1 Introduction

The application of polymers is on the rise due to their advantages over conventional materials (e.g., metals). Polymers are light in weight, corrosion-resistant, and are easily produced at low cost (Aguilar and San Román 2019; Khutoryanskiy and Georgiou 2018; Nezakati et al. 2018; Yuan et al. 2012). While several advancements have been made in polymer technology, their physical properties can be further enhanced by the addition of different fillers such as metal particles, semiconductors, organic and inorganic particles, carbon, ceramics and fibers among others (Abd-Elnaiem et al. 2020; Hussein et al. 2018; Prasher et al. 2006). Polymers modified with different fillers have enhanced structural, optical, thermal, electrical, mechanical and magnetic properties. This widens the scope of their applications ranging from structural units in the construction industry to aerospace and medical applications (Huang et al. 2003). Polymethyl methacrylate (PMMA), similar to other polymers, is transparent, compatible with the human body, has low weight, and possesses excellent optical clarity and good weathering behavior (Logakis et al. 2011). However, PMMA is brittle and insulating hence it cannot be deployed for applications where ductility or conductivity is an important consideration. Several studies have been conducted to enhance the mechanical properties of PMMA (Landry et al. 1992; Ash et al. 2002; Chen et al. 2007; Stipho 1998; Layek et al. 2010; Zhang et al. 2018; Pahlevanzadeh et al. 2019; Münker et al. 2018). Among them are blending and copolymerization (Logakis et al. 2011) which have significantly enhanced the toughness of the polymer.

Multi-Walled Carbon Nanotubes (MWCNTs) are considered as one of the most important nano-reinforcing materials because of their high modulus (~ 1 TPa) and tensile strength (~ 63 GPa) emanated from the strong bonds between the carbon atoms (Meysami et al. 2017; Zhi et al. 2008; Huang et al. 2011; Deep and Mishra 2018; Mir et al. 2016; Faraguna et al. 2018; Mittal 2014). They have been widely deployed for reinforcement of polymer with enhanced physical and mechanical properties (Faraguna et al. 2018). The degree of enhancement depends on how well CNTs disperse in the matrix and the subsequent interfacial interactions between them. A study showed that the presence of 1 wt% MWCNTs improved the thermal stability and thermal conductivity of PMMA/MWCNTs composites (Deep and Mishra 2018). Blond et al. (2006) enhanced Young’s modulus and strength of poly(vinyl alcohol) based composites by 3.7 and 3.9, respectively, by adding 1 wt% of CNTs. In this case, CNTs provide surfaces for better stress transfer. In the simulation results of Arash et al. (2014), the Young’s modulus of PMMA/CNT polymer composite increased from 3.9 to 6.85 GPa when 5 wt% CNTs was incorporated. Mir et al. reported a higher electrical percolation threshold for PMMA/MWCNTs composites compared to other published data (Mir et al. 2016). The strengthening efficiency of MWCNTs can also be improved  by coating the surfaces with surfactants or compatibilizers (Mittal 2014). Due to the interesting properties of the developed PMMA/MWCNTs composites, they are being deployed for several applications in aerospace, automobile, sports, bone-bonding ability, hydrogen gas permeation, electronics sectors among others (Münker et al. 2018; Nakason et al. 2005; Kumar et al. 2010; Yuen et al. 2008; Nawar and El-Mahalawy 2019; Su and Wang 2007; Wang et al. 2019).

Currently, limited reports on electrical and mechanical properties of PMMA/MWCNTs composites are available in the literature. The present investigation aims to extend the previous studies by investigating the influence of low concentrations (0–0.5 wt%) of MWCNTs on the mechanical and electrical properties of PMMA/MWCNTs composites synthesized by a solution casting method. Various mechanical parameters of the PMMA/MWCNTs composites such as tensile strength, elongation, tear resistance, fracture energy, impact strength, and hardness were investigated. Moreover, the electrical conductivity as a function of temperature was studied and the value of the percolation threshold was determined and discussed. In addition, a mechanism describing the influence of MWCNTs on electrical and mechanical properties of PMMA/MWCNTs composites was proposed.

