Preparation and application of a dual light-responsive triblock terpolymer

Article abstract of DOI:10.1021/ma3006476

Reported are the preparation and application of a triblock terpolymer poly(ethylene oxide)-ONB-poly[2-(perfluorooctyl)ethyl methacrylate]-block- poly(2-cinnamoyloxyethyl methacrylate) (PEO-ONB-PFOEMA-b-PCEMA). Here PEO is water-soluble, PCEMA is photo-cro

Full text of DOI:10.1021/ma3006476

Article
pubs.acs.org/Macromolecules
Preparation and Application of a Dual Light-Responsive Triblock
Terpolymer
Muhammad Rabnawaz and Guojun Liu*
Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
S
* Supporting Information
ABSTRACT: Reported are the preparation and application of
a triblock terpolymer poly(ethylene oxide)-ONB-poly[2-
(perfluorooctyl)ethyl methacrylate]-block-poly(2-cinnamoylox-
yethyl methacrylate) (PEO-ONB-PFOEMA-b-PCEMA). Here
PEO is water-soluble, PCEMA is photo-cross-linkable,
PFOEMA is of low surface tension, and ONB denotes a
photocleavable o-nitrobenzyl unit at the junction of the PEO
and PFOEMA blocks. To prepare the copolymer, a macro-
initiator bearing an ONB unit between PEO and the initiating
site was first synthesized. FOEMA and 2-trimethylsilyloxyethyl
methacrylate (HEMA-TMS) were then sequentially poly-
merized by atom transfer radical polymerization (ATRP) to yield the triblock copolymer PEO-ONB-PFOEMA-b-P(HEMA-
TMS). Removal of the trimethylsilyl groups from the P(HEMA-TMS) block and cinnamation of the resultant poly(2-
hydroxyethyl methacrylate) block with cinnamoyl chloride resulted in the targeted triblock copolymer. PEO-ONB-PFOEMA-b-
PCEMA formed micelles in tetrahydrofuran/water at the water volume fraction of 80%, in which only the PEO block was
soluble. Upon photolysis, the micellar PCEMA cores were cross-linked and the micellar coronal PEO chains were cleft, resulting
in the precipitation of cross-linked PFOEMA-b-PCEMA nanoparticles. Films of the nanoparticles were water- and oil-repellent
due to the exposure of the initially hidden PFOEMA block.
reported by Zhao and co-workers,30 this represents the second
I. INTRODUCTION
Light-responsive block copolymers have various applications.1
report on block copolymers possessing dual light responses.
The targeted polymer consisted of poly(ethylene oxide)-
For example, the photo-cross-linking of poly(2-cinnamoylox-
yethyl methacrylate)- or PCEMA-bearing micelles2−6 has
ONB-poly[2-(perfluorooctyl)ethyl methacrylate)-block-poly(2-
cinnamoyloxyethyl methacrylate) (PEO-ONB-PFOEMA-b-
allowed their structural stabilization to yield “permanent”
nanostructures including nanofibers,3,4,7 nanotubes,8−10 nano-
PCEMA or P1). Here PEO is water-soluble, PFOEMA is of
low surface tension,31 PCEMA is photo-cross-linkable,2,32 and
spheres,2 and hollow nanospheres.11−13 The photocleavage of a
ONB denotes a photocleavable o-nitrobenzyl unit. Also
diblock copolymer at its block junction in a thin film and the
reported is the preparation of micelles from P1 in THF/
subsequent removal of the minority block by solvent extraction
water mixtures at a water volume fraction f H O of 80%, in which
2
yielded membranes with controlled and uniformly sized
permeating nanochannels.14−22 Furthermore, photoinduced
only the PEO block was soluble. Photolysis caused the PCEMA
domains to cross-link and the PEO coronas to cleave, resulting
in precipitation of cross-linked PFOEMA-b-PCEMA nano-
particles. Coatings made of these nanoparticles were strongly
water- and oil-repellent due to the exposure of the originally
masked or hidden PFOEMA block.
While the copolymer composition disclosed in this paper is
new, atom transfer radical polymerization (ATRP),33−35
reversible addition−fragmentation chain transfer polymer-
ization (RAFT),36,37 and anionic polymerization38,39 have
been used to prepare copolymers containing fluorinated blocks.
Such diblock copolymers have included poly[oligo(ethylene
oxide) monomethyl ether acrylate]-block-poly(1H,1H-perfluor-
dissociation of block copolymer micelles has been tested in
vitro for triggering reagent release from micelle carriers.23−25
Last but not the least, block copolymers bearing azobenzene
units that undergo a photoinduced reversible trans−cis
isomerization have been studied extensively for their potential
applications in optical information storage and other areas.1
Reported in this paper is the synthesis of a triblock
copolymer that bears a photo-cross-linkable block and a
photocleavable block junction. Upon photolysis, the triblock
copolymer undergoes not only cross-linking but also cleavage.
