Nanoporous Structures from PS-b-PMMA-b-PtBA Triblock Copolymer

Subscriber access provided by University of Glasgow Library

Article

Nanoporous Structures from PS-b-PMMA-b-PtBA Triblock Copolymer and Selective Modification for Ultrafiltration Membranes Sungmin Park, Taesuk Jun, Hye Rin Yoon, Seongjun Jo, Jong Hak Kim, Chang Y. Ryu, and Du Yeol Ryu ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00050 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Polymer Materials

Nanoporous Structures from PS-b-PMMA-b-PtBA Triblock Copolymer and Selective Modification for Ultrafiltration Membranes Sungmin Park,†,‡ Taesuk Jun, † Hye Rin Yoon, † Seongjun Jo,† Jong Hak Kim, † Chang Y. Ryu, ‡,*and Du Yeol Ryu†,*

†

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu,

Seoul 03722, Korea. ‡

Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, NY 12180, US.

*E-mail: [email protected] (C.Y.R.), [email protected] (D.Y.R.) ABSTRACT: A simple approach to fabricating nanoporous structures and their functionality is demonstrated using a triblock copolymer of polystyrene-b-poly(methyl methacrylate)-b-poly(tert-butyl acrylate) (PS-bPMMA-b-PtBA), where a continuous type morphology is set in PS matrix to form the cylinders consisting of the PMMA and minor PtBA blocks. For directional and uniform nanochannels at the interfaces, a perpendicular orientation of cylinders was exploited near two interfaces of air/polymer and polymer/neutral substrate, sandwiching the random orientation of cylinders in interior of film. Non-degradative, selective swellingdeswelling process of cylindrical (PMMA-b-PtBA) blocks generates nanopores as an effective route to precisely tune the pore size. Further, a simple hydrolysis of tBA units functionalizes the nanopore surfaces and walls into poly(acrylic acid) layers. We demonstrate the pH-responsive water permeability of nanoporous membranes and their active switching with respect to biomolecules such as Bovine Serum Albumin (BSA), suggesting a feasible functional platform to fabricate a stimuli-responsive ultrafiltration membrane using a tunable multiblock copolymer. Keywords : Triblock copolymer, Self-assembly, Hydrolysis, Non-degradative nanopore generation, Functional membrane ACS Paragon Plus Environment

1

ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

INTRODUCTION Achieving a well-defined nanoporous structure has been a challenging issue for high performance application such as tissue scaffolds,1,2 water purification,3-5 ion channel6-8 and drug delivery,9,10 because the implementations are associated with the pore size uniformity, interconnectivity, porosity, and interfacial engineering, etc.11,12 Block copolymer (BCP) has been an attractive candidate for fabrication of nanoporous structures with narrow size distribution due to the inherent assembly feature, providing highly-ordered periodic alignment of lamellae, cylinders, spheres and double gyroid morphologies.13-17 Owing to specific feature sizes of 10 – 50 nm and minor phase removal chemistries, the nanoporous templates from the BCP self-assembly are classified into the regime of ultrafiltration (UF) membrane technology.18-22 A variety of BCP systems have been explored to improve the membrane performance in terms of transport and separation properties. A challenging application of BCP films was demonstrated by Kim and coworkers using cylinder-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films, where the PMMA block was selectively removed in order to create nanopores that filtrate human rhinovirus type 14 (HRV-14) with a specific size of 30 nm.3,19 Hillmyer, Cussler, and coworkers introduced nanoporous cylinders made of PS-bpoly(lactide) (PS-b-PLA) films, which were prepared during the solvent-casting and selective removal of PLA block in a basic condition.23 Unlike the above degradative methods for pore generation, Wang and coworkers reported the straightforward nanopore generation from PS-b-poly(2-vinylpiridine) (PS-b-P2VP) films via selective swelling of the P2VP block with solvent vapor and deswelling process.24,25 They further developed nanoporous membranes in the similar manner using a triblock copolymer of polyisoprene-b-PS-b-P2VP (PI-bPS-b-P2VP), which reveals higher permeability than PS-b-P2VP films due to the accelerated swelling process of PI block.26 Several coworkers of us also used a non-degradative approach to fabricating nanopores from PSb-PMMA films, where the tunable nanopores were achieved by controlling the swelling of the PMMA block and deswelling process.27 An attractive feature of these non-degradative approaches highlights that the polymer chains are undamaged, retaining the structural stability of the BCP films.