2 Experimental Details

2.1 Materials

The PMMA (Mw ~ 15,000 by GPC) samples were obtained from Sigma-Aldrich Company while the MWCNTs (95%), having an outer diameter of 5–15 nm, inside diameter of 3–5 nm and length of 50 µm, were purchased from US Research nanomaterials, INC., Houston, USA.

2.2 Preparation of PMMA/MWCNTs Composites

The MWCNTs have grain size distribution within the range of 12–14 nm (Fig. 1). The preparation of the PMMA/MWCNTs composites followed the procedure depicted in Fig. 2a. Solution A was made from 2 g of pure PMMA dissolved in 10 ml of Chloroform (CHCl3) and then stirred with a magnetic stirrer at 60 °C for 1 h to obtain a grade solution of PMMA. Similarly, solution B was prepared from MWCNTs dissolved in 10 ml of CHCl3 and agitated ultrasonically at 30 °C for 10 min. Thereafter, solutions A and B were mixed and stirred vigorously for 1 h at 60 °C. The final mixture was then thoroughly sonicated until a stable, black-colored solution of PMMA/MWCNTs composite was formed. The PMMA/MWCNTs samples were prepared with 0.1, 0.3 and 0.5 wt% MWCNTs. They were then cast on the Petri dish and kept at 60 °C in an oven for 24 h to ensure the solvent was completely removed. The optical images of the prepared PMMA/MWCNTs thin sheet samples are presented in Fig. 2b. The PMMA sample without MWCNTs was considered as the control.

Fig. 1
figure 1

Normal distribution histogram for MWCNTs nanoparticles

Full size image
Fig. 2
figure 2

a A schematic illustration of the fabrication procedure, and b optical images of the PMMA/MWCNTs composites (sample dimension is 2 cm × 2 cm)

Full size image

2.3 Characterizations of PMMA/MWCNTs Composites

The thicknesses of the PMMA and PMMA/MWCNTs composites were measured with an electronic digital micrometer (IDM) of ± 0.001 sensitivity. The IDM cycled up and down automatically at the push of a button, providing an accurate and repeatable reading at the end of each cycle. The sheet samples were ~ 115 µm thick.

The infrared spectra of the sheet samples were recorded with SHIMADZU FTIR-7600 in the range of 400–4000 cm−1. The Fourier-transform infrared spectroscopy (FTIR) spectra gave information about the chemical structures of pure PMMA and PMMA/MWCNTs composites. The surface morphology of the composites was investigated by JEOL JSM-5600LV scanning electron microscopy (SEM, Tokyo Japan). The SEM images were analyzed for the grain size and distribution using image processing software (ImageJ, Version 1.37).

The tensile test was conducted at room temperature (~ 30 °C) using Instron 4467 tensile tester following the ASTMD-882 standard (D882-18 2018). Sheet samples of dimensions 10 mm × 80 mm × 0.115 mm were tested at an extension rate of 5 mm/min while the Young modulus and tensile strength were obtained from the stress–strain curves. Tear resistance (tear strength) is another mechanical property associated with material failure and is normally quoted in terms of tear force per unit thickness N/mm (Kaufman 2001). Tear strengths of the samples were determined using Universal Electronic Dynamometer according to ASTM D-1922 standard. The sample (100 mm long by 60 mm wide) has 50 mm cut at the center. The experiments were conducted at 180 mm/min extension rate. A pendulum impact tester was used to measure the force required to propagate slit at a fixed distance to the edge of the test sample. Shore D hardness was conducted to measure the hardness of 170 mm × 10 mm × 4 mm sheet samples. Most of the numerical values of the mechanical properties were estimated by taking averages of three readings.