Thus, this is a multiresponsive block copolymer that changes
several of its properties or several of its substructure units when
subjected to one single stimulus. While there have been many
reports on multiply stimulable block copolymers that are
responsive to different stimuli,26−29 reports on multiresponsive
block copolymers have been rare.30 Aside from the system
Received: March 29, 2012
Revised: June 14, 2012
Published: June 22, 2012
© 2012 American Chemical Society
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temperature, then passed through an alumina column using THF as
eluant to remove ligated copper, and subsequently concentrated to 2.0
mL via rotary evaporation. This mixture was then added into 20 mL of
diethyl ether to precipitate out the polymer. The precipitate was
redissolved in 2.0 mL of THF, and the resultant solution was added
into another 20 mL of diethyl ether to precipitate the polymer. This
redissolution and precipitation procedure was repeated once more
before the polymer was dried under vacuum for 24 h to yield 0.61 g of
product in 86% yield. 1H NMR (CDCl3, 500 MHz): δ 4.4 (br,
−COOCH2), δ 3.4−3.8 (br, CH2), δ 2.3 (br, CH2), and δ 0.8−1.4 (br,
CH3) ppm.
obutyl acrylate) or PEOA-b-PFBA,40 poly(butyl methacrylate)-
block-poly(perfluoroalkyl acrylate),41 poly(4-fluorostyrene)-
block-poly(methyl acrylate) and poly(1H,1H-perfluorooctyl
acrylate)-block-poly(methyl methacrylate),42 PEOA-b-PFOEA
[poly(perfluorooctylethyl acrylate)],43 and poly(styrene)-block-
poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate).44 Triblock
copolymers reported have included poly(ethylene oxide)-
block-polystyrene-block-poly(perfluorohexylethyl acrylate)45
and poly[4-methyl-4-(4-vinylbenzyl)morpholin-4-ium chlor-
ide]-block-polystyrene-block-poly(pentafluorophenyl-4-vinyl-
benzyl ether).46 Triblock copolymers have also been prepared
through different monomer addition sequences from EOA,
benzyl acrylate, and FBA40 as well as from EGA, butyl or 2-
ethylhexyl acrylate, and FOEA.43
FOEMA Polymerization Kinetics. The procedure discussed
above for the synthesis of PEO-ONB-PFOEMA was used, except
that samples at ∼0.05 mL each were taken from the reaction mixture
at 1.0, 2.0, and 3.0 h. The crude mixture was exposed to air in a vial
placed in liquid nitrogen to stop the reaction. Subsequently 0.5 mL of
1
CDCl3 was added for H NMR analysis. The decrease in the area of
the FOEMA peak at 4.4 ppm was compared with that of the PEO
main peak at 3.4−3.8 ppm to determine the FOEMA conversion. For
SEC analysis, samples were prepared by initially passing the crude
mixture over a short pad of alumina to remove residual copper and
evaporating the solvent and then dissolving the crude mixture into
DMF at 5 mg/mL.
PEO-ONB-PFOEMA-b-PHEMA. PEO-ONB-b-PFOEMA was
placed into a dialysis tube with a cutoff molecular weight of 12 000
g/mol and dialyzed against distilled THF. The solvent was changed 4
times over 36 h to remove low molecular weight impurities. The
dialyzed PEO-ONB-b-PFOEMA sample was dried (0.68 g, 5.9 × 10−2
mmol), transferred into a two-neck flask, and subsequently redissolved
in a solvent mixture of anisole/TFT (4.0 mL, v/v = 1/1). After 20 min
of stirring at room temperature, HEMA-TMS monomer (0.80 mL, 3.8
× 10−1 mmol), bipyridine ligand (35 mg, 2.2 × 10−1 mmol), and
CuBr2 (2.0 mg, 8.1 × 10−3 mmol) were added. CuBr (15.1 mg, 1.05 ×
10−1 mmol) was added only after the system was purged with N2. The
flask was subjected to four freeze−pump−thaw−N2 refill cycles and
was immersed into a preheated oil bath at 65 °C. After 3 h, the
reaction flask was quenched in liquid N2 and opened to introduce air.
Ligated copper was removed by passing the crude mixture through a
short pad of alumina using THF as eluant. At the end of elution,
methanol and water were added to reach volume ratios of 3/0.5/0.1
for THF, methanol, and water, respectively. The TMS group was
removed after the mixture was stirred overnight at room temperature.
The crude polymer was dissolved into THF (4.0 mL) and precipitated
from 50 mL of diethyl ether. This was repeated another two times.
The product was subsequently centrifuged at 3900 rpm (2600g) for 10
min to yield a compact precipitate, which was dried under vacuum for
16 h, yielding 0.71 g of polymer in 82% yield. 1H NMR (CD3OD, 500
MHz): δ 4.2 (br, −COOCH2), δ 3.8 (br, CH2OH), δ 3.4−3.7 (br, −
CH2CH2O), and δ 0.9−1.4 ppm (br, −CH3).
II. EXPERIMENTAL SECTION
Materials. 2-Trimethylsiloxyethyl methacrylate (HEMA-TMS) was
synthesized following a literature method47 and was distilled over
calcium hydride before use. Prior to use, poly(ethylene oxide)
monomethyl ether (Aldrich, Mn = 5000 g/mol) was vacuum-dried
for 3 days at 55 °C, pyridine (ACS reagent, Fisher Scientific) was
refluxed and distilled over CaH2 under nitrogen, and tetrahydrofuran
(THF) was distilled over sodium and a small amount of
benzophenone. Cinnamoyl chloride (98%, Aldrich), 5-hydroxy-2-
nitrobenzaldehyde (98%, Aldrich), p-toluenesulphonyl chloride
(99.0%, TCI), 2-bromoisobutyryl bromide (98%, Aldrich), triethyl-
amine (>99.5%, Sigma-Aldrich), α,α,α-trifluorotoluene or TFT (99+
%, Acros), anisole (99%, Sigma-Aldrich), CuBr (Aldrich, 99.999%),
CuBr2 (Aldrich, 99.999%), bipyridine (Acros, 99+%), and poly-
(ethylene oxide) monomethyl ether or PEO-550 (Fluka, Mn = 550 g/
mol) were used as received.