ACS Paragon Plus Environment

2

Page 3 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Furthermore, it is indisputable to utilize chemical functionality in such nanopores to capture or filtrate target molecules or compounds, which has been demonstrated in the recent UF membrane applications. In particular, the pH-responsive selectivity to biomolecules was demonstrated by Peinemann, Abetz, and coworkers using an asymmetric type membrane consisting of amphiphilic PS-b-poly(4-vinylpyridine) (PS-b-P4VP) films based on the nonsolvent induced phase separation (NIPS) method.28 Phillip, Boudouris, and coworkers also reported functional membranes derived from triblock copolymer of PI-b-PS-b-poly(N,N-dimethylacrylamide) (PI-b-PSb-PDMA) based on the NIPS method, where the DMA units were hydrolyzed into acrylic acid (AA) units to provide the pH-responsive size selectivity.29 Despite all these efforts for nanoporous membranes, the approaches to chemical modification in on-demand directional and uniform nanopores imposed more difficulty in postfunctionalization of BCP films as well as the desired orientation of nanochannels. In this study, we utilized a PS-b-PMMA-b-poly(tert-butylacrylate) (PS-b-PMMA-b-PtBA) that was tailored to set a continuous cylindrical morphology of PMMA-b-PtBA in PS matrix, because the protecting tBA units in the minor end-block are a versatile precursor to apply to other functionalities. For directional and uniform nanochannels at the interfaces, we exploited a perpendicular orientation of cylinders near two interfaces of air/polymer and polymer/neutral substrate, while maintaining the random orientation in the interior of films. Nondegradative method for nanopore generation such as selective swelling-deswelling process of cylindrical (PMMA-b-PtBA) blocks and create tunable nanopores, and the subsequent hydrolysis of tBA units functionalizes the nanopore surfaces and walls into PAA layers. The pH-responsive performance of nanoporous BCP membranes due to functional PAA layers was implemented in terms of water permeability and selectivity with respect to a biomolecule of Bovine Serum Albumin (BSA).

3


Page 4 of 23

EXPERIMENTAL Synthesis and sample preparation PS-b-PMMA-b-PtBA triblock copolymer was synthesized by sequential anionic polymerization of styrene (S), methyl methacrylate (MMA) and tert-butyl acrylate (tBA) in tetrahydrofuran (THF) solvent using secbutyllithium (1.4 M in cyclohexane, Aldrich) as an initiator; this synthesis was performed at -78 oC in the presence of LiCl (high purity, Aldrich) under purified argon atmosphere. The number-averaged molecular weight (Mn), characterized by size-exclusion chromatography (SEC), was measured to be 85 000 g/mol with narrow dispersity (Đ = Mw/Mn) less than 1.05. Volume fractions of PS (ϕPS = 0.720), PMMA (ϕPMMA = 0.196), and PtBA (ϕPtBA = 0.084) were determined by 1H NMR using the mass densities of each block (1.05, 1.184 and 1.02 g/cm3 for PS, PMMA and PtBA, respectively). A hydroxyl-terminated P(S-r-MMA) from the prior study27 was used for the interfacial neutrality toward the BCP using 1 wt% solution in toluene,30 because the PS-b-PMMA-b-PtBA with minor end-PtBA block (8.4 %) is presumed to work as a cylinder-forming (PMMA-minor) PS-b-PMMA. The BCP films were prepared directly onto the neutral substrate by spin-coating using BCP solutions (5 wt%) in toluene to get the film thickness of 200 nm. The film thickness was measured by ellipsometry (SE MG-1000, Nanoview Co.) at an incidence angle of 70o, where a Cauchy’s equation was applied to nanoporous BCP films to correlate with the refractive index (n). The BCP films were subjected to thermal annealing at 170 oC for 48 h under vacuum. A swelling-deswelling process was used to control over the pore size, in which the BCP films were treated with acetic acid for various periods of time, then immediately rinsed with ethanol for 30 min. Finally, the films were dried under vacuum at room temperature. To functionalize the nanopore surfaces and walls, the films were treated (or hydrolyzed) with 3M HCl solution for 48 h; this process allows the hydrolysis of tBA units into poly(acrylic acid) (PAA) layers.


4

Page 5 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Characterization of BCP Films. Grazing incidence small angle X-ray scattering (GISAXS) experiments were performed at the 9A and 4C beamlines at Pohang Accelerator Laboratory (PAL), Korea, using the similar operating conditions of the prior report.31 Incidence angle (αi) was varied from 0.090 to 0.140 o, below and above the critical angle (0.113 o) to probe the structures from surface to entire films, respectively. Surface and cross-sectional morphologies of BCP films were examined by field-emission scanning electron microscopy (JEOL-7001F, JEOL) with a semi-in-lens detector at an accelerating voltage of 5.0 kV. A contact angle meter (CAM-101, KSV Instrument Ltd.) was used in a static mode to measure water contact angle (WCA) of BCP films with different periods of hydrolysis.

Membrane performance. To assemble a composite ultrafiltration (UF) membrane, nanoporous BCP films were delaminated from the substrate using a 5 wt% HF solution and transferred to the top of macroporous supportive membrane which is a 150-μm thick poly(vinylidene fluoride) (PVDF; Millipore) film. A lab-made membrane test cell (Figure S1) was devised to measure the pH-responsive water permeability of nanoporous BCP membranes and selectivity with respect to Bovine Serum Albumin (BSA). The cell was set at a 500-ml working volume and an effective membrane area of 0.636 cm2. The water flux was measured at each pressure of 0.5, 1.0, 1.5, and 2.0 bar by varying pH from 2.0 to 10.5. A BSA disperse solution (Sigma-Aldrich) was filtered through nanoporous BCP films to characterize pH-responsive selectivity, where the concentrations of BSA in feed solution and the filtrates were analyzed at the maximum absorption of λ = 590 nm by UV-Vis absorption spectroscopy (Lambda 750S, Perkin Elmer) using a Bradford assay.32 RESULTS AND DISCUSSION

5


Page 6 of 23

Figure 1. (a) Top and bottom view SEM images of 200-nm thick PS-b-PMMA-b-PtBA film. The film was prepared directly onto a substrate modified with a random copolymer composed of S and MMA (for neutrality), then the BCP films were subjected to thermal annealing at 170 oC for 48 h under vacuum. (b) Cross-sectional SEM image of the BCP film. (c) 2D GISAXS pattern of the BCP film. The inset shows the primary and higher-order peaks scanned along the 2θf at a constant αf = 0.240o correspond to the characteristic peak ratios of q/q* = 1:√3: (√4): √7.

