- Email: [email protected]
PVAc blends
ARTICLE IN PRESS Physica B 403 (2008) 3547– 3552
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Spectroscopic and thermal studies of PS/PVAc blends I.S. Elashmawi a,, N.A. Hakeem a, E.M. Abdelrazek b a b
Spectroscopy Department, Physics Division, National Research Centre, Giza, Dokki, Cairo, Egypt Physics Department, Faculty of Science, Mansoura University, 35516, Mansoura, Egypt
a r t i c l e in fo
abstract
Article history: Received 20 February 2008 Received in revised form 22 April 2008 Accepted 22 May 2008
Polystyrene and polyvinyl acetate (PS/PVAc) films were blended with different contents using casting method. The effect of PS content on PVAc blends was investigated by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), Ultra violet and visible studies (UV/VIS), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Significant changes in FT-IR, XRD and DSC analysis are observed which reveals an interactions between the two polymers and PS/PVAc blends had good or certain miscibility. XRD scans show some changes in the intensity and the height of the amorphous halos with increased PS. UV/VIS analysis revealed that the optical band gap decreases with increasing content of PS from 5 to 4.11 eV. A single glass transition temperature for each blend was observed, this DSC results supported that the miscibility existed in the blend. The apparent activation energy (E) of the blends was evaluated using TGA analysis. The value of E was increased with the increase of PS content. & 2008 Elsevier B.V. All rights reserved.
Keywords: Polymer blends Miscibility FT-IR X-ray diffraction UV-visible DSC Optical energy gap
1. Introduction Polymer blends have attracted the attention of materials researchers, because the structural and some physical properties of polymers can be modified to a specific requirement by blending two or more polymers [1–4]. Blending involves physical mixing of polymers and allows one to create a new material having some of the desired properties of each component. There has been considerable interest in the study of polymer blends because of their importance in academic and technical aspects. Particularly, much attention has been paid to miscibility and phase behavior in polymer blends [5–7]. Many techniques can be used to study the miscibility and phase behavior of polymer blends. In order to investigate the miscibility and phase behavior of polymer blends, differential scanning calorimetry (DSC) has been frequently used for the determination of the glass transition temperature (Tg) [8]. A miscible polymer blend would exhibit a single transition between Tg of the two components. With increasing immiscibility there is a broadening of the transition, whereas an incompatible system would be marked by separate transitions of the polymer components in the blends [9].
The miscibility of the binary blends was investigated earlier. The miscibility of PVAc with PCL (polycaprolactone) was studied using DSC and a single transition temperature was observed for the complete range [10]. The miscibility of PVAC and PVC was also investigated by DSC and their miscibility depends on the solvent used during solution blending [11]. FT-IR spectroscopy is a rapidly expanding area in polymer miscibility determination and has provided much information over the years on molecular vibrations. Recent interest in vibrational spectroscopy has been focused on instrumentation, method development and vibrational analysis. The rapid development of FT-IR has revolutionized the applications of infrared spectroscopy and over recent years has been used to elucidate the interactions present in blends and from this information the blend miscibility has been deduced [12]. In this work, the objective is to investigate the miscibility of PS/PVAc blends and to examine phase behavior and spectroscopic studies with various composition ratios of these blends. PS was chosen as one of the blend components in view of its lack of crystallinity and its miscibility with PVAc.
2. Experimental 2.1. Samples preparation
Corresponding author. Tel.: +20 502536150.
E-mail addresses: [email protected], [email protected] (I.S. Elashmawi). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.05.024
Polystyrene (PS) from BDH chemicals Ltd. (Poole, England) and polyvinyl acetate (PVAc) from Aldrich chemical co. Ltd.
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(Gillingham, England) were used as received. Pure acetone was used as a common solvent for the two polymers at 50 1C. After complete dissolution, the blend was prepared by casting onto a Petri dish glass, then left to evaporate the solvent slowly. The resulting PS/PVAc films were then dried in a vacuum oven at 50 1C for three days to ensure the removal of the solvent traces. Blends of PS/PVAc were prepared in different weight concentration (0/100, 25/75, 50/50, 75/25 and 100/0) using a casting technique. The thickness of the films was in the range of 100–130 mm. 2.2. Measurements FT-IR measurements were carried out using the single beam FT-IR (FT-IR-430, Jasco, Japan). FT-IR spectra of the samples were obtained in the spectral range of 4000–400 cm1. X–ray diffraction scans were obtained using DIANO corporation-USA equipped using Cu-Ka radiation (l ¼ 1.540 A˚, the tube operated at 30 kV, the Bragg angle (2y) in the range of 51–351. Ultraviolet–visible (UVVIS) absorption spectra were measured in the wavelength region of 210–600 nm using spectrophotometer (V-570 UV/VIS/ NIR, Jasco, Japan). The differential scanning calorimetry of the prepared films was carried out using an equipment type (Shimadzu DSC–50) from room temperature to 350 1C with a heating rate of 10 1C/min. A Perkin-Elmer (US, Norwalk, CT) TGA-7 was used for the thermogravimetric analysis of the samples. A small amount (5–10 mg) of the sample was taken for the analysis and the samples were heated from 50 to 600 1C at a rate of 10 1C min1 in nitrogen atmosphere.
