Local Controllable Laser Patterning of Polymers Induced by Graphene

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Local Controllable Laser Patterning of Polymers Induced by Graphene Material Liang Wen, Tao Zhou, Jihai Zhang, and Aiming Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09504 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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ACS Applied Materials & Interfaces

Local Controllable Laser Patterning of Polymers Induced by Graphene Material Liang Wen, Tao Zhou,* Jihai Zhang, and Aiming Zhang State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute, Sichuan University, Chengdu 610065, China *Corresponding author. Tel.: +86-28-85402601; Fax: +86-28-85402465; E-mail address: [email protected]

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Abstract: Graphene has been successfully applied to the field of polymer laser patterning. As an efficient 1064 nm near-infrared (NIR) pulsed laser absorber, the graphene prepared by the mechanical exfoliation of only 0.005 wt% (50 ppm) endowed polymer materials with a very good performance of NIR pulsed laser patterning. Optical microscopy observed that the generated black patterns came from the local discoloration of the polymer surface subject to the laser irradiation, and the depth of the discoloration layer were determined within 221-348 µm. The X-ray photoelectron spectroscopy confirmed that the polymer surface discoloration was contributed from the local carbonization of polymers caused by graphene due to its high photothermal conversion capacity. Raman depth imaging successfully detected that the generated carbon in the discoloration layer was composed of the amorphous carbon and the complex sp/sp2-carbon compounds containing C≡C or conjugated C=C/C≡C structures. This study also provides a simple guideline to fabricate laser-patterning polymer materials based on graphene. We believe that graphene has broad application prospects in the field of polymer laser patterning. Importantly, this work opens up a valuable feasible direction for the practical application of this new carbon material. Keywords: Graphene; laser patterning; polymer; near-infrared pulsed laser; carbonization


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1. Introduction Graphene has been a fascinating material since it was found in 2004.1 Graphene has many excellent characteristics, such as a large theoretical specific surface (2630 m2g-1)2, high intrinsic mobility (200,000 cm2v-1s-1),3 high Young’s modulus (~1.0 TPa),4 and good thermal conductivity (~5000 Wm-1K-1).5 After many unremitting efforts of numerous scientists and researchers, the development and applications of graphene materials have a great achievement, showing us an enormous prospect in many fields. In recent years, people believe that the graphene is one of the promising revolutionary materials. However, we also recognized that the large-scale industrial applications of graphene closely related to people's daily life still lacks and requires a very long way to go. In this paper, a novel application area for graphene material was developed—the polymer laser patterning, which is closely connected with our daily life. Since 2009, the interaction between graphene and laser has attracted the attention of scientists, and the rapid development has also been obtained over recent years.6 Laser can be used to prepare and process graphene.7-9 For an example, Han et al8 successfully obtained monolayer graphene by laser irradiation. For this reason, laser can also be used to fabricate the patterned graphene in a single-step.10-13 Those patterned graphene demonstrated a well conductivity for the electrical application. In addition, as a semiconductor saturable absorber mirror, graphene has been proved to be appropriate for Q-switched and mode-locked lasers.14-17 In this field, graphene was used for an ideal saturable absorber, outperforming traditional semiconductor materials in many aspects. Importantly, the direct light propulsion of graphene on a bulk scale was achieved by Zhang et al18 in 2015. Their findings provided



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an exciting opportunity to realize the bulk-scale light manipulation with potential applications, such as the solar sail and the space transportation directly driven by sunlight. In summary, above achievements illustrate the remarkable interaction between graphene and laser. Moreover, the laser absorption of graphene is certainly strong. This raises us a question, whether graphene can be used in our research field—the laser patterning of polymers. Laser patterning (or laser marking) is an important component in the field of laser processing technology. It has been widely used in surface patterning of metals, ceramics, plastics and other materials.19-24 At present, the laser patterning is usually employed to fabricate logos, images, texts, two-dimensional codes, or barcode on the material surface, ensuring the identification and the trace-ability of various goods.25-27 Compared with traditional printing, laser patterning is high efficient, fine, more environmental and flexible. Therefore, it is expected to replace the conventional printing in many areas in the future. Nowadays, the near-infrared (NIR) pulsed laser system with λ=1064 nm is most widely used in the field of laser patterning. This is because the laser equipment of 1064 nm has the lowest cost. Moreover, compared to the blue laser (λ=355 nm) and green laser (λ=532 nm), 1064 nm NIR pulsed laser is less harmful to the human. As is known, laser patterning with 1064 nm has been very popular in the field of metal and ceramic materials. The most important reason is that laser patterning for these materials is relatively easy due to the inherent high absorption of 1064 nm NIR laser.20,

