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Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films
Polymer Degradation and Stability xxx (2012) 1e10
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Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films J. Tripathi a, *, S. Tripathi b, J.M. Keller c, K. Das d, T. Shripathi b a
Dept. of Physics, St. Aloysius Institute of Technology, Mandala Road, Tilhari, Jabalpur, M.P. 482001, India UGC-DAE Consortium for Scientific Research, Indore, M.P., India c Dept. of PG Stud. & Res. in Physics, Rani Durgawati Vishwavidyalya, Jabalpur, India d Dept. of Physics, St. Aloysius College, Jabalpur, M.P., India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2012 Received in revised form 23 October 2012 Accepted 8 November 2012 Available online xxx
We report the observations on optical and structural properties of thermally treated undoped and doped poly(vinyl) alcohol (PVA) films as a function of doping and annealing temperature. The crystalline peaks of both PVA and Rhodamine B are seen in the X-ray diffraction (XRD) of the as prepared samples where the crystallinity of Rhodamine B increases with doping. At 2 wt% doping, Rhodamine B crystals segregate and are seen at the surface. However, after annealing only PVA peaks remain and Rhodamine B becomes amorphous as seen from XRD and micro-Raman measurements. It is shown how the thermal energy imparted to host PVA and dopant Rhodamine B molecules results in the movement of molecules, breaking the bonds inside the material. The intensity and frequency of Raman active modes is observed to be modified with increasing temperature due to structural changes in polymer bonds. It is also observed that thermal annealing leads to a shift in the optical absorption edge towards lower energies. This is attributed to the modified electronic states. The overall results are interpreted in terms of modifications in structural and optical properties. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Doping Annealing PVA Rhodamine B XRD FTIR
1. Introduction Polymeric materials fall under the category of condensed matter and their properties can easily be tailored by doping, by applying thermal stresses, thermal fluctuations (heating) or upon irradiation [1e6]. Poly(vinyl) alcohol (PVA) is such an important polymer and has found wide range of applications owing to its novel properties like thin film forming ability with less number of defects, emulsifying and adhesive properties along with good resistant to oil coating, grease and solvent and transparency. An increasing trend of scientific interest is nowadays seen, devoted to the understanding and utilization of the properties of polymers, their blends, composites and reinforced polymers etc. This is mainly driven by the need to fabricate new generations of advanced organic structures showing unique optical, mechanical and structural behaviour under specific conditions. In these fabrication processes, several parameters (purity, mixing ability, solubility, temperature and post fabrication annealing etc.) play important roles [7e9]. For example, annealing PVA can result in softening or melting of the material if
* Corresponding author. Tel.: þ91 94258 60675; fax: þ91 761 2602152. E-mail address: [email protected] (J. Tripathi).
performed at higher temperatures (melting point of PVA ¼ 180 C). But when this annealing is carried out at elevated but comparatively lower temperatures of 45 Ce105 C, it may be expected that the properties will drastically change without any softening or melting of host or dopant material. On the other hand, modification in the overall properties can be seen. In past, several studies have been performed on doped polymeric systems such as doping of laser dyes (Rhodamine B and Rhodamine 6G) [10e13]. Although a rough interpretation can be the correlation of temperature with the change in structural properties, yet a deeper understanding of the mechanism behind the temperature induced modification is required and further improvement in the understanding of PVA doped with Rhodamine B system can be achieved by incorporating the analysis of optical and structural properties. Also, an understanding of polymer crystallinity is important because the mechanical properties of polymers are dependent on the crystalline nature and degree of crystallinity [14e17]. Polymer crystalline regions are much stiffer and stronger than amorphous regions [18,19]. As a result of the difference between the amorphous and crystalline arrangements of polymer chains, the X-ray diffraction patterns of the two phases are very different. The amorphous phase contains only short range order and therefore no diffraction peak appears as against the long range order
0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 1. XRD patterns of as prepared PVA with different Rhodamine B doping percentages: (i) pure and (ii) 2 wt%.
crystalline polymer. In the crystalline phase, the repeating lamellar chains provide a regular structure, thus the diffraction pattern will contain sharp, prominent signature peaks, the position of which depends on the exact spacing between chains. Considering all these important phenomena, in the present study, the results are presented on PVA samples containing systematic variation of Rhodamine B doping in the as prepared and annealed conditions (annealed at 45 C and 65 C) and techniques such as X-ray diffraction (XRD), micro-Raman, UVeVis and FTIR spectroscopy were employed for characterization. 2. Experimental details For the present investigation, six sets of samples of different Rhodamine B doping were prepared. For this, following process
was adopted. 5 grams (g) quantity of poly(vinyl) alcohol (PVA) with molecular weight of w44 g/mol (Thomas Baker Chemical, Mumbai, India) was dissolved in 50 ml of water. In a separate beaker, 5 g of Rhodamine B dye (Burgoyne, Burbidges and Co. Mumbai, India) was taken as a solution with water and was put in separate beakers to prepare a number of doped solutions. In each of these beakers, as per the amount of doping required, different wt% of Rhodamine B (0.00, 0.01, 0.05, 0.1, 0.5, 1 and 2 wt%) were mixed well in the PVA solutions. All these solutions were then magnetically stirred for around 15 min till they were completely mixed. The homogeneous solution so obtained were then poured on plane glass plates and left for drying to yield samples in film form. Such films were then used for different measurements. To determine the structural properties, XRD experiments were performed on Bruker D advance spectrometer equipped with 2.2 kW sealed X-ray tube with Cu Ka source. The qe2q scans were done using 40 kV, 40 mA settings of the X-ray tube in the range of 2q ¼ 10e90 covering all the main peaks of Rhodamine B and PVA. The X-rays were detected using a fast counting detector based on Silicon strip technology (Bruker LynxEye detector). More information on structural properties and bonding was obtained from micro-Raman measurements performed on an integrated Raman system (Horiba Jobin Yvon HR800, Edison, NJ). The excitation wavelength from HeeNe laser was 632.2 nm. The diameter of laser spot on the sample surface is w1e2 mm and the wave number accuracy is 0.3 cm1. A notch filter is used to suppress the Reighley light. The optical absorbance of the prepared PVA films were measured at normal incidence at room temperature using a double beam UVeVIS scanning spectrophotometer (Systronics 2202, Ahmedabad, India) in the wavelength range of 200e1000 nm with a spectral bandwidth of w2 nm in the absorbance mode. The FTIR experiments were done on Bruker VERTEX 70/70v FTIR spectrometer to find detailed information on thermally induced changes in bonding. First, the background was scanned and then polymer films were mounted on the holder. Spectra were recorded in the wave number region of 400e4000 cm1 in transmission mode (32 scans) with a resolution of 4 cm1. All the measurements were performed at room temperature.
