Adhesive-free adhesion between heat-assisted plasma-treated fluoropolymers (PTFE, PFA) and plasma-jet-treated polydimethylsiloxane (PDMS) and its application.

Conventional low-temperature plasma treatment was reported to minimally improve the adhesion property of polytetrafluoroethylene (PTFE), whereas heat-assisted plasma (HAP) treatment significantly improved the same. An unvulcanized rubber was previously used as an adherent for PTFE. This study aimed to achieve strong adhesive-free adhesion between PTFE and vulcanized polydimethylsiloxane (PDMS) rubber. As-received vulcanized PDMS rubber did not adhere to HAP-treated PTFE, and as-received PTFE did not adhere to vulcanized rubber of plasma-jet (PJ) treated PDMS rubber; however, HAP-treated PTFE strongly adhered to vulcanized PJ-treated PDMS rubber, and both PTFE and PDMS exhibited cohesion failure in the T-peel test. The surface chemical compositions of the PTFE and PDMS sides were determined using X-ray photoelectron spectroscopy. The strong PTFE/PDMS adhesion was explained via hydrogen and covalent bond formation (C-O-Si and/or C(=O)-O-Si) between hydroxyl (C-OH) or carboxyl (C(=O)-OH) groups of the HAP-treated PTFE. This process was also applied to adhesive-free adhesion between a tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) and PDMS; subsequently, a translucent PFA/PDMS assembly with strong adhesion was realized together with the PTFE/PDMS assembly. Strong adhesive-free adhesion between fluoropolymers (PTFE, PFA) and vulcanized PDMS rubber without using any adhesives and graft polymer was successfully realized upon plasma treatment of both the fluoropolymer and PDMS sides. Additionally, a PDMS sheet, which was PJ-treated on both sides, was applied to strongly adhere fluoropolymers (PTFE, PFA) to materials such as metal and glass. PJ-treated PDMS was used as an intermediate layer rather than a strong adhesive, achieving PTFE/PDMS/metal and PTFE/PDMS/glass assemblies. The PTFE/PDMS, PDMS/metal, and PDMS/glass adhesion strengths exceeded 2 N/mm.


Results
Surface chemical composition analysis of plasma-treated PDMS and plasma-treated PTFE using X-ray photoelectron spectroscopy (XPS). Figure 1a-1c shows the XPS spectra of the PDMS samples before and after plasma-jet (PJ) treatment. The intensity of the peak indexed to CH 3 decreased after PJ treatment, resulting in C-Si bond scission and desorption of CH 3 on the PJ-treated PDMS surface (Fig. 1a). No peaks indexed to C-O (286.5 eV) 34,35 , C=O (288.0 eV) 34,35 , or O-C(=O) (289.2 eV) 34,35 were observed for the PJ-treated PDMS surface (Fig. 1a). The intensity of the O1s peak increased and shifted to higher binding energy after PJ treatment, indicating an oxidation reaction on the PJ-treated PDMS surface (Fig. 1b). The intensity of the Si2p peak also increased and shifted to higher binding energy after PJ treatment, also indicating an oxidation reaction on the PJ-treated PDMS surface (Fig. 1c). Furthermore, peak resolution of O1s-XPS and Si2p-XPS spectra of as-received and PJ-treated PDMS samples was conducted. Figure 1d,1e shows peak resolution of the O1s-XPS spectra of the PDMS samples before and after PJ treatment. Peaks 1 and 2 indicate Si-O-Si and Si-OH, respectively 36,37 . The as-received PDMS sample had the ratio Si-OH: Si-O-Si = 9:91. This also indicated that the surface of the as-received PDMS sample used in this study originally contained ca. 10% Si-OH. The PJ-treated PDMS sample had the ratio Si-OH: Si-O-Si = 13:87. These results indicated that Si-OH increased after PJ treatment. These results also suggested that Si-OH increased after PJ treatment. Previous articles 15,36,37 reported Si-C scission and CH 3 desorption upon plasma treatment, followed by formation of Si-O and/or Si-OH upon reaction between Si radicals and oxygen atoms or OH − ions in plasma. These reports are consistent with our study. Figure 2a,2b shows the XPS spectra of the PTFE samples before and after HAP treatment. The as-received PTFE sample exhibited only a peak indexed to CF 2 at ca. 292 eV, as shown in Fig. 2a. However, the HAP-treated PTFE sample had peaks indexed to not only fluorine-containing functional groups (CF 3 , CF 2 , C-F) 13,34,35,38 at ca. 292 eV but also oxygen-containing functional groups (O-C=O, C=O, C-O) 34,35 at ca. 289-286 eV and carbon groups (C-C, C=C) 39 at ca. 286-284 eV, as shown in Fig. 2a. Additionally, the as-received PTFE sample exhibited no peaks, whereas the HAP-treated PTFE sample exhibited a broad peak at ca. 531-537 eV, as shown in Fig. 2b. These results indicated that the PTFE surface was oxidized upon HAP treatment. Furthermore, peak resolution was conducted for C1s-XPS spectra of as-received and HAP-treated PTFE samples. Figure 2c,2d shows peak resolution of the C1s-XPS spectra of the PTFE samples before and after HAP treatment. Peaks 1-8 indicate CF 3 , CF 2 , C-F, O-C=O, C=O, C-O, C-C, and C=C, respectively. samples before and after HAP treatment, calculated from the C1s-XPS spectra shown in Fig. 2c,2d. These results indicated that both C-C crosslinking and the generation of oxygen-containing functional groups occurred on the HAP-treated PTFE surface.
