Electrical, thermal and structural properties of

Electrical, thermal and structural properties of plasticized waste cooking oil-based polyurethane solid polymer electrolyte Rahmatina Mohd Huzaizi, Syuhada Mohd Tahir, and Kamisah Mohamad Mahbor

Citation: AIP Conference Proceedings 1901, 090002 (2017); View online: https://doi.org/10.1063/1.5010520 View Table of Contents: http://aip.scitation.org/toc/apc/1901/1 Published by the American Institute of Physics

Electrical, Thermal and Structural Properties of Plasticized Waste Cooking Oil-based Polyurethane Solid Polymer Electrolyte Rahmatina Mohd Huzaizia), Syuhada Mohd Tahirb) and Kamisah Mohamad Mahborc) Universiti Teknologi MARA, Cawangan Pahang Kampus Jengka, 26400 Bandar Tun Abdul Razak Jengka, Pahang, Malaysia. b)

Corresponding author: [email protected] a) [email protected] c) [email protected]

Abstract. Waste cooking oil-based polyol was synthesized using epoxidation and hydroxylation methods. The polyol was combined with 4,4-diphenylmethane diisocyanate to produce polyurethane (PU) to be used as polymer host in solid polymer electrolyte. 30 wt% LiClO4 was added as doping salt and two types of plasticizers were used; ethylene carbonate (PU-EC) and polyethylene glycol (PU-PEG). The SPE films were characterized using Fourier transform infrared spectroscopy, electrochemical impedance spectroscopy, differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The highest conductivity achieved was 8.4 x 10-8 S cm-1 upon addition of 10 wt% EC. The XRD results showed a decrease of crystalline peaks in PU-EC and the increase in PU-PEG. DSC results revealed that the films; PU, PU-EC and PU-PEG had glass transition temperatures of 159.7, 106.0 and 179.7 oC, respectively. The results showed that the addition of EC increased the amorphous region and the free volume in the SPE structure, thus resulted in higher ionic conductivity.

INTRODUCTION Solid polymer electrolytes (SPEs) are polymers incorporated with metal salt to produce electrical properties. They are widely studied due to the vast applications in electrochemical devices such as batteries, fuel cells, sensors and supercapacitors [1]. They are flexible, leakage-free and easy to process. Polyurethane is one of the promising polymer hosts for SPEs. Polyurethane is a block copolymer consists of soft and hard segments made up of polyol and diisocyanate, respectively. The soft segment acts as polymeric solvent to solvate the cation while the hard segment acts as physical crosslink sites to give the dimensional stability to polyurethane structure [2,3]. The polyol component of polyurethane is currently being synthesized using bio-based materials derived from vegetable oils. Various vegetable oils have been studied such as palm kernel, sunflower, castor and soybean oils [1,2,4,5]. However, due to the competition with food chain supply, interest had shifted to the synthesis of polyol using waste or used cooking oil as raw material. Methods used usually by introducing the hydroxyl groups on the unsaturated site. Examples of the methods were hydroxylation, epoxidation and ring opening and transesterification [6-9,24]. However, these previous works focused on using the waste cooking oil-based polyol to synthesize polyurethane for applications such as foam, adhesive, coating and furniture. None had been used for application as polymer electrolyte. In this work, polyurethane SPE was synthesized via solution casting method using waste cooking oil-based polyol reacted with 4,4’-diphenylmethane diisocyanate (MDI). Waste cooking oil was converted into polyol using one-pot epoxidation and hydroxylation process. The effect of adding organic plasticizers as chain extenders on the structural, thermal and electrical properties of SPE were investigated.

Advanced Materials for Sustainability and Growth AIP Conf. Proc. 1901, 090002-1–090002-6; https://doi.org/10.1063/1.5010520 Published by AIP Publishing. 978-0-7354-1589-8/$30.00

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EXPERIMENTAL Synthesis of Waste Cooking Oil-Based Polyol Waste cooking oil (WCO) sample was filtered and heated at 110 ºC to remove water and solid contaminants. The pre-treated waste cooking oil was then used to synthesize polyol by using one-pot epoxidation and hydroxylation [8]. First, 40 g of pre-treated WCO and 5 g formic acid were mixed until the mixture became homogeneous in a roundbottom flask. Next, the flask was immersed in water bath at 10-15 ºC. Then, 80 g of 35% hydrogen peroxide was added dropwise into the mixture while stirring. After that, the temperature was raised to 50 ºC and continuously stirred for five hours. The mixture was then allowed to cool to room temperature. Next, 50 mL of distilled water followed by 5 mL of 11.6 M of hydrochloric acid were added to the mixture and temperature was raised to 80 ºC for another five hours. After that, the product was extracted by using petroleum ether, and the upper layer was taken and washed with sodium carbonate, followed by distilled water and sodium chloride. The ether was finally removed over anhydrous sodium sulphate in rotary evaporator at 45 ºC to get the WCO-based polyol.