A 200-nm thick PS-b-PMMA-b-PtBA film was prepared directly onto the neutral substrate modified with a hydroxyl-terminated P(S-r-MMA), then the BCP film was thermally annealed at 170 oC for 48 h under vacuum. Note that no thermal deprotection (or crosslinking) of tBA units occurs at 170 oC, as confirmed by an isothermal TGA result (Figure S2). Figure 1a and 1b show the top and bottom view SEM images of the BCP film, and crosssectional view, respectively, where the PMMA and minor end-PtBA blocks were selectively removed via UV (λmax = 254 nm) irradiation and the subsequent rinsing in acetic acid for structural analysis.33 For the bottom view SEM image, the BCP film was delaminated on the surface of 5 wt% HF solution and reversely transferred to a


6

Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fresh Si substrate, followed by the short-time plasma etching with O2/Ar (5/1 in volume ratio) gas mixture; this process trims an ultrathin brush layer (~ 5 nm) to enhance the phase contrast. Similar with a cylinder-forming (PMMA-minor) PS-b-PMMA, hexagonally packed pores on the surface and bottom indicate the cylinders oriented perpendicular to two interfaces of air/polymer and polymer/substrate, which are attributed to the balanced interfacial interactions considering the negligible fraction of end-PtBA block. However, the interconnectivity observed in a cross-sectional SEM image represents the random orientation in the interior of films. Figure 1c displays 2D GISAXS pattern of the BCP film, in which αf and 2θf are the exit angles of X-ray beam along the out-of-plane scattering normal to the sample surface and along the in-plane scattering normal to the incidence plane, respectively. To ensure the structure of entire film in comparison with the SEM images, incidence angle (αi) was set at 0.120o, which is larger than the critical angle (0.115o) for PS-b-PMMA-b-PtBA film. The two elliptical patterns (marked with blue arrows) arise from the random orientation of cylinders in the film interior, and the primary and higher-order peaks scanned along the 2θf at a constant αf = 0.240o correspond to the characteristic peak ratios of q/q* = 1:√3: (√4): √7 (inset), where q = (4π/λ) sin θf, indicative of the perpendicular orientation of cylinders near two interfaces of air/polymer and polymer/substrate. For a thermally annealed film of PS-b-PMMA-b-PtBA on a modified substrate, results manifest that the balanced interfacial interactions are limited near two interfaces due to nearly equal surface energy between the two blocks at air/polymer interface and a neutrality toward PS and PMMA blocks at polymer/substrate interface. However, these interactions dissipate with distance from the interfaces, leading to the desired random orientation of cylinders that may protect the BCP films against plastic fracture normal to unidirectional loads.19 An average diameter (D) of cylinders, composed of the PMMA and minor end-PtBA blocks, is calculated to be 25.7 nm by

𝐷=

4𝑑 √3 [ (1 √3 2𝜋

0.5

− 𝜙PS )]

(1)


7


Page 8 of 23

where d = 40.02 nm and ϕPS = 0.72 are the inter-lattice distance (d-spacing)34 of cylinders by d = 2π/q* (using q* = 0.157 nm-1 obtained from the line scan) and PS (matrix) volume fraction, respectively. For nanopore generation, the BCP films were treated with acetic acid for various periods of time, then immediately rinsed with ethanol. During this non-degradative process, the acetic acid selectively solubilizes hexagonally packed cylinders that contain end-PtBA block, then poly(meth)acrylates (or PMMA-b-PtBA) chains shrink to the nanopore surfaces and walls of PS matrix upon rinsing with a non-solvent of ethanol, leading to well-defined cylindrical nanopores.

Figure 2. Top view SEM images of PS-b-PMMA-b-PtBA films immersed into acetic acid for swelling time of (a) 30 s and (b) 60 min, followed by immediate deswelling with ethanol. (c) Effective pore diameter (Deff) and thickness ratio (t/to) of nanoporous films with increasing period of swelling process, where the Deff is calculated from t/to of PS-bPMMA-b-PtBA films. (d) Schematic views of nanopore gereration, where t and to denote swollen and initial thicknesses of BCP films, respectively, in hexagonal surface area (S) of a unit cell. ACS Paragon Plus Environment

