REGULAR CONTRIBUTED ARTICLES S. Ramesh 1 *, A. K. Arof 2 1 Faculty of Engineering & Science, Universiti Tunku Abdul Rahman, Kuala Lumpur, Malaysia 2 Physics Department, University of Malaysia, Kuala Lumpur, Malaysia The Evaluation of Miscibility of Poly(vinyl Chloride) and Poly(ethylene Oxide) Blends by DSC, Refractive Index and XRD Analyses The miscibility of poly(vinyl chloride) (PVC) and poly(ethylene oxide) (PEO) blends was investigated by using differential scanning calorimetry (DSC), refractive index measurements and x-ray diffraction (XRD) analysis. The negative Flory-Huggins interaction parameter, v 12 from the melting point depression of PEO by the blending of PVC shows that the interaction between PVC/PEO occurs. A refractive index measurement was performed to show that the miscibility of PVC/PEO blends occurs not only in solid form but also in solution form. The linear variation with composition of the refractive index of the polymer blend solutions in THF at 28 8C, 40 8C and 50 8C indicates the miscibility of PVC with PEO. The shift in peaks in XRD of PEO by the blending of PVC is a characteristic evidence of complexation between PVC and PEO. 1 Introduction By definition, any physical mixture of two or more different polymers or copolymers that are not linked by covalent bonds is a polymer blend. Polymer blends are gaining practical importance and scientific interest, as a result of the current emphasis on modifying existing synthesized polymers, rather than manufacturing new ones (Linares and Acosta, 1997). Polymer blends often exhibit more desirable characteristics than individual homopolymers (Uno et al., 2008). A polymer blend may be obtained by several methods like melt-mixing, co-precipitation and kcasting (Crispim et al., 1999). A polymer blend is used as a matrix polymer in order to attain high ionic conductivity (Kim and Sun, 2001; Anand et al., 1999; Morita et al., 1996). The incorporation of poly (ethylene glycol) (PEG) into PEO-LiCF 3 SO 3 system gives rise to a maximum conductivity value of 3 10 3 Scm 1 at 25 8C (Ito et al., 1987). Poly(ethylene oxide) (PEO)/Polyethylenimine (PEI) blends were more conductive than the pure PEI systems (Tanaka et al., 2001). The conductivities of the boroxine polymers (BP)/(PEO) blends are enhanced over pure PEO systems (Yang * Mail address: Ramesh T. Submramaniam, Faculty of Engineering & Science, Universiti Tunku Abdul Rahman, Setapak, 53300 Kuala Lumpur, Malaysia E-mail: ramesh@utar.edu.my et al., 2001). The PEO/polyacrylonitrile (PAN) blend film is more likely to be a gel electrolyte than a plasticized PEO-salt electrolyte (Choi et al., 2000). The conductivity enhancement can be attributed to the increase in the amorphous regions responsible for the ionic conduction. The miscibility between the constituents of a polymer mixture is an important factor in the development of new materials based on polymer blends (Neiro et al., 2000; MacCallum and Smith, 2000; Koh et al., 1998). The basic rule governing miscibility is negative free energy of mixing (Brown et al., 1994). Polymer polymer miscibility, at the limit of high molecular weights, generally requires an exothermic heat of mixing because the entropy of mixing is very small. Two polymers may contain units capable of some type of specific interaction and/or have repulsive intramolecular interactions between groups that promote exothermic mixing (Neiro et al., 2000; Pielichowski and Hamerton, 2000). There are various methods in determination of the miscibility of polymers such as thermal analysis, electron microscopy, viscosimetry, inverse gas chromatography etc (Hourston et al., 1997; Haiyang et al., 2000). In this study, the miscibility of polyvinyl chloride (PVC)/ PEO blends was investigated by several techniques. DSC measurements were carried out to analyze the thermal properties of blends of different compositions. Refractive index measurement has been used as a quantitative method to study the miscibility of polymer blends. XRD method can be used to show the complexation that takes place between polymer blends. 2 Experimental PVC (Fluka) and PEO (Aldrich) were used as received for blend preparation. The blends of PVC/PEO with different weight ratios have been prepared by mixing with tetrahydrofuran (THF) that was obtained from J. T. Baker. The DSC measurements were carried out with a Mettler Toledo DSC 821 calorimeter. The samples were cut from films that were prepared by casting from THF solution at ambient temperature. The samples were heated from room temperature to 3508C at a heating rate of 108C min 1 in a nitrogen atmosphere. In this work, the refractive indexes of the blended solutions (PVC/PEO with THF as solvent) have been measured 354 Ó Carl Hanser Verlag, Munich Intern. Polymer Processing XXIV (2009) 4
