Scanning electron microscopy as a valuable tool to optimize the properties of the polymer/clay nanocomposites

Abstract The current research utilizes a low voltage scanning electron microscopy (LV-SEM) with an electron beam along with low loading energy of lower than 2.2 KeV to minimize damage and specimen surface charging. The NovaSEM, is used as an efficient tool in the current study due to the high resolution information it can gather as images and its high magnification nanometers. Polyamide 12 (PA12), as a polymer matrix, and Cloisite 30B (C30B) nanoclay, as a filling material, were the materials tested in this study. From the results obtained, CBS was found to be a significant and valuable tool for certain complex tasks when studying and analyzing polymer/clay interfaces. CBS in conjunction with beam deceleration in a LV-SEM was used to map the C30B clay distribution on PA12 particles and within PA12-nanoclay nanocomposites manufactured from the latter’s clay distribution particles within the polymer particles’ surfaces. This SEM experimentation has demonstrated that using the clay’s air plasma design preceding the composite preparation resulted in removing the large clay assemblages. The plasma treatment has improved the interfacial adhesion and dispersion in the nanoclay/PA12 composite, resulting in similar maximum stress values which were both higher than the pure PA12. Thus, the mechanical tests exhibited performance enhancement for the resulting composites defined in the present work, and the enhancement of this method can be identified via SEM imaging.


