Preparation of Stable Wetting Surface by Hyperthermal Hydrogen

Article pubs.acs.org/JPCC

Preparation of Stable Wetting Surface by Hyperthermal Hydrogen Induced Cross-Linking of Poly(acrylic acid) on Poly(chloro‑p‑xylylene) Film Hong Shao,† Zhoukun He,† Keqin Xu,† Xin Hu,† Yuanlin Zhou,§ Changyu Tang,*,† Jun Mei,† Maobing Shuai,*,‡,§ Woon-ming Lau,† and David Hui∥ †

Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu Development Center of Science and Technology, China Academy of Engineering Physics, Chengdu, 610207, China ‡ Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang, 621907, China § State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang, 621000, China ∥ Department of Mechanical Engineering, University of New Orleans, New Orleans, Louisiana 70148, United States S Supporting Information *

ABSTRACT: Enhancing surface wetting is very critical for various applications of polymer films. Although existing modification methods (e.g., UV radiation and plasma treatments) can improve the wetting of polymer films by inserting hydrophilic groups, the resultant polymer surface is unstable and shows strong hydrophobic recovery in a short time (less than 1 day) due to the rearrangement of the polymer chains. Herein, we report a new approach to prepare stable wetting surface by cross-linking hydrophilic poly(acrylic acid) (PAA) molecules on poly(chloro-pxylylene) (PPXC) films via hyperthermal hydrogen induced crosslinking (HHIC) treatment. With the HHIC treatment, the polar functionalities of PAA (e.g., −COOH) can be preserved through selective cleavage of C−H bonds and subsequent cross-linking of resulting carbon radicals generated on PAA and PPXC chains. HHIC-treated PAA−PPXC film shows an excellent wetting stability, of which WCA and surface energy stay almost the same over 40 days. The improved wetting stability of PPXC film is attributed to the controllable HHIC reaction without undesirable side reaction (e.g., the scission of polymer chain backbone), which effectively restricts the rearrangement of PAA chains on the surface. Besides, the improved wetting stability by our approach results in reliable adhesion between silver ink and polymer films. Thus, fixing hydrophilic molecules on hydrophobic polymer surface by HHIC treatment could be an alternative approach to conventional surface treatments for preparation of stable wetting surface on polymer films.

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INTRODUCTION

electrolyte and the difficulty in building functional electrodes on the PPXC film.9,10 Enhancing the surface wetting of polymer films has been proved to be a good way to address the above issues.11−14 Principally, this goal can be achieved by introducing various hydrophilic groups (e.g., −COOH and −OH) onto the polymer film surface to improve its surface energy. Various methods, such as depositing hydrophilic coating,15 wet chemical treatment (e.g., chemical oxidation and photografting),16,17 ultraviolet (UV) irradiation,18 and plasma treatment,19 have been employed to increase the surface wetting of polymer films. But these methods still have some vital disadvantages. For instance, the weak noncovalent

As petroleum-based polymer films were widely used in the fields of food and electronic packing, substrates for printing, water treatment, and biomedical health, their surface properties have received more and more attention.1−4 The common polymer film surface often exhibits low surface energy and high hydrophobicity because of its hydrocarbon structure and the absence of hydrophilic groups. This surface characteristic can lead to poor surface wetting and adhesion failure of the coating on the films, thereby seriously limiting the wide applications of polymer films.5,6 For example, poly(chloro-p-xylylene) (PPXC) film prepared by chemical vapor deposition (CVD) polymerization of chloro-p-xylylene, is often used for the encapsulation of semiconductor and bioimplant devices due to its excellent moisture barrier, anticorrosion, and biological compatibility.7,8 However, its surface hydrophobicity leads to poor wetting with © XXXX American Chemical Society

Received: September 21, 2016 Revised: November 29, 2016 Published: November 30, 2016 A

