Synthesis and self-assembly of stimuli-responsive amphiphilic block copolymers based on polyhedral oligomeric silsesquioxane

Article abstract of DOI:10.1002/pola.27287

A novel POSS-containing methacrylate monomer (HEMAPOSS) was fabricated by extending the side chain between polyhedral oligomeric silsesquioxane (POSS) unit and methacrylate group, which can efficiently decrease the steric hindrance in free-radical polymerization of POSS-methacrylate monomer. POSS-containing homopolymers (PHEMAPOSS) with a higher degree of polymerization (DP) can be prepared using HEMAPOSS monomer via reversible addition-fragmentation chain transfer (RAFT) polymerization. PHEMAPOSS was further used as the macro-RAFT agent to construct a series of amphiphilic POSS-containing poly(N, N-dimethylaminoethyl methacrylate) diblock copolymers, PHEMAPOSS-b-PDMAEMA. PHEMAPOSS-b-PDMAEMA block copolymers can self-assemble into a plethora of morphologies ranging from irregular assembled aggregates to core-shell spheres and further from complex spheres (pearl-necklace-liked structure) to large compound vesicles. The thermo- and pH-responsive behaviors of the micelles were also investigated by dynamic laser scattering, UV spectroscopy, SEM, and TEM. The results reveal the reversible transition of the assembled morphologies from spherical micelles to complex micelles was realized through acid-base control. 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2669-2683 Stimuli-responsive amphiphilic diblock copolymers of PHEMPOSS-b-PDMAEMA with a high degree of polymerization of the POSS unit were synthesized via RAFT polymerization. The self-assembled morphologies of PHEMAPOSS-b-PDMAEMA in aqueous solution can be tuned from irregularly assembled aggregates into spherical core-shell micelle and from pearl-necklace-liked structure into large compound vesicles by varying the length of hydrophilic PDMAEMA block chain. Copyright

Full text of DOI:10.1002/pola.27287

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Synthesis and Self-Assembly of Stimuli-Responsive Amphiphilic Block
Copolymers Based on Polyhedral Oligomeric Silsesquioxane
Lizhi Hong, Zhenghe Zhang, Yuan Zhang, Weian Zhang
Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China
University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
Correspondence to: W. Zhang (E-mail: wazhang@ecust.edu.cn)
Received 9 May 2014; accepted 10 June 2014; published online 25 June 2014
DOI: 10.1002/pola.27287
ABSTRACT: A novel POSS-containing methacrylate monomer
(HEMAPOSS) was fabricated by extending the side chain
between polyhedral oligomeric silsesquioxane (POSS) unit and
methacrylate group, which can efficiently decrease the steric
hindrance in free-radical polymerization of POSS-methacrylate
monomer. POSS-containing homopolymers (PHEMAPOSS)
with a higher degree of polymerization (DP) can be prepared
using HEMAPOSS monomer via reversible addition–fragmenta-
tion chain transfer (RAFT) polymerization. PHEMAPOSS was
further used as the macro-RAFT agent to construct a series of
amphiphilic POSS-containing poly(N, N-dimethylaminoethyl
methacrylate) diblock copolymers, PHEMAPOSS-b-PDMAEMA.
PHEMAPOSS-b-PDMAEMA block copolymers can self-
assemble into a plethora of morphologies ranging from irregu-
lar assembled aggregates to core-shell spheres and further
from complex spheres (pearl-necklace-liked structure) to large
compound vesicles. The thermo- and pH-responsive behaviors
of the micelles were also investigated by dynamic laser scatter-
ing, UV spectroscopy, SEM, and TEM. The results reveal the
reversible transition of the assembled morphologies from
spherical micelles to complex micelles was realized through
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acid-base control. 2014 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2014, 52, 2669–2683
KEYWORDS: amphiphiles; block copolymers; reversible addition
fragmentation chain transfer (RAFT); self-assembly; stimuli-
sensitive polymers
and drug delivery.7,8,17–19 These stimuli-responsive polymers
also could form single, dual-, and multistimuli-responsive
assemblies such as pH,20 temperature,21 pH/temperature,22
pH/ions,23 light/redox,24 light/temperature,25 and pH/tem-
perature/ light26,27 responsive micelles. Among them, poly(N,
N-dimethylaminoethyl methacrylate) (PDMAEMA), a typical
dual-responsive polymer,28–30 is a weak polybase with pKa of
7.0–7.8, and has a lower critical solution temperature (LCST)
at the range of 25–95 ꢀC in its aqueous solution depending
on the pH, concentration, topological structure, and the
molecular weight of PDMAEMA.31–33 Meanwhile, the LCST of
PDMAEMA aqueous solution is also directly influenced by
pH because the tertiary amine groups can be protonated or
deprotonated at low or high pH, respectively. Thus
PDMAEMA could be used for the preparation of drug- and
gene-delivery systems by several groups.34–37 Nevertheless,
there are few reports on the stimuli-responsive and assem-
bly behavior of PDMAEMA containing inorganic components,
especially for PDMAEMA block copolymers.
INTRODUCTION The self-assembly of amphiphilic block
copolymers has attracted extensive interests in the past sev-
eral decades, since they can self-assemble into a rich variety
of morphologies including spheres, rods, lamellae, vesicles,
and others.1–4 These assembled morphologies could endow
them with the potential application in many fields such as
nanoreactors, catalysis, and drug delivery systems.5–9 It is
generally acknowledged that self-assembled morphologies of
amphiphilic copolymers in selective solution are governed by
three components: interfacial energy between the core and
the solvent, the stretching of the core-forming block, and the
repulsive interactions among coronal chains.1,10,11 Thus, the
morphologies could be determined by the following
parameters: the composition and concentration of the copol-
ymer, the overall molecular weight, the nature of the com-
mon solvent, the solvent/nonsolvent ratio, the pH value,
ions, etc.12–16
More recently, a growing number of stimuli-responsive poly-
mers and their assemblies have been investigated due to the
need to develop smart materials which have advantageous
applications such as biotechnology, nanotechnology, catalysis,
Polyhedral oligomeric silsesquioxane (POSS) molecules, with
the general formula (RSiO1.5 n
)
(n ꢁ 6), have an inorganic
Additional Supporting Information may be found in the online version of this article.
