Nanoporous Films with Superior Resistance to Protein Adsorption by

Jul 6, 2016 - recognized that good fouling resistance can be achieved by providing the ... surface with the PEO enrichment and exhibit superior resist...
0 downloads 0 Views 6MB Size
Download PDF
Article pubs.acs.org/IECR

Nanoporous Films with Superior Resistance to Protein Adsorption by Selective Swelling of Polystyrene-block-poly(ethylene oxide) Hao Yang, Leiming Guo, Zhaogen Wang, Nina Yan, and Yong Wang* State Key Laboratory of Materials-Oriented Chemical Engineering, National Synergetic Innovation Center for Advanced Materials, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, Jiangsu, People’s Republic of China ABSTRACT: Poly(ethylene oxide) (PEO) is well-known for its excellent resistance to the nonspecific adsorption of proteins. However, it remains challenging to fabricate surfaces with uniform and robust coverage of PEO chains in a simple and efficient way. Simply by treating the block copolymer polystyrene-block-poly(ethylene oxide) (PS-b-PEO) in ethanol, we obtain nanoporous polymeric films with superior resistance to protein adsorption. The pores are nondestructively formed via the selective swelling-induced pore generation mechanism in which PEO microdomains embedded in PS matrix are selectively swollen in ethanol and collapsed in the subsequent air drying. Pore sizes and porosities can be typically tuned in the ranges ∼20−50 nm and ∼20−70%, respectively, by changing swelling durations and temperatures. Interestingly, the PEO chains are simultaneously migrated on the film surface and pore walls in the selective swelling-induced pore forming process. Atomic force microscopy examinations reveal that PEO chains are solvated when the film is wetted with water, closing the pores in the film. Thus, produced SEO films adsorb much less proteins than PS films without PEO on the surface, and also demonstrate a better fouling resistance than many reported PEO-covered surfaces prepared by other methods.

1. INTRODUCTION Antifouling surfaces are essential in a diversity of fields such as pharmaceutical technology, implant materials, biosensors, and membrane separation.1−3 A number of strategies have been established to fabricate antifouling surfaces by changing the surface chemical and physical properties to reduce the interactions between pollutants and surfaces.4,5 It is well recognized that good fouling resistance can be achieved by providing the surface with bounded water.6,7 Consequently, hydrophilic groups are generally utilized as the antifouling moieties to produce hydrated surfaces. As one of the most frequently used hydrophilic modifiers, poly(ethylene oxide) (PEO, also known as poly(ethylene glycol) (PEG)) has been extensively studied in antifouling surface modifications.8,9 PEO is an electrically neutral, hydrophilic polymer and is well-known for its excellent protein resistance because of its strong hydrophilicity, large excluded volume, and unique coordination with surrounding water molecules in aqueous media.10,11 Although the exact mechanism of the protein resistance of PEO is not yet fully understood, many studies have verified that PEO chains are adsorbed on the material surface to produce a hydration layer on the material surface. The hydration layer forms a steric hindrance to resist the protein adsorption.12 PEO chains are attached onto the substrate surfaces either physically, e.g. by adsorption13,14 or blending,15 or chemically, e.g. by grafting.16,17 However, many of these techniques of surface modification suffer from specific limitations and drawbacks. Physical adsorption frequently encounters the issues of inhomogeneous surface covering, © 2016 American Chemical Society

poor adhesion, and stability. Chemical grafting typically requires a high surface activity for the surface to be modified and the PEO-containing precursors as well.18,19 Moreover, tedious, cumbersome pretreatment, reaction, and purification operations are generally involved.20 Therefore, it remains highly demanded for effective and facile strategies to uniformly and tightly adhere PEO chains onto substrate surfaces. Recently, we discovered that nanoporous films can be easily prepared by swelling of block copolymers (BCPs) with selective solvents to the minority blocks, which is known as “selective swelling-induced pore generation”.21,22 Interestingly, simultaneously to the pore formation, the minority blocks migrate and are enriched on the pore walls, producing films with surfaces homogeneously covered by the minority blocks. BCPs are composed of two or more thermodynamically incompatible homopolymer chains covalently linked together, and tend to phase separate but only limit to the microscale, which is called “microphase separation”.23−25 Through this selective swelling-induced pore-making process, the phaseseparated BCP films can be nondestructively converted to nanoporous structures. Our previous works were exclusively focused on block copolymers of polystyrene-block-poly(2vinylpyridine) (PS-b-P2VP) and polystyrene-block-poly(4vinylpyridine) (PS-b-P4VP), and analyses on surface compoReceived: Revised: Accepted: Published: 8133

