Micelle-to-vesicle morphological transition via light-induced rapid hydrophilic arm detachment from a star polymer

Article abstract of DOI:10.1039/c2cc16363b

Micelle-to-vesicle morphological transition has been achieved by light-induced rapid hydrophilic arm detachment from a star polymer. This provides a remote and clean method to control morphology transition of polymeric assemblies. The Royal Society of Che

Full text of DOI:10.1039/c2cc16363b

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Micelle-to-vesicle morphological transition via light-induced rapid
hydrophilic arm detachment from a star polymerw
Jin-Zhi Du,a Hong-Yan Long,a You-Yong Yuan,b Meng-Meng Song,c Liang Chen,b
Hong Bic and Jun Wang*b
Received 13th October 2011, Accepted 29th November 2011
DOI: 10.1039/c2cc16363b
Micelle-to-vesicle morphological transition has been achieved
by light-induced rapid hydrophilic arm detachment from a star
polymer. This provides a remote and clean method to control
morphology transition of polymeric assemblies.
molecules (e.g. H2O2,11 DTT12 and DNA),13 which can stimulate
specific interactions. For example, Hubbell et al. have reported
an oxidation-responsive vesicles to micelles transition for a poly-
(ethylene glycol)-b-poly(propylene sulfide) block copolymer.11
Kataoka and co-workers have demonstrated micelles to vesicles
transition of polyion complex micelles (PICmicelles) via
reduction-induced degradation of the polymer.12 Gianneschi
et al. have produced programmable morphological transitions
of biomacromolecules (e.g. DNA and enzyme) containing
polymeric assemblies by utilizing the specific interactions
between the molecules.13
Precise control over the morphology of polymeric self-assemblies
currently receives great attention from basic research to practical
applications.1 Morphologies of the polymeric assemblies (e.g.
spherical micelle, worm-like micelle and vesicle) in aqueous
solution are dictated by the relative molecular size of the hydro-
philic and hydrophobic blocks of the constituent amphiphiles,
while change of the relative ratio of hydrophilic and hydrophobic
components can lead to morphological change of the assembly.2
There has been wide interest in the transformation studies between
single polymer chains and self-assembled aggregates under various
external stimuli, including temperature,3 pH,4 light5 and others.6
Of particular interest is to study the transitions between
different morphologies of the self-assemblies in response to an
applied external stimulus. Such stimulus-responsive morphol-
ogy switching has been observed in the assemblies of small
surfactants7 and polymers, and the latter has been pioneered
by Eisenberg et al. on an examination of ion-induced morphol-
ogical changes in ‘‘crew-cut’’ aggregates of amphiphilic block
copolymers.8 Thereafter, a few interesting strategies have been
utilized to manipulate the morphological switching of polymeric
assemblies. For example, Grubbs et al.9 and O’Reilly et al.10 have
independently reported that thermally induced amphiphilicity
change can drive the transition of micelles to vesicles of thermo-
responsive block copolymers, but their studies have also shown
that the thermo-induced morphological transition proceeds at a
slow rate and requires a long time to reach equilibrium. Another
strategy with potency to induce the morphological transitions
of polymeric assemblies is achieved by the addition of other
Herein, we introduce the concept of light-triggered morpho-
logical transition via rapid hydrophilic arm detachment from
an amphiphilic star polymer. The model polymer ECE is
composed of one hydrophobic poly(e-caprolactone) (PCL)
block and two hydrophilic poly(ethylene glycol) (PEG) blocks
with the molecular weights of 2000 (mPEG45) and 750 (mPEG16),
respectively. As shown in Scheme 1, mPEG45 is connected to the
polymer through a photolabile 2-nitrobenzyl (NB) ester bond and
can rapidly detach from the polymer upon UV irradiation.
Initially, the polymer self-assembles into micelles in water due
to the higher PEG weight fraction, but upon UV irradiation the
PEG weight fraction decreases significantly due to mPEG45
detachment, which induces programmed micelle to vesicle
morphological transition.
a CAS Key Laboratory of Soft Matter Chemistry and Department of
Polymer Science and Engineering, University of Science and
Technology of China, Hefei, Anhui 230026, P.R. China
b School of Life Sciences, University of Science and Technology of
China, Hefei, Anhui 230027, P.R. China.
E-mail: jwang699@ustc.edu.cn; Fax: +86 551 3600402
c College of Chemistry and Chemical Engineering, Anhui University,
Hefei 230039, China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc16363b
Scheme 1 (a) Chemical structure and UV-induced cleavage of the
star polymer ECE and (b) schematic illustration of UV irradiation
triggered micelle to vesicle transition.
c
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The ECE star polymer was synthesized through a multiple
reaction procedure as illustrated in ESI.w First, a NB derivative
(5-methoxy-2-nitro-4-(oxiran-2-ylmethoxy)phenyl) methanol (1)
was prepared and conjugated with poly(ethylene glycol) succinate
(mPEG45–COOH) to produce an epoxide-functionalized
PEG (2). Then, the epoxide ring was opened by sodium azide
to generate azido and hydroxyl groups at the end of the
polymer (3). The azido group reacted with a-propargyl-o-
acetyl-poly(e-caprolactone) (4) via a Huisgen 1,3-dipolar cyclo-
addition reaction (click chemistry) to produce the diblock
copolymer mPEG45–NB–PCL (5). The remaining hydroxyl
group was then modified with freshly prepared mPEG16–COCl
to generate the star polymer ECE (6). Details of the charac-
terization data are outlined in ESI.w
