Preparation of Polyoxymethylene/Exfoliated Molybdenum Disulfide Nanocomposite through Solid-State Shear Milling

In this paper, the solid-state shear milling (S3M) strategy featuring a very strong three-dimensional shear stress field was adopted to prepare the high-performance polyoxymethylene (POM)/molybdenum disulfide (MoS2) functional nanocomposite. The transmission electron microscope and Raman measurement results confirmed that the bulk MoS2 particle was successfully exfoliated into few-layer MoS2 nanoplatelets by the above simple S3M physical method. The polarized optical microscope (PLM) observation indicated the pan-milled nanoscale MoS2 particles presented a better dispersion performance in the POM matrix. The results of the tribological test indicated that the incorporation of MoS2 could substantially improve the wear resistance performance of POM. Moreover, the pan-milled exfoliated MoS2 nanosheets could further substantially decrease the friction coefficient of POM. Scanning electron microscope observations on the worn scar revealed the tribological mechanism of the POM/MoS2 nanocomposite prepared by solid-state shear milling. The tensile test results showed that the pan-milled POM/MoS2 nanocomposite has much higher elongation at break than the conventionally melt-compounded material. The solid-state shear milling strategy shows a promising prospect in the preparation of functional nanocomposite with excellent comprehensive performance at a large scale.


Introduction
The two-dimensional molybdenum disulfide (MoS 2 ) has attracted extensive attention due to its excellent tribological performance and potential electrical and optical performance [1].The weak layered structure of MoS 2 gives it its perfect lubrication property [2], which makes the MoS 2 particles widely used as additives in lubricating oils, greases, and solid materials [3].However, the morphology of the MoS 2 particle has a great influence on its ultimate tribological performance.It was found that the exfoliated nanoscale MoS 2 (such as fullerene-like, tube-like, and platelet-like) showed better tribological performance than the conventional bulk MoS 2 [4,5].Hu et al. [6] investigated the tribological properties of liquid paraffin filled with nano-ball MoS 2 , nano-slice MoS 2 , and bulk MoS 2 , respectively.The results showed that nano-ball MoS 2 showed the better tribological performance, and nano-slice MoS 2 performed the best lubrication property at high rotation speed in the hydrodynamic lubrication region.In recent years, the ultrathin nanoscale MoS 2 has been reported to fabricate some field effect transistors due to its thickness-dependent bandgap [7] and is also used as a catalyst for the hydrogen evolution reaction [8].As a result, the nano-MoS 2 substance has been considered a promising candidate for various future electric devices.As a consequence, the preparation of nano-MoS 2 has attracted increasing attention [9].
The bulk MoS 2 is mainly composed of Mo-S-Mo sandwich layers, where every molybdenum sheet is interacted between two sulfur sheets.It was found that there are three The MoS 2 was first mixed with POM pellets in a high-speed mixer, and the loading of MoS 2 was fixed at 15 wt%.Then, the pan-milling equipment was applied to pulverize and mill the mixture with a rotation speed of 20 rpm.The discharged co-powders were collected for the next milling cycle.The heat generated during pan-milling was removed by the circulating water.A small quantity of the milled POM/MoS 2 co-powders was collected for characterization every 10 milling cycles.After 40 milling cycles were finished, the obtained POM/MoS 2 co-powders were firstly dried in an oven at 60 • C for 12 h and then used as the master batch and diluted to the MoS 2 content at 2 wt% by adding the dried neat POM and 0.5 wt% of the antioxide agent IRGANOX 245 to avoid mechanochemical degradation.Subsequently, the well-mixed mixtures with 2 wt% MoS 2 were extruded in a twin-screw extruder (Φ = 25 mm, L/D = 33, Chenguang Research Institute of Chemical Industry, China) with a screw rotation speed of 120 rpm at 180 • C, and the cooled extrudates were cut into pellets and dried.For purposes of comparison, the pristine MoS 2 particles were also well mixed with the dried neat POM and 0.5 wt% of the antioxide agent IRGANOX 245 in a high-speed mixer and then simply melt-compounded in the above-mentioned twin-screw extruder under the same conditions.

Preparation of POM/MoS 2 Composite Samples for Tensile and Tribological Tests
The dumbbell-shaped specimens with a dimension of 150 mm × 20 mm × 4 mm (L × W × T) for the tensile test were prepared by using a MA500II injection molding machine (Ningbo Haitian Co., Ltd., Ningbo, China) with an injection speed of 50 mm/s at 180 • C. The samples for the tribological test were first compression-molded into sheets at 180 • C with 20 MPa and then cut into samples with a dimension of 30 mm × 7 mm × 6 mm (L × W × T).

