Block copolymer route towards poly(vinylidene fluoride)/poly(methacrylic acid)/nickel nanocomposites

Article abstract of DOI:10.1039/c3ra40365c

PVDF-based block copolymers have been employed as precursors for the construction of PVDF/PMAA/Ni nanocomposites. New poly(tert-butyl methacrylate)-block-poly(vinylidene fluoride)-block-poly(tert-butyl methacrylate) (PtBMA-b-PVDF-b-PtBMA) triblock copolymers were synthesized via atom transfer radical polymerization (ATRP) of tBMA from chlorine-terminated PVDF macroinitiators. The alternating crystalline-amorphous lamellar nanostructure and the spherulitic microstructure indicate the dominant role of crystallization of the PVDF segments during structure formation. The polar β-crystalline phase of PVDF has been detected within the block copolymer films. Hydrolysis of the tBMA segments and subsequent backfilling of the remaining polymer template with nickel through electroless metal deposition generated PVDF/PMAA/Ni nanocomposites. The β-polymorph was preserved during hydrolysis and electroless plating, as well as the lamellar morphology.

Full text of DOI:10.1039/c3ra40365c

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PAPER
Block copolymer route towards poly(vinylidene
fluoride)/poly(methacrylic acid)/nickel
Cite this: RSC Advances, 2013, 3,
7938
nanocomposites3
Vincent S. D. Voet,a Daniel Hermida-Merino,b Gerrit ten Brinke*a and Katja Loos*a
PVDF-based block copolymers have been employed as precursors for the construction of PVDF/PMAA/Ni
nanocomposites. New poly(tert-butyl methacrylate)-block-poly(vinylidene fluoride)-block-poly(tert-butyl
methacrylate) (PtBMA-b-PVDF-b-PtBMA) triblock copolymers were synthesized via atom transfer radical
polymerization (ATRP) of tBMA from chlorine-terminated PVDF macroinitiators. The alternating crystal-
line–amorphous lamellar nanostructure and the spherulitic microstructure indicate the dominant role of
crystallization of the PVDF segments during structure formation. The polar b-crystalline phase of PVDF has
been detected within the block copolymer films. Hydrolysis of the tBMA segments and subsequent
backfilling of the remaining polymer template with nickel through electroless metal deposition generated
PVDF/PMAA/Ni nanocomposites. The b-polymorph was preserved during hydrolysis and electroless
plating, as well as the lamellar morphology.
Received 23rd January 2013,
Accepted 11th March 2013
DOI: 10.1039/c3ra40365c
www.rsc.org/advances
research9–14 due to the wealth of potential applications.
Fluorinated block copolymers containing PVDF segments are
Introduction
Similar to other fluorinated polymers, poly(vinylidene fluoride)
(PVDF) demonstrates outstanding thermal, physical and
chemical stability.1 However, PVDF is exceptional, due to the
reported ferroelectric properties of its b-crystalline phase,2–5
originating from the orientation of the strong dipole in the
of particular interest, as potential precursors for ordered
nanostructured materials with excellent properties. However,
such well-architectured copolymers cannot be readily poly-
merized by controlled radical polymerization15 or sequential
living anionic polymerization.16 Nevertheless, a few investiga-
tions report on the preparation of PVDF-containing block
copolymers with reasonable chain lengths and polydispersity.
For example, trichloromethyl-terminated PVDF-co-PHFP copo-
lymers, prepared via an emulsion copolymerization of vinyli-
dene fluoride (VDF) and hexafluoropropylene (HFP), were used
as macroinitiators for atom transfer radical polymerization
(ATRP) of styrene (S) and methacrylate (MMA) to prepare
(PVDF-co-PHFP)-b-PS and (PVDF-co-PHFP)-b-PMMA respec-
tively.17 A similar ATRP strategy was adopted to synthesize
PS-b-PVDF-b-PS.18 Chlorine-terminated PVDF, synthesized via
a radical polymerization with chloromethyl benzoyl peroxide,
initiated the ATRP of styrene in order to obtain PVDF-based
triblock copolymers. The same group also reported the
preparation of triblock copolymers composed of (PVDF-co-
PHFP) and ionic liquid segments.19 A telechelic fluoropolymer
acted as the macro-chain transfer agent in the reversible
addition–fragmentation transfer (RAFT) polymerization of
imidazolium methacrylate monomer. Recently, (PVDF-co-
PTFP)-block-oligo(VAc) was prepared through sequential con-
trolled radical polymerization.20 First, a fluorinated xanthate
was successfully involved as chain transfer agent for macro-
molecular design via the interchange of xanthates (MADIX)
polymerization of VDF and 3,3,3-trifluoropropene (TFP). The
–CH2–CF2– units along the polymer chain.