Fig. 2. XRD patterns of 45 C annealed PVA with different Rhodamine B doping percentages: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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3. Results and discussion 3.1. XRD measurements Fig. 1 represents the XRD patterns of as prepared pure (i) and 2 wt% doped (ii) PVA, where a characteristic PVA XRD peak at 2q ¼ 19.66 is observed. Using Scherrer formula, we have also found the average grain size of PVA present in the sample [20]. Scherrer formula for determining the broadening of X-ray diffraction peaks due to small grain size is given as:
B ¼
Kl Lcos q
where, l is the wavelength of the X-ray and L is the average grain size measured in a direction parallel to surface of the specimen and K is a constant (K z 0.9). Taking into account the full width at half maxima (FWHM) of the main PVA peak using peak fitting, we have calculated average grain size, which came out to be w4.03 nm exhibiting semicrystalline nature of the polymer in agreement with the reported literature [21,22]. The details of doping in as prepared PVA (unannealed) and its influence on crystallinity have been reported elsewhere [23]. It was discussed that due to the growth of the new planes caused by the rearrangement of molecules as well as due to incorporation of Rhodamine B results in a peak close to this PVA position [24]. Figs. 2 and 3 illustrate the XRD patterns where drastic modification in the structural properties can easily be seen on
Fig. 4. Degree of crystallinity as a function of annealing for PVA with different Rhodamine B doping percentages.
annealing doped as well as undoped samples. On comparing Fig. 2(i) with Fig. 3(i) (annealed pure PVA), one can notice slight enhancement in crystallinity. The PVA peak at 2q w 19 becomes slightly sharper giving an average crystallite size of w5.16 nm at 45 C and
Fig. 3. XRD patterns of 65 C annealed PVA with different Rhodamine B doping percentages: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%. Table 1 XRD parameters of annealed Rhodamine B doped PVA for different doping percentages. Annealed at 45 C Sample
Pure PVA PVA þ 0.05 wt% Rhodamine B PVA þ 0.1 wt% Rhodamine B PVA þ 0.5 wt% Rhodamine B PVA þ 1 wt% Rhodamine B PVA þ 2 wt% Rhodamine B
Annealed at 65 C
2q
hkl
d spacing (nm)
Average crystallite size (nm)
2q
hkl
d spacing (nm)
Average crystallite size (nm)
19.52 19.66 19.62 19.64 19.63 19.62
100 100 100 100 100 100
0.454 0.451 0.452 0.451 0.452 0.452
5.16 3.33 3.66 3.40 3.87 3.75
19.56 19.7 19.58 19.69 19.55 19.61
100 100 100 100 100 100
0.453 0.450 0.453 0.450 0.453 0.452
3.68 4.07 4.38 4.53 4.55 4.60
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 5. Micro-Raman spectra of as prepared PVA with different Rhodamine B doping percentages (in the range of 750e1545 cm1): (i) pure and (ii) 2 wt% (iii) represents the micro-Raman spectrum of pure Rhodamine B showing the presence of only fluorescence background.
w3.68 at 65 C, which was w4.6 nm in the as prepared case. However, when doped samples are annealed surprisingly all the Rhodamine B peaks disappear, demonstrating purely amorphous nature of Rhodamine B, which is in contrast with the crystalline nature of unannealed pure Rhodamine B powder and also to the unannealed Rhodamine B doped in PVA host. This breaking of crystalline grains into purely amorphous grains may be understood in terms of thermally induced change in the bonding. When thermal energy is imparted to the crystalline region, the lattice vibrates through phonon interaction and thus the possibility of bond breaking or new bond formation comes into existence. A surprising feature of this interaction in the present case is the intact nature of host PVA crystalline region, which shows only a slight increase in crystallite size, without any changes in peak position or lattice spacing. Further modifications in the structural properties of as prepared pure PVA can be observed by annealing it to relatively higher temp of 65 C as depicted in Fig. 3. Here also, except 2 wt% doped sample, all the sample show no signs of crystallinity in Rhodamine B along with unaltered PVA peak shape. The average crystallite size, identification of crystalline planes and d spacing are reported in Table 1. Fig. 4 shows the plots of degree of crystallinity vs. temperature for each Rhodamine B doping percentage. At lower doping, the changes are clearly observable, while at higher doping the curve points are overlapping within the error bar indicating that at these doping wt%, degree of crystallinity does not vary much with annealing temperature. However, an overall increasing pattern follows with temperature, independent of Rhodamine B concentration. This can be correlated with the molecular structure of PVA. In the as prepared case, Rhodamine B inclusion acts as the foreign seed for crystallite nucleation. Resulting material is a heterogeneous mixture of Rhodamine B and PVA crystalline and amorphous particles. More and more addition of such molecules produces a higher degree of crystallization. But after the addition of higher amount with annealing, there is no further crystallization of the host and Rhodamine B may start segregating at the surface. Thus, one observes saturation in the degree of crystallinity (see Fig. 4).
Fig. 6. Micro-Raman spectra of 45 C annealed PVA with different Rhodamine B doping percentages (in the range of 750e1545 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 7. Micro-Raman spectra of 45 C annealed PVA with different Rhodamine B doping percentages (in the range of 2750e3100 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
3.2. Micro-Raman measurements The micro-Raman spectra of as prepared PVA as a function of doping is shown in Fig. 5 along with the spectrum of pure Rhodamine B [see Fig. 5(iii)]. The effect of doping on the properties of as prepared unannealed PVA has been reported in our previous publication [25]. The observation on 45 C and 65 C annealed samples using microRaman experiments are almost same as those of as prepared samples the only slight differences appear at higher doping (see Figs. 6e9). In presence of fluorescent dopant material, the micro-Raman peaks are superimposed on a broad intense fluorescence background as seen in
Fig. 5(iii) where Rhodamine B shows a very high intensity due to fluorescence without the appearance of any peak and therefore the data has to be background subtracted before assigning the peaks [26]. The strong PVA peak at w2750e3500 cm1 disappears due to the inclusion of fluorescent Rhodamine B molecule in PVA matrix and does not reappear at any doping wt%. On the other hand, the features as the range w760e1545 cm1 also exhibit modifications as a function of doping and in accordance with as prepared samples, show the appearance of new peaks at higher amount of doping however, in this case, these peaks are not much strong and are seen only for the highest doping wt%.