Characterization
The scanning electron microscope (SEM) morphology of the S 3 M (milled) and conventionally melt-compounded (unmilled) POM/MoS 2 composites was observed on an Inspect F Scanning electron microscope (FEI Company, Hillsboro, OR, USA) with an accelerating voltage of 20 kV.Before observation, the worn scar of samples after the friction and wear tests and also the fractured surface of samples after tensile tests were coated with a thin gold layer to prevent charging on the surface.The polarized light microscope (PLM) morphology was observed by a DM2500p microscope (Leica Camera AG, Wetzlar, Germany).Before observation, the samples were meticulously prepared by cryogenically ultramicrocutting them into 20 µm slices from the injection-molded sheet using an ultramicrotome machine.The milled and unmilled POM/MoS 2 composites were melted at 180 • C and controlled by the THMSG600 heating and cooling stage (Linkam Scientific Instruments, Salfords, UK).The X-ray diffraction (XRD) analysis of the milled and unmilled POM/MoS 2 composites was performed using a DX-1000 diffractometer (Dandong Fangyuan Instrument Co., Ltd., Dandong, China).The CuKα generator system was operated at 40 kV and 25 mA, and the scanning 2θ ranged from 5 • to 35 • .The transmission electron microscope (TEM) was performed on a Tecnai G2 F20 electron microscope (FEI Company, Hillsboro, OR, USA) with an accelerating voltage of 200 kV.The injection-molded samples of milled and unmilled POM/MoS 2 composites were cryogenically ultramicrocut into 80-100 nm thin slices at −100 were then placed on the copper grids for observation.The Raman measurements were conducted on the pristine MoS 2 and POM/MoS 2 co-powders with 40 milling cycles by using a LabRAM HR Laser Raman spectrometer (HORIBA Company, Palaiseau, France) at room temperature with an excitation wavelength of 532 nm.The friction and wear tests were conducted using a MC-200 friction and abrasion testing machine (Beijing Guance Testing Instrument Co., LTD., Beijing, China) with a block-on-ring arrangement at room temperature with a rotation of 120 rpm and a load of 200 N for 60 min.The schematic diagram of the friction and wear testing experiment is shown in Figure 1a.The wear loss was determined by the wear scar width (Figure 1b).The friction torques (T) were recorded every second, and the friction coefficient (µ) was defined by an equation of µ = T/MR, where µ was the friction coefficient, T was the average friction torque (Nm), M was the load (N), and R was the radius of the steel ring (m), respectively.
diffractometer (Dandong Fangyuan Instrument Co., Ltd., Dandong, China).The CuKα generator system was operated at 40 kV and 25 mA, and the scanning 2θ ranged from 5° to 35°.The transmission electron microscope (TEM) was performed on a Tecnai G2 F20 electron microscope (FEI Company, Hillsboro, OR, USA) with an accelerating voltage of 200 kV.The injection-molded samples of milled and unmilled POM/MoS2 composites were cryogenically ultramicrocut into 80-100 nm thin slices at −100 °C using a LEICA EM FC6 frozen ultramicrotome.The POM/MoS2 thin films were then placed on the copper grids for observation.The Raman measurements were conducted on the pristine MoS2 and POM/MoS2 co-powders with 40 milling cycles by using a LabRAM HR Laser Raman spectrometer (HORIBA Company, Palaiseau, France) at room temperature with an excitation wavelength of 532 nm.The friction and wear tests were conducted using a MC-200 friction and abrasion testing machine (Beijing Guance Testing Instrument Co., LTD., Beijing, China) with a block-on-ring arrangement at room temperature with a rotation of 120 rpm and a load of 200 N for 60 min.The schematic diagram of the friction and wear testing experiment is shown in Figure 1a.The wear loss was determined by the wear scar width (Figure 1b).The friction torques (T) were recorded every second, and the friction coefficient (μ) was defined by an equation of μ = T/MR, where μ was the friction coefficient, T was the average friction torque (Nm), M was the load (N), and R was the radius of the steel ring (m), respectively.The tensile property measurement of the milled and unmilled POM/MoS2 composites was performed on injection-molded dumbbell-shaped specimens using a 5567 type of Instron Universal Testing machine (Instron Company, Buckinghamshire, UK) with a cross-head rate of 10 mm/min at room temperature.The tensile property measurement of the milled and unmilled POM/MoS 2 composites was performed on injection-molded dumbbell-shaped specimens using a 5567 type of Instron Universal Testing machine (Instron Company, Buckinghamshire, UK) with a crosshead rate of 10 mm/min at room temperature.The morphology development of POM/MoS 2 co-powders prepared at different milling cycles was observed by SEM, and the results are shown in Figure 2. As can be seen, the MoS 2 particles are adhered to the ball-like POM particles through simple physical mixing (in a high-speed mixer) before pan-milling treatment.The milled POM/MoS 2 co-powder particles with low milling cycles (10) show a sheet structure under the effect of the very strong compression, shear, and hoop stretching stress fields generated by pan-Polymers 2024, 16, 1334 5 of 14 milling.It is noted that the sheet-like POM powder particles possess a relatively higher specific surface area and benefit from sufficient contact with MoS 2 particles.Moreover, the MoS 2 particles have the opportunity to be imbedded into the POM matrix under strong compression stress.As a consequence, it is promising to achieve good dispersion of MoS 2 filler particles in the POM matrix by using this strategy.With increasing milling cycles, the POM/MoS 2 co-powders keep the same sheet structure, but the size of the sheet particles becomes much smaller and thinner, indicating the high efficiency of the S 3 M technology.The final size of the POM/MoS 2 co-powder particles with 40 milling cycles could further decrease to 200 µm with the effect of the continuous pulverization and mixing provided by S 3 M.The morphology development of POM/MoS2 co-powders prepared at different milling cycles was observed by SEM, and the results are shown in Figure 2. As can be seen, the MoS2 particles are adhered to the ball-like POM particles through simple physical mixing (in a high-speed mixer) before pan-milling treatment.The milled POM/MoS2 co-powder particles with low milling cycles (10) show a sheet structure under the effect of the very strong compression, shear, and hoop stretching stress fields generated by pan-milling.It is noted that the sheet-like POM powder particles possess a relatively higher specific surface area and benefit from sufficient contact with MoS2 particles.Moreover, the MoS2 particles have the opportunity to be imbedded into the POM matrix under strong compression stress.As a consequence, it is promising to achieve good dispersion of MoS2 filler particles in the POM matrix by using this strategy.With increasing milling cycles, the POM/MoS2 co-powders keep the same sheet structure, but the size of the sheet particles becomes much smaller and thinner, indicating the high efficiency of the S 3 M technology.The final size of the POM/MoS2 co-powder particles with 40 milling cycles could further decrease to 200 μm with the effect of the continuous pulverization and mixing provided by S 3 M.

