusters
micelles
a Determined by 1H NMR (kDa); b obtained by SEC (kDa); cobtained by ZetaSizer Nano series instrument (mV). PEO: poly(eth-
ylene oxide); TBA: 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl methacrylate; DEA: 2-(diethylamino)ethyl methacrylate:
TBEA: 2-((((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)carbonyl)oxy)ethyl methacrylate.
2.1 Synthesis of Block Copolymers P1-P6. The synthetic
route to TBA, TBEA monomer and RAFT agent is provided in
Figure S1. Block copolymers P1-P6 were synthesized through
1B and D are magnified TEM and SEM images of a tetrapod
polymersome, revealing tetrahedrally distributed hollow pods.
Figure 1E is the tetrahedral model of a tetrapod polymersome,
showing the edge length (D) of 307 ± 30 nm and the distance
between the center and the vertices (L) of 171 ± 31 nm, as de-
termined by SEM. TEM confirms that the membrane thickness
(M) of the hollow pods is 36.2 ± 1.8 nm. SEM also confirms
the pod width (W) of the tetrapod polymersome of 68 ± 10 nm.
The D of the tetrapod polymersomes is consistent with the dy-
namic light scattering (DLS) analysis in Figure S17 where the
hydrodynamic diameter (Dh) is 275 nm (Rh = 138 nm) and the
polydispersity (PD) is 0.079.
1
a RAFT polymerization (Figure S2). The corresponding H
NMR spectra of TBA monomers, RAFT agent, copolymers P1-
P6 and size exclusion chromatography (SEC) study of P1-P6
are shown in Figures S3-S15. Typical SEC and NMR results of
block copolymers P1-P6 are presented in Table 1.
2.2 Self-Assembling PEO113-b-P(TBA61-stat-DEA23) Co-
polymer (P3) into Tetrapod Polymersomes. Copolymer P3
was self-assembled in the mixture of DMF/H2O (1/2, v/v) at an
initial concentration (Cini) of 2.0 mg/mL in DMF. Then DMF
was removed by dialyzing against deionized water for 48 h.
Transmission electron microscopy (TEM) and scanning elec-
tron microscopy (SEM) were applied to reveal the morphology
of tetrapod polymersomes (Figure 1 and Figure S16). Figure 1A
and C confirmed many tetrapod polymersomes with four hol-
low pods that are, in nature, elongated polymersomes. Figure
The structure of the tetrapod polymersomes is further con-
firmed by gold decoration as they are excellent scaffolds for the
growth of gold nanoparticles due to the DEA moieties (Figure
S18).46,47 TEM analysis of the gold-decorated tetrapod poly-
mersomes (AuNP@TP) clearly revealed the subtle structure of
tetrapod polymersomes (Figure S18B and C). Those
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(pod length), pod width and membrane thickness of the hollow
pods of the tetrapod polymersomes, respectively.
AuNP@TP may have potential applications in catalysis, photo-
thermal therapy and photoacoustic imaging, etc.19
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2.3 Formation Mechanism of Tetrapod Polymersomes.
To unravel the possible formation mechanism of tetrapod poly-
mersomes, we studied various factors and found that the most
important ones were the Cw of the mixture solvent, the molecu-
lar structure of the block copolymers, and the type of the or-
ganic solvent.
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2.3.1 Effect of Cw on the Formation of Tetrapod Poly-
mersomes. Usually, Cw rising can lead to a change of the co-
rona repulsion of polymersomes and lead to a fusion pro-
cess.48,49 To test this hypothesis, we studied the effect of Cw by
tracing the formation process via UV-Vis spectroscopy, DLS
and electron microscopy.
Firstly, UV-Vis spectroscopy was used to monitor the varia-
tion of the turbidity of the solution during the self-assembly.
The turbidity rises rapidly when the Cw is ca. 10% (Figure S19),
indicating the onset of the self-assembly. Then, DLS was used
to monitor the size change of the intermediate nanostructures
‘frozen’ by adding a small amount of solution into a large
amount of water.50 The
Dh remains ca. 200 nm until the Cw rises
to 85%, then the Dh rises rapidly as the Cw further increases
(Figure S20A). Finally, TEM and SEM analyses revealed the
corresponding shapes. Spherical polymersomes appear at lower
Cw (e.g., 10%, 36% and 67%; Figure 2A and Figure S21) and
their diameter is 166 ± 11 nm. When the Cw rises to 95%, the
spherical polymersomes are fused with each other to form dipod
and tripod polymersomes (Figure 2B and Figure S22). At Cw of
100%, tetrapod polymersomes appear (Figure 2C). Figure 2D
shows a proposed formation process of the tetrapod poly-
mersomes preferably formed in the mixture solvent with higher
solubility parameter (δ, Table S2).