May 25, 2016 July 5, 2016 July 6, 2016 July 6, 2016 DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140

Article

Industrial & Engineering Chemistry Research

scanning electron microscope (FESEM, Hitachi S-4800) operated at an accelerating voltage of 5 kV. Prior to examination, the samples were sputter-coated with a thin layer of platinum to enhance the conductivity of samples. We measured at least 50 pores on the surface SEM image of each sample using the software Nanomeasurer. As most of the pores are in the near-spherical or elliptical shape, we considered the short axis of the pores as the pore diameters, and the averaged values were reported. We note that thus determined pore sizes are rough numbers as these pores are not in a uniform, spherical shape and only the pores on the film surface are taken into account. The thickness and refractive index of the SEO films before and after swelling were measured using an ellipsometer (Complete EASE M-2000U, J. A. Woollam) with an incidence angle of 65°. The optical parameters of the film samples were obtained by fitting the ellipsometric data using the Cauchy model. The porosities of the SEO films are determined from the change in refractive index by applying the formula31

sitions by X-ray photon spectroscopy and surface wetting revealed the progressive enrichment and existence of P2VP or P4VP chains on the surface of the produced nanoporous films.26−28 Inspired by the previous works, we expect that selective swelling of block copolymers with PEO as the minority blocks will produce nanoporous films with PEOcovering surfaces, and thus produced films may exhibit excellent resistance to protein adsorption. Such a method to prepare fouling-resistant surfaces is extremely simple and highly controllable as it involves only a solvent swelling and air drying and no chemical reactions are required. Moreover, the PEO blocks will uniformly exist on the film surfaces, which is highly demanded for stable and robust fouling resistance, as they are covalently bonded to the majority blocks composed of the film matrix. It should be noted that the block copolymers of PEO and polystyrene have been previously used to prepared ultrafiltration membranes by a technique combining both the self-assembly of block copolymers and nonsolvent induced phase separation.29 The obtained membranes are expected to exhibit excellent fouling resistance as polar blocks such as the PEO chains are segregated on the membrane surfaces.30 However, this method requires use of concentrated BCP dopes and the produced membranes are typically several hundreds of micrometers in thickness. There is still a need for strategies to establish thin coating layers of fouling resistance on different substrates. In this work, we fabricate nanoporous films with PEOenriched surfaces by selective swelling of PS-b-PEO block copolymers. We first explore the swelling behavior of the copolymer under various swelling temperatures and durations, and then investigate the protein adsorption of the nanoprous films prepared under different swelling conditions. As expected, the produced nanoporous films have an intrinsically hydrophilic surface with the PEO enrichment and exhibit superior resistance to bovine serum albumin (BSA) adsorption. Compared to other methods for the fabrication of antifouling surfaces with PEO, the present one reported in this work is distinguished for its near-zero protein adsorption, long-standing resistance to protein adsorption, and extreme simplicity in the fabrication process.

neff 2 = n2 2(1 − P) + n12P

(1)

where n1, n2, and neff are the refractive indices of air (n1 = 1), the solid block copolymer, and porous block copolymer, respectively, and P is the porosity of the SEO membranes. Xray photoelectron spectroscopy (XPS) was utilized to reveal the distribution of PEO chains on the surface of the membrane. XPS spectra were obtained with an ESCALAB 250 XPS system (Thermo Scientific) at room temperature by using an Al Kα Xray source (hν = 1486.6 eV). The surface topography and surface roughness of an SEM film prepared by swelling at 60 °C for 4 h in both the dried and wet states were obtained from atomic force microscopy (AFM, Dimension Icon, Bruker). For the AFM examination of the film in the wet state, a few droplets of deionized water were first deposited on the film surface and a waiting period of about 30 min was allowed for PEO chains to be solvated in water before AFM imaging. 2.3. Preparation of the Nanoporous SEO Films. SEO solutions with a concentration of 1 wt % were prepared by dissolving SEO in chloroform. PTFE filters with a nominal pore size of 0.22 μm were used to filter SEO solutions three times to weed out any big aggregates which may exist in the solutions. The SEO solutions were then spin-coated onto cleaned silicon wafers at 2000 rpm for 30 s, which were subsequently immersed in different swelling solvents to carry out the swelling-induced pore-making process at preset temperatures for desired durations. 2.4. Protein Adsorption Tests. BSA was used as the model protein to investigate the antifouling properties by the static adsorption method of SEO films prepared under different swelling conditions. Each sample with an area of 1.5 cm × 1.5 cm was soaked in 0.1 g/L BSA solutions (pH 7.4 in PBS, 1.5 mL) at room temperature for 24 h to ensure saturated adsorption. The amount of the adsorbed protein was determined by comparing the concentrations of protein solutions before and after adsorption with an UV−visible spectrophotometer (NanoDrop 2000C, Thermo). To visualize the protein adsorption on these samples, BSA-FITC was used in the adsorption tests, and a fluorescence microscope (FM, Olympus IX73) was used to observe the adsorbed samples with appropriate filter sets and 50× magnification.

