Effect of -C(CF3)2- on the surface energy of main-chain liquid crystalline and crystalline polymers

Article abstract of DOI:10.1021/jp003103c

Novel fluorinated main-chain liquid crystalline/crystalline polymers were prepared through thin film polymerization. Two polymer systems were studied: one from 2,6-acetoxynaphthoic acid (ANA), acetoxyacetanilide (AAA), and 4,4a?2-(hexafluoroisopropylidene)bis(benzoic acid) (6F acid) and the other from ANA, hydroquinone diacetate (HQAT), and 6F acid. The surface energy was estimated using contact angles of water, glycerol, and diiodomethane. A small amount of -C(CF₃)₂- in the main chain lowered the surface energy, and the fluorocarbons were preferentially enriched at the air-polymer interface, causing low surface energy and large water contact angles. These results agreed with XPS data. Since the -O-Ar-O- unit in the HQAT moiety is more rigid than the -O-Ar-N- unit in the AAA moiety, LC texture formed more easily in the ANA/HQAT/6F acid system than in the ANA/AAA/6F acid system. Contrarily, the fluorocarbons enriched more preferentially at the surface in the ANA/AAA/6F acid system than in the ANA/HQAT/6F acid system. Moreover, the hydrogen bonding originating from the amide group hindered further decrease of surface energy with an increase in 6F acid content in the ANA/AAA/6F acid system.

Full text of DOI:10.1021/jp003103c

J. Phys. Chem. B 2001, 105, 4145-4150
4145
Effect of -C(CF3)2- on the Surface Energy of Main-Chain Liquid Crystalline and
Crystalline Polymers
Kui-Xiang Ma and Tai-Shung Chung*
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, and Department of
Chemical & EnVironmental Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent,
Singapore 119260
ReceiVed: August 28, 2000; In Final Form: December 12, 2000
Novel fluorinated main-chain liquid crystalline/crystalline polymers were prepared through thin film
polymerization. Two polymer systems were studied: one from 2,6-acetoxynaphthoic acid (ANA), acetoxy-
acetanilide (AAA), and 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) (6F acid) and the other from ANA,
hydroquinone diacetate (HQAT), and 6F acid. The surface energy was estimated using contact angles of
water, glycerol, and diiodomethane. A small amount of -C(CF3)2- in the main chain lowered the surface
energy, and the fluorocarbons were preferentially enriched at the air-polymer interface, causing low surface
energy and large water contact angles. These results agreed with XPS data. Since the -O-Ar-O- unit in
the HQAT moiety is more rigid than the -O-Ar-N- unit in the AAA moiety, LC texture formed more
easily in the ANA/HQAT/6F acid system than in the ANA/AAA/6F acid system. Contrarily, the fluorocarbons
enriched more preferentially at the surface in the ANA/AAA/6F acid system than in the ANA/HQAT/6F acid
system. Moreover, the hydrogen bonding originating from the amide group hindered further decrease of surface
energy with an increase in 6F acid content in the ANA/AAA/6F acid system.
1. Introduction
demonstrated that the fluorinated side chains had a profound
influence on the composition-depth profiles. The longer the
fluorinated side chains, the thicker the fluorine-rich layer in the
surface.
Fluorine’s small size, large electronegativity, low polariz-
ability, and large fluorine-fluorine repulsion lead to many
interesting properties of fluoro polymers.1 The low intermo-
lecular forces present in fluorinated polymers have been
recognized to account for the relatively low surface energy. As
a result, fluorine-containing polymers are generally difficult to
wet with liquids; moreover, these surfaces tend to have
nonadhesive character and low coefficients of friction.2 Due to
the unique nonwetting property, fluorinated polymers have been
used for imparting oil- and water-resistance applications.3
It has been reported and accepted that the low surface energy
of a fluoro-containing polymer is caused by segregation of the
fluorinated segment to the surface, which creates a hydrophobic
surface. Grinten et al.4 investigated a typical UV-curable resin
mixed with fluorine-ended monomers through photopolymeri-
zation and observed surface segregation. The fluoro monomer
was preferentially enriched at the air interface when the curable
mixture was coated on a polar substrate, whereas the monomer
was found to preferentially reside at both interfaces when the
mixture was coated on an apolar substrate.