A common
technique to yield an increase of the polar b-phase involves
mechanical stretching of a-phase films at suitable tempera-
tures.3 Additionally, the b-polymorph can be obtained directly
from solution by casting from the proper (polar) solvent,4,6 or
by the incorporation of nanoclays.7 Its ferroelectric behavior,
together with the high thermal resistance and chemical
inertness, designates PVDF as a functional material that can
be applied in membranes, mechanical actuators, sensors and
multiferroic composites.8
Block copolymers, consisting of two or more covalently
linked polymer blocks, are able to microphase separate on a
scale related to the size of the copolymer chains (10–100 nm),
typically leading to lamellar, cylindrical or spherical morphol-
ogies. This spontaneous formation of nanostructured materi-
als by block copolymer self-assembly has stimulated extensive
aDepartment of Polymer Chemistry, Zernike Institute for Advanced Materials,
University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
E-mail: k.u.loos@rug.nl; g.ten.brinke@rug.nl
bNetherlands Organization for Scientific Research, DUBBLE Beamline at ESRF, 6 rue
Jules Horowitz, BP 220, 38043, CEDEX 9, Grenoble, France
Electronic supplementary information (ESI) available: Additional DSC, kinetic
3
analysis, SAXS, SEM and WAXS data is included. See DOI: 10.1039/c3ra40365c
7938 | RSC Adv., 2013, 3, 7938–7946
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resulting fluorinated copolymer with xanthate end group was
involved in the MADIX of vinyl acetate (VAc), leading to diblock
copolymers. Despite the successful synthesis of PVDF-based
block copolymers, the self-assembly of these fluorinated block
copolymers into ordered nanostructures has hardly been
investigated.
obtained via the selective hydrolysis of a gyroid-forming block
copolymer.41 Recently, we adopted a similar strategy to
generate PVDF/nickel nanocomposites.42 First, a series of PS-
b-PVDF-b-PS triblock copolymers that self-assemble into
ordered
lamellar
nanostructures
were
synthesized.
Subsequently, nanoporous PVDF was prepared by selective
etching of the PS domains, and finally, electroless metal
plating resulted in PVDF/Ni nanohybrids. The lamellar
morphology and ferroelectric b-polymorph, both originating
from the block copolymer phase separation, were preserved
within the composite. Considering the ferroelectric properties
of PVDF and the ferromagnetic behavior of nickel, the PVDF/
Ni nanohybrid is a potential multiferroic material. Other
multiferroic PVDF-based nanocomposites, composed of a
piezoelectric polymer and magnetostrictive compound, have
already demonstrated considerable strain-coupled magneto-
electric effects,43–46 i.e. the appearance of an electric polariza-
tion upon applying a magnetic field, or the appearance of a
magnetization upon applying an electric field. The self-
assembled lamellar structure of the fabricated PVDF/Ni
composite may greatly enhance the magnetoelectric effect
due to the optimal laminated configuration and intimate
contact between the piezoelectric and magnetostrictive
phase.47,48 Multiferroic magnetoelectric materials may find
application in transducers, sensors and memory devices.
Here, we present the successful synthesis of new PVDF-
based block copolymers: poly(tert-butyl methacrylate)-block-
poly(vinylidene fluoride)-block-poly(tert-butyl methacrylate)
(PtBMA-b-PVDF-b-PtBMA), via functional benzoyl peroxide
initiated polymerization of VDF and subsequently ATRP of
tBMA from the PVDF macroinitiator. Subsequently, the phase
separation of these semicrystalline triblock copolymers into
ordered nanostructures is investigated. In addition, we report
the selective hydrolysis of the PtBMA blocks to poly(-
methacrylic acid) (PMAA), followed by the electroless metal
deposition onto the resulting polymer template, leading to
lamellar PVDF/PMAA/Ni nanocomposites. The hydrophilic
nature of the PMAA pores facilitates the complete penetration
of the plating reagents into the polymer template, leading to a
higher extent of nickel insertion compared to the earlier
reported PVDF/Ni composites from PS-b-PVDF-b-PS precur-
sors.42
Considering the crystallinity of poly(vinylidene fluoride),
PVDF-containing block copolymers are called semicrystalline
(or crystalline–amorphous). The phase behavior of this special
class of copolymers becomes more complex due to the
incorporation of the crystallizable blocks.12,21–23 The final
morphology depends on the competition between crystal-
lization and microphase separation driven by block incompat-
ibility, or more specifically, between three phase transitions,
i.e. the correlation between the crystallization temperature of
the crystalline block Tc, the glass transition temperature of the
amorphous block Tg and the order–disorder transition
temperature TODT. For unconfined crystallization (TODT , Tc
. Tg), the phase separation between both blocks is driven by
crystallization of the crystallizable block. As a consequence,
alternating crystalline–amorphous lamellar nanodomains with
a spherulitic microstructure are obtained, regardless of the
copolymer composition.24,25
Block copolymers can be used as precursors for ordered
nanoporous materials with desired functionality and control-
lable pore sizes, provided that one of the blocks is thermally,
(photo)chemically, or otherwise selectively degradable.26 This
concept was introduced and demonstrated already three
decades ago.27 Two main requirements for obtaining such
porous nanostructured materials are the physical accessibility
of the etchable component to the degradation process (e.g.
