Response Sensitivity of a Gating Membrane Related to Grafted

Jan 8, 2016 - In the present study, the relationship between the permeable properties of stimuli-responsive gating membranes and the structural proper...
0 downloads 5 Views 1MB Size
Download PDF
Article pubs.acs.org/IECR

Response Sensitivity of a Gating Membrane Related to Grafted Polymer Characteristics Hidenori Ohashi,† Xueqin Chi,† Hidenori Kuroki,†,‡ and Takeo Yamaguchi*,†,‡ †

Chemical Resources Laboratory, Tokyo Institute of Technology, R1-17, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan ‡ Kanagawa Academy of Science and Technology, R1-17, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan ABSTRACT: In the present study, the relationship between the permeable properties of stimuli-responsive gating membranes and the structural properties of surface-grafted polymers was investigated. The grafting initiation rate and the resultant characteristics of the grafted polymers were altered by controlling the grafting temperature in acid-assisted plasma-induced graft polymerization. The grafted polymers were isolated from the membranes to investigate their structural properties, including molecular weight and grafting density. The molecular weights of the grafted polymers were sufficiently high to fill the pore in the polymer swollen state. Therefore, the grafting density is strongly associated with the open/close gating performance of the membranes. A higher grafting density enhances both the pore-close and pore-open performances, while a lower grafting density impairs both open and close performances. The hydrodynamic dimensions and the structures of the grafted polymers, as well as membrane pore size, were considered when investigating the underlying mechanism of this applicative phenomenon.

1. INTRODUCTION Systems of stimuli-responsive channels in living organisms inspire the synthesis of artificial gating materials, particularly for applications such as separation and refinement processes, controlled delivery of drugs, and chemical- and biosensors.1−4 A number of synthetic gating materials have been developed so far through the grafting of stimuli-responsive polymers onto pore walls in porous substrates (e.g., nanopore films, porous capsules, and membranes).5−19 The volume of a stimuliresponsive grafted polymer can be changed by polymerswelling/shrinking in response to specific stimuli (e.g., temperature,11,12 pH,19,20 electric field,21,22 magnetic field,23,24 light,25,26 and specific chemical signals16−18,27). Changes to polymer volumes in nano- and microsized pores regulate poreopening/closing, leading to the control of molecular weight cutoff of dextran, and the permeability changes of molecules (ions, vitamin B12, and lysozyme) by diffusion retarding effect of swollen polymer clogging pores.15,16 In our previous studies, we developed thermal- and ion-responsive gating membranes prepared by the plasma-induced graft polymerization (PIGP) technique; these membranes exhibited ten- to hundred-fold changes in their water permeability upon their response to stimuli.15,16,28 The analysis of grafted polymers isolated from their porous substrates suggested that such dramatic permeability changes were strongly related to the properties of the polymers grafted onto the pores.28 However, although the structural properties of the grafted polymers definitely affected the gating functions of the membranes, the relationship between them remains unclear. Particularly, the influence of molecular weight and grafting density on swelling/shrinking behaviors inside the pores needs to be clarified to achieve the desired clear open/close gating functions. While temperatures of approximately 80 °C are required to induce the initiation process of conventional PIGP to cleave peroxide groups to produce alkoxy radicals in aqueous solution, © XXXX American Chemical Society

recently, pH was found to be another initiation-determining factor.29 The cleavage of peroxide groups can be induced by the presence of acid (acid-assisted PIGP) even at 25 °C, providing a much wider reaction-temperature window than the conventional thermal method; therefore, acid-assisted PIGP could change the initiation and grafting rates when using a sulfonic acid containing monomer by altering the required temperature.29 As the grafting density is directly related to the initiation reaction rate, it is thus controllable by acid-assisted PIGP. Accordingly in the present study, gating membranes with stimuli-responsive grafted polymers having different grafting densities and molecular weights were fabricated by changing the reaction temperatures and time. A thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) and a track-etched porous polycarbonate (PC) substrate were used as a grafted polymer and a porous substrate, respectively.28 PNIPAM was chosen because its gating properties can be investigated by simply changing the solution temperature, while the PC substrate was chosen because it can be decomposed under high-alkalinity conditions by hydrolysis of its ester bonds. Utilizing the substrate’s characteristics, the grafted polymers were isolated from the membranes and were analyzed. Then, the relationship between the grafted polymer characteristics and the pore-gating functions, in conjunction with the swelling/ shrinking of the grafted polymers, was investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. A track-etched polycarbonate membrane (Cyclopore, thickness 11 ± 1 μm, porosity 17%, BET specific Received: November 16, 2015 Revised: January 6, 2016 Accepted: January 8, 2016