Chapman et al. synthesized and characterized fluorinated
poly(amide urethane) block copolymers, which were fluorinated
on both side chain and main chain.5-8 Their studies demon-
strated that poly(amide urethanes) with tertiary polyamide soft
blocks containing fluorinated side chains could phase-segregate
giving a surface of exposed fluorocarbons, shown by the low
critical surface tension. The existence of fluorine in the main
chain appeared not to have significant influence on the interfacial
properties if there were fluorinated side chains. Their results
Kim et al. synthesized urethane graft copolymers with both
perfluoroalkyl- and silicone-containing side chains and inves-
tigated their surface properties.9,10 The lowest surface energies
were found to be 9-12 mJ/m2, which was dependent strongly
on perfluoroalkyl content but not on trimethylsilyloxy content.
Together with XPS analysis, their results confirmed that
perfluoroalkyl group was more preferentially enriched to the
outermost layer than the silicone-containing group.
Ober et al. studied the surface properties of a series of block
polymers synthesized from tetrahydropyranyl methacrylate and
fluorinated methacrylates.11-13 They found that the length of
fluorocarbon units in the side chain gave different liquid
crystalline phases, which affected their critical surface tension
and surface stability. The semifluorinated liquid crystal polymer
with smectic B packing had the most stable surface.13
A thin film polymerization technique has recently been
developed for LCP studies.14,15 This technique has some
advantages over the bulk synthesis: for example, it consumes
extremely small amounts of monomers, and the evolution of
morphological changes and liquid crystal formation during the
entire polycondensation reaction can be observed. Comparing
the thin film polymer of 73/23 (mole ratio) p-acetoxybenzoic
acid (ABA)/2,6-acetoxynaphthoic acid (ANA) to the commercial
Vectra A-950 LCP (Hoechst Celanese), we found that their
reaction kinetics followed the same second order and the
activation energies of the polycondensation reaction for both
systems were quite similar.16 In addition, the FTIR spectrum,
the total surface energy, and its Lewis acid and base components
of the thin film copolymer were identical to those of the
* To whom correspondence should be addressed. Fax: 65-8748388.
E-mail: chencts@nus.edu.sg.
10.1021/jp003103c CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/13/2001

4146 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Ma and Chung
TABLE 1: Surface Tension Parameters (in mJ/m2) of
commercial LCP.17 These results indicated that the polymeric
film synthesized by the thin film polymerization technique is
suitable for surface energy and contact angle measurements, as
polymers rather than oligomers obtained after the thin film
polymerization.
Testing Liquids
parameter
water
glycerol
diiodomethane
γ+
γ-
γAB
γLW
γ
25.5
25.5
51.0
21.8
72.8
3.92
57.4
30.0
34.0
64.0
0.0
0.0
0.0
50.8
50.8
The purpose of the present study was to investigate the effect
of -C(CF3)2- on the surface energy of main-chain liquid
crystalline (LCPs) and crystalline polymers. LCPs were chosen
because of their unique anisotropic mechanical properties,
excellent chemical resistance, and high thermal stabilities.18-22
The incorporation of fluorine atoms into thermotropic LCPs may
alter their physical properties for other medical applications.23
Two series of aromatic thermotropic LCPs were prepared: one
from 2,6-acetoxynaphthoic acid (ANA), acetoxyacetanilide
(AAA), and 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)
(6F acid) and the other from ANA, hydroquinone diacetate
(HQAT), and 6F acid. A thin film polymerization technique
was utilized to prepare copolymers samples. The formation of
liquid crystalline/crystalline was examined under a polarized
microscope (PLM). Surface properties of the resulting copoly-
mers were investigated with the aid of a contact angle goni-
ometer and X-ray photoelectron spectroscopy (XPS).