solvent, heat, UV) and the stability of the matrix material left
after degradation, to prevent pore collapse.28 In recent years,
many studies reported the fabrication of porous nanomaterials
from block copolymer precursors, using various etching
techniques, like UV irradition,29 ozonolysis,30 reactive ion
etching,31 amphiphile extraction32,33 and thermolysis.34
Selective hydrolysis is another well-exploited technique to
generate porous structures. For example, this method has
been exploited to completely degrade a polymer block such as
poly(lactic acid),35 or to selectively cleave polystyrene grafts
from a poly(3-hexylthiophene) backbone.36 Additionally, par-
tial hydrolysis of poly(tert-butyl acrylate) segments in PtBA-
b-PS block copolymers generates a porous polymer nanos-
tructure with hydrophilic poly(acrylic acid) nanochannels.37
Nanoporous materials from self-assembled block copoly-
mer structures can act as templates for functional nanocom-
posites. Backfilling of nanopores in the polymer template will
lead to the fabrication of such nanohybrid materials.
Commonly used methods for backfilling are sol–gel synth-
esis,38 electrochemical plating39 and electroless plating.33 The
latter technique involves the autocatalytic deposition of metal
onto the template surface, and allows the uniform coating of
metals onto complex shapes and into small pores.40 For
example, a PS/Ni gyroid nanohybrid has been prepared
through electroless metal plating of a nanoporous PS matrix,
Experimental
Materials
Tert-butyl methacrylate (tBMA, Aldrich, 98%) was dried over-
night in a nitrogen atmosphere over CaH2 and condensed at
room temperature (1026 mbar). Oxalyl chloride (Acros, 98%),
4-(chloromethyl)benzoic acid (Acros, 98%), lithium peroxide
(Li2O2, Acros, 95%), vinylidene fluoride (VDF, Synquest Labs,
98%), copper(I) chloride (CuCl, Acros, 99.99%), 1,1,4,7,7-
pentamethyldiethylenetriamine (PMDETA, Acros, 99%), tri-
fluoroacetic acid (TFA, Acros, 99%), tin chloride (SnCl2, Acros,
98%), hydrochloric acid (HCl, Merck, 37%), palladium
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chloride (PdCl2, Aldrich, 60% Pd basis), nickel sulfate
(NiSO4?6H2O, Aldrich, 99%), citric acid trisodium salt
(Na3C6H5O7, Aldrich, 98%), lactic acid (SAFC, 85%), borane
dimethylamine complex (DMAB, Aldrich, 97%) and ammo-
nium hydroxide (NH4OH, Aldrich, 29% NH3 basis) were used
as received. All solvents used were of analytical grade.
After reacting at 75 uC for a desired amount of time, the dark-
brown mixture was cooled down to room temperature using a
water bath and subsequently precipitated in MeOH–H2O (100
mL, 1 : 1). The solid was collected by filtration and washed
with MeOH–H2O (1 : 1) and methanol. Reprecipitation was
carried out from DMF in MeOH–H2O (1 : 1), and the collected
off-white solid was dried in vacuum at 40 uC. 1H NMR (400
MHZ, acetone-d6, d): 2.94 (t, –CF2CH2–CF2CH2–, head-to-tail),
Synthesis of 4-(chloromethyl)benzoyl peroxide
Oxalyl chloride (5.4 mL, 63 mmol) and a few drops of
anhydrous DMF were added to a stirred solution of 4-
(chloromethyl)benzoic acid (10 g, 59 mmol) in 50 mL of
anhydrous DCM at 0 uC. After reacting for 2 h at room
temperature, the solvent was removed by rotary evaporation.
The remaining yellow residue was immediately dissolved in
100 mL n-hexane–EtOH (1 : 1). The resulting solution was
slowly added via a droplet funnel to a rapidly stirred 50 mL
aqueous solution of Li2O2 (3.5 g, 75 mmol) at 0 uC. After
reacting for 2 h at room temperature, the reaction mixture was
diluted with 250 mL chloroform and washed twice with 100
mL H2O. The aqueous phase was extracted twice with 50 mL
chloroform. The combined organic phases were dried over
MgSO4 and chloroform was subsequently removed by rotary
evaporation. The remaining white solid was recrystallized from
chloroform, yielding white needle-shaped crystals. 1H NMR
(400 MHz, DMSO-d6, d): 8.12 (d, 4H, –ArH), 7.76 (d, 4H, –ArH),
4.95 (s, 4H, –CH2Cl).