DOI: 10.1021/acs.jpcc.6b09547 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C interaction between the coating and the polymer film often causes poor adhesion due to the absence of reactive groups from the polymer film surface.20,21 Wet chemical treatment and ultraviolet (UV) light polymerization can effectively generate hydrophilic groups which are covalently bonded onto the polymer film surface, but their harsh reactive conditions inevitably lead to serious shrinkage and damage on the surface.22,23 Plasma treatment can provide a “dry and mild” chemical process to quickly introduce hydrophilic groups (e.g., −COOH and −OH) onto the polymer film; thus, it has been a prevalent method to improve the wetting of polymer films.24−26 Unfortunately, the modified polymer surface is usually unstable and is easily reverted back to its original hydrophobic state from hydrophilic state within 1 day.10,27 This undesirable hydrophobic recovery behavior was widely observed in various modified polymer films,28 which seriously degrades the lifetime and performance of the modified films. Surface hydrophobic recovery of modified polymer could be attributed to the molecular chain reorientation process, in which the polymer chains with hydrophilic groups diffuse from the surface into the bulk of the polymer and the nonmodified polymer chains diffuse to the surface.29 Restricting the chain mobility by cross-linking has been proved to be an important and effective strategy to reduce the hydrophobicity recovery.30 UV radiation and plasma treatment can induce cross-linking reaction on the polymer film surface, but meanwhile, they will lead to molecular chain degradation (cleavage of C−C bonds) due to the uncontrolled or high energy projectiles arising from these approaches.31 The chain degradation generates lowmolecular-weight polymers, which easily move and rearrange.32 Thus, these methods have limited ability to restrict chain mobility and to prevent hydrophobic recovery of the modified polymer surface. Recently, a new technology called the hyperthermal hydrogen induced cross-linking (HHIC) has been developed for achieving a mild surface cross-linking reaction on polymer materials without any physical damage (molecular main chain degradation) to the surface.33−36 It involves the bombardment of surfaces with energy-controlled hydrogen projectiles to selectively cleave C−H bonds from organic molecules without breaking other bonds (e.g., C−C bonds in polymer backbone), since the energy transfer for H2 collisions is more efficient with H than with C.37,38 Consequently, the resulting carbon radicals generated from the organic molecules can combine together and form the cross-linking of C−C bonds. Thus, HHIC treatment is expected to effectively restrict chain mobility of hydrophilic polymer and to prevent surface hydrophobicity recovery. Although protons were employed to selectively cleave the C−H bonds in our earlier work, this ion bombardment method is not practical, because most polymer materials are electrically insulating and have a surface charging problem under charged species bombardment. The surface charging problem can further limit the ion flux density and the surface reaction throughput. Alternatively, neutral hyperthermal hydrogen molecules with a kinetic energy of 10−20 eV were developed to overcome the above issue, which are generated by transferring the kinetic energy of voltage accelerated protons (primary ions) to hydrogen molecules through collisions. Using this technology, we demonstrate successfully that hydrophilic poly(acrylic acid) (PAA) molecules can be cross-linked on surface-inert PPXC film without damage on both PAA and PPXC film and, thus, significantly improve the surface wetting of PPXC film, which shows a good stability due to the

restriction of chain mobility by well-controlled cross-linking between PAA molecules and PPXC. We also demonstrate the surface adhesion and barrier properties of the modified PPXC films.

