Soft nanoparticles assembled from linear poly(ethylene glycol) and linear brush polydimethylsiloxane diblock copolymers

Article abstract of DOI:10.1002/pola.27111

A series of novel amphiphilic diblock copolymers composed of hydrophilic linear poly(ethylene glycol) (PEG) and linear brush hydrophobic polydimethylsiloxane (PDMS) were synthesized. Three different molecular weights of monomethyl ether PEG were initially functionalized with 2-bromoisobutyryl bromide to afford macroinitiators suitable for atom-transfer radical polymerization. The macroinitiators were characterized by gel permeation chromatography, ¹H and ¹³C nuclear magnetic resonance spectroscopic analysis and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy. The three different molecular weight macroinitiators were then chain extended with monomethacryloxypropyl-terminated PDMS and photoactive 2-(methylacyloyloxy)ethyl anthracene-9-carboxylate in different molar ratios to afford a series of photoresponsive amphiphilic diblock copolymers with high conversions. Self-assembly of these linear-linear brush diblock copolymers in N,N-dimethylformamide afforded nanoparticles with hydrodynamic diameters (d_H) ranging from 41 to 268 nm, as determined by dynamic light scattering analysis. Crosslinking and stabilization of the nanoparticles was achieved via [4+4] photodimerization of the anthracene moieties upon exposure to UV radiation at 365 nm with the reverse reaction studied at a wavelength of 254 nm. Transmission electron microscopy revealed that the self-assembled nanoparticles and their crosslinked derivatives had spherical morphologies.

Full text of DOI:10.1002/pola.27111

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Soft Nanoparticles Assembled from Linear Poly(ethylene glycol) and
Linear Brush Polydimethylsiloxane Diblock Copolymers
1
,2
2
2
1,2
1,2
Andri Halim, Timothy D. Reid, Jing M. Ren, Qiang Fu, Paul A. Gurr,
2
1
1,2
Anton Blencowe, * Sandra E. Kentish, Greg G. Qiao
1
Department of Chemical and Biomolecular Engineering, Cooperative Research Centre for Greenhouse Gas Technologies,
University of Melbourne, Parkville, Victoria 3010, Australia
2
Department of Chemical and Biomolecular Engineering, Polymer Science Group, University of Melbourne, Parkville, Victoria
3010, Australia
Correspondence to: G. G. Qiao (E-mail: gregghq@unimelb.edu.au)
Received 1 November 2013; accepted 21 December 2013; published online 18 February 2014
DOI: 10.1002/pola.27111
ABSTRACT: A series of novel amphiphilic diblock copolymers
composed of hydrophilic linear poly(ethylene glycol) (PEG) and
linear brush hydrophobic polydimethylsiloxane (PDMS) were
synthesized. Three different molecular weights of monomethyl
ether PEG were initially functionalized with 2-bromoisobutyryl
bromide to afford macroinitiators suitable for atom-transfer
radical polymerization. The macroinitiators were characterized
copolymers in N,N-dimethylformamide afforded nanoparticles
with hydrodynamic diameters (d ) ranging from 41 to 268 nm,
H
as determined by dynamic light scattering analysis. Crosslink-
ing and stabilization of the nanoparticles was achieved via
[414] photodimerization of the anthracene moieties upon
exposure to UV radiation at 365 nm with the reverse reaction
studied at a wavelength of 254 nm. Transmission electron
microscopy revealed that the self-assembled nanoparticles and
1
13
by gel permeation chromatography, H and C nuclear mag-
netic resonance spectroscopic analysis and matrix-assisted
laser desorption ionization time-of-flight mass spectroscopy.
The three different molecular weight macroinitiators were then
chain extended with monomethacryloxypropyl-terminated
PDMS and photoactive 2-(methylacyloyloxy)ethyl anthracene-9-
carboxylate in different molar ratios to afford a series of photo-
responsive amphiphilic diblock copolymers with high conver-
sions. Self-assembly of these linear–linear brush diblock
their crosslinked derivatives had spherical morphologies.
V
C
2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2014, 52, 1251–1262
KEYWORDS: anthracene; atom transfer radical polymerization
(ATRP); block copolymers; poly(dimethylsiloxane); poly(ethyl-
ene glycol); polysiloxanes; self-assembly
INTRODUCTION The last decade has seen nanotechnology
variety of biomedical applications including the preparation
of biocompatible conjugates, surface modification of drug
delivery vectors, and as scaffolds for tissue engineering.
emerging as one of the most significant areas in research
1
,2
6
and development in both academia and industry.
Nanoscience” has developed into a multidisciplinary field,
“
PEG has also been significant in the development of mem-
branes for CO2 separation as it possesses a high affinity
ranging from fundamental in-depth studies to the fabrication
of next-generation functional materials for a wide range of
applications. “Soft” materials such as micelles and vesicles
have been extensively used in the biomedical field, whereas
the application of such materials to separation technologies
7
–9
towards CO2.
Additionally, PEG is readily available, inex-
pensive, and easily produced on an industrial scale. Polydi-
methylsiloxane (PDMS) is a class of hydrophobic rubbery
polymer that has been utilized in many areas as a result of
1
0–12
3
,4
its high stability, biocompatibility, and flexibility.
In the
has been less explored.
gas separation field, PDMS is known as one of the most per-
meable polymers, which results from the flexibility of the
Poly(ethylene glycol) (PEG) is a water-soluble polymer that
has attracted much attention as a result of its biocompatibil-
1
3–15
siloxane backbone.