A

DOI: 10.1021/acs.iecr.5b04332 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research surface area 3.99 m2/g, these data were measured for the samples of the same product, nominal pore size is 0.2 μm (+0%/−20%), nominal pore density is 3 × 108 pores/cm2), purchased from the SPI Supplies Division of Structure Probe, Inc., was used as the porous substrate. The membrane was cut into 5.5 cm × 5.5 cm-sized pieces, washed in hexane overnight, and then dried in a vacuum oven prior to use. The Nisopropylacrylamide monomer (kindly provided by Kohjin Co. Ltd., Japan) was recrystallized from an acetone/hexane mixture (25/75, v/v) to remove the polymerization inhibitor prior to use. All other chemicals were purchased from Wako Pure Chemical Industries, Ltd., Japan, and were used as received. Water purified by a reverse osmosis membrane using a Milli-Q system was used throughout this work. 2.2. Preparation of PC-g-PNIPAM Membrane. The grafted membrane was prepared using the PIGP techniqueperoxide method described in our previous reports.29 Briefly, a PC substrate was irradiated by an argon plasma (10 Pa, 30 W, 13.56 MHz) for 1 min and stored in dry, dark conditions for 2 days until the free radicals disappeared. The treated substrate was immersed in a 5 wt % NIPAM aqueous solution containing 5 wt % sodium dodecyl sulfate (SDS) (pH = 2, carefully adjusted using a sulfuric acid aqueous solution), subjected to nitrogen gas purging for 10 min, and incubated in a shaking water bath at the designated temperature (Tgr = 40 or 80 °C). As PNIPAM achieves swollen/shrunken states at temperatures lower/higher than the lower critical solution temperature (LCST), the difference in their states during polymerization may affect the propagation step of the grafting. In the present study, temperatures of 40 and 80 °C, both higher than the LCST (32 °C) of PNIPAM were chosen as grafting temperatures Tgr. In the two temperature conditions, both grafting PNIPAMs grow in shrunken state, and thus, the relationship between the grafting temperature and the characteristics of the resulting grafted polymers can be discussed. The graft polymerization was stopped at a prescribed time (tgr) by exposing the solution to open air. After grafting, the membrane was washed using an ethanol/water solution (50/50, v/v) for 24 h to remove the residual monomers, SDS, and nongrafted homopolymers. Subsequently, the fabricated PC-g-PNIPAM membrane was dried at 50 °C in a vacuum oven. The degree of grafting was evaluated by the filling ratio (ϕdry, %), which is defined as the ratio of graft polymer volume to the total pore volume of the substrate. The PNIPAM density ρPNIPAM is assumed to be 1.0 g/cm3 and ϕdry was calculated by eq 1. ϕdry =

(wdry − wPC)/ρPNIPAM AlϕPC

due to the hydrolysis of the polycarbonate ester bond upon exposure to the strong-alkaline solution, while the grafted PNIPAM remained. The isolated PNIPAM was precipitated as a solid due to the salting-out effect; therefore, it was easily separated from the reaction solution by filtration. The solidified polymer was then washed with a 5 M sodium chloride aqueous solution. The isolated PNIPAM was subjected to gel permeation chromatography (GPC) analysis (Hitachi pump L-7100, Hitachi Ltd.; Shodex Oven A0−30, Showa Denko K.K.; Shodex RI-71, Showa Denko K.K.; Shodex Asahipak GF7M HQ, KD-806M, and KD-807 columns, Showa Denko K.K.). Dimethylformamide containing 0.01 M LiBr was used as the eluent at a flow rate of 0.5 mL min−1. The GPC chromatogram was analyzed by Chromato-PRO-GPC data processing software (Run Time Corporation) based on the calibration curve of a polystyrene standard solution (TSK, Tosoh Corporation). It should be noted that molecular weights of higher than 8.8 × 106 may not have been precisely determined as these weights exceeded the linear range of the calibration curve. However, this upper molecular-weight range comprised less than 1% of the total polymer mass. The grafting density (σ, chains nm−2) of the grafted polymer on the pore surface is defined by eq 2 σ=