with an Abbe s refractometer and a thermostated water circulation system at 28 8C, 40 8C and 50 8C. X-ray diffraction analysis was performed using a Shimadzu XD-5 Diffractometer using Cu K radiation of wavelength k = 1.5418 Å in the 2h range between 108 and 808. The samples were scanned using slit width 0.05 mm and receiving slit 0.2 mm. 3 Results and Discussion 3.1 Differential Scanning Calorimetry (DSC) Differential Scanning Calorimetry (DSC) is used to study the miscibility of polymer blends obtained through several methods, including those cast. The depression of the melting point, T M of the crystalline polymer is an important characteristic to demonstrate the miscibility of a semi-crystalline/amorphous polymer blend system. The DSC thermogram of pure PEO is shown in Fig. 1. For pure PEO, the main melting peak, T M1 was observed at 71.48C. There is also a secondary melting peak, T M2 at 45.68C. Marentette et al. (1998) reported that the T M1 value for PEO varies from 69 to 78 8C. Fig. 2 and Fig. 3 show the DSC thermograms of PVC/PEO blends. Since T M1 is the crucial parameter for miscibility studies, the DSC results of PVC/PEO blends in this section will emphasize on the main melting point only. The melting depression in PVC/PEO blends indicates that the blends are miscible. The T M depression of PEO with the increase of PVC content can be observed from the DSC thermograms. The melting point depression of a polymer in a miscible blend can be used to determine the Flory-Huggins interaction parameter, v 12 (Marentette and Brown, 1998; Huang and Goh, 2002). The Flory-Huggins interaction parameter, v 12 is determined using the Nishi-Wang equation: 1 T o m 1 T 0 ¼ RV 2 v m;bl DH u V 12 u 2 ; 1 Fig. 1. DSC thermogram of pure PEO where T o m and To m;bl are melting temperatures of the polymer in the pure state and in the blend, respectively, R is the gas constant and u is the volume fraction of PVC in the blend. V i is the molar volume of polymer repeating unit which is defined as atomic weight per density (V 1 = 46.1 cm 3 mol 1 for PVC; V 2 = 40.5 cm 3 mol 1 for PEO). DH u is the enthalpy for 100% crystalline PEO as 190 J g 1 (Neiro et al., 2000). The melting point temperatures used should be equilibrium temperatures. However, Nishi and Wang (1975) concluded that Fig. 2. DSC thermogram of PVC/PEO (30:70) Fig. 3. DSC thermogram of PVC/PEO (40:60) Intern. Polymer Processing XXIV (2009) 4 355
the absence of equilibrium had only small influence on the melting point depression. Consequently, a plot of the left-hand side of the above equation as a function of u 2 pvcshould be linear with v 12 be extracted from the slope. The application of the Nishi-Wang equation assumes that the entropic contributions are practically negligible and that v 12 is independent of the composition of the blend (Huang and Goh, 2002). The calculated v 12 value for the PVC/PEO blend is 0.12. Neiro et al. (2000) and Huang and Goh (2002) also found negative Flory-Huggins interaction parameter, v 12 from the melting point depression results that demonstrate the interaction between polymers. The negative v 12 value in the present study shows that PVC/PEO blends are miscible and thermodynamically stable in the melt (Neiro et al., 2000). 