Enhanced water vapor barrier property of poly(chloro-p-xylylene) film by formation of dense surface cross-linking layer via hyperthermal hydrogen treatment

Abstract

Polymer barrier materials have been increasingly used in many applications such as metal anti-corrosion, electronic packaging, and cultural relic protection, but they have poor resistance to water vapor compared to inorganic barrier materials. Herein, we demonstrate the first application of hyperthermal hydrogen induced cross-linking (HHIC) technology to improve dramatically the water vapor barrier properties of poly(chloro-p-xylylene) (PPXC) films by building a dense and intact surface cross-linking layer. With the HHIC treatment, a dense cross-linked layer is formed on the surface of the PPXC film, which serves as a dense barrier layer to water vapor diffusion. The water vapor transmission rate of the PPXC film sharply decreases from 8.4 × 10⁻¹⁶ to 2.1 × 10⁻¹⁶ g cm cm⁻² s⁻¹ Pa by 75% after 5 min HHIC treatment. Due to the advantage of selective cleavage of C–H bonds by HHIC treatment, the desired chloride groups and original physical properties (e.g. mechanical strength and light transmittance) were well preserved.

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1. Introduction

Isolating moisture-sensitive substances from exposure to water vapor, a process which can effectively enhance their lifetime, is greatly needed in many applications such as metal anti-corrosion,¹ electronic packaging,^2,3 food packaging⁴ and cultural relic protection.⁵ Polymer barrier materials have been increasingly used in these applications due to their excellent mechanical flexibility, low stress, low cost and the ease of processing. For example, one of the commonly used polymer barrier materials is poly(chloro-p-xylylene) (PPXC), which can be conformed onto a desirable protected substrate by chemical vapor deposition (CVD) polymerization of chloro-p-xylylene.⁶ However, compared to the inorganic barrier materials, polymer barrier materials have poor resistance to water vapor because of the low package density of the polymer chains and their low crystallinity.⁷

To improve the water vapor barrier properties of polymer films, various methods, such as the filling of impermeable nano-sheets into polymer, the incorporation of chemical cross-linkers, annealing treatment, and vapor deposition of dense inorganic coating have been developed.^8–11 The basic principle of these methods to improve the water vapor barrier property is decreasing the water vapor diffusion through the polymer film by building a denser barrier structure in the polymer matrix. But these methods still have some vital disadvantages. For example, although the incorporation of sheet-like nano-fillers (e.g. clay and graphene) into polymer composite shows good ability to improve the barrier properties, the aggregation of nano-fillers and their random dispersion easily lead to the degradation in both the barrier and mechanical properties of the polymer composites.^11,12 Annealing at high temperature and incorporation of chemical cross-linker can be used to decrease the water vapor diffusion by the formation of a dense package of polymer chains, but they often lead to the increased brittleness of the bulk polymer film.^13,14 Deposition of inorganic coating^15,16 (e.g. silica and alumina) is a surface modification way to build a barrier layer which can keep the original mechanical properties of the polymer films. However, the method not only involves expensive materials and complicated processing (e.g. high temperature exposure and multi-step coating), but also the coating is prone to delaminating and cracking due to the poor interface adhesion between the coating and the polymer film.^17–19

Achieving cross-linking on the polymer surface rather than in polymer bulk may overcome the above issues. This design not only can build up a permanent and solid barrier layer on the polymer film surface, but also can keep the original physical properties including the mechanical properties of the bulk polymer film. Although current surface modification methods such as UV radiation and plasma treatment can induce polymer surface cross-linking reactions, the usual uncontrolled or high energy projectiles arising from these methods often lead to the degradation of the polymer backbone (C–C bond) and hence the difficulty in the formation of a dense surface cross-linking layer.^20–22 Thus, these methods were rarely reported to be used to enhance the vapor barrier properties of polymer films.

