Dielectric and thermal properties of magnesium oxide/poly(aryl ether ketone) nanocomposites

2.1 Materials and methods

The commercial PAEK, (grade G-PAEK 1200 P) in powder form was purchased from M/s Gharda chemicals, Mumbai, India. The G-PAEK 1200 P was a gray color powder with a melting point of 372°C, density of 1.3 g/cm³, and crystallinity of >40%. Magnesium oxide (MgO) nanofiller was prepared and developed in-house by the sol-gel process. The gel consisting of the solution of magnesium nitrate and urea was prepared in an aqueous system in a ratio of 1:5. In this gel network magnesium nitrate species was uniformly surrounded by the fuel molecules. Then this gel matrix was heated at a temperature of 550°C as the microexplosions of species take place leading to the formation of nanometric metal oxide particles. The stabilized gels consist of highly porous metal oxide matrix, upon grinding and dispersion it produces nano-sized metal oxide fillers having dimensions of 25–35 nm and having a density of 3.58 g/cm³.

2.2 Nanocomposite fabrication

The procedure for the preparation of pure PAEK and PAEK/MgO composite is explained as below:

To obtain pure PAEK polymer matrix, the PAEK powder was initially dried for 4 h in an oven maintained at 150°C. Then 1000 g of PAEK was weighed and fed to the twin screw extruder (Omega 20 Steer Engineering, Bengaluru make) by maintaining temperature at eight different zones from 340°C to 395°C to bring the material into molten state. The extrudate from the extruder was air cooled, dried and passed through a pelletizer.

To obtain 2 wt% of PAEK/MgO nanocomposite 980 g of pre-dried G-PAEK powder and 20 g of MgO nanopowder suspended in 300 ml acetone were taken (MgO suspension). In the first step MgO suspension and only 200 g of dried G- PAEK powder was manually mixed using mortar and pestle, and dried to remove the solvent. This resultant mixture together with the remaining 780 g G-PAEK powder is thoroughly mixed in an industrial dry mixer. Finally, this mixture was fed to a twin screw extruder for material to get molten and mixed well by maintaining temperature at eight different zones from 340°C to 395°C. The extrudate from the extruder was air cooled, dried and passed through a pelletizer.

Similarly, for 4 wt% of PAEK/MgO nanocomposite 960 g of dried G-PAEK powder and mixed 40 g of MgO nanopowder suspended in 300 ml acetone were taken. The suspension was mixed with 200 g of pre-dried G-PAEK powder. The mixture was mixed with mortar and pestle and dried to remove the solvent. The mixture was then mixed with the remaining amount of G-PAEK powder, i.e. 760 g. For 6 wt% of PAEK/MgO nanocomposite 940 g of G-PAEK powder and 60 g of MgO powder and for 10 wt% of PAEK/MgO nanocomposite 900 g of G-PAEK powder and 100 g of MgO powder were appropriately added to obtain the pellets. Similar sub-mixture procedure was used, i.e. MgO powder suspended in acetone was mixed with 200 g of G-PAEK, dried to remove the solvent and then mixed with the remaining G-PAEK powder. The PAEK/MgO nanocomposite pellets obtained by this process were fed into the injection molding machine maintained at a temperature of 385°C (Mallik Engineering, Mumbai, India). Then the pellets were ejected from the mold cavity. The size of the molded disc was about 110 mm diameter×3 mm thick. These discs were finished by abrading with emery papers, cleaned with acetone and dried. Finally, they are used for the characterization of the dielectric and thermal properties.

2.3 Morphological studies

Morphological analysis of PAEK and PAEK/MgO nanocomposites was conducted using Philips XL 20 scanning electron microscope (SEM). For SEM of nanocomposites, a small piece of the sample was cut from the disc specimen manually, polished with successively finer grades of emery paper followed by a cloth to remove scratches. The samples were coated with a thin layer of gold to make the sample electrically conducting. The dispersion of MgO nanoparticles in PAEK matrix was also observed using transmission electron microscopy (TEM) (Joel Ltd., Tokyo, Japan) 100 CXII system with a standard magnification of 360–320,000×.