Fig. 8. Micro-Raman spectra of 65 C annealed PVA with different Rhodamine B doping percentages (in the range of 750e1545 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 9. Micro-Raman spectra of 65 C annealed PVA with different Rhodamine B doping percentages (in the range of 2750e3100 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
As discussed earlier, the XRD patterns of 45 C annealed sample show amorphous nature of the dopant as a result of thermal treatment, but at 2 wt% doping, some amount of Rhodamine B is also present in crystalline form. Similarly in micro-Raman spectra, this modified form of Rhodamine B produces new bonds with the surrounding molecules and is seen in the presence of new peaks. However, when as prepared/doped/undoped samples are thermally treated, amorphization of Rhodamine B dominates at all doping wt% and there is no appearance of any new peaks. As seen in corresponding AFM and SEM images (not shown here) [25], at this elevated temperature, there is no segregation of crystallites on the surface and no evidence of any kind of ordered structure is found. Thus one can say that after annealing at 65 C, in all samples, the sample surface become rough and the dopant completely amorphazizes breaking the bonds of host material. But this breaking does not influence the entire host matrix completely because XRD patterns still show crystalline regions in the PVA material. The reports by C. C. Yang et al., I. Yu Prosanov et al. and S. Selvasekarapandian et al. discuss the vanishing of fundamental PVA peak upon annealing or upon doping some foreign materials with simultaneous presence of some additional Raman peaks [27e29]. Although a precise explanation is not available, Prosanov et al. have tried to discuss the effect of annealing on PVA. They have correlated the bond modifications in PVA with the cross-linking of polymer chains when thermal treatment is carried out. Using the Raman scattering model, they conclude that heating leads to destruction of existing band features. This situation also implies to our present case of pure PVA thermally annealed at 45 C and 65 C, while in doped, annealed samples, apart from the above discussed factors, the interaction of Rhodamine B molecules with the surrounding PVA molecules adds to the overall bond structure modification. Thus one can say that micro-Raman spectroscopy has provided useful information about the bond structure and interaction between Rhodamine B and PVA molecules. The molecule aggregates of Rhodamine B get trapped in the region of the host and
show detectable differences in micro-Raman spectra as the polymer environment influences the physical structure of dopant molecules. The broadening and disappearance of PVA peaks along with appearance of new peaks is an evidence of increasing disorderness in the sample with increasing doping and provides important information about physical and chemical modification in the sample as a consequence of thermal treatment. 3.3. UVeVis measurements In order to further investigate the effect of annealing on the optical properties, the as prepared as well as annealed undoped/ doped samples were subjected to UVeVis measurements under
Fig. 10. UVeVis spectra of as prepared PVA with different Rhodamine B doping percentages [(ahn)2 vs. hn plots]: (i) pure and (ii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 11. UVeVis spectra of 45 C annealed PVA with different Rhodamine B doping percentages [(ahn)2 vs. hn plots]: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
similar conditions providing a good comparison between as prepared and annealed samples. Figs. 10e14 show the plots of direct and indirect allowed transitions vs. incident energy in as prepared 45 C and 65 C annealed samples also including the extrapolated linear fits to obtain direct and indirect bandgaps respectively. The absorption edge in all these spectra show pronounced effect of thermal treatment, where a comparatively gradual slope is observed along with modifications in the features below this absorption edge. These are quite different from those of bulk untreated PVA (not shown here), suggesting strong modifications in the optical properties arising from bond breakage as
a result of thermal treatment. Since these measurements are bulk sensitive, they can be better correlated with earlier discussed bulk sensitive experimental observations using micro-Raman and XRD. All the spectra shown in Figs. 10e14 represent the change in absorption edge of PVA as a function of doping and annealing. PVA exhibits both direct as well as indirect interband transitions between valence band and conduction bands. The absorption edge serves as a measure of optical bandgap energy and the coefficient of absorption can be converted into optical bandgap via the relation [30,31]:
ahn ¼ C hn Eg
n
Fig. 12. UVeVis spectra of 45 C annealed PVA with different Rhodamine B doping percentages [(ahn)1/2 vs. hn plots]: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 13. UVeVis spectra of 65 C annealed PVA with different Rhodamine B doping percentages [(ahn)2 vs. hn plots]: (i) pure, (ii) 0.05 (iii) 0.1 (iv) 0.5 (v) 1 and (vi) 2 wt%.
where C is a constant, hn is the photon energy and Eg is the bandgap and n is an integer that can take different values depending on the type of interband allowed/forbidden transitions. For example, n can be 1/2 (direct allowed), 2/3 (direct forbidden), 2 (indirect allowed) and 3 for indirect forbidden transitions [30,31]. The absorption edges are fitted in the region where pure PVA absorption edge lies. Since Rhodamine B shows a complex spectrum, it is easier to observe the changes in bandgap of PVA. Here one can see that there are drastic changes at and around the absorption edge after annealing. This supports the argument of bond structure modifications discussed previously [25]. After annealing at 45 C, corresponding bandgap values show decreasing trend. While pure unannealed PVA showed an indirect bandgap of w5.17 eV, after 2 wt% doping it reduces to 3.49 eV and after 2 wt%
doping plus annealing, it reduces to w3.12 at 45 C and w3.17 at 65 C. At the same time, direct bandgap does not show pronounced change at 65 C as compared to that of 45 C annealed sample indicating changes only in some of the energy band structures instead of modifying whole of the band structure at once. However direct as well as indirect bandgaps, both show a decreasing pattern after doping and in each case, the values are below the values for pure untreated PVA. The observations are in close agreement with those from micro-Raman measurements. 3.4. FTIR measurements Fig. 15 represents the FTIR spectra of as prepared PVA without doping and with 2 wt% doping, while Figs. 16 and 17 depict the FTIR
Fig. 14. UVeVis spectra of 65 C annealed PVA with different Rhodamine B doping percentages [(ahn)1/2 vs. hn plots]: (i) pure, (ii) 0.05 (iii) 0.1 (iv) 0.5 (v) 1 and (vi) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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C
annealed PVA with different Rhodamine B doping
Fig. 15. FTIR plots of as prepared PVA with different Rhodamine B doping percentages: (i) pure and (ii) 2 wt%.
