Polypropylene Lab Report

Abstract

In this work, the mechanical and barrier properties were examined for Polypropylene (PP) film in which the surface of the film was modified by Oxygen plasma treatment. The PP film was treated in various intervals of time of 60 s, 120 s, 180 s, 240 s and 300 s with three various RF power settings of 7.2 W, 10.2 W, 29.6 W. The contact angle was measured to characterize the wettability. The oxygen functional groups were generated on the surface of oxygen modified PP which was observed by Fourier transform infrared spectroscope and it was resulted in the improvement of wettability. The surface morphology and roughness of the PP films before and after the oxygen plasma treatment was analyzed by Atomic Force Microscopy (AFM). It was found that the roughness of

the PP film was increased with treatment time and RF Power and the roughness value was increased from 1.491 nm to 7.26 nm. The mechanical properties like tensile strength and the barrier properties like oxygen transmission rate (OTR), Water-vapour transmission rate (WVTR) were also calculated. The results showed that, there was a reduction of tensile strength from 6 MPa to 1.350 MPa because of etching and degradation of polypropylene. The OTR was increased from 1851.2 to 2248.92 cc/m2/24hrs and the WVTR also increased from 9.6 to 14.24 g/m2/24 hrs. Finally, it was observed that an increase in treatment time and plasma rate resulted in higher hydrophilicity of the polymer because of low etching rate at the beginning and the exponential increase of etching rate and there was a great variation in mechanical properties and trifling changes occurred in barrier properties.
Keywords: Polypropylene (PP), Oxygen plasma treatment, contact angle, AFM, Mechanical, Barrier property
Introduction
Polymers like polyethylene, polypropylene, polystyrene and poly (ethylene terephthalate) are the most extensively used packaging materials because they are available in large quantities at low costs [1]. Polypropylene (PP) films have their application in industry and are used in many packaging applications. It has excellent physical properties, chemical resistance, good mechanical and barrier properties. These polymers are generally characterized by low surface energy and hence poor adhesive properties [2]. If the surface of the polymer does not have the desired properties for particular application, then it leads to failure of the material and the device or system containing it [3]. By activating the surface of the polymer results in good adhesion between polymer and coating [4]. This can be achieved by plasma treatment. In this technique, it is possible to modify the surface properties of polymers without affecting the bulk properties. Without surface modification, it’s owed to the lack of adhesion [2]. Recently, a number of techniques have been utilized for modifying the surface properties of polymers such as chemical [………..28-30?], thermal, mechanical and plasma treatments. The use of plasma treatment appears to be suitable for packaging and the treatment time is short to achieve significant change in the properties governed by surface characteristics [5]. Plasma refers to a partially or wholly ionized gas composed essentially of photons, ions and free electrons as well as atoms in their fundamental or excited states possessing a net neutral charge [6].
Plasma treatment is also one of the effective technologies to achieve the modification of surface characteristics of polymeric materials. The interactions between the plasma and the surface molecules of polymers lead to the surface phenomena such as etching, crosslinking and activation [7]. Plasma treatments definitely offer many advantages over conventional approaches, since they are characteristically dry processes that do not generate chemical waste [8]. Depending on the conditions and the plasma species, surface properties of polymers, such as morphology, hydrophobicity and adhesion can be altered [9]. It can also influence on the mechanical and barrier property of the polymers. By the plasma treatment, the reactive functional group and also the surface roughness were introduced in the polymers. This surface roughness will improve the adhesion and the mechanical performance of the matrix of the film. These actions were happened due to the chemical interaction and also by the mechanical interlocking mechanism [2, 28].