3.2 Refractive Index Measurements In order to confirm the results obtained from the previous method on the miscibility of polymer blend under study, the variations of the refractive index of the polymer blend solutions has been measured. When light photons are transmitted through a material, they lose some of their energy, and as a result, the speed of light is reduced and the beam of light changes direction. The relative velocity of light passing through a medium is expressed by the optical property called the index of refraction (Smith, 1993). The refractive index value, n of a medium is defined as the ratio of the velocity of light in vacuum, c, to the velocity of light in the medium considered, v: n ¼ c v : The variations of the refractive index with composition of the PVC/PEO blend solutions in THF at different temperatures are depicted in Figs. 4 to 6. From these results, it can be observed that the value of refractive index for the composition decreases with temperature. For example, in PVC/PEO (50 : 50) blend, the values of refractive index are 1.4051, 1.4018 and 1.3940 at 288C, 408C and 508C respectively. At higher temperatures, an increase in light velocity of polymer material is observed, thus explaining the decrease in refractive index value. Fig. 4. The variation of refractive index with composition of PVC/ PEO blends in THF at 288C Fig. 5. The variation of refractive index with composition of PVC/ PEO blends in THF at 408C Fig. 6. The variation of refractive index with composition of PVC/ PEO blends in THF at 508C From these figures, it is also clearly evident that the variation is linear, showing a single phase in the blends at 288C, 408C and 508C. This study confirms that PVC/PEO blends are not only miscible at room temperature, but also at higher temperatures. Therefore, refractive index technique can be used for rapid investigation of miscibility of the blends (Rajulu et al., 1999). 3.3 X-ray Diffraction Analysis (XRD) The structure of polymer electrolytes is related to ionic conductivity. It has been proposed that in polymer electrolyte systems, ionic conductivity takes place in amorphous regions (Ramesh and Arof, 2001). X-ray diffraction (XRD) measurements were conducted for PVC blend based polymer electrolytes to examine the nature of crystallinity and to investigate the occurrence of complexation. X-ray diffractograms of pure PVC, pure PEO and PVC/ PEO (70/30) blend are compared in Fig. 7. PEO is a crystalline material and the Bragg reflection angles are in agreement with those reported by Kim and Kim (1999). The X-ray diffractogram of pure PEO shows two prominent peaks at 2h = 19.58, 23.88. These peaks are shifted to 2h = 19.18, 23.58 in the PVC/PEO (70/30) blend. This behavior demonstrates that complexation has occurred between PVC and PEO. There is a decrease in the relative intensity of the apparent peaks appearing at 2h = 19.18, 23.58 in PVC/PEO (70/30) blend. The intensity of the sharp peak at 2h = 23.88 for pure 356 Intern. Polymer Processing XXIV (2009) 4
4 Conclusion Fig. 7. XRD patterns for (a) pure PVC, (b) pure PEO and (c) PVC/ PEO (70/30) blend PEO reduced about 67% in PVC/PEO (70/30) blend. The decrease in intensity of XRD reflection peaks of PEO by the blending of PVC is mainly due to the dilution effect of PVC (Jacob et al., 1997). 3.4 Relation between Polymer Structure and Miscibility The linear variation in refractive index with composition, the melting depression and negative v 12 values observed are connected with the donor-acceptor interaction between the chlorine atoms of PVC, as weak acceptor species and oxygen atoms of PEO as donor species. Electrostatic interactions have also been considered to explain the miscibility in polymer blends. In miscible blends containing PVC, the chlorine atoms interact with atoms of another polymer that have non-bonded electrons, like oxygen or nitrogen. Polymers containing proton donor groups are found to be miscible with those containing proton acceptor groups. This is due to the presence of a specific interaction like hydrogen bonding. PVC is a weak proton-donating polymer; its -hydrogen atoms can form hydrogen bonding with proton accepting polymers, such as PEO. The above considerations could be utilized to explain the miscibility of PVC with PEO. We performed the current studies to correlate the miscibility of PVC/PEO blend in solid state by using DSC and in solution by using refractive index methods. XRD reveals that PVC disrupts the crystalline nature of PEO in PVC/PEO blend. The shift in peaks in XRD is a characteristic evidence of complexation between PVC and PEO. These analyses concluded that the PVC/ PEO blends are miscible. These analyses