28, 29

However, the laser absorption

(1064nm) of polymers is intrinsically weak. Consequently, NIR laser patterning for most of the polymers, such as polypropylene (PP), polyethylene (PE), and nylons, always has difficulties. That is to say, after laser “bombardment”, no changes can be observed on the


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surface of polymer materials. Fortunately, after years of research, scientists discovered that adding the laser absorbent in polymers successfully solved this problem. For examples, such as antimony doped tin oxide and bismuth oxide, metal oxides are good laser absorbents for polymers, which have the best performance.30-32 These metal oxides have a very strong absorption for 1064 nm NIR laser. During the laser irradiation, metal oxide absorbents quickly converted the laser energy into heat to produce a locally ultra-high temperature, causing the instantaneous carbonization of polymers. Subsequently, the black patterning is formed on the light-colored polymer surface. Although metal oxide absorbents addressed the issue of polymer laser patterning, the potential hazard of the metal elements on the human body cannot be ignored. In this study, for the first time, graphene has been successfully applied to the field of polymer laser patterning. Based on the extensive used polypropylene (PP) material, the graphene doped polymer is endowed with a very good performance of NIR pulsed laser patterning. Compared to the conventional absorbents, as a pure carbon material, graphene has many advantages of environmentally friendly, efficient, non-toxic, and so on. We believe that graphene has broad application prospects in the field of polymer laser patterning, and its industrial applications for polymer products will be easier. 2. Experimental 2.1. Materials Graphene was purchased from Sichuan Jinlu Group (China). This graphene was prepared by the mechanical exfoliation of planetary ball milling method. Polypropylene (PP, T30S, melt flow rate: 2.6 g/10min) was a commercial product from the Lanzhou Petrochemical



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Company (China). High-density polyethylene (HDPE, TR144, melt flow rate: 0.18 g/10min) was purchased from Maoming Company of Sinopec (China). Polycarbonate (PC, Infino SC-1220R, melt flow rate: 22.0 g/10min) was produced by Samsung Chemical Company (Korea). Acrylonitrile-butadiene-styrene copolymers (ABS, PA-747S, melt flow rate: 0.5 g/10 min) was provided by Chi Mei Corporation (Taiwan). Polystyrene (PS, GPPS-500, melt flow rate: 5.0 g/10min) was purchased from PetroChina Company Limited. Analytical grade titanium dioxide (TiO2) was obtained from the ChengDu Kelong Chemical Reagent Company (China). All the materials were used as received without any further treatment. 2.2. Characterization of graphene 2.2.1. Raman spectroscopy The graphene was characterized by Horiba Jobin Yvon laser Raman analyzer LabRAM HR 800 (Horiba Jobin Yvon, France) equipped with a frequency-doubled Nd:YAG 532.1 nm laser. The laser of the Raman spectrometric analyzer, integrated with a confocal microscope, was operated at 5 mW in order to minimize photo-degradation of the graphene. 2.2.2. Atomic force microscope (AFM) AFM tests were carried out using a Bruker MultiMode 8 AFM (Bruker, USA) with a Bruker Nanoscope V controller in the peak force tapping mode. AFM images were recorded and processed using Bruker Nanoscope 8.15 and Nanoscope Analysis 1.20 software respectively. Before the tests, a small amount of graphene was diluted in ethanol (about 0.1 mg in 10 mL). After 1 h of sonication (JY 98-IIIN, Ningbo Scientz Biotechnology Co., Ltd., China), a drop of the dispersion liquid (almost be transparent) was dropped on a mica sheet. After that, the mica sheet with the liquid was dried for 1 h in a vacuum oven at 60 °C


