Engineering low surface energy polymers through molecular design: Synthetic routes to fluorinated polystyrene-based block copolymers

Article abstract of DOI:10.1039/b200891b

New narrow polydisperse polystyrene-based block copolymers comprising fluorinated aromatic substituents in the side chains were synthesized following two different synthetic routes: (i) TEMPO-mediated controlled radical polymerization of fluorinated styre

Full text of DOI:10.1039/b200891b

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Engineering low surface energy polymers through molecular design:
synthetic routes to fluorinated polystyrene-based block copolymers
Luisa Andruzzi,a Emo Chiellini,*a Giancarlo Galli,a Xuefa Li,b Seok H. Kangb and
Christopher K. Oberb
aDipartimento di Chimica e Chimica Industriale, Universita` di Pisa, 56126 Pisa, Italy.
E-mail: chlmeo@dcci.unipi.it
bDepartment of Materials Science and Engineering, Cornell University, Ithaca, NY 14853,
USA
Received 24th January 2002, Accepted 18th March 2002
First published as an Advance Article on the web 16th April 2002
New narrow polydisperse polystyrene-based block copolymers comprising fluorinated aromatic substituents
in the side chains were synthesized following two different synthetic routes: (i) TEMPO-mediated controlled
radical polymerization of fluorinated styrene monomers; (ii) A sequence of polymer modification reactions on
anionically formed polystyrene-block-polyisoprene block copolymers. Using the former route, AB diblock and
ABA triblock copolymers were obtained in which a block of polystyrene bearing a fluorocarbon chain
substituent with a range of nCF2 groups (n ~ 4, 6, or 8) was incorporated as B block. Following the latter
route, polystyrene-block-polyisoprene AB diblock copolymers were prepared by anionic polymerization and
then modification by introduction of fluorinated tails with different numbers of CF2 groups (n ~ 6 or 8). The
thermal behavior as well as the bulk microstructure of the fluorinated block copolymers were investigated and
the effect of the chemical structure on the properties was evaluated. All the samples showed a tendency to form
layered mesophases in the bulk, and increasing the length of the fluorocarbon tail of the side chain enhanced
the degree of order of the mesophase from a disordered smectic (n ~ 4 or 6) to ordered pseudohexagonal
smectic (n ~ 8). Contact angles of block copolymer films were measured using water as the wetting medium.
Values of advanced contact angles up to 130u (120u receded angles) were found which were quite constant over
prolonged immersion times in water.
result in the formation of a stable, uniformly organized array
Introduction
of trifluoromethyl groups at the surface of the polymer film
which could be used for the production of high performance
non-stick coatings. Control of polydispersity of block copo-
lymers is a necessary prerequisite for the macromolecular
engineering of polymers with suitable properties for these
applications.
Fluorination of organic compounds and polymers brings about
dramatic changes in their physical properties with respect to
the corresponding fully hydrogenated materials. In general,
perfluorinated materials show low intramolecular and inter-
molecular interactions which lead to low cohesive energy and
therefore to low surface energy properties.1 Moreover, fluori-
nated polymers possess good thermal stability as well as great
chemical stability as a consequence of the perfluorination of the
macromolecular chain. Other special characteristics of fluori-
nated polymers are represented by a low relative permittivity,2
a low refractive index,3 hydrophobicity and lipophobicity,4,5
and a low friction coefficient.6
Due to these unique properties, fluorinated polymers may
find diverse applications in electronics,7 optics,8 biomaterials,9
lubricants and surfactants,10 and coatings;11 for more extensive
references see Ref. 12. In particular, because of the special
surface behavior of fluorinated polymers, the introduction of
fluorine atoms on a polymer chain may represent a strategy for
producing materials to be used in low surface energy, non-stick
coatings.
Bearing these ideas in mind, in this work we prepared two
classes of new narrow polydisperse block copolymers bearing
fluorocarbon segments in the side chains of one polymer block
for orderly assembly in the bulk. We followed two different
synthetic routes, the former is the controlled radical poly-
merization of fluorinated styrene monomers, and the latter is
the polymer modification of anionically prepared polystyrene-
block-polyisoprene block copolymers with fluorocarbon
chains. In general, the adopted experimental schemes were
able to provide block copolymers with a fluorinated block in
which the chemical structure could be controlled, and reliably
correlated with the ultimate bulk and surface properties. The
relative ease of preparation can be taken as a useful requisite
for scaling up the process leading to fluorinated materials for
advanced applications.
The surface behavior, as well as the stability of the surface
properties towards reconstruction in water of fluorinated
liquid-crystalline block copolymers has been the subject of
recent studies.13–16 The interplay between the microphase
separation of the incompatible blocks and the liquid-crystalline
organization of the mesogenic molecules may be a useful means
for producing highly hydrophobic, low adhesion surfaces
where the fluorinated block component of the block copolymer
can be selectively and efficiently driven towards the surface
of a polymer coating. The liquid-crystalline assembly of the
fluorinated mesogenic side groups of the polymer can in fact
Experimental section
Starting materials
1H,1H,2H,2H-Perfluorohexanol (99%), 1H,1H,2H,2H-perfluoro-
octanol (99%), 1H,1H,2H,2H-perfluorodecanol (99%), 4-chloro-
methylstyrene (95%), ethyl 4-hydroxybenzoate (99%), triphenyl
phosphine, diethyl azodicarboxylate (DEAD), 2,2,6,6-tetramethyl-
piperidin-1-yloxy radical (TEMPO), 2-fluoro-1-methylpyridi-
nium toluene-4-sulfonate (FMPTS), 9-borabicyclo[3.3.1]nonane
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This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b200891b

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Scheme 1 Reaction scheme for the preparation of fluorinated side-chain block copolymers by controlled radical polymerization.
