Strongly amphiphilic wetting behavior for polyurethanes with polyoxetane soft blocks having -CF2H terminated side chains

Article abstract of DOI:10.1016/j.polymer.2014.03.011

Two novel polyoxetanes with -CF₂CF₂H (4F) or -(CF₂)₃CF₂H (8F) terminated side chains were synthesized and characterized by ¹H NMR spectroscopy, DSC and GPC. 4F and 8F diols were incorporated in polyurethanes IPDI-BD(40)/4F-8.2 (U-4F-8.2) and IPDI-BD(40)/8F-5.8 (U-8F-5.8), where isophorone diisocyanate (IPDI) and 1,4-butane diol (BD) comprise the hard block (40 wt%) and 4F-8.2 or 8F-5.8 are soft blocks with M_n in kDa. Surface characteristics were evaluated using TM-AFM, XPS and dynamic contact angle (DCA) measurements. In contrast to U-4F-8.2, TM-AFM reveals an interesting phase separated surface morphology for U-8F-5.8 apparently driven by higher F side chain content. A model is proposed to account for contact angle measurements that show reversible, strongly amphiphilic wetting with θ_adv > 100° and θ_rec < 40°. Resistance to surface phase separation and related studies suggest the -CF₂CF₂H moiety is an important candidate for expanding the range of functional groups employed in surface modifiers.

Full text of DOI:10.1016/j.polymer.2014.03.011

Polymer 55 (2014) 2170e2178
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Strongly amphiphilic wetting behavior for polyurethanes with
polyoxetane soft blocks having eCF2H terminated side chains
*
Pinar Kurt, Asima Chakravorty, Xiaomei Zeng, Kenneth J. Wynne
Department of Chemical and Life Science Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel polyoxetanes with eCF2CF2H (4F) or e(CF2)3CF2H (8F) terminated side chains were synthe-
sized and characterized by 1H NMR spectroscopy, DSC and GPC. 4F and 8F diols were incorporated in
polyurethanes IPDI-BD(40)/4F-8.2 (U-4F-8.2) and IPDI-BD(40)/8F-5.8 (U-8F-5.8), where isophorone dii-
socyanate (IPDI) and 1,4-butane diol (BD) comprise the hard block (40 wt%) and 4F-8.2 or 8F-5.8 are soft
blocks with Mn in kDa. Surface characteristics were evaluated using TM-AFM, XPS and dynamic contact
angle (DCA) measurements. In contrast to U-4F-8.2, TM-AFM reveals an interesting phase separated
surface morphology for U-8F-5.8 apparently driven by higher F side chain content. A model is proposed
to account for contact angle measurements that show reversible, strongly amphiphilic wetting with
qadv > 100ꢀ and qrec < 40ꢀ. Resistance to surface phase separation and related studies suggest the
eCF2CF2H moiety is an important candidate for expanding the range of functional groups employed in
surface modifiers.
Received 10 January 2014
Received in revised form
2 March 2014
Accepted 5 March 2014
Available online 19 March 2014
Keywords:
Polyurethane
Fluorous polyoxetane
Surface science
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
and microscale roughness and near-superhydrophobic wetting
[25,26].
Modification of surfaces with fluorous moieties has been widely
studied to obtain both hydrophobicity and oleophobicity. Much is
known about surfaces with terminal eCF3 groups especially poly-
acrylates [2e8]. Semifluorinated moieties with eight (e(CF2)7CF3,
C8F) or more perfluoro-side chain carbons form enthalpically sta-
bilized phases that prevent surface reorganization. Such systems
are characterized by low critical surface tensions (ca. 8 mJ/m2) and
In continuing development of surface modifiers based on fluo-
rous copolyoxetanes [27e30], we sought to expand the palette of
side chains beyond those having eCF3 termination so as to tailor
further soft block functionality. This led to interest in replacing a e
CF3 terminal fluorine by eCF2H. Sixty years ago, Ellison observed 5ꢀ
lower contact angles for monolayers of acids having eCF2H end
groups compared to eCF3 analogs [31]. Replacement of terminal e
CF3 by eCF2H in side chains resulted in substantially increased
surface tensions, emphasizing the importance of the terminal
group in determining nanosurface (<1 nm) properties [31,32].
Lower contact angles and increased surface tensions were attrib-
uted to terminal CeH/X hydrogen bonding due to decreased
electron density on the terminal hydrogen creating a partial posi-
tive charge.
For polystyrene-b-semifluorinated block copolymers, the pres-
ence of eCF2H terminated side chains resulted in 21ꢀ lower
advancing contact angles (qadv) compared to eCF3 terminated an-
alogs [33]. Additional studies on liquid crystalline moieties also
showed surface energy increases as a result of replacement of eCF3
by eCF2H end groups [32,34].
low contact angle hysteresis (qD
¼
qadv
ꢁ
qrec) [3,9e12]. The dis-
covery that C8F and C10F moieties degrade to perfluorooctanoic
acid (PFOA) [13,14], which is bioaccumulative, greatly attenuated
commercial production [14,15]. This finding stimulated a search for
alternatives, such as placing C4F and C6F groups at the end of long
alkyl chains [16].
Our interest in eCF3 terminated side chains has focused on
copolyoxetane soft blocks. These studies led to novel surface
properties including contraphilic wetting, where the dry surface is
hydrophilic but the wetted surface is hydrophobic [17,18]. With two
identical side chains, polyoxetanes are semicrystalline [19e24].
