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Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
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Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior M. Sachin, Reshma Haridass, B.T.S. Ramanujam ⇑ Department of Sciences, Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Coimbatore 641112, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 29 December 2019 Received in revised form 13 January 2020 Accepted 17 January 2020 Available online xxxx Keywords: Polymer nanocomposites Electrical conductivity Electroactive phase Structure development Thermal properties
a b s t r a c t Polyvinylidene fluoride (PVDF)-20 wt% poly (methyl methacrylate) blend has been synthesized by solution blending method. Expanded graphite has been synthesized by microwave irradiation of commercial expandable graphite. With varying concentration of expanded graphite (ExGr) in PVDF-20 wt% PMMA, series of blends are synthesized. The electrical percolation threshold has been observed to be less than 1 wt% ExGr which is due to the formation of graphite nanosheets. The conducting blend system exhibits typical percolation behavior. The thermogravimetric analysis shows two step degradation in PVDF-20 wt % PMMA blend system. This is due to the individual degradation of PMMA and PVDF main chains respectively. The thermal stability of the blend system with 1 wt% ExGr incorporation in PVDF-20 wt% PMMA is increased by 11 °C corresponding to main chain degradation of PVDF. The result proves existence of strong interaction between ExGr particles and the polymer. The DSC analysis shows very clearly the existence of electroactive gamma phase both in PVDF-20 wt% PMMA with and without ExGr particles as there is an additional high temperature melting peak apart from that of the melting peak at 167 °C. This result is confirmed through XRD also. The blend system is characterized by XRD, TGA, DSC and FTIR. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
1. Introduction Polymer blending is a versatile technique in which the properties of individual polymers are harnessed to achieve cost reduction, improvement in mechanical properties, to improve flexibility of the polymer blend films etc. [1]. The final morphologies of a blend system have immense effect on the end properties which is decided by the miscibility of the blend system. Miscible blends such as PVDF-PMMA are being extensively focused due to the enhanced chemical resistance, toughness etc. along with complete miscibility in the entire concentration range [2]. Polyvinylidene fluoride (PVDF) is a semi crystalline thermoplastic polymer which exists in five different polymorphs such as a, b, c, d and e [3]. Out of five different polymorphs of PVDF, the nonpolar a phase is abundant and most stable. The b, c and d phases are electroactive. Out of the three electroactive phases, b and c phases are extensively observed in various PVDF based systems ⇑ Corresponding author. E-mail address: [email protected] (B.T.S. Ramanujam).
[4]. The b phase has largest spontaneous polarization due to trans-planar zigzag conformation. The net dipole moment of cphase is lesser than that of b-phase. However the generation of electroactive phases by simple means poses great challenge. Extensive reports are there in the literature regarding high voltage poling [5] and uniaxial mechanical stretching at elevated temperature [6] to generate electroactive b-phase. Thus PVDF up on generation of electroactive phases exhibit piezoelectric, ferroelectric and pyroelectric behavior. There are various methods reported in the literature for the generation of electroactive beta phase of PVDF such as electrospinning of PVDF solution in a particular solvent, incorporation of nanofillers such as clay particles [7] or Titania [8] in to the PVDF matrix. PMMA is a transparent amorphous polymer extensively used in ophthalmic application such as making contact lenses. Electrically conducting polymer composites or conducting blends are important class of materials due to their potential applications in fuel cell bipolar plates, EMI shielding devices, sensors and actuators etc. [8]. In order to tailor make electrical conductivity, conducting fillers such as graphite are incorporated into the
https://doi.org/10.1016/j.matpr.2020.01.353 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353
2
M. Sachin et al. / Materials Today: Proceedings xxx (xxxx) xxx
insulating polymers. The concentration of fillers at which the electrical conductivity shoots up to many orders is known as electrical percolation threshold. It is important to reduce the electrical percolation threshold as minimum as possible so that the processing of conducting composites or blends become easy. In this regard, conducting nanofillers such as graphene [9] CNT [10] etc. are incorporated into the insulating polymer matrix. Pawde et al. [11] have investigated the structural, thermal, optical properties of PVDF-PMMA blend prepared by melt compounding followed by solution casting technique. Liu et al. [12] have investigated graphite oxide driven miscibility of PVDF/PMMA blends through rheological measurements. Incorporation of graphite oxide enlarges the window of miscibility of the blends as the authors proved that graphite oxide acts as entanglement points. Martins et al. [13] have reported the electrical percolation threshold in PVDF-MWCNT composites to be 1.2 wt%. Li et al. [14] have investigated the ferroelectric phase diagram of PVDF/ PMMA blend. Chiu et al. [4] have investigated in detail the effect of graphite nanoplatelet incorporation into PVDF and PVDF/PMMA blend with regard to crystallization and electrical percolation. They have reported that the ternary composite exhibit lower electrical resistivity compared to that of binary composites. After going through the literature, hardly any systematic study of the electrical properties, origin of electroactive phase and the associated thermal characterization of sonicated expanded graphite incorporated PVDF/PMMA blends is reported. Hence in this work starting from the synthesis of expanded graphite, the structure development and electrical and thermal properties of PVDF/PMMA/ExGr ternary system are reported. 2. Experimental 2.1. Materials PVDF (Grade Solef 1006-MFI-30–40 g/10 min at 230 °C/2.16 Kg) has been procured from Solvay Solexis, France. Expandable graphite (grade 3772) has been kindly provided by Asbury Carbons Inc., USA. PMMA-injection grade (IH830C-MFI-2.0 g/10 min by ISO1133 method) has been procured from L.G. MMA Corp. South Korea. N, N-Dimethylacetamide (DMAc) (AR grade) has been procured from S.D-fine chemicals, India.
