Polymer Membranes with Vertically Oriented Pores Constructed by 2D

May 17, 2016 - Polymer membranes with well-controlled and vertically oriented pores are of great importance in the applications for water treatment an...
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Polymer Membranes with Vertically Oriented Pores Constructed by 2D Freezing at Ambient Temperature Hong-Qing Liang,† Ke-Jia Ji,‡ Li-Yun Zha,‡ Wen-Bing Hu,*,‡ Yang Ou,† and Zhi-Kang Xu*,† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Key Laboratory of High Performance Polymer Materials and Technology, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: Polymer membranes with well-controlled and vertically oriented pores are of great importance in the applications for water treatment and tissue engineering. On the basis of twodimensional solvent freezing, we report environmentally friendly facile fabrication of such membranes from a broad spectrum of polymer resources including poly(vinylidene fluoride), poly(L-lactic acid), polyacrylonitrile, polystyrene, polysulfone and polypropylene. Dimethyl sulfone, diphenyl sulfone, and arachidic acid are selected as green solvents crystallized in the polymer matrices under two-dimensional temperature gradients induced by water at ambient temperature. Parallel Monte Carlo simulations of the lattice polymers demonstrate that the directional process is feasible for each polymer holding suitable interaction with a corresponding solvent. As a typical example of this approach, poly(vinylidene fluoride) membranes exhibit excellent tensile strength, high optical transparence, and outstanding separation performance for the mixtures of yeasts and lactobacilli. KEYWORDS: porous membrane, vertical pores, directional freezing, temperature gradient, Monte Carlo simulation

1. INTRODUCTION Porous materials with oriented pores have exerted a tremendous fascination on academia and industries because of their wide applications in precise separation,1 organic electronics,2 microfluidics,3 and tissue engineering.4 Materials with pores vertically oriented to the surface are much more outstanding, because these materials are greatly potential as advanced separation membranes. Traditional membranes always possess tortuous porous structures and broad pore distributions which greatly limit the enhancement of transportation/permeation and the improvement of selectivity.5,6 In contrast, membranes with vertically oriented pores can perfectly solve the problems and provide an opportunity for breaking the trade-off between transportation/permeation and selectivity. Up to now, the most commonly used techniques for fabricating such membranes include track etching,7 anodization,8 lithographic microfabrication,9 self-assembly,10 and microphase separation of block copolymers.11,12 Although the track etched polycarbonate and anodized aluminum oxide membranes are commercially available, the former still suffers from low pore density (∼15%) and the latter is difficult in large-scale preparation. Self-assembly processes offer a promising alternative to fabricate vertically oriented pores with different sizes and morphologies,13,14 but the disadvantages should also be concerned, such as the required careful design of the chemistry and the difficulty in alignment of the self-assembled domains. Therefore, it remains as a big challenge to develop more © 2016 American Chemical Society

suitable methods not only facile and efficient but also adaptable enough for numerous polymers to prepare these membranes with vertical pores. Recently, directional freezing has been explored as a unique route to fabricate novel porous materials,15−19 which may overcome the existing limitations of those conventional processes. In this approach, a polymer solution is first directionally frozen to induce the oriented crystallization of solvent. Then the oriented solvent crystals act as templates to form shaped pores in the polymer matrix. Water is a commonly used solvent for the directional freezing, as its ice crystals are cheap and environment-friendly templates for water-soluble materials. Zhang et al. slowly lowered poly(vinyl alcohol) aqueous solutions into liquid nitrogen to fabricate oriented scaffolds.15 Wu et al. prepared poly(ethylene glycol) cryogels with oriented porous structures by the directional freezing and subsequently cryopolymerization.20 Vickery et al. fabricated graphene−polymer nanocomposites with highly ordered 3D architectures via the directional freezing of aqueous dispersions.21 However, ice crystals are only applicable as templates for water-soluble polymers or particles that can be homogeneously dispersed in water, while almost all separation membranes are prepared from water-insoluble polymers. Received: March 11, 2016 Accepted: May 17, 2016 Published: May 17, 2016 14174

DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181

Research Article

ACS Applied Materials & Interfaces

different polymer concentrations. After degassing air bubbles, a solution was quickly poured into the mold, which was preheated in an oven. The solution was sealed between the two plates. A Teflon film with a circular opening (diameter = 10 cm) was inserted between the two plates to adjust membrane thickness. The mold was then vertically put into a reservoir, and water with different temperatures (4, 15, and 30 °C) was added into the reservoir at a rate of 4 cm min−1. After total solidification, the nascent membrane was taken out of the mold and immersed in extraction solvent. A wet membrane was formed as soon as the diluent was completely extracted. To gain a dry membrane, the wet one was washed with an ethanol−hexane sequence, and then dried in vacuum for 24 h at 60 °C. The preparation conditions for the six polymer membranes were documented in Table S3. 2.3. Recovery of Crystallizable Solvent. DMSO2 was recovered by recrystallization from water. The nascent membrane was immersed in 20 mL of water at 70 °C for 6 h to extract DMSO2 inside the membrane. After filtration, the obtained DMSO2 aqueous solution was further concentrated at 80 °C and then cooled at 0 °C for 24 h. Pure DMSO2 were obtained by filtration. DPSO2 and arachidic acid were recovered by nonsolvent precipitation method. The nascent membrane was first dissolved in ethanol at 50 °C to totally extract the crystallizable solvent. Then a big amount of water was added to induce the crystallizable solvent precipitate from the solution. Finally, the crystallizable solvent was recovered by filtration. 2.4. Simulations. The dynamical crystallization process of solvent in the polymer system was simulated by the Monte Carlo method. In the simulation, the lattice model is a box of 128 × 128 × 32 with periodic boundaries. We then put 1152 polymer chains (chain length of 128 sites) and 172032 diluent solvent (length of 2 sites) into the box, thus the polymer volume fraction is 30%. Two perpendicular temperature gradients are performed onto the polymer/solvent system, which are controlled by changing moving rates in the ydirection and temperature increments from 1 in the z-direction. Here, moving rates in the y-direction represents the water flow rate, whereas z value denotes the temperature gradient between the bottom and top surfaces. A metropolis sampling algorithm was used in this simulation. The intrinsic growth habit of DMSO2 was simulated using growth morphology model in the Material Studio 7.0 software. The crystal structure of DMSO2 was first optimized using Forcite tutorial and the COMPASS force field. Then the crystal morphology of DMSO2 was calculated by applying the growth morphology method in the Morphology tutorial. The quality was set as ultrafine, and the used energy method was COMPASS. 2.5. Membrane Characterization. Morphologies of the membrane cross-section and surface were examined by FESEM (Hitachi S4800, Japan) with an accelerating voltage of 10 kV. Porosity and pore size were evaluated using a mercury injection apparatus (Auto Pore IV9500, Micromeritics, USA). Attenuated total reflectance FTIR spectroscopy (FTIR/ATR, Nicolet 6700, Nicolet. Co., USA) was performed on the membrane to analyze the PVDF crystal phase. Mechanical properties were measured from a stress−strain test using a tensile test instrument (RGM-4000, Shenzhen REGER Instrument Co. Ltd., China). The test was carried out at a strain rate of 5 mm/min at 20 °C and a relative humidity of 74%. UV−vis spectrometry (UV2450, Shimadzu, Japan) was used to characterize the light transmission of the porous membranes. 2.6. Cell Separation. The separation process was performed in a homemade dead-end filtration system under a pressure of 0.02 MPa. A membrane sample (diameter = 12 mm) with vertical pores was mounted in the permeation module. Before filtration, the membrane was sterilized in 75% alcohol for 10 min and rinsed by physiological saline three times to completely replace the air captured in the membrane pores and wet the membrane surfaces. Yeasts and lactobacilli were mixed and dispersed in physiological saline. The cell density of yeasts and lactobacilli in the feed solution was 2.5 × 105 and 1.1 × 107 cfu mL−1, respectively. The cell density of yeasts and lactobacilli was obtained by cell counting via a hemocytometer and from OD600 values by an UV−vis spectrophotometer (UV-2450, Shimadzu, Japan), respectively. The relationship between the cell

