Inimer graft-copolymerized poly(vinylidene fluoride) for the preparation of arborescent copolymers and surface-active copolymer membranes

Article abstract of DOI:10.1021/ma048894l

A novel graft copolymer was synthesized via graft copolymerization of an inimer, 2-(2-bromoisobutyryloxy)ethyl acrylate (BIEA), with ozone-pretreated poly(vinylidene fluoride) (PVDF) (the PVDF-g-PBIEA copolymer). Porous membranes could be fabricated from the copolymer solution by phase inversion. The BIEA polymer (PBIEA) side chains allowed the initiation of atom transfer radical polymerization (ATRP) of functional monomers on the copolymer and the membrane surface. An arborescent copolymer was prepared via ATRP of sodium 4-styrenesulfonate (NaSS) initiated from the PVDF-g-PBIEA copolymer side chains (PVDF-g-PBIEA-ar-NaPSS copolymer). In comparison with the PVDF-g-PBIEA-ar-NaPSS copolymer membrane cast in water by phase inversion, the copolymer membrane cast in 1 M aqueous NaCl solution had enriched NaPSS side chains on the surface and larger pore size. The PVDF-g-PBIEA-ar-PPEGMA membrane was prepared via the surface-initiated ATRP of poly(ethylene glycol) methacrylate (PEGMA) on the porous PVDF-g-PBIEA membrane. Protein adsorption studies indicated that the PVDF-g-PBIEA-ar-PPEGMA membrane exhibited substantially improved antifouling properties. Thus, the PVDF-g-PBIEA membranes with the active ATRP inimer repeat units on the surface offers opportunities for the functionalization of membranes via surface molecular design.

Full text of DOI:10.1021/ma048894l

7240
Macromolecules 2004, 37, 7240-7249
Inimer Graft-Copolymerized Poly(vinylidene fluoride) for the
Preparation of Arborescent Copolymers and “Surface-Active” Copolymer
Membranes
Gu a n gqu n Zh a i, E. T. Ka n g,* a n d K. G. Neoh
Department of Chemical and Biomolecular Engineering, National University of Singapore,
Kent Ridge, Singapore 119260
Received J une 4, 2004; Revised Manuscript Received J uly 5, 2004
ABSTRACT: A novel graft copolymer was synthesized via graft copolymerization of an inimer, 2-(2-
bromoisobutyryloxy)ethyl acrylate (BIEA), with ozone-pretreated poly(vinylidene fluoride) (PVDF) (the
PVDF-g-PBIEA copolymer). Porous membranes could be fabricated from the copolymer solution by phase
inversion. The BIEA polymer (PBIEA) side chains allowed the initiation of atom transfer radical
polymerization (ATRP) of functional monomers on the copolymer and the membrane surface. An
arborescent copolymer was prepared via ATRP of sodium 4-styrenesulfonate (NaSS) initiated from the
PVDF-g-PBIEA copolymer side chains (PVDF-g-PBIEA-ar-NaPSS copolymer). In comparison with the
PVDF-g-PBIEA-ar-NaPSS copolymer membrane cast in water by phase inversion, the copolymer
membrane cast in 1 M aqueous NaCl solution had enriched NaPSS side chains on the surface and larger
pore size. The PVDF-g-PBIEA-ar-PPEGMA membrane was prepared via the surface-initiated ATRP of
poly(ethylene glycol) methacrylate (PEGMA) on the porous PVDF-g-PBIEA membrane. Protein adsorption
studies indicated that the PVDF-g-PBIEA-ar-PPEGMA membrane exhibited substantially improved
antifouling properties. Thus, the PVDF-g-PBIEA membranes with the active ATRP inimer repeat units
on the surface offers opportunities for the functionalization of membranes via surface molecular design.
1. In tr od u ction
architecture, and specific functionality.25-27 More sig-
nificantly, polymers prepared by ATRP have a living end
functionality that can reinitiate further polymerization
mediated by transition metals. ATRP has been em-
ployed to design and obtain PVDF-based block copoly-
mers. For example, the poly(styrene)-b-poly(vinylidene
fluoride)-b-poly(styrene) (PS-b-PVDF-b-PS) triblock co-
polymers were synthesized from the bromide-termi-
nated PVDF macroinitiator (the Br-PVDF-Br telechelic
polymer).28 The poly(methyl methacrylate)-b-poly(vi-
nylidene fluoride) (PMMA-b-PVDF) diblock copolymers
and poly(methyl methacrylate)-b-poly(vinylidene fluo-
ride)-b-poly(methyl methacrylate) (PMMA-b-PVDF-b-
PMMA) triblock copolymers were synthesized from
iodine-terminated monofunctional and bifunctional PVDF
macroinitiators (PVDF-I and I-PVDF-I telechelic poly-
mers, respectively).29 To functionalize the PVDF mem-
branes by controlled radical polymerization, vinylbenzyl
chloride (VBC) was first graft copolymerized on the
PVDF membrane surface via electron beam preirradia-
tion. Subsequent ATRP of styrene was initiated from
the benzyl chloride groups immobilized on the PVDF
membrane surfaces.30 The ATRP of poly(ethylene glycol)
methacrylate initiated directly by the secondary -CF2-
group of the PVDF main chains has also been re-
ported.31 Other reports indicate that the C-F bond
cannot undergo homolytic cleavage to initiate ATRP of
the monomers involved.28-30,32-35
Poly(vinylidene fluoride) (PVDF) has attracted con-
siderable attention in recent decades because it is
capable of exhibiting ferroelectric, piezoelectric, and
pyroelectric properties.1 As a membrane material, PVDF
has been intensively studied for its application in
ultrafiltration and microfiltration membrane,2-4 protein
adsorption, immobilization and separation,5-7 waster
water treatment,8,9 proton conduction,10-12 and stimuli-
responsive controlled releases.13-16 Graft copolymeri-
zation is a frequently adopted strategy in order to endow
such specific functionalities to the existing PVDF ho-
mopolymers or commercial PVDF membranes. The
radicals or radical-generating moieties are produced on
the PVDF main chains or membrane surfaces initially,
followed by the free-radical polymerization of the func-
tional monomers alongside the PVDF main chains or
on the membrane surface. The approaches to producing
radicals or radical-generating moieties on the existing
membranes have included irradiation by electron
beam9-12 and ion beam17,18 and activation by plasma.19-22
An alternative strategy involved the pretreatment of
PVDF membrane in ethanolic KOH solution to form the
unsaturated CdC bond on the membrane surface. Graft
copolymer chains can also form on the PVDF membrane
surface in the presence of benzyl peroxide and functional
monomers.23 On the other hand, peroxide initiators can
be introduced onto the PVDF backbone by treatment of
PVDF in solution with ozone. Subsequent graft co-
polymerization with a functional monomer gives rise to
PVDF with functional side chains.24
The physical properties of the arborescent copolymers
can differ to a great extent from those of their linear
analogues. For example, water-soluble arborescent poly-
styrene (PS) copolymers had been constructed on the
basis of the iterative grafting of end-functional polysty-
ryllithium (PS-Li) chains onto reactive poly(chloroethyl
vinyl) (PCEV) backbones, followed by chain extension
of the external PS branches via living cationic polym-
erization of the protected hydrophilic vinyl ethers,
As a controlled radical polymerization technique,
atom transfer radical polymerization (ATRP) facilitates
the control over the molecular weight, macromolecular
* To whom all correspondence should be addressed: Fax (65)-
6779-1936; e-mail cheket@nus.edu.sg.
