Durability Improvement of Concentrated Polymer Brushes by Multiscale Texturing

Concentrated polymer brushes (CPBs) are promising soft-material coatings for improving tribological properties under severe sliding conditions, even in the macroscopic scale. Therefore, they are expected to be applied to mechanical sliding components. However, the durability of CPBs has remained challenging for industrial applications. Previous studies revealed that applying a groove texture to the CPB substrate is effective in improving the durability of CPBs. In order to achieve further improvement of durability of CPBs, we attempted to apply nano-periodic structures, whereas the groove texture applied in previous studies has widths and depths in micrometres. In this study, the effect of the nano-periodic structure in addition to the groove texture applied to the CPB substrate on the durability of CPB is investigated. The results demonstrate a significant improvement in the durability of CPBs by up to 90% compared with non-textured CPB when an appropriate nano-periodic structure is applied (i.e. a nano-periodic structure oriented parallel to the groove texture).


Introduction
Polymer brushes are a kind of polymer thin films in which polymers are grafted onto a substrate with high density and form a brush shape structure; they have been extensively studied as tribo-materials [1][2][3][4][5][6][7][8]. In particular, polymer brushes with a normalised graft density higher than 10% is known as a 'concentrated polymer brush (CPB)' [9]. CPBs 1 3 99 Page 2 of 11 in good solvents have been reported to exhibit ultralow friction in both micro [10] and macro [11][12][13][14] scale sliding; therefore, their application to mechanical components such as bearings and oil seals is expected. The challenge of applying CPBs for actual industrial applications is their durability against friction. In particular, under the frictional condition where less solvent is provided to the contact area, the tribological properties and durability of CPBs will degrade [12]. An effective way to expand the potential use of CPBs in various sliding applications is to provide the solvent to the contact area while maintaining a swollen condition. Applying a surface texture to the substrate is one of the key methods to improve the durability of CPBs. Surface texturing is a surface modification technique to improve anti-wear and frictional properties by the geometrical modification of surfaces to be slid [15][16][17][18][19]. A previous study demonstrated that applying surface textures onto the substrate of CPBs effectively improved their durability; furthermore, when a groove texture, which acts as a path for lubricants, was applied, the durability of CPBs increased by 36% [20]. In this study, in order to achieve further improvement of the durability of CPBs, we applied nano-periodic structures to the substrate of CPBs in addition to groove surface textures, similar to the previous study [20]. Nano-periodic structures were fabricated based on the following processes: when a metal surface was irradiated by a femtosecond laser with its laser fluence compatible with the energy required for the laser abrasion of the metal used, a striped groove structure with a period shorter than the wavelength of the incident laser light was formed on the metal surface [21,22]. This period can be controlled by the laser fluence [21], wavelength [23], and number of pulses [23] of the femtosecond laser. The striped groove was formed orthogonal to the polarisation of the incident light such that the direction of the striped groove can be controlled via laser polarisation. For example, when a femtosecond laser with parallel polarisation in the scanning direction was scanned on an AISI 52100 surface, as shown in Fig. 1a, a nano-periodic structure (shown in Fig. 1b) was formed. By changing the polarisation of the femtosecond laser, nano-periodic structures with different orientations against the direction of laser scanning were formed, as shown in Fig. 2.
In this study, we investigated the combined effect of nanoperiodic structures that correspond to a surface roughness (Ra) of 0.02 μm and a groove texture on the friction durability of CPBs swollen by an ionic liquid. The phenomena at sliding surfaces are discussed herein.