∑ Ni ∑ (MiNi) M ∑ Ni = n = Spore M nSpore M nSpore

(2)

where ∑(MiNi) and ∑Ni represent the total mass and total number, respectively, of grafted polymers on the surface of one pore. Spore (nm2) represents the BET surface area of one pore, while Mn (g mol−1) is the number-averaged molecular weight of the grafted polymers. Number of grafted polymer having molecular weight Mi in one pore (molecular number of grafted polymer per pore) can be calculated by using the following equation. number of grafted polymer per pore =

wdry − wPC Ni An poreM n N

(3)

where npore represents pore density (pores cm−2). Ni/N is number fraction of grafted polymer having molecular weight Mi. 2.4. Analysis of Stimuli-Responsive Behavior via Water Permeability Measurements. The stimuli-responsive gating behavior of the PC-g-PNIPAM membrane was observed by water permeability measurement using the apparatus described in a previous study.28 An appropriate pressure (0.1−1.0 kgf cm−2) was applied across the membrane to force the pure water to pass through the membrane. Each measurement was conducted at least 30 min after the system reached its specified temperature to ensure thermal equilibrium. The permeation flux J (m3 m−2 s−1) was calculated from the volume of the permeate per unit time per unit area. To eliminate the effect derived from the difference in the applied pressure ΔP and in the water viscosity μ at different temperatures, the permeability coefficient (Lp, m3 m−2 s−1/ (kgf cm−2)) was employed to evaluate the membrane permeability. Lp is calculated by eq 4:

× 100 (1)

, where A (cm2), l (cm), and ϕPC (−) are membrane area, thickness, and porosity of PC substrate, respectively. The terms wdry (g) and wPC (g) represent dry weight of grafted membrane and that of porous PC substrate before grafting, respectively. The obtained membrane was analyzed by Fourier transform infrared (FT-IR) spectroscopy using a JASCO FT/IR-6200 spectrophotometer. 2.3. Isolation and Analysis of Graft Polymers. The isolation of the grafted PNIPAM from the PC-g-PNIPAM membrane was conducted via a previously developed method.28 In brief, a PC-g-PNIPAM membrane was immersed in a 5 M sodium hydroxide aqueous solution and kept in an 80 °C shaking water bath for 3 days. The PC substrate decomposed

Lp =

J ΔP(μ25 /μT )

(4)

where ΔP (kgf cm−2) represents the pressure applied across the membrane, and μ25 (Pa s) and μT (Pa s) represent the media B

DOI: 10.1021/acs.iecr.5b04332 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research viscosities at 25 °C and at the temperature when measuring, respectively. The permeability coefficient for pure water of each membrane was measured within a temperature range from 25 to 38 °C. 2.5. Calculation of Graft Polymer Dimensions. To estimate the conformation of the graft polymer, the average Flory radius (Rf) of the grafted PNIPAM chain was calculated.30−33 The Rf of a homopolymer at the equilibrium state depends on the chain length and the solvent conditions, as defined by eq 5 R f = aN ν

(5)

Figure 1. Change in the filling ratio of the PC-g-PNIPAM membrane as a function of grafting time. The membrane was fabricated at 40 or 80 °C.

where a is the effective segment length corresponding to the effective segment length of the backbone and is assumed to be 0.3 nm for PNIPAM.34−36 N is the number of segments and can be calculated based on the Mw obtained from the GPC analysis in the present study. Also, ν is the factor decided by the solvent; generally, ν ≈ 3/5 in a good solvent and ν ≈ 1/3 in a poor solvent. Waters at 25 and 38 °C are regarded as a good solvent and a poor solvent, where free PNIPAM behaves like a random coil chain and uniform hard sphere, respectively.37,38 The dimensions, i.e., the thicknesses of the grafted PNIPAM in its shrunken state dshrink (m) were estimated via the membrane water permeability by assuming that uniform polymer layers were formed on the pore surface. The pore diameters of the membrane can be calculated using the Hagen− Poiseuille equation (eq 6) rp =

Figure 2. FT-IR spectra of the PC substrate and PC-g-PNIPAM membranes with different filling ratios. The peaks at 1650 and 1550 cm−1 are characteristic of PNIPAM.

