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Series of Liquid Separation System Made of Homogeneous Copolymer Films with Controlled Surface Wettability Moo Jin Kwak, Myung Seok Oh, Youngmin Yoo, Jae Bem You, Jiyeon Kim, Seung Jung Yu, and Sung Gap Im* Department of Chemical and Biomolecular Engineering and KI for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Exquisite surface wettability control of separation system surface is required to achieve separation of liquids with low surface tension difference. Here, we demonstrate a series of surface-energy-controlled homogeneous copolymer films to control the surface wettability of polyester fabric, utilizing a vapor-phase process, termed as initiated chemical vapor deposition (iCVD). The homogeneous copolymer films consist of a hydrophobic polymer, poly(2,4,6,8-tetramethyl2,4,6,8-tetravinylcyclotetrasiloxane), pV4D4, and a hydrophilic polymer, poly(4-vinylpyridine), p4VP. Because the mixing of two or more components is always favorable in vapor phase, the iCVD process allows the formation of homogeneous copolymers from two immiscible, hydrophilic/hydrophobic monomer pairs, which is highly challenging to achieve in liquid phase. Simply by tuning the flow rate ratio of monomer pairs, a series of homogeneous copolymers with systematically controlled surface energy were formed successfully. The fabricated separation system could separate water (surface energy = 72.8 mJ/m2), glycerol (64 mJ/m2), ethylene glycol (48 mJ/m2), and olive oil (35.1 mJ/m2) sequentially with excellent selectivity, just by choosing a copolymer-coated polyester fabric with proper surface energy. Considering the small differences in the surface tension of the liquids used in this work, the surface-energy-controlled separation system can be a powerful tool to separate various kinds of liquid mixtures.

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INTRODUCTION

surface of separation system with exquisite surface wettability control. Here, we demonstrate a series of homogeneous copolymer films with controlled surface wettability deposited on polyester fabric for the application to separation system for liquid mixture. The surface energy of the fabric could be tuned systematically by depositing a series of homogeneous copolymers with various compositions on the surface via a vapor-phase method, termed as initiated chemical vapor deposition (iCVD). The solvent-free nature and the mild process condition of the iCVD enables the deposition of polymer films onto various vulnerable substrates such as paper, fabric, and membranes without damaging them.13 The complicated surface topography of the substrates can be maintained due to the conformal nature of the iCVD polymer films.14 Most importantly, because the mixing of two or more components in vapor phase is always thermodynamically favorable,15,16 even monomer pairs with extremely different surface energies can also be blended homogeneously in vapor phase inside the iCVD chamber, which permits the formation of homogeneous copolymers with any desired functional

An efficient liquid separation system has been investigated extensively due to its great importance in various fields such as wastewater treatment, oil−water separation, and others.1−4 Generally, liquid separation system harnesses the difference of surface tension of each liquid component in the mixture.5 Therefore, it is easier to separate a liquid component from others with larger surface tension, and thereby, most of the liquid separation systems developed so far have been applied to the liquid mixture with larger difference of surface tension (>42.8 mJ/m2).4,6−8 In such cases, the surface of the separation system was controlled in a way that it possesses either superhydrophilicity and oleophobicity or superhydrophobicity and oleophilicity. For example, a separation system with superhydrophilic and underwater superoleophobic surface was fabricated using nanocomposite structure by TiO2 particle coating followed by UV-light irradiation.6,9 Also, surface with superhydrophobic/oleophilic property was made from carbonbased three-dimensional (3D) network,10 hydrophobic polymer coating,11 and GO(graphene oxide) coated nanostructure.12 However, only a few reports can be found to achieve an efficient method for separating liquids with similar surface tension. To separate liquid mixture with close surface tension values, it is required to secure the capability of engineering the © 2015 American Chemical Society