γp
γd
γ
51.0
21.8
72.8
26.4
37.0
63.4
2.3
48.5
50.8
analysis was carried out with a TA Instruments differential
scanning calorimetry (DSC) 2920. For DSC, samples were
heated from 50 to 390 °C at a rate of 10 °C/min. For modulated
DSC (MDSC), samples were heated from 50 to 390 °C at a
rate of 2 °C/min and modulated at ( 1 °C every 30 s. All the
experiments were run under nitrogen atmosphere. Inherent
viscosity (IV) of the thin film copolymers was tested by capillary
rheometer in 3,5-bis(trifluoromethyl)phenol (BTMP) solution
(0.1 g/dL).
Contact angles were determined using a Rame´-Hart contact
angle goniometer (model 100-22) at room temperature. Liquid
droplets were laid by a Gilmont microsyringe onto the surfaces
of the thin film copolymers. Advancing and receding contact
angles were measured with a tilting base. Surface energies were
calculated using both the geometric mean and the Lifshitz-
van der Waals-acid-base (LWAB) method. Each data point
is the average of 20 measurements. Deionized water, glycerol,
and diiodomethane were chosen as the testing liquids and their
surface energies are given in Table 1.
2. Experimental Section
2.1. Monomers. Monomer ANA was prepared by acetylation
of 2,6-hydroxynaphthoic acid (HNA) with acetic anhydride in
refluxing toluene in the presence of pyridine, followed by
recrystallization of ANA in methanol. AAA was obtained by
acetylation of p-aminophenol with acetic anhydride in NaOH
solution, followed by recrystallization in a mixture of methanol
and hexane with a volume ratio of 50:50. NMR confirmed the
success of acetylation. Monomers HQAT and 6F acid were
purchased from Aldrich and used as received. The melting points
of ANA, AAA, HQAT, and 6F acid were 226, 157, 121, and
272 °C, respectively.
X-ray photoelectron spectroscopy (XPS) characterization was
done with a V. G. Scientific ESCALAB 220I-XL spectrometer
with monochromated Mg KR X-ray. The XPS data were
acquired ata takeoff angle of 90° unless stated otherwise.
3. Surface Energy Analysis
2.2. Thin Film Polymerization. Monomer mixture with a
certain mole ratio was placed on a microglass slide, followed
by depositing several drops of acetone to dissolve monomers,
to yield a thin layer of reactant mixture attached to the
microglass slide after acetone evaporation. The microglass slide
containing monomers was sandwiched by another glass slide
and spaced about 0.5 mm by aluminum foil. The monomers
were attached to the top slide. The spacer provides the channel
for the removal or release of acetic acid during polymerization.
The whole set was then wrapped by aluminum foil with the
glass slide containing the monomer facing down and heated on
a digital hot plate at 300 °C (1% for 3 h. After reaction, the
whole package was removed from the hot plate and cooled under
ambient temperature (∼25 °C). The resulting thin film copoly-
mer on the top microglass slide was used for contact angle
measurements.
Two approaches are employed to calculate the surface energy
of a solid material: the geometric mean method24,25 and the
three-liquid method.29-31 These two methods are the most
widely used for surface energy analyses even though they have
received many critiques.26-28,32-34
The geometric mean method estimates surface energy of a
solid (γS) as follows:
γL(1 + cos θ) ) 2(γdSγLd)1/2 + 2(γSpγpL)1/2
(1)
d
p
where γL, γL , and γL are surface energies of the test liquid
and γSd and γSp can be calculated from measured contact angles
(θ). The superscript d refers to the London dispersion forces
and p refers to polar forces, including all the interactions
established between the solid and liquid, such as Keesom
dipole-dipole, Debye dipole-induced dipole, and hydrogen
bonding, etc.
