Hybrid nanocomposites based on poly aryl ether ketone, boron carbide and multi walled carbon nanotubes: evaluation of tensile, dynamic mechanical and thermal degradation properties

4.1 Morphological interpretation

Figure 1A–C shows the SEM images of B₄C nanopowder and MWCNTs. The micrograph clearly reveals the spherical structure of B₄C nanopowder and the rod-like structure of MWCNT and F-MWCNT. As the nanofillers are in powder form, they are seen as agglomerated particles in the SEM morphology. TEM studies show the type of distribution of the nanofillers in the base matrix (14). Figure 2A–E shows the TEM micrographs explaining the distribution of individual and hybrid nano reinforcements such as 0.75FC, 0.75B, 0.75BFC in PAEK. Figure 2A shows the dispersion of 0.75% F-MWCNT reinforcement. The TEM morphology depicts that the reinforcement F-MWCNT is in nanometer scale and suggests good dispersion in PAEK. It indicates the outer diameter and inner diameter of MWCNT as 20 and 16 nm, respectively.

Figure 1:

SEM images of (A) B₄C nanopowder (B) MWCNT (C) F-MWCNT.

Figure 2:

TEM image of (A) 0.75FC (B) 0.75B (C) 0.75B at 100 nm scale (D) 0.75BFC (E) higher magnified TEM image of 0.75BFC.

Figure 2B reveals the morphology of 0.75% B₄C nanofillers in the matrix. The TEM image shows that the B₄C nanopowder is separated and dispersed throughout the matrix. The shape and particle size of the B₄C nanopowder can be measured from the micrograph. It is found that the average particle size of B₄C ranges from 30 to 60 nm. The B₄C nanoparticle is clearly visible by its shape and size. TEM images typically provide a small sampling for the overall material. As the B₄C particle size is of nanometer level, TEM is the best analysis method to know the information about the particle size. For large sampling for the overall material the TEM image of 100 nm scale is shown in Figure 2C.

Figure 2D depicts the TEM image of 0.75% of B₄C and F-MWCNT nanofillers in the polymer PAEK. It is found that the outer diameter of F-MWCNT is same as that of the individual composite based on MWCNT. Similarly B₄C particle size is also not varied by the hybrid inclusions. The higher magnified TEM morphology of 0.75BFC is shown in Figure 2E. From the morphological analysis, it can be inferred that the twin screw extrusion process enabled the nanofillers to distribute properly in PAEK.

4.2 Fourier transform infrared spectroscopy (FTIR)

FTIR analysis can be employed to ensure whether the surface treatment or functionalization of filler is successfully carried out.

Figure 3A shows the FTIR spectra of PAEK, MWCNT, F-MWCNT, B₄C and TB₄C. IR spectra of PAEK give the following details. The band at 1654 cm⁻¹ shows the C=O stretching. The peak at 1585 cm⁻¹ shows the benzene ring plane R-O-R vibration spectrum. At 1305 cm⁻¹ R-CO-R stretching within the benzene ring plane vibration spectrum is seen. The CH bending of the aryl ketone structure within the benzene ring plane is at 1157cm⁻¹. R-CO-R of the symmetric stretching is obtained at 925 cm⁻¹ (15). For B₄C nanopowder FTIR shows two strong bands at 1581 cm⁻¹ and at 1024 cm⁻¹ which shows C-B-C and B-C stretching (16, 17). The broad form of B-C, C-C and BC clusters in the vibration spectrum shows the presence of amorphous materials (17). The band at 3432 cm⁻¹ and 1455 cm⁻¹ indicate the O-H and B-O stretching, respectively (17, 18). The FTIR spectra for silane coupling agents indicates C-H stretching at 2942 and 2843 cm⁻¹, CH₂-O-CH₂ stretching at 1193 cm⁻¹ (19, 20). The Si-O-CH₃ stretching is seen at 1088 cm⁻¹ and epoxy ring at 1255 and 908 cm⁻¹ (21, 22). The 1156 cm⁻¹ band indicates Si-OH stretching which is absent in the FTIR spectra of untreated B₄C nanopowder. B-C, Si-O-Si stretching occurs in the 1102 cm⁻¹ band. CH-CH₂ stretching intensity reduced at 2922 and 2656 cm⁻¹ after silane treatment. C-B-C stretching occurs at the 1648 cm⁻¹ band. Bi-O-Si stretching occurs at bands 1410, 926, 851 cm⁻¹ (3). These data clearly suggests that silane functional group is attached to B₄C nanopowder.