3. Results and discussion Fig. 1. FT-IR absorption spectra of PS/PVAc blends.
3.1. FT-IR spectroscopic analysis If two polymers are completely incompatible, each individual polymer does not recognize, in infrared spectral terms, the existence of the other in the blend. On the other hand, if the polymers are compatible, there should be considerable differences between the infrared spectrum of the blend and the spectra of the pure components. These differences would be derived from chemical interactions resulting in band shifts and broadening. Fig. 1 depicts the FT-IR absorption spectra in the range 4000–400 cm1 of PS/PVAc blends with different blend ratios. For pure PVAc, the vibrational bands observed at 2927 and 2854 cm1 are ascribed to O–CH3 (ester group) asymmetric stretching and symmetric stretching vibrations, respectively. The intense band at around 1736 cm1 represents the CQO stretching band of an unconjugated ester. At 1373 cm1, a prominent band is evident, here the CH3 (CQO) group strongly absorbs acetate esters; these acetate esters show a corresponding weak band at 629 cm1. The strong band at 1243 cm1 and the band at 1122 cm1 are ascribed to C–O–C symmetric stretching and C–O stretching vibrations, respectively. Also the peak at 947 cm1 is ascribed to CH bending vibrations and there is a significant band at 606 cm1 which is assumed to be linked to CH3 (CQO) group [13–15]. It is known that, PS consists of alternating methylene and methane groups. However, each repeat unit in PS contains a pendant benzene ring. The spectrum of pure PS is shown in Fig. 1. The main PS characterizing bands are observed. The methylene (CH2) asymmetric and symmetric stretching bands are observed at 2924 and 2852 cm1. There is a group of aromatic C–H stretches around 3026 cm1 and benzene ring modes are found at 1600 and 1491 cm1. The out-of-plane C–H bending mode of the aromatic
ring is shown at 756 cm1 and the ring-bending vibrational band appears at 698 cm1 [16]. These last two bands confirm that PS contains a monosubstituted benzene ring. For all blends, a triple split band appearance is identified at 3465, 3535 and 3637 cm1 as shown in Fig. 2. This is the C–H stretching band, typical C–H stretching being usually present as a distinctively large band above 3000 cm1. Some bands are disappeared in the blends and the intensity of some bands was changed. All results data suggest that homogeneous polymer composites are formed over all the blend compositions.
3.2. X-ray diffraction analysis The measurement of XRD diffraction scans of the polymer blends is also used as a criterion to determine its miscibility. If two polymers have low compatibility then each polymer would have its own crystal region in the blend films and the X-ray scans of the samples would be expressed as simply mixed scans of the two polymers with the same ratio as those for blending [17]. XRD diffraction scans of PS blends with different PVAc contents are depicted in Fig. 3. All the blend compositions show a broad and diffuse peak, which indicates the amorphous nature of the blends. Fig. 3(a) shows two broad peaks at angles 2y15.071 and 22.71, which reveal the amorphous nature of the PVAc polymer. The observed scans of pure PS (as shown in Fig. 3e) exhibits an amorphous character of two halos of approximately equal intensities centered at 2y ¼ 111 and 23.51. The first halo may be attributed to the size of the side group which corresponds to an
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Fig. 2. FT-IR absorption spectra of PS/PVAc blends in the range of 2750–4000 cm1. Fig. 3. X-ray diffraction scans for pure PVAc, pure PS and their blends.
approximately hexagonal ordering of the molecular chains. The second amorphous halo corresponds to Van der Waals distances [18]. Changing in the intensity and the height of the two halos with increase of PS content was observed. This indicates that the miscibility existed in the blends. The outcome, deduced from XRD, is consistent with that of FT-IR and DSC. 3.3. Optical spectra analysis Fig. 4 shows the optical absorption measurements carried out in the spectral range (210–600 nm) for the present system. The observed spectrum of pure PVAc has a small absorption band (shoulder) at about 263 nm. This band is attributed to the carbonyl group in polymeric macromolecule; the intensity of the absorption peak slightly increases with increasing the concentration of PS. This may be due to the increase of the number of carbonyl groups of the PVAc macromolecules. On the other hand, the observed spectra have a sharp edge about 250 and 290 nm. The position of the edge is slightly shifted towards higher wavelength side suggests the miscibility of the blend between the two polymers. The nature of the optical transition involved in the blends can be determined on the basis of the dependence of absorption coefficient (a) on photon energy (hn). The total absorption could be due to the optical transition which is fitted to the relation [19]: ahn ¼ Bðhn Eg Þn
(1)
where Eg is the optical energy gap between the bottom of the conduction band and the top of the valance band at the same
Fig. 4. UV–visible absorption spectra of PS/PVAc blends.
