# Membranes of Polyvinylidene Fluoride and  PVDF Nanocomposites with Carbon Nanotubes  via Immersion Precipitation

Authors: Gaurav Mago; Dilhan M. Kalyon; Frank T. Fisher
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Microporous polyvinylidene fluoride (PVDF) and PVDF nanocomposite membranes were prepared via an isothermal immersion precipitation method using two different antisolvents (ethanol and water). The structure and morphology of the resulting membranes were investigated by wide angle X-ray diffraction (WAXD), Fourier transform infrared  spectroscopy (FTIR), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The effects of the type of the antisolvent and the presence of multiwalled carbon nanotubes (MWNTs) on membrane morphology and the crystal structure developed within the membranes were studied. The crystallization of the PVDF upon immersion precipitation occurred predominantly in theα-phase when water is used as the antisolvent or in the absence of the carbon nanotubes. On the other hand, β-phase crystallization of the PVDF was promoted upon the use of ethanol as the antisolvent in conjunction with the incorporation of the MWNTs. The morphology and the total crystallinity of the PVDF membranes were also affected by the incorporation of the MWNTs and the antisolvent used, suggesting that the microstructure and the ultimate properties of the PVDF membranes can be engineered upon the judicious selection of crystallization conditions and the use of carbon nanotubes.


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## Body

## 1. Introduction

The physical properties of semicrystalline polymers
depend upon the processing conditions [1] and can be strongly
influenced by the presence of nanoparticles, which affect the crystallization
behavior [2] and the resulting crystal
morphology developed within the processed sample. Polyvinylidene fluoride
(PVDF) is a semicrystalline thermoplastic polymer with five possible polymorphs
[3]. Among these polymorphs, more
attention has been paid to the β-phase due to its piezoelectric, ferroelectric,
and pyroelectric properties [4–7]. Since the increase in the
β-phase of PVDF has been associated with a greater piezoelectric coefficient
(due to an enhancement of the electromechanical coupling coefficient), a
variety of experimental techniques have been developed to induce β-phase
formation in PVDF [8, 9]. For example, Matsushige and Takemura showed that crystallization
from themelt at pressures which exceed 350 MPa led to the formation of the β-form
of PVDF [10]. Uniaxial or biaxial drawing
of PVDF films has also been shown to induce an α-β transition [11–13]. In addition, a number of
reports also indicate that nanoclays and carbon nanotubes can induce the
β-crystal formation in PVDF nanocomposites prepared via melt processing or
solution processing [14–17]. Recently, Dillon et
al. used a coprecipitation method to induce the β-crystal structure for
PVDF samples reinforced with nanoclays [18].Such organic-inorganic polymer nanocomposites have
attracted wide interest in the research community as researchers investigate
the addition of inorganic nanoparticles to impart multifunctionality to the
host polymer system, for example, simultaneous enhancement of multiple properties
such as electrical and thermal conductivity, mechanical toughness, and
dielectric constants of composites [19–24]. Such nanocomposites can also
prove to be useful for molecular separations. For example, Merkel et al. found that physical dispersion
of nonporous, nanoscale, fumed silica particles in glassy amorphous
poly(4-methyl-2-pentyne) enhanced both membrane permeability and selectivity
for large organic molecules over small gas molecules such as hydrogen[25]. In this regard, immersion
precipitation is a commonly used technique to prepare PVDF membranes for
microfiltration and ultrafiltration [26–32]. Membranes from electrospun
polymer nanocomposites can also be used for sensing applications [33, 34]. The incorporation of carbon
nanotubes into PVDF is especially attractive because of the significant
increase in the electromechanical coefficient to enable the use of the
resulting nanocomposites as actuators for artificial muscles and sensors for
vibration control [35–37].While immersion precipitation has been used for a
number of pure polymer systems [38–44], lacking are studies of the
effect of MWNTs and other nanoparticles on the morphology and crystal structure
of PVDF nanocomposite membranes prepared using this technique. Here, we report
the utilization of the immersion precipitation technique for the preparation of
PVDF nanocomposite membranes. The effects of the incorporation of MWNTs on the
crystallization behavior of PVDF was investigated when ethanol and water, respectively, were used as the antisolvent in the immersion precipitation process .


## 2. Experimental

### 2.1. Materials

Powdered PVDF (Kynar 741) was obtained from Arkema  Inc., Philadelphia, PA, USA. As reported by the manufacturer, its weight average molecular
weight is 250,000 and its density is 1.78 g/cm3. The MWNTs (trade
name: MWNT-A-P) were purchased from Sunnano, Jiangxi,  China . As reported by the manufacturer, the diameter of the MWNTs was 10–30 nm, and the
average bulk density was 1.5 g/cm3. To examine the size and shape
distributions of the MWNTs samples, an LEO 1550 scanning electron microscope
(SEM) operated at 15 kV was used. Figure 1 shows a typical scanning electron
micrograph of MWNTs. The scale bar is 1 μm. The solvent used was HPLC grade (99.9%) N,N-dimethylformamide (DMF) from Sigma-Aldrich  (St Louis., MO, USA). The antisolvents
used were distilled water and ethanol (A.C.S grade 99.98% purity from Pharmco Inc. (Brookfield, CT, USA).Figure 1
SEM micrograph of MWNTs.


