Ultrathin, Biomimetic, Superhydrophilic Layers of Cross-Linked Poly

School of Materials Science and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, P. R. China. ‡ College of Materi...
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Ultrathin, Biomimetic, Superhydrophilic Layers of Cross-Linked Poly(phosphobetaine) on Polyethylene by Photografting Biao Yang,*,† Xiaobo Duan,† and Jijun Huang*,‡ †

School of Materials Science and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, P. R. China College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China

‡

S Supporting Information *

ABSTRACT: Ultrathin, biomimetic, superhydrophilic hydrogel layers, composed of cross-linked poly(2-methacryloyloxyethyl phosphorylcholine), are formed on low-density polyethylene films via ultraviolet-initiated surface graft polymerization. The layers are 19−58 nm thick as revealed by electron microscopy and have three-dimensional networks; the unique network structure, along with its zwitterionic nature, rather than surface roughness results in superhydrophilicity, that is, the water contact angle around 5°. This superhydrophilicity depends on a variety of factors, including the concentration of the monomer and cross-linker, the type of reaction solvents, the reaction and drying time, the intensity of UV light, and the way of measurement of water contact angles. Superhydrophilicity is obtained under a fixed ratio (e.g., 1/1) of the monomer to cross-linker, a reaction time over 120 s, a short drying time, (75%) ethanol as the reaction solvent, and low-intensity UV light, largely because these factors together generate optimal three-dimensional networks of cross-links.

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INTRODUCTION Superhydrophilicity of polymeric surfaces, defined generally as having a water contact angle below 10°, is crucial to active packaging and biomedical applications.1,2 Water harvesting, self-cleaning, antifogging, antibiofouling, artificial joints, bioimplants (such as vascular grafts, heart valves, artificial hearts, catheters, breast implants, and contact lenses), and microfluidic devices all require polymeric surfaces to be superhydrophilic to minimize friction, wear, protein adsorption, and lipid and cell adhesion.3−18 There are various approaches to rendering superhydrophilic polymeric surfaces. Ion irradiation and plasma treatment in conjunction with subsequent modification of surface chemistry are often employed to achieve superhydrophilicity, particularly for nonpolar or weakly polar polymers such as poly(ether ether ketone) (PEEK), poly(methyl methacrylate) (PMMA), poly(styrene), poly(ethylene terephthalate), polycarbonate, polyimide, and poly(vinylidene fluoride).15,16,19−25 Aligned conductive nanofibers and nanowires based on polyaniline and polypyrrole have been shown to exhibit superhydrophilicity.26,27 Superhydrophilicity may also be achieved in certain polymer/SiO2 nanocomposites formed by layer-by-layer assembly or atom transfer radical polymerization (ATRP).28−30 Alteration in pH, temperature, electrical potential, and oxidation−reduction state may lead to superhydrophilicity of polymeric systems.5,31−35 Formation of hydrophilic polymer (e.g., polyelectrolytes and poly(acrylic acid)) brushes on a polymer substrate via ATRP is an effective way to give rise to superhydrophilicity.3,36−38 Recently, linear polymeric brushes, based on 2-(methacryloyloxy) ethyl © XXXX American Chemical Society

phosphorylcholine (MPC), have been formed via ATRP on hydroxy-functionalized polyolefins to achieve superhydrophilicity.39 In combination with other methods such as plasma processing, ultraviolet (UV)-initiated surface graft polymerization has been reported to lead to superhydrophilic polymeric surfaces.17 With hydrophilic monomers, such as acrylic acid, methacrylic acid, poly(ethylene glycol) monomethacrylate, and MPC, this method, often combined with heating, argon plasma pretreatment, sophisticated post-treatment (e.g., reaction with NaOH) or postfunctionalization, has been employed to enhance the hydrophilicity of generic polyethylene by reducing the water contact angle up to 15°.11,13,40−44 However, the sole use of this approach (i.e., without using an additional method) to generate superhydrophilicity has not been reported on hydrophobic polymeric substrates such as polyethylene. While they all achieve superhydrophilic, polymeric surfaces, these approaches have disadvantages such as time costliness, sophisticated polymerization (e.g., ATRP), expensive equipment, and limited scalability and material availability.16,17,22,24,39,45 Additionally, each requires multiple techniques simultaneously. For example, fabrication of superhydrophilic PMMA or PEEK microfluidics requires lithography, polymer plasma etching, and selective plasma deposition simultaneously.16 Therefore, a generic, inexpensive, efficient, and scalable approach is desired. Here, we report UV light-initiated surface Received: August 4, 2014 Revised: December 20, 2014

