Fluorinated star-shaped block copolymers: Synthesis and optical properties

Article abstract of DOI:10.1002/pola.28056

Tetrakis(4-(1-bromoethyl)phenyl)silane is synthesized and utilized to initiate the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) to generate bromo-terminated four-armed PMMA macroinitiators, which further initiate the ATRP of methylacryloyloxyl-2-hydroxypropyl perfluorooctanoate (FGOA) to create fluorinated star-shaped block copolymers PMMA-b-poly(FGOA)s with fluorine content ranging from 0 to 31.7 wt %. The polymerizations are well controlled with the polydispersity indices <1.30. The polymers readily dissolve in common organic solvents and show good film-formation. Compared with the nonfluorinated sample, the fluorinated films exhibit significantly increased water contact angles owing to the enrichment of fluorine on the surface. The enhanced hydrophobicity is advantageous for the optical stability when the devices work under a moist environment. Moreover, the films possess high thermo-optic coefficients, tunable refractive indices, and extremely low birefringence coefficients because of the presence of bulky and rigid tetraphenylsilane core and star-shaped topological structure, showing potential application in optical waveguide devices.

Full text of DOI:10.1002/pola.28056

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Fluorinated Star-Shaped Block Copolymers: Synthesis
and Optical Properties
Jing Qu, Xinran Luo, Zhonggang Wang
Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian
116024, China
Correspondence to: Z. Wang (E-mail: zgwang@dlut.edu.cn)
Received 1 December 2015; accepted 10 January 2016; published online 8 February 2016
DOI: 10.1002/pola.28056
ABSTRACT: Tetrakis(4-(1-bromoethyl)phenyl)silane is synthe-
sized and utilized to initiate the atom transfer radical polymer-
ization (ATRP) of methyl methacrylate (MMA) to generate
bromo-terminated four-armed PMMA macroinitiators, which
further initiate the ATRP of methylacryloyloxyl-2-hydroxypropyl
perfluorooctanoate (FGOA) to create fluorinated star-shaped
block copolymers PMMA-b-poly(FGOA)s with fluorine content
ranging from 0 to 31.7 wt %. The polymerizations are well con-
trolled with the polydispersity indices <1.30. The polymers
readily dissolve in common organic solvents and show good
film-formation. Compared with the nonfluorinated sample, the
fluorinated films exhibit significantly increased water contact
angles owing to the enrichment of fluorine on the surface. The
enhanced hydrophobicity is advantageous for the optical sta-
bility when the devices work under moist environment.
a
Moreover, the films possess high thermo-optic coefficients,
tunable refractive indices, and extremely low birefringence
coefficients because of the presence of bulky and rigid tetra-
phenylsilane core and star-shaped topological structure, show-
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ing potential application in optical waveguide devices.
2016
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2016, 54, 1969–1977
KEYWORDS: atom transfer radical polymerization; block copoly-
mer; fluorinated; optical property; star polymer
gyration radius, excellent solubility and low viscosity2,5,7 as
well as low optical birefringence since the star-shaped topol-
ogy can effectively inhibit the crystallinity and orientation of
segments. Especially, the narrow molecular weight distribu-
tion are of great advantage in reducing the line width rough-
ness and line edge roughness of small features for the
fabrication of optical waveguide devices.8
INTRODUCTION Star-shaped polymers are a new class of special
macromolecules attracting great attention because of their fasci-
nating topological architecture with multiple chains stretching
out from a single core,1–7 and some of them have exhibited
promising applications in passive optical waveguide materials,8
microelectronic devices,9 biomedical materials,10,11 solid-state
Lithium-Ion battery,12 and so on. Different from conventional
branched or hyper-branched polymers with many branches ran-
domly linking to the polymer backbone, star-shaped polymers
have regularly branched structure, well-controlled molecular
weight, and narrow molecular weight distribution achieved by
means of living radical polymerization,4,13,14 living anionic poly-
merization,15 living cationic polymerization,16 and “click” chem-
istry17 techniques though various synthetic strategies, including
core-first, arm-first and the mixed use of the two methods.18,19
Relatively, the core-first approach has apparent advantage since,
in this way, the multifunctional core compound initiates the poly-
merization of monomers to generate arms, so the number of
arms are conductive to be predefined.
However, for optical waveguide devices, the optical stability
of the polymer layer under the humid service condition has
been always great concern because the absorbed water will
greatly alter refractive index and increase optical transmis-
sion loss.20–23 It is well known that fluoropolymers possess
low surface energy and high hydrophobicity,24 but most fluo-
ropolymers have poor solubility in common organic solvents.