Fig. 17. FTIR plots of 65 percentages.
spectra of pure and doped PVA thermally treated at 45 C and 65 C. Main FTIR peaks are assigned to different bond vibrations such as valence CeH bond vibrations in the range 2800e3000 cm1 and a ratio of syndio- and isotactic sequences in PVA in the range 850e916 cm1 are similar to that in pure unannealed PVA samples. The effect of doping in as prepared PVA is discussed in detail earlier [23]. Among the four possible types of deformation vibrations of shear mode, pendular mode, fan mode and twist mode, one can see the appearance of shear mode at w1465 cm1 matching well with the standard values reported in literature [32,33]. Due to very weak intensity dominated by high Rhodamine B fluorescence background, pendular mode is not observed. The other type, fan and twist modes are seen at w1150e1350 cm1. During PVA thermal annealing well below its melting point of 180 C, many changes in the spectra are observed. Band intensities are varied along with small changes in the peak positions and disappearance of some small intensity absorption bands. Also, in these annealed samples, a systematic decrease of intensity is observed with thermal treatment. It is caused by little bit of dehydration and oxidation of PVA molecules. Some relatively weak bands remain unclassified.
4. Conclusion The authors have systematically investigated structural and optical properties of PVA samples with and without doping of different wt% of Rhodamine B dye. The micro-Raman measurements provided a clear picture of intermixing between the host and dopant molecules leading to the breakage of bonds at the interface between the dopant and its surroundings during fabrication. The structure of PVA shows both direct and indirect bandgaps which reduce slightly as the doping is increased from 0 wt% to 2 wt%, whereas in annealed samples, it is already reduced slightly even in pure form. It is also found that the variation in intensity and frequency of Raman active modes depend on annealing temperature and are a signature of bond modifications in the samples. The results show the presence of small size of PVA crystallites surrounding the comparatively bigger size of Rhodamine B particles as revealed by XRD observations. In these structures, it is also seen that thermal treatment leads to a shift of the optical absorption edge to lower energies as the electronic states are modified with change in degree of crystallinity and bonding structure. Acknowledgements The author S. Tripathi is thankful to Board of Research in Nuclear Sciences, BARC, Mumbai project (No.2009/34/52/BRNS) for providing research fellowship. We thank Dr. V. Sathe (Micro Raman) and Dr. M. Gupta (XRD), UGC-DAE CSR, Indore for their help in respective measurements. References
Fig. 16. FTIR plots of 45 C annealed PVA with different Rhodamine B doping percentages: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
[1] Dutcher JR, Marangoni AG, editors. Soft materials structure and dynamics. New York: Marcel Dekker Publications; 2005. [2] Tao X, editor. Smart fibres, fabrics and clothing. Cambridge, England: Woodhead Publishing Limited; 2000. [3] Teraoka I. Polymer solutions: an introduction to physical properties. John Wiley & Sons; 2002. [4] Utracki LA, editor. Polymer blends handbook. USA: Kluwer Academic Publishers; 2002. [5] Doi M. Introduction to polymer physics. Oxford: Oxford University Press; 1995. [6] Chiellini E, Corti A, Sarto GD, D’Antone S. Polymer Degradation and Stability 2006;91(12):3397. [7] Mohsin M, Hossin A, Haik Y. Material Science and Engineering A 2011;528:925. [8] Fonseca C, Pereiia JM, Benavente R, Cerrada ML, Bello A, Pkrez E. Polymer 1995;36(9):1887.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
10
J. Tripathi et al. / Polymer Degradation and Stability xxx (2012) 1e10
[9] Erbil HY. Vinyl acetate emulsion polymerization and copolymerization with acrylic monomers. USA: CRC Press Ltd; 2000. [10] Saini GSS, Kaur S, Tripathi SK, Mahajan CG, Thanga HH, Verma AL. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2005;61(4):653. [11] Somasundaram G, Ramalingam A. Journal of Photochemistry and Photobiology A: Chemistry 1999;125(1e3):93. [12] El-Agramy AM, Henaish BA. Polymer Degradation and Stability 1993;40(1):13. [13] Muto J, Higuchi F. Physics Letters A 1983;96(2):101. [14] Flores A, Ania F, Calleja FJB. Polymer 2009;50:729. [15] Calleja FJB. Trends in Polymer Science 1994;2:419. [16] Petch NJ. Iron Steel Instruments 1953;173:25. [17] Mishra S, Bajpai R, Katare R, Bajpai AK. Journal of Material Science: Materials and Methods 2006;17:1305. [18] Storcks KH. Journal of American Chemical Society 1938;60:1753. [19] Ferry JD. Viscoelastic properties of polymers. USA: John Wiley and Sons Publications; 1980. [20] Cullity BD. Elements of X-ray diffraction. 2nd ed. New York: Addison-Wesley Publication; 1978. [21] Nasar G, Khan MS, Khalil U. Journal of Pakistan Materials Society 2009;3(2):67.