Photoinduced cleavage of the NB ester linkage of the
polymer is crucial to our design. Therefore, in the first place
we studied the photolabile property of the NB ester linkage of
the ECE polymer. The polymer solution was subjected to UV
irradiation at 10 mW cmÀ2 (lmax = 365 nm) for 15 min.
Unless stated otherwise, this irradiation condition was used
throughout the study. The GPC profiles before and after UV
irradiation were recorded and compared, as shown in Fig. 1.
From the GPC data, one could find that upon UV irradiation
two overlapped peaks (b) were observed at elution times
similar to the control diblock copolymer mPEG16-b-PCL37
(c) and mPEG45–COOH (d), indicating the UV-induced cleavage
of the NB ester bond. It should be noted that the slight shoulder
in trace a and c may be caused by the presence of a small portion
of PEG diol in commercially available mPEG.14 The cleavage of
the NB linkage led to the formation of a derivative of 2-nitroso-
benzaldehyde (Scheme 1), and resulted in changes in the UV-vis
spectra (Fig. S7, ESIw).15
Cleavage of the NB ester bond in ECE causes the detach-
ment of mPEG45, which will make the weight fraction of PEG
in the polymer decrease from 41.8% to 17.2%. It is known
that the hydrophilic and hydrophobic weight fractions have a
great influence on the morphology of assembly and the lower
hydrophilic fraction favours vesicle formation.2,16 This implies
that UV-induced detachment of mPEG45 may trigger the
morphology transition of the ECE micelle. In order to clarify
this, we investigated the particle size change of the ECE micelle
after UV irradiation using dynamic light scattering (DLS)
measurement. Before UV irradiation, the diameter of the ECE
Fig. 2 Time-dependent variations of (a) diameter and (b) PDI of the
ECE assemblies at 65 1C without UV irradiation (~) or with UV
irradiation for 15 min (m).
micelle was mainly around 30 nm. After exposure to UV light at
365 nm for 15 min, the micelle solution was stirred at 65 1C and
the size change was monitored using DLS. As shown in Fig. 2a,
the particle size showed dramatic variations with the extension
of incubation time. Stirring the solution by 20 h significantly
increased the diameter of the assemblies to a maximum of 460 nm.
Correspondingly, the polydistribution index (PDI) of the
assemblies also increased from 0.20 to 0.49 (Fig. 2b), implying
that some large aggregates might be formed during heating.
However, with time prolonged particle size of the aggregates
deceased rapidly, reaching 143 nm by 90 h and then kept
constant. The PDI profile showed a similar change trend and
eventually stood at 0.13. On the contrary, without UV irradia-
tion, the ECE micelle showed minimal change in both the
diameter and PDI when heated at the same temperature for 123 h.
The morphological transition of the micelle was further
visualized by transmission electron microscopy (TEM) analyses.
Compared with the initial micelles (Fig. 3a), the micelles without
UV irradiation did not alter their morphology when heated at
65 1C for 123 h, just showing very slight increase in particle size
(Fig. S8A, ESIw). On the contrary, the UV irradiated micelles
showed time-dependent morphology evolution. After 24 h of
stirring at 65 1C, larger aggregates were formed due to micelle
association (Fig. S8B, ESIw). With extending the time to 48 h,
the micelles were still spherical, but with significantly increased
diameter (Fig. 3b). These results were in good agreement with
the DLS analyses as shown in Fig. 2. It is known that after UV
irradiation, the weight fraction of PEG in the resultant diblock
copolymer was significantly reduced to 17.2%. Upon heating,
the PCL segments reorganized and the shorter PEG chain might
not be able to stabilize the micelle any longer, thus the micelles
fused with each other to form larger particles to reduce the
surface tension.17 Upon further extending the incubation time to
123 h, polymeric vesicles were observed in the solution, with
an average diameter of about 120 nm (Fig. 3c). The particles
Fig. 1 GPC profiles of the ECE polymer before (a) and after (b) UV
irradiation. (c) and (d) are GPC profiles of control polymer mPEG16
-
b-PCL37 and mPEG45–COOH, respectively.
c
1258 Chem. Commun., 2012, 48, 1257–1259
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Key Laboratory of Supramolecular Structure and Materials
(SKLSSM201117) and the 211 Project of Anhui University.
Notes and references
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Fig. 3 TEM images of the assemblies after different treatments. (a) ECE
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In conclusion, we have developed a novel approach to
manipulating the morphological transition from micelle to
vesicle of polymeric assemblies under stimulus. In this method,
UV irradiation triggered a hydrophilic chain detachment,
making the polymer suitable for vesicle formation in principle,
while heating facilitated the reorganization of the hydrophobic
segment to promote vesicle formation in practice. The merit of
such an approach is that the resultant vesicles can be preserved
without any cross-linking. Besides, this concept is also applic-
able to polymers of other structures and compositions as long
as they can satisfy the stimuli-triggered amphiphilicity changes.
This work was supported by the National Basic Research
Program of China (973 Programs, 2010CB934001 and
2009CB930301), the National Natural Science Foundation of
China (51125012), the Fundamental Research Funds for the
Central Universities (WK2070000008), Open Project of State
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c
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Chem. Commun., 2012, 48, 1257–1259 1259