Strength Evaluation of Functionalized MWCNT-Reinforced Polymer Nanocomposites Synthesized Using a 3D Mixing Approach

The incorporation of carboxyl functionalized multi-walled carbon nanotube (MWCNT- COOH) into a polymethyl methacrylate (PMMA) has been investigated. The resultant tensile and flexural mechanical properties have been determined. In this paper, a novel synthesis process for a MWCNT-reinforced polymer nanocomposite is proposed. The proposed method significantly eliminates the most challenging issues of the nano-dispersed phase, including agglomeration and non-homogeneous mixing within a given matrix material, and also resolves the issues occurring in conventional mixing processes. The results of scanning electron microscopy support these claims. This 3D-mixing process is followed by an extrusion process, using a twin-screw extruder for pristine MWCNT, and a compression molding process for COOH-MWCNT, to prepare test specimens for experimentally determining the mechanical properties. The test specimens are fabricated using 0.1, 0.5, and 1.0 wt.% MWCNT, with a remaining PMMA phase. The testing is conducted according to ASTM D3039 and ASTM D7264 standards. Significant improvements of 25.41%, 35.85%, and 31.75% in tensile properties and 18.27%, 48%, and 33.33% in flexural properties for 0.1, 0.5, and 1.0 wt.% COOH-MWCNT in PMMA, respectively, compared to non-functionalized MWCNTs, were demonstrated. The highest strength was recorded for the nanocomposite with 0.5 wt.% f-MWCNT content, indicating the best doping effect at a lower concentration of f-MWCNT. The proposed CNT-PMMA nanocomposite may be found suitable for use as a scaffold material in the domain of bone tissue engineering research. This type of research possesses a high strength requirement, which may be fulfilled using MWCNT. Furthermore, this analysis also shows a significant amount of enhancement in flexural strength, which is clinically required for fabricating denture bases.


Introduction
The nanocomposite is a multiphase solid material where one of the phases possesses a nano level dimension. The final product does not need to be on the nanoscale, but can be macroscopic in size [1]. Based on the matrix material, nanocomposite materials are divided into three distinct groups: ceramic matrix nanocomposites (CMNC), metal matrix of a CNT-based composite mainly depends on the size, dose, duration, testing systems, and functionalization used. The surface modification improves their solubility and biocompatibility and alters their cellular interaction-reduced cytotoxic effects. Ma et al. [29] investigated the efficiency of CNTs as a reinforcement for polypropylene (PP) for biocompatible applications. The type of functional group may be the key factor affecting the mechanical properties and biocompatibility of PP nanocomposites compared to neat PP.
Hamit and Mehmet [30] investigated the tensile and bending strength of a composite produced in the form of epoxy adhesive layers by using a hand lay-up method in which aluminum, mica, and ceramic particles were added into epoxy as a structural adhesive by 2, 4 and 6 wt.% and achieved good enhancement in tensile strength and bending strength for 4 wt.% and 2 wt.%, respectively. Adin and Okumus [31] presented the microstructural characterizations of martensitic stainless steel and mild/low carbon steel welded by friction welding using an optical microscope (OM), X-ray diffractometer (XRD), and scanning electron microscope (SEM). Satish et al. [32] presented the synthesis of multi walled carbon nanotubes (MWCNT's)/E glass fiber and their morphology, and the thermal behavior with different percent loadings of MWCNTs was inspected. The results showed that a clear improvement in the thermal stability of the composites increased with increasing MWCNTs content. Rouway et al. [33] performed homogeneous mixing using the Mori-Tanaka approach for CNT and GNP nanofillers, and then the matrix was used with alfa and E-glass isotropic fibers. They investigated the elastic properties with the influence of the volume fraction Vf and the aspect ratio AR of the nanofiller. The enhancement in properties was reported for CNT and GNP-reinforced nanocomposites, as well as a further increase in the volume fraction and aspect ratio of the nanofillers, Young's modulus E increases and Poisson's ratio ν decreases. Parnian and D'Amore [34] presented a novel process to prepare CNT-reinforced filaments using the sol gel method to improve the mechanical, thermal, and electrical properties with the homogeneous dispersion of CNTs in a dilute polymer solution. The optimizing the filament can also reasonably improve the orientation of CNT in the filament.
Based on the above-cited literature, it is quite evident that a PMMA-based nanocomposite has a vital role to play in various domains of the biomedical field. Moreover, the two main problems related to the synthesis of nanocomposites are related to the agglomeration of CNTs and the lack of interfacial adhesion. A 3D mixer is ideal for the homogeneous mixing of powder; however, this may not be the case with different bulk density, proportions of mixing, or shape and particle size. It is often observed that the particles with different densities or sizes form clusters on the outer periphery of the mixer or at the centre of the mixing mass. This is resolved by a rotary or shaking motion, which provides the required homogeneity. A 3D mixer exhibits an inversion motion rather than a rotary motion. Since there is no rotating blade inside, there is no shear force being applied to the material. The major problems of agglomeration can be solved by using a novel 3D mixing method. The characterization images presented in a later section indicate that the novel mixing process is free of agglomeration. In order to increase the interfacial adhesion, the authors have utilized functionalized MWCNT, achieving promising results, as detailed in the following sections.