The Charpy impact test is a standardized high strain-rate test for measuring the amount of energy absorbed by a material during fracture. The test was conducted following ASTM D256 standard on bar-shaped specimens of dimension 170 mm × 10 mm × 4 mm). The Impact Strength (I.S.) was evaluated from Eq. (1):

$${\text{I}} . {\text{S}} .= \frac{{{\text{F}} . {\text{E}}. - 0.09{\text{J}}}}{a \times b}$$
(1)

where F.E. is the fracture energy, a and b are the length and width of the tested sample, respectively.

The electrical resistivity (ρ) of the samples was measured within a temperature range of 303–393 K with the electrical measurement setup shown in Fig. 3 using Eq. (2):

$$\rho = \frac{RA}{L}$$
(2)

where R is the sample resistance (Ω), A is the cross-sectional area (mm2) and L is the thickness of the specimen (mm). The PMMA/MWCNTs samples were 0.11 mm thick circular sheet of radius 0.55 cm. The dc electrical conductivity \(\left( {\sigma_{\text{dc}} } \right)\) was determined as the reciprocal of the resistivity using the following equation:

Fig. 3
figure 3

Schematic diagram of the electrical circuit (left-hand side) used for the dc resistivity measurement. The sample holder and electrodes are depicted on the right-hand side

Full size image
$$\sigma_{\text{dc}} = \frac{1}{\rho }$$
(3)

In addition, the activation energies for the conduction (Ea) were estimated from the plot of ln(σ) versus l000/T according to the Arrhenius equation:

$$\sigma_{\text{dc}} = A_{0} e^{{\frac{{ - E_{\text{a}} }}{{k_{\text{B}} T}}}}$$
(4)

where kB is the Boltzmann’s constant (1.38 × 10−23 J/K) and A0 is the pre-exponential factor.

3 Results and Discussions

The FTIR spectra of the pure PMMA sample, Fig. 4 curve (a), has absorption peaks around 2992 cm−1 and 2949 cm−1 corresponding to C–H asymmetric stretching in CH3 and CH2, respectively. The vibrational band at 2847 cm−1 is due to the C–H symmetric stretching in CH3. The characteristic band for the pure PMMA is observed at 1721 cm−1 which corresponds to the C=O stretching band. The vibrations due to deformation modes of CH3 groups appear at 1489 cm−1, 1434 cm−1 and 1385 cm−1. Medium bands at 1268 cm−1 and 1238 cm−1 indicate the presence of C–O stretching modes. The band at 1189 cm−1 is assigned to CH3 wagging mode while the two bands at 1141 cm−1 and 1062 cm−1 are due to the CH3 twisting.

Fig. 4
figure 4

FTIR of (a) pure PMMA and (b) PMMA + 0.5 wt% MWCNTs composites

Full size image

Figure 4 curve (b) shows the FTIR spectrum of PMMA/0.5 wt% MWCNTs composite. The observed vibration modes due to C–C stretching appear at 985 cm−1 and 969 cm−1. The peaks at 912 cm−1 and 840 cm−1 are assigned to CH2 rocking while the peaks at 809 and 749 are due to the C=O in the plane and out-of-plane bending, respectively. The C–H stretching vibration, which might be associated with the carbon of ethylene, is between 2850 and 3000 cm−1. The peak at 1728 cm−1 is assigned to the COO. The relative intensity of the hydrogen-bonded carbonyl group at 1815 cm−1 increased with the MWCNTs content. The number of hydrogen atoms in the O–CH3 groups of the composite film remained constant. The intensity of the carbonyl molecular group peaks increased when MWCNTs were added to PMMA. The covalent interaction and hydrogen bonds between PMMA and MWCNTs are desired to inhibit phase separation.