2.35
(t,
–CF2CH2–CH2CF2–,
tail-to-tail),
1.88
(m,
–CH2C(CH3)(COOtBu)–), 1.48 (m, –CH2C(CH3)(COOtBu)–),
1.09 (m, –CH2C(CH3)(COOtBu)–).
Preparation of block copolymer films
Solvent annealing was applied to obtain a phase separated
block copolymer nanostructure. A 1.5% w/w solution of
PtBMA-b-PVDF-b-PtBMA in DMF (or NMP) was stirred for at
least
2 h at room temperature and the solution was
subsequently poured in a glass Petri dish. The solvent was
allowed to slowly evaporate at 45 uC and the film was annealed
in the solvent vapor for at least one week, yielding a slightly
yellow transparent film with a thickness of ca. 50 mm.
Hydrolysis of block copolymer films
Hydrolysis of the PtBMA-b-PVDF-b-PtBMA film was performed
to remove the tert-butyl moieties of tBMA. The film was
submerged in 10 mL of TFA. After 16 h, the TFA was decanted,
and the hydrolyzed film was rinsed with methanol. The
resulting film was immediately subjected to the electroless
nickel plating procedure.
Synthesis of chlorine-terminated PVDF
A typical procedure for the polymerization of vinylidene
fluoride is as follows. A solution of 4-(chloromethyl)benzoyl
peroxide (1.5 g, 4.5 mmol) in 300 mL of anhydrous acetonitrile
was added to a pressure reactor (Parr Instruments, model
4568). The vessel was closed and purged with nitrogen for 30
min to degas the mixture. Subsequently, the reactor was
charged with 20 bar of VDF, heated to 90 uC and stirred at 700
rpm. After reacting for 15 min, the vessel was cooled down to
room temperature and depressurized. The reaction mixture
was cooled to 0 uC, and the precipitate was collected by
filtration. The remaining solid was washed with acetonitrile
and chloroform, and finally dried in vacuum at room
temperature to yield a white solid. 1H NMR (400 MHz,
DMSO-d6, d): 8.01 (d, –ArH), 7.61 (d, –ArH), 4.84 (s, –CH2Cl),
4.64 (m, –COOCH2CF2–), 2.87 (t, –CF2CH2–CF2CH2–, head-to-
tail), 2.23 (t, –CF2CH2–CH2CF2–, tail-to-tail).
Preparation of PVDF/PMAA/Ni nanocomposites
Electroless metal plating was applied for nickel deposition
onto the polymer substrate. For surface sensitization, the
hydrolyzed film was immersed into a solution of SnCl2 (0.1 M)
and HCl (0.1 M) in MeOH–H2O (1 : 1) for 1 h, and the surface
of the pores adsorbed Sn2+. The sensitized film was rinsed
with MeOH–H2O (1 : 1) and soaked into a solution of PdCl2
(1.4 mM) and HCl (0.25 M) in MeOH–H2O (1 : 1) for 1 h. The
surface was activated by a redox reaction (Sn2+ + Pd2+ A Sn4+
+
Pd0) to exchange Sn2+ adsorbed on the surface into Pd0. After a
rinse with MeOH–H2O (1 : 1), the activated film was immersed
into an aqueous electroless nickel plating bath composed of
NiSO4?6H2O (40 g L21), Na3C6H5O7 (20 g L21), lactic acid (10 g
L
21) and DMAB (1 g L21). The metallic palladium functions as
a catalyst for the reduction of Ni2+. The pH of the nickel bath
was adjusted to 7 using an aqueous solution of NH4OH (1.0
M), and the plating was performed at room temperature for 1
h. The nickel plated film was rinsed with H2O and dried in
vacuum at room temperature.
Synthesis of PtBMA-b-PVDF-b-PtBMA
A typical procedure for the atom transfer radical polymeriza-
tion of tBMA is as follows. Chlorine-terminated PVDF (0.39 g,
0.025 mmol) and CuCl (50 mg, 0.50 mmol) were added to a
dried Schlenk tube sealed with a rubber septum, followed by a
degassing procedure (i.e. evacuating and backfilling three
times with nitrogen). 5.0 mL of anhydrous DMF was added via
a degassed syringe to dissolve the PVDF macroinitiator.
PMDETA (0.31 mL, 1.5 mmol) was added via a degassed
syringe and the solution was stirred for at least 10 min to let
the dark green colored catalyst–ligand complex form.