2.4 Electrical measurements

The volume resistivity (ρ_v) measurement was performed using an Eltel-Automatic Resistivity Test Set kit type-ADTR-2K (EL TEL, Bengaluru, India) and tetex bridge was used to measure the capacitance (C) and dissipation factor (tan δ) over a temperature range of 30°C–230°C. From the measured capacitance, dielectric constant (ε_r) is calculated for each sample. These experiments were carried out according to ASTM D 257 and ASTM D 150, respectively.

2.5 Differential scanning calorimetry

The effect of MgO nanoparticles on the melting characteristics of PAEK was measured using differential scanning calorimetry (TA Instruments, DSC model 821, USA) operated under a nitrogen atmosphere at a heating rate of 10°C/min in the temperature range of 50°C–400°C.

2.6 Thermo-mechanical analysis

Thermo-mechanical analyzer (TMA) (TA Instruments, TA450, USA) was used to assess the penetration and dimensional changes of PAEK/MgO nanocomposites over a temperature range of 25°C–350°C. It consists of a highly sensitive linear variable differential transformer, low mass furnace and a precise motor for probe loading. Samples were run at a heating rate of 10°C/min from room temperature to 350°C under nitrogen atmosphere. The experiments were carried out in accordance with ASTM E 1131.

Thermal conductivity studies were carried out by the hot transient method as per ASTM 22007 Part 2, 2008 using TPS 500 model thermal conductivity analyzer. The penetration probe and general probe were used for TMA evaluation of PAEK/MgO nanocomposites. The area of penetration was calculated by triangle approximation. The penetration probe has a contact diameter of 0.89 mm and the expansion probe has a diameter of 2.54 mm. The probe was subjected to a load of 0.5 N and the experiment was carried out as per ISO 1359 Part 3.

The CTE behavior of PAEK nanocomposites was carried out using the expansion probe method (TA instruments Q400 make, USA). The diameter of expansion probe was 3 mm and the measurements were made at an applied force of 0.5 N under nitrogen atmosphere with a flow of 50 ml/min. The dimensional change with temperature (μm/°C) was measured for all the nanocomposites.

3.1 Dispersion of MgO nanoparticles in PAEK

The dispersion of MgO nanofiller in PAEK polymer was observed using TEM and the image is shown in Figure 1. The TEM image of the PAEK/MgO nanocomposites showed that the particles are in the nanometric dimensions of ~35 nm. The metal nitrate and the urea solution were allowed for more than 48 h which enhances the nanoparticle size range and its distribution. The uniformity of the distribution of MgO nanoparticles in PAEK matrix polymer was examined by SEM images. Figure 2 (A–D) represents the SEM micrographs of 0, 2, 6 and 10 wt% MgO-filled PAEK nanocomposites, respectively. These micrographs show a homogenous morphology. No localized domains and no phase segregation were observed even at the nanometer scale, implying that MgO nanoparticles were well dispersed in the PAEK matrix without any MgO aggregation. In the present study the experiments are carried out for the fabrication of unmodified MgO nanoparticles in PAEK matrix. In future work, modification of MgO nanoparticles is carried out and the results are compared.

Figure 1:

TEM image of MgO nanoparticles.

Figure 2:

SEM images of PAEK-MgO nanocomposites: (A) 0 wt%, (B) 2 wt%, (C) 4 wt% and (D) 6 wt% of MgO nanoparticles.

3.2 Dielectric properties

The dielectric property of the material generally depends on the mechanism of interaction of the molecules with the applied electric field. Figure 3 shows the effect of frequency on dielectric constant (ε_r) of the PAEK/MgO nanocomposites.

Figure 3:

Effect of frequency on dielectric constant for PAEK-MgO nanocomposites.