Poly[bis(trifluoroethoxy)oxetane] is interesting by virtue of cold
crystallization that results in spontaneous formation of nanoscale
Since characteristic hydrophobicity and oleophobicity are
reduced, little attention has been paid to systems with eCF2H ter-
minal groups. An exception is the work of Kunzler, where eCF2H
terminated side chains increased the miscibility of methacrylate
* Corresponding author.
E-mail address: kjwynne@vcu.edu (K.J. Wynne).
http://dx.doi.org/10.1016/j.polymer.2014.03.011
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

P. Kurt et al. / Polymer 55 (2014) 2170e2178
2171
end-capped dimethyl siloxane intermediates with hydrophilic
2.3. Polyoxetane diols
monomers. This permitted bulk radical polymerization with hy-
drophilic monomers, such as dimethylacrylamide and resulted in
transparent, high water content hydrogels [35].
4F and 8F polyols were prepared by cationic ring opening
polymerization method described previously [22,37]. An example
follows for the synthesis of 4F. A three necked round bottom flask
was placed in a cooling bath and a nitrogen purge (45 min) CH2Cl2
(5e7 mL) was added. Butane diol (0.23 g, 2.54 mmol) and BF3$Et2O
(0.73 g, 5.13 mmol) were injected sequentially via syringe. A
refrigeration system (PolyScience Model 912) was used to cool the
solution (ꢁ9 ꢀC). 4F monomer (5.54 g, 25.7 mmol) in 7.02 g CH2Cl2
was added. After stirring overnight (13 h), the solution was warmed
to ambient temperature and washed sequentially with aqueous 3%
HCl and 3% NaCl. The CH2Cl2 solution containing 4F polyoxetane
was added drop wise to MEOH:H2O (3:1, v:v) for precipitation. The
fluorous diol layer was separated and placed in a vacuum oven for
Extending a preliminary study [36], we report herein poly-
urethanes containing soft blocks derived from polyoxetane diols 4F
and 8F (Fig. 1). Effects of the longer CF2 sequence are seen clearly by
TM-AFM surface morphological studies for a polyurethane with an
8F-based soft block compared to a 4F analog. Dynamic contact
angle measurements (DCA, Wilhelmy plate) reveal new details for
wetting behavior that contrast with previously investigated eCF2H
systems [31e34]. This wetting behavior is discussed along with a
model for high contact angle hysteresis. Other characterization
studies expand knowledge of solid state (DSC) and surface science
(XPS).
solvent evaporation (40 ꢀC, 36 h). 1H NMR (300 MHz, CDCl3,
d:
ppm): 6.09e6.06, 5.91e5.88, 5.74e5.70 (eCF2H, 1H, t), 3.81, 3.75,
3.72 (eCH2CF2e, 2H, t), 3.42 (eCH2Oe, 2H, s), 3.22e3.14 (main
chain eCH2eCeCH2e, 4H, d), 0.91 (eCH3, 3H, s).
A number of 4F polyoxetane diol preparations were carried out
with monomer-catalyst ratios of 10:1, 20:1 and 100:1 (Table S1). 8F
polyoxetane diols were prepared following the same procedure
using only a 10:1 monomerecatalyst ratio (Table S1). 4F-8.2 and 8F-
5.8 (designated by approximate Mn in kDa) were chosen for poly-
urethane synthesis.
2. Experimental section
2.1. Materials
2,2,3,3-Tetrafluoropropan-1-ol and 2,2,3,3,4,4,5,5-octafluoro-1-
ol were generously provided by Daikin Industries, Yodogawa,
Japan. 3-Bromomethyl-3-methyl oxetane (BrOx) was a gift from
OMNOVA Solutions, Akron, OH. Isophorone diisocyanate (IPDI),
tetrabutylammonium bromide (TBAB), boron trifluoride dietherate
(BF3$Et2O), dibutyltin dilaurate catalyst (T-12) and trifluorotoluene
(TFT, 99þ%) were from Aldrich. Butane diol (BD), tetrahydrofuran
(THF) and dimethylformamide (DMF) were obtained from Acros
Organics (99þ%). BrOx was vacuum distilled at 85 ꢀC/5 mmHg. IPDI,
BD, fluorinated alcohols, catalysts and organic solvents were used
as received.
2.4. Polyurethanes
Polyurethanes with 40 wt% hard block were made by the con-
ventional soft block first procedure [1]. Designations are IPDI-
BD(40)/4F-8.2 and IPDI-BD(40)/8F-5.8, where IPDI-BD hard block
content is indicated in parenthesis followed by the soft block with
the Mn in kDa. Shortened designations are U-4F-8.2 and U-8F-5.8,
respectively. An example for U-4F-8.2 synthesis follows.
2.2. Monomers
Polyol 4F-8.2 (3.6 g, 0.36 mmol) in 1.23 g THF was added to a
three-necked round bottom flask containing IPDI (1.75 g,
7.58 mmol). Dibutyltin dilaurate catalyst (4 drops, 10 wt% in THF)
was added and the solution was heated to 70 ꢀC under nitrogen
purge. The progress of the reaction was followed by FT-IR, i.e., the
growing carbonyl peak at 1716 cmꢁ1. After 4 h, the carbonyl peak
remained unchanged indicating completion of prepolymer forma-
tion. 1,4-Butane diol (0.65 g, 7.21 mmol) in 5.4 g THF was added
drop wise followed by heating at 70 ꢀC until complete disappear-
ance of eNCO peak at 2267 cmꢁ1 (ca. 3 h). The mixture was cooled
to room temperature and added drop wise to methanol/water (1:3)
to affect polyurethane precipitation. Residual solvent was evapo-
rated under vacuum at 60 ꢀC for 48 h. Polyurethane U-8F-5.8 wad
prepared similarly using trifluorotoluene (TFT) as solvent. IPDI-
BD(40)-3F-3.4 designated U-3F-3.4 was reported previously [1].