Subsequently, tBMA (2.84 mL, 17.5 mmol) was added via a
degassed syringe, and the reaction mixture was immediately
subjected to at least four freeze–pump–thaw cycles to degas.
Characterization
Gel permeation chromatography (GPC) was performed in DMF
(1 mL min21) with 0.01 M LiBr on a Viscotek GPCMAX
equipped with model 302 TDA detectors, using two columns
(PSS-Gram-1000/30, 10 m 30 cm). Molecular weights were
calculated relative to polystyrene according to universal
calibration using narrow disperse standards (Polymer
Laboratories). 1H nuclear magnetic resonance (1H NMR)
spectra were recorded on a 400 MHz Varian VXR operating
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at room temperature. Differential scanning calorimetry (DSC)
was carried out using a TA Instruments Q1000 in a nitrogen
(chloromethyl)benzoyl peroxide as an initiator, has been
reported recently.42 Due to the absence of termination through
disproportionation, the benzoyl peroxide initiated polymeriza-
tion of vinylidene fluoride resulted in chlorine-terminated
PVDF with polydispersities in the range of 1.2–1.4.
Considering the phenylmethyl chlorine end-groups and the
reasonably narrow dispersity, these PVDF homopolymers are
suitable to be employed as macroinitiators for atom transfer
radical polymerization.
Previously, we prepared PS-b-PVDF-b-PS triblock copolymers
through ATRP of styrene from these PVDF macroinitiators.42
ATRP has proven to be a versatile polymerization technique,
effective in providing a controlled polymerization environment
for a wide range of vinyl monomers including styrenes,
(meth)acrylates, (meth)acrylamides and acrylonitriles.15 In
this study, we report the synthesis of new PtBMA-b-PVDF-
b-PtBMA triblock copolymers, using a similar ATRP approach
as for PS-b-PVDF-b-PS.
atmosphere and with a heating/cooling rate of 10 uC min21
.
Polarized optical microscopy (POM) was conducted on a Zeiss
Axiophot. Fourier transform infrared (FTIR) measurements
were performed in attenuated total reflection mode, using a
Specac Golden Gate accessory with diamond top-plate on a
Bruker IFS88 spectrometer equipped with a MCT-A detector at
a resolution of 4 cm21. Wide-angle X-ray scattering (WAXS)
and Small-angle X-ray scattering (SAXS) were performed at the
Dutch–Belgium Beamline (DUBBLE) station BM26B of the
European Synchrotron Radiation Facility (ESRF) in Grenoble,
France.49,50 The sample–detector distance of the SAXS set-up
was ca. 6 m, while the X-ray wavelength was 1.03 Å. The
scattering vector q is defined as q = 4p/lsinh with 2h being the
scattering angle. The patterns were collected at room
temperature. Transmission electron microscopy (TEM) was
carried out on a Philips CM12 transmission electron micro-
scope operating at an accelerating voltage of 120 kV. TEM
samples were prepared as follows: ultrathin sections (ca. 80
nm) of a solvent-cast block copolymer film embedded in epoxy
resin (Epofix, Electron Microscopy Sciences) were microtomed
using a Leica Ultracut UCT-ultramicrotome equipped with a
35u Diatome diamond knife at room temperature, and
subsequently placed on copper grids. Scanning electron
microscopy (SEM) was carried out on a JEOL 6320F field
microscope operating at 3 kV. Prior to imaging, the specimens
were coated with 6 nm Pt/Pd (80 : 20). Energy-dispersive X-ray
The chlorine-terminated PVDF, used to initiate the con-
trolled radical polymerization of tert-butyl methacrylate, has a
molecular weight (Mn,PVDF) of 15.7 kg mol21 and polydispersity
(PDI) of 1.29.42 Three block copolymers were prepared through
ATRP of tBMA from this macroinitiator, using CuCl–PMDETA
1
as catalyst–ligand complex (Table 1). The H NMR spectra of
the PVDF homopolymer and PtBMA-b-PVDF-b-PtBMA are
depicted in Fig. 1. The spectrum of the triblock copolymer
(Fig. 1b) demonstrates the characteristic signals (assigned in
the experimental section) for poly(tert-butyl methacrylate) in
addition to the typical head-to-tail and tail-to-tail signals for
poly(vinylidene fluoride). The number average molecular
weight (Mn,PtBMA) and weight fraction (fPtBMA) of the tBMA
block were calculated by comparing the integrals of PtBMA
and PVDF, using the predetermined Mn,PVDF. The monomer
conversion (Conv) was determined from 1H NMR spectra of
the reaction mixture, by comparing the integrals of tBMA and
PtBMA.
(EDX) spectroscopy was performed on
a Philips XL-30
environmental scanning electron microscope.