Characterization. Size exclusion chromatography (SEC) was
performed at 70 °C on a Waters 515 system equipped with a Waters
2410 refractive index detector. The three columns were packed by
American Polymer Standards Corporation with 5 μm AM 1000, 10
000, and 100 000 Å gels. The system was calibrated using
monodisperse polystyrene (PS) standards. The eluent used was
dimethylformamide (DMF) containing tetrabutylammonium bromide
1
at 2.5 g/L with a flow rate of 0.9 mL/min. H NMR measurements
PEO-ONB-PFOEMA-b-PCEMA. PEO-ONB-PFOEMA-b-PHEMA
(0.20 g, 1.1 × 10−2 mmol containing 2.8 × 10−1 mmol of hydroxyl
groups) was dissolved into 4.0 mL of dry pyridine and stirred for 30
min before cinnamoyl chloride (302 mg, 1.8 mmol, 6.5 mol equiv) was
added. After stirring the mixture in the dark overnight at room
temperature, the reacted mixture was centrifuged at 3900 rpm (2600g)
for 10 min to settle the pyridinium salt. The supernatant was
concentrated to ∼2.5 mL via rotary evaporation and added into 50 mL
of diethyl ether to precipitate the polymer. The precipitate was briefly
dried before it was redissolved into 3.0 mL of THF. The resultant
solution was added into another 50 mL of diethyl ether to precipitate
the polymer. This procedure was repeated once again. The resultant
solid was dried at room temperature in a vacuum oven overnight to
yield 0.20 g of polymer in 82% yield. 1H NMR (CDCl3, 500 MHz): δ
7.8−7.2 (aromatic protons), δ 4.25 (br, COOCH2CH2CF2), δ 4.2 (br,
COOCH2), δ 4.05 (br, −CH2−), δ 3.5−3.6 (br, −CH2CH2O), δ 2.4
(br, CH2CF2), δ 1.8−2.2 (br, CH2), and δ 0.8−1.4 ppm (br, CH3).
PEO-Br. The PEO-Br macroinitiator was prepared by reacting
poly(ethylene oxide) monomethyl ether with 2-bromoisobutyryl
were performed on Bruker Avance-300, Avance-400, or Avance-500
instruments using deuterated pyridine-d5, methanol-d4, or chloroform-
d3 as solvents and a 3 s relaxation delay.
Macroinitiator. The ATRP macroinitiator PEO-ONB-Br bearing
an ONB unit between PEO and the initiating site was synthesized in
three steps following a literature method.18 The overall yield for the
1
macroinitiator was 40%, and this product was characterized by H
NMR in CDCl3: δ 8.21 (d, J = 9 Hz, 1H), δ 7.22 (s,1H), δ 6.98 (dd,
1H, J = 9.0 and 2.2 Hz), δ 5.64 (s, 2H), δ 4.24 (t, J = 5 Hz, 2H), δ
3.8−3.4 (br, −OCH2CH2), δ 3.36 (s, 3H), and δ 2.01 (s, 6H) ppm.
PEO-ONB-PFOEMA. PEO-ONB-Br (0.30 g, 5.6 × 10−2 mmol) and
FOEMA (0.31 mL, 9.6 × 10−1 mmol) were mixed in a two-neck flask.
To this mixture were added anisole (0.7 mL), TFT (0.7 mL),
bipyridine (17.6 mg, 1.13 × 10−1 mmol), and CuBr2 (1.2 mg, 5.3 ×
10−3 mmol). The flask was purged with N2 before CuBr (8.14 mg, 5.6
× 10−2 mmol) was added under N2 flow. The flask was degassed by
three freeze−pump−thaw−N2 refill cycles before it was immersed in a
preheated oil bath at 85 °C. The polymerization was quenched after 1
h by immersing the reaction flask into a liquid nitrogen bath and the
introduction of air. The crude mixture was warmed to room
1
bromide using the protocol used to prepare PEG-ONB-Br. H NMR
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initiating site was first synthesized.18,19 This was followed by
the sequential ATRP of FOEMA and 2-trimethylsilyloxyethyl
methacrylate (HEMA-TMS) to yield the triblock copolymer
PEO-ONB-PFOEMA-b-P(HEMA-TMS). The targeted block
copolymer was obtained after the cleavage of the trimethylsilyl
group and the cinnamation of the resultant PHEMA block with
cinnamoyl chloride.2
(300 MHz, CDCl3): δ 4.24 (CH2OCO, 2H), δ 3.35−3.80 (br,
−CH2CH2−), δ 3.38 (3H, OCH3), δ 2.03 ppm (CH3, 6H).
PEO-b-PHEMA. The polymerization procedure was similar to that
used to prepare PEO-ONB-PFOEMA-b-PHEMA from PEO-ONB-
PFOEMA-Br, except the macroinitiator used was PEO-Br. The crude
product was filtered through an aluminum pad to remove the catalyst,
was dissolved into THF (2.0 mL), and then added into 40 mL of
diethyl ether to precipitate the polymer. This procedure was repeated
once. The polymer was centrifuged at 3900 rpm (2600g) and dried
under vacuum for 24 h, producing 0.31 g of pure sample in 60% yield.
