Article pubs.acs.org/IECR
Effect of Dose on Mole Percentages of Polymer Brush and Root Grafted onto Porous Polyethylene Sheet by Radiation-Induced Graft Polymerization Ryo Ishihara,† Shoichiro Uchiyama,† Hidekazu Ikezawa,† Shinsuke Yamada,‡ Hideyuki Hirota,‡ Daisuke Umeno,† and Kyoichi Saito*,† †
Department of Applied Chemistry and Biotechnology, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, Japan INOAC Corporation, 1-16-30, Sen-nen, Atsuta-ku, Aichi, Japan
‡
ABSTRACT: Glycidyl methacrylate (GMA) was graft-polymerized onto a porous sheet ( 75% porosity and 1.6-μm average pore size) previously irradiated with an electron beam. The resultant grafted poly-GMA chain can be classified as a polymer brush extending from the pore surface toward the pore interior and a polymer root invading the polymer matrix. The boundary crossing the pore/matrix interface can be detected in two independent ways: molar conversion of the epoxy group into a sulfonic group providing a plateau in the molar conversion versus reaction time curve and molar conversion exhibiting a breakthrough point in the swelling ratio versus molar conversion curve. A higher irradiation dose was found to lead to a higher mole percentage of polymer brush in the graft chain. The dose range from 20 to 200 kGy corresponded to the mole percentage range of polymer brush from 6% to 15%.
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matrices.21 The mole percentages of the polymer brush and root govern the performance characteristics, namely, the protein adsorptivity and liquid permeability, of the resultant porous hollow-fiber membranes.20,21 The properties of a graft chain appended onto a porous trunk polymer by radiation-induced graft polymerization have remained unclear because the isolation of a graft chain from a porous trunk polymer by chemical treatment is difficult and the characterization of a graft chain by instrumental analysis is ineffective. At present, materials functionalized by radiationinduced grafting under reproducible irradiation conditions have a wide range of applications including metal recovery,26−28 ultrapure water production,29 protein purification,23,30 and enzyme immobilization.31 However, to precisely design and manufacture sophisticated porous polymeric materials, graft chain characterization is essential. Recently, we suggested a simple method for determining the mole percentages of a polymer brush and a polymer root grafted onto a porous polyethylene sheet.32 The principle of the method is based on the fact that a hydrophobic graft chain, namely, a poly(glycidyl methacrylate) chain, reacts in a shrunken conformation with its reactants, namely, sodium sulfite and trimethylammonium chloride, dissolved in water, resulting in the preferential introduction of ion-exchange groups into the polymer brush. We demonstrated that a boundary crossing between the polymer brush and root can be detected on the basis of the swelling behavior of the porous sheet induced by the introduction of an ion-exchange group into the polymer root. By this novel method, the mole
INTRODUCTION Porous materials have been utilized in various fields because of their large surface areas1−5 and wide pore size ranges.6−8 By modifying existing porous polymeric materials, functional pore surfaces can be prepared while retaining the porous structure. Such modification methods include radiation-,9−15 plasma-,16,17 and photoinduced18,19 graft polymerization. Among these approaches, radiation-induced graft polymerization is advantageous over other grafting methods in that it is applicable to existing porous polymers with various shapes and qualities as trunk polymers for grafting.20 Porous polymers consist of pores and matrices; for example, a porous polyethylene sheet produced by INOAC Corporation for the removal of particulates in liquids has a porosity of 75% and an average pore size of 1.6 μm. Graft chains are appended over porous trunk polymers because both electron beams and γ-rays of high energy penetrate the entire volume of the porous trunk polymer. In the case of pre-irradiation grafting, radicals produced uniformly throughout a porous polymer react with a vinyl monomer to form a graft chain through growth and termination processes. Therefore, the graft chain can be classified into two categories according to the formation site in the porous polymer: a polymer brush extending from the pore surface toward the pore interior and a polymer root invading the depth of the polymer matrix.20,21 Thus far, we have prepared ion exchangers for protein recovery from biological fluids based on porous polyethylene hollow-fiber membranes22 using radiation-induced graft polymerization and subsequent chemical modifications. The ionexchange groups such as a sulfonic acid group (−SO3H) and trimethylammonium chloride group [−N(CH3)3Cl] introduced into the polymer brush capture proteins through electrostatic interactions,23−25 whereas those introduced into the polymer root prevent pore size reduction through the swelling of © 2013 American Chemical Society
Received: Revised: Accepted: Published: 12582
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Figure 1. Graft polymerization of GMA onto a porous polyethylene sheet and introduction of sulfonic acid groups.
