Styrene-vinyl pyridine diblock copolymers: Synthesis by RAFT polymerization and self-assembly in solution and in the bulk

Article abstract of DOI:10.1002/pola.25935

Reversible addition-fragmentation chain transfer (RAFT) polymerization along with benzyl dithiobenzoate chain transfer agent was employed for the controlled preparation of four diblock copolymers of styrene (St, less polar monomer) and 2- or 4-vinyl pyrid

Full text of DOI:10.1002/pola.25935

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Styrene–Vinyl Pyridine Diblock Copolymers: Synthesis by RAFT
Polymerization and Self-Assembly in Solution and in the Bulk
1
1
2
3
Mirela Zamfir, * Costas S. Patrickios, Franck Montagne, Clarissa Abetz,
3
4
4
Volker Abetz, Liat Oss-Ronen, Yeshayahu Talmon
1
Department of Chemistry, University of Cyprus, PO Box 20537, 1678 Nicosia, Cyprus
2
Division of Nanotechnology and Life Sciences, Swiss Center for Electronics and Microtechnology SA (CSEM SA),
CH 2002 Neuch aˆ tel, Switzerland
3
Institute of Polymer Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502 Geesthacht, Germany
4
Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
Correspondence to: C. S. Patrickios (E-mail: costasp@ucy.ac.cy)
Received 12 September 2011; accepted 6 January 2012; published online 10 February 2012
DOI: 10.1002/pola.25935
ABSTRACT: Reversible addition-fragmentation chain transfer
RAFT) polymerization along with benzyl dithiobenzoate chain
copolymers formed larger micelles than the 2VP-bearing ones,
(
a result of the greater 4VP-toluene incompatibility as compared
to the 2VP-toluene one. Finally, films cast from chloroform solu-
tions of the diblocks were investigated in terms of their bulk
morphologies using transmission electron microscopy. While
transfer agent was employed for the controlled preparation of
four diblock copolymers of styrene (St, less polar monomer)
and 2- or 4-vinyl pyridine (2VP or 4VP, more polar monomers):
St161-b-2VP48, St161-b-2VP121, St161-b-4VP76, and St161-b-4VP107
,
the 2VP-containing block copolymer self-assembled into a
where the subscripts indicate the experimentally determined
degrees of polymerization for each block. These diblock copoly-
mers and their common St homopolymer precursor were char-
spherical morphology, the 4VP-containing one with comparable
composition and molecular weight formed a cylindrical struc-
ture, manifesting the greater 4VP-St incompatibility as com-
pared to that of the 2VP-St pair. VC 2012 Wiley Periodicals, Inc. J
acterized in terms of their molecular weights and compositions
1
using gel permeation chromatography and
H
NMR
Polym Sci Part A: Polym Chem 50: 1636–1644, 2012
spectroscopy, respectively. All four diblock copolymers self-
assembled in dilute toluene solutions to form reverse spherical
micelles, which were characterized using atomic force micros-
copy and cryogenic transmission electron microscopy. Both mi-
croscopy techniques revealed that the 4VP-bearing diblock
KEYWORDS: bulk morphologies; diblock copolymers; living radi-
cal polymerization; micelles; reversible addition-fragmentation
chain transfer polymerization; self-assembly; self-organization;
styrene; 2-vinyl pyridine; 4-vinyl pyridine
INTRODUCTION Block copolymers (BCs) are important self-
assembling systems that can assume a wide diversity of
nanometer-scale morphologies, including lamellar, hexago-
nally packed cylindrical and body-centered cubic structures,
due to the incompatibility and the connectivity constraints
between the chemically distinct segments. This self-organiza-
tion property of the BCs along with their large size has led
to diverse applications in solution, in bulk, and in thin
decades were focused on developing alternate techniques for
BC synthesis.
Controlled radical polymerization techniques, such as nitro-
5
,6
xide-mediated polymerization (NMP), atom transfer radical
7
,8
polymerization (ATRP), and reversible addition-fragmenta-
9
tion chain transfer (RAFT) polymerization, are now avail-
able and provide themselves as facile and convenient means
for the synthesis of block, graft, star, hyperbranched, tele-
1
–3
films.
Up to now, the technique most frequently used for
1
0
chelic, and cyclic (co)polymers. These polymers can be
readily reactivated for chain extension with the same or dif-
ferent monomer since they retain their active end groups.
RAFT polymerization possesses distinct advantages over the
two other controlled radical polymerization techniques,
including the absence of particular limitations in the reaction
the synthesis of BCs has been living anionic polymerization.
However, anionic polymerization has to be carried out in
complete exclusion of moisture and air and often at very low
temperature. Furthermore, this polymerization technique is
restricted to a relatively small number of monomers. In
order to overcome these limitations, efforts in the past two
4
*Present address: Precision Macromolecular Chemistry Group, Institut Charles Sadron, UPR22-CNRS, 23, rue du Loess, BP 84047,
67034 Strasbourg Cedex 2, France.
V
C
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conditions, such as temperature and solvent, and, most
importantly, the compatibility with a wide range of monomer
types (methacrylates, acrylates, styrenics, and acrylamides)
and monomer functionality (carboxylic acid, carboxylic acid
1
1
salt, hydroxyl, or tertiary amine groups).