Organic solvents such as 1,4-dioxane and benzene, are suitable options to fabricate polymer membranes with oriented pores.22−25 Kim et al. reported poly(vinylidene fluoride) (PVDF) membranes with large-area and through-thickness porosity by using 1,4-dioxane crystals as templates.22 Ma et al. studied poly(L-lactic acid) (PLLA) scaffolds with oriented, gradient, and microtubular structures via the directional freezing of PLLA/benzene systems.23 However, both water and organic solvents need a relatively low freezing temperature to induce satisfactory orientation of the crystallized templates, because of their low melting points (water 0 °C, benzene 5.5 °C, and 1,4-dioxane 11.8 °C). Consequently, solvent crystals here also need to be removed by extraction or freeze-drying at low temperature, in order to prevent localized polymer redissolution at high temperature, e.g., room temperature. All the demands make the directional freezing process complicated and energy-consuming, which greatly hinders the development of water-insoluble polymer membranes with vertically oriented pores for practical applications. Here, we report a room-temperature bidirectional freezing process which can be applied into most of the commonly used water-insoluble polymers including poly(vinylidene fluoride) (PVDF), poly(L-lactic acid) (PLLA), and polyacrylonitrile (PAN), polysulfone (PSf), polystyrene (PS), and polypropylene (PP). Three crystallizable solvents with high melting point are adopted on the basis of the polarity of the polymers, i.e., dimethyl sulfone (DMSO2) for polar polymers (PVDF, PLLA, and PAN), diphenyl sulfone (DPSO2) for low polar PSf and PS, and arachidic acid for PP. The directional freezing and solvent extraction process can be easily conducted at relatively high temperatures, e.g., around room temperature, because of the high melting point of the crystallizable solvents (arachidic acid 75.4 °C, DMSO2 109 °C, DPSO2 128 °C). Moreover, the crystallizable solvents can be recovered conveniently by recrystallization or nonsolvent precipitation, which means our process is environmentally friendly. The results from experiments and Monte Carlo simulations indicate that the vertical pores can be precisely manipulated by polymer concentration and temperature gradient. The as-prepared membranes are supposed to own potential applications in various areas, such as fine separation, tissue engineering, lithium ion battery separator, and support for forward osmosis membrane. Herein, it is proved that PVDF membranes with vertical pores can precisely separate aqueous mixtures of yeasts and lactobacilli with high efficiency and permeation.

2. EXPERIMENTAL SECTION 2.1. Materials. PVDF (Mn = 110 000 g mol−1, Solef 6010, Solvay, Belgium), PSf (Mn = 22 000 g mol−1, Udel P-3500 LCD, Solvay, Belgium), PAN (Mn = 80 000 g mol−1, Anqing Petroleum Chemical Co., China), PLLA (melting index =3−5 g per 10 min, Zhejiang Hisun Biomaterials Co., China), PS (Mn = 235 000 g mol−1, MWD = 2.89, Zhenjiang Chiemei Chemicals, China), and PP (T300, melting index= 3 g per 10 min, Yangzi Petrochemical, Nanjing, China) were dried to constant weight before use. DMSO2 (99%, Dakang Chemicals Co., China), diphenyl sulfone (99%, Aladdin Chemistry Co., Ltd., USA), and arachidic acid (98%, TCI Chemicals Co., Japan) were adopted as crystallizable solvents. Ethanol and hexane were supplied by Sinopharm Chemical Reagent Co. Ltd., and used without further purification. 2.2. Fabrication of Membranes with Vertically Oriented Pores. Porous membranes were prepared with a homemade mold, which consists of two plates, one is made from stainless steel and the other from glass. Polymer/solvent mixtures were heated at a designated high temperature to form homogeneous solutions with 14175

DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic preparation procedure for polymer membranes with vertical pores via directional freezing. Polymer solution after heating was sealed inside a homemade mold consisting of stainless steel plate and glass plate. The mold was then perpendicularly put into a reservoir, and water was gradually added into the reservoir. (b) Temperature profiles in variation of time of the inner surface of stainless steel plate and/or glass plate. The water temperature is 30 °C. (c) Dimensionless temperature profiles of stainless steel plate along the y-direction. Dimensionless position ζ = 0 denotes the initial position of the stainless steel plate contacting with water, whereas ζ = 1 means the top position of the stainless steel plate. The total length of the stainless steel is 30 cm. The water temperature is 30 °C and the flow rate is 4 cm min−1. (d) Schematic representation of the bidirectional freezing process of solvent molecule. density and OD600 values of lactobacilli was established according to the National food safety standard food microbiological examination (GB 4789.35−2010), which shows the lactobacillus cell density of 3.89 × 108 cfu mL−1 at OD600 = 1.0.

membranes with vertical pores. A homogeneous polymer solution dissolved at high temperature was first sealed inside a mold, which has two plates of different materials, i.e., stainless steel and glass. The mold was then perpendicularly put into a reservoir and water with controlled temperature was gradually poured into the reservoir at a certain rate. Once the water is added, dual temperature gradients are produced on the mold along y- and z-direction, respectively. The temperature gradient along the z-direction arises from the disparity in the thermal conductivity of stainless steel and glass (Table S1). Typical evolution curves are shown in Figure 1b for temperature of the inner faces of the two plates upon cooling by water at 30 °C. It was calculated by solving the one-dimensional unsteady state heat conduction problem with analytical solution. The inner face of the stainless steel plate cools down much faster than that of the glass plate, as stainless steel is more thermally conductive than glass. A temperature gradient is thus formed in the polymer solution along the z-direction. This gradient maintains relatively constant upon further cooling the two plates, which ensures a relatively constant solidification velocity for the

3. RESULTS AND DISCUSSION 3.1. Preparation, Mechanism, and Monte Carlo Simulation. Traditional unidirectional freezing process always causes the nucleation of ice to occur randomly on the coldfinger surface, which severely hinders the scale-up fabrication of oriented structures aimed for larger applications. Bai et al. reported a new bidirectional freezing technique by covering the coldfinger with a polydimethylsiloxane (PDMS) wedge having different slopes.26 Larger porous scaffolds with highly controlled, ordered structures can thus be obtained through a proper control of nucleation of ice crystals and growth under dual temperature gradients. In this work, we developed a bidirectional freezing process to construct vertically oriented pores in water-insoluble polymers. Figure 1a schematically represents the fabrication procedure for polymer 14176

DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181

Research Article

ACS Applied Materials & Interfaces

y = 0 with the addition of temperature gradient. Upon further cooling, the through-thickness pores come out gradually along the y-direction, and the pore size also increases simultaneously. The morphologies change a little after 6000 Monte Carlo cycles. At the same time, the crystallinity first increases with the increase of cycles (Figure S1), and then becomes constant after 6000 cycles, indicating a complete crystallization. Although the pore shape mismatches with the real crystal growth habit of the solvent due to the statistical nature of Monte Carlo simulation, the through-thickness pores still indicate that the solvent molecules tend to aggregate across the membranes thickness under the temperature gradients. It confirms the feasibility of the bidirectional freezing process for fabricating membranes with vertically oriented pores. We then set the mixing interaction parameter B between polymer and solvent as variable to simulate the directional freezing process of different polymer/solvent systems. In these cases, a small B means a strong interaction between polymer and solvent. It can be seen in Figure 3a−c that the oriented

