FTIR and DSC studies of mechanically 
deformed β-PVDF films
Authors: S. Lanceros-Mendez, J. F. Mano, A. M. 
Costa, and V. Hugo Schmidt
This is an Accepted Manuscript of an article published in Journal of Macromolecular Science, Part 
B: Physics in 2001, available online: https://www.tandfonline.com/10.1081/MB-100106174.
S. Lanceros-Mendez, J. F. Mano, A. M. Costa, and V. Hugo Schmidt, “FTIR and DSC studies of
mechanically deformed β-PVDF films,” Journal of Macromolecular Science, Part B: Physics 40,
517-527 (2001). doi: 10.1081/MB-100106174.
Made available through Montana State University’s ScholarWorks 
scholarworks.montana.edu 
FTIR AND DSC STUDIES OF MECHANICALLY
DEFORMED 
-PVDF FILMS
S. Lanceros-Me´ndez,1 J. F. Mano,2,* A. M. Costa,2
and V. H. Schmidt3
1Physics Department, Minho University, 4710 Braga, Portugal
2Polymer Engineering Department, Minho University,
4800-058 Guimara˜es, Portugal
3Physics Department, Montana State University, Bozeman, MT 59717
ABSTRACT
Films of semicrystalline poly(vinylidene fluoride) (PVDF) in the β-
phase were studied by Fourier transform infrared (FTIR) spectroscopy and
differential scanning calorimetry (DSC). The main goal of this study was to
improve the understanding of the structural changes that occur in β-PVDF
during a mechanical deformation process. FTIR spectroscopy was used to
examine the structural variations as a function of strain. DSC data allowed
measurement of the melting temperatures and enthalpies of the material be-
fore and after deformation, providing information about the changes in the
crystalline fraction. After the molecular vibrations were assigned to the corre-
sponding vibrational modes, we investigated the energy and intensity varia-
tions of these vibrations at different deformations. A reorientation of the
chains from perpendicular to parallel to the stress direction was observed to
occur in the plastic region. During the deformation, a decrease in the degree
of crystallinity of the material was observed, but the thickness of the lamellae
did not change significantly.
Key Words: Mechanical anisotropy; Plastic deformation; PVDF; Vibrational
modes.
INTRODUCTION
Poly(vinylidene fluoride) (PVDF) has been intensely investigated because
of its interesting ferroelectric properties and technological applications. PVDF
can exist in several crystalline phases [1]. Of the five known modifications, the
α- and β-phases are most common. The α-form, the more common and stable,
arises usually from the melt when PVDF crystallizes in quiescent conditions. The
β-form offers the highest piezo-, pyro-, and ferroelectric properties. To obtain β-
PVDF, a technological process involving stretching and poling of extruded thin
sheets of the polymer is used, resulting in permanent polarization. Uniaxial
stretching provides the alignment of molecular chains. An applied electric field
up to 100 kV/mm, at temperatures around 100°C, causes the net polarization of
PVDF, which is maintained once the material is cooled to room temperature [2].
Generally, this transformation does not give rise to samples composed com-
pletely of one polymorphic modification, but small amounts of the other phases
are always present. The infrared spectra allow the various forms to be clearly
distinguished by identifying some characteristic bands with the corresponding
phases.
Deformation processes are intimately associated with the production of
these kinds of films, which have applications as sensors and actuators. Such
oriented films are often subjected to further stresses during service, and morpho-
logical changes may occur mainly in the direction perpendicular to the preferred
orientation of the chains, in which the mechanical properties are weaker.
In this study, Fourier transform infrared (FTIR) experiments were carried
out on β-PVDF films with different deformations induced at room temperature.
Such information may be relevant in identifying changes in the anisotropy, de-
gree of crystallinity, crystalline phase, and other morphological changes that oc-
cur during deformation. Complementary information was also obtained by per-
forming differential scanning calorimetry (DSC) studies of the same samples.
The correlation between different properties (e.g., mechanical) and the mi-
crostructure of polymeric systems has been studied for many years. The present
work constitutes a preliminary study within a broader project that intends to
follow and understand the changes at the microstructural level of relevant plastic
materials (with originally different morphologies) during deformation using a
series of complementary techniques, including FTIR, DSC, atomic force micros-
copy, X-ray spectroscopy, and dielectric and mechanical relaxation spectros-
copies.
EXPERIMENTAL PROCEDURE
The starting material was a 28-µm thick commercial PVDF film from Mea-
surements Specialties, Incorporated (Fairfield, NJ). The material was deformed
to different degrees in the direction perpendicular to the initial chain orientation.