value of wavenumber and B is a constant related to the properties of the conduction and valance bands. On the basis of Eq. (1) and the value of n ¼ 12 for allowed direct transitions, Eg can be determined.
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Fig. 5. The dependence of (ahn)1/2 verses the photon energy (hn) of PS/PVAc blends.
Table 1 The values of optical energy gap (Eg) and the activation energy (E) for PS/PVAc blends PS: PVAc
Fg (eV)
E (J/mol)
0:100 25:75 50:50 75:25 100:0
5.00 4.81 4.76 4.21 4.11
69.03 82.37 89.45 99.58 129.2
The plot of (ahn)1/2 versus the photon energy hn shows a linear behavior which are presented in Fig. 5. Each linear portion indicates an optical energy gap Eg. The values of Eg are listed in Table 1. The data in this table show that the band gap decreases with increasing the content of PS from 5 to 4.11 eV. The existence and variation of optical energy gap Eg may be explained by invoking the occurrence of local cross linking within the amorphous phase of PVAc, in such a way as to increase the degree of ordering in these parts. It is noticed that the curves are characterized by the presence of an exponentially decay tail at low energy [20]. These results indicate the presence of a well defined p-p* transition associated with the formation of conjugated electronic structure [21].
Fig. 6. DSC thermograms of PS/PVAc blends.
3.4. Differential scanning calorimetry analysis DSC is one of the most convenient method to determine the miscibility and thermal properties of the polymer blends. Measurement of the glass transition temperature, Tg, (from the DSC thermograms) of the polymer blends is used to determine its miscibility [22]. A miscible polymer blend possesses a homo-
geneous amorphous phase and hence will exhibit a single glass transition temperature (Tg) between the Tgs. of the two polymers. The DSC curves of PS/PVAc blends are shown in Fig. 6. The thermograms show that the Tg values for pure PS and pure PVAc are about 95 and 38 1C [23], respectively. The curves of the other samples also show one Tg in the temperature range from 38 to
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Fig. 7. Glass transition temperature, Tg (K) as a function of weight fraction of PS.
95 1C and Tg value increases with increasing the PS content. This results supports that PS/PVAc blends have miscibility in the amorphous state and this is due to the interaction occurs between PS and PVAc. There are several classical equations that correlate the glass transition temperature of a miscible blend system with its composition [24–26]. These equations are expressed by using a modified Fox [24] equation 1 T gðblendÞ
¼
w1 w2 þ T g1 T g2
Fig. 8. TGA thermograms of weight loss as a function of temperature for PS/PVAc blends.
Table 2 Weight loss (%) at different decomposition temperatures for different polymer blends of various ratios of PS/PVAC Polymer blends PS/PVAc
Temperature (1C) at different weight loss
(2)
where w1 and w2 are the weight fractions and Tg1, Tg2 are the respective glass transition temperatures of the homopolymers. The glass transition temperature estimated using Fox equation and Tg from the DSC curves (experimental) for the studied blends as a function of PVAc content are shown in Fig. 7. In addition, when the PS content is increased, the melting point of the blends decreased slightly due to favorable interactions between the two polymers and the miscibility take place between PS and PVAc. No endothermic peaks appear in the scans, indicating that crystalline regions do not exist in our samples. This results data were correlated to X-ray analysis.