(9-BBN) in tetrahydrofuran solution and 1,1,2-trichlorotrifluoro-
ethane were purchased from Aldrich and used without further
purification. Styrene was washed several times with a 30%
NaOH aqueous solution and then distilled over CaH2 (bp 45 uC/
30 mmHg). Benzoyl peroxide (BPO) and azobis(isobutyroni-
trile) (AIBN) were recrystallized from methanol. Diglyme was
distilled over sodium (bp 62 uC/17 mmHg). Thionyl chloride
was distilled under a nitrogen atmosphere. Pyridine was
refluxed over KOH and distilled under a nitrogen atmosphere.
THF was refluxed over Na/K alloy and then distilled under a
nitrogen atmosphere.
The synthetic procedures for the preparation of fluorinated
polystyrene-based block-copolymers are illustrated in Schemes
1 and 2. Experimental details are given below for the pre-
paration of one representative block copolymer sample for
each copolymer class.
A. Controlled radical polymerization of fluorinated styrenes
(Scheme 1)
Preparation of polystyrene macroinitiator poly(1). 2.0 g
(19 mmol) of freshly distilled styrene (1), 30 mg (13 mmol)
Scheme 2 Reaction scheme for the preparation of fluorinated side-chain block copolymers by polymer modification.
J. Mater. Chem., 2002, 12, 1684–1692
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of BPO, 29 mg (19 mmol) of TEMPO and 9.0 g (32 mmol)
of FMPTS were degassed by several freeze–thaw cycles in a
glass vial which was then sealed off in vacuo and kept at 125 uC
for 22 hours. The polymer poly(1) was purified by repeated
precipitations from dichloromethane solution into methanol
and finally dried in vacuo for several hours at room temperature
(70% yield).
PhCH2O), 6.3–7.6 (22.6H, aromatic).19F-NMR (CDCl3, d in
ppm from CF3CO2H): 26 (3F, CF3), 238 (2F, CH2CF2), 248
(2F, CF2), 252 (2F, CF2).
Preparation of homopolymers. In a typical homopolymer-
ization experiment, 0.5 g of fluorinated monomer (2), 5 mg of
AIBN and 1 mL of anhydrous benzene were degassed by
several freeze–thaw cycles in a glass vial that was then sealed
off in vacuo. The polymerization was carried out at 60 uC
for 24 hours. The polymer poly(2) was purified by repeated
precipitations from chloroform solution into methanol and
finally dried in vacuo for several hours at room temperature
(55% yield).
1H-NMR (CDCl3, d in ppm from TMS): 0.23 and 0.40 (not
integrable, TEMPO methyl), 1.3–2.5 (3H, CH2CH), 6.2–7.4
(5H, aromatic).
Preparation of 4-(1H,1H,2H,2H-perfluorohexyl)oxymethyl-
styrene (2). A mixture of 10.0 g (21 mmol) of 1H,1H,2H,2H-
perfluorohexanol and 80 mL of 50 wt% aqueous NaOH
solution was vigorously stirred at room temperature for an
hour (for 1H,1H,2H,2H-perfluorodecanol stirring at 100 uC
was necessary). Then 1.0 g (3 mmol) of tetrabutylammonium
hydrogensulfate (TBAH) in 80 mL of CH2Cl2 was added.
Subsequently, 6.5 g (44 mmol) of 4-chloromethylstyrene was
added dropwise to the suspension. After stirring for 15 hours
at 40 uC, the organic layer was separated, washed with dilute
HCl, with water to neutrality and finally dried over Na2SO4.
After evaporation of the solvent an oily residue was obtained
that was purified by double elution on silica gel with hexane–
ethyl acetate (30/1 v/v) as the eluent. 7.1 g of pure (2) (56%
yield, GLC purity w 99%) as a transparent viscous liquid was
obtained.
FT-IR (film on KBr, n in cm21): 3050–3020 (n C–H
¯
aromatic), 2930 (n CH2), 1620–1450 (n CLC aromatic),
1250–1030 (n C-O-C 1 n C-F), 708 (d C–H aromatic), 658
(d CF2). 1H-NMR (CDCl3, d in ppm from TMS): 1.3–2.7 (5H,
CH2CH 1 CH2CF2), 3.8 (2H, OCH2), 4.5 (2H, PhCH2O),
6.3–7.6 (4H, aromatic).
B. Polymer modification of block copolymers (Scheme 2)
Anionic copolymerization of styrene and isoprene. Polystyr-
ene-block-polyisoprene block copolymers PS-PIa (x ~ 400,
y ~ 150) and PS-PIb (x ~ 640, y ~ 115) were synthesized
by living anionic polymerization according to a literature
procedure.17
FT-IR (KBr pellet, n in cm21): 3060–3025 (n C-H aromatic),
¯
Hydroboration of polystyrene-block-polyisoprene. 3.0
g
2920 (n CH2), 1630–1450 (n CLC aromatic 1 vinyl), 1210–1030
(n C-O-C 1 n C-F), 706 (d C-H aromatic), 666 (v CF2).1H-
NMR (CDCl3, d in ppm from TMS): 2.4 (2H, CH2CF2), 3.7
(2H, OCH2), 4.4 (2H, PhCH2O), 5.2 and 5.7 (2H, CH2L), 6.7
(1H, CHL), 7.2–7.5 (4H, aromatic). 19F-NMR (CDCl3, d in
ppm from CF3CO2H): 26 (3F, CF3), 238 (2F, CH2CF2), 249
(2F, CF2), 251 (2F, CF2). Anal. (C15H13F9O): % calcd.