Deformed samples were prepared using a T30K JJ instruments (Looyd In-
struments, Southampton, UK) tensile testing machine. The 13-mm wide PVDF
films were fixed (distance between clamps 40 mm) and pulled apart at a constant
speed of 2 mm/min. This strain rate is small enough to warrant negligible tem-
perature elevation. Samples with different deformations in the range from 35%
to 140% (break point) were prepared and studied.
Infrared spectra were obtained from 400 to 6000 cm−1 using polarized light
in perpendicular ⊥ and parallel 
 modes. In the parallel mode, the electric vector
of the light is parallel to the prior chain orientation (and perpendicular to the
draw direction); in the perpendicular mode, the electric vector of the light is
perpendicular to the initial chain orientation. FTIR analysis was performed in
transmission at room temperature on a Bruker 66V spectrometer (Ettlinger, Ger-
many), with a resolution of 1 cm−1.
Thermal analysis was carried out with a Setaram DSC 131 differential
scanning calorimeter (Caluire, France) at a heating rate of 10°C/min under nitro-
gen atmosphere. Samples were heated from room temperature to 250°C. Melting
temperatures and enthalpies were determined at the onset of the peaks and from
the peak areas, respectively.
RESULTS AND DISCUSSION
The anisotropy of the original β-PVDF films is observed in the stress-strain
mechanical experiments carried out in the two main directions (Fig. 1). In the
longitudinal direction, the film shows a typical brittle behavior with a higher
Figure 1. Stress-strain curves for β-PVDF in the two main directions obtained at 2 mm/min.
Longitudinal means that the stress has the same direction as the original preferred chain orientation,
and transversal means that the deformation is perpendicular to the preferred chain direction.
ultimate stress and a lower strain at break than for the transversal direction.
Therefore, the material enables higher deformations in the transversal direction.
The anisotropic behavior of the β-PVDF films was also analyzed by FTIR.
First, the spectra of the virgin sample were analyzed in both polarization direc-
tions to assign some modes or spectral regions to specific molecular vibrations.
Second, the effects on these molecular modes and on the different crystalline
phases produced by the application of a well-defined strain were analyzed. These
results were further compared with the DSC data, which gives complementary
information on the effect of the strain on the modification within the crystalline
fraction of the polymer.
Identification of Vibrational Modes and Spectral Regions
400–1500-cm−1 Region
In Fig. 2, the FTIR transmission spectra of virgin PVDF with the electric
field vector both parallel and perpendicular to the chain direction are shown in
the 400–1500-cm−1 region. The observed pattern originates from oscillations of
large parts of the skeleton and/or the skeleton and attached functional groups.
Below 1500 cm−1, most single bonds absorb at similar frequencies, and the vibra-
tions couple. Nevertheless, some modes and spectral regions have been identified
Figure 2. FTIR spectra of the original PVDF with polarized light. In the PVDF ⊥ mode, the
electric vector of the light is perpendicular to the chain orientation; in PVDF 
, the electric vector
of the light is parallel to the chain orientation. Some typical vibration regions are also shown.
with the help of the literature [3–9]. The results are summarized in Table 1 and
Fig. 2, respectively.
The identified regions are rich in information on the conformational iso-
merism of the chain, providing information on the α- and β-phase content.
Though the virgin sample was prepared to obtain the β-phase, some characteris-
tic bands of the crystalline α-phase have been observed at 763, 615, and 490
cm
−1
. The absorption band at 763 cm−1 is related to a rocking vibration [5]. The
band at 615 cm−1 is assigned to a mixed mode of CF2 bending and CCC skeletal
vibration; this peak is parallel to the chain axis [5,7]. The 490-cm−1 band is
related to bending and wagging vibrations of the CF2 group ascribed to the α-
PVDF polymorph [6,7].
Characteristic bands from the β-phase have been identified at 840, 745,
510, and 445 cm−1. The band at 840 cm−1 is assigned to a mixed mode of CH2
rocking and CF2 asymmetric stretching vibration; this mode is parallel to the
chain axis [3,4,7]. The band at 745 cm−1 is assigned to a rocking mode [4]. The
510-cm−1 band is assigned to a CF2 bending mode; this mode is perpendicular to
the chain axis and parallel to the polar b-axis [3,4,7]. The mode at 445 cm−1, also
characteristic of the β-phase, is parallel to the a-axis [4].
Table 1. Characteristic Bands with Specific Vibrational Modes and Crystalline Phases
Experimental
Wavenumber,
cm−1 Group Vibration Comments Ref.