BCs of styrene (St) and 4-vinyl pyridine (4VP) or 2-vinyl pyri-
dine (2VP) represent an important class of polymers, as they
can be used for the compatibilization of blends of the corre-
sponding homopolymers via interfacial BC adsorption, ulti-
mately leading to the enhancement of the mechanical properties
1
2,13
of the blend,
or for the fabrication of functional nanostruc-
14,15
tures due to their potential use as templates and scaffolds.
Another attractive application of this type of BCs is the forma-
tion of organic ultrathin nanoporous PSt-b-P4VP membranes
for separation purposes (water treatment and filtration of bio-
logical entities), by a solution casting process, followed by the
immersion of the cast film into a precipitant for the BC, which
involves exchange of solvent with nonsolvent in the solution
film. This procedure leads to a thin and dense isoporous layer
on top of a sponge-like porous support made of the same mate-
FIGURE 1 Chemical structures and names of the main
reagents used for the diblock copolymer synthesis.
calcium hydride (CaH , 90–95%), 2,2-diphenyl-1-picrylhydra-
2
zyl hydrate (DPPH, 95%), S-(thiobenzoyl)thioglycolic acid
(
99%), benzyl mercaptan (99%), diethyl ether (99.7%) silica
gel (60 Å, 70–230 mesh), n-hexane (96%), N,N-dimethylfor-
mamide (DMF, 99.8%), and N,N-dimethylacetamide (DMAc,
1
6
rial. Moreover, silicon-based nanoporous membranes with a
precise control of the size and size-distribution of the nano-
pores, as well as the final thickness of the membranes, were
produced by a proprietary process combining PSt-b-P2VP BC
0
HPLC, ꢁ99.9%) were purchased from Aldrich, Germany. 2,2 -
Azobis(isobutyronitrile) (AIBN, 95%), ethanol (99.6%), and
deuterated chloroform (CDCl ) were purchased from Merck,
3
1
7
lithography and standard microfabrication techniques.
Germany. Tetrahydrofuran (THF, 99.8%, both HPLC and rea-
gent grade) and methanol (99.9%) were purchased from
Labscan, Ireland. The synthesis of BDTB is described in a fol-
lowing paragraph.
Only a few papers have been published on the investigation of
the synthesis of St-VP BCs using RAFT polymerization. In the
1
8
first such study, Yuan and coworkers described the prepara-
tion of various St-4VP-St or 4VP-St-4VP triblock copolymers
using dibenzyl trithiocarbonate as chain transfer agent (CTA).
These BCs, having molecular weights (MWs) in the range of
Methods
The monomers were passed through basic alumina columns
to remove the inhibitors and any other acidic impurities. Sub-
sequently, the DPPH free radical inhibitor was added to the
monomers to prevent their undesired thermal polymeriza-
ꢀ
1
1
0
8,000–29,900 g mol and 4VP mol fractions from 0.32 to
.55, were characterized in terms of their self-assembly behav-
1
9
ior in water. Zhang et al. reported the preparation of nano-
composites of montmorillonite (MMT) and PSt-b-poly(quater-
nized 4VP) (PSt-b-QP4VP) of number–average molecular weight
tion. Afterward, CaH was also added, over which the mono-
2
mers were stirred overnight to remove the last traces of
moisture. This was followed by monomer vacuum distillation
prior to their polymerization. The AIBN radical initiator was
recrystallized twice from ethanol. The polymerization solvent,
ꢀ
1
n
(M ) equal to 7400 g mol , previously synthesized using RAFT
polymerization, followed by quaternization of the 4VP units
using CH I. Recently, a facile strategy for the preparation of mul-
3
DMF, was dried over CaH and filtered through syringe filters
2
timorphologies and a variety of nanostructures in solution was
attained by the RAFT polymerization of St using a trithiocarbon-
prior to use. Figure 1 displays the chemical structures and
the names of the monomers, the initiator, and the monofunc-
tional CTA used for the diblock copolymer synthesis.
ꢀ
1
ate-terminated P4VP macroRAFT CTA (M ¼ 10,400 g mol ) in
n
methanol (selective solvent for P4VP), by varying the feed molar
2
0
Syntheses
ratios and controlling the polymerization conditions.
Synthesis of Benzyl Dithiobenzoate (BDTB)
BDTB was synthesized according to the published literature
Herein, we report the synthesis by RAFT copolymerization of
four well-defined linear amphiphilic diblock copolymers of St
and 4VP or 2VP, obtained by stepwise monomer addition,
using benzyl dithiobenzoate (BDTB) as CTA. Following stand-
ard MW and composition characterization, the resulting BCs
were thoroughly characterized in terms of their self-assem-
bly behavior in solution and in the bulk.