8

Page 9 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As seen in top view SEM image analysis of PS-b-PMMA-b-PtBA films immersed into acetic acid for 30 s (Figure 2a) and 60 min (Figure 2b), longer swelling the BCP films in acetic acid produces larger and clearer nanopores. However, their pore sizes are not quantifiable because greater diameters on surface images arise from the end-rings of tapered nanopores. Keeping in mind that the diameter of nanopores are tunable with the selective swelling of cylinders (and immediate deswelling with ethanol), the average pore size formed during swellingdeswelling process is reflected in thickness variation of the BCP films. Figure 2c shows the effective pore diameter (Deff) of nanoporous films with increasing period of swelling process, where the pore diameters are calculated from the thickness ratios of PS-b-PMMA-b-PtBA films. Film thicknesses were measured by ellipsometry and confirmed by AFM height difference after scratching off BCP films from substrate using a plastic razor blade. Since an increase in film thickness during swelling-deswelling process is caused by partial plastic deformation to generate nanopores throughout the BCP films, as shown in Figure 2d, the Deff of nanopores is calculated by 4 𝑡 Deff =√3𝜋 𝑆(1 − 𝑜⁄𝑡)

(2)

where t and to denote swollen and initial thicknesses of BCP films, respectively, in hexagonal surface area (S) of a unit cell. Despite the defects, it is pressumable that a hexagonal unit cell retains throughout the BCP film as the characteristic peak ratios of q/q* = 1:√3: (√4): √7 were identified along the in-plane direction from 2D GISAXS pattern (Figure 1c). The Deff significantly increases to 4.5 nm at the early period (30 s) of swelling, then it gently approaches to 10.4 nm at 30 min, further maintaining a plateau value of 10.5 nm for longer swelling. For subsequent nanopore functionalization, we set a nanoporous precursor as the BCP film having Deff = 11.3 nm, taken at 60 min of swelling. It should be pointed out that the nanopore generation during swelling process is associated with the selective solubility of the PMMA block in acetic acid rather than that of the end-PtBA block.


9


Page 10 of 23

Figure 3. (a) Acid catalyzed transesterification from PS-b-PMMA-b-PtBA to PS-b-PMMA-b-PAA, where the tBA (red color) units are hydrolyzed into AA (blue color) units in 3M HCl sulution at 50 oC. (b) Static water contact angle (WCA) of nanoporous BCP films with increasing hydrolysis time using a nanoporous precursor film taken at 60 min of swelling. The red symbol indicates 92.6o of WCA for a thermally annealed BCP film with no nanopores. Two insets display the cross-section view images of each WCA measurement. (c) Deff of nanoporous BCP films with increasing hydrolysis time. (d) Schematic illustration of nanopore generation and functionalization process for the BCP films. ACS Paragon Plus Environment

10

Page 11 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3a depicts an acid catalyzed transesterification reaction from PS-b-PMMA-b-PtBA to PS-b-PMMA-bPAA, where the tBA (red color) units were hydrolyzed into AA (blue color) units.35 A nanoporous precursor films were gently treated in 3M HCl solution at 50 oC to convert the minor end-PtBA block in nanopores into PAA block, while maintaining the PMMA block intact (Figure S3). Hydrolysis time dependence on surface property of nanoporous BCP film is shown in Figure 3b by static water contact angle (WCA). From a thermally annealed BCP (with no nanopores) film to nanoporous precursor film (taken at 60 min of swelling in acetic acid), a remarkable drop in WCAs from 92.6o (marked with red color) to 76.4o during swelling-deswelling process reflects the surface reconstruction by relocation of PMMA block from cylinders to the surface of PS matrix by burying the minor end-PtBA block, because the PMMA block (71.6o) is hydrophilic than PtBA block (88o) despite an increase in nanoporosity.36 For better description, Figure 3d demonstrates the simple schematic illustration of nanopore generation and functionalization process for the BCP films. Along with subsequent hydrolysis of end-PtBA block, the WCA decreases to the minimum of 64.8o for 110h because the nanopore surfaces and walls are converted into hydrophilic PAA layers. Figure 3c shows Deff of nanoporous BCP films with increasing hydrolysis time in 3M HCl solution, in which the pore diameters were calculated from the thickness ratios of PS-b-PMMA-b-PtBA films. Unlike the nanopore generation during swelling-deswelling process, the Deff decreases to 7.0 nm with increasing hydrolysis time. As the removal of tBA units in the minor end-PtBA block (8.4 %) is so negligible as to correlate with a decrease in film thickness, it is more plausible to take account the change in molecular interactions. Hence, Flory-Huggins interaction parameter (χ) between the two blocks can be expressed as a function of the difference between solubility parameters (δi) of components by χ~

V 𝑅𝑇

(𝛿1 − 𝛿2 )2

(3)

where V and R denote a molar volume and the ideal gas constant, respectively. Considering δPMMA = 19.88, δPtBA = 18.46, and δPAA = 24.09 J1/2/cm3/2, the chemical modification from PtBA into PAA block will lead to a


11


Page 12 of 23

remarkable increase in χ between the two minor blocks. In addition to an increase in segregation power between PMMA and PAA blocks, the minor PAA block becomes soluble in water via hydrogen bonding. Therefore, it can be speculated that larger population of AA units after hydrolysis enhances microphase separation between the PMMA and PAA blocks, leading to PAA block stretched toward core of nanopores (Figure 3d). Since the above results demonstrate the control over nanopores and functionalization (to Deff = 7.0 nm) based on a nanoporous precursor film having the largest Deff = 11.3 nm, it would be unlimited to smaller nanopore and further functionalization.