The Dispersion of MoS2 Particles in POM Matrix
The dispersion behavior of the incorporated MoS2 particles in the conventionally unmilled and milled POM/MoS2 composite was observed using PLM.The results are shown in Figure 3. Obviously, for the conventionally melt-compounded sample (Figure 3a), the opaque black areas represent the MoS2 particles, which basically appear to have a size in the range of microns.There is a heavy agglomeration of MoS2 particles occurring in the POM matrix, demonstrating the poor dispersion effect of twin-screw extrusion processing.This could be due to the high surface energy and interaction potential of the MoS2 particles.Comparatively, for the pan-milled sample (Figure 3b

The Dispersion of MoS 2 Particles in POM Matrix
The dispersion behavior of the incorporated MoS 2 particles in the conventionally unmilled and milled POM/MoS 2 composite was observed using PLM.The results are shown in Figure 3. Obviously, for the conventionally melt-compounded sample (Figure 3a), the opaque black areas represent the MoS 2 particles, which basically appear to have a size in the range of microns.There is a heavy agglomeration of MoS 2 particles occurring in the POM matrix, demonstrating the poor dispersion effect of twin-screw extrusion processing.This could be due to the high surface energy and interaction potential of the MoS 2 particles.Comparatively, for the pan-milled sample (Figure 3b), the dispersion of MoS 2 particles in the POM matrix is substantially improved.The individual MoS 2 particle cannot be clearly identified due to the great reduction in filler particle size after milling.The above results again verify the high efficiency of S 3 M technology.This is because the MoS 2 particles are efficiently pulverized and imbedded into the POM matrix under the strong compression, shear, and hoop stretching stresses induced by pan-milling.Thus, the S 3 M strategy could be applied to effectively solve the aggregation problem of the pristine bulk MoS 2 particles in the POM matrix.
ticle cannot be clearly identified due to the great reduction in filler particle size after milling.The above results again verify the high efficiency of S 3 M technology.This is because the MoS2 particles are efficiently pulverized and imbedded into the POM matrix under the strong compression, shear, and hoop stretching stresses induced by pan-milling.Thus, the S 3 M strategy could be applied to effectively solve the aggregation problem of the pristine bulk MoS2 particles in the POM matrix.