Adhesion strength between plasma-treated fluoropolymers and PDMS. Table 2 shows the sample preparation conditions and adhesion strengths of the PTFE/PDMS assembly. When the as-received PDMS or PTFE samples were used, the adhesion strength of the PTFE/PDMS assembly was 0 N/mm. Thus, the vulcanized PDMS sample required PJ treatment to strongly adhere to plasma-treated PTFE, although unvulcanized natural rubber (NR) containing hydrophilic SiO 2 powder did not require plasma treatment. When both sides of PDMS and PTFE were plasma-treated, the adhesion strength of the PTFE/PDMS assembly was 2.6 N/mm, which indicated drastic increase of adhesion strength. The load-displacement curve of the PTFE/PDMS assembly was shown in Supplementary Information Fig. S1(a). Figure 3 shows the photographs of the PTFE/PDMS assembly during  the T-peel test, where both sides of the PDMS and PTFE sheets were plasma-treated. Both the PDMS and PTFE sheets were extended during the T-peel test (Fig. 3a). Figure 3b shows that PDMS which was strongly adhered to the PTFE surface was transferred to the PTFE side after the peel test. Figure 4 shows the XPS spectra and schematic of the peeled surfaces of the PTFE/PDMS assembly sample. Si was detected on both the PTFE and PDMS sides ( Fig. 4a,4b). C-H but not CF 2 was detected on the PDMS side, while the peak indexed to C-H had a higher intensity than that indexed to CF 2 on the PTFE side ( Fig. 4c,4d). Thus, most of the PTFE surface was covered with PDMS after the peel test and a small amount of CF 2 was detected at 292.5 eV but not at 291.8 eV, because the PTFE sheet was extended during the T-peel test because of strong adhesion of the interface between PTFE and PDMS, after which bulk PTFE appeared on the surface, as illustrated in Fig. 4g. These results indicated cohesion failure of both PDMS and PTFE occurred when HAP-treated PTFE adhered strongly to PJ-treated PDMS. In summary, the adhesion strength of PTFE/PDMS interface was higher than breaking strengths of both PDMS and PTFE. For comparison, when PTFE was plasma-treated at low temperature (below 100 °C), the PTFE was thermally compressed to PJ-treated PDMS, and the PTFE/PDMS adhesion strength was 0.1 N/mm, which was extremely low. The effect of heating during plasma treatment was also confirmed during this study. HAP treatment was essential for obtaining high adhesion strength in the PTFE/PDMS assembly. Rather than PTFE, tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) sheet was HAP-treated at 19.1 W/cm 2 before preparing a PFA/PDMS assembly by thermal compression of the PJ-treated PDMS and HAP-treated PFA sheets. The adhesion strength of the PFA/PDMS assembly, in which the thicknesses of the PFA and PDMS sheets were 0.1 and 2 mm, respectively, exceeded 2 N/mm, which indicated strong adhesive-free adhesion between the PDMS and PFA sheets. Figure 5 shows the photographs of PTFE/PDMS and PFA/PDMS assemblies on a paper showing the school badge of Osaka University. Although the badge was not observed under the PTFE/PDMS assembly, it was clearly observed under the PFA/PDMS assembly. The optical transparency of the PFA/PDMS assembly would allow observation of liquid flow in a PFA/PDMS assembly hose and/or tube, which would have high flexibility, chemical resistance, and weather resistance.