3-Methyl-3-(2,2,3,3,-tetrafluoropropoxymethyl)oxetane and
3-methyl-3-(2,2,3,3,4,4,5,5-octafluoropentyloxymethyl)-oxetane
are designated 4F and 8F monomers (Fig. 1). Their preparation
was carried out by nucleophilic substitution using phase transfer
catalysis (TBAB) [37e39]. Scheme S1 illustrates the reaction of
BrOx with fluorinated alcohols. As a specific example for 4F
monomer, BrOx (41.25 g, 250 mmol), 4F alcohol (46.2 g,
350 mmol) and TBAB (5 g, 0.0125 mmol) were heated to 60 ꢀC in
20 mL water. Aqueous KOH (15.8 g, 87%) in water (20 mL) was
added drop wise over 1 h. The solution was heated to 75 ꢀC with
stirring for 72 h. 4F monomer was extracted with dichloro-
methane, the solution dried with MgSO4, and the product freed of
solvent with a rotovap. GCeMS showed the presence of a small
amount of BrOx. Short path distillation gave >99% 4F monomer
(b.p. 85 ꢀC/3.3 mmHg). 1H NMR (300 MHz, CDCl3,
d: ppm): 6.09e
6.06, 5.91e5.88, 5.74e5.70 (eCF2H, 1H, t), 4.48e4.34 (oxetane
ring, eCH2e, 4H, d), 3.92e3.91, 3.87e3.87, 3.83e3.82 (eCH2CF2e,
2H, t), 3.63 (eCH2Oe, 2H, s), 1.30 (eCH3, 3H, s). 8F monomer was
prepared and purified in an analogous manner (b.p. 65 ꢀC/
0.8 mmHg).
2.5. Molecular weights
Fluorous diol molecular weights (Mn) were determined by re-
action with trifluoroacetic anhydride (TFAA) for end group analysis.
1H NMR spectra for 4F monomer, 4F-8.2 polyol and post-TFAA are
shown in Fig. 2. GPC was also used for polyol molecular weight
determinations (Table 1).
CH2(CF2)xCF2H
2.6. DSC
O
CH2(CF2)xCF2H
O
HO
O
A TA-Q 1000 (TA instruments) temperature Modulated Differ-
ential Scanning Calorimetry (MDSC) was used for determination of
thermal transitions at a heating rate of 3 ꢀC/min. and ꢂ0.5 ꢀC
modulation at 60 s.
H
n
O
Fig. 1. 4F (x ¼ 1) and 8F (x ¼ 3) monomers and polyoxetane diols.

2172
P. Kurt et al. / Polymer 55 (2014) 2170e2178
Fig. 2. 1H NMR spectra and assignments: A, 4F monomer; B, polyoxetane 4F-8.2; C, 4F-8.2 polyol-TFAA.

P. Kurt et al. / Polymer 55 (2014) 2170e2178
2173
Table 1
Molecular weight data for 4F-8.2 and 8F-5.8. 1H NMR peak assignments for
a,
b, and
g
are in Fig. 2.
DPa
Mn (kDa)a
Mn (kDa)
Mwb (kDa)
PDIb
b
Sample
Peak
eCF2H
a
b
a/b
l2
l1
a
þ
b
l1
l2
a
þ
b
l2
l1
4F-8.2
8F-5.8
Shift/ppm
Integral area
Shift/ppm
6.1e5.7
1.0
6.1e5.7
1.0
4.28
0.1
4.28
0.21
4.38
0.09
4.38
0.06
e
1.03
0.17
1.03
0.39
0.85
0.16
0.85
0.35
e
e
e
e
e
e
4.8
5.0
8.5
8.7
1.8
1.7
1.1
e
40
e
38
e
36
e
8.6
e
8.2
e
7.7
e
Integral area
3.5
15
16
17
5.5
5.8
6.2
a
DP: degree of polymerization by 1H NMR end group analysis; The integral area of reference peak d in main chain for: 4F-8.2, 3.08; for 8F-5.8, 2.81.
GPC.
b
2.7. Coatings
after DCA analysis to assess whether water contamination had
occurred [41].
For contact angle measurements, cover slips (Corning,
24 ꢃ 40 ꢃ 0.5 mm) were dip coated from 10 wt% polyurethane
solutions in THF and manipulated to give an even distribution
during solvent evaporation. Slides were stored at room tempera-
ture for 4 h and then at 60 ꢀC overnight under reduced pressure.
3. Results and discussion
3.1. Polyoxetane monomers and diols
4F and 8F monomers (Fig. 1) were prepared by nucleophilic
substitution (Williamson synthesis) using bromomethyl(methyl)
oxetane (Scheme S1) [42]. The 1H NMR spectrum of 4F monomer
with assignments is shown in Fig. 2A. The characteristic peaks for e
CF2H consist of a triplet of triplets centered at 6.08, 5.90, 5.72 ppm.
This set of peaks was noted by Kunzler for eCF2H terminated
siloxane side chains [35]. The high field shift for the terminal proton
reflects a strongly acidic nature. The 1H NMR spectrum of 8F
monomer with assignments is shown in Fig. S1A.