### 2.2. Membrane Preparation

First, the PVDF powder and the MWNTs were mixed
separately with DMF. The polymer solution was prepared by heating a mixture of
PVDF and solvent (1 : 4 in weight ratio) at 70°C for 2 hours. The MWNT/DMF premix
was mixed using a hot plate with magnetic stirrer at 50°C for approximately 3 hours.
The final mixture was prepared by adding the contents of the MWNT/DMF premix to
the PVDF solution at 70°C with continuous stirring. The result was a mixture
that is approximately 5 wt% MWNT-PVDF in DMF (from here onwards, the loading of
MWNTs is in wt% unless otherwise stated). This solution was cast on a glass
plate and then first dried in a vacuum oven at 50°C for approximately 2 minutes
for solvent pre-evaporation; and then the membrane was coagulated using ethanol
or water antisolvents. After complete coagulation, the membranes were
transferred to a vacuum oven for drying overnight at 50°C. A similar technique
was followed to prepare pure PVDF membranes for comparison purposes. Thickness
of the membranes was 100–200 μm. The
morphology of the dried membranes was examined using an LEO 1550 SEM at 20 kV.
The top surface of the membranes was sputter-coated with gold before analysis.


### 2.3. Membrane Characterization

Differential scanning calorimetry (DSC) studies were conducted using a TA Instruments (New Castle, DE, USA)
DSC model Q1000 for pure PVDF and PVDF nanocomposite samples under a dryN2 environment. The sample weight was kept at approximately 3 mg. During DSC
analysis, the precipitated and dried samples were ramped from 25°C to 200°C at
a rate of 10°C/min, then maintained at isothermal conditions for 5 minutes at
200°C. The specimens were then cooled at a rate of 10°C/min to ambient. The
nominal melting temperature (Tm,p) was defined as the peak of the
melting endotherm during first heating from 25 to 200°C, and the nominal
crystallization temperature (Tc,p) was defined as the peak of the
crystallization exotherm upon cooling from 200 to 25°C. Wide angle X-ray
diffraction (WAXD) data were collected at room temperature by positioning the
membranes on a quartz sample holder using a Rigaku Miniflex diffractometer in
conjunction with a CuKα radiation source (λ=0.154 nm) operated at
30 kV. The X-ray diffractograms were collected in the scan range, 2θ of 5–50° at the scan
speed of 1°min−1 and using a step size of 0.04°. Fourier
transform infrared (FTIR) spectra of the membranes were recorded on a JASCO
ATR/FTIR-460 Plus over a range of 1000–550 cm−1 with a resolution of 4 cm−1.


## 2.1. Materials

Powdered PVDF (Kynar 741) was obtained from Arkema  Inc., Philadelphia, PA, USA. As reported by the manufacturer, its weight average molecular
weight is 250,000 and its density is 1.78 g/cm3. The MWNTs (trade
name: MWNT-A-P) were purchased from Sunnano, Jiangxi,  China . As reported by the manufacturer, the diameter of the MWNTs was 10–30 nm, and the
average bulk density was 1.5 g/cm3. To examine the size and shape
distributions of the MWNTs samples, an LEO 1550 scanning electron microscope
(SEM) operated at 15 kV was used. Figure 1 shows a typical scanning electron
micrograph of MWNTs. The scale bar is 1 μm. The solvent used was HPLC grade (99.9%) N,N-dimethylformamide (DMF) from Sigma-Aldrich  (St Louis., MO, USA). The antisolvents
used were distilled water and ethanol (A.C.S grade 99.98% purity from Pharmco Inc. (Brookfield, CT, USA).Figure 1
SEM micrograph of MWNTs.


## 2.2. Membrane Preparation

First, the PVDF powder and the MWNTs were mixed
separately with DMF. The polymer solution was prepared by heating a mixture of
PVDF and solvent (1 : 4 in weight ratio) at 70°C for 2 hours. The MWNT/DMF premix
was mixed using a hot plate with magnetic stirrer at 50°C for approximately 3 hours.
The final mixture was prepared by adding the contents of the MWNT/DMF premix to
the PVDF solution at 70°C with continuous stirring. The result was a mixture
that is approximately 5 wt% MWNT-PVDF in DMF (from here onwards, the loading of
MWNTs is in wt% unless otherwise stated). This solution was cast on a glass
plate and then first dried in a vacuum oven at 50°C for approximately 2 minutes
for solvent pre-evaporation; and then the membrane was coagulated using ethanol
or water antisolvents. After complete coagulation, the membranes were
transferred to a vacuum oven for drying overnight at 50°C. A similar technique
was followed to prepare pure PVDF membranes for comparison purposes. Thickness
of the membranes was 100–200 μm. The
morphology of the dried membranes was examined using an LEO 1550 SEM at 20 kV.
The top surface of the membranes was sputter-coated with gold before analysis.