concluded that the PVC/PEO blends are miscible. References Anand, J., et al., Spectral, Thermal and Electrical Properties of Poly(o- and m-toluidine)-pvc Blends Prepared by Solution Blending, Eur. Polym. J., 35, 499 507 (1999) Brown, J. L., et al.: Chemistry The Central Science, 6 th Edition, Prentice-Hall, USA (1994) Choi, B. K., et al., Ionic Conduction in PEO-PAN Blend Polymer Electrolytes, Electrochim. Acta., 45, 1371 1374 (2000) Crispim, E. G., et al., Solvent Effects on the Miscibility of Poly- (methyl methacrylate)/poly(vinyl acetate) Blends: I: Using Differential Scanning Calorimetry and Viscometry Technique, Polymer, 40, 5129 5135 (1999) Haiyang, Y., et al., Viscometric Investigations on the Intrinsic Viscosity of PMMA affected by Polymer-Polymer Interactions in Binary System, Eur. Polym. J., 36, 949 956 (2000) Hourston, D. J., et al., Modulated Differential Scanning Calorimetry- VII: Interfacial Macromolecular Diffusion in Core-shell Latex Particles, Thermochim. Acta, 294, 23 31 (1997) Huang, X. D., Goh, S. H., Miscibility of C 60 -end-capped PEO with PVC, Polymer, 43, 1417 1421 (2002) Ito, Y., et al., Ionic Conductivity of Electrolytes Formed from PEO- LiCF 3 SO 3 Complex with Low Molecular Weight Poly(ethylene glycol), J. Mater. Sci., 22, 1845 1849 (1987) Jacob, M. M. E., et al., Effect of PEO Addition on the Electrolytic and Thermal Properties of PVDF-LiClO 4 Polymer Electrolytes, Solid State Ionics, 104, 267 276 (1997) Kim, D. W., Sun, Y. K., Electrochemical Characterization of Gel Polymer Electrolytes Prepared with Porous Membranes, J. Power Sources, 102, 41 45 (2001) Kim, J. Y., Kim, S. H., Ionic Conduction Behaviour in Network Polymer Electrolytes Based on Phosphate and Polyether Copolymer, Solid State Ionics, 124, 91 99 (1999) Koh, K. A., et al., Miscibility of Tetramethyl Polycarbonate With Syndiotactic Polystyrene, Eur. Polym. J., 34, 1229 1231 (1998) Linares, A., Acosta, J. L., Tensile and Dynamic Mechanical Behavior of Polymer Blends on PVDF, Eur. Polym. J., 33, 467 473 (1997) MacCallum, J. R., Smith, J. S. G., A Novel Method for Producing Miscible Polymer Blends, Eur. Polym. J., 36, 491 494 (2000) Marentette, J. M., Brown, G. R., The Crystallization of PEO in Blends with Neat and Plasticized PVC, Polymer, 39, 1415 1427 (1998) Morita, M., et al., Effects of Crown Ethers on the Electrochemical Properties of Polymeric Solid Electrolytes Consisting of Poly(ethylene oxide)-grafted Poly(methyl methacrylates), Solid State Ionics, 86 88, 401 405 (1996) Neiro, S. M. S., et al., Miscibility of PVC/PEO Blends by Viscosimetric, Microscopic and Thermal Analyses, Eur. Polym. J., 36, 583 589 (2000) Nishi, T., Wang, T. T., Melting Point Depression and Kinetic Effects of Cooling on Crystallization in PVDF-PMMA Mixtures, Macromolecules, 8, 909 915 (1975) Pielichowski, K., Hamerton, I., Compatible Poly(vinyl chloride)/ chlorinated Polyurethane Blends: Thermal Characteristics, Eur. Polym. J., 36, 171 181 (2000) Rajulu, A. V., et al., Miscibility of PVC/PMMA Blend by the Ultrasonic and Refractive Index Method, Eur. Polym. J., 35, 1183 1186 (1999) Intern. Polymer Processing XXIV (2009) 4 357
Ramesh, S., Arof, A. K., Structural, Thermal and Electrochemical Cell Characteristic of Poly(vinyl chloride) Based Polymer Electrolytes, J. Power Sources, 99, 41 47 (2001) Smith, W. F.: Foundation of Material Science and Engineering, 2 nd Edition. MacGraw-Hill, Singapore (1993) Tanaka, R., et al., Lithium Ion Conductivity in Polyoxyethylene/Polyethylenimine Blends, Electrochim. Acta, 46, 1709 1715 (2001) Uno, T., et al., Ionic Conductivity and Thermal Property of Solid Hybrid Polymer Electrolyte Composed of Oligo(ethylene oxide) Unit and Butyrolactone Unit, J. Power Sources, 178, 716 722 (2008) Yang, Y., et al., Blended Lithium Ion Conducting Polymer Electrolytes Based on Boroxine Polymers, Solid State Ionics, 140, 353 359 (2001) Date received: February 27, 2009 Date accepted: June 24, 2009 Bibliography DOI 10.3139/217.2275 Intern. Polymer Processing XXIV (2009) 4; page 354 358 ª Carl Hanser Verlag GmbH & Co. KG ISSN 0930-777X You will find the article and additional material by entering the document number IPP2275 on our website at www.polymer-process.com 358 Intern. Polymer Processing XXIV (2009) 4