Effect of PMMA addition on characterization and morphology of PVDF.

INTRODUCTION

Blending of polymers has given a new direction for developing novel materials. It is an easy and inexpensive method of modifying various properties of the polymers. Polymer blending is a widely used technique to improve some physical properties of homopolymers. Blending of two polymers may either result in miscible or immiscible system. Characteristics and morphology of blends of polyvinylidene fluoride and polymethylmethacrylate (PVDF/PMMA) have been investigated by various techniques such as X-ray, Fourier transform infrared (FTIR), optical, calorimetric, microscopic, and electric measurements [1-6]. PVDF and PMMA are completely miscible in the melt and exhibit a lower critical solution temperature around 330[degrees]C.

PVDF is a semicrystalline polymer which has drawn both scientific and technological attention because of the useful pyroelectric [7] and piezoelectric [8] properties. It is also one of the rare polymers that exhibit diverse crystalline forms, having at least five phases known as [alpha], [beta], [gamma], [delta], and [epsilon] phases [9-11]. The polar phases [beta] and [gamma] are technologically the most interesting because of its better pyroelectric and piezoelectric properties. PVDF is of interest because of its typical fluoropolymer characteristics and its miscibility with PMMA, which is commonly used as advanced material. There have been several studies regarding the miscibility of PMMA and PVDF [12-14].

Also, PMMA has received great attention due to its optical properties and its possible use in nonlinear optics. PMMA is a plastic widely used for its stiffness and clarity in various industrial applications. It can be used as a good keeper for rare earth and garnets which has wide technological applications [15]. The structure characteristics of PMMA have been the subject of several investigations [16-18].

PVDF/PMMA blends have been studied extensively, mainly in relation to PVDF piezoelectric properties [19, 20]. Much attention has been paid to problems such as miscibility of the amorphous phase, crystallization of PVDF in various phases, and molecular origin of PVDF/PMMA interactions. Moreover, blending with PMMA was described as an original way to force PVDF to crystallize into the piezoelectric phase, which is thermodynamically unstable in pure material [6].

The aim of the present work was an attempt to investigate the effect of addition of PMMA on the crystallization and morphology of PVDF phases. Spectroscopic, thermal, and morphological methods are particularly useful in such investigations. The results are demonstrated by FTIR, X-ray, UV-visible, differential thermal analysis (DTA), and SEM spectroscopy.

EXPERIMENTAL

Samples Preparation

PVDF pellet (SOLEF 1008, Solvay, Belgium) with average molecular weight 5.3 x [10.sup.5] and PMMA with an average molecular weight of 1.2 x [10.sup.5] were used. Appropriate amounts of PVDF and PMMA were dissolved in tetrahydrofuran (THF). After complete dissolution and at suitable viscosity, the blend was prepared by casting onto a glass Petri dish, then left to evaporate the solvent slowly. The resulting PVDF/PMMA films were then dried in a vacuum oven at 60[degrees]C for 3 days to ensure the removal of the solvent traces. The blends of PVDF/PMMA were prepared in different weight concentration (100/0, 80/20, 60/40, 50/50, 60/40, 80/20, and 0/100). The thickness of the films was in the range of 110-140 [micro]m.

[FIGURE 1 OMITTED]

Measurements

X-ray diffraction scans were obtained using DIANO corporation-USA equipped using Cu K[alpha] radiation ([lambda] = 1.540 [Angstrom], the tube operated at 30 kV, the Bragg angle (2[theta]) in the range of 10-50[degrees], step size = 0.1 and step time 1 sec). The FTIR measurements were carried out using the single beam FTIR spectrometer (FTIR-430, Jascow, Japan). The FTIR spectra of the samples were obtained in the spectral range of 2000-400 [cm.sup.1] with scanning speed of 2 mm/sec. Ultra violet and visible (UV/VIS) absorption spectra were measured in the wavelength region of 200-900 nm using spectrophotometer (V-570 UV/VIS/NIR, Jasco, Japan). The DTA of the prepared films was carried out using an equipment type (Shimadzu DTA-50) from room temperature to 300[degrees]C at a heating rate of 10[degrees]C/min.

The morphology of the blends was characterized by scanning electron microscope using (JEOL 5300, Tokyo, Japan), operating at 30 kV accelerating voltage. Surface of the samples were coated with a thin layer of gold (3.5 nm) by the vacuum evaporation technique to minimize sample charging effects due to the electron beam.