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DOI: 10.1021/la5031137 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Schematic for UV-initiated surface graft polymerization on LDPE. was removed with a tweezer and washed with (95%) ethanol for a minute to eliminate the unreacted reaction solution. The upper, grafted LDPE film was further cleaned with ethanol/water (1/1 in v/ v) at 40 °C under (300 W) sonication for four times with 30 min each time. The cleaned, grafted LDPE film was then dried under vacuum (0.1 MPa) at room temperature for various times, that is, 30, 60, 90... 240 min, and stored at a desiccator (CaCl2 as the desiccant) prior to further characterization. Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) Spectroscopy. The grafted layer of the LDPE films was examined using a Nicolet iN10 MX Fourier transform infrared spectrometer (Thermo Fisher Scientific Inc., USA) connected with an auxiliary optical bench iZ10 in which an ATR element was located. The spectra were collected from 500 to 4000 cm−1. X-ray Photoelectron Spectroscopy (XPS). Analysis of surface composition of the grafted layer was performed with a Thermo Scientific ESCALab 250Xi X-ray photoelectron spectrometer using 200 W monochromated Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10−10 mbar. Typically the hydrocarbon C 1s line at 284.8 eV from adventitious carbon was used for energy referencing. For each composition, three spots were examined to ensure that the obtained information was representative. Transmission Electron Microscopy (TEM). The thickness of the ultrathin, cross-linked poly(MPC) layer was evaluated with either a JEOL 2010 or a JEOL 1011 transmission electron microscope operated at 120 kV (for JEOL 2010) or 100 kV (for JEOL 1011). To obtain the ultrathin sections for TEM observation, a grafted LDPE film was embedded in an epoxy system to form a sandwich structure at a rubbery mold. After the epoxy resin cured overnight, the sandwiched film was trimmed with a glass knife at −140 °C on a Leica RM 2265 microtome and subsequently cryo-microtomed at a setup thickness of 50 nm. The ultrathin sections were collected onto carbon-coated copper grids. The thickness of the cross-linked poly(MPC) layer was calculated based on the darkest region in between the epoxy system, as poly(MPC) has phosphor element in the structure, scattering the electron beam the most in the material. Contact Angle Measurement. Sessile drop contact angle measurement was performed with DI-water using an automatic video-based contact angle device, OCA35 (DataPhysics Instruments GmbH, Germany). A water drop of 2 μL was placed on the cleaned, grafted LDPE films with a syringe. Static contact angle values were determined from dynamic video files that captured at 15.6 frames/s with the software provided by the manufacturer. Typically, at least five different points were measured and averaged. All measurements were performed at room temperature and humidity of 18−22%. Prior to the measurement, the blocks of 50 × 6 mm2 (length × width) were cut off from the middle part of the (7 × 7 cm2) grafted LDPE films. Atomic Force Microscopy (AFM). The surface of the grafted LDPE film was examined with a SPM Multimode N3 instrument (Veeco Digital Instruments, Santa Barbara, CA) under tapping mode

graft polymerization as such an approach to creating a superhydrophilic biomimetic hydrogel layer, composed of poly(2-methacryloyloxyethyl phosphorylcholine) (poly(MPC)) with three-dimensional (3D) networks formed via cross-linking, on generic low-density polyethylene (LDPE) films. As the molecular structure of poly(MPC) resembles that of the lipid of cell membranes, the hydrogel layer makes LDPE attractive to certain applications of biomedical engineering. Through systematically tuning key variables, this approach allows us to achieve superhydrophilic LDPE without using an additional method, such as plasma treatment or ATRP.