further probe sonicated together for 30 min and then poured in a Petri dish and heated at 60 °C till all the solvent molecules are evaporated. Thus the uniform film formed has been used for further characterization. The amount of expanded graphite according to the calculation is incorporated in to the blend and series of conducting blend films are made. 2.4. Measurement of electrical resistance and thermal properties The electrical resistance of series of blend film has been measured using source meter model 2450 from Keithley instruments. A specially constructed brass cell is used for holding the film intact in order to eliminate contact resistance. The electrical resistance has been measured for three times for each sample and the average is used for conductivity calculation. Thermogravimetric analyzer (Model: SDT Q600) from TA instruments has been used for studying the thermal stability of blend systems. The heating rate of 10 °C per minute under the nitrogen atmosphere has been maintained. The melting and crystallization behavior of PVDF-PMMA blend system has been studied through differential scanning calorimetry under nitrogen atmosphere with 10 °C to be the heating and cooling rates using the model Q20 from TA instruments, USA. The data reported corresponds to heating and cooling cycle in the second cycle as first cycle is used to destroy the sample’s previous history. 2.5. Structural characterization and FTIR analysis The structure development in polymer blends has been understood through X-ray diffraction patterns obtained for various compositions of blend films using X’Pert PRO model from Panalytical instruments. The confirmation of electroactive phases has been done through FTIR analysis under ATR mode for the blend system. The Model Shimadzu Miracle 10 is used for FTIR analysis. 2.6. Scanning electron microscopy analysis The surface morphology of synthesized expanded graphite is analyzed using FESEM (Model: FEI Quanta FEG 200). 3. Results and discussion
2.2. Preparation of expanded graphite
3.1. FESEM analysis of expanded graphite
Expanded graphite has been prepared by microwave irradiation by keeping 0.1–0.2 gm of commercial expandable graphite inside an alumina crucible and kept inside a domestic microwave oven (Model-LGMH2342W-Out put power 800 W) and the period of irradiation of microwave is 10 s. This results in the formation of fluffy expanded graphite which has been probe sonicated in DMAc solvent to produce graphite nanosheets.
Fig. 1a and b depict digital photographs of expandable graphite before and after microwave irradiation. It is clear that after microwave irradiation, more fluffy expanded graphite has been formed. The increase in volume is due to the bursting of acid molecules which are held between the graphite planes in expandable graphite which eventually pushes the planes increasing expansion along c-direction of graphite and hence porous structure is formed. Fig. 1c shows the FESEM picture of microwave irradiated expanded graphite which has more porous structure.
2.3. Synthesis of PVDF/PMMA/ExGr blends Required quantity of PVDF (Keeping total blend amount to be 1 gm) is first dissolved in 8 ml of N-N-dimethylacetamide at 60 °C using mechanical stirrer with hot plate. Similarly PMMA of required quantity according to weight fraction calculation has been dissolved in DMAc separately at 60 °C and mixed with the dissolved PVDF and the mixture was mixed for about an hour. Expanded graphite of required amount in DMAc is probe sonicated using a probe sonicator (ENUP-500 model from LIFE-CARE EQUIPMENTS) for thirty minutes in an intermittent manner to ensure dispersion of graphite nanosheets in the solvent. This dispersed graphite nanosheets are mixed with PVDF/PMMA solution and
3.2. DC conductivity analysis of PVDF-PMMA-ExGr blends Fig. 2 depicts the variation of electrical conductivity of PVDF20 wt% PMMA-x wt% ExGr (x = 0, 0.5, 1, 1.5, 2). A particular blend composition (PVDF-20 wt% PMMA) has been chosen from the point of view of easy formation of film. The electrical conductivity is enhanced more than three orders when the loading of ExGr is changed from 0.5 wt% to 1 wt%. Thus it is clear that electrical percolation threshold for this system lies below 1 wt%. This is due to the fact that the conductive network formation between graphite nanosheets is formed. Below and at 0.5 wt% ExGr there will not
Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353
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Fig. 2. DC electrical conductivity variation of PVDF-20 wt% PMMA- x wt% ExGr (x = 0, 0.5,1, 1.5, 2) blend film.