2. EXPERIMENTAL SECTION 2.1. Materials. Polystyrene-block-poly(ethylene oxide) (PSb-PEO, abbreviated as SEO below, Mn(PS) = 42 000 g mol−1, Mn(PEO) = 11 500 g mol−1, polydispersity index (PDI) = 1.07) was purchased from Polymer Source Inc., Canada, and was used as received. PS homopolymer (Mn = 18 000 g mol−1, Mw = 18 100 g mol−1) was purchased from Sigma-Aldrich and used as received. Silicon wafers cleaned with ethanol under ultrasonication were used as the substrates on which the BCP solutions were spin-coated. All organic solvents including chloroform, methanol, ethanol, n-propanol, n-butanol, and nhexanol were of analytical grade and supplied by local suppliers. Bovine serum albumin (BSA, Aladdin, Fraction V), with a molecular weight of 67 kDa, was dissolved in phosphate buffered saline (PBS, at the physiological pH 7.4) to prepare protein solutions with the concentration of 0.1 g/L, which were to characterize the antifouling performances of the SEO films. BSA labeled with fluorescein isothiocyanate (BSA-FITC, Beijing NobleRyder Technology Co., Ltd.) was used for fluorescent characterization in the static adsorption tests. 2.2. Characterization. The surface and cross-sectional morphologies of SEO films were examined with a field emission 8134

DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140

Article

Industrial & Engineering Chemistry Research

3. RESULTS AND DISCUSSION 3.1. Pore Formation in SEO Films by Selective Swelling. We spin-coated 1 wt % chloroform solutions of SEO onto silicon substrates. The films were then immersed in hot ethanol to perform the swelling treatment. The as-coated SEO films exhibited a nonporous morphology with a dense surface. After immersion in ethanol at 70 °C for 4 h and subsequent air drying at room temperature, irregular pores appeared in the films (Figure 1a,b). These pores are largely in

walls as they are covalently linked to the PS chains which constituent the matrix of the films. In the selective swelling-induced pore generation, the swelling degrees determine the morphology of the produced porous materials. Therefore, the parameters influencing the swelling degree of block copolymers in solvents are expected to play roles in this pore-forming process. In sections 3.2 and 3.3, we will investigate the effects of swelling duration and temperature on the morphology of the SEO films. 3.2. Effect of Swelling Durations. We examined the morphology transitions of the SEO films swelling-treated in ethanol at 65 °C for different durations. As shown in Figure 2a−e, there is a general trend that the pores are turned to be increased in numbers and in sizes with increased swelling durations. With a brief swelling for 30 min, irregular nanopores with an average pore size of 22 nm sporadically appeared on the film surface (Figure 2a). More pores were formed and their sizes were increased to approximately 31 nm when the swelling duration was extended to 1 h (Figure 2b). Pores were formed throughout the entire film surface and more pores were merged with their neighbors to form an interconnected network when the swelling duration reached 4 h and the pore size was averaged to be 34 nm (Figure 2c). With further extending the swelling duration up to 15 h, the films maintained an interconnected porous morphology but exhibited increased pore sizes (41 and 49 nm for the swelling durations of 8 and 15 h, respectively). In this pore-forming process, the films experience significant volume expansion with numerous pores formed in the films at no sacrifice of any component in the BCP because of the nondestructive nature of this pore-making method. However, the volume expansion only occurs in the direction perpendicular to the film surface as vertical stretching because the films are tightly adhered to the substrate and the deformation in the direction along the film surface is thus prohibited.32 The thickness of the SEO film is also a good indicator of the swelling degree due to the vertical stretch of the PS matrix in the swelling process. Before swelling, the thickness of the ascoated SEO film was measured as 110 nm by ellipsometry. The thickness was remarkably increased to 231 nm, indicating a porosity of 60% considering that the increase in thickness was proportional to the pore formation after a swelling duration of 30 min. The increase in the film thickness was accounted for 120%. With extending the swelling duration up to 15 h, the film kept increasing in thickness but at a much slower rate. For instance, its thickness was slightly increased to 264 nm (porosity = 70%) from 230 nm when the swelling duration was greatly prolonged from 30 min to 15 h. Therefore, we understand that the vertical stretching of the films mostly takes place in the initial stage within 30 min upon immersion in ethanol although very a few pores appear on the film surface in this period as shown in Figure 1a. 3.3. Effect of Swelling Temperatures. The swelling temperatures play a significant role in the process of selective swelling-induced pore generation as temperature affects the interaction between the swelling solvent and the polymer as well as the mobility of the polymer chains. Here we investigate the morphologies of SEO films subjected to swelling in ethanol at different temperatures. After treatment in ethanol at 50 °C for 4 h, the SEO film retained its initial nonporous morphology despite the slight increase in membrane thickness. When the swelling temperature was increased to 60 °C, pores were formed in the film (Figure 3a) and both the number and size of