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EXPERIMENTAL SECTION

Materials. LDPE films were homemade with a blown film extrusion system (Labtech Engineering Company, Ltd.) using LDPE pellets (LD 607 with a melt flow index of 7.5 g/10 min) purchased from the SINOPEC-Beijing. The setup temperatures of the single screw extruder from the feeding zone to the die were 170, 190, 210, and 210 °C, respectively. The obtained LDPE films have a thickness of 35−43 μm and crystallinity of 32.6% measured by differential scanning calorimetry. 2-Methacryloyloxyethyl phosphorylcholine (MPC, ≥96%), purchased from Nanjing Letian Institute of Science and Technology Development, China, N,N′-methylenebis(acrylamide) (MBA, chemically pure), benzophenone (BP, chemically pure), and ethanol (75% and 99.7%), purchased from the Sinopharm Chemical Reagent Co., Ltd., China, were all used without further purification. Deionized water was produced with a Millipore water system. UV-Initiated Surface Graft Polymerization. The graft polymerization was carried out at room temperature. First, square-shaped LDPE films (7 × 7 cm2 in size) were immerged into (95%) ethanol at 60 °C under magnetic stirring for 6 h and then removed for vacuum drying prior to use. Typically, a 200 μL reaction solution, composed of 0.5 M MPC, 0.5 M MBA, and 0.15 M BP at a desired ratio, was transferred on a corner of a cleaned LDPE film; another cleaned LDPE film was slightly touched the reaction solution with a tweezer, followed by being slowly lifted down so that a sandwich structure was formed without air bubbles. The UV light of a wavelength of 254 nm was then shone on the sandwich system to initiate surface graft polymerization and to maintain the reaction for up to 240 s before moving away the grafted LDPE films in termination of the reaction. The intensity of the UV light was controlled by adjusting the height (e.g., typically 20 cm) of a single-wavelength (254 nm), low-pressure UV light lamp (purchased from Beijing Aerospace Hongda Optoelectronics Technology Co., Ltd.) above the upper LDPE film and monitored by an intensity meter. The lamp had an output power of 300 W, and was made of a “Z” shaped tube (150 cm in length and 25 mm in diameter) folded into three-equal-parallel portions with a wall-to-wall distance of 1.5 cm. The effect of thermal radiation of the lamp on the reaction was negligible, as the sandwich structure was situated on a platform equipped with water cooling. After the grafting, the upper LDPE film B

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Langmuir in the air. A scan size of 2 × 2 or 5 × 5 μm2 was applied. Grafted LDPE specimens of 7 × 7 mm2 were used.

Apparently, N and O elements originated from both MPC and MBA, while P element came exclusively from MPC. More importantly, the intensity of N, O, and P elements increases with the reaction time, while the intensity of C element decreases. Since XPS typically analyzes surface composition of top 1−10 nm depth, these observations confirm occurrence of the graft polymerization and thickness growth. Figure 2 indicates that both the amount of MPC and MBA and the elemental ratios of P/C and P/O increase with the reaction time while the ratio of MBA/MPC remains nearly constant (ca. 0.75) (see Table S1, Supporting Information). The thickness of the grafted layer was measured by TEM (Figure 3). Under typical reaction conditions, the grafted layer

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RESULTS AND DISCUSSION A prudent design is essential to the superhydrophilic LDPE. We believe that the 3D networks of poly(MPC), which differ from linear polymer chains previously reported for polyethylene and other systems,3,6,39,40,43 are the key to superhydrophilicity. With our method, a homogeneous reaction solution, comprising MPC as the hydrophilic monomer, MBA as the cross-linker, BP as the photoinitiator, and ethanol or a mixture of ethanol with water as the reaction solvent, was inserted into two LDPE films to form a sandwich structure (Figure 1). Under UV irradiation, the photoinitiator abstracted hydrogen atoms of LDPE macromolecular chains to create surface free radicals.41 Subsequently, these free radicals initiated graft copolymerization, giving rise to an ultrathin hydrogel layer of cross-linked poly(MPC). Such a layer would not alter the macroscopic and microscopic profile of polymeric surfaces, which is important to various applications especially in biomedical engineering. Our method is efficient and scalable in generating superhydrophilicity compared with other approaches, because the graft polymerization was completed within a few minutes. Despite these advantages, some variables need to be better controlled to achieve superhydrophilicity, including the thickness of the grafted layer, the intensity of UV light irradiation, the concentrations of MPC and MBA, the type of reaction solvents, the reaction time, and the way of measuring the water contact angle. The structure of 3D networks of cross-linked poly(MPC) on the LDPE was examined by ATR-FTIR (Figure S1, Supporting Information). Irrespective of the reaction time, the typical spectra of grafted polyethylene appear without showing any features of the 3D networks of poly(MPC), which is likely due to the fact that the cross-linked poly(MPC) layer is too thin to be detected. The peak around 2300−2400 cm−1 is ascribed to carbon dioxide.46 To further verify whether the graft polymerization occurred on the surface of LDPE, XPS was employed to show surface composition of the graft layer. Figure 2 shows that the grafted layer is mainly composed of C, P, N, and O elements.

Figure 3. TEM photomicrographs for the grafted LDPE film with an ultrathin, biomimetic layer composed of cross-linked poly(MPC) at various reaction times: (a) 60 s, (b) 120 s, and (c) 240 s. Ethanol was used as the reaction solvent and the specimens were dried under vacuum at 25 °C for 240 min.

of cross-linked poly(MPC) has a thickness around 19−58 nm, depending primarily on the reaction time. However, such a thickness does not exhibit a linear relationship with the reaction time, because the thickness reaches 21 ± 2 nm at a reaction time of 60 s but 55 ± 3 nm at 240 s. At the molecular level, the graft polymerization (i.e., cross-linking) occurred first on the surface of LDPE wherein surface free radicals were created.