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(vacuum degree = -0.09 MPa). 2.2.3. Ultraviolet-Visible-Infrared (UV-vis-IR) The NIR absorbance of graphene powder was investigated using UV-vis-IR spectroscopy at room temperature. The UV-vis-IR spectrophotometer (UV-3600, Shimadzu, Japan), integrated with integrating sphere was employed to carry out the experiments in the region of 350-1800 nm. Before the test, the graphene powder was placed in the sample chamber and compacted. 2.3. Preparation of the graphene doped Polypropylene Polypropylene (PP) was, firstly, mechanical mixed with the graphene. The amount of the graphene was 0 wt% (neat PP), 0.0025 wt%, 0.005 wt%, 0.01 wt%, 0.02 wt%, and 0.1 wt% by weight of PP. Then, the graphene doped PP were prepared using a laboratory twin-screw extruder (Φ20) around 190 °C. After that, PP plates with the thickness of 3 mm were prepared using a laboratory injection molding machine, and the injection processing temperature was also 190 °C. 2.4. Parameters determination of laser patterning Laser-patterning experiments were performed on an optical fiber pulsed (λ=1064 nm) MK-GQ10B laser machining system (Mike Laser Technology Co., Ltd., China). The maximum laser power of this system was up to 10 W (it can be adjusted from 0 W to 10 W). The pulsed laser frequency of the system was ranged from 20 kHz to 100 kHz, and the scanning speed was up to 4000 mm/s. The laser machining system was controlled by the software (EzCAD 2.0), which was convenient to generate electronic geometric patterns and accurately control of the working parameters. The entire process of laser patterning for the



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graphene doped PP material is illustrated in Scheme 1. The working parameters of NIR pulsed laser patterning are composed of the laser power, the pulse frequency, and the scanning speed. In our experiments, the scanning speed of the laser beam was fixed at 1000 mm/s. The electronic vector image in the form of a coordinate system was designed via EzCAD 2.0 software. As shown in Scheme 1, each square represents a different laser energy. The horizontal axis corresponds to the pulse frequency of the laser (20, 30, ..., 90, 100 kHz), and the vertical axis is the laser power ranged from 1 W to 10 W (10, 20, ..., 90, 100%; for examples, 30%=3W, 70%=7W). Subsequently, ninety laser energy windows (4×4 cm2) are successfully obtained. After NIR pulsed laser irradiation, the laser-energy windows which formed a legible black pattern were the desired laser-patterning windows, and the laser patterning performance of the graphene doped PP at the different parameters were evaluated.

Scheme 1. A schematic illustration of the laser patterning for the graphene doped polypropylene (PP). 2.5. Characterizations of polymer patterning plates


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2.5.1. Scanning electron microscopy (SEM) The surfaces of polymer patterning plates before and after laser irradiation were analyzed using a field-emission scanning electron microscopy (JSM-7500F, JEOL, Japan) with an acceleration voltage at 5 kV. Before SEM observation, the surfaces of the samples were coated with gold to prevent the accumulation of static electric fields. 2.5.2. Optical microscope The surfaces and cross sections of polymer patterning plates after laser irradiation were observed by an optical microscope (PH-100, Phoenix Optics, China) with the transmission mode. Before the experiments, the cross sections of the samples were repeatedly polished to be smooth using an abrasive paper (grain size = 2000 mesh). 2.5.3. X-ray photoelectron spectroscopy (XPS) The surface chemical elements of polymer patterning plates before and after laser irradiation were analyzed by X-ray photoelectron spectroscopy (XSAM-800, KRATOS, UK) operating at the monochromatic Al Kα achromatic X-ray source (hv=1486.6 eV). 2.5.4. Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy The ATR FTIR spectra of the surface of polymer patterning plates in the region of 4000−650 cm-1 were collected using a Nicolet iS50 spectrometer equipped with a smart iTR accessory and the deuterated triglycine sulfate (DTGS) detector. The resolution of the spectra was 4 cm-1, and the scans of each FTIR spectrum were 20. 2.5.5. Micro-Raman and Raman depth imaging The Raman depth imaging (100×100 µm) and the corresponding micro-Raman spectra at different depth from the polymer surface after laser patterning were recorded on a DXRxi



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micro-Raman imaging spectrometer (Thermo Fisher Scientific, USA). The scan spacing of Raman depth imaging was 10 µm. The 532 nm laser was used as the excitation source, with the laser power of 5 mW and the slit width of 50 µm. 2.5.6. Degree of whiteness The degree of whiteness of polymer plates before the laser patterning was tested by an automatic whiteness meter (WSD-3, Beijing Kangguang Optical Instruments Co., Ltd., China). The light source was a halogen lamp, and the diameter of the test hole was 15 mm. Before the test, to ensure the surface of polymer plates was clean. And the obtained degree of whiteness was blu-ray whiteness according to ISO 2470. 3. Results and discussion 3.1. Characterization of the graphene

Figure 1. (a) AFM image of the graphene and the corresponding line profile extracted from the position at the white line. (b) Raman spectrum of the graphene. The top image of Figure 1(a) is the typical AFM of the used graphene, and the bottom is the corresponding line profile extracted from the position at the white line. From the line