Received: March 5, 2015 Revised: April 20, 2015 Published: April 20, 2015 3441

DOI: 10.1021/acs.chemmater.5b00842 Chem. Mater. 2015, 27, 3441−3449

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Chemistry of Materials groups, regardless of the miscibility of the monomers.17,18 Furthermore, by simply controlling the flow rate of each vaporized monomer in the iCVD system, the surface composition and, thus, the surface energy of homogeneous copolymer films could be controlled systematically. Recently, Gleason and co-workers showed a set of homogeneous copolymer films consisting of hydrophilic monomer, hydroxyethyl methacrylate (HEMA), and hydrophobic monomer, perfluorodecyl acrylate (PFDA), using the iCVD process and its application to antifouling surfaces.19,20 In this work, we synthesized a series of copolymer films made from the blending of a hydrophobic monomer, 2,4,6,8tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4), and a hydrophilic monomer, 4-vinylpyridine (4VP), via iCVD process to generate a series of copolymer films with controlled surface wettability. The water contact angle (CA) of the iCVD copolymer of p(V4D4-co-4VP) was measured to characterize the surface wettability. The surface energy of the p(V4D4-co4VP) copolymer films was also calculated using Van Oss− Chaudhury−Good (OCG) equation. The interfacial energy of the corresponding surfaces with applied liquids with various surface tensions was also obtained from the interfacial energy equation.21 The analysis confirmed that the copolymer films with controlled chemical compositions were successfully deposited via iCVD process. Also, the wetting behavior of liquids on the surface-energy-controlled polyester fabric was well consistent with the calculated interfacial energy values. The copolymer films with designed surface energies were conformally deposited onto polyester fabric, which could separate liquid mixture, driven by the difference in interfacial energy between the fabric surface and the given liquid. The surface-energy-controlled separation system could efficiently separate liquid mixture even with small surface tension differences of liquids. Four liquids with representative surface tension values were selected in this work; water (surface energy = 72.8 mJ/m2), glycerol (64 mJ/m2), ethylene glycol (EG, 48 mJ/m2), and olive oil (35.1 mJ/m2). By applying the polyester fabric with proper surface energy, all four liquid components can be separated sequentially from the mixture, demonstrating the unique advantages of the developed separation system using the composition-controlled copolymer series, which is enabled by the iCVD process.

respectively. In the case of the deposition of p(V4D4-co-4VP), the flow rates of V4D4 and TBPO were set to 0.53 and 0.67 sccm, and the flow rate of 4VP was varied from 6.3 to 8.99 sccm, respectively, to obtain various copolymers with different surface compositions. In all cases, the filament temperature was kept at 180 °C. The reactor pressure and substrate temperature of three kinds of copolymers were kept at 220 mTorr and 33 °C. To deposit pV4D4, the reactor pressure and substrate temperature were kept at 200 mTorr and 38 °C. For the deposition of p4VP, the reactor pressure and substrate temperature were kept at 400 mTorr and 25 °C. All the process parameters of the series of copolymer films synthesized in this work was summarized in Table 1. The target thickness of the iCVD copolymer and homopolymer films were set to 200 nm and monitored in situ by a He−Ne laser (JDS Uniphase) interferometer. Table 1. Summary of the Icvd Process Conditions for the Deposition of pV4D4, p4VP, and p(V4D4-co-4VP) with Three Different Compositions Deposition condition Monomer and initiator flow rate

Controlled polymeric surface via iCVD

V4D4 (sccm)

4VP (sccm)

TBPO (sccm)

Pressure (mtorr)

Substrate Temperature (°C)

p4VP Copolymer 1 Copolymer 2 Copolymer 3 pV4D4

0.53 0.53 0.53 0.53

2.38 8.99 7.54 6.30 -

0.60 0.67 0.67 0.67 0.45

400 220 220 220 200

25 33 33 33 38

Analysis of the Film Characteristics. Fourier transforminfrared (FT-IR) spectra of the pV4D4, p4VP, and p(V4D4-co4VP) with three different compositions were obtained using ALPHA FT-IR in absorbance mode (Bruker Optics). A total of 64 scans were collected and averaged for each spectrum. The Xray photoelectron spectroscopy (XPS) results for the pV4D4, and p(V4D4-co-4VP) with three different compositions were obtained using Sigma Probe Multipurpose XPS (Thermo VG Scientic) with a monochoromatized Al Kα source. Atomic force microscope (AFM) images of the p4VP, p(V4D4-co-4VP) with three different compositions, and pV4D4 were also taken using a scanning probe microscope (SPM) (XE-100, Park system) at a scan size of 10 × 10 μm. The water, EG, and diiodomethane (DIM) contact angles of the polymer-coated Si wafers were measured using Contact Angle Analyzer (Phoenix 150, SEO, Inc.). Scanning electron microscopy (SEM) (Nova 230, FEI) was used to monitor the surface image of the polyester fabric. Surface free energy and interfacial energy of the four samples were calculated using the Van Oss−Chaudhury−Good (OCG) and interfacial energy equations, respectively. In Supporting Information Figure S3, an unpaired student’s t test was performed using statistical analyses with GraphPad Prism 6 software (GraphPad Software Inc., La Jolla, CA, U.S.A.) for comparing the surface energy of copolymers. A p < 0.001 or 0.0001 meant statistically significant. Chemical Stability Test and the Liquid Separation. The iCVD copolymer films with different compositions coated on Si wafer substrates were soaked in deionized (DI) water, glycerol (99%, Junsei), EG (99.8%, Aldrich), and olive oil for 5, 10, and 15 h. After soaking the samples for a designated time, the iCVD polymer films were rinsed thoroughly with DI water