21
procedure. In particular, 3.22 mL of benzyl mercaptan
(
3.41 g, 27.5 mmol) was added dropwise over a period of 30
min to a well-stirred dilute alkaline solution containing
about two equivalents of NaOH (2 g in 200 mL H O, 50
2
mmol) and 5.3 g (25 mmol) of S-(thiobenzoyl)thioglycolic
acid, keeping the reaction mixture at room temperature.
After 15 h, BDTB separated out as a heavy, dark red oil and
was extracted with ether (1 ꢂ 300 mL, 1 ꢂ 200 mL). The
organic extracts were separately washed with 0.1 N aqueous
NaOH (3 ꢂ 200 mL) and water (3 ꢂ 200 mL), and then
EXPERIMENTAL
Materials
Styrene (St, purity > 99%), 2-vinyl pyridine (2VP, purity
>
97%), 4-vinyl pyridine (4VP, purity >95%), basic alumina,
dried over anhydrous MgSO . After the evaporation of the
4
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solvent, the remaining product was purified by column chro-
tributions (MWD). For the PSt precursor and the 2VP-con-
taining BCs, GPC was performed on a Polymer Laboratories
chromatograph equipped with an ERC-7515A refractive index
detector and a PL Mixed ‘‘D’’ column, calibrated using poly
(methyl methacrylate) (PMMA) MW standards. The mobile
phase was THF delivered at a flow rate of 1 mL min using a
Waters 515 isocratic pump. The MW calibration curve was
based on eight narrow MWD linear PMMA standards (MWs of
matography (silica gel, eluent: n-hexane). The purified CTA
1
was stored in the freezer in the dark. H NMR (300 MHz,
ꢃ
CDCl , 25 C): d ¼ 4.6 ppm (2H, s, ACH APh), 7.29–7.99
3
2
1
3
ꢃ
ppm (10H, m, Ph); C NMR (CDCl , 25 C): d ¼ 227.2 ppm
3
ꢀ
1
(
C ¼¼ S), 144.7 ppm, 134.92–132.4 ppm, 129.27–126.87 ppm
(
Ph), 42.26 ppm (ASCH Ph). Yield: 81%.
2
Synthesis of Linear PSt MacroRAFT Agent
8
3
50, 2810, 4900, 11,550, 30,530, 60,150, 138,500, and
42,900 g mol ) also from Polymer Laboratories. Since the
The synthetic procedure followed for the preparation of the PSt
macroRAFT CTA with a targeted degree of polymerization (DP)
equal to 300 is described in detail below. A mixture of St (25
mL, 22.72 g, 0.218 mol), BDTB (0.177 g, 0.728 mmol), and AIBN
ꢀ1
P4VP blocks are insoluble in THF, their resultant BCs were an-
ꢀ1
alyzed in DMAc with LiCl as an additive (0.05 mol L ) at 50
ꢃ
ꢀ1
C, at a flow rate of 1 mL min employing a set of two PSS
(74.5 mg, 0.454 mmol) were added into a 50-mL Schlenk tube
1
0 l GRAM-Gel columns (3000 and 1000 Å, 8 ꢂ 300 mm
equipped with a magnetic stirring bar and sealed with a rubber
septum. After the mixture was degassed by three freeze-evacu-
ate-thaw cycles, pure nitrogen gas was introduced into the tube
which was subsequently immersed in an oil bath thermostated
each), and calibrated using PSt MW standards. A Knauer RI
differential refractometer was used as a concentration detec-
tor; 20 lL of polymer solutions (0.2 wt % in the mobile
phase) were injected using a Rheodyne 7725i manual injec-
tion valve. Eluograms were flow-rate corrected using the
method of internal standard (250 ppm diethylene glycol).
ꢃ
at 110 C for 5 h. The polymerization was stopped by placing
the tube in an ice bath. The tube was opened and the rather
solid reaction mixture was dissolved in THF. The resulting THF
solution was then slowly poured into an excess of methanol
under stirring in order to precipitate the polymer. The precipi-
tate was collected by filtration and dried in a vacuum oven at
room temperature for 36 h. The PSt macroRAFT CTA was
n w
Apparent molar mass averages, M and M , were based on PSt
calibration and were calculated using the WinGPC software
package (PSS GmbH, Mainz, Germany).
Proton and Carbon Nuclear Magnetic Resonance
ꢀ
1
1
13
obtained at 66% yield and had an Mn(GPC) ¼ 16,800 g mol
( H and C NMR) Spectroscopy
and a polydispersity index (PDI) ¼ M /M ¼ 1.4.
w
n
All BCs and their PSt precursor were also characterized by
1
H NMR spectroscopy in terms of their composition and pu-
The polymerization reaction was followed kinetically. Sam-
ples were withdrawn at different reaction times. The mono-
rity using a 300-MHz Avance Bruker NMR spectrometer
equipped with an Ultrashield magnet and using deuterated
1
mer-to-polymer conversion was determined by H NMR
3
chloroform (CDCl ) as solvent. The BDTB CTA was also char-
spectroscopy from the ratio of the areas of the peaks of the
olefinic protons of the monomer (5.18, 5.7, 6.6 ppm) divided
by that of the aliphatic protons of the polymer (1.35–1.77
ppm), whereas the polymer MWs and PDIs were character-
ized by gel permeation chromatography (GPC).