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profile, it can be observed that the thickness of the graphene is 2 nm. As is known, the thickness of the single-layered graphene is 0.34 nm.33 That is to say, the number of the graphene layers used here is around 5. Figure 1(b) is the typical Raman spectrum of the graphene. In general, the D peak at around 1350 cm-1 is assigned to the defects or symmetry-broken sites in graphene, and the G peak at around 1569 cm-1 is attributed to the ordered sp2-hybridized C–C bonds. The 2D peak located at 2717 cm-1 is considered as the fingerprint of graphene layers.34 For the graphene used here, the D peak is very weak, which means that the defect of graphene is few. At the same time, the intensity ratio of the G peak and 2D peak (IG/I2D) is 2.9, and the full-width at half-height maximum (FWHM) of the 2D peak is about 78 cm-1. As is known, the graphene is known as monolayer when IG/I2D is lower than 0.7 and the FWHM of the 2D peak is lower than 45 cm-1.35 Similarly, when the IG/I2D is around 0.7-1.3 and the FWHM of 2D peak is 45-60 cm-1, the graphene has bilayer. And if IG/I2D and the FWHM of 2D peak are greater than 1.3 and 60 cm-1, the graphene is considered to be few-layer. Therefore, the graphene used in our study owns few layers, which is consistent with the result of AFM. Figure S2 in the Supporting Information shows the typical UV-vis-IR spectrum of the used graphene in the region of 350-1800 nm, and an obvious NIR absorbance is observed around 1064 nm. In fact, graphene has a significant absorption within the whole band from 350 nm to 1800 nm.



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Figure 2. Digital photographs of the graphene doped PP plates after NIR pulsed laser patterning. The amount of graphene added: (a) 0 wt% (neat PP); (b) 0.0025 wt%; (c) 0.005 wt%; (d) 0.01 wt%; (e) 0.02 wt%; (f) 0.1 wt%. 3.2. Performance of laser patterning at different parameters (laser energy) For a successful laser patterning of polymers, the basic principle is that the area of polymer materials surface has the local discoloration after laser irradiation, appearing black or gray patterns against the light-colored polymer matrix. And the surface area of no laser irradiation appears no color change. Figures 2(a)-2(f) are the digital photographs of the graphene doped PP plates after laser patterning, in which the amount of graphene added are 0 wt% (neat PP), 0.0025 wt%, 0.005 wt%, 0.01 wt%, 0.02 wt%, and 0.1 wt%, respectively. In our experiments, the scanning rate of NIR pulsed laser patterning was fixed at 1000 mm/s.


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Figure 3. The degree of whiteness for pure PP plate and graphene doped PP plates before laser patterning. The squares are the typical background color of PP plates (captured by digital camera) After laser irradiation, no patterns are generated on the neat PP plate (Figures 2(a)) due to no absorption of 1064 nm NIR of neat PP material. Surprisingly, as illustrated in Figures 2(b)-2(f), all the graphene doped PP plates show a perfect performance of NIR pulsed laser patterning. To be specific, all of the laser-energy windows (4×4 cm2) form the legible black patterns in Figures 2(b)-2(f). It is noted that PP doped with only 0.0025 wt% graphene (25 ppm) still obtains a good laser patterning performance, and this is beyond our expectations. With the increase of the added graphene from 0.0025 wt% to 0.1 wt%, the black color of patterns in the same laser-energy windows (the same parameters) becomes deeper and deeper. However, it also can be observed that the background color of the graphene doped PP plates was gradually darkened, thereby reducing the contrast and sharpness of the patterns. To show the background color change in a quantitative way, tests of degree of whiteness for polymer plates (before laser patterning) were carried out. As shown in Figure 3, the degree of



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whiteness for neat PP is 28.2. However, after adding graphene, the degree of whiteness is decreased from 20.1 to 3.3 with the increase of graphene from 0.0025 wt% to 0.1 wt%. Digital photographs of the graphene doped PP plates before laser patterning are also provided in Figure S1 in the Supporting Information. Considering from the background color of the graphene doped PP plates, Figures 2(b)-2(d) are obviously better than Figures 2(e) and 2(f). Moreover, as shown in Figures 2(b)-2(d), the contrast and sharpness of black patterns in Figures 2(c) and 2(d) is better than that of Figures 2(b). Therefore, in this work, the most suitable amount of graphene is 0.005-0.010wt% (five hundred thousandths to one ten thousandths, 50-100 ppm), which not only maintains a light background of PP material (the corresponding degree of whiteness is 17.5-12.9), but also produces black patterns with the high contrast and sharpness. In addition, as shown in Figures 2(c), the patterns have a dependence on the laser power and the pulse frequency, because the color and the resolution are obviously enhanced and deepen with the increase of the laser power from 1 W (10%) to 10 W (100%); moreover, the resolution is slightly improved with the enhancement of the pulse frequency. Overall, for the PP plates doped with 0.005-0.01 wt% graphene, the performance can meet the requirements of various industry applications in the field of polymer laser patterning when the laser power is within 3-10 W and the pulse frequency is 20-100 kHz.


