Structure and permeability of porous films of poly(hydroxy ethyl methacrylate)

Porous films of poly(hydroxy ethyl methacrylate) have been made by photo‐polymerisation of solutions of the monomer. The permeability of homogeneous films depends on the cross‐link density and the concentration and composition of the polymerisation solvent. A membrane of minimum water permeability, in which the transport mechanism is principally diffusion, results from a cross‐link concentration of ∼9.4 mol‐% and a monomer concentration of ∼75% in an aqueous ethylene glycol solvent (20% ethylene glycol). More porous (macroporous) films were prepared by first cooling the solution such that the solvent freezes and the monomer occupies the interstices between the solvent crystals. After polymerisation and removal of the solvent, a porous structure results. The rate of freezing, the profile of the thermal gradient, and the solvent and crosslinker concentrations control the permeability of this film. Slow cooling gives random pores (K ≃6 × 10−15 cm2), but rapid, directional cooling from one face gives oriented pores with K ≃10−12 cm2. A 6‐order change in K results from varying the monomer concentration from 40 to 85% in a 4:1 water: ethylene glycol solvent, and a 3‐order change from increasing the ethylene glycol content of the solvent from 0 to 40% at a fixed monomer concentration.