Introduction
The need for morphological analysis is increasing due to the development of nanocomposite materials.The interface between organic/inorganic materials and nano-structures of the nanocomposite materials systems makes them one of the most difficult nano-scale materials to characterize for structure.Organic polymeric structure like thermoplastic systems are often employed because they offer benefits in terms of process performance and flexibility (Alim et al., 2020;Martin et al., 2020;Mathis et al., 2022;Wu et al., 2021).Additionally, polymeric systems are cost-effective and can be easily manufactured into the desired shape.Furthermore, composite materials can be made by combining the polymer matrix with fillers like fibers, particles, and nanoparticles to give it properties that neither the matrix nor the additive phase can provide on their own (Alkaron et al., 2023;Fegade et al., 2022;Sun et al., 2023).Therefore, polymer composites are utilized in various industrial applications, including textiles, food packaging, aerospace, pharmaceutical, chemical processes, automotive, and construction.However, aspects like the kind of constituents, volume proportion of elements, form and arrangement of inclusions, and the interface between the matrices and alloying elements all contribute to improving the properties of composite materials and filling materials.Thus, nanometer-scale inclusion composites (Sahoo et al., 2014) have been an important method for achieving considerable property improvement because of the size of the interface region.With nano-examination, the link between properties and structures is more complicated than it is for micro-sized composite (Müller et al., 2017).It has been demonstrated that the size, shape, concentration of local inclusions, and morphology of each phase, as well as the organization of phases from the nanoscale to microscopic levels, all have an effect on the properties of nano-composite materials (Agubra et al., 2013).This relationship between shape and properties has been seen in a wide range of artificial and organic polymeric nano-composite systems (Choi et al., 2019;Omanović-Mikličanin et al., 2020;Srivastava & Manna, 2022).
Nano-particle combinations' mechanical properties, viscoelastic behavior, and crystallinity are significantly influenced by the shape of the additions.The viscoelastic characteristics of composites depend on a strong bond between both the additive and matrix materials.Consequently, any trend that intensifies this interaction, such as a decrease in addition size, an increase of around 50% in the optimization fraction, or exfoliation of the dispersion of nanoparticles, would lead to an improvement in the mechanical properties (Guo et al., 2014;Rais-Rohani & Rouhi, 2017).Moreover, the distribution of local crystallization can be dramatically altered when reinforcements or fillers are altered, although the aggregate quality of the polymeric matrix's crystallinity does not change greatly.Thus, the considerable effects of admixture interaction and matrix material alteration caused by compounds, together with the concentration and shape of admixtures have a major influence on the mechanical properties of nanocomposites (Kumar et al., 2021;Tjong, 2006).
Polymer and nanoclay composites (Ismail et al., 2019;Luo & Daniel, 2003;Rao & Pochan, 2007;Saharudin et al., 2020) are the mostly frequently used in existing research.Furthermore, various innovative and hybrid materials have been developed with certain properties that are better than conventional composite materials.This is because the existing composites are acquired with a minimized modified filler on the nanoclay content when compared to the conventional system and the unique features of nanoclay.Thus, studying the interface and the degree of dispersion of the nanoclay within the polymer matrix is important when optimizing the optimal properties of the resultant nanocomposite.The interface and dispersion of the nanoclay within the polymer matrix have a significant influence on the final properties of the nanocomposites (Kausar et al., 2022).This is because the poor interface and the dispersion of the nanofilling materials, i.e., nanoclay, can result in the produced parts having decreased mechanical properties.Therefore, morphological studies can be used to identify the optimal nanoclay loading and its dispersion in the polymer matrix to achieve the desired properties.Various components are used in the nanoclay morphologies' process, based on factors such as clay type, processing method, polymer matrix type, clay modification, clay weight fraction associated with the polymer, and the incorporation between polymer chain and the nanoclay (Guo et al., 2018;Kahraman et al., 2023;Murugesan & Scheibel, 2020).
Since the development of electron microscopes in the 1930s, scanning transmission electrons have become a vital tool in several disciplines, ranging from forensics, industrial manufacturing, and even the biological sciences to materials engineering.When microscopic information about the surface or surrounding area of a specimen is needed, scanning electron microscopy (SEM) becomes essential.
The morphologies can vary between different scales or configurations within the same composite structure, and this variability can further affect the properties of nanomaterials.This variation is important for the overall performance of the composite.This performance is largely due to the multiple parts in each structural level of the overall composite structure, each of which covers a different element of mechanical characteristics.While nano clays are arranged in a number of co-matrix structures in plant fibers, natural silk has a combination of flexible disorder phases and higher hardness orderly phases.Given the complex patterns different phases have at the nano to the microscopic level, the aggregation of nanoclay particles and the difficulties of improving the composites' final properties, if the nanoclay had aggregated component characteristics and local morphology variation are both crucial in creating a remarkable array of mechanical properties.
The current study mainly focused on the novel technique employed, in which the air plasma treatment method was processed with this approach to treat the organically modified nanoclay.Furthermore, this study uses low-voltage scanning electron microscopy (LV-SEM) to optimize polymer/clay nanocomposites.LV-SEM can generate excellent topographical images up to a few nanometers at a higher resolution.Field-emission gun-scanned transmission electron microscopy like the NovaSEM is an effective tool in the nanocomposites research.Moreover, decreased specimen charging and degradation were achieved by using an electron beam with low-loading energy.
Additionally, two separate detectors were employed to visualize the samples: an Everhart-Thornley Detector (ETD) for low-power secondary electron photography and a concentric side scatter detector (CBS) for high-magnification visualization.To visualize the dispersion of nanoclay on particles and evaluate clay dispersion without the need for transition electron microscopy, CBS is utilized in conjunction with an electron beam.For those reasons, SEM is recommended as a valuable tool to study the effects of using air plasma etching of the clay before preparing the composites.Thus, the aim in the current study is to optimize the mechanical tests of polymer nanocomposite materials that underwent plasma treatment, and which revealed that the characteristics of the resulting composites, correspond to previous works using SEM.Furthermore, tensile tests were undertaken to investigate the performance of the resulting composites defined in the present work, and evaluating the importance of this method was determined via SEM imaging.

Materials
The materials used in the experimentation process are Polyamide 12 (PA12) powder purchased from EOS; it is also known as Nylon-12.PA12 has significant properties such as better chemical resistance, low density, thermal stability, effective mechanical properties, high resistance, rigidity, pressure resistance, and flexibility.Moreover, PA12 is considered the most frequently utilized laser sintering powder.This is due to its preferred round shaped particles which can be easily flow during sintering.However, the powder contains 50% utilized powder recycled from the previous laser sintering process with a virgin powder of 50%.Furthermore, the nanoclay includes Cloisite 30B (C30B) acquired from the Southern Clays company.C30B is an organically modified Natural Na ±MMT.However, the organoclay used in this research was modified with an organic modifier, i.e., methyl-tallow bis-2-hydroxyethyl-ammonium cations, to enhance its compatibility with the organic polymer matrix-PA12.