8μ25 lLp ε

(6)

where l (m) is the membrane thickness and ε is the porosity of the substrate. By subtracting the pore diameter with the grafting polymer rp,graft (m) by that without the grafting polymer rp0 (m), as in eq 7, the polymer thickness dshrink can be calculated. dshrink = rp0 − rp,graft

cm−1 are assigned to the stretching of CO and the aromatic CC bond of the substrate, respectively. Peaks appeared at 1650 and 1550 cm−1 after grafting, corresponding to amide I and amide II of the amide groups of PNIPAM, and proportionally increased with the filling ratio, indicating the successful grafting of PNIPAM onto the PC substrate. 3.2. Characteristics of Grafted PNIPAM Polymer. To analyze the characteristics of the grafted polymers on the fabricated PC-g-PNIPAM membrane, the grafted PNIPAM was isolated by hydrolysis of the PC substrate. In the previous research,28 it was proven that PNIPAM grafted polymer was not hydrolyzed by the alkaline treatment, probably because PNIPAM is in shrunken condition in 5 M NaOH and is protected from hydrolysis. Figures 3a and b display the GPC chromatograms of PNIPAM isolated from PC-g-PNIPAM membranes fabricated at 40 and 80 °C, respectively. The molecular weight distribution of the grafted polymers derived from the GPC chromatograms is shown in Figure 4. In the present study, by connecting the appropriate GPC columns, linearity between retention time and logarithm of molecular weight can be retained up to ca. 107 g/mol, enabling a quantitative analysis for isolated polymers. The main characteristics of the grafted polymers are summarized in Table 1 and Figure 5. The polydispersion of the grafted polymers was the common feature between membranes fabricated at different temperatures. When the filling ratio ϕdry was increased, molecular weight distribution peak shifted to a higher molecular weight range, and the fraction of higher molecular weights increased. This indicates the growth of the polymer chain with the elongation of the reaction times. These results are in good agreement with previously reported results.28

(7)

Using the pore radii with/without the grafted polymer, the filling ratio of the wet shrunken grafted polymer ϕwet,shrink (%), which is the ratio of the wet shrunken grafting polymer volume to the total pore volume of the substrate, and the water content of wet shrunken grafting polymer wshrink (%), can be calculated by the following formulas: ϕwet,shrink = 100 ×

wshrink = 100 ×

π (rp0 − rp,graft)2 πrp0 2

(8)

ϕwet,shrink − ϕdry ϕwet,shrink

(9)

3. RESULTS AND DISCUSSION 3.1. Preparation of PC-g-PNIPAM Membrane. Figure 1 displays the change in filling ratio of the PC-g-PNIPAM membrane at 40 and 80 °C when the grafting time is increased. At each temperature, the filling ratio increased systematically with the grafting time proving that the reproducible membranes were made. The grafting rate was higher at 40 °C. These tendencies are similar to that observed for a previously investigated monomer containing sulfonic acid.29 Figure 2 compares the FT-IR spectra of a virgin PC substrate and PC-gPNIPAM at different filling ratios. The peaks at 1770 and 1600 C

DOI: 10.1021/acs.iecr.5b04332 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 5. Grafting time dependence of (a) number-averaged molecular weight and (b) grafting density of grafted PNIPAM fabricated by acidassisted PIGP at 40 and 80 °C.

