Synthesis of N-vinylcarbazole-N-vinylpyrrolidone amphiphilic block copolymers by xanthate-mediated controlled radical polymerization

Article abstract of DOI:10.1139/V09-160

Amphiphilic block copolymers poly(N-vinylcarbazole)-b-poly(N- vinylpyrrolidone) (PNVK-b-PNVP) were prepared by xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization. Both the PNVK and PNVP macroinitiators and the resulting block copolymers had molecular weights close to theoretical values, predicted for efficient initiation, in the range of M_n = 30 000 to 90 000. The block copolymers dissolved in several organic solvents but, depending on their composition, in methanol formed either micelles or large aggregates, as confirmed by dynamic light scattering. The presence of globular aggregates was confirmed by tapping mode atomic force microscopy.

Full text of DOI:10.1139/V09-160

228
Synthesis of N-vinylcarbazole–N-vinylpyrrolidone
amphiphilic block copolymers by xanthate-
mediated controlled radical polymerization
Chih-Feng Huang, Jeong Ae Yoon, and Krzysztof Matyjaszewski
Abstract: Amphiphilic block copolymers poly(N-vinylcarbazole)-b-poly(N-vinylpyrrolidone) (PNVK-b-PNVP) were pre-
pared by xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization. Both the PNVK and
PNVP macroinitiators and the resulting block copolymers had molecular weights close to theoretical values, predicted for
efficient initiation, in the range of Mn = 30 000 to 90 000. The block copolymers dissolved in several organic solvents
but, depending on their composition, in methanol formed either micelles or large aggregates, as confirmed by dynamic
light scattering. The presence of globular aggregates was confirmed by tapping mode atomic force microscopy.
Key words: amphiphilic block copolymer, RAFT, living radical polymerization, micelle.
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Resume : On a prepare des copolymeres a bloc amphiphiles poly(N-vinylcarbazole)-b-poly(N-vinylpyrrolidone) (PNVK-b-
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PNVP) par une reaction de polymerisation de transfert de chaıne avec addition-fragmentation reversible (TCAR) catalysee
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par le xanthate. Les valeurs des poids moleculaires des deux macroinitiateurs PNVK et PNVP ainsi que des copolymeres a
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bloc qui en ont resulte etaient proches des valeurs theoriques prevues pour une initiation efficace, de l’ordre de Mn =
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30,000 a 90,000. Les copolymeres a bloc se dissolvent dans plusieurs solvants organiques; toutefois, dans le methanol, sui-
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vant leur composition, il y a formation de micelles ou de gros agregats deceles par la diffusion dynamique de la lumiere.
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La presence d’agregats globulaires a aussi ete confirmee par la spectroscopie en mode de forces atomiques.
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Mots-cles : copolymere a bloc amphiphile, transfert de chaıne avec addition-fragmentation reversible (TCAR), polymerisa-
tion radicalaire vivante, micelle.
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[Traduit par la Redaction]
in medical, pharmaceutical, and cosmetic applications, due
to its low toxicity and good biocompatibility.3 This indicates
that PNVK-b-PNVP could be an interesting conductive pol-
ymer which could be processed in water. The nanophase
separation of the amphiphilic block copolymer could result
in a conductive PNVK segment embedded in an insulating
PNVP matrix. There are only a few reports on water proc-
essable conductive block copolymers because of the difficul-
ties with synthesis.4 To prepare well-defined amphiphilic
block copolymers, a controlled/living polymerization techni-
que is required.5 Although both NVK and NVP can be poly-
merized by radical mechanisms, the controlled radical
polymerizations of NVK and NVP have not been yet thor-
oughly studied due to the low reactivity of non-conjugated
vinyl monomers.
Introduction
The preparation and properties of amphiphilic block
copolymers, with hydrophilic and hydrophobic blocks, is
currently an area of active study. Amphiphilic block copoly-
mers form various supramolecular structures such as mi-
celles, cylinders, or vesicles through self-organization and
self-assembly and are being evaluated in numerous applica-
tions including emulsifiers, dispersants, and surfactants.1 Po-
tential uses also include microcontainers for delivery of
hydrophobic materials or templates for syntheses of materi-
als with nanosized features.