Figure 4. (a) 2D GISAXS patterns of PS-b-PMMA-b-PtBA film, measured αi = 0.120o after swelling-deswelling process (left) and subsequent hydrolysis in 3M HCl solution at 50 oC for 48 h (right). The black and blue arrows indicate the characteristic peak ratios of q/q* = 1:√3: (√4): √7 and two elliptical patterns, respectively. (b) d-spacing (d = 2π/q*) and FWHM of BCP films during swelling–deswelling process and subsequent hydrolysis. The dashed line denotes d-spacing (40.0 nm) of a thermally annealed BCP film for comparison.


12

Page 13 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4a shows 2D GISAXS patterns of PS-b-PMMA-b-PtBA films, measured at αi = 0.120o after swelling (in acetic acid for 60 min)-deswelling process (left) and subsequent hydrolysis in 3M HCl solution at 50 oC for 48h (right). As marked by black and blue arrows, the characteristic peak ratios of q/q* = 1:√3: (√4): √7 and two elliptical patterns, respectively, are relatively consistent with those displayed in Figure 1c. In addition, Figure 4b displays d-spacing (d = 2π/q*) and FWHM of BCP films during swelling-deswelling process and subsequent hydrolysis, which are evaluated along the 2θf at a constant αf = 0.240o from each 2D GISAXS pattern. A consistent d-spacing of BCP film was measured during nanopore generation and functionalization process, compared with a d-spacing (40.0 nm, denoted by a dashed line) of thermally annealed BCP film. However, a little increase (~ 10%) in FWHM occurs due to partial plastic deformation during nanopore generation, and it maintains during functionalization process. These results indicate that the cylinder-forming BCP films are retained intact in lateral direction of film geometry during whole processes, confirming the structural stability of functional nanopores.

13


Page 14 of 23

Figure 5. (a) Water flux of composite UF membrane as a function of hydrostatic pressure applied normal to the nanoporous BCP surface, where pH is discretely varied from 2.0 to 10.5. (b) pH dependent water flux of the nanoporous BCP membrane at a constant pressure of 1.0 and 2.0 bar. The dotted red line indicates pKa for PAA. (c) pH-responsive separation performance of nanoporous BCP membrane using BSA disperse solution at a pressure of 1.0 bar. Two open symbols indicate the blocked points of feed solutions. The left inset displays a SEM image of blocked membrane measured at pH 4.5. The right inset shows UV-Vis absorption spectra of a feed solution with BSA and the filtrates through a composite UF membrane at different pH conditions. (d) Schematic illustration of BSA release behavior through a composite UF membrane at pIBSA = 4.7. ACS Paragon Plus Environment

14

Page 15 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To evaluate functionality of nanoporous BCP films in terms of transport and separation properties, a composite UF membrane was assembled with a macroporous PVDF film (150-μm thick with a pore diameter of 0.22 μm), as described in the prior report.27 A functional nanoporous BCP film (prepared after hydrolysis for 48h) was transferred onto the top of a supportive membrane (PVDF), immediately after delamination from the substrate using a 5 wt% HF solution. Figure 5a shows water flux of composite UF membrane as a function of hydrostatic pressure applied normal to the nanoporous BCP surface, where the pH of solutions was controlled by the different mixing ratios between potassium dihydrogen phosphate (KH2PO4) and disodium hydrogen phosphate (Na2HPO4). A linear increase in water flux with increasing pressure indicates a typical water transport performance of a wellprepared nanoporous membrane up to 2.0 bar. At a constant pressure of 1.0 and 2.0 bar, however, the water flux of the membrane harnessed with functional nanopores decreases with increasing pH of solutions (associated with the ionic strength in buffer solutions), as shown in Figure 5b. The pH-responsive transport performance of nanoporous BCP membrane measures to be 164.6 L/m2h·bar at pH = 2.0 and it decreases to 48.5 L/m2h·bar at pH = 10.5, as an open functional membrane system that still retains the transport property. A decrease in water flux at basic (or high pH) condition reflects that the deprotonated or negatively charged (‒) PAA chains stretch normal to the wall in order to minimize the charge repulsion among AA units, whereas at acidic (or low pH) condition, the reduced hydrodynamic volume of neutral PAA layers on the walls produces larger pore sizes. In other words, the hydrodynamic conformation of PAA chains attached to the nanopore wall can be precisely tuned by controlling over pH. It should be noted that a prior study on BCP membranes prepared by NIPS showed very low water flux (0.6 L/m2h·bar) at high pH due to the closed pores in an irregular film structure.29, Our results, however, demonstrate that the nanoporous BCP film at the highest pH = 10.5 still exhibit a water flux of 48.5 L/m2h·bar, because the cylindrical nanopores are not collapsed but retained intact during nanopore generation and functionalization process, further confirming the non-degradative structural stability despite a local conformational change of PAA chains. Interestingly, a discontinuity-like drop in pH-dependent water flux, (observed between pH = 4.0 and pH