Recently, a new technology called the hyperthermal hydrogen induced cross-linking (HHIC) technology has been developed for achieving a mild surface cross-linking reaction on polymer materials containing C–H bonds (such as poly(ethylene oxide), poly(acrylic acid), and butyl rubber) without any physical damage to the surface.^21,23,26 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 is more efficient for H₂ collisions with H, than for those with C. Consequently, the resulting carbon radicals generated from the organic molecules can combine together and form the cross-linking of C–C bonds.²⁴ It is also an extremely clean process because no other additive such as chemical cross-linkers, initiators, and catalysts are required.^25,26 Here, we demonstrate the first application of HHIC technology to improve dramatically the water vapor barrier properties of PPXC film by building a dense surface cross-linking layer. With the HHIC treatment, the dense cross-linking layer is only formed on the surface of the PPXC film, which serves as a dense barrier layer to water vapor diffusion. In addition, the original mechanical characteristics and desirable functionalities (e.g. chloride atom for chemical- and moisture-resistance) of the PPXC film can be effectively preserved.

2. Experimental

2.1 Materials

Poly(chloro-p-xylylene) film with a thickness of 110 μm was provided by China Academy of Engineering Physics. PPXC films were prepared by chemical vapor deposition polymerization of chloro-p-xylylene dimer. The dimer was sublimed in the vaporization chamber at 170 °C followed by pyrolying into a gaseous monomer at 690 °C. The reactive gas was then flowed into a deposition chamber and formed a film on the substrate via a free-radical polymerization.²⁷ Alcohol, acetone, and distilled water were purchased from Kelong Chemical Company (Chengdu, China). Before HHIC treatment, the PPXC films were rinsed with acetone and alcohol for several times followed by drying with nitrogen gas.

2.2 Hyperthermal hydrogen induced cross-linking of PPXC films

A homebuilt system (Fig. 1S†) equipped with an electron cyclotron resonance (ECR) microwave plasma source (87.5 mT, 2.45 GHz) was used for HHIC cross-linking of the PPXC films. In our system, the exposing surface area of sample under hyperthermal hydrogen can be as large as ∼200 cm², which is determined by the aperture plate area for proton transport. The penetration thickness of hyperthermal hydrogen into polymer surface can be controlled by the average energy of hyperthermal hydrogen. 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 6.0 × 10⁻⁴ Pa. Subsequently, hydrogen gas was introduced inside the reactor until a pressure of ∼9.9 × 10⁻² Pa was reached and 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 by using an applied potential difference of −100 V with a current of ∼160 mA. The protons extracted were accelerated into the drift zone, which is a 60 cm-long electric field-free compartment. There, 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 capable of selectively breaking the C–H bonds were effectively generated.²⁸ Residual electrons and positive ions were blocked by two grids immediately above the sample with an applied voltage of −150 V and +200 V, respectively. The PPXC films were exposed to hyperthermal molecular hydrogen for 2 to 20 min.

2.3 Characterizations

Fourier transform infrared (FTIR) measurements. The pristine and HHIC-treated PPXC films were studied by FTIR (Nicolet-iS10, Thermo Fisher Company, USA) in the range of 4000–400 cm⁻¹ under an 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 take-off angle of 90°. The Spectra were analyzed by XPSViewer software.

Contact angle measurements. The contact angles of the PPXC films were measured by a contact angle goniometer (DSA-25, Siberhegner China). Samples were first loaded onto the stage and drops (3 μL) of distilled water were placed on the specimens. The reported static angles were calculated by averaging the measurements from both the left and right sides of the droplet. At least 6 measurements on each sample were made to give the average value.

Atomic force microscopy (AFM). The surface Young's modulus of the PPXC films were examined by atomic force microscopy (SPI4000, Seiko Instruments) with a maximum indentation depth of 50 nm. Commercial silicon nitride tips were used as the indentors in the AFM indentation experiments. The average value of surface modulus was obtained from five replicate samples.

Ellipsometry. After HHIC treatment, the refractive index of surface cross-linking layer of PPXC film is much different from that of bulk film. A EX2 2300 ellipsometer was employed to roughly evaluate the thickness of surface cross-linking layer at the single laser wavelength of 632.8 nm and a constant incidence angle of 70°.At least 5 measurements on each sample were made to give the average thickness value.