polymer solution. The mean temperature gradient is calculated as 0.25 °C μm−1, by assuming the thickness is 200 μm for the polymer solution in the mold. In addition, a small temperature gradient is also established along the y-direction on the x−y plane of the stainless steel plate with the addition of water into the reservoir. Figure 1c shows the dimensionless temperature profiles for the stainless steel plate along its y-direction with the gradual addition of water. The slope of the profile at the solid−liquid interface was used to estimate the temperature gradient in the y-direction. It is calculated as only 3.1 × 10−3 °C μm−1, which can be negligible when compared with that in the z-direction. Therefore, although the overall temperature gradient is not perfectly perpendicular to the surfaces of the polymer solution, the solvent crystals still grow directionally in the z-direction, whereas the small temperature gradient in the y-direction promotes the ordered nucleation of solvent crystals. It is reasonable to envisage that addition of water into the reservoir promotes the crystal growth of solvent molecules in the zdirection with uniform propagation of nucleation along the ydirection. Figure 1d shows the schematic representation of the directional freezing process of solvent crystals. The nucleation of solvent crystals start from the position where the stainless steel plate first contacts with water and propagates along the ydirection with the addition of water at a certain rate. Simultaneously, the directional freezing takes place from the inner surface of the stainless steel plate to the inner surface of the glass plate because of the temperature gradient in the zdirection. Monte Carlo simulations were then conducted to confirm the bidirectional freezing process.27,28 In the initial state, a polymer/solvent system with 30% polymer fraction is put into a box of 128 × 128 × 32 with periodic boundaries. The mixing interaction parameter B was set as 0.3, which was used to depict the interaction strength between polymer and solvent. The two perpendicular temperature gradients were controlled by changing temperature increments in the z-direction and moving rates in the y-direction. A temperature gradient of z*0.06 was first conducted onto the system along the z-direction in the plane of y = 0. Then the temperature gradient moved along the y-direction with a rate of 5 sites per 200 cycles. Figure 2 shows that small through-thickness pores first appear near the plane of

Figure 3. (a−c) Monte Carlo simulation results for systems with different mixing interactions (B) between polymer and solvent: (a) B = 0.3; (b) B = 0.1; (c) B = −0.1. The polymer volume fraction is 30%. The temperature gradient is z × 0.06 in the z-direction. The moving rate is 4 sites per 200 cycles in the y-direction. (d-i) SEM images of six water-insoluble polymer membranes with vertically porous structure: (d) PVDF, (e) PLLA, (f) PS, (g) PSf, (h) PAN, (i) PP. The preparation conditions are listed in Table S3.

pores gradually disappear with the increase of the interaction between polymer and solvent (decrease of B), indicating that the solvent molecules are difficult to aggregate across the membrane thickness upon cooling under a strong polymer/ solvent interaction. We conclude that a polymer/solvent system with poor interaction is ideal for fabricating polymer membranes with vertically oriented pores. 3.2. Morphologies and Properties of Membranes with Vertical Pores. Herein, six polymer/crystallizable solvent systems were adopted to fabricate membranes with vertically oriented pores, including PVDF/DMSO2, PLLA/DMSO2, PAN/DMSO2, PS/DPSO2, PSf/DPSO2, and PP/arachidic acid. All the systems possess proper polymer/solvent interactions (Table S2), which is beneficial for the bidirectional freezing process. DMSO2, DPSO2, and arachidic acid are all crystallizable solvents with high melting temperature (arachidic acid (75.4 °C), DMSO2 109 °C, DPSO2 128 °C), which

Figure 2. Morphology evolution with Monte Carlo cycles: (a) Snapshot of initial configuration of polymer solution. The red parts represent the diluent and the blue parts represent polymer chains. The polymer volume fraction here is 30%. (b-d) Variation of morphology with time upon addition of a temperature gradient of z × 0.06: (b) 1000, (c) 3000, (d) 5000, and (e) 7000 cycles. The moving rate in the y-direction is 5 sites per 200 cycles. 14177

DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181

Research Article

ACS Applied Materials & Interfaces facilitates the fabrication of vertically oriented pores by bidirectional freezing process at ambient temperature (Table S3). As Figure 3d−i shows, membranes with well-controlled vertically oriented pores are successfully fabricated inspired by the bidirectional freezing mechanisms and the Monte Carlo simulations. Figure 3d, e, h indicates that the pore size of the membranes prepared with 20 wt % polymer concentration decreases in the order of PLLA (∼4.9 μm), PVDF (∼4.6 μm) and PAN (∼0.15 μm), when using DMSO2 as solvent. One of the reasons is the increased interaction between the polymer and the solvent (Table S2). The fabricated membranes with vertically oriented pores can be as large as 10 cm (Figure S2), which is only limited by the mold dimensions. Furthermore, the crystallizable solvents exhibit lower toxicity than the common liquid solvents such as N,N-dimethylformamide, and are solid at room temperature. It facilitates the recovery process by simply recrystallization or nonsolvent precipitation, which greatly reduces the potential pollution and cuts down the production costs. The recovery efficiency of DMSO2 is 22.0% by recrystallization,29,30 whereas those of DPSO2 and arachidic acid can be as high as 87.3 and 82.4% by nonsolvent precipitation method, respectively. All the results demonstrate that the ambient-temperature bidirectional freezing process of crystallizable solvents can be facilely and versatilely applied to the large-scale fabrication of membranes with well-controlled vertical pores for water-insoluble polymers. We then adopt PVDF/DMSO2 as a model system to conduct a detailed analysis. We denote the membrane surface contacting the stainless steel plate as bottom surface, and that contacting the glass plate as top surface, because the DMSO2 crystals grow directionally from the stainless steel plate to the glass plate. It can be seen in Figure 4a−d that the pores are extremely vertical to the membrane surfaces with a pore size of 4.6 μm from the cross-sectional view and a narrow pore size distribution (Figure S3). Particularly, the bottom surface also exhibits an oriented dendrite structure. It is resulted from the oriented nucleation process along the y-direction under the temperature gradient.31 The top surface shows a quite different morphology with a rectangular-like shape. Growth morphology model was used to simulate the crystal habit of DMSO2. It is found that the (110) and (020) face are the dominant crystal facets of DMSO2, resulting in a rectangular-like cross-section (Figure S4). This is in accordance with the optical observation of DMSO2 crystals (Figure S5). The oriented morphologies are following the interfacial instability theory.15,32 The directional freezing of a PVDF/DMSO2 solution causes constitutional supercooling at the solid/liquid interface, leading to the instability of interfacial morphology. The instability wavelength, which defines the primary periodicity of the templated structure in Figures 4b, c, is governed by competition between the interfacial concentration gradient and the interfacial energy. According to the theory of Kurz and Fisher,33 the instability wavelength λ to stabilize a planar interface during mass diffusion can be described as follows ⎛ D ΓΔT0 ⎞1/4 −1/2 ⎟ G λ = 4.3⎜ ⎝ kV ⎠

Figure 4. (a−d) SEM images of PVDF membranes with vertical pores prepared with polymer concentration of 20 wt %. The water temperature is 30 °C and the flow rate is 4 cm min−1. (a) Crosssectional morphology, (b) enlarged cross-sectional morphology, (c) bottom surface morphology, (d) top surface morphology. The direction of the arrow represents the y-direction, which is the direction of water flow. (e) Transmission of PVDF membranes with and without vertical pores. The insets are digital images of PVDF membrane with vertical pores (left) and with randomly oriented pores (right), respectively. (f) Porosity and pore size of PVDF membranes prepared with different polymer concentrations. (g) Tensile strength and elongation of PVDF membranes with vertical pores prepared with different polymer concentrations. The water temperature is 30 °C and the flow rate is 4 cm min−1.

as well as the pore spacing after extraction. For those vertical pores, the pore spacing is just equal to the pore size (4.6 μm) along the z-direction. It is quite different from the porous structure along the y-direction which exhibits dendrite morphology. Herein, we only consider the primary spacing of the crystal trunk along the y-direction, as the secondary dendrite growth is much more complicated. According to eq 1, the pore spacing is inversely proportional to the temperature gradient, which is just consistent with Figure 4b, c. The primary spacing in the y-direction (113 μm) is much larger than the spacing in the z-direction, ascribed to a much smaller temperature gradient. Moreover, the oriented porous structures can be facilely manipulated by adjusting water flow rate, polymer concentration, and cooling temperature. The water flow rate is related to the nucleation rate in the y-direction. A higher water flow rate results in a faster growth velocity V, and a smaller instability wavelength according to eq 1. It indicates that more pores with small size are template in the membranes, when the flow rate is fast. Increasing the polymer concentration leads to the solution viscosity increases and then a small diffusion coefficient k. As a result, the instability wavelength (pore spacing) decreases with increasing polymer concentration. Meanwhile, the water temperature affects the temperature difference on the polymer surfaces. Herein, the water temperature for bidirectional freezing is 4, 15, and 30 °C, respectively, which is much higher than those reported in other literatures,11,19 and thus can be processed much more easily. It reveals that water bath with lower temperature causes a larger temperature difference (Figure S6). Thus, a larger temperature gradient is formed, causing smaller pore spacing. All these trends are in consistent with the SEM images and the Monte