Materials and Methods
The synthesis procedure, testing procedure, and characterization technique are presented in this section. The tensile strength and flexural strength are determined experimentally for different mixing ratios of functionalized CNTs with the PMMA matrix.
PMMA is a synthetic resin delivered from the polymerization of methyl methacrylate, commonly known as acrylic. It must be preheated before micro compounding in a vacuum oven at 60 • C for 1 h to remove its moisture content. PMMA, in its fine powder form, was obtained from N. Shashikant Co. Mumbai (Mumbai, India); its properties are shown in Table 1. CNTs are tubes with nanometer measurements and micron lengths comprised of hollow cylinders of graphene with remarkable electronic and mechanical properties. This carbon nanotube has high carbon purity and length-to-diameter ratio. The maximum advantage of CNTs as the filler has been limited because of poor interfacial interaction and van der Waals connection among CNTs and the polymer lattice. To resolve such issues, the CNTs are made chemically more reactive by functionalizing them [35]. These approaches can be simply divided into chemical (covalent) and physical (noncovalent) functionalization as interactions between active materials and CNTs. Carboxyl-modified multi-walled carbon nanotubes were utilized for better dispersion of multi-walled carbon nanotubes into the polymer matrix. MWCNT-COOH was prepared by the oxidation of pristine (p) MWCNTs in a concentrated H 2 SO 4 /HNO 3 mixture. Into a flask equipped with a condenser, p-MWCNTs, HNO 3 , and H 2 SO 4 were added under vigorous stirring. Before the reaction, the flask was immersed in an ultrasonic bath for 10 min. Then the mixture was stirred for 100 min under reflux; the oil bath temperature was increased gradually from 90 • C to 133 • C. After cooling to room temperature, the reaction mixture was diluted with deionized water and then vacuum-filtered through filter paper. The dispersion, filtering, and washing steps were repeated until the pH of the filtrate reached 6-7. The filtered solid was dried under vacuum for 24 h at 60 • C, yielding MWCNT-COOH [36,37]. Carboxyl (COOH) is a functionalized group of carboxylic acid that strongly interacts with many organic and inorganic materials. MWCNTs were procured from Platonic Nanotech Pvt Ltd. (Jharkhand, India). The properties of MWCNTs are shown in Table 2.

The 3D Mixing Process
Homogeneous dispersion of nanoparticles in a polymer by using traditional compounding techniques is very difficult due to the strong tendency of fine particles to agglomerate. A 3D mixer is ideal for the homogeneous mixing of powder; it may be more difficult with different bulk density, or proportions of mixing, or shape and particle size. The classical tumbling motion ensures that every particle of powder changes its position and mixes at every angle of motion, which reduce the clustering of materials with different densities on the outer periphery of the mixer, or at the center of the mixing mass [33]. The important parameters of a 3D mixture are rotor speed, mixing time, rotary direction, and filler loading. The interactions between these parameters are also vital. To evaluate this, the mixing parameters are changed simultaneously, and the data are analyzed using a visual inspection of the response of mixture. The results reveal that mixing time and rotor speed strongly affect the overall performance of the prepared composite. The experiment was performed for 0.1, 0.5, and 1 wt.% of MWCNT in PMMA polymer. The operating parameters for the 3D mixing process are speed, direction, and time, as shown in Table 3.