temperature of 80 °C resulted in a higher grafting density σ than that for temperature of 40 °C, even in the case of a low filling ratio, as shown in Table 1 and Figures 4 and 5. This implies that a faster initiation process occurs in the beginning of graft polymerization at 80 °C than at 40 °C. In our previous study, the initiation process of the PIGP is found to be started by a decomposition of a peroxide group.29 The peroxide decomposition rate is faster at higher temperature; thus, the consumption of initiators is faster and the resultant grafting polymer becomes more crowded at 80 °C. Moreover, the most pronounced difference is that the molecular weight of the graft polymer fabricated at 40 °C is much higher than that at 80 °C. Though this seems contradictory to the idea that the polymer elongation reaction rate should be faster at higher temperatures; however, the lower molecular weight can be explained by a faster termination reaction and/or a slower monomer diffusion. The termination processes, combination and disproportion reaction, may affect the molecular weight distribution. Further research is required to clarify the details of chain propagation behaviors at different temperatures. Nevertheless, the above results prove that the PIGP method can graft polymers possessing different characteristics onto the porous membrane substrate. 3.3. Comparison of Stimuli-Responsive Gating Performance. Water permeability measurements were employed to examine the stimuli-responsive behavior of PC-g-PNIPAM membranes fabricated at different temperatures. Figure 6 compares the gating behavior of membranes fabricated at 40 and 80 °C. The two membranes have similar filling ratios, i.e., comparable degrees of grafting; when the grafting density varies with the grafting temperature, the molecular weight of the grafted polymer simultaneously changes. A filling ratio of approximately 12% was chosen because a filling ratio between 10% and 20% is reported to exhibit clear pore open/close functionality.28 For each membrane, the permeability coefficient changed at the LCST due to the swelling/shrinking behavior of the grafted PNIPAM in the substrate pore. There were some differences in the gating behavior between membranes fabricated at the two different temperatures.

Figure 3. GPC chromatogram of grafted PNIPAM isolated from PC-gPNIPAM membranes with different filling ratios fabricated via acidassisted PIGP at (a) 40 and (b) 80 °C.

Figure 4. Molecular weight distribution of grafted PNIPAM isolated from PC-g-PNIPAM membranes with different filling ratios fabricated by acid-assisted PIGP at (a) 40 and (b) 80 °C.

On the other hand, differences were observed among the grafted polymers fabricated at different temperatures. A grafting

Table 1. GPC Analysis Results for Grafted PNIPAM Polymers Isolated from PC-g-PNIPAM Membranes Fabricated at Different Temperatures Tgr (°C) 40

80

a

ϕdrya (%) 11 27 53 13 27 40

tgr (h)

Mn (g mol−1)

Mw (g mol−1)

Mw/Mn (−)

σb (nm−2)

0.5 2 3 1 4 8

× × × × × ×

× × × × × ×

2.6 2.1 2.2 3.7 2.4 3.2

0.003 0.004 0.009 0.012 0.013 0.022

1.1 2.0 1.9 3.5 6.6 5.9

6

2.8 4.2 4.1 1.3 1.6 1.9

10 106 106 105 105 106

6

10 106 106 106 106 106

Filling ratio of grafted polymer. bGrafting density of polymer chain calculated using eq 2. D

DOI: 10.1021/acs.iecr.5b04332 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 7. Schematic representation of the gating behaviors of PC-gPNIPAM membranes with differing graft polymer characteristics. Figure 6. Permeation coefficient (Lp) change as a function of temperature. The dashed line represents the permeability of the PC substrate. The solid gray and black lines represent the Lp of stimuliresponsive gating PC-g-PNIPAM membrane fabricated at 40 and 80 °C, respectively.

that the above assumption of similar water contents for the two grafted polymers may be different; the water contents of the diluted grafted polymers could be more than that of the crowded grafted polymers. The diluted polymers could form thicker layer on the pore surface with the more water contents inducing smaller permeability. This is another possible description for the permeation behavior. The water contents of the grafted polymer fabricated at 40 °C (wshrink,40) and that at 80 °C (wshrink,80) by calculating with dshrink,40 and dshrink,80 are 76% and 56%, and the order of these values is similar to that of linear PNIPAM in shrunken state, 66%.38 In a polymer swollen condition, 2Rf,swell is larger than the pore radius of the substrate (100 nm); that is to say, the pore is filled with sufficiently long grafted polymers for both membranes. However, the Lp for a membrane fabricated at 40 °C is smaller than that of a membrane fabricated at 80 °C. The cause of this disparity is considered to be related to the low grafting density of membranes fabricated at 40 °C, which is only one-quarter of that of membranes fabricated at 80 °C. Because of its low grafting density, the structure of the polymer layer, especially near the pore surface, seems to be loose and provides more space between the polymer chains for water permeation. The description that grafted polymers can contain interstices between them is well suited with the first description in polymer shrunken state. On the basis of the above discussions, a schematic of the observed phenomena is illustrated in Figure 7. In the shrunken state, the grafted polymer collapses to make a concentrated polymer layer on the surface, opening the pore. Scarcely grafted polymers forms mushroom structure and the height determines the pore size, whereas densely grafted polymers form thinner layers of higher water permeability, hence leading a better poreopen performance. On the other hand, in the swollen state, grafting polymers stretch, closing the pore. A larger grafting density provides less space between polymer chains near the pore surface, resulting in a better pore-close performance. The gating membrane fabricated by PIGP at 80 °C realized high grafting densities with sufficiently high molecular weight, resulting in higher gating sensitivity.15,28 The grafted polymers