100/0 25/75 50/50 75/25 0/100
10%
20%
30%
40%
50%
60%
70%
278 270 273 278 270
305 282 287 278 282
320 290 295 286 290
330 295 307 295 296
340 300 314 307 303
352 305 328 310 308
360 320 340 330 316
PVAc is less than the weight loss of the PS. Thus, it may be concluded that the addition PS polymer is more stable against thermal decomposition than the pure PVAc. 3.6. Determine the activation energy
3.5. Thermogravimetric analysis TGA is widely used to investigate the thermal decomposition of polymers and to determine the kinetic parameters such as activation energy and order of reaction. These parameters can be used to give a better understanding of the thermal stability of polymer blends. Fig. 8 shows TGA thermograms of weight loss as a function of temperature for pure PS, pure PVAc and their blends with a heating rate of 5 1C/min in the temperature range from 50 to 600 1C. It is clear that, the initial weight loss for all the samples occurs at 70–90 1C due to the moisture evaporation and it is stable up to 120 1C, above which the solvent evaporated. The major weight losses are observed in the range of 270–360 1C for all the samples. This may be correspondent to the structural decomposition of the polymer blends. Table 2 summarizes the percentage weight loss at different decomposition temperatures of PS/PVAc polymer blends taken from the TGA thermograms. It can be seen that (from the thermograms) pure PS and PVAc polymers lose about 5% of weight at 256 and 250 1C, respectively. This behavior indicates that the initial decomposition reaction for PS begins at a slightly shifted towards lower temperature than PVAc. By increasing the heating temperature from 270 to 360 1C, the loss in weight of the
The methods used to calculating kinetic parameters from TGA data are classified into two groups: integral and differential methods. The most suitable method has, however, not yet been clarified. For both methods, the basic equation for the fraction of conversion, a, for a weight loss system, is given by: a¼
wo w wo wf
(3)
where w, wo and wf are the acual, initial and final weight of the samples, respectively. The activation energy for the thermal decomposition of the present samples depends on the residual mass that can be calculated using integral equation of Coats and Redfern [27]: " # 1 ð1 aÞ1n R 2RT E 1 0:434 (4) log ¼ log 2 DE E RT T where, T is the absolute temperature in Kelvin, E is the activation energy in J/mol, R is the universal gas constant (8.3136 J/mol K) and n is the order of reaction. For n ¼ 1, Eq. (4) reduces to: logð1 aÞ R 2RT E log 1 0:434 (5) ¼ log DE E RT T2
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temperature for each blend was observed, its value increased upon increasing the content of PS, this finding suggests that this blend system is only miscible. From UV/VIS data, the position of the edge was slightly shifted towards higher wavelength side suggests the miscibility of the blend. The optical gap decreases with increasing the content of PS from 5 to 4.11 eV. It is clear that from TGA studies, the initial weight loss occurs at 70–90 1C due to the moisture evaporation and it was stable up to 120 1C, the major weight losses were observed in the range of 270–360 1C due to the structural decomposition of the polymers. The values of the apparent activation energy, E, increases with increase PS content from 69.01 to 129.2 J/mol. References
Fig. 9. log [log(1a)/T2] against reciprocal absolute temperature of PS/PVAc blends.
From this method by plotting the dependence of log [log(1a)/T2] versus 1000/T for each sample, we obtain straight lines as shown in Fig. 9. The apparent activation energies are calculated from the slopes of the lines using the expression: E ¼ 2:303R slope
(6)
The values of the apparent activation energy, E, of the blends are listed in Table 1. From this table, it clears that, the values of the activation energy increases with the increase of PS content.
4. Conclusions In this study, we prepared PS/PVAc blends by casting method. The results data suggest that homogeneous polymer composites are formed over all the blend compositions. Various types of the bands for the two polymers and its blends were assigned in FT-IR spectra. XRD scans show a broad and diffuse peak, which indicates the amorphous nature of the blends. A single glass transition
[1] M.J. Folkes, P.S. Hope, Polymer Blends and Alloys, Chapman and Hall, London, 1993. [2] L.A. Utracki, Text Book of Polymer Alloys and Blends, Hanser, New York, 1990. [3] O. Olabisi, L. Robeson, M.T. Shaw, Polymer–Polymer Miscibility, Academic Press, New York, 1979. [4] M. Sivakumar, R. Subadevi, S. Rajendran, H.-C. Wu, N.-L. Wu, Eur. Polym. J. 43 (2007) 4466. [5] Y. Huang, C. Xiao, Polymer 48 (2007) 371. [6] V.H. Mareau, R.E. Prudhomme, Macromolecules 36 (2003) 675. [7] R.S. Porter, L.H. Wang, Polymer 33 (1992) 2019. [8] J.W. Park, S.S. Im, Polymer 44 (2003) 4341. [9] C.G. Biliaderis, A. Lazaridou, I. Arvanitoyannis, Carbohydr. Polym. 40 (1999) 29. [10] G. Sivalingam, R. Karthik, Giridhar Madras Polymer Degradation and Stability 84 (2004) 345e351. [11] D.E. Bhagwagar, C.J. Serman, P.C. Painter, M.M. Coleman, Macromolecules 22 (1989) 4656. [12] A. Garton, Poly. Eng. Sci. 23 (1983) 663. [13] L. Sharma, T. Kimura, Polym. Adv. Technol. 