The aim of this study is to synthesize amphiphilic block
copolymers using N-vinylcarbazole and N-vinylpyrrolidone
(PNVK-b-PNVP). PNVK, a carbazole-containing polymer,
is a photoconductive material with good charge-transport
properties that has found applications as a photoreceptor,
light-emitting diode, and photorefractive material.2 On the
other hand, PNVP, the water-soluble block, has been used
In general, block copolymers can be prepared by three
routes: sequential monomer addition;6 chain-end coupling of
separately prepared blocks;7 or transformation of chain ends
of polymers prepared by one mechanism into initiating moi-
Received 5 September 2009. Accepted 14 October 2009. Published on the NRC Research Press Web site at canjchem.nrc.ca on
10 February 2010.
This article is part of a Special Issue dedicated to Professor M. A. Winnik.
C. Huang. Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA; Department of
Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan.
J.A. Yoon and K. Matyjaszewski.1 Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213,
USA.
1Corresponding author (e-mail: km3b@andrew.cmu.edu).
Can. J. Chem. 88: 228–235 (2010)
doi:10.1139/V09-160
Published by NRC Research Press

Huang et al.
229
Scheme 1. Synthesis of PNVK-b-PNVP amphiphilic block copolymers via xanthate-mediated RAFT polymerization.
Table 1. Compositions and molecular weights of macroinitiators and block copolymers.
g
Entry
C1a
C2b
B1c
Compositionf
PNVK31
Mn, theory
6200
5200
39 200
70 900
32 400
Mn, NMR
—
Mn, GPC
6300h
PDI
1.28
1.28
1.35
1.52
1.35
PNVP46
—
5300i
PNVK31-b-PNVP269
PNVK31-b-PNVP573
PNVP46-b-PNVK139
35 800
69 200
32 100
35 400i
88 000i
21 700i
B2d
B3e
a[NVK]/[EX]/[AIBN] = 43/1/0.5, 66 wt% of 1,4-dioxane to NVK, 60 8C, 4 h, conversion 71.0%.
b[NVP]/[EX]/[AIBN] = 150/1/0.35, 20 wt% of anisole to NVP, 60 8C, 4.5 h, conversion 30.1%.
c[NVP]/[C1]/[AIBN] = 600/1/0.5, 33 (v/v)% of anisole to NVP, 60 8C, 3.1 h, conversion 50.3%.
d[NVP]/[C1]/[AIBN] = 600/1/0.5, 33 (v/v)% of anisole to NVP, 60 8C, 20.9 h, conversion 98.1%.
e[NVK]/[C2]/[AIBN] = 150/1/0.5, 50 wt% of anisole to (NVK + C2), 60 8C, 26.2 h, conversion 93.9%.
fHomopolymer compositions were based on molecular weights measured by GPC. Block copolymer compositions
were based on their macroinitiator composition and the NMR spectra of block copolymers.
gMolecular weights based on monomer conversion.
hGPC in THF, PS standards.
iGPC in DMF, PMMA standards.
eties for the chain propagation of the second monomer.8 Pre-
vious attempts to prepare well-defined PNVK or PNVP ho-
mopolymers using one of the CRP methods has involved a
degenerative transfer radical mechanism,9 ATRP,10 or Co-
mediated polymerization.11 Only a few studies describe
chain extension from either PNVK or PNVP homopolymers
to prepare block copolymers.12 Regardless of the potentially
interesting properties, block copolymers of PNVK and
PNVP (PNVK-b-PNVP) have not been yet prepared, since
the synthesis of well defined PNVK-b-PNVP block copoly-
mers is challenging, owing to the low reactivity of both
NVK and NVP. The low reactivity of NVK and NVP and
the low stability of the propagating radicals indicate that a
degenerative transfer process, such as reversible addition-
fragmentation chain transfer (RAFT) polymerization, could
be best suited for conducting a controlled polymerization.