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cage-like core surrounded by organic corner groups which
could be reactive or unreactive.38–40 POSS molecule and its
derivatives have a diameter in the range 1–3 nm and could
be thought of as the smallest existing silica nanopar-
ticles.41,42 Recently, with the development of controlled/liv-
ing radical polymerization, a variety of POSS-containing
polymers with well-defined structures have also been syn-
thesized.43–45 These POSS-containing hybrid polymers can be
directly prepared using POSS-containing initiator or reversi-
ble addition–fragmentation chain transfer (RAFT) agent via
living polymerization technique, and their self-assembly
behavior was further studied. Li et al.23 synthesized hemi-
telechelic amphiphilic POSS-containing hybrid PDMAEMA
(HT-POSS-PDMAEMA) via atom transfer radical polymeriza-
tion (ATRP), and found HT-POSS-PDMAEMA can self-
assemble into a hierarchical structure in aqueous solution.
Zheng et al.45 have also synthesized POSS-capped poly(e-cap-
rolactone) (PCL) via ring-opening polymerization (ROP) of e-
caprolactone with 3-hydroxypropylheptaphenyl POSS as the
initiator, and the hemi-telechelic amphiphile can be used to
incorporate into epoxy resin to prepare the nanostructured
thermosets. Besides, Chang and coworkers46 have synthe-
sized a series of organic/inorganic hybrid star PCLs through
coordinated ROP of e-caprolactone using POSS as the initia-
tor, and the star PCL can form inclusion complexes with a-
and c-cyclodextrin. Li et al.36 synthesized an amphiphilic
star-shaped POSS-containing PDMAEMA via ATRP, and it is
the first report of a cationic POSS-containing material for
gene delivery. Additionally, hemitelechelic (tadpole-shaped),
ditelechelic, and multitelechilic POSS-containing polymers
could be easily constructed by a combination of living poly-
merization and coupling reaction such as click chemistry and
hydrosilylation coupling. We have prepared POSS-containing
telechelic hybrid poly(acrylic acid) by a combination of ATRP
and click chemistry, and studied their self-assembly behav-
iour in water.47–49 Cheng et al.50,51 have reported a facile
approach to prepare amphiphiles with POSS nanoparticle as
the head group and a polymer chain as the tail via the com-
bination of ROP and thiol-ene click chemistry. These func-
tional POSS can serve as versatile nanobuilding blocks to be
incorporated into other molecular structures for the bottom-
up construction of mesoscopic nanomaterials via self-
assembly.
PMAPOSS with a relatively high degree of polymerization
(DP) of POSS unit via the living anionic polymerization, the
synthesis and self-assembly of POSS-containing block copoly-
mers still have not been well studied, especially in solution.
This is because the POSS-containing monomers used in pre-
vious researches are the commercial monomers, which has a
great steric hindrance in radical polymerization, especially
for living radical polymerization such as ATRP58,59 and RAFT
polymerization.60,61
In this contribution, we extended the length of the side chain
between POSS unit and methacrylate group to prepare a
novel POSS-containing methacrylate monomer (HEMAPOSS),
which could efficiently overcome the steric hindrance of
POSS unit in free-radical polymerization, and obtained a high
DP of POSS-containing hybrid polymers. The kinetic process
of HEMAPOSS RAFT polymerization was evaluated by Gel
Permeation Chromatography (GPC) and 1H NMR. PHEMA-
POSS was further used as the macro-RAFT agent to construct
a series of POSS-containing amphiphilic PDMAEMA diblock
copolymers, PHEMAPOSS-b-PDMAEMA. The self-assembly
behavior of amphiphilic PHEMAPOSS-b-PDMAEMA block
copolymers was investigated by transmission electron
microscopy (TEM), atomic force microscopy (AFM), scanning
electron microscopy (SEM), and dynamic light scattering
(DLS). Finally, the dual stimuli-responsiveness of assembled
aggregates was also studied by varying pH and temperature.
EXPERIMENTAL
Materials
Aminoisobutyl POSS was purchased from Hybrid Plastics
Company. N, N-dimethylaminoethyl methacrylate (DMAEMA,
Aladdin, 99%) was passed through a column of basic alumi-
num oxide to remove inhibitors shortly before polymeriza-
tion. The RAFT agent cumyl dithiobenzoate (CDB) was
synthesized according to the literature.62 Azobisisobutyroni-
trile (AIBN) was purified by recrystallization from ethanol.
Tetrahydrofuran (THF) and toluene were distilled from a
purple sodium ketyl solution, respectively. Dichloromethane
(DCM) and triethylamine (TEA) were separately dried over
calcium hydride and distilled before use. Other regents and
solvents in analytical grade were obtained from Aladdin and
used as received unless otherwise noted.
As an important kind of well-defined polymers, POSS-
containing hybrid block copolymers also have been explored
using POSS-based monomers. Pyun and Matyjaszewski52,53
first reported the synthesis of homopolymers, triblock
copolymers, and star-block copolymers of hybrid polymers
from a POSS methacrylate monomer via ATRP. He’s group54–
CHARACTERIZATION
Nuclear Magnetic Resonance Spectroscopy (NMR)
A Bruker 400 MHz NMR spectrometer instrument was used
to acquire the proton NMR spectra. The samples were dis-
solved with deuterated CDCl3 and measured with tetrame-
thylsilane (TMS) as an internal reference.
56
synthesized well-defined amphiphilic diblock and triblock
copolymers of poly(ethylene glycol) as a hydrophilic block
and poly(methacrylisobutyl POSS) (PMAPOSS) as a hydro-
phobic block, and the micelle formation, gelation, and rheo-
logical performance were further studied. Although POSS-
containing block copolymers have been developed by several
groups, even if Hayakawa and coworkers57 recently synthe-
sized diblock copolymers of PS-b-PMAPOSS and PMMA-b-
Gel Permeation Chromatography
The molecular weight and molecular weight distribution of
PHEMAPOSS homopolymers and PHEMAPOSS-b-PDMAEMA
diblock copolymers were determined with a GPC (Waters
1515). A series of monodisperse polystyrene standards were
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used for calibration, and THF was used as the eluent at a
flow rate of 1 mL/min.