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EXPERIMNETAL SECTION Deposition of the Polymeric Thin Film via iCVD Process. Poly(2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclo-tetrasiloxane) (pV4D4), poly(4-vinylpyridine) (p4VP), and poly(2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetra-siloxane-co-4-vinylpyridine) (p(V4D4-co-4VP)) layers were deposited using iCVD process onto various substrates such as slide glass, Si wafer, and polyester fabric. Two monomers, V4D4 (98%, Wandachem, China) and 4VP (95%, Aldrich, U.S.A.), and the initiator, tert-butyl peroxide (TBPO, 98%, Aldrich, U.S.A.) were used for polymerization in the iCVD process. All the chemicals in this work were used as purchased without any further purification. The vaporized monomers and initiator were introduced into an iCVD reactor (Daeki Hi-Tech co, Ltd.). To obtain the desired flow rates, TBPO and 4VP were kept at room temperature and V4D4 was heated to 60 °C. In the case of the deposition of pV4D4, the flow rates of V4D4 and TBPO were set to 0.53 and 0.45 standard cubic centimeters per minute (sccm), respectively. For the deposition of p4VP, the flow rates of 4VP and TBPO were set to 2.38 and 0.6 sccm, 3442


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Figure 1. (a) Schematic of the iCVD procedure for the deposition of copolymer films from monomers, V4D4 and 4VP and initiator, TBPO. (b) Illustration of synthetic scheme of p(V4D4-co-4VP).

series of copolymer films with various composition of V4D4/ 4VP, thus various surface free energies could also be obtained. In the case of the homopolymer, pV4D4 or p4VP, same iCVD procedure was performed only with single monomer component input of either V4D4 or 4VP into the iCVD system. Table 1 summarized the iCVD deposition conditions for copolymers with three different compositions along with the pV4D4 and p4VP homopolymers. Each process parameter such as monomer and initiator flow rate, chamber pressure, and substrates temperature substantially affects the iCVD polymerization, thus the properties of final copolymer films.22 For the copolymerization via iCVD, the difference in the vapor pressure of each monomer plays a critical role in determining the composition of the copolymer film. In the iCVD process, the polymerization rate is directly affected by the concentration of adsorbed monomers on target substrate,23 and monomers with lower vapor pressure−less volatile one−tend to be adsorbed more than those with higher vapor pressure at the same process temperature. Therefore, the ratio of input flow rate of each vaporized monomers will be different from the composition of the final iCVD-synthesized copolymer. The vapor pressures of V4D4 and 4VP are 40 mTorr and 1680 mTorr at 25 °C, respectively,24 indicating that the concentration of the amount of the adsorbed V4D4 is definitely much higher than that of 4VP at the same substrate temperature. Therefore, it is possible to control the composition of the final copolymer film by tuning the flow rate of 4VP with fixed V4D4 flow rate, as shown in Table 1. The difference in the vapor pressures between V4D4 and 4VP also affects the operating pressure in iCVD. At the pressure higher than 220 mTorr, adsorption rate of V4D4 is too high and a condensation of V4D4 vapor was observed, resulting in a thin, liquid-phase V4D4 film on the substrate, which is critically detrimental to form uniform polymer film. However, pressures lower than 200 mTorr, 4VP was too volatile and it could not be adsorbed sufficiently on the target substrate. In consideration of this difference, the iCVD for copolymer films was operated at the process pressure of 220 mTorr during the deposition, which is slightly higher than the system pressure

and purged with N2 gas. The water contact angles (CA) of the coated Si wafers before and after the chemical stability test were measured. Separation of liquid mixture was carried out using a modified filter apparatus equipped with iCVD-polymer-coated polyester fabric (filtration area is 2.5 cm2). Two kinds of liquid mixture pairs were applied for each separation: water/glycerol and glycerol/EG mixture. Before the liquid separation, water and EG were dyed with Janus green B (Aldrich) and orange water paint, respectively, for better visualization. Then, 20 mL of the solvent mixture was poured into the coated polyester fabric, and the separation of liquid mixture driven by gravity was monitored. Polyester fabric coated with iCVD copolymers of different compositions were used for each separation procedure.