Methods
The current study mainly focused on the air plasma treatment model.This was proposed as a novel method that handles the nanoclay, which is organically modified and referred to as C30B before having the PA12 added.The time required for plasma exposure of the selected clay to an air plasma required was 30 minutes, and the percentages of clay were measured as 3%, 5%, and 7%, which were added to the PA12 powder.Additionally, the dry mixing technique was utilized to combine the clay platelets and polymers particles using an 800 rpm magnetic stirrer for 30 minutes then ultrasonication for another 30 minutes.The composite samples' fabrication was processed using a hot press, and these samples were melted at a temperature of 185 to 195 degrees Celsius, as per the method in our previous study (Almansoori et al., 2017).Hence, the processed samples were tested mechanically, and to characterize the resulting composite, the tensile test was used in the present work.

Plasma treatment (PT)
Plasma treatment (PT) was utilized in this research due to the low-temperature modification on the surface, which is suitable for the process.The C30B clay powders with a weight of ≈ 3 grams were positioned in thin layers in a small glass-petri dish.Three petri dishes were placed in a cylindrical glass vacuum-chamber of the plasma chamber.Once the evacuation was done in the chamber, the plasma utilized a maximum power of 100W power for 1000 seconds.Next, air gas plasma was generated in a cylindrical chamber and was processed by an electric field occurring between electrodes.At the same time, the oxygen molecules present in the process were activated and parted into electrons, ions, atoms, reactive species, and radicals.Hence, the plasma generator was turned on during the achieved work pressure of 3 mbar or less.Nevertheless, the pressure varied moderately after the plasma generation.The method taken place was repeated twice on each powder quantity.Subsequently, the plasma system acquired fresh gas during the elimination of contaminants, and the treated powder was stored after being removed it from the chamber in a sealed glass container.

Scanning Electron Microscopy (SEM)
A low-voltage SEM Nova NanoSEM was utilized in the research for the clay morphological observation in the plasma treatment that was produced utilizing PA12 composites along with both untreated and treated C30B clays.An electron beam, along with low voltage of below 2.0 KeV, was utilized in this study to minimize specimen damage and surface charging.Furthermore, no metal coating was utilized on the polymer particle surface, and the secondary electron images were gathered via an Everhart-Thornley Detector (ETD) utilized for higher magnification imaging with chemical contrast.Additionally, CBS with a beam deceleration conjunction was used to outline the nanoclay distribution, which occurred with polymer particles, to facilitate the assessing the clay dispersion on the surface of the polymer particles.

Tensile test
Specimens made from PA12, 3% treated and untreated clay/PA12, and 5% treated and untreated clay/PA12 composites were tested using a Hounsfield Tensometer to study their tensile test properties.The tests were carried out according to the British Standard ISO-527.The tests were conducted at a maximum load of 10kN, at a loading speed of 5 mm/min with a 5N as a preloading.