Fig. 3. TGA analysis of PVDF-20 wt% PMMA and PVDF-20 wt% PMMA-1 wt% ExGr blend film.
[15]. It can be seen that almost 11 °C enhancement in the onset of degradation for PVDF chains due to the incorporation of 1 wt% ExGr particles in to the blend system. Also between 434 °C and 445 °C, the PMMA chains degrade slowly due to ExGr particles. The result clearly proves the interaction between the ExGr particles and the polymer chains.
3.4. XRD analysis
Fig. 1. Digital pictures of (a) expandable graphite (b) expanded graphite (c) FESEM of expanded graphite.
be enough contacts between the filler particles and hence the conductivity lies in the insulating range. The variation of electrical conductivity follows typical percolation behavior. 3.3. Thermogravimetric analysis The thermal stability of PVDF-20 wt% PMMA with and without 1 wt% ExGr incorporation has been studied and depicted in Fig. 3. The blend system exhibits two step degradation. The first step denotes the degradation of PMMA chains and the second step refers to the degradation of PVDF chains as reported elsewhere
The structural changes taking place in the polymer because of the processing method employed are depicted in Fig. 4 as shown below. The solution casted pure PVDF film exhibits XRD peaks at 2 Theta positions 18.5° (0 2 0), 20.3° (110/101) and a broad peak at 39° (2 1 1) which are the characteristic peaks of monoclinic gamma phase of PVDF as reported by Cai et al. [16]. Further it has been reported that 20.1° is also one of the characteristic peak of gamma phase of PVDF apart from 20.3° [17]. Though, incorporation of graphite up to 1 wt% did not shift the peak corresponding to that of gamma phase in our study, higher loading of ExGr results in disappearance of broad peak at 39°. This results show that there can be a switch over of gamma phase of PVDF to beta phase as the peak corresponding to that of gamma phase is broadened which depends on the loading of expanded graphite particles. However the compositional dependence of electroactive phase of PVDF will be the focus of our future work. In the present work, it is proved that the gamma phase of PVDF is formed. The confirma-
Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353
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Fig. 4. XRD analysis PVDF, PVDF-20 wt% PMMA, PVDF-20 wt% PMMA-1 wt% ExGr and PVDF-20 wt% PMMA-2 wt% ExGr blend film.
tion of electroactive phases cannot be done through XRD alone but DSC analysis will give insight about the polymorphs of PVDF. It has been reported that gamma crystals melt at higher temperature than non polar alpha crystals and electroactive beta crystals melt at slightly lower temperature than that of alpha crystals [16]. 3.5. DSC analysis Fig. 5a and b depict the DSC curves of PVDF-20 wt% PMMA and PVDF-20 wt% PMMA-1 wt% ExGr. It is clear from the DSC melting curves that there are two peaks at 167 °C and 173 °C respectively in PVDF-20 wt% PMMA blend. The former corresponds to the melting temperature of alpha crystals and the later is associated with gamma crystals [18]. The total area under the melting curve in Fig. 5a is 32.74 J/g. The result shows that there exists mixture of alpha and gamma phases. Up on incorporation of 1 wt% ExGr in PVDF-20 wt% PMMA, there exists high temperature melting peak corresponding to gamma crystals. However the area under the melting peak is increased to 49.43 J/g. This result suggests that gamma crystals are induced more with ExGr incorporation as the XRD result does not show any enhancement in the alpha phase reflections. Also there exist 3.4 °C enhancements in the crystallization temperature which suggests that ExGr particles act as nucleating sites. The DSC analysis of pure PVDF film is shown in Fig. 5c which clearly proves that the melting temperature of alpha crystal lies at 170.9 °C. The melting peak of alpha crystal for blends is lesser than that of pure PVDF. Similar trend has been reported elsewhere [11]. This is due to the presence of amorphous PMMA. 3.6. FTIR analysis FTIR analysis of pure PVDF film, PVDF-20 wt% PMMA both un annealed and annealed at 120 °C for five hours and PMMA film is depicted in Fig. 6. Since incorporation of graphite does not alter the phase of PVDF as depicted in DSC and XRD analyses only the FTIR of PVDF, PMMA and PVDF-20 wt% PMMA blend films are shown. The characteristic bands which confirm electroactive gamma phase in PVDF and blends are at 833 cm 1 and 1234 cm 1 as reported elsewhere [16]. These bands are absent in PMMA film. Not all the IR bands are assigned since only the confirmatory bands for the existence of gamma crystals are shown by arrows. The other bands are associated with alpha phase of PVDF. Thus the DSC results are further confirmed through FTIR analysis which are corroborating with XRD results. The complete assignments of bands can be found in ref 17.