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RESULTS AND DISCUSSION In this study, a series of homogeneous copolymer thin films made of a hydrophobic polymer, pV4D4, and a hydrophilic polymer, p4VP were synthesized using iCVD process (Figure 1a). Vaporized reactants of initiator, TBPO (orange diamond in Figure 1a), and two monomers, V4D4 (blue square) and 4VP (green square), are all introduced into the iCVD chamber to form a copolymer thin film of p(V4D4-co-4VP). Figure 1b shows the chemical structure of the formed copolymer from V4D4 and 4VP. Due to the considerable difference in surface tension between two monomers, the V4D4 and 4VP monomers are not miscible to each other, only to form an emulsion in liquid phase as shown in Figure S1 in Supporting Information. However, since the mixing of two or more components is thermodynamically always favorable in vapor phase, the V4D4 and 4VP monomers are readily miscible in vapor phase where the effect of surface tension is negligible.15 Therefore, a homogeneous copolymer film could be formed without phase segregation in this vapor-phase polymerization method, even though the monomer mixture in liquid phase is immiscible.15,16 Moreover, by controlling the relative input amount of V4D4 and 4VP vapors into the iCVD chamber, a 3443


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Figure 2. Chemical composition analysis of various kinds of homopolymer and copolymer films by iCVD process: (a) FT-IR spectra of (1) p4VP, (2) copolymer 1, (3) copolymer 2, (4) copolymer 3 and (5) pV4D4, (b) XPS high-resolution scan N 1s (left) and Si 2p (right), and (c) calculated surface chemical composition from the XPS analysis.

Figure 3. AFM images of iCVD pV4D4, p4VP, and copolymer 1, 2, 3 films on Si wafer; (a) p4VP, (b) copolymer 1, (c) copolymer 2, (d) copolymer 3, (e) pV4D4.

asymmetric Si−O−Si stretching peaks related to the cyclic siloxane rings in a network configuration, and the strong SiCH3 symmetric bending peak at 1260 cm−1.26 By tuning the flow rate of 4VP, the composition of the copolymers could be controlled. As shown in the FT-IR spectra, the intensity of the characteristic peaks representing p4VP decreased gradually, which is highly correlated with the decrease of p4VP flow rate in the iCVD copolymers 1−3, marked with black/red dash line in Figure 2a. The observation clearly demonstrates that the concentration of adsorbed 4VP on the target substrate changed systemically by tuning the flow rate of 4VP with the fixed flow rate of V4D4, which ultimately led to the change of the fraction of 4VP in the copolymer film.

used to deposit pV4D4 homopolymer. Analogously, the temperature of the target substrate was kept at 33 °C, which is in between the temperatures used to deposit p4VP and pV4D4 homopolymers; At the substrate temperature of 25 °C, condensation of V4D4 vapor was observed, while at 38 °C almost no adsorption of 4VP monomers occurred. Figure 2 shows the FT-IR and XPS spectra of various polymer and copolymer thin films by iCVD process. In the FTIR spectra (Figure 2a), peaks at 1596 and 1415 cm−1, marked with red dash line, indicate the vibration of pyridine ring in p4VP. Also, peaks at 800−802 cm−1 correspond to the methyl group attached to pyridine ring.25 In case of pV4D4, two representative pV4D4-characteristic peaks are shown by black dash lines in Figure 2a: the peak at 1075 cm−1 representing the 3444