Figure 5a, b, c show the SEM micrographs of the composites with 0.1, 0.3, and 0.5 wt% MWCNTs loading, respectively. Accordingly, the grains have a hemispherical shape with an average size between 6.5 and 13.5 μm. The MWCNTs (the bright spots) were present as small clusters whose distribution was fairly uniform. As the MWCNTs ratio increased, dispersion worsened and large agglomerates were observed at 0.5 wt% MWCNTs loading.

Fig. 5
figure 5

Top-view SEM micrographs of PMMA/MWCNTs composites at a 0.1 wt% b 0.3 wt% and c 0.5 wt% MWCNTs

Full size image

According to Table 1 and Fig. 6, pure PMMA is very brittle with a tensile strength of 8.16 MPa and elongation of ~ 1%. However, addition of 0.1 wt% MWCNTs raised the tensile strength, elongation, Young modulus, tear strength, and hardness by 269%, 88%, 96%, 20%, and 10%, respectively. At 0.5 wt% MWCNTs, the tensile strength, elongation, tear strength, and hardness increased by 397%, 567%, 89%, and 27%, respectively, whereas Young modulus decreased by 26% compared to the PMMA sample. The results obtained for 0.1 wt% MWCNTs composite clearly illustrate the importance of good MWCNTs dispersion at low volume fractions. It also shows that effective reinforcement is limited by the maximum achievable volume fraction where good dispersion can be maintained. Mechanical properties have shown an increase in all the parameters investigated for a low content of carbon nanotubes. The calculated F.E. and I.S. for the composites are given in Table 1. The I.S. increased from 5.96 to 8.12 kg/m2 while the F.E. fluctuates between 0.16 and 0.19 J as the concentration of the filler increased from 0 to 0.5 wt%, respectively. The maximum and minimum fracture energy were obtained at 0.5 wt% and 0.3 wt% MWCNTs, respectively.

Table 1 Mechanical properties of PMMA and PMMA/MWCNTs composites
Full size table
Fig. 6
figure 6

Stress-strain curves of pure PMMA and PMMA/MWCNTs composites

Full size image

It has been established that surface modification improves MWCNTs dispersion and filler/matrix interfacial bonding. Hydrogen bonds provide important interfacial forces in the composite system. This bond can be intramolecular and intermolecular (such as OH–OH and OH–COOH). Carbonyl groups, available in polymer macromolecules, strongly accept hydrogen to form hydrogen bonds which consequently enhanced the Young modulus of the composites. The increase in tear strength with increased MWCNTs content (Table 1) can be attributed to high tensile strength and Young’s modulus of the filler. The strength of the C–C in-plane bonds along the cylinder axis provides strong resistance to any failure including large out-of-plane distortion.

Furthermore, the hardness of the composites increased with MWCNTs (Table 1) which is associated with the uniform dispersion of the fillers within the PMMA matrix. The MWCNTs have a high aspect ratio, high modulus, and high strength, and the loading force can be transferred to the nanotubes. The load transfer also depends on the mechanism of interfacial shear stress between the fiber and the matrix, which can be micromechanical interlocking, chemical bonding, and Van Der Waals force (Du et al. 2007). In addition, MWCNTs raised the Young Modulus of the composites from 7.85 to 16.35 MPa at 0.3 wt % loading. Usually, a higher concentration of MWCNTs leads to a higher Young modulus, for example, ~ 1 GPa reported by Makireddi et al. (2015) at 10 wt% MWCNT and 4 GPa reported by Jindal et al. (2015) at 5 wt% MWCNTs. The large difference in the moduli could be attributed to MWCNTs concentration as well as the diameter of the fillers which lied in the range of 5–15 nm for the present study and 40–60 nm used elsewhere (Makireddi et al. 2015).