HCH@C(CH3)A), 5.61 (s, 1H, HCH@C(CH3)A), 4.36 (t, 4H,
AOOC(CH2)2OCOA), 2.68 (t, 4H, HOOC(CH2)2COOA), and
1.95 (s, 3H, H3CC(COOA)CH2).
Critical Micelle Concentration (CMC)
Synthesis of HEMA-COCl
CMC measurements were estimated by fluorescence spec-
troscopy on Hitachi FL-4500 using pyrene as a probe.
A Schlenk flask (250 mL) with a magnetic stirring bar was
connected to a standard Schlenk line system and was
degassed and then refilled with highly pure nitrogen three
times. Four gram of HEMA-COOH and 30 mL of anhydrous
DCM were introduced into the flask. Freshly distilled thionyl
chloride (25 mL) was added dropwise into the mixture solu-
tion over 30 min under nitrogen atmosphere. Then the reac-
tion mixture was heated to 40 ꢀC at reflux for about 1 h.
The solvent was removed by distilling at atmospheric pres-
sure and then under vacuum to produce HEMA-COCl, which
was directly used in next step.
Transmission Electron Microscopy
TEM images were taken on a JEOL JEM1400 instrument
operated at an accelerating voltage of 100 KV. The samples
were made by directly dropping a drop of PHEMAPOSS-b-
PDMAEMA aqueous solution (0.25 mg/mL) onto a copper
grid (300 mesh) coated with a carbon film, and the sample
was allowed to dry at room temperature.
Atomic Force Microscopy
AFM observation was carried out on a Veeco’s NanoScope IV
of Digital Instruments using Tapping Mode. The measure-
ments were prepared by spin-coating the aggregate solutions
(0.25 mg/mL) onto freshly cleaved mica surfaces and then
dried naturally at room temperature for 24 h.
Synthesis of HEMAPOSS Monomer
Aminoisobutyl-POSS (7.6 g, 8.7 mmol) was dissolved in
50 mL of anhydrous THF and 2.4 mL of anhydrous TEA was
added. The fresh HEMA-COCl was dissolved in 30 mL of
anhydrous THF and was added slowly (about 1 h) into the
ꢀ
Scanning Electron Microscopy
above POSS solution at 0 C under nitrogen atmosphere, and
SEM was performed using a Hitachi S4800 electron micro-
scope at an acceleration voltage of 1 kV. The aggregate solu-
tion (0.25 mg/mL) was directly dropped onto a freshly
cleaved mica and then dried at room temperature. Before
the measurement, the sample was sputtered by gold.
the mixture was stirred for 12 h at room temperature. A
white precipitate (triethylamine hydrochloride) was removed
by filtration and the resulting solution was concentrated
under vacuum at room temperature. The residue was puri-
fied by a silica gel chromatography with ethylacetate/petro-
leum ether (1:2, v/v) as the eluent. A white solid was
obtained (7.6 g, yield 80.5%). 1H NMR analysis: 1H NMR
Turbidity Measurement
Transmittance experiments were carried out with a UV–visi-
ble spectrophotometer (UV-2450, Shimadzu, Japan). Trans-
mittance at 500 nm was monitored. Heating scans were
performed between 30 and 80 ꢀC at a scanning rate of 0.1
ꢀC/min.
(400 MHz, CDCl3),
d
(TMS, ppm): 6.13 (s, 1H,
HCH@C(CH3)A), 5.60 (s, 1H, HCH@C(CH3)A), 4.35 (s, 4H,
AOCO(CH2)2OCOA), 3.26–3.20 (m, 2H, ACH2ANHCOA),
2.72–2.69 (m, 2H, ANHCOACH2A), 2.50–2.44 (m, 2H,
ACH2CH2COOA), 1.95 (s, 3H, H3CC(COOA)CH2), 1.88–1.82
(m,
7H,
ASiAH2CCH(CH3)2),
1.61
(m,
2H,
Dynamic Light Scattering
ASiACH2CH2CH2ANHA), 0.96 (d, 42H, ASiACH2CH(CH3)2),
0.61 (d, 16H, ASiCH2CH(CH3)2, ASiCH2CH2CH2ANHA).
DLS measurements were performed by using a BECKMAN
COULTER Delasa Nano C particle analyzer at a fixed angle of
165ꢀ. All measurements were repeated at least three times
and the average result was accepted as the final hydrody-
namic diameter (Dh).
Preparation PHEMAPOSS Homopolymer
In a typical experiment, HEMAPOSS monomer (1 g, 0.92
mmol), CDB (250.2 lL, 20 mg/mL CDB toluene solution,
0.0184 mmol) and AIBN (98.5 lL, 10 mg/mL AIBN toluene
solution, 0.006 mmol) and 0.6 mL toluene were placed in a
dry glass tube equipped with a magnetic stirring bar. The
mixture was degassed by at least three freeze-pump-thaw
cycles. After the tube was flame-sealed under vacuum, it was
stirred in an oil bath at 65 ꢀC for 48 h. The reaction was
stopped by plunging the tube into liquid nitrogen. After pre-
cipitating into a solvent mixture (methanol/acetic ether 5 6/
1, volume ratio) three times, the product was dried under
Synthesis of HEMA-COOH
2-Hydroxyethyl methacrylate (HEMA; 6.1 mL, 0.05 mol) was
dissolved in anhydrous THF in a schlenk flask (250 mL)
with a magnetic stirring bar at room temperature under
nitrogen. Succinic anhydride (6 g, 0.06 mol), 12 mL of pyri-
dine, and 4-dimethylamiopryidine (0.49 g, 0.004 mol) were
added, and then the reaction mixture was stirred for 24 h at
ꢀ
40 C under nitrogen. After cooling to room temperature, the
vacuum at 30 ꢀC for 24
Mn 5 25,400 g/mol, Mw/Mn 5 1.12.
h to afford PHMEAPOSS.