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Figure 4. Optical microscope photographs of both the polymer surface (left) and the cross section (right) at the areas of the patterns with laser-energy windows of (80 kHz, 80%) in Figures 2(c), 2(d), and 2(e). The amount of the graphene added: (a) 0.005 wt%; (b) 0.01 wt%; (c) 0.02 wt%. Optical microscope with transmission mode was employed to observe the microscopic features of the patterns formed. The laser-energy windows at (80 kHz, 80%) of Figures 2(c), 2(d), and 2(e) were chosen, and the corresponding amounts of the graphene added were 0.005 wt%, 0.01 wt%, and 0.02 wt%, respectively. Figures 4(a)-4(c) are the optical microscope photographs of both the polymer surface (left) and the cross section (right) at the areas of the patterns. From the left photographs of the polymer surface, it can be clearly observed that the patterns are actually composed of many regularly arranged black points, and each point corresponds to an exact location of the laser spot irradiation. The black color of these points comes from the discoloration of the polymer surface induced by NIR pulsed laser. In addition,



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the average diameter of these points is 24.6 µm, which is close to the laser spot size of the used laser patterning system. The right photographs are the cross section cut from the A-A position. As shown in Figure 4, the depth of the discoloration layer can be conveniently determined, and the corresponding values are 221 µm, 292 µm, and 348 µm for the graphene addition of 0.005 wt%, 0.01 wt%, and 0.02 wt%, respectively (Table 1). It also indicates that the depth of the discoloration layer is proportional to the amount of graphene doped in PP material. Table 1. The depth of discoloration layer determined from Figure 4 The amount of graphene

Depth of discoloration layer (µm)

0.005 wt%

0.01 wt%

0.02 wt%

221

292

348

In the present study, SEM experiments were also performed for the patterns of the polymer surface in the same laser-energy windows as Figure 4. These SEM images are provided in Figure S4 in the Supporting Information. The surface morphologies of the neat PP plate with no laser patterning is shown in Figure S4(a), and Figures S4(b)-(d) are the surface morphology of PP plates doped with 0.005 wt%, 0.01 wt%, and 0.02 wt% graphene after laser patterning. However, compared with the neat PP plate of no laser patterning, the surface morphology of the graphene doped PP plates after laser patterning have no obvious change. This indicates that laser did not produce any damage to the PP surface. This phenomenon is quite different from previous studies.24, 28, 32 For an example, for metals, such as the stainless steel, Qi et al28 found the generated patterns were composed of the evaporated part and the melted part. From SEM photographs of the metal surface, the gullys, valleys, and other morphology changes caused by laser radiation were clearly observed. Another example, Shin


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et al24 also observed the pit and rim on the surface of ABS resin after laser patterning. In their study, the size and depth of the pits are nearly proportional to the laser energy. 3.3. The discoloration mechanism caused by laser To find out the specific reasons for PP discoloration caused by the graphene after laser patterning, the XPS analysis of neat PP, PP doped with 0.005 wt%, 0.01 wt%, and 0.02 wt% graphene after laser irradiation was carried out. The same as the previous section, the laser-energy windows at (80 kHz, 80%) were chosen for XPS experiments. The XPS spectra of these four samples are illustrated in Figure 5. The results shows that the surface carbon contents of neat PP, PP doped with 0.005 wt%, 0.01 wt%, and 0.02 wt% graphene are 83.1%, 85.2%, 86.0%, and 90.0%, respectively. This clearly indicates the carbonization occurs on the polymer surface after laser patterning. Moreover, with the increase of the amount of graphene, the carbonization degree is gradually enhanced.