Crystal Structure of S 3 M POM/MoS2 Co-Powders
Figure 4 shows the XRD patterns of pristine MoS2, neat POM, and POM/MoS2 co-powders with different milling cycles.As can be seen, the diffraction patterns of MoS2 mainly appear in three peaks at 14.6°, 32.8°, and 33.7° in the detected 2θ range, which correspond to the (002), (100), and (101) crystal planes of 2H-MoS2 [24], respectively.The diffraction peaks located at 22.8° and 34.6° are attributed to the (100) and ( 105) crystal plane of POM, respectively [25].As can be seen, the diffraction peak intensities of MoS2 and POM decrease sharply after 10 milling cycles, indicating the breakage of original crystal crystallites and distortion of the three-dimensional crystalline order of MoS2 after the S 3 M process [26].In addition, the diffraction peak intensities of MoS2 (002) and POM (100) show a decreasing tendency with further increase of milling cycles.According to the Scherrer equation: where D is the crystalline size (nm), λ is the X-ray wavelength in nanometer (nm), β is the full width at half maximum (FWHM), k is the scherrer constant, and θ is the diffraction angle.Figure 4 shows the XRD patterns of pristine MoS 2 , neat POM, and POM/MoS 2 copowders with different milling cycles.As can be seen, the diffraction patterns of MoS 2 mainly appear in three peaks at 14.6 • , 32.8 • , and 33.7 • in the detected 2θ range, which correspond to the (002), (100), and (101) crystal planes of 2H-MoS 2 [24], respectively.The diffraction peaks located at 22.8 • and 34.6 • are attributed to the (100) and (105) crystal plane of POM, respectively [25].As can be seen, the diffraction peak intensities of MoS 2 and POM decrease sharply after 10 milling cycles, indicating the breakage of original crystal crystallites and distortion of the three-dimensional crystalline order of MoS 2 after the S 3 M process [26].In addition, the diffraction peak intensities of MoS 2 (002) and POM (100) show a decreasing tendency with further increase of milling cycles.According to the Scherrer equation: where D is the crystalline size (nm), λ is the X-ray wavelength in nanometer (nm), β is the full width at half maximum (FWHM), k is the scherrer constant, and θ is the diffraction angle.Accordingly, the crystalline size can be calculated by using the full width at half maximum (FWHM) of the diffraction peak and diffraction angle.Apparently, the crystalline size of MoS2 decreases with increasing milling cycles, indicating the constant milling treatment could destroy the inner-ordered stacking of MoS2.Hence, the layered structures of MoS2 are expected to be exfoliated using the S 3 M strategy, which will be further confirmed by the following TEM characterization.Moreover, the diffraction profiles of MoS2 particles with different milling cycles show similar featured peaks, implying that the 2H crystal type of MoS2 does not change after pan-milling.Accordingly, the crystalline size can be calculated by using the full width at half maximum (FWHM) of the diffraction peak and diffraction angle.Apparently, the crystalline size of MoS 2 decreases with increasing milling cycles, indicating the constant milling treatment could destroy the inner-ordered stacking of MoS 2 .Hence, the layered structures of MoS 2 are expected to be exfoliated using the S 3 M strategy, which will be further confirmed by the following TEM characterization.Moreover, the diffraction profiles of MoS 2 particles with different milling cycles show similar featured peaks, implying that the 2H crystal type of MoS 2 does not change after pan-milling.As a comparison, the morphology of MoS2 particles in the milled POM/MoS2 nanocomposite was observed using TEM, and the results are shown in Figure 6.As can be seen from Figure 6a, a small number of larger-size MoS2 particles can still be observed, while the size is much smaller than that of the original pristine particles.Meanwhile, a large quantity of nanoscale particles with a dimension of about hundreds of nanometers can be clearly identified.Figure 6b-d show the magnified morphologies of these nanoscale particles.As can be seen, there are few-layer nanosheets of MoS2 particles formed in the polymer matrix, clearly indicating the successful exfoliation of bulk MoS2 particles taking advantage of the very strong three-dimensional shear stress field of pan-milling.This is a breakthrough in the preparation of the exfoliated MoS2 in its solid As a comparison, the morphology of MoS 2 particles in the milled POM/MoS 2 nanocomposite was observed using TEM, and the results are shown in Figure 6.As can be seen from Figure 6a, a small number of larger-size MoS 2 particles can still be observed, while the size is much smaller than that of the original pristine particles.Meanwhile, a large quantity of nanoscale particles with a dimension of about hundreds of nanometers can be clearly identified.Figure 6b-d show the magnified morphologies of these nanoscale particles.As can be seen, there are few-layer nanosheets of MoS 2 particles formed in the polymer matrix, clearly indicating the successful exfoliation of bulk MoS 2 particles taking advantage of the very strong three-dimensional shear stress field of pan-milling.This is a breakthrough in the preparation of the exfoliated MoS 2 in its solid state by only adopting a simple physical strategy.Some details could be further known from Figure 6b-d.For Figure 6b, the center region of particles appears translucent, implying only a few layers overlapping one another.The particle edge is transparent, indicating a much thinner structure in this area [8]. Figure 6c shows the TEM image of a larger particle (in small magnification).It can be seen that the center area appears opaque, while the edge region is translucent, indicating that these particles are partially exfoliated.Figure 6d further shows the magnified edge region of the larger particle, and it appears nearly transparent, apparently with a high exfoliation occurring here.It is noted that this region is weakly combined with the larger particle and seems to be separated from the larger one.Therefore, it can be speculated that these exfoliated nano-MoS 2 were crushed and fractured on the surface of larger particles and finally delaminated from them via a S 3 M process.In this section, Raman spectroscopy, which is widely applied to investigate the two-dimensional material for thickness identification, was used to further evaluate the exfoliation of MoS2 particles after pan-milling.Figure 7 shows the Raman spectra of pristine MoS2 and milled POM/MoS2 nanocomposite with 40 milling cycles.It can be seen that there are two main peaks clearly presented, which are corresponding to E 1 2g (377.2 cm −1 , in-plane vibration of two S atoms with respect to Mo atom) and A1g (404.0 cm −1 , out-of-plane vibration of S atoms) [27] (as shown in Figure 7b).Here, it is noted that the frequency of E frequency shifts in a high wavenumber direction due to the exfoliation of layered structure, which can be attributed to the influence of neighboring layers on the effective restoring force on atoms and the increase in dielectric screening of long-range Coulomb  In this section, Raman spectroscopy, which is widely applied to investigate the twodimensional material for thickness identification, was used to further evaluate the exfoliation of MoS 2 particles after pan-milling.Figure 7 shows the Raman spectra of pristine MoS 2 and milled POM/MoS 2 nanocomposite with 40 milling cycles.It can be seen that there are two main peaks clearly presented, which are corresponding to E 1 2g (377.2 cm −1 , in-plane vibration of two S atoms with respect to Mo atom) and A 1g (404.0 cm −1 , out-of-plane Polymers 2024, 16, 1334 9 of 14 vibration of S atoms) [27] (as shown in Figure 7b).Here, it is noted that the frequency of E 1 2g increases after 40 milling cycles.It has been proven that the E 1 2g frequency shifts in a high wavenumber direction due to the exfoliation of layered structure, which can be attributed to the influence of neighboring layers on the effective restoring force on atoms and the increase in dielectric screening of long-range Coulomb interactions [28].As a consequence, the frequency difference between the A 1g and E 1 2g decreases from 26.8 cm −1 to 25.2 cm −1 after S 3 M treatment, further verifying that the bulk MoS 2 particles are successfully exfoliated into few-layer nanosheets of MoS 2 [29].