porous sheet is hereafter referred to as the SA(ds, x) sheet. The molar conversion of epoxy groups into SA groups, x, is defined as
percentages of polymer brush and root grafted onto a polyethylene porous sheet was determined as a function of the degree of glycidyl methacrylate (GMA) grafting.32 For radiation-induced grafting, the irradiation dose of the electron beam or γ-ray is a critical parameter because the radicals produced by the irradiation govern the properties of the graft chain over the polymer matrix. However, the dependence of the mole percentages of polymer brush and root on dose has not yet been clarified. The objectives of this study were two-fold: (1) to determine the mole percentages of a polymer brush and root grafted onto a porous polyethylene sheet as functions of dose and (2) to correlate the mole percentage of the polymer brush with protein adsorptivity and liquid permeability. Here, the sulfonic acid group was selected as the ion-exchange group introduced into the graft chain.
molar conversion of epoxy groups into SA groups (x , %) =
swelling ratio =
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W1 − W0 × 100 W0
(2)
where W2 is the mass of the SA(ds, x) sheet and 104 and 142 are the molecular masses of NaHSO3 and GMA, respectively. The swelling ratios of the SA(ds, x) sheet with respect to the GMA(ds) sheet in the wet state, defined as volume of SA(ds, x) sheet volume of GMA(ds) sheet
(3)
were evaluated from measurements of the diameters and thicknesses of the porous sheets with a micrometer caliper and a film thickness gauge, respectively. Measurement of Liquid Flux. The SA(ds, x) sheet was cut into 13-mm-diameter disks. Each disk was packed into a commercially available empty cylindrical cartridge that was 13 mm in diameter and 65 mm in length (reservoir, 6 mL capacity; Varian Co.). The resultant SA(ds, x)-disk-packed cartridge is hereafter referred to as the SA(ds, x) cartridge. To determine the liquid permeability of the porous sheet, 50 mM MgCl2 aqueous solution was forced to flow through the porous sheet using a syringe pump at a constant flow rate. The pressure required to maintain a constant flow rate of the liquid was measured. The liquid flux of the porous sheet was evaluated according to
EXPERIMENTAL SECTION Materials. A porous polyethylene sheet, manufactured by INOAC Corporation, Nagoya, Japan, was used as the trunk polymer for grafting. This sheet had a thickness of 2 mm, an average pore size of 1.6 μm, and a porosity of 75%. Glycidyl methacrylate (GMA, CH2CCH3COOCH2CHOCH2) was purchased from Nacalai Tesque Co., Kyoto, Japan, and used without further purification. Lysozyme (pI = 11, Mr = 14000) was purchased from Tokyo Kasei Co., Tokyo, Japan. Other reagents were of analytical grade or higher. Grafting of Epoxy-Group-Containing Vinyl Monomer. Glycidyl methacrylate was graft-polymerized by a preirradiation method, as shown in Figure 1. Briefly, the porous sheet was irradiated with an electron beam in a nitrogen atmosphere at ambient temperature. A cascade-type accelerator (Dynamitron, Radiation Dynamics, Ltd.) was operated at a beam energy of 2 MeV and a current of 20 mA. The doses ranged from 20 to 200 kGy. The irradiated porous sheet was immersed in 5% (v/v) GMA/methanol at 277 K. After a prescribed reaction time of up to 36 h, the sheet was removed and washed repeatedly with N-dimethylformamide. The GMAgrafted porous sheet is hereafter referred to as the GMA(ds) sheet, where ds in parentheses denotes the dose of GMA grafting. Degree of grafting (dg) is defined as degree of GMA grafting (dg, %) =
[(W2 − W1)/104] × 100 [(W1 − W0)/142]
liquid flux at 298 K and 0.1 MPa (m/h) flow rate = cross‐sectional area of porous sheet
(4)