Strong adhesive-free adhesion between PTFE and other types of materials (metal and glass) via PJ-treated PDMS. As described in the Introduction section, it was previously reported that plasma-treated PDMS could strongly adhere to several materials such as metal and glass [27][28][29][30] . We attempted adhesive-free  Table 2. Sample preparation conditions and the adhesion strengths of PTFE/PDMS assembly. "×" denotes no plasma treatment, and "○" denotes plasma treatment. *PJ treatment. **HAP treatment. ***Cohesion failure of PDMS rubber and/or PTFE sheet during T-peel test.  Supplementary Information Fig. S2(a)-(c). We then prepared three-layer assemblies: PTFE/PDMS/metal and PTFE/PDMS/glass, as shown in Fig. 6. It was shown that strong adhesive-free adhesion between fluoropolymers (PTFE, PFA) and other types of materials such as metal and glass was readily realized by PJ treatment of both sides of a PDMS sheet.

Discussion
In this study, vulcanized PDMS rubber was employed as an adherent for fluoropolymers, rather than unvulcanized rubber, which contains hydrophilic SiO 2 powder containing Si-OH groups. Although unvulcanized rubber adhered strongly to HAP-treated fluoropolymers without PJ treatment of the rubber in a previous study 14 , vulcanized rubber did not adhere to HAP-treated fluoropolymers without PJ treatment in the present study. When a vulcanized PDMS rubber was subjected to PJ treatment, we realized strong adhesive-free adhesion between the HAP-treated fluoropolymer and vulcanized PDMS. It was clear that this was achieved by plasma treatment of both sides of the fluoropolymer and vulcanized rubber. Figure 7 shows a proposed model for strong adhesive-free adhesion as well as a preparation procedure for a two-layer assembly such as PTFE/PDMS and a three-layer assembly such as PTFE/PDMS/Cu. Firstly, a PTFE sheet is HAP-treated above 200 °C; oxygen-containing functional groups (C(=O)-OH, C-OH) are then generated and surface hardening occurs on the PTFE surface via HAP treatment. Secondly, a PDMS sheet is PJ-treated,  that cohesion failure of PDMS occurred when PJ-treated PDMS and PJ-treated Cu were thermally compressed, it is reasonable to assume that not only hydrogen bonds but also covalent bonds (Si-O-Cu) are formed. This technique for achieving strong adhesive-free adhesion between PDMS and fluoropolymer adds several functions, such as high chemical resistance, water and oil repellency, antifouling and sliding properties, to PDMS, and elasticity to fluoropolymers. Additionally, three-layer assemblies such as PTFE/PDMS/metal and PTFE/ PDMS/glass were successfully prepared by PJ treatment of both sides of a PDMS sheet. This demonstrated that PJ-treated PDMS can adhere fluoropolymers to several materials such as metal and glass as desired. In summary, this technique for achieving adhesive-free adhesion between PDMS combined with fluoropolymer will expand the application range of PDMS and fluoropolymers will be useful in various fields, especially the medical and food industries, in which adhesives are unsuitable.

Methods
Materials. An addition-crosslinked PDMS sheet (KE541U + C25A/B, HATADA, hardness 40°) was cut into 35 mm × 50 mm pieces, which was used as a silicone rubber specimen. A PDMS sheet with a thickness of 2 mm was mainly used, but a PDMS sheet with a thickness of 0.5 mm was also used for realization at high optical transparency. Commercially available PTFE sheet (NITOFLON ® No. 900UL, Nitto Denko; thickness: 0.2 mm) was cut into 45 mm × 70 mm pieces, which was used as a fluoropolymer specimen. The color of PTFE sheet is white and PTFE has no optical transparency due to its high crystallinity. Conversely, the PFA sheet originally has optical transparency as well as PDMS sheet. Therefore, the PFA sheet (AF-0100, DAIKIN INDUSTRIES; thickness: 0.1 mm) was used only when high transparency was required. Copper foil (Cu, 99.9%, CU-113263, Nilaco Corporation; thickness: 0.050 mm) was cut into 30 mm × 25 mm pieces, which was used as a metal foil specimen. A pure copper plate (Cu, 99.96%, CU-113381, Nilaco Corporation; thickness: 0.20 mm) was also cut into 30 mm × 25 mm pieces, which was used as a Cu plate specimen. Stainless steel foil (SUS304, TS200-200-005, IWATA MFG; thickness: 0.05 mm) was cut into 30 mm × 25 mm pieces, which was used as a SUS foil specimen. A stainless steel plate (SUS430, HSO531, Hikari; thickness: 0.5 mm) was also cut into 30 mm × 25 mm pieces, which was used as a SUS plate specimen. A glass slide (S7213, Matsunami Glass Ind.) with 76 mm × 26 mm × 1 mm was used without cutting.