15


Page 16 of 23

= 6.0 as indicated in Figure 5a and 5b), is caused by the fact that the deprotonated PAA chains on the nanopore surfaces and walls dominate the system above an acidity constant (pKa,PAA = 4.3) of PAA.37 Figure 5c demonstrates pH-responsive separation performance of nanoporous BCP films using bovine serum albumin (BSA) disperse solution at 1.0 bar, in which the right inset shows UV-Vis absorption spectra of a feed solution with BSA and the filtrates through a composite UF membrane at different pH conditions. The BSA release (mole%) was calculated based on the Beer-Lambert law by, BSA release (mole%) =

𝐴𝑓 𝐴𝑜

× 100

(4)

where Ao and Af are the absorption intensities measured at 590 nm for a feed solution and the filtrates, respectively. The BSA release drastically decreases to 26.1%, as the pH of solutions increases from 2.0 to 4.0, indicating that the pore size in this regime decreases with increasing pH. At pH = 4.5 above pKa,PAA, the BSA disperse solution is entirely impenetrable to the BCP membrane, presumably because the positively charged (+) BSA infiltrates and blocks up the nanopore surfaces and walls composed of (‒) PAA chains, as displayed by a SEM image (the left inset of Figure 5c). More interestingly, the BSA is detected again as 23.9% only at pH = 4.7 corresponding to isoelectric point of BSA (pIBSA = 4.7), indicating a selective response of functional membrane to the neutral BSA, as depicted in Figure 5d. It can be also speculated that the neutral BSA is prone to pass through the nanopores surrounded by (‒) PAA chains. The other blocking behavior when pH = 6 (denoted by the open symbol) would be attributed to the repulsion of (‒) BSA against the nanopore surfaces and walls composed of (‒) PAA chains, rather than the reduced pore size.

CONCLUSIONS A stimuli-responsive UF membrane technology has been demonstrated by taking advantage of a continuous cylindrical morphology for directional and uniform nanochannels at the interfaces. A triblock copolymer assembly of PS-b-PMMA-b-PtBA was designed for a smart membrane structure, where the perpendicular ACS Paragon Plus Environment

16

Page 17 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cylinders consisting of the PMMA and minor PtBA blocks sandwich the random orientation of cylinders in interior of films due to the receding interfacial interactions from the interfaces. Nanopores throughout the BCP film were generated by non-degradative, selective swelling-deswelling process of cylindrical PMMA-b-PtBA blocks, which are precisely tunable in the pore size by controlling swelling period. The tBA units in the minor end-PtBA block were hydrolyzed into AA units to functionalize the nanopore surfaces and walls into PAA layers, and the conversion was traced by a decrease in WCA as a function of hydrolysis time. In contrast to an increase in Deff during nanopore generation (swelling-deswelling process), the Deff decreases to 7.0 nm with increasing hydrolysis time, maintaining the structural stability of functional nanopores during whole processes. The pH-responsive water permeability of nanoporous BCP membranes manifests that the water flux decreases as the solution property changes from acidic to basic conditions. Such a discontinuity-like drop in pH dependent water flux is attributed to the change in hydrodynamic conformation of PAA chains undergoing pKa,PAA = 4.3. Particularly in solution at pH = 4.7 corresponding to pIBSA, the functional membranes from nanoporous BCP films release the BSA as 23.9 %, whereas at above pH = 4.5, the BSA disperse solution is entirely impenetrable to the BCP membrane, demonstrating the pH-responsive selectivity with respect to a protein dispersion. Our results suggest a non-degradative approach using a cylinder-forming multiblock copolymer to well-defined nanoporous BCP membranes. Using directional nanopores for post-functionalization,38 this approach may be applied to other functionalities for biomolecular separation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.xxxxxxx.

17


Page 18 of 23

Description of the lab-made membrane test cell, isothermal TGA results of PS-b-PMMA-b-PtBA, and static WCA for 170-nm thick PMMA films (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (C.Y.R.). *E-mail: [email protected] (D.Y.R.). ORCID Chang Y. Ryu : 0000-0002-8013-0328 Du Yeol Ryu: 0000-0002-0929-7934 Author Contributions S. P., T. J. and, H. R. Y. contributed equally to the article.

ACKNOWLEDGMENTS This research was supported by the NRF grants (2017R1A2A2A05001048, 2017R1A4A1014569) funded by the Ministry of Science, ICT & Future Planning (MSIP), and funding (20163030013960) from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE), Korea. CR also acknowledges funding in part by the NSF grant (DMR Polymers-1308617).

REFERENCES (1) Druecke, D.; Langer, S.; Lamme, E.; Pieper, J.; Ugarkovic, M.; Steinau, H. U.; Homann, H. H. Neovascularization of Poly(ether ester) Block-copolymer Scaffolds in vivo: Long-term Investigations using Intravital Fluorescent Microscopy. J. Biomed. Mater. Res., Part A 2004, 68A, 10-18.