Determination of Equilibrium Binding Capacity of SA Sheet. Lysozyme solution (0.2 g/L), whose pH had been adjusted to 7.4 with a 20 mM phosphate buffer, was forced to flow through an SA(ds, 15) cartridge at a constant flow rate of 150 mL/h. The effluent flowing through the cartridge was continuously sampled with fraction vials. The lysozyme concentration of each fraction was determined by UV absorption spectrometry at 280 nm. The adsorption procedure was continued until the lysozyme concentration of the effluent reached that of the feed. The equilibrium binding capacity of the SA(ds, 15) sheet for lysozyme was calculated from the breakthrough curve (i.e., the change in protein concentration of the effluent with effluent volume) as
(1)
where W0 and W1 are the masses of the trunk and GMA-grafted porous sheets, respectively. dg was set at 120% for the GMAgrafted porous sheets prepared at various doses. Introduction of a Sulfonic Acid Group into the Graft Chain. The epoxy group of the polymer chain grafted onto the trunk polymer was converted into a sulfonic acid (SA) group. The GMA(ds) sheet was immersed in a mixture of 1 M Na2SO3 and 0.5 M NaHSO3 aqueous solutions at 353 K. The resultant
equilibrium binding capacity (mg/g) =
∫0
Ve
C0 − C dV W2 (5)
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where C0 and C are the lysozyme concentrations of the feed and effluent, respectively, and V and Ve are the effluent volume and the effluent volume when C reaches C0, respectively.
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RESULTS AND DISCUSSION Reaction Degree and Swelling Ratio. We prepared GMA-grafted porous sheets with the same degree of GMA of 120% by controlling the reaction time [i.e., GMA(ds) sheets] from porous sheets irradiated at doses ranging from 20 to 200 kGy. The epoxy groups of the graft chains of the GMA(ds) sheet were converted into SA groups using Na2SO3/NaHSO3 aqueous solution. The time courses of the molar conversion of epoxy groups into SA groups, x, defined by eq 2, are shown in Figure 2. The curves exhibit similar features with a plateau: x
Figure 2. Time course of molar conversion of epoxy groups into SA groups for a GMA(ds) sheet.
Figure 4. Swelling ratio of SA(ds, x) sheet to GMA(ds) sheet as a function of molar conversion of epoxy groups into SA groups: (a) real case and (b) model case.
initially increased with increasing reaction time and subsequently reached a plateau before increasing again. The plateau value of x gradually increased with increasing dose, as shown in Figure 3.
was first observed by our group,32 using water as the solvent during functionalization, is shown in Figure 4b. Here, the following assumptions were made: The introduction of the SA group into the polymer brush does not cause the porous polymeric material to swell, whereas the introduction of the SA group into the polymer root induces swelling. Below x1 in this figure, the functionalization occurs exclusively in the polymer brush of the graft chain; therefore, no swelling of the porous polymeric material can be observed. Above x2 in this figure, the curve is approximately linear because the polymer root of the graft chain is functionalized. Thus, the extrapolation of the straight line toward the horizontal axis (i.e., zero swelling) produces the x intercept. This x intercept, x*, is referred to as the “boundary molar conversion (BMC),” where boundary means the boundary between the polymer brush and root. The value of BMC is equivalent to the mole percentage of the polymer brush of the graft chain. The BMC obtained from Figure 4a was inserted in Figure 3 as a function of dose. The BMC evaluated from the swelling degree was found to be in good agreement with the plateau value evaluated using the reaction degree. BMC reflects the average image of a graft chain with a molecular mass distribution in the length direction. Graft chains are classified into two categories: polymer brushes extending from the pore surface toward the pore interior and polymer roots invading the polymer matrix. BMC is a measure indicating the crossing of the interface between the pore and matrix. In addition, because the final molar conversion of epoxy groups into SA groups
Figure 3. Molar conversion at plateau and critical molar conversion versus dose of electron beam.