The highest tensile strength of 40.52 MPa was obtained at 0.5 wt% MWCNTs compared to 8.16 MPa observed for the control. This is close to 55.6 MPa for the same composition reported elsewhere (Deep and Mishra 2018) as well as the result of Jia et al. (1999). However, the Young modulus in the present study is lower than 100 MPa obtained for PMMA functionalized graphene/polyvinylidene fluoride nanocomposites (Layek et al. 2010). This high value buttresses the point that functionalized surfaces enhance mechanical properties. The effects of concentration of MWCNTs below 0.1 wt%, aspect ratio, and interfaces miscibility on the mechanical properties of PMMA/MWCNTs composites have been reported elsewhere (Kim and Jo 2008; Mu et al. 2018; Mammeri et al. 2012).

The behavior of σdc for PMMA/MWCNTs composites as a function of MWCNTs concentrations and temperature is presented in Fig. 7. The increase in σdc with temperature is similar to the behavior of semiconductors. At room temperature, the σdc has a value of 2.45 × 10−10 S cm−1 reveals the insulating behavior of the PMMA polymer. However, σdc rose to 2.1 × 10−8, 7.54 × 10−5 and 1.33 × 10−4 S cm−1 for 0.1, 0.3 and 0.5 wt% MWCNTs loading, respectively. The magnitude of σdc at 0.5 wt% loading was 6 orders higher than the control. The observed increase in σdc is due to the increased carrier concentration occasioned by the addition of MWCNTs (Avilés et al. 2019; Mir et al. 2016; Chayad et al. 2015). The presence of MWCNTs as fillers in the PMMA matrix creates extensive conductive networks that facilitate electrons transport in the composite causing current to flow even in the absence of direct contacts between the nanotubes. This mechanism termed the quantum mechanical tunneling effect, allows electrons to move through an insulator between conductive elements (MWCNTs). The MWCNTs network showed a classical percolating network behavior with a low percolation threshold.

Fig. 7
figure 7

Plot of ln(σdc) against (103/T) for PMMA and PMMA/MWCNTs composites

Full size image

The numerical value of Ea was estimated from the slope of the straight lines fitted into the Arrhenius equation (Fig. 7). These values were estimated to be 0.19, 0.37, 1.07 and 1.32 eV for samples containing 0, 0.1, 0.3 and 0.5 wt% MWCNTs, respectively. The increase in the activation energy is in good agreement with published work (Das et al. 2011) and is most likely due to the stress relaxation of polymer chains (Hsu et al. 2017). This behavior can also be due to the reduction in impurity density occasioned by the agglomeration of MWCNTs in the localized states near the valence and conduction bands.

Figure 8 describes the mechanism of mechanical strengthening and electrical conductivity enhancement in the composites. The MWCNTs fillers dispersed randomly within the PMMA matrix which increased in density with the concentration of the fillers (Fig. 8a). The concentration reaches a critical ratio beyond which MWCNTs agglomerated. The significant increase in elongation (Table 1) is due to the strong interfacial interaction between the PMMA matrix and MWCNTs. This facilitates the formation of bridge cracking and prevents premature failure of the composites. The toughening mechanism of the composites can be described in three stages. In the first stage, a tensile load is applied to the composite beyond the elastic limit of the matrix leading to cracking of the matrix. Further propagation of the crack causes crack initiation in the MWCNTs bundles and subsequent bridging (Fig. 8b). In the second stage, the MWCNTs start aligning in the direction of stress thereby enabling the maximum load to be suspended without failure of the fillers. In the third stage, as the tensile load extends beyond the tensile limit of MWCNTs, the crack propagates rapidly within the fillers. Ultimately, most of the MWCNTs are disrupted and failure of the polymer occurs (Fig. 8c).

Fig. 8
figure 8

Schematic illustration of mechanical strengthening (a, b, and c) and enhancement of electrical properties (d, e, and f) in PMMA/MWCNTs composites

Full size image

Furthermore, applying the electrical field at an elevated temperature over the composites leads to alignment of MWCNTs in the direction of the electric field (Fig. 8d–e). The polarization formed a symmetrical and close macro network (Fig. 8f). The fillers gradually congregated into bundles forming conductive networks in the PMMA matrix leading to the observed increase in electric conductivity. At a higher concentration of MWCNTs, the higher connection is expected to increase electric conductivity. Accordingly, addition of MWCNTs converted PMMA from an insulator to a conductive plastic.