(1)

where D is the diffusion coefficient, Γ is the ratio of the surface energy to the latent heat of fusion, ΔT0 is the equilibrium under cooling, k is the partition coefficient, V is the growth velocity, and G is the temperature gradient. The instability wavelength can be used to predict the primary spacing of DMSO2 crystals 14178

DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a) Transmission of lactobacilli and (b) permeability during separating cell mixtures of yeasts and lactobacilli by PVDF membranes with vertical pores prepared with different polymer concentrations. The water temperature is 30 °C and the flow rate is 4 cm min−1. Yeasts and lactobacilli were dispersed in physiological saline and mixed with a volume ratio of 1:1. The yeast density is 2.5 × 105 cfu mL−1, while the lactobacillus density is 1.1 × 107 cfu mL−1. (c) Recyclability for cell separation by PVDF membranes with vertical pores. The membranes were prepared with 30 wt % polymer concentration in water bath at 30 °C. (d−f) SEM images of top surface of the membranes with vertical pores (d) before cell separation, (e) after cell separation, and (f) after back washing. (g−i) Schematic of cell separation process: (g) before cell separation, (h) after cell separation, and (i) after back washing.

ascribed to the smallest pore size (1.5 μm) and the lowest porosity (58.2%). Mechanical properties are much important when the membranes are applied for separation, such as ultrafiltration and microfiltration, as it directly determines the long-term performance and back-washing capability.17 Figure 4g displays the tensile strength and elongation of the PVDF membranes prepared from different polymer concentrations. The tensile strength increases with the increase of polymer concentration. Nevertheless, the elongation shows an opposite trend. When the polymer concentration is 35 wt %, the membranes possess excellent tensile strength as high as 6.3 ± 0.4 MPa and a moderate elongation of 36.9 ± 4.8%, which are higher than those reported in other literature.37,38 It is ascribed to the small pore size and the low porosity of the membranes. The excellent mechanical properties of membranes with vertical pores are beneficial to the application in separation and tissue engineering. 3.3. Cell Separation by Using PVDF Membranes with Vertical Pores. All the properties mentioned above endows the membranes with tremendous potentials for practical applications, such as precise separation, organic electronics, tissue engineering, lithium ion battery separator, and supporting layer for forward osmosis membrane. In this work, the PVDF membranes with vertical pores were applied for precise separation of cell mixtures. The oriented porous

Carlo simulation results (Figures S7−S12). It is notable that we can prepare membranes with a series of thicknesses (25, 50, 100, and 200 μm) by easily changing the thickness of the inserted Teflon film (Figure S13). The adjustable oriented pores endow the membranes with tunable properties, which is beneficial for applications in various fields. Optical transparence is an important property for the porous membrane especially when used in the area of organic electronics.34,35 Figure 4e shows that the membranes with randomly oriented pores are almost opaque with a transmission of only 5% (Figure S14), which were prepared with 20 wt % PVDF/DMSO2 solution in the symmetric cooling condition using two steel plates as cooling medium. This is due to the light scattering at the interfaces of randomly oriented polydomains. On the other hand, the porous membranes with vertical pores are semitransparent in the wavelength range of visual lights, as the light can directly pass through the vertical pores. What’s more, the light transmission can be tunable by changing the pore size and porosity.36 Figure 4f indicates both porosity and pore size decrease with the increase of polymer concentration for the membrane preparation, leading to a decrease of light transmission. The membranes prepared with 20 wt % PVDF possess the largest pore size (4.6 μm) and the highest porosity (66.6%), resulting in the highest light transmission (50%). Meanwhile, the membranes prepared with 35 wt % PVDF exhibit the lowest transmission (30%) 14179

DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181

Research Article

ACS Applied Materials & Interfaces

preparation process. The membrane pores can be finely tuned by polymer concentration, water temperature and flow rate. PVDF membranes holding vertical pores exhibit high optical transmission and excellent mechanical strength. Moreover, the membranes can efficiently separate cell mixtures of yeasts and lactobacilli with high permeability and selectivity. This work provides a facile approach to prepare large-scale membranes of water-insoluble polymers with vertically oriented pores at ambient conditions.