## 2.3. Membrane Characterization

Differential scanning calorimetry (DSC) studies were conducted using a TA Instruments (New Castle, DE, USA)
DSC model Q1000 for pure PVDF and PVDF nanocomposite samples under a dryN2 environment. The sample weight was kept at approximately 3 mg. During DSC
analysis, the precipitated and dried samples were ramped from 25°C to 200°C at
a rate of 10°C/min, then maintained at isothermal conditions for 5 minutes at
200°C. The specimens were then cooled at a rate of 10°C/min to ambient. The
nominal melting temperature (Tm,p) was defined as the peak of the
melting endotherm during first heating from 25 to 200°C, and the nominal
crystallization temperature (Tc,p) was defined as the peak of the
crystallization exotherm upon cooling from 200 to 25°C. Wide angle X-ray
diffraction (WAXD) data were collected at room temperature by positioning the
membranes on a quartz sample holder using a Rigaku Miniflex diffractometer in
conjunction with a CuKα radiation source (λ=0.154 nm) operated at
30 kV. The X-ray diffractograms were collected in the scan range, 2θ of 5–50° at the scan
speed of 1°min−1 and using a step size of 0.04°. Fourier
transform infrared (FTIR) spectra of the membranes were recorded on a JASCO
ATR/FTIR-460 Plus over a range of 1000–550 cm−1 with a resolution of 4 cm−1.


## 3. Results and Discussion

The morphology and the crystallinity of the PVDF and
MWNT-PVDF nanocomposite membranes, prepared by the immersion precipitation
technique, were investigated as a function of the incorporation of the MWNTs
and the type of antisolvent used. As described below, the crystallization of
the PVDF upon immersion precipitation occurred predominantly in theα-phase
when water is used as the antisolvent or in the absence of the carbon
nanotubes, while β-phase crystallization was favored upon the use of ethanol as
the antisolvent in conjunction with the incorporation of the MWNTs.


### 3.1. Effect of MWNTs and Antisolvent on the PVDF Crystal Structure

The results of WAXD analysis of PVDF and PVDF nanocomposite membranes
prepared using ethanol as the antisolvent are shown in Figure2. From the
integration of the area under the crystalline peaks, it can be seen that the
pure PVDF membrane contains predominantly major crystalline peaks at the 2θ values of 18.6°, 20.3°, and 27°. These peaks
are attributed to the crystal planes associated with the α-phase of PVDF [18, 41]. It is significant that the
addition of 5% MWNTs into PVDF promotes the crystallization of the PVDF in the
β-polymorph. This is indicated from the WAXD patterns of 5% MWNT-PVDF
nanocomposite membranes which exhibit the distinct feature of containing a
sharp peak at 20.6° which is attributed specifically to the presence of the β
polymorph of PVDF [14–18]. In addition to this peak,
the 5% MWNT-PVDF nanocomposite samples also exhibit large reductions in the
areas under the peaks associated with the α polymorph that occur at the 2θ values of 18.6° and 27°. The WAXD pattern for
pure MWNTs (Figure 3) shows major peaks at 28.5°, 39.5°, and 40.6°
which indicate that the crystalline peaks arising from the presence of the
MWNTs are not overlapping with the crystalline peak associated with β polymorph
which occurs at 20.6°.Figure 2
WAXD of PVDF and PVDF
nanocomposite membranes prepared using ethanol as the antisolvent.Figure 3
WAXD of pure MWNT sample.To further elucidate the effects of the concentration of the MWNTs onβ-phase formation, PVDF membranes were prepared using 2 and 10 wt% loadings of
MWNTs as shown in Figure 4. A decrease in the α-peak area with increase in MWNT
loadings can clearly be seen in the figure. It was found that the incorporation
of MWNTs at both the 2 and 10 wt% levels continue to promote the crystallization
of PVDF in the β-polymorph. The WAXD patterns of 2 and 10% MWNT-PVDF
nanocomposites both exhibit a single sharp peak at 20.6° (Figure 4).
Furthermore, the effect of the polymer concentration in the premix was also
investigated by using a 10% PVDF/DMF solution (compared to the 20% PVDF
solution used previously); this premix solution was then used to prepare 5%
MWNT-PVDF nanocomposite membranes using ethanol as the antisolvent. X-ray data
from these samples also indicated the crystallization of the PVDF predominantly
in the β-phase (not shown here). This again suggests that the β-phase formation
in MWNT-PVDF membranes is principally due to the presence of the nanotubes and
ethanol as the antisolvent.Figure 4
WAXD of nanocomposite membranes with different loadings of MWNTs prepared
using ethanol as the antisolvent.By comparison, the WAXD results obtained from PVDF and 5% MWNT-PVDF
membranes prepared using water as the antisolvent during the precipitation
process are shown in Figure5. Here, it can be seen that the major peaks in
both membranes correspond to the α-form of PVDF and remain largely unchanged
upon the incorporation of the MWNTs. Additional WAXD results on membranes
prepared using 2 and 10 wt% MWNTs (not shown here) show similar behavior. The
difference in crystal structure attained as a function of antisolvent can be
attributed to the fact that ethanol gives rise to a relatively slow rate of
polymer coagulation for PVDF, whereas water as the anitsolvent results in rapid
coagulation of PVDF [26, 40, 42, 45]. Upon addition of ethanol
solid-liquid, demixing occurs in the crystallizable segments of the polymer,
which enables polymer crystallization to take place due to slow precipitation
of the polymer [42]. In the case of water, the
rate of polymer precipitation is very fast, leading to a relatively fast rate
of crystallization, which should result
in a lowered degree of crystallinity [38, 45, 46]. The principle reason that
the rate of precipitation increases for the water/PVDF system should be
associated with the rapid rate of interdiffusion of the solvent and the
nonsolvent [45]. It is interesting to note
that the nanocomposite membranes prepared using water as the antisolvent did
not exhibit the β-phase (Figure 5) which can again be related to the slow
coagulation rate with ethanol. Figure 3 shows that major α-peaks in PVDF
membranes are transformed to β-peak upon the incorporation of the MWNTs (5%).
As discussed further in Section 3.3, the presence of the MWNTs increases the
nucleation rate for crystallization (with MWNTs acting as heterogeneous nuclei
sites), giving rise to an increase in the crystallinity of the membranes versus
virgin PVDF and as seen here promotes crystallization in the β phase.Figure 5
WAXD of PVDF and PVDF
nanocomposite membranes prepared using water as the antisolvent.The crystal phases within the membranes were further verified using FTIR
analysis. The corresponding IR absorption band characteristics of theα-phase
are 766 and 796 cm−1, whereas for β-phase peaks in the IR spectra
are located at 511 and 840 cm−1 [20, 41]. Figure 6 shows the FTIR
spectra for the pure PVDF and PVDF nanocomposite membranes prepared using
ethanol and water as the antisolvent. It can be seen that 5% MWNT-PVDF membrane
prepared using ethanol antisolvent consists primarily of β-phase crystals (840 cm−1), along with small amount of α-phase crystals as well (the
α-peaks at 766 and 796 cm−1 have low intensity). FTIR spectra of
pure PVDF membrane prepared with ethanol antisolvent indicate predominantly the
α-crystal phase. Similarly, the FTIR results for membranes prepared with water
antisolvent (both the pure PVDF and the 5% MWNT-PVDF nanocomposite samples) are
dominated by the presence of α-crystals (peaks at 766 and 796 cm−1).
These results along with WAXD data confirm the enhancement of β-phase crystal
in 5% MWNT-PVDF membrane prepared with ethanol antisolvent.Figure 6
FTIR spectra of (A) 5% MWNT-PVDF
(ethanol antisolvent), (B) pure PVDF (ethanol antisolvent), (C) 5% MWNT-PVDF
(water antisolvent), and (D) pure PVDF (water antisolvent) membranes.


