Continuous Gradient Nanoporous Film Enabled by Delayed

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Continuous gradient nanoporous film enabled by delayed directional diffusion of solvent and selective swelling Di Tan, Qian Li, Baisong Yang, Xin Wang, Shiqi Hu, Zhengzhi Wang, Yifeng Lei, and Longjian Xue Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00328 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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Continuous gradient nanoporous film enabled by delayed directional diffusion of solvent and selective swelling

Di Tan†, Qian Li†, Baisong Yang†, Xin Wang†, Shiqi Hu†, Zhengzhi Wang‡, Yifeng Lei†, Longjian Xue†* †School

of Power and Mechanical Engineering, The Institute of Technological Science, Wuhan

University, South Donghu Road 8, 430072, Wuhan, China ‡Department

of Engineering Mechanics, School of Civil Engineering, Wuhan University, South

Donghu Road 8, 430072, Wuhan, China *email:

[email protected]

Abstract Nature-inspired porous structures are highly desired in the fields of new materials, sustainable energy, biological & chemical science, etc. Here, a new strategy for the fabrication of continuous, gradient nanoporous polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) film is established. The continuous nanopore gradient along the direction of film thickness (~120 μm) is achieved via delayed directional diffusion of dynamic binary solvent of ethanol/water and selective swelling of P2VP domains. Ethanol in binary solvent diffuses into the film from one side to another, which is retarded by the water gate as water is concentrated at the film surface. The delayed diffusion matches the swelling rate of P2VP domains, forming the continuous nanopore gradient normal to the film surface.

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INTRODUCTION Gradient porous structures have been widely found in living organisms showing various biological functions. The branched tracheas in animal lungs maximize the efficiency of O2/CO2 exchange1,2. The gradient pores in bones balance the mechanical properties3 and function of mass transportation4. Besides the life science, gradient porous structures can find applications in chemical science5,6, mechanical engineering7-9, energy harvesting and storage10-12, material separation13, etc. For example, for water purification, with proper pore size and film thickness, gradient porous films can keep the purification efficiency and the durability at the same time. In field of clean energy, for example, during the salinity gradient power generating, the pore size gradient can promote the material exchange and increase the efficiency of power generating. The technique to fabricate micro- and nano-scaled gradient porous structures are therefore highly desired. Several approaches have been proposed to prepare gradient porous structures, including gradient particle stacking11,14,15, thermal-16-18, pressure-19 and diffusion-20-22 induced gradients. In stacking method, micro-15 and nano-11,14 particles with several diameters were assembled via layer-by-layer placing14 or centrifugation11,15 and served as the template to generate pores. Because of the particles used, the gradient in pore size is not continuous, but stepwise. Recently, a new method for in situ generation of nanoparticles with gradually changed size by thermopolymerization of resol in the progressive downward gelating solution was successfully established23. The method can be used to prepare large area membranes with low-cost phenolics and the as-prepared membranes showed 20-80 times higher water permeance than commercial ones with similar rejections. The phase separation of immiscible polymers16 and the in-situ polymerization17,18 of monomers under gradient temperature fields have been used to realize 2


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the continuous gradient porous structures. As the temperature gradient is difficult to realize in microscale or even smaller, the temperature induced gradient usually presents in a bulk material or along the XY plane of a membrane. Making use of the enormous change of bubble size of critical CO2 under a small pressure gradient19, gradient pores in micro-scaled film have been developed. In order to prepare nanoscale gradient porous structures, methods of diffusion-regulated chemical reactions20,21 and microphase separation of block copolymers22,24 were proposed. In diffusion-regulated chemical reactions, one of the reactants diffuses though another reactant from one side to another, resulting in a gradient of reaction extent. Depends on the reaction type, the crosslinking degree or the content of the product forms the gradient. As the diffusion process is quite slow, the resulted gradient could be very well controlled. Using block copolymer polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP), porous films with inter interconnected inner-pores or vertical pores of homogeneous pores size throughout the film have been widely investigated13,25-29. Asymmetric porous film composed of a surface layer of vertical pores and interconnected inner pores was also realized in the PS-b-P2VP film24,30-34. The gradient composition in the mixed solvent during evaporation, where the non-solvent dimethylformamide translates into a reduction of incompatibility between the diblock copolymer and the mixed-solvent at the film surface, leads to the increase of the coarsening of the phase-separated structure from the top surface towards the bottom of the film, forming an asymmetric structure. However, the realization of gradient pores in nanoscale remains challenging. In this contribution, a simple fabrication strategy is proposed to prepare continuous nanoporous structures with gradient porosity in diblock copolymer film with thickness of ~120 3


