Membrane Formation and Modification - American Chemical Society

Chapter 15

Preparation and Properties of Surface-Modified Polyacrylonitrile Hollow Fibers 1

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A . Higuchi , P. Wang , T.-M. Tak , T. Nohmi , and T. Hashimoto 1

Department of Industrial Chemistry, Seikei University, Tokyo 180-8633, Japan Chia Yiu Enterprise Corporation, Tokyo 160, Japan Department of Natural Fiber Science, Seoul National University, Suwon 440-774, South Korea Research Laboratory, Nohmi Bosai Ltd., Tokyo 160, Japan 2

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Downloaded by UNIV OF OTTAWA on June 13, 2013 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0744.ch015

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A controlled reaction on the surface of polyacrylonitrile hollow fibers was developed to change the C N unit to a C N H unit. Ion -exchange measurements and IR spectra for the chemically modified fibers suggested the existence of N H units on the surface of the modified fibers. The modified fibers had a smaller molecular weight cut-off than the unmodified fibers. Serum albumin was immobilized on the surface of the modified and unmodified fibers using the succinimide reaction. Ultrafiltration experiments for optical resolution of racemic amino acids were also performed using the immobilized albumin membranes. The immobilized albumin membranes prepared from chemically modified hollow fibers demonstrated efficient optical resolution of racemic phenylalanine. 2

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Surface reactions of active reagents on membranes are useful modifications to introduce various functional groups such as S 0 H , N H , C O O H or O H (1-18). For example, Nabe et al prepared five different chemically modified membranes and investigated protein fouling using these membranes (1). Membranes modified by direct sulfonation had the lowest surface energy and the shortest grafted chain length and exhibited the highest volumetric flux with B S A solution (1). In previous studies (2-5), we developed several surface reactions on polysulfone hollow fibers. A negatively charged group, -CH2CH2CH2SO3", and a hydroxide group, -CH(CH )CH OH, were introduced on the surface of the polysulfone hollow fibers via Friedel-Crafts reactions. It was demonstrated that the modified hollow fibers showed excellent properties for the anti-absorption of proteins and solutes compared to those of unmodified and conventionally sulfonated fibers (2-5). Recently, special attention has focused on affinity membranes for the purification of proteins and optical isomers for biotechnology applications (19-22). Affinity membranes are generally made from porous membranes activated by the 3

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© 2000 American Chemical Society

In Membrane Formation and Modification; Pinnau, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

229 covalent attachment of affinity ligands to their interior pore surfaces. When membranes made from conventional polymers such as polyacrylonitrile or polysulfone are selected as the base materials, the membranes must be activated. One of the most promising methods for activation of the membranes is surface modification. This paper describes the surface reaction of polyacrylonitrile (PAN) hollow fibers with hydrazine and Raney nickel by a one-step reaction that introduces an N H unit on the surface. Bovine serum albumin (BSA) was subsequently immobilized on the surface of the modified fibers using the succinimide reaction. Optical resolution of racemic phenylalanine was also investigated in ultrafiltration experiments using the immobilized B S A hollow fiber membranes. 2


Experimental Materials. Membranes used for the chemical modification were commercially available polyacrylonitrile ultrafiltration hollow fibers (HC-1, Asahi Chemical Co., Ltd., molecular weight cut-off = 13,000 g/mole). The substitution ratio of the amine residue was estimated to be 1/100 from ion exchange capacity experiments of the membranes. The fibers have internal and external skin layers and two layers of finger-like macrovoids in their cross-section. The inside and outside diameters of the fibers were approximately 0.8 mm and 1.4 mm, respectively. Water flux of the fibers in mixtures with polyethylene glycol (PEG) [Mw = 20,000 g/mole] was 2.8 m /m *day at a pressure difference of 100 kPa. The rejection for PEG was 40% using a feed solution concentration of 0.1 wt%. B S A (F-V, Lot. 0125R) purchased from the Armour Pharmaceutical Co. was used in this study. Phenylalanine, hydrazine hydrate, Raney nickel, ethanol, 1,6dimethylsuberimidate dihydrochloride (DMS), polyethylene glycol, glucose, and maltose purchased from Tokyo Chemical Industry Co., Ltd., were reagent grade and were used without further purification. Ultrapure water was used throughout the experiments. 3