Preparation of Waste Cooking Oil-Based Polyurethane Solid Polymer Electrolytes Polymer electrolyte films were prepared using solution casting method. Firstly, the WCO-based polyol, 10% (w/w) plasticizer, 30% (w/w) LiClO4 and MDI were separately dissolved in acetone. Next, polyol and MDI with weight ratio of 80:20 (OH:NCO) were mixed to form urethane prepolymer. Then, plasticizer was added as chain extender, followed by LiClO4 as doping salt. The mixture was stirred continuously until it became homogeneous and cast into Petri dish. The solvent was slowly evaporated at room temperature for five days. Lastly, the sample was put in an oven at 50 ºC for three hours. The films obtained were yellow in colour and rigid. The composition of each polymer electrolyte film; polyurethane solid polymer electrolyte without plasticizer (PU SPE), polyurethane solid polymer electrolyte with ethylene carbonate (PU SPE EC) and polyurethane solid polymer electrolyte with polyethylene glycol (PU SPE PEG) are shown in the following Table 1. TABLE 1. The composition of polymer electrolyte films Salt (% w/w) EC (% w/w)

PU SPE PU SPE EC PU SPE PEG

30 30 30

0 10 0

PEG (% w/w)

0 0 10

Characterizations FTIR spectra were recorded using Perkin Elmer Spectrum 100 FTIR spectrometer in the range of 450-4000 cm-1 wavenumbers. The thermal stability was analyzed using Netzsch DSC model 214 Polyma with heating temperature from 20 Ԩ to 250 Ԩ at scanning rate of 10 Ԩ /min under nitrogen atmosphere. Approximately 3 mg of the film samples were used for each measurement. The T g of the sample was taken at mid-point of the endothermic peak using STARe software. The crystallinity of the sample was characterized using XRD model D-5000 Siemen. The data was collected at diffraction angle 2θ from 5̊ to 60° at the rate of 0.04 s-1. The ionic conductivity was analyzed by impedance spectroscopy with a high frequency resonance analyzer (HFRA; Solartron 1260, Schlumberger), with applied frequencies ranging from 1 MHz to 0.1 Hz at the voltage of 1000 mV. The measurement was taken at room temperature using 1 cm2 of film sample that was sandwiched between two stainless steel block electrodes.

RESULTS AND DISCUSSION Fourier Transform Infrared Spectroscopy Figure 1 shows the Fourier transform infrared spectra of three polyurethane SPE samples. The formation of urethane linkage (-NHCOO-) is confirmed with the presence of: (1) peaks for secondary amine (N-H) hydrogen bonded to oxygen in ether at 3370 to 3600 cm-1 [9], (2) peaks for O-C=O ester at around ~1300 cm-1 and ~1200 cm-1 and (3) peaks for carbamate (C-N) at 1620 to 1640 cm-1 [2]. The addition of plasticizers, EC or PEG, resulted in the

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%T

absence of band around 2279 cm-1 which confirmed the absence of free NCO group. This indicates that plasticizers act as a chain extender that extends the soft segment and helps the completion of urethane reaction [1,10]. The effect of complexation with Li+ in the added salt could be observed at three regions: (1) broad peaks for N-H stretching at higher wavenumber at 3370 to 3600 cm-1 due to generation of free N-H and less hard-hard segment hydrogen bonded N-H left, (2) small peaks of C=O symmetric stretching around ~1740 cm-1 and (3) sharp peaks of C-O-C at higher wavenumber, 1070 to 1090 cm-1, due to overlapping free, Li+ bonded and hydrogen bonded ether [4,11].