This present study used RF oxygen plasma for the surface modification of polypropylene. By surface activation and slight etching, the oxygen plasma promotes surface modification. The surface chemical changes were observed by FTIR. The contact angle and the surface morphology were analyzed by goniometer and AFM. The mechanical and barrier properties of the untreated and treated PP samples were carried out and the impact of surface modification on mechanical and barrier properties were analyzed.
Materials and Methods
Materials
The film used for this study was a commercial Polypropylene (PP) film with a density of 0.92 g/cm3 and thickness of 0.048 ± 003 mm which was purchased from Jayanthi Plastics, Erode. Tamil Nadu. Double distilled water was used as a test liquid for contact angle measurements. Ethanol and acetone were procured from sigma Aldrich, India.
Sample Preparation
The samples for oxygen plasma treatment were prepared in the dimension of 20 X 20 cm. The samples were cut into different dimensions for further studies. The samples were washed with ethanol and then with acetone. It was dried and kept under vacuum for 24 hrs.
Oxygen plasma treatment
The oxygen plasma treatment was carried out by Harrick plasma equipment (model Make and country). The principle of the equipment is, when a gas is passed under suitable low pressure, it is subjected to a high frequency oscillating electromagnetic field and the gas contains the accelerated ions collide with the gas molecules ionizing them and forming plasma. The sample should be placed in the reaction chamber. The gas processed at low pressure of 200-600 mtorr and the low flow rates of 5 – 10 SCFH (Standard cubic feet per hour) are subjected to RF (radio frequency) electromagnetic radiation at 8-12 MHz creating plasma, at near ambient temperatures, within the chamber. Vacuum pump was operated with minimum pump speed of 1.4 m3/h and ultimate total pressure was of 200 mtorr. It was operated in various RF power settings of low, medium and high such as 7.2 W, 10.2 W, 29.6 W.
Contact angle studies
The contact angle measurements were carried out by a DGD – DX Goniometer (GBX, Romans – sur–Isere, France), equipped with the DIGIDROP Image analysis software. It was carried out at room temperature (23 ± 2 °C) by sessile drop method. Before the measurement, the samples were dried in a vacuum oven for 24 h at 50 ºC. 5 µL of MilliQ grade water drop was placed with a micro syringe on the sample. The contact angle was measured within 5 s. The average contact angle on each polymer film was determined at five different locations and the experimental uncertainty was within ±1º.
AFM
The morphology and surface roughness of the treated and untreated Polypropylene (PP) samples were examined by atomic force microscope (AFM, Agilent 5500, USA). It was happened in an ambient atmosphere at room temperature. Tapping mode was used for sample imaging and the scanning range was 5 µm × 5 µm. The Probes were silicon tips with a spring constant of 20 -80 N/m. The resonance frequency lies in the range of 250 – 300 kHz and the images were analyzed by using the Nanoscope image processing software.
FTIR Analysis
The surface chemical composition of the untreated and oxygen plasma treated PP films were observed by attenuated total reflection – Fourier transform infrared spectroscope (ATR – FTIR, Thermo Nicolet Nexus 670). These spectra were investigated between the wavenumber ranging from 4000 to 650 cm-1 with 16 scans at a resolution of 4 cm-1.
Mechanical Properties
The Tensile strength of the Polypropylene (PP) films before and after the plasma treatment were investigated using Universal Testing Machine (UTM, H10KS, Tinius Olsen, UK) at 23 ºC according to the ASTM D638 standard. The samples with dimensions of 150 mm x 25 mm x 0.048 mm (thickness) with a gauge length of 25 mm at a cross speed of 10 mm/min. The tensile strength was expressed in MPa.
Oxygen Transmission Rate (OTR)
The Oxygen Transmission Rate (OTR) of the PP films were calculated using an OXTRAN Oxygen permeability Tester (MOCON, Minneapolis, MN).