Ring opening polymerization (ROP) was carried out using
BF3$Et2O catalyst and BD as co-catalyst to give 4F polyoxetane diols
as transparent, viscous liquids (Scheme S1) [43,44]. Information on
monomer to catalyst ratios and resulting molecular weights is
provided in Table S1. Monomer/catalyst ratios of 100/1 gave rela-
tively high molecular weight 4F polyoxetanes (Mn > 15 kDa) while
lower ratios resulted in Mn < 10 kDa. This catalyst/co-catalyst
method followed Desai [45], Malik [43], Fujiwara [37] and Jutier
[46]. Alternatively, Kawakami [47a] and more recently Moller
[47b,48] used BF3 alone for ROP. From the results reported below, it
is apparent that the latter method is advantageous for control of
end group chemistry.
A low molecular weight polyol was sought for polyurethanes.
This influenced the choice of 4F-8.2 and 8F-5.8 polyoxetane diols
for polyurethanes reported herein. The designations are based on
the number of fluorines in the side chains (Fig. 1) and approximate
number average molecular weights in kDa (Table 1). 1H NMR
spectra and peak assignments for 4F-8.2 polyoxetane are shown in
Fig. 2B. The peaks for monomer ring methylene groups e and 3-
methyl groups d shift to high field (f) in the 4F-8.2 polyoxetane.
8F monomer and 8F polyoxetanes were prepared in a similar
manner. A 10/1 ratio of BF3$Et2O/BD co-catalyst was used to obtain
a relatively low molecular weight copolyoxetane suitable for a soft
block (Table S1). The 1H NMR spectra of 8F monomer and 8F-5.8
polyoxetane (selected for polyurethane synthesis) are shown with
assignments in Figs. S2A and B, respectively.
2.8. X-ray photoelectron spectroscopy (XPS)
Spectra were obtained using a Thermo Fisher Scientific ESCALAB
250 X-ray photoelectron spectrometer having monochromatized Al
K
a X-rays and low energy electron flood gun charge neutralization.
X-ray spot size for these acquisitions (90ꢀ TOA) was on the order of
500 mm. Pressure in the analytical chamber during spectral
acquisition was less than 2 ꢃ 10ꢁ8 Torr; pass energy for survey
spectra was 150 eV. The data were analyzed with the Thermo
Avantage software (v4.40). Calibration of C1s binding energies to
correct for the energy shift caused by charging was done by
assuming that the lowest energy peak (285.0 eV) was for aliphatic
carbon moieties. For peak fitting, the program PeakFit, version 4.12,
was used for C1s spectra assuming 100% Gaussian peaks.
2.9. Atomic force microscopy (AFM)
Topological and morphological analyses were carried out using
a Dimension-3100 (Digital Instruments, CA) atomic force micro-
scope with a NanoScope V controller. Tapping mode imaging was
performed in air using microfabricated silicon cantilevers (40 N/m,
Veeco, Santa Barbara, CA). A tapping force corresponding to a set-
point ratio rsp of 0.8 was used; rsp ¼ Aexp/Ao, where Ao is free
oscillation amplitude and Aexp is the experimental oscillation
amplitude. Images were analyzed by using NanoScope v710r1
software.
2.10. Wetting behavior
Dynamic contact angles (DCA) were obtained using a Cahn
Model 312 Analyzer (Cerritos, CA). The surface tension of the probe
liquid (Nanopure water) was checked before each measurement
and was typically 72.6 ꢂ 1 dyn/cm. Beakers used for DCA analysis
were cleaned by soaking in an isopropanol/potassium hydroxide
base bath for at least 24 h, rinsing with Nanopure water and
treatment with a gas/oxygen flame.
End group analysis followed a previously reported procedure
[37,49]. Fig. 2C shows structures for end group trifluoromethyl es-
ters. One analysis provides Mn based on methylene peaks associ-
DCA measurements were based on the Wilhelmy plate method
[40]. A coated slide is attached to the electrobalance via a clip. The
ated with oxetane (
a) and butoxy (b) end groups (Fig. 2C, insert).
The integral ratio is 1.1 indicating the BD co-catalyst competes
a:b
stage speed was 150
receding test segments was 10 s. Resulting force versus distance
m
m/s and dwell time between advancing and
for end group sites.
A second end group analysis is based on g
1, the methyl end
curves (fdc’s) were used to calculate advancing (qadv) and receding
group peak upfield from the main chain methyl peak d for 4F-8.2
(Fig. 2B,
1). This peak shifts downfield ( 2, 0.85 ppm) from the
main chain methyl peak d after TFAA addition (Fig. 2C). Comparison
of integrals for (
) and g2 peaks to main chain peaks gives
(qrec) contact angles. Five cycles (w3 min/cycle) in succession were
g
g
obtained to study any change in wetting behavior on exposure to
water. The surface tension of the test water was also determined
a
þ
b

2174
P. Kurt et al. / Polymer 55 (2014) 2170e2178
number average molecular weights of 8.6 and 8.2 kDa, respectively
(Table 1), while
1 gives 7.7 kDa.
g
Molecular weight determinations were also obtained by GPC for
several 4F polyols (Table S1). Table 1 shows molecular weights by
end group analysis and GPC for 4F-8.2 and 8F-5.8. Fair agreement is
found between the two methods, although Mn is systematically
lower for GPC measurements.
Thermal analysis by modulated DSC gave glass transition tem-
peratures of ꢁ46 ꢀC for 4F-8.2 and ꢁ56 ꢀC for 8F-5.8 (Fig. 3). The
10 ꢀC lower Tg for 8F-5.8 was surprising considering the longer
fluorous side chain. No thermal transitions were observed at higher
temperatures confirming the amorphous state above Tg of these
viscous liquid diols.