For this study, we selected a xanthate-type mediating agent,
since it was successfully employed for RAFT polymeriza-
tion of less active monomers in previous studies.12d
tillation under reduced pressure. N-vinylcarbazole (NVK,
97%, Sigma-Aldrich) and 2,2’-azobis(isobutyronitrile)
(AIBN, 99%, Sigma-Aldrich) were recrystallized twice from
methanol. All solvents were purified by distillation prior to
use. S-(2-Ethyl propionate)-O-ethyl xanthate (EX)13 was
synthesized according to the literature procedure. Briefly,
potassium O-ethyl xanthate (4.75 g, 2.9 Â 10–2 mol) was
dissolved in 25 mL of ethanol with ethyl 2-bromopropionate
(4.74 g, 5.3 Â 10–2 mol) for 20 h at room temperature. The
crude product was purified by extraction with diethyl ether –
water followed by column chromatography using hexane –
ethyl acetate 95:5 (v/v) as eluent to give a yellow oil. Yield:
1
3.0 g (51%). H NMR (300 MHz, CDCl3 , ppm) d: 4.63 (q,
2H), 4.37 (q, 1H), 4.20 (q, 2H), 1.56 (d, 3H), 1.41 (t, 3H),
1.28 (t, 3H).
RAFT polymerization of NVK (C1)
3.41
g of NVK (17.6 mmol), 91.0 mg of EX
(0.410 mmol), 34.0 mg of AIBN (0.207 Â 10–3 mmol), and
8 mL of anisole were charged to a Schlenk flask. The molar
ratio of NVK–EX–AIBN was 43:1:0.5. The mixture was de-
oxygenated by three freeze–pump–thaw cycles. The flask
was placed in an oil bath thermostated at 60 8C for a re-
quired time period. Samples were withdrawn via a syringe
for the measurement of monomer conversion and molecular
Experimental
Materials
N-Vinylpyrrolidone (NVP, 99%, Sigma-Aldrich) was
dried over anhydrous magnesium sulfate and purified by dis-
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230
Can. J. Chem. Vol. 88, 2010
Fig. 1. Kinetic plots, evolution of molecular weights and polydispersities with conversion, and GPC traces for the synthesis of B1 and B2
(a, b, and c) and B3 (d, e, and f). Dashed lines (- - -) in (a) and (d) are least square linear fits. Solid lines (—) in (b) and (e) represent
theoretical molecular weights vs. monomer conversion. Red triangles in (b) and (e) represent Mn values calculated based on block copoly-
1
mer using H NMR.
Table 2. Hydrodynamic diameters (nm) measured in various solvents at room temperature.
Solvent
Anisole
CHCl3
THF
DCa
4.3
4.8
7.5
B1 (PNVK31-b-PNVP269
315 (1.751)
7.86 (0.417)
)
B2 (PNVK31-b-PNVP573
2377 (0.540)
12.9 (0.441)
14.4 (0.304)
)
B3 (PNVP46-b-PNVK139
10.3 (0.425)
9.34 (0.140)
9.67 (0.170)
)
9.21 (0.390)
MeOH
33.0
44.0 (0.129)
64.0 (0.334)
774 + precipitation
Note: Average volume distribution. Numbers in parentheses are coefficients of variation.
aDielectric constant.
weight of polymer by GPC with DMF or THF as eluent.
The reaction was quenched by placing the flask in an ice
bath, exposing to air, and diluting with THF to provide the
PNVK–EX macromolecular chain transfer agent (C1). The
crude polymer was purified by filtering through an alumina
column followed by precipitation in hexane. Conversion =
preparation of C1. Conversion = 30% (by NMR); Mn, theor
5200 g/mol, Mn, DMF GPC = 5300 g/mol (PDI = 1.28).
=
Chain extension of PNVK (C1) by RAFT polymerization
of NVP forming a block copolymer
3.04 mL of NVP (3.18 g, 28.6 mmol), 0.300 g of C1
(Mn, THF GPC = 6300 g/mol, PDI = 1.28, 47.6 Â 10–3 mmol),
3.90 mg of AIBN (23.7 Â 10–3 mmol), and 1.56 mL of ani-
sole were charged to a Schlenk flask. The mole ratio of
NVP–C1–AIBN was 600:1:0.5. The mixture was deoxygen-
ated by three freeze–pump–thaw cycles and then was placed
in an oil bath thermostated at 60 8C for the required time
period. Monomer conversion and molecular weight were
tracked by GPC with DMF as eluent. At the desired con-
version, the reaction was quenched and the polymer was
purified by filtering through an alumina column followed
by precipitation in hexane. B1 conversion = 50% (by
GPC); Mn, theor = 39 200 g/mol, Mn, DMF GPC = 35 400 g/mol
71% (by NMR); Mn,
= 6200 g/mol, Mn,
=
theor
TFH GPC
6300 g/mol (PDI = 1.28).