To examine the optical morphology of the entire polycon-
densation reaction, the above process was also conducted in
situ under a hot staged polarizing light microscope, PLM
(Olympus BX50, Tokyo, Japan), and the details of experiments
have been published elsewhere.14
The three-liquid approach expresses the total surface energy
as the sum of the Lifshitz-van der Waals (γLW) and acid-
base (γAB) components:
2.3. Polymer Characterizations. The thin film copolymers
with different monomer compositions were scratched from glass
slides and characterized without any further treatment. Infrared
measurements were conducted on a Perkin Elmer FTIR spec-
trometer spectrum 2000, where samples were mixed with KBr
for IR spectrum. A Perkin Elmer thermogravimetric analyzer
(TGA 7) was used to investigate the thermal stability of the
thin film copolymers, at a scanning rate of 20 °C/min. Thermal
+
-
γ ) γLW + γAB ) γLW + 2 γ γ
(2)
x
where γ+ is the Lewis acid (electron acceptor) component and
γ- is the Lewis base (electron donor) component of surface
energy. The LW component includes London dispersion forces,
Keesom dipole-dipole, and Debye induction forces. Therefore,
the total surface energy and its components, γLW, γ+ , γ- and

Effect of -C(CF3)2- on Surface Energy
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4147
Figure 1. Monomer formulas.
Figure 3. TGA results of thin film copolymers (after 3-h reaction at
300 °C).
Figure 4. Surface energy comparisons between geometric mean
method and LWAB methods.
Figure 2. FTIR spectra of monomers and thin film copolymers for
80/10/10 ANA/AAA/6F acid system: (a) monomers and (b) thin film
copolymer after 3-h reaction at 300 °C.
TABLE 2: Contact Angles of ANA/AAA/6F Acid Thin Film
Copolymers (reacted at 300 °C for 3 h)
γ
AB, can be determined from the following equation if three
monomer
testing liquids are used:
ratios
contact angles
water
glycerol diiodomethane
90/5/5
advancing angle 93.3 ( 1.6 84.8 ( 2.0
receding angle 74.3 ( 2.2 56.7 ( 1.7
44.9 ( 1.3
24.0 ( 1.2
50.7 ( 1.8
30.6 ( 1.8
50.9 ( 2.0
31.1 ( 2.3
53.1 ( 2.1
30.6 ( 1.7
γ (1 + cos θ) ) 2( γLWγLLW
+
γ+γ- + γ-γ+) (3)
x
x
x
L
S
S
L
S
L
80/10/10 advancing angle 92.5 ( 1.6 87.4 ( 2.0
receding angle 73.7 ( 1.5 59.0 ( 1.8
4. Results and Discussion
60/20/20 advancing angle 91.9 ( 1.7 89.7 ( 2.0
receding angle 76.6 ( 2.0 70.4 ( 2.0
4.1. Thin Film Copolymerization with Monomer Compo-
sitions of ANA/AAA/6F Acid and ANA/HQAT/6F Acid. Two
series of aromatic thermotropic LCPs were prepared: one from
ANA, AAA, and 6F acid and the other from ANA, HQAT,
and 6F acid. Four monomer molar compositions were chosen:
90/5/5, 80/10/10, 60/20/20, and 40/30/30. Figure 1 depicts these
monomer structures.
ANA and AAA are two of the most commonly used
monomers for synthesizing main-chain thermotropic LCPs. For
example, two commercially available LCP products, Vectra A
and B, produced by Hoechst Celanese, are made from 27/73
ANA/ABA (p-acetoxybenzoic acid) and 60/20/20 ANA/AAA/
TA (terephthalic acid), respectively. ANA has a side-step
(crankshaft) naphthoic ring and a large L/D chain characteristic
which make ANA potentially acting as a mesogenic unit.
Monomers containing hydroquinone yielded the first thermo-
tropic LCP because of its straightness and rigidity35 and have
been used extensively for new LCPs.21,22,36 In this study, AAA
and HQAT also provide equal moles of acetoxy groups to react
with carboxyl groups in 6F acid. Besides ester groups being
formed in the resultant copolymers, AAA moiety also creates
amide-bridging groups.
40/30/30 advancing angle 90.9 ( 1.6 84.5 ( 1.6
receding angle 76.1 ( 1.7 61.7 ( 1.8
is νCdO of the CH3COO- group. After 3-h reaction at 300 °C,
the bands at 1679 and 1761 cm-1 almost completely disappear,
indicating that nearly all acetoxy and carboxyl groups have been
consumed and a substantial number of ester groups form at band
of 1735 cm-1 as shown in Figure 2b. These spectra confirm
that copolymers are obtained after thin film polymerization.