Specimens and Lubricants
A disc (φ 24 mm × t 7.9 mm) and cylinder (φ 6 mm × l 8 mm) made of AISI 52100 (hardness: HRC 61) purchased from test materials Co., Ltd., JP, were used as test specimens. The roughness of the cylinder Ra was 0.03 μm. Nanoperiodic structures and surface textures were applied to the disc specimens by femto-and picosecond lasers (PiCooLs, L.P.S. Works, JP). The detailed texture patterns are summarised in Sect. 2.2. To prepare CPBs based on poly (methyl methacrylate) (PMMA), the disc, which was used as the substrate for the CPBs, was cleaned via ultrasonic treatment in acetone/hexane (1:1, v/v), chloroform, and 2-propanol for 30 min in each. Then, it was treated with a UV-ozone cleaner (PC440, Meiwafosis Co., Ltd., JP) for 30 min just before use. Subsequently, a thin silica layer that was approximately 1 nm thick was deposited onto the discs by immersing the discs in an ethanol solution containing tetraethoxysilane (0.03 mol/L) and 28% aqueous ammonia solution (0.24 mol/L) overnight, followed by washing with ethanol for 30 min via ultrasonic treatment. A silane coupling agent (2-bromoisobutyryloxy) propyltrimethoxysilane (BPM) was used as an initiator to treat the silica-coated discs for the subsequent graft polymerisation. During the polymerisation, the discs were immersed in a solution containing BPM, 28% aqueous ammonia solution, and ethanol at the weight ratio of 1:10:89, respectively, overnight. This was followed by washing with ethanol for 30 min via ultrasonic treatment. Methyl methacrylate (MMA) was purified by passing it through neutral alumina to remove any radical inhibitors before use. The CPB samples were prepared via surfaceinitiated atom transfer radical polymerisation (SI-ATRP) at the high pressure of 400 MPa [24]. The BPM-immobilised discs were placed in a fluoroplastic vessel containing a solution of MMA (4.7 mol/L), ethyl 2-bromoisobutyrate (EBIB) (24 μmol/L), Cu(I)Br (15 mmol/L), Cu(II)Br 2 (1.7 mmol/L), and 4,4′-dinonyl-2,2′-bipyridine (34 mmol/L) in anisole (50 wt.%). The closed vessel was covered with an Al bag and was placed in a high-pressure reaction system (Syn Corporation Ltd., JP) that was equipped with a high-pressure vessel, an automated high-pressure pump using water as the pressure medium, and a circulating thermostat bath. Polymerisation was conducted at 60 °C and 400 MPa for 4 h. Afterwards, the solution was analysed via 1 H nuclear magnetic resonance (NMR) spectroscopy to estimate the monomer conversion and gel permeation chromatography (GPC) in order to determine the number-average molecular weight (M n ) and polydispersity index for the free polymer produced in solution, which was used as an indicator of the graft polymer [24]. NMR spectra were recorded at 400 MHz on a JNM-ECS400 spectrometer (JEOL, JP) in CDCl 3 at ambient temperature. The GPC measurements were conducted on a GPC-101 (Showa Denko K.K., JP) equipped with a guard column (Shodex KF-G), two 30-cm mixed columns (Shodex KF-806L), and a differential refractometer. Tetrahydrofuran was used as the eluent at the flow rate of 0.8 mL/min. The M n and polydispersity index values were estimated using Table 1 Molecular structure of PMMA and MEMP-TFSI the calibration data obtained by the PMMA standards (Polymer Laboratories Ltd., USA, M p = 1.31 × 10 3 − 1.64 × 10 6 ) and complemented by the polystyrene standards (Polymer Laboratories Ltd., USA, M p = 1.93 × 10 3 −1.32 × 10 7 ). The CPB-grafted discs were washed with tetrahydrofuran several times using a shaking apparatus and ultrasonic treatment. The thickness of the CPBs under dry conditions was approximately 1000 nm, as measured using a spectroscopic ellipsometer with a rotating compensator (M-2000U, J. A. Woollam, USA) equipped with D 2 and QTH lamps. The CPBs were swollen using an ionic liquid, i.e. N-(2-methoxyethyl)-N-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (MEMP-TFSI, Kanto Chemical Co., Inc., JP), which is a good solvent for PMMA. A previous study showed that the thickness of CPBs after swelling with a good solvent increased by more than two times [14]. Therefore, in this study, the thickness of the CPBs was estimated to increase to above 2,000 nm via swelling by MEMP-TFSI. The molecular structures of PMMA and MEMP-TFSI are shown in Table 1. The thickness of the CPBs was controlled by the polymerisation time. In this study, the CPBs grafted on the textured discs were prepared in the same lot. MEMP-TFSI of volume 300 μL was applied onto a dried CPB disc and stored for 96 h under low vacuum conditions. The swollen CPB discs used in the friction test were unmodified.

Surface Texture
In this study, we applied nano-periodic structures and a groove texture to AISI 52100 discs. A groove texture with a depth of 300 nm, width of 10 μm, and pitch of 110 μm was applied to the AISI 52100 disc substrate. The femtosecond laser for the nano-periodic structure was scanned with a pitch of 10 μm along the groove texture. This laser scanning induced undulation on the surface, corresponding to Ra = 0.11 μm. Three types of orientations of the nano-periodic structures, namely 0°, 45°, and 90°, against the scanning direction of the femtosecond laser, were prepared; these test samples are referred to hereinafter as Nano 0, Nano 45, and Nano 90, respectively. The pitch and corresponding roughness of the nano-periodic structures were 400 nm and 20 nm, respectively. The M n value and polydispersity index of PMMA (free polymer simultaneously produced during the SI-ATRP) were 1.70 × 10 6 and 1.26, respectively. The dry thickness of the CPBs on these textured steel discs was 1102 nm, and the graft density and normalised (dimensionless) graft density were 0.46 chains/nm 2 and 0.26, respectively. The topographic images captured via Atomic Force microscopy (AFM) measurements are shown in Fig. 3.