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Figure 5. XPS analysis of the laser-energy windows at (80 kHz, 80%) after laser patterning. (a) Neat PP; (b) PP doped with 0.005 wt% graphene; (c) PP doped with 0.01 wt% graphene. (d) PP doped with 0.02 wt% graphene. In the Supporting Information, Figure S5 illustrates the corresponding high-resolution C (1s) XPS spectra of the same laser-energy windows (80 kHz, 80%) after laser patterning. Compared with neat PP, the asymmetry of C (1s) peaks for PP doped with 0.005 wt%, 0.01 wt%, and 0.02 wt% graphene is observed. So, the peak separation process was carried out for each C (1s) peak using the curve fitting method. In Figures S5(b)-(d), not only the main peaks of C (1s) at 284.8 eV are determined, but also the small shoulder peaks at 286.4 eV are fitted. According to the literature, the peak at 286.4 eV is assigned to the carbon element of C–O groups,32,

36

and therefore, this shows that new C–O groups are generated on the

polymer surface after laser patterning. We also calculated the relative molar content of the carbon element of C–O groups (Table S1 in the Supporting Information), which enhances from 4.27% to 5.51% when the content of graphene is increased from 0.005 wt% to 0.02 wt%. We think the new generated C–O groups on the PP surface probably come from the slight oxidization owing to the laser thermal effect and the presence of air. The ATR FTIR experiments of the same laser-energy windows (80 kHz, 80%) for neat PP, PP doped with 0.005 wt%, 0.01 wt%, and 0.02 wt% graphene after laser patterning were also performed, which are particularly suitable for the study of the polymer surface. Figure S6 provides these ATR FTIR spectra. It can be observed that new small bands at 1739 cm-1 appears when the content of graphene exceeds 0.01 wt%. These new bands are attributed to the stretching vibration of C=O groups,37 which is the characteristic peak of polymer oxidation products. The evidence discussed above confirms that the reason of PP discoloration is the PP instantaneous carbonization due to the graphene absorbing laser energy to produce a high


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temperature. The temperature rise of PP matrix is certainly the first criterion for the carbonization. As discussed in our previous study,30 the carbonization temperature of polymers is usually higher than 500 °C. However, to test this actual instantaneous temperature of the matrix is unavailable due to no effective test methods. This is explained by the facts that, during the laser patterning, the role of NIR pulsed laser on polymers is only confined within the range of the laser beam focus. For our laser patterning system, the focus spot of laser beam is only 25 µm (diameter), a very small region. In addition, the moving speed of laser beam is extremely fast during laser patterning. Thus, the real-time monitoring of the actual instantaneous temperature caused by laser is very difficult. Here, we offer a simple way to theoretically estimate the instantaneous high temperature of PP (see the Supporting Information). The instantaneous temperature of PP matrix caused by NIR pulsed laser irradiation is roughly calculated within 745-1475 °C, which is high enough to cause a rapid carbonization of PP. With the increase of graphene content, more laser energy is absorbed by graphene to convert into more heat, producing a higher instantaneous temperature. Higher temperature means a higher carbonization degree for polymers. Thus, the carbonization degree of PP surface is gradually enhanced with the increase of graphene content, which has been approved by XPS analysis in Figure 5. Similarly, a higher instantaneous temperature also raises the thermal effect on PP surface, resulting in the increase of PP oxidation products (C–O and C=O groups) when the graphene content increasing. In Figure S3 in the Supporting Information, we provide UV-vis-IR spectra of PP plates doped with 0.005 wt%, 0.01 wt%, and 0.02 wt% graphene before laser patterning. The thickness of these polymer plates is 3 mm, and UV-vis-IR spectra are collected in the



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region of 350-1800 nm. As expected, with the increase of graphene content from 0.005 wt% to 0.02 wt%, the enhancement of NIR absorbance of polymer plates around 1064 nm is observed (the decrease of transmittance in Figure S3). This also indirectly proves more graphene can absorb more NIR laser energy. Scheme 2 is a simple schematic illustration to show the role of graphene for PP discoloration during the laser patterning. In the irradiation process of 1064nm NIR pulsed laser, graphene strongly absorbs the laser energy and converted into heat to produce a local high temperature. The PP instantaneous carbonization occurs around the graphene, forming the distinct black patterns on the PP material surface at the macro level.