The swelling ratios of the SA(ds, x) sheets to the respective GMA(ds) sheet for various doses are shown in Figure 4a as a function of x. The curves exhibit a breakthrough behavior: The SA(ds, x) sheet initially remained unchanged in volume, then started to swell from a molar conversion specific to the dose, and linearly increased in volume. The relationship between swelling ratio in the wet state and molar conversion exhibiting a “breakthrough” behavior, which 12584
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The SA-group-containing polymer brush can exclusively capture lysozyme (pI 11) dissolved in a buffer (pH 7.4) because lysozyme with a molecular mass of 14000 cannot invade the polymer-root-incorporated matrix. The breakthrough curves in permeation mode (i.e., protein concentration of the effluent versus effluent volume) shifted to the right as the dose increased. The equilibrium binding capacity of the SA(ds, 15) sheet for lysozyme, defined by eq 5, is shown in Figure 7 as a
reached almost 100%, the BMC in this study was equivalent to the mole percentage of the polymer brush of the poly-GMA graft chain prepared. The mole percentage of the polymer brush was found to increase with increaseing dose. This can be explained by the consumption of vinyl monomers induced by diffusion and grafting, as illustrated in Figure 5. Electron-beam irradiation
Figure 5. Formation mechanism of polymer brushes and roots grafted onto a porous polyethylene sheet. Figure 7. Dependence of the equilibrium binding capacity of an SA(ds, 15) sheet on dose for lysozyme.
with a higher dose produces a higher density of radicals in the polymer matrix; the higher density of radicals promotes grafting in a shallow domain in the polymer matrix, resulting in a longer and higher-density polymer brush. In contrast, a lower dose allows the monomer to deeply invade the polymer matrix, resulting in a shorter and lower-density polymer brush. The polyethylene matrix employed here had a thickness of about 1 μm and three-dimensionally connected pores about 2 μm in size. Dose is one of readily controllable parameters for designing functional porous sheets by radiation-induced graft polymerization. Effects of Dose on Liquid Permeability and Protein Adsorptivity. When the SA group is introduced as a strongly acidic cation-exchange group for the epoxy group of the polymer brush grafted onto the porous sheet, the extension of the polymer brush induced by mutual electrostatic repulsion reduces the pore size, that is, the liquid flux of the porous sheet. The liquid flux of the SA(ds, 15) sheet is shown in Figure 6 as a function of dose. The liquid flux decreased with increasing dose because of the longer and higher-density polymer brush of the porous sheet.
function of dose. The equilibrium binding capacity increased with increasing dose. This is due to the fact that a higher mole percentage of the polymer brush produces a three-dimensional space for the higher-capacity adsorption of the protein. The nonlinear relationship between the liquid protein adsorptivity or liquid flux and dose can be explained by the fact that the density of radicals produced by the dose governs the length and density of the polymer brush.
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CONCLUSIONS Porous polymeric materials can be modified into functional porous polymers capable of collecting metal ions and proteins and immobilizing enzymes. Radiation-induced graft polymerization readily enables a useful uniform modification over porous polymers. As a precursor monomer, an epoxy-groupcontaining vinyl monomer, glycidyl methacrylate (GMA), can be conveniently used for the subsequent introduction of ionexchange and chelating groups, as well as hydrophobic and affinity ligands. Polymer chains grafted onto the porous polymer are classified as polymer brushes and roots. The former extends from the pore surface toward the pore interior, whereas the latter invades the polymer matrix. The mole percentage of polymer brush governs the liquid permeability and protein adsorptivity of the functional porous sheet. Here, the effects of the dose of electron-beam irradiation on the mole percentages of the poly-GMA brush and root grafted onto a porous polyethylene sheet were clarified. The boundary crossing the interface between the pore and matrix was found to be detectable by the following two independent values: a plateau in the time course of the molar conversion of epoxy groups into sulfonic acid (SA) groups and a breakthrough point in the swelling ratio of the SA-group-introduced porous sheet to the GMA-grafted porous sheet. The results showed that a higher dose induced a higher mole percentage of polymer brush, namely, a longer and higher-density polymer brush.
Figure 6. Dependence of the liquid flux of an SA(ds, 15) sheet on dose for 50 mM MgCl2. 12585
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The SA-group-containing polymer brush extends by the mutual electrostatic repulsion of SA groups, thereby decreasing the flux of the liquid permeating through the pores and increasing the binding capacity of the protein entangled in the polymer brush. The liquid permeability and protein adsorptivity of the functional porous sheet can be balanced by controlling the irradiation dose by pre-irradiation grafting.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +81 43 290 3439. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ABBREVIATIONS BMC = boundary molar conversion dg = degree of GMA grafting GMA = glycidyl methacrylate SA group = sulfonic acid group
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