Tribological Performance of POM/MoS2 Nanocomposite
The friction coefficient and wear loss of the milled nanocomposite, conventionally unmilled composite, and neat POM are shown in Figure 8.As can be seen, the pan-milled POM/MoS2 nanocomposite shows the lowest friction coefficient, while the conventionally melt-compounded POM/MoS2 composite presents an increase in friction coefficient when compared with neat POM, indicating that the exfoliated nano-MoS2 particles could really remarkably decrease the friction coefficient, while the pristine bulk MoS2 particles have the most negative influence on the friction coefficient.On the other hand, the S 3 M-processed and conventionally prepared composites present a lower wear loss than the neat POM, indicating the incorporation of MoS2 could effectively improve the abrasion property of POM.

Tribological Performance of POM/MoS 2 Nanocomposite
The friction coefficient and wear loss of the milled nanocomposite, conventionally unmilled composite, and neat POM are shown in Figure 8.As can be seen, the pan-milled POM/MoS 2 nanocomposite shows the lowest friction coefficient, while the conventionally melt-compounded POM/MoS 2 composite presents an increase in friction coefficient when compared with neat POM, indicating that the exfoliated nano-MoS 2 particles could really remarkably decrease the friction coefficient, while the pristine bulk MoS 2 particles have the most negative influence on the friction coefficient.On the other hand, the S 3 M-processed and conventionally prepared composites present a lower wear loss than the neat POM, indicating the incorporation of MoS 2 could effectively improve the abrasion property of POM.
In order to deeply understand the effect of incorporated MoS 2 on the friction and wear behaviors of POM, the morphology of the worn surface of neat POM, milled, and conventionally unmilled composites was investigated, and the results are shown in Figure 9.As can be seen, the worn surface of the neat POM appears to have obvious scratch grooves and some small debris.Many investigations [30,31] indicated that the wear mechanism of neat POM is governed by adhesion wear.Hence, the worn surface is plowed by its hard counterpart (spinning steel ring).Meanwhile, the plastic deformation may occur due to the lower hardness of the POM matrix.Additionally, the transfer film cannot be formed in neat POM [30], as a consequence of the higher wear loss of neat POM.The worn surface of S 3 M-processed POM/MoS 2 nanocomposite appears the smoothest, and there are only some shallow scratch grooves, which can be observed.The improved wear resistance of the milled POM/MoS 2 nanocomposite can be attributed to the formation of a 2H-MoS 2 transfer film on the counterpart surface [32].The worn surface of conventionally melt-compounded Polymers 2024, 16, 1334 POM/MoS 2 composite appears roughest, and the obvious plow-like gaps can be identified clearly.This large-size debris in the scratch grooves could probably be caused by the big MoS 2 agglomerates plowed out in the POM matrix during the test.Additionally, there are also large quantities of MoS 2 particles observed on the worn surface.This confirms that the bulk 2H-MoS 2 particles easily form the transfer film, and sliding then occurs on the MoS 2 lubrication film, which can possibly explain the reason why the conventionally prepared POM/MoS 2 composite could present a relatively lower wear loss.
ence between the A1g and E 1 2g Raman modes (c).