Figure 3:

(A) FTIR spectra of PAEK, MWCNT, F-MWCNT, B₄C and TB₄C. (B) Schematic representation of the interaction between PAEK and COOH modified MWCNT.

In the FTIR spectrum of MWCNT, the O-H stretch took place at 3400 cm⁻¹. The peak at 1630 cm⁻¹ refers the backbone stretching of carbon nanotube. The carboxyl group (COOH) stretching occurs at 1720 cm⁻¹ band during -COOH modification of MWCNT (23). At 1720 cm⁻¹ in MWCNT the carboxyl group stretching is missing. But in F-MWCNT FTIR spectra the carboxyl group peak is at 1720 cm⁻¹ band. The broad absorption band at 3432 cm⁻¹ in F-MWCNT shows that the oscillation of carboxyl group. In the F-MWCNT the vibration at 3432 cm⁻¹ suggests the O-H stretch from carboxyl group indicating -COOH functional groups are clearly attached on nanotubes (24).

When pristine MWCNT is introduced, the peak at 1654 cm⁻¹ in PAEK slightly shifted to 1652 cm⁻¹ which shows that it has very little interaction with PAEK. A good shift is observed when functionalized MWCNT (F-MWCNT) was incorporated in the PAEK. The band shifted to 1644 cm⁻¹ manifesting good interaction between the reinforcement and the matrix. This proves that the -COOH modification provides stronger interfacial interaction between the polymer matrix and the CNT. It was reported by Roy (25) that hydrogen bonding will be formed between -COOH and ketone groups. Figure 3B shows the scheme of this interaction.

4.3 Tensile properties

The tensile properties of PAEK with individual and hybrid loadings of nanofillers such as treated and untreated B₄C nanopowder, MWCNT and F-MWCNT are shown in Table 2. The stress-strain curves of PAEK and different composites are delineated in Figure 4. The tensile strength and modulus of neat PAEK is 100 MPa and 2442 MPa, respectively. In the individual effect it is observed that the tensile strength of 0.75B reduced to 97.7 MPa, but the modulus enhanced to 2989 MPa which is evident from the increase in the slope of the initial linear portion of the stress-strain curve. Even a minor loading of B₄C (0.75%) made the composite brittle as evident from the low elongation at break of 5.6%. The earlier studies (3) exhibited similar reduction in the mechanical properties of the composites based on HDPE. It is observed that the tensile strength and modulus of 0.75C is 102.6 MPa and 2503 MPa, respectively, which shows that there is no significant improvement in the tensile properties compared to PAEK. A remarkable increase in tensile strength is obtained in 0.75FC. It is found that the tensile strength is 108 MPa and elongation at break is 25% which made the composite to exhibit a ductile failure. This reveals that the functionalization of MWCNT improved the tensile properties.

Figure 4:

Stress strain curves of PAEK and nanocomposites.

The tensile properties of hybrid composites, 1.5BFC, 2.25BFC and 3BFC are indicated in Table 2. There is remarkable reduction in the tensile properties. It is clearly seen from the graph that as the weight percentage of the hybrid nanofillers in the polymer increased beyond 0.75%, the tensile strength reduced. In order to analyze the effect of treatment and functionalization of nanofillers 0.375 wt% each of B₄C and MWCNT were made use of. In 0.75BC the tensile strength and elongation at the break reduced to 95.2 MPa and 4.5%, respectively. The composite 0.75BFC indicated the tensile strength and modulus value of 101 MPa and 3011 MPa, respectively. However the elongation at the break increased to 23.5%. Among the hybrid composites 0.75TBFC shows good tensile properties. The tensile strength and modulus of 0.75TBFC was obtained as 104.8 MPa and 3262 MPa, respectively, with 14% elongation at the break. This clearly reveals that both treated and untreated B₄C in hybrid composites have comparable tensile modulus at 0.75% loading of nanofillers. During extrusion of the 0.75TBFC composite there was a color change in the composite as the strands emerged from the die. This may be due to entrapment of volatiles generated during silane surface treatment which are difficult to remove. As the properties of 0.75TBFC and 0.75BFC are comparable it was decided to carry out the experiments using untreated B₄C and F-MWCNT.