Results and discussions
SEM micrographs were utilized in the research to observe the plasma-exposed particles on the morphological surface variations.Furthermore, several variations between PA12 and nanoclay particles were analyzed in the represented SEM images.The first investigation under SEM showed that the PA12 particles are usually found to have a rounded shape with a size of 60-100 μm, which includes the LS powder standard specification as shown in Figure 1 These particles are normally coated by nanosized particles found to be white in color that are TiO 2 , which helps to enhance the powder whiteness as it cannot be recognized by the ETD detector (Verbelen et al., 2016).
Thus, Figure 1 gives a general view of the PA12 particles (image (a)), while image (b) on the right represents the surface morphology of a single PA12.Furthermore, PA12 particles shown in Figure 1(a) with semi-circular shape become stuck together in the process of developing large particles and the remaining particles split because of the manufacturing process and powder history due to the powder including 50% reused powder from the laser sintering process along with 50% virgin powder.
The images were obtained with an ETD detector due to the material usage and powder general view.Figure 1(b) the SEM image shows a large PA12 particle with a height of about 120 μm and width of about 80 μm.This comes from the particles joining during processing or from previous sintering.However, both images show a poor surface morphology of the poorly conductive PA12 particles and a low landing voltage was observed with the ETD detector as shown in Figure 1 (a  & b).
Thus, the utilization of a CBS detector in this research has close imaging capability for the surface morphology and more morphology contrast as seen in Figure 2 (a and b).The SEM image of PA12 particle in Figure 2 (a) obtained by the high resolution detector-CBS detector shows a solid, rough and non-porous surface morphology.Furthermore, titanium dioxide (TiO 2 ), in the manufacturing of PA12 powder, was also observed using the CBS detector at a higher magnification.The distribution of TiO 2 powder on the PA12 powder particles' surface that was used to maximize the whiteness and flowability, as shown in Figure 2b.The TiO 2 particles appear whiter because TiO 2 has a higher refractive index thus it has a higher capacity for reflecting more electrons (Almansoori et al., 2018).Nanoclay (Closite 30B) was utilized in the current research and was also investigated with the ETD detector and CBS detector as illustrated in Figure 3(a),(b).The main use of the nanoclay particles in the research has various benefits, depending on the dispersion level and the distribution that occurs when generating the nanocomposites.However, in several cases, it does not generate nanocomposites with certain effective properties for different reasons and most important is the poor dispersion of the clay within the polymer matrix (Kenned et al., 2021).Thus, nanoclays were investigated using SEM with two different detectors; ETD (Image a) to provide an overview of the powder and CBS (Image b) to detect the size and shape of the powder particles as per in Figure 3  As can be clearly observed in SEM image (a), the nanoclay has varied sizes and shapes which is because nanoclay particles tends to aggregate into large and undistributed particles.At a higher magnification and using the CBS detector, the SEM image in Figure 4(b) shows the platelet nanoclay structure.The distribution, shape and size of nanoclay particles have an impact on the properties of the resultant composite, therefore, it is important to obtain an image the nanoclay particles before adding to the polymer matrix.
We utilized the CBS detector in this research for the manufacturing of the polymer composite, as the ETD was not able to recognize the clay particles presented from the tiny particles, known as PA12-small in SEM, and shown in Figure 4 The untreated nano-platelets consisted of micron-size shaped particles in irregular forms.Most of the aggregated particles were oval and large or round.
The clay particles in Figure 4 (after plasma treatment) tended to minimize the large agglomerates to build smaller-sized agglomerates with clear platelet shapes, verifying that the particles countered to the plasma action.Therefore, these findings were included in the hot stage microscopic imaging during the PA12 particles' heating process once the PA12-powder coalescence with the untreated C30B powder had accumulated in comparatively large non-uniformly distributed particles, thereby minimizing the contact area that adversely impacted the final properties of the products (Almansoori et al., 2017).(Almansoori et al., 2019).This is because PA12 is an organic polymer, whilst the clay (C30B) particles are a mineral.Thus, the color differences result from the different chemical compositions of organic materials and minerals.
Three different clay loadings were investigated using SEM-CBS detector as shown in Figure 6(a-f).The CBS detector is a valuable tool for heterogeneous materials.Furthermore, the TiO 2 and clay can be easily recognized on the PA12 particles as per Figure 6(a-f).Therefore, it allows the optimization of the nanoclay/polymer composite and the obtained values on the best case and worst case.The worst result achieved with clay was 7% (Figure 6e-f), and the best result was 3% (Figure 6a-b).Notably, the SEM of 3%, 5%, and 7% nanoclay in Figures 6b, 6d, and 6f did not provide good results.This can be attributed to the nonhomogeneous mixing which occurred from the dry mixing.However, dry mixing is a quick, and easy method which is affordable when medium to high volume production is required.Therefore, the 3% nanoclay was chosen and the results compared with and without plasma treatment.It is clear that plasma treatment has led to the reduction the aggregation of nanoclay, as per Figure 7, showing the C30B SEM micrographs of the nanoclay before and after the plasma treatment.The C30B nanoclay exposed a minimization in agglomeration because of the plasma action, as exhibited in SEM images utilizing the advantages of the high resolution CBS detector.Meanwhile, the treated clay particles were well-dispersed and minimized agglomerations, which resulted in a maximization of the contact area and enhanced the interaction of the polymer matrix and clay.This impact was expected to enhance the properties of the final products.Futhermore, a few small aggregated treated clay particles were even found at low clay concentrations.However, based on the obtained results of the 3% clay/PA12 composites the plasma-treated composites show better results than the untreated ones.Additionally, the better distribution of particles with plasma treatment is illustrated in Figure 8, which shows the etched clay distribution that occurred within the polymer particles was more efficient and consistent when compared to the non-etched clay.Finally, the result shown using air plasma etching for clay was efficient and successful as since the large clay assemblages, as in the picture, did not occur.The results obtained via SEM were confirmed in the tensile tests.Thus, tensile tests were employed in the present work to verify the effectiveness of the plasma treatment.Table 1, gives the mechanical properties of the nanoclay/PA12 composites that were also enhanced because of the plasma treatment effects that occurred on the nanoclay.This occurred because of the dispersion and better distribution of nanoclay presented in the polymer particles.Table 1) presents the results of the tensile tests conducted on Polyamide 12 (PA12) and nanoclay/PA12 composites both with and without plasma treatment.The mechanical properties evaluated include the elastic modulus, maximum stress, and maximum elongation.
Starting with the elastic modulus, it was observed that the treated 3% nanoclay/PA12 composite exhibited the highest value (1011.7 MPa), followed by the treated 5% nanoclay/PA12 composite (998.3MPa).Meanwhile, the untreated composites, both at 3% and 5% nanoclay loading, showed lower elastic moduli (971.7 MPa and 967 MPa, respectively) but higher than the PA12 samples.Moreover, the results of treated samples showed less variation than the untreated.This can be  attributed to the action of the plasma treatment.The behavior observed can be attributed to the distribution and dispersion of nanoclay particles within the PA12 matrix (Agubra et al., 2013;Al-Jumaili et al., 2023;Almansoori et al., 2017).The presence of well-dispersed nanoclay particles enhances the interfacial adhesion between the filler and the matrix, resulting in a stiffer composite (Agubra et al., 2013;Borić et al., 2019).This can be attributed to the plasma treatment having reduced some agglomeration or having altered the particle-matrix interaction, thereby leading to an improved modulus of elasticity (Almansoori et al., 2019).
Moving on to the maximum stress, both untreated and plasma-treated 3% nanoclay/PA12 composites exhibited almost similar values (43 MPa and 43.7 MPa, respectively).However, with the 5% composites, the treated one shows a slightly higher maximum stress (40.3 MPa) compared to its plasma-treated counterpart (41.2 MPa).The maximum stress is influenced by factors such as filler dispersion, interfacial bonding, and stress transfer efficiency.The plasma treatment might have improved the interfacial adhesion and dispersion in the 3% nanoclay/PA12 composite, resulting in similar maximum stress values which are both higher than the pure PA12.However, with the 5% composite, the effect of plasma treatment was not enough to alter the particle distribution due to the higher clay concentration leading to unchanged or slightly reduced values compared to the untreated ones.
Considering the maximum elongation, both untreated and plasma-treated nanoclay/PA12 composites show reduced values compared to the pure PA12.This behavior can be attributed to the inherent stiffness of the nanoclay filler, which restricts the polymer chains' mobility and hinders plastic deformation.However, the plasma treatment, as observed in both 3% and 5% composites, showed better maximum elongation values which can be attributed to the strong interaction between the plasma treated nanoclay and polymer matrix (Almansoori et al., 2019).
In summary, the plasma-treated nanoclay/PA12 composites demonstrated a higher elastic modulus, while the untreated composites showed a slightly lower modulus, likely due to altered particle-matrix interaction.The maximum stress values were similar for the untreated and plasmatreated 3% nanoclay/PA12 composites but were higher than pure PA12, indicating improved interfacial adhesion with the plasma treatment.However, the untreated 5% composite had a slightly higher maximum stress, suggesting potential effects on particle distribution.Both untreated and plasma-treated composites had reduced maximum elongation compared to PA12, primarily due to the stiffness of the nanoclay filler, while the plasma treatment showed better elongation values, potentially due to polymer matrix changes.Thus, the tensile test confirm those obtained by the SEM images.Furthermore, the obtained results confirmed the validity of the SEM images and findings.The variation in the tensile results can be attributed to different reasons such as the non-homogeneous distribution of nanoclay on polymer particles found mainly in high clay concentration (5% nanoclay).Thus, at low clay concentration the variation in results were less than at higher values.Additionally, the plasma treatment has resulted in minimized error percentages as shown in Table 1.