Figure 1. Electron microscopy studies of tetrapod polymersomes
evolved from PEO113-b-P(TBA61-stat-DEA23) (P3) with DMF as
organic solvent: (A, B) TEM images; (C, D) SEM images; (E) Tet-
rahedral model of a tetrapod polymersome. D, L, W and M repre-
sent the edge length, distance between the center and the vertices
Figure 2. Formation process of tetrapod polymersomes from PEO113-b-P(TBA61-stat-DEA23) (P3, Cini = 2.0 mg/mL in DMF) as monitored
by SEM: (A) Cw = 67%; (B) Cw = 95%; (C) Cw = 100%; (D) Illustration of the fusion process. On the top are the water content (Cw) and the
corresponding solubility parameter (δ) of the solvent.
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Figure 3. Electron microscopy studies of micelles, cylinders, tetrapod polymersomes and micelle clusters self-assembled from copolymers
P1 to P5 with DMF as organic solvent (Cw = 100% after dialysis) to investigate the contribution of TBA and DEA moieties to the final
tetrapod polymersomes morphology: (A-E) SEM images; (F-J) TEM images; (K) schematic illustration of spherical complex micelles, cyl-
inders, tetrapod polymersomes and micelle clusters. (A) and (F) micelles from P1; (B) and (G) cylinders with two solid heads from P2; (C)
and (H) tetrapod polymersomes fused by elongated traditional polymersomes which are self-assembled from P3; (D) and (I) micelle clusters
fused by individual micelles which are self-assembled from P4, (E) and (J) micelle clusters from P5.
Copolymer P3 can be molecularly dissolved in DMF due to
their similar δ. Initially, there are amphiphilic PEO segments,
hydrophobic TBA and DEA segments, and amphiphilic DMF
molecules in the solution. Upon adding water, δ increases, and
the hydrophobic TBA and DEA segments prefer to stay with
DMF molecules as they are immiscible with water, while DMF
molecules are gradually squeezed from TBA and DEA when
water was added into the solution. Therefore, when the Cw rises
to 10% (δ = 27.1), traditional spherical polymersomes are
formed due to micro-phase separation of the block copolymers
in the mixture solution. When the Cw rises to 95% (δ = 46.7),
the spherical polymersomes trend to aggregate and further fuse
with each other. Consequently, dipod and tripod polymersomes,
and eventually tetrapod polymersomes are formed when the Cw
is 100% (δ = 47.9).
S23A), which is consistent with SEM studies considering their
dry (SEM) and wet (DLS) states. Please note that these complex
micelles are different from conventional micelles with a hydro-
phobic core and hydrophilic coronas as the core of the complex
micelles is much bigger than traditional one, suggesting that this
bigger core consists of both hydrophobic and hydrophilic moi-
eties.51,52 For simplicity, we still use ‘micelle’ to describe this
nanostructure in this paper.
PEO113-b-P(TBA41-stat-DEA23) (P2) has similar length of
rigid TBA chain to P1, shorter TBA than P3 but the same length
of flexible DEA chain as P3. P2 can self-assemble into cylin-
ders with two solid heads (Figure 3B, G). The cylinders have a
length of 430 ± 140 nm and a diameter of 68 ± 10 nm, as
determined by SEM (Figure 3B). Comparing P1 with P2, the
formation of cylinders from P2 indicates that the flexible DEA
facilitates the elongation of self-assemblies. Comparing P3 with
P2, P3 can self-assemble into tetrapod polymersomes (Figure
3C, H), suggesting that more TBA can provide better balance
between the hydrophobicity and the chain mobility for the for-
mation of tetrapod polymersomes.
2.3.2 Effect of Molecular Structure of Block Copolymers
on the Formation of Tetrapod Polymersomes. A series of
block copolymers (P1-P6) with different contents of TBA and
DEA were designed, synthesized and self-assembled. The fol-
lowing are the self-assembly results studied by DLS, TEM and
SEM, which will be discussed in Section 2.3.5 in detail.
PEO113-b-PTBA65 (P4) with the similar length of TBA to P3
but without DEA can self-assemble into micelle clusters (Figure
3D, I) with a width (W) of 329 ± 36 nm (by SEM in Figure
3D). Their Dh is 333 nm with a PD of 0.052 (Figure S23C).