[22] Bama GK, Devi PI, Ramachandran K. Structural and thermal properties of PVDF/PVA blends. Journal of Material Science 2009;44:1302. [23] Tripathi J, Keller JM, Das K, Tripathi S, Fatima A, Shripathi T. Applied Surface Science 2012;261:481. [24] Faraga AAM, Yahia IS. Synthetic Metals 2011;161:32. [25] Tripathi J, Keller JM, Das K, Tripathi S, Shripathi T. Journal of Physics and Chemistry of Solids 2012;73:1026. [26] Zhang ZM, Chen S, Liang YZ, Liu ZX, Zhang QM, Ding LX, et al. Journal of Raman Spectroscopy 2009;41(6):659. [27] Yang CC, Lin CT, Chiu SJ. Desalination 2008;233:137. [28] Prosanov IYu, Matvienko AA. Physics of the Solid State 2010;52(10):2203. [29] Selvasekarapandian S, Hema M, Kawamura J, Kamishima O, Baskaran R. Proceedings of the 3rd International Conference on Physics of Solid State Ionics (ICPSSI-3), Journal of Physical Society of Japan 2010;79(Suppl. A):163. [30] Tauc J. Amorphus and liquid semiconductors. p. 159. New York: Plenum; 1974. [31] Pankove JI. Optical processes in semiconductors. Prentice-Hall, Inc; 1971. [32] Mansur HS, Oréfice RL, Mansur AAP. Polymer 2004;45(21):7193. [33] Mansur HS, Oréfice RL, Pereira MM, Lobato ZIP, Vasconcelos WL, Machado LJC. Spectroscopy 2002;16(3e4):351.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films J. Tripathi a, *, S. Tripathi b, J.M. Keller c, K. Das d, T. Shripathi b a
Dept. of Physics, St. Aloysius Institute of Technology, Mandala Road, Tilhari, Jabalpur, M.P. 482001, India UGC-DAE Consortium for Scientific Research, Indore, M.P., India c Dept. of PG Stud. & Res. in Physics, Rani Durgawati Vishwavidyalya, Jabalpur, India d Dept. of Physics, St. Aloysius College, Jabalpur, M.P., India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2012 Received in revised form 23 October 2012 Accepted 8 November 2012 Available online xxx
We report the observations on optical and structural properties of thermally treated undoped and doped poly(vinyl) alcohol (PVA) films as a function of doping and annealing temperature. The crystalline peaks of both PVA and Rhodamine B are seen in the X-ray diffraction (XRD) of the as prepared samples where the crystallinity of Rhodamine B increases with doping. At 2 wt% doping, Rhodamine B crystals segregate and are seen at the surface. However, after annealing only PVA peaks remain and Rhodamine B becomes amorphous as seen from XRD and micro-Raman measurements. It is shown how the thermal energy imparted to host PVA and dopant Rhodamine B molecules results in the movement of molecules, breaking the bonds inside the material. The intensity and frequency of Raman active modes is observed to be modified with increasing temperature due to structural changes in polymer bonds. It is also observed that thermal annealing leads to a shift in the optical absorption edge towards lower energies. This is attributed to the modified electronic states. The overall results are interpreted in terms of modifications in structural and optical properties. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Doping Annealing PVA Rhodamine B XRD FTIR
1. Introduction Polymeric materials fall under the category of condensed matter and their properties can easily be tailored by doping, by applying thermal stresses, thermal fluctuations (heating) or upon irradiation [1e6]. Poly(vinyl) alcohol (PVA) is such an important polymer and has found wide range of applications owing to its novel properties like thin film forming ability with less number of defects, emulsifying and adhesive properties along with good resistant to oil coating, grease and solvent and transparency. An increasing trend of scientific interest is nowadays seen, devoted to the understanding and utilization of the properties of polymers, their blends, composites and reinforced polymers etc. This is mainly driven by the need to fabricate new generations of advanced organic structures showing unique optical, mechanical and structural behaviour under specific conditions. In these fabrication processes, several parameters (purity, mixing ability, solubility, temperature and post fabrication annealing etc.) play important roles [7e9]. For example, annealing PVA can result in softening or melting of the material if
* Corresponding author. Tel.: þ91 94258 60675; fax: þ91 761 2602152. E-mail address: [email protected] (J. Tripathi).
performed at higher temperatures (melting point of PVA ¼ 180 C). But when this annealing is carried out at elevated but comparatively lower temperatures of 45 Ce105 C, it may be expected that the properties will drastically change without any softening or melting of host or dopant material. On the other hand, modification in the overall properties can be seen. In past, several studies have been performed on doped polymeric systems such as doping of laser dyes (Rhodamine B and Rhodamine 6G) [10e13]. Although a rough interpretation can be the correlation of temperature with the change in structural properties, yet a deeper understanding of the mechanism behind the temperature induced modification is required and further improvement in the understanding of PVA doped with Rhodamine B system can be achieved by incorporating the analysis of optical and structural properties. Also, an understanding of polymer crystallinity is important because the mechanical properties of polymers are dependent on the crystalline nature and degree of crystallinity [14e17]. Polymer crystalline regions are much stiffer and stronger than amorphous regions [18,19]. As a result of the difference between the amorphous and crystalline arrangements of polymer chains, the X-ray diffraction patterns of the two phases are very different. The amorphous phase contains only short range order and therefore no diffraction peak appears as against the long range order
0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 1. XRD patterns of as prepared PVA with different Rhodamine B doping percentages: (i) pure and (ii) 2 wt%.
crystalline polymer. In the crystalline phase, the repeating lamellar chains provide a regular structure, thus the diffraction pattern will contain sharp, prominent signature peaks, the position of which depends on the exact spacing between chains. Considering all these important phenomena, in the present study, the results are presented on PVA samples containing systematic variation of Rhodamine B doping in the as prepared and annealed conditions (annealed at 45 C and 65 C) and techniques such as X-ray diffraction (XRD), micro-Raman, UVeVis and FTIR spectroscopy were employed for characterization. 2. Experimental details For the present investigation, six sets of samples of different Rhodamine B doping were prepared. For this, following process
was adopted. 5 grams (g) quantity of poly(vinyl) alcohol (PVA) with molecular weight of w44 g/mol (Thomas Baker Chemical, Mumbai, India) was dissolved in 50 ml of water. In a separate beaker, 5 g of Rhodamine B dye (Burgoyne, Burbidges and Co. Mumbai, India) was taken as a solution with water and was put in separate beakers to prepare a number of doped solutions. In each of these beakers, as per the amount of doping required, different wt% of Rhodamine B (0.00, 0.01, 0.05, 0.1, 0.5, 1 and 2 wt%) were mixed well in the PVA solutions. All these solutions were then magnetically stirred for around 15 min till they were completely mixed. The homogeneous solution so obtained were then poured on plane glass plates and left for drying to yield samples in film form. Such films were then used for different measurements. To determine the structural properties, XRD experiments were performed on Bruker D advance spectrometer equipped with 2.2 kW sealed X-ray tube with Cu Ka source. The qe2q scans were done using 40 kV, 40 mA settings of the X-ray tube in the range of 2q ¼ 10e90 covering all the main peaks of Rhodamine B and PVA. The X-rays were detected using a fast counting detector based on Silicon strip technology (Bruker LynxEye detector). More information on structural properties and bonding was obtained from micro-Raman measurements performed on an integrated Raman system (Horiba Jobin Yvon HR800, Edison, NJ). The excitation wavelength from HeeNe laser was 632.2 nm. The diameter of laser spot on the sample surface is w1e2 mm and the wave number accuracy is 0.3 cm1. A notch filter is used to suppress the Reighley light. The optical absorbance of the prepared PVA films were measured at normal incidence at room temperature using a double beam UVeVIS scanning spectrophotometer (Systronics 2202, Ahmedabad, India) in the wavelength range of 200e1000 nm with a spectral bandwidth of w2 nm in the absorbance mode. The FTIR experiments were done on Bruker VERTEX 70/70v FTIR spectrometer to find detailed information on thermally induced changes in bonding. First, the background was scanned and then polymer films were mounted on the holder. Spectra were recorded in the wave number region of 400e4000 cm1 in transmission mode (32 scans) with a resolution of 4 cm1. All the measurements were performed at room temperature.