Friction Durability Test
To analyse the durability of CPBs on surface-modified discs against friction, we performed friction tests with increasing applied loads. A reciprocating friction tester (SRV4, Optimol, DE) with a cylinder-on-disc geometry was used, and the applied load was increased from 5 to 70 N by 1 N every 1 min. The detailed conditions of the friction test are summarised in Table 2. In this study, the load when the friction coefficient exceeded 0.03 was defined as the 'limit load' of the CPBs and used as the durability index. We tested the CPBs on each textured disc three times.
The reciprocating friction tests were conducted in both the perpendicular and parallel directions against a groove texture. The nano-periodic structures comprised three orientation types; therefore, the tests were classified into six categories (3 different orientation samples × 2 sliding directions). Herein, we denote the result of the sliding test as 'sample name-sliding direction'. For example, when Nano 0 is slid to perpendicular to the groove texture, it is denoted as 'Nano 0-Per (instead of Pll in the case of sliding to the parallel direction)'. The test combinations are shown in Table 3.

Surface Analyses
The macro image of the wear track was captured using a laser microscope (VK-X 150, KEYENCE, JP). Nanoand microscale observations were conducted using AFM (S-image, SII, JP) in MEMP-TFSI, and the resultant residual polymers on the wear tracks were evaluated from force-distance curve measurements. A pyramidal Si cantilever (SI-DF20, Hitachi High-Tech, JP) with the spring constant of 20 N/m and tip radius of 10 nm was used. A 100 × 100 μm topographic image was obtained with the load of 300 nN and a scan rate of 0.3 Hz The force-distance curve was obtained with the indentation speed of 2 μm/s. The relationship between force and distance and the amount of polymers at the wear track is shown in Fig. 4.
The force-distance curve shows the behaviour where (i) the CPB was worn out; (ii) less polymers exist, allowing the sharp cantilever to penetrate into the CPB [25]; and (iii) polymers exist sufficiently. As shown in Figs. 4 (i) and (iii), we measured the force curves of the AISI 52100 substrate and unworn CPB as references, respectively. The adhesion shown in Fig. 4 (ii) was derived from the adhesion force between the tip and the chain segments of the polymers, when the tip penetrated the polymer brushes. The details of the analysis are published elsewhere [20].

Friction Durability of Textured CPB Surfaces
The representative frictional behaviours of each CPB on the non-textured and textured substrates are shown in Fig. 5. The limit load where the friction coefficient exceeded 0.03 in each test is summarised in Fig. 6. The limit loads were 26.0 N for non-textured, 44.3 N for Nano 0-Per, 43.3 N for Nano 45-Per, 31.3 N for Nano 90-Per, 49.3 N for Nano 0-Pll, 40.6 N for Nano 45-Pll, and 33.6 N for Nano 90-Pll. The Table 3 Specifications of combination of nano-periodic structures and groove texture and sliding direction Fig. 4 Assessment of amount of polymer remaining on disc specimens surface-modified samples exhibited increased durability as follows: 70% for Nano 0-Per, 67% for Nano 45-Per, 20% for Nano 90-Per, 90% for Nano 0-Pll, 56% for Nano 45-Pll, and 29% for Nano 90-Pll compared with non-textured samples. For the case where the nano-periodic structure was oriented 0° and 45° against the groove texture, the durability improved significantly; meanwhile, that oriented at 90° indicated a moderate increase in durability.

Mechanism of Durability Improvement by Multi-Scaled Surface Texturing
To clarify the mechanism of wear and exfoliation of CPBs, it is important to analyse the surface being worn rather than after it has worn out. Therefore, in addition to the duration friction tests, we performed a test that halted when the friction coefficient exceeded 0.06 and observed the surface. For the 0° and 45° orientations of the nano-periodic structures shown in Fig. 7a, b, respectively, the exfoliation of the polymers starting from the edge of the groove texture was not observed; however, the exfoliation from the edge of the texture was observed in the 90° orientation of the nano-periodic structure. This result indicates that the appropriate nanoperiodic structure is effective in suppressing the exfoliation starting from the edge of the groove texture. Notably, the wear of the CPB occurred from the centre of the reciprocating surface, whereas the sliding condition of the edge of the reciprocating was more severe owing to the sliding speed.
To discuss the detailed mechanism of the wear of CPBs on textured substrates, we performed force-distance curve measurements on the wear track of the surface halfway during the friction test (at the same point as shown in Fig. 7). The topographic images and force-distance curves of Nano  Fig. 8a, b, and c, respectively. In the cases of Nano 0-Per and Nano 45-Per shown in Fig. 8a, b, respectively, the force curves at both inside (blue and black circles) and outside (red and green circles) the groove texture suggest the wear of polymers. The 3D topographic images are shown in Supporting Information (S-1). In the case of Nano 90-Per shown in Fig. 8c, the exfoliation of polymers starting from the edge of the groove texture was observed.
The topographic images and force-distance curves of Nano 0-Pll, Nano 45-Pll, and Nano 90-Pll are shown in Fig. 9a, b, and c, respectively. Groove structures were not indicated from these results. In the cases of Nano 0-Pll and Nano 45-Pll shown in Fig. 9a, b, respectively, the force-distance curves suggest the wear of polymers. Note that the inside and outside the wear track could not be distinguished from the topographic images. In the case of Nano 90-Pll shown in Fig. 9c, the exfoliation of polymers occurred at many points.