14 (2003) 392. [14] R. Baskaran, S. Selvasekarapandian, N. Kuwata, J. Kawamura, T. Hattori, Mater. Chem. Phys. 98 (2006) 55. [15] S. Selvasekarapandian, R. Baskaran, O. Kamishima, J. Kawamura, T. Hattori, Spectrochimica. Acta. Part A 65 (2006) 1234. [16] P. Kuivalainnen, H. Stubb, H. Isotlo, Phys. Rev. B 32 (1985) 7900. [17] W.C. Lai, W.B. Liau, T.T. Lin, Polymer 45 (2004) 3073. [18] G. Bodor, Structural Investigation of Polymers, Ellis Horwood Limited, England, 1991, p. 304. [19] N.F. Mott, E.A. Davis, Electronic Process in Non-Crystalline Materials, Clarendon Press, Oxford, 1979. [20] J. Bardeen, Phys. Rev. 75 (1949) 169. [21] W.R. Salaneck, C.R. Wu, J.L. Bredas, J.J. Ritsko, Chem. Phys. Lett. 127 (1986) 88. [22] M.M. Coleman, J. Zarian, J. Polym. Sci. Polym. Phys. Ed. 17 (1979) 837. [23] J.E. Mark, Polym. Data Handbook, Oxford Uni. Press, Oxford, 1999. [24] T.G. Fox, Bull. Amer. Phys. Soc. 1 (1956) 123. [25] M. Gordon, J.S. Taylor, J. Appl. Chem. 2 (1952) 495. [26] T.K. Kwei, J. Polym. Sci. Polym. Lett. Ed. 22 (1984) 307. [27] A.W. Coats, J.P. Redfern, Nature 201 (1984) 681.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Spectroscopic and thermal studies of PS/PVAc blends I.S. Elashmawi a,, N.A. Hakeem a, E.M. Abdelrazek b a b
Spectroscopy Department, Physics Division, National Research Centre, Giza, Dokki, Cairo, Egypt Physics Department, Faculty of Science, Mansoura University, 35516, Mansoura, Egypt
a r t i c l e in fo
abstract
Article history: Received 20 February 2008 Received in revised form 22 April 2008 Accepted 22 May 2008
Polystyrene and polyvinyl acetate (PS/PVAc) films were blended with different contents using casting method. The effect of PS content on PVAc blends was investigated by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), Ultra violet and visible studies (UV/VIS), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Significant changes in FT-IR, XRD and DSC analysis are observed which reveals an interactions between the two polymers and PS/PVAc blends had good or certain miscibility. XRD scans show some changes in the intensity and the height of the amorphous halos with increased PS. UV/VIS analysis revealed that the optical band gap decreases with increasing content of PS from 5 to 4.11 eV. A single glass transition temperature for each blend was observed, this DSC results supported that the miscibility existed in the blend. The apparent activation energy (E) of the blends was evaluated using TGA analysis. The value of E was increased with the increase of PS content. & 2008 Elsevier B.V. All rights reserved.
Keywords: Polymer blends Miscibility FT-IR X-ray diffraction UV-visible DSC Optical energy gap
1. Introduction Polymer blends have attracted the attention of materials researchers, because the structural and some physical properties of polymers can be modified to a specific requirement by blending two or more polymers [1–4]. Blending involves physical mixing of polymers and allows one to create a new material having some of the desired properties of each component. There has been considerable interest in the study of polymer blends because of their importance in academic and technical aspects. Particularly, much attention has been paid to miscibility and phase behavior in polymer blends [5–7]. Many techniques can be used to study the miscibility and phase behavior of polymer blends. In order to investigate the miscibility and phase behavior of polymer blends, differential scanning calorimetry (DSC) has been frequently used for the determination of the glass transition temperature (Tg) [8]. A miscible polymer blend would exhibit a single transition between Tg of the two components. With increasing immiscibility there is a broadening of the transition, whereas an incompatible system would be marked by separate transitions of the polymer components in the blends [9].
The miscibility of the binary blends was investigated earlier. The miscibility of PVAc with PCL (polycaprolactone) was studied using DSC and a single transition temperature was observed for the complete range [10]. The miscibility of PVAC and PVC was also investigated by DSC and their miscibility depends on the solvent used during solution blending [11]. FT-IR spectroscopy is a rapidly expanding area in polymer miscibility determination and has provided much information over the years on molecular vibrations. Recent interest in vibrational spectroscopy has been focused on instrumentation, method development and vibrational analysis. The rapid development of FT-IR has revolutionized the applications of infrared spectroscopy and over recent years has been used to elucidate the interactions present in blends and from this information the blend miscibility has been deduced [12]. In this work, the objective is to investigate the miscibility of PS/PVAc blends and to examine phase behavior and spectroscopic studies with various composition ratios of these blends. PS was chosen as one of the blend components in view of its lack of crystallinity and its miscibility with PVAc.