3.2.2 Effect of temperature on dielectric constant

From Figure 4, it can be seen that the dielectric constant (ε_r) of nanocomposites almost remains constant ~2.9 irrespective of the filler loading up to 150°C. Beyond 150°C ε_r increases rapidly with an increase in temperature. The variation of ε_r may depend on several factors such as: (i) polarizability of the moieties on the chain, (ii) pendant groups hanging on the chain, (iii) orientation of the groups and their interaction with the surrounding groups, (iv) segmentation movement of polymeric chain, (vi) their synchronization with the applied frequency, and (vii) accompanied losses. The PAEKs are nonpolar in nature and have aromatic rings coupled with the ether and ketone groups which may tend to exhibit lower polarizability of the chain and stiffness of the ketone groups against the flexibility of ether linkages. Therefore, the capacitive currents arising due to the polarization of dipoles may be limited which holds the charges. This property may make the nanocomposites more suitable for power transformer cable insulation applications. The polymer chain as well as pendant groups of PAEK does not exhibit high polarizability at the temperature below 100°C, hence the ε_r variation is not high. Due to the inclusion of metal oxide nanofillers into PAEK, there might be slight reduction in free volume. Beyond 100°C there is an increase in ε_r as shown in Figure 4, which might be due to the expanding nature of the PAEK polymer.

Figure 4:

Effect of temperature on dielectric constant for PAEK-MgO nanocomposites.

3.2.3 Effect of frequency on tan δ

The effect of frequency on tan δ of PAEK/MgO nanocomposites at room temperature is shown in Figure 5 as a function of MgO nanofiller concentration. The PAEK/MgO nanocomposites showed an increase in tan δ values with increase in frequency, but at frequencies greater than 750 Hz the tan δ values decrease with an increase in frequency. At frequencies lower than 750 Hz tan δ is higher in PAEK/MgO nanocomposites, which might be due to lower energy level activations of charge carriers occurring due to dipole relaxations at these frequencies. With increase in frequency above 750 Hz the overall rigidity of the back bone chains of PAEK and its nanocomposites may lead to lower dipolar orientation losses which might result in lower tan δ. As seen, tan δ decreases with increasing content of MgO nanofillers. This effect may reduce the interaction between polymer chains and polar pendant interactions with the applied electric field. The moisture present in the nanocomposite samples may also contribute to higher losses in this frequency range.

Figure 5:

Effect of frequency on tan δ for PAEK-MgO nanocomposites.

3.2.4 Effect of temperature on tan δ

Figure 6 shows the effect of temperature on tan δ of PAEK/MgO nanocomposites. It has been observed that the tan δ of the nanocomposites are fairly constant up to 150°C, and then afterwards it increases with increase in temperature. The polymer PAEK and its nanocomposites have shown very low values of tan δ ranging from 0.0001 at room temperature to 0.0025 at 150°C. This behavior is attributed to the presence of the low polarizable component and high ketone component in PAEK molecule as compared to conventional PEEK polymer available in the market. This makes the molecular chains in PAEK more rigid and compact leading to higher melting point and higher thermal stability at these temperatures. Generally, lower tan δ indicates the suitability of the polymeric composites for dielectric applications such as power transformer windings. Another reason for low values of tan δ may be the nonsynchronization of the polarization process and applied oscillating electric potential within the nanocomposite system as observed by Ref. [12]. Polarization in the polymer composite is due to orientation, atomic and electronic polarization. With the application of an electric field the polarization of the molecules takes place and once the potential is reversed the depolarization process is not able to follow the change in the field which results in dielectric loss within the material.

Figure 6:

Effect of temperature on tan δ for PAEK-MgO nanocomposites.

The resulting polarization and its nonsynchronization with the changing potential, leads to dielectric losses known as dielectric relaxation. The dielectric relaxation in PAEK/MgO nanocomposites might depend on the range of energy loss mechanisms (α, β and γ relaxation) and temperature. The α relaxation is a high energy range loss mechanism which may be due to the glass transition and the polymer chain movements within the nanocomposites [13, 14]. Beta relaxation is a medium energy range loss mechanism, may be due to conformational flips of the cyclic units in the chain and also due to rotation of polar groups around the main chain of the polymeric system while γ relaxation is the low energy range loss whose variation may be due to the segmental motion of polymeric chains. Increase in tan δ at temperatures above 200°C may be due to contaminating species like moisture, presence of polar gases due to decomposition, partial discharges of polar species in the voids and cavities, thermally excited and rheological movement of ions and species within the system.