Fig. 4. Polyurethanes U-4F-8.2 (x ¼ 1) and U-8F-5.8 (x ¼ 3), highlighting eCH2e A, B,
and C assigned to XPS binding energies in Table S2.
Table 2
Molecular weight, thermal transitions, and calculation of phase mixing in U-3F-3.4,
U-4F-8.2 and U-8F-5.8.
a
Polyurethane
Mw
Soft block
Tg (ꢀC)
Polyol
Tg (ꢀC)
Soft block in
pure soft block (%)
3.2. Polyurethanes
(kDa)
U-3F-3.4b
U-4F-8.2
U-8F-5.8c
37.5
36.1
nd
ꢁ37
ꢁ25
ꢁ46
ꢁ51
ꢁ46
ꢁ56
84
77
89
IPDI/BD(40)-(4F-8.2), where 40 is the IPDI/BD wt% and 4F Mn is
in kDa, was prepared in THF/DMF while trifluorotoluene was used
for IPDI/BD(40)-(8F-5.8). The two polyurethanes are designated U-
4F-8.2 and U-8F-5.8 (Fig. 4). GPC for U-4F-8.2 gave Mw ¼ 36.1 kDa
and Mn ¼ 21.2 (PDI ¼ 1.7). U-8F-5.8 was not be characterized by GPC
due to poor solubility in THF.
a
GPC.
b
Previously published [1].
c
Not determined due to insolubility in THF.
Thermograms for the two polyurethanes are shown in Fig. 4. The
Tg for the soft block shifts upward by 21 ꢀC in U-4F-8.2 while the
shift in Tg for P-8F-5.8 diol to U-8F-5.8 is 10 ꢀC. Based on the dif-
ference in Tg’s, the Fox equation (Eq. (1)) may be used to estimate
the percent soft block in the pure soft block domain (Table 2).
contain images for U-8F-5.8 at two setpoint ratios. Columns C and D
have images for U-4F-8.2 at the same two setpoint ratios. Rows 1, 2,
and 3 present images for increasing areas.
A 1 ꢃ 1
mm phase image for U-8F-5.8 at relatively hard tapping
(rsp 0.7, Fig. 5-B1) shows nano-domains ranging in size from the
lower limit of TM-AFM (w20 nm) to w80 nm. By the conventional
interpretation of phase imaging, the lighter color (yellow in the
web version) domains are assigned to hard block aggregates
[52,53]. A phase image for U-8F-5.8 at somewhat lighter tapping
(rsp 8.0, Fig. 5-A1) barely resolves near surface phase separation
suggesting a steep morphological gradient at the nanosurface.
À
Áꢁ1
À
Á
À
Áꢁ1
Tg
¼ wSOFT Tg SOFT ꢁ1 þ wHARD Tg HARD
Calculations were based on the Tg of pure hard block as 85 ꢀC [1].
(1)
Based on % soft block in the pure soft block domain obtained from
the Fox equation there is more phase mixing for U-4F-8.2 than for
U-8F-5.8. Less phase mixing for U-8F-5.8 is attributed to minimal
intermolecular interactions and low solubility parameter for the e
(CF2)4e sequence counteracting terminal eCF2H hydrogen
bonding. Phase mixing is also greater for U-4F-8.2 than for U-3F-
3.4, which has trifluoroethoxymethyl side chains [1]. Phase mixing
is ascribed to a gain in enthalpy due to terminal eCF2H hydrogen
bonding and the smaller influence of the short e(CF2)2e group.
Because of low molecular symmetry, hard block endotherms for
IPDI polyurethanes are not always observed or have low enthalpies
[50,51]. Endotherms for hard block melting are not seen at <100 ꢀC
for U-4F-8.2 or U-8F-5.8 (Fig. 3).
The 10 ꢃ 10
mm phase image for U-8F-5.8 (Fig. 5-B2, rsp 0.7)
reveals interesting fringed circular microdomains surrounded by a
field of nanodomains. The corresponding image at rsp 0.8, Fig. 5-A2,
shows that the microdomains are comprised of clusters of nano-
domains as only the near surface nano-components are imaged.
Imaging at 50
mm (Fig. 5-B3) shows that the microdomains are
fairly uniform in size (w3
mm) and evenly distributed. Some groups
of these domains form chain-like rows. The corresponding images
3.3. Surface morphology
Tapping Mode Atomic Force Microscopy, TM-AFM, revealed a
complex surface morphology at the microscale and nanoscale. Fig. 5
shows phase images for U-4F-8.2 and U-8F-5.8. Columns A and B
Fig. 5. TM-AFM phase images: Left (columns A, B), U-8F-5.8; Right (columns C, D), U-
Fig. 3. DSC for polyols and polyurethanes: A, 4F-8.2; B, 8F-5.8; C, U-4F-8.2; D, U-8F-5.8.
4F-8.2.

P. Kurt et al. / Polymer 55 (2014) 2170e2178
2175
Table 3
at rsp 0.8, Fig. 5-A3, again reflect a gradient for these microscale
Atomic percentages of elements in U-4F-8.2 and U-8F-5.8.
domains with fainter features.
Surface phase separation for U-8F-5.8 is reminiscent of micro-
scale phase separation in a polyurethane having a block-copolyox-
etane soft block with trifluoroethoxymethyl (3F) and PEG-like side
chains [30]. A model for synergistic 3F/urethane hard block self-
aggregation to form Janus-like nanostructures was proposed by
Zhang [30]. An analogous version of the model for U-8F-5.8 is
shown in Fig. 6. Synergistic nanoscale demixing driven by fluorous
8F-5.8 soft block and hard block phase separation is proposed to
account for the nanoscale features that aggregate to form micron
scale domains (Fig. 5).