10.1021/ma048894l CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/17/2004

Macromolecules, Vol. 37, No. 19, 2004
Inimer Graft-Copolymerized PVDF 7241
F igu r e 1. Schematic illustration of the process of ozone pretreatment and graft copolymerization of PVDF with inimer BIEA,
preparation of “surface-active” PVDF-g-PBIEA membrane by phase inversion, the molecular functionalization of the PVDF-g-
PBIEA graft copolymer via ATRP of NaSS, preparation of the electrolyte-responsive membrane from PVDF-g-PBIEA-ar-NaPSS
copolymer by phase inversion, and surface-initiated ATRP of PEGMA on the PVDF-g-PBIEA membrane.
removed by molecular sieve (20 nm). Triethylamine and
2-bromoisobutyl bromide were purchased from Aldrich Chemi-
cal Co. of Milwaukee, WI, and were used as received. Poly-
(ethylene glycol) methacryate (PEGMA) (MW ) 360) was also
obtained from Aldrich Chemical Co. The inhibitor in PEGMA
was removed by column chromatography. The anionic mono-
mer, sodium 4-styrenesulfonate (NaSS), the catalysts, copper-
(I) chloride (CuCl) and copper(II) chloride (CuCl2), and the
ligand for the ATRP, bipyridine (bPy), were also purchased
from Aldrich Chemical Co. They were used as received. The
solvents, N-methyl-2-pyrrolidone (NMP) and dimethyl
sulfoxide (DMSO), were obtained from Merck Chemical Co.
and Lab-Scan Asia Ltd. of Bangkok, Thailand, respectively.
They were used as received.
leading to polystyrene cores surrounded by a dense poly-
(vinyl ether) shell. Such arborescent copolymers can be
made water-soluble and to behave like unimolecular
nanoparticles in aqueous media by deprotection of the
hydroxyl functions of the vinyl ether units.36,37
In this work, a novel PVDF graft copolymer with
“living” side chains, from graft copolymerization of an
ATRP inimer with the ozone-pretreated PVDF in solu-
tion, was prepared. ATRP of a functional monomer,
initiated from the repeat units of the inimer side chains,
gave rise to an arborescent copolymer. Both the initial
PVDF-inimer graft copolymer and the arborescent
copolymer could be cast into microporous membranes.
The graft copolymer membrane with the active ATRP
inimer repeat units on the surface could be further
functionalized via surface-initiated ATRP. The reactions
involved are illustrated schematically in Figure 1.
2.2. Syn t h esis of t h e In im er , 2-(2-Br om oisob u t yr yl-
oxy)eth yl Acr yla te (BIEA). The inimer, 2-(2-bromoisobu-
tyryloxy)ethyl acrylate (BIEA), with the structure shown
below, was synthesized according to the procedures reported
in the literature.38
2. Exp er im en ta l Section
2.1. Ma ter ia ls. Poly(vinylidene fluoride) (PVDF, Kynar
K-761) powders (MW ) 441 000) were obtained from Elf
Atochem of North America Inc. They were used as received.
2-Hydroxyethyl acrylate (HEA) was obtained from Fluka
Chemie of Neu-Ulm, Germany. The water content in HEA was
About 200 mL of dry THF, 20 mL of HEA (0.17 mol), and
25 mL of triethylamine (0.17 mol) were added into a 500 mL

7242 Zhai et al.
Macromolecules, Vol. 37, No. 19, 2004
flask, equipped with a magnetic follower and kept in an ice
bath. 28 mL of 2-bromoisobutyl bromide (0.23 mol) was added
into the flask dropwise through a funnel. After the addition,
the flask was sealed, and the mixture was left to react at room
temperature for 14 h. After the reaction, the mixture was
washed with 3 × 200 mL of water to remove the THF, HEA
residue, 2-bromoisobutylcarboxylic acid, and triethylammo-
nium chloride. After drying over anhydrous MgSO4, the
remaining yellow liquid was distilled under vacuum at 75 °C
for about 30 min. About 30 g of a yellow viscous liquid was
obtained. Details on the polymerization of the inimer had been
reported.39,40
2.2. Th er m a lly In d u ced Gr a ft Cop olym er iza tion of
BIEA w ith th e Ozon e-P r etr ea ted P VDF (P VDF -g-P BIEA
Cop olym er ). The preparation of PVDF-g-PBIEA copolymer
was carried out in a similar approach as reported earlier for
the graft copolymerization of vinyl monomer with the ozone-
pretreated PVDF.41 The PVDF powders were dissolved in NMP
to form a 7 wt % solution. A continuous O3/O2 mixture stream
was bubbled through 30 mL of the PVDF/NMP solution at
room temperature (∼25 °C) for about 15 min, leading to a
peroxide content of about 10-4 mol/g.41 The O3/O2 mixture
stream was generated from an Azcozon RMU16-40EM ozone
generator. The flow rate was adjusted to 300 L/h to result in
an ozone concentration of about 0.027 g/L in the gaseous
mixture. After the ozone preactivation, the polymer solution
was cooled in an ice bath. An argon stream was introduced
for about 30 min to degas the ozone and oxygen dissolved in
the solution.
