Mechanical, Dielectric and Thermal Decomposition properties of Ethylene co-vinyl acetate Polymer Composites with Modified Chitosan Filler

doi:10.21203/rs.3.rs-5279728/v1

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Mechanical, Dielectric and Thermal Decomposition properties of Ethylene co-vinyl acetate Polymer Composites with Modified Chitosan Filler

https://doi.org/10.21203/rs.3.rs-5279728/v1

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The mechanical and dielectric properties of melt mixed EVA composites with polyaniline modified chitosan as fillers were investigated. A dependence on the filler concentration was observed for the mechanical properties such as tensile strength and Young’s modulus. The composite with higher concentration of filler showed 16.57 MPa and 25.26 MPa respectively. However, the presence of EVA matrix decreased the dielectric properties of the developed composites. In order to better understand the thermal decomposition and further analyse the recycling of the developed composites, kinetic parameters and thermodynamic properties of EVA/chitosan-g-PANi composites were determined. This was carried out on the basis of non-isothermal thermogravimetric experiments conducted at four different heating rates under N₂ atmosphere. The Ea values obtained was strong function of conversion rates which ranged from 138–267 kJ/mol and 150–280 kJ/mol for KAS and Friedman methods respectively. Moreover, enthalpy, entropy and Gibbs energy changes were computed and these results would help design a process for the recycling of synthetic polymers.

Composites

Dielectrics

Mechanical Properties

Degradation kinetics

Thermodynamic properties

Composite materials are explored for the past couple of decades owing to their tuneable nature and reliability. Among the various kinds of composites, the lion-share is taken up by polymer composites. By changing the filler/reinforcement interesting changes to the existing properties of the raw polymers are introduced. These composites are made by maintaining ecology–economy–technology balance in order to enhance their properties [1, 2]. The material choice for a precise application is carried out by analysing the performance under different conditions viz; mechanical, thermal or electrical.

On account of the biocompatibility, non-toxic nature, remarkable physical properties and inexpensiveness, materials derived from nature such as cellulose, chitosan and lignin are the forward runners as reinforcements in composites [3, 4]. Among them chitosan is chosen as filler as an attempt to tackle the large marine waste predicament among the coastal areas of India. Chitosan aka Poliglusam is the more reactive deacetylated product of chitin that’s derived from the exoskeleton of crustaceans [5]. However, the lack of mechanical strength and short shelf life makes it imperative to modify chitosan. Chemical modification via grafting technique is a robust tool to develop tailored materials for advanced applications[6].

EVA, a copolymer of ethylene and vinyl acetate can be a sensible option as a matrix for grafted composites owing to its cost effectiveness, processability, excellent mechanical properties & long shelf life [7]. The potential of modified chitosan with polymers such as ethylene co-vinyl acetate (EVA) leads to the development of performance enhanced hybrid materials or composites. These polysaccharidic materials accord a crystalline nature to the otherwise amorphous EVA [8].

Literature depicts that the materials modified with polyaniline show remarkable performance in water treatment, bio-applications together with a modified electrode, sensor applications and microbial fuel cells [6, 9]. In this study, chitosan grafted with polyaniline (PANi) through oxidative-radical copolymerization using ammonium persulfate as initiator is used as a modified filler to fabricate a composite with ethylene-co-vinyl acetate (EVA) as the matrix. Albeit EVA being a widely used material for material development, not many research groups work with composites involving melt-mixed EVA and CS-based fillers. Previous literature from our group extensively evaluated the thermal degradation kinetics of the developed composites [6]. Through the present study composites are expected to show improved properties than that of the individual matrix and filler.

CS with 82% degree of deacetylation was obtained from India Sea Foods, Kochi, Kerala, India. Ethylene-co-vinyl acetate (EVA-40) (vinyl acetate content 40%) was procured from Sreenivasa polymers, Chennai, India. Other chemicals such as aniline (Merck), ammonium persulfate (APS) (Merck), hydrochloric acid (HiMedia Laboratories), and acetic acid (Spectrum reagents and Chemicals, Cochin) of AR grade were used as such.

2.1. Mechanical studies

From the stress-strain anaylsis tensile strength, elongation at break and Young’s modulus were determined. Tensile properties of dumb shell shaped specimens of EVA and the composites were evaluated using a ‘Shimadzu Autograph AG-X series’ Universal Testing Machine (UTM) at a cross-head speed of 50 mm/min. The length between the jaws at the start of each test was fixed to 40 mm. All tensile data were obtained for three replicates specimens and averaged data are included into the final results. A digital thickness gauge was used to measure the thickness of the narrow portion of the specimen.