3016 CH2 Symmetric stretching 5, 9
2978 CH2 Asymmetric stretching 5, 9
1453 CH2 In-plane bending or 8, 9
scissoring
1335 CH2 Out-of-plane bending 8
(wagging or twisting)
840 CH2, CF2 CH2 rocking and CF2 β-Phase (out-of-phase 3, 4, 7
asymmetric stretching combination)
763 In-plane bending or α-Phase 5
rocking
745 In-plane bending β-Phase 4
or rocking
677 Presence of head-to-head 6
and tail-to-tail configu-
rations
615 CF2, CCC CF2 bending and CCC α-Phase (out of phase) 5, 7
skeletal vibration
600 β-Phase 4
510 CF2 Bending β-Phase 3, 4, 7
490 CF2 Bending and wagging α-Phase (in-phase 6, 7
combination)
445 β-Phase 4
Irregularities of head-to-tail addition, leading to defect structures, can occur
during polymerization for several reasons [9]. In polymers of the vinylidene
class, sequence isomerism may occur. The transmission band at 677 cm−1 points
to the presence of head-to-head and tail-to-tail configurations [6]. Such defects
are produced during the polymerization process and reduce the dipole moment
of the all-trans conformation. This peak is observable in both polarization direc-
tions and is relatively unaffected by the deformation process. The transmittance
values of this peak were taken as the reference for calculation of relative changes
in the intensities of the other peaks as the ratio of transmittance is not dependent
on the sample thickness [10]. This methodology has been used to monitor the
relative amount of β- and α-crystalline phases and amorphous phase [11].
CH Stretching Vibrations
In many simple compounds, the CH2 group gives rise to two frequencies in
the 2800–3100-cm−1 zone. The asymmetric and symmetric stretching vibrations
of the CH2 group in the virgin sample are located, respectively, at 3016 cm−1 (νa
CH2) and 2978 cm−1 (νs CH2) (Fig. 3) [5,9]. Symmetric vibrations are generally
weaker than asymmetric vibrations since the former lead to less of a change in
dipole moment. These frequencies vary by only a few wavenumbers from com-
pound to compound [8,9].
Effect of the Applied Stress
Samples with different levels of deformations were prepared and studied
by FTIR and DSC. Deformations of 30%, 35%, 37.5%, 40%, 42.5%, 50%, 60%,
62.5%, 67.5%, 72.5%, 75%, 110%, 130%, and 140% (break point) were achieved
by interrupting the transverse experiment shown in Fig. 1 on reaching the desired
values. Note that, at such strain levels, the materials are in the plastic region
(Fig. 1), and therefore the deformations are highly irreversible on releasing the
stress.
The samples were first analyzed by FTIR in both polarizations. The idea
was to follow the variations of the absorbance and energy of the identified modes
and spectral regions with the applied deformation.
An unexpected and drastic effect was observed between the virgin film and
the film deformed with the lowest strain: The FTIR spectra of the deformed
samples for a given polarization direction seemed to be identical to the spectra
of the undeformed samples in the other polarization. That is, a polarization
switching seemed to occur (Figs. 4 and 5).
Figure 3. FTIR spectra of different samples of PVDF in the C−H mode region. Comparison of
three different deformations and virgin sample in both polarization directions: (a) the electric vec-
tor of the light is perpendicular to the chain orientation; (b) the electric vector of the light is
parallel to the chain orientation.
Such inversion of behavior from longitudinal to transversal direction and
vice-versa suggests changes of morphologies within the crystalline fraction such
that the chains in the crystalline phase switch their direction by 90°. This inver-
sion of the spectra occurs even for the lowest deformed sample (ε = 0.35), rela-
tive to the virgin one, (i.e., in the early plastic region). Moreover, the results in
Figs. 4 and 5 show that, with increasing strain, the changes are less pronounced.
Therefore, we may conclude that the inversion of polarization is not a progres-
sive process, and it occurs possibly near the yield point of the specimens. A
complete characterization of this process would need more morphological infor-
mation than could be provided by small-angle X-ray scattering (SAXS) and
wide-angle X-ray scattering (WAXS) experiments.