1H NMR (CD3OD, 500 MHz): δ 4.2 (br, −COOCH2), δ 3.8−3.4 (br,
−OCH2CH2), and 1.0−1.4 ppm (br, −CH3).
PEO-ONB-Br. PEO-ONB-Br was synthesized following a
literature method using reactions depicted in Scheme 1.18 The
Scheme 1. Reactions Used To Synthesize PEO-ONB-Br
PEO-b-PCEMA. The cinnmation reaction of PEO-b-PHEMA into
PEO-b-PCEMA was carried out following the procedure described
above for the cinnamation of PEO-ONB-PFOEMA-b-PHEMA. PEO-
b-PCEMA was obtained in 88% yield. 1H NMR (CDCl3, 500 MHz): δ
7.8−7.2 (aromatic protons), δ 4.25 (br, COOCH2), δ 4.15 (br,
−CH2CO−), δ 3.5−3.6 (br, −CH2CH2O), δ 1.8−2.2 (br, CH2), and δ
0.8−1.4 ppm (br, CH3).
PEO-ONB-PFOEMA-b-PCEMA Micelles. PEO-ONB-PFOEMA-b-
PCEMA, or P1 (2 mg), was dissolved into 2.0 mL of THF and stirred
for 4 h at room temperature. Water (8.0 mL) was added at 6−7 drops/
min to the solution until f H O reached 80%. The final solution had a
2
concentration of ∼0.1 mg/mL and was stirred at 500 rpm at room
temperature until analysis.
TEM Measurements. Transmission electron microscopy (TEM)
specimens were prepared by aero-spraying samples via a homemade
atomizer onto cellulose-coated copper grids.48 The specimens were
further dried under vacuum for 4 h before staining by OsO4 vapor for
1.5 h. The specimens were analyzed using a Hitachi H-7000
instrument operated at 75 kV.
commercially available 5-hydroxy-2-nitrobenzaldehyde was first
reduced to yield 3-hydroxymethyl-4-nitrophenol. The latter was
then reacted with α-methoxy-ω-toluenesulfonyl-PEO to yield
PEO bearing a terminal hydroxyl group or PEO-ONB-OH.
This hydroxyl group was further reacted with 2-bromoisobu-
tyryl bromide to yield the targeted macroinitiator.
AFM Measurements. Specimens were prepared by aero-spraying
samples onto freshly cleft mica surfaces. Tapping mode atomic force
microscopy (AFM) was performed using a Veeco Multimode
microscope equipped with a Nanoscope IIIa controller. The silicon
cantilevers used had a force constant and an oscillating frequency of
∼40 N/m and ∼300 kHz, respectively.
1
Figure 1 shows a H NMR spectrum of PEO-ONB-Br in
Micelle Photolysis. A P1 solution (3.0 mL at 0.06 mg/mL in
1
CDCl3 and its peak assignments. A H NMR spectrum for
THF/water at f H O = 80%) was irradiated in a 1.00 cm thick Hellma
2
PEO-ONB-OH and the integrated intensities for the peaks are
given in Figure S1 of the Supporting Information. The intensity
ratio between the peaks at 3.5−3.8 ppm for the main chain
methylene protons and the benzyl methyene was determined to
be 228. Based on the molar mass of 5.0 × 103 g/mol for PEO,
this ratio should be 226. The agreement between these two
numbers suggested that PEO was quantitatively labeled by 3-
hydroxymethyl-4-nitrophenol. This was important because the
incomplete end-capping of PEO by 3-hydroxymethyl-4-nitro-
phenol and the subsequent capping of PEO-OH by 2-
bromoisobutyryl bromide would yield eventually PEO-b-
PFOEMA-b-PCEMA. Unlike PEO-ONB-PFOEMA-b-PCEMA,
PEO-b-PFOEMA-b-PCEMA would not photocleave.
SEC analysis of PEO-ONB-Br was performed using DMF
containing tetrabutylammonium bromide as the mobile phase,
an eluant that we used routinely. Figure 2 shows the SEC trace
for PEO-ONB-Br. It consisted of a main peak and a small
impurity peak at 26 min. The polydispersity of the main peak
was 1.04 in terms of polystyrene standards. Both of these peaks
were essentially identical to those observed for the precursory
PEO sample. Thus, the unknown impurity existed in the
original PEO sample and was not introduced by us.
quartz cell under stirring. The light used was a focused beam that had
passed through a 300 nm cutoff filter from a 500 W mercury lamp in
an Oriel 6140 lamp housing powered by an Oriel 6128 power supply.
To monitor the reaction progress, UV absorption spectra were
recorded at different times.
For the study of reaction progress by SEC, P1 (16 mg) was
dissolved into 0.40 mL of THF before water was added at a rate of 6−
7 drops/min until the total volume reached 2.0 mL. This was followed
by the addition of PEO-550 (11.8 mg) as an internal standard. At
predesignated times, 0.15 mL of the irradiated sample was collected,
dried in a vacuum oven, and then dissolved in 0.02 mL of DMF for
SEC analysis.