Testing was performed at 23 ºC under the condition of 0% RH at 1 atm with the standard of ASTM D3985. Measurements were taken at three times and the average value was calculated. All specimens were conditioned at ambient conditions.

Water-vapour Transmission Rate (WVTR)

The Water vapor Transmission Rate (WVTR) of the treated and untreated PP samples was calculated by Mocon Permatran, according to the standard of ASTM F 1249 – 90. The tests were carried out at 35 ºC under the condition of 100% RH. It was repeated for three times and the average mean values were reported. All specimens were conditioned at ambient conditions.

Results and Discussions

AFM analysis

The surface morphology and the surface roughness of the untreated and treated PP samples were investigated by AFM. The Figure 1 represents the AFM images in a three dimensional view of untreated and treated samples in a time of 60s with various power rates. The scan size of all the images is 5×5 μm2. Evidently, the oxygen plasma treatment modified the surface morphology of the PP

films.

Fig 1: AFM Images untreated and Oxygen Plasma treated PP films: (a) Untreated PP; (b) Treated PP of 60s of 7.2W; (c) Treated PP of 60s of 10.2W; (d) Treated PP of 60s of 29.6W
[[Please give the making in the Figs a, b, c, d]]
In the untreated PP sample, most of the surfaces were fairly even and randomly dispersed all over the surface. After plasma treatment the surface becomes rougher and the annular feature shape transformation could be observed. The topographical changes occurred by the chemical reaction of oxygen plasma. After the oxygen plasma treatment on PP, it was observed that all the peaks are with average size within the area. The cone like structures on the samples for the various treatment times with different power was due to the etching effect and oxidative reactions of oxygen plasma which was applied. Additionally, the change in surface roughness and the surface topography can be enumerated by the surface parameter, mainly the Root mean square roughness value (RMS) [10]. The roughness was increased because of the degradation and surface modification of polymer and the crystalline regions will remain intact, while the amorphous regions are etched away after the plasma treatment [11].

Fig 2: RMS values of Plasma treated PP samples with various Power rate and treatment time
The RMS values for all the samples with different parameters are shown in Figure 2. The smoother surface was designated by lower rms values. The rms value of the untreated PP sample was 1.49 nm. After the plasma treatment of 60 s in 7.2W this value was increased to 2.705 nm and the roughness value was increased to 3.321 nm for the sample of 60 s in 10.2W. The higher roughness value of 7.26 nm was observed for the sample at a power rate of 29.6 W for 300 s. It clearly indicates that the roughness of PP increases with increasing power rate as well as the treatment time. The surface morphologies became more complicated with the duration of oxygen plasma treatment [12]. It can be seen that the plasma treated PP samples show higher RMS values compared to that of the untreated PP. From the results, the oxygen plasma sturdily influence on the PP surface by removing the top layer of the PP surface. This occurrence may relate with the physical or chemical removal of molecules, chain scission and degradation process [13]. The oxygen plasma treatment eliminates the few layers on the surface of the polypropylene film and it increases the surface roughness and also it improved the wettability and bonding strength. The adhesion sturdily depends on the polar functional groups attached to the PP surface [14]. The oxygen plasma mainly etches the amorphous region than the crystalline and it may be due to the higher energy bonding. After the plasma treatment the amorphous regions are etched which leads to rougher surface [15]. The etching of the surface on the polymer films is also due to the bombardment of energetic particles such as electrons, ions, radicals, neutrals, excited atoms and UV-vis radiations [8].
Therefore, the oxygen plasma treatment could chemically and physically modify the outermost surface of the PP film which results in increase in surface roughness and changes in surface morphology.
Chemical composition of the oxygen plasma treated PP surface
The surface chemical modification induced by the oxygen plasma was characterized by ATR – FTIR [29, 30]. Figure 3 shows the ATR – FTIR spectra of the untreated and the oxygen plasma treated PP films of 60 s for 6.2 W, 10.2 W and 29.6 W. It clearly indicates the evolution of the polar groups on the film surface resulting from the interaction between PP surface and the oxygen plasma. These polar groups mainly consists of carboxyl, carbonyl and hydroxyl groups [24-26]. The hydrophilic nature of the PP surface increases because of the strong contribution of polar groups. By equalizing the height of the transmittance at 2922 cm-1 the spectra are normalized and it represents the CH2 group in the main polypropylene chain. The FTIR spectrum shows a peak in the range of 3000 – 2800 cm-1. The CH3 asymmetric and symmetric stretching vibrations can be attributed to the peaks at 2956 and 2872 cm-1. By CH2 asymmetric and symmetric stretching vibrations, the peaks are at 2920 and 2842 cm-1.