Figure 2. XPS spectra of the virgin LDPE and grafted versions obtained at different reaction times. Ethanol was used as the reaction solvent and the specimens were dried under vacuum at 25 °C for 240 min. C

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Figure 4. Contact angle as a function of the reaction time for the systems with different reaction solvents: (a) ethanol and (b) 75% ethanol. In both cases, a contact time of 10 s was used. Evolution of the contact angle and the base diameter of water drops with the contact time for the systems under different reaction times and solvents: (c) ethanol, a reaction time of 210 s, and (d) 75% ethanol, a reaction time of 150 s. In both cases, a drying time of 30 min under vacuum was employed. The lower optical image compares the ungrafted LDPE film (left) with a grafted LDPE film (right) covered on beakers with warm water after 10 min.

the intensity of UV light used significantly affect the contact angle (Figure S2, Supporting Information). Without the crosslinker, the contact angle of grafted LDPE by linear macromolecular chains of poly(MPC) reaches ∼57°; although the grafted layer of linear poly(MPC) macromolecules greatly enhanced hydrophilicity by reducing the contact angle from 90° to 57°, such linear poly(MPC) chains were insufficient to lead to superhydrophilicity as previously reported for polyethylene,7,11,40,43 which is likely to arise from the low reactivity of MPC and nonhomogeneous reaction. This insufficiency drove us to apply cross-linking of poly(MPC).18 With cross-linker MBA incorporated, superhydrophilicity is achieved at the concentration of 0.375 M of MBA. A further increase in the concentration shows a minimal effect on the contact angle. A feed ratio of MBA to MPC was maintained as 1/1 (in mol), while the cross-linked poly(MPC) layer exhibits 0.75/1 (in mol) in composition for MBA/MPC shown in Table S1 in the Supporting Information. Thus, a high cross-link density (i.e., a small mesh size of the 3D networks) appears to be essential to achieving superhydrophilicity. With a fixed ratio of MBA/MPC and a constant drying time, a low irradiation intensity, 4400 μw/cm2, is necessary to lead to superhydrophilicity, when the reaction time is over 150 s. The intensity of UV light plays a subtle role in determining hydrophilicity, as we believe that a low intensity allows graft polymerization/cross-linking to occur more uniformly than

Subsequently, the thickness of the grafted layer increased with time as the free radicals on newly formed macromolecular chains further induced graft polymerization. Concomitantly, the grafted layer became more uniform with time because of diffusion and availability of more grafting points. The ultrathin, grafted layer, composed of 3D networks of poly(MPC), determines the hydrophilicity of the LDPE. Figure 4 shows effects of the reaction time on the contact angle under different drying times. Regardless of the type of reaction solvents, the contact angle decreases considerably with the reaction time; it remains nearly unchanged at the reaction time over 150 s for ethanol as the reaction solvent, while with 75% ethanol as the reaction solvent the contact angle starts to level off at the reaction time of over 90 s but to gradually increase after 150 s. This difference, caused by the reaction solvent, may be interpreted from the fact that BP has a higher efficiency in initiating free radicals in a more polar solvent.47 Thus, superhydrophilicity is achieved when the reaction time falls in a range of 150−240 s for ethanol as the reaction solvent. In the case of 75% ethanol as the reaction solvent, a range of reaction times of 90−180 s result in superhydrophilicity. Such superhydrophilicity depends slightly on the drying condition, as a longer drying time (i.e., lower humidity) slightly increases the contact angle. In addition to the reaction time, drying time, and reaction solvent on superhydrophilicity, the concentration of MBA and D