C 47.37, H 3.42, F 45.00; % found C 47.51, H 3.29, F 45.1.
(9 mmol of vinyl and methylvinyl groups) of PS-PIa was
vacuum dried overnight at 50 uC in a flask equipped with a
septum. After cooling, 150 mL of freshly distilled THF that
had been degassed by repeated freeze–pump–N2 purge cycles
was transferred into the flask via a cannula. The solution
was cooled to 215 uC and 22 mL (11 mmol) of a 0.5 M
THF solution of 9-BBN, which had previously been degassed
by repeated freeze–pump–N2 purge cycles, was added. The
solution was stirred for 24 hours at room temperature and
then cooled to 225 uC before 2 mL of anhydrous methanol
was injected into the solution. The solution was stirred for an
additional 30 min and 2 mL (39 mmol) of 6 M NaOH (purged
with N2 for 30 min) was added. After an additional 10 min,
4 mL of 30% H2O2 (purged with N2 for 30 min) was added.
Precipitation occurred and stirring became difficult. After 2
additional hours at 225 uC, the temperature of the solution
was slowly raised to room temperature. The suspension was
stirred for 30 min and then heated to 55 uC for 1 hour to form a
homogeneous solution. After cooling to room temperature, the
solution phase separated. The lower layer was frozen in a liquid
N2–propan-2-ol bath and the upper organic layer was slowly
poured into 800 mL of 0.25 M NaOH aqueous solution. The
polymer precipitated was filtered, washed with 0.25 M NaOH
aqueous solution, and dissolved in 60 mL of THF. The polymer
solution was then reprecipitated into 0.25 M NaOH aqueous
solution and stirred overnight. The polymer was reprecipitated
three more times. Finally the polymer PS-PIOHa was washed
with water and dried under vacuum (95% yield). FT-IR (KBr,
Preparation of AB diblock copolymer poly1-block-poly2.
100 mg of polystyrene macroinitiator poly(1) and 1.0 g of
fluorinated monomer (2) were degassed by several freeze–thaw
cycles in a glass vial which was then sealed off in vacuo and kept
at 125 uC for 24 hours. The diblock copolymer poly1-block-
poly2 formed was purified by repeated precipitations from
chloroform solution into methanol and extraction with boiling
acetone and finally dried in vacuo for several hours at room
temperature (70% yield).
FT-IR (film on KBr, n in cm21): 3050–3020 (n C-H aro-
¯
matic), 2930 (n CH2), 1620–1450 (n CLC aromatic), 1250–1030
(n C–O–C 1 n C–F), 702 (d C–H aromatic), 658 (v CF2).1H-
NMR (CDCl3, d in ppm from TMS): 1.3–2.7 (6.6H, CH2CH 1
CH2CF2), 3.8 (2.0H, OCH2), 4.5 (2.0H, PhCH2O), 6.3–
7.6 (4.7H, aromatic).19F-NMR (CDCl3, d in ppm from
CF3CO2H): 26 (3F, CF3), 238 (2F, CH2CF2), 248 (2F, CF2),
252 (2F, CF2).
Preparation of ABA triblock copolymer poly1-block-poly2-
block-poly1. 100 mg of poly1-block-poly2 diblock copolymer
and 1.5 mL of freshly distilled styrene (1) were degassed by
several freeze–thaw cycles in a glass vial which was then sealed
off in vacuo and kept at 125 uC for 3 hours. The triblock
copolymer poly1-block-poly2-block-poly1 formed was purified
by repeated precipitations from chloroform solution into
methanol and extraction with boiling acetone and finally
dried in vacuo for several hours at room temperature (70%
yield).
n in cm21): 3500 (broad, n O–H), 2980–2840 (n CH2), 1680–
1450 (n CLC aromatic), 1300–1100 (n C–O–C), 710 (d C–H
aromatic).
¯
Synthesis of semifluorinated acyl chlorides. A solution of 4.0 g
(0.048 mol) of DEAD in 8 mL of dry diethyl ether was slowly
added at room temperature under vigorous stirring to a
mixture of 7.0 g (0.042 mol) of ethyl 4-hydroxybenzoate, 11.6 g
(0.042 mol) and triphenyl phosphine, 20.0 g (0.043 mol) of
1H,1H,2H,2H-perfluorodecanol and 500 mL of dry diethyl
ether. The reaction mixture was stirred at room temperature
for 72 hours. The precipitate was filtered off and the solvent
evaporated under vacuum. The crude product was purified
FT-IR (film on KBr, n in cm21): 3050–3020 (n C–H
¯
aromatic), 2930 (n CH2), 1620–1450 (n CLC aromatic),
1250–1030 (n C-O-C 1 n C-F), 702 (d C-H aromatic), 658
(v CF2).1H-NMR (CDCl3, d in ppm from TMS): 1.3–2.7
(16.2H, CH2CH 1 CH2CF2), 3.8 (2.0H, OCH2), 4.5 (2.0H,
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by silica gel column chromatography with ethyl acetate–hexane
(3 : 1 v/v) as the eluent. 15.6 g (25% yield) of pure ethyl
4-(1H,1H,2H,2H-perfluorodecyloxy)benzoate as a white solid
was obtained.
performed with a Mettler DSC-30 instrument. Samples of
5–15 mg were used with 5–20 uC min21 scan rate. The phase
transition temperatures of the polymers were taken as the
maximum temperature in the DSC enthalpic peaks. The glass
transition temperature was taken as the half-devitrification
temperature.