solvent was evaporated under vacuum and the residue was
dissolved in DCM followed by washing it three times with
0.1 M HCl solution. The organic phase was dried over anhy-
drous magnesium sulfate overnight and filtered. After evapo-
ration of the solvent, the remaining HEMA-COOH product
was dried under vacuum at room temperature. A viscous liq-
uid was obtained (10.9 g, yield 95%) and confirmed by 1H
Synthesis of PHEMAPOSS-b-PDMAEMA Block Copolymers
A representative example of the synthesis of the PHEMA-
POSS-b-PDMAEMA block copolymers is as follows: PHEMA-
POSS (0.3 g, 0.0061 mmol), DMAEMA (0.8 g, 5.1 mmol), and
AIBN (33 lL, 10 mg/mL AIBN THF solution, 0.006 mmol)
1
NMR. H NMR (400 MHz, CDCl3), d (TMS, ppm): 6.13 (s, 1H,
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SCHEME 1 Synthesis of POSS-containing HEMAPOSS monomer.
and 2 mL THF were placed in a dry glass tube equipped
with a magnetic stirring bar. The mixture was degassed by at
least three freeze-pump-thaw cycles. After the tube was
flame-sealed under vacuum, it was stirred in an oil bath at
65 ꢀC for 12 h. The reaction was stopped by plunging the
tube into liquid nitrogen. The solution was diluted into
10 mL by using THF and dialyzed in water at least three
times to completely remove the DMAEMA monomer. To
obtain the pure block copolymers, stepwise precipitation
was further carried out to remove the dead polymer, PHE-
MAPOSS macro-RAFT agent. The final product was dried by
freeze-drying for two days. Mn 5 59,100 g/mol, Mw/
Mn 5 1.20.
in previous research are commercial, which has a high steric
hindrance in free radical polymerization of POSS-based
monomers. Here, we fabricated a novel POSS-containing
methacrylate monomer (HEMAPOSS) by extending the side
chain between POSS unit and methacrylate group, which
would efficiently decrease the steric hindrance of POSS unit
in free-radical polymerization of POSS-based monomers, and
the POSS-containing homopolymer (PHEMAPOSS) with a
higher DP could be achieved.
The synthetic procedure for the single long-armed POSS-
based monomer, namely HEMAPOSS is illustrated in Scheme
1. We selected HEMA to synthesize the POSS-containing
methacrylate monomer, since the hydroxyl group of HEMA is
easily converted to other functional groups. Here, the
hydroxyl group of HEMA was converted into a carboxylic
acid to produce HEMA-COOH using succinic anhydride,
which is also helpful for the extension of the side chain of
POSS-based monomer (1H NMR spectrum of HEMA-COOH is
shown in Supporting Information Fig. S1). To improve the
reaction efficiency between HEMA-COOH and POSS-NH2, the
carboxyl group of HEMA-COOH was converted into acyl chlo-
ride to prepare HEMA-COCl via the acylation reaction
between HEMA-COOH and SOCl2. Finally, the POSS-
containing methacrylate monomer (HEMAPOSS) was success-
fully synthesized via the amide reaction between POSS-NH2
and freshly prepared HEMA-COCl, which is confirmed by 1H
NMR (Supporting Information Fig. S2).
Self-Assembly of PHEMAPOSS-b-PDMAEMA in Aqueous
Solution
In a typical process, PHEMAPOSS45-b-PDMAEMA286 (10 mg)
block copolymer was first dissolved in 10 mL of dioxane
(common solvent). The solution was stirred overnight and
gradually dialyzed against the mixture solvent of dioxane
with ultrapure water (selective solvent) for 3 days. The self-
assembly solution was dialyzed at least three times using
ultrapure water to make sure to remove dioxane completely.
The pH in the experiment was adjusted by using hydrochlo-
ric acid and sodium hydroxide solution.
RESULTS AND DISCUSSION
Synthesis of HEMAPOSS Monomer
Although some POSS-containing hybrid polymers with advan-
tageous properties have been reported, there are still many
challenges in the development of well-defined POSS-contain-
ing hybrid polymers. This is because POSS monomers used
Preparation of PHEMAPOSS Homopolymer
In recent years, RAFT polymerization was found and widely
used in construction of well-defined polymers.44 Here, RAFT
polymerization of HEMAPOSS was carried out in toluene
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SCHEME 2 Synthesis of PHEMAPOSS and PHEMAPOSS-b-PDMAEMA successively via RAFT Polymerization.
using the CDB as a transfer agent, AIBN as the initiator, and
the molar feed ratio of HEMAPOSS/CDB/AIBN is 50/1/0.33
(see Scheme 2). The polymerization conversion at intervals
a living manner and followed pseudo-first-order kinetics.
The relationships of number-average molecular weights, PDI
and the molecule weight calculated by 1H NMR versus the
monomer conversion are shown in Figure 3. It can be seen
that the molecular weights calculated by 1H NMR linearly
increases with the conversion, but it differs from the
number-average molecular weights determined by GPC (see
also Table 1). This should be noted that the molar masses
evaluated by GPC with PS standards are underestimated
owing to the relatively compact cage structure of HEMA-
POSS, which has a very different solution behavior with PS
standard in THF.63,64
1
was measured by H NMR.
Figure 1 shows the evolution of GPC curves with reaction
time. With increasing reaction times, the curves shift to
lower elution time whereas the PDI keeps <1.12. The kinetic
plot of the RAFT polymerization of HEMAPOSS is shown in
Figure 2. A very satisfactory linear relationship exists
between ln[1/(1 2 x)] and reaction time even up to high
conversions (around 60%), indicating a constant concentra-
tion of propagating chains during the RAFT polymerization
of HEMAPOSS. Thus, the polymerization of HEMAPOSS is in
1
Figure 4 shows the H NMR spectrum of PHEMAPOSS45, the
peaks at d 5 0.97 ppm (o) and 0.62 ppm (m, g) are ascribed
FIGURE 1 Evolution of GPC curves for the RAFT polymeriza-
tion of HEMAPOSS at 65 ꢀC in toluene with different reaction
times. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 2 Pseudo-first-order kinetic plot for the RAFT polymer-
ization of HEMAPOSS using CDB as a RAFT agent at 65 ꢀC in
toluene.