Fig 3: ATR – FTIR spectrum of Untreated and Oxygen plasma treated PP samples
The two peaks at 1459 and 1377 cm-1 are also shown in the ATR – FTIR spectrum. The peak at 1459 cm-1 are caused by CH2 scissor vibrations or by CH3 asymmetric vibrations. From the ATR – FTIR spectrum, in the range of 1200 – 750 cm-1, the untreated polypropylene film shows several small peaks. The peak at 1168 cm-1 can be attributed to C – C asymmetric stretching. The peak at 996 cm-1 is due to CH3 asymmetric rocking vibrations. The CH3 asymmetric rocking and C – C asymmetric stretching vibrations is due to the peak at 972 cm-1. The peak at 902 cm-1 is due to the C – C asymmetric and symmetric stretching. The CH2 rocking vibrations are due to the peaks at 842 and 809 cm-1 [27].
[[Where is the discussion and observation for the generation of polar groups like hydroxyl, carboxyl etc. due to oxygen plasma treatment? It is a crucial evidence which is required because whole of the investigation and its justification are based on this observation.]]
Contact angle studies
The wettability of a surface is determined by the contact angle (º). In a flat surface, if the affinity of a liquid drop increases, the contact angle decreases. The poor wetting is determined by the high contact angle. A water wettable surface may indicate its hydrophilic property [16]. To understand the wetting properties of PP film, the value of contact angle was measured for both untreated and treated PP samples. The contact angle above 90º corresponds to hydrophobic surfaces. Lower the contact angle, more is the hydrophilic surface [17]. The contact angles were measured immediately after the oxygen plasma treatment on PP with 7.2 W, 10.2 W and 29.6 W and exposure time of 60 s - 300 s. In the figure 4, it represents the water contact angle versus the time of oxygen plasma treatment with different RF Power settings on Polypropylene films. The contact angle for the untreated PP film was 74.56 º. In 60 s of 7.2 W, the water contact angle was greatly decreased from 74.56 º to 60º. A significant decrease in water contact angle was observed after the oxygen plasma treatment in PP film. The value of the water contact angle was still decreased to 58.20º when the time was 120 s. The contact angle was shifted lowery when the treatment time was increased. This inferred that, if the exposure time is increased, the value of contact angle was decreases [16]. The contact angle depends on both treatment time and discharge power. This result signifies that, higher the plasma power, more noticeable decrease of contact angle value was observed. This may be due to the presence of oxygen functional groups on the surface of the plasma treated sample and it becomes hydrophobic to hydrophilic.

Fig 4: Contact angle values for Untreated and Treated PP Samples
These functional groups are non-polar in nature and hence increases the surface energy of PP resulting in the decrease of contact angle value. The contact angle was further decreased with increase in time. The contact angle value of the samples treated at 10.2 W and 29.6 W at various intervals of time, indicates the etching and degradation occurred in the PP sample. The contact angle was greatly decreased when the plasma power was increased [18]. The decreasing of contact angle value proves that the hydrophilicity of the PP film increases by the oxygen plasma treatment. It may by the appearance of polar functional groups on the PP film surface after the oxygen plasma treatment [16] and the polymer crystallinity may also be affected [19]. The contact angle was decreased with increased plasma power and treatment time. It was observed that higher the power of the plasma treatment and time, lower the contact angle.
Studies on Mechanical Property
The effects of oxygen plasma treatment on tensile strength of the PP samples were carried out. The Figure 5 represents the tensile strength of the untreated and the oxygen plasma treated PP samples with different times and with different RF Power settings. The tensile strength of the PP samples totally depends upon the plasma exposure time and plasma power rate.

Fig 5: Tensile Strength of Untreated and Treated PP samples
As compared to the untreated polymers, the tensile strength of the oxygen plasma treated polymers were decreased. Moreover, higher tensile strength reduction was observed for longer treatment times [20]. The tensile strength of the untreated PP sample was 6 MPa. After the plasma treatment of 60 s, the tensile strength of the oxygen plasma treated PP sample was 4.680 MPa. It was greatly decreased when compared to the untreated sample. When the time of the plasma treatment increases, the tensile strength of the PP film considerably decreased. All the values of the oxygen plasma treated samples at 7.2 W were lower than the untreated one. The mechanical properties of the polymeric films are influenced mainly by the plasma power and not by the treatment time [16]. After the RF power setting was increased to 10.2 W there was a notable decrease in tensile strength. In 60 s of 10.2 W the tensile strength was 3.480 MPa. It observed that, the Tensile strength decreases, when the time increases. Even in the 29.6 W power rate, the tensile strength for all PP samples was decreased rapidly. The minimum value was obtained of 2.2 MPa in 29.6 W of 300 s. It was observed that, when there is increase in power, the rate of etching increases. Because of this, there was a decrease in tensile strength. The detraction in the tensile strength was due to the plasma etching. Therefore the oxygen plasma treatment could alter the mechanical properties of the PP film with the change of plasma treatment time and plasma energy.
Effect of plasma treatment on Oxygen Transmission Rate
In a packaging material, oxygen permeability is one of the most important property for deciding its suitability for various applications [21]. OTR values were measured for both untreated and treated PP samples and the values were represented in the Figure 6 and also in the Table 1. The average OTR value for the untreated PP sample was 1851.2 cc/m2/24hrs. There was a slight decrease in OTR in the treated PP sample with 7.2 W of 60 s. It was observed that no significant differences in the values.