Wide angle X-ray diffraction (WAXD) patterns were
obtained at the Cornell High Energy Synchrotron Source
˚
(CHESS) (l ~ 1.55 A) using a charge coupled detector. The
X-ray intensity and the distance between the sample and the
detector were adjusted according to experimental needs. Films
of thickness #1 mm cast from 5 wt% trifluorotoluene solutions
were analyzed.
Contact angles were measured using a NRL Contact Angle
Goniometer Model 100-00 (Rame´-Hart Inc.) at 20 uC. The
contact angle values were averaged over four measurements.
The advanced contact angle was read by injecting 4 mL liquid
drop. The receded contact angle was measured by removing
3 mL of liquid from the droplet. Single-layer polymer films were
prepared by casting 5 wt% trifluorotoluene polymer solutions
on glass slides which were allowed to evaporate slowly for a
few days and then annealed for 24 hours in a vacuum oven at
30 uC (poly1-block-poly2 to poly1-block-poly4) or at 110 uC
(poly1-block-poly5 and poly1-block-poly6). Double-layer poly-
mer films were prepared by casting a top layer of a 2 wt%
fluorinated block copolymer trifluorotoluene solution onto
a bottom layer cast from a 3 wt% SEBS (styrene-ethylene-
butadiene rubber, Kraton G1652M from Shell) CHCl3 solu-
tion. The solvent was allowed to evaporate slowly for a few
days and subsequently the film was annealed at 110 uC for
24 hours in a vacuum oven.
1H-NMR (CDCl3, d in ppm from TMS): 1.4 (3H, CH3), 2.6
(2H, CH2CF2), 4.3 (4H, 2CH2), 6.9 (2H, aromatic), 8.0 (2H,
aromatic).
A mixture of 7.0 g (0.011 mmol) of ethyl 4-(1H,1H,2H,2H-
perfluorodecyloxy)benzoate, 0.9 g (0.017 mmol) of KOH,
80 mL of water and 110 mL of ethanol was refluxed for 5 hours.
The reaction solution was then acidified with 37% HCl and
extracted with CH2Cl2. The organic solution was finally washed
with 5% NaHCO3, 37% HCl and water. After evaporation of the
solvent, 6.2 g (97% yield) of 4-(1H,1H,2H,2H-perfluorodecyl-
oxy)benzoic acid was obtained (mp 201–204 uC). 1H-NMR
(DMSO-d6, d in ppm from TMS): 2.6 (2H, CH2CF2), 4.3 (2H,
CH2O), 7.0 (2H, aromatic), 8.0 (2H, aromatic). Finally, a
mixture of 1.5 g (2.6 mmol) of 4-(1H,1H,2H,2H-perfluoro-
decyloxy)benzoic acid and 5 mL (42 mmol) of SOCl2 was
stirred under nitrogen atmosphere at 40 uC for 1 hour and then
refluxed for 4 hours. The excess SOCl2 was stripped off under
vacuum and 1.5 g (96% yield) of 4-(1H,1H,2H,2H-perfluoro-
decyloxy)benzoyl chloride (6) was obtained (mp 70–72 uC).
FT-IR (KBr pellet, n in cm21): 3100–3050 (n C–H aromatic),
¯
2980–2840 (n CH2), 1780 (n CLO), 1680–1450 (n CLC
aromatic), 1240–1090 (n C-O-C 1 n C-F), 648 (v CF2).
1H-NMR (CDCl3, d in ppm from TMS): 2.6 (2H, CH2CF2), 4.3
(2H, CH2O), 7.0 (2H, aromatic), 8.0 (2H, aromatic). Anal
(C17H8ClF17O2): % calcd. C 33.85, H 1.33, Cl 5.89, F 53.61; %
found C 33.72, H 1.39, Cl 76.05, F 53.9.
Side-chain esterification of block copolymers. 0.3 g (0.8 mmol
of CH2OH groups) of block copolymer PS-PIOHa was
dissolved in 3 mL of anhydrous THF and 0.08 g (1 mmol)
of pyridine. Then a solution of 0.6 g (1 mmol) of (6) in 2 mL of
anhydrous THF was slowly injected through a rubber septum
into the reaction flask. The reaction mixture was kept at 50 uC
for 24 hours, after which 1 mL of anhydrous methanol was
injected into the flask to convert the excess acid chloride to
ester. The polymer solution was poured into a large amount of
a methanol–water (1 : 1 v/v) solution. The polymer poly1-
block-poly6 precipitate was filtered and dissolved in THF and
reprecipitated into methanol three times. Finally the polymer
was dried overnight at 60 uC in a vacuum oven (90% yield).
Results and discussion
Synthesis by controlled radical polymerization
The TEMPO-mediated controlled radical polymerization
(CRP) was chosen to prepare the first class of fluorinated
polystyrene-based block copolymers. This method involves the
combined use of a conventional free radical initiator, such as
benzoyl peroxide (BPO) or azobis(isobutyronitrile) (AIBN),
and a stable free radical such as 2,2,6,6-tetramethylpiperidin-
1-yloxy (TEMPO).18–22 The latter functions as a reversible
capping agent of the active site on the growing polymer chain
and regulates the growth of the chain. The molar mass increase
is linear with conversion and polymers with narrow poly-
dispersities can be obtained. Furthermore, due to the ‘‘living
character’’ of CRP, block copolymers can be prepared by
sequential polymerization of different monomers. However,
the difficulty in the reversible dissociation of the polymer
chain-TEMPO adduct restricts the versatility of the method.