In the tensile strength of the composite, the properties and structure of the filler matrix interface play an important role. Boron carbide is a hard ceramic particle. By the incorporation of boron carbide, the composite tensile strength reduces and becomes brittle as the reinforcement percentages increases. This is due to the lack of proper bonding between boron carbide and base matrix PAEK. The advantage by the inclusion of boron carbide in the matrix is that the modulus of the composite increased. In the present work the strength of the hybrid composite increased at room temperature when silane treatment was carried out on boron carbide.

4.4 Dynamic mechanical analysis (DMA)

To study the thermo mechanical behavior of polymer composites at elevated temperature, DMA is considered to be a powerful tool (26).

4.4.1 Storage modulus (E′)

Storage modulus (E′) is regarded as the ability of the material to store energy during deformation and it is an indicator of the elastic characteristics of viscoelastic materials. Figure 5A represents the variation of E′ with the temperature of PAEK, individual and hybrid nano composites. In comparison with PAEK it is found that there is an enhancement in the storage modulus for 3BFC, 0.75TBFC and 0.75BFC. The individual nano composite 0.75FC indicates almost the same storage modulus compared to the base matrix. There is a decrease in the storage modulus (E′) for 0.75BC and 0.75B. This can be attributed to the poor interaction between hard B₄C particles in the matrix. The variation of the storage modulus at different temperatures is depicted in Figure 5B. It is clear from the plot that at room temperature the effect of B₄C treatment shows good results. The storage modulus of 0.75 TBFC is found to be highest amongst all the composites with a value of 1350 MPa. The effect of silane treatment was, however, found to be less influential in maintaining the storage modulus at elevated temperature of 150°C. At this temperature the highest modulus was obtained for 0.75BFC, 1050 MPa in comparison with 950 MPa for 0.75TBFC. This indicates that the acid modification of MWCNT is contributing much more than silane treatment of B₄C towards the stiffness at higher temperature.

Figure 5:

(A) Variation of storage modulus with temperature for PAEK and nanocomposites. (B) Variation of storage modulus of PAEK and nanocomposites at 50°C, 75°C, 100°C, 125°C, 150°C, and 175°C. (C) Variation of normalized storage modulus of individual and hybrid nanocomposites at 50°C, 75°C, 100°C, 125°C, 150°C, 175°C.

The normalized storage modulus is defined as the ratio of the storage modulus of the composite to the storage modulus of the matrix at the same temperature (27). The influence of individual and hybrid reinforcement in the base matrix PAEK at different temperatures are studied and illustrated in Figure 5C. It is evident from the figure the individual composite 0.75FC shows an increase in the normalized storage modulus in the entire temperature range. In the case of the hybrid effect of loadings, at room temperature, 0.75TBFC shows an increase in the normalized storage modulus compared to 0.75BFC. As the temperature increases, 0.75BFC exhibits higher normalized storage modulus compared to 0.75TBFC. This explains that at higher temperature the 0.75TBFC composite is not stable due to the decomposition of silanes at higher temperatures. The higher normalized storage modulus value for 0.75BFC further indicates the hindrance offered by the nanofillers to the movement of polymer molecules at elevated temperatures of 150°C and 175°C. The contribution of functionalization of MWCNT in the 0.75BFC plays an important role for the stiffness of the composite at higher temperature which is evident from the graph. From the trace of the sample 2 and 4, it is seen that functionalization of MWCNT contributes greatly to maintaining higher stiffness at 175°C.

4.4.2 Loss modulus (E″)

Loss modulus (E″) or dynamic loss modulus represents the viscous response and tendency to dissipate energy as heat (28). The variation of loss modulus with temperature for individual and hybrid reinforcement in PAEK is shown in Figure 6. Amongst the individual nano composites, 0.75FC exhibits higher energy dissipation compared to 0.75B. It is observed from the graph that 0.75BFC manifest a higher value of loss modulus compared to individual and other hybrid composites. Thus 0.75BFC dissipates maximum energy as heat and indicate high viscous response. In the case of 0.75TBFC composite, viscous dissipation was poor as compared to 0.75BFC. It was reported that high loss modulus indicates the restriction on the segmental mobility of the polymer chain (29).