1
13
acterized by H and C NMR spectroscopy to confirm its
structure and purity.
Transmission Electron Microscopy (TEM)
TEM measurements were carried out using an FEI Tecnai G2
F20 transmission electron microscope operated at 200 kV in
bright-field mode. All the samples were cast from 5 wt %
solutions in chloroform, following a standard procedure
Synthesis of Linear Block Copolymers PSt-b-PVP
Into a 25-mL Schlenk tube, 1.7 g of PSt macroRAFT CTA (0.101
mmol), 2VP or 4VP (1.75 mL, 1.71 g, 16.3 mmol), AIBN (10.4
mg, 0.063 mmol), and DMF (3.5 mL) were added. After the
mixture was degassed by three freeze-evacuate-thaw cycles,
and placed under an inert nitrogen atmosphere, the tube was
3
described below. The BC was dissolved in CHCl and trans-
ferred into a Teflon dish inside a desiccator rich in solvent
vapors. The overall evaporation (casting) procedure lasted
for a period of two weeks. The samples were taken out of
the crucibles and transferred in a vacuum oven in order to
completely remove any traces of solvent and were treated
thermally. The samples were dried for 8 h under vacuum at
ꢃ
immersed in an oil bath thermostated at 80 C. After the pre-
scribed polymerization time (8 h for St161-b-2VP , 19 h for
48
St161-b-2VP121, 20 h for St161-b-4VP , and 45 h for St161-b-
76
4VP107; subscripts denote the experimentally determined
ꢃ
room temperature and then heated up to 40 C using a rate
degrees of polymerization), the tube was cooled down at room
temperature immediately. The reaction mixtures were diluted
with THF for the 2VP-containing BCs or with DMF for the 4VP-
containing BCs, and the resulting BC solutions were slowly
poured into excess of n-hexane to precipitate the produced
BCs. After being separated by filtration and dried in a vacuum
oven at room temperature for 24 h, the pure BCs were
obtained as light pink glassy materials.
ꢃ
ꢀ1
of 10 C h , where they were stored for another 24 h. The
last step involved their storage at 60 C for 18 h. Afterward,
ꢃ
the oven was left to cool down to room temperature, the
vacuum was released and the samples were taken out.
Microtoming was performed at room temperature using a
Leica Ultracut UCT microtome equipped with a Diatome Dia-
ꢃ
mond Knife-Ultra 35 , the final sample thickness being
approximately 50 nm.
Characterization
Gel Permeation Chromatography (GPC)
All BCs and their PSt precursor were characterized using
GPC to determine their MWs and their molecular weight dis-
Cryogenic-Transmission Electron Microscopy (cryo-TEM)
Vitrified specimens of BC solutions in toluene (concentra-
tions 0.2–1.0 w/v%) were prepared in the following manner.
1
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SCHEME 1 Synthetic route followed for the preparation of the St-2VP and St-4VP diblock copolymers via RAFT polymerization.
A drop of the solution was applied to a perforated carbon
film supported on a TEM copper grid in a controlled envi-
ronment vitrification system (CEVS). The CEVS is a closed
chamber, which allows the maintaining of a constant temper-
Styrene Homopolymer
ꢃ
The polymerization of St was performed at 110 C in the
bulk, with BDTB as the RAFT CTA and AIBN as the initiator
([St] /[CTA] /[AIBN] ¼ 300:1:0.625), to yield a polymer
0
0
0
ꢃ
ꢀ1
ature (25 C for our experiments) and 100% saturation of
n
having an M value equal to 16,800 g mol and a PDI of
the desired solvent (toluene in the present case), to prevent
evaporation of the solution from the grid. Then, the drop
was blotted to create a thin film (<300 nm) and immedi-
ately plunged into liquid nitrogen. The specimens were
transferred to a Gatan 626 cooling holder via its transfer sta-
1.4. The successful formation of the linear homopolymer pre-
cursor was also confirmed by H NMR spectroscopy. In par-
1
ticular, resonance signals assigned to the aliphatic protons of
the hydrocarbon backbone of the polymer appeared in the
range of 1.35–1.77 ppm, whereas the intensity of the peaks
due to the St olefinic protons at 5.18, 5.7, 6.6 ppm was
greatly reduced.
ꢃ
tion and equilibrated in the microscope at about –180 C.
The samples were observed in a FEI T12 G2 transmission
electron microscope operated in 120 kV low dose mode to
minimize electron-beam radiation damage. We used Gatan
US1000 high-resolution cooled-CCD camera to record the
images at a nominal underfocus of 1–2 lm using the Digital
Micrograph software package.