Results and discussion
Synthesis
PtBMA-b-PVDF-b-PtBMA triblock copolymers were synthesized
via functionalized benzoyl peroxide initiated polymerization of
vinylidene fluoride, followed by atom radical transfer poly-
merization of tert-butyl methacrylate from the resulting PVDF
macroinitiator (Scheme 1).
Thermal analysis reveals the glass transition temperature of
PtBMA segments at 131 uC and the melting endotherm of
PVDF at 171 uC in the heat flow curve of the synthesized block
copolymer (Fig. S1b, ESI ), indicating phase separation
3
between the blocks. The double melting endotherm of PVDF
Functionalized benzoyl peroxides have been demonstrated
to act as effective initiators for the preparation of poly(viny-
lidene fluoride) and other fluoropolymers,18,51 and the
synthesis of a library of end-functionalized PVDF, using 4-
(Fig. S1a, ESI ) is a common phenomenon in semicrystalline
3
polymers and is ascribed to melting, recrystallization and
remelting during the DSC heating process.52
Scheme 1 Synthesis route towards the 4-(chloromethyl)benzoyl peroxide initiator, chlorine-terminated PVDF macroinitiator and PtBMA-b-PVDF-b-PtBMA triblock
copolymer.
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Table 1 Conditionsa and characteristics of PtBMA-b-PVDF-b-PtBMA
Convb (%)
Mn,PtBMA (kg mol21
)
fPtBMA
c
c
Entry
[CuCl] (M)
[PMDETA] (M)
t (h)
1
2
3
0.10
0.10
0.05
0.30
0.30
0.05
2.0
3.0
3.0
15
17
20
12.2
18.5
21.4
0.44
0.54
0.58
a
b
c
[PVDF] = 5 mM; [tBMA] = 3.5 M; T = 75 uC. Determined by 1H NMR of reaction mixture. Determined by 1H NMR of precipitated product.
To investigate the controlled behavior of the atom transfer
radical polymerization method, a kinetic analysis has been
performed. The semilogarithmic plot of monomer conversion
microstructure (Fig. 2). This confirms the dominant role of
crystallization during the structure formation. Moreover, the
remaining triblock copolymers 1 and 3 (Table 1) demonstrate a
similar morphology in TEM. The obtained contrast arises from
the electron dense crystalline PVDF layers (dark gray)
alternated by the less electron dense amorphous layers (light
gray), since no additional staining was applied to the
microtomed sample prior to imaging. The alternating crystal-
line–amorphous lamellar morphology was confirmed by small-
angle X-ray experiments. The SAXS intensity profile of PtBMA-
b-PVDF-b-PtBMA 2 (Fig. 3b) demonstrates a diffraction pattern
with a q ratio of 1 : 2, indicative for a lamellar nanostructure.
The characteristic domain spacing, calculated from the value
of the first-order reflection q* = 0.12 nm21, satisfies 52 nm,
and corresponds to the length scale observed with TEM
(Fig. 3a).
The crystalline phase behavior of the triblock copolymers
has been explored using wide-angle X-ray scattering. The
WAXS pattern of the PtBMA-b-PVDF-b-PtBMA copolymer
(Fig. 4a2) reveals a diffraction reflection at q = 14.2 nm21
(0.44 nm), that arises from the (110) and (200) b-crystal
planes.53,54 The diffraction pattern demonstrates a clear
difference with the WAXS intensity profile of the PVDF
macroinitiator (Fig. 4a1), that reveals diffraction peaks at q =
12.5 nm21 (0.50 nm), 13.0 nm21 (0.48 nm) and 14.0 nm21
(0.45 nm). These peak positions correspond respectively to the
(100), (020) and (110) a-crystal planes. Hence, the crystalline
phase of the homopolymer can be identified as predominantly
a-polymorph, while the self-assembled block copolymer film
versus time (Fig. S2a, ESI ) suggests minimal contribution of
3
termination reactions during ATRP. Moreover, the linear
relationship between molecular weight and conversion (Fig.
S2b, ESI ) indicates indeed a controlled radical polymerization
3
of tert-butyl methacrylate when initiated by the chlorine-
terminated PVDF.
Phase behavior
The crystallization temperature of PVDF (Tc = 143 uC) exceeds
the glass transition of tert-butyl methacrylate (Tg = 131 uC), as
demonstrated by thermal analysis. As a result, the amorphous
segments are rubbery during crystallization (Tg
, Tc).
Furthermore, a disordered melt state is observed during
temperature-dependent small-angle X-ray experiments (Fig.
S3, ESI ), indicating that the block incompatibility is relatively
3
small (TODT , Tc). As a consequence, crystallization is expected
to be the dominant self-organizing mechanism, giving rise to
crystalline–amorphous lamellar nanodomains within
a
spherulitic superstructure. Indeed, spherulites are observed
at the microscale with a diameter of 30–40 mm, as displayed in
the polarized optical microscope image (Fig. 2).