The pod length of the micelle clusters is 55 ± 9 nm (by TEM
in Figure 3I).
PEO113-b-PTBA45 (P1) with shorter TBA chain than PEO113
-
b-P(TBA61-stat-DEA23) (P3) but without DEA can self-assem-
ble into solid complex micelles with a diameter of 106 ± 12
nm, as revealed by SEM images in Figure 3A, F. The Dh of the
micelles is 140 nm with a polydispersity (PD) of 0.035 (Figure
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PEO113-b-PTBA80 (P5) with longer TBA chain than P3 but
a diameter of 92 ± 11 nm (by SEM, Figure S25). The Dh of
the micelles is 125 nm with a PD of 0.105 (Figure S23E).
These results indicate that relatively low chain mobility of TBA
is essential for the multipod structures.
without DEA moiety can also self-assemble into micelle clus-
ters (Figure 3E, J and Figure S24) with a width of (W) 322 ±
27 nm (by SEM in Figure 3E), which is consistent with the Dh
of 339 nm and PD of 0.086 by DLS (Figure S23D). The pod
length of the micelle clusters is 71 ± 12 nm according to the
TEM images (Figure S24), which is 29.1% longer than that of
the micelle clusters from P4, suggesting that longer TBA chain
facilitates the pod growth.
Figure 3K shows the schematic models of the micelle, cylin-
der with two solid heads, tetrapod polymersome and micelle
cluster and their building copolymers. Figure S17 and Figure
S23 show the DLS study and ζ potentials of those nanostruc-
tures from P1-P6. Compared with other nanostructures self-as-
sembled from P1-P6 in DMF/water, the tetrapod polymersomes
from P3 have the lowest negative ζ potential (-11.2 mV), as a
result of the highest content of positively charged DEA at pH
6.7.
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Poly(ethylene oxide)-block-poly[2-((((4-(4,4,5,5-tetrame-
thyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)carbonyl)oxy)ethyl
methacrylate] (PEO113-b-PTBEA69, P6) has the similar molecu-
lar structure to P4, but has a longer side chain (see Figure S2)
than P4. P6 can self-assemble into solid spherical micelles with
Figure 4. Formation process of micelle clusters from PEO113-b-PTBA80 (P5, Cini = 2.0 mg/mL in DMF) as monitored by SEM with a similar
fusion mechanism to the tetrapod polymersomes: (A) Spherical micelles (Cw = 12%); (B) Tripod micelles (Cw = 36%); (C) Multipod micelles
(Cw = 50%); (D) Micelle clusters (Cw = 100%). (E) Proposed fusing process. On the top are the water content (Cw) and the corresponding
solubility parameter (δ) of the solvent.
Considering both tetrapod polymersomes and micelle clus-
ters are multipod structures, we studied the formation process
of micelle clusters from P5 in a similar way. Figure S20B shows
that the Dh keeps growing from 270 nm to 330 nm when the Cw
rises from 10% to 100%. SEM and TEM images of the frozen
intermediates with the Cws of 12%, 36%, 50% and 100% are
shown in Figure 4 and Figure S26, respectively. When the Cw
is 12% (δ = 27.6), spherical micelles are formed (Figure 4A and
Figure S26A). As the Cw rises to 36% (δ = 33.1), the solid mi-
celles are fused to tripod or tetrapod micelles (Figure 4B and
Figure S26B). After the Cw rises to 50% (δ = 36.4), multipod
micelles are formed (Figure 4C and Figure S26C). When the Cw
rises to 100% (δ = 47.9), final micelle clusters are formed
(Figure 4D and Figure S26D). Figure 4E shows a proposed fu-
sion process. The micelle clusters are fused from micelles while
the tetrapod polymersomes are fused from traditional poly-
mersomes. Both multipod structures share the similar fusion
mechanism.
2.3.3 Stability of the Tetrapod Polymersomes and Micelle
Clusters. The stability with respect to long-term ageing, soni-
cation53 and temperature are studied. First, upon one year age-
ing, the tetrapod polymersomes still keep the same Dh and the
original tetrapod structures, as proven by DLS, SEM and TEM
studies in Figure S27A, B and C. While the Dh of the micelle
clusters decreased 29% (from 339 to 240 nm) after one year
(Figure S27D). SEM and TEM studies (Figure S27E, F) of the
micelle clusters also confirmed that, after one year ageing, their
size became 28% smaller (from 322 to 231 nm) and their pod
length became 35% shorter (from 85 to 55 nm).