Tribological Performance of POM/MoS2 Nanocomposite
The friction coefficient and wear loss of the milled nanocomposite, conventionally unmilled composite, and neat POM are shown in Figure 8.As can be seen, the pan-milled POM/MoS2 nanocomposite shows the lowest friction coefficient, while the conventionally melt-compounded POM/MoS2 composite presents an increase in friction coefficient when compared with neat POM, indicating that the exfoliated nano-MoS2 particles could really remarkably decrease the friction coefficient, while the pristine bulk MoS2 particles have the most negative influence on the friction coefficient.On the other hand, the S 3 M-processed and conventionally prepared composites present a lower wear loss than the neat POM, indicating the incorporation of MoS2 could effectively improve the abrasion property of POM.In order to deeply understand the effect of incorporated MoS2 on the friction and wear behaviors of POM, the morphology of the worn surface of neat POM, milled, and conventionally unmilled composites was investigated, and the results are shown in Figure 9.As can be seen, the worn surface of the neat POM appears to have obvious scratch grooves and some small debris.Many investigations [30,31] indicated that the wear mechanism of neat POM is governed by adhesion wear.Hence, the worn surface is plowed by its hard counterpart (spinning steel ring).Meanwhile, the plastic deformation may occur due to the lower hardness of the POM matrix.Additionally, the transfer film cannot be formed in neat POM [30], as a consequence of the higher wear loss of neat POM.The worn surface of S 3 M-processed POM/MoS2 nanocomposite appears the smoothest, and there are only some shallow scratch grooves, which can be observed.The improved wear resistance of the milled POM/MoS2 nanocomposite can be attributed to the formation of a 2H-MoS2 transfer film on the counterpart surface [32].The worn surface of conventionally melt-compounded POM/MoS2 composite appears roughest, and the obvious plow-like gaps can be identified clearly.This large-size debris in the scratch grooves could probably be caused by the big MoS2 agglomerates plowed out in the POM matrix during the test.Additionally, there are also large quantities of MoS2 particles observed on the worn surface.This confirms that the bulk 2H-MoS2 particles easily form the transfer film, and sliding then occurs on the MoS2 lubrication film, which can possibly explain the reason why the conventionally prepared POM/MoS2 composite could present a relatively lower wear loss.Based on the above results, a friction and wear mechanism could be proposed.Figure 10 demonstrates the sliding process of the POM/MoS2 composite.In the conventionally prepared POM/MoS2 composite, there are large-size MoS2 particles (in the range of 1~30 μm) dispersed in POM, which play a primary role in separating the frictional pairs.Besides, the two-dimensional MoS2 generally possesses an extremely high strength perpendicular to the thickness direction [33], which could probably plow out the POM in the sliding process, leading to the obvious scratch grooves.However, the pan-milled POM/MoS2 nanocomposite appears to have the smoothest worn surface, indicating that the exfoliated nano-MoS2 particles could improve the anti-wear performance.The size of the exfoliated nano-MoS2 particle (in the range of nanometers) is smaller than the surface roughness (0.8 μm) of the steel ring used.Thus, the ultrathin MoS2 particles can easily enter the contact area of the steel ring and then prevent the POM matrix from being worn [34].The overall friction coefficient μ in a tribological test can be divided into two parts, i.e., the adhesion term μa and the plowing term μp.Certainly, the friction coefficient μ can be defined by the equation μ = μa + μp [35].Obviously, the lowest friction coefficient of the milled POM/MoS2 nanocomposite can be attributed to the lower ratio of plowing due to the smoothest worn surface.Comparatively, the obvious scratches of the conventionally prepared POM/MoS2 composite mean a higher ratio of plowing term, Based on the above results, a friction and wear mechanism could be proposed.Figure 10 demonstrates the sliding process of the POM/MoS 2 composite.In the conventionally prepared POM/MoS 2 composite, there are large-size MoS 2 particles (in the range of 1~30 µm) dispersed in POM, which play a primary role in separating the frictional pairs.Besides, the two-dimensional MoS 2 generally possesses an extremely high strength perpendicular to the thickness direction [33], which could probably plow out the POM in the sliding process, leading to the obvious scratch grooves.However, the pan-milled POM/MoS 2 nanocomposite appears to have the smoothest worn surface, indicating that the exfoliated nano-MoS 2 particles could improve the anti-wear performance.The size of the exfoliated nano-MoS 2 particle (in the range of nanometers) is smaller than the surface roughness (0.8 µm) of the steel ring used.Thus, the ultrathin MoS 2 particles can easily enter the contact area of the steel ring and then prevent the POM matrix from being worn [34].The overall friction coefficient µ in a tribological test can be divided into two parts, i.e., the adhesion term µ a and the plowing term µ p .Certainly, the friction coefficient µ can be defined by the equation µ = µ a + µ p [35].Obviously, the lowest friction coefficient of the milled POM/MoS 2 nanocomposite can be attributed to the lower ratio of plowing due to the smoothest worn surface.Comparatively, the obvious scratches of the conventionally prepared POM/MoS 2 composite mean a higher ratio of plowing term, certainly resulting in a higher coefficient.As a result, the S 3 M-processed POM/MoS 2 nanocomposite can have much better tribological performance.

Mechanical Performance of POM/MoS2 Nanocomposite
Figure 11 shows the mechanical performance of different samples.It can be seen that the tensile strength of the milled and unmilled POM/MoS2 composites is slightly lower than that of neat POM, possibly due to the degradation effect of MoS2.Obviously, the slight decrease would have little influence on the ultimate application of S 3 M-processed POM/MoS2 nanocomposite.Compared with the conventionally prepared POM/MoS2 composite and neat POM, the elongation at break of the S 3 M-processed POM/MoS2 nanocomposite increases remarkably.Very clearly, the excellent comprehensive performance of POM/MoS2 nanocomposite can be obtained using S 3 M technology.In order to deeply understand the influence of the S 3 M process on the mechanical performance, the fractured surface of the sample after the tensile test was observed by SEM, and the results are shown in Figure 12.As can be seen, the fractured surface of S 3 M-processed POM/MoS2 nanocomposite is uneven, and there are some deep dimples there, which can absorb a substantial amount of energy during a tensile test.On the other hand, the dot-like MoS2 particles appear nearly invisible, possibly due to the nanoscale dispersion, and the extremely small, rigid MoS2 particles would lead to local

Mechanical Performance of POM/MoS2 Nanocomposite
Figure 11 shows the mechanical performance of different samples.It can be seen that the tensile strength of the milled and unmilled POM/MoS2 composites is slightly lower than that of neat POM, possibly due to the degradation effect of MoS2.Obviously, the slight decrease would have little influence on the ultimate application of S 3 M-processed POM/MoS2 nanocomposite.Compared with the conventionally prepared POM/MoS2 composite and neat POM, the elongation at break of the S 3 M-processed POM/MoS2 nanocomposite increases remarkably.Very clearly, the excellent comprehensive performance of POM/MoS2 nanocomposite can be obtained using S 3 M technology.In order to deeply understand the influence of the S 3 M process on the mechanical performance, the fractured surface of the sample after the tensile test was observed by SEM, and the results are shown in Figure 12.As can be seen, the fractured surface of S 3 M-processed POM/MoS2 nanocomposite is uneven, and there are some deep dimples there, which can absorb a substantial amount of energy during a tensile test.On the other hand, the dot-like MoS2 particles appear nearly invisible, possibly due to the nanoscale dispersion, and the extremely small, rigid MoS2 particles would lead to local In order to deeply understand the influence of the S 3 M process on the mechanical performance, the fractured surface of the sample after the tensile test was observed by SEM, and the results are shown in Figure 12.As can be seen, the fractured surface of S 3 M-processed POM/MoS 2 nanocomposite is uneven, and there are some deep dimples there, which can absorb a substantial amount of energy during a tensile test.On the other hand, the dot-like MoS 2 particles appear nearly invisible, possibly due to the nanoscale dispersion, and the extremely small, rigid MoS 2 particles would lead to local crazes, which benefit yielding and plastic deformation under stress fields [36].As a result, the S 3 Mprocessed nanocomposite can have higher elongation at break.The fractured surface of the unmilled POM/MoS 2 composite also appears uneven, but the dimples have almost disappeared.There are some large MoS 2 particles that can be identified, and there are also some holes caused by the pulling out of MoS 2 particles upon fracture, indicating the bad compatibility between MoS 2 and POM.In addition, these large MoS 2 particles would lead to stress concentration.As a result, the conventionally prepared POM/MoS 2 composite shows poor mechanical properties.
Polymers 2024, 16, x FOR PEER REVIEW 13 of 15 crazes, which benefit yielding and plastic deformation under stress fields [36].As a result, the S 3 M-processed nanocomposite can have higher elongation at break.The fractured surface of the unmilled POM/MoS2 composite also appears uneven, but the dimples have almost disappeared.There are some large MoS2 particles that can be identified, and there are also some holes caused by the pulling out of MoS2 particles upon fracture, indicating the bad compatibility between MoS2 and POM.In addition, these large MoS2 particles would lead to stress concentration.As a result, the conventionally prepared POM/MoS2 composite shows poor mechanical properties.