Figure 6:

Variation of loss modulus of PAEK and nanocomposites.

4.4.3 Damping factor (tan δ)

The damping factor or tan δ regarded as a dimensionless number is defined as the ratio of loss modulus (E″) to storage modulus (E′). Figure 7 shows the variation of the damping factor with the temperature for different individual and hybrid reinforcement in PAEK. If the matrix and reinforcement shows good interfacial bonding, it results in the reduction in the peak of the tan δ curve. The T_g of the composite plays an important role in the tan δ analysis. The positive shift in the T_g value shows good mechanical stability of the composite. At a particular frequency the broadening of tan δ curve indicates the hindrance to the slippage of the polymer molecules (30).

Figure 7:

Variation of mechanical damping factor of PAEK and nanocomposites.

Compared to base matrix all individual and hybrid composites indicate a reduction in the tan δ peak value. In the individual effect it is observed that the damping factor is less for 0.75B compared to 0.75FC. In the hybrid effect it is found that the damping factor of 0.75TBFC is marginally lower than that of 0.75BFC and other hybrid composites. This indicates high elasticity and noise absorption of the 0.75TBFC at temperatures below 150°C.

Due to silane treatment better bonding between boron carbide and PAEK takes place in 0.75TBFC which causes the lowering of tan δ peak value. But positive right side shift of the peak (corresponding to T_g) was not seen compared to 0.75BFC. This indicates that the 0.75TBFC composite becomes flexible at higher temperatures like 150°C and 175°C. Elevation of T_g is taken as a measure of interfacial interaction. In the case of 0.75BFC broadening of the peak is seen which can be attributed to restricted relaxation (27). The composite with higher value of T_g indicates that the nanofillers impede the mobility of polymer chains. For the hybrid effect of reinforcement, it is found from the graph that the 0.75BFC shows maximum positive T_g shift compared to other composites and 5°C to base matrix PAEK. So 0.75BFC is stable and rigid compared to other composites. It shows a good interaction between the reinforcement and matrix which require higher temperature to initiate the slippage of coiled polymer chains past the nanofillers. The peak value obtained is lower for 3BFC composite due to higher reinforcement percentage of nanofillers but they are comparatively ineffective in reinforcing the polymer as there is no evidence of positive shift of the relaxation peak.

4.5 Thermogravimetric analysis (TGA)

The loading of fillers in the polymer can improve the resistivity and their interaction helps in delaying the degradation of the polymer composite (31). Thermal decomposition studies of polymer composites used in radiation resistant application is of paramount importance as the service temperature is expected to be high. TGA analysis is expected to throw light on the influence of B₄C and MWCNT on the decomposition of PAEK. The thermal degradation parameters of PAEK, 0.75B, 0.75FC, 0.75TBFC and 0.75BFC are presented in Table 3. T_onset represents the temperature at which the degradation is initiated and T_max the temperature at which the weight loss is maximum. T_0.1 and T_0.4 indicate the temperature at 10% and 40% weight loss, respectively. The yield at 800°C is also reported.

The loss of weight with temperature for the various samples is delineated in Figure 8A. It is clearly seen that all the individual and hybrid composites decomposed in a single step. It is observed from Table 3 and Figure 8A that among all the composites, 0.75B and 0.75BFC shows useful results. T_onset of both the samples are shifted to 554°C from 530°C of PAEK. In 0.75TBFC the onset degradation temperature is 540.5°C. It can be inferred that the presence of untreated B₄C helps much more than the treated counterpart in delaying the degradation. The major finding is that the temperature at 40% weight loss for 0.75B and 0.75BFC is almost 100°C higher than that of PAEK. At 800°C it is found that around 58% material remains for both 0.75B and 0.75BFCwhich is 6% more than that of neat PAEK.

Figure 8:

(A) Dynamic TG profiles of PAEK and nanocomposites. (B) The derivative thermogram of PAEK and nanocomposites.

Figure 8B shows the derivative thermogravimetric curves. It gives information on the temperature at which maximum weight loss happens (32). From the plot it is observed that 0.75B and 0.75BFC show good results as they indicate a positive right side shift for the peak of the curve. It is found that for both the samples around 10°C a positive right side shift has occurred compared to PAEK. This shift is only 2°C for TBFC suggesting the inability of treated B₄C in delaying the degradation.