The formation of amphiphilic block
5
ity and low toxicity. PEG has been widely employed in a
copolymers (BCPs) can be achieved by covalently linking
Additional Supporting Information may be found in the online version of this article.
*Present address: Anton Blencowe, Mawson Institute, Division of Information Technology, Engineering and the Environment, Uni-
versity of South Australia, South Australia 5095, Australia
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hydrophilic PEG with hydrophobic PDMS. Soft nanoparticles
made of amphiphilic BCPs have found many applications as
ticles that were further stabilized by photocrosslinking via
[414] photodimerization. The self-assembled nanoparticles
and their crosslinked derivatives were characterized to
determine their size, morphology, and molecular structure.
The well-defined linear–linear brush diblock copolymers dis-
played various structures, from spherical to crew-cut aggre-
gates or large compound micelles (LCMs), with unique
particle sizes.
1
5–17
nanocarriers and nanoreactors.
Previous studies have
also reported the utilization of amphiphilic BCPs to prepare
1
8,19
crosslinked hydrogels for tissue engineering applications.
Several approaches have been reported to combine PEG and
PDMS; mainly through covalently linking linear PEG with lin-
2
0–22
ear PDMS to form a diblock copolymer,
PEG with linear PDMS to form a triblock copolymer,
end-capping linear PDMS with linear PEG to form a triblock
end-capping linear
2
3,24
and
EXPERIMENTAL
1
0,25,26
copolymer.
Other examples include a random-grafted
2
7
Materials
PEG/PDMS copolymer and a commercially available grafted
PEG along a PDMS backbone. To the best of the authors’
knowledge, no studies have reported the synthesis of a well-
defined PEG/PDMS-grafted BCPs. This may arise from the dif-
ficulty in conducting the reaction under controlled living radi-
cal polymerization conditions.
2
-Bromoisobutyryl bromide (BIBB, 98%), copper(I) bromide
CuBr, 98%), sodium trifluoroacetate (NaTFA, 98%), anhy-
drous tert-butanol (t-BuOH, 99.5%), anthracene-9-carboxylic
(
0
00 00
acid
PMDETA, 99%), 1 kDa PEG monomethyl ether (MeOPEG
and 5 kDa PEG monomethyl ether (MeOPEG ) were pur-
(99%),
N,N,N ,N ,N -pentamethyldiethylenetriamine
(
1
),
5
BCP self-assembly has attracted significant interest as it has
been identified as one of the key future strategies to prepare
nanostructures with well-defined composition and struc-
chased
from
Aldrich
and
used
as
received.
5 600–800
Monomethacryloxypropyl-terminated PDMS (M
w
Da) (PDMS-MA, 95%, Gelest) was stirred with basic alumina
99%, Scharlau) for 16 h and filtered prior to use. 2-
2
8,29
(
tures.
Past studies have mainly focused on the self-
Hydroxyethyl methacylate (HEMA, 98%, Aldrich) was stirred
for 16 h with inhibitor remover (Aldrich) and filtered prior
to use. Dichloromethane (DCM, AR), methanol (MeOH, AR),
DMF (AR), chloroform (AR), sodium hydroxide (NaOH, 97%),
and sodium chloride (NaCl, 97%) were purchased from
Chem-Supply and used as received. Tetrahydrofuran (THF,
HPLC grade, RCI Labscan) was distilled from sodium benzo-
phenone ketyl under argon. Triethylamine (TEA, 99%, Ajax)
was distilled from calcium hydride (95%, Aldrich) under
assembly of linear–linear PEG/PDMS diblock copolymers where
2
0,22,30,31
the formation of micelles and vesicles was reported.
These self-assembled structures have the potential to be uti-
3
1
lized as nanocarriers in drug delivery system.
The stability of nanoparticles is a crucial condition for appli-
3
2
cations such as controlled drug delivery and release. With-
out crosslinking, the nanoparticles might disintegrate upon
changes in temperature or solution composition. Several
approaches have been utilized to stabilize polymeric nano-
argon. Deuterated chloroform (CDCl
dimethyl sulfoxide (d -DMSO, 99.9%) were purchased from
Cambridge Isotope Laboratories. Trans-2-[3-(4-tert-butyl-
phenyl)-2-methyl-2-propenylidene]-malononitrile (DCTB,
Santa Cruz Biotechnology), 10 kDa PEG monomethyl ether
(MeOPEG10, Creative PEGWorks), thionyl chloride (SOCl , AR,
Merck), hydrochloric acid (HCl, 37% in H O, Scharlau), and
, 98 wt%, Scharlau)
, 99.9%) and deuterated
3
3
3–35
36,37
particles including core
and shell crosslinking.
6
There has been a growing interest in stabilizing nanopar-
ticles via photocrosslinking as this provides good temporal
3
8
and spatial control over the process. Light-induced cross-
linking also provides a stabilization route that avoids
2
3
2
unwanted by-products. Past studies have utilized couma-
2
3
9
40
rin and cinnamate groups to incorporate photochromic
moieties into the nanoparticles. Limited studies have utilized
anthracene moieties to stabilize nanoparticles. Compared to
coumarin and cinnamate groups, the [414] photodimeriza-
tion of anthracene groups provides stability at elevated tem-
magnesium sulfate anhydrous (MgSO
were used as received.