Typical monomers that are polymerized by TEMPO-mediated
CRP are styrene and its derivatives.23–25 More recently, this
technique has been successfully extended to alkyl methacry-
lates26 and other vinyl monomers, such as vinylpyridine.27
2-Fluoro-1-methylpyridinium toluene-4-sulfonate (FMPTS)
can be used in combination with TEMPO to provide faster
reaction rates than TEMPO alone and to reduce the self-
polymerization of styrene.28,29 The organic acid would decrease
the concentration of the nitroxide radicals, thus increasing the
number of active growing polymer chains to be involved in
the propagation step.
FT-IR (film on KBr, n in cm21): 3100–3050 (n C–H
¯
aromatic), 2980–2840 (n CH2), 1716 (n CLO), 1680–1450 (n
CLC aromatic), 1250–1040 (n C–O–C 1 n C–F), 710 (d C–H
aromatic), 648 (v CF2).1H-NMR (CDCl3, d in ppm from
TMS): 0.6–2.4 (15.0H, aliphatic), 2.5–2.6 (2.0H, CH2CF2), 3.8–
4.4 (4.0H, COOCH2 1 PhCH2O 1 CH2OH (unreacted)), 6.3–
7.2 (13.3H, styrenic), 7.4–8.0 (4.0H, benzoate).
Characterization. 1H-NMR and 19F-NMR spectra were
recorded with Varian Gemini 200 and Varian Gemini VXR
300 spectrometers, respectively. Either deuterated chloroform
or deuterated chloroform–1,1,2-trichlorotrifluoroethane (2 : 1
v/v) was used as a solvent. Infrared spectra were taken with a
Perkin Elmer 1600 Series FTIR spectrophotometer. Spectra
were recorded on KBr pellets or on polymer films cast from
solution on KBr plate.
Gel permeation chromatography (GPC) was carried out
using a Jasco PU-1580 liquid chromatograph equipped with
four PL gel 5 mm Mixed-C columns, a Jasco 830-RI refractive
index detector and a Perkin Elmer LC75 UV detector. Mole-
cular weights were quoted with respect to monodisperse
polystyrene standards (Mn ~ 500–120 000). Chloroform was
used as solvent and the GPC operated at 1 mL min21. 1 wt%
polymer solution volumes of 20 ml were used for GPC
measurements.
In this work, styrene (1) and 4-(1H,1H,2H,2H-perfluoro-
alkyl)oxymethylstyrenes (2)–(4) were selected for the pre-
paration of fluorinated polystyrene-based block copolymers
(Scheme 1). The fluorinated monomers were synthesized by a
phase-transfer catalyzed etherification of fluorinated alcohols
with 4-chloromethylstyrene in the presence of tetrabutyl-
ammonium hydrogensulfate (TBAH) as a phase transfer
catalyst, by following a modification of a literature procedure.6
Differential scanning calorimetry (DSC) measurements were
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Table 1 Characterization data of block polymers from CRP
b
Degree of polymerisation
Mn /kg mol21
Mw/Mn
x
y
z
a
a
b
Compn.FB (wt %)
Sample
Poly(1)
Poly1-block-poly2
Poly1-block-poly3
Poly1-block-poly4
Poly1-block-poly2-block-poly1
Poly1-block-poly3-block-poly1
Poly1-block-poly4-block-poly1
4
18
18
n.d.
110
65
1.10
1.30
1.28
n.d.
1.37
1.35
1.40
0
88
87
92
50
49
19
40
40
40
40
40
40
40
–
75
60
110
75
60
110
–
–
–
–
240
260
1270
140
aDetermined by GPC in chloroform, with PS calibration. bWeight composition in fluorinated block (FB) and degree of polymerization, deter-
mined by 1H-NMR.
In fact, it was necessary to preform the fluorinated alkoxide by
reaction with a 50% aqueous NaOH solution prior to nucleo-
philic substitution of 4-chloromethylstyrene. 1H,1H,2H,2H-
Perfluorohexanol and 1H,1H,2H,2H-perfluorooctanol were
kept under vigorous stirring at room temperature, whereas
1H,1H,2H,2H-perfluorodecanol was reacted at 100 uC for
two hours in order to achieve quantitative formation of
the corresponding alkoxide. The purification of the reaction
products was complicated by the presence of unreacted
4-chloromethylstyrene and distyryl by-products, which had
to be carefully separated from the monomers in order to
avoid cross-linking side-reaction during the polymerization.
Monomers (2)–(4) were used for the preparation of AB
diblock and ABA triblock copolymers composed of a poly-
styrene block (block A) and a fluorinated polystyrene block
(block B) (Scheme 1). This was achieved by using a ‘‘living’’
TEMPO-terminated polystyrene poly(1) as one macroinitiator
(Mn ~ 4 000, x ~ 40, and Mw/Mn ~ 1.10) to prepare diblock
copolymers poly1-block-poly2 to poly1-block-poly4 that in
turn were used as TEMPO-terminated ‘‘living’’ block copoly-
mers to polymerize styrene, thereby yielding the corresponding
triblock copolymers, poly1-block-poly2-block-poly1 to poly1-
block-poly4-block-poly1 (Scheme 1).
The polystyrene macroinitiator poly(1) was prepared in bulk
at 125 uC in the presence of the BPO–TEMPO–FMPTS system.
The polymerization of the fluorinated styrenes was performed
at 125 uC in either bulk (liquid monomers (2) and (3)) or in
diglyme solution (solid monomer (4)).