Fig. 5. DSC analysis of (a) PVDF-20 wt% PMMA (b) PVDF-20 wt% PMMA-1 wt% ExGr (c) pure PVDF film.
4. Conclusion PVDF-20 wt% PMMA solution processed blend film using DMAc as the solvent has electroactive gamma phase. Incorporation of sonicated expanded graphite in PVDF-20 wt% ExGr results in the generation of more electroactive gamma phase which is reflected
Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353
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Acknowledgements The authors are grateful to Asbury Carbon Inc., USA for providing expandable graphite as free sample. Special thanks to Dr. M. Sivakumar, Amrita Vishwa Vidyapeetham for extending his lab facilities to carry out some part of this work. References
Fig. 6. FTIR analysis of PMMA, PVDF, PVDF-20 wt% PMMA annealed at 120 °C for five hours and PVDF-20 wt% PMMA un annealed blend films.
through the area under the melting peak in DSC. The formation of electroactive gamma phase is confirmed through WAXD and DSC analysis. The blend film has mixture of electroactive gamma and non polar alpha phases. The blend system exhibits electrical percolation threshold less than 1 wt%. The sonicated ExGr particle act as nucleating sites as there exists 3.4 °C enhancement in the crystallization temperature. The TGA analysis shows that the thermal stability of main chain of PVDF is increased by 11 °C. The result suggests that the composite film can be used for energy harvesting applications.
CRediT authorship contribution statement M. Sachin: Investigation, Data Curation, Writing - original draft. Reshma Haridass: Methodology, Validation. B.T.S. Ramanujam: Conceptualization, Writing - review & editing.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[1] C. Huang, L. Zhang, Miscibility of poly (vinylidene fluoride) and atactic poly (methyl methacrylate), J. Appl. Polym. Sci. 92 (2004) 1–5. [2] V.N. Bliznyuk, A. Baig, S. Singamanemi, A.A. Pud, A,, K., Y., Fatyeyeva,, G., S., Shaporal,, Effects of surface and volume modification of poly (vinylidene fluoride) by polyaniline on the structure and electrical properties of their composites, Polymer 46 (2005) 11728–11736. [3] A.J. Lovinger, Ferroelectric polymers, Science 220 (1983) 1115–1121. [4] F.-C. Chiu, Y.-J. Chen, Evaluation of thermal, mechanical, and electrical properties of PVDF/GNP binary and PVDF/PMMA/GNP ternary nanocomposites, Compos. A: Appl. Sci. Manuf. 68 (2015) 62–71. [5] G.H. Kim, S.M. Hong, Y. Seo, Piezoelectric properties of poly (vinylidene fluoride) and carbon nanotube blends: b-phase development, Phys. Chem. Chem. Phys. 11 (2009) 10506–10512. [6] R.P. Vijayakumar, D.V. Khakar, A. Misra, Studies on a to b phase transformations in mechanically deformed PVDF films, J. Appl. Polym. Sci. 117 (2010) 3491–3497. [7] A.C. Lopes, C.M. Costa, C.J. Tavares, I.C. Neves, S. Lanceros-Mendez, Nucleation of the electroactive c phase and enhancement of the optical transparency in low filler content poly (vinylidene fluoride)/clay nanocomposites, J. Phys. Chem. C 115 (2011) 18076–18082. [8] T.A. Ezquerra, J.C. Canalda, A. Sanz, A. Linares, On the electrical conductivity of PVDF composites with different carbon-based nanoadditives, Colloid Polym. Sci. 292 (2014) 1989–1998. [9] B. T. S. Ramanujam, P. K. Annamalai, in Vijay Kumar Thakur, Manju Kumar Thakur, Asokan Pappu (Eds.), Hybrid polymer composite materials: Applications, Wood head publishers, UK, 2017, pp. 1–29. [10] B.T.S. Ramanujam, S. Radhakrishnan, in: Trends and Applications in Advanced Polymeric Materials, Scrivener Publishers, USA, 2017, pp. 127–143. [11] S.M. Pawde, K. Deshmukh, Investigation of the structural, thermal, mechanical, and optical properties of poly (methyl methacrylate) and poly (vinylidene fluoride) blends, J. Appl. Polym. Sci. 114 (2009) 2169–2179. [12] D. Liu, W. Li, N. Zhang, T. Huang, J. Yang, Y. Wang, Graphite oxide-driven miscibility in PVDF/PMMA blends: assessment through dynamic rheology method, European Polymer Journal 96 (2017) 232–247. [13] J.N. Martins, T.S. Bassani, G.M.O. Barra, R.V.B. Oliveira, Electrical and rheological percolation in poly (vinylidene fluoride)/multi-walled carbon nanotube nanocomposites, Polym. Int. 60 (2011) 430–435. [14] M. Li, N. Stingelin, J.J. Michels, M.J. Spijkman, K. Asadi, K. Feldman, P.W.M. Blom, D.M. De Leeuw, Ferroelectric phase diagram of PVDF:PMMA, Macromolecules 45 (2012) 7477–7748. [15] W. Li, L. Hong, Y.