2. Experimental 2.1. Samples preparation
Corresponding author. Tel.: +20 502536150.
E-mail addresses: [email protected], [email protected] (I.S. Elashmawi). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.05.024
Polystyrene (PS) from BDH chemicals Ltd. (Poole, England) and polyvinyl acetate (PVAc) from Aldrich chemical co. Ltd.
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(Gillingham, England) were used as received. Pure acetone was used as a common solvent for the two polymers at 50 1C. After complete dissolution, the blend was prepared by casting onto a Petri dish glass, then left to evaporate the solvent slowly. The resulting PS/PVAc films were then dried in a vacuum oven at 50 1C for three days to ensure the removal of the solvent traces. Blends of PS/PVAc were prepared in different weight concentration (0/100, 25/75, 50/50, 75/25 and 100/0) using a casting technique. The thickness of the films was in the range of 100–130 mm. 2.2. Measurements FT-IR measurements were carried out using the single beam FT-IR (FT-IR-430, Jasco, Japan). FT-IR spectra of the samples were obtained in the spectral range of 4000–400 cm1. X–ray diffraction scans were obtained using DIANO corporation-USA equipped using Cu-Ka radiation (l ¼ 1.540 A˚, the tube operated at 30 kV, the Bragg angle (2y) in the range of 51–351. Ultraviolet–visible (UVVIS) absorption spectra were measured in the wavelength region of 210–600 nm using spectrophotometer (V-570 UV/VIS/ NIR, Jasco, Japan). The differential scanning calorimetry of the prepared films was carried out using an equipment type (Shimadzu DSC–50) from room temperature to 350 1C with a heating rate of 10 1C/min. A Perkin-Elmer (US, Norwalk, CT) TGA-7 was used for the thermogravimetric analysis of the samples. A small amount (5–10 mg) of the sample was taken for the analysis and the samples were heated from 50 to 600 1C at a rate of 10 1C min1 in nitrogen atmosphere.
3. Results and discussion Fig. 1. FT-IR absorption spectra of PS/PVAc blends.
3.1. FT-IR spectroscopic analysis If two polymers are completely incompatible, each individual polymer does not recognize, in infrared spectral terms, the existence of the other in the blend. On the other hand, if the polymers are compatible, there should be considerable differences between the infrared spectrum of the blend and the spectra of the pure components. These differences would be derived from chemical interactions resulting in band shifts and broadening. Fig. 1 depicts the FT-IR absorption spectra in the range 4000–400 cm1 of PS/PVAc blends with different blend ratios. For pure PVAc, the vibrational bands observed at 2927 and 2854 cm1 are ascribed to O–CH3 (ester group) asymmetric stretching and symmetric stretching vibrations, respectively. The intense band at around 1736 cm1 represents the CQO stretching band of an unconjugated ester. At 1373 cm1, a prominent band is evident, here the CH3 (CQO) group strongly absorbs acetate esters; these acetate esters show a corresponding weak band at 629 cm1. The strong band at 1243 cm1 and the band at 1122 cm1 are ascribed to C–O–C symmetric stretching and C–O stretching vibrations, respectively. Also the peak at 947 cm1 is ascribed to CH bending vibrations and there is a significant band at 606 cm1 which is assumed to be linked to CH3 (CQO) group [13–15]. It is known that, PS consists of alternating methylene and methane groups. However, each repeat unit in PS contains a pendant benzene ring. The spectrum of pure PS is shown in Fig. 1. The main PS characterizing bands are observed. The methylene (CH2) asymmetric and symmetric stretching bands are observed at 2924 and 2852 cm1. There is a group of aromatic C–H stretches around 3026 cm1 and benzene ring modes are found at 1600 and 1491 cm1. The out-of-plane C–H bending mode of the aromatic
ring is shown at 756 cm1 and the ring-bending vibrational band appears at 698 cm1 [16]. These last two bands confirm that PS contains a monosubstituted benzene ring. For all blends, a triple split band appearance is identified at 3465, 3535 and 3637 cm1 as shown in Fig. 2. This is the C–H stretching band, typical C–H stretching being usually present as a distinctively large band above 3000 cm1. Some bands are disappeared in the blends and the intensity of some bands was changed. All results data suggest that homogeneous polymer composites are formed over all the blend compositions.