structure with low tortuosity exhibits low transport barrier, which is ideal for efficient biological transport and separation. Yeasts and lactobacilli were chosen as the model cells. It is known that yeasts (S. cerevisiae) are elliptical cells with diameter of 4−6 μm and lactobacilli (S. thermophilus) are spherical cells with an average diameter of 0.7 μm.1 The membranes prepared with 20 wt % PVDF concentration were first applied to filtrate yeast cells only to evaluate the retention capability. Almost 100% yeasts are removed from the feed solution, indicating perfect retention to these cells because they are larger than the pore size of the membrane (Figure S15). Therefore, the cell mixtures of yeasts and lactobacilli can be precisely separated using PVDF membranes with suitable vertical pores. Yeast cells are expected to be completely removed by the size sieving effect of the membranes, resulting in a permeation ratio of nearly zero. Although some lactobacillus cells can link together to form a chainlike joint structure and enlarge the aggregate size, a high lactobacillus transmission of 87.4% has been obtained when using the membranes prepared with 20 wt % concentration (Figure 5a). The permeation ratio of lactobacillus cells significantly decreases when using membranes prepared with high polymer concentration. The filtration flux also shows a similar trend. Figure 5b indicates that the permeability decreases from 3.8 × 103 m3 m−2 h−1 bar−1 to 6.0 m3 m−2 h−1 bar−1. It is stemmed from that the pore size of the membrane (1.5 μm when 35 wt % PVDF was used to prepare the membranes) is too close to the size of lactobacillus cells. What’s more, the cell separation performance outstands from the commercial PVDF porous membranes with tortuous structure (Figure S16), which exhibit a small permeability of 3.8 m3 m−2 h−1 bar−1 and a low lactobacillus transmission of 22%. We also investigated the recyclability of the membranes for cell separation (Figure 5c−i). First, physiological saline was filtrated through the membrane. The cell filtration step was then performed in the same condition (Figure 5d, g). During the filtration of cell mixture, a filter cake gradually forms on the membrane surface (Figure 5e, h). As can be seen from Figure 5e, the membrane surface is almost totally covered by the cells, leading to a reduced pore size and a declined permeation flux. Then a back washing was conducted from the opposite side of the membranes using physiological saline under 0.05 MPa (Figure 5f, i). The effects of back washing can be efficiently performed onto the filter cake, due to the vertical porous structure. Figure 5f shows that the filter cake is almost removed from the membrane surface. As a consequence, the flux can be recovered with a flux recover ratio (FRR) higher than 90%. It can be concluded that the PVDF membranes with vertical pores can be used for efficiently separation of cell mixtures with high separation resolution and permeability, as well as excellent recovery ability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03071. Physical properties for materials used, photograph of large-scale PVDF membrane with vertical pores, pore size distribution of PVDF membranes with vertical pores, effect of polymer concentration, cooling temperature and water addition rate on the vertically oriented pores, SEM images of PVDF membranes with vertical pores with different thicknesses, optical images of the feed and filtrate solutions on a hemocytometer (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is financially supported by the National Natural Science Foundation of China (Grant no. 21174124). We also thank Dr. Qing-Yun Wu (Ningbo University) and Dr. Ling-Shu Wan (Zhejiang University) for their useful suggestion and discussion to this work.

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REFERENCES

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4. CONCLUSION Ambient-temperature bidirectional freezing process has been successfully applied to prepare large-scale polymer membranes with well controlled vertically oriented pores by using crystallizable solvents with high melting point. PVDF, PLLA, PAN, PS, PSf, and PP are all suitable polymers for these membranes by adopting DMSO2, DPSO2, and arachidic acid as crystallizable solvent, respectively. The generality has been evidenced by dynamic Monte Carlo simulations of parallel processes. What’s more, the used solvents can be facilely recovered which facilitates the environment-friendly issue of the 14180

DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b03071 ACS Appl. Mater. Interfaces 2016, 8, 14174−14181