4
Instrumentation
Gel permeation chromatography (GPC) was performed on a
Shimadzu liquid chromatography system fitted with a Shi-
madzu RID-10A refractive index detector (633 nm) and a
Shimadzu SPD-10A UV–Vis detector (230 and 390 nm), using
4
1
peratures. Additionally, photodimerization of coumarin and
cinnamate groups may be easily reversed under certain con-
3
8
ditions, which may be undesirable for certain applications.
4
6
three Phenomenex Phenogel columns (500, 10 , 10 Å poros-
In this study, we report the synthesis of soft nanoparticles
assembled from linear PEG linked with a photocross-linkable
PDMS brush. The photocross-linkable diblock copolymers
were synthesized by chain extension of functionalized linear
PEG with monomethacryloxypropyl-terminated PDMS
ꢀ
ity; bead size, 5 mm) maintained at 50 C. Chloroform was
used as the eluent at a flow rate of 1 mL/min. Astra soft-
ware (Wyatt Technology) was used to process the data and
determine molecular weight characteristics based on a con-
ventional column calibration using poly(ethylene oxide)
1
13
(
PDMS-MA) and photoactive 2-(methylacyloyloxy)ethyl
(PEO) standards. H and C NMR spectroscopic analysis
was performed on a Varian Unity Plus 400 MHz spectrome-
ter operating at 400 and 100 MHz, respectively, using the
deuterated solvent resonance as reference. Matrix-assisted
laser desorption ionization time-of-flight mass spectroscopy
(MALDI ToF MS) was performed on a Bruker Autoflex III
anthracene-9-carboxylate (MEAC) via atom-transfer radical
polymerization (ATRP) in different molar ratios. Self-
assembly of these diblock copolymers was investigated in
N,N-dimethylformamide (DMF), which is a selective solvent
for the PEG block, and resulted in the formation of nanopar-
1
252
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TABLE 1 Molecular Weight Characterization of MeOPEG Macroinitiator P1
NMR
a
MALDI-ToF
b
GPC
c
c
d
Macroinitiator
M
n
(kDa)
M
n
(kDa)
M
n
(kDa)
PDI
Degree of Functionalization (%)
P1
1
5
1.1
5.2
1.1
5.1
1.6
5.7
1.07
1.13
1.26
>99
>99
>99
P1
P110
10.5
10.8
13.6
a
1
c
Number-average molecular weight calculated from H NMR spectro-
scopic analysis.
Number-average molecular weight and PDI measured from GPC-UV
analysis (230 nm) relative to PEO standards.
b
d
Number-average molecular weight determined by MALDI ToF MS.
Degree of functionalization determined by MALDI ToF MS.
Mass Spectrometer operating in positive/linear mode; the ana-
lyte was dissolved in DCM at concentrations of 10 mg/mL,
whereas the matrix (DCTB) and cationization agent (NaTFA)
were dissolved in MeOH at concentrations of 10 and 1 mg/mL,
respectively, and then mixed in a ratio of 10:1:1. A 0.3 mL ali-
quot of this solution was then spotted onto a ground steel tar-
get plate, and the solvent was allowed to evaporate prior to
analysis. Flex Analysis (Bruker) was used to analyze the data.
Dynamic light scattering (DLS) measurements were performed
on a Wyatt DynaPro NanoStar fitted with a 120 mW Ga–As
laser operating at 658 nm; 100 mW was delivered to the sam-
argon. The reaction mixture was refluxed under argon for 90
min and the excess SOCl was removed in vacuo (0.1 mbar).
2
The resulting residue was azeotroped with benzene (3 3
10 mL), redissolved in anhydrous THF (40 mL), and cooled
ꢀ
to 0 C. TEA (2.4 mL, 32.7 mmol) was added, followed by
the dropwise addition of HEMA (1.15 mL, 9.5 mmol) in
anhydrous THF (30 mL). The reaction solution was stirred at
ꢀ
0 C for 1 h, warmed to room temperature, and stirred for
20 h, and then filtered and concentrated. The crude product
was redissolved in DCM (100 mL), washed with 2 M HCl (2
3 100 mL), 2 M NaOH (4 3 100 mL), saturated NaCl (100
ꢀ
ple cell. Analysis was performed at an angle of 90 and a con-
mL), distilled water (2 3 100 mL), dried (MgSO ), filtered,
4
ꢀ
stant temperature of 25 6 0.01 C. UV–Vis analysis was
and concentrated in vacuo (2 mbar). The product was then
ꢀ
performed on a Shimadzu UV–Vis Scanning Spectrophotometer
dried in vacuo (0.1 mbar) at 30 C for 16 h to yield MEAC as
1
(
UV-2101 PC) using quartz cuvettes. Transmission electron
a pale yellow solid, 2.88 g (83%; purity ( H NMR), >90%).
microscopy (TEM) images were taken using a Tecnai TF30
transmission electron microscope (FEI, Eindhoven, The Neth-
erlands) operating at 200 kV. Images were acquired digitally
with a Gatan US1000 2k 3 2k CCD Camera (Pleasanton, CA).