Moreover, some perfluoropolymers and their copolymers are
conductive to crystallize, resulting in opacity, optical anisot-
ropy, and large birefringence.25 In this regard, the design
and synthesis of star-shaped fluorinated polymers provide
us the possibility to create new optical materials with the
combination of favorable merits of star-shaped polymers and
fluoropolymers to fulfill the stringent requirements of optical
materials.
In comparison with the linear polymers of the same molar
mass, the peculiar topological architecture of star–shaped
polymers leads to a low hydrodynamic volume, small mean
Additional Supporting Information may be found in the online version of this article.
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Based on the above considerations, the present work was
undertaken to synthesize a new tetrafunctional compound
tetrakis(4-(1-bromoethyl)phenyl)silane (BTES). Using BTES
as an initiator, the two-step atom transfer radical polymer-
ization (ATRP) of methyl methacrylate (MMA) and
and the crude product was washed with deionized water and
methanol. Further purification was carried out by recrystalliza-
tion in toluene to yield white crystals. Yield: 75%. FTIR (KBr,
cm21): 3068 and 3015 (arom. CAH), 2956, 2923, and 2853
(aliph. CAH), 1597 and 1499 (arom. C@C), 1439, 1107, and
732 (SiAC). 1H NMR (400 MHz, CDCl3/tetramethylsilane (TMS,
ppm): d 7.5–7.44 (d, 8H), 7.23–7.16 (d, 8H), 2.72–2.60 (q, 8H),
1.30–1.19 (t, 12H). 13C NMR (400 MHz, CDCl3/TMS, ppm): d
145, 137, 132, 127, 29, 15.
methylacryloyloxyl-2-hydroxypropyl
perfluorooctanoate
(FGOA) leads to a series of novel star-shaped fluorinated
block copolymers with fluorine contents over a wide range
from 0 to 31.7 wt%. The fluorinated films are expected to
have excellent resistance to moisture, while the incorpora-
tion of bulky and rigid tetraphenylsilane core can reduce the
orientation degree of segments, and therefore is expected to
greatly decrease the birefringence of polymer films. The syn-
thesis and characterization of BTES initiator, star-shaped
PMMA macroinitiators and fluorinated star-shaped block
copolymers PMMA-b-poly(FGOA)s as well as the influence of
star-topology and fluorine content on hydrophobicity, surface
morphology, and optical properties of fluorinated copolymer
films are studied in detail.
Synthesis of BTES
TES (1.545 mmol; 0.69 g), 1.1 g N-bromosuccinimide (6.18
mmol), 0.011 g benzoyl peroxide (0.045 mmol), and 50 mL
CCl4 were charged into a 250 mL three-neck flask equipped
with a thermometer, a mechanical stirrer, and condenser
under nitrogen atmosphere. The system was heated to the
refluxing temperature, and reacted at this temperature for
6 h. After cooling to room temperature, the mixture was fil-
trated and the filtrate was concentrated by vacuum distilla-
tion. The resulting solid was purified by recrystallization
twice in mixed solvent of toluene and n-hexane to give white
crystals. Yield 47%. Elemental analysis: calculated for
C32H32Br4Si, C 50.29%, H 4.22%; found, C 50.36%, H 4.29%.
IR (KBr, cm21): 3068 and 3015 (CAH arom.), 2958, 2924,
and 2856 (CAH aliph.), 1598 and 1499 (C@C arom.), 1439,
1107, and 732 (SiAC), 1261 and 592 (ACABr). 1H NMR
(400 MHz, CDCI3/TMS, ppm): 7.57–7.48 (d, 8H), 7.48–7.41
(d, 8H), 5.26–5.16 (q, 4H), 2.10–1.98 (d, 12H). 13C NMR
(400 MHz, CDCl3/TMS, ppm): d 145, 137, 134, 126, 49, 27.
EXPERIMENTAL
Materials
Tetrachlorosilane (SiCl4), p-bromoethylbenzene, n-butyl lith-
ium, and perflurooctanoic acid (PFOA) were purchased from
J&K-Chemical and used without further purification. Diethyl
ether was purified by refluxing over sodium with the indica-
tor benzophenone complex. Carbon tetrachloride and N,
N-dimethylformamide (DMF) were from Fuyu Fine Chemical
Industry and were purified by reduced pressure distillation
prior to use. MMA purchased from Tianjin Guangfu Fine
Chemical was washed with 5% aqueous NaOH solution to
remove the inhibitor, and then washed with distilled water,
dried over CaCl2, and distilled twice over CaH2 under
reduced pressure before use. N,N,N’,N’’,N’’-pentamethyldiethy-
lenetriamine (PMDETA) was purchased from Aldrich and
used as received. Copper (I) bromide (CuBr) provided by
Sinopharm Chemical Reagent was purified by stirring over-
night over CH3COOH at room temperature followed by wash-
ing the solid with ethanol before drying at 40 8C in vacuo for
1 day. Glycidyl methacrylate (GMA) was from Guangzhou
Yuanmao Chemical Corp. Benzoyl peroxide from Sinopharm
Chemical Reagent was recrystallized with chloroform and
methanol. FGOA was prepared by reacting PFOA with GMA
according to the procedure described in our previous arti-
cle.26 Other chemical reagents are of reagent grade and used
as received.