The reaction was followed kinetically. The monomer conver-
sion and the polymer Mn and PDI in the samples extracted
at various reaction times were determined. Figure 2 presents
the collected kinetic data in different plots. In particular, Fig-
ure 2 shows (a) the temporal evolution of St conversion (cal-
1
culated from the H NMR spectra), (b) the dependence of Mn
Atomic Force Microscopy (AFM)
on monomer conversion, (c) the temporal evolution of M
n
,
All the samples were characterized using AFM in the tapping
mode with a Veeco Dimension 3100 microscope using silicon
cantilevers (NSC15) from MikroMasch. Topography images
were zero-order flattened using a standard algorithm from
the Nanoscope III software. The preparation of micellar solu-
tions and deposition on silicon substrates were performed as
follows. The P2VP-containing BCs were dissolved in toluene
at room temperature at a concentration of 0.2 wt %. On the
other hand, the P4VP-containing BCs were only partially
soluble in toluene at room temperature and were, therefore,
the MW at the peak maximum (M ) and the PDI, and (d) the
p
pseudo-first-order kinetic plot for monomer conversion. The
linearity of the M variation versus monomer conversion and
n
of the ln([M] /[M]) versus time, are both good indications of
0
the controlled character of the polymerization. Figure 2(b)
shows that the Mn values as determined by GPC were
slightly lower (by ꢄ10%) than those expected on the basis
of monomer conversion. This indicates that no CTA deactiva-
tion took place, which would have led to higher M values as
n
ꢃ
compared to the theoretical ones; it also indicates some ex-
heated up to 80 C for 30 min and then cooled down to
perimental errors in the determination of M by GPC, in the
n
room temperature under stirring. All micellar solutions were
1
determination of monomer conversion by H NMR spectros-
filtered through PTFE syringe filters with 0.45-lm pore
2
copy, and in the determination of the mass of the CTA used
for the polymerization.
diameters. Prior to micelle deposition, 1 cm silicon sub-
strates were cleaned in a piranha solution (H O :H SO , 1:3
2
2
2
4
ꢃ
v/v %) at 120 C for 30 min, rinsed in boiling deionized
water for 20 min and dried with nitrogen (Caution: Piranha
solution is a hazardous oxidizing agent and must be handled
with extreme care). Micellar films were spin-coated at 4000
rpm for 30 s in a chamber of controlled atmosphere (T ¼ 21
Diblock Copolymers
The four diblock copolymers of this investigation, St161-b-
2
VP , St161-b-2VP121, St161-b-4VP , and St161-b-4VP107, were
48 76
synthesized by chain-extending the same PSt macroRAFT CTA
ꢃ
with 2VP or 4VP in DMF, using AIBN as initiator at 80 C for
8
ꢃ
C and 60% of relative humidity).
1
to 45 h. Typical H NMR spectra of (A) the PSt macroRAFT
CTA, (B) the BC St161-b-2VP , and (C) the BC St161-b-4VP107
48
RESULTS AND DISCUSSION
are shown in Figure 3. The VP content in the copolymers was
calculated from the ratio of the peak area of the ‘‘c’’ protons
at position 3 of the pyridine ring (8.3–8.4 ppm), divided by
the area of peaks ‘‘b’’ corresponding to all backbone
Amphiphilic BCs of St and 2VP or 4VP were synthesized
via BDTB-mediated RAFT polymerization, according to
Scheme 1.
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FIGURE 2 Results of the kinetic study of styrene RAFT homopolymerization. (a) Temporal evolution of the conversion of styrene,
(
b) dependence of M
n
on monomer conversion, (c) temporal evolution of the MWs and the PDIs, and (d) pseudo-first-order kinetic
ꢃ
plot of the polymerization. [St]
0
/[AIBN]
0
/[CTA]
0
¼ 300:0.625:1, temperature ¼ 110 C, bulk polymerization.
methylene and methine protons (1.4–1.8 ppm). Using this co-
polymer composition and the GPC-determined M of the PSt
bimodal MWD with the high MW shoulder indicating partial
recombination of the polymer chains.
n
homopolymer precursor, another estimation of the copolymer
MW was made. Table 1 lists the yields, the size, and the com-
position characteristics of the starting homopolymer and its
daughter diblock copolymers.
The greater polarity and reactivity of 4VP over that of 2VP
are the likely reasons for the more extensive side reactions
The controlled nature of the polymerizations in this study is
also demonstrated in Figure 4 which displays the GPC traces
of the St homopolymer and the two St-2VP BCs produced
using the former as a macroRAFT CTA. It may be observed
that the GPC traces of the BCs were shifted to shorter elu-
tion times and the MWDs were broadened relative to the
parent PSt macroRAFT CTA (corresponding to increases in
the PDI values from the original value of 1.4 for the parent
St homopolymer to 1.7 for the BCs, Table 1). This increase in
PDI reflects the tailing toward lower MWs in the MWDs of
the BCs, resulting from deactivated polymer generated dur-
2
2,23
ing the synthesis of the macroRAFT CTA.
produced by an unfavorable high concentration of radicals
responsible for substantial extent of termination
This can be
a
2
4
reactions.