3.2. X-ray diffraction analysis The measurement of XRD diffraction scans of the polymer blends is also used as a criterion to determine its miscibility. If two polymers have low compatibility then each polymer would have its own crystal region in the blend films and the X-ray scans of the samples would be expressed as simply mixed scans of the two polymers with the same ratio as those for blending [17]. XRD diffraction scans of PS blends with different PVAc contents are depicted in Fig. 3. All the blend compositions show a broad and diffuse peak, which indicates the amorphous nature of the blends. Fig. 3(a) shows two broad peaks at angles 2y15.071 and 22.71, which reveal the amorphous nature of the PVAc polymer. The observed scans of pure PS (as shown in Fig. 3e) exhibits an amorphous character of two halos of approximately equal intensities centered at 2y ¼ 111 and 23.51. The first halo may be attributed to the size of the side group which corresponds to an
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Fig. 2. FT-IR absorption spectra of PS/PVAc blends in the range of 2750–4000 cm1. Fig. 3. X-ray diffraction scans for pure PVAc, pure PS and their blends.
approximately hexagonal ordering of the molecular chains. The second amorphous halo corresponds to Van der Waals distances [18]. Changing in the intensity and the height of the two halos with increase of PS content was observed. This indicates that the miscibility existed in the blends. The outcome, deduced from XRD, is consistent with that of FT-IR and DSC. 3.3. Optical spectra analysis Fig. 4 shows the optical absorption measurements carried out in the spectral range (210–600 nm) for the present system. The observed spectrum of pure PVAc has a small absorption band (shoulder) at about 263 nm. This band is attributed to the carbonyl group in polymeric macromolecule; the intensity of the absorption peak slightly increases with increasing the concentration of PS. This may be due to the increase of the number of carbonyl groups of the PVAc macromolecules. On the other hand, the observed spectra have a sharp edge about 250 and 290 nm. The position of the edge is slightly shifted towards higher wavelength side suggests the miscibility of the blend between the two polymers. The nature of the optical transition involved in the blends can be determined on the basis of the dependence of absorption coefficient (a) on photon energy (hn). The total absorption could be due to the optical transition which is fitted to the relation [19]: ahn ¼ Bðhn Eg Þn
(1)
where Eg is the optical energy gap between the bottom of the conduction band and the top of the valance band at the same
Fig. 4. UV–visible absorption spectra of PS/PVAc blends.
value of wavenumber and B is a constant related to the properties of the conduction and valance bands. On the basis of Eq. (1) and the value of n ¼ 12 for allowed direct transitions, Eg can be determined.
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Fig. 5. The dependence of (ahn)1/2 verses the photon energy (hn) of PS/PVAc blends.
Table 1 The values of optical energy gap (Eg) and the activation energy (E) for PS/PVAc blends PS: PVAc
Fg (eV)
E (J/mol)
0:100 25:75 50:50 75:25 100:0
5.00 4.81 4.76 4.21 4.11
69.03 82.37 89.45 99.58 129.2
The plot of (ahn)1/2 versus the photon energy hn shows a linear behavior which are presented in Fig. 5. Each linear portion indicates an optical energy gap Eg. The values of Eg are listed in Table 1. The data in this table show that the band gap decreases with increasing the content of PS from 5 to 4.11 eV. The existence and variation of optical energy gap Eg may be explained by invoking the occurrence of local cross linking within the amorphous phase of PVAc, in such a way as to increase the degree of ordering in these parts. It is noticed that the curves are characterized by the presence of an exponentially decay tail at low energy [20]. These results indicate the presence of a well defined p-p* transition associated with the formation of conjugated electronic structure [21].
Fig. 6. DSC thermograms of PS/PVAc blends.
3.4. Differential scanning calorimetry analysis DSC is one of the most convenient method to determine the miscibility and thermal properties of the polymer blends. Measurement of the glass transition temperature, Tg, (from the DSC thermograms) of the polymer blends is used to determine its miscibility [22]. A miscible polymer blend possesses a homo-
geneous amorphous phase and hence will exhibit a single glass transition temperature (Tg) between the Tgs. of the two polymers. The DSC curves of PS/PVAc blends are shown in Fig. 6. The thermograms show that the Tg values for pure PS and pure PVAc are about 95 and 38 1C [23], respectively. The curves of the other samples also show one Tg in the temperature range from 38 to
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Fig. 7. Glass transition temperature, Tg (K) as a function of weight fraction of PS.
95 1C and Tg value increases with increasing the PS content. This results supports that PS/PVAc blends have miscibility in the amorphous state and this is due to the interaction occurs between PS and PVAc. There are several classical equations that correlate the glass transition temperature of a miscible blend system with its composition [24–26]. These equations are expressed by using a modified Fox [24] equation 1 T gðblendÞ
¼
w1 w2 þ T g1 T g2
Fig. 8. TGA thermograms of weight loss as a function of temperature for PS/PVAc blends.