-M. Zhang, Preparation and investigation of PVDF/PMMA/TiO2 composite film, J. Mater. Sci. 44 (2009) 2977–2984. [16] X. Cai, T. Lei, D. Sun, L. Lin, A critical analysis of the a, b and c phases in poly (vinylidene fluoride) using FTIR, RSC Adv. 7 (2017) 15382–15389. [17] M. Kanik, O. Aktas, H.S. Sen, E. Durgun, M. Bayindir, Spontaneous high piezoelectricity in poly(vinylidene fluoride) nanoribbons produced by iterative thermal size reduction technique, ACS Nano 8 (2014) 9311–9323. [18] C. Tsonos, N. Soin, G. Tomara, B. Yang, G.C. Psarras, A. Kanapitsas, E. Siores, Electromagnetic wave absorption properties of ternary poly (vinylidene fluoride)/magnetite nanocomposites with carbon nanotubes and graphene, RSC Adv. 6 (2016) 1919–1924.
Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior M. Sachin, Reshma Haridass, B.T.S. Ramanujam ⇑ Department of Sciences, Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Coimbatore 641112, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 29 December 2019 Received in revised form 13 January 2020 Accepted 17 January 2020 Available online xxxx Keywords: Polymer nanocomposites Electrical conductivity Electroactive phase Structure development Thermal properties
a b s t r a c t Polyvinylidene fluoride (PVDF)-20 wt% poly (methyl methacrylate) blend has been synthesized by solution blending method. Expanded graphite has been synthesized by microwave irradiation of commercial expandable graphite. With varying concentration of expanded graphite (ExGr) in PVDF-20 wt% PMMA, series of blends are synthesized. The electrical percolation threshold has been observed to be less than 1 wt% ExGr which is due to the formation of graphite nanosheets. The conducting blend system exhibits typical percolation behavior. The thermogravimetric analysis shows two step degradation in PVDF-20 wt % PMMA blend system. This is due to the individual degradation of PMMA and PVDF main chains respectively. The thermal stability of the blend system with 1 wt% ExGr incorporation in PVDF-20 wt% PMMA is increased by 11 °C corresponding to main chain degradation of PVDF. The result proves existence of strong interaction between ExGr particles and the polymer. The DSC analysis shows very clearly the existence of electroactive gamma phase both in PVDF-20 wt% PMMA with and without ExGr particles as there is an additional high temperature melting peak apart from that of the melting peak at 167 °C. This result is confirmed through XRD also. The blend system is characterized by XRD, TGA, DSC and FTIR. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
1. Introduction Polymer blending is a versatile technique in which the properties of individual polymers are harnessed to achieve cost reduction, improvement in mechanical properties, to improve flexibility of the polymer blend films etc. [1]. The final morphologies of a blend system have immense effect on the end properties which is decided by the miscibility of the blend system. Miscible blends such as PVDF-PMMA are being extensively focused due to the enhanced chemical resistance, toughness etc. along with complete miscibility in the entire concentration range [2]. Polyvinylidene fluoride (PVDF) is a semi crystalline thermoplastic polymer which exists in five different polymorphs such as a, b, c, d and e [3]. Out of five different polymorphs of PVDF, the nonpolar a phase is abundant and most stable. The b, c and d phases are electroactive. Out of the three electroactive phases, b and c phases are extensively observed in various PVDF based systems ⇑ Corresponding author. E-mail address: [email protected] (B.T.S. Ramanujam).