Fig 6: Oxygen Transmission Rate of Untreated and Treated PP samples
After the RF power increased to 10.2 W, there was a major difference in the OTR values with the untreated and the treated PP samples of 7.2 W and the value was noted as 1891.65 cc/m2/24hrs for 60 s. The value was further increased when the exposure time increases. There was no doubt, the value was increased more in all the exposure time in 29.6 W.
The maximum value of 2248.92 cc/m2/24hrs was observed in 29.6 W of 300 s. It was observed that the OTR increased with increase of energy and exposure time. The gas permeability in polymers is the combined effect of diffusion and solubility via voids of gaps present in between the segments of a polymer chain. The results are most important and showed that, if the plasma exposure time and power rate increases, the value of OTR also increases.
Effect of plasma treatment on Watervapour Transmission Rate
The WVTR measurements allow determining the influence of the surface treatment on the transport properties such as permeability and diffusivity of water through the film.

Fig 7: Watervapour Transmission Rate of Untreated and Treated PP samples
The water permeability of the polymer film depends on the solubility parameter linked to the affinity between the polymer material and permeant and on the water diffusivity, related mainly to the structure and particularly to the density connected to the degree of crystallinity and to the stiffness of the film [22]. The effect of WVTR in untreated and plasma treated PP samples was represented in the Figure 7. The changes were observed the WVTR was increased after the plasma treatment in various power settings and time. The WVTR for the untreated PP sample was 9.6 g/m2/24 hrs. At 7.2 W of 60 s, the value was observed as 9.62 g/m2/24 hrs. Consistently, the value was increased slightly in other exposure times of 7.2W. The oxygen plasma treatment strongly increases the water vapor permeability for 10.2 W and 29.6 W. The highest permeability of 14.24 g/m2/24 hrs was obtained at 300 s of 29.6 W. This is because of the polymer structure degradation. The Watervapour permeation is the polymers are governed by vapor pressures and concentration gradients [23]. For the water absorption and water vapor permeability different process are responsible although it involves the penetration of water molecules through a polymer. From the results, it was observed that the water vapor permeability was considerably increased, if the power rate and exposure time increased.
Conclusion
In this study, the PP films were used for surface modification by oxygen plasma treatment with various treatment times and with different plasma energy. The oxygen plasma treatment generated the oxygen containing polar functional groups on the film surface and modifies the surface chemical composition which was confirmed by the FTIR spectroscopy. The film surface degradation after plasma treatment causes a change in surface topography, which results in the increase of surface roughness and it can be noticed by AFM analysis. The plasma treatment also increases the surface wettability and enhanced the hydrophilicity of the PP film and it causes a decrease in the contact angle values that can be attributed to the different mechanisms. The oxygen plasma treatment conditions were adopted to provide structural changes in the outermost layer of the polymer which induced significant changes in mechanical and barrier properties. When the plasma treatment time and the plasma energy increase, the mechanical properties were reduced, and the gas and water-vapour transmission rate was increased due to the etching or degradation of polymers. From the practical point of view, the plasma treatment time and the plasma energy were found to be the most important role to decide the property of the surface modified polymers.
Acknowledgements