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μm. The selective swelling of P2VP domains in microphase-separated films by ethanol, which is good solvent for P2VP and non-solvent for PS, generates nanopores in the film. The gradient in nanoporous structure was generated by the synergistic effects of directional diffusion of solvents and dynamic content of binary solvent. The dynamic content of binary solvent slowed down the diffusion rate of ethanol to match the swelling rate of P2VP domains, resulting in a continuous gradient of swelling extent. The following complete removal of solvent collapses the swollen P2VP domains forming gradient nanopores with pore diameters decreasing from 43.6±7.5 nm to several nm. The method does not need any specific equipment or any harsh experimental skills, therefore, it could be easily adapted to other systems to prepare gradient structures.

EXPERIMENTAL SECTION Materials. The asymmetric diblock copolymer, polystyrene-block-poly (2-vinyl pyridine) (PSb-P2VP) with Me=101 kg/mol for PS block and 29 kg/mol for P2VP block and a polydispersity index of 1.09 was purchased from Polymer Source Inc., Canada. Tetrahydrofuran (THF) was purchased from Sigma Aldrich. The ethanol of 99.9% was purchased from China National Pharmaceutical Group. Polydimethylsiloxane (PDMS) elastomer kits (Sylgard 184) were purchased from Dow Corning (MI, USA). The Al sheets were purchased from JiangSu JinHai Aluminum Co.,Ltd and polished by electropolishing in HClO4/ethanol binary solvent with 25vol-% HClO4 and 75vol-% C2H5OH. Silicon wafers were purchased from China RDMICRO AG. Deionized water was prepared by Millipore Direct-Q 3 water purifier. All the chemicals were used without further purification. Fabrication. The PS-b-P2VP films were prepared by solution casting. 30μl 100 mg/ml solution 4


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of PS-b-P2VP/THF was dropped on a clean silicon sheets (10mm*10mm) and then dried in a THF atmosphere by covering a glass petri dish over the sample. After the complete drying for 12h, the PS-b-P2VP films were sandwiched between two PDMS sheets (figure1a) and annealed at 200 ℃ and 1 mbar vacuum for 48 h in a vacuum oven (VO200, Memmert GmbH Co. KG, Germany) for microphase separation, followed by a nature cooling to room temperature. The PDMS sheets are prepared by pouring the mixed PDMS precursor (ratio of 10:1 prepolymer/crosslinker) in a 7 mm thick mold made by two glass slide and cured at 120 °C for 2 h in an oven. The resulted PS-b-P2VP films were sealed in a home-made PDMS mold, with only one side exposing to open air (figure 1b). The sealed PS-b-P2VP films were then swelled in pure ethanol or binary solvent at 60 ℃ for various periods in a closed chamber. To gain the continuous gradient nanoporous film, a binary solvent of ethanol mixed with 1 wt% of water was used. The directional swelling was conducted at 60 ℃ in an open weighing bottle with the diameter of 25 mm for 1h (figure 1b). The swelled PS-b-P2VP films was dried in vacuum for 3h to remove the solvents. Characterization. Surface microstructures were characterized by field emission scanning electron microscope (FESEM) (SIGMA, Zeiss AG, Germany and MIRA 3 LMH, Tescan AG, Czech Republic). The cross-section samples were prepared by breaking them in liquid nitrogen. All the samples were sprayed platinum at 10mA for 90s before SEM test. The microphase separation of PS-b-P2VP film was tested by transmission electron microscope (TEM) (JEOL2100F, JEOL Ltd., Japan). PS-b-P2VP film was cut with microtome and P2VP domains were stained with iodine, appearing darker in TEM images. The modulus at the cross section of samples was tested by atomic force microscopy (Nano wizard4, JPK AG, Germany) with a 10μm SiO2 sphere probe. The cross-section samples were cut by scalpel with two glass slide 5