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Chemistry of Materials XPS analysis of elemental scan was also performed to monitor the change in the composition of copolymers. Since each V4D4 and 4VP monomer contains a characteristic element of Si for V4D4 and N for 4VP, the XPS high resolution scans of N 1s and Si 2p can show the change of the surface composition of the series of iCVD copolymers. The XPS peak intensity of N 1s gradually increased from pV4D4 to p4VP while that of Si 2p gradually decreased (Figure 2b). The XPS result clearly indicates that decreasing the vapor flow of 4VP monomer led to the decrease in the fraction of 4VP in the copolymer film, which is shown as the decrease of relative intensity of N 1s and increase in that of Si 2p in XPS spectra. The XPS result is fully consistent with the observation in the FT-IR analysis. Additionally, the surface composition of each copolymer was estimated using the atomic fraction data from the quantitative XPS survey scan,27 with the consideration that the copolymer contains characteristic elements of four Si atoms in V4D4, and one N atom in 4VP repeating unit in the copolymer, respectively; The calculated p4VP fraction at the surface of the copolymer 1−3 were 69.2%, 63.7%, and 44.3%, respectively (Figure 2c). These XPS and FT-IR results confirms that it is possible to make copolymer thin films with various controlled compositions by simply tuning the process parameters in iCVD process. The surface morphologies of the 200 nm-thick p4VP, pV4D4, and the copolymers were observed via AFM, as shown in Figure 3. The AFM images of homopolymers, pV4D4 and p4VP showed a highly smooth surface morphology with root-mean-square (RMS) roughness (Rq) of 0.9 and 0.61 nm for pV4D4 and p4VP, respectively. In cases of copolymer films, the surface became slightly rough, but Rq of each copolymer film was no larger than 1.5 nm, indicating that the surface was still extremely smooth. Moreover, the AFM image did not show any apparent grain-like morphology or phase segregation, strongly inferring that the copolymer is homogeneous without any noticeable phase segregation between V4D4 and 4VP. The AFM images show the ability of iCVD process to deposit homogeneous copolymer films even from immiscible monomer pairs, which had been difficult in conventional liquid phase synthesis methods. Figure 4a shows changes of contact angles of iCVD pV4D4, p4VP, and copolymer films with DI water (black line), ethylene glycol (EG) (red line) and diiodomethane (DIM) (blue line). In case of p4VP, the EG contact angle could not be measured since p4VP film was soluble in EG solvent. Except for this case, no solvent-related damage was observed, because of the highly cross-linked pV4D4 fraction in the copolymer films of p(V4D4co-4VP). As can be expected, pV4D4 was hydrophobic with the highest water contact angle of 97.4°. p4VP was hydrophilic (water contact angle of 43.1°). Regardless of liquids used, the static contact angles increased steadily with the increase of hydrophobic V4D4 fraction. For instance, copolymer 1, with 69.2% of p4VP, has a water contact angle of 69.5°, EG contact angle of 42.5°, and DIM contact angle of 24.7°, while pV4D4 homopolymer has water contact angle of 97.4°, EG contact angle of 73.2°, and DIM contact angle of 49.3°. The decrease of hydrophilic p4VP fraction in the copolymer film means that surface of copolymer film becomes more hydrophobic. Figure 4b shows the optical microscope images of various water droplets on polymer and copolymer thin films synthesized by iCVD process. The contact angle analysis demonstrates clearly that the controllability of the composition of the homogeneous

Figure 4. (a) Contact angle values of iCVD pV4D4, p4VP, and copolymer films on Si wafer with three kinds of solvents with different surface tension, water (black), EG (red), and DIM (blue). Note that p4VP is soluble to EG and no contact angle data was shown in the graph. (b) Optical microscope images of water droplets on the five kinds of iCVD pV4D4, p4VP, and copolymer films on Si wafer.

copolymer films made it possible to tune the surface wettability of the copolymer surfaces to various solvents with different surface tensions. The capability of the iCVD method to form a homogeneous copolymer films from monomer pairs with extreme difference can further widen the range of the controllable wettability of the copolymers. Surface free energy and the interfacial energy information were extracted based on the contact angle measurement data. Surface energy, work of adhesion, and interfacial energy can be determined by measuring the contact angles of liquids with different surface tensions.21 The surface energy is determined as the sum of all polar and apolar intermolecular forces on the surface of a material. Generally, the surface energy consist of polar and apolar components;28,29

γ = γ LW + 2 γ +γ −

(1)

, where γ, γLW, γ+, and γ− represent the total surface energy, apolar (Lifshitz-van der Waals) component, polar electronacceptor (Lewis acid) component, and the electron-donor (Lewis base) component of surface energy, respectively. The surface energy components of solid polymers and copolymers with various compositions could be calculated from the contact angle values using the Van Oss−Chaudhury−Good (OCG) equation, γL(1 + cos θ ) = 2 γS LW γL LW + 2 γS +γL− + 2 γS−γL+ (2)

, where the subscript L and S correspond to solid and liquid, respectively and θ is the contact angle between the solid and the liquid.29 The OCG equation illustrates the relationship between the contact angle and the surface free energy of solid and liquid, including the whole apolar and polar parts. From the measured contact angle values and the known surface energy information on the liquids (γL, γLLW, γL+ and γL−) used in Figure 4, all the necessary surface free energy information on solid, γs, γsLW, γs+ and γs− can be obtained by use of eq 1 and 2. For the calculation of the iCVD polymer set in this work, the contact 3445