2.2. Dielectric Studies

Dielectric measurements were carried out using a GmBH Alpha impedance analyzer by Novo-Control Technologies at room temperature. The dielectric measurements were collected over a wide range of frequency (10 ^− 2 to 10 ⁷ Hz).

2.3. Kinetic evaluation using thermogravimetric analysis

One of the key aspects that help determine the characteristic of the developed materials is the assessment of the Arrhenius parameters and kinetics. The International Confederation for Thermal Analysis and Calorimetry (ICTAC) tender recommendations for the evaluation of kinetic parameters using the data acquired by thermal methods of analysis [10]. Literatures show that the isoconversional methods can give significant activation energy values in the wide range of conditions. The decomposition features of the composites were studied using a Hitachi STA 7200 thermogravimetric analyzer (TGA). In order to predict the kinetic and thermodynamic parameters two model free isoconversional methods were employed by making a number of patterns at different heating rates (β) ie, β = dT/dt = constant. The extent of conversion (α) was derived from the data of weight loss using the reported methods [11]. Among the two techniques employed Friedman uses the differential route and the other, Kissinger-Akahira-Sunose (KAS), uses the integral route to calculate the kinetic parameters.

Friedman’s (FR) method applies the logarithm of conversion rate as a function of the reciprocal temperature at different degrees of conversion and can be represented as,

$$\:\text{ln}\left[\begin{array}{c}d\alpha\:\\\:\stackrel{-}{\text{d}\text{t}}\end{array}\right]=\text{ln}\left[\text{A}\text{f}\left({\alpha\:}\right)\right]-\frac{{E}_{a}}{RT}$$

Where, f(α) is the reaction model, α is the conversion rate, Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 Jmol^− 1K^− 1).

$$\:-\text{ln}\left(\frac{\beta\:}{{T}^{2}}\right)=\frac{{E}_{a}}{RT}-\text{ln}\left(\frac{AR}{{E}_{a}}\right)$$

2.4. Thermodynamic Parameters

Thermodynamic parameters, such as, ΔH, ΔG and ΔS, can be determined using the Ea and A values calculated from FR and KAS methods according to the following equations

ΔH$\:\:={E}_{a}-RT$ (3)

ΔG = $\:{E}_{a}+R{T}_{p}{ln}\left(\frac{{K}_{B}{T}_{P}}{hA}\right)$ (4)

ΔS = $\:\:\frac{\varDelta\:H-\varDelta\:G}{{T}_{P}}$(5)

where K_B (1.3819 x 10–23 J/K) and h (6.6269 x 10–34 Js) are the Boltzmann and Planck constants, respectively. In order to reduce interaction effects, the value Tp (the DTG peak temperature) value was calculated based on the lowest heating rate i.e., 5°C/min [12].

3.1. Mechanical Properties

The stress-strain plots give the different mechanical properties like tensile strength, elongation at break, and Young’s modulus of the developed composites (Fig. 1). The composites with both chitosan and PANi grafts showed lesser tensile strength than the raw EVA. Raw EVA has a tensile strength of ~ 27 MPa. The chitosan loaded composites initially show an increase of the mechanical properties. Further, a decrease in the values is observed after 6% addition of the filler to the matrix. This may be attributed to the inadequate interaction between the matrix and filler at higher filler concentration. The values of the tensile properties are denoted in Table 1. Another reason for the decline in tensile values may be the agglomeration of the filler at higher loading that in turn makes the composite less mechanically viable. A similar trend was observed by Sonia et al for composites of EVA with cellulose[13].

Table 1

Tensile strength, elongation at break, and Young’s modulus of EVA/chitosan (EC) composites
Composite	TS at break (MPa)	Elongation at break %	Young’s Modulus (MPa)
EC1	8.19 ± 0.6	408 ± 8.4	25.08 ± 5.84
EC2	11.75 ± 0.49	527 ± 8.1	22.31 ± 8.74
EC3	13.95 ± 0.48	602 ± 9.9	28.40 ± 3.27
EC4	12.53 ± 0.51	545 ± 8.7	27.04 ± 9.83
EC5	9.24 ± 0.21	398 ± 13.4	20.39 ± 13.7

It was observed that with progressive increase in filler concentration, all mechanical properties such as tensile strength (TS) at break, tensile modulus (TM) and elongation at break (EB) as percentage (%) increases for the prepared composite systems.