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FIGURE 4 1H NMR spectrum of PHEMAPOSS45. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 3 Evolution of molecular weight and polydispersity
with conversion for the RAFT polymerization of HEMAPOSS at
65 ꢀC in toluene. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
merization process of HEMAPOSS follows pseudo-first-order
kinetics at the conversion <60%.
to the methyl protons and methylene protons of POSS units,
respectively. The signals of the phenyl protons of the RAFT
agent still can be identified in the insert enlarged spectrum.
We also can calculate the molecular weight from 1H NMR
spectrum by comparing the peaks at 0.62 and 7.81 ppm
(aromatic protons of CDB). The average degrees of polymer-
ization (DP) on of PHEMAPOSS and the molecular
Preparation of Amphiphilic PHEMAPOSS-b-PDMAEMA
Diblock Copolymers
PHEMAPOSS-b-PDMAEMA amphiphilic diblock copolymers
(ABCPs) with different PDMAEMA block length were synthe-
sized using PHEMAPOSS45 as a macro-RAFT agent, and the
polymerization results were listed in Table 2. Their GPC
curves are shown in Figure 5, and the eluograms shift to
lower elution volume with increasing the length of
weight were calculated as DP 5 I0.6/(8*I7.81
)
and
Mn,PHEMAPOSS,NMR 5 I0.6/(8*I7.81) 3 MHMEAPOSS 1 MCDB, where
the MHEMAPOSS and MCDB are the molecular weights of HEMA-
POSS monomer and CDB, respectively. Here “I” represents
the area of a peak, and we first set I0.6 to 16.00. The result
shows the well-defined homopolymer PHEMAPOSS has a DP
of 45, and Mn,PHEMAPOSS,NMR of 48,900 g/mol. Based on the
above GPC and NMR results, we have successfully synthe-
sized the PHEMAPOSS with a higher DP by extending the
side chain of the POSS-based monomer, moreover, the poly-
1
PDMAEMA chain. The H NMR spectrum of PHEMAPOSS45-b-
PDMAEMA286 is shown in Figure 6. Besides the signals from
the main chain of block copolymers, the peaks at 2.30 ppm
are assigned to methyl protons of PDMAEMA block, which
also can be used to estimate the molecular weight of
PDMAEMA block. The average polymerization degree of
PDMAEMA (DPPDMAEMA) and the molecule weight of diblock
copolymers (Mn,PHEMAPOSS-b-PDMAEMA,NMR) can be calculated
according to the peaks at 0.6 ppm of methylene protons next
to silicon (ASiACH2CH2CH2A) with the peaks at 2.3 ppm of
the dimethyl protons as follows: The DPPDMAEMA is deter-
mined as DPPDMAEMA 5 (16*I2.3)/(6*I0.6) 3 DPPHEMAPOSS, and
the Mn,PHEMAPOSS-b-PDMAEMA,NMR is determined as Mn,PHEMA-
POSS-b-PDMAEMA,NMR 5 DPPDMAEMA 3 Mn,DMAEMA 1 Mn,PHEMAPOSS,
NMR, where the Mn,PHEMAPOSS, NMR is the molecular weight of
PHEMAPOSS.
TABLE 1 Conditions and Results of the RAFT Polymerization of
HEMAPOSS Using CDB as RAFT Agent
Time
Samplesa (h)
Conversion
(%)
1023Mn,
1023Mn,
PDIc
b
c
NMR
GPC
1
2
3
4
5
6
7
1
2
0.38
6.3
0.48
3.7
\
\
\
\
4
17.1
32.1
48.4
61.4
68.8
9.6
7.52
9.84
10.7
12.3
13.3
1.05
1.11
1.12
1.12
1.12
Self-Assembly of PHEMAPOSS-b-PDMAEMA in Aqueous
Solution
7
17.7
26.5
33.7
37.7
The self-assembly behaviors of hybrid polymers based on
POSS have attracted considerable interest in recent years,
and some interesting self-assembly morphologies have been
obtained, which is obviously different with the pure organic
amphiphilic polymers.40,65,66 However, the self-assembly of
POSS-containing amphiphilic block copolymers in selective
solution have not been well studied, due to the absence of
the suitable POSS-containing block polymers. Here, to well
investigated the self-assembly behavior of POSS-containing
11
17
22
a
In toluene, [M]0 5 0.3 mol/L; [CTA]/[AIBN] 5 3.
Calculated as Mn,NMR 5 [M]0/[CTA]0 3 MHEMAPOSS 3 x 1 MCDB, [M]0 and
b
[CTA]0 are, respectively, the initial concentrations of the HEMAPOSS
and CDB agent; MHEMAPOSS is the molecular weight of HEMAPOSS,
MCDB is the molecular weight of RAFT agent and x is the conversion.
c
Measured by GPC calibrated with PS standards.
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TABLE 2 PHEMAPOSS-b-PDMAEMA ABCPs Synthesized Successively via RAFT Polymerization
[HEMAPOSS]:[CDB]b
or [M]/[Macro-CTA]b
fHEMAPOSS
(wt %)a
23
a
23
c
Samplesa
10
M
n, NMR
10
M
PDId
n, GPC
PHEMAPOSS45
50:1
61:1
48.9
25.4
28.5
30.4
35.2
43.2
59.1
1.12
1.15
1.16
1.21
1.21
1.20
1.22
100.0%
90.0%
82.0%
74.6%
68.2%
52.1%
30.8%
PHEMAPOSS45-b-PDMAEMA34
PHEMAPOSS45-b-PDMAEMA68
PHEMAPOSS45-b-PDMAEMA106
PHEMAPOSS45-b-PDMAEMA145
PHEMAPOSS45-b-PDMAEMA286
PHEMAPOSS45-b-PDMAEMA699
54.3
59.7
65.7
71.7
93.8
133:1
206:1
332:1
830:1
1784:1
158.7
104.4
a
c
The final composition (DP and the mass fraction of PHEMAPOSS in
the block copolymer) and Mn,NMR were determined from the integration
of 1H NMR spectra.