### 3.2. Effect of MWNTs and Antisolvent on Morphology of PVDF Membrane

SEM images of the top surface of pure PVDF and 5% MWNT-PVDF membranes
precipitated from ethanol are shown in Figure7. The effect of MWNTs on the
membrane structure is significant and gives rise to the decrease in crystal
size and to an increase of the microporosity of the PVDF membranes. From Figure
7(a), it can be seen that a sponge-like structure is formed to constitute the
pure PVDF membrane. While a clear skin surface can be seen in the image, the
structure of the membrane appears dominated by spherulitic crystallites in the
20–40 μm diameter range.SEM of (a) PVDF and (b) 5% MWNT-PVDF nanocomposite membranes prepared using ethanol as the antisolvent.
(a)
PVDF(b)
5% MWNT-PVDFBy comparison, the incorporation of the MWNTs has a profound effect on
the distributions of crystallite shapes, sizes, and porosity. The morphological
features shown in Figure7(b) suggest that MWNTs are acting as heterogeneous
nucleating agents, the concentration of which determines the number of nuclei.
The increase of the nucleation rate and the crystallization of the PVDF on the
nanotube surfaces eliminate the impinging of the relatively large spherulitic
crystallites observed with pure PVDF (see Figure 7(a)). Upon the incorporation
of the MWNTs, a porous superstructure consisting of shish-kebab-like
crystalline entities, forming a loose network, is observed. It also appears
that the nanotubes are providing interconnections to form a continuous and open
structure. The interconnectivity of the membrane will likely provide additional
mechanical strength to the membrane, possibly increasing the utility of such
piezoelectric membranes.On the other hand, Figure8 shows the PVDF and 5% MWNT-PVDF membranes
prepared using water as the antisolvent. The top surface of the PVDF membrane
shows a dense “skin” layer, which appears to be nonporous [38, 45]. The skin is composed of
intersecting polygonal plates with clear boundaries; similar structures with
crystalline polygonal plates have been observed in Nylon-6 membranes prepared
via the immersion precipitation in water [39, 46]. As noted earlier, the
formation of the skin layer and lack of an interconnected pore structure upon
the use of water as the antisolvent is likely due to the rapid precipitation
that occurs when water is used as the antisolvent, associated with the high
rate of interdiffusion of DMF and water [45], where the rate of
interdiffusion depends on the value of the solubility parameters of the solvent
and antisolvent. In addition, small micropores can be seen on the surface of
the membrane. Similar effects on morphology have been observed in
mica-intercalated Nylon-6 nanocomposite membrane prepared by phase inversion
method using water as the antisolvent [39]. As shown in Figure 8(b), the
incorporation of MWNTs results in a similar morphology, albeit with smaller
polygonal regions making up the surface of the membrane.SEM images of (a) PVDF and (b) 5% MWNT-PVDF nanocomposite membranes prepared using water as the antisolvent.
(a)
PVDF(b)
5% MWNT-PVDFThese initial results, together with the WAXD results presented in
Section3.1, indicate that the overall crystal morphology and porosity of the
PVDF nanocomposite membranes are strongly influenced by both the antisolvent
used to prepare the membrane as well as the presence of the MWNTs. Further
analysis of the effects of these parameters on the crystallinity, porosity, and
the availability of the targeted β crystalline phase in PVDF nanocomposite
membranes is necessary to allow the optimization of the microstructure and
associated ultimate properties of such membranes.