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EXPERIMENTAL SECTION Materials. Poly(chloro-p-xylylene) (PPXC) film with a thickness of 110 μm was provided by China Academy of Engineering Physics. Poly(acrylic acid) (PAA, Mw = 450000 g/ mol) was purchased from Sigma-Aldrich Company (U.S.A.). Alcohol, acetone, distilled water, and hydrogen gas were purchased from Kelong Chemical Company (Chengdu, China). Preparation of PAA Coating on PPXC Film. PAA solution (10 mg/mL) was prepared by dissolving PAA powder in alcohol. The resultant solution was spin-coated on the surface of PPXC film at a speed of 4000 rpm for 30 s to create a thin PAA film. Hyperthermal Hydrogen Induced Cross-Linking of PAA onto PPXC Films. A home-built system equipped with an electron cyclotron resonance (ECR) microwave plasma source (87.5 mT, 2.45 GHz) was used for cross-linking PAA coated PPXC films. In a typical HHIC process, the samples were placed on a stage in the bottom of the reactor. After the sample was loaded, the whole system was pumped down to a base pressure of about 7.4 × 10−4 Pa. Subsequently, hydrogen gas was introduced into the reactor until a pressure of ∼9.9 × 10−2 Pa was reached, and then the pressure was maintained throughout the whole experiment process. Protons from the hydrogen plasma produced in the top of the reactor were then partly extracted with an applied potential difference of −100 V and a current of ∼160 mA. The protons extracted were accelerated into a 60 cm long electric field-free drift zone, where they underwent serial but random collisions with hydrogen molecules fed into the system. Through energy transfer from protons to molecular hydrogen during collisions, molecular hydrogen projectiles with appropriate kinetic energy which is capable of selectively breaking the C−H bonds were effectively generated.39,40 Residual electrons and positive ions were then blocked by two grids immediately above the sample with an applied voltage of −150 and +200 V, respectively. The PAA-coated PPXC films were exposed to hyperthermal hydrogen for 0 to 180 s. Characterizations. Contact Angle Measurements. The water contact angle (WCA) of the PPXC film was measured by a contact angle goniometer (DSA-25, Kruss, Germany) with 3 μL of distilled water droplets. The reported static angles were calculated by averaging the measurements from both the left and right sides of the droplet. Six data points were collected at six different positions on the film surface and were used for calculating the final average values. The surface energies of films were calculated by measuring the contact angles using both water and methyl iodide as the testing liquids with known surface energy. Fourier Transform Infrared (FTIR) Measurements. The pristine and PAA-coated PPXC films were studied by FTIR (Nicolet-iS10, Thermo Fisher Company, U.S.A.) in the range of 4000−400 cm−1 under an attenuated total reflectance (ATR) mode. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded on a Kratos AXIS Ultra instrument using a monochromatic Al Kα photon source (1486.6 eV). Sample data were collected at a takeoff angle of 90°, and then the spectra were analyzed by XPS viewer software. B


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The Journal of Physical Chemistry C

Figure 1. Schematic illustration of coating and cross-linking of PAA on the PPXC film.

Figure 2. AFM images of pristine PPXC film (a) and PAA-coated PPXC films with different HHIC treatment time: (b) 0, (c) 30, (d) 60, (e) 90, and (f) 180 s.

exposing area of 50 cm2 and the cross-linking side of film faced to the wet chambers (90% RH). Adhesion Test. Circuit patterns were prepared by inkjet printing of silver ink on PPXC film using a piezoelectric auto drop printer (DMP-2800, Micro Drop Technologies GmbH) equipped with a 100 μm nozzle. The adhesion between Circuit patterns (dried silver ink) and PPXC film was evaluated by cross-cut tape tests according to GBT9286−98 standard.

Atomic Force Microscopy (AFM). The surface Young’s modulus of the PAA-coated PPXC films were examined by an AFM (SPI4000, Seiko Instruments) with a maximum indentation depth of 50 nm. Commercial silicon nitride tips were used as an indentor in the AFM indentation experiments. The average value of surface modulus was obtained from five replicate samples. Water Vapor Transmission Rate (WVTR) Measurements. WTVR was measured by an instrument (W3−330, Labthink, China) with a lowest detection limit of 0.01 g/(m2·day) according to GB1037−88 (electrolytic sensor method). The films were clamped between wet and dry chambers with an

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RESULTS AND DISCUSSION Coating and Cross-Linking of PAA on the PPXC Film. To improve the wetting properties of PPXC film surface, PAA C


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Figure 4. Surface modulus (measured by AFM) of PAA-coated PPXC films with various HHIC treatment time.