Extrusion and Injection Molding Process
Before compounding, the mixer was preheated/dried at 80 • C in a vacuum oven for 6 h to remove moisture and to enable the uniform distribution of PMMA and CNTs particles, which will subsequently influence the nanocomposite homogeneity [6,35]. As shown in Figure 1, PMMA/MWNT nanocomposites were prepared with a co-rotating twin-screw extruder. The effect of the processing conditions and extruder characteristics on the dispensability of nanocomposites is investigated. Dried pellets of polymers were mixed in a container and melt-blended in an intermeshing, co-rotating, twin-screw extruder made by Thermo Scientific, followed by injection molding. The mixture is fed into the hopper, and it moves towards the barrel at a temperature of 240 • C, melting before reaching the bottom of the barrel. The mixture is fed into the barrel, which houses a twin co-rotating screw rotating at a speed of 50 rpm. An injection molding machine is used for sample preparation. It is crucial to set proper parameters, particularly regarding thermal and shear loading. It is also necessary to avoid the degradation of the nanotube structure. The nozzle contains heating oil , which maintains the temperature of the compounded melt at around 230 °C. For production, an injection mold with a central ejector, which had exchangeable plates according to requirements, was used for testing samples. The holding pressure time was 35 s. The mold was  An injection molding machine is used for sample preparation. It is crucial to set proper parameters, particularly regarding thermal and shear loading. It is also necessary to avoid the degradation of the nanotube structure. The nozzle contains heating oil, which maintains the temperature of the compounded melt at around 230 • C. For production, an injection mold with a central ejector, which had exchangeable plates according to requirements, was used for testing samples. The holding pressure time was 35 s. The mold was cooled to a temperature of 50 • C for both halves of the injection mold. The compounded materials discharged from the micro compounder are accumulated inside the cylinder (nozzle). Pressure shaping is a notable assembling process for a composite. Specifically, the advancement of high-strength sheet molding compounds motivated the widespread adoption of the compression molding process in automotive and appliance applications [38][39][40]. In compression molding, the remeasured volume of the powder mixture is set directly into a heated mold cavity. The temperature of the mold is kept at more than 280 • C. The quantity of charge (molding material) put into the mold is calculated by considering the parameters mentioned in Table 4. The samples were prepared by cutting/trimming them from compression-molded nanocomposite sheets, as per the ASTM D3039 [41] and ASTM D7264 [42] standards, as shown in Figure 2. The specimens were prepared as per ASTM standard regarding the size of specimen for a tensile test, having an overall dimension length of 165 mm, width of 19 mm, and thickness of 3.3 mm; the grip length, gauge length, and gauge width were 125 mm, 50 mm, and 13 mm, respectively. The samples were prepared by cutting/trimming them from compression-molded nanocomposite sheets, as per the ASTM D3039 [41] and ASTM D7264 [42] standards, as shown in Figure 2. The specimens were prepared as per ASTM standard regarding the size of specimen for a tensile test, having an overall dimension length of 165 mm, width of 19 mm, and thickness of 3.3 mm; the grip length, gauge length, and gauge width were 125 mm, 50 mm, and 13 mm, respectively.