Mechanical properties test. The stress–strain curves of PPXC films were measured on an electro-mechanical universal testing machine (WDW-10, Jinan New Century Company, China) at room temperature with a relative humidity of 50 ± 3%. The specimens were tested with a crosshead speed of 5 mm min⁻¹. Four parallel measurements were carried out for each sample.

UV/vis absorption measurements. Light transmittance spectra of the PPXC films with different HHIC treatment time were measured by UV/vis spectrophotometer (UV-1800PPXC, Mapada Co, Ltd., China) in the range of 300–800 nm to examine if the HHIC physically damaged the PPXC film surface.

Water vapor transmission rate (WVTR) measurements. Water vapor transmission rate was measured by an instrument (W3-330, Labthink, China) with a lower detection limit of 0.01 g m⁻² per day according to GB1037-88 (electrolytic sensor method). The PPXC film was clamped between wet and dry chambers with an exposing area of 50 cm² and the cross-linking side of film faced to the wet chambers (90% RH). Water vapor permeated the film under the drive of humidity gradient and was detected by an electrolytic sensor in the dry chamber. The measurements were conducted at 37 °C for 10 hours to reach a complete vapor diffusion balance. Five parallel measurements were carried out for each sample. The WVTR was used to calculate the water vapor permeability (WVP) by following equation:

Statistical analysis. The significance of difference in tensile strength and WVP of PPXC films during various HHIC treatment time were assessed by Excel software. For data comparison, unpaired and two-tailed T tests were used.

3. Results and discussion

3.1 Design strategy

Dissociation–diffusion model is commonly used to describe the permeation mechanism of the gas through the polymer medium. The water vapor permeability in materials is mainly determined by the solubility and the diffusivity (P = S × D, where P, S and D represent permeability, solubility and diffusivity, respectively).²⁹ According to the above theory, HHIC technology is designed to decrease water vapor permeability of PPXC films based on the following considerations (Fig. 1): (1) in this approach, energy-controlled hyper-thermal hydrogen can selectively break the C–H bonds on the PPXC film to produce carbon radicals via physical collision between hyper-thermal hydrogen and hydrogen atoms from the C–H structure and results in the cross-linking by carbon radicals combination on the film surface (see details in ESI†). The resultant cross-linked layer would exhibit much lower diffusivity than bulk polymer and can serve as a dense barrier layer; (2) this mild modification approach can avoid the mechanical damage and the introduction of many defects into the cross-linking layer; (3) chloride atoms bonded to benzene rings, which enable poly(chloro-p-xylylene) to own a better moisture barrier property than common poly(p-xylylene),³⁰ can be well preserved during the HHIC treatment.

Fig. 1 The schematics for surface cross-linking of PPXC film induced by hyperthermal hydrogen collision.

3.2 Surface cross-linking reaction of PPXC film

Fig. 2 shows the ATR-FTIR spectra of PPXC films before and after the HHIC treatment. Pristine PPXC films display two types of C–H stretching vibration: the peak at 3020 cm⁻¹ is assigned to the stretching vibration of C–H on the benzene ring, and the other peaks at 2926 and 2860 cm⁻¹ are assigned to the C–H groups on the –CH₂–.³¹ The intensities of these C–H peaks decrease with HHIC treatment time. It indicates that some C–H bonds on the PPXC backbone are cleaved via hyperthermal hydrogen bombardment, a process which can generate carbon radicals for cross-linking. However, the minimal peak intensity of C–H is obtained at 5 min HHIC treatment, after which the peak intensity of C–H increases with the HHIC treatment time. In the meanwhile, the characteristic multiple peaks of benzene ring ranging from 1700 to 1400 cm⁻¹ monotonically decrease with the HHIC treatment time. These results suggest the formation of new C–H bonds through the addition of hydrogen to the benzene rings and the loss of the rigid aromatic structure in the PPXC backbone. Thus, the abstract and addition of hydrogen on the PPXC molecules are present simultaneously, which may affect the structure and barrier properties of the surface cross-linking layer.

Fig. 2 ATR-FTIR spectra of (a) pristine PPXC film, and HHIC-treated PPXC films for (b) 2, (c) 5, (d) 10, and (e) 20 min, respectively. The peak intensities of C–H and benzene rings in FTIR spectra were normalized with respect to the C–Cl peak; (B) the peak intensities of C–H and benzene ring with various HHIC treatment time.