The GPC traces of St161-b-4VP76 and St161-b-4VP107 BCs (not
shown) were recorded using DMAc as solvent, due to the
2
5
insolubility of P4VP in THF. The peaks were shifted to
ꢀ
1
ꢀ1
higher MWs, from 16,800 g mol to 26,300 g mol , and
2
1
ꢀ
1
1
5,900 g mol , while the PDIs of the BCs increased from
.4 to 1.6 and 1.7, respectively. The GPC traces revealed a
FIGURE 3 H NMR spectra of (A) the PSt macroRAFT CTA, (B)
3
St161-b-2VP48, and (C) St161-b-4VP107, all recorded in CDCl .
1
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TABLE 1 Monomer Conversions, Polymer Compositions, Molar Masses and PDIs for the St Homopolymer and St-VP BCs
Synthesized in This Study
M
n
c
1
d
Sample
CTA
VP content (mol %)
Time (h)
Conv (%)
GPC
H NMR
PDI
Yield (%)
a
St161
BDTB
St161
St161
St161
St161
0
18
8
70
28
58
77
83
16,800
21,000
22,300
26,300
25,900
–
1.44
1.71
1.71
1.55
1.68
66
71
76
73
76
b
St161-b-2VP48
23
43
32
40
21,800
29,500
24,700
28,000
b
St161-b-2VP121
19
20
45
b
St161-b-4VP76
b
St161-b-4VP107
a
ꢃ
c
1
Polymerization at 110 C in the bulk and at [CTA]
0
/[St]
0
/[AIBN]
0
¼ 1/
Conversions were determined from the H NMR spectra.
Gravimetric yield.
d
3
b
00/0.625.
ꢃ
Second-stage polymerization at 80 C in DMF and at [CTA]
0
/[VP]
0
/
[
AIBN]
0
¼ 1/160/0.625.
in the block copolymerizations of the former compared to
those of the latter, leading to bimodal MWDs in the diblocks
with 4VP and only a tail in those with 2VP. These side reac-
tions were more pronounced in the present investigation,
where relatively high MWs were targeted in order to afford
readily microphase-separating block copolymers. The deacti-
vated polymer in the GPC traces in Figure 4 is PSt with just
a small number of 2VP units.
The P4VP-containing BCs exhibited greater micellar diame-
ters and heights, 40 and 10 nm for St161-b-4VP76 and 59
and 17 nm for St161-b-4VP107, as compared to the P2VP-con-
taining ones. This was due to the greater polarity of the
P4VP block than the P2VP block, leading to a greater incom-
patibility with toluene and a stronger drive for (reverse)
micellization in toluene in the case of the PSt-P4VP BCs as
compared to the PSt-P2VP ones. The BC with the longer
solvophobic P4VP block formed larger (diameter and height)
reverse micelles, as expected.
2
7
Self-Assembly Behavior in Solution and in the Bulk
Reverse Micelles in Toluene
In addition to their larger size, the PSt-P4VP-based micelles
presented a more heterogeneous spatial distribution on the
silicon substrate than the PSt-P2VP-based ones. This is prob-
ably due to stronger interactions between the former type
of micelles themselves, arising from their larger size in solu-
tion, rather than the stronger affinity of the 4VP block for
the silicon substrate, as the present aggregates are reverse
micelles with PVP cores shielded from the substrate by the
PSt shells.
It is well known that amphiphilic BCs form micelles or
reverse micelles when dissolved in selective solvents. Regu-
lar micelles are formed in aqueous or in high polarity sol-
vents, while reverse micelles are formed in nonpolar or in
2
6
low-polarity solvents. In this study, the low-polarity selec-
tive solvent toluene was chosen, and, therefore, reverse
micelles were expected to form. Toluene solutions of all four
BCs were characterized using AFM and cryoTEM. The results
are presented and discussed below.
For the cryo-TEM imaging, BC solutions in toluene (concen-
trations 0.2–1.0 w/v %) were deposited onto perforated
For the AFM measurements, BC solutions in toluene were
deposited onto a silicon substrate, and let dry. The recorded
images are presented in Figure 5. The reverse micelles
formed were characterized in terms of their height (h in
nm), mean diameter (/ in nm), and adsorption density (in
2
number of micelles per lm ). Due to the spreading of the
micelles upon their adsorption onto the substrate, and due
to characterization in the dry state, it is expected that h ꢅ
/. Both of the 2VP-containing diblock copolymers formed
spherical micelles with the same characteristics in terms of
the h and the / values, 3 and 23 nm, respectively, with dif-
ferences being observed only for the density of the deposited
micelles [Fig. 5(a,b)]. Thus, in the case of the St161-b-2VP121
BC, comprising a higher content of insoluble core block, the
2
density of the micelles was found to be lower (150/lm ) as
2
compared to St161-b-2VP48 (224/lm ). This was most likely
due to the fact that a longer solvophobic block resulted in
greater aggregation numbers, producing a smaller number of
larger micelles.