copolymer). After the membrane had detached from the glass
plates, it was extracted in a second bath of doubly distilled
water at 70 °C for 15 min. This procedure was to stabilize the
pore structure and to refine the pore size distribution. The
purified MF membrane was dried by pump under reduced
pressure for the subsequent characterization.
2.5. Su r fa ce-In itia ted ATRP of P EGMA on P VDF -g-
P BIE A MF m em b r a n es (P VDF -g-P BIE A-a r -P P E GMA
Mem br a n e). For the surface-initiated ATRP of the mac-
romonomer, PEGMA, on the PVDF-g-PBIEA MF membrane,
15 mL of doubly distilled water, 2 mL of PEGMA, and 1 piece
of the PVDF-g-PBIEA MF membrane of about 1 cm × 1 cm in
area were added into a 25 mL single-necked round-bottom
flask. A purified argon stream was introduced to degas the
mixture for about 15 min. 158 mg of 2,2′-bipyridine, 13.5 mg
of CuCl2, and 40 mg of CuCl were added to the solution. The
polymerization reaction was allowed to proceed for 1 h with
stirring. After the reaction, the membrane was washed with
copious amount of doubly distilled water over a period of about
2 h, followed by drying under reduced pressure. The process
of the surface-initiated ATRP of PEGMA on the PVDF-g-
PBIEA membrane is also shown schematically in Figure 1.
2.6. F ou r ier Tr a n sfor m In fr a r ed (F TIR) a n d Nu clea r
Ma gn etic Reson a n ce (NMR) Sp ectr oscop ies. After disper-
sion in KBr, the FTIR spectra of the copolymers were mea-
sured on a Bio-Rad FTS 135 FT-IR spectrophotometer. Each
spectrum was collected by cumulating 16 scans at a resolution
1
of eight wavenumbers. H NMR spectroscopy of the PVDF-g-
PBIEA copolymer was measured on
a Bruker ARX 300
Two grams of BIEA monomer and 12 mL of NMP were then
added into the reaction mixture. After an additional 15 min
of argon purging, the temperature of the water bath was raised
to 60 °C to induce the decomposition of peroxide groups on
the PVDF chains and to initiate the graft copolymerization of
BIEA. After 6 h of reaction, the reaction mixture was cooled
in an ice bath, and the resultant PVDF-g-PBIEA copolymer
was precipitated in an excess amount of absolute ethanol. After
filtering, the copolymer was redissolved in NMP and repre-
cipitated in ethanol. The copolymer was washed in an excess
volume of ethanol for 48 h. The solvent was changed every 8
h. The copolymer sample was recovered and dried by pump
under reduced pressure for subsequent characterization. The
processes of ozone preactivation of PVDF and thermally
induced graft copolymerization of BIEA are shown schemati-
cally in Figure 1.
2.3. Syn th esis of th e Ar bor escen t Cop olym er (P VDF -
g-P BIEA-a r -Na P SS) via Atom Tr a n sfer Ra d ica l P olym -
er iza tion (ATRP ). 0.5 g of PVDF-g-PBIEA copolymer powder
was introduced into 15 mL of DMSO. The temperature of the
thermostated bath was increased to 50 °C to facilitate the
dissolution, which took about 2 h. After the complete dissolu-
tion, the water bath was cooled to room temperature. An argon
stream was introduced to degas the DMSO solution for 30 min.
2 g of sodium 4-styrenesulfonate (NaSS), 158 mg of 2,2′-
bipyridine, 13.5 mg of CuCl2, and 40 mg of CuCl were added
to the solution. After the addition and dissolution, the solution
became a brown liquid. The mixture was sealed and allowed
to react at 40 °C for 6 h. At the end of the reaction, the reaction
mixture was cooled in an ice bath, and the copolymer was
precipitated in excessive ethanol. After recovery by filtration,
the copolymer was washed in an excess volume of ethanol for
48 h. The solvent was changed every 8 h. The resulting
arborescent copolymer (PVDF-g-PBIEA-ar-NaPSS) was recov-
ered and dried by pump under reduced pressure for subse-
quent characterization. The preparation of the arborescent
PVDF-g-PBIEA-ar-NaPSS copolymer is also shown schemati-
cally in Figure 1.
instrument at room temperature with deuterated DMSO as
the solvent.
2.7. Th er m ogr a vim etr ic An a lysis (TGA). The thermal
stability of the copolymers was studied by thermogravimetric
analysis (TGA). The samples were heated from room temper-
ature to about 700 °C at a rate of 10 °C/min under a dry
nitrogen atmosphere in a Du Pont Thermal Analyst 2100
system, equipped with a TGA 2050 thermogravimetric thermal
analyzer.
2.8. X-r a y P h otoelectr on ic Sp ectr oscop y (XP S). The
surface composition of the membranes was measured by XPS.
XPS measurements were carried out on a Kratos AXIS HSi
spectrometer, under conditions similar to those reported in the
earlier work.41
2.9. Sca n n in g Electr on Micr oscop y (SEM). The surface
morphology of the MF membranes was studied by scanning
electron microscopy (SEM), using a J EOL 6320 scanning
electron microscope. The membranes were mounted on the
sample studs by means of double-sided adhesive tapes. A thin
layer of palladium was sputtered onto the membrane surface
prior to the SEM measurement. The measurements were
performed at an accelerating voltage of 15 kV.