Contact Angle Measurements. The P1 particles that were
photolyzed for 3 h should have a PFOEMA corona. The photolyzed
particles were settled from the solvation medium by centrifugation for
10 min at 2500 rpm (1250g). The settled particles (∼1.0 mg) were
then stirred with 10 mL of methanol for 1 h before the particles were
settled by centrifugation for 10 min at 1250g. The methanol rinsing
step was repeated thrice, and the final particles (∼0.75 mg) were
redispersed in TFT (2.0 mL). Two drops of this dispersion were then
dispensed onto a glass plate to cover an area of ∼5−6 mm2. After TFT
evaporation, another 2 drops were applied to the same area. This
procedure was repeated once more. The particulate film was allowed
to dry for 14 h before contact angle measurements using 5 μL of
testing liquids.
PEO-ONB-PFOEMA-Br. PEO-ONB-PFOEMA-Br was syn-
thesized by ATRP of FOEMA using PEO-ONB-Br as the
macroinitiator. FOEMA was polymerized at different temper-
atures and using different solvents such as toluene, hexa-
fluorobenzene, TFT, and mixtures of TFT and toluene as well
as different ligands such as bipyridine and N,N,N′,N″,N″-
III. RESULTS AND DISCUSSION
To prepare PEO-ONB-PFOEMA-b-PCEMA, a PEO-ONB-Br
macroinitiator bearing an ONB unit between PEO and an
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1
Figure 1. H NMR spectra of PEO-ONB-Br (top) and PEO-ONB-PFOEMA (bottom) determined in CDCl3.
The prepared PEO-ONB-PFOEMA-Br was filtered through
an alumina column to remove ligated copper and was purified
by repeated precipitation from diethyl ether. Figure 1 also
1
shows a H NMR spectrum for PEO-ONB-PFOEMA. From
the comparison between the intensities of the PEO and
PFOEMA peaks and based on the repeat unit number of 113
for PEO, the PFOEMA repeat unit number was calculated to be
12.
Figure 2 also shows an SEC trace for PEO-ONB-PFOEMA-
Br or PEO-ONB-PFOEMA. The polydispersity index Mw/Mn
in terms of PS standards was low at 1.06. While the apparent
Mn for PEO-ONB-Br in terms of PS standards was 13 300 g/
mol, the value for PEO-ONB-PFOEMA was ∼15 000 g/mol.
This small increase was most likely due to the poor solubility
and the compact conformation of the PFOEMA block in DMF.
PEO-ONB-PFOEMA-b-PHEMA. HEMA was not directly
polymerized using PEO-ONB-PFOEMA-Br because the
PFOEMA block was insoluble in polar solvents, which
solubilized PHEMA. Thus, a detour was taken by first
polymerizing HEMA-TMS using PEO-ONB-PFOEMA-Br in
TFT/anisole at v/v = 1/1, in which both the short PFOEMA
and P(HEMA-TMS) blocks were soluble. PHEMA was
obtained via the hydrolysis of P(HEMA-TMS) under mild
conditions in the presence of water and methanol in THF.
At the beginning of the HEMA-TMS polymerization, the
reaction mixture was foaming, probably due to bubble
stabilization by PEO-ONB-PFOEMA-Br. Foaming gradually
disappeared with time. The reaction progress was again
Figure 2. Comparison of SEC traces of PEO-ONB-Br, PEO-ONB-
PFOEMA, and PEO-ONB-PFOEMA-b-PCEMA.
pentamethyldiethylenetriamine. These experiments eventually
established that the use of TFT/anisole at v/v = 1/1 as the
solvent and bipyridine as the ligand yielded samples with the
lowest polydispersity.
FOEMA polymerization in TFT/anisole at v/v = 1/1 was
1
followed by analyzing samples taken at different times by H
NMR and SEC. Our NMR analyses indicated that the
monomer conversions at the polymerization times of 1, 2,
and 3 h were 67%, 90%, and 100%, respectively. Figure S2 in
the Supporting Information compares the SEC traces of
samples taken at these times. At 1 h, the diblock copolymer
exhibited a symmetric peak and the small impurity peak at 26
min originally seen in the PEO sample. At 2 h, the peak at 26
min increased in intensity. At 3 h, an extra peak appeared at
25.2 min. The high molecular weight peaks might be due to the
coupling between different chains at the latter stages of
polymerization. Thus, PEO-ONB-PFOEMA was prepared
using a FOEMA polymerization time of 1 h.
1
monitored by H NMR and SEC analysis of samples taken at
different times. The optimized polymerization time was 3 h at
65 °C when bipyridine was used as the ligand.
PEO-ONB-PFOEMA-b-PHEMA was freed of the ligated
copper again by filtration through an alumina column and was
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1
Figure 3. H NMR spectra of PEO-ONB-PFOEMA-b-PHEMA measured in CD3OD (top) and PEO-ONB-PFOEMA-b-PCEMA measured in
CDCl3 (bottom).
Table 1. Characteristics of P1 at Different Stages of Preparation
sample
PEO-ONB-Br
SEC Mw (g/mol)
SEC Mw/Mn
NMR l/m/n
NMR Mn
l
m
n
a
14 000
15 000
38 000
1.04
1.06
1.10
5 000
113
113
113
PEO-OBN-PFOEMA-Br
113/12
11 500
18 200
12
12
P1
113/12/25
25
a
Calculated based on the supplier’s nominal molecular weight of 5000 g/mol for PEO.