Figure 1. SEM images of SEO films subjected to swelling in ethanol at 70 °C for 4 h: (a, b) surface SEM images with large magnifications, (c) large-field surface SEM image, and (d) cross-sectional SEM image.

the near-spherical or elliptical shapes, and their pore sizes can be roughly estimated to be 50 nm by considering the pore widths of elliptical pores as their pore diameters. The large-field SEM image shown in Figure 1c reveals that these pores are uniformly distributed throughout the film surface. Moreover, as shown in the cross-sectional SEM image (Figure 1d), the pores are interconnected with each other, penetrating the entire membrane thickness. The pores are formed through the mechanism of selective swelling-induced pore generation.20,21 The volume fraction of PEO in the BCP is approximately 21.5%, and the PEO chains form cylinders randomly distributed in the PS matrix in the ascoated films according to the phase diagram of BCP.27 The pore generation in PS-b-PEO membranes by selective swelling the PEO domains at temperatures below the bulk glass transition temperature of PS can be explained as a transformation from one equilibrium structure into another equilibrium structure. In the swelling process, ethanol is selectively enriched in PEO cylinders as ethanol is a good solvent to PEO but a nonsolvent to PS. With the uptake of ethanol in PEO cylinders osmotic pressure is accumulated and PEO cylinders are expanded in size, leading to the plastic deformation of the glassy PS matrix. The expanding PEO cylinders are contacted and merged with their neighbors, resulting in a continuous phase of swelling PEO chains distributed in the PS matrix. When the SEO films are removed from the swelling bath with the evaporation of ethanol the deformed PS chains are frozen as they are in the glassy state. Meanwhile, the swollen PEO chains collapse, producing pores along the positions where the expanding PEO chains initially occupy. The collapsed PEO chains are covered along the pore 8135

DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140

Article

Industrial & Engineering Chemistry Research

Figure 2. SEM surface images of SEO films subjected to swelling in ethanol at 65 °C for (a) 30 min, (b) 1 h, (c) 4 h, (d) 8 h, and (e) 15 h, respectively. These images have the same magnification and the scale bar corresponding to 200 nm is given in (a). (f) Thicknesses and porosities of films prepared with different swelling durations.

while the PS matrix has less resistance to plastic deformation at higher temperatures, and consequently the expanding PEO cylinders occupy larger sizes and enlarged pores are formed in the subsequent drying process. There shall be an appropriate temperature range for the selective swelling-induced pore generation process. When the swelling temperature is below the range, the BCP films will remain nonporous. However, the swelling temprerature should not exceed the glass transition temperature (Tg) of the PS blocks in the swelling solvent to keep the PS chains still in the glassy state. Otherwise, the framework of the film will be collapsed.34 3.4. Migration of PEO on the Film Surface. During the swelling process, PEO chains migrate to the film surface and adhere to the pore walls. To confirm the migration of PEO chains onto the film surface, we used XPS to analyze the surface compositions of the SEO films after swelling in ethanol with different durations. The peak at the binding energy of 284.8 eV is assigned to C 1s in PS (both C−H and C−C), and the peak at 286 eV is from the C 1s of PEO (C−O).35 Therefore, we used the area ratio of the deconvolved C 1s (C−O) peak from PEO over the C 1s peak from SEO (C 1s (C−O)/C 1s) to indicate the contents of PEO chains on the film surface. As can be seen in Figure 4, the relative intensities of C 1s (C−O) to C 1s (C−C and C−H) are varied with swelling durations. The SEO film prior to swelling had a ratio of C 1s (C−O) to C 1s as low as ca. 10.5%. After the film was swelling treated in ethanol