Second, after sonication for 15 min at 25 oC, the tetrapod pol-
ymersomes keep the original tetrapod structures and are only
3.6% smaller than before, as proven by DLS, SEM and TEM
studies (Figure S28A, B and C). In stark contrast, the Dh of the
micelle clusters keeps decreasing during sonication and finally
becomes 27% smaller (from 339 to 246 nm) after 15 min (Fig-
ure S28D). SEM and TEM studies (Figure S28E, F) of the mi-
celle clusters also confirm that after sonication for 15 min, their
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size becomes 19% smaller (from 322 to 260 nm) and their pod
length becomes 29% shorter (from 85 to 60 nm).
length, 189 ± 20 nm (head), 143 ± 17 nm (middle) in diameter
by SEM).
The above results confirmed that the final fusion-induced
higher-order nanostructures also depend on the starting build-
ing blocks for fusion, which can be designed by tuning the or-
ganic solvents and other self-assembly conditions, even based
on the same polymers.
Third, we further evaluated the effect of the change in tem-
perature (higher than Tg, PTBA) on the morphology of both tetra-
pod polymersomes and micelle clusters. A homopolymer,
PTBA55, was synthesized to evaluate the Tg of PTBA segment
1
in the copolymers. Its H NMR spectrum and SEC trace were
shown in Figure S29A and B, and its Tg of 45 oC was measured
2.3.5 Mechanism Insight into Tetrapod Polymersomes.
The roles of PEO, PTBA and PDEA in copolymers 1-5 are dif-
ferent during the self-assembly. Hydrophilic PEO chains form
the coronas of polymersomes and other nanostructures, while
the roles of hydrophobic and rigid TBA, and hydrophobic and
flexible DEA are more complicated, as discussed below.
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by DSC (Figure S29C). So, both the tetrapod polymersomes
o
and micelle clusters are treated at 70 C. The tetrapod poly-
mersomes keep the same size and the original tetrapod struc-
o
tures even after heating at 70 C for 15 h, as proven by DLS,
SEM and TEM studies (Figure S30A, B and C). In contrast, the
Dh of the micelle clusters keeps decreasing during the heating
process and finally becomes 33% smaller (from 339 nm to 228
nm) after 15 h (Figure S30D), and the SEM and TEM studies
(Figure S30E, F) confirmed that they turned into spherical mi-
celles (230 ± 16 nm, Figure S30E, F) at 70 oC after 15 h.
To reveal the effect of the hydrophobicity of TBA segment,
we compared the self-assembly behavior between P1 (PEO113
-
b-PTBA45) and P5 (PEO113-b-PTBA80). P5 self-assembles into
spherical complex micelles at low Cw. As the Cw rises, the mi-
celles from P5 are further fused to higher-ordered structures
such as tripod, multipod micelles and micelle clusters (Figure
4) due to more hydrophobic TBA content than P1 that only
forms micelles (Figure 3A, F).54 Considering their similar chain
mobility of TBA moieties at room temperature, this significant
difference mainly relates to the higher hydrophobicity of P5
than P1, and suggests that there is a threshold of hydrophobicity
from TBA essential for the ‘pro-fusion’ and the birth of the mul-
tipod structures.
The above results confirm that the tetrapod polymersomes
are much more stable than the micelle clusters against long-
term ageing, sonication and heating at temperatures above the
Tg of the hydrophobic segments, suggesting that tetrapod poly-
mersomes with a membrane composed of flexible PDEA seg-
ments and relatively rigid PTBA are more prone to reaching the
thermodynamic state than micelle clusters without flexible
PDEA during self-assembly.
To disclose the effect of the chain mobility of TBA segment,
we designed copolymer P6 (Figure S2), which is more mobile
than P5 due to its longer flexible side chain than P5. P5 forms
spherical micelles with a less thermodynamically stable core
than that of P6 as the TBA chains are less mobile than the
TBEA chains. Consequently, the P5 micelles are prone to fus-
ing into tripod micelles, multipod micelles, and micelle clusters.
Since the chain mobility also depends on the temperature, TBA
should be relatively rigid when the temperature is below its Tg
o
(45 C, Figure S29C). However, above the Tg, TBA chains
should become mobile, which could be beneficial for the for-
mation of thermodynamically stable structures and the fusion
process. To test this hypothesis, the micelle clusters from P5
Figure 5. TEM (A) and SEM (B) images of peapod polymersomes
self-assembled from PEO113-b-P(TBA61-stat-DEA23) (P3) in
THF/water. The final Cw is 100% after dialysis.
o
o
were heated up to 70 C (>> 45 C) for 15 h (Figure S30). As
expected, they turned into spherical micelles. These results sug-
gest that the low chain mobility of the TBA at room temperature
can provide ‘pro-fusion’ force by forming thermodynamically
unstable or metastable building blocks for the subsequent fu-
sion.