1
H NMR (400 MHz, CDCl , d, ppm): 1.99–2.00 (t, J 5 1.2 Hz,
3
CH , H ), 4.60–4.62 (m, CH , H ), 4.87–4.90 (m, CH , H ),
5
3
c
2
d
2
e
.64–5.65 (quin, CH, H ), 6.20–6.25 (quin, CH, H ), 7.47–7.54
a
b
(m, ArH, H ), 8.02–8.10 (m, ArH, H ), 8.55 (s, ArH, H ) (Sup-
h g f
1
3
porting Information Fig. S3). C NMR (100 MHz, CDCl
ppm): d 18.3, 62.5, 63.3, 125.0, 125.4, 126.2, 127.0, 128 (d),
1
3
, d,
Synthesis of a-Methyl, x-Bromo-functionalized PEG
Macroinitiator P1
C
29.7, 130.9, 135.9, 167.0, and 169.3 (Supporting Informa-
MeOPEG (10.0 g, 2.0 mmol, 1 equiv.) was dissolved in anhy-
5
tion Fig. S4).
drous DCM (80 mL) under argon. The solution was cooled to
ꢀ
0
3
C followed by the dropwise addition of TEA (4.50 mL,
Synthesis of PEG/PDMS Diblock Copolymers P2
PDMS-MA (0.30 g, 0.429 mmol, 8 equiv.), P15 (0.279 g,
0
equiv.) were dissolved in t-BuOH (1.85 mL) at 60 C to
afford a macromonomer concentration of 0.02 M. An aliquot
2.3 mmol, 16 equiv.) and BIBB (6.88 g, 30.0 mmol, 15
ꢀ
equiv.). The mixture was stirred for 1 h at 0 C. The crude
product was then washed with 1 M HCl (2 3 80 mL), 1 M
NaOH (4 3 80 mL), saturated NaCl (80 mL), dried (MgSO4),
filtered, and concentrated in vacuo (2 mbar). The dried prod-
uct was redissolved in chloroform (80 mL), passed through
basic alumina, and concentrated in vacuo (2 mbar). The
.054 mmol, 1 equiv.) and MEAC (53.7 mg, 0.161 mmol, 3
ꢀ
(
0
0.2 mL) was taken (t ) to monitor macromonomer and
MEAC conversion. Bipyridine (16.7 mg, 0.107 mmol, 2
equiv.) was then added to the mixture and the solution was
transferred to a Schlenk tube (oven-dried at 110 C for 24
h). The mixture was then subjected to four freeze–pump–
thaw cycles (thaw cycles conducted at 60 C) and the
ꢀ
desired product was then dried in vacuo (0.1 mbar) at 25 C
ꢀ
for 16 h to yield P1 as a white solid, 5.76 g (isolated yield,
5
34%). The synthetic procedure was repeated for the func-
ꢀ
tionalization of MeOPEG and MeOPEG (Table 1).
1
10
Schlenk tube was then immersed again in liquid N . Once
2
1
H NMR (400 MHz, CDCl , d, ppm): 1.89 (s, CH , H end-
the solution was frozen, CuBr (15.4 mg, 0.107 mmol, 2
equiv.) was added and one more freeze–pump–thaw cycle
was completed. The Schlenk tube was then equilibrated at
room temperature for 10 min with stirring to ensure homo-
3
3
h
3 a 2 2
group), 3.24 (s, CH , H end-group), 3.41–3.69 (m, CH CH O,
H , H , H , H , and H ), 4.23–4.25 (m, CH CH OCO, H end-
b
c
d
e
f
2
2
g
1
3
group) (Supporting Information Fig. S1). C NMR (100 MHz,
ꢀ
CDCl , d, ppm): 30.2, 57.0, 57.9, 64.9, 67.9, 69.5, 69.7, 71.2,
geneity and then heated at 90 C for 48 h. After cooling to
3
and 170.7 (Supporting Information Fig. S2).
room temperature, an aliquot (0.2 mL) was taken (t ) to
4
8
determine the macromonomer and MEAC conversion. The
reaction mixture was then diluted with DCM and passed
through a plug of basic alumina to remove the copper cata-
lyst. The filtrate was concentrated in vacuo (2 mbar) before
Synthesis of MEAC
Anthracene-9-carboxylic acid (2.22 g, 10.0 mmol) and thionyl
chloride (SOCl ) (30 mL) were added to a dried flask under
2
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TABLE 2 Synthesis and Characterization of PEG/PDMS Diblock Copolymers
Conversion
a
Theor
d
NMR
e
GPC
f
Polymer
Molar
(PDMS-MA/
M
n
M
n
M
n
b
c
f
(
P2PEG-MW/PDMS-MW
)
Macroinitiator
Ratio
MEAC)
(kDa)
(kDa)
(kDa)
PDI
P21/5
P1
P1
P1
P1
P1
1
1
5
5
5
1:5:3
95/>99
92/>99
94/>99
96/>99
92/>99
92/>99
90/>99
88/>99
5.7
6.0
19.7
29.2
13.0
17.4
22.2
58.9
63.1
64.0
2.00
2.28
2.23
2.17
2.06
2.15
2.06
1.90
P21/8
1:10:3
1:4:3
8.6
9.8
P25/4
8.7
9.7
P25/7
1:8:3
11.8
14.1
18.5
21.6
25.1
12.3
17.0
19.5
24.0
28.5
P25/9
1:12:4
1:11:4
1:16:4
1:21:5
P210/9
P210/12
P210/15
P110
P110
P110
a
d
The first subscript refers to PEG-MW in kDa and the second refers to
PDMS-MW in kDa.
Number-average molecular weight based on macromonomer conver-
sion and initial feed molar ratio.
b
e
1
Molar ratio of [P1]:[PDMS-MA]:[MEAC].