Block copolymers poly1-block-poly2 and poly1-block-poly3
showed an increased molar mass with respect to the macro-
initiator poly(1) and a relatively low polydispersity (Mw/Mn #
1.3) consistent with the occurrence of a CRP process (Table 1).
They also appeared free from residual unreacted macroinitiator
(see GPC trace in Fig. 1). Poly1-block-poly4 could not be
analyzed by GPC because of its limited solubility in common
Fig. 2 GPC traces of triblock copolymer poly1-block-poly4-block-
poly1 before (dashed line) and after (solid line) extraction with boiling
acetone.
organic solvents for analysis. The actual copolymer structure
was confirmed by 19F-NMR and 1H-NMR analyses. The
1H-NMR data were also used to evaluate the copolymer
composition and number average molar mass.
Relatively narrow polydispersities were also exhibited by
the triblock copolymers (Mw/Mn # 1.4). One typical GPC
curve is reported in Fig. 1. Minor amounts of ‘‘dead’’ poly-
styrene were selectively removed from triblock copolymers
by extraction with boiling acetone (Fig. 2). All these findings
proved that CRP was indeed successful in preparing different
architectures of block copolymers with controlled block length
ratios and narrow polydispersities of block lengths.
Homopolymers poly(2) to poly(4) were easily prepared by
conventional free radical initiation in the presence of AIBN as
a radical initiator at 60 uC to be used as model polymers in
terms of both molecular characteristics and bulk properties
(Table 2).
Synthesis by polymer modification
The anionic block copolymerization of styrene and isoprene
followed by polymer chemical modification was the chosen
method to prepare a second class of fluorinated side-chain
block copolymers with narrow molar mass distributions. The
overall synthetic procedure is outlined in Scheme 2. Polysty-
rene-block-polyisoprene PS-PI parent block copolymers were
Table 2 Characterization data of homopolymers from free radical
polymerization (AIBN)
a
a
Sample
Mn /kg mol21
Mw/Mn
Poly(2)
Poly(3)
Poly(4)
21
15
n.d.
2.5
2.5
n.d.
Fig. 1 GPC traces of the polystyrene macroinitiator, poly(1), and the
derived diblock and triblock copolymers, poly1-block-poly3 and poly1-
block-poly3-block-poly1, respectively.
aBy GPC in chloroform, with PS calibration.
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Table 3 Characterization data of block copolymers from polymer
modification
greater proportion of longer perfluorocarbon side-chain units.
In this copolymer, the higher molar mass GPC peak was
attributed to the formation of micelles in solution while
the lower molar mass peak was assigned to isolated block
copolymer macromolecules (Fig. 3). This was in agreement
with what is reported in the literature for polystyrene-block-
polyisoprene block copolymers having different fluoroalkyl
tails in the side chains.16
Following this route, the modification with fluorinated
moieties of the side chains of the isoprene block enabled the
creation of fluorinated AB block copolymers in which the block
length ratio and the narrow polydispersity of the starting
copolymer were retained.
a
c
Mn
kg mol21 Mn
Mw/ % Side chain Compn.FB
a
Sample
modificationb (wt%)
PS-PIOHa
PS-PIOHb
54
83
1.10 100
1.10 100
–
–
Poly1-block-poly5a 64
Poly1-block-6a n.d.
Poly1-block-poly6b 82
1.12
n.d.
1.12
96
94
95
64
69
54
aDetermined by GPC in THF, with PS calibration. bBy 1H-NMR.
cWeight composition in fluorinated block (FB), determined by 1H-
NMR.
prepared by anionic polymerization of styrene and isoprene
according to a well established procedure.17 Two copolymer
samples were prepared in which the polystyrene–polyisoprene
block length ratio was 40 000 : 10 000 in PS-PIa and 65 000 :
8 000 in PS-PIb. In each case the polydispersity was rather low
(Mw/Mn ~ 1.10). The 1,2 to 3,4 isoprene units molar ratio was
Thermal studies of fluorinated block copolymers
The phase transition behavior of the block copolymers was
investigated by DSC and the results are summarized in Table 4.
All the samples were first heated to a higher temperature than
the glass transition temperature (Tg), or the isotropization
temperature (Ti) when detected, in order to eliminate effects of
the thermal history on sample transitions. The DSC data
collected in the table were taken from the second heating scan.
Homopolymers poly(2) to poly(4) showed a glass transition
temperature that increased with increasing length of the
fluorocarbon group from 14 uC to 50 uC. The glass transition
was detected as a weak increase in the heat capacity which
might be explained by the low mobility of the fluorinated
segments that are assembled in an organized structure. Poly(4)
also underwent a first order transition at 67 uC because of an
intermediate phase transition between mesophases (see below).
In no case was isotropization of the mesophase detectable
by DSC. Optical microscopy observations revealed weakly
birefringent textures but no definite isotropization tempera-
ture.
The DSC curves of diblock copolymers poly1-block-poly2
to poly1-block-poly4 displayed the glass transition of the
fluorinated block as a weak increase in heat capacity (Tg ~ 17–
52 uC), but did not show the glass transition of the polystyrene
block, probably due to the low styrene content. Instead,
triblock copolymers poly1-block-poly2-block-poly1 to poly1-
block-poly4-block-poly1 exhibited only the glass transition of
the polystyrene block at 106 uC but not the glass transition
1
40 : 60 as determined by H-NMR.