18

Page 19 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2) Lin, Z.; Cao, S.; Chen, X.; Wu, W.; Li, J. Thermoresponsive Hydrogels from Phosphorylated ABA Triblock Copolymers: A Potential Scaffold for Bone Tissue Engineering. Biomacromolecules 2013, 14, 22062214. (3) Yang, S. Y.; Ryu, I.; Kim, H. Y.; Kim, J. K.; Jang, S. K.; Russell, T. P. Nanoporous Membranes with Ultrahigh Selectivity and Flux for the Filtration of Viruses. Adv. Mater. 2006, 18, 709-712. (4) Wang, Y.; Li, F. An Emerging Pore-Making Strategy: Confined Swelling-Induced Pore Generation in Block Copolymer Materials. Adv. Mater. 2011, 23, 2134-2148. (5) Surwade, S. P.; Smirnov, S. N.; Vlassiouk, I. V.; Unocic, R. R.; Veith, G. M.; Dai, S.; Mahurin, S. M. Water Desalination using Nanoporous Single-layer Graphene. Nat. Nanotechnol. 2015, 10, 459. (6) Jain, T.; Rasera, B. C.; Guerrero, R. J. S.; Boutilier, M. S. H.; O'Hern, S. C.; Idrobo, J.-C.; Karnik, R. Heterogeneous Sub-continuum Ionic Transport in Statistically Isolated Graphene Nanopores. Nat. Nanotechnol. 2015, 10, 1053-1057. (7) Yoo, S.; Kim, J.-H.; Shin, M.; Park, H.; Kim, J.-H.; Lee, S.-Y.; Park, S. Hierarchical Multiscale Hyperporous Block Copolymer Membranes via Tunable Dual-phase Separation. Sci. Adv. 2015, 1, e1500101 (8) Xie, M.; Wang, J.; Wang, X.; Yin, M.; Wang, C.; Chao, D.; Liu, X. The High Performance of Polydopamine-coated Electrospun Poly(ether sulfone) Nanofibrous Separator for Lithium-ion Batteries. Macromol. Res. 2016, 24, 965-972. (9) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Polymeric Systems for Controlled Drug Release. Chem. Rev. 1999, 99, 3181-3198. (10) Yang, S. Y.; Yang, J.-A.; Kim, E.-S.; Jeon, G.; Oh, E. J.; Choi, K. Y.; Hahn, S. K.; Kim, J. K. Single-File Diffusion of Protein Drugs through Cylindrical Nanochannels. ACS Nano 2010, 4, 3817-3822. (11) Lee, A.; Elam, J. W.; Darling, S. B. Membrane Materials for Water Purification: Design, Development, and Application. Environ. Sci.: Water Res. Technol. 2016, 2, 17-42. (12) Darling, S. B. Perspective: Interfacial Materials at the Interface of Energy and Water. J. Appl. Phys. 2018, 124, 030901.

19


Page 20 of 23

(13) Thurn-Albrecht, T.; Schotter, J.; Kästle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Ultrahigh-Density Nanowire Arrays Grown in Self-Assembled Diblock Copolymer Templates. Science 2000, 290, 2126-2129. (14) Hashimoto, T.; Tsutsumi, K.; Funaki, Y. Nanoprocessing Based on Bicontinuous Microdomains of Block Copolymers: Nanochannels Coated with Metals. Langmuir 1997, 13, 6869-6872. (15) Chan, V. Z.-H.; Hoffman, J.; Lee, V. Y.; Iatrou, H.; Avgeropoulos, A.; Hadjichristidis, N.; Miller, R. D.; Thomas, E. L. Ordered Bicontinuous Nanoporous and Nanorelief Ceramic Films from Self Assembling Polymer Precursors. Science 1999, 286, 1716-1719. (16) Crossland, E. J. W.; Nedelcu, M.; Ducati, C.; Ludwigs, S.; Hillmyer, M. A.; Steiner, U.; Snaith, H. J. Block Copolymer Morphologies in Dye-Sensitized Solar Cells: Probing the Photovoltaic Structure−Function Relation. Nano Lett. 2009, 9, 2813-2819. (17) Hsueh, H.-Y.; Chen, H.-Y.; She, M.-S.; Chen, C.-K.; Ho, R.-M.; Gwo, S.; Hasegawa, H.; Thomas, E. L. Inorganic Gyroid with Exceptionally Low Refractive Index from Block Copolymer Templating. Nano Lett. 2010, 10, 4994-5000. (18) Peinemann, K.-V.; Abetz, V.; Simon, P. F. W. Asymmetric Superstructure Formed in a Block Copolymer via Phase Separation. Nat. Mater. 2007, 6, 992-996. (19) Yang, S. Y.; Park, J.; Yoon, J.; Ree, M.; Jang, S. K.; Kim, J. K. Virus Filtration Membranes Prepared from Nanoporous Block Copolymers with Good Dimensional Stability under High Pressures and Excellent Solvent Resistance. Adv. Funct. Mater. 2008, 18, 1371-1377. (20) Phillip, W. A.; Amendt, M.; O’Neill, B.; Chen, L.; Hillmyer, M. A.; Cussler, E. L. Diffusion and Flow Across Nanoporous Polydicyclopentadiene-Based Membranes. ACS Appl. Mater. Interfaces 2009, 1, 472-480. (21) Wang, Z.; Yao, X.; Wang, Y. Swelling-induced Mesoporous Block Copolymer Membranes with Intrinsically Active Surfaces for Size-selective Separation. J. Mater. Chem. 2012, 22, 20542-20548. (22) Jackson, E. A.; Hillmyer, M. A. Nanoporous Membranes Derived from Block Copolymers: From Drug Delivery to Water Filtration. ACS Nano 2010, 4, 3548-3553. ACS Paragon Plus Environment