Surface mechanical analysis by AFM (Fig. 3) was used to evaluate the state of the surface cross-linking layer. Compared to the pristine PPXC film, the surface modulus of the HHIC-treated PPXC film increase with HHIC treatment time and increase by nearly 50% for a 5 min treatment. This result corresponds to the improved cross-linking degree of the PPXC molecules via the coupling of the resultant C radicals generated by the cleavage of C–H.³² That is, the surface cross-linking layer was formed and become denser with HHIC treatment. However, the surface modulus of PPXC film starts to decrease when the HHIC treatment time is over 5 min. It should be attributed to the fact that some of the rigid aromatic rings on the PPXC film decrease and transform to physically flexible saturated rings under excessive HHIC treatment due to the hydrogenation reaction of benzene rings (see FTIR result). The loss of the rigid aromatic structure on the PPXC film surface can lead to the decrease of surface mechanical strength. Although FTIR results show the amount of rigid benzene rings decrease over all the time, surface modulus of PPXC film do not decrease monotonically with treatment time. It implies that the surface modulus of HHIC-treated PPXC is dominated by the surface cross-linking reaction in the first 5 min and then by hydrogenation reaction of benzene rings. It is noted that normally, the addition reaction of benzene ring with hydrogen is difficult to carry out and often needs a catalyst and high temperature (e.g. ∼300 °C).^33,34 However, in the HHIC reaction, the hydrogen can be easily introduced into the benzene rings at room temperature, indicating the high reactivity of hyperthermal hydrogen with respect to the common hydrogen.^35,36 Thus, to obtain dense surface cross-linking layer on PPXC film, it is necessary to control the HHIC treatment time.

Fig. 3 Surface modulus (measured by AFM) of PPXC films with various HHIC treatment time.

XPS spectra were used to determine the change of components and functionalities on the PPXC films after HHIC treatment. Pristine PPXC film (Fig. 4) presents C, Cl, and O peaks at 284.7, 200.3, and 535.6 eV, respectively. After HHIC treatment, there were no obvious changes in the peak intensities of C and Cl components on the PPXC film. It means that C–C and C–Cl bonds were not broken under hyperthermal hydrogen bombardment. Particularly with the selective cleavage bond advantage of HHIC technology, preserving well the Cl atoms in the PPXC film has big significance, because Cl atoms bonded on the benzene ring are very critical to the moisture- and chemical-resistant properties of the PPXC film. It is noted that pure PPXC molecules generally do not contain any O component, so the weak O-peak observed in the XPS spectrum probably comes from air oxidation during the fabrication processing of PPXC film.

Fig. 4 XPS spectra of PPXC films with different HHIC treatment time: (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 min, respectively.

3.3 Surface morphology and physical properties of HHIC treated PPXC film

Fig. 5 shows the surface morphology of PPXC films (measured by AFM) with various HHIC treatment time. There was no obvious change in roughness of PPXC films (ranging from 11.3 to 14.5 nm) after HHIC treatment, indicating that the mild HHIC reaction does not destroy the polymer surface. The unique characteristic facilitates the formation of a dense surface cross-linking layer to barrier water vapor diffusion. Ellipsometer measurement (Fig. 2S†) shows the cross-linking layer on PPXC film forms shortly after 2 min HHIC treatment and its thickness (30–40 nm) does not further change with treatment time. The limited penetrating depth of hyperthermal hydrogen into polymer surface means that the bulk structure and physical properties of PPXC film would not be affected by hyperthermal hydrogen bombardment. The original physical properties (such as mechanical and optical properties) of the HHIC-treated PPXC films were further checked by measurements. The stress–strain curves in Fig. 6 show that the yield strength of the PPXC film with different HHIC treatment time is not significantly different (P > 0.05). In addition, the light transmittances of HHIC-treated PPXC films (Fig. 7) were studied by a UV/vis spectrophotometer. There is no obvious change in light transmittances ranging from 300 to 800 nm in the wavelength for all PPXC films with different HHIC treatment time. These results further demonstrate that the surface cross-linking reaction induced by hyperthermal bombardment does not change the overall physical properties of the PPXC film. This significant advantage of HHIC treatment over the traditional radiation cross-linking methods (e.g. UV and plasma treatments) is attributed to the unique design for selectively cleaving C–H bonds with energy-controllable hydrogen projectiles. In contrast, the regular plasma and UV treatment often sharply increase the polymer surface roughness by etching and degrading.²²

Fig. 5 AFM images of PPXC films with various HHIC treatment time: (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 min, respectively.