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sandwiched. The loading force is 1μN and the loading rate is 0.5μm/s. The moduli were calculated from force displacement curves by Hertz/Sneddon model, which is built in the data processing software. The water contact angles were tested on OCA25 (Dataphysics AG, Germany). The thermal induced microphase separation of block copolymer was conducted in vacuum oven (VO200, Memmert AG, Germany). The chemicals were weighed by balance (ME204, Mettler Toledo AG, Switzerland). The pore size was analyzed and calculated from the SEM images by ImageJ software. The projected area of the pores at the cross section was identified as the pore size and the proportion of projected area of pores to total area was identified as the porosity. The diffusion depth was also determined from SEM image of cross section. After the diffusion of ethanol and the followed drying process, the area ethanol has reached showed different morphology from that it hasn’t reached. The border of the two areas was then considered as the diffusion front.

RESULTS AND DISCUSSION Gradient nanopores Diblock copolymers offer the chance to produce nanoporous films because of their ability to self-assemble into uniformly sized nanodomains which can be transformed into nanopores subsequently9,29. The molecular weight, block ratio, Flory-Huggins interaction parameter χ between blocks and boundary conditions, etc. determine the resulted microphaseseparated structures of diblock copolymers35. Here, we chose diblock copolymer PS-b-P2VP with MPS =101 kg/mol and MP2VP =29 kg/mol to fabricate gradient nanoporous films8,9. χ for P2VP/PS pair is ~0.09 at 298K, a typical value for complete incompatibility36. The P2VP block, which has a volume fraction of 21% in PS-b-P2VP, forms cylinder phase dispersed in the 6


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continuous phase of PS in bulk state (Figure S1)9. The period of microphase separated structure was calculated to be ~51nm. PS-b-P2VP film with thickness of ~120 μm between two polydimethylsiloxane (PDMS) sheets was annealed at 200 °C for 48h, followed by cooling down to room temperature naturally (Figure 1a). The resulted film was then sealed in a PDMS mold with the configuration as depicted in Figure 1b. The sealed PS-b-P2VP film was then annealed at 60 ℃ in binary solution of ethanol/H2O in an open container for 1h (Figure 1b). For the convenience, the film surface contacting binary solvent and the sealed side are termed as surface SA and SB, respectively, in the following text. Because of the strong affinity between P2VP and ethanol, the P2VP domains expanded, distorted and contacted with neighboring domains transforming into network-like structure37. As ethanol can only diffuse from surface SA into PS-b-P2VP film through P2VP domains, gradient swelling formed through the film. The following drying procedure at ambient condition collapsed the swollen P2VP domains into continuous pores29. Porous structure with gradient in pore size and density from SA to SB across the PS-bP2VP film was examined by SEM (Figure 1c). The magnified SEM images (bottom row in Figure 1c) clearly show the differences in pore size and porosity across whole film. The pores close to SA have a mean diameter of 43.6±7.5 nm and gradually reduced to 22.3±6.8 nm at SB (Figure 2a). Meanwhile, the porosity (P), defined as the ratio of pore areas to the whole detected area, is 12.0±0.2% at SA, slightly smaller than the volume fraction of P2VP (Figure 2b). Close to SB, P reduced to ~2.3±0.7%. Both the pore size and P clearly confirmed the continuous nanogradient along the thickness direction of the film. For comparison, PS-b-P2VP film was also treated without the synergistic effect by immersing the unsealed film in the binary solvent for 1h, where both surfaces of PS-b-P2VP film directly contacted the solvent, followed by the 7