Calculated as Mn,PHEMAPOSS-b-PDMAEMA, NMR 5 DP PDMAEMA 3 Mn,DMAE-
MA 1 Mn,PHEMAPOSS, NMR, where the Mn,PHEMAPOSS,
weight of PHEMAPOSS.
d
is the molecular
NMR
b
The feed molar ratio between the chain transfer agent and the initiator
Measured by GPC against polystyrene standards.
AIBN was maintained at 1:0.33.
amphiphilic block copolymers in solution, we synthesized a
series of amphiphilic PHEMAPOSS-b-PDMAEMA block copoly-
mers with different PDMAEMA block lengths. According to
previous researches,1,10,67 the assembled morphology of
amphiphilic block copolymers is mainly dependent on the
ratio of hydrophilic and hydrophobic blocks. In this case, we
mainly studied the influence of the ratio of hydrophilic and
hydrophobic blocks on the self-assembled behavior of amphi-
philic PHEMAPOSS-b-PDMAEMA block copolymers. First, we
studied the self-assembled morphologies formed from PHE-
MAPOSS-b-PDMAEMA with the molar ratio of hydrophobic/
hydrophilic block (PHEMAPOSS/PDMAEMA) of around 1/
15.5, 1/6.4, and 1/3.2 (System I). To further explore the
assembled morphologies with shorter hydrophilic chain, we
decreased the chain length of hydrophilic PDMAEMA block
in ABCPs, and the molar ratio of hydrophobic/hydrophilic
block (PHEMAPOSS/PDMAEMA) is around 1/2.4, 1/1.5, and
1/0.8 (System II).
In this study, dioxane is chosen as the common solvent and
water as selective solvent in which PHEMAPOSS block is
insoluble. The CMC of ABCPs in water at pH 5 7 was meas-
ured using pyrene as a probe by fluorescence spectropho-
tometry. Supporting Information Figure S3 shows three CMC
values of ABCPs in aqueous solution. Obviously, with the
increase of the length of PDMAEMA segment, the CMC value
increased remarkably, since water is a good solvent for
PDMAEMA chains. The self-assembly morphology of PHEMA-
POSS-b-PDMAEMA ABCPs was investigated by TEM, AFM,
SEM, and DLS.
System I
Self-assembled morphologies from ABCPs with the molar
ratio of hydrophobic/hydrophilic block (PHEMAPOSS/
PDMAEMA) of around 1/15.5, 1/6.4, and 1/3.2.
First, we synthesized three ABCPs of PHEMAPOSS45-b-
PDMAEMA699
,
PHEMAPOSS45-b-PDMAEMA286
,
and
FIGURE 5 GPC curves of PHEMAPOSS45 and PHEMAPOSS-b-
PDMAEMA. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
FIGURE
6
1H
NMR
spectrum
of
PHEMAPOSS45-b-
PDMAEMA286
.
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FIGURE 7 TEM images [scale bar, 500 nm (a), 200 nm (b)] of PHEMAPOSS45-b-PDMAEMA699 self-assembled aggregates in aque-
ous solution.
PHEMAPOSS45-b-PDMAEMA145, and the molar ratio of hydro-
phobic/hydrophilic block (PHEMAPOSS/PDMAEMA) is
around 1/15.5, 1/6.4, and 1/3.2, respectively.
at the range 33–48 nm in their TEM image [Figs. 8(a,b) and
9(a,b)]. The spherical micelles were further confirmed by
AFM (Supporting Information Figs. S5 and S6), where diame-
ter of the micelles is at the range 65–116 nm, which is quite
larger than that observed in the TEM images. We can attrib-
ute this phenomenon to the following reason: because of the
high repeat units of the hydrophilic chain, the micelles have
a relatively long corona in the aqueous solution, and the
shell resulting from the shrinkage of the corona is taken into
consideration in the AFM measurement, while the TEM
images only exhibit the PHEMAPOSS core. Additionally, it is
worth to mention that the height (about 45 nm) exhibited in
Supporting Information Figs. S5c and S6c, respectively,
exported from Supporting Information Figs. S5a and S6a is
in good agreement with the diameter observed in TEM
images. This also confirms the assembled morphology is a
typical core-shell structure.
Figure 7 presents the TEM images of PHEMAPOSS45-b-
PDMAEMA699. From Figure 7(a,b), we can find that there are
some irregular assembled aggregates formed, and the den-
sity of these aggregates are not uniform, where the dark
domains formed by the POSS units. The dimension of the lit-
tle dots in assembled aggregates is about 8.5 nm. Apparently,
the hydrophilic group dimension becomes so large that the
aggregates of PHEMAPOSS are embedded in the PDMAEMA
matrix, which is very different from the typical core-shell
micelles with a hydrophobic block as the core and a hydro-
philic block as the shell in the ABCPs. The morphology of
these aggregates characterized by AFM (shown in Supporting
Information Fig. S4) is relatively in good agreement with
that shown in TEM images. It is mentionable that the height
of the micelles (the height is 8.8 nm shown in Supporting
Information Fig. S4c) exported from Supporting Information
Fig. S4a is in accordance with the dimension of the dark
dots observed in the TEM images. This further suggests the
aggregates are the loose structure, where POSS domain
mixed with PDMAEMA chains. This result is similar to that
of our group’s previous study50 in self-assembly of hemi-
telechelic poly(acrylic acid) (POSS-PAA), where the chain of
hydrophilic PAA is also very longer compared to the hydro-
phobic POSS moiety. Their self-assembly morphologies are
not the typical core-shell micelles, and the density is nonuni-
form in a single aggregate.
System II
Self-assembled morphologies from ABCPs with the molar
ratio of hydrophobic/hydrophilic block (PHEMAPOSS/
PDMAEMA) of around 1/2.4, 1/1.5, and 1/0.8. To further
explore the effect of the molar ratio of hydrophobic/hydro-
philic block morphologies, we decreased the chain length of
PDMAEMA block, and we designed the molar ratio of the
hydrophobic/hydrophilic block (PHEMAPOSS/PDMAEMA) of
around 1/2.4, 1/1.5, and 1/0.8.