2.10. Cop olym er Com p osition An a lysis. The carbon and
bromide elemental contents were determined by the Mi-
croanalysis Laboratory, National University of Singapore.
Taking into account the stoichiometries of the graft and the
fluoropolymer chains, the bulk graft concentration of the
PVDF-g-PBIEA copolymer, defined as the number of BIEA
repeat units per repeat unit of PVDF, or the ([-BIEA-]/[-
CH2CF2-])bulk ratio, can be calculated from eq 1:
([-BIEA-]/[-CH2CF2-])bulk ) 2[Br]/([C] - 9[Br]) (1)
where the factors 2 and 9 are introduced to account for the
fact that there are 2 and 9 carbon atoms per repeat unit of
PVDF and BIEA polymer, respectively. The graft concentration
of NaSS in the PVDF-g-PBIEA-ar-NaPSS arborescent copoly-
mer, or the ([-NaSS-]/[-CH2CF2-])bulk ratio, is estimated
from eq 2:
2.4. F a br ica tion of Micr ofiltr a tion (MF ) Mem br a n es.
The MF membranes of the PVDF-g-PBIEA and PVDF-g-
PBIEA-ar-NaPSS copolymers were prepared by phase inver-
sion. The copolymer powders were dissolved in NMP to a
concentration of 12 wt % at room temperature. The copolymer
solution was first cast onto a glass plate. The glass plate was
subsequently immersed in doubly distilled water (and also in
1 M aqueous NaCl solution for the PVDF-g-PBIEA-ar-NaPSS
([-NaSS-]/[-CH2CF2-])bulk ) 2[S]/([F])
(2)
where the factor 2 accounts for the fact that there are two

Macromolecules, Vol. 37, No. 19, 2004
Inimer Graft-Copolymerized PVDF 7243
fluorine atoms per repeat unit of the PVDF polymer and one
sulfur atom per repeat unit of the NaPSS polymer, respec-
tively.
2.11. P r otein Ad sor p tion Assa y. To assess the effect of
surface-initiated ATRP of PEGMA from the PVDF-g-PBIEA
membrane on the resistance of the latter to protein fouling,
protein adsorption assays were performed on both the PVDF-
g-PBIEA membrane and the PVDF-g-PBIEA-ar-PPEGMA
membrane (time of polymerization ) 1 h). The membranes
(about 1 cm × 1 cm) were hydrated in methanol for 30 min
initially, followed by washing three times with the phosphate-
buffer saline (0.01 M PBS, pH ) 7.4). The membranes were
subsequently equilibrated in PBS solution for 1 h prior to the
protein adsorption experiment. For the protein adsorption, the
membranes were immersed in the PBS containing 2 mg/mL
of γ-globulin for 24 h at room temperature. After the protein
adsorption, the membranes were washed with copious amount
of PBS and doubly distilled water. The membranes were dried
by pumping under reduced pressure at room temperature. The
surface composition of the membranes after protein adsorption
was analyzed by XPS, using the N 1s signal as a marker for
the adsorbed protein.
3. Resu lts a n d Discu ssion
F igu r e 2. FTIR spectra of the (a) PVDF homopolymer, (b)
PVDF-g-PBIEA copolymer, (c) PVDF-g-PBIEA-ar-NaPSS co-
polymer, and (d) NaPSS homopolymer.
3.1. P r ep a r a tion of th e P VDF -g-P BIEA Gr a ft
Cop olym er . 3.1.1. P r ep a r a tion of th e In im er , 2-(2-
Br om oisobu tyr yloxy)eth yl Acr yla te (BIEA). The
initiator monomer or the AB*-type “inimer” (A: poly-
merizable group; B*: initiator moiety) is designed for
the synthesis of hyperbranched polymers because the
AB* inimer can become involved in polymerization and
initiation simultaneously.42,43 For the addition polym-
erization, “A” typically represents the CdC double bond,
while a variety of initiator moieties can be introduced
as “B*”. Examples of “B*” include an azo group for
conventional radical polymerization,44 an alkoxy or
chloroalkyl group for living cationic polymerization,42,45
an organolithium group for anionic living polymeriza-
tion,46 a nitroxide group for stable free radical polym-
erization,47 an alkyl halide group for transition metal-
catalyzed atom transfer radical polymerization
(ATRP),38-40 a dithioester group for photoinitiated ad-
dition-fragmentation chain transfer process,48 and a
trimethylsiloxyl group for group transfer polymerization
(GTP).49
peroxide groups decomposed into radicals under thermal
induction to initiate the chain polymerization of the
vinyl monomer to produce the graft copolymer.24,51 On
the basis of the activation energy and Arrhenius coef-
ficient reported in the literature, the half-life of this kind
of peroxide groups is estimated to be about 45 min at
60 °C.41 Thus, in this study, the time of polymerization
was set at 6 h, both to increase the graft concentration
of the BIEA side chains and to consume the peroxide
residues completely.
Elemental analysis results indicate that the ([Br]/
[C])bulk ratio of the copolymer is about 0.02. The bulk
graft concentration of the BIEA polymer (PBIEA) in
PVDF-g-PBIEA, defined as the number of BIEA repeat
units per repeat unit of PVDF, or the ([-BIEA-]/[-CH2-
CF2-])bulk ratio, is estimated by eq 1 (Experimental
Section) to be about 0.05. Figure 2a,b shows the respec-
tive FTIR spectra of the pristine PVDF homopolymer
and the PVDF-g-PBIEA copolymer. For the pristine
PVDF homopolymer, the characteristic absorption band
in the wavenumber region of 1120-1350 cm-1 is as-
sociated with the -CF2- group of the PVDF main
chains.50 A new absorption band at the wavenumber of
about 1740 cm-1, attributable to the ester carbonyl
group,50 has appeared after graft copolymerization with
BIEA. Thus, the FTIR spectroscopic results confirm the
presence of the BIEA side chains, in agreement with
the results of the elemental analysis.