Synthesis of Star-Shaped PMMA Macroinitiators
The 50 mL Schlenk flask was flame-dried under vacuum
prior to use, which was degassed under vacuum at room
temperature and back-filled with nitrogen three times. The
freshly distilled MMA and DMF were deoxygenated by purg-
ing with nitrogen for 1 h, respectively. Then, 38.2 mg BTES
(0.05 mmol), 69.3 mg PMDETA (0.4 mmol), 28.7 g CuBr (0.2
mmol), 2 g MMA (20 mmol), and 20 mL DMF were added.
The feed molar ratio of BTES:CuBr:PMDETA:MMA is
1:4:8:400. The system was sealed and then degassed three
times by freezing and thawing. The polymerizations were
conducted at 30 8C under nitrogen atmosphere for 300, 150,
130, and 30 min to obtain four bromo-terminated macroini-
tiators PMMA-1, PMMA-2, PMMA-3, and PMMA-4, respec-
tively. After cooling to room temperature, the polymerization
was stopped by exposing the system to air. The solution was
diluted with acetone and passed a neutral alumina column
to remove the catalyst, and then precipitated in methanol
and dried at 80 8C under vacuum to obtain white solid.
Synthesis of Tetrakis(4-ethylphenyl)silane (TES)
A solution of 32.0 g p-bromoethylbenzene (163.6 mmol) in
anhydrous diethyl ether (70 mL) was stirred at 278 8C under
dry N2 and treated with 65.5 mL solution of n-butyllithium
(2.5 M in hexane, 164 mmol) added dropwise over 1 h. The
resulting mixture was stirred at 278 8C for 0.5 h, and then
4.70 mL SiCl4 (40.93 mmol) in 20 mL diethyl ether was added.
The mixture was stirred at room temperature overnight; 40 mL
1 N aqueous HCl was added cautiously into the system to stop
the reaction. The solvent was removed by rotary evaporation,
Synthesis of Fluorinated Star-Shaped Block Copolymers
Four-armed star-shaped fluorinated block copolymers were
synthesized from fluorinated monomer FGOA using the
bromo-terminated star-shaped PMMA macroinitiator by
ATRP with a molar ratio of bromo-terminated polymer:
CuBr:PMDETA 5 1 : 4 : 8. The four block copolymers were
prepared in the similar procedure, so only the synthesis of
FPMMA-1 is described here as an example. 0.326 g FGOA
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(0.59 mmol), 0.618 g PMMA-1 (0.10 mmol), 0.0143 g CuBr
(0.10 mmol), and 0.347 g PMEDTA (0.20 mmol) and 10 mL
DMF were added in a predried Schlenk flask. The polymer-
ization was carried out at 30 8C for 72 h. After cooled to
room temperature and exposed the catalyst to air, the reac-
tion mixture was diluted with acetone and passed through a
column of neutral alumina to remove the catalyst. The fil-
trate was concentrated under reduced pressure and then
precipitated in excess methanol. After filtered, the solid was
washed with diethyl ether repeatedly to remove the possible
unreacted FGOA monomer. The product was dried at 80 8C
under vacuum to constant weight.
olinium gallium garnet prism. For each sample, at least three
pieces of films were measured and their values were aver-
aged. The refractive indices nTE and nTM for the transverse
electric (TE) and transverse magnetic (TM) modes of the
films were measured by using a Sairon SPA-4000 prism cou-
pler at the wavelengths of 1310 and 1550 nm, respectively,
with the accuracy of 0.001 and the resolution of 6 0.0005.
The birefringences (Dn) were calculated as the difference
between nTE and nTM. The curves of refractive index as a
function of temperature were obtained by measuring the
refractive indices at different sample temperatures from 25
to 85 8C. Their slopes are thermo-optic coefficients.