1
purified by repeated precipitation. A H NMR spectrum was
obtained in CD3OD, which dissolved only the PEO and
PHEMA blocks and not the PFOEMA block. Figure 3 shows
The SEC trace of PEO-ONB-PFOEMA-b-PCEMA or P1 is
included Figure 2. Despite its apparent width, the polydispersity
of the peak including the high-molecular-weight shoulder was
only 1.10. Having difficulty believing in this number, we
reanalyzed this sample on another SEC system using
chloroform as the eluant, and the SEC trace is shown in
Figure S3. The peak was narrow, and the polydispersity based
on PS standards was again low at 1.10. Thus, the apparent
width of this peak might be due to the good resolution of the
1000 and 10 000 Å columns used in the molecular weight range
of P1.
P1 Micelles. P1 micelles were prepared in several steps.
First, P1 was dissolved into THF. While THF is a good solvent
for PCEMA, it does not dissolve high-molecular-weight
PFOEMA or PEO. Despite this, it dissolved the PFOEMA
and PEO blocks in P1 due to their low molecular weights.
Second, water, a selective solvent for PEO, was added to a
1
the H NMR spectrum together with peak assignments. Peaks
at 4.2 and 3.8 ppm corresponded to ethylene protons of the
hydroxyethyl group, confirming the production of the PHEMA
block. Our quantitative analysis indicated that the EO/HEMA
molar ratio was 113/25 or the repeat unit number was 25 for
the PHEMA block.
PEO-ONB-PFOEMA-b-PCEMA. Reacting the hydroxyl
groups of PEO-ONB-PFOEMA-b-PHEMA with an excess of
cinnamoyl chloride in pyridine at room temperature yielded
PEO-ONB-PFOEMA-b-PCEMA.2,49,50 The resultant polymer
was easily purified by repeated precipitation into diethyl ether
1
and was analyzed by H NMR in CDCl3. Figure 3 also shows
an NMR spectrum for PEO-ONB-PFOEMA-b-PCEMA
together with peak assignments. All of the anticipated peaks
for PEO, PFOEMA, and PCEMA were observed. A quantitative
peak integral analysis confirmed l/m/n = 113/12/25.
volume fraction f H O of 80% to yield micelles with PEG as the
2
corona and PFOEMA and PCEMA as the core.
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Figure 4. AFM topography images of P1 micelles (a), photolyzed P1 micelles (c), and TEM image of the P1 micelles (b). The micelles were aero-
sprayed from THF/water at f H O = 80%.
2
The aqueous micellar solution was atomized or aero-sprayed
using a home-built device48 onto freshly cleft mica for tapping-
mode AFM analysis and on a cellulose-covered grid for TEM
analysis. Before TEM analysis, the PCEMA block of the sample
was stained by OsO4. Aero-spraying was used because it sped
up solvent evaporation from the atomized liquid droplets.
Under these conditions, THF should have evaporated as it
traveled from the spraying nozzle to the silicon wafer and water
should have evaporated within ∼3 s after the landing of the
atomized aqueous droplets. This fast solvent evaporation was to
minimize the chances for a morphological transition of the
micelles during specimen preparation.
Absorption Characteristics of ONB and PCEMA. To
facilitate the choice of photolysis conditions that would cross-
link PCEMA and cleave PEO simultaneously, the absorption
properties of PEO-ONB-Br, P1, and PEG-b-PCEMA were
compared. Here PEO-b-PCEMA was prepared using PEG-Br as
the macroinitiator for HEMA-TMS polymerization. The TMS
groups were then removed, and the resultant PHEMA block
was cinnamated. The number of repeat units for the PCEMA
block of PEO-b-PCEMA was also 25.
Compared in Figure 5 are the UV absorption spectra of
PEO-ONB-Br, PEO-b-PCEMA, and P1 in distilled THF. Since
Figure 4a,b shows AFM height and TEM images of the aero-
sprayed P1 micelles. The particles seemed to have a bimodal
distribution. The smaller particles had average AFM and TEM
diameters of 31 5 and 18 4 nm, respectively, while the
larger particles had AFM and TEM diameters of 47 7 and 33
6 nm, respectively.
Our suspicion is that the smaller particles were core−shell−
corona spherical micelles, where PCEMA, PFOEMA, and PEG
formed the core, shell, and corona, respectively. The AFM
diameter was larger than the TEM diameter because AFM
probed the whole particles including PEO and PFOEMA layers,
and TEM only probed the OsO4-stained PCEMA core. Also,
the AFM diameter should have some contribution from the
finite size of the tip used.
Figure 5. UV absorption spectra of PEO-ONB-Br (a), PEG-b-PCEMA
(b), and P1 (c).
We initially suspected that the larger particles were vesicles
because the PCEMA core chains of the simple core−shell−
corona micelles at an average repeat unit number of 25 could
PEO should absorb negligibly at wavelengths >260 nm, the
maxima at 306 and 274 nm should result from ONB and
CEMA absorption, respectively. A quantitative analysis
indicated that the ONB group in PEO-ONB-Br had a molar
extinction coefficient ε of 8.2 × 103 M−1 cm−1 at 306 nm. The
molar extinction coefficient of CEMA units at their absorption
maximum has been reported to be 2.8 × 104 M−1 cm−1 by Guo
et al.2 and 2.1 × 104 M−1 cm−1 by Marusich et al.52 Using 2.5 ×
104 M−1 cm−1, the average for the two numbers, and the
absorbance ratio of 0.237 determined from Figure 5 for PEO-b-
PCEMA at 306 and 274 nm, we calculated a ε value of 5.9 ×
103 M−1 cm−1 for CEMA absoprtion at 306 nm. Since there
were 25 CEMA units and only 1 ONB unit per P1 chain, the
ONB group should contribute insignificantly to P1 light
absorption at wavelengths shorter than 306 nm. A comparison
of curves b and c of Figure 5 also concluded ONB would
absorb light dominantly at wavelengths longer than 320 nm.