Scheme 2. Schematic illustration of the role of graphene for PP discoloration during the 1064 nm NIR pulsed laser patterning. To further study the chemical structure of the carbon generated in the PP discoloration layer after laser patterning, the black pattern of PP doped with 0.01 wt% graphene in the laser-energy window at (80 kHz, 80%) was selected to carry out the micro-Raman nondestructive depth analysis (also called as Raman depth imaging). The result of the


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micro-Raman depth analysis is illustrated in Figure 6(a). The analysis depth was 100 µm from the polymer surface, and the analysis width was also 100 µm. The scan spacing of Raman depth imaging was 10 µm. The Raman image actually reflects the differences in the chemical composition of the discoloration layer with the depth from 0 µm to 100 µm. As is known, Raman depth imaging is a scanning process. Before starting the experiments, the Raman spectrum in the upper-right corner of the image is selected as the reference spectrum. After the scanning finished, software (OMNICxi) automatically calculates the degree of correlation between the scanned spectra and the reference spectrum using a mathematical algorithm. Spectra that are exactly like the reference spectrum have a correlation value of 1.00 and are displayed in the color at the top of the scale. Meanwhile, spectra that are completely different from the reference spectrum have a correlation value of 0 and are displayed in the color at the bottom of the scale. In Figure 6(a), the left color bar shows the relationship between the color and the correlation value. For examples, the correlation value of 1.00 corresponds to red color (the top), and the correlation value of 0 corresponds to blue color (the bottom). Therefore, in this 100 µm×100 µm area, if the collected Raman spectrum at the given position is exactly like the reference spectrum, the position is rendered in red. On the contrary, if the collected spectrum at the given position is totally different from the reference spectrum, the color of these positions is rendered in blue. And if the correlation value of the collected spectrum is between the two situations described above, the position is rendered in green. As shown in Figure 6(a), Raman depth imaging successfully detects the distribution of the different chemical composition in the discoloration layer, mainly represented by red and



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green regions. Figure 6(c) is the typical Raman spectrum of the red regions. Compared with the typical Raman spectrum of PP plate doped with 0.01 wt% graphene before laser patterning (Figure 6(b)), Figure 6(c) appears a new broad diffusion band in the region of 1000-2000 cm-1 (centered around 1580 cm-1), which is assigned to the amorphous carbon.38 That is to say, the red regions in the discoloration layer contain a number of the newly formed amorphous carbon after NIR pulsed laser irradiation. Thus, it direct proves the carbonization of PP materials caused by laser, which is similar with previous reports.30 In addition, the red regions close to polymer surface demonstrate a horizontal layered structure, and the average depth of this red layer from the surface is 33.1 µm. Figure 6(d) is the typical Raman spectrum of the green regions. It is noted that not only a small broad band presents in the region of 1000-2000 cm-1, but also a big broad band around 2058 cm-1 appears. As is known, for Raman spectroscopy, the broad bands of 2058 cm-1 is attributed to C≡C or conjugated C=C/C≡C (e.g., the diyne links, –C≡C–C≡C–).39, 40 Therefore, in the green regions of the discoloration layer, it not only generates a small amount of the amorphous carbon, but also forms a lot of the complex sp/sp2-carbon compounds containing C≡C or conjugated C=C/C≡C structures.


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Figure 6. Results of the micro-Raman depth imaging and the corresponding micro-Raman spectra at different depth (the amount of graphene doped in PP material is 0.01 wt%). (a) the image of micro-Raman depth analysis after laser patterning; (b) the typical Raman spectrum of PP doped with 0.01 wt% graphene before laser patterning; (c) the typical Raman spectrum of the red regions; (d) the typical Raman spectrum of the green regions. Why did it generate a large number of complex carbon compounds in green regions after laser patterning? This arouses our interest. In Figure 6(a), the depth range of green regions is around 40.0-90.0 µm, and the green regions are generally divided into the columnar structures in the vertical direction (Z) by red or orange regions. The average depth of the center point of the columnar green regions is approximately 78.5 µm. After careful investigation, we found this depth is just the focus position of the laser beam of our laser patterning system. That is to say, the columnar green regions are the position that is directly exposure to the laser focus spot during laser patterning. Thus, the green regions are the



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location formed by the direct irradiation of the focused laser spot. Because the center of focused laser spot has the highest energy, in the present of the graphene, the huge light and heat make the PP material instantly carbonized and generate a lot of complex carbon compounds. And the area around the laser focus spot is subjected to the heat passed from the green regions (the indirect effect), only resulting in the generation of the amorphous carbon (represented by the red regions). This also explains why the green regions are surrounded by the red regions. Meanwhile, the distance between two axes of columnar green regions is 55.8 µm, which is very close to the distance between two black points (53.2 µm) in the left images of Figure 4. So, in the projection of Z direction, the columnar green regions exactly correspond to two black points in Figure 4. As discussed in the previous section, the patterns are composed of regularly arranged black points. Combined with the results of Raman depth imaging, it can be easily concluded that the black color of those points actually springs from the complex sp/sp2-carbon compounds containing C≡C or conjugated C=C/C≡C structures. 3.4. Application of laser patterning In this section, to demonstrate the practical application effect of the graphene for polymer laser patterning, the electronic vector image of a panda (Figure 7(a)) was used to pattern onto the central location of PP plates. Here, the parameters of laser patterning were set at (80 kHz, 80%) with a scanning speed of 1000 mm/s, and the PP plate doped with 0.005 wt % graphene was chosen. In Figure 7, the photos were captured using a digital camera (Cannon EOS 7D). As expected, after laser irradiation, the PP plate only doped with 0.005 wt% graphene shows a clear black pattern with the high resolution (Figure 7(c)), showing a very good application