Preparation of Polymer Films
RESULTS AND DISCUSSION
The films of fluorinated star-shaped block copolymers were
prepared from their 20 wt % DMF solutions, which were fil-
tered through a 0.22-mm Teflon filter and spin coated onto a
silicon wafer. The resulting films were dried at 60 8C for
0.5 h and 120 8C for 4 h to remove the residual solvent
under vacuum.
Synthesis and Characterization of Tetrafunctional
Initiator (BTES)
As shown in Scheme 1, the designed initiator (BTES) con-
tains a wholly aromatic tetraphenylsilane core. The r-p con-
jugation between the electron-deficient silicon atom and
benzene ring results in a electron-withdrawing effect on the
halogenated a-carbon, which is advantageous for the activa-
tion of initiator for ATRP polymerization. Specifically, p-bro-
moethylbenzene was lithiated by n-butyllithium, and then
reacted with SiCl4 via nucleophilic reaction to yield TES. The
bromination of TES by N-bromosuccinimide gave the target
tetraphenylsilane-based tetrafunctional initiator (BTES).
Measurements
Fourier transform infrared spectra (FTIR) were recorded
using a Nicolet 20XB FTIR spectrophotometer in the 400–
4000 cm21 region. Samples were prepared by dispersing the
complexes in KBr and compressing the mixtures to form
disks. Sixty-four scans were signal-averaged with a resolu-
tion of 2 cm21 at room temperature. 1H and 13C NMR spec-
tra were carried out by an INOVA-400 NMR spectrometer
(Varian) using DCCl3 or acetone-d6 as solvent, and TMS as
internal standard. Elemental analyses were determined with
an Elementar Vario EL III elemental analyzer. Gel permeation
chromatography (GPC) analyses were carried out on a PL-
GPC220 system using polystyrene (PS) as standard and tet-
rahydrofuran (THF) as the eluent. DSC curves were meas-
ured by a NETZCH DSC 204 thermal analyzer using N2 as a
purge gas (20 mL/min) at a heating rate of 10 8C/min.
About 5-10 mg sample was put into the aluminum pan for
DSC measurement. XPS measurements were performed in
both survey and high resolution mode on a Thermo ESCA-
LAB 250, with a Monochromatic Al Ka (hm 5 1486.6 eV)
X-ray light source of 150 W at 15 kV. Spectra were acquired
at a takeoff angle of 08, which is defined as the angle
between the sample surface normal and the optical axis of
the photoelectron spectrometer. The survey spectra from 0
to 1000 eV were collected with a pass energy of 50 eV, while
C1s, F1s, and O1s high resolution spectra were acquired with
a pass energy of 20 eV. The atomic force microscopy (AFM)
images were recorded with a PicoPlus II microscope (Agi-
lent) at room temperature in tapping mode. Contact angles
of deionized water were measured on an OCAH200 contact
angle goniometer (Dataphysics) at room temperature. For
each sample, two films were used for contact angle measure-
ments; 3 2 4 drops on different position for one sheet were
tested, and the obtained values were averaged. The standard
deviation of contact angle was found to be within 6 28.
Refractive index and birefringence were measured at room
temperature on a Sairon SPA-4000 prism coupler with a gad-
The chemical structures of BTES and its precursor TES were
confirmed by FTIR, 1H NMR and 13C NMR spectroscopy. In
the FTIR spectra (Fig. S1, Supporting Information), the
absorptions at 1439, 1107, and 732 cm21 are characteristics
of the CASi bond in tetraphenylsilane core. The bands at
2800–2960 cm21 are attributed to aliphatic CAH bonds. For
BTES, the stretching vibrations of CABr at 1261 and
592 cm21 indicates the occurrence of bromination reac-
1
tion.27,28 In the H NMR spectrum of TES (Fig. 1), the signals
of two protons in benzene ring appear at 7.46–7.16 ppm,
whereas the peaks at 2.66 and 1.26 ppm are assigned to the
protons of ACH2A and ACH3 groups, respectively. After bro-
mination, the chemical shifts of protons in benzene ring
move to 7.57–7.41 ppm, and the protons of ACHBrA and
ACH3 groups appear at 5.21 and 2.04 ppm, respectively. The
13C NMR spectra of TES and BTES (Fig. S2, Supporting Infor-
mation) reveal that, compared with TES, after bromination,
the former signals belonging to the ethyl carbon at 29 and
15 ppm shift to 49 and 27 ppm, respectively.