From the TEM images of PHEMAPOSS45-b-PDMAEMA106
assembled aggregates in water in Figure 10(a,b), we found
not only spherical core-shell micelles (30–55 nm), but also
the pearl-necklace-like morphology formed from the linked
spherical aggregates. This observation was also confirmed by
AFM image (Supporting Information Fig. S7). With further
decreasing the chain length of PDMAEMA, PHEMAPOSS45-b-
PDMAEMA68 has a much clearer pearl-necklace-like mor-
phology shown in Figure 11 and Supporting Information Fig-
ure S8, but we found PHEMAPOSS45-b-PDMAEMA68 has a
For PHEMAPOSS45-b-PDMAEMA286 and PHEMAPOSS45-b-
PDMAEMA145
, we can find they have a similar self-
assembled morphology, although the chain length of hydro-
philic PDMAEMA of PHEMAPOSS45-b-PDMAEMA286 is about
2 times to that of PHEMAPOSS45-b-PDMAEMA145. The mor-
phologies shown in the Figures 8 and 9 are both typical
core-shell spherical micelles, which both possess a diameter
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FIGURE 8 TEM images [scale bar, 200 nm (a), 50 nm (b)] of PHEMAPOSS45-b-PDMAEMA286 self-assembled aggregates in aqueous
solution.
more compacted structure, where the single spherical aggre-
gate has merged with the adjacent one and the edge almost
could not be discerned. For the PHEMAPOSS-b-PDMAEMA
block copolymers with a shorter PDMAEMA hydrophilic
chain, the slow kinetics in dilute solution hinder assembled
nanostructures from reaching global equilibrium state to
form pearl-necklace-like aggregates, due to the slow chain
exchange between micelles. The similar assembled aggre-
gates were also observed by other groups.68–72
1/0.8. Surprisingly, the morphology of the micelles changed
dramatically and large compound vesicles (LCVs) which have
both inner and outer hydrophilic surfaces were obtained in
Figure 12. The size of the LCVs is in the range of 297–
372 nm in TEM image [Fig. 12(a,b)], but it is about 450 nm
in AFM image (Supporting Information Fig. S9a). Here, the
size of LCVs in AFM image is not much larger than that in
TEM image compared with that in the core-shell spherical or
pearl-necklace-like structure, which means the difference may
be attributed to the tip convolution effects. Additionally, the
height of the LCVs observed in AFM (Supporting Information
Fig. S9c) is 296.5 nm, which is smaller than the size in TEM
images. We can attribute this to the hollow structure among
the LCVs that collapsed on the mica, which also confirms that
LCVs were obtained successfully. Additionally, the assembled
LCVs of PHEMAPOSS45-b-PDMAEMA34 in aqueous solution
were further revealed by SEM images (Supporting Information
Fig. S10), which is consistent with the TEM and AFM results.
Additionally, some single spheres coexist in these two self-
assembled morphologies, which indicating the spheres
formed in the self-assemble process first, and then they fur-
ther involve adhesive collisions, resulting in the formation of
a pearl-necklace-liked structure.73,74
We further decreased the length of the soluble PDMAEMA
block to prepare PHEMAPOSS45-b-PDMAEMA34, where the
molar ratio of hydrophobic/hydrophilic reaches to about
FIGURE 9 TEM images [scale bar, 200 nm (a), 50 nm (b)] of PHEMAPOSS45-b-PDMAEMA145 self-assembled aggregates in aqueous
solution.
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FIGURE 10 TEM images [scale bar, 200 nm (a), 50 nm (b)] of PHEMAPOSS45-b-PDMAEMA106 self-assembled aggregates in aque-
ous solution.
In our previous studies of hemi-telechelic and telechelic
amphiphilic POSS-containing hybrid PAAs, the POSS moieties
are only located at the end of polymer chains, moreover, the
POSS content is quite lower in these hybrid polyelectrolytes,
thus the self-assembly of POSS-containing hybrid polyelectro-
lytes could not be well studied. However, in this work, we
constructed a series of amphiphilic POSS-containing hybrid
block copolymers, and systematically studied the influence of
the content of POSS on the assembled morphologies. Based
on above AFM and TEM results, we found there are mainly
four assembled morphologies formed from PHEMAPOSS-b-
PDMAEMA block copolymers in water, which are dependent
on the molar ratio of hydrophobic/hydrophilic block (Scheme
3). For PHEMAPOSS45-b-PDMAEMA699 with the longest hydro-
philic PDAEMA chain (fPHEMAPOSS 5 30.8%), it self-assembled
into some irregular aggregates with some dark dots. This
structure is similar to that of our previous study in hemi-
telechelic POSS-PAA, which is rarely found from the self-
assembly of the pure organic amphiphilic block copolymers.
With decreasing PDMAEMA chain length, PHEMAPOSS45-b-
PDMAEMA286 (fPHEMAPOSS 5 52.1%) and PHEMAPOSS45-b-
PDMAEMA145 (fPHEMAPOSS 5 68.2%) can form typical core-
shell spherical micelles where the hydrophobic PHEMAPOSS
block as the core and hydrophilic PDMAEMA block as the
shell. With further decreasing PDMAEMA chain length, PHE-
MAPOSS45-b-PDMAEMA106 (fPHEMAPOSS 5 74.6%) and PHEMA-
POSS45-b-PDMAEMA68 (fPHEMAPOSS 5 82.0%) self-assembled
into pearl-necklace-like aggregates from the single core-shell
micelles.
Finally,
PHEMAPOSS45-b-PDMAEMA34
(fPHEMAPOSS 5 90.0%), formed a LCV and this is rarely reported
from the self-assembly of hybrid block copolymers.
The self-assembled aggregates of PHEMAPOSS-b-PDMAEMA
in aqueous solution (0.25 mg/mL) were also studied by DLS.
Figure 13 shows the intensity-weighted hydrodynamic diam-
eter distributions and hydrodynamic diameter of PHEMA-
POSS-b-PDMAEMA ABCPs self-assembly aggregates in
aqueous solution at pH 5 7. In system I, the diameter of the
FIGURE 11 TEM images [scale bar, 200 nm (a), 50 nm (b)] of PHEMAPOSS45-b-PDMAEMA68 self-assembled aggregates in aqueous
solution.