FIGURE 4 GPC traces of the St161 macroRAFT CTA and the
daughter diblock copolymers St161-b-2VP48 and St161-b-2VP121
.
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FIGURE 5 Topographic tapping-mode AFM images (2 lm ꢂ 2 lm scan size) of the reverse micelles formed by the BCs cast from
toluene solution (c ¼ 0.2 w/v %) (a) St161-b-2VP48 (3 nm), (b) St161-b-2VP121 (3 nm), (c) St161-b-4VP76 (10 nm), and (d) St161-b-4VP107
(
17 nm). The values in parentheses indicate the micelle heights measured by AFM.
carbon films supported by copper grids, vitrified in liquid
nitrogen, and imaged at about –180 C in the TEM. This
with the former exhibiting diameters of around 58 nm, and
the latter having diameters of 22 nm. In most cases the
micelles were globular and polydisperse in diameter; how-
ever, some more elongated micelles were also observed.
ꢃ
method allows direct imaging of the morphologies produced
by the BCs in the bulk of the solution. Samples of the recorded
images are presented in Figure 6. Micellar aggregates are
clearly seen in the vitrified solution. ‘‘C’’ denotes the carbon
part of the perforated carbon films. The results are very simi-
lar to those obtained using AFM. In particular, the PSt-P4VP
BCs formed larger reverse micelles than the PSt-P2VP ones,
Bulk Morphologies
BCs comprising repeating units of sufficient mutual incom-
patibility and possessing high enough MW microphase sepa-
rate in the bulk and form composition-dependent
FIGURE 6 Cryo-TEM images of the reverse micelles formed by the BCs in toluene solutions. The letter ‘‘C’’ denotes the perforated carbon
film. (a) St161-b-2VP48 (c ¼ 1.0 w/v %), (b) St161-b-2VP121 (c ¼ 0.2 w/v %), (c) St161-b-4VP76 (c ¼ 1.0 w/v %), and (d) St161-b-4VP107 (c ¼ 1.0 w/v %).
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FIGURE 7 TEM images of (a) St161-b-2VP48, (b) and (c) St161-b-4VP76, cast from chloroform. The inset at the top-left corner of (a) is
a 4 ꢂ 4 magnification of the area indicated toward the bottom-right corner of (a).
morphologies. Films cast from chloroform solutions of BCs
tive solvent toluene to form reverse micelles. The copoly-
mers with blocks comprising the more polar 4VP units
formed larger micelles than their 2VP-based counterparts of
similar size and compositions, manifesting the greater driv-
ing force for self-assembly in the case of the former type of
block copolymers. In the bulk, the diblock copolymer with
the more PSt-incompatible P4VP block was able to micro-
phase separate into hexagonally packed cylinders, while the
similar diblock with the less PSt-incompatible P2VP block
formed a spherical structure.
St161-b-2VP48 and St -b-4VP , having similar M values of
161
76
n
ꢀ
1
around 25,000 g mol , were investigated in terms of their
bulk morphology using TEM, and the recorded images are
displayed in Figure 7. While St161-b-2VP48 formed a spheri-
cal structure [Fig. 7(a)], the St161-b-4VP76 diblock copolymer
formed hexagonally packed cylinders of P4VP in a PSt ma-
trix, coexisting with layered structures [Figs. 7(b,c)], the lat-
ter corresponding to cylinders lying in the plane of the sec-
2
8,29
tion.
The alternating light and dark bands suggest a
layered morphology; the presence of light (PSt) channels
extending into the dark (P4VP) layers demonstrate that this
morphology is not lamellar. The spots in a hexagonal array
correspond to an end-on view of the cylinders and the lay-
ered structures correspond to a side view normal to the cyl-
inder axis. The experimental value of the diameter of one
cylinder was found to be 40 nm which compares favorably
with the value of 38.3 nm calculated considering fully
stretched P4VP blocks of number–average degree of poly-
The authors wish to thank the European Commission for fund-
ing this work within the FP7 project SELFMEM (grant agree-
ment no. NMP3-SL-2009-228652). We are also grateful to Dr.
P.F.W. Simon of the Helmholtz-Zentrum Geesthacht, for kindly
recording for us the GPC traces for the P4VP-containing BCs.
Furthermore, we are indebted to Dr. E. Kesselman of the Techn-
ion Israel Institute of Technology for her valuable suggestions
and assistance with the cryo-TEM characterization of the sam-
ples. Finally, we are grateful to the A.G. Leventis Foundation for
helping establish the NMR infrastructure at the University of
Cyprus.
3
0
merization of 76 (¼2 ꢂ 76 ꢂ 0.252 nm ). However, given
the BC PDI of almost 1.6, this would not necessarily imply
fully stretched P4VP chains. The sphere formation in the St-
2
VP BC can be attributed to the lower polarity and weaker
2
9
polarizability of 2VP than 4VP, leading to a smaller incom-
patibility with St in the former case.
REFERENCES AND NOTES
1
Thomas, E. L.; Anderson, D. M.; Henkee, C. S.; Hoffman, D.