### 3.3. Effects of MWNTs and Antisolvent on Melting and Crystallization Behavior of PVDF Membrane

DSC analysis of pure PVDF and PVDF nanocomposite membranes was conducted
to examine the effect of MWNTs and antisolvents on the degree of crystallinityXc of the membranes upon precipitation and the
melting/crystallization temperatures observed upon the heating and the
subsequent cooling of the membranes. Representative DSC heating and cooling
curves are shown in Figures 9 and 10, respectively, while the results for four
samples tested at each condition are summarized in Table 1. The heat of fusion
(ΔHm) data obtained for
nanocomposite samples was corrected due to the presence of MWNTs in the sample
(such that ΔHcorrected=ΔHmeasured/Wp,
where Wp is the weight fraction of polymer in the sample).
Similarly, the heat of crystallization (ΔHc) data for PVDF
nanocomposite samples was also corrected for true heat of crystallization of PVDF.Table 1
Thermal analysis for PVDF and PVDF nanocomposite membranes.
SampleAntisolventΔHm, J/gXcm (%)Tm,p (°C)ΔHc, J/gXcc (%)Tc,p (°C)PVDFEthanol51.32±1.0148.77±0.59168.03±0.3941.5±0.5639.67±0.54132.4±0.785% MWNT-PVDFEthanol56.6±1.6254.41±1.24166.13±0.6241.33±2.1139.27±1.89142.89±3.02PVDFWater44.9±1.0642.9±1.05168.03±0.3041.16±1.0539.26±0.99133.48±0.395% MWNT-PVDFWater41.93±1.5240.09±1.45165.41±0.9838.35±2.2736.67±2.17136.41±0.50Tm,p= nominal melting temperature, Tc,p= nominal
crystallization temperature, ΔHm= heat of fusion,  ΔHc= heat of crystallization, Xcm= crystallinity from melting endotherm, Xcc= crystallinity from crystallization exotherm.
Average values and 95% confidence intervals are reported.DSC endotherms of PVDF and 5% MWNT-PVDF nanocomposite
membranes scanned at 10°C/min (a) using
ethanol as the antisolvent and (b) using water as the antisolvent.
(a)(b)DSC exotherms for PVDF and 5% MWNT-PVDF nanocomposite membrane scanned at 10°C/min
(a) using ethanol antisolvent and (b) using water antisolvent.
(a)(b)There are significant differences in the heat of fusion values for the
four cases (with and without the MWNTs, and with ethanol or water). The
use of ethanol as the antisolvent versus water gives rise to a greater heat of
fusion as shown in Table1. The percent crystallinity of PVDF and PVDF
nanocomposites, based on either the melting (Xcm) or crystallization (Xcc) DSC scans, can be obtained from the
expressions:
(1)Xcm=ΔHm*100ΔH100%,crystalline,Xcc=ΔHc*100ΔH100%,crystalline, where ΔH100%,crystalline is the heat of fusion of pure
crystalline PVDF, which is reported to be 104.6 J/g [14, 45], giving rise to a
significantly higher rate of precipitation in water and thus restricting the
degree of crystallinity of PVDF.The role played by the nanotubes in the crystallization process is
interesting and again depends on which antisolvent is utilized during the
precipitation. With ethanol, the favorable relatively slow rate of
precipitation allows the MWNTs to act as nucleating agents to induce an
increase in the nucleation and enables crystallization to take place to a
greater extent. The presence of the MWNTs further increases the degree of the
crystallinityXcm of PVDF (Table 1), which is consistent with
the SEM micrographs showing a decrease in the size of the crystallites upon the
incorporation of the MWNTs (Figure 7(b)). On the other hand, the MWNTs
incorporated into the PVDF membranes upon using water as the antisolvent have
no favorable effect on the crystallization of PVDF and are even detrimental, that
is, a reduction in the heat of fusion and hence the degree of crystallinity is
observed. It appears that during the rapid precipitation process associated
with water as the antisolvent, the MWNTs presumably remain as clusters and
immobilize some of the PVDF, preventing their crystallization and thus reducing
the degree of crystallinity Xcm of PVDF upon precipitation.The data associated with the crystallization peak temperature further
reinforce this understanding associated with the roles of the rate of
precipitation and the effects of the MWNTs (Table1). The nominal
crystallization temperature (Tc,p) was found to increase upon the
addition of MWNTs, which is attributed to the heterogeneous nucleation induced
by the presence of the MWNTs and the associated increase in the rate of
nucleation/crystallization [47]. The increase in Tc,p is
significantly higher in the case of nanocomposite membranes prepared using
ethanol. In contrast, a small change in crystallization temperature and a
comparatively small decrease in polymer crystal size in nanocomposite membranes
are observed when water is used as the antisolvent. In addition, little change
is observed in the percent crystallinity Xcc based on the DSC cooling scans.