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Table 2. Summary of the Calculated Surface Free Energies and the Interfacial Energies of the Polymeric Surfaces via iCVD Interfacial energy γSL (mJ/m2)

Surface free energy components of various copolymers (mJ/m2) Controlled polymeric surface via iCVD

γs

p4VP Copolymer 1 Copolymer 2 Copolymer 3 pV4D4

61.61 45.79 43.02 39.26 36.56

γs

LW

46.15 45.15 42.61 39.26 36.56

γs

γs

Pa

15.46 0.64 0.41 0 0

+

N/Ab 0.0085 0.0057 0c 0c

γs

−

N/Ab 12.38 7.50 4.90 0.95

Water

Glycerol

EG

Olive oil

14.81 19.40 26.46 31.19 43.08

3.31 16.20 18.92 21.42 26.0

2.16 10.43 12.10 13.64 16.75

16.2 1.3 0.8 0.1 0

The total polar surface energy (γsP) is related to each polar term of the surface energy, γs+ and γs−, using the equation, γpS = 2(γ+S γ−S )1/2. bThe values could not be calculated because p4VP was soluble in EG. cNegative values is often considered to be zero21 a

interfacial energy. Therefore, decreasing the p4VP fraction in the copolymer, meaning the decrease of polar part values, resulted in a gradual increase in the interfacial energy between polymer film and the polar solvents. On the other hand, interfacial energy of olive oil, a nonpolar liquid on the iCVD polymer films showed the opposite tendency. In the case of using nonpolar liquid, where all γL+ and γL− are zero, the polar part of eq 4 does not contribute to the total interfacial energy, leading to a steady decrease of interfacial energy from copolymer 1 to pV4D4. Generally, the low interfacial energy means the better attraction between two phases, solid and liquid.33 The quantitative analysis of the interfacial energy indicates that the systematically controlled surface free energy of the iCVD polymer films greatly affected the interaction between solid and liquid phases, thus the wettability of the iCVD polymer films various liquids. In Figure 5, the difference of wetting behavior of various kinds of liquids, water (surface tension of 72.8 mJ/m2), glycerol

angles of three different liquids, water, DIM and EG were obtained as shown in Figure 4a. The surface free energy information on solvents available from literature is summarized in Table S1 in Supporting Information.30 The calculated surface free energies of iCVD copolymers 1−3 and pV4D4 are summarized in Table 2. Interfacial energy is defined as the free energy at the interface between two phases, resulting from the attraction differences in each phase. Interfacial energies of the iCVD polymers and copolymers with various types of liquids with different surface tensions can also be calculated using well-known Young’s equation coupled with the information in Table 2.21 Young’s equation represents the relationship between surface free energy of solid, liquid, two phases and contact angle,

γS = γSL + γLcos θ

(3)

, where γSL indicates the interfacial energy between solid and liquid. From eq 2 and 3, γSL is given by γSL = γS + γL − 2 γS LW γL LW − 2 γS +γL− − 2 γS−γL+ (4)

The calculated γSL values were also shown in Table 2. In Table 2, left side of the table demonstrates the surface free energy information on the iCVD copolymer 1 to 3 and pV4D4. In the case of p4VP, the contact angle values from only two kinds of liquid, water and DIM were available, since p4VP was soluble in EG. Therefore, the surface free energy of p4VP film was calculated using the contact information from water and DIM, with the modified OCG equation.31 The detailed procedure to calculate the surface energy of the solid surface with the modified OCG equation is described in Supporting Information. The calculated total surface free energy (γs) of the p4VP film was 61.61 mJ/m2, the highest among all the tested iCVD polymer films, meaning that p4VP film is the most hydrophilic film. The decrease of the composition of hydrophilic p4VP fraction from 100% to 0% led to a gradual decrease in all surface free energy components. Previous report indicated that the more hydrophilic surface tend to have the higher surface free energy,32 which is fully consistent with our observation. The right side in Table 2 presents the interfacial energy data of the iCVD p4VP, copolymer 1 to 3 and pV4D4 surfaces with different 4VP fractions contacting with four kinds of liquids, water, glycerol, EG, and olive oil. In case of p4VP, the interfacial energy between p4VP and each liquid was calculated using the modified interfacial energy equation. The detailed procedure of calculation is also described in Supporting Information. As mentioned above, the interfacial energy equation (eq 4) includes apolar and polar parts of surface free energy. In cases of polar solvents such as water, glycerol, and EG, the polar part of eq 4 play a significant role in the total

Figure 5. Droplets of water (dyed blue), glycerol, EG (dyed orange) and olive oil on bare and iCVD-polymer-coated polyester fabrics with different surface energy.