TEM and SAXS measurements have been performed to
further investigate the nanoscale structure of the triblock
copolymer films. Fig. 3a depicts the TEM micrograph of
PtBMA-b-PVDF-b-PtBMA 2 cast from DMF solution, revealing
the predicted lamellar nanostructure inside the spherulitic
Fig. 1 1H NMR spectra of (a) chlorine-terminated PVDF in DMSO-d6 and (b)
PtBMA-b-PVDF-b-PtBMA triblock copolymer in acetone-d6.
Fig. 2 POM image of the spherulitic microstructure in PtBMA-b-PVDF-b-PtBMA
2 cast from DMF solution.
7942 | RSC Adv., 2013, 3, 7938–7946
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Fig. 3 (a) TEM image and (b) SAXS pattern of the alternating crystalline–amorphous lamellar nanostructure in PtBMA-b-PVDF-b-PtBMA 2 cast from DMF solution.
stimulated the formation of the polar b-polymorph of PVDF,
well-known for its ferroelectric properties. The presence of the
poly(tert-butyl methacrylate) layers is expected to stimulate the
nucleation of the PVDF chains in the all-trans conformation
resulting in the b-crystalline phase,42 similar to the effect
demonstrated in well-dispersed layered silicates embedded in
PVDF.7,55 Additionally, we suggest the strong influence of the
cast solution. This influence is clearly demonstrated by
comparing the WAXS patterns of PtBMA-b-PVDF-b-PtBMA
films cast from NMP and DMF (Fig. 4b). The intensity profile
of the film cast from DMF reveals mainly the b-polymorph
(Fig. 4b2), while the film cast from NMP demonstrates a
mixture of a- and b-crystalline phase (Fig. 4b1). Although both
solvents are highly polar, demonstrating equal dipole
moments of 3.8 D,56 the lower boiling point of DMF will
result in a higher vapor pressure during the solvent annealing
at 45 uC, therefore stimulating the formation of the polar
b-polymorph to a higher extent.
route towards a nanoporous PVDF template with hydrophilic
pores. In order to achieve this, hydrolysis with TFA has been
applied to the PtBMA-b-PVDF-b-PtBMA films cast from DMF
solution.
The conversion of the hydrolysis reaction can be determined
by infrared spectroscopy. FTIR spectra of the block copolymer
films before and after hydrolysis are represented in Fig. 5a and
5b respectively. The disappearance of the tert-butyl double
absorption band at 1393/1366 cm21 demonstrates the quanti-
tative removal of the tert-butyl groups. Moreover, the CLO
absorption band at 1719 cm21 is broadened and shifted to
1700 cm21 due to the formation of carboxylic acid moieties,
and the appearance of the broad band between 2500 and 3500
cm21 due to the O–H vibration supports those findings.
The glass transition at 131 uC of the amorphous PtBMA
segments disappears in the heat flow curve of the hydrolyzed
film (Fig. S1c, ESI ), confirming the complete removal of tert-
3
butyl groups by applying the TFA hydrolysis treatment. The Tg
of poly(methacrylic acid) is reported to be 228 uC,57 and is
therefore not visible in the depicted DSC curve.
Hydrolysis
The selective removal of tert-butyl moieties from PtBMA
segments in the triblock copolymer provides a convenient
In an attempt to study the bulk morphology, the treated
films were cleaved after drying, and subsequently investigated
Fig. 4 (a) WAXS pattern of (1) chlorine-terminated PVDF, (2) PtBMA-b-PVDF-b-PtBMA film (cast from DMF solution), (3) PMAA-b-PVDF-b-PMAA film (after hydrolysis),
and (4) PVDF/PMAA/Ni nanocomposite; and (b) WAXS pattern of (1) PtBMA-b-PVDF-b-PtBMA film cast from NMP solution, and (2) PtBMA-b-PVDF-b-PtBMA film cast
from DMF solution.
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Fig. 5 FTIR spectra of (a) PtBMA-b-PVDF-b-PtBMA 2 cast from DMF solution and
(b) PMAA-b-PVDF-b-PMAA after hydrolysis of the triblock copolymer.
by scanning electron microscopy. Despite the successful
hydrolysis of PtBMA-b-PVDF-b-PtBMA to PMAA-b-PVDF-
b-PMAA, no nanoporous structure was detected (Fig. S4,
ESI ). Presumably, the lamellar matrix is not able to support
3
the resultant nanoporous structure after removal of the tert-
butyl groups, leading to the collapse of lamellar domains after
drying the films. Therefore, to circumvent the structure
collapse, the wetted samples were immediately subjected to
the electroless metal deposition process after the methanol
rinsing step, without further drying.