PS-PI block copolymers were then modified, prior to attach-
ment of fluorinated side groups, by a hydroboration reaction
of the side-chain double bonds with 9-BBN.30 Hydrolysis with
H2O2–OH2 of the borane intermediate led to copolymers
PS-PIOH containing hydroxy terminated side chains. Finally,
the attachment of the semifluorinated side groups was carried
out by esterification with a fluorinated acid chloride 5 or 6
in anhydrous pyridine. The fluorinated acid chlorides were
synthesized from ethyl 4-hydroxybenzoate after a Mitsunobu
reaction31 with the fluorinated alcohol of choice (Scheme 2).
In each modification step the extent of substitution of the
isoprene side chain was practically quantitative (Table 3). In
particular, the FT-IR spectra of the final modified block
copolymers showed that the hydroxy absorption of PS-PIOH
in the 3500 cm21 region had completely vanished with the
concurrent appearance of very prominent ester group absor-
bances at 1716 cm21 and 1195 cm21. The attachment of the
fluorinated side substituent was confirmed by 19F-NMR and
1H-NMR analyses, which also permitted evaluation of the
block copolymer composition (Table 3).
Three modified block copolymers were prepared bearing
two different fluorinated alkyl tails, poly1-block-poly5a with a
perfluorooctyl tail and poly1-block-poly6a-b with a perfluoro-
decyl tail respectively. Poly1-block-poly6a and poly1-block-
poly6b possessed different block length ratios (Table 3).
The GPC traces of the hydroxylated block copolymer
PS-PIOHa and the respective fluorinated block copolymers
poly1-block-poly5a and poly1-block-poly6a are reported in
Fig. 3. Solution aggregation of the fluorinated blocks was
observed for the poly1-block-poly6a sample that contained a
Table 4 Thermal (by DSC) and structural (by WAXD) data of
fluorinated polymers
a
b
Tg
uC
/
Tg
uC
/
Tia
uC
/
DHia / dc
J g21 (¡0.3 A) (¡1 A)
Ld
˚
˚
Sample
Poly(1)
Poly(2)
Poly(3)
Poly(4)
Poly1-block-poly2
Poly1-block-poly3
Poly1-block-poly4
Poly1-block-poly2-
block-poly1
Poly1-block-poly3-
block-poly1
Poly1-block-poly4-
block-poly1
–
85
–
–
–
22
26
–
13
16
18
13
16
18
13
14
17
50
17
17
52
–
–
–
n.d.
n.d.
n.d.
n.d. n.d.
n.d. n.d.
n.d.e n.d.e 35
n.d. n.d.
n.d. n.d.
22
26
n.d.f n.d.f 34
n.d. 106
n.d. n.d.
n.d. n.d.
n.d. n.d.
21
25
33
n.d. 106
n.d. 106
16
18
Poly1-block-poly5a 232
Poly1-block-poly6a 220
Poly1-block-poly6b 220
101
100
100
207
210
210
1.1
0.4
0.4
34
40
40
21
23
23
aGlass transition temperature and isotropization temperature (and
enthalpy) of the fluorinated block. bGlass transition temperature of
the polystyrene block. cMeasured smectic interlayer spacing. dCalcu-
lated length of the polymer side chain. eTransition between meso-
phases at 67 uC (DH ~ 1.5 J g21). Transition between mesophases
Fig. 3 GPC traces of hydroxylated block copolymer PS-PIOHa and
derived diblock copolymers poly1-block-poly5a and poly1-block-
poly6a.
f
at 68 uC (DH ~ 0.7 J g21).
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Fig. 4 DSC traces of homopolymer poly(1), derived diblock copolymer
poly1-block-poly4 and homopolymer poly(4) (10 uC min21).
Fig. 6 WAXD patterns at 25 uC of homopolymers poly(3) (top) and
poly(4) (bottom).
Fig. 5 DSC traces of diblock copolymer poly1-block-poly5a
(10 uC min21).
calculated by molecular models in the extended planar confor-
mation with the fluorinated tail in a twisted zig-zag (helical)
33
conformation of span p ~ 2.59 A are listed in Table 4. A
˚
of the fluorinated block due to the high molar content of
styrene. Poly1-block-poly4 and poly1-block-poly4-block-poly1
also formed mesophases quite similar to the respective
homopolymer poly(4). Representative DSC traces are shown
in Fig. 4.
The side-chain modified block copolymers poly1-block-
poly5, poly1-block-poly6a-b were found to have a glass
transition due to the polystyrene block at about 100 uC
(detected on heating) and a glass transition for the fluorinated
block in the range 230 to 220 uC (detected on cooling) as is
shown in Fig. 5. Furthermore, they gave rise to a broad
mesophase up to a Ti of 210 uC.
These observations strongly suggested that the chemically
incompatible polymer blocks were microphase separated in
each block copolymer architecture, whether AB diblock or
ABA triblock. The mesophase order of the fluorinated side
groups was not disrupted in the separated domains. These
findings were confirmed by a detailed investigation of the
morphology of the block copolymers by small angle X-ray
scattering (SAXS) and transmission electron microscopy
(TEM). This topic will be the subject of a forthcoming paper.
comparison between d and L suggested that the side groups
were organized in a double layer structure in each polymer.