Fig. 5 shows AFM images for U-4F-8.2 at the same acquisition
conditions used for U-8F-5.8. In contrast to the latter, images for U-
4F-8.2 are largely devoid of morphological features. At hard tapping
(rsp 0.7) dispersed dot-like 20e50 nm features signal phase sepa-
rated domains. These features remain isolated without aggregation
seen for U-8F-5.8. A 500 ꢃ 500 nm image for U-4F-8.2 was taken on
a different sample (Fig. S2). Nanoscale phase separation was found
similar to that for U-8F-5.8 (Fig. 5-B1). At the nanoscale, the phase
separated features (20e50 nm) are quite similar to polyurethanes
with fluorous [27,54] and conventional polyether [55,56] soft
blocks.
Atom
U-4F-8.2
U-8F-5.8
Calc. U
(at%)
Calc. soft
block (at%)
Obs.
(at%)
Calc. U
(at%)
Calc. soft
block (at%)
Obs. (at%)
F1s
16.7
15.8
63.4
3.8
28.6
14.3
57.1
e
19.40
15.96
59.34
2.71
23.1
13.4
59.6
3.9
40
10
50
24.30
15.31
57.4
O1s
C1s
N1s
2.02
for U-3F-3.4, a moderate nanosurface enrichment of F at% is found.
The nitrogen at% (2.71) indicates the presence of near surface hard
block but supports soft block surface concentration, as the bulk N at
% (3.8) is attenuated by w30%.
Unexpectedly, at% F for U-8F-5.8 is only slightly higher than the
bulk (Table 3). On the other hand, the N at% is 2.02 at% while 3.9 at%
is calculated for the bulk. Surface concentration of the soft block is
indicated, but a complex interplay of soft block molecular weight
difference and nanosurface structure and morphology precludes a
simple interpretation.
Fig. 7 shows high resolution C1s XPS spectra for U-4F-8.2 and U-
8F-5.8. The U-8F-5.8 hydrocarbon CeCeC peak at 285 eV [61,62] is
used as a reference for binding energies. The C1s binding energies
assignments given in Table S2 are guided by literature values for
carbon in similar chemical environments [32,63e66].
The peak at 293.1 eV is unique for U-8F-5.8 and is assigned to the
eCF2CF2e unit in the side chain. Binding energies for urethane
carbonyl eC(O)eNe, eCH2eCF2eCF2e and eCF2H are superposed
in a broad peak at 290.6 eV. Previously, it was shown that eCeCe
Oe binding energies depend on the nature of the polyether [61].
Assignments for eCeCeOe A, B, and C environments (Fig. 2) are
made based on this previous study and studies on fluorinated
polyethers [66].
The absence of aggregated microscale features for U-4F-8.2 re-
inforces the conclusion from DSC that demixing is driven more
strongly for U-8F-5.8 by the higher fluorous content of the 8F-side
chain. Furthermore, the surface features for U-8F-5.8 result in a
higher root mean square roughness. For comparable 50 ꢃ 50
mm
scans, Rq is 30.4 nm for U-8F-5.8 compared to 8.7 nm for U-4F-8.2.
3.4. X-ray photoelectron spectroscopy
Atomic percentages of carbon, fluorine, nitrogen and oxygen
were analyzed by XPS at a 90ꢀ TOA (Table 3). Previous studies have
shown that the soft blocks in polyurethanes tend to be concen-
trated in the nanosurface, that is, the outermost 1e2 nm [57e60].
This trend is also observed for polyoxetane soft blocks containing
side chains terminated by eCF3, viz., U-3F-3.4 [27]. For U-3F-3.4 at a
takeoff angle of 55ꢀ, 18.1 at% F was found compared to the bulk
value of 15.4 at% and to the 3F soft block 25.1 at%. The moderate F at
% nanosurface enrichment is consistent with phase mixing noted
above (Table 2).
Curve fitting was used to facilitate binding energy assignments
(Experimental section). The insert in Fig. 7 shows a six peak fit for
U-8F-5.8. Peak areas are provided in Table S2. Phase mixing and
superposition of binding energies precludes an accurate analysis of
contributions.
3.5. Dynamic contact angles (Wilhelmy plate)
Contact angles for U-4F-8.2 and U-8F-5.8 were obtained for
comparison with previously studied longer, rigid rod analogs hav-
ing eCF3 and CF2H termini [32e34] and with U-3F-3.4 [27]. As
At a 90ꢀ TOA, the 19.4 at% F for U-4F-8.2 may be compared with
the bulk value (16.7) and that calculated for the soft block (28.6). As
Fig. 6. Model for surface nanoscale phase separation. Phase image Fig. 5-B1.
Fig. 7. C1s XPS spectra for U-4F-8.2 and U-8F-5.8. Assignments are shown for U-8F-5.8.

2176
P. Kurt et al. / Polymer 55 (2014) 2170e2178
Table 4
described previously, dynamic contact angles qadv and qrec are ob-
tained from force distance curves (fdc’s) via Eq. (2) [41].
Polyurethane dynamic contact angles for cycles 1 and 5.
Polyurethane
U-3F-3.4
Cycle
qadv
qrec
F ¼ m=g ¼ P
g
cos
q
(2)
1
5
1
5
1
5
102
98
108
103
108
106
45
46
40
40
31
30
Where F is the force derived from respective mass (m) changes on
immersion and emersion, g is the gravitational constant, is the
liquid surface tension, and is the contact angle. Extrapolating an
U-4F-8.2
U-8F-5.8
g
q
fdc to the point of maximum (or minimum) initial mass upon im-
mersion eliminates the need for a buoyancy correction to F.