RAFT polymerization of NVP (C2)
10.0 mL of NVP (10.4 g, 93.7 mmol), 139 mg of EX
(0.626 mmol), 4.1 mg of AIBN (0.220 mmol), 2.5 mL of ani-
sole were charged to a Schlenk flask. The mole ratio of NVP–
EX–AIBN was 150:1:0.35. The mixture was deoxygenated by
three freeze–pump–thaw cycles. The flask was placed in an
oil bath thermostated at 60 8C for the desired time period.
The rest of the procedure was the same as that for the
Published by NRC Research Press

Huang et al.
231
Fig. 2. DLS profile for PNVK-b-PNVP block copolymer (B1, B2, and B3) in chloroform and methanol. Blue line: PNVP block, Red line:
PNVK block.
1
Fig. 3. H NMR spectra of B1 in different solvents, toluene-d8, MeOH-d4, and chloroform-d.
(PDI = 1.35). B2 conversion = 98% (by GPC); Mn, theor
70 900 g/mol, Mn, DMF GPC = 88 000 g/mol (PDI = 1.52).
=
merization of NVP from PNVK described above. B3 con-
version = 94% (by GPC); Mn, = 32 400 g/mol,
Mn, DMF GPC = 21 700 g/mol (PDI = 1.35).
theor
Chain extension of PNVP (C2) by RAFT polymerization
of NVK
Analyses
1.09 g of NVK (5.65 mmol), 0.200 of C2 (Mn, DMF
5300 g/mol, PDI = 1.28, 37.7 Â 10–3 mmol), 3.10 mg of
AIBN (18.9 Â 10–3 mmol), and 2.60 mL of anisole were
charged to a Schlenk flask. The mole ratio of NVK–C2–
AIBN was 150:1:0.5. The mixture was deoxygenated by
three freeze–pump–thaw cycles. The flask was placed in an
oil bath thermostated at 60 8C for the required time. The rest
of the procedure was the same as that for the block copoly-
=
The molecular weights and molecular-weight distributions
were measured using GPC (Polymer Standards Services,
5
3
2
˚
columns (guard, 10 , 10 , and 10 A)) with DMF (containing
5 mmol/L LiBr) as eluent at 50 8C or with THF as eluent at
35 8C. The flow rate was kept at 1.00 mL/min and a differ-
ential refractive index (RI) detector (Waters, 2410) was
used. Toluene was used as the internal standard to correct
for any fluctuation in eluent flow rate. The molecular weight
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232
Can. J. Chem. Vol. 88, 2010
Fig. 4. 3D images of height mode AFM at different conditions. B1: (a), (b), and (c), B3: (d), (e), and (f).
Fig. 5. 3D height mode AFM images (left column) and phase mode
AFM images (right column) of B1 (a, b) and B2 (c, d) after drop
casting of DMF solution and drying. Phase image size: 0.6 mm Â
0.6 mm.
quency of 40 N/m and 330 kHz, respectively. The height and
phase images were acquired simultaneously at a set-point ra-
tio (A/A0), in the range of 0.7–0.9, where A and A0 refer, re-
spectively, to the ‘‘tapping’’ and ‘‘free’’ cantilever amplitude.
Results and discussions
Synthesis
The synthesis of PNVK-b-PNVP block copolymers was
conducted sequentially via the two alternative pathways
shown in Scheme 1.
First, macromolecular chain transfer agents (CTAs),
i.e., macroinitiators, were prepared by RAFT polymeriza-
tions using EX as a CTA. The molecular weights of the
macroinitiators were measured by GPC and summarized
in Table 1. The molecular weight of C1 measured by
GPC (in THF using PS standards) (Mn = 6300, DP = 31)
agreed well with the value estimated from monomer con-
version (D[M]/[EX]0 = 43, DP = 31 at 71% conversion),
assuming quantitative initiation. However, when the molec-
ular weight was measured by GPC in DMF, a much
smaller value (Mn = 3800, DP = 19) was obtained, presum-
ably due to the formation of more compact polymer coils
in DMF. A similar observation concerning a much smaller
molecular weight of PS measured in DMF GPC, was re-
ported previously.14 The molecular weight of C2, deter-
mined by GPC (in DMF, PMMA standards), corresponded
well to the theoretical molecular weight estimated from
monomer conversion (DP = D[M]/[EX]0) with the assump-
tion of the quantitative initiation efficiency. The molecular
weight of the macroinitiators measured by GPC (in DMF)
compared well with the molecular weight calculated by
and the molecular-weight distribution were determined with
a calibration curve based on linear PMMA (for DMF GPC
line) or PS (for THF GPC line) standards using GPCWin
software. Conversions were determined by NMR on a
Bruker AvanceTM 300 MHz instrument or by an internal
standard addition, injecting a known amount of monomer
into reaction mixtures and following the peak area changes
in GPC traces. Particle size and size distribution were meas-
ured by dynamic light scattering (DLS) on High Perform-
ance Particle Sizer, Model HP5001 from Malvern
Instruments, Ltd. DLS measurements provide average diam-
eter, Dav, and size distribution index, CV (coefficient of var-
iation), which is defined as CV = (S Â 100)/Dav, where Dav
is the mean diameter and S is the size standard deviation.