Fig. 2. XRD patterns of 45 C annealed PVA with different Rhodamine B doping percentages: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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3. Results and discussion 3.1. XRD measurements Fig. 1 represents the XRD patterns of as prepared pure (i) and 2 wt% doped (ii) PVA, where a characteristic PVA XRD peak at 2q ¼ 19.66 is observed. Using Scherrer formula, we have also found the average grain size of PVA present in the sample [20]. Scherrer formula for determining the broadening of X-ray diffraction peaks due to small grain size is given as:
B ¼
Kl Lcos q
where, l is the wavelength of the X-ray and L is the average grain size measured in a direction parallel to surface of the specimen and K is a constant (K z 0.9). Taking into account the full width at half maxima (FWHM) of the main PVA peak using peak fitting, we have calculated average grain size, which came out to be w4.03 nm exhibiting semicrystalline nature of the polymer in agreement with the reported literature [21,22]. The details of doping in as prepared PVA (unannealed) and its influence on crystallinity have been reported elsewhere [23]. It was discussed that due to the growth of the new planes caused by the rearrangement of molecules as well as due to incorporation of Rhodamine B results in a peak close to this PVA position [24]. Figs. 2 and 3 illustrate the XRD patterns where drastic modification in the structural properties can easily be seen on
Fig. 4. Degree of crystallinity as a function of annealing for PVA with different Rhodamine B doping percentages.
annealing doped as well as undoped samples. On comparing Fig. 2(i) with Fig. 3(i) (annealed pure PVA), one can notice slight enhancement in crystallinity. The PVA peak at 2q w 19 becomes slightly sharper giving an average crystallite size of w5.16 nm at 45 C and
Fig. 3. XRD patterns of 65 C annealed PVA with different Rhodamine B doping percentages: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%. Table 1 XRD parameters of annealed Rhodamine B doped PVA for different doping percentages. Annealed at 45 C Sample
Pure PVA PVA þ 0.05 wt% Rhodamine B PVA þ 0.1 wt% Rhodamine B PVA þ 0.5 wt% Rhodamine B PVA þ 1 wt% Rhodamine B PVA þ 2 wt% Rhodamine B
Annealed at 65 C
2q
hkl
d spacing (nm)
Average crystallite size (nm)
2q
hkl
d spacing (nm)
Average crystallite size (nm)
19.52 19.66 19.62 19.64 19.63 19.62
100 100 100 100 100 100
0.454 0.451 0.452 0.451 0.452 0.452
5.16 3.33 3.66 3.40 3.87 3.75
19.56 19.7 19.58 19.69 19.55 19.61
100 100 100 100 100 100
0.453 0.450 0.453 0.450 0.453 0.452
3.68 4.07 4.38 4.53 4.55 4.60
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 5. Micro-Raman spectra of as prepared PVA with different Rhodamine B doping percentages (in the range of 750e1545 cm1): (i) pure and (ii) 2 wt% (iii) represents the micro-Raman spectrum of pure Rhodamine B showing the presence of only fluorescence background.
w3.68 at 65 C, which was w4.6 nm in the as prepared case. However, when doped samples are annealed surprisingly all the Rhodamine B peaks disappear, demonstrating purely amorphous nature of Rhodamine B, which is in contrast with the crystalline nature of unannealed pure Rhodamine B powder and also to the unannealed Rhodamine B doped in PVA host. This breaking of crystalline grains into purely amorphous grains may be understood in terms of thermally induced change in the bonding. When thermal energy is imparted to the crystalline region, the lattice vibrates through phonon interaction and thus the possibility of bond breaking or new bond formation comes into existence. A surprising feature of this interaction in the present case is the intact nature of host PVA crystalline region, which shows only a slight increase in crystallite size, without any changes in peak position or lattice spacing. Further modifications in the structural properties of as prepared pure PVA can be observed by annealing it to relatively higher temp of 65 C as depicted in Fig. 3. Here also, except 2 wt% doped sample, all the sample show no signs of crystallinity in Rhodamine B along with unaltered PVA peak shape. The average crystallite size, identification of crystalline planes and d spacing are reported in Table 1. Fig. 4 shows the plots of degree of crystallinity vs. temperature for each Rhodamine B doping percentage. At lower doping, the changes are clearly observable, while at higher doping the curve points are overlapping within the error bar indicating that at these doping wt%, degree of crystallinity does not vary much with annealing temperature. However, an overall increasing pattern follows with temperature, independent of Rhodamine B concentration. This can be correlated with the molecular structure of PVA. In the as prepared case, Rhodamine B inclusion acts as the foreign seed for crystallite nucleation. Resulting material is a heterogeneous mixture of Rhodamine B and PVA crystalline and amorphous particles. More and more addition of such molecules produces a higher degree of crystallization. But after the addition of higher amount with annealing, there is no further crystallization of the host and Rhodamine B may start segregating at the surface. Thus, one observes saturation in the degree of crystallinity (see Fig. 4).
Fig. 6. Micro-Raman spectra of 45 C annealed PVA with different Rhodamine B doping percentages (in the range of 750e1545 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 7. Micro-Raman spectra of 45 C annealed PVA with different Rhodamine B doping percentages (in the range of 2750e3100 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
3.2. Micro-Raman measurements The micro-Raman spectra of as prepared PVA as a function of doping is shown in Fig. 5 along with the spectrum of pure Rhodamine B [see Fig. 5(iii)]. The effect of doping on the properties of as prepared unannealed PVA has been reported in our previous publication [25]. The observation on 45 C and 65 C annealed samples using microRaman experiments are almost same as those of as prepared samples the only slight differences appear at higher doping (see Figs. 6e9). In presence of fluorescent dopant material, the micro-Raman peaks are superimposed on a broad intense fluorescence background as seen in
Fig. 5(iii) where Rhodamine B shows a very high intensity due to fluorescence without the appearance of any peak and therefore the data has to be background subtracted before assigning the peaks [26]. The strong PVA peak at w2750e3500 cm1 disappears due to the inclusion of fluorescent Rhodamine B molecule in PVA matrix and does not reappear at any doping wt%. On the other hand, the features as the range w760e1545 cm1 also exhibit modifications as a function of doping and in accordance with as prepared samples, show the appearance of new peaks at higher amount of doping however, in this case, these peaks are not much strong and are seen only for the highest doping wt%.