## 3.1. Effect of MWNTs and Antisolvent on the PVDF Crystal Structure

The results of WAXD analysis of PVDF and PVDF nanocomposite membranes
prepared using ethanol as the antisolvent are shown in Figure2. From the
integration of the area under the crystalline peaks, it can be seen that the
pure PVDF membrane contains predominantly major crystalline peaks at the 2θ values of 18.6°, 20.3°, and 27°. These peaks
are attributed to the crystal planes associated with the α-phase of PVDF [18, 41]. It is significant that the
addition of 5% MWNTs into PVDF promotes the crystallization of the PVDF in the
β-polymorph. This is indicated from the WAXD patterns of 5% MWNT-PVDF
nanocomposite membranes which exhibit the distinct feature of containing a
sharp peak at 20.6° which is attributed specifically to the presence of the β
polymorph of PVDF [14–18]. In addition to this peak,
the 5% MWNT-PVDF nanocomposite samples also exhibit large reductions in the
areas under the peaks associated with the α polymorph that occur at the 2θ values of 18.6° and 27°. The WAXD pattern for
pure MWNTs (Figure 3) shows major peaks at 28.5°, 39.5°, and 40.6°
which indicate that the crystalline peaks arising from the presence of the
MWNTs are not overlapping with the crystalline peak associated with β polymorph
which occurs at 20.6°.Figure 2
WAXD of PVDF and PVDF
nanocomposite membranes prepared using ethanol as the antisolvent.Figure 3
WAXD of pure MWNT sample.To further elucidate the effects of the concentration of the MWNTs onβ-phase formation, PVDF membranes were prepared using 2 and 10 wt% loadings of
MWNTs as shown in Figure 4. A decrease in the α-peak area with increase in MWNT
loadings can clearly be seen in the figure. It was found that the incorporation
of MWNTs at both the 2 and 10 wt% levels continue to promote the crystallization
of PVDF in the β-polymorph. The WAXD patterns of 2 and 10% MWNT-PVDF
nanocomposites both exhibit a single sharp peak at 20.6° (Figure 4).
Furthermore, the effect of the polymer concentration in the premix was also
investigated by using a 10% PVDF/DMF solution (compared to the 20% PVDF
solution used previously); this premix solution was then used to prepare 5%
MWNT-PVDF nanocomposite membranes using ethanol as the antisolvent. X-ray data
from these samples also indicated the crystallization of the PVDF predominantly
in the β-phase (not shown here). This again suggests that the β-phase formation
in MWNT-PVDF membranes is principally due to the presence of the nanotubes and
ethanol as the antisolvent.Figure 4
WAXD of nanocomposite membranes with different loadings of MWNTs prepared
using ethanol as the antisolvent.By comparison, the WAXD results obtained from PVDF and 5% MWNT-PVDF
membranes prepared using water as the antisolvent during the precipitation
process are shown in Figure5. Here, it can be seen that the major peaks in
both membranes correspond to the α-form of PVDF and remain largely unchanged
upon the incorporation of the MWNTs. Additional WAXD results on membranes
prepared using 2 and 10 wt% MWNTs (not shown here) show similar behavior. The
difference in crystal structure attained as a function of antisolvent can be
attributed to the fact that ethanol gives rise to a relatively slow rate of
polymer coagulation for PVDF, whereas water as the anitsolvent results in rapid
coagulation of PVDF [26, 40, 42, 45]. Upon addition of ethanol
solid-liquid, demixing occurs in the crystallizable segments of the polymer,
which enables polymer crystallization to take place due to slow precipitation
of the polymer [42]. In the case of water, the
rate of polymer precipitation is very fast, leading to a relatively fast rate
of crystallization, which should result
in a lowered degree of crystallinity [38, 45, 46]. The principle reason that
the rate of precipitation increases for the water/PVDF system should be
associated with the rapid rate of interdiffusion of the solvent and the
nonsolvent [45]. It is interesting to note
that the nanocomposite membranes prepared using water as the antisolvent did
not exhibit the β-phase (Figure 5) which can again be related to the slow
coagulation rate with ethanol. Figure 3 shows that major α-peaks in PVDF
membranes are transformed to β-peak upon the incorporation of the MWNTs (5%).
As discussed further in Section 3.3, the presence of the MWNTs increases the
nucleation rate for crystallization (with MWNTs acting as heterogeneous nuclei
sites), giving rise to an increase in the crystallinity of the membranes versus
virgin PVDF and as seen here promotes crystallization in the β phase.Figure 5
WAXD of PVDF and PVDF
nanocomposite membranes prepared using water as the antisolvent.The crystal phases within the membranes were further verified using FTIR
analysis. The corresponding IR absorption band characteristics of theα-phase
are 766 and 796 cm−1, whereas for β-phase peaks in the IR spectra
are located at 511 and 840 cm−1 [20, 41]. Figure 6 shows the FTIR
spectra for the pure PVDF and PVDF nanocomposite membranes prepared using
ethanol and water as the antisolvent. It can be seen that 5% MWNT-PVDF membrane
prepared using ethanol antisolvent consists primarily of β-phase crystals (840 cm−1), along with small amount of α-phase crystals as well (the
α-peaks at 766 and 796 cm−1 have low intensity). FTIR spectra of
pure PVDF membrane prepared with ethanol antisolvent indicate predominantly the
α-crystal phase. Similarly, the FTIR results for membranes prepared with water
antisolvent (both the pure PVDF and the 5% MWNT-PVDF nanocomposite samples) are
dominated by the presence of α-crystals (peaks at 766 and 796 cm−1).
These results along with WAXD data confirm the enhancement of β-phase crystal
in 5% MWNT-PVDF membrane prepared with ethanol antisolvent.Figure 6
FTIR spectra of (A) 5% MWNT-PVDF
(ethanol antisolvent), (B) pure PVDF (ethanol antisolvent), (C) 5% MWNT-PVDF
(water antisolvent), and (D) pure PVDF (water antisolvent) membranes.