Fig. 6 Stress–strain curves of PPXC films with different HHIC treatment time: (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 min, respectively.

Fig. 7 Light transmittance spectra of PPXC films with different HHIC treatment time: (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 min, respectively. The inset shows the photographs of PPXC films.

3.4 Water vapor barrier properties of HHIC-treated PPXC films

The WVPs of the PPXC films with different HHIC treatment time are shown in Fig. 8. The significance of difference in WVPs (P < 0.05) was observed from 2 to 10 min HHIC treatment. Compared to the pristine PPXC film, The WVPs of the PPXC film decreases with HHIC treatment time and sharply decreases from 8.4 × 10⁻¹⁶ to 2.1 × 10⁻¹⁶ g cm cm⁻² s⁻¹ Pa by 75% at 5 min treatment. In the HHIC reaction, the Cl atoms in the PPXC film were well preserved (as shown in XPS analysis) so that its original water vapor solubility should be kept. Thus, the dramatically decreased WVP of PPXC film should be attributed to the formation of the dense surface cross-linking layer, which obviously decreased the water vapor diffusivity through the PPXC film. Both FTIR and AFM results also show that the most optimal surface cross-linking layer formed at 5 min HHIC treatment.

Fig. 8 Water vapor permeation of PPXC films with different HHIC treatment time.

Unexpectedly, the water vapor transmittance of the PPXC film increases gradually after 5 min HHIC treatment, a result which correlates with the structure change of the cross-linking layer evidenced by FTIR and AFM tests. In the cross-linking layer, some of the rigid benzene rings disappeared and transformed to a physically flexible saturated structure (as indicated by surface modulus test) after excessive HHIC treatment. Compared to rigid benzene ring structure, the new formation of saturated rings can increase the dynamic motion of chain segment and free volume of PPXC molecules, which leads to the decrease of water vapor barrier properties of the cross-linking layer.^8,37–39 Thus, enhancing the water vapor barrier properties of the PPXC film by HHIC technology requires optimizing the reaction time well.

4. Conclusions

HHIC technology was employed to successfully improve the water vapor barrier properties of PPXC film by building a permanent and dense surface cross-linking layer. With the HHIC treatment, the dense cross-linking layer is only formed on the surface of the PPXC film, which serves as a dense barrier layer to water vapor diffusion. The WVP of the PPXC film sharply decreases from 8.4 × 10⁻¹⁶ to 2.1 × 10⁻¹⁶ g cm cm⁻² s⁻¹ Pa by 75% for 5 min HHIC treatment. In addition, due to the advantage of selective cleavage of C–H bonds, the desired Cl groups and original physical properties (e.g. mechanical strength and light transmittance) were well preserved by HHIC treatment. HHIC approach is also very environmental and cost-effective because the HHIC reaction is operated at room temperature and the equipment consumes no thermal energy and reactive chemical additives (e.g. cross-linker, initiator, and catalyst). Consumption of a small of amount of hydrogen gas and little side-reaction enable this approach to produce no toxic waste. In future, industrial-scale applications for a high-throughput surface modification of polymer films can be further exploited by adding a roll-to-roll sample-feedthrough to the apparatus, to take advantage of the high flux of hyperthermal hydrogen.

Acknowledgements

We would like to express our sincere thanks to financial support from advanced functional polymer coating program, CAEP funding (2013B0302058), the platform funding (213zxfk03) from Southwest University of Science and Technology, the National Natural Science Foundation of China (21104028) and the 2014 Competitive Grant Program of Oversea Returnees Research Projects.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07557b

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