Number-average molecular weight determined by H NMR spectro-
c
1
PDMS-MA and MEAC conversions were calculated from H NMR spec-
troscopic analysis.
scopic analysis.
f
Number-average molecular weight and PDI measured by GPC-UV trace
(230 nm) relative to PEO standards.
ꢀ
drying in vacuo (0.1 mbar) at 45 C for 16 h to yield product
P25/4 as an opaque tacky solid, 0.35 g (isolated yield, 65%).
The polymer composition was varied by changing the macro-
initiator, monomer, and MEAC feed ratio (Table 2).
Vis spectroscopy was again used to monitor the progress of
the photocleavage process.
Imaging of Self-assembled Nanoparticles and Their
Photocross-linked Derivatives
1
H NMR (400 MHz, CDCl , d, ppm): 0.04–0.09 (m, H ), 0.51–
.55 (m, H ), 0.86–1.04 (m, H , H ), 1.25–1.35 (m, H ), 1.64 (br
), 3.38–4.12 (m, H
H ), 7.44 (br s, H ), 7.99 (br s, H ), and 8.43 (br s, H ) (Fig. 2).
3
h
TEM samples were prepared by drop-coating the self-
assembled nanoparticles in DMF (0.167 mg/mL) and their
photocrosslinked derivatives in chloroform (0.167 mg/mL)
onto copper grids (10 mL). The samples were then blotted
and negatively stained by uranyl acetate aqueous solution (2
wt %) before imaging was conducted.
0
g
d
j
i
f
s, H ), 1.78 (br s, H
c
), 3.38 (s, H
a
b
), 3.87 (br s,
e
l
k
m
Self-assembly of PEG/PDMS Diblock Copolymers
The solvents used for the self-assembly studies, chloroform,
and DMF, were first filtered through 0.45 mm PTFE filters
RESULTS AND DISCUSSION
(
Labquip Technologies). Copolymers were dissolved in chlo-
roform (30 mg/mL) prior to dropwise addition of DMF to
affect dissolution (0.167 mg/mL). DLS was then employed to
measure the hydrodynamic diameters of the self-assembled
nanoparticles. DLS was also utilized to measure the hydrody-
namic diameters of the unimers prior to self-assembly. For
the control experiment, copolymers were dissolved in chloro-
form (0.167 mg/mL).
ATRP was employed to synthesize photoresponsive amphi-
philic linear PEG and brush PDMS diblock copolymers
(Scheme 1). Initially, three different molecular weights of
monomethyl ether PEG (MeOPEG) were functionalized to
afford a-methyl, x-bromo-functionalized PEG macroinitiators
P1 suitable for ATRP. The macroinitiators were characterized
via GPC, nuclear magnetic resonance (NMR) spectroscopic
analysis, and MALDI ToF MS. The PEG macroinitiators P1
were subsequently chain extended with varying amounts of
PDMS-MA macromonomer and photoactive MEAC to afford a
series of novel photoresponsive amphiphilic copolymers with
various degrees of polymerization (DP).
Photocrosslinking of PEG/PDMS Diblock Copolymers
After self-assembly, brush diblock copolymer samples were
photocrosslinked by exposure to UV radiation with a wave-
length of 365 nm over a period of 4 h. UV–Vis spectros-
copy was used to monitor the progression of the [414]
photodimerization. Samples were concentrated in vacuo
Synthesis of a-Methyl, x-Bromo-functionalized PEG
Macroinitiator
(
0.1 mbar) to remove DMF and then diluted with chloro-
form for postcrosslinking DLS analysis. Samples were also
diluted below their CMCs to further confirm the extent of
crosslinking.
Functionalization of MeOPEG with initiating moieties was
achieved through reaction with BIBB to afford a-methyl, x-
bromo-functionalized PEG macroinitiator P1 with quantita-
tive end-group conversion (Scheme 1). In this study, three
different molecular weights of MeOPEG were functionalized
and the resulting macroinitiators (P1 , P1 , and P110) were
Photocleavage of PEG/PDMS Diblock Copolymers
To initiate photocleavage, samples were exposed to UV radia-
tion with a wavelength of 254 nm over a period of 20 h. UV–
1
5
obtained in moderate yields after a rigorous purification
1
254
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SCHEME 1 Synthetic outline for the preparation of soft nanoparticles assembled from linear PEG and linear brush PDMS diblock
copolymers. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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5
FIGURE 1 MALDI ToF mass spectra of (a) 5 kDa MeOPEG and (b) its bromo-functionalized derivate, P1 , recorded in linear/positive
mode using DCTB and NaTFA as the matrix and cationization agent, respectively. The numbers on the peaks represent the number
of EO repeat units (n, m/z 5 44). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
process. A large excess of BIBB was utilized to ensure quan-
titative functionalization of MeOPEG.
m/z 5 150 (Fig. 1(b)) relative to their precursors (Fig. 1(a)),
which corresponds to the addition of the bromoisobutyrate
end-group. The observed mass values were found to corre-
late well with the sodium salts of the desired macroinitiator
1
H NMR spectroscopic analysis of P1 revealed a new reso-
5
H
nance at d 1.9 ppm (Supporting Information Fig. S1) corre-
sponding to the methyl groups of the bromoisobutyrate end-
group upon esterification. NMR spectroscopic analysis also
P1
5
, resulting from the use of NaTFA as the cationization
and P110 also showed
agent. MALDI ToF MS spectra of P1
1
similar results (Supporting Information Figs. S6 and S7). All
of the MALDI ToF MS spectra revealed series of peaks sepa-
rated by ca. m/z 5 44, corresponding to the repeat unit of
PEG (Fig. 1(a,b)). Determination of the number-average
confirmed the formation of the PEG macroinitiators P1 and
1
P110 (Supporting Information Fig. S5).