RESULTS AND DISCUSSION

X-ray Diffraction

X-ray diffraction (XRD) scans was a useful tool to examine the influence of PMMA contents on the crystalline structure of PVDF in the samples. Figure 1 shows the XRD scans of pure PVDF, pure PMMA, and their blends. The interplanar distance (d values), 2[theta], and the assignments of all the major peaks are listed in Table 1. The following general observations are made on the basis of Fig. 1 and Table 1.

From Fig. 1, it is clear that for pure PVDF, the diffraction peak observed at 2[theta] = 23.5[degrees], corresponding to the plans (101) is due to crystalline structure of PVDF [beta]-phase. The diffraction peaks at 2[theta] = 21.8[degrees] and 45.7[degrees], corresponding to the plans (110) and (220), are due to crystalline structure of PVDF [alpha]-phases. Pure PMMA exhibit an amorphous feature which is characterized by two amorphous halos (a large hump) centered at 2[theta] = 16[degrees] and a small hump at 2[theta] = 35[degrees] with no sharp peaks.

From the diffraction scans and with increase PMMA content, it is found that: (i) There is a decrease progressively in the relative intensity (without disappear) of the peak appearing at 23.5[degrees], i.e. the blends also exhibit [beta]-crystals of PVDF, (ii) The amorphous halo becomes more and more obvious, this gives a clear indication of complexation of the two polymers blend, i.e. reduces the long-rang order in PMMA [21], and (iii) For the blends in which the PMMA was higher than 80 wt%, a broad peak near at about 45.7[degrees] was characteristic of the combined (201 and 111) reflections of the [beta]-phase resulting from molecular defects caused by head-head and tail-tail sequences [22]. However, as the PMMA content was higher than 80 wt% of PMMA, this diffraction peak disappeared. Thus XRD analysis reveals that the blends take place based on the influence of PMMA content on PVDF blends.

[FIGURE 2 OMITTED]

There was decrease in the relative intensity (height of the peak) of the apparent peaks (2[theta] = 23.5[degrees]) with increasing PMMA content. These results can be interpreted by considering by the Hodge et al. [23] criterion which establishes a correlation between the height of the peak and the degree of crystallinity.

FTIR Analysis

Figure 2 shows FTIR vibrational spectra of PVDF/PMMA films blend from 2000 to 400 [cm.sup.1]. Characteristic absorption bands were identified in these spectra and assigned by comparison with the literature values found for PVDF, PMMA, and their blends. Our assignments for these spectra are listed in Tables 2 and 3. According to characteristic absorption bands of [alpha]-phase at (484, 1074, 1257, and 1729 [cm.sup.1]), [beta]-phase at (512, 840, 877, and 1409 [cm.sup.1]) for pure PVDF are indicated in Fig. 2 [24, 25]. The vibrational band at 512 [cm.sup.1] corresponds to bending vibrations mode of C[F.sub.2] dipoles, characteristic of TT (trans) conformation of the ferroelectric [beta]-phase of PVDF.

The vibrational bands at 987 and 1455 [cm.sup.1] are assigned to O-C[H.sub.3] bending and stretching deformation of PMMA. The bands at 1712 and 1250 [cm.sup.1] are assigned to stretching frequency of C=O of PMMA. The absorption band appearing at 854 [cm.sup.1] is assigned to the characteristic frequency of vinylidene compound. It is clear that the stretching frequency at 1712 [cm.sup.1] which corresponds to C=O of pure PMMA, is shifted to 1729 [cm.sup.1] in the blends. This shift observed in the carbonyl stretching frequencies of blends when compared to pure PMMA is due to specific interaction between the carbonyl groups of PMMA and the C[H.sub.2] groups of PVDF and indicates the formation of blends. This is agreement with results reported by Colemann and Painter [26] for PMMA/PVDF binary blends.

To investigate the influence of the PMMA chains on the formation of ferroelectric crystalline [beta]-phase of PVDF, we have plotted the intensity corresponding to the band at 877 [cm.sup.1] as a function of PMMA content. As seen in Fig. 3, it gradually decreases as PMMA is added to PVDF.