Notably, σdc can be increased sharply for the case of electrical percolation when  the concentration of filler exceeds a critical value called the percolation threshold (Pc). The presence of MWCNTs in the insulating PMMA matrix utilized to a continuous network of electrical connections in the insulator matrix. The discontinuity in the electrical property could be explained in terms of the metal filler concentrations via the percolation theory. As a sufficient ratio of conductive filler is loaded into a polymer matrix, the composite transforms from an insulator to a conductor due to the continuous linkages of filler particles. Accordingly, no significant change in σdc occurs until the Pc of filler loading is reached. For a concentration of conductive filler greater than Pc, the value of σdc significantly increases due to the formation of a network of chains of conductive particles that span the composite (Sancaktar and Bai 2011). In addition, reducing filler size leads to a decrease in Pc, because inter-particle contacts increase with a rise in the amount of conductive filler (Pothukuchi et al. 2004). Other parameters that can affect the value of Pc are chemical nature of the composite components, preparation method, the crystal structure, thermal and electrical conductivity, dimension, geometry, and distribution of the fillers in the composite (Tee et al. 2007). Although a very large amount of filler is required for conduction, the polymer material’s strength and elasticity may be degraded. It is therefore important to use the minimum quantity of conductive filler to achieve the required degree of electrical performance (Li et al. 1993). It can be observed from Fig. 7, the value of \(\sigma_{\text{dc}}\) at room temperature at various concentrations of MWCNTs is between 1.4 × 10−12 and 2.1 × 10−10 S/cm. This small change in conductivity reveals that the concentration of conductive filler in the prepared PMMA/MWCNTs composites less than Pc. Therefore, based on the classic theory of percolation, Pc can be obtained by the power-law relationship in polymeric materials by the following equation (Snarskii et al. 2020):

$$\sigma_{\text{dc}} = \sigma_{0} \left( {P_{\text{C}} {-}P } \right)^{ - q} \;{\text{for}}\;P_{\text{C}} > P$$
(5)

where \(\sigma_{0}\) is the conductivity at q = 0, P is the concentration of the conductive phase, and q is the critical exponent. The exponent q is the critical exponent and is related to network dimensional aspects. According to Eq. 5, the best linear fitting of log(σdc) versus log(Pc − P) gives a value of Pc, q and \(\sigma_{0}\) at room temperature for PMMA/MWCNTs composites around 0.55 wt%, 2.06, and 4.5 × 10−13 S/cm, respectively. The estimated value of PC means the concentrations of all prepared samples are less than Pc. In the present study, the value of q is closed to 2 which is normally associated with three-dimensional networks (Snarskii et al. 2020).

4 Conclusion

PMMA/MWCNTs composites were prepared by direct mixing technique while their mechanical and electrical properties were examined at 0.1, 0.3, and 0.5 wt% MWCNTs. The mechanical and electrical properties of the synthesized PMMA/MWCNTs composites were significantly enhanced. For instance, addition of 0.5 wt% MWCNTs enhanced the tensile strength, elongation, tear strength, hardness, fracture energy, and impact strength by 397%, 567%, 89%, 27%, 12%, and 36%, respectively, whereas Young modulus decreased by 26% compared to the control sample. Both the dc electrical conductivity and activation energy were significantly improved. The estimated percolation threshold was found to be ~ 0.55 wt% MWCNTs. According to FTIR analysis, the carboxyl acid which bonded to the surfaces of MWCNTs strongly influenced dispersion performance. This study demonstrated the huge opportunity embedded in MWCNTs as fillers in composite materials. Moreover, we have proposed a mechanism to simulate the effects of MWCNTs on the electrical and mechanical properties of PMMA/MWCNTs composites.