Control of pore size of polycarbonate membrane with straight pores by poly(acrylic acid) grafts Yoshihiro Ito, Seigo Kotera, Masahiro Inaba, Kenji Kono and Yukio Imanishi* Department of Polymer Chemistry, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto, 606 Japan (Received 9 August 1989; revised 8 February 1990; accepted 13 February 1990) The pH-dependent change of the pore size of poly(acrylic acid)-grafted, straight pored polycarbonate membrane was investigated.Peroxide groups were generated on the surfaceof the polycarbonate membrane by glow discharge, and the membrane was heated in the aqueous solution of acrylic acid to initiate the graft copolymerization. The density and the length of graft chains were dependent on the conditions in the glow-dischargetreatment and the monomer concentration in the graft copolymerization, respectively. The water permeability to the poly(acrylicacid)-grafted membrane increased sharply in pH regions below 4, representing the expansion of pores. However, very high densities of graft chains or very long graft chains restricted the mobility of poly(acrylic acid) chains, and the pore size became pH-independent. (Keywords: poly(acrylic acid); polycarbonate membrane; surface grafting; water permeation; pore size; conformation) INTRODUCTION The permeation and release control of substances has recently been investigated on the basis of the change of conformation, chain extension, hydrogen bonding and hydrophilicity of polyelectrolytes. Molecular systems using the swelling behaviour of polyelectrolyte gels 1'2, the transition of specific secondary conformations of polypeptide electrolytes3'4, the formation of polyelectro- lyte complexes 5-7, the solubility-dependent interactions with liposomes 8 and the extension/shrinkage of poly- electrolyte grafts 9'1° have been reported. These molecular systems in combination with enzymes suitable for molecular sensing could be applied for drug-delivery systems that are responsive to biological substances 11-13. In the present investigation, the control of pore size by changing the length and the density of poly(acrylic acid) chains, which are grafted on polycarbonate membrane with straight pores of uniform size, was investigated on the basis of water permeability. EXPERIMENTAL Materials Commercial straight pored polycarbonate membrane (Nuclepore) was used; the membrane diameter was 25mm, the membrane thickness was 10/~m, the maximum pore size was 0.2 #m, the pore density was 3 x l0 s cm -2. Acrylic acid (extra pure, Nacalai Tesque, Co., Kyoto, Japan) was purified by vacuum distillation. 1,1-Diphenyl-2-picrylhydrazil (DPPH) and rhodamin 6G (practical grade) were purchased from Wake Pure Chem., Co. (Osaka, Japan) and used without further purification. Triply distilled water was used for permeation experiments. The buffer solution used for the * To whomcorrespondenceshould be addressed permeation experiments was 5 mM tris(hydroxymethyl) aminomethane/hydrochloric acid. Toluene and N,N- dimethylformamide (DMF) were purified by distillation and vacuum distillation, respectively. Graft copolymerization 14 The polycarbonate membrane was placed in an ion-coater (Eiko Eng., Co., Mite, Japan) and glow- discharged under 0.2 mmHg atmosphere with 6 mA electric current for a definite time. The glow-discharged membrane was immediately transferred to aqueous solution of acrylic acid, and the mixture was heated at 60°C for 2 h under a nitrogen atmosphere. After the graft polymerization, the polycarbonate membrane was washed with deionized water using ultrasonic cleaner until the pH of the wash remained unchanged. The membrane was then subjected to the permeation experiment. Determination of peroxides formed on the membrane 1 s The glow-discharged membrane was immersed in a toluene solution of DPPH (1.116x 10 -4 M), and the mixture was heated under the same conditions as in the graft copolymerization. After cooling the mixture, the amount of DPPH consumed was determined with 521.5 nm absorption (the molar extinction coefficient, 1.05 x 104 M cm- 1). The u.v. determination was corrected for the amount of DPPH non-specifically adsorbed to the glow-discharged membrane, which was taken to be the same as that of non-specific adsorption to untreated membrane. Determination of poly(acrylic acid) graft 16 Rhodamine 6G was dissolved in phosphate-buffered solution (pHil, 0.1M Na2PO4) and extracted with toluene. Poly(acrylic acid)-grafted membrane was 0032-3861/90/1121574)5 © 1990Butterworth-Heinemann Ltd. POLYMER, 1990, Vol 31, November 2157