plasma treatments) is attributed to the unique design in the HHIC process. The surface wetting of modified PPXC film was examined by WCA measurement (Figure 3a). After spin-coating treatment, the WCA of PPXC film surface significantly decreases from 84 to 23° (Figure S1), indicating an improved surface wetting arising from the formation of the hydrophilic PAA layer. After HHIC treatment, the WCA of the PAA-coated PPXC film does not change nearly within 60 s and then obviously increases with increasing treatment time (up to 180 s). The WCA change of the modified PPXC films could be induced by the chemical structure change in the PAA layer during HHIC treatment. The high resolution C 1s XPS spectra (Figure 3b) were employed to confirm the change of functionalities on the PAA-coated PPXC films with different HHIC treatment time. Compared to the untreated PAA-coated PPXC film, C−O (286.5 eV) and CO (289.8 eV) peaks from the hydrophilic COOH groups on PAA do not change obviously at the HHIC treatment time of 30 and 60 s, but become weak and even disappear after longer treatment time (90 and 180 s). These results demonstrate that COOH groups on PAA were not damaged in a certain treatment time (within 60 s) due to the high selectivity of hyperthermal hydrogen.34,42 However, excessive hyperthermal hydrogen bombardment (over 60 s) leads to the loss of hydrophilic COOH groups and consequently degrades the wetting of modified PPXC film. Surface mechanical measurement by AFM was used to confirm the surface cross-linking reaction of PAA on PPXC films induced by hyperthermal hydrogen bombardment. Figure 4 shows the surface modulus of PAA-coated PPXC films as a function of HHIC treatment time. The surface modulus of the PAA-coated PPXC film increases with HHIC treatment time and reaches a maximum value of 5.43 GPa at a 60 s treatment, indicating a constantly improved cross-linking degree of the PAA layer. However, the surface modulus of PAA-coated PPXC film starts to decrease when the HHIC treatment time is over 60 s. This result is probably attributed to the damage of crosslinking layer arising from excessive hyperthermal hydrogen bombardment. To check whether PAA layer was firmly cross-linked on PPXC film, PAA-coated PPXC films before and after HHIC treatment were washed with alcohol (as a solvent for PAA) and

Figure 3. (a) WCA of PAA-coated PPXC film as a function of HHIC treatment time. (b) High-resolution C 1s XPS spectra for PAA-coated PPXC films with different HHIC treatment time.

solution was spin-coated on the PPXC film surface to form a hydrophilic PAA layer (38 nm, measured by ellipsometer), followed by HHIC treatment (Figure 1). In the HHIC treatment, the hyperthermal hydrogen projectiles were employed to selectively cleave the C−H bonds but no other bonds on the PAA and PPXC surfaces by collision-driven chemistry reaction.33,41 Finally, the PAA layer is expected to covalently cross-link with the PPXC film by coupling between generated carbon radicals on these hydrogen-atom abstracted molecules. Figure 2 shows the AFM images of a pristine and a PAA-coated PPXC film after HHIC treatment with different time. The PAA-coated PPXC film surface seems smoother than the pristine one (Figure 2a and Figure 2b), indicating that hydrophilic PAA formed a homogeneous thin film covering the PPXC film. The roughness of the PAA-coated PPXC film before and after HHIC treatment (Figure 2c−e) is almost the same, suggesting that HHIC treatment does not physically destroy the PAA and the underlying film bulk. This significant advantage over traditional radiation modification methods (e.g., D


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Figure 5. (a) WCA change of PAA-coated PPXC films with different HHIC treatment time before and after alcohol washing for 3 h. FTIR spectra of PAA-coated PPXC films with various HHIC treatment time (b) before and (c) after alcohol washing for 3 h. The spectral regions of CO peaks are expanded for clear observation.

degree of PAA layer. However, the WCAs of the films with treatment time of 60 s do not change obviously after solvent washing. This result indicates that PAA can fully cross-linked themselves and also with PPXC film under sufficient hyperthermal hydrogen bombardment. Figure 5b,c shows the FTIR spectra of HHIC-treated PAA-coated PPXC film before and after alcohol washing, respectively. The peak at 1710 cm−1

then were subjected to WCA measurements and ATR-FTIR. The WCA of the PAA-coated PPXC film without HHIC treatment (Figure 5a) increases to about 84° after solvent washing due to the completed dissociation of the PAA layer by alcohol. The WCA of PAA-coated PPXC films with HHIC treatment time of 30 s also obviously increases after solvent washing, which is attributed to the relatively low cross-linking E