[4]. The b phase has largest spontaneous polarization due to trans-planar zigzag conformation. The net dipole moment of cphase is lesser than that of b-phase. However the generation of electroactive phases by simple means poses great challenge. Extensive reports are there in the literature regarding high voltage poling [5] and uniaxial mechanical stretching at elevated temperature [6] to generate electroactive b-phase. Thus PVDF up on generation of electroactive phases exhibit piezoelectric, ferroelectric and pyroelectric behavior. There are various methods reported in the literature for the generation of electroactive beta phase of PVDF such as electrospinning of PVDF solution in a particular solvent, incorporation of nanofillers such as clay particles [7] or Titania [8] in to the PVDF matrix. PMMA is a transparent amorphous polymer extensively used in ophthalmic application such as making contact lenses. Electrically conducting polymer composites or conducting blends are important class of materials due to their potential applications in fuel cell bipolar plates, EMI shielding devices, sensors and actuators etc. [8]. In order to tailor make electrical conductivity, conducting fillers such as graphite are incorporated into the
https://doi.org/10.1016/j.matpr.2020.01.353 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353
2
M. Sachin et al. / Materials Today: Proceedings xxx (xxxx) xxx
insulating polymers. The concentration of fillers at which the electrical conductivity shoots up to many orders is known as electrical percolation threshold. It is important to reduce the electrical percolation threshold as minimum as possible so that the processing of conducting composites or blends become easy. In this regard, conducting nanofillers such as graphene [9] CNT [10] etc. are incorporated into the insulating polymer matrix. Pawde et al. [11] have investigated the structural, thermal, optical properties of PVDF-PMMA blend prepared by melt compounding followed by solution casting technique. Liu et al. [12] have investigated graphite oxide driven miscibility of PVDF/PMMA blends through rheological measurements. Incorporation of graphite oxide enlarges the window of miscibility of the blends as the authors proved that graphite oxide acts as entanglement points. Martins et al. [13] have reported the electrical percolation threshold in PVDF-MWCNT composites to be 1.2 wt%. Li et al. [14] have investigated the ferroelectric phase diagram of PVDF/ PMMA blend. Chiu et al. [4] have investigated in detail the effect of graphite nanoplatelet incorporation into PVDF and PVDF/PMMA blend with regard to crystallization and electrical percolation. They have reported that the ternary composite exhibit lower electrical resistivity compared to that of binary composites. After going through the literature, hardly any systematic study of the electrical properties, origin of electroactive phase and the associated thermal characterization of sonicated expanded graphite incorporated PVDF/PMMA blends is reported. Hence in this work starting from the synthesis of expanded graphite, the structure development and electrical and thermal properties of PVDF/PMMA/ExGr ternary system are reported. 2. Experimental 2.1. Materials PVDF (Grade Solef 1006-MFI-30–40 g/10 min at 230 °C/2.16 Kg) has been procured from Solvay Solexis, France. Expandable graphite (grade 3772) has been kindly provided by Asbury Carbons Inc., USA. PMMA-injection grade (IH830C-MFI-2.0 g/10 min by ISO1133 method) has been procured from L.G. MMA Corp. South Korea. N, N-Dimethylacetamide (DMAc) (AR grade) has been procured from S.D-fine chemicals, India.
further probe sonicated together for 30 min and then poured in a Petri dish and heated at 60 °C till all the solvent molecules are evaporated. Thus the uniform film formed has been used for further characterization. The amount of expanded graphite according to the calculation is incorporated in to the blend and series of conducting blend films are made. 2.4. Measurement of electrical resistance and thermal properties The electrical resistance of series of blend film has been measured using source meter model 2450 from Keithley instruments. A specially constructed brass cell is used for holding the film intact in order to eliminate contact resistance. The electrical resistance has been measured for three times for each sample and the average is used for conductivity calculation. Thermogravimetric analyzer (Model: SDT Q600) from TA instruments has been used for studying the thermal stability of blend systems. The heating rate of 10 °C per minute under the nitrogen atmosphere has been maintained. The melting and crystallization behavior of PVDF-PMMA blend system has been studied through differential scanning calorimetry under nitrogen atmosphere with 10 °C to be the heating and cooling rates using the model Q20 from TA instruments, USA. The data reported corresponds to heating and cooling cycle in the second cycle as first cycle is used to destroy the sample’s previous history. 2.5. Structural characterization and FTIR analysis The structure development in polymer blends has been understood through X-ray diffraction patterns obtained for various compositions of blend films using X’Pert PRO model from Panalytical instruments. The confirmation of electroactive phases has been done through FTIR analysis under ATR mode for the blend system. The Model Shimadzu Miracle 10 is used for FTIR analysis. 2.6. Scanning electron microscopy analysis The surface morphology of synthesized expanded graphite is analyzed using FESEM (Model: FEI Quanta FEG 200). 3. Results and discussion
2.2. Preparation of expanded graphite
3.1. FESEM analysis of expanded graphite
Expanded graphite has been prepared by microwave irradiation by keeping 0.1–0.2 gm of commercial expandable graphite inside an alumina crucible and kept inside a domestic microwave oven (Model-LGMH2342W-Out put power 800 W) and the period of irradiation of microwave is 10 s. This results in the formation of fluffy expanded graphite which has been probe sonicated in DMAc solvent to produce graphite nanosheets.