Conclusion
The current study has mainly focused on employing a novel technique where the air plasma treatment method was processed with this approach to treat the organically modified nanoclay.Furthermore, LV-SEM has been used in the study to optimize polymer/clay nanocomposites.In addition, a Low-Voltage SEM Nova NanoSEM was utilized in the research for the clay morphological observation in the plasma treatment produced utilizing, PA12 composites along with both untreated and treated nanoclays.
Therefore, the utilization of air plasma etching when researching the clay composite generation provided efficient results in reducing the large clay accumulations, seen via SEM images.In the obtained results, CBS was found as to be a significant and valuable tool for certain complex tasks when studying and analyzing polymer/clay interfaces.Moreover, CBS with beam deceleration conjunction was used in this research to outline the nanoclay distribution that occurred in polymer particles, and that enabled the assessment of clay dispersion in the absence of transmission electron microscopy.The experimentation with SEM has demonstrated the utilization of air plasma on the design of the clay preceding composite preparation attained by removing the large clay assemblages.The mechanical tests exhibited an enhancement in the features of the resulting composites as defined previously, and the enhanced aspects are explained clearly via the SEM imaging.
For future work, the recommendation is to undertake further testing, for instance the TGA, and DSC tests in to investigate the thermal properties of PA12 and composites.Furthermore, studying the effect of plasma treatment on the thermal properties is also recommended.