the pores kept increasing when the temperature was further increased to 70 °C with a fixed swelling duration of 4 h (Figure 1 and Figure 3b−e). Specifically, for the film swelling at 60 °C, sporadic pores with a diameter of 25 nm began to emerge (Figure 3a). The membrane thickness was measured to be 196 nm, indicating a porosity of 51%. When the temperature was increased to 62 °C, more isolated pores emerged at the surface (Figure 3b) and the membrane thickness was correspondingly increased to 232 nm, corresponding to a porosity of 57%. Further increasing the swelling temperature to 64 °C led to a film thickness of 248 nm and a porosity of 64%. Moreover, the originally isolated pores at the surface began to be connected with each other and the mean pore diameter was determined as 37 nm. For the SEO film swelling at 66 °C, more interconnected pores with an average diameter of 43 nm were observed the surface (Figure 3d). The film thickness and porosity were 267 nm and 67%, respectively. The threedimensionally interconnected porous structure of the SEO film became much more obvious at the swelling temperature of 70 °C (thickness = 294 nm and porosity = 72%). It is clear that higher swelling temperatures lead to stronger swelling degrees and consequently higher porosities.33 This is because elevated temperatures enhance the interaction between ethanol and both the PS and PEO blocks on one hand and promote the segmental mobility of both blocks on the other. Therefore, the PEO chains exhibit stronger swelling degrees 8136

DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140

Article

Industrial & Engineering Chemistry Research

capacity of BSA adsorption of the as-coated SEO film was 1867 ng/cm2, which was much less than that of the PS homopolymer film which exhibited an adsorption capacity as high as 4933 ng/ cm2. The above discrepancy is attributed to the enrichment of PEO chains on the surface of SEO films as confirmed by XPS analysis. PEO chains are highly hydrophilic and can form hydration shells as a natural barrier to prevent protein adsorption in aqueous systems. For the swelling duration of 30 min, 1 h, 4 h, and 8 h, the SEO films exhibited progressively reduced BSA adsorption of 1067, 267, 133, and 133 ng/cm2, respectively. With the SEO film prepared by swelling for 10 h, the BSA adsorption was even undetected, indicating the superior resistance of the nanoporous SEO films to BSA adsorption. Compared to many PEO-covered surfaces prepared by other methods, the nanoporous SEO films enabled by selective swelling exhibit a better resistance to protein adsorption. George et al.36 investigated the protein adsorptions on spin-coated SEO films with the morphology of vertically oriented PEO cylinders embedded in PS matrix and the protein adsorption was found to be varied within 500−1200 ng/cm2 depending on the molecular weight of SEO, which is severalfold higher than that of nanoporous SEO films prepared in this work with a swelling duration higher than 4 h. Bosker and co-workers14 fabricated surfaces covered with PEO brushes by forming thin films of SEO on liquid surfaces via the complicated Langmuir−Blodgett technique followed by transferring the thin films on PS substrates. Although the surfaces were densely covered with PEO brushes, their minimal protein adsorption was still higher than ∼100 ng/cm2. Moreover, our method is also advantageous in terms of convenience in the fabrication process and the stability of the resulted foulingresistant surfaces. It should be noted that the SEO films after selective swelling are nanoporous in the dried state as revealed by SEM imaging. However, this nanoporous morphology is expected to change when the films are exposed to aqueous surroundings as PEO is highly water-soluble. To demonstrate it, we compare the morphology of the swelling-treated SEO film in the dried and wet states obtained by AFM. As shown in Figure 6a, the dried film shows a nanoporous surface which is similar to the SEM results. In stark contrast, after the film is wetted by water these pores mostly disappear and the film has a much denser and smoother surface (Figure 6b). Moreover, the surface becomes much smoother with the average surface roughness reduced from 13.6 nm in the dried state to 5.7 nm in the wet state. The transition from a nanoporous morphology in the dried state to a dense one by wetting with water is due to the solvation effect of the PEO chains attached on the pore walls. The PEO chains take the extending conformation in water. Considering that the PEO block in the SEO copolymer has a molecular weight of 11 500 g mol−1 and a fully stretched such PEO chain will have a length more than 50 nm, the solvated PEO chains will easily block the pores with sizes in the range of approximately 20−50 nm. Usually, a porous surface is expected to adsorb more proteins than a nonporous, dense surface with the same chemistry because the former has bigger surface area and the trapping effect of the pores. However, the nanoporous morphology of the SEO films prepared in our work is not a problem as it will transform to be a nonporous one upon exposure to water and behave as a smooth surface covered with PEO chains. The excellent resistance to protein adsorption of the SEO films treated by selective swelling is arisen from the homogeneous

Figure 3. Surface SEM images of SEO films after swelling in ethanol for 4 h at (a) 60, (b) 62, (c) 64, and (d) 66 °C, respectively. These images have the same magnification, and the scale bar corresponding to 200 nm is given in (a). (e) Thicknesses and porosities of SEO films subjected to ethanol swelling for 4 h at different swelling temperatures.