Many models have been proposed for the changes of microstructure during
deformation. It is usually accepted that, in semicrystalline polymers, yielding is
Figure 4. FTIR spectra of β-PVDF samples with different deformations. Comparison of two
different deformations (40% and 75%) in perpendicular mode with a virgin sample in both polar-
izations.
associated with interchain sliding, chain segmental motions, and chain reconfor-
mation [12]. Slip can occur between the crystalline lamellae, which slide by each
other, and within the individual lamellae by a process comparable to glide in
monoatomic crystals. The last—and dominant—process leads to molecular ori-
entation since the slip direction within the crystal is along the axis of the mole-
cule. As plastic flow continues, the slip direction rotates systematically toward
the tensile axis. The plastic deformation of semicrystalline polymers is a kinetic,
Figure 5. FTIR spectra of β-PVDF samples with different deformations. Comparison of two
different deformations (40% and 75%) in parallel mode with a virgin sample in both polarizations.
thermoactivated process caused by shear stress [13]. Ultimately, the slip direction
(i.e., the molecular axis) coincides with the tensile axis, and the polymer is then
oriented and resists further extension.
The changes in the microstructure during deformation depend on the chemi-
cal nature of the material and on the initial morphology. In the case of the β-
PVDF films studied in this work, the result suggests that yielding and further
plastic drawing induce a complete reorientation of the crystal lamellae. There
are models for deformation that are consistent with such features. For example,
morphological changes in semicrystalline polymers during stress application
have been studied by Flory and Yoon [14] and Takahashy and coworkers [15],
who proposed that plastic deformation occurs by fusion and recrystallization of
the crystal phase present in the semicrystalline polymer. This model was used to
explain a local 90° reorientation of the polymeric segments in the crystalline
structure of PVDF [16], as seen by creep and stress-strain experiments.
The inversion effect observed in the FTIR experiments now provides a
method for identifying the modes corresponding to the amorphous part of the
polymer: These modes should not depend on the orientation of the sample or on
the polarization switching that corresponds to a reorientation of the crystalline
part of the polymer. The modes appearing, for example, at 600, 745, and 948
cm
−1
are thus assigned to the amorphous component of the material.
Differential Scanning Calorimetry Results
DSC was used to characterize sections of previously deformed β-PVDF
films. Only one endothermic peak was detected in the films (onset temperatures
in the temperature range from 150°C to 160°C), corresponding to the crystal
melting. The DSC traces did not exhibit multiple melting endotherms that corre-
sponded to different crystalline phases. Consequently, we can conclude that the
β-phase was predominant in all the samples. Nevertheless, there was a very small
amount of α-phase present, as observed in the infrared spectra. The film orienta-
tion (deformation, in our case) changes the characteristic transition enthalpy, but
not the melting temperature.
The degree of crystallinity was measured as the ratio between ∆Hm and
∆H0, where ∆Hm is the melting enthalpy of the material under study and ∆H0 is
the melting enthalpy of totally crystalline material (∆H0 = 104.50 J/g for PVDF)
[17,18]. It is to be noticed that the melting enthalpy that is reported is not explic-
itly assigned to the β-form of PVDF. In fact, to our knowledge, the true melting
enthalpy for such material has never been measured.
As can be seen in Table 2, the melting enthalpy and consequently the de-
gree of crystallinity of PVDF decrease with increasing applied deformation. This
indicates that the deformation process leads to partial destruction of the crystal-
line structure.
Table 2. Melting Temperature (Onset) and Enthalpy Measured
by Differential Scanning Calorimetry for Samples Deformed up
to Different Strains
Strain, % Tm, °C ∆Hm, J/g Crystallinity, %
0 157.9 50.0 47.9
30 156.8 44.9 43.0
60 157.7 43.5 41.6
140 157.9 37.4 35.8
However, the melting temperature was found to be constant for all samples,
within the experimental error. It is known, for semicrystalline polymers in gen-
eral and particularly for PVDF [19], that the melting temperature is related with
the lamellae thickness. The results shown in Table 2 suggest that the strong
changes in the morphology during the film drawing were not accompanied by
changes in the crystalline lamellae thickness.
CONCLUSIONS
Films of semicrystalline PVDF in the β-phase were studied by FTIR and
DSC to understand better the structural changes occurring during the deforma-
tion process. FTIR spectra showed a reorientation of the chains from perpendicu-
lar to parallel to the stress orientation within the plastic region. A decrease in the
degree of crystallinity due to the deformation was observed. On the other hand,
the thickness of the lamellae seemed to remain practically unchanged.
ACKNOWLEDGMENT
We thank Jose´ Luis Ribeiro and Luı´s Vieira for their help with the FTIR
measurements. The work was partially supported by Fundac¸a˜o para a Cieˆncia e
Tecnologia (PRAXIS/P/CTM/14171/1998) and by a NSF grant (DMR-9520251).
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