Below the LCST and when the grafted PNIPAM was in the swollen state, the PC-g-PNIPAM membrane fabricated at 40 °C exhibited a larger permeability than that fabricated at 80 °C. Above the LCST and when the grafted PNIPAM was in the shrunken state, the PC-g-PNIPAM membrane fabricated at 40 °C exhibited a smaller permeability than the equivalent membrane fabricated at 80 °C. To explain this phenomenon, Table 2 compares the properties of the membranes used for the water permeability measurements. Weight-averaged molecular weights and grafting densities for the two membranes are estimated from the information presented in Table 1. The Flory radius Rf,swell and Rf,shrink of the equivalent linear polymer in a swelling and shrinking state, the grafting layer thickness dshrink calculated by permeation data, and the resultant water content of the grafted polymer wshrink in a shrunken state were calculated to identify the state of their grafted polymer chains. In a shrunken polymer, the ratio of projected area of independent shrunken grafted polymer (π × Rf,shrink2) to the allowed area for one grafted polymer (1/σ) for both of the grafted polymers fabricated at 40 and 80 °C are 0.9 and 1.8, respectively. This means that both the grafted polymers are in the transition or mixed state of mushroom and brush structures (the theoretical transition point is unity), and thus, we can assume that the two grafting polymers in shrunken state would have similar water contents. Under this assumption, the lower permeability due to the grafted polymers fabricated at 40 °C can be explained as follows. The long and diluted polymer chains on the pore walls form inhomogeneous structures of mushrooms with interstices, whereas the short and crowded polymers rather form thinner homogeneous brush-layer structure. And the height of the mushroom fabricated at 40 °C is larger than that of brush-layer fabricated at 80 °C. This leads smaller pore size for the grafted polymers fabricated at 40 °C as schematically shown in Figure 7 (lower entries). Note

Table 2. Graft Polymer Characteristics and Dimensions in Swollen and Shrunken States Tgr (°C) 40 80

ϕdry (%)

Mw (g mol−1)

σ (nm−2)

2Rf,swella (nm)

2Rf,shrinka (nm)

dshrinkb (nm)

wshrinkc (%)

12 13

2.8 × 10 1.3 × 106

0.003 0.012

260 170

18 14

33 18

76 56

6

a Rf,swell is the Flory radius of a linear polymer chain in swollen state. bdshrink is the thickness of the wet polymer layer (in shrunken state) derived from the water permeation test. cwshrink is the water content of the wet polymer layer (in shrunken state) calculated by ϕdry and ϕwet,shrink. The homogeneous grafted layer was assumed to be formed on the pore surface.

E

DOI: 10.1021/acs.iecr.5b04332 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research fabricated at 80 °C have more crowded structure and intuitively should show lower on/off performance. Nevertheless, from the above possible reasons, the gating membranes exhibit the opposite tendency and it is of interest. We believe this knowledge will be valuable for the design and fabrication of further smart materials with clear-cut gating functions.