Homopolymers poly(2) through poly(4) were taken as simple
model polymers of the fluorinated block in the corresponding
block copolymers. The X-ray diffraction patterns at room
temperature of poly(3) and poly(4) are shown in Fig. 6. Poly(2)
and poly(3) containing shorter fluorocarbon tails, (CF2)4 and
(CF2)6 respectively, formed a disordered smectic mesophase
(Fig. 6 top). On the other hand, poly(4) containing a longer
(CF2)8 tail gave rise to an ordered (pseudo-hexagonal) smectic
mesophase (Fig. 6 bottom). For this last homopolymer the
detection of up to 4 orders of sharp low-angle reflections
evidenced the existence of a long range correlation between the
smectic layers, which modulated the electronic density along
the layer normal.32
The diblock and triblock copolymers displayed X-ray
diffraction patterns similar to their corresponding homopoly-
mers, as illustrated in Fig. 7. No significant changes of the
smectic layer spacing were detected in the copolymers (d ~
˚
˚
34 A for poly1-block-poly4, d ~ 33 A for poly1-block-poly4-
block-poly1), which also confirmed that the block copolymers
were phase separated and gave rise to the same kind of smectic
mesophase as the respective homopolymers. In the latter block
copolymer, a diffuse halo was also detected at intermediate
diffraction angles originating from the amorphous polystyrene
X-Ray studies of fluorinated block copolymers
Wide-angle X-ray diffraction measurements were carried out
at room temperature on film specimens aimed at gaining
further information on the bulk microstructure of the
synthesized polymers.
˚
block (periodicity of 10–11 A) (Fig. 7 bottom).
Representative X-ray patterns of block copolymers poly1-
block-poly5a and poly1-block-poly6a are shown in Fig. 8. The
˚
former presented a disordered smectic mesophase (d ~ 34 A),
while the latter formed a pseudo-hexagonal smectic mesophase
˚
(d ~ 40 A). Therefore, the lengthening of the fluorinated
tail from (CF2)6 to (CF2)8 segments enhanced the order of
the mesophase by favoring in-plane correlations of the side
chains.
All the polymers exhibited one wide-angle reflection (per-
˚
iodicity of about 5 A) and one or more low-angle reflections
pointing to the existence of a smectic mesophase.32 This was
formed by the assembly of the side groups in a two-dimensional
structure probably driven by the separation at the molecular
level of their different chemical constituents. The smectic layer
spacings (d) measured and the lengths (L) of the side groups
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Fig. 9 Advanced (filled symbols) and receded (open symbols) contact
angles (¡3u) of films from block copolymers poly1-block-poly2 (r),
poly1-block-poly3 (+), and poly1-block-poly4 (&) as a function of
immersion time in water.
angles with water as the wetting liquid were carried out as
a function of the immersion time in water. The films were
prepared as single-layer films by casting from dilute (2–5 wt%)
polymer solutions onto glass slides. Double-layer films of
copolymers poly1-block-poly5a and poly1-block-poly6a were
prepared by casting a top-layer of fluorinated block copolymer
onto a SEBS film bottom-layer. This was meant to check for
the possibility of achieving surface performance comparable
to pure block copolymer films, by using polymer films with a
smaller amount of fluorinated material.
Dry films from block copolymers poly1-block-poly2 to
poly1-block-poly4 showed values of the advanced contact
angle in the range 94–111u that depended on the length of the
fluorinated side-chain tail (Fig. 9). Introduction of the longer
tails resulted in higher contact angles. This was indicative of
a relatively high hydrophobicity of the polymer film surfaces.
In all cases the receded contact angles were lower by 5–6u,
which also pointed to the formation of quite stable fluorinated
surfaces. Both advanced and receded contact angles remained
essentially unaffected over a 10 day exposure to water.
Single-layer dry films of poly1-block-poly5a and poly1-block-
poly6a showed large advanced contact angles of about 120u
and 130u, respectively (Fig. 10). These values were about 6–10u
greater than the receded angles. Thus, the contact angles for
the latter class of block copolymers were significantly greater
than those measured for the former class, which might be due
to a higher order at the film surface in the latter polymers. Both
advanced and receded contact angles were quite constant over
a 25 day exposure to water, consistent with the presence of
non-reconstructing hydrophobic surfaces.
Fig. 7 WAXD patterns at 25 uC of diblock copolymer poly1-block-
poly4 (top) and triblock poly1-block-poly4-block-poly1 copolymer
(bottom).
Double-layer dry films also showed rather high advanced
contact angles of about 119u and 123u for poly1-block-poly5a
Fig. 8 WAXD patterns at 25 uC of diblock copolymers poly1-block-
poly5a (top) and poly1-block-poly6a (bottom).
Surface properties of block copolymer films
Fig. 10 Advanced (filled symbols) and receded (open symbols) contact
angles (¡3u) of single-layer films from block copolymers poly1-block-
poly5a ($), poly1-block-poly6a (&) as a function of immersion time
in water.
In order to assess the wettability of the surface of the polymer
films and their stability towards reconstruction upon immer-
sion in water, measurements of advanced and receded contact
J. Mater. Chem., 2002, 12, 1684–1692
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Acknowledgement
The authors thank the Italian Ministry of the Education,
University and Research (PRIN 2001034479) and the Office of
Naval Research and the National Science Foundation,
Division of Materials Research, for partial support of this
work. The Cornell Center for Materials Research and the
Cornell High Energy Synchrotron Source (CHESS) are also
gratefully acknowledged for the use of their facilities.
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Following our strategy for engineering polymers as suitable
materials for hydrophobic non-stick coatings, we prepared two
classes of new fluorinated polystyrene-based block copolymers
with low polydispersity by controlled radical polymerization
and polymer modification. The TEMPO-mediated CRP turned
out to be a rather simple and straightforward preparation
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side chains. On the contrary, the polymer modification scheme
involved a greater number of more complex synthetic steps, the
anionic polymerization for the preparation of the parent
copolymers to be modified was the initial stage.
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nature of the fluorinated side chains and the phase separation
at the molecular level of the different incompatible aromatic-
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