The spheres in Figure 7(a) are rather small and blurred due
to the relatively low MW and high polydispersity of the
St161-b-2VP48 BC. It is noteworthy that the 2VP volume frac-
tion of this BC of 23% was expected to lead to cylinder,
rather than sphere, formation. Thus, the observed spherical
structure is apparently a nonequilibrium frozen morphology.
In contrast, the St161-b-4VP76 BC, comprising the more PSt-
incompatible P4VP block, and a 4VP volume fraction of 32%,
further away from that for sphere formation (12%), readily
equilibrated to cylinders.
Nature 1988, 334, 598–601.
2 Bates, F. S. Science 1991, 251, 898–905.
3
Hamley, I. W. Prog. Polym. Sci. 2009, 34, 1161–1210.
4
Hsieh, H. L.; Quirk, R. P. Anionic Polymerization: Principles
and Practical Applications; Marcel Dekker: New York, 1996.
5
Sciannamea, V.; Jer oˆ me, R.; Detrembleur, C. Chem. Rev.
ꢀ
2008, 108, 1104–1126.
6 Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001,
101, 3661–3688.
7 Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001,
1
8
9
01, 3689–3745.
Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921–2990.
CONCLUSIONS
Rizzardo, E.; Chiefari, J.; Mayadunne, R.; Moad, G.; Thang, S.
RAFT step-wise copolymerization was employed for the syn-
thesis of diblock copolymers of St and VP, two with 2VP and
the other two with 4VP. In solution, all four diblock copoly-
mers were able to self-assemble in the nonpolar and selec-
Macromol. Symp. 2001, 174, 209–212.
10 Matyjaszewski, K. Macromol. Symp. 2001, 174, 51–67.
1
1 Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005,
58, 379–410.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 1636–1644
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JOURNAL OF
POLYMER SCIENCE
ARTICLE
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1
2 Dai, C. A.; Dair, B. J.; Dai, K. H.; Ober, C. K.; Kramer, E. J.;
Hui, C. Y.; Jelinski, L. W. Phys. Rev. Lett. 1994, 73, 2472–2475.
3 Dai, C. A.; Jandt, K. D.; Dhamodharan, R.; Slack, N. L.; Dai, K. H.;
Davidson, W. B.; Kramer, E. J. Macromolecules 1997, 30, 549–560.
21 Severac, R.; Lacroix-Desmazes, P.; Boutevin, B. Polym. Int.
2002, 51, 1117–1122.
1
22 Goto, A.; Sato, K.; Tsujii, Y.; Fukuda, T.; Moad, G.; Rizzardo,
E.; Thang, S. H. Macromolecules 2001, 34, 402–408.
1
2
4 Cho, Y. H.; Yang, J. E.; Lee, J. S. Mater. Sci. Eng. 2004, 24,
93–295.
2
3 Goto, A.; Fukuda, T. Macromolecules 1997, 30, 5183–5186.
2
4 Wong, K. H.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H.
1
2
5 Yun, S. H.; Yoo, S. I.; Junc, S. C.; Sohn, B. H. Chem. Mater.
006, 18, 5646–5648.
Polymer 2007, 48, 4950–4965.
2
1
5 Varshney, S. K.; Zhong, X. F.; Eisenberg, A. Macromolecules
993, 26, 701–706.
1
2
6 Peinemann, K. V.; Abetz, V.; Simon, P. F. W. Nat. Mater.
007, 6, 992–996.
2
6 Gohy, J.-F. Adv. Polym. Sci. 2005, 190, 65–136.
1
7 Krishnamoorthy, S.; Pugin, R.; Brugger, J.; Heinzelmann, H.;
2
7 Liu, Y.; Lor, C.; Fu, Q.; Pan, D.; Ding, D.; Liu, J.; Lu, J.
Hinderling, K. Adv. Funct. Mater. 2006, 16, 1469–1475.
8 Yuan, J. J.; Ma, R.; Gao, Q.; Wang, Y.-F.; Cheng, S.-Y.; Feng,
L.-X.; Fan, Z.-Q.; Jiang, L. J. Appl. Polym. Sci. 2003, 89,
J. Phys. Chem. C 2010, 114, 5767–5772.
1
2
8 Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.;
Wiley: New York, 1989.
1017–1025.
1
9 Zhang, B.-Q.; Chen, G.-D.; Pan, C.-Y.; Luan, B.; Hong, C.-Y.
29 Zha, W.; Han, C. D.; Lee, D. H.; Han, S. H.; Kim, J. K.; Kang,
J. H.; Park, C. Macromolecules 2007, 40, 2109–2119.
J. Appl. Polym. Sci. 2006, 102, 1950–1958.
2
0 Wan, W.-M.; Hong, C.-Y.; Pan, C.-Y. Chem. Commun. 2009,
30 Hiemenz, P. C. Polymer Chemistry: The Basic Concepts;
Marcel Dekker: New York, 1984; p. 6.
5883–5885.
1
644
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 1636–1644