(64 mJ/m2), EG (48 mJ/m2) and olive oil (35.1 mJ/m2) with different surface tension and interfacial energy on iCVD copolymer 1−3 and pV4D4 films conformally coated porous polyester fabric surface was clearly demonstrated. As mentioned above, one of the advantages of the iCVD process is the preservation of the original porous microstructure of fabric. The SEM image indicated that the surface morphology of the 3446


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an excellent long-term solvent stability; the contact angle variation was less than 3° and no apparent dissolution or delamination of the iCVD polymer film from the polyester fabric was observed even after the 15-h incubation in water, glycerol, EG, and olive oil. To check the capability of selective filtration of the developed separation system, the polyester fabric coated with each iCVD polymer film was placed at the connection part in the device shown in Figure 6. Two sets of liquid mixture, glycerol/water

iCVD polymer-coated polyester fabric was practically identical to its original structure (Figure S2 in Supporting Information). The bare polyester fabric was hydrophilic enough for all liquids used in this work to wet readily the bare polyester fabric. In case of the same polyester fabric coated with copolymer 1 (p4VP fraction of 69.3%), only water (γSL = 19.40 mJ/m2) was nonwettable on coated surface and other solvents, glycerol (γSL = 16.20 mJ/m2), EG (γSL = 10.43 mJ/m2), and olive oil (γSL = 1.3 mJ/m2) could completely wet on the polyester fabric. In the case of more hydrophobic copolymer 3 (p4VP content of 44.3%) coated polyester fabric, glycerol (γSL = 21.42 mJ/m2) remained unwet on the polyester fabric but EG (γSL = 13.64 mJ/m2) was still wettable to the fabric. For a polyester fabric coated with pV4D4 (no p4VP content), the most hydrophobic surface among the polymer films in this work, droplets of water (γSL = 43.08 mJ/m2), glycerol (γSL = 26.0 mJ/m2), and EG (γSL = 16.75 mJ/m2) were nonwettable on coated polyester fabric surface. Only olive oil (γSL = 0 mJ/m2) with the lowest surface tension could permeate the fabric. It is well-known that the hydrophilicity of a surface is highly dependent upon the surface morphology as well as the surface chemical composition. Generally, roughness of the surface tends to amplify the hydrophilic and hydrophobic nature of the surface, especially in Wenzel’s condition.34 In other words, a moderately hydrophilic liquid forming contact angle greater than 0° on smooth surface of a solid material can penetrate through a porous surface of same solid material, due to the substantial difference in surface morphology.35 Therefore, the wettability difference of a liquid on porous surface might be far larger than that on smooth surface. However, as far as a fixed surface morphology was applied to monitor the wettability of liquid on the polymer film to make the contribution of morphology-dependent surface free energy all the same, the calculated interfacial energy of liquids on polymer films deposited on smooth surface can also be used as a criteria to compare the wetting behavior of liquids on the polymer films on rough, porous surfaces. For the liquids used in this work, the contact angle values determining if the liquid is wettable to the polyester fabric coated with iCVD polymer films is in the range of 60−65° on smooth Si wafer, which is fully consistent with the previous observations.34,36 According to the observation, we also found that liquids with the interfacial energy below 16.20 mJ/m2 on smooth surface completely permeated through the porous polyester fabric. In contrast, solvents with higher interfacial energy than 16.75 mJ/m2 on smooth surface remained nonwettable. Coupled with the superior conformal coverage of the iCVD process, the wetting behavior of various kinds of liquids could be controlled systemically by utilizing the advantageous characteristics of the iCVD process, by applying the homogeneous copolymer films onto porous polyester fabric. The polyester fabrics coated with surface-energy-controlled iCVD polymer films were applied to selective separation of various kinds of liquid mixtures with different surface tensions. The tunable wettability of the polyester fabrics to various liquids enabled the selective permeation of a liquid from the mixture. To achieve a reliable performance of the liquid separation, the developed separation system must retain an excellent chemical stability against various kinds of liquids. The long-term stability of the iCVD polymer films against various liquids was monitored by incubating the polymer films in liquids for designated time interval (Figure S4 in Supporting Information). The iCVD pV4D4 and copolymer 1−3 showed