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FIGURE 12 TEM images [scale bar, 200 nm (a), 50 nm (b)] of PHEMAPOSS45-b-PDMAEMA34 self-assembled aggregates in aqueous
solution.
three block copolymers measured by DLS are all larger than
those in AFM or TEM images, which can be explained by the
highly stretched conformation of the PDMAEMA chains at
pH 5 7 in aqueous solution, and the DLS data directly
reflects the dimension of micelles in solution. Whereas, in
system II, with the decreasing of the length of the hydro-
philic chain, the diameter observed in DLS is more close to
the diameter of the AFM images. It can be seen that the
hydrodynamic diameter (176.6 nm) of PHEMAPOSS45-b-
PDMAEMA68 has a broad distribution, which attributed to
the pearl-necklace-like structure and a small number of
AFM images, because the very short soluble chain contrib-
utes little to the hydrodynamic diameter.
Thermoresponsive Behavior of PHEMAPOSS-b-PDMAEMA
Block Copolymers
It is well known that thermoresponsive polymers can
undergo a solubility transition by varying the temperature in
water due to the reversible disruption of hydrogen bonds
between the polymer and water. Thermoresponsive micelles
containing PDMAEMA that involve a coil-to-globule transition
have been reported by many groups.26,28,31 Here, the repre-
sentative block copolymer of PHEMAPOSS45-b-PDMAEMA286
in the systems was chosen to study the dual stimuli-
responsive behaviors, since the mass fraction of
spheres.
Additionally,
the
hydrodynamic
diameter
(436.4 nm) of PHEMAPOSS45-b-PDMAEMA34 with the short-
est soluble block chain is in good agreement with that in
SCHEME 3 Self-assembly of PHEMAPOSS-b-PDMAEMA in water with the decreasing length of hydrophilic PDMAEMA
chain.
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FIGURE 13 Intensity-weighted hydrodynamic diameter distri-
butions of PHEMAPOSS-b-PDMAEMA self-assembled aggre-
gates in aqueous solution at pH 5 7. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
FIGURE 14 Temperature dependence of the hydrodynamic
diameter of PHEMAPOSS45-b-PDMAEMA286 solution at differ-
ent pH. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
PHEMAPOSS/PDMAEMA in PHEMAPOSS45-b-PDMAEMA286 is
roughly equal to 1/1 (the mass fraction of PDMAEMA is
52.1%). DLS and UV spectroscopy were performed to study
the thermal behavior of the micelles formed by PHEMA-
POSS45-b-PDMAEMA286. As seen from Figure 14, the solu-
tions (0.25 mg/mL) at pH 5 7 and pH 5 9 (deprotonation)
were conducted in the experiments, respectively. There is no
abrupt change of the hydrodynamic diameter in both condi-
tions, but a broaden collapse trꢀansition appears under heat-
ing the solution from 25 to 85 C, which is attributed to the
shrinkage of the corona of the micelles upon heating. The
temperature dependence of transmittance was also studied
by UV and was shown in Supporting Information Figure S11.
It can be seen that the transmittance of solution almost does
not change with the increasing temperature, which is agree-
able with DLS results. Furthermore, TEM was also used to
characterize the self-assembled morphology of PHEMA-
POSS45-b-PDMAEMA286 at 60 ꢀC in aqueous solution
(pH 5 7), as shown in Supporting Information Figure S12.
There is no apparent change in the morphology, compared
with that of the self-assembly at room temperature [Fig.
8(a)], and they are still typical core-shell spherical micelles,
which is in accordance with DLS and UV results.
matically, resulting in the abrupt decrease of the hydrody-
namic diameter. In the second stage, with the increase of the
pH, the diameter of the micelles almost does not change in
solution. However, in the third stage, the bimodal distribu-
tion can clearly see in Supporting Information Figure S13,
which indicates the micelles aggregated into complex
micelles.
To further confirm the structural change of the micelles due
to the stimuli-responsive behavior of PDMAEMA, the self-
assembled morphology of PHEMAPOSS45-b-PDMAEMA286
under high pH was characterized by TEM and SEM, respec-
tively [Fig. 16(b) and Supporting Information Fig. S14]. The
results showed the assembled micelles obviously shrunk and
aggregated into
a
complex micelles under high pH
(pH 5 12.5), compared with that of the original assembled
morphology at pH 5 7 [Figs. 8(a) and 16(a)], which means
pH-Responsive Behavior of PHEMAPOSS-b-PDMAEMA
Block Copolymers
PDMAEMA is a pH-responsive polymer and the tertiary
amine groups along its main chain can be charged or
uncharged at low or high pH, respectively. DLS was used to
study the pH-responsive behavior of PHEMAPOSS45-b-
PDMAEMA286, and the result was shown in Figure 15. It can
be seen that with increasing the pH of the micelles solution,
the diameter of the micelles goes through three stages:
sharply decrease, keep constant and sharply increase. In the
first stage, when the pH increased, the amino groups of
PDMAEMA were deprotonated and the corona shrunk dra-
FIGURE 15 Hydrodynamic diameter of PHEMAPOSS45-b-
PDMAEMA286 in aqueous solution at different pH.
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FIGURE 16 SEM image [scale bar 500 nm, pH 5 7 (a)] and TEM images [scale bar 200 nm; pH 5 12.5 (b), pH 5 7 (c)] of PHEMA-
POSS45-b-PDMAEMA286 self-assembled aggregates in aqueous solution. Schematic illustration of the reversible assembly mor-
phology transition of the micelles through acid-base control (d). [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
ACKNOWLEDGMENTS
the PDMAEMA segment is fully deprotonated and the shell
has collapsed under high pH. Surprisingly, when we adjusted
the alkaline self-assembled solution to pH 5 7, the complex
This work was financially supported by the National Natural
Science Foundation of China (No.51173044 and 21074035),
micelles can transfer back to the original spherical morphol-
Research Innovation Program of SMEC (No.14ZZ065), and the
ogy [Fig. 16(c)]. Thus, the reversible assembly morphology
Project-sponsored by SRF for ROCS, SEM. W. Z. also acknowl-
transition of the micelles was realized through acid-base
edges the support from the Fundamental Research Funds for
the Central Universities.
control.
CONCLUSIONS
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