Surface modification of YS-20 with polydopamine for improving the tribological properties of polyimide composites

Recently, great effort has been devoted to prepare various reinforce fillers to improve polymer performances, but ignoring the importance of raw polymer powders which are indispensable parts of hot-pressed polymer composites. Herein, we engineer raw polyimide (PI) powders with the assistance of polydopamine (PDA) in aqueous solutions. After the modification, polymer powders change from hydrophobic to hydrophilic, which makes it is possible to further modification of polymer powders in liquid phase. During the curing process of modified polymer powders, the partial dehydration of the catechol groups and crosslinking of PDA via C-O-C bonds are confirmed. Based on the features of PDA, a non-destructive mixing method is utilized to realize homogeneous dispersion of multi-walled carbon nanotubes (MWCNTs) in polymer matrix. In comparison with ball milling method, this way can preserve the integrated innate structure of MWCNTs effectively. Besides, by taking full advantage of the reducing and metal-coordination capability of PDA, Cu2+ is successfully loaded onto the surfaces of polymer powders. The related characterizations demonstrate that Cu2+in situ converts to metallic copper rather than copper oxide during the hot pressing process. The tribological properties of corresponding polymer composites are also studied. These results indicate that modifying polymer powders with PDA is multi-profit and presents practical application prospect.


Introduction
Polymer composites are among the most rapidly growing classes of materials that have been designed for various industrial applications because of their superior properties, such as easy manufacturability, low density, corrosion resistance, and excellent specific strength. A lot of efforts have been devoted to obtain the composite with high comprehensive performances and the integration of nano-materials into the polymer matrix is a facile and effective route. For instance, Xiao et al. [1] prepared hollow boron nitride microbeads through a salt-temple method and epoxy composites exhibited outstanding thermal performance due to the interconnected networks. Zhang et al. [2] synthesized hydroxyapatite nanorods with controlled size and achieved better mechanical properties of polyurea by in situ polymerization. Min et al. [3] fabricated aminefunctionalized graphene nanosheets and the tribological performance of polyimide (PI) was successfully improved because of the covalent bonds between graphene nanosheets and PI. Despite the studies which utilize nano-materials to enhance the performances of polymer composites have received much attention, obtaining homogenous dispersion of nano-materials in polymer matrix is still a crucial step and full of challenges.
There are many novelty ways to reduce abrasion of friction pairs [4,5], particularly, avoiding thermal and stress concentration caused by nanoparticle agglomerations is important to reduce friction and wear when polymer composites are applied for tribology field. For the composites prepared by means of hot press molding technique, ball milling treatment of raw polymer powders, and reinforcing fillers is a common method to improve the dispersibility of nanoparticles, even though it may damage and chop the structure of nanoparticles to some extent [6]. Mixing raw polymer powders and reinforcing fillers in liquid phase is benign and alternative, but most of raw polymer powders are hydrophobic which restrict the use of water as a dispersion medium. Though hydrophilic modification of polymer powders is rarely reported, it is still a direction with application prospect.
Composites prepared by means of hot press molding technique generally relate to high curing temperature, for example it is 350 °C at least for PI. Under such condition, some metal ions will be oxidized to form corresponding metal oxides and then well-dispersed nanoparticles can be realized. Thus the performances of polymer can be improved owning to the incorporation of well-dispersed reinforcing fillers by this in situ synthetic method. However, the interaction between polymer powders and metal ions is limited due to the inertia feature of polymer powders. Moreover, hydrophobicity also makes it difficult to mix polymer powders with metal ion precursors in aqueous solution. In order to chelate the metal ions, carboxyl or polyhydroxyl structure is necessary and fortunately, these groups are also hydrophilic. Therefore, modifying polymer powder with carboxyl or polyhydroxyl structure is multi-profit and it is of practical significance to find such a simple but universal way.
Recently, inspired by mussel chemistry, biopolymer polydopamine (PDA) has been widely used as a versatile surface modification material due to its extraordinary wet-adhesion properties [7]. The molecules of PDA contain high concentrations of amine and catechol functional groups, which endow PDA with good hydrophilicity, firm adhesion, and the ability to chelate metal cations [8][9][10]. For polymer composites, the deposited PDA layer on the surfaces of reinforcing fillers commonly has two functions. On the one hand, the layer can act as a bridge to improve the interfacial bonding strength between filler and polymer matrix [11]. On the other hand, PDA coating is proven to be an efficient secondary reaction platform, which leads to tailoring of hybrid structure [12]. However, most of current works concentrate on preparing novelty fillers or modifying existing fillers to enhance hotpressed polymer composites. It is noteworthy that raw polymer powders are also vital parts of ultimate polymer composites. Thereby more attention toward the treatment of polymer powders is reasonable and necessary.
Herein, we engineered raw PI powders with the assistance of PDA in aqueous solutions. After the modification, polymer powders changed from hydrophobic to hydrophilic. Based on the features of PDA, a non-destructive method was explored to homogeneously mix multi-walled carbon nanotubes (MWCNTs) and PI powders by introducing MWCNTs into dopamine solution. Compared to ball milling method, this method was milder and more efficient to improve the dispersibility of nanoparticles in polymer matrix. Otherwise, PI powders loaded with copper ions on their surfaces were also received by the addition of copper chloride into the dopamine solution. Related characterization revealed that the copper ions were in situ converted to metallic copper rather than copper oxide during hot pressing process. The tribological properties of PI composites prepared with modified polymer powders were further studied and the experimental results indicated the great application value of this method.