20

Page 21 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Phillip, W. A.; O’Neill, B.; Rodwogin, M.; Hillmyer, M. A.; Cussler, E. Self-assembled Block Copolymer Thin Films as Water Filtration Membranes. ACS Appl. Mater. Interfaces 2010, 2, 847-853. (24) Yin, J.; Yao, X.; Liou, J.-Y.; Sun, W.; Sun, Y.-S.; Wang, Y. Membranes with Highly Ordered Straight Nanopores by Selective Swelling of Fast Perpendicularly Aligned Block Copolymers. ACS Nano 2013, 7, 9961-9974. (25) Guo, L.; Wang, Z.; Wang, Y. Perpendicular Alignment and Selective Swelling-Induced Generation of Homopores of Polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) Triblock Terpolymer. Macromolecules 2018, 51, 6248-6256. (26) Wang, Z.; Wang, Y. Highly Permeable and Robust Responsive Nanoporous membranes by Selective Swelling of Triblock Terpolymers with a Rubbery Block. Macromolecules 2015, 49, 182-191. (27) Ahn, H.; Park, S.; Kim, S.-W.; Yoo, P. J.; Ryu, D. Y.; Russell, T. P. Nanoporous Block Copolymer Membranes for Ultrafiltration: A Simple Approach to Size Tunability. ACS Nano 2014, 8, 11745-11752. (28) Nunes, S. P.; Behzad, A. R.; Hooghan, B.; Sougrat, R.; Karunakaran, M.; Pradeep, N.; Vainio, U.; Peinemann, K.-V. Switchable pH-Responsive Polymeric Membranes Prepared via Block Copolymer Micelle Assembly. ACS Nano 2011, 5, 3516-3522. (29) Mulvenna, R. A.; Weidman, J. L.; Jing, B.; Pople, J. A.; Zhu, Y.; Boudouris, B. W.; Phillip, W. A. Tunable Nanoporous Membranes with Chemically-tailored Pore Walls from Triblock Polymer Templates. J. Membr. Sci. 2014, 470, 246-256. (30) Ham, S.; Shin, C.; Kim, E.; Ryu, D. Y.; Jeong, U.; Russell, T. P.; Hawker, C. J. Microdomain Orientation of PS-b-PMMA by Controlled Interfacial Interactions. Macromolecules 2008, 41, 6431-6437. (31) Park, S.; Kim, Y.; Lee, W.; Hur, S.-M.; Ryu, D. Y. Gyroid Structures in Solvent Annealed PS-b-PMMA Films: Controlled Orientation by Substrate Interactions. Macromolecules 2017, 50, 5033-5041. (32) Ernst, O.; Zor, T. Linearization of the Bradford Protein Assay. J. Visualized Exp.: JoVE 2010, 1918. (33) Kim, K.; Park, S.; Kim, Y.; Bang, J.; Park, C.; Ryu, D. Y. Optimized Solvent Vapor Annealing for LongRange Perpendicular Lamellae in PS-b-PMMA Films. Macromolecules 2016, 49, 1722-1730.

21


Page 22 of 23

(34) Gong, J.; Ahn, H.; Kim, E.; Lee, H.; Park, S.; Lee, M.; Lee, S.; Kim, T.; Kwak, E.-A.; Ryu, D. Y. Rapid Structural Reorganization in Thin Films of Block Copolymer Self-assembly. Soft Matter 2012, 8, 3570-3575. (35) Wu, T.; Gong, P.; Szleifer, I.; Vlček, P.; Šubr, V.; Genzer, J. Behavior of Surface-Anchored Poly(acrylic acid) Brushes with Grafting Density Gradients on Solid Substrates: 1. Experiment. Macromolecules 2007, 40, 8756-8764. (36) Barbey, R.; Lavanant, L.; Paripovic, D.; Schuwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Polymer Brushes via Surface-initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and Applications. Chem. Rev. 2009, 109, 5437-5527. (37) Peez, R. F.; Dermody, D. L.; Franchina, J. G.; Jones, S. J.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Aqueous Solvation and Functionalization of Weak-acid Polyelectrolyte Thin Films. Langmuir 1998, 14, 4232-4237. (38) Liu, E. Y.; Jung, S.; Yi, H. Improved Protein Conjugation with Uniform, Macroporous Poly(acrylamideco-acrylic acid) Hydrogel Microspheres via EDC/NHS Chemistry. Langmuir 2016, 32, 11043-11054.


22

Page 23 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

23

Nanoporous Structures from PS-b-PMMA-b-PtBA Triblock Copolymer

Recommend Documents