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performance (also see the movie S1 in the Supporting Information). However, in Figure 7(b), for the neat PP material, no patterns are produced due to no absorption of NIR pulsed laser. The real-time comparison of NIR pulsed laser patterning process for neat PP and PP doped with 0.005 wt% graphene is provided in the movie S2 in the Supporting Information.

Figure 7. (a) The original electronic vector image of the panda; (b) laser patterning on the neat PP material; (c) laser patterning on PP doped with 0.005 wt% graphene; (d) laser patterning on PP/TiO2 composite doped with 0.005 wt% graphene. The parameters of laser patterning were set at (80 kHz, 80%) with a scanning speed of 1000 mm/s. In general, people often used titanium dioxide (TiO2) to whiten plastics for light-colored plastic products in the industry. So, the laser patterning performance of the graphene for these light-colored polymers is also important and should be evaluated. Therefore, we prepared PP/TiO2 composite doped with 0.005 wt% graphene, and the amount of TiO2 in the composite is 0.2 wt% by weight of PP. After adding TiO2, as shown in Figure 7(d), the background of



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the PP plate changes to pure white, and the composite still demonstrates a good laser patterning performance; moreover, the contrast of patterns is further improved against the white background. The real-time laser patterning process of PP/TiO2 composite doped with 0.005 wt% graphene is also shown in the movie S3 in the Supporting Information. In this study, the graphene has also been successfully applied to other polymers, including PE, PS, ABS, and PC. The digital photographs of the patterns of these polymers are provided in Figure S7 in the Supporting Information. As expected, patterns with high resolution and high contrast are also obtained. 4. Conclusions In this work, we report the graphene prepared by the mechanical exfoliation has been successfully applied to the field of polymer laser patterning. The most important finding of this study is that as an efficient 1064 nm NIR pulsed laser absorber, only 0.005 wt% graphene (five hundred thousandths, 50 ppm) endows polymer materials with a very good performance of NIR pulsed laser patterning. Not only, it produces a clear pattern with the high resolution and contrast, but also the light-colored background of the polymer matrix is retained. It is found that the generated black pattern comes from the local discoloration of the polymer surface subject to the laser irradiation. We also confirm that the discoloration mechanism of laser patterning is the local carbonization of the polymer caused by the instantaneous high temperature due to the graphene absorbing laser energy to produce a huge heat. More graphene leads to a higher carbonization degree for polymers. Furthermore, after laser patterning, two main carbonization products are detected, including the amorphous carbon and the complex sp/sp2-carbon compounds containing C ≡ C or conjugated C=C/C ≡ C


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structures. This study also provides a simple guideline to fabricate laser-patterning polymer materials based on graphene. Compared to the conventional absorbers, as a pure carbon material, the graphene is more efficient, environmentally friendly, and non-toxic. We believe that graphene has broad application prospects in the field of polymer laser patterning. Most importantly, this work opens up a valuable feasible direction for the practical application of this new carbon material. Supporting Information. Digital photographs of graphene doped PP plates before laser patterning, UV-vis-IR of graphene powder, UV-vis-IR of graphene doped PP plates, SEM of formed patterns of the polymer surface, high-resolution C (1s) XPS spectra of patterns after laser patterning and the corresponding statistical Table, ATR-FTIR spectra of formed patterns, digital photographs of the patterns of HDPE, PC, ABS, and PS after laser patterning, theoretical estimation of the instantaneous temperature, and supplementary movies S1, S2, and S3 captions. Real-time movie S1 of laser patterning Real-time movie S2 of laser patterning Real-time movie S3 of laser patterning Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51473104), and State Key Laboratory of Polymer Materials Engineering (Grant Nos. sklpme2014-3-06, sklpme2016-3-10). References



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For TOC only


Local Controllable Laser Patterning of Polymers Induced by Graphene

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