Fig. 1a and b depict digital photographs of expandable graphite before and after microwave irradiation. It is clear that after microwave irradiation, more fluffy expanded graphite has been formed. The increase in volume is due to the bursting of acid molecules which are held between the graphite planes in expandable graphite which eventually pushes the planes increasing expansion along c-direction of graphite and hence porous structure is formed. Fig. 1c shows the FESEM picture of microwave irradiated expanded graphite which has more porous structure.
2.3. Synthesis of PVDF/PMMA/ExGr blends Required quantity of PVDF (Keeping total blend amount to be 1 gm) is first dissolved in 8 ml of N-N-dimethylacetamide at 60 °C using mechanical stirrer with hot plate. Similarly PMMA of required quantity according to weight fraction calculation has been dissolved in DMAc separately at 60 °C and mixed with the dissolved PVDF and the mixture was mixed for about an hour. Expanded graphite of required amount in DMAc is probe sonicated using a probe sonicator (ENUP-500 model from LIFE-CARE EQUIPMENTS) for thirty minutes in an intermittent manner to ensure dispersion of graphite nanosheets in the solvent. This dispersed graphite nanosheets are mixed with PVDF/PMMA solution and
3.2. DC conductivity analysis of PVDF-PMMA-ExGr blends Fig. 2 depicts the variation of electrical conductivity of PVDF20 wt% PMMA-x wt% ExGr (x = 0, 0.5, 1, 1.5, 2). A particular blend composition (PVDF-20 wt% PMMA) has been chosen from the point of view of easy formation of film. The electrical conductivity is enhanced more than three orders when the loading of ExGr is changed from 0.5 wt% to 1 wt%. Thus it is clear that electrical percolation threshold for this system lies below 1 wt%. This is due to the fact that the conductive network formation between graphite nanosheets is formed. Below and at 0.5 wt% ExGr there will not
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Fig. 2. DC electrical conductivity variation of PVDF-20 wt% PMMA- x wt% ExGr (x = 0, 0.5,1, 1.5, 2) blend film.
Fig. 3. TGA analysis of PVDF-20 wt% PMMA and PVDF-20 wt% PMMA-1 wt% ExGr blend film.
[15]. It can be seen that almost 11 °C enhancement in the onset of degradation for PVDF chains due to the incorporation of 1 wt% ExGr particles in to the blend system. Also between 434 °C and 445 °C, the PMMA chains degrade slowly due to ExGr particles. The result clearly proves the interaction between the ExGr particles and the polymer chains.
3.4. XRD analysis
Fig. 1. Digital pictures of (a) expandable graphite (b) expanded graphite (c) FESEM of expanded graphite.
be enough contacts between the filler particles and hence the conductivity lies in the insulating range. The variation of electrical conductivity follows typical percolation behavior. 3.3. Thermogravimetric analysis The thermal stability of PVDF-20 wt% PMMA with and without 1 wt% ExGr incorporation has been studied and depicted in Fig. 3. The blend system exhibits two step degradation. The first step denotes the degradation of PMMA chains and the second step refers to the degradation of PVDF chains as reported elsewhere
The structural changes taking place in the polymer because of the processing method employed are depicted in Fig. 4 as shown below. The solution casted pure PVDF film exhibits XRD peaks at 2 Theta positions 18.5° (0 2 0), 20.3° (110/101) and a broad peak at 39° (2 1 1) which are the characteristic peaks of monoclinic gamma phase of PVDF as reported by Cai et al. [16]. Further it has been reported that 20.1° is also one of the characteristic peak of gamma phase of PVDF apart from 20.3° [17]. Though, incorporation of graphite up to 1 wt% did not shift the peak corresponding to that of gamma phase in our study, higher loading of ExGr results in disappearance of broad peak at 39°. This results show that there can be a switch over of gamma phase of PVDF to beta phase as the peak corresponding to that of gamma phase is broadened which depends on the loading of expanded graphite particles. However the compositional dependence of electroactive phase of PVDF will be the focus of our future work. In the present work, it is proved that the gamma phase of PVDF is formed. The confirma-
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Fig. 4. XRD analysis PVDF, PVDF-20 wt% PMMA, PVDF-20 wt% PMMA-1 wt% ExGr and PVDF-20 wt% PMMA-2 wt% ExGr blend film.