Figure 3 shows the thermal gravimetric analysis
(TGA) curves of the PVDF homopolymer and the PVDF-
g-PBIEA copolymer. The PVDF homopolymer exhibits
only one thermal decomposition step, commencing at
about 470 °C. On the other hand, the PVDF-g-PBIEA
copolymer exhibited a two-step thermal decomposition
process. The first main weight loss occurs at about 300
°C, attributable to degradation of the PBIEA side
chains, while the second weight loss commences at
about 470 °C, attributable to the decomposition of the
PVDF main chains. The TGA curve also shows that the
weight loss during the first decomposition step is about
14 wt %, which translates into a ([-BIEA-]/[-CH2CF2-
])bulk ratio of about 0.06. This ratio is comparable to that
obtained from the elemental analysis. Both the FTIR
As a controlled radical polymerization technique,
ATRP is versatile to a variety of monomers containing
appropriate functional groups. In this study, the ATRP
inimer, BIEA, was prepared via the esterification of
HEA with 2-bromoisobutyryl bromide in dry THF and
in the presence of triethylamine to neutralize the
hydrogen bromide produced. The Fourier transform
infrared (FTIR) spectra of HEA and BIEA were com-
pared. After the esterification reaction, the absorption
band at the wavenumber of about 3600 cm-1 50
associ-
,
ated with the hydroxyl group of HEA, has disappeared
almost completely, and the absorption band at the
wavenumber of about 1740 cm-1, associated with the
ester carbonyl group,50 has been enhanced. The results
indicate that esterification reaction has occurred be-
tween HEA and 2-bromoisobutyryl bromide to form the
AB* inimer, BIEA. The broad absorption band in the
wavenumber region of 1100-1400 cm-1 is attributable
to the dimethyl group of the initiator moiety.50
3.1.2. Ozon e P r etr ea tm en t a n d Th er m a lly In -
d u ced Gr a ft Cop olym er iza t ion of P VDF w it h
BIEA: P VDF -g-P BIEA Cop olym er . Ozone pretreat-
ment of the PVDF solution was adopted to generate
peroxide groups along the polymer main chain. The

7244 Zhai et al.
Macromolecules, Vol. 37, No. 19, 2004
transfer to the brominated monomer during graft co-
polymerization probably has not occurred to a signifi-
cant extent, as the process can lead to cross-linking. The
PVDF-g-PBIEA copolymer remains soluble and can be
readily cast into porous membranes (see below).
3.2. P VDF -g-P BIEA Mem br a n es Ca st by P h a se
In ver sion . The PVDF-g-PBIEA microporous mem-
branes were fabricated by phase inversion in doubly
distilled water at room temperature from 12 wt % NMP
solutions of the PVDF-g-PBIEA copolymer. The mor-
phology and surface chemical composition of the mem-
brane were analyzed by SEM and XPS, respectively.
Figure 5a,b shows the respective SEM images, obtained
at a magnification of ×2000, for the air and glass
substrate sides of the PVDF-g-PBIEA membranes.
Thus, the PVDF-g-PBIEA copolymer can be cast into a
microporous membrane by the phase inversion tech-
nique. Cross-sectional SEM images also indicate that
the membrane is symmetric, although the porosity and
pore size distribution for the two surfaces can differ
substantially.
Figure 6a-c shows respectively the XPS wide-scan,
Br 3d, and C 1s core-level spectra of the PVDF-g-PBIEA
membrane. Carbon, bromine, fluorine, and oxygen
signals are detected in the wide-scan spectrum of the
PVDF-g-PBIEA membrane. The Br 3d core-level spec-
trum was resolved into Br 3d3/2 and Br 3d5/2 peak
components with an energy separation of 1.05 eV. The
Br 3d5/2 peak component with a binding energy at about
70.5 eV is characteristic of that of covalently bonded
bromine.53 The surface graft concentration of the PVDF-
g-PBIEA membrane, or the ([-BIEA-]/[-CH2CF2-
])surface ratio, is determined to be about 0.07 from eq 1,
based on the XPS-derived ([Br]/[C])surface ratio of about
0.03. The ([-BIEA-]/[-CH2CF2-])surface ratio is slightly
higher than the bulk graft concentration of the PVDF-
g-PBIEA copolymer, indicating that surface enrichment
of the BIEA side chains occurs to some extent during
the phase inversion process of the PVDF-g-PBIEA/NMP
copolymer solution in water.
F igu r e 3. (a) TGA weight loss curves of (1) PVDF homopoly-
mer, (2) PVDF-g-PBIEA copolymer ([-BIEA-]/[-CH2CF2-]bulk
)
0.05), and (3) PVDF-g-PBIEA-ar-NaPSS copolymer
([-NaSS-]/[-CH2CF2-]bulk ) 0.22). (b) TGA derivative curves
of (1) the PVDF-g-PBIEA copolymer and (2) the PVDF-g-
PBIEA-ar-NaPSS copolymer.
The C 1s core-level spectrum of the PVDF membrane
was curve-fitted with peak components for the -CH2-
and -CF2- species, at the BE of about 286.0 and 290.5
eV, respectively (Figure 6d).54 The molar ratio of the
two species, determined from the spectral area ratio of
the two peak components, is about 1.06, in good agree-
ment with the theoretical value of 1:1. The C 1s core-
level spectrum of the PVDF-g-PBIEA membrane (Figure
6b) was curved-fitted with five species, adopting the
following strategy. The peak components of approxi-
mately equal intensity with BE’s at 286.0 and 290.5 eV
are attributed to the -CH2- and -CF2- species, respec-
tively, of the PVDF main chains. The peak components
with BE’s at about 284.6 and 288.8 eV are associated
respectively with the neutral -CH2- and the OdC-O
species of the BIEA side chains. The peak component
with the BE at about 286.5 eV arises from combined
contributions of the C-O species and the C-Br termi-
nus of the BIEA side chains. For the comparison of the
C 1s core-level spectra of the PVDF and PVDF-g-PBIEA
membranes, it is clear that the peak intensity of the
-CF2- species of the PVDF main chains has decreased
substantially, after grafting of the BIEA side chains.