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Chemical Modifications. The polyacrylonitrile hollow fibers were dipped in a solution of 75 ml ethanol, 0.25 g Raney nickel, 25 ml hydrazine hydrate at different temperatures (50 - 80°C) and reaction times (0.5 - 23 hours). The anticipated product by the reaction is shown in Figure 1. After the reaction, the modified fibers were washed in water at 50°C for 1 hour and, thereafter, washed three more times in fresh water of 50°C each for 15 minutes under ultrasonic treatment generated by an ultrasonic cleaner (200 W, 40 KHz, Kaijo Denki Co.). B S A Immobilization. The modified fibers were immersed in D M S solution (0.015 g/50 ml of H 0 ) for 5 minutes at 25°C, which was adjusted to pH 10.0 using 0.1 Ν NaOH solution. After the excess DMS solution on the surface of the hollow fibers was removed, the membranes were dipped in the B S A solution (0.813 g/ 50 ml of H 0 ) for 30 minutes at 25°C and subsequently for 18 hours at 4°C. The solution was then adjusted to pH 8.5 by adding 0.1 Ν NaOH solution. The amount 2

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230 of B S A bound on the membranes was estimated from the decrease of B S A concentration in the solution where the membranes were immersed. The concentration of B S A was measured by HPLC using a U V detector. After BSA immobilization (Figure 2), the immobilized B S A fibers were washed in water for 1 hour and stored in water at 4°C. Transport Measurements. The ultrafiltration apparatus used in this study was described in previous papers (2-5). The pressure difference across the fibers, Δρ, was 0.4 kg/cm or 0.8 kg/cm . The ultrafiltration measurements were performed at 25±0.5°C. The rejection was calculated from the concentration ratio of solute in the feed solution, Cf, and the concentration of solute in the permeate, C ,: 2

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p

R = (l-C /Cf)xl00(%)

(1)


p

The concentration of glucose (Mw = 180 g/mole), maltose (Mw = 342 g/mole), polyethylene glycol) (PEG) 1000 (Mw - 950-1,050 g/mole), P E G 2000 (Mw ~ 1,800-2,200 g/mole), PEG 4000 (Mw ~ 3,000 g/mole), P E G 6000 (Mw ~ 7,8009,000 g/mole), and PEG 20,000 (Mw ~ 15,000 g/mole) was measured by an HPLC system using the refractive index as a detector (ERC-7510, Erma Optical Works, Ltd.). Optical Resolution of Amino Acid. Optical resolution of phenylalanine was also performed in this study. After the racemic phenylalanine permeated through the immobilized BSA membranes, the concentration of the D - and L-phenylalanines in the filtrate were determined using HPLC (880-PU, UV-970, Jasco Co.) with a Crownpak column [CR(+), Daicel Chemical Industries, Ltd.]. The detection limit of phenylalanine was less than 0.0001 m M in this system. The separation factor, a, is defined as, [JD/JL]/[Cf d(D)/C ed(L)], where C ( L ) and C (D) are the concentrations of the L-isomer or D-isomer in the feed, respectively, and J and J are the fluxes of the L-isomer or D-isomer, respectively. Because the feed solution used in this study was the racemic phenylalanine solution (i.e., Cf d(D) = Cf d(L)) and the flux of the solute was directly related to the concentration in the permeate (i.e., J / J = Cp(D)/C (L) where C (L) and C (D) were the concentrations of the L isomer or D-isomer in the permeate, respectively), α could be reduced to the concentration ratio of the D-isomer to L-isomer in the permeate. The standard deviations for the flux and rejection measurements were 10% and 20% for four independent experiments. ee

fe

feed

feed

L

ee

D

L

p

P

D

ee

P

Results and Discussion Chemical Modification. Polyacrylonitrile (PAN) hollow fibers were chemically modified on their surface according to the experimental procedures described above. The anticipated product of the reaction with the hollow fibers and hydrazine hydrate is shown in Figure 1. The mechanism of the reaction was postulated from the reaction of nitriles with hydrazine hydrate and Raney nickel and from the color



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NH2

NHz

NHNH2

Figure 1. Reaction scheme of surface-modified fibers.

NHNH2

NHNH2

polyacrylonitrile hollow

Modified P A N f Nik + C H O C * C H ^ C O C H j 3

+

.

=»

NH

+ 2

II

NH

2

II

I Modified PANT NHC ( C H \ C O C H , + C H , O H ?

(BS^-NH =>

.

2

+NH

2

+

NH

2

I Modified PAN^NHC ( C H , )

Membrane Formation and Modification - American Chemical Society

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