The authors would like to thank Dr. Ashis Kumar Sen and Mr. Sajeesh Parayil, Department of Mechanical Engineering, IIT Madras for providing the Harrick plasma facility to do the Oxygen plasma treatment. The authors thank Dr.M.V.Panchagnula and Mr. Nachiketa Janardhan, IIT Madras for extending their support for contact angle measurements. We also acknowledge Mr. S.M.Suresh Kumar and Mr. P.Prabhunathan for FTIR characterization and also the Crystal growth Centre, Anna University for the AFM characterization.
References
1. S.K. Pankaj, C. Bueno-Ferrer, N.N. Misra, L. O’Neill, Alfonso Jim´enez, Paula Bourke and P.J. Cullen, Innovative Food Sci. Emerg. Technol. 21 (2013)107 – 113
2. Gye hwa shin, Yeon Hee lee, Jin sil lee, Young soo Kim, Won seok choi and hyun Jin park, J. Agric. Food. Chem. 50 (2002) 4608-4614
3. Allan S. Hoffman, Chin. j. Polym. Sci, 13 (1995) 3
4. Alenka Vesel, Miran Mozetic, Vacuum, (2012) 86
5. Q.F. Wei, W.D. GAO, D.Y. Hou and X.Q. Wang, Appl. Surf. Sci. 245 (2005) 16–20
6. Chaozong Liua, Naiyi Cuib, Norman M.D. Brown and Brian J. Meenan, Surf. Coat. Technol. 185 (2005) 311 – 320
7. Claudia Riccardi, Ruggero Barni, Elena Selli, Giovanni Mazzone, Maria Rosaria Massafra, Bruno Marcandalli and Giulio Poletti, Appl. Surf. Sci. 211 (2003) 386–397
8. Yang, L, Chen, J, Guo, Y and Zhang Z, Appl. Surf. Sci. 255 (2009) 4446–4451
9. Alan P. Kauling , Gabriel V. Soares , Carlos A. Figueroa , Ricardo V.B. de Oliveira , Israel J.R. Baumvol , Cristiano Giacomelli and Leonardo Miotti, Mater. Sci. Eng C. 29 (2009) 363–366
10. Sanchis.M.R, V. Blanes, M. Blanes, D. Garcia and R. Balart, Eur. Polym. J. 42 (2006) 1558–1568
11. T. Jacobs, R. Morent, N. De Geyter, T. Desmet, P. Dubruel and C. Leys, Surf. Coat. Technol. 205 (2010) 2256–2261
12. Chengshuang Zhang, Ping Chen, Baolei Sun, Wei Li, Baichen Wang and Jing Wang, Appl. Surf. Sci, 254 (2008) 5776–5780
13. K. Navaneetha Pandiyaraj, V. Selvarajan, R.R. Deshmukh and Changyou Gao, Appl.Surf. Sci. 255 (2009) 3965–3971
14. Vijayalakshmi.K.A, M.Mekala, C. P. Yoganand and K. Navaneetha Pandiyaraj, Int. J. Polym. Sci. (2011)
15. Khalaf Ibrahim Khaleel, Awatif Sabir Jasim, Mohamad Abdul Kareem Ahmed , Y. K. Vijay and Subodh Srivastava, Int. J. Rec. Res. Rev. 3 (2012) 9
16. Siriporn Theapsak, Anyarat Watthanaphanit and Ratana Rujiravanit, ACS Appl. Mater. Inter. 4 (2012) 2474−2482
17. Labay.C, Canal.C and Garcia – Celma.M.J, Plasma Chem Plasma Process, 30 (2010) 885 – 896
18. Sutida Paisoonsin, Orathai Pornsunthorntawee and Ratana Rujiravanit, Appl. Surf. Sci. 273 (2013) 824– 835
19. Kim K. S, Ryu C. M, Park C. S, Sur G. S and Park C.E, Polymer, 44 (2003) 6287–6295
20. Kiran H Kale and A. N Desai, Ind. J. Fib. Tex. Res. 36 (2011) 289-299
21. Misra, N. Tiwari, B. Raghavarao and K P. Cullen, Food Eng. Rev, 3 (2011) 1–12
22. Nadine Tenn, Nadège Follain, Kateryna Fatyeyeva, Jean-Marc Valleton, Fabienne Poncin-Epaillard and Nicolas Delpouve, Stéphane Marais, J. Phys. Chem. C 116, (2012) 12599−12612
23. Chaiwong.C, P. Rachtanapun, P. Wongchaiya, R. Auras and D. Boonyawan, Surf. Coat. Tech. 204 (2012) 2933–2939
24. H. Drnovska, L.Lapcik, V. Bursikova, J. Zemek, AM and Barros- Timmons, Colloid. Polym. Sci. 281 (2003) 1025–33
25. M. Lehocky, H. Drnovska, B. Lapcikova, AM. Barros-Timmons, T. Trindade and M. Zembala, Colloid Surf A-Physicochem. Eng. Asp. 222 (2003) 125–31
26. AB. Ortiz-Maga´n, MM. Pastor-Blas, TP Ferra´ndiz-Go´mez, C. Morant-Zacare´s and JM. Martı´n-Martı´nez, Plasmas Polym. 6, (2001) 81–105
27. Socrates G. Infrared and Raman Characteristic Group Frequencies – Tables and Charts (3rd edn). John Wiley & Sons: West Sussex, 2001

28. Dibyendu S. Bag, Santi N. Ghosh and Sukumar Maiti, European Polym. J. 34 (1998) 855-861
29. Dibyendu S. Bag, V. Pradeep Kumar and Sukumar Maiti, J. Appl. Polym. Sci. 71 (1999) 1041-1048
30. Dibyendu S. Bag, V. Pradeep Kumar and Sukumar Maiti, Angew. Makromol. Chemie, 249 (1997) 33-46