UV-Visible Analysis

UV-visible absorption spectra of PMMA/PVDF blend are shown in Fig. 4. The absorption edges were observed around 240 to 290 nm. The sharp absorption edge for PVDF indicates the semicrystalline nature of PVDF. A shift in band edges toward the higher wavelengths with different absorption intensity for PMMA doped PVDF was observed. These shifts indicate the formation of inter/intra between PMMA and PVDF that are in consistence with X-ray, FTIR, and DTA results. Also, the shift in absorption edge in the films reflects the variation in the optical energy band gap, [E.sub.g].

It is clear that some blends exhibit a well-defined window of wavelength range 290-350 nm. A sharp and a maximum height of this window are noticed at the blend (50/50). The present optical window can be used as an optical sensor or band pass filter for the wavelength range 290-350 nm in the UV and VIS regions.

[FIGURE 3 OMITTED]

The relation between the absorption coefficient [alpha] and the optical energy band gap obeys the classical Tauc's expression. The optical energy band gap for an indirect transition can be determined by using the following relation [27]:

[E.sub.g] = hv - ([alpha]hv/b)[.sup.1/2] (1)

where h is plank's constant, v is the photon frequency, B is a constant, and [alpha] is the absorption coefficient, which can determined as a function of photon frequency using the equation:

[alpha] = [A/d] x 2.303 (2)

where A is the absorbance and d is the thickness of the sample. The plot of ([alpha]hv)[.sup.1/2] versus the photon energy hv at room temperature shows a linear behavior which are presented in Fig. 5a and b. Each linear portion indicates an optical band gap, [E.sub.g], which can be considered as an evidence for indirect allowed transition. Figure 6 displays the PMMA content dependence of [E.sub.g]. It is clear that the optical energy band gap decreases with increasing PMMA content. The existence and variation of optical energy gap, [E.sub.g], may be explained by invoking the occurrence of local crosslinking within the amorphous phase of PMMA and PVDF.

Differential Thermal Analysis

The thermal behavior of PVDF/PMMA blends was studied using DTA from room temperature to 300[degrees]C. DTA thermograms (after smoothing) are shown in Fig. 7. A sharp endothermic peak is attributed to the melting temperature ([T.sub.m]). In DTA thermograms, the original PVDF showed a single melting peak at 167.11[degrees]C which could be ascribed to the presence of [alpha]-phase crystals of PVDF, according to the results of FTIR and X-ray studies. It can be seen that the melting temperature of PVDF/PMMA blends depresses markedly with increasing PMMA content. For example, the [T.sub.m] of PMMA/PVDF (80/20) blends become 150.91[degrees]C, being 16.2[degrees]C lower than that of pure PVDF.

The area under crystalline melting endothermic peaks is correlated to the degree of crystallinity. If it is assumed that all PVDF crystal form present has the same heat of fusion [DELTA][H.sub.c] = 104.7 J/g for completely crystalline PVDF sample [28], then [DELTA][H.sub.f] (measured directly from DTA thermograms) can be related to the crystallinity of the sample by the following equation:

Crystallinity = [[[DELTA][H.sub.f]]/[[DELTA][H.sub.c]]] x 100 (3)

The calculated values of the degree of crystallinity and the values of the melting temperature of the blends from DTA thermograms are listed in Table 4.

It is observed that, the degree of crystallinity and the melting temperature for the blends are reduced with increasing PMMA content. These results data accords with FTIR and X-ray finding. The dramatic decrease in the crystallinity and the melting is related to a good miscibility between PVDF and PMMA. Such is legitimate in that PMMA is largely amorphous and does not contribute to the heat of fusion.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Morphology

To investigate fully the effect of PMMA content, the morphology of PMMA/PVDF blends were studied using scanning electron microscope (SEM). Figure 8a-g shows the SEM micrographs of the surface for PMMA/PVDF blends at magnification 10,000 times. The micrograph in Fig. 5a for pure PVDF is characterized by normal crystalline domains uniformly shaped with equal size. The presence of PMMA leads to changes in the surface morphology (Fig. 8b-g). The blend in Fig. 8b gave rise to crystalline domains with coarse spherulitic structure. From these micrographs it can be seen that the spherulites will increase with the addition of PMMA, up to 60 wt% of PMMA, which gives rise to extended crystalline regions, as shown in Fig. 8e. This is due to the fact that the PMMA segregated into interlamellar or intercrystalline regions of PVDF. These spherulites are a very useful feature for piezoelectric and other applications.