Fig. 8. Micro-Raman spectra of 65 C annealed PVA with different Rhodamine B doping percentages (in the range of 750e1545 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 9. Micro-Raman spectra of 65 C annealed PVA with different Rhodamine B doping percentages (in the range of 2750e3100 cm1): (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
As discussed earlier, the XRD patterns of 45 C annealed sample show amorphous nature of the dopant as a result of thermal treatment, but at 2 wt% doping, some amount of Rhodamine B is also present in crystalline form. Similarly in micro-Raman spectra, this modified form of Rhodamine B produces new bonds with the surrounding molecules and is seen in the presence of new peaks. However, when as prepared/doped/undoped samples are thermally treated, amorphization of Rhodamine B dominates at all doping wt% and there is no appearance of any new peaks. As seen in corresponding AFM and SEM images (not shown here) [25], at this elevated temperature, there is no segregation of crystallites on the surface and no evidence of any kind of ordered structure is found. Thus one can say that after annealing at 65 C, in all samples, the sample surface become rough and the dopant completely amorphazizes breaking the bonds of host material. But this breaking does not influence the entire host matrix completely because XRD patterns still show crystalline regions in the PVA material. The reports by C. C. Yang et al., I. Yu Prosanov et al. and S. Selvasekarapandian et al. discuss the vanishing of fundamental PVA peak upon annealing or upon doping some foreign materials with simultaneous presence of some additional Raman peaks [27e29]. Although a precise explanation is not available, Prosanov et al. have tried to discuss the effect of annealing on PVA. They have correlated the bond modifications in PVA with the cross-linking of polymer chains when thermal treatment is carried out. Using the Raman scattering model, they conclude that heating leads to destruction of existing band features. This situation also implies to our present case of pure PVA thermally annealed at 45 C and 65 C, while in doped, annealed samples, apart from the above discussed factors, the interaction of Rhodamine B molecules with the surrounding PVA molecules adds to the overall bond structure modification. Thus one can say that micro-Raman spectroscopy has provided useful information about the bond structure and interaction between Rhodamine B and PVA molecules. The molecule aggregates of Rhodamine B get trapped in the region of the host and
show detectable differences in micro-Raman spectra as the polymer environment influences the physical structure of dopant molecules. The broadening and disappearance of PVA peaks along with appearance of new peaks is an evidence of increasing disorderness in the sample with increasing doping and provides important information about physical and chemical modification in the sample as a consequence of thermal treatment. 3.3. UVeVis measurements In order to further investigate the effect of annealing on the optical properties, the as prepared as well as annealed undoped/ doped samples were subjected to UVeVis measurements under
Fig. 10. UVeVis spectra of as prepared PVA with different Rhodamine B doping percentages [(ahn)2 vs. hn plots]: (i) pure and (ii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 11. UVeVis spectra of 45 C annealed PVA with different Rhodamine B doping percentages [(ahn)2 vs. hn plots]: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
similar conditions providing a good comparison between as prepared and annealed samples. Figs. 10e14 show the plots of direct and indirect allowed transitions vs. incident energy in as prepared 45 C and 65 C annealed samples also including the extrapolated linear fits to obtain direct and indirect bandgaps respectively. The absorption edge in all these spectra show pronounced effect of thermal treatment, where a comparatively gradual slope is observed along with modifications in the features below this absorption edge. These are quite different from those of bulk untreated PVA (not shown here), suggesting strong modifications in the optical properties arising from bond breakage as
a result of thermal treatment. Since these measurements are bulk sensitive, they can be better correlated with earlier discussed bulk sensitive experimental observations using micro-Raman and XRD. All the spectra shown in Figs. 10e14 represent the change in absorption edge of PVA as a function of doping and annealing. PVA exhibits both direct as well as indirect interband transitions between valence band and conduction bands. The absorption edge serves as a measure of optical bandgap energy and the coefficient of absorption can be converted into optical bandgap via the relation [30,31]:
ahn ¼ C hn Eg
n
Fig. 12. UVeVis spectra of 45 C annealed PVA with different Rhodamine B doping percentages [(ahn)1/2 vs. hn plots]: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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Fig. 13. UVeVis spectra of 65 C annealed PVA with different Rhodamine B doping percentages [(ahn)2 vs. hn plots]: (i) pure, (ii) 0.05 (iii) 0.1 (iv) 0.5 (v) 1 and (vi) 2 wt%.
where C is a constant, hn is the photon energy and Eg is the bandgap and n is an integer that can take different values depending on the type of interband allowed/forbidden transitions. For example, n can be 1/2 (direct allowed), 2/3 (direct forbidden), 2 (indirect allowed) and 3 for indirect forbidden transitions [30,31]. The absorption edges are fitted in the region where pure PVA absorption edge lies. Since Rhodamine B shows a complex spectrum, it is easier to observe the changes in bandgap of PVA. Here one can see that there are drastic changes at and around the absorption edge after annealing. This supports the argument of bond structure modifications discussed previously [25]. After annealing at 45 C, corresponding bandgap values show decreasing trend. While pure unannealed PVA showed an indirect bandgap of w5.17 eV, after 2 wt% doping it reduces to 3.49 eV and after 2 wt%
doping plus annealing, it reduces to w3.12 at 45 C and w3.17 at 65 C. At the same time, direct bandgap does not show pronounced change at 65 C as compared to that of 45 C annealed sample indicating changes only in some of the energy band structures instead of modifying whole of the band structure at once. However direct as well as indirect bandgaps, both show a decreasing pattern after doping and in each case, the values are below the values for pure untreated PVA. The observations are in close agreement with those from micro-Raman measurements. 3.4. FTIR measurements Fig. 15 represents the FTIR spectra of as prepared PVA without doping and with 2 wt% doping, while Figs. 16 and 17 depict the FTIR
Fig. 14. UVeVis spectra of 65 C annealed PVA with different Rhodamine B doping percentages [(ahn)1/2 vs. hn plots]: (i) pure, (ii) 0.05 (iii) 0.1 (iv) 0.5 (v) 1 and (vi) 2 wt%.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
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C
annealed PVA with different Rhodamine B doping
Fig. 15. FTIR plots of as prepared PVA with different Rhodamine B doping percentages: (i) pure and (ii) 2 wt%.