Discussion
First, we consider the effect of the nano-periodic structure on the CPB substrate. Ramakrishna et al. reported the effect of the nanoscale surface roughness on the lubrication properties of semi-diluted polymer brushes [26]. They reported that the polymer brushes on spherical surfaces with a radius of 6 nm resulted in a higher friction owing to the less-stretched polymer chains, resulting in a smaller repulsion force between the polymer brush and the counter surface. In our study, we did not observe an increase in the friction coefficient when a nano-periodic structure was applied. The radius of the nanoperiodic surface in this study is estimated to be 202 nm [the details of the calculation are described in the Supporting Information (S-2)]. The thickness of the CPB after swelling is estimated to be above 2000 nm, which is much larger than the radius of the nano-periodic structure. Furthermore, the curved surface of the nano-periodic structure continuously appeared in contrast to the sparse appearance of the embedded particles in a previous study [26]. Therefore, it can be hypothesised that polymer chains grafted on a certain asperity are affected by the steric hindrance with other polymer chains on the neighbouring asperity. This causes the polymer brushes to stretch vertically. Based on this hypothesis, the nano-periodic structure induces an increase in the grafted area and leads to a denser polymer brush layer than that on a flat surface. Further, the graft density at the nano-periodic structure surface (0.46 chains/nm 2 ) is larger than that at the flat surface (0.35 chains/nm 2 [20]). This increase in the graft density contributed to better durability of the CPB. Another aspect of the effect of multiscale textures is the reduction in the contact area. Previous studies have reported that the nanotextured surface of soft materials reduces the contact area at the interface, thus reducing the adhesion and friction compared with flat analogues [27][28][29]. Hence, the multiscale textures in this study are also effective in reducing the contact area, which will reduce adhesion and friction and consequently improve durability.
Subsequently, we consider the effect of the orientation of the groove texture and nano-periodic structures. A previous study reported that the exfoliation of polymers starts from the edge of the groove texture [20]. The results of the limit loads shown in Fig. 6 indicate that sliding parallel to the groove texture tends to yield better durability than sliding perpendicular to the same orientation of the nano-periodic structures. In addition, the exfoliation of the CPB was observed for Nano 90-Per by AFM measurements, as shown in Fig. 8c. Therefore, we consider that the edge of groove texture worked as the origin of exfoliation when the groove slid vertically, as mentioned in a previous study [20]. Focussing on the effect of the orientation of the nano-periodic structures and the groove textures, the parallel orientation of the nano-periodic structure along the groove texture tends to yield better durability than the perpendicular orientation analogue. A previous study verified that the supply of an ionic liquid, which is a good solvent, to the contact region plays an important role in maintaining the CPB [12]. From the laser microscope image shown in Fig. 7, it is found that the wear of the CPB started from the centre of the reciprocating direction. This indicates that the  Fig. 10 Progress ratio of durability of surface-modified sample compared with that of non-textured CPB sample CPB wear was caused by the starvation of MEMP-TFSI. Based on this mechanism of wear of the CPB, we consider that the orientation of the nano-periodic structure along with the groove texture acts as a lubricant path at the nanoscale and contributes to providing the MEMP-TFSI to the sliding area, which prevents the starvation of the MEMP-TFSI and improves the friction durability.
The comparison of the progress ratio of durability of the surface-modified sample with that of the non-textured CPB sample is summarised in Fig. 10. The data for the samples having a groove texture are based on previous research [20], in which the data were obtained under exactly the same test conditions. The result demonstrates that the application of nano-periodic structures together with a groove texture to the CPB substrate significantly improved the durability of the CPB.

Conclusion
Applying nano-periodic structures and a groove texture to the CPB substrate effectively improved the durability of the CPB through nano-periodic structures, which increased the graft density and induced the formation of a semi-diluted polymer layer at the outermost surface. The results demonstrated an improvement in the durability of the CPB by up to 90% compared with that of the non-textured sample. To achieve such improvements, the nano-periodic structure must be oriented along the groove texture and the laser track, which is formed during the fabrication of nano-periodic structures.