Table 2 Weight loss (%) at different decomposition temperatures for different polymer blends of various ratios of PS/PVAC Polymer blends PS/PVAc
Temperature (1C) at different weight loss
(2)
where w1 and w2 are the weight fractions and Tg1, Tg2 are the respective glass transition temperatures of the homopolymers. The glass transition temperature estimated using Fox equation and Tg from the DSC curves (experimental) for the studied blends as a function of PVAc content are shown in Fig. 7. In addition, when the PS content is increased, the melting point of the blends decreased slightly due to favorable interactions between the two polymers and the miscibility take place between PS and PVAc. No endothermic peaks appear in the scans, indicating that crystalline regions do not exist in our samples. This results data were correlated to X-ray analysis.
100/0 25/75 50/50 75/25 0/100
10%
20%
30%
40%
50%
60%
70%
278 270 273 278 270
305 282 287 278 282
320 290 295 286 290
330 295 307 295 296
340 300 314 307 303
352 305 328 310 308
360 320 340 330 316
PVAc is less than the weight loss of the PS. Thus, it may be concluded that the addition PS polymer is more stable against thermal decomposition than the pure PVAc. 3.6. Determine the activation energy
3.5. Thermogravimetric analysis TGA is widely used to investigate the thermal decomposition of polymers and to determine the kinetic parameters such as activation energy and order of reaction. These parameters can be used to give a better understanding of the thermal stability of polymer blends. Fig. 8 shows TGA thermograms of weight loss as a function of temperature for pure PS, pure PVAc and their blends with a heating rate of 5 1C/min in the temperature range from 50 to 600 1C. It is clear that, the initial weight loss for all the samples occurs at 70–90 1C due to the moisture evaporation and it is stable up to 120 1C, above which the solvent evaporated. The major weight losses are observed in the range of 270–360 1C for all the samples. This may be correspondent to the structural decomposition of the polymer blends. Table 2 summarizes the percentage weight loss at different decomposition temperatures of PS/PVAc polymer blends taken from the TGA thermograms. It can be seen that (from the thermograms) pure PS and PVAc polymers lose about 5% of weight at 256 and 250 1C, respectively. This behavior indicates that the initial decomposition reaction for PS begins at a slightly shifted towards lower temperature than PVAc. By increasing the heating temperature from 270 to 360 1C, the loss in weight of the
The methods used to calculating kinetic parameters from TGA data are classified into two groups: integral and differential methods. The most suitable method has, however, not yet been clarified. For both methods, the basic equation for the fraction of conversion, a, for a weight loss system, is given by: a¼
wo w wo wf
(3)
where w, wo and wf are the acual, initial and final weight of the samples, respectively. The activation energy for the thermal decomposition of the present samples depends on the residual mass that can be calculated using integral equation of Coats and Redfern [27]: " # 1 ð1 aÞ1n R 2RT E 1 0:434 (4) log ¼ log 2 DE E RT T where, T is the absolute temperature in Kelvin, E is the activation energy in J/mol, R is the universal gas constant (8.3136 J/mol K) and n is the order of reaction. For n ¼ 1, Eq. (4) reduces to: logð1 aÞ R 2RT E log 1 0:434 (5) ¼ log DE E RT T2
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temperature for each blend was observed, its value increased upon increasing the content of PS, this finding suggests that this blend system is only miscible. From UV/VIS data, the position of the edge was slightly shifted towards higher wavelength side suggests the miscibility of the blend. The optical gap decreases with increasing the content of PS from 5 to 4.11 eV. It is clear that from TGA studies, the initial weight loss occurs at 70–90 1C due to the moisture evaporation and it was stable up to 120 1C, the major weight losses were observed in the range of 270–360 1C due to the structural decomposition of the polymers. The values of the apparent activation energy, E, increases with increase PS content from 69.01 to 129.2 J/mol. References
Fig. 9. log [log(1a)/T2] against reciprocal absolute temperature of PS/PVAc blends.
From this method by plotting the dependence of log [log(1a)/T2] versus 1000/T for each sample, we obtain straight lines as shown in Fig. 9. The apparent activation energies are calculated from the slopes of the lines using the expression: E ¼ 2:303R slope
(6)
The values of the apparent activation energy, E, of the blends are listed in Table 1. From this table, it clears that, the values of the activation energy increases with the increase of PS content.
4. Conclusions In this study, we prepared PS/PVAc blends by casting method. The results data suggest that homogeneous polymer composites are formed over all the blend compositions. Various types of the bands for the two polymers and its blends were assigned in FT-IR spectra. XRD scans show a broad and diffuse peak, which indicates the amorphous nature of the blends. A single glass transition
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