Tapping mode AFM experiments were carried out using a
Multimode Nanoscope V system (Veeco instruments). The
measurements were performed in air using commercial Si
cantilevers with a nominal spring constant and resonance fre-
1
end group analysis of H NMR spectra. For example, for
another PNVP macroinitiator prepared under the same con-
ditions as C2, Mn was calculated by end group analysis to
be 4400 g/mol, and was measured by GPC to be
4500 g/mol (in DMF, PMMA standards). Furthermore,
good agreement of experimental and theoretical molecular
weights indicated that the number of new chains generated
by decomposition of AIBN, coupling with CTA, and trans-
fer process was small.
Chain extensions from the prepared macroinitiators were
Published by NRC Research Press

Huang et al.
233
performed by RAFT polymerization of the second monomer.
The molecular weight and polydispersity of each block co-
polymer and the block copolymer composition determined
by NMR are summarized in Table 1. Figure 1 displays the
kinetic plots, the linear change of molecular weight with
conversion, and the evolution of GPC traces over time for
each block copolymer. The chain extension reactions were
carried out to over 90% monomer conversion for both path-
ways. The molecular weights of B1 and B2 measured by
GPC (in DMF, PMMA standards) were very similar to the
calculated molecular weights based on monomer conversion
the spectrum measured in chloroform-d, a good solvent for
both blocks, peaks from both PNVK block (4.5–8 ppm) and
PNVP block (0.5–4.5 ppm) are well resolved. However, the
peaks of PNVK block disappear when measured in MeOH-
d4, because of the insolubility of the PNVK block. Toluene-
d8 was selected as a nonpolar solvent instead of anisole. No
appreciable peak was observed, proving the insolubility of
the B1 to toluene owing to the large portion of polar PNVP
block.
AFM studies
1
and H NMR analyses. THF could not be used as GPC elu-
The phase separation in block copolymers was also ob-
served by tapping mode AFM (Fig. 4). The block copoly-
mers B1 and B3 were dissolved in chloroform (1 mg/mL)
and solutions were drop-cast onto silicon wafers (1 cm Â
1 cm), followed by drying under vacuum at room tempera-
ture overnight. The films of block copolymers were studied
by AFM. Globular morphologies with dimensions of 37 nm
and 49 nm were observed for B1 and B3. Although the di-
lute chloroform solutions of B1 and B3 did not contain ag-
gregates (cf. DLS studies), they formed aggregates as
concentration increased during the solvent evaporation. The
polymer films were then annealed at 160 8C (Figs. 4b and
4e) and 190 8C (Figs. 4c and 4f), i.e., above the Tg of
PNVP block (*120 8C) under vacuum. Because the anneal-
ing was conducted below the Tg of the PNVK block
(*210 8C), phase separation could not be enhanced by ther-
mal annealing. The aggregates, formed during the film for-
mation, relaxed, as seen from smoothed surfaces after
annealing.