## 3.2. Effect of MWNTs and Antisolvent on Morphology of PVDF Membrane

SEM images of the top surface of pure PVDF and 5% MWNT-PVDF membranes
precipitated from ethanol are shown in Figure7. The effect of MWNTs on the
membrane structure is significant and gives rise to the decrease in crystal
size and to an increase of the microporosity of the PVDF membranes. From Figure
7(a), it can be seen that a sponge-like structure is formed to constitute the
pure PVDF membrane. While a clear skin surface can be seen in the image, the
structure of the membrane appears dominated by spherulitic crystallites in the
20–40 μm diameter range.SEM of (a) PVDF and (b) 5% MWNT-PVDF nanocomposite membranes prepared using ethanol as the antisolvent.
(a)
PVDF(b)
5% MWNT-PVDFBy comparison, the incorporation of the MWNTs has a profound effect on
the distributions of crystallite shapes, sizes, and porosity. The morphological
features shown in Figure7(b) suggest that MWNTs are acting as heterogeneous
nucleating agents, the concentration of which determines the number of nuclei.
The increase of the nucleation rate and the crystallization of the PVDF on the
nanotube surfaces eliminate the impinging of the relatively large spherulitic
crystallites observed with pure PVDF (see Figure 7(a)). Upon the incorporation
of the MWNTs, a porous superstructure consisting of shish-kebab-like
crystalline entities, forming a loose network, is observed. It also appears
that the nanotubes are providing interconnections to form a continuous and open
structure. The interconnectivity of the membrane will likely provide additional
mechanical strength to the membrane, possibly increasing the utility of such
piezoelectric membranes.On the other hand, Figure8 shows the PVDF and 5% MWNT-PVDF membranes
prepared using water as the antisolvent. The top surface of the PVDF membrane
shows a dense “skin” layer, which appears to be nonporous [38, 45]. The skin is composed of
intersecting polygonal plates with clear boundaries; similar structures with
crystalline polygonal plates have been observed in Nylon-6 membranes prepared
via the immersion precipitation in water [39, 46]. As noted earlier, the
formation of the skin layer and lack of an interconnected pore structure upon
the use of water as the antisolvent is likely due to the rapid precipitation
that occurs when water is used as the antisolvent, associated with the high
rate of interdiffusion of DMF and water [45], where the rate of
interdiffusion depends on the value of the solubility parameters of the solvent
and antisolvent. In addition, small micropores can be seen on the surface of
the membrane. Similar effects on morphology have been observed in
mica-intercalated Nylon-6 nanocomposite membrane prepared by phase inversion
method using water as the antisolvent [39]. As shown in Figure 8(b), the
incorporation of MWNTs results in a similar morphology, albeit with smaller
polygonal regions making up the surface of the membrane.SEM images of (a) PVDF and (b) 5% MWNT-PVDF nanocomposite membranes prepared using water as the antisolvent.
(a)
PVDF(b)
5% MWNT-PVDFThese initial results, together with the WAXD results presented in
Section3.1, indicate that the overall crystal morphology and porosity of the
PVDF nanocomposite membranes are strongly influenced by both the antisolvent
used to prepare the membrane as well as the presence of the MWNTs. Further
analysis of the effects of these parameters on the crystallinity, porosity, and
the availability of the targeted β crystalline phase in PVDF nanocomposite
membranes is necessary to allow the optimization of the microstructure and
associated ultimate properties of such membranes.