An important step after dynamic contact angle (DCA) analysis
comprises measuring post-test water surface tension. Diffusion and
surface migration of water immiscible contaminants such as cyclics
or surfactants changes water surface tension and hence contact an-
gles. This simple test provides important information that differen-
tiates whether changes for contact angles after repeated immersion
are due to surface reorganization or water contamination [41].
Unlike polysiloxane coatings reported earlier [41] minimal
contamination was observed for polyurethanes with 3F-3.4, 4F-8.2,
and 8F-5.8 soft blocks. Thus, except for slight water contamination
by U-3F-3.4, contact angles are used with confidence to assess
relative wetting behavior and extent of surface reorganization
during immersion.
low respective coverages [69,70]. For example, on a hydrophobic
surface, a low area fraction of homogeneously distributed polar
moieties “pins” water at the three phase contact line and dispro-
portionately decreases qrec
.
3.5.3. Dynamic heterogeneity
Unlike rigid, heterogeneous surfaces, “soft surfaces” comprised
of flexible chains can display high contact angle hysteresis due to
rapid surface reorganization during immersion. Enthalpic in-
teractions of polar groups with water drive interfacial chain reor-
ganization at the nanosurface [71e74]. At longer immersion times
contact angles can change as polar groups slowly diffuse to the
surface [75]. Both slow and rapid processes are driven by minimi-
zation of interfacial free energy.
XPS showed that the 3F soft block is surface concentrated, while
DSC points to a low soft block Tg and a significant degree of hard
block/soft block phase mixing. Contact angle hysteresis for U-3F-3.4
reflects “dynamic heterogeneity” due to the presence of a nano-
surface soft block that rapidly undergoes enthalpically driven
reorganization. Water hydrogen bonding to the eCH2e group
adjacent to eCF3 was proposed to account for this observation
[18,63]. Water H-bonding to ether oxygen (side chain, and main
chain) as well as to near surface hard block amide oxygen is also
likely.
Fig. 8 shows the first cycle fdc’s for U-4F-8.2, U-8F-5.8, and U-3F-
3.4. Contact angles are summarized in Table 4 for first and fifth cycles
and in Table S3 for all five cycles. The total immersion time after five
cycles is w15 min. For U-3F-3.4 qadv is 102ꢀ (Fig. 8, curve C) which is in
the >100ꢀ range expected for a fluorous polymer at the aireliquide
solid interface [27]. A 4ꢀ decrease in qadv was observed after several
fdc cycles (Table 4). Part of this decrease (w1e2ꢀ) is due to slight
watercontamination that was not entirelyeliminated despite several
reprecipitations. The receding contact angle is 46ꢀ thereby giving a
contact angle hysteresis of 52ꢀ (qD
for high contact angle hysteresis are considered below.
¼
qadv
ꢁ
qrec). Sources that account
3.5.1. Roughness
In contrast to prior studies for e(CF2)7CF2H systems where qadv
decreases 10e20ꢀ compared to eCF3 analogs [32e34], U-4F-8.2 has
a 6ꢀ higher initial qadv (108ꢀ) and a 6ꢀ lower qrec (40ꢀ) than U-3F-3.4
(Fig. 8, curve A, Table 4). U-4F-8.2 has an initial contact angle
hysteresis (qD) of 68ꢀ. After five cycles, qD was 63ꢀ due mainly to a
decrease in qadv. This decrease is ascribed to water adsorption
driven by hydrogen bonding. After drying, initial force distance
curves are obtained.
Effects of uniform rugosity on contact angles are well under-
stood [10,67,68]. Increasing roughness generally increases qadv
Receding contact angles (qrec) determine contact angle hysteresis
and are important in determining both hydrophobic and lyophobic
.
character. TM-AFM 50 ꢃ 50
mm images for U-4F-8.2 in Fig. 5 show
Rq is 8.7 nm. This low level of nanoscale roughness is unlikely to
influence contact angles.
U-8F-5.8 also has a 6ꢀ higher initial qadv of 108ꢀ with force dis-
tance curve B closely paralleling that for U-4F-8.2. The receding
contact angle is uniquely low (qrec 31ꢀ) compared to U-3F-3.4 and
U-4F-8.2 (Fig. 8, Table 4), which results in the highest qD of 76ꢀ. Over
five immersion/emersion cycles, contact angle hysteresis for U-8F-
5.8 is more stable than U-4F-8.2, perhaps due to better phase
separation.
3.5.2. Static heterogeneity
Johnson and Dettre showed that for smooth, rigid surfaces, hy-
drophilic and hydrophobic area fractions changed contact angles at
For both U-4F-8.2 and U-8F-5.8, dynamic heterogeneity facili-
tated by a low Tg and driven by enthalpically favored hydrogen
bonding accounts for high contact angle hysteresis. In contrast, for
block copolymers with e(CF2)7CF2H and similar side chain termini
qD of 7e19ꢀ were reported [32,33]. Microscale roughness may
contribute to the uniquely low qrec for U-8F-5.8 (31ꢀ). Further
studies including angle resolved XPS or ATR-IR spectroscopy may
clarify the origin of the high qD
.
Models for wetting behavior of surfaces having A, longer e
(CF2)nCF3 terminated side chains and B, analogous e(CF2)nCF2H side
chains are shown in Fig. 9. Fig. 9A is applicable to long side chain
fluorous methacrylates investigated by Katano [2] and Takahara [3]
and poly(styrene-b-semifluorinated isoprene) block copolymers
reported by Ober [33]. Surfaces with e(CF2)7CF3 (and longer)
terminated side chains are characterized by high contact angles and
Fig. 8. Dynamic contact angle force distance curves: A. U-4F-8.2 (e e e); B, U-8F-5.8 (ꢄ
ꢄ ꢄ); C, U-3F-3.4 (dd).