Specimen Testing and Characterization
The properties of polymers are not only determined by their chemical structure, but also by the processing method, which determines the orientation of the filler in the final products [43]. The tensile properties of injection molded PMMA/MWCNT specimens were measured with a universal tension test machine at ambient conditions (room temperature and humidity) with a crosshead speed of 0.1 mm/min, according to the ASTM

Specimen Testing and Characterization
The properties of polymers are not only determined by their chemical structure, but also by the processing method, which determines the orientation of the filler in the final products [43]. The tensile properties of injection molded PMMA/MWCNT specimens were measured with a universal tension test machine at ambient conditions (room temperature and humidity) with a crosshead speed of 0.1 mm/min, according to the ASTM D3039 standard [41]. The flexural properties of nanocomposites were determined with the threepoint bending test apparatus with a crosshead displacement rate of 1 mm/min, as per the ASTM D7264 standard [42]. The crucial parameters that determine the effects of fillers on the properties of composites are: filler size, shape, and aspect ratio, along with and fillermatrix interactions. The mechanical properties of the prepared specimens were evaluated with tensile and flexural tests, as shown in Figure 3. The dispersion and distribution of CNTs within the prepared nanocomposites can be observed using SEM [44][45][46][47]. The SEM images were obtained using SEM JSM-6010LA, Jeol LTD (Akishima, Tokyo, Japan).

Microstructural Assessment
SEM images of the powder form of PMMA/MWCNT and fractured structure of 0.5 wt.% MWCNT filled PMMA nanocomposites are shown in Figure 4. In Figure 4c-e, the long and undamaged MWCNTs show that there is no breakage of MWCNTS during handling, which indicates that the 3D mixing process provided a positive impact on nanocomposite handling. It also provides the maximum advantage of the L/D ratio for property enhancement.
The morphological form of the tube is uniform, and the presence of clots and clumps around the MWCNT contain white spots. The white patches and dots are not representative of carbon nanotubes, but rather the polymer matrix used in the synthesis process. The size of the CNT's diameter is estimated from an SEM image with 1000× magnification.

Microstructural Assessment
SEM images of the powder form of PMMA/MWCNT and fractured structure of 0.5 wt.% MWCNT filled PMMA nanocomposites are shown in Figure 4. In Figure 4c-e, the long and undamaged MWCNTs show that there is no breakage of MWCNTS during handling, which indicates that the 3D mixing process provided a positive impact on nanocomposite handling. It also provides the maximum advantage of the L/D ratio for property enhancement.
The morphological form of the tube is uniform, and the presence of clots and clumps around the MWCNT contain white spots. The white patches and dots are not representative of carbon nanotubes, but rather the polymer matrix used in the synthesis process. The size of the CNT's diameter is estimated from an SEM image with 1000× magnification.

Mechanical Assessment
As per ASTM standards for each combination, five specimens were tested, and their tensile and flexural test results are reported for 0.1, 0.5, and 1.0 wt.% MWCNT/PMMA, respectively. Figure 5 shows the stress-strain curves of the COOH-MWCNT/PMMA composite specimens, as a measurement of the evolution of the mechanical properties of the PMMA composites with f-MWCNT. The graph of tensile stress increased in a linear manner at the initial phase, showing the elastic deformation stage of the material. Later, the stress reaches its maximum value and starts decreasing up to the complete failure state. The presented graph clearly indicates that a combination of a linear and nonlinear regime is found when evaluating the tensile stress of the given specimen. Figure 5 exhibits a linear trend until it reaches a strain value of 1%, which tends to become nonlinear with the increase in the strain rate. The reason for this nonlinearity can be attributed to the behavioral aspect of the nanocomposite and the amount of reinforcement. A similar trend is also observed when the amount of reinforcement in the PMMA matrix is increased up to 1%.

Microstructural Assessment
SEM images of the powder form of PMMA/MWCNT and fractured structure of 0.5 wt.% MWCNT filled PMMA nanocomposites are shown in Figure 4. In Figure 4c-e, the long and undamaged MWCNTs show that there is no breakage of MWCNTS during handling, which indicates that the 3D mixing process provided a positive impact on nanocomposite handling. It also provides the maximum advantage of the L/D ratio for property enhancement.
The morphological form of the tube is uniform, and the presence of clots and clumps around the MWCNT contain white spots. The white patches and dots are not representative of carbon nanotubes, but rather the polymer matrix used in the synthesis process. The size of the CNT's diameter is estimated from an SEM image with 1000× magnification.