## 3.3. Effects of MWNTs and Antisolvent on Melting and Crystallization Behavior of PVDF Membrane

DSC analysis of pure PVDF and PVDF nanocomposite membranes was conducted
to examine the effect of MWNTs and antisolvents on the degree of crystallinityXc of the membranes upon precipitation and the
melting/crystallization temperatures observed upon the heating and the
subsequent cooling of the membranes. Representative DSC heating and cooling
curves are shown in Figures 9 and 10, respectively, while the results for four
samples tested at each condition are summarized in Table 1. The heat of fusion
(ΔHm) data obtained for
nanocomposite samples was corrected due to the presence of MWNTs in the sample
(such that ΔHcorrected=ΔHmeasured/Wp,
where Wp is the weight fraction of polymer in the sample).
Similarly, the heat of crystallization (ΔHc) data for PVDF
nanocomposite samples was also corrected for true heat of crystallization of PVDF.Table 1
Thermal analysis for PVDF and PVDF nanocomposite membranes.
SampleAntisolventΔHm, J/gXcm (%)Tm,p (°C)ΔHc, J/gXcc (%)Tc,p (°C)PVDFEthanol51.32±1.0148.77±0.59168.03±0.3941.5±0.5639.67±0.54132.4±0.785% MWNT-PVDFEthanol56.6±1.6254.41±1.24166.13±0.6241.33±2.1139.27±1.89142.89±3.02PVDFWater44.9±1.0642.9±1.05168.03±0.3041.16±1.0539.26±0.99133.48±0.395% MWNT-PVDFWater41.93±1.5240.09±1.45165.41±0.9838.35±2.2736.67±2.17136.41±0.50Tm,p= nominal melting temperature, Tc,p= nominal
crystallization temperature, ΔHm= heat of fusion,  ΔHc= heat of crystallization, Xcm= crystallinity from melting endotherm, Xcc= crystallinity from crystallization exotherm.
Average values and 95% confidence intervals are reported.DSC endotherms of PVDF and 5% MWNT-PVDF nanocomposite
membranes scanned at 10°C/min (a) using
ethanol as the antisolvent and (b) using water as the antisolvent.
(a)(b)DSC exotherms for PVDF and 5% MWNT-PVDF nanocomposite membrane scanned at 10°C/min
(a) using ethanol antisolvent and (b) using water antisolvent.
(a)(b)There are significant differences in the heat of fusion values for the
four cases (with and without the MWNTs, and with ethanol or water). The
use of ethanol as the antisolvent versus water gives rise to a greater heat of
fusion as shown in Table1. The percent crystallinity of PVDF and PVDF
nanocomposites, based on either the melting (Xcm) or crystallization (Xcc) DSC scans, can be obtained from the
expressions:
(1)Xcm=ΔHm*100ΔH100%,crystalline,Xcc=ΔHc*100ΔH100%,crystalline, where ΔH100%,crystalline is the heat of fusion of pure
crystalline PVDF, which is reported to be 104.6 J/g [14, 45], giving rise to a
significantly higher rate of precipitation in water and thus restricting the
degree of crystallinity of PVDF.The role played by the nanotubes in the crystallization process is
interesting and again depends on which antisolvent is utilized during the
precipitation. With ethanol, the favorable relatively slow rate of
precipitation allows the MWNTs to act as nucleating agents to induce an
increase in the nucleation and enables crystallization to take place to a
greater extent. The presence of the MWNTs further increases the degree of the
crystallinityXcm of PVDF (Table 1), which is consistent with
the SEM micrographs showing a decrease in the size of the crystallites upon the
incorporation of the MWNTs (Figure 7(b)). On the other hand, the MWNTs
incorporated into the PVDF membranes upon using water as the antisolvent have
no favorable effect on the crystallization of PVDF and are even detrimental, that
is, a reduction in the heat of fusion and hence the degree of crystallinity is
observed. It appears that during the rapid precipitation process associated
with water as the antisolvent, the MWNTs presumably remain as clusters and
immobilize some of the PVDF, preventing their crystallization and thus reducing
the degree of crystallinity Xcm of PVDF upon precipitation.The data associated with the crystallization peak temperature further
reinforce this understanding associated with the roles of the rate of
precipitation and the effects of the MWNTs (Table1). The nominal
crystallization temperature (Tc,p) was found to increase upon the
addition of MWNTs, which is attributed to the heterogeneous nucleation induced
by the presence of the MWNTs and the associated increase in the rate of
nucleation/crystallization [47]. The increase in Tc,p is
significantly higher in the case of nanocomposite membranes prepared using
ethanol. In contrast, a small change in crystallization temperature and a
comparatively small decrease in polymer crystal size in nanocomposite membranes
are observed when water is used as the antisolvent. In addition, little change
is observed in the percent crystallinity Xcc based on the DSC cooling scans.


## 4. Conclusions

MWNT-PVDF nanocomposite membranes were prepared using ethanol and water
antisolvents. The desirableβ-phase  crystallization of PVDF was promoted using
MWNTs and ethanol as the antisolvent. The crystal size of the polymer was found
to decrease due to addition of MWNTs, using ethanol, while the degree of
crystallinity and the crystallization temperature increased. Further tests to
correlate the mechanical and piezoelectric properties of the membranes as a
function of the processing parameters are ongoing. These results indicate that
the microstructure and the ultimate properties of the PVDF membranes can be
engineered upon the judicious selection of crystallization conditions and the
use of carbon nanotubes, with applications in the engineering of surfaces and
structures for targeted applications such as sensors, filters, and artificial
muscles.



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*Source: 759825-2008-05-13.xml*