Synthesis and Characterization of Star-Shaped PMMA
Macroinitiators
The bromo-terminated star-shaped PMMA macroinitiators
with different molecular weights were prepared in DMF at
30 8C using BTES as an initiator, CuBr and PMDETA as cata-
lysts. The polymerizations were carried out at a relatively
low temperature to avoid the self-polymerization of MMA.29
The four PMMA macroinitiators with incremental molecular
weights obtained with the polymerization time of 300, 150,
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SCHEME 1 Synthesis routes to tetrakis(4-(1-bromoethyl)phenyl)silane (BTES), star-shaped PMMA macroinitiators and fluorinated
block copolymers.
130, and 30 min are named as PMMA-1, PMMA-2, PMMA-3,
and PMMA-4, respectively.
characteristic bands of PMMA chains can be found. For
example, the absorptions at 2996–2850 cm21 are associated
to the methyl and methylene groups, while the strong peak
at 1732 cm21 corresponds to the carbonyl stretching vibra-
tion of ester group. The 1H NMR spectrum of PMMA-4 is
shown in Figure 2. The weak peaks of phenyl protons of tet-
raphenylsilane core are found at around 7.57 ppm and 7.48
ppm. The peaks at around 1.25–0.75 ppm and 2.02–1.75
ppm are corresponded to the protons of methyl and methyl-
ene groups of PMMA backbone, respectively. The methyl pro-
tons of COOCH3 locate at 3.59 ppm.
The representative FTIR spectrum of PMMA-4 is presented
in Fig. S3 (Supporting Information). Besides the absorptions
of SiAC bond and phenyl groups derived from BTES, the
The number averaged polymerization degree of each PMMA
arm (DPn) and the number average molecular weights (Mn)
of four-armed PMMA macroinitiators are calculated from the
peak areas of COOCH3 and phenyl according to eqs 1 and 2,
respectively.
ACOOCH =3
3
DPnðNMRÞ5
(1)
(2)
A
phenyl=4
MnðNMRÞ543100:123DPn1764:30
where ACOOCH3 and Aphenyl are the peak areas of COOCH3 and
phenyl, respectively; 100.12 is the molecular weight of MMA
unit, whereas 764.30 is the molecular weight of BTES core.
1
FIGURE 1 1H NMR spectra of initiator BTES and its precursor TES.
The data of DPn and Mn calculated from the H NMR spectra
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FIGURE 3 Dependence of ln([M]0/[M]t) and Mw/Mn on polymer-
ization time of MMA at 30 8C using BTES as initiator with
[M]0:[I]0:[CuBr]0:[PMEDTA]0 5 400:1:4:8.
The GPC traces of bromo-terminated star-shaped PMMA
macroinitiators are illustrated in Figure 4, which are mono-
modal with the relatively low polydispersity indices ranging
from 1.12 to 1.24 (Table 1), reflecting the well-controlled
polymerization characteristic. On the other hand, it is seen
that the data of Mn obtained from GPC are significantly
larger than those by 1H NMR method. This deviation is
rational since the number averaged molecular weight
derived from GPC is only a relative value to the linear PS
standard, whereas the hydrodynamic volumes of star-shaped
polymers and are much different from that of linear PS.30,31
FIGURE 2 1H NMR spectra of star-shaped PMMA macroinitiator
(PMMA-4) and PMMA-b-Poly(FGOA) copolymer (F-PMMA-4).
for the star-shaped PMMA macroinitiators are presented in
Table 1. Their Mn values are in the range from 6800 to
25,000 g/mol.
In addition, the polymerization kinetics of MMA monomer ini-
tiated by BTES was studied by plotting the monomer conver-
sion (Ln[M]0/[M]t) against the polymerization time at 30 8C
(Fig. 3). The samples were obtained under the same polymer-
ization conditions with different polymerization time. The
ratio of [I]0:[Cu(I)Br]0:[PMDETA]0:[M]0 is 1:4:8:400, where
[I]0, [Cu(I)Br]0, [PMDETA], and [M]0 refer to the charged
molar concentrations of BETS, Cu(I)Br, PMDETA, and MMA
monomer, respectively, while [M]t is the remaining molar con-
centrations of MMA monomer at time t. As shown in Figure 3,
the variation of ln([M]0/[M]t) with polymerization time fol-
lows a good linear relationship. Therefore, the concentration
of the propagating radicals in the system is constant and the
kinetics obey a first-order dependence on [M]0, indicative of a
controlled-manner polymerization.