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Table 1. C1s Moiety Distribution of HHIC- and Air PlasmaTreated PPXC Films with Various Aging Time C1s relative contribution (%) sample

aging time

C−O

OC

air plasma treatment

fresh 1 week 1 month fresh 1 week 1 month

11.4 ± 1.1 1.5 ± 0.6 0.2 ± 0.8 24.5 ± 0.8 24.3 ± 1.4 24.5 ± 1.2

9.3 ± 0.8 8.5 ± 0.5 0.5 ± 0.4 10.3 ± 1.2 10.2 ± 0.9 10.2 ± 1.0

HHIC treatment

chemical compositions with HHIC treatment time shown in FTIR results shows a similar trend to that in the XPS results (Figure 3b), and once again, it indicates that controlling HHIC treatment time is critical for preserving the desired hydrophilic COOH groups. After solvent washing, the characteristic peak (CO) of PAA disappears in the films at the HHIC treatment time of 30 s but remains at a HHIC treatment time of 60 s. It suggests that PAA has been stably cross-linked with the underlying PPXC film and meanwhile its hydrophilic groups can be well preserved at the treatment time of 60 s. This FTIR result is also in good agreement with the WCA results for modified films after solvent washing. Besides, it is observed that the WVTR of PAA−PPXC film after HHIC treatment obviously decreases from 0.48 to 0.18 g/(m2·day) (as shown in Figure 6). The improved barrier property should be attributed to the formed cross-linking layer that can act as a dense barrier layer for blocking water vapor.43 This result additionally proves that hyperthermal hydrogen projectiles can penetrate the PAA layer to reach the underlying film and subsequently induce the cross-linking of the PPXC film. Overall, the above results suggest that HHIC treatment can successfully cross-linking PAA and form a hydrophilic layer stably bonding to PPXC film without destroying the desired COOH groups from PAA at an optimized treatment time (60 s). The optimized condition facilitates to take full advantage of the hydrophilicity of PAA molecules to enhance the surface wetting of PPXC film. Stable Surface Wetting of PAA-Coated PPXC Film via HHIC Treatment. Seeking an appropriate method to prepare a stable wetting surface is a very important subject for long-term application of modified polymer films. For comparison, the variation of WCA and surface energy for the air plasma and HHIC-treated PPXC films with aging time are shown in Figure 7, respectively. After air-plasma treatment, WCA of the PPXC film decreases from 84 to 18°, indicating an improved surface energy arising from introducing oxidized functionalities on the polymer surface. But the air-plasma treated PPXC film surface is not stable and undergoes a hydrophobic recovery process accompanying a quick increase of WCA within 1 week. Accordingly, XPS results show that the oxidized functionalities components (C−O and CO) on plasma-treated PPXC film surface significantly decrease within 1 week compared to its fresh counterpart (Table 1 and Figure S2). This hydrophobicity recovery behavior is attributed to the rotation of hydrophilic functionalities into bulk polymer arising from the polymer chain rearrangement, which is often observed in the plasmatreated polymer films.44 Compared to nonmodified polymers, plasma treatments with high energy projectiles often induce the degradation of main macromolecular chains,32 which further promotes polymer chain rearrangement due to lower steric hindrance from short molecular chains generated by degrada-

Figure 6. Water vapor transmission rate of PAA-coated PPXC films with various HHIC treatment time.

Figure 7. WCA (a) and surface energy (b) for the air plasma-treated and the HHIC-treated PPXC films with various aging time at room temperature with a relative humidity of ∼50%.

corresponding to CO of PAA could be observed in the films with HHIC treatment time of less than 60 s and disappears in the samples with treatment time of 90 and 180 s. The change of F


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Figure 8. Silver ink patterns printed on (a) pristine PPXC, (b) air plasma-treated PPXC, (c) air plasma-treated PPXC aged for 7 days, (d) HHICtreated PAA-coated PPXC, (e) HHIC-treated PAA-coated PPXC aged for 40 days. (f) HHIC-treated PAA-coated PPXC with silver patterns showing good flexibility. (a1)−(e1) are optical images (5×) of silver structures on PPXC films (a−e).