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Langmuir does a high intensity, thus leading to a little less rough layer. Cross-linking played a critical role in achieving superhydrophilicity of polyethylene not only for MPC but also for other hydrophilic monomers such as acrylic acid. Cross-linking reaction considerably enhanced adhesion of the grafted layer onto polyethylene, and made coverage of graft chains uniform offsetting the effects of nonuniformity of graft points on superhydrophilicity as a result of sole introduction of hydrophilic monomers; without a cross-linker, photografting of acrylic acid occurred faster but in a less controlled fashion, resulting in formation of clusters of various sizes. These clusters were detrimental to attaining superhydrophilicity.48 It is challenging to accurately measure the contact angle below 10°, because wetting is a dynamic process.2,49 Thus, to better understand superhydrophilicity of the grafted LDPE, it is important to know how the contact angle was measured in the process (Figure 4). The contact angle decreases considerably with the contact time, while the base diameter of the water drop increases significantly. Both the contact angle and base diameter exhibit a minimal change for the system with ethanol as the reaction solvent, when the contact time exceeds 10 s. However, for the system with 75% ethanol, the base diameter still increases moderately though the contact angle decreases only slightly. Thus, a contact time of 10 s was used for all measurements. Such a contact time allows us to accurately appreciate the roles of various parameters determining superhydrophilicity. The stability of the superhydrophilicity is fairly good in that it was maintained after a superhydropilic LDPE film was stored for 10 days in a deccicator. The water contact angle of this film increased to about 12° after storage in the same deccicator for a month largely because of the increased moisture content. To demonstrate potential applications of our approach in active packaging, we compared an ungrafted LDPE film with the grafted counterpart after the films were covered onto beakers with warm water for 10 min. The grafted LDPE still keeps its transparency without any condensed water drops in direct contrast to the ungrafted counterpart. This distinct difference arises from the fact that a thin, uniform water layer was formed on the grafted LDPE film due to its superwettability caused by superhydrophilicity while the condensed water drops on the ungrafted LDPE counterpart brought about multiple refraction and scattering leading to fuzziness. The 3D networks of cross-linked poly(MPC), together with their zwitterionic nature and the hydrophilicity of cross-linker MBA rather than the roughness of the grafted layer, are dominant factors in rendering superhydrophilicity. The surface topology of the ungrafted and grafted LDPE was examined in detail with AFM as shown in Figure 5 and Supporting Information Figures S3−S5. Irrespective of the reaction solvent, the grafted LDPE shows a little smoother surface than does the ungrafted counterpart, implying a slight reduction in surface roughness. A longer reaction time seems to result in a slightly smoother surface. Such a reduction in roughness did not occur at the micrometer scale as seen from the AFM images (Figures S4 and S5, Supporting Information), but should have taken place at a sub50 nm scale in consideration of the thickness of the grafted layer. Thus, surface roughness of the LDPE film was maintained macroscopically before and after graft polymerization. This change in roughness implies that it is 3D networks of crosslinked poly(MPC) and their zwitterionic nature together with the hydrophilic nature of MBA that are predominant in

Figure 5. Comparison of surface topology revealed by AFM between the ungrafted LDPE film and the grafted counterparts obtained in different reaction solvents at various reaction times: (a) ungrafted LDPE film, (b) grafted LDPE film via ethanol at a reaction time of 210 s, (c) grafted LDPE film via 75% ethanol at a reaction time of 210 s, (d) grafted LDPE film via ethanol at a reaction time of 240 s, and (e) grafted LDPE film via 75% ethanol at a reaction time 240 s. In all cases, a 2 × 2 μm2 area was scanned.

determining superhydrophilicity, and that the role of surface roughness is secondary.

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CONCLUSIONS

We have demonstrated that superhydrophilic LDPE is achieved by UV-initiated surface graft polymerization of a zwitterionic monomer of MPC with a hydrophilic cross-linker of MBA at room temperature under appropriate conditions. The graft polymerization generated an ultrathin biomimetic hydrogel layer composed of 3D networks of poly(MPC) on the surface of LDPE. The biomimetic layer had a thickness of 21−55 nm (and thus did not macroscopically alter surface roughness), and led to a water contact angle below 10° under optimal conditions. The obtained superhydrophilicity, thickness, and hydrogel structure were controlled by the type of reaction solvents, the concentration ratio of MPC to MBA, the reaction time, and the intensity of UV light. The superhydrophilicity of the grafted LDPE was believed to result mainly from the 3D networks of poly(MPC) and their zwitterionic nature, but not from surface roughness. At the molecular level, such superhydrophilicity was mainly attributed to the least undisturbed hydrogen bonding between water molecules, that is, much more free water molecules around the zwitterionic structure,50 in addition to the unique configuration of 3D networks at sub10 nm scale. All these characteristics distinguish our approach from those previously reported about hydrophilic monomers such as acrylic acid used for UV induced surface graft polymerization on a substrate like LDPE. Thus, our approach opens up new avenues for creating superhydrophilic polymeric surfaces required for numerous applications particularly in packaging and biomedical engineering. E

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ASSOCIATED CONTENT

S Supporting Information *

Additional FTIR spectra, contact angle and atomic force microscopy characterization, and XPS results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(B.Y.) E-mail: [email protected]. *(J.J.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support by both Beijing Municipal Natural Science Foundation (2132018) and the National Natural Science Foundation of China (51473007).

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REFERENCES

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DOI: 10.1021/la5031137 Langmuir XXXX, XXX, XXX−XXX