Micelle Photolysis. Based on the absorption characteristics
not be stretched to 33
6 nm. This was, however, not
supported by the TEM or the AFM results. If they were
vesicles, the larger particles would have appeared in the TEM
image as a circle with a dark rim and a gray center. Instead, the
larger circles in Figure 4b appeared increasingly dark toward the
center, suggesting that they were solid particles. Also, some
vesicles would normally collapse after solvent evaporation
leaving behind a Kippah-like structure.51 This Kippah-like
structure was not seen by AFM in Figure 4a.
While the nature of this study does not demand a detailed
clarification of chain packing in the larger particles, we suspect
that the larger particles were still core−shell−corona particles
formed from polymer chains with a PCEMA block that was
substantially longer than 25 units. Despite its apparent low
polydispersity index, P1 probably possessed substantial
composition heterogeneity or a fairly large distribution in
FOEMA to CEMA repeat unit ratio m/n.
of the PCEMA and ONB, P1 micelles in THF/water at f
=
H2O
80% were photolyzed using light generated by a high-pressure
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Scheme 2. The ONB Rearrangement and PEO Cleavage Reaction
Figure 6. Comparison of UV absorption spectra of P1 at different photolysis times (a). Also shown are the variations in the relative absorbances at
274 and 306 nm as a function of irradiation time (b).
Figure 7. Comparison of SEC traces of P1 containing PEO-550 at photolysis times of 0, 10, 20, and 180 min (a). Also shown is the increase in the
PEO-5k peak relative to that of the PEO-500 peak as a function of photolysis time (b).
mercury lamp and filtered by a 300 nm cutoff filter. Most of the
light below 300 nm was removed to hopefully match the rates
of ONB cleavage and PCEMA cross-linking. The micellar
solution was clear before irradiation. Turbidity developed
within 5 min and intensified with further irradiation. After 3 h,
the particles settled from the solvation medium after stirring
was stopped, suggesting the cleavage of the PEO block and
photoinduced particle precipitation. Also, the settled particles
were redispersed into CDCl3 to yield a cloudy solution. Since
PFOEMA-b-PCEMA bearing un-cross-linked PCEMA would
have dissolved in this solvent to yield a clear solution, the
cloudiness suggested the retention of the micellar structure in
CDCl3 and the cross-linking of the PCEMA core.
The PEO chains were cleaved from the PFOEMA block
because of a Norrish II rearrangement (Scheme 2) of ONB.53
PCEMA was cross-linked due to the dimerization of CEMA
units of different P1 chains.2 The photolysis processes were
monitored by analyzing samples collected at different
irradiation times using UV and SEC.
as a function of the photolysis time. The two curves almost
coincided, with both curves showing rapid initial absorbance
decrease that was followed by little change between 2 and 3 h.
The observed absorbance decrease pattern was reasonable as
the rate of a reaction depended on the amount of light
absorbed by the reacting species. As the conversion increased
and less reactant remained, the amount of light absorbed by the
reactant per unit time decreased, and the reaction rate
decreased.
The almost identical rate of absorbance decrease at 274 and
306 nm should not be surprising because the PCEMA
absorption dominated over ONB absorption at 306 nm as
well. Thus, UV absorption analysis at these two wavelengths
only allowed the monitoring of the rate of CEMA
disappearance with photolysis.
Meanwhile, SEC was better suited for monitoring PEG
cleavage. As more sample was required for SEC analysis, the
photolyzed polymer solution in this case had a higher initial
concentration of 8 mg/mL. To quantify the amount of P1 lost
or PEG formed, a PEO sample with Mn = 550 g/mol or PEO-
550 was added into the photolysis mixture as an internal
standard. This oligomer was used as the internal standard
because it would remain inert during photolysis, and its peak
Figure 6a compares the UV absorption spectra of a P1
solution at 0.06 mg/mL in THF/water at f
= 80% after it
H2O
was irradiated for different time periods. Plotted in Figure 6b
are the changes in the relative absorbances at 274 and 306 nm
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Figure 8. Photographs of H2O droplets impregnated with rhodamine B (a, b) and CH2I2 droplets (c, d) on films of P1 micelles (a, c) and
photolyzed P1 micelles (b, d). Also shown are the variations in the contact angles of H2O and CH2I2 droplets as functions of PEG cleavage (e).
appeared at 32 min, a position well-resolved from that of P1
and the cleft PEO block.
however, have been increased by using a filter with a shorter
cutoff wavelength such as at 270 nm.
Solvent was removed from the photolyzed samples via rotary
evaporation. They were then immediately redispersed into
DMF, filtered, and analyzed by SEC. Figure 7 compares SEC
traces of P1 irradiated for 0, 10, 20, and 180 min, respectively.
Before irradiation, the peaks eluting at 25.5 and 32 min
corresponded to P1 and PEO-550, respectively. After 10 min of
irradiation, a shoulder emerged at ∼24 min on the higher
molecular weight side and the intensity of the P1 peak
decreased. This suggested the production of higher molecular
weight species due to the photo-cross-linking of PCEMA.