for 30 min, the ratio was increased to 14.2% indicating the enrichment of PEO chains on the film surface. The ratios of C 1s (C−O) to C 1s were further increased to 20.5, 34.8, and 49.6% when the swelling durations were extended to 1, 4, and 10 h, respectively, indicating that more PEO chains migrated to the surface along with the increase of the swelling degree. The migration of the PEO chains onto the film surface occurs in the swelling process but is terminated after air-drying while PEO and PS chains both lose their mobility, impeding the recovery of PEO chains back to the original positions. 3.5. Resistance to Protein Adsorption. To have a direct impression of the resistance to protein adsorption of the produced SEO films, we first investigated their adsorption of fluorescently labeled BSA. Films of PS homopolymer were also used as a control. Uniform and intense fluorescence was observed on the film surface of of the PS homopolymer (M1 in Figure 5a). However, only a few fluorescent spots can be observed on the surface of the as-coated SEO film (M2 in Figure 5a). For the SEO film subjected to ethanol swelling at 65 °C for 30 min, fluorescent signals were nearly invisible (M3 in Figure 5a). Further increasing the swelling durations to 1, 4, 8, and 10 h, the fluorescence intensities were too low to be observed. To quantitatively evaluate the resistance of SEO films to protein adsorption, we tested the adsorption capacity of these films. They were immersed in BSA solutions to achieve the adsorption equilibrium at room temperature for 24 h. As shown in Figure 5b, all of the nanoporous SEO films showed much lower protein adsorption compared to the PS homopolymer film regardless of the swelling duration. The 8137

DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140

Article

Industrial & Engineering Chemistry Research

Figure 4. XPS C 1s spectra of SEO films after swelling treatment in ethanol at 65 °C for (a) 0 h, (b) 30 min, (c) 1 h, (d) 4 h, and (e) 10 h, respectively.

covering of densely packed PEO blocks on the film surface as we demonstrated above. For the films prepared at longer swelling durations, more PEO chains are enriched on the surface, leading to stronger resistance to protein adsorption. Therefore, less proteins are adsorbed on porous SEO films prepared at longer swelling durations.

selective swelling and subsequent collapse of the PEO microdomains embedded in PS matrix. The pores are interconnected with each other, forming a continuous phase distributed in the PS matrix. The average pore sizes can be flexibly tuned typically in the range ∼20−50 nm by changing the swelling durations and temperatures as they determine the swelling degrees of the copolymer. Interestingly, the hydrophilic PEO chains migrate to the film surface in the swelling process and are fixed on the surface in the subsequent drying. As revealed by AFM imaging, these PEO chains are solvated in water, leading to the morphology transition from nanoporous to nonporous for the swelling-treated SEM films upon exposure to water. With the presence of PEO chains on the film surface,

4. CONCLUSIONS Nanoporous PS-b-PEO films are facilely prepared via the selective swelling-induced pore-making strategy, which simply requires immersion in warm alcohols for hours followed by air drying. Nanopores are nondestructively formed as a result of 8138

DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140

Industrial & Engineering Chemistry Research

■

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-8317 2247. Fax: +86-25-8317 2292. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Basic Research Program of China (2015CB655301), the Natural Science Foundation of Jiangsu Province (BK20150063), and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged.