Materials
MWCNTs with an average length of 10-30 μm were supplied by Nanjing XFNANO Materials ech. Co. Ltd. Copper chloride dehydrate was purchased from Tianjin Fuchen Chemical Reagents Factory. 3-Hydroxytyramine hydrochloride (DA) was purchased from Shanghai Energy Chemical Company. Tris(hydroxymethyl) aminomethane (TRIS, 99%) was provided by Kermel Chemical Reagent Plant. PI powders (YS-20) were used with particles size less than 75 μm. Water used throughout the experiment was deionized.

Preparation of modified PI powders
Firstly, 0.4 g DA and 1 ml Tris-HCl buffer solution were dissolved in 200 ml of deionized water. Then 10 g YS-20 were added into the aqueous solution by continuous stirring for 24 h. The suspension was filtered and washed several times with deionized water to remove unreacted DA. Finally, the obtained powders (PDA-YS20) were dried at 60 °C for 12 h. YS-20 loaded with MWCNTs and Cu 2+ were also prepared by addition of 0.2 g MWCNTs and 0.3 g CuCl 2 ·2H 2 O into the DA solution, respectively.

Preparation of PI composites
The composites were prepared by means of hot press molding technique. Briefly, 4 g modified polyimide powders were compressed and heated to 350 °C in a mold with a dimensional of 2 mm × 35 mm × 40 mm. The pressure was kept at 10 MPa for 1.2 h and then the specimens were cooled down at room temperature naturally. Figure 1 presented the schematic diagram of an overall manufacturing procedure for making PI composites with various dopamine-modified YS-20.

Tribological tests
Friction and wear tests were carried out using a ballon-disk tribometer under the ambient temperature. The counterpart ball was GCr15 with a diameter of 6 mm. The applied load was 20 N and the rotation speed was 300 r/min. The continuous friction lasted for 2 h and each friction test was conducted three times.

Characterization
A transmission electron microscope (TEM, Tecnai TF20) was used to visually observe the particle structure. The surface morphologies of nanoparticles and wear tracks were observed by a field-emission scanning electron microscope (FE-SEM, JSM-6701F, JEOL) and a scanning electron microscope (SEM, JSM-5600LV). Fourier transform infrared (FTIR) spectra were collected on a Bruker IFS 66v/s spectrometer. X-ray photoelectron spectroscopy (XPS) characterization was carried out on an ESCALAB 250xi spectrometer. Thermal analysis was performed on a thermo-gravimetric analysis instrument (TGA, STA 449 F3). Wear volumes of different wear tracks were measured by MicroXAM-3D noncontact surface mapping profiler. Ball milling of polymer powders were performed with GRINDER GT200. The duration is 2 min and the frequency was 1,000 rpm.