P. Kurt et al. / Polymer 55 (2014) 2170e2178
2177
low qD (Fig. 9A). Such systems are enthalpically stabilized by the
formation of ordered phases driven by side chain crystallization.
Surfaces having e(CF2)7CF2H and similar side chain termini have
lower qadv but retain low qD in the 7e19ꢀ range [32,33]. The lower
qadv was ascribed to the dipolar nature of the eCF2H end group that
enhances hydrogen bonding depicted in Fig. 9B-i [33]. The forma-
tion of ordered phases stabilizes the eCF2H surface against reor-
ganization on short time scales, but over the course of weeks
contact angles decrease [33].
qadv. Thus, we return to the conclusion suggested above that a
combination of phase separated morphology and microscale
roughness accounts for low qrec. With a longer eCF2CF2CF2CF2e
unit in the side chain, it seems counter intuitive that a wet state for
U-8F-5.8 similar to that depicted by Fig. 9D-i would result in a
lower receding contact angle. Apparently, surface microscale sur-
face roughness increases dynamic heterogeneity and amplifies qrec
.
Further work is required to elucidate this interesting result.
A model for surfaces with a single eCF3 terminal group as in U-
3F-3.4 is shown in Fig. 9C. These surfaces have fairly high qadv
(w106ꢀ), though not as high as well ordered surfaces having, longer
e(CF2)7CF3 terminated side chains (Fig. 9A). The amorphous nature
of the soft blocks, which are too short to form ordered side chain
phases results in low qrec and high contact angle hysteresis
4. Conclusion
In continuing development of surface modifiers based on fluo-
rous copolyoxetanes [27e30], we sought to expand the palette of
side chains to include copolyoxetanes with eCF2H terminal groups.
Extending a preliminary study [36], U-4F-8.2 and U-8F-5.8 poly-
urethanes containing soft blocks with eCF2H terminated side
chains are reported (Fig. 3). Effects of the longer CF2 sequence for U-
8F-5.8 are seen by TM-AFM that reveals “blooming” of
chrysanthemum-shaped phase separated microfeatures. In
contrast, U-4F-8.2 surface structure is minimal at the micron scale,
while typical polyurethane hard block/soft block domains are seen
at the nanoscale.
(
qD ¼ 52ꢀ).
Fig. 9D shows a model for the wetting behavior of U-4F-8.2.
Upon immersion, hydrogen bonding of eCF2CF2H to the side chain
ether oxygen (depicted) and ether and other polar moieties (e.g.,
carbonyl) is proposed (Fig. 9D-i). The latter is consistent with
thermal analysis that suggests only moderate soft block-hard block
phase separation.
A model paralleling that for U-4F-8.2 (Fig. 9D) is suggested for
U-8F-5.8. Due to the longer e(CF2)4e unit in the side chain, this
model might imply that U-8F-5.8 would have a higher qadv but this
is not observed. Neither does this nanoscale model explain the low
Low Tg’s for the 4F and 8F soft blocks mean that side chains are
not constrained by ordered domains previously described for e
(CF2)nCF2H systems (Fig. 9B) [32,33]. Dynamic contact angle mea-
surements show reversible amphiphilic wetting with in q
adv > 100ꢀ
Fig. 9. Wetting models for dry (d) and immersed (i) surfaces for side chains terminated with: A rigid eCF3; B, rigid eCF2H; C, flexible eCF3 and D, flexible eCF2CF2H. H-bonding
interactions are depicted; terminal eCF2H hydrogen bonding is highlighted and circled; For D-d, 4F-ether H-bonding is designated with an arrow while shaded semicircles highlight
eCF2CF2e surface concentration driven by eCF2H hydrogen bonding to side chain oxygen.

2178
P. Kurt et al. / Polymer 55 (2014) 2170e2178
[24] Jiang W-C, Huang Y, Gu G-T, Meng W-D, Qing F-L. Appl Surf Sci 2006;253(4):
and q
rec < 40ꢀ. A model for the strongly amphiphilic nature of U-4F-
2304e9.
8.2 and U-8F-5.8 is proposed (Fig. 9D). In the dry state, e(CF2)ne
moieties are projected to the surface by virtue of enthalpically
favored inward-directed H-bonding of eCF2H with resultant high
qadv. In contrast, in the wet state, strong hydrogen bonding of e
CF2H to water “submerges” hydrophobic e(CF2)ne moieties.
Strongly amphiphilic character is shown by high contact angle
hysteresis (ꢅ60ꢀ). Importantly, amphiphilic behavior is reversible.
The AFM morphological study discussed above, which revealed
minimal microscale phase separation for U-4F-8.2 (Fig. 5), inspired
replacement of eCF3 by eCF2CF2H to generate copolyoxetane soft
blocks with 4F and quaternary side chains, as described in a pre-
liminary report [76]. Phase separation that resulted in sequestra-
tion of quaternary charge for the eCF3 analog was avoided but
surface concentrated quaternary function was retained. Thus, the e
CF2CF2H moiety is an important candidate for expanding the range
of functional groups employed in surface modifiers. This adds a
new layer of interest to Kunzler’s finding that eCF2CF2H side chains
are useful in compatibilizing methacrylate/silicones [35].
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The author thanks the National Science Foundation, Division of
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