(8) Shtanko, N. I.; Kabanov, V. Y.; Apel, P. Y.; Yoshida, M.; Vilenskii, A. I. Preparation of permeability-controlled track membranes on the basis of ’smart’ polymers. J. Membr. Sci. 2000, 179, 155. (9) Chu, L. Y.; Li, Y.; Zhu, J. H.; Wang, H. D.; Liang, Y. J. Control of pore size and permeability of a glucose-responsive gating membrane for insulin delivery. J. Controlled Release 2004, 97, 43. (10) Liu, G. J.; Lu, Z. H.; Duncan, S. Porous membranes of polysulfone-graft-poly(tert-butyl acrylate) and polysulfone-graft-poly(acrylic acid): Morphology, pH-gated water flow, size selectivity, and ion selectivity. Macromolecules 2004, 37, 4218. (11) Park, Y. S.; Ito, Y.; Imanishi, Y. Permeation control through porous membranes immobilized with thermosensitive polymer. Langmuir 1998, 14, 910. (12) Chu, L. Y.; Li, Y.; Zhu, J. H.; Chen, W. M. Negatively thermoresponsive membranes with functional gates driven by zippertype hydrogen-bonding interactions. Angew. Chem., Int. Ed. 2005, 44, 2124. (13) Lokuge, I.; Wang, X.; Bohn, P. W. Temperature-controlled flow switching in nanocapillary array membranes mediated by poly(Nisopropylacrylamide) polymer brushes grafted by atom transfer radical polymerization. Langmuir 2007, 23, 305. (14) Lue, S. J.; Chen, C.-H.; Shih, C.-M.; Tsai, M.-C.; Kuo, C.-Y.; Lai, J.-Y. Grafting of poly(N-isopropylacrylamide-co-acrylic acid) on microporous polycarbonate films: Regulating lower critical solution temperatures for drug controlled release. J. Membr. Sci. 2011, 379, 330. (15) Ito, T.; Hioki, T.; Yamaguchi, T.; Shinbo, T.; Nakao, S.; Kimura, S. Development of a molecular recognition ion gating membrane and estimation of its pore size control. J. Am. Chem. Soc. 2002, 124, 7840. (16) Ito, T.; Yamaguchi, T. Controlled release of model drugs through a molecular recognition ion gating membrane in response to a specific ion signal. Langmuir 2006, 22, 3945. (17) Kuroki, H.; Ito, T.; Ohashi, H.; Tamaki, T.; Yamaguchi, T. Biomolecule-Recognition Gating Membrane Using Biomolecular Cross-Linking and Polymer Phase Transition. Anal. Chem. 2011, 83, 9226. (18) Ohashi, H.; Ebina, S.; Yamaguchi, T. Logistic gate-like permeable property of gating membrane with ion-recognition polyampholyte. Polymer 2014, 55, 1412. (19) Mika, A. M.; Childs, R. F.; Dickson, J. M.; McCarry, B. E.; Gagnon, D. R. A new class of polyelectrolyte-filled microfiltration membranes with environmentally controlled porosity. J. Membr. Sci. 1995, 108, 37. (20) Maeda, T.; Takenouchi, M.; Yamamoto, K.; Aoyagi, T. CoilGlobule Transition and/or Coacervation of Temperature and pH Dual-Responsive Carboxylated Poly(N-isopropylacrylamide). Polym. J. 2009, 41, 181. (21) Kwon, I. C.; Bae, Y. H.; Kim, S. W. Electrically erodible polymer gel for controlled release of drugs. Nature 1991, 354, 291. (22) Kulkarni, R. V.; Setty, C. M.; Sa, B. Polyacrylamide-g-AlginateBased Electrically Responsive Hydrogel for Drug Delivery Application: Synthesis, Characterization, and Formulation Development. J. Appl. Polym. Sci. 2010, 115, 1180. (23) Himstedt, H. H.; Yang, Q.; Qian, X. H.; Wickramasinghe, S. R.; Ulbricht, M. Toward remote-controlled valve functions via magnetically responsive capillary pore membranes. J. Membr. Sci. 2012, 423, 257. (24) Yang, Q.; Himstedt, H. H.; Ulbricht, M.; Qian, X. H.; Wickramasinghe, S. R. Designing magnetic field responsive nanofiltration membranes. J. Membr. Sci. 2013, 430, 70. (25) Suzuki, A.; Tanaka, T. Phase-transition in polymer gels induced by visible-light. Nature 1990, 346, 345. (26) Yamaguchi, H.; Kobayashi, Y.; Kobayashi, R.; Takashima, Y.; Hashidzume, A.; Harada, A. Photoswitchable gel assembly based on molecular recognition. Nat. Commun. 2012, 3, 603. (27) Miyata, T.; Asami, N.; Uragami, T. A reversibly antigenresponsive hydrogel. Nature 1999, 399, 766. (28) Kuroki, H.; Ohashi, H.; Ito, T.; Tamaki, T.; Yamaguchi, T. Isolation and analysis of a grafted polymer onto a straight cylindrical