The molar ratio of the -CF2- species to the total carbon
species in the PVDF-g-PBIEA membrane is determined
to be about 0.28, significantly lower than about 0.48 for
the membrane prepared from the PVDF homopolymer.
F igu r e 4. 1H NMR spectrum of the PVDF-g-PBIEA copoly-
mer.
spectra and the TGA results confirm that the BIEA have
been graft-copolymerized with the ozone-pretreated
PVDF main chains.
The chemical structure of the PVDF-g-PBIEA copoly-
mer is shown in Figure 4 and verified by 1H NMR
spectroscopy. The chemical shifts at 2.9 and 2.2 ppm
are attributable to the CH2 species of PVDF, arising
from the head-to-tail (ht) and head-to-head (hh) stereo-
regularities. On the other hand, the chemical shifts at
4.2-4.4 and 1.9-2.0 ppm are assigned to the -CH2CH2-
species and the isobutyryl species of the PBIEA side
chains, respectively.52 The 1H NMR and the TGA results
of the PVDF-g-PBIEA copolymer also indicate that

Macromolecules, Vol. 37, No. 19, 2004
Inimer Graft-Copolymerized PVDF 7245
F igu r e 5. SEM micrographs of the membranes cast from the 12 wt % NMP solution of corresponding copolymer by phase
inversion: (a) air side and (b) substrate (glass plate) side of PVDF-g-PBIEA membrane cast in water; (c) air and (d) substrate
side of PVDF-g-PBIEA-ar-NaPSS membrane cast in water; (e) air and (f) substrate side of PVDF-g-PBIEA-ar-NaPSS membrane
cast in 1 M aqueous NaCl solution.
F igu r e 6. XPS wide-scan, Br 3d, and C 1s core-level spectra of the PVDF-g-PBIEA membrane and C 1s core-level spectrum of
the PVDF membrane. Both membranes are cast from their corresponding 12 wt % NMP solution in doubly distilled water by the
phase inversion technique.
3.3. Th e P VDF -g-P BIEA-a r -Na P SS Ar bor escen t
Cop olym er via ATRP . The BIEA side chains of the
PVDF-g-PBIEA copolymer and on the PVDF-g-PBIEA
membrane surface can function as the ATRP macro-
initiators because the tertiary C-Br moiety of each
BIEA repeat unit will be able to initiate the ATRP of
functional monomers.25 The ATRP of substituted sty-
renic monomers had been well investigated using the
Cu/bPy catalyst system.55 In specific, the ATRP of
sodium 4-styrenesulfonate (NaSS) has been carried out
in the presence of the Cu(I)/bPy catalyst system in a
water/methanol mixed medium to produce polyelectro-
lytes dissolved in solution or grafted on the silica
particle surface.56,57 As an amphiphilic polyelectrolyte,
the NaSS polymer (NaPSS) can switch its conformation
from an extended to a coiled state in an aqueous
medium upon the addition of a low molecular weight
electrolyte because the latter can shield the electrostatic
repulsion among the sulfonate anions alongside the
NaPSS chains. Thus, NaPSS exhibits an electrolyte-

7246 Zhai et al.
Macromolecules, Vol. 37, No. 19, 2004
F igu r e 7. XPS wide-scan, C 1s, S 2p, and Na 1s core-level spectra of the PVDF-g-PBIEA-ar-NaPSS membranes cast from the 12
wt % NMP solution by phase inversion in doubly distilled water and in 1 M aqueous NaCl solution.
responsive behavior (the polyelectrolyte effect).58-60
large weight loss in the temperature range of 300-480
°C, attributable to the decomposition of the PBIEA and
NaPSS graft chains. Figure 3b shows the derivative of
the weight curves as a function of the temperature for
the PVDF-g-PBIEA and PVDF-g-PBIEA-ar-NaPSS co-
polymers. For the PVDF-g-PBIEA copolymer, the de-
rivative curve contains two major peaks at temperatures
of 310 and 490 °C, attributable to the PBIEA side chains
and PVDF main chains, respectively. However, for the
PVDF-g-PBIEA-ar-NaPSS copolymer, these two peaks
diminish, and a new decomposition peak emerges at the
temperature of about 425 °C, attributable to the NaPSS
side chains. The NaPSS content of the copolymer cannot
be determined unambiguously from the weight loss
curve of the copolymer. Attempts were made to improve
the bulk graft concentration of NaPSS in the PVDF-g-
PBIEA-ar-NaPSS copolymer by increasing the NaSS
monomer concentration in the reaction mixture or
extending the time of polymerization from 6 to 12 h. It
was found that the solubility of the PVDF-g-PBIEA-ar-
NaPSS copolymer in NMP was compromised by the high
content of NaPSS. Therefore, the ATRP of NaSS initi-
ated from PVDF-g-PBIEA was not carried out at a
higher monomer feed ratio or for a longer polymeriza-
tion time.
The arborescent copolymer with the NaPSS side
chains, or the PVDF-g-PBIEA-ar-NaPSS copolymer, was
prepared via solution polymerization of NaSS, initiated
by the BIEA repeat units of the PVDF-g-PBIEA copoly-
mer and catalyzed by the CuCl/CuCl2/bPy system in
DMSO. The sulfur, fluorine, and carbon contents of the
PVDF-g-PBIEA-ar-NaPSS copolymer were determined
by elemental analysis. The ([S]/[F])bulk ratio of the
copolymer was determined to be about 0.11. Thus, the
bulk graft concentration, defined as the number of NaSS
repeat units per PVDF repeat unit, or the ([-NaSS-]/
[-CH2CF2-])bulk ratio, is determine to be about 0.22 (eq
2, Experimental Section). The FTIR spectra of the
PVDF-g-PBIEA-ar-NaPSS copolymer and NaPSS ho-
mopolymer are shown and compared in Figure 2. After
copolymerization with NaSS, a new absorption band in
the wavenumber region of 1050 cm-1, associated with
the sulfonate groups of the NaPSS side chains,50 ap-
pears in the FTIR spectrum of the PVDF-g-PBIEA-ar-
NaPSS copolymer. On the other hand, the absorption
band in the wavenumber region of 1100-1300 cm-1
corresponding to the aromatic C-H in-plane bending
mode,50 also serves to indicate that the NaPSS side
chains have been incorporated into the copolymer.
,
The TGA curve of the PVDF-g-PBIEA-ar-NaPSS
copolymer was compared to that of PVDF-g-PBIEA
polymer in Figure 3. Figure 3a shows the weight loss
curves of the PVDF homopolymer, the PVDF-g-PBIEA
copolymer, and the PVDF-g-PBIEA-ar-NaPSS copoly-
mer. In comparison with the PVDF-g-PBIEA copolymer,
the PVDF-g-PBIEA-ar-NaPSS copolymer exhibits a
3.4. Electr olyte-Respon sive Mem br an e Cast fr om
th e P VDF-g-P BIEA-a r -Na P SS Cop olym er . The mor-
phology, surface composition, and the electrolyte-
responsive nature of the PVDF-g-PBIEA-ar-NaPSS
membranes were investigated. Figure 7 shows the
respective XPS wide-scan, C 1s, S 2p, and Na 1s core-
level spectra of the PVDF-g-PBIEA-ar-NaPSS mem-

Macromolecules, Vol. 37, No. 19, 2004
Inimer Graft-Copolymerized PVDF 7247
branes cast from a 12 wt % NMP solution of the PVDF-
g-PBIEA-ar-NaPSS copolymer by phase inversion in
doubly distilled water and in 1 M aqueous NaCl solu-
tion. In the wide-scan spectra of the two membranes,
not only are the C 1s, F 1s, and O 1s signals detected,
the Na 1s and S 2p signals are also discernible. For both
membranes, the intensity of the Br 3d signal for the
PVDF-g-PBIEA-ar-NaPSS membrane has been reduced
considerably, in comparison with that of the Br 3d signal
for the PVDF-g-PBIEA membrane. The membrane cast
in 1 M aqueous NaCl solution exhibits higher Na 1s and
S 2p signal intensities compared to those of the mem-
brane cast in water. The XPS-derived surface elemental
composition, or the ([C]:[Na]:[S]:[Br])surface ratio, deter-
mined from the respective spectral peak areas, shifted
from 100:1.3:2.0:0.3 to 100:4.0:4.6:0.8 when the ionic
strength of the casting bath was increased from about
0 (for doubly distilled water) to 1 M (for the 1 M aqueous
NaCl solution). Thus, the surface composition data
indicate a significant surface enrichment of the PBIEA-
g-NaPSS side chains for the membrane cast in 1 M
aqueous NaCl solution.
revealed a “through pore” structure of the membrane,
although the pore size differs on both sides of the
membrane.
The nature of the NaPSS side chain as a polyelectro-
lyte accounts for the electrolyte-responsive character-
istics of the surface chemical composition and morphol-
ogy of the PVDF-g-PBIEA-ar-NaPSS copolymer mem-
brane. When the PVDF-g-PBIEA-ar-NaPSS polymer
solution was cast into doubly distilled water, the strong
electrostatic repulsion among the sulfonate anions
alongside the NaPSS chains on the newly formed
membrane surface prohibited extensive migration of the
NaSS side chains from the bulk of the polymer solution
to the outer layer of the membranes. On the other hand,
when cast into 1 M aqueous NaCl solution, the electro-
static repulsion was mostly shielded by the electrolyte
ions in the casting bath (polyelectrolyte effect), allowing
more extensive migration of the NaPSS side chains to
the membrane surface, including the surface of the
pores.
3.5. F u n ction a liza tion of th e P VDF -g-P BIEA
Mem br an e via Su r face-In itiated ATRP of th e P oly-
(eth ylen e glycol) Meth a cr yla te (P EGMA): An ti-
fou lin g P VDF -g-P BIEA-a r -P P EGMA Mem br a n e.
For the porous membrane cast from the PVDF-g-PBIEA
copolymer, each repeat unit of the PBIEA side chains
on the membrane and pore surfaces can initiate the
ATRP of a functional monomer to endow specific surface
functionalities to the as-cast PVDF-g-PBIEA mem-
branes. Poly(ethyl glycol) (PEG) and poly(ethylene
oxide) (PEO) have attracted considerable attention
because they can impart antifouling, nonadhesive, and
biocompatible characteristics to the materials.62,63 ATRP
of the PEGMA macromonomer has been extensively
studied during the past few years because the process
allows the preparation of well-defined PEGMA polymer
(PPEGMA).64-66 The Cu/bPy catalyst system can also
result in a controlled fashion in the ATRP of methacry-
late monomer;66,67 therefore, in this study, PEGMA was
graft copolymerized directly onto the PVDF-g-PBIEA
membrane surface, via ATRP in an aqueous medium
and catalyzed by the CuCl/CuCl2/bPy system, with the
intention of improving the antifouling properties of the
PVDF-g-PBIEA membrane.
The C 1s core-level spectra of the PVDF-g-PBIEA-ar-
NaPSS membranes cast in doubly distilled water and
in 1 M aqueous NaCl solution were curve-fitted using
the approach similar to that used for the PVDF-g-
PBIEA membrane, except that the peak component with
BE at about 286.5 eV is deemed to have resulted from
a combined contribution from the C-O, C-Br, and
C-SO3 species.61 From the C 1s spectra, it is clear that
the intensity of the -CH- species, associated with the
PBIEA-g-NaPSS side chain, increases while that of the
(-CF2-)PVDF species decreases when the ionic strength
of the casting bath is increased to 1 M. The XPS-derived
([-CF2-]PVDF/[-CH-])surface ratio, determined from the
corresponding C 1s peak component spectral area ratio,
decreases from 1.1 to 0.86, suggesting that more PBIEA-
g-NaPSS side chains have migrated from the bulk to
the surface during the phase inversion process in 1 M
aqueous NaCl solution. The S 2p core-level spectrum of
the PVDF-g-PBIEA-ar-NaPSS membrane, cast in dou-
bly distilled water, was curve-fitted with S 2p1/2 and S
2p3/2 peak components with an energy separation of 1.2
eV, as shown in Figure 7.
The microporous morphology of the PVDF-g-PBIEA-
ar-NaPSS membranes is also dependent on the electro-
lyte concentration. Figure 5c-f shows the respective
SEM micrographs, obtained at a magnification of ×2000,
for the air side and substrate side of the PVDF-g-
PBIEA-ar-NaPSS membranes cast in doubly distilled
water and in 1 M aqueous NaCl solution. With the
increase in ionic strength of the casting bath, the
morphology and pore structure do not change signifi-
cantly on the substrate side. However, significant dif-
ferences in the surface morphology and pore structure
are observed on the air side of the PVDF-g-PBIEA-ar-
NaPSS membrane. The membrane cast in 1 M aqueous
NaCl solution exhibits a much larger pore size and a
less uniform pore size distribution, in comparison to
those of the membrane cast in doubly distilled water.
When cast in the aqueous NaCl medium, the membrane
surfaces is enriched by the NaPSS chains. Pores of
larger size are formed to minimize the interfacial
interaction between the membrane surface and the
aqueous electrolyte medium, as larger pores correspond
to smaller specific surface area. The cross-sectional SEM
images of the PVDF-g-PBIEA-ar-NaPSS membranes
Figure 8a shows the XPS wide-scan and C 1s core-
level spectra of the PVDF-g-PBIEA-ar-PPEGMA mem-
brane after 1 h of surface-initiated ATRP of PEGMA.
Compared to those of the PVDF-g-PBIEA membrane
(see Figure 6), significant changes in the XPS wide-scan
and C 1s core-level spectra of the PVDF-g-PBIEA-ar-
PPEGMA membrane are observed. The F 1s signals
associated with the underlying PVDF main chains are
no longer discernible in the wide-scan spectrum, indi-
cating that the thickness of the PEGMA polymer film
on the PVDF-g-PBIEA membrane surface, after a 1 h
of polymerization, has exceeded the sampling depth of
the XPS technique (∼7.5 nm for organic matrix68). The
same result is also deduced from the changes in the C
1s core-level line shape of the PVDF-g-PBIEA-ar-
PPEGMA membrane. The C 1s core-level spectrum of
the PVDF-g-PBIEA-ar-PPEGMA membrane was curve-
fitted using the similar approach as that for the PVDF-
g-PBIEA membrane. However, the two species associ-
ated with the PVDF main chains, (-CH2-)PVDF and
(-CF2-)PVDF with BE’s at 285.8 and 290.5 eV, respec-
tively, have disappeared completely. On the other hand,
the peak component with a BE of about 286.2 eV,

7248 Zhai et al.
Macromolecules, Vol. 37, No. 19, 2004
of the two membranes are also compared in Figure 8.
The ([N]/[C])surface ratios, determined from the respective
peak component area ratios, were about 0.02 and 0.16
for the PVDF-g-PBIEA-g-PPEGMA membrane and
PVDF-g-PBIEA membrane, respectively. These results
indicate that the antifouling property of the PVDF-g-
PBIEA membrane has been improved substantially by
the surface-initiated ATRP of PEGMA.
4. Con clu sion s
PVDF with grafted ATRP inimer, BIEA, side chains
allowed the preparation of arborescent copolymer. Both
the PVDF-g-PBIEA and its arborescent copolymer of a
polyelectrolyte, NaPSS, could be readily cast into mi-
croporous membranes with surface-enriched graft chains
via phase inversion in an aqueous medium. The surface
composition and morphology of the PVDF-g-PBIEA-ar-
NaPSS membranes exhibited electrolyte-responsive be-
havior. The BIEA units on the PVDF-g-PBIEA mem-
branes could serve as the macroinitiators for the surface-
initiated ATRP to further functionalize the membrane.
The inimer graft copolymerization technique provided
an alternative approach to the functionalization of
membranes both at the molecular level (prior to the
casting of membrane) and at the membrane surface
level. Because of the end functionality associated with
the ATRP technique, such membrane surface exhibited
a “living” character, allowing for additional surface-
initiated reactions to introduce well-defined functional
polymers and moieties. The porous membrane with
active or “living” surfaces thus provides exciting op-
portunities for the direct molecular design and engi-
neering of the membrane and pore surfaces.
F igu r e 8. (a) XPS wide-scan and C 1s core-level spectra of
the PVDF-g-PBIEA-ar-PPEGMA membrane (time of polym-
erization ) 1 h); XPS wide-scan and N 1s core-level spectra of
(b) the PVDF-g-PBIEA membrane and (c) PVDF-g-PBIEA-ar-
PPEGMA membrane after a 24 h of γ-globulin adsorption.
Refer en ces a n d Notes
attributed to the C-O species of the PBIEA and
PPEGMA side chains, has been greatly enhanced.
Similar to that of the PVDF-g-PBIEA-ar-NaPSS mem-
brane cast from the PVDF-g-PBIEA-ar-NaPSS copoly-
mer, the intensity of the Br 3d component has been
greatly reduced after the surface-initiated ATRP of
PEGMA with the PVDF-g-PBIEA membrane. The ([Br]/
[C])surface ratio has decreased from 0.03 to about 0.02,
arising probably from the irreversible termination and
halogen exchange of the propagating radicals during the
surface-initiated ATRP process.69-71
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