3.2.5 Effect of temperature on volume resistivity

Figure 7 shows the effect of temperature on log volume resistivity (ρ_v) for PAEK/MgO nanocomposites. From the figure three distinct regions in ρ_v are observed: (i) 25°C to about 100°C, (ii) about 100°C–175°C and (iii) about 175°C–225°C. The obtained ρ_v for the PAEK nanocomposites is above 1×10¹⁶ indicating they are more insulating in nature. This might be due to the absence of polarizable moieties in the matrix at low temperature resulting in very low leakage current flow from one end to another end of the electrode. In regions (i) and (ii) ρ_v decreases insignificantly with increasing temperature up to about 100°C after which it decreases significantly with increasing temperature. This decrease in ρ_v may be due to the appearance of moisture molecules that are gaseous and polar in nature. In region (iii) the volatile and dissociation products in the nanocomposites contribute to the formation of polar moieties which generate leakage currents, thereby reducing the ρ_v. In PAEK family perfect chain packing is difficult and this is manifested by maximal achievable crystallinity of <50%. The PAEK polymers are considered to have good thermal stability which disallows the formation of degradation products. Hence, PAEK/MgO nanocomposites are able to maintain high volume resistivities in the measured range of temperatures.

Figure 7:

Effect of temperature on ρ_v for PAEK-MgO nanocomposites.

3.3 Differential calorimetric analysis

The differential scanning calorimetry (DSC) heating curves corresponding to PAEK and PAEK/MgO nanocomposites are shown in Figure 8. In DSC heating curves, two distinct thermal zones are observed. The first zone is in the region near 300°C where the polymer starts to soften at the onset melting temperature (T_m^on), and the polymer completely melts at 350°C and is designated as the melting temperature (T_m). It was observed that the peak value, T_m is slightly deviated with the addition of MgO nanoparticles. Existing literature on the subject indicates that the thermal properties of PEEK polymer composites largely depend upon the type of fillers, processing conditions, and matrix-filler interactions [8, 17, 18]. In the present study, even though there is no significant change in the T_m values, drastic change in the T_m^on was observed with increase in the filler loading of MgO. In pure PAEK polymer, the melting starts around 325°C whereas, for the PAEK/MgO nanocomposites with 2 wt% filler loading melting starts around 350°C. The melting temperature does not change unless a polymer is chemically modified at the molecular level [19]. In the present study, not much difference is observed in the melting temperature of PAEK/MgO nanocomposites, this might be due to the physical interaction of nanofiller phase with PAEK. Because of this, the molecular nature of the polymer was unaffected. However, MgO nanofillers can effectively retard the segmental movement of PAEK polymer chains thereby increasing the onset melting temperature of the composites. Therefore, the semi-crystalline aromatic polymers such as PAEK can maintain good strength even at high temperatures.

Figure 8:

DSC plot of PAEK-MgO nanocomposites.

3.4 Thermo-mechanical analysis

Dimensional stability of the composite material at high temperature regions is an important characteristic of the dielectric material during its service in the power equipment. Figure 9A,B shows the plot of thermo-mechanical analysis (TMA) for PAEK and PAEK/MgO nanocomposites. The penetration probe provides data on the softening properties of thermoplastics. This probe is placed at the geometric center of the top portion of the square sample. Due to the influence of the heat and the applied force on the penetration probe, the sample sags uniformly without diminishing in its sectional thickness. As opposed to PAEK/MgO nanocomposites, the PAEK sample shows appreciable penetration of the probe indicating material softening.

Figure 9:

TMA plots using (A) penetration probe and (B) expansion probe for PAEK-MgO nanocomposites.