2.3.4 Effect of Organic Solvent during Self-Assembly. To
investigate the effect of the type of organic solvent on the self-
assembly behavior, P3 was dissolved in THF (rather than DMF)
with a Cini of 2.0 mg/mL, then water was added (vwater/vTHF
=
2/1). After dialyzing against deionized water for 48 h, peapod
polymersomes rather than tetrapod polymersomes were formed,
as confirmed by TEM and SEM images in Figure 5. In nature,
those peapod polymersomes are cylindrical multicompartment
polymersomes with an average length of 1201 ± 297 nm, and
an average width of 143 ± 17 nm.
DEA can regulate the pod numbers as an ‘anti-fusion’ force
because when it is statistically copolymerized with TBA, its
high flexibility can promote the chain mobility of TBA to afford
thermodynamically metastable building blocks, which is not
beneficial for the subsequent fusion. Consequently, less pods of
tetrapod polymersomes from P3 than micelle clusters from P5
are obtained. However, once several polymersomes approached
together, DEA can still act as the binder due to its intrinsic chain
flexibility to promote their adhesion, leading to thermodynam-
ically stable tetrapod polymersomes, as confirmed by the sta-
bility tests in Section 2.3.3 (Figure S30A, B and C). In addition,
DEA can be considered as a promotor of hollow structures since
spherical hollow polymersomes are formed from P3 whereas
spherical micelles are formed from P5 at low Cw.
The evolution of peapod polymersomes was also monitored
by varying the water content. Figure S31 shows the variation of
the turbidity of the THF/water solution during the self-assembly
process. When the Cw is less than 25%, the turbidity is zero.
When the Cw rises to 30%, the turbidity increases sharply, indi-
cating the onset of the self-assembly. TEM and SEM were used
to study the intermediate morphologies. As shown in Figure
S32, when the Cw is 36%, spherical complex micelles (325 ± 21
nm by SEM) are formed. When the Cw rises to 67%, those mi-
celles are fused to cylinders with two hollow heads (718 ± 131
nm in length, 199 ± 17 nm (head), 176 ± 15 nm (middle) in
diameter by SEM). Finally, when the Cw is 100%, those cylin-
ders are fused into peapod polymersomes (1201 ± 297 nm in
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Scheme 2. Tetrapod Polymersomes and Micelle Clusters Based
solvent composition during self-assembly. Overall, our findings
open up a new avenue for creating tetrapod polymersomes as
well as other higher-order nanostructures with precisely defined
spatial compartments.
on the Balance between ‘Pro-fusion’ and ‘Anti-fusion’ Forces a
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Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website.
Experimental section and characterization of TBA monomer,
and block copolymers P1-P6; DLS and Zeta potential studies,
turbidity study, TEM images, SEM images of the self-assem-
blies and their formation process, CLSM images, etc. (Figures
S1-S33).
AUTHOR INFORMATION
a The ‘pro-fusion’ force originates from the hydrophobicity and
the low chain mobility of TBA (red segment), which facilitates the
thermodynamically unstable or metastable building blocks for the
subsequent fusion. The ‘anti-fusion’ force originates from the high
chain mobility of DEA (green segment) as it is in favor of the ther-
modynamically metastable building blocks for fusion.
Corresponding Author
*E-mail: jzdu@tongji.edu.cn.
Notes
The authors declare no competing financial interest.
Either tetrapod polymersomes or micelle clusters are a matter
of balance between ‘pro-fusion’ and ‘anti-fusion’ forces
(Scheme 2). The maximum pod number is mainly governed by
the spatial hindrance, and the corona repulsion.48,49,55 Tetrapod
polymersomes consist of less fused building blocks for fusion
(spherical polymersomes) than micelle clusters, because of the
larger size of the spherical polymersomes than the micelles. To
further verify this hypothesis, we compared the fusion pro-
cesses between the polymersomes from P3 and the micelles
from P4. Both copolymers have the same pro-fusion TBA con-
tent, but P3 contains anti-fusion DEA while P4 doesn’t. As ex-
pected, P3 forms tetrapod polymersomes with less pods, while
P4 forms micelle clusters with more pods.
ACKNOWLEDGMENT
This work is supported by NSFC (21925505 and 21674081). J.D.
is the recipient of a 5-year National Science Fund for Distinguished
Young Scholars.
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