4000

3600

3200

2800

2400

2000

1600

1800

1400

1200

1000

800

600

450

cm-1

FIGURE 1. FTIR spectra of waste cooking oil-based polyurethane solid polymer electrolytes TABLE 2. The wavenumbers of functional groups in waste cooking oil-based polyurethane solid polymer electrolytes Functional Groups Wavenumber cm-1 PU SPE PU SPE EC PU SPE PEG

C=O N-H C-N C-O-C NCO

1742.16 3370.86 1628.51 1073.25 2279.03

1744.47 3392.33 1627.80 1092.38 -

1743.12 3523.53 1637.18 1069.77 -

Ionic Conductivity The ionic conductivity was obtained from the calculation using the equation of ߪ ൌ ݈Ȁሺܴ௕ ‫ܣ‬ሻ where ݈ is the thickness of the polyurethane films, A is the surface area in cm2 and Rb is the bulk resistance value. The ionic conductivity value for PU SPE (without plasticizer) is 2.29 x 10 -10, PU SPE EC is 8.40 x 10-8 Scm-1 and PU SPE PEG is 6.15 x 10-10, as shown in Table 3 below. Adding plasticizer was found to increase the ionic conductivity of SPE. This is because plasticizer acted as chain extender that increases the volume of soft segment [4]. The interaction between the added plasticizer and polymer chains also reduced the cohesive forces between polymer chains, thus resulted in higher ionic conductivity [12,13].

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TABLE 3. Ionic conductivity values for waste cooking oil-based polyurethane solid polymer electrolytes Polyurethanes Solid Polymer Electrolytes Ionic conductivity (S.cm-1)

2.29 x 10-10 8.40 x 10-8 6.15 x 10-10


Good plasticizer must have high dielectric constant and low viscosity to promote dissociation of salt by lowering the Coulomb force between anion and cation in salt [14,15]. This led to more free lithium ions in the electrolyte system. Plasticizer with higher molecular weight produced higher crystallinity in SPE and thus lower ion diffusivity [16]. These facts explained the higher ionic conductivity observed in PU SPE EC compared to PU SPE PEG. The properties of both plasticizers used in this study are detailed in Table 4. Plasticizer

TABLE 4. Dielectric constant and molecular weight values for plasticizers Dielectric constant Molecular weight

EC PEG

89.6 8.3

88.06 6000

Thermal Analysis DSC was performed to determine the glass transition temperature (Tg) of the synthesized electrolyte films. Table 5 shows the values of Tg for PU SPE with and without plasticizer. The Tg values above room temperature for all three samples indicated the glassy state of PU SPE at room temperature [4]. The addition of EC in PU SPE EC reduced the Tg value to 106Ԩ compared to PU SPE with Tg 159.7 ԨǤThis agreed well with EIS result which showed that PU SPE EC has higher ionic conductivity. The shift of Tg to lower value showed the plasticization effect that decreased the dipole-dipole interaction between polymer chains and disrupted the hydrogen bond urethane soft segment. These resulted in enhanced free volume and thus provided better percolation pathway for the lithium ions [1,17]. The addition of PEG increased Tg value of PU SPE PEG. Addition of plasticizer is known to decrease in both thermal and mechanical properties, but lower had more effect than higher molecular weights and concentrations [21]. High molecular weight PEG have greater chain length thus producing networks with lower cross-linking densities and higher average molecular weight between two consecutive cross-links [22]. The crystallizability first reduced with an increase in the molecular weight of PEG, but increased thereafter with a further increase in the molecular weight of PEG [23]. TABLE 5. Glass transition temperature for waste cooking oil-based polyurethane solid polymer electrolytes Samples Glass Transition Temperature, Tg (Ԩሻ


159.7 106.0 179.7

Structural Analysis The XRD analysis was used to determine the structure, crystallization and complex formation of polyurethane solid polymer electrolytes [18]. Figure 2 shows the XRD diffractograms for the synthesized SPE samples. The addition of EC reduced the crystalline peaks which mean more amorphous regions. While the addition of PEG increased the intensity of the crystalline peaks but more broadness in hump around 2θ = 20°. Generally, the presence of amorphous region and reduction of crystalline region would give high ionic conductivity. Increase in amorphicity means; (1) greater ionic diffusivity since ions can move freely due to the lower energy barrier, and (2) more flexible backbones that increase the chain mobility and segmental motion, thus improve ions transportation [2,19].

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FIGURE 2. XRD diffractograms for waste cooking oil-based polyurethane solid polymer electrolytes

CONCLUSIONS SPE films using EC and PEG as plasticizers were prepared and characterized. The effects of plasticization on electrical, thermal and structural properties of the films were visible and highly correlated. FTIR results proven the complexation occurred. PU SPE EC produced highest ionic conductivity. This result supported by the lower Tg value observed in DSC and more amorphocity in XRD. The effects of plasticization were highly dependent on the dielectric constant and molecular weight of chosen plasticizer.

ACKNOWLEDGEMENT The authors would like to thank Universiti Teknologi MARA and Ministry of Higher Education, Malaysia for research grant no RAGS/1/2014/SG01/UITM//3.

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