hydrophilic semipermeable membranes based on copolymers of acrylonitrile and hydroxy - c 2 - c 4 - alkyl esters of acrylic acid are provided . the membranes have a substantial water regain at equilibrium , a wide range of ultrafiltration rates , and low solute adsorption by ion - exchange or hydrophobic interaction mechanisms . the membranes are particularly suitable for processing protein containing fluids , especially biological fluids such as blood or plasmas .