TGA was used to characterize the thermostability of the thin
film copolymers. Figure 3 shows TGA curves in air of ANA/
AAA/6F acid and ANA/HQAT/6F acid thin film copolymers,
both with an 80/10/10 monomer mole ratio. From the curves,
the 5% weight loss for the ANA/AAA/6F acid and ANA/HQAT/
6F acid copolymers are 509 and 490 °C, respectively. These
high temperatures further confirm that the thin film polymeri-
zation products are copolymers, without any oligomers remain-
ing. Both DSC and MDSC were used to analyze the thin film
copolymers. However, no obvious transition peaks were found.
The inherent viscosity of ANA/HQAT/6F acid copolymers
with an 80/10/10 monomer ratio was tested in 3,5-bis(trifluo-
romethyl)phenol at 25 °C. The IV value is 2.5, indicating that
the thin film product is indeed a copolymer. The IV value is
also acceptable in terms of LCP synthesis,21,37 and even higher
values could be obtained if increasing reaction temperature or
prolonging reaction time.15
4.2. Characterizations of Thin Film Copolymers. Figure
2 shows typical FTIR spectra of the monomer mixtures and
thin film copolymers, using 80/10/10 ANA/AAA/6F acid
reaction system as an example. In Figure 2a, the band at 1679
cm-1 is νCdO of the -COOH group and the band at 1761 cm-1

4148 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Ma and Chung
TABLE 3: Contact Angles of ANA/HQAT/6F Acid Thin
TABLE 6: Surface Energies of ANA/HQAT/6F Acid Thin
Film Copolymers Using the Two-Liquid Geometric Mean
Method (reacted at 300 °C for 3 h)
Film Copolymers (reacted at 300 °C for 3 h)
monomer
γd
γp
γ
ratios
contact angles
water
glycerol diiodomethane
(mJ/m2) (mJ/m2) (mJ/m2)
90/5/5
advancing angle 89.1 ( 1.2 78.9 ( 2.0
receding angle 72.4 ( 2.3 55.6 ( 1.9
47.2 ( 1.7
29.3 ( 2.5
53.5 ( 2.1
32.3 ( 2.4
58.3 ( 1.8
36.7 ( 2.0
60.6 ( 2.3
39.0 ( 2.0
90/5/5
water-diiodomethane
glycerol-diiodomethane
average
33.95
37.14
35.55
29.63
33.14
31.38
28.06
29.51
28.78
27.00
29.16
28.08
1.87
0.00
0.94
2.92
0.00
1.46
1.51
0.00
0.76
1.21
0.00
0.60
35.82
37.14
36.48
32.55
33.14
32.55
29.57
29.51
29.54
28.21
29.16
28.69
80/10/10 advancing angle 88.1 ( 1.4 84.9 ( 1.9
receding angle 76.5 ( 1.7 63.3 ( 1.7
80/10/10 water-diiodomethane
glycerol-diiodomethane
average
60/20/20 water-diiodomethane
glycerol-diiodomethane
average
40/30/30 water-diiodomethane
glycerol-diiodomethane
average
60/20/20 advancing angle 94.6 ( 2.1 89.5 ( 2.0
receding angle 79.0 ( 1.6 68.9 ( 2.3
40/30/30 advancing angle 96.8 ( 1.9 88.4 ( 2.0
receding angle 82.8 ( 2.1 67.1 ( 2.3
TABLE 4: Surface Energies of ANA/AAA/6F Acid Thin
Film Copolymers Using the Two-Liquid Geometric Mean
Method (reacted at 300 °C for 3 h)
γd
γp
γ
(mJ/m2) (mJ/m2) (mJ/m2)
acid content from 5 to 30 mol % in the ANA/AAA/6F acid
system, the advancing angle of diiodomethane increases from
45° to 53°, whereas the advancing angles of water decrease very
slightly. Similar trends are found for diiodomethane in the ANA/
HQAT/6F acid system as tabulated in Table 3. However,
increases in the advancing angles of water and glycerol are
noticed for the ANA/HQAT/6F acid copolymers. Both copoly-
mers show nonpolar surfaces because of their large water contact
angles. The increasing contact angles of both water and
diiodomethane with increasing 6F acid content in the ANA/
HQAT/6F acid system imply that the copolymer surfaces have
been fluorinated. Similar results were reported for other
copolymers by Kim10 and Chapman.5
Tables 4 and 5 summarize the surface energy results for the
ANA/AAA/6F acid system using the two-liquid geometric mean
method and three-liquid Lifshitz-van der Waals-acid-base
(LWAB) method, respectively, while Tables 6 and 7 list results
for the ANA/HQAT/6F acid system. Both methods yield similar
values and trends on the overall surface energy.