Figure 6. Photographs of a modified filter apparatus equipped with polyester fabrics coated with iCVD copolymer films. (a) Permeation behavior of water/glycerol mixture on the polyester fabric: pV4D4 (left) and coated with copolymer 1 (middle) and bare (right). Water was dyed in blue for better visualization. (b) Permeation behavior of glycerol/EG on the polyester fabric coated with iCVD polymers: pV4D4 (left), copolymer 3 (middle), and copolymer 1 (right). EG was dyed in orange for better visualization.

(dyed blue) pair, and glycerol/EG (dyed orange) pair were prepared. Using the surface-energy-controlled polyester fabrics, selective solvent filtration was successfully demonstrated. Figure 6a shows still-shot photograph images of the selective solvents filtration of water/glycerol mixture before mixing using the developed separation system. As expected, the polyester fabric coated with copolymer 1 could pass the glycerol selectively without leaking water. On the other hand, pV4D4 coated polyester fabric blocked the permeation of both liquids. In the case of bare fabric, both liquids permeated through. Analogously, Figure 6b shows another example of selective liquid filtration of glycerol/EG before mixing. In this case, the polyester fabric coated with copolymer 3 could selectively filter EG, the liquid with lower surface tension, without passing glycerol, the liquid with higher surface tension. The observation clearly demonstrated that the selective liquid filtration was successfully achieved by harnessing the difference of the surface tension of liquid. The optimal surface composition of polyester fabric can be designed and fabricated with respect to each mixture of liquid pairs. The iCVD process is capable of controlling a wide range of surface energy with excellent sensitivity by introducing a series of surface-energy-tuned homogeneous copolymers. Therefore, we anticipate this simple, one-step process is highly versatile to extend the use of the developed separation system for further industrial applications. 3447


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CONCLUSION In summary, a series of homogeneous copolymer system were fabricated from hydrophobic V4D4 and hydrophilic 4VP components. The V4D4 and 4VP mixture was immiscible to each other and the synthesis of homogeneous copolymer in conventional liquid phase methods is highly challenging. Here, a vapor-phase method, iCVD was used to form homogeneous copolymer films and various kinds of copolymer films with a wide range of surface compositions were demonstrated. Because the copolymer films were composed of hydrophobic and hydrophilic monomers, the surface energy of each copolymer film can be systematically controlled by tuning the input ratio of the monomers. FT-IR spectroscopy and XPS analysis clearly demonstrated that the chemical composition of the copolymer films were controllable. The AFM image analysis indicated that the polymer film was fairly homogeneous without any apparent phase segregation at its surface. The contact angle measurement and its quantitative analysis using OCG equation and interfacial energy equation were performed to show that the variation of the surface energy was also fully consistent with the corresponding surface composition. The developed copolymer series was applied onto a polyester fabric to control the wettability of various kinds of common liquids with their own surface tensions. Due to the superior conformal nature of the iCVD process, the copolymer films with controlled surface energy were successfully incorporated onto the polyester fabric without altering its porous microstructure. The polyester fabric coated with the surface-energy-controlled iCVD copolymer films could filter a component of liquid selectively from the mixture. The experimental results clearly illustrate the possibility of synthesizing the copolymer films with desired chemical composition for the application to liquid separation. The highly cross-linked V4D4 component made the developed copolymer films chemically robust against various kinds of organic solvents that will be used commonly for the filtration purpose. Therefore, this surface modification process with various advantages mentioned above can be importantly adapted to many industrial applications.

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0031350) and by Graphene Materials and Components Development Program of MOTIE/KEIT (10044412, Development of basic and applied technologies for OLEDs with graphene).

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ASSOCIATED CONTENT

S Supporting Information *

Photograph of V4D4, 4VP monomer, and mixture. Surface components of liquids and SEM images of bare and copolymer coated polyester fabric. Statistical analysis for surface energy of copolymers and pV4D4. Chemical stability test and calculation procedure of p4VP surface energy and interfacial energy. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00842.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by part by the Advanced Biomass R&D Center (ABC) of Global Frontier Project funded by the Ministry of Science, ICT and Future Planning (ABC-20113448


Article

Chemistry of Materials (36) Choi, W.; Tuteja, A.; Chhatre, S.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H. Adv. Mater. 2009, 21, 2190.

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