The WAXS intensity profile of the hydrolyzed film (Fig. 4a3)
is similar to the pattern of the initial block copolymer film
(Fig. 4a2), with a diffraction reflection at q = 14.2 nm21 (0.44
nm) corresponding to the (110)/(200) b-crystal planes. Hence,
regardless of the nanostructure collapse upon drying, the
b-crystalline phase is conserved during the acidic hydrolysis
treatment.
Nickel deposition
Electroless metal plating has been performed on the hydro-
lyzed films, in order to insert nickel inside the polymer
template. The high affinity between the aqueous plating
solutions and the hydrophilic polymer template is beneficial
for successful metal deposition. Immediately after the hydro-
lysis treatment the films were subjected to the sensitization
step of the plating process, to prevent the collapse of the
lamellar matrix, observed upon drying.
Fig. 6 displays TEM micrographs of the electroless plated
films. The images demonstrate the successful deposition of
nickel inside the polymer template. Nickel completely pene-
trated the pores in the polymer matrix, resulting in a lamellar
PVDF/PMAA/Ni nanocomposite. The hydrophilic nature of the
PMAA-coated pores facilitates the complete penetration of the
water-based plating reagents into the polymer network and
enables the successful metal deposition. The obtained
contrast arises from the electron dense nickel layers (black)
alternated by the less electron dense polymer layers (white),
since no additional staining was applied prior to imaging. The
Fig. 6 TEM images of the PVDF/PMAA/Ni nanocomposite with a lamellar
morphology, obtained after nickel plating of the polymer template.
original lamellar morphology is clearly preserved within the
obtained polymer/nickel nanohybrid. In fact, the TEM images
of the nanocomposite (Fig. 6) look similar to the micrograph
of the block copolymer nanostructure (Fig. 3a).
The chemical composition of the PVDF/PMAA/Ni nanocom-
posite has been investigated by energy-dispersive X-ray
spectroscopy. Therefore, the nickel plated films were cleaved,
and the EDX spectrum of the cross section is depicted in Fig. 7.
The carbon (0.28 keV), oxygen (0.53 keV) and fluorine (0.68
keV) peaks correspond to the presence of PVDF and PMAA
within the composite. The signals at 0.85 and 7.48 keV
represent nickel, demonstrating the successful penetration of
nickel inside the pores of the polymer matrix. The spectrum
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the nucleation of the PVDF chains in the all-trans conforma-
tion, resulting in the b-crystalline phase.
The tert-butyl moieties of the tBMA domains were selec-
tively removed by acidic hydrolysis using TFA. Subsequently,
backfilling of the polymer template with nickel was achieved
by applying electroless metal deposition. The resulting PVDF/
PMAA/Ni nanohybrid material demonstrated
a lamellar
morphology, originating from the block copolymer phase
separation. Furthermore, the b-polymorph of PVDF is con-
served within the hydrolyzed polymer template and the
nanocomposite. In comparison to the earlier reported PVDF/
Ni nanocomposites fabricated from PS-b-PVDF-b-PS precur-
sors, a higher extent of nickel insertion was accomplished, due
to the hydrophilic PMAA-coated pores that facilitate the
complete penetration of the water-based plating reagents into
the polymer network.
Fig. 7 EDX spectrum of the PVDF/PMAA/Ni nanocomposite, obtained after
electroless nickel plating of the polymer template.
Acknowledgements
This study was funded by the Netherlands Organization for
Scientific Research (NWO) via a VIDI innovational research
grant. We kindly acknowledge Joop Vorenkamp for his
assistance with FTIR experiments, Evgeny Polushkin for SEM
measurements and Sergey Punzhin for EDX analysis. The
authors are grateful to Gert Alberda van Ekenstein for POM
measurements and valuable discussion regarding thermal
analysis. Beam time on the DUBBLE of ESRF in Grenoble has
been made available by NWO, and we thank Wim Bras and
Giuseppe Portale for their experimental assistance.
demonstrates a higher extent of nickel with respect to the
earlier reported PVDF/Ni nanocomposites,42 assigned to the
hydrophilic nature of the PMAA pores. Additionally, the full
WAXS pattern of the nanohybrid (Fig. S5, ESI ) demonstrates
3
typical diffraction reflections at higher q values, confirming
the presence of nickel.
The WAXS intensity profile of the obtained nanocomposite
is displayed in Fig. 4a4. The diffraction pattern is comparable
to the profiles of the original block copolymer and the
hydrolyzed film. Again, the pattern demonstrates the presence
of the b-polymorph of PVDF. Thus, the crystalline phase of the
block copolymer film is preserved during both the hydrolysis
treatment and the electroless nickel deposition. The PVDF/
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