Conclusions
The solid-state shear milling (S 3 M) technology was adopted to prepare the POM/MoS2 nanocomposite.As a comparison, the conventional melt-compounding method was also performed to prepare the POM/MoS2 composite in a twin-screw extruder.The dispersion and exfoliation of MoS2 particles, tribological properties, and mechanical performance of the above-prepared POM/MoS2 composites were comparatively investigated.The results show that the S 3 M strategy has a much better dispersion and exfoliation effect on the MoS2 particles than the traditional melt-compounding method.Under the effect of the very strong three-dimensional shear stress field induced by S 3 M, the pristine bulk MoS2 particles were pulverized into nanoscale particles and particularly efficiently exfoliated to few-layer 2H-MoS2 nanosheets at a large scale, which is verified by TEM, Raman, and XRD measurements.The dispersion of MoS2 particles in the POM matrix has accordingly improved substantially.On the contrary, the simple melt-compounding extrusion does not have any influence on the dispersion and exfoliation of MoS2 particles, and there is heavy agglomeration of filler particles in the matrix due to the poor shear stress field of the twin-screw extruder.Correspondingly, the S 3 M-processed POM/MoS2 nanocomposite shows substantially better tribological and mechanical properties than the traditionally melt-compounded material.Although incorporation of MoS2 could improve the anti-wear performance of POM, the S 3 M-processed nanocomposite shows a significantly lower friction coefficient due to nanoscale MoS2 decreasing the plowing effect.Meanwhile, the successfully exfoliated MoS2 nanosheets of S 3 M could substantially enhance the elongation at break of the POM/MoS2 composite.Therefore, the S 3 M strategy could show a very promising prospect in the preparation of POM/MoS2 functional nanocomposites with excellent comprehensive performance.

Conclusions
The solid-state shear milling (S 3 M) technology was adopted to prepare the POM/MoS 2 nanocomposite.As a comparison, the conventional melt-compounding method was also performed to prepare the POM/MoS 2 composite in a twin-screw extruder.The dispersion and exfoliation of MoS 2 particles, tribological properties, and mechanical performance of the above-prepared POM/MoS 2 composites were comparatively investigated.The results show that the S 3 M strategy has a much better dispersion and exfoliation effect on the MoS 2 particles than the traditional melt-compounding method.Under the effect of the very strong three-dimensional shear stress field induced by S 3 M, the pristine bulk MoS 2 particles were pulverized into nanoscale particles and particularly efficiently exfoliated to few-layer 2H-MoS 2 nanosheets at a large scale, which is verified by TEM, Raman, and XRD measurements.The dispersion of MoS 2 particles in the POM matrix has accordingly improved substantially.On the contrary, the simple melt-compounding extrusion does not have any influence on the dispersion and exfoliation of MoS 2 particles, and there is heavy agglomeration of filler particles in the matrix due to the poor shear stress field of the twin-screw extruder.Correspondingly, the S 3 M-processed POM/MoS 2 nanocomposite shows substantially better tribological and mechanical properties than the traditionally melt-compounded material.Although incorporation of MoS 2 could improve the antiwear performance of POM, the S 3 M-processed nanocomposite shows a significantly lower friction coefficient due to nanoscale MoS 2 decreasing the plowing effect.Meanwhile, the successfully exfoliated MoS 2 nanosheets of S 3 M could substantially enhance the elongation at break of the POM/MoS 2 composite.Therefore, the S 3 M strategy could show a very promising prospect in the preparation of POM/MoS 2 functional nanocomposites with excellent comprehensive performance.Institutional Review Board Statement: Not applicable.

Figure 1 .
Figure 1.The schematic diagram of wear testing experiment for wear tester (a) and worn surface of sample (b); the shape and dimensions of tensile specimen (c).

Figure 1 .
Figure 1.The schematic diagram of wear testing experiment for wear tester (a) and worn surface of sample (b); the shape and dimensions of tensile specimen (c).

Figure 3 .
Figure 3.The PLM images of POM/MoS2 composite prepared by different method at 180 °C: conventionally melt-compounding (a) and S 3 M method (b).

Figure 3 .
Figure 3.The PLM images of POM/MoS 2 composite prepared by different method at 180 • C: conventionally melt-compounding (a) and S 3 M method (b).