Figure
Figure 1.Two SEM images of polyamide 12 (PA12) with two different magnifications.(a) 500×shows the general view of PA12 particles, and (b) is the 1500× image showing a single PA12 particle.

Figure
Figure 2. (A) illustrated with the utilized CBS detector for the surface morphology to identify close imaging, and (b) distribution of TiO2 on the PA12 particles surface, used to maximize the whiteness and flowability.

Figure
Figure 3. (A) representation of nanoclay using ETD detector (b) using the CBS detector.

Figure 4 .
Figure 4. Illustration of treated nanoclay showing the reduction of the nanoclay agglomeration.

Figure 5
Figure5represents the outcome of the CBS detector as shown in images (c and d) compared to images (a and b) that easily recognized the clay particles.This represented the composite images with CBS and is considered the effective beam deceleration presented in conjunction with the CBS detector that was utilized due to the high contrast level between the polymer and clay.Therefore, the white particles shown are clay particles, and the darker background indicates the PA12 materials(Almansoori et al., 2019).This is because PA12 is an organic polymer, whilst the clay (C30B) particles are a mineral.Thus, the color differences result from the different chemical compositions of organic materials and minerals.

Figure
Figure 5. Illustration of composite ETD SEM images (a and b) where it is hard to recognize the nanoclay particles and CBS SEM images (c and d) of 5% polymer composites where the clay particles can be distinguished from the tiny particles (PA12).The white tiny particles are the nanoclay.

Figure
Figure 6.The outcome from CBS detector detecting the clay particles and the PA12 materials at three clay loadings 3% (images a and b), 5% (images c and d), 7% (images e and f).

Figure 7 .
Figure 7. Illustration of the 3% treated clay are recognized easily on the PA12 particles at two different magnifications.

Figure 8 .
Figure 8. Distribution of nanoclay particles with plasma treatment.