From Table 2, the prepared EVA/chitosan-g-PANi composites show tensile strength up to 16.57 MPa with the increase in the filler addition (Table 2). The composites with modified filler, chitosan-g-PANi, has shown improved strength considerably due to the interactions between the filler and matrix indicating better mixing leading to the formation of interfacial area[14]. This can further lead to the effective distribution of stress across the composite. Also, with the increase of the filler content, the elongation at break and the Young’s modulus calculated from the initial linear region of the stress-strain plot increases which indicates better miscibility of the matrix and the filler and the formation of a strong interface between the matrix and the filler.

Table 2

Tensile strength, elongation at break, and Young’s modulus of EVA/chitosan-g-PANi (ECP) composites
Composite	TS at break (MPa)	Elongation at break %	Young’s Modulus (MPa)
ECP1	8.87 ± 0.45	413 ± 6.3	13.36 ± 1.46
ECP2	12.19 ± 0.46	537 ± 5.1	18.63 ± 1.74
ECP3	14.47 ± 0.54	653 ± 5.5	22.17 ± 4.66
ECP4	16.57 ± 0.89	723 ± 4.4	25.26 ± 6.21
EVA0	27.22	-	-

3.2. Dielectric Properties

Ethylene-co-vinyl acetate (EVA) based composites display interesting dielectric properties that make them appropriate for diverse electrical and electronic applications. The dielectric properties of EVA-based composites are influenced by a variety of factors, such as the matrix properties, the type and concentration of fillers or additives, and the processing techniques utilised It is to be noted that the results presented are aimed only at room temperature measurements.

The dielectric constant ε’, or permittivity, is a measure of the energy stored in a sample during a cyclic electric excitation. The energy stored is usually in the form of a non-uniform dipole distribution or ionic charge layers. With increased filler addition, dielectric constant is increased at any measured frequency. There is an observed decrease in the dielectric constant value for the maximum concentration of filler which is attributed to the agglomeration of the filler at higher concentration. For neat EVA or any composite, dielectric constant is found to decreased with the increase of frequency [15, 16]. Due to the presence of EVA, some charge carriers present in fillers get confined and is not able to freely discharge at the electrodes which results in space charges accumulating at the interface of conducting filler and insulating matrix combined with microscopic distortion of applied electric field. This phenomenon is termed as Maxwell-Wagner-Sillers (MWS) interfacial polarization. This effect is observed for composites that contain EVA as matrix in the literature [17].

From Fig. 3, the tan δ of all composites remained at low values and showed no drastic variation over the entire frequency range indicative of the low leakage current and energy loss in the composites.

3.3. Kinetic and Thermodynamic Parameters

Over a period of decades, the prediction of reaction mechanism and kinetic parameters have become imperative for the sustenance and disposal of materials of polymer decent. The rate of decomposition can be configured in terms of temperature, extent of conversion and pressure nonetheless the popular methods for kinetics regard the rate to be a function of temperature and extent of conversion. The temperature dependence of the isoconversional rate can be used to evaluate the values of activation energy without assuming or determining any particular form of the reaction model [6, 11, 18]. A comparative study on the thermal decomposition of EC and ECP composites using Flynn-Wall-Ozawa and Kissinger models has been reported by the group previously [6]. In this work a better understating of the kinetic and thermodynamic parameters using isoconversional methods that utilizes the integral and differential pathways is facilitated.

From Fig. 4, it is evident that for both the methods the Ea values show a dependency to the α values. The Ea vs α values were evaluated over a range of 0.1–0.9 with a step of 0.1. Both the methods depict separate Ea values at each conversion. The non-linearity of the activation energy values indicated the decomposition of the developed composited to undergo multistep kinetics. Also, the numerical values of the activation energies also showed variations (Table 3). The Ea values determined by FR method is higher than the values from KAS which is likely due to the approximation of temperature integral used for the derivations of the kinetic methods.