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complete drying at ambient condition. SEM revealed homogeneous pores across the film thickness (Figure S2a) that the mean diameter and porosity are constant values of 31.1±2.0 nm and 11.8±1.1% (Figure 2a and 2b), respectively. This kind of film is therefore referred as homogeneous film in the following text. As the porosity close to SA is identical to that of the homogeneous film but the mean pore size is larger than that of homogeneous film, we assume some neighboring pores close to SA merged together forming larger pores, increasing the mean size of the pores. A structured material (for instance, porous material) shows greatly reduced elastic modulus as compared to the bulk material. The elastic modulus of a porous material is also considered as effective elastic modulus (Eeff) which is the property of a structure38. The Eeffs along the thickness direction of the films were examined by AFM with a spherical probe. The generation of pores inside the PS-b-P2VP film greatly reduced the Eeff from 299.8±7.7 to 109.2±9.7 MPa (Figure S3). There is no gradient in the homogeneous film, confirmed the homogeneity of the pores throughout the film. In contrast, Eeff of gradient film increased from 100.4±2.1 MPa at SA to 381.3±36.5 MPa at SB, showing a clear, continuous gradient in the film treated with directional swelling (Figure 2c). Eeff of a porous structure is a function of the elastic modulus of the bulk material (E0) and the porosity P of the structure via following equation39: 𝐸𝑒𝑓𝑓 = 𝐸0 (1 ― 𝑎𝑃)(1 ― 𝑃)

(1)

where a is the coefficient depending on Poisson’s ratio. The dependence of Eeff on P could be reproduced very well with the model (Figure 2d). For the gradient nanoporous PS-b-P2VP film, the calculated E0 is 290.7 MPa and a is 0.064. The calculated bulk modulus of PS-b-P2VP film E0 =290.7MPa matches very well to tested result (299.8±7.7MPa). The well-fitted theoretical results with experimental results indicate the continuous pore size gradient inside the films. 8


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Influence of surface nanostructure As the penetration of binary solvent starts at the film surface and proceeds into it, the phase-separated structure on the film surface is critical to generate gradient pores with open ends at both surfaces. The influence of surface nanostructure of the film was therefore investigated by altering the surface energy of the substrate that the film contacted during thermal annealing40. Based on our previous studies, we chose PDMS-aluminum (with native oxide layer) and PDMS-PDMS pairs to sandwich the PS-b-P2VP film during thermal treatments. The polar block P2VP segregated to aluminum oxide forming a thin layer of P2VP on the surface SAl41,42, confirmed by TEM image (Figure 3a). Moreover, there are some locations, the P2VP layer is connected to the beneath P2VP cylinder phases. In contrast, PS enriches on SPDMS (Figure 3b). The surface induced enrichment of corresponding block was further investigated with water contact angle (WCA). WCA of SAl is 74.7°, confirming the main component of P2VP (WCAP2VP=78.6°) (Figure 3c). The SPDMS showed a WCA around 92.1°, similar to that of a flat polystyrene (PS) film (WCAPS=97.6°), indicating the enrichment of PS on surface. Both WCA measurements confirmed the surface-induced microphase separation on SPDMS and SAl. Different morphologies on surfaces of SAl and SPDMS were generated during the treatment in pure ethanol (Figure 3d). When the film was prepared by sandwiching between two PDMS sheets, both film surfaces were SPDMS. After the treatment in pure ethanol, pores were generated on the surface. Before the treatment in ethanol, the film surface facing PDMS (SPDMS) is featureless (Figure S4a). Treated in ethanol for 2 min, nano-bumps with some tiny dimples appeared on the surface (Figure S4b). From the corresponding phase image, many black dots 9