The hypothesis that the globular morphology was gener-
ated by aggregation of polymers during the drying process
was supported by another experiment. This time, block co-
polymers B1 and B2, with the same PNVK block but differ-
ent chain length of PNVP, were dissolved in DMF, a good
solvent for both blocks. Since DMF evaporates at a slower
rate than chloroform, it should provide sufficient time for
the copolymers to attain lower-energy morphologies. The
height and phase mode AFM images of B1 and B2 in
Fig. 5 were measured after drop casting and drying of DMF
in a vacuum oven at room temperature. The average domain
sizes of B1 and B2, obtained by 2D isotropic power spectral
density analysis of height images, were 28 and 33 nm, re-
spectively, indicating that the increase in the size of the do-
mains is correlated with the dimensions of the PNVP blocks
(DP = 269 vs. DP = 573).
ent for PNVP because it is isorefractive with PNVP. The
molecular weight of B3, calculated from the H NMR spec-
1
trum, was very close to that based on monomer conversion,
assuming quantitative initiation. The values differed from
the Mn by GPC (in DMF, PMMA standard) which can be
attributed to the compact structure of the PNVK block
(83 wt% in B3) in DMF, as was the case with the PNVK
macroinitiator. The observed first-order kinetic plots and the
very high efficiency of the chain extensions suggested the
negligible contribution of possible side reactions such as for-
mation of NVP dimers or elimination of xanthate end
groups.12e,15 This could be attributed to the simple structure
of xanthate (EX), use of purified reagents, and the mild pol-
ymerization temperature (60 8C).
DLS and NMR studies
Particle-size distribution (volume vs. hydrodynamic diam-
eter (Dh)) of each prepared block copolymer was studied by
dynamic light scattering (DLS). Table 2 summarizes particle
size distributions in various solvents having different polar-
ities (represented in Table 2 by dielectric constant). The
DLS profiles in two representative solvents, chloroform and
methanol, are presented in Fig. 2.
All block copolymers displayed Dh values smaller than
15 nm in medium polarity solvents (chloroform and THF),
implying dissolution of the block copolymers without for-
mation of aggregates. However, when methanol was used as
the solvent, the average particle size was much larger, rang-
ing from 44 to 64 nm for B1 and B2, and resulted in larger
aggregates (Dh > 700 nm) for B3 (some precipitation was
visually observed). Methanol, a non-solvent for PNVK and
a good solvent for PNVP, resulted in formation of micelles
with PNVP as the major block in the outer shell (B1 and
B2). With PNVK as the major block (B3), larger aggregates
were formed, accompanied by precipitation. Assuming full
stretching of PNVP chains, the calculated size of micelles
for B1 and B2 should range from 150 nm to 390 nm (DP Â
(0.25 nm) Â 2), respectively. Observed smaller diameters
suggest coiled chains of PNVP, due to a less dense micelle
structures (Fig. 2, insets). In contrast to methanol, anisole is
a good solvent for PNVK because of aromatic interactions
and a poor solvent for PNVP because of its weak hydrogen-
bonding ability. As a result, while B3 dissolved as individ-
ual chains (Dh = 10.3 nm), B1 and B2, with PNVP as the
major block, formed very large aggregates sized over
300 nm (cf. Table 2).
Conclusion
Amphiphilic block copolymers with a hydrophobic PNVK
block and a hydrophilic PNVP block were synthesized by
xanthate-mediated RAFT polymerization by preparation of
PNVK or PNVP macroinitiators and sequential chain exten-
sions with the second monomer, i.e., NVP or NVK. The re-
sulting block copolymers had low polydispersity and the
1
molecular weight, measured by GPC and H NMR, agreed
well with the theoretical values. The particle-size distribu-
tions of the prepared block copolymers were measured in
various solvents with different polarities. Block copolymers
B1 and B2, with longer PNVP blocks, formed micelles in
methanol whereas B3, with PNVK as the major block,
The change of solubility of the block copolymer in selec-
1
tive solvents was further confirmed by H NMR measure-
1
ments. The H NMR spectra of B1 are shown in Fig. 3. In
Published by NRC Research Press

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Can. J. Chem. Vol. 88, 2010
Domski, G. J.; Rose, J. M.; Coates, G. W.; Bolig, A. D.;
Brookhart, M. Prog. Polym. Sci. 2007, 32 (1), 30. doi:10.
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formed larger aggregates and precipitates. AFM measure-
ments of film samples revealed the block copolymers aggre-
gated during the drying of films, even though the initial
solution did not contain aggregates. Annealing the film sam-
ples at a temperature between the Tg of PNVK (210 8C) and
PNVP (120 8C) yielded a smoother surface than before an-
nealing but did not enhance the phase separation.
Acknowledgements
C.-F. Huang acknowledges the National Science Council
and National Chiao Tung University (NSC-096–2120-M-
009–009 and NSC-096–2917-I-564–131) for postdoctoral
fellowship support.
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