■

REFERENCES

(1) Rana, D.; Matsuura, T. Surface Modifications for Antifouling Membranes. Chem. Rev. 2010, 110, 2448−2471. (2) Banerjee, I.; Pangule, R. C.; Kane, R. S. Antifouling Coatings: Recent Developments in the Design of Surfaces that Prevent Fouling by Proteins, Bacteria, and Marine Organisms. Adv. Mater. 2011, 23, 690−718. (3) Lee, A.; Elam, J. W.; Darling, S. B. Membrane Materials for Water Purification: Design Development and Application. Environ. Sci.: Water Res. Technol. 2016, 2, 17−42. (4) Zhao, J.; Zhao, X. T.; Jiang, Z. Y.; Li, Z.; Fan, X. C.; Zhu, J. A.; Wu, H.; Su, Y. L.; Yang, D.; Pan, F. S.; Shi, J. F. Biomimetic and Bioinspired Membranes: Preparation and Application. Prog. Polym. Sci. 2014, 39, 1668−1720. (5) Chen, H.; Yuan, L.; Song, W.; Wu, Z. K.; Li, D. Biocompatible Polymer Materials: Role of Protein-Surface Interactions. Prog. Polym. Sci. 2008, 33, 1059−1087. (6) Tiraferri, A.; Kang, Y.; Giannelis, E. P.; Elimelech, M. Superhydrophilic Thin-Film Composite Forward Osmosis Membranes for Organic Fouling Control: Fouling Behavior and Antifouling Mechanisms. Environ. Sci. Technol. 2012, 46, 11135−11144. (7) Chen, S. F.; Li, L. Y.; Zhao, C.; Zheng, J. Surface Hydration: Principles and Applications toward Low-Fouling/Nonfouling Biomaterials. Polymer 2010, 51, 5283−5293. (8) Banerjee, I.; Pangule, R. C.; Kane, R. S. Antifouling Coatings: Recent Developments in the Design of Surfaces That Prevent Fouling by Proteins, Bacteria, and Marine Organisms. Adv. Mater. 2011, 23, 690−718. (9) Lowe, S.; O’Brien-Simpson, N. M.; Connal, L. A. Antibiofouling Polymer Interfaces: Poly(Ethylene Glycol) and Other Promising Candidates. Polym. Chem. 2015, 6, 198−212. (10) Gombotz, W. R.; Guanghui, W.; Horbett, T. A.; Hoffman, A. S. Protein Adsorption to Poly(Ethylene Oxide) Surfaces. J. Biomed. Mater. Res. 1991, 25, 1547−1562. (11) Alconcel, S. N. S.; Baas, A. S.; Maynard, H. D. FDA-Approved Poly(Ethylene Glycol)-Protein Conjugate Drugs. Polym. Chem. 2011, 2, 1442−1448. (12) Amiji, M.; Park, K. Prevention of Protein Adsorption and Platelet-Adhesion on Surfaces by PEO-PPO-PEO Triblock Copolymers. Biomaterials 1992, 13, 682−692. (13) Liu, V. A.; Jastromb, W. E.; Bhatia, S. N. Engineering Protein and Cell Adhesivity Using PEO-Terminated Triblock Polymers. J. Biomed. Mater. Res. 2002, 60, 126−134. (14) Bosker, W. T. E.; Iakovlev, P. A.; Norde, W.; Cohen Stuart, M. A. BSA Adsorption on Bimodal PEO Brushes. J. Colloid Interface Sci. 2005, 286, 496−503. (15) Leung, B. O.; Hitchcock, A. P.; Brash, J. L.; Scholl, A.; Doran, A. An X-ray Spectromicroscopy Study of Protein Adsorption to Polystyrene-Poly(ethylene oxide) Blends. Langmuir 2010, 26, 14759−14765. (16) Sharma, S.; Desai, T. A. Nanostructured Antifouling Poly(Ethylene Glycol) Films for Silicon-Based Microsystems. J. Nanosci. Nanotechnol. 2005, 5, 235−243.

Figure 5. (a) Representative fluorescence microscopy images of PS homopolymer membrane (M1) and SEO membranes with different swelling times (M2−M7 for 0, 30 min, 1 h, 4 h, 8 h, and 10 h, respectively). (b) Corresponding histogram of BSA adsorption on these films. These fluorescent images all have the same magnification, and the scale bar corresponding to 400 μm is given in (M1).

Figure 6. AFM surface topography of SEO film prepared by swelling in ethanol at 65 °C for 4 h in the dried (a) and wet (b) states. The scan size is 1 μm × 1 μm.

the swelling-treated SEO films exhibit excellent resistance to protein adsorption, and better resistance is obtained for films prepared at a longer swelling durations as more PEO chains are enriched on the surfaces. The BSA adsorption of the film produced by swelling in ethanol at 65 °C for 8 h is as low as 133 ng/cm2, which is more than 37 times lower than that of PS homopolymer film without the presence of PEO chains on the surface. Even further, the BSA adsorption on the film produced with a longer swelling duration of 10 h cannot be detected, demonstrating a better fouling resistance than PEO-covering surfaces prepared by other methods. In addition, thus-produced SEO nanoporous films are expected to find interesting applications as membranes for ultrafiltration because their uniform pores and fouling-resistant surfaces will ensure both good selectivity and stabilized permeability. 8139

DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140

Article

Industrial & Engineering Chemistry Research (17) Dong, B. Y.; Jiang, H. Q.; Manolache, S.; Wong, A. C. L.; Denes, F. S. Plasma-Mediated Grafting of Poly(Ethylene Glycol) on Polyamide and Polyester Surfaces and Evaluation of Antifouling Ability of Modified Substrates. Langmuir 2007, 23, 7306−7313. (18) Liu, B. C.; Chen, C.; Li, T.; Crittenden, J.; Chen, Y. S. High Performance Ultrafiltration Membrane Composed of PVDF Blended with Its Derivative Copolymer PVDF-g-PEGMA. J. Membr. Sci. 2013, 445, 66−75. (19) Ju, H.; McCloskey, B. D.; Sagle, A. C.; Kusuma, V. A.; Freeman, B. D. Preparation and Characterization of Crosslinked Poly(Ethylene Glycol) Diacrylate Hydrogels as Fouling-Resistant Membrane Coating Materials. J. Membr. Sci. 2009, 330, 180−188. (20) Zhang, F.; Liu, S. W.; Zhang, Y.; Chi, Z. G.; Xu, J. R.; Wei, Y. A Facile Approach to Surface Modification on Versatile Substrates for Biological Applications. J. Mater. Chem. 2012, 22, 17159−17166. (21) Wang, Y.; He, C. C.; Xing, W. H.; Li, F. B.; Tong, L.; Chen, Z. Q.; Liao, X. Z.; Steinhart, M. Nanoporous Metal Membranes with Bicontinuous Morphology from Recyclable Block-Copolymer Templates. Adv. Mater. 2010, 22, 2068−2072. (22) Wang, Y.; Li, F. B. Pore-Making Strategy: Confined SwellingInduced Pore Generation in Block Copolymer Materials. Adv. Mater. 2011, 23, 2134−2148. (23) Wang, Z.; Yao, X.; Wang, Y. Swelling-Induced Mesoporous Block Copolymer Membranes with Intrinsically Active Surfaces for Size-Selective Separation. J. Mater. Chem. 2012, 22, 20542−20548. (24) Sun, W.; Wang, Z. G.; Yao, X. P.; Guo, L. M.; Chen, X. Q.; Wang, Y. Surface-Active Isoporous Membranes Nondestructively Derived from Perpendicularly Aligned Block Copolymers for SizeSelective Separation. J. Membr. Sci. 2014, 466, 229−237. (25) Guo, L. M.; Wang, Y. Nanoslitting of Phase-Separated Block Copolymers by Solvent Swelling for Membranes with Ultrahigh Flux and Sharp Selectivity. Chem. Commun. 2014, 50, 12022−12025. (26) Darling, S. B. Directing the Self-Assembly of Block Copolymers. Prog. Polym. Sci. 2007, 32, 1152−1204. (27) Abetz, V.; Simon, P. F. W. Phase Behaviour and Morphologies of Block Copolymers. Adv. Polym. Sci. 2005, 189, 125−212. (28) Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics - Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41, 525−557. (29) Hahn, J.; Filiz, V.; Rangou, S.; Clodt, J.; Jung, A.; Buhr, K.; Abetz, C.; Abetz, V. Structure Formation of Integral-Asymmetric Membranes of Polystyrene-Block-Poly(ethylene oxide). J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 281−290. (30) Hahn, J.; Clodt, J. I.; Filiz, V.; Abetz, V. Protein Separation Performance of Self-Assembled Block Copolymer Membranes. RSC Adv. 2014, 4, 10252−10260. (31) Choy, T. C. Effective Medium Theory: Principles and Application; Oxford University Press: New York, 1999. (32) Yin, J.; Yao, X.; Liou, J.-Y.; Sun, W.; Sun, Y.-S.; Wang, Y. Membranes with Highly Ordered Straight Nanopores by Selective Swelling of Fast Perpendicularly Aligned Block Copolymers. ACS Nano 2013, 7, 9961−9974. (33) Wang, Y.; Tong, L.; Steinhart, M. Swelling-Induced Morphology Reconstruction in Block Copolymer Nanorods: Kinetics and Impact of Surface Tension during Solvent Evaporation. ACS Nano 2011, 5, 1928−1938. (34) Yan, N. N.; Wang, Y. Reversible Switch between the Nanoporous and the Nonporous State of Amphiphilic Block Copolymer Films Regulated by Selective Swelling. Soft Matter 2015, 11, 6927−6937. (35) Wang, Z. L.; Xu, J. T.; Du, B. Y.; Fan, Z. Q. Preparation and Characterization of V-Shaped PS-b-PEO Brushes Anchored on Planar Gold Substrate through the Trithiocarbonate Junction Group. J. Colloid Interface Sci. 2012, 384, 29−37. (36) George, P. A.; Donose, B. C.; Cooper-White, J. J. Selfassembling polystyrene-block-poly(ethylene oxide) copolymer surface coatings: Resistance to protein and cell adhesion. Biomaterials 2009, 30, 2449−456.

8140

DOI: 10.1021/acs.iecr.6b02008 Ind. Eng. Chem. Res. 2016, 55, 8133−8140