tion of electroactive phases cannot be done through XRD alone but DSC analysis will give insight about the polymorphs of PVDF. It has been reported that gamma crystals melt at higher temperature than non polar alpha crystals and electroactive beta crystals melt at slightly lower temperature than that of alpha crystals [16]. 3.5. DSC analysis Fig. 5a and b depict the DSC curves of PVDF-20 wt% PMMA and PVDF-20 wt% PMMA-1 wt% ExGr. It is clear from the DSC melting curves that there are two peaks at 167 °C and 173 °C respectively in PVDF-20 wt% PMMA blend. The former corresponds to the melting temperature of alpha crystals and the later is associated with gamma crystals [18]. The total area under the melting curve in Fig. 5a is 32.74 J/g. The result shows that there exists mixture of alpha and gamma phases. Up on incorporation of 1 wt% ExGr in PVDF-20 wt% PMMA, there exists high temperature melting peak corresponding to gamma crystals. However the area under the melting peak is increased to 49.43 J/g. This result suggests that gamma crystals are induced more with ExGr incorporation as the XRD result does not show any enhancement in the alpha phase reflections. Also there exist 3.4 °C enhancements in the crystallization temperature which suggests that ExGr particles act as nucleating sites. The DSC analysis of pure PVDF film is shown in Fig. 5c which clearly proves that the melting temperature of alpha crystal lies at 170.9 °C. The melting peak of alpha crystal for blends is lesser than that of pure PVDF. Similar trend has been reported elsewhere [11]. This is due to the presence of amorphous PMMA. 3.6. FTIR analysis FTIR analysis of pure PVDF film, PVDF-20 wt% PMMA both un annealed and annealed at 120 °C for five hours and PMMA film is depicted in Fig. 6. Since incorporation of graphite does not alter the phase of PVDF as depicted in DSC and XRD analyses only the FTIR of PVDF, PMMA and PVDF-20 wt% PMMA blend films are shown. The characteristic bands which confirm electroactive gamma phase in PVDF and blends are at 833 cm 1 and 1234 cm 1 as reported elsewhere [16]. These bands are absent in PMMA film. Not all the IR bands are assigned since only the confirmatory bands for the existence of gamma crystals are shown by arrows. The other bands are associated with alpha phase of PVDF. Thus the DSC results are further confirmed through FTIR analysis which are corroborating with XRD results. The complete assignments of bands can be found in ref 17.
Fig. 5. DSC analysis of (a) PVDF-20 wt% PMMA (b) PVDF-20 wt% PMMA-1 wt% ExGr (c) pure PVDF film.
4. Conclusion PVDF-20 wt% PMMA solution processed blend film using DMAc as the solvent has electroactive gamma phase. Incorporation of sonicated expanded graphite in PVDF-20 wt% ExGr results in the generation of more electroactive gamma phase which is reflected
Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353
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Acknowledgements The authors are grateful to Asbury Carbon Inc., USA for providing expandable graphite as free sample. Special thanks to Dr. M. Sivakumar, Amrita Vishwa Vidyapeetham for extending his lab facilities to carry out some part of this work. References
Fig. 6. FTIR analysis of PMMA, PVDF, PVDF-20 wt% PMMA annealed at 120 °C for five hours and PVDF-20 wt% PMMA un annealed blend films.
through the area under the melting peak in DSC. The formation of electroactive gamma phase is confirmed through WAXD and DSC analysis. The blend film has mixture of electroactive gamma and non polar alpha phases. The blend system exhibits electrical percolation threshold less than 1 wt%. The sonicated ExGr particle act as nucleating sites as there exists 3.4 °C enhancement in the crystallization temperature. The TGA analysis shows that the thermal stability of main chain of PVDF is increased by 11 °C. The result suggests that the composite film can be used for energy harvesting applications.
CRediT authorship contribution statement M. Sachin: Investigation, Data Curation, Writing - original draft. Reshma Haridass: Methodology, Validation. B.T.S. Ramanujam: Conceptualization, Writing - review & editing.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Please cite this article as: M. Sachin, R. Haridass and B. T. S. Ramanujam, Polyvinylidene fluoride (PVDF)-poly(methyl methacrylate) (PMMA)-expanded graphite (ExGr) conducting polymer blends: Analysis of electrical and thermal behavior, Materials Today: Proceedings, https://doi.org/10.1016/j. matpr.2020.01.353