Synthesis and Characterization of Star-Shaped
Fluorinated Block Copolymers
Four bromo-terminated star-shaped PMMA macroinitiators
with different molecular weights were used to continuously
initiate the ATRP polymerization of the FGOA, respectively, to
generate four star-shaped fluorinated block copolymers
TABLE 1 Molecular Weights and Distributions of Star-Shaped
PMMA Macroinitiators
1H NMR
GPC
Mn
Mn
Mw
a
Sample
(kg/mol)
DPMMA
(kg/mol)
(kg/mol)
Mw/Mn
PMMA-1
PMMA-2
PMMA-3
PMMA-4
24.9
15.4
14.1
6.8
60.39
36.43
33.38
15.14
44.5
34.0
25.3
12.9
49.5
38.2
29.9
16.0
1.13
1.12
1.18
1.24
FIGURE 4 GPC curves of star-shaped PMMA macroinitiators
and the PMMA-b-Poly(FGOA) copolymers.
a
Number averaged polymerization degree of PMMA arm.
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TABLE 2 Molecular Weights and Distributions of Fluorinated
Star-Shaped Block Copolymers
1H NMR
GPC
F
Mn
Mn
Mw
a
(wt%) (kg/mol) DPFGOA (g/mol) (kg/mol) Mw/Mn
F-PMMA-1 4.16
F-PMMA-2 10.3
F-PMMA-3 13.8
F-PMMA-4 31.7
27.1
19.4
19.2
16.7
1.03
1.82
2.27
4.44
52.0
38.3
36.2
37.2
65.9
44.1
47.1
44.0
1.27
1.12
1.30
1.18
a
Number averaged polymerization degree of poly(FGOA) segment.
PMMA-b-poly(FGOA)s (Scheme 1). Compared with the non-
fluorinated PMMA-4 sample, the FTIR spectrum of the fluori-
nated F-PMMA-4 (Supporting Information Fig. S3) shows a
new absorption at 1782 cm21 corresponding to the ester
bond of poly(FGOA) segment,26 indicating that FGOA mono-
mer has indeed participated in the copolymerization. The
absorption of stretching vibration of C-F bond is overlapped
with that of CAOAC linkage, but the band at 1100–
1250 cm21 in F-PMMA-1 sample becomes broadened and
enhanced, indicating that FGOA monomer has indeed partici-
pated in the copolymerization. The absorption of stretching
vibration of CAF bond is overlapped with that of CAOAC
linkage, but the band at 1100–1250 cm21 in F-PMMA-1 sam-
ple becomes broadened and enhanced.
FIGURE 5 DSC thermograms of star-shaped PMMA-b-Poly(-
FGOA) copolymers.
ane, DMF, N,N-dimethylacetamide, and N-methylpyrrolidone,
reflecting the typical characteristic of star-shaped polymers.
The DSC traces of the star-shaped block copolymers are
shown in Figure 5. It is seen that the glass transition tempera-
tures slightly decrease with the increase of fluorine content.
The similar phenomena have also been observed for other flu-
orinated polymers.32 The reason is contributed to that the
long fluorinated side groups act to open up that polymer
chains to generate large free volume, leading to an increased
motion ability of polymer segments.
The representative 1H NMR spectrum of F-PMMA-4 is pre-
sented in Figure 2. The peak at 3.68 ppm is attributed to the
protons of AOCH3 group in PMMA segment, whereas the
newly emerged peaks at 4.1–5.2 ppm are assigned to the pro-
tons of ACH2CHOHCH2A linkage in poly(FGOA) segment, and
the signal at 2.73 ppm is attributed to the proton of hydroxyl
in FGOA unit. Since the number averaged polymerization
degrees (DPn) of PMMA segment in star-shaped macroinitia-
tors have been obtained by 1H NMR spectra (Table 1), thus,
the average number of FGOA unit per arm can be readily cal-
culated according to the ratio of the peak area of
ACH2CHOHCH2A linkage to that of AOCH3 group. The DPns
of poly(FGOA) segment and the number average molecular
weights of the copolymers derived from1H NMR spectra are
presented in (Table 2). As can be seen, the DPn values of
FGOA segment per arm for the four copolymers are 1.03,
1.82, 2.27, and 4.44, respectively. Accordingly, their fluorine
contents are in the range from 4.16 to 31.7 wt %.
The GPC traces of the star-shaped fluorinated block copoly-
mers are shown in Figure 4. Relative to the PMMA macroini-
tiators, the curves of the copolymers apparently shift toward
left, indicating the increase of molecular weight. Moreover,
all the samples exhibit quite narrow molecular weight distri-
bution with polydispersities in the range from 1.12 to 1.30
(Table 2).
The fluorinated star-shaped copolymers obtained can readily
dissolve in most common solvents such as acetone, THF, diox-
FIGURE 6 XPS survey spectrum (a) and high resolution C1s
spectrum (b) for F-PMMA-2 film.
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and 291 eV, respectively. The assignments above are consist-
ent with the previously reports of other fluorinated poly-
mers.26,34 In addition, the peak at 290 eV is attributed to
carbonyl carbon of ester group adjacent to (CF2)6CF3 group,
whereas that at 288 eV is assigned to the carbonyl carbon of
ester group in PMMA segments.26 It is found that the meas-
ured F/C atomic ratios are significantly higher than the theo-
retical values according to the charged amount of fluorinated
monomer in the polymerization system. For example, the
theoretical F/C ratio in FPMMA-2 is 0.126, whereas the
measured value calculated from the peak areas of fluorine
and carbon signals is as high as 0.324, increased by about
1.9-fold, indicating that the fluorinated segments have quite
strong segregation ability from the bulk to the surface. The
highly hydrophobic fluorinated groups act as an effective
shield against the absorption of water molecules on the poly-
mer films.
FIGURE 7 AFM images (2 3 2 lm2) of films for PMMA macroi-
nitator (a) and F-PMMA-4 (b). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
Hydrophobicity and Optical Properties of Star-Shaped
Fluorinated Block Copolymer Films
AFM measurements for star-shaped non-fluorinated and flu-
orinated PMMA films were performed in tapping mode to
investigate the variation of surface morphology and directly
observe the aggregated fluorinated segments on the topmost
surface. As shown in Figure 7, the surface of nonfluorinated
PMMA film is very smooth and homogeneous. However, with
the introduction of fluorine, large amounts of domains
appear on the AFM image. The similar images have also
been observed in other fluorinated polymer films.26,35,36 Tak-
ing account of the XPS results, it is deduced that these
domains are due to the aggregation of fluorinated groups on
the surface.
PMMA is a transparent amorphous thermoplastic material
widely employed in a variety of important industrial fields
such as the fabrication of optical devices. However, the
PMMA chains contain large amount of polar ester groups.
The strong hydrophilic nature of PMMA leads to an apparent
change of optical parameters upon moving the device to a
moisture environment.20–23,33 Therefore, its optical stability
has to be improved to meet the increasing property require-
ment in optical applications. The introduction of highly
hydrophobic fluorinated groups can be an effective way to
enhance the resistance of PMMA material to corrosion of
moisture.
The effect of fluorine on the hydrophobicity of the polymer
films was studied by the measurements of water contact
angle (h). The surface tensions csv of the films were calcu-
lated by one-liquid method according to the eq 3:37
rffiffiffiffiffiffi
The fluorine information on the surface of fluorinated PMMA
films was examined by XPS method. The XPS survey spec-
trum and high-resolution C1s spectrum of the representative
F-PMMA-2 recorded at the takeoff angle of 08 are shown in
Figure 6. The peaks at 688, 534, and 283 eV are assigned to
the fluorine, oxygen, and carbon elements, respectively.
Moreover, in the high-resolution C1s spectrum, the signals of
CF3 and CF2 groups are found at the binding energies of 293
2
csv
lv2csv
e2bðc
Þ
11cos h52
(3)
clv
where b is an empirical parameter and is 0.0001247 (mN/m)22
,
while csv and clv denote the tensions of the solid-air and liquid-
air interfaces, respectively. h is the contact angle of liquid.
TABLE 3 Refractive Indices and Birefringences of Star-Shaped
Copolymer Films
Wavelength (1310 nm)
᭝na
Wavelength (1550 nm)
᭝na
Sample
nTE
nTM
nTE
nTM
PMMA-1
1.4763 1.4758 0.0005 1.4753 1.4753 0.0001
F-PMMA-1 1.4737 1.4734 0.0003 1.4721 1.4721 0.0000
F-PMMA-2 1.4715 1.4713 0.0002 1.4699 1.4700 0.0001
F-PMMA-3 1.4708 1.4708 0.0000 1.4694 1.4693 0.0000
F-PMMA-4 1.4619 1.4615 0.0004 1.4609 1.4609 0.0000
FIGURE 8 Plots of contact angles of water and surface tensions
a
Birefringence, ᭝n 5 nTE 2 nTM
.
as a function of fluorine content of copolymer films.
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FIGURE 9 Refractive index versus temperature at TE mode and TM mode for (᭿) PMMA-1 at 1310 nm; (
(᭡) F-PMMA-4 at 1310 nm; (᭢) F-PMMA-4 at 1550 nm.
•) PMMA-1 at 1550 nm;
The results in Figure 8 show that the fluorinated star-
shaped polymers exhibit remarkably enhanced hydrophobic-
ity. For example, the water contact angle of the non-
fluorinated film is only 818. With the introduction of fluorine,
the h value remarkably increases to 1058. Correspondingly,
the surface tensions csv of the fluorinated samples decrease
from 34.8 to 19.9 mN/m. The improved hydrophobicity is
favorable for the optical stability when the optical devices
are moved from dry to wet environment. In addition, it is
found that, when the fluorine content exceeds 10 wt %, the
increase of contact angles and decrease of csv values with
fluorine content become rather slow, implying that the
enrichment of fluorine on the surface has approached satura-
tion. Thus, for the star-shaped PMMA polymers, the excellent
hydrophobicity can be achieved by the incorporation of small
amount of fluorine. This property is also desirable for the
practical application based on the cost-saving consideration
since the fluorinated monomers are usually are very
expensive.
fringence of polymer films. Indeed, as shown in Table 3, the
birefringence coefficients of all the polymers at 1310 nm are
<0.0005. Surprisingly, it is found that their birefringence
coefficients at 1550 nm are close to zero, which are even
lower than the measurement accuracy of the prism coupler
employed.
However, the refractive indices of polymer films at different
temperatures from 20 to 85 8C are also measured. For both
the nonfluorinated and fluorinated polymers, the relationship
between refractive indices and temperatures exhibit good
straight lines (Fig. 9). From the slops of lines the thermo-
optical coefficients of polymer films were calculated, and the
data are listed in Table S1 (Supporting Information). It is
seen that the absolute values of the thermo-optical coeffi-
cients of polymer films are in the range from 1.44 3 1024 to
1.59 3 1024, which are about a magnitude order larger than
that of inorganic materials.38 In addition, it is noted that the
absolute values of the thermo-optical coefficients exhibit a
growing trend with the increase of fluorine content. For
polymer materials, the thermal-optical coefficient is propor-
tional to volume coefficient of thermal expansions (a),39
whereas the a value is positively correlated to the free vol-
ume of polymer. As mentioned in the discussion of glass
transition temperature, for the fluorinated star-shaped poly-
mers, the long fluorinated side groups act to open up that
polymer chains to generate large free volume. Therefore, the
end-capping of star-shaped PMMA with poly(FGOA) seg-
ments leads to an increased thermo-optical coefficients. High
thermo-optic coefficients of polymer films are important for
applications in the active optical switches and wavelength fil-
ter devices.
The refractive indices, birefringences, and thermo-optic coef-
ficients of the star-shaped polymer films were measured
with a prism coupler at 1310 and 1550 nm, which wave-
lengths are the most common used for optical communica-
tions. The data in Table 3 show that the refractive indices of
the fluorinated star-shaped copolymers range from 1.4763 to
1.4619 with the incremental fluorine content, indicating that
refractive index of these copolymers can be tuned and con-
trolled through variations of the length of poly(FGOA)
segments.
Birefringence refer to the difference between the refractive
indices at TE and TM modes (᭝n 5 nTE – nTM), which
reflects the optical anisotropy of a material. Low birefrin-
gence is required for optical communications in order to
reduce polarization dependent loss.38 In this work, the
designed polymers have star-shaped topology with a rigid
and bulky tetraphenylsilane core which is expected to effec-
tively hinder the orientation of polymer chains or segments
along the surface of substrate, and thereby decrease the bire-
CONCLUSIONS
In this work, a tetrafunctional ATRP initiator, BTES, was synthe-
sized and characterized. Then, in the presence of CuBr/PMDETA,
BTES was used to initiate the ATRP of MMA to generate
bromine-terminated star-shaped four-armed PMMA macroinitia-
tor, which further initiated polymerization of FGOA to create
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ARTICLE
13 X. Y. Huan, D. L. Wang, R. Dong, C. L. Tu, B. S. Zhu, D. Y.
Yan, X. Y. Zhu, Macromolecule 2012, 45, 5941–5947.
novel star-shaped fluorinated block copolymers PMMA-b-poly(-
FGOA)s with narrow molecular weight distributions (ꢀ1.30) and
varied fluorine contents from 0 to 31.7 wt%. The fluorinated
copolymers exhibit excellent solubility in common organic sol-
vent and good film-formation. The fluorine information, mor-
phology, and hydrophobicity of film surface were investigated by
the measurements of XPS, AFM, water contact angle, and calcula-
tion of surface tension. It is found that the surface tensions of
polymer films remarkably decrease from 34.8 to 19.9 mN/m
after end-capping of the arms with fluorinated segments. The
copolymer films have adjustable refractive indices over a range
of 1.4619–1.4763, and extremely low birefringences. In addition,
they show quite high thermo-optic coefficients of around 1.5 3
10248C21, exhibiting the advantage in decreasing the power con-
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