Fig. 17. FTIR plots of 65 percentages.
spectra of pure and doped PVA thermally treated at 45 C and 65 C. Main FTIR peaks are assigned to different bond vibrations such as valence CeH bond vibrations in the range 2800e3000 cm1 and a ratio of syndio- and isotactic sequences in PVA in the range 850e916 cm1 are similar to that in pure unannealed PVA samples. The effect of doping in as prepared PVA is discussed in detail earlier [23]. Among the four possible types of deformation vibrations of shear mode, pendular mode, fan mode and twist mode, one can see the appearance of shear mode at w1465 cm1 matching well with the standard values reported in literature [32,33]. Due to very weak intensity dominated by high Rhodamine B fluorescence background, pendular mode is not observed. The other type, fan and twist modes are seen at w1150e1350 cm1. During PVA thermal annealing well below its melting point of 180 C, many changes in the spectra are observed. Band intensities are varied along with small changes in the peak positions and disappearance of some small intensity absorption bands. Also, in these annealed samples, a systematic decrease of intensity is observed with thermal treatment. It is caused by little bit of dehydration and oxidation of PVA molecules. Some relatively weak bands remain unclassified.
4. Conclusion The authors have systematically investigated structural and optical properties of PVA samples with and without doping of different wt% of Rhodamine B dye. The micro-Raman measurements provided a clear picture of intermixing between the host and dopant molecules leading to the breakage of bonds at the interface between the dopant and its surroundings during fabrication. The structure of PVA shows both direct and indirect bandgaps which reduce slightly as the doping is increased from 0 wt% to 2 wt%, whereas in annealed samples, it is already reduced slightly even in pure form. It is also found that the variation in intensity and frequency of Raman active modes depend on annealing temperature and are a signature of bond modifications in the samples. The results show the presence of small size of PVA crystallites surrounding the comparatively bigger size of Rhodamine B particles as revealed by XRD observations. In these structures, it is also seen that thermal treatment leads to a shift of the optical absorption edge to lower energies as the electronic states are modified with change in degree of crystallinity and bonding structure. Acknowledgements The author S. Tripathi is thankful to Board of Research in Nuclear Sciences, BARC, Mumbai project (No.2009/34/52/BRNS) for providing research fellowship. We thank Dr. V. Sathe (Micro Raman) and Dr. M. Gupta (XRD), UGC-DAE CSR, Indore for their help in respective measurements. References
Fig. 16. FTIR plots of 45 C annealed PVA with different Rhodamine B doping percentages: (i) pure, (ii) 0.01, (iii) 0.05 (iv) 0.1 (v) 0.5 (vi) 1 and (vii) 2 wt%.
[1] Dutcher JR, Marangoni AG, editors. Soft materials structure and dynamics. New York: Marcel Dekker Publications; 2005. [2] Tao X, editor. Smart fibres, fabrics and clothing. Cambridge, England: Woodhead Publishing Limited; 2000. [3] Teraoka I. Polymer solutions: an introduction to physical properties. John Wiley & Sons; 2002. [4] Utracki LA, editor. Polymer blends handbook. USA: Kluwer Academic Publishers; 2002. [5] Doi M. Introduction to polymer physics. Oxford: Oxford University Press; 1995. [6] Chiellini E, Corti A, Sarto GD, D’Antone S. Polymer Degradation and Stability 2006;91(12):3397. [7] Mohsin M, Hossin A, Haik Y. Material Science and Engineering A 2011;528:925. [8] Fonseca C, Pereiia JM, Benavente R, Cerrada ML, Bello A, Pkrez E. Polymer 1995;36(9):1887.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003
10
J. Tripathi et al. / Polymer Degradation and Stability xxx (2012) 1e10
[9] Erbil HY. Vinyl acetate emulsion polymerization and copolymerization with acrylic monomers. USA: CRC Press Ltd; 2000. [10] Saini GSS, Kaur S, Tripathi SK, Mahajan CG, Thanga HH, Verma AL. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2005;61(4):653. [11] Somasundaram G, Ramalingam A. Journal of Photochemistry and Photobiology A: Chemistry 1999;125(1e3):93. [12] El-Agramy AM, Henaish BA. Polymer Degradation and Stability 1993;40(1):13. [13] Muto J, Higuchi F. Physics Letters A 1983;96(2):101. [14] Flores A, Ania F, Calleja FJB. Polymer 2009;50:729. [15] Calleja FJB. Trends in Polymer Science 1994;2:419. [16] Petch NJ. Iron Steel Instruments 1953;173:25. [17] Mishra S, Bajpai R, Katare R, Bajpai AK. Journal of Material Science: Materials and Methods 2006;17:1305. [18] Storcks KH. Journal of American Chemical Society 1938;60:1753. [19] Ferry JD. Viscoelastic properties of polymers. USA: John Wiley and Sons Publications; 1980. [20] Cullity BD. Elements of X-ray diffraction. 2nd ed. New York: Addison-Wesley Publication; 1978. [21] Nasar G, Khan MS, Khalil U. Journal of Pakistan Materials Society 2009;3(2):67.
[22] Bama GK, Devi PI, Ramachandran K. Structural and thermal properties of PVDF/PVA blends. Journal of Material Science 2009;44:1302. [23] Tripathi J, Keller JM, Das K, Tripathi S, Fatima A, Shripathi T. Applied Surface Science 2012;261:481. [24] Faraga AAM, Yahia IS. Synthetic Metals 2011;161:32. [25] Tripathi J, Keller JM, Das K, Tripathi S, Shripathi T. Journal of Physics and Chemistry of Solids 2012;73:1026. [26] Zhang ZM, Chen S, Liang YZ, Liu ZX, Zhang QM, Ding LX, et al. Journal of Raman Spectroscopy 2009;41(6):659. [27] Yang CC, Lin CT, Chiu SJ. Desalination 2008;233:137. [28] Prosanov IYu, Matvienko AA. Physics of the Solid State 2010;52(10):2203. [29] Selvasekarapandian S, Hema M, Kawamura J, Kamishima O, Baskaran R. Proceedings of the 3rd International Conference on Physics of Solid State Ionics (ICPSSI-3), Journal of Physical Society of Japan 2010;79(Suppl. A):163. [30] Tauc J. Amorphus and liquid semiconductors. p. 159. New York: Plenum; 1974. [31] Pankove JI. Optical processes in semiconductors. Prentice-Hall, Inc; 1971. [32] Mansur HS, Oréfice RL, Mansur AAP. Polymer 2004;45(21):7193. [33] Mansur HS, Oréfice RL, Pereira MM, Lobato ZIP, Vasconcelos WL, Machado LJC. Spectroscopy 2002;16(3e4):351.
Please cite this article in press as: Tripathi J, et al., Degradation study on structural and optical properties of annealed Rhodamine B doped poly(vinyl) alcohol films, Polymer Degradation and Stability (2012), http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.003