Sample preparation by plasma treatment. Prior to PJ treatment, the PDMS sheets were washed sequentially with acetone (99.5%, Kishida Chemical) and pure water for 1 min each in an ultrasonic bath (US-4R, AS-ONE). The washed PDMS sheets were then dried using an air gun containing N 2 gas (99.99%, Iwatani Fine Gas). The washed and dried PDMS sheets were then plasma-treated using open-air-type PJ treatment equipment (Tough Plasma FPE-20, FUJI CORPORATION), but not HAP-treated. The gap between the irradiation hole and the surface of the PDMS sheet was 10 mm; a mixture of N 2 gas (99.99%, Iwatani Fine Gas) and air gas (N 2 / O 2 = 79%/21%, Iwatani Fine Gas) was used as a process gas for plasma generation, and the flow rates of N 2 and air gases were 29.7 and 0.3 L/min, respectively. The PDMS sheet was placed on a movable stage and fixed using double-sided polyimide tape (10-mm width, No. 4390, 3 M Japan), then the stage was moved at 8 mm/s during PJ treatment. The number of scan operations was only one in this study. To confirm whether the PJ treatment conditions were suitable, two PJ-treated PDMS sheets were prepared under the same PJ treatment conditions; the adhesion strength of the PDMS/PDMS assembly was then measured using a T-peel test. Cohesion failure of the PDMS occurred during the T-peel test, as shown in Supplementary Information Fig. S2(d). It was confirmed that these PJ conditions were suitable for PDMS.
It was previously reported that low-temperature plasma treatment barely improved the adhesion properties of PTFE, whereas HAP treatment significant improved them 12,13 . On the basis of these reports, the surface of the PTFE sheet was modified via HAP treatment in the present study. The detailed surface treatment procedure for a PTFE sheet using HAP has been previously reported 13 ; hence, it is described only briefly here. Prior to HAP treatment, PTFE sheets as well as PDMS sheets were washed with acetone and pure water using an ultrasonic bath (US-4R, AS-ONE) then dried using an air gun containing N 2 gas (99.99%, Iwatani Fine Gas). The washed and dried PTFE sheets were then HAP-treated at 19.1 W/cm 2 for 600 s using He gas (99.99%, Iwatani Fine Gas) at atmospheric pressure in a custom-made chamber system (Meisyo Kiko) [12][13][14] . During HAP treatment, the surface temperature of the PTFE samples was measured with a digital radiation thermometer system (FT-H40K and FT-50A, Keyence); it was confirmed that the maximum surface temperature exceeded 200 °C during HAP treatment at 19.1 W/cm 2 for 600 s. To confirm whether the HAP treatment conditions were suitable, an HAP-treated PTFE sheet and unvulcanized NR containing hydrophilic SiO 2 powder were prepared; the adhesion strength of the PTFE/NR assembly was then measured using a T-peel test. Cohesion failure of the NR occurred during the T-peel test, as shown in Supplementary Information Fig. S2(e). It was thus confirmed that these HAP conditions were suitable for PTFE 14 .
A glass slide was used without washing or PJ treatment. Cu foils, Cu plate, SUS foils, and SUS plates as well as PDMS sheets were washed in an ultrasonic bath (US-4R, AS-ONE) then dried using an air gun containing N 2 gas (99.99%, Iwatani Fine Gas). The washed and dried Cu foils, Cu plate, SUS foils, and SUS plates were also PJ-treated to clean their surfaces. The gap between the irradiation hole and the surface of the Cu foils, Cu plate, SUS foils, or SUS plates was 10 mm. The stage was moved at 0.8 mm/s during PJ treatment for Cu foils, Cu plate, SUS foils, or SUS plates. The number of scan operations was five for Cu foils, Cu plate, SUS foils, and SUS plates.