Mechanical Assessment
As per ASTM standards for each combination, five specimens were tested, and their tensile and flexural test results are reported for 0.1, 0.5, and 1.0 wt.% MWCNT/PMMA, respectively. Figure 5 shows the stress-strain curves of the COOH-MWCNT/PMMA composite specimens, as a measurement of the evolution of the mechanical properties of the PMMA composites with f-MWCNT. The graph of tensile stress increased in a linear manner at the initial phase, showing the elastic deformation stage of the material. Later, the stress reaches its maximum value and starts decreasing up to the complete failure state. The presented graph clearly indicates that a combination of a linear and nonlinear regime is found when evaluating the tensile stress of the given specimen. Figure 5 exhibits a linear trend until it reaches a strain value of 1%, which tends to become nonlinear with the increase in the strain rate. The reason for this nonlinearity can be attributed to the behavioral aspect of the nanocomposite and the amount of reinforcement. A similar trend is also observed when the amount of reinforcement in the PMMA matrix is increased up to 1%. The stress-strain curves for non-functionalized and f-MWCNT/PMMA nanocomposites are shown in Figure 6, and the impact of wt.% on tensile strength is highlighted in Figure 7.  The stress-strain curves for non-functionalized and f-MWCNT/PMMA nanocomposites are shown in Figure 6, and the impact of wt.% on tensile strength is highlighted in Figure 7.    That is, 3D mixing could play a synergetic role, not only in improving the dispersion of MWCNTs in the PMMA matrix, but also in strengthening the reinforcement due to the hyperactive branched polymer structure of the functionalized polyamide molecules [48]. The effect on tensile strength is 24.56%, 35.95%, and 31.75% for 0.1, 0.5, and 1.0 wt.% COOH-MWCNT in PMMA, respectively.
Similarly for flexural testing, five samples of each mixing combination were tested, and the results are shown in Figure 8. Flexural strength significantly increases with the increasing amount of MWCNT up to 0.5 wt.%. The comparative stress-strain curves of non-functionalized and carboxyl functionalized MWCNT in PMMA polymer are shown in Figure 9. It is observed that the flexural stress increases for 0.1 and 0.5 wt.% and then decreases for 1.0 wt.%. This shows a sharp initial increase, followed by a slightly decreasing trend. The comparative stress-strain curves of non-functionalized and carboxyl functionalized MWCNT in PMMA polymer are shown in Figure 9. It is observed that the flexural stress increases for 0.1 and 0.5 wt.% and then decreases for 1.0 wt.%. This shows a sharp initial increase, followed by a slightly decreasing trend.

Conclusions
In the present study, polymer composites reinforced with different concentrations of carbon nanotubes (CNTs) were developed and experimentally investigated. The main mechanical properties of carboxyl functionalized multi-walled carbon nanotube (COOH-MWCNT) composites are significantly improved with the addition of f-MWCNTs. The tensile and flexural strength of MWCNT/PMMA nanocomposites increase with an increase in weight percentages of f-MWCNT up to 0.5 wt.%, and then it starts to decrease. Moreover, the adequacy of the process was validated, followed by an SEM examination in which uniform distribution was observed. Based on the results, we can conclude that the small amount of f-MWCNT is sufficient to upgrade the mechanical properties of nanocomposites. The property enhancement is due to the beneficial f-MWCNT properties, such as large contact surface area and phenomenal load absorbance. The fluctuation of properties at higher MWCNT percentages makes the nanocomposite harder, leading to both reduced flexibility and decreased tensile and flexural properties. Additionally, the novel mixing process, followed by a compression molding process, yielded a promising improvement of 25.41%, 35.85%, and 31.75% in tensile properties and 18.27%, 48%, and 33.33% in flexural properties for 0.1, 0.5, and 1.0 wt.% COOH-MWCNTs in PMMA compared to non-functionalized MWCNTs. The proposed nanocomposite is suitable for bone tissue engineering and for use as a scaffold material because the MWCNT phase meets the requirements of high strength, while PMMA provides the property of biodegradability.