MALDI ToF MS and NMR spectroscopy were used to deter-
mine the molecular weights of the MeOPEG macroinitiators
P1 (Fig. 1(b), Supporting Information Fig. S1, and Table 1).
MALDI ToF MS spectra of the functionalized MeOPEG
macroinitiators revealed a series of peaks shifted by about
molecular weights (M ) via MALDI ToF MS provided values
n
that corresponded well with those determined by NMR spec-
troscopy (Table 1). GPC analysis was performed on the MeO-
PEG macroinitiators P1, and their molecular weights and
1
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mation of the presence of anthracene moieties within the
copolymers was determined using GPC analysis coupled with
UV detection at a wavelength of 390 nm owing to a strong
series of absorbance peaks between 320 and 400 nm.
1
H NMR spectroscopic analysis of the copolymers (Fig. 2)
revealed characteristic methylene and methyl proton
resonances (ca. d 5 1.8 and 1.0 ppm) associated with the
H
methacrylate polymer backbone. The peak shifts to lower
1
retention times in the GPC traces (Fig. 3) and H NMR spec-
troscopic analysis confirmed the synthesis of the PEG/PDMS
diblock copolymers. The integral ratios of the PEG repeat
unit (ca. d 5 3.6 ppm), PDMS repeat unit (ca. d 5 0.7 ppm),
H
H
and the total molecular weight of the anthracene moieties
NMR
were used to calculate the Mn
values (Table 2). The Mn
values determined from NMR spectroscopy corresponded
well to the theoretical values calculated from conversion and
molar feed ratios (Table 2). M values obtained from the GPC
n
UV traces differed significantly from those determined by
NMR spectroscopy and theoretical molecular weight calcula-
tions. This most likely arises from inaccuracies caused by
using a conventional calibration method with PEO standards
for copolymers containing both PEG and PDMS moieties.
Unfortunately, GPC coupled with multiangle light scattering
detection could not be used as the polymers did not show
sufficient peak intensities.
1
FIGURE 2 H NMR spectrum (CDCl
3
) of PEG/PDMS diblock
copolymer P25/9. The asterisks denote resonances correspond-
ing to the double bond of unreacted PDMS-MA macromono-
mer. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
molecular weight distributions were determined relative to a
conventional column calibration with PEO standards. Despite
PEG having a strong positive RI signal in chloroform, to be
consistent, the UV traces were used for the analysis of all
polymers, as the PDMS components in the following section
have either a negative RI signal in chloroform or are isore-
1
H NMR spectroscopic analysis of the copolymers revealed
alkene proton resonances that correspond to small amounts
(<8%) of unreacted PDMS-MA macromonomer (Fig. 2).
These trace amounts of unreacted macromonomers were not
removed as we have previously shown that the association
of unreacted PDMS-MA with high-density brushes makes the
fractive in THF. The M values determined from GPC analysis
35
n
complete removal difficult. Closer examination of the GPC
were slightly higher than those calculated from MALDI ToF
MS and NMR spectroscopy, which possibly results from the
inaccuracy of the standard calibration method with PEO
standards (Table 1).
traces of copolymers revealed peaks at about 31 min, which
corresponded to a combination of unreacted PDMS-MA and
unfunctionalized PDMS (Fig. 3). The presence of the unfunc-
tionalized PDMS in the PDMS-MA was detected via MALDI
ToF MS although the amount of unfunctionalized PDMS
could not be quantified as a result of possible preferential
ionization during analysis (Supporting Information Fig. S8).
Synthesis of PEG/PDMS Diblock Copolymers
Photoresponsive amphiphilic copolymers were synthesized
via ATRP with a-methyl, x-bromo-functionalized PEG P1 as
the macroinitiators, PDMS-MA as the macromonomer, and
MEAC as the photoactive monomer. Cu(I)/PMDETA was uti-
lized as the catalyst/ligand system for the copolymers based
Self-assembly of PEG/PDMS Diblock Copolymers
Self-assembly of the linear–linear brush diblock copolymers
was conducted in DMF, which is a selective solvent for the PEG
on the 1-kDa macroinitiator P1 , whereas Cu(I)/bipyridine
1
block. The intensity-average hydrodynamic diameters (d ) of
H
was used for copolymers based on the 5- and 10-kDa macro-
the PEG/PDMS nanoparticles formed from self-assembly in
DMF were determined via DLS (Supporting Information Fig.
S9) and ranged from 41 to 268 nm (Table 3). Correlation func-
tion plots revealed smooth profiles for all samples, implying a
continuous distribution in the measured samples (Supporting
Information Fig. S9). Control experiments were conducted in
chloroform to measure the size of the unimers prior to self-
assembly, as chloroform is a good solvent for both blocks
initiators P1 and P110, respectively. ATRP of PDMS-MA and
5
1
MEAC was initiated with P1 using THF as a solvent at vari-
ous macromonomer to macroinitiator ratios to yield copoly-
mers with high PDMS-MA conversions, up to 95% (Table 2);
ꢀ
the reaction was conducted at 60 C. Longer PEG chain mac-
roinitiators (P1 and P1 ) required a more polar solvent, t-
5
10
ꢀ
BuOH, and higher temperature (90 C) to yield copolymers
with high PDMS-MA conversions, up to 96% (Table 2). MEAC
conversions were observed to be >99% for all the synthe-
sized copolymers, as MEAC was a smaller molecule which
resulted in less steric constraints. Additionally, a smaller
quantity of MEAC was used to ensure full conversion. Confir-
(
Table 3 and Supporting Information Fig. S10).