Figure 4 .
Figure 4.The XRD patterns of pristine MoS2, neat POM and POM/MoS2 co-powders prepared at different milling cycles.

Figure 4 .
Figure 4.The XRD patterns of pristine MoS 2 , neat POM and POM/MoS 2 co-powders prepared at different milling cycles.

3. 1 . 4 .
The Microscopic Morphology of MoS 2 in the POM Matrix The dispersion morphology and crystallization structure of pan-milled POM/MoS 2 nanocomposite were well explored, but the ultimate microscopic morphology of MoS 2 particles after S 3 M processing is still not clear.Figure 5a,b show the SEM images of the pristine bulk MoS 2 particles.As can be seen, most of the pristine MoS 2 particles show a size distribution in the micron range, and some particles have a length dimension up to 20 µm.Meanwhile, the stacking layered structure can be clearly observed (Figure 5b, enlarged image), and the thickness of some large-size particles could achieve 1 µm.The morphology of MoS 2 in the conventionally unmilled composite is observed by TEM (Figure 5c,d), and the dark area indicates the MoS 2 particles.As can be seen, the thickness stays at the micron scale.MoS 2 exists as pristine bulk agglomerates in the POM matrix, and the poor shear stress field of the twin-screw extruder could not induce the structural change of the pristine MoS 2 particles.Polymers 2024, 16, x FOR PEER REVIEW 8 of 15

Figure 5 .
Figure 5.The SEM images of pristine MoS2 (a,b); The TEM images of the conventionally melt-compounded POM/MoS2 composite (c,d).

Figure 5 .
Figure 5.The SEM images of pristine MoS 2 (a,b); The TEM images of the conventionally meltcompounded POM/MoS 2 composite (c,d).

1 2g
increases after 40 milling cycles.It has been proven that the E 1 2g

Polymers 2024 , 15 Figure 7 . 1 2g
Figure 7.The Raman spectra of the pristine MoS2 and POM/MoS2 nanocomposite after 40 milling cycles (a), atomic displacement of the E 1 2g and A1g Raman active model (b) and frequency difference between the A1g and E 1 2g Raman modes (c).

Figure 8 .
Figure 8.The friction coefficient and wear loss of milled composite, conventionally unmilled composite and neat POM.

Figure 7 .
Figure 7.The Raman spectra of the pristine MoS 2 and POM/MoS 2 nanocomposite after 40 milling cycles (a), atomic displacement of the E 1 2g and A 1g Raman active model (b) and frequency difference between the A 1g and E 1 2g Raman modes (c).

Figure 8 .
Figure 8.The friction coefficient and wear loss of milled composite, conventionally unmilled composite and neat POM.

Figure 8 .
Figure 8.The friction coefficient and wear loss of milled composite, conventionally unmilled composite and neat POM.

Polymers 2024 ,
16,  x FOR PEER REVIEW 12 of 15 certainly resulting in a higher coefficient.As a result, the S 3 M-processed POM/MoS2 nanocomposite can have much better tribological performance.

Figure 10 .
Figure 10.The schematic diagram of sliding process for the conventionally melt-compounded POM/MoS2 composite and S 3 M-processed POM/MoS2 composite.

Figure 10 .
Figure 10.The schematic diagram of sliding process for the conventionally melt-compounded POM/MoS 2 composite and S 3 M-processed POM/MoS 2 composite.

3. 3 .
Figure11shows the mechanical performance of different samples.It can be seen that the tensile strength of the milled and unmilled POM/MoS 2 composites is slightly lower than that of neat POM, possibly due to the degradation effect of MoS 2 .Obviously, the slight decrease would have little influence on the ultimate application of S 3 M-processed POM/MoS 2 nanocomposite.Compared with the conventionally prepared POM/MoS 2 composite and neat POM, the elongation at break of the S 3 M-processed POM/MoS 2 nanocomposite increases remarkably.Very clearly, the excellent comprehensive performance of POM/MoS 2 nanocomposite can be obtained using S 3 M technology.

Figure 10 .
Figure 10.The schematic diagram of sliding process for the conventionally melt-compounded POM/MoS2 composite and S 3 M-processed POM/MoS2 composite.

Figure 12 .
Figure 12.The SEM images of the fractured surface of different samples after tensile test: neat POM (a), S 3 M-processed POM/MoS2 nanocomposite (b) and conventionally melt-compounded POM/MoS2 composite (c).

Figure 12 .
Figure 12.The SEM images of the fractured surface of different samples after tensile test: neat POM (a), S 3 M-processed POM/MoS 2 nanocomposite (b) and conventionally melt-compounded POM/MoS 2 composite (c).
and S.Y.; Writing-review and editing, Y.C.; Visualization, S.F. and S.Y.; Supervision, Y.C.; Project administration, Y.C. and H.Z.; Funding acquisition, Y.C.All authors have read and agreed to the published version of the manuscript.Funding: International Science & Technology Innovation Cooperation Project of Sichuan Province (24GJHZ0037), International Science & Technology Cooperation Project of Chengdu (2021-GH03-00009-HZ), Program for Featured Directions of Engineering Multi-disciplines of Sichuan University (2020SCUNG203) and Program of Innovative Research Team for Young Scientists of Sichuan Province (22CXTD0019).
• C using a LEICA EM FC6 frozen ultramicrotome.The POM/MoS 2 thin films