effectively restricts the rearrangement of polymer chains on the surface. Thus, HHIC treatment could be an alternative approach to conventional air plasma treatment for enhancing the wetting of polymer film surface. Furthermore, the effect of wetting stability of the modified PPXC film on printability and adhesion of ink was studied. Figure 8 shows the circuit patterns printed with a water-based silver ink on untreated, air plasma-treated, and HHIC-treated PPXC films, respectively. Silver ink cannot completely wet the pristine hydrophobic PPXC films, but it can be printed as continuous lines on fresh PPXC film after either air plasma or HHIC treatment due to the improved wetting for polymer films (Figure 8b,d). However, the strong hydrophobicity recovery of air plasma-treated polymer films (aged for 1 week at room temperature) leads to poor silver ink printability and results in the formation of separated ink dots on the film (Figure 8c1). However, the silver ink still remains good printability on HHIC treated PAA-coated PPXC film aged for 40 days (Figure 8e), which is attributed to the stable wetting of the modified films. This result also means that the HHICtreated polymer film would have longer shelf lifetime for utilization. Thus, our treatment method is meaningful for actual industry application. A tape delaminating test was employed to evaluate the adhesive strength between the cured silver ink and the polymer substrate, which is responsible for the remaining area of the ink layer after the tape pulling. Figure 9 shows that the adhesion between ink and PPXC films can be significantly improved by over ∼2.6× via both air plasma and HHIC treatment due to the introduction of the oxidized functionalities to strongly bond with silver ink (particles). However, the adhesion between the cured silver ink and the air plasma-treated PPXC film aged over

Figure 9. Adhesion evaluation results for silver inks printed on (a) pristine, (b) air plasma-treated, (c) HHIC-treated PAA-coated PPXC films before and after 40 day aging.

tion. However, both WCA and surface energy of PAA-coated PPXC film (by 60 s HHIC treatment) change little over one month, indicating that a highly stable wetting surface is prepared by HHIC treatment. HHIC-treated PAA-coated PPXC film surface contains more oxidized functionalities (24.5 ± 0.8% for C−O and 10.3 ± 1.2% for CO, as listed in Table 1), and its functionalities content nearly keeps constant within our observing time, which well agrees with the WCA and surface energy results. The formation of highly stable wetting surface in PPXC film should be due to the well controllable hyperthermal hydrogen induced surface crosslinking reaction with few byproduct reactions (e.g., the scission of polymer chain backbone), which does not lead to the degradation of main macromolecular chains and, thus, G


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40 days largely decreases due to the hydrophobicity recovery as indicated in WCA measurement (Figure 7a). In contrast, the cured silver ink still keeps robust adhesion (90−95% of the remaining area after tape pulling) on the HHIC treated PAA− PPXC. This result demonstrates that the excellent wetting stability of HHIC-treated PAA−PPXC film surface can well contribute to the reliable adhesion between silver ink and polymer films.

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CONCLUSION Hydrophilic PAA molecules were successfully cross-linked on the surface-inert PPXC film without damaging both PAA and PPXC film by the HHIC treatment of only 60 s because of the unique selectivity of hyperthermal hydrogen. The surface wetting of the resultant PPXC film is significantly improved by grafting hydrophilic PAA molecules. Compared with the conventional air-plasma treatment, HHIC-treated PAA−PPXC film shows an excellent wetting stability, of which WCA and surface energy stay almost the same over 40 days. The improved wetting stability of PPXC film is attributed to the controllable surface cross-linking reaction without the scission of polymer chain backbone under hyperthermal hydrogen bombardment, which effectively restricts the rearrangement of polymer chains on the surface. Besides, the improved wetting stability results in reliable adhesion between silver ink and polymer films. Thus, fixing hydrophilic molecules on hydrophobic polymer surface by HHIC treatment could be an alternative approach to conventional air plasma treatment for preparation of stable wetting surface on polymer films.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09547. Profiles of water droplet on pristine PPXC film and PAAcoated PPXC film with HHIC treatment (Figure S1), and XPS spectra of air plasma-treated PPXC film (Figure S2; PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Changyu Tang: 0000-0002-2874-8745 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to express our sincere thanks to financial support from the National Natural Science Foundation of China (51573217, 21504106, and 51573172) and advanced functional polymer coating program.

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REFERENCES

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Preparation of Stable Wetting Surface by Hyperthermal Hydrogen

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