Figure 4 shows the plot of intensity-average hydrodynamic
diameters (d ) of assembled particles versus the molecular
weight of poly(PDMS-MA) brush block for various PEG block
H
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___
FIGURE 3 GPC ultraviolet (UV) traces at 230 ( ) and 390 nm (---) of PEG/PDMS diblock copolymers and their precursor macroini-
tiators P1 and PDMS-MA macromonomer. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.
com.]
molecular weights. All three series (P2 , P2 , and P210)
show an increasing trend in dH with an increase in pol-
y(PDMS-MA) brush block molecular weight. Interestingly, the
imum micelle diameter, which was based on twice the calcu-
lated contour length of each copolymer (Table 3). Therefore,
it is proposed that different assembled structures are formed
in this case.
1
5
P2 series has a much greater increase relative to the other
1
series. Furthermore, the overall dH decreased with an
increase in the PEG block molecular weight from P21 to
P210 series, which is not consistent with that normally
In amphiphilic BCP self-assembly, LCMs or crew-cut aggre-
gates are formed when BCPs with one block that is far
shorter than the other blocks are assembled in selective sol-
4
2,43
observed for ordinary BCP self-assembly.
4
2,43
43
vents for the shorter block.
For example, Eisenberg
For the P210 series with PEG M of 10.5 kDa, the d
n
H
of the
observed such aggregates for the assembly of polystyrene-b-
self-assemblies increased from 41 to 47 nm as the molecular
weight of the poly(PDMS-MA) block increased from 9 to 15
kDa. Judging from the molecular weight of each block and
the size of the particles, the assembly structure is likely to
be that of spherical micelles with the poly(PDMS-MA) brush
block forming the core and PEG block forming the protective
poly(acrylic acid) (PS200-b-PAA ) in a selective solvent for
4
PAA. In these LCMs, PAA is located on the surface of the par-
ticles as well as within the particles as isolated domains.
Similarly, the nanoparticles formed from the P21 series of
linear–linear brush diblock copolymers observed in this
study are likely to form similar LCMs. Interestingly, the parti-
cle size increased substantially to 268 nm when the molecu-
lar weight of poly(PDMS-MA) was increased from 5 to 8 kDa
in the P21 series self-assemblies. This behavior is the first
coronal shell. In comparison, for the P2 series with PEG Mn
1
of 1.1 kDa, the assembled particle sizes were significantly
larger (e.g., 151 and 268 nm) relative to the theoretical max-
1
258
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TABLE 3 Intensity-Average Hydrodynamic Diameters of PEG/PDMS Diblock Copolymer Self-assemblies
Photocrosslinked
Self-assemblies
(DMF)
Assemblies
(CHCl
Unimers (CHCl
3
)
3
)
Theoretical Maximum
a
b
Polymer
PDMS (wt %)
d
H
(nm)
PDI
d
H
(nm)
PDI
d
H
(nm)
PDI
Micelle Diameter (nm)
c
P21/5
63
78
36
50
60
41
52
58
0.8
0.9
0.8
1.0
1.0
1.2
1.3
1.3
0.03
0.03
0.02
0.02
0.02
0.03
0.02
0.03
151
268
72
0.17
0.17
0.10
0.14
0.15
0.04
0.04
0.07
165
275
78
0.20
0.24
0.26
0.24
0.24
0.18
0.17
0.19
20
c
P21/8
20
c
P25/4
74
c
P25/7
80
88
74
c
P25/9
85
90
74
c
P210/9
P210/12
P210/15
41
54
142
d
42
54
143
d
47
50
145
a
c
wt % PDMS calculated from number-average molecular weight meas-
Contour length was calculated based on the stretched PEG backbone
1
ured by H NMR spectroscopic analysis.
b
and stretched grafted PDMS side chain.
d
Theoretical maximum micelle diameter was calculated as twice of
Contour length was calculated based on the stretched PEG backbone
each copolymer’s contour length, representing the maximum possible
diameter for spherical micelles.
and stretched grafted PDMS backbone.
such observation for LCMs formed from linear–linear brush
BCPs. TEM images of P21/5 showed a spherical morphology
and diameters of 50–220 nm (Fig. 5(a)), which is in agree-
was utilized for self-assembly in an aqueous NaCl solution.
When the DP of the PEG branch was reduced, a substantial
increase in particle size (or aggregation number) was
observed. The reduction in the number of PEG-grafted side
chains also reduced the stabilization capacity of the particles,
resulting in an increase in particle size. This reported trend
is similar to the trend observed in this study when the
molecular weight of PEG block is reduced.
ment with the results of DLS (d 5 151 nm, polydispersity
H
index [PDI] 5 0.17). The spherical morphology of the par-
ticles and the relatively polydisperse system is in agreement
with the reported LCM morphology.