YS-20 modified with PDA
The DA molecules were strongly oxidized in a basic environment and catechol was oxidized to benzoquinone, followed by the self-polymerization in a manner of melanin formation [13,14]. PI powders were hydrophobic and most of them floated on water as shown in Fig. 2(a). After the deposition of PDA, the color of YS-20 altered from bright yellow to dark gray and PDA-YS20 displayed better compatibility with deionized water. Notably, this method not only improved the hydrophilicity of YS-20, but also applied to other common polymer powders, such as UHMWPE and PEEK powders ( Fig. 2(a)). The FTIR spectra of asreceived YS-20 and PDA-YS20 were shown in Fig. 2 | https://mc03.manuscriptcentral.com/friction PDA particles were treated at high temperature by the same heating procedure which was used to prepare polymer composite under the protection of inert gas. TEM images indicated PDA particles were spherical and calcination had little effect on the morphologies due to lower heating rate (Figs. 2(b) and 2(c)). Yu et al.
[18] reported the existence of distinct graphite-like nanostructure in the carbonized PDA particles but it was not observed in the magnified high-resolution image (Figs. 2(b) and 2(c)). One possible reason was that 350 °C was not enough to produce graphite-like structure. Figure 2(e) presented the Raman spectrum of PDA particles. Two dominating peaks appeared at 1,345 and 1,575 cm -1 , corresponding to D band (catechol stretch) and G band (catechol deformation) [19]. TG analysis (Fig. 2(f)) showed the residue weight of PDA particles was about 80 % with the temperature from 25 to 350 °C and it was about 40% when the temperature up to 800 °C . The high residue weight implied the superior thermal stability of PDA particles. In the FTIR spectra ( Fig. 2(g)), carbonized PDA showed a broad band in the range of 1,500-1,000 cm -1 compared to that of PDA particles. According to the work of Zou et al. [20], the broad band was mainly consisted of C-OH, C-O, and C-O-C, indicating the partial dehydration of the catechol groups on adjacent PDA molecules [21]. Combined with the above analysis, it was speculated that the deposited PDA on the surfaces of YS-20 partially cross-linked via C-O-C bonds ( Fig. 2(h)) and could maintain their original morphologies during the curing process. The degradation of some functional groups also existed.
The pure PI bulk materials prepared with YS-20 were orange and transparent while they were black and opaque when PDA-YS20 were used as the raw powders (pPI). Figure 3(a) displayed the fracture surfaces of PI and the composite. The pure PI matrix showed smooth and clean surfaces, which was attributed to brittle fracture. By contrast, the fractured surfaces of pPI became more rough (Fig. 3(d)), suggesting the incorporation of PDA could inhibit the crack propagation in polymer matrix [22]. Mean friction coefficient and wear volume of PI and pPI were displayed in Figs. 3(b) and 3(e). It was obvious that modifying YS-20  [23], were observed on both wear tracks. Much pits and bulges came out on the wear tracks of pPI and the surfaces were coarser. To take full advantage of PDA and to enhance the tribological performances of PI composites, two facile and efficient improvements were made to PDA-YS20.