Surface chemical composition analysis. XPS measurements were performed using a scanning XPS spectrometer (Quantum-2000, Ulvac-Phi) with a monochromated Al-Kα source. All the XPS spectra were obtained below 5 × 10 −6 Pa. The photoelectron take-off angle was 45°, and the X-ray irradiation area was Ø100 μm. Narrow scan XPS spectra of Si2p, C1s, O1s, and F1s were collected at 95-115 eV, 275-300 eV, 525-545 eV, and 680-700 eV, respectively, with a pass energy of 23.50 eV and a step size of 0.05 eV. The cumulative Scientific RepORts | (2018) 8:18058 | DOI:10.1038/s41598-018-36469-y number of measurements was three. During an XPS measurement, the samples were irradiated with a low-speed electron beam and an Ar ion beam to achieve charge neutralization. The obtained XPS spectra of the PDMS samples were referenced to the peak indexed to C-Si-O-Si and/or C-H at 284.6 eV 36,37 , and the obtained XPS spectra of as-received and HAP-treated PTFE samples were referenced to peaks indexed to -CF 2 -at 292.5 eV 34,35 and 291.8 eV 13,38 , respectively. The main peak of -CF 2 -in the C1s-XPS spectra shifted toward lower binding energy for plasma-treated PTFE due to surface charging 13,40 . Adhesion strength test for a two-layer assembly. Firstly, the HAP-treated PTFE sample was placed on the PJ-treated PDMS sheets in a mold such that the plasma-treated surfaces faced each other. Secondly, the PTFE/PDMS assembly was compressed without adhesive at 180 °C and 10 MPa for 10 min using a hot-pressing machine (AH-2003, AS-ONE). Thirdly, the PTFE/PDMS assembly was restored to room temperature. Fourthly, the adhesion strength of the PTFE/PDMS assembly was measured using a T-peel test by combining of a digital force gauge (ZP-200N, Imada) and an electric-driven stand (MX-500N, Imada). The T-peel test was conducted at room temperature of 25 ± 5 °C, and the humidity was not controlled. The sweep rate was 60 mm/min. Finally, the average adhesion strength was calculated by dividing the average tensile strength by the width of the PTFE (ca. 10 mm). To verify the reproducibility, three samples were prepared under the same conditions. A PFA/PDMS assembly was also prepared, and its adhesion strength was measured as for the PTFE/PDMS assembly.
Adhesion strength test for a three-layer assembly. The first to third steps for a three-layer assembly were the same as those for a two-layer assembly. Fourthly, a PJ-treated PDMS/PTFE sample was placed on a PJ-treated Cu plate in a mold so that the plasma-treated surfaces faced each other. Fifthly, the PTFE/PDMS/ Cu assembly was compressed without adhesive at 180 °C and 5 MPa for 10 min using a hot-pressing machine (AH-2003, AS-ONE). Sixthly, the PTFE/PDMS/Cu assembly was restored to room temperature. Seventhly, the PTFE and PDMS sheet on the Cu plate was cut to a width of 10 mm. Eighthly, the Cu plate was fixed on the electric-driven stand (MX-500N, Imada) using two stainless steel bars, then both the PTFE and PDMS sheets were simultaneously grasped and pulled up when measuring the adhesion strength of the PDMS/Cu interface with a 90° peel test. The 90° peel test was conducted under the same conditions of temperature and humidity as T-peel test. The sweep rate was also 60 mm/min. When the adhesion strength of the PTFE/PDMS interface was measured using a 90° peel test, the Cu plate was fixed on the electric-driven stand using two stainless steel bars, and then only the PTFE sheet was pulled up. The PTFE/PDMS/SUS430 assembly was prepared as for the PTFE/PDMS/Cu assembly. The PFA/PDMS/glass and PTFE/PDMS/glass assemblies were prepared as for the PTFE/PDMS/Cu assembly except for the fifth step, in which the pressure was decreased from 5 to almost 0 MPa (the empty weight of the PFA/PDMS or PTFE/PDMS assembly) because the glass slide was easily broken. The adhesion strengths of PFA/PDMS/glass, PTFE/PDMS/glass, and PTFE/PDMS/SUS430 were measured as for the PTFE/PDMS/Cu assembly. Peeling tests were conducted at least two times for each three-layer assembly to verify the reproducibility.