4. CONCLUSIONS The influence of the characteristics of graft polymers on the gating behavior of the stimuli-responsive gating membrane PCg-PNIPAM was investigated. Polymers of differing characteristics were successfully grafted onto track-etched PC substrates using a wide range of temperatures for the acid-assisted PIGP method. The difference in the characteristics of the grafted polymers were confirmed upon their isolation and subsequent GPC analysis. The thermoresponsive behavior of each membrane was examined though water permeability measurements over the temperature range of 25−38 °C. Through comparison of the characteristics of the grafting polymers and their stimuli-responsive gating behaviors, a high grafting density was found to be advantageous for both pore open/close performances when the molecular weights of the grafted polymers were sufficiently high to fill the pore in their swollen state. Under such conditions, a high grafting density diminishes the space between the tethered chains, and water permeation through the space is suppressed in the polymer’s swollen state, thus enhancing the pore-close performance. This decrease in space also forms thinner homogeneous layer of grafted polymer in shrunken state, thus enhancing the poreopen performance.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel.: +81-45-9245254. Fax: +81-45-924-5253. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Numbers 15H02315 and 26820336.

■

REFERENCES

(1) Kikuchi, A.; Okano, T. Intelligent thermoresponsive polymeric stationary phases for aqueous chromatography of biological compounds. Prog. Polym. Sci. 2002, 27, 1165. (2) He, D.; Susanto, H.; Ulbricht, M. Photo-irradiation for preparation, modification and stimulation of polymeric membranes. Prog. Polym. Sci. 2009, 34, 62. (3) Wandera, D.; Wickramasinghe, S. R.; Husson, S. M. Stimuliresponsive membranes. J. Membr. Sci. 2010, 357, 6. (4) Kuroki, H.; Tokarev, I.; Minko, S. Responsive Surfaces for Life Science Applications. Annu. Rev. Mater. Res. 2012, 42, 343. (5) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y. pH-sensitive gating by conformational change of a polypeptide brush grafted onto a porous polymer membrane. J. Am. Chem. Soc. 1997, 119, 1619. (6) Peng, T.; Cheng, Y. L. Temperature-responsive permeability of porous PNIPAAm-g-PE membranes. J. Appl. Polym. Sci. 1998, 70, 2133. (7) Liang, L.; Shi, M. K.; Viswanathan, V. V.; Peurrung, L. M.; Young, J. S. Temperature-sensitive polypropylene membranes prepared by plasma polymerization. J. Membr. Sci. 2000, 177, 97. F

DOI: 10.1021/acs.iecr.5b04332 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research pore in a thermal-responsive gating membrane and elucidation of its permeation behavior. J. Membr. Sci. 2010, 352, 22. (29) Chi, X.; Ohashi, H.; Yamaguchi, T. Plasma-Induced Graft Polymerization Inside Pores of Porous Substrates Assisted by an Infiltration Agent in Acidic Conditions. Plasma Processes Polym. 2014, 11, 306. (30) Flory, P. J. The Configuration of Real Polymer Chains. J. Chem. Phys. 1949, 17, 303. (31) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: New York, 1953; p 519. (32) Degennes, P. G. Conformations of Polymers Attached to an Interface. Macromolecules 1980, 13, 1069. (33) Viduna, D.; Limpouchová, Z.; Procházka, K. Monte Carlo simulation of polymer brushes in narrow pores. J. Chem. Phys. 2001, 115, 7309. (34) Plunkett, K. N.; Zhu, X.; Moore, J. S.; Leckband, D. E. PNIPAM chain collapse depends on the molecular weight and grafting density. Langmuir 2006, 22, 4259. (35) Zhu, X.; Yan, C.; Winnik, F. M.; Leckband, D. End-grafted lowmolecular-weight PNIPAM does not collapse above the LCST. Langmuir 2007, 23, 162. (36) Zhu, P. W. Effects of sodium dodecyl sulfate on structures of poly(N-isopropylacrylamide) at the particle surface. J. Phys. Chem. B 2015, 119, 359. (37) Wang, X.; Wu, C. Light-scattering study of coil-to-globule transition of a poly(N-isopropylacrylamide) chain in deuterated water. Macromolecules 1999, 32, 4299. (38) Wang, X. H.; Qiu, X. P.; Wu, C. Comparison of the coil-toglobule and the globule-to-coil transitions of a single poly(Nisopropylacrylamide) homopolymer chain in water. Macromolecules 1998, 31, 2972.

G

DOI: 10.1021/acs.iecr.5b04332 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX