Effect of Electron Beam Irradiation on Poly(vinylidene fluoride) Films at the Melting Temperature
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Effect of Electron Beam Irradiation on Poly(vinylidene fluoride)
Films at the Melting Temperature
Youn Mook Lim, Phil Hyun Kang, Sang Mann Lee, Sung Sik Kim , Joon Pyo Jeun,
Chan Hee Jung, Jae Hak Choi, Young Moo Lee , and Young Chang Nho
Radiation Application Research Division, Advanced Radiation Technology Institute-Jeongeup, Korea Atomic Energy
Research Institute, Jeongeup-si, Jeonbuk 580-185, Korea
School of Environmental and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Korea
School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Korea
Received November 1, 2005; Accepted May 19, 2006
Abstract: The radiation-induced changes taking place in poly(vinylidene fluoride) (PVDF) films exposed to
electron beam irradiation were investigated in correlation with the applied doses. Films were irradiated in air at
o
the melting temperature (174 C) using a universal electron beam accelerator at doses in the range 50 200
kGy. Various properties of the melt-irradiated PVDF films were studied using ATR-FTIR spectroscopy,
differential scanning calorimetry (DSC), X-ray diffraction and scattering, and universal mechanical testing.
The unirradiated PVDF film was used as a reference. Electron beam irradiation at the melting temperature
induced changes in the physical, chemical, thermal, structural, and mechanical properties of the PVDF films;
such changes varied depending on the irradiation dose.
Keywords: PVDF film, electron beam irradiation, crosslinking, mechanical properties, X-ray diffraction
Introduction chain session, or evolution of hydrogen depending upon
1) the chemical and physical nature of the polymer and the
Fluorinated polymers possess excellent thermal and type of radiation. Radiation crosslinking and radiolysis of
chemical stability and mechanical properties [1]. Among fluorinated polymers have been reviewed in Refs. [1] and
these materials, poly(vinylidene fluoride) [PVDF; (CF2- [9], respectively. The increased crystallinity of irradiated
CH2)n] has attracted a considerable degree of attention in PVDF was first reported by Bhateja [10], who found that
many areas of research and for industrial use [2] because the crystallinity increased appreciably in proportion to
of its favorable electrical properties, resistance to the radiation dose up to 500 kGy after electron beam
weathering durability, biocompatibility, and processibility radiation. Other researchers [11,12] have also claimed
[3]. Investigation of PVDF’s morphology and piezoelec- that X-ray and γ-radiation causes the increased crystal-
tric properties has become a subject of active research linity of PVDF, but the cause remains somewhat unclear.
during the past four decades [4-7]. PVDF is a semi- The effect of electron beam radiation on the inherent
crystalline polymer that exhibits at least three different properties of PVDF has been studied on various occa-
crystalline forms. Application of a high electric field sions [10-16]. However, some of the reported results,
affects the polarization of the PVDF film through the particularly those related to the crystallinity, crosslink-
alignment of hydrogen and fluorine ions according to ing, and melting temperature, are contradictory and, as a
their respective electrical polarities. result, further clarification studies are necessary.
Irradiation by ionizing radiation induces changes in the
chemical structure and physical properties of polymers
[8]. Such changes may arise from crosslinking, main Experimental
To whom all correspondence should be addressed. Materials and Electron Beam Irradiation
(e-mail: ycnho@kaeri.re.kr) The PVDF films were purchased from Goodfellow Cam-590 Youn Mook Lim, Phil Hyun Kang, Sang Mann Lee, Sung Sik Kim, Joon Pyo Jeun, Chan Hee Jung, Jae Hak Choi, Young Moo Lee, and Young Chang Nho
Table 1. Conditions under which Irradiation was Performed equipped with a DLaTGS detector. Transmission and
Accelerating voltage 1.0 MeV ATR spectra were recorded from 16 scans at a resolution
Beam current / Power 2.5 mA / 4 kW of 4 cm-1. The spectra weremeasured in the wavenumber
-1
Distance from film 10 cm range from 4000 to 500 cm and analyzed using com-
Conveyor speed 10 m/min mercial software. The heat of fusion and melting tem-
Dose per pass 5 kGy perature were determined using a Perkin-Elmer Differ-
Dose range 50 200 kGy ential Scanning Calorimeter (DSC-7). Typical samples
Atmosphere air
o weighing 3 5 mg were used. Thermograms were ob-
Temperature 175 C
tained from the first heating run in a temperature range
o o
from 25 to 200 C at a heating rate of 10 C/min under a
nitrogen atmosphere. The degree of crystallinity was
calculated using the expression
where Hf is the heat of fusion for the tested sample, and
is 104.7 J/g [18], the heat of fusion for the 100 %
crystalline sample. Wide-angle X-ray diffraction (WAXD)
patterns were recorded at room temperature using a
D/Max-rA rotating-anode X-Ray Diffractometer (Rigaku
Electrical Machine Co., Japan) equipped with a CuKα
tube and Ni filter. The diffraction patterns were deter-
o
mined over a range of diffraction angles (2θ = 10 30 )
at 40 kV and 70 mA. The small-angle X-ray scattering
(SXAS) experiments were conducted using a synchrotron
X-ray radiation source [4C1 beam line; energy: 2 GeV;
150 mA; wavelength: 1.608 ; 2-d Position Sensitive
Figure 1. Gel fraction as a function of radiation dose.
Detector (PSD)] at the Pohang Accelerator Laboratory,
Korea.
bridge (England). The films has a thickness of 100 µm The tensile strength of the irradiated films was mea-
3
and a density of 1.76 g/cm . The irradiation experiments sured using a universal mechanical tester (Instron, model
were performed using a conventional electron beam 4443, USA) at room temperature (n=5). Dumbbell-
accelerator (ELV-4, Korea) located at EB-tech. The irra- shaped specimens [50-mm long; 28-mm neck; 4-mm
diation conditions are given in Table 1. wide (ASTM D882)] were used. The cross-head speed
was fixed at 50 mm/min.
Determination of Gel Fraction
After irradiation, the melt-crosslinked PVDF films were
weighed. The gel fraction was determined by extraction Result and Discussion
with N,N-dimethylacetamide (DMA) at 50 oC for 100 h
[17]. The insoluble portion of the films, which consisted Gel Fraction
o
of crosslinked PVDF, were dried for 48 h at 80 C and The gel fraction as a function of theradiation dose is
then weighed. The gel fraction is defined as shown in Figure 1. As the radiation dose increased grad-
ually, the gel fraction increased. The PVDF film, cross-
linked at room temperature, has a gel fraction of 1 21
%. After being crosslinked in the melt state, the gel
fraction increased to 40 65 %. This phenomenon is due
where Wi is the initial weight of the PVDF film after to an increase in the degree of crosslinking from electron
irradiation and Wd is the weight of the dried insoluble beam with full disentan-glement of the polymer chain at
portion after extraction with DMA. the melting temperature.
Characterization Thermal and X-ray Analysis
The ATR-FTIR spectroscopic investigations were per- The melting temperature and crystallinity of irradiated
formed using a Bruker TENSOR 37 spectrophotometer PVDF as a function of the radiation dose at the meltingEffect of Electron Beam Irradiation on Poly(vinylidene fluoride) Films at the Melting Temperature 591
Table 2. Peak Areas of PVDF Films for Each Lattice Plane as
a Function of Radiation Dose
Radiation Peak area (%)
Dose (kGy) (110) (020) (100)
0 26.59 10.84 8.74
50 18.03 9.97 7.53
100 16.86 9.10 6.54
150 16.28 9.09 6.21
200 15.90 8.29 4.48
Figure 3. WAXD curves of irradiated PVDF films.
Figure 2. Crystallinity and melting temperature as a function
of radiation dose.
temperature are shown in Figure 2. During irradiation,
the melting temperature fell gently, whereas the crys-
tallinity decreased rapidly with doses in the range 100
200 kGy. Kusy and Turner [19] believed that the decr- Figure 4. SAXS curves of irradiated PVDF films.
eased melting temperature resulted from radiation-
induced crystal defects, suggesting that irradiation cannot in this case, the lamellar structure. Polymer lamellar
result in the formation of perfect crystallites. The collect- stacks are composed of alternating crystalline and
ed X-ray scattered intensities at wide angles and those of amorphous layers. One lamellar stack then can be con-
small angles contain structural information of different sidered as a one-dimensional array, with periodic length
length scales in real space. The analysis of the ranges from several nanometers to tens of nanometers.
wide-angle X-ray intensities is rather straightforward. The Lorentz-corrected X-ray scattered intensities at
The raw intensity curves were analyzed using a appropriate small angles reveal a periodic length scale
conventional ‘Curve Fit’ program to decompose the between the individual lamellar layers.
individual peaks from the intensity curves. The crystal- The WAXD curves are shown in Figure 3. Irradiation
linity index (cc) was then estimated using the following not only caused a decrease in the diffraction intensity but
equation: also the broadening of peaks. If the diffraction peak
broaden, it can be expected that the mean crystallite size
will decrease. This situation may arise because some
χ
small crystallites form in the amorphous regions of
PVDF during irradiation. Summation of these small
where Ic is the integrated intensity of the crystal peak and crystallites with the original crystallites causes the mean
It is the total scattered intensity. size of the crystallite to decrease. The percentages of the
The analysis of small-angle intensities, on the other peak area are shown in Table 2 for each lattice plane as a
hand, is not straightforward; it requires mathematical function of the radiation dose.
manipulation to gain some useful structural information, The effects of irradiation on the SAXS curves of PVDF592 Youn Mook Lim, Phil Hyun Kang, Sang Mann Lee, Sung Sik Kim, Joon Pyo Jeun, Chan Hee Jung, Jae Hak Choi, Young Moo Lee, and Young Chang Nho
Figure 5. Lamellar long-spacing of PVDF films as a function
of the radiation dose at the melting temperature.
Figure 7. Effect of irradiation dose on the tensile strength of
PVDF films.
crystallinity of the single crystal can be explained by
considering that the radiation induced small crystallites
in the middle of the amorphous region. Because there is
no amorphous region in a single crystal, irradiation can
damage the crystalline structure, causing an inevitable
decrease in the crystallinity. Figure 5 shows the lamellar
long-spacings of PVDF films as a function of the
radiation dose at the melting temperature. As the crystal
decreased with irradiation, the lamellar long-spacing
decreased accordingly.
ATR-FTIR Analysis
Figure 6 shows ATR-FTIR spectra of the irradiated
PVDF films along with that of its corresponding unirra-
Figure 6. ATR-FTIR spectra of unirradiated and irradiated diated sample. The unirradiated film is characterized by
PVDF films. the presence of symmetric and asymmetric stretching
-1
vibrations of the CH2 groups at 2985 and 3025 cm ,
-1
in Figure 4 confirm the correctness of the above con- respectively. The strong band at 1210 cm is assigned
jecture. The scattering peaks shifted to larger diffraction the CF2 groups. The spectra of the irradiated films did
angles with increasing dose, indicating that the long not show any major changes in the main absorption
period increases accordingly. The density of the amor- bands relative to those of the unirradiated film. However,
3
phous region was 1.68 g/cm , but that of theα-crystallite two adjacent bands at 1454 and 1646 cm-1 could be iden-
3
phase was 1.92 g/cm in PVDF [20]. When some molec- tified. The former is assigned to the C=C bond resulting
ular chains in the middle of an amorphous region trans- from dehydrofluorination (removal of HF) and subse-
late into an ordered arrangement to form small crystal- quent crosslinking, whereas the latter is assigned to C=O
lites, summation of these small crystallites with the origi- (carbonyl) groups resulting from the formation of
nal crystallites causes the mean size of the long period to hydroperoxide radicals initiated by irradiation in air.
decrease.
Compared with the increase in crystallinity in semi- Mechanical Property
crystalline PVDF under a low dose, Lovinger [20] found The effect of the irradiation dose on a mechanical
that the crystallinity in an irradiated PVDF single crystal property, i.e., tensile strength, of the PVDF films is given
decreased with increasing the dose. The decrease in the in Figure 7. The tensile strength of PVDF films de-Effect of Electron Beam Irradiation on Poly(vinylidene fluoride) Films at the Melting Temperature 593
nd
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cy in PVDF films to form predominant crosslinked struc- (1985).
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