Table 3

The energy of activation according to conversion degree for KAS and FR methods of ECP composites
	KAS	FR
α	Ea (kJ/mol)
0.1	138.51	150.66
0.2	142.01	195.26
0.3	232.06	277.15
0.4	250.79	272.22
0.5	258.18	280.24
0.6	262.55	277.40
0.7	264.30	268.07
0.8	265.13	269.59
0.9	268.70	277.50
Average	231.36	252.01

Additional to the activation energies the pre-exponential factor, change in enthalpy, Gibbs free energy, and entropy corresponding to the α values of the developed ECP composites were computed for KAS and FR methods and depicted in Table 4.

Table 4

Pre-exponential factor and other thermodynamic parameters for ECP composite obtained by using activation energy deduced from KAS and FR methods
	α	Pre-exponential factor, A (s^− 1)	Enthalpy, ∆H (kJ/mol)	Gibbs free energy, ∆G (kJ/mol)	Entropy, ∆S (J /mol)
KAS	0.1	3.94E + 08	135.77	310.51	-529.80
	0.2	5.64E + 07	138.63	361.57	-547.73
	0.3	1.79E + 14	228.42	414.08	-423.85
	0.4	3.61E + 15	247.06	425.81	-399.09
	0.5	1.16E + 16	254.41	431.35	-389.46
	0.6	2.34E + 16	258.73	435.07	-383.75
	0.7	3.12E + 16	260.45	437.48	-381.44
	0.8	3.61E + 16	261.23	439.50	-380.32
	0.9	6.62E + 16	264.76	442.85	-375.37
	Average	1.91E + 16	227.72	410.91	-423.42
Friedman	0.1	2.40E + 09	147.92	317.72	-514.79
	0.2	1.33E + 11	191.88	388.53	-483.15
	0.3	2.00E + 17	273.50	433.61	-365.50
	0.4	9.82E + 16	268.50	434.94	-371.61
	0.5	3.85E + 17	276.47	440.19	-360.36
	0.6	2.57E + 17	273.58	440.77	-363.83
	0.7	6.01E + 16	264.21	438.72	-375.99
	0.8	8.17E + 16	265.70	440.78	-373.52
	0.9	3.05E + 17	273.55	445.62	-362.67
	Average	1.54E + 17	248.37	420.10	-396.83

The E_a values obtained from all of the chosen models were used to compute the values of ΔH, ΔG, and ΔS. To reduce interaction effects, the Tp value was calculated based on the lowest heating rate. The reason is that increasing heating rates results in more interaction between sample components. It can be observed from the table that the values of ΔH and ΔG leans towards positive values. This is indicative of the utilization of external energy for the thermal decomposition of the composites. The negative values of ΔS implies the formation of active complexes

A thorough investigation of the mechanical, dielectric and thermal properties of the developed EVA/chitosan-g-PANi composites were conducted. The mechanical properties of the formed composites depended mainly on the concentration and the nature of the fillers. The trend observed was that the properties increased with the increase of the filler quantity. Albeit, the elongation depended mainly on the influence of the EVA matrix. For the dielectric values, a decrease in dielectric constant with increase in frequency was noted implying the free discharge of charge carriers due to the presence of EVA. The thermal decomposition kinetics of the formed ECP composites has been investigated using TGA technique under nitrogen atmosphere. The energy of activation values estimated using KAS and FR isoconversional methods indicated the non-linear nature of thermal decomposition. Also, the Ea values from FR is slightly on the higher side which is due to the lack of approximations and to the presence of experimental noise during the calculation of Ea with FR method. In addition to Ea values, pre-exponential factor and thermodynamic parameters such as ΔH, ΔG, and ΔS were also evaluated which indicated the endothermic nature of thermal degradation by forming active complexes of the formed EVA/chitosan-g-PANi. The present study can be counted as preliminary stage which further aids to decrease the waste disposal via land-filling by providing an alternative end-of-life route.

Acknowledgments

The authors gratefully acknowledge Rashtriya Uchchatar Shiksha Abhiyan (RUSA) and Kerala

State Council for Science Technology and Environment (KSCSTE) for financial support.

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No competing interests reported.

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Mechanical, Dielectric and Thermal Decomposition properties of Ethylene co-vinyl acetate Polymer Composites with Modified Chitosan Filler

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Mechanical studies

2.2. Dielectric Studies

2.3. Kinetic evaluation using thermogravimetric analysis

2.4. Thermodynamic Parameters

3. Results and Discussion

3.1. Mechanical Properties

3.2. Dielectric Properties

3.3. Kinetic and Thermodynamic Parameters

Conclusion

Declarations

References

Additional Declarations

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