[FIGURE 6 OMITTED]

PMMA/PVDF 80/20 as shown in Fig. 8f have distinct longitudinal shape not spheres, and the spherulites disappear. These distinct longitudinal shapes has been attributed to a strong increase of lamellar twisting period and to a decreased radial growth rate in amorphous regions with higher PMMA content [29]. There are small differences that are visible on micrographs of PVDF/PMMA blend containing 80 wt% of PMMA compared to morphology of virgin PMMA in Fig. 8g. No crystalline structure is clearly observed on the surface of virgin PMMA and the surface is rather rough. This is very similar to that observed by Omastova and Simon [30].

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

CONCLUSIONS

XRD analysis reveals that the blends take place based on the influence of PMMA content on PVDF blends. Characteristic absorption bands from FTIR spectrum were identified and assigned by comparison with the literature values found for PVDF/PMMA blends. The shift of C=O observed in the carbonyl stretching frequencies of blends is due to specific interaction between the carbonyl groups of PMMA and the C[H.sub.2] groups of PVDF. The change in the UV-visible spectrum is due to complex formation which can be reflected in the form of decrease in the optical energy gap. The DTA thermograms depicts that the addition of PMMA decreased the melting temperature and the degree of crystallinity. Morphology of PMMA/PVDF blends shows crystalline domains uniformly shaped with spherulites. The morphology becomes sharper and contains a longitudinal shape note spheres for PMMA/PVDF (80/20).

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I.S. Elashmawi, N.A. Hakeem

Spectroscopy Department, Physics Division, National Research Centre, Giza, Egypt

Correspondence to: I.S. Elashmawi; e-mail: [e-mail]

TABLE 1. The values of 2[theta] and respective d spacing were observed
in the X-ray diffraction scans.

PMMA/PVDF  2[theta] ([degrees])  d ([Angstrom])  Assignment     Plane

0/100      21.8                  4.72            [alpha]-phase  110
           23.5                  3.21            [beta]-phase   101
           45.7                  2.26            [alpha]-phase  220
20/80      23.4                  4.82            [beta]-phase   101
           45.7                  2.36            [alpha]-phase  220
40/60      23.8                  4.63            [beta]-phase   101
           45.7                  2.23            [alpha]-phase  220
50/50      23.9                  4.53            [beta]-phase   101
           45.7                  2.21            [alpha]-phase  220
60/40      23.9                  4.82            [beta]-phase   101
           45.7                  2.21            [alpha]-phase  220
80/20      23.9                  4.72            [beta]-phase   101

TABLE 2. Characteristic absorption frequencies for PVDF observed in
Fig. 2.

Peak position
([cm.sup.1])   Phase    Assignment

 484           [alpha]  C[F.sub.2] bending and wagging
 512           [beta]   C[F.sub.2] bending
 840           [beta]   C[H.sub.2] rocking and C[F.sub.2] asymmetric
                          stretching
 877           [gamma]  C[F.sub.2] symmetric stretching
1074           [alpha]  C[H.sub.2] wagging deformation
1257           [alpha]  C[F.sub.2] symmetric stretching
1409           [beta]   C[H.sub.2] wagging deformation
1729           [alpha]  CF out of plane deformation

TABLE 3. Characteristic absorption frequencies for PMMA observed in Fig.
2.

Peak position ([cm.sup.1])  Assignment

 752                        Wagging deformation (-C[H.sub.3])
 987                        Bending (C[H.sub.3]-O)
1193                        Skeletal chain
1250                        Stretching deformation (C=O)
1455                        Stretching (C[H.sub.2]) and Stretching
                            asymmetric (O-C[H.sub.3])
1480                        Bending asymmetric (C[H.sub.2])
1712                        Stretching (C=O)

TABLE 4. The values of the melting temperature and the degree of
crystallinity.

PMMA/PVDF (wt%)  Melting point ([degrees]C)  Crystallinity %

0/100            167.11                      48.12
20/80            162.53                      44.24
40/60            158.29                      39.61
50/50            155.03                      37.84
60/40            153.29                      33.20
80/20            150.91                      30.50