For the ANA/AAA/6F acid system, with an increase in 6F
acid content from 5 to 10% (mol), the total surface energy
decreases abruptly from 37 to 34 mJ/m2. Further increase in 6F
acid content from 10 to 30 mol % results in little change in the
surface energy, which is maintained at about 33 mJ/m2. For
the ANA/HQAT/6F acid system, the situation changes. Its total
surface energy decreases continuously with an increase in the
content of 6F acid monomer. Figure 3 compares the surface
energies calculated from both the two-liquid geometric mean
method and the three-liquid LWAB method for both ANA/
AAA/6F acid and ANA/HQAT/6F acid systems. The agreement
of surface energy calculated from these two methods is excellent
with about 1% difference.
90/5/5
water-diiodomethane
glycerol-diiodomethane
average
36.58
35.97
36.28
32.60
32.96
32.78
32.33
31.81
32.07
30.63
33.48
32.05
0.72
0.00
0.36
1.31
0.00
0.65
1.47
0.00
0.73
1.96
0.00
0.98
37.30
35.97
36.63
33.90
32.96
33.43
33.79
31.81
32.80
32.59
33.48
33.04
80/10/10 water-diiodomethane
glycerol-diiodomethane
average
60/20/20 water-diiodomethane
glycerol-diiodomethane
average
40/30/30 water-diiodomethane
glycerol-diiodomethane
aVerage
Investigation based on a polarized microscope indicates that
60/20/20 and 40/30/30 ANA/AAA/6F acid and 40/30/30 ANA/
HQAT/6F acid copolymers are amorphous, while 90/5/5 and
80/10/10 ANA/AAA/6F acid and 80/10/10 and 60/20/20 ANA/
HQAT/6F acid undergo morphological evolution during reaction
as follows: generation of liquid crystal phase, coalescence of
liquid crystal domains, formation of schlieren texture, annihila-
tion of disclinations, and appearance of crystalline texture. Only
90/5/5 ANA/HQAT/6F acid yields a polymer with the liquid
crystalline texture. The above observation implies that -O-
Ar-O- units in the HQAT moiety have greater possibility to
form LC than that -O-Ar-N- units in the AAA moiety. This
was coincided with He and Windle’s prediction.38 Comparing
the rotation potential of the amide with that of the ester, they
found the potential barrier between the trans and cis barriers
was higher for the amide than for the ester. However, the energy
difference between trans and cis conformations is more
important in influencing the persistence length (or mesogenicity)
and it was found larger in the ester group than in the amide
group. In other words, the ester group in polyester will have a
longer persistence length that is favorable to LC formation.
4.3. Contact Angles and Surface Energies of the Thin Film
Copolymers. Tables 2 and 3 summarize all the advancing and
receding contact angles as well as their standard deviations of
the thin film copolymers for both systems as a function of
monomer ratio, respectively. Since the advancing contact angle
represents more precisely the equilibrium contact angle than
the receding angle, only the advancing contact angle will be
discussed hereafter. Table 2 indicates that at an increase in 6F
Negative square root values of the Lewis acid components
(γ+) of surface energies are found in this study. According to
Good and van Oss,31 these negative values could be assumed
to be zero when they are small, which leads to a vanishing value
of the donor-acceptor component. As a result, the acid-base
interactions, γAB, for these two polymer systems are trivial and
the total surface energies are mainly contributed from γLW
.