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were found, indicating the forming of new phase on the surface. The further treatment with ethanol for longer time, for instance 8 min, holes appeared clearly on the surface (Figure S4c). It suggests that the treatment with solvents selective to P2VP, such as ethanol here, leads to reorientation of P2VP cylinders, though the top surface is covered with PS. Similar phenomenon was also reported by treating the PS-b-P2VP film with aqueous acid solutions of HCl and acetic acid43,44. From the shape of the pores, it is quite clear that some neighboring pores merged together forming peanut-shape or elongated pores on the surface (Figure 3d). It confirms again the merging of pores close to the film surface contacting solvent (SA) (Figure 2a). While this merging effect seems to be less obvious in the gradient porous film, we assume the generation of pore gradient may introduce extra stress in the film and cause more neighboring pores to merge together. The treatment of 1h generated pores with similar porosity on both surfaces. However, the SAl facing solvent ethanol had no pores on the surface. The P2VP blocks on surface was very well swelled by ethanol, but cannot insert stress into the underlying continuous PS layer to open pores. After drying, the swelled P2VP layer collapsed into nano-bumps on the surface, but not holes (Figure 3d). Therefore, the PS-b-P2VP film, which was prepared by sandwiching between PDMS and Al and treated with SPDMS facing ethanol, was solid on the SAl side (Figure S5), though there was still a sharp gradient structure across the film. When the treatment with ethanol started from SAl, no pore was generated throughout the film. In contrast, pores with open ends at both surfaces were generated in the PS-b-P2VP film with two SPDMS surfaces. It therefore suggests that the phase separation morphology on the film surface is crucial for the generation of continuous pores with open ends at both surfaces.

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Directional diffusion and swelling As the pores are generated by the drying of ethanol-swelled P2VP domains, the time for diffusion and swelling of ethanol in P2VP domains are critical to form the pore gradient. Using the method described in Figure 1b, the directional diffusion of ethanol (60 ℃) started from SA and the diffusion front was monitored (The area ethanol has diffused into showed a much rougher topography as compared to the area ethanol has not reached, as shown in Figure S6). The diffusion rate of ethanol in PS-b-P2VP film was evaluated to be ~8 μm/min that ethanol can go through a 120 μm-thick film in ~15 min (Figure 4a). The directional diffusion is a quite quick process, surprisingly. The swelling process was investigated by monitoring the pore size on the film surface exposed to ethanol (SA) since the initiation of diffusion and swelling of ethanol in P2VP domains happed simultaneously on the film surface (Figure S7). Within the first hour, the pore size grew linearly proportional to the swelling time showing a swelling rate of 0.3nm/min. Afterwards, the swelling rate slowed down to about 0.05 nm/min, and the pores kept growing for the next 3 hours (Figure 4b). In order to get large pores at SA and therefore a large gradient across the film, the swelling process needs at least 60 min. In a 60 min treatment, however, the difference in swelling time of two surfaces is only 15 min. It therefore resulted in a pore-size difference of only 3.8 nm between two surfaces. Moreover, the gradient pores existed only close to SB and the first ~75% of the film was consisted of homogeneous pores (Figure S8). The fast diffusion of ethanol hindered the formation of continuous gradient throughout the film. While a lower temperature of ethanol could slow down the diffusion, the swelling rate will also be reduced synchronously. It therefore suggests the diffusion rate must be inhibited in order to match the swelling process. 11


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Dynamic binary solvent for controlled diffusion and swelling Binary solvent of ethanol mixed with water were used to balance the procedure of swelling and diffusion. The interaction between a solvent and a polymer can be evaluated by FloryHuggins interaction parameter χsp45 𝑉

χsp = 𝑅𝑇(𝛿𝑠 ― 𝛿𝑝)2 +0.34

(2)

where V, R, T, δs and δp are the molar volume of solvent, the gas constant and Kelvin temperature, the solubility parameter of solvent and polymer, respectively. When χsp

Continuous Gradient Nanoporous Film Enabled by Delayed

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