PDA-YS20 loaded with MWCNTs (pCNT-YS20)
MWCNTs were ideal reinforcements to develop high performance polymer composites owning to their excellent optical, thermal, and mechanical properties [24]. It was a consensus that the distribution state of MWCNTs had a great influence on the polymer properties [25]. Ball milling was a common mixing method, however, some issues such as flattening of polymer powders, shortening of MWCNTs length, and MWCNTs agglomeration have also raised [6,26]. In this work, when MWCNTs were put into the DA solution, the non-destructive modification of MWCNTs and homogeneous mixing of MWCNTs with YS-20 were received simultaneously by continuous stirring. YS-20 loaded with MWCNTs were also obtained by ball milling method and they were named as mCNT-YS20. Figures 4(a)-4(d) showed the surface morphologies of pCNT-YS20 and mCNT-YS20. It was found that the surfaces of YS-20 were rough and irregular. For mCNT-YS20, one part of the MWCNTs was distributed relatively uniform on the surfaces of   YS-20, while the other part was agglomerated. For pCNT-YS20, the agglomeration has been alleviated and most MWCNTs were tightly bound to YS-20. Tight binding represented better interfacial affinity between MWCNTs and polymer powders, thus the stress transfer from polymer matrix to reinforcement fillers in cured polymer was strengthened followed by enhanced mechanical and tribological performances of polymer composites. Raman spectroscopy was a widely used method to determine the defect structures of carbon-based materials. In Fig. 4(e), two remarkable bands at 1,345 and 1,586 cm -1 could be observed, which was resulted from the D-band and G-band of graphitic carbon [27]. The intensity ratio of D-band to G-band (I D /I G ) was employed to assess the degree of structural defects in pristine MWCNTs and ball-milled MWCNTs (mCNT). The I D /I G ratio slightly shifted from 1.19 for pristine MWCNTs to 1.27 for mCNT, indicating the partially damaged structure after ball milling. The Raman peaks of MWCNTs and PDA particles overlapped to some extent, so the Raman spectra was not a true reflection of the defects of PDA modified MWCNT (pCNT) and it was not supplied here. From the TEM images in Figs. 4(f)-4(h), it was obvious that the pristine MWCNTs possessed tubular and curving structure with an average diameter about 10 nm. After the ball milling, pristine MWCNTs fractured and their length decreased marked by the red arrows in Fig. 4(g), which was consistent with higher ratio of I D /I G . As shown in Fig. 4(h), the surfaces of pCNT were wrapped with a 3 nm thick continuous amorphous layer, demonstrating a successful deposition of PDA. Figures 5(a) and 5(b) depicted the mean friction coefficient and wear volume of PI, PI composites prepared with mCNT-YS20 (mCNT-PI), and PI composites prepared with pCNT-YS20 (pCNT-PI). It was obvious the wear resistance of mCNT-PI and pCNT-PI is both better than that of pure PI, which confirmed the positive wear reducing effect of MWCNTs. As to the worn surface of mCNT-PI (Fig. 5(c)), there were apparent furrows and holes. The stress bearing capacity of MWCNTs became weaker since their integrated innate structure was destroyed by ball milling. Otherwise, ball milling reduce the aspect ratio of MWCNTs, which impeded the formation of percolated network in polymer matrix and decreased the interfacial bonding [28]. Thus the steel counterface penetrated the polymer matrix more easily and more scratches appeared on the worn surface. Besides, the friction stress was mainly concentrated at the matrix-filler www.Springer.com/journal/40544 | Friction interface due to the superior mechanical strength of MWCNTs. Micro-cracks propagated along the agglomeration of MWCNTs, resulting in a large volume loss on the worn surface [29]. For the worn surface of pCNT-PI (Fig. 5(d)), it was much smoother and flatter. The non-destructive mixing method not only preserved the strength of MWCNTs, but also improved their dispersion state in polymer matrix, thus a better tribological performance of polymer composites was achieved.