Because of containing a large amount of aromatic rings and
>CdO groups in polymeric chains, both ANA/AAA/6F acid
TABLE 5: Surface Energies of ANA/AAA/6F Acid Thin Film Copolymers Using the Three-Liquid LWAB Method (reacted at
300 °C for 3 h)
γ+ (mJ/m2)
γ- (mJ/m2)
γ
AB (mJ/m2)
γ
LW (mJ/m2)
γ (mJ/m2)
90/5/5
water-glycerol-diiodomethane
water-glycerol-diiodomethane
water-glycerol-diiodomethane
water-glycerol-diiodomethane
0.00
0.00
0.00
0.00
2.80
4.58
6.12
4.59
0.00
0.00
0.00
0.00
37.09
33.90
33.79
32.56
37.09
33.90
33.79
32.56
80/10/10
60/20/20
40/30/30

Effect of -C(CF3)2- on Surface Energy
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4149
TABLE 7: Surface Energies of ANA/HQAT/6F Acid Thin Film Copolymers Using the Three-Liquid LWAB Method (reacted at
300 °C for 3 h)
γ
+ (mJ/m2)
γ- (mJ/m2)
γ
AB (mJ/m2)
γ
LW (mJ/m2)
γ (mJ/m2)
90/5/5
water-glycerol-diiodomethane
water-glycerol-diiodomethane
water-glycerol-diiodomethane
water-glycerol-diiodomethane
0.00
0.00
0.00
0.00
3.39
7.09
4.29
2.62
0.00
0.00
0.00
0.00
35.82
32.29
29.56
28.21
35.82
32.29
29.56
28.21
80/10/10
60/20/20
40/30/30
TABLE 8: XPS Atomic Concentration of the Thin Film
Copolymers
systems, as shown in Tables 4 and 5, may be explainable with
the aid of XPS data. The difference may mainly arise from two
factors: different chain rigidities between AAA and HQAT
moieties and hydrogen bonding induced by the amide group
because of the AAA moiety. Since the rigidity of -O-Ar-
N- units in AAA is lower than that of -O-Ar-O- units in
HQAT, the pendant CF3 segments in the former system has a
greater possibility to be able to preferential reside at the air-
polymer interface than the latter system. Therefore, the atomic
concentrations of F element measured by XPS are greater in
the ANA/AAA/6F acid system than that in the ANA/HQAT/
6F acid system, as shown in Table 8. It is very interesting to
point out that the atomic concentrations of F element in ANA/
HQAT/6F acid copolymers are close to their bulk values,
especially for 60/20/20 and 40/30/30 monomer ratios, implying
the embedding of -C(CF3)2- groups in the bulk possibly caused
by the high rigidity of -O-Ar-O- units in HQAT.
experimental XPS (AT%)
theoretical values
monomer
ratios
C
O
N
F
C
O
N
F
ANA/AAA/6F Acid
79.00 14.06 0.57 6.37 82.77 14.61 0.37 2.25
90/5/5
80/10/10 72.28 13.90 2.00 11.84 81.02 13.87 0.73 4.38
60/20/20 73.45 10.88 1.78 13.89 77.78 12.50 1.39 8.33
40/30/30 73.41 10.86 1.72 14.01 74.83 11.26 1.99 11.92
ANA/HQAT/6F Acid
90/5/5
80.95 14.06
/
/
/
/
4.98 82.77 14.98
6.77 81.02 14.60
9.38 77.78 13.89
11.89 74.83 13.25
/
/
/
/
2.25
4.38
8.33
80/10/10 78.90 14.33
60/20/20 78.78 11.84
40/30/30 75.70 12.41
11.92
and ANA/HQAT/6F acid copolymers show the Lewis base
(electron donor) character.