For the P2 series with a PEG block of M of 5.2 kDa, a simi-
5
n
lar trend to that of the P210 series was observed; however,
the particle sizes are bigger. Therefore, these particles are
likely to represent a transition from spherical micelles to
LCMs. Again, particles with spherical morphologies were
observed in TEM images (Fig. 5(c)).
Photocrosslinking of PEG/PDMS Diblock Copolymers
Crosslinking of the soft nanoparticles was conducted to cova-
lently stabilize the structure and prevent disassembly in
good solvents for both the PEG and the PDMS blocks. All of
the samples were photocrosslinked using UV radiation. The
anthracene moieties incorporated into the PDMS segment of
the BCP are photosensitive and photodimerize upon expo-
4
4
Eckert and Webber observed similar aggregation behavior
when a random polystyrene-alt-maleic anhydride-graft-PEO
4
1
sure to UV radiation at wavelengths >300 nm (Scheme 2).
The [414] photodimerization of anthracene forms an ortho-
substituted benzene molecule. Once dimerized, it does not
absorb light at wavelengths >300 nm, making the reaction
4
5
easy to monitor spectrophotometrically. It is well known
that the dimerization predominantly occurs in a “head-to-
4
1,46
head” arrangement as shown in Scheme 2.
Samples containing self-assemblies in DMF were exposed to
UV radiation with a wavelength of 365 nm for 4 h and then
they were tested for their UV absorbance between 300 and
420 nm to monitor the progress of the photodimerization
(
Fig. 6 and Supporting Information Fig. S11). A decrease in
UV absorption is indicative of anthracene moieties losing
their conjugated structure owing to dimerization. The extent
of crosslinking was found to vary between 81 and 91%
(Table 4). A small fraction of nondimerized anthracene moi-
eties residing within the dense PDMS core were unable to
couple, possibly as a result of steric hindrance within the
H
FIGURE 4 Intensity-average hydrodynamic diameter (d ) of
self-assembled nanoparticles as a function of poly(PDMS-MA)
number-average molecular weight determined from NMR spec-
troscopic analysis. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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FIGURE 5 TEM images of (a) P21/5, (b) photocrosslinked P21/5, (c) P25/7, and (d) photocrosslinked P25/7. Scale bars 5 200 nm.
core. However, 100% dimerization is not required to effect
successful stabilization of the assemblies.
ing Information Fig. S12). Dilution of the samples below
their CMCs (Supporting Information Table S1) showed no
evidence of dissociation which further confirmed successful
crosslinking of the nanoparticles (Supporting Information
Fig. S13).
Samples were concentrated to remove DMF, and then diluted
with chloroform to verify the extent of crosslinking; chloro-
form is a good solvent for both blocks. From DLS analysis,
similar single peaks were detected, which implied that all of
the self-assembled copolymers had been crosslinked. Addi-
tionally, correlation function plots revealed smooth profiles,
verifying no aggregation in the measured samples (Support-
DLS analysis revealed that the dH values were found to
increase slightly from the uncrosslinked values recorded for
all of the crosslinked nanoparticles. Rearrangement of the
anthracene moieties to undergo crosslinking possibly forces
SCHEME 2 Mechanism for photodimerization and photocleavage anthracene of molecules.
1
260
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TABLE 4 Extent of Photodimerization of the Anthracene
Groups for more than 4 h
Extent of
Polymers
Photodimerization (%)
P21/5
91
87
81
86
86
86
88
82
P21/8
P25/4
P25/7
P25/9
P210/9
P210/12
P210/15
FIGURE 6 UV–Vis spectra of P25/4 upon exposure to UV radia-
tion (365 nm) for 4 h. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
from spherical micelles to LCMs. The synthetic approach out-
lined in this article is a viable strategy to synthesize stable
soft nanoparticles that can be utilized for different applica-
tions such as additives for membrane to improve their gas
separation performance.
the chains into a larger and less well-packed structure, lead-
ing to a slight increase in particle size (Table 3). The photo-
crosslinking also led to an increase in PDI, implying a more
dispersed nanoparticle size distribution (Table 3).
The reversibility of the crosslinked anthracene moieties was
investigated by monitoring the UV absorption of the photo-
crosslinked P2 under irradiation at a lower wavelength of
ACKNOWLEDGMENTS
The authors acknowledge the funding provided by the Austra-
lian Government through the CRC Program to support this
research. The authors wish to acknowledge the University of
Melbourne’s 2013 Early Career Researcher Grant (Q. Fu). The
authors also acknowledge the Electron Microscopy Unit at the
Bio21 institute (The University of Melbourne) for assistance
with TEM. Q. Fu is the recipient of an Australian Research Coun-
cil Super Science Fellowship (FS 110200025). G. G. Qiao is the
recipient of an Australian Research Council Future Fellowship
2
1
54 nm for 20 h. Results indicated that on average only
5% of the dimerized anthracene moieties were converted
back to the unimolecular state (Supporting Information Table
S2 and Supporting Information Fig. S14). These results are
in agreement with the previously published results which
suggest that the presence of the carbonyl functionality inhib-
4
7,48
its the complete photocleavage process.
(
FT 110100411). A. Halim acknowledges Qiwyn Yong for prac-
TEM images of the crosslinked P21/5 and P25/7 nanopar-
ticles (Figure 5(b,d)) revealed nanoparticles with spherical
morphologies of 70–240 and 60–100 nm in diameters,
which is again in agreement with the results from DLS
tical assistance.
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