PDA-YS20 loaded with Cu 2+ (pCu-YS20)
Bio-inspired catechol-based compounds were typical organic ligand for metal-coordination [30]. Thus a co-deposition strategy could be realized by the addition of Cu 2+ into the DA solution. During the continuous stirring, DA chelated Cu 2+ to self-polymerize and deposited onto the surfaces of YS-20 at the same time. The element mapping of modified PDA-YS20 (Fig. 6(a1)) indicated the relatively homogeneous dispersion of Cu 2+ . Similarly, this loading method was applicable to other metal cations, such as Ni 2+ and Zn 2+ (Figs. 6(a2) and 6(a3)). XPS was performed to identify the surface chemical valence state of Cu 2+ before and after hot pressing process. As elucidated in Figs. 6(b) and 6(c), the high-resolution Cu 2p spectrum of PDA after the incorporation of Cu 2+ (PDA-Cu) displayed two distinct signals for Cu 2p 1/2 and Cu 2p 3/2 . The binding energy at 952.4 and 932.4 eV were assigned to zero valent copper (Cu 0 ), while the Cu 2p 1/2 at 954.6 eV and Cu 2p 3/2 at 934.7 eV were attributed to Cu 2+ [31,32]. The existence of Cu 0 was related to the strong reducing capability of PDA, which contained many catechol moieties and secondary amino groups [12,33]. The satellite peaks at 963 and 944 eV was because of the open-shell 3d character of Cu 2+ [34]. After the carbonization, the peak intensity of Cu 2+ weakened, suggesting high temperature could enhance the reducibility of PDA and more Cu 2+ could convert into metallic copper. The residual satellite peaks in Fig. 6(c) were might due to the surface oxidation of copper when the sample was exposed to air. The XRD patterns of PI composites were investigated to further reveal structural changes of the Cu 2+ after curing. In Figs. 6(e) and 6(f), both PI and pPI showed broad diffraction peaks at ~ 22° while a new weak peak appeared at 43.2° for PI that prepared with pCu-YS20. The peak corresponded to the (111) of metallic copper, also indicating the existence of Cu 0 [35]. Details of morphology evolution of PDA-Cu were characterized by TEM. In Fig. 7(a), the boundaries of PDA particles disappeared and more PDA particles connected together in a sheet-like structure. Du et al. [36] pointed out that adhesion ability of PDA could be significantly enhanced after coordination. Yang et al. [37] suggested that metal ions would complex with dopamine oligomers and produce stacked aggregates by coordination chemistry. These phenomena could be indirectly reflected in Fig. 6(d), the PDA were fluffy after freeze drying, while PDA-Cu easily assembled into huge sticky aggregates. In high-resolution TEM images ( Fig. 7(b)), the crystal lattice fringes with   [38]. After carbonized, more metallic copper nanoparticles with high dispersity were observed and the particle sizes were about 5.42 nm. The excellent particle uniformity was because of this in situ synthetic method. In Fig. 7(f), it was clearly recognized two distinct lattice fringes and the lattice plane spacing was about 0.18 and 0.21 nm, which was in agreement with (200) and (111) planes of metallic copper, respectively [39]. The analysis described above demonstrated that Cu 2+ in situ converted to metallic copper rather than copper oxide during the hot pressing process and a well-dispersed state of reinforcing filler was obtained.
There were two ways to prepare YS-20 particles loaded with Cu 2+ . The one way was to stir YS-20 in the mixed solution of DA and Cu 2+ . The other way was to stir PDA-YS20 in the solution of Cu 2+ (p-Cu-YS20). TG analysis of two different YS-20 particles was performed in Fig. 7(c). The results confirmed residue weight of pCu-YS20 was higher than that of p-Cu-YS20, implying more Cu 2+ loading in pCu-YS20. The XRD pattern of p-Cu-PI also showed a peak at 43.2°, indicating the formation of Cu 0 (Fig. 7(d)).     Fig. 8(e1)) was irregular with different thickness and some uncompacted wear debris was found on the transfer film ( Fig. 8(e2)). The uncompacted wear debris increased the roughness of transfer film and was easy to fall off from the counterpart ball, which would aggravate abrasive wear. For pCu-PI, the wear debris was sheet like (Fig. 8(f1)) and the transfer film was clean as well as compacted (Fig. 8(f2)). The successful load of Cu 2+ and the conversion from Cu 2+ to metallic copper contributed to strengthen the tribofilm, thus the wear resistance of PI composites could be reinforced.

Conclusions
In summary, we engineered YS-20 with the assistance of PDA in aqueous solutions. After the modification, polymer powders changed from hydrophobic to hydrophilic, which make it was possible to further modify polymer powders in liquid phase. During the curing process, the partial dehydration of the catechol groups and crosslinking of PDA via C-O-C bonds were confirmed from our experimental results. By taking full advantage of PDA features, two different manners were developed to improve the tribological properties of PI composites. The one way was to add MWCNTs into the DA solution, which could achieve non-destructive and homogeneous dispersion of MWCNTs in polymer matrix. The other way was to add Cu 2+ into the DA solution, which in situ converted to metallic copper rather than copper oxide during the hot pressing process owning to the reducing capability of PDA. These two efficient and simple ways indicated that modifying polymer powders with PDA was multiprofit and presented practical application prospect.