4.4. XPS Study of the Thin Film Copolymers. To confirm
our hypothesis that the variation of surface energy with 6F acid
content is due to preferential enrichment of bulky -C(CF3)2-
at the air-polymer interface because of chain conformation and
packaging, XPS at a takeoff angle of 90° was employed to study
the interface chemistry. For the ANA/AAA/6F acid system,
Table 8 shows that experimental atomic concentrations of F
element at interface far exceed the corresponding to theoretic
(bulk) values. The F content at the interface increases rapidly
from 6.37 to 11.84% when the 6F acid concentration increases
from 5 to 10 mol %. Interestingly, the C% and O% at the
interface decrease with increasing 6F acid in this composition
range, suggesting preferential surface segregation or enrichment
of fluorocarbons against C and O at the interface. With further
increase in 6F acid in the copolymers, the experimental F value
varies, in a much slower pace, from 11.8 to 14.0%, while there
are little changes in C% and O% content at the surface.
For the ANA/HQAT/6F acid system, the atomic concentration
of F element at the surface increases almost linearly with an
increase in the 6F acid monomer ratio. These XPS results agree
well with their surface energy values, which decreased consis-
tently with an increase in fluorine content. A smaller takeoff
angle (30°) was also utilized to re-confirm the surface chemistry
profiles on an 80/10/10 ANA/HQAT/6F acid thin film copoly-
mer. It is well-accepted that the takeoff angle corresponds to
the depth of the polymer surface at the air-polymer interface:
10,39 the smaller the takeoff angle, the nearer to the surface. The
experimental atomic concentrations of F element taken at 30°
and 90° are 9.71 and 6.77%, respectively. These results clearly
verify that the nearer to the surface, the more fluorine exists,
indicating the pendant CF3 segments originating from 6F acid
is preferentially enriched to the outermost layer at the air-
polymer interface against other elements. In other words, the
presence of the bulky -C(CF3)2- groups in the bis(benzoic
acid) moiety hinders intrasegmental mobility and thus stiffens
the backbone chains. The bulky -C(CF3)2- groups also inhibit
intersegmental chain packing, thus leading themselves to
preferentially enrich at the air-polymer interface in order to
reduce the overall Gibbs free energy.
Even though the ANA/AAA/6F acid copolymer has greater
fluorine enrichment at the air-polymer interface than the ANA/
HQAT/6F acid copolymer, they follow different patterns with
an increase in the 6F acid content. The surface energy of the
former drops significantly when the 6F acid content increases
from 5 to 10% (mol), but it then becomes invariant with further
increasing in 6F acid content, whereas the latter system
decreases continuously. This is may be due to the fact that the
amide groups in the ANA/AAA/6F acid copolymer play an
important role on surface energy because amide induces strong
intermolecular hydrogen bonding. Evidence of strong hydrogen
bonding can be observed from the water contact angles, shown
in Table 3. Even with the interference of nonpolar perfluoro-
carbon moiety, the water contact angle still shows a clear
decreasing trend with an increase in the amide content, which
is consistent with the 6F acid content. In contrast, the ANA/
HQAT/6F acid system does not have hydrogen bonding and its
water contact angle tends to increase with increasing the 6F
acid content.
5. Conclusions
Thin film polymerization technique was utilized to synthesize
two series of liquid crystalline/crystalline copolymers with -
C(CF3)2- in the main chain. Surface energies were found in
the range of 28-36 mJ/m2 depending on the monomer
concentrations and reaction systems. For the ANA/AAA/6F acid
system, an increase in 6F acid content from 5 to 10% (mol),
the surface energy decreased. But a further increase in 6F acid
content resulted in little change in the surface energy and it
was maintained at about 33 mJ/m2. For the ANA/HQAT/6F acid
system, the surface energy decreased continuously with an
increase in the content of 6F acid. XPS data were in agreement
with the surface energy behavior of the two series of copoly-
mers. The difference between the ANA/AAA/6F acid and ANA/
HQAT/6F acid systems can be explained through higher chain
rigidity and the existence of hydrogen bonding in the former
system.
The different relationships between surface energy and 6F
acid content for ANA/AAA/6F acid and ANA/HQAT/6F acid
Acknowledgment. The authors express their gratitude to the
Institute of Materials Research and Engineering (IMRE),

4150 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Ma and Chung
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