It is important that, at high temperatures, penetration into the material indicates how well the material has softened and sticks to the conductor. High penetration indicates that the material is softening and a dimensional change has taken place. From Figure 8, we can observe that T_g of the PAEK and PAEK/MgO nanocomposites is ~153°C the variation in the penetration probe senses the dimensional change of material and returns to the normal conditions. With further increase in temperature above T_g, the PAEK/MgO nanocomposites have maintained their dimensional stability with variation in the force applied by the penetration probe. This behavior in nanocomposite is due to the filling effect of MgO which provides the dimensional compactness at T_g and beyond T_g. From Figure 9A it is also observed that the effects of the dimensional stability using a penetration probe has a much smaller contact diameter and is able to penetrate more, hence the softening is much pronounced in these plots. Figure 9B shows the TMA plot for PAEK and PAEK/MgO nanocomposites using the expansion probe method. In this method, the probe rests on the surface of the test specimen under low loadings. Initially, as the temperature increases from room temperature, the sample expands and the probe is pushed up. This results in the expansion of the sample which can be measured. In this method also, the observed T_g is ~153°C. At T_g, the PAEK matrix exhibits a significant change in slope due to an increase in its rate of expansion, whereas the rate of expansion is lower for PAEK/MgO nanocomposites due to the filling effect of the MgO nanofiller. The onset temperature of this change in expansion behavior is due to the T_g of PAEK nanocomposites. On the other hand, voids are typically not desired in the polymer composites used for dielectric applications. They create a suitable space for allowing moisture, air and other polar contaminants in the material. Under an applied electric potential the presence of voids leads to partial discharges, increases the dielectric loss, and degrades the dielectric leading to ultimate failure. Hence, it would be desirable to have the polymer composites compact and devoid of air gaps so that they can perform well as dielectrics. In the present study, a high degree of packing devoid of pores would have facilitated the reduction of the softening of composite at the tested temperatures.

3.5 Thermal conductivity

Normally polymers are poor conductors of heat. In order to improve the thermal conductivity (γ) of PAEK, MgO nanoparticles were selected whose γ was 38 W/mK [20], thereby enhancing the overall γ of the nanocomposite. The γ values of the PAEK/MgO nanocomposites are listed in Table 2. From Table 2, it was observed that, γ and diffusivity have been increased with increase in MgO loading and the improvement of γ for the studied range of MgO content is about 12.7%. Thermal conductivity is a kinetic property determined by the contributions from the vibration, rotational and electronic degrees of freedom. In the tested range of temperature range 300°C, the conduction in MgO is essentially considered as ionic in nature [16], which depends on magnesium cation vacancies, lower activation energy during the conduction process, and crystalline surfaces. As MgO nanofillers are considered as high temperature insulators heat transfer within the composite system occurs at a higher rate due to increased γ. From the solid state theory, an increase in temperature causes an increase in the intermolecular vibrations, which is a dominant mechanism for heat transfer. In the present study, 10 wt% nanocomposites has high value of thermal conductivity which means that heat travels easily through the material. With the increase in filler loading thermal diffusivity increases by 16% and is maximum at 10 wt% nanofiller loading. This signifies that at 10 wt% nanocomposite will experience a quick rise in the temperature. In contrast, pure PAEK exhibits a high value of specific heat, thus it requires more heat to raise the temperature of the material, but with the increase in filler loading this effect slightly decreases.

3.6 Coefficient of thermal expansion

The CTE values of PAEK/MgO nanocomposites are listed in Table 3. The CTE is a quantitative assessment of the expansion of a material over a temperature interval. In designing a structural material, dimensional stability with temperature is a very crucial factor. From Table 3, it is observed that the CTE was reduced by the addition of MgO nanofillers after T_g, i.e. between temperature 250°C and 260°C as compared to pure PAEK. The CTE is almost constant with the addition of MgO nanofillers when the temperature is above and below T_g. The reason for higher CTE above T_g as compared to below T_g may be due to the greater segmental mobility of PAEK polymer chains in the amorphous domains.

Lowering of CTE may be due to several factors: (i) reduction in free volume, which reduces the expansion of PAEK polymer, (ii) hindrance of segmental movement of polymer chains within the composite material, and (iii) increase in volume of the polymer chains. Several other researchers have shown that, in ceramic polymer composites the ceramic particles are surrounded by tightly bound (constrained) and loosely bound (unconstrained) polymer chains. With the addition of more reinforcement particles, unconstrained polymer chains get converted to constrained chains as a result of reduction in the inter-particle distance. This effect retards the thermal expansion of the polymer composites [8, 21]. In the present study, during composite fabrication, the MgO nanoparticles might be randomly aligned in both lateral and horizontal directions within the PAEK matrix which prevents the expansion of PAEK matrix. It was reported that the bulk CTE of MgO is only 13.5°C [19], which is much less than the pure PAEK. This may also be a probable reason for the reduction in CTE upon MgO nanoparticles inclusion. Nanoparticles provide a larger surface area than conventional microparticles and hence are more effective in reducing the CTE of polymer matrices [22].