Although less noticeable, the shoulder on the lower-molecular-
weight side at 27.8 min had also grown, suggesting PEO-5k
formation. At 20 min, the P1 peak had fully disappeared,
leaving behind a very small peak at ∼26 min, while the PEO-5k
peak eluting at 27.8 nm became distinct. Further photolysis up
to 180 min did not eliminate the small peak at 26 min but
increased the intensity of the peak eluting at 27.8 min.
No peak was seen for the PFOEMA-b-PCEMA fragments
after 20 min photolysis because this part became locked into a
cross-linked micellar structure. Cross-linked particles with
PFOEMA corona were not dispersed into DMF, the eluant
used for SEC analysis, and were mostly removed during sample
filtration. As mentioned before, the small peak at ∼26 min must
have been due to an impurity present in the original PEO
precursor.
PEO-Cleft Particles. After a P1 solution at 8 mg/mL in
THF/water at f H O = 80% was irradiated for 3 h, the solution
2
became turbid. This solution was diluted to ∼1 mg/mL with
THF/water at f H O = 80% and then pushed through a 3.1 μm
2
filter to remove large aggregates. The smaller particles were
then aero-sprayed for AFM analysis. Such an image is shown in
Figure 4c.
The bimodal distribution of the particles was retained and is
evident in Figure 4c. The major difference between the particles
in Figures 4a and 4c was that the particles in Figure 4c had
formed aggregates and those in Figure 4a were mostly
individual. The particles had aggregated because they were
free of the PEO corona and were not readily dispersible in
THF/water.
The particles after photolysis were centrifuged, separated
from the supernatant, redispersed into methanol under
vigorous stirring, and settled via centrifugation. This methanol
rinsing step was repeated several times to remove the
photocleft PEO chains. The particles were dried and then
1
dispersed into CDCl3 to yield a turbid solution. H NMR
analysis indicated the absence of any PEO peaks despite the
solubility of PEO in CDCl3 (Figure S4). This thus suggested
the complete removal of the PEO coronal chains during
photolysis. No PCEMA signals were observed either because
the PCEMA chains were cross-linked and not mobile. The
presence of the PFOEMA block in the photolyzed sample was,
however, confirmed by 19F NMR analysis (Figure S5).
Visual evidence for PEO cleavage was that the H2O and
CH2I2 contact angle change on films made of P1 micelles
before and after their photolysis. Micellar P1 films were
prepared by casting onto glass plates P1 micellar solutions at
The Peakfit program was used to resolve the peak of the P1
sample that was irradiated for 10 min into three peaks
consisting of cross-linked P1, P1, and photocleaved PEO-5k.
The areas of the peak for PEO-5k and PEO-550 were calculated
using the Peakfit program to yield the area ratio A5k/A550
,
where A5k and A550 represent the areas corresponding to PEO-
5k and PEO-550, respectively. Since the PEO-5k peak was free
of interference from P1 at other irradiation times, the
determination of A5k/A550 was straightforward. Figure 7b
shows how A5k/A550 varied with photolysis time.
Figure 7b clearly shows much less PEO was cleft in the first
10 min than in the next 10 min. This was probably due to the
fact that PCEMA cross-linking dominated initially. It was only
after the CEMA concentration and its absorption had decreased
that the ONB rearrangement took off. The A5k/A550 value
leveled off after 120 min, suggesting the completion of the ONB
rearrangement reaction at that point.
f H O = 80% and photolyzed P1 micellar solutions dispersed in
2
TFT, respectively. Figure 8a−d compares the shapes of H2O
and CH2I2 droplets on different films. While H2O and CH2I2
contact angles on the micellar films were 52° and 32°,
respectively, the values increased to 154° and 136° on films of
the photolyzed samples. These large contact angles were
possible only if the particle surfaces were enriched by PFOEMA
and the particulate films were rough.39,54,55
Two AFM images of films cast from a TFT dispsersion of a
photolyzed nanoparticle sample are shown in Figure S6. The
root-mean-square roughness of the films at the probed scale of
3.1 and 10 000 μm2 from the two images were 8.5 and 154 nm,
respectively. Thus, the films were indeed rough.
A comparison between Figures 6b and 7b further suggested
that the cleavage reaction seemed to go faster than the cross-
linking reaction. The PCEMA cross-linking rate could,
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preparing better micellar films because this structural rearrange-
ment should not have changed the observed H2O and CH2I2
contact angle variation trends for films made of micelles and
photolyzed micelles.
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ATRP has been used to produce a triblock copolymer PEO-
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1
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2
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cleft the PEG corona, yielding particles bearing expoesd
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TFT yielded films that were both superhydrophobic and
oleophobic. The water and oil repellence of films of the cross-
linked micelles improved as the degree of PEO cleavage
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ASSOCIATED CONTENT
■
S
* Supporting Information
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AUTHOR INFORMATION
■
(32) Zhou, Z. H.; Liu, G. J.; Hong, L. Z. Biomacromolecules 2011, 12,
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Notes
The authors declare no competing financial interest.
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■
NSERC of Canada is thanked for financially sponsoring this
research and Ian Wyman is thanked for proofreading the
manuscript. G.L. acknowledges the Canada Research Chairs
program for a Tier I Canada Research Chair Position in
Materials Science.
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