Studies on the influence of ultraviolet (u.v.) exposure in air showing significant increases on track etching rates of irradiated polycarbonate film, first reported by Humphrey and Crawford, have been extended to include other environmental gases, namely oxygen, carbon dioxide and nitrogen in addition to air. Detailed studies of the effects of 'aging' on etching characteristics are also reported. The processes of pore formation and growth have been monitored by measurement of the a.c. conductivity across an irradiated film during the etching process. This procedure has sensitivity enough to follow the breakthrough and enlargement of a single pore. The etching has been carried out at temperatures lower than heretofore reported. The investigation has demonstrated the following. (1) For a given condition of u.v. exposure, the presence of oxygen is essential and more benificial than air and the other gases. (2) Since the u.v.- oxygen treatment has little or no effect on the bulk general attack rate, VG, cone angles as small as 3 x10-4 degrees have been observed for fission-fragment tracks at low temperatures of etching (6.4°C). These cone angles increase with increasing etching temperature. (3) Oxygen compared with air as the environmental gas during the u.v. exposure tends to stabilize tracks against undesirable 'thermal' effects below 100°C. (4) Etching at low temperature makes it possible to obtain smaller initial pore sizes (< 100 Å) than heretofore possible with fission-fragments. (5) Exposureof an irradiated film to (a) u.v.- oxygen, (b) high-energy electrons-oxygen and (c) X-rays-oxygen helps to decrease this lower-limit pore size. (6) Aging irradiated polycarbonate films for more than several days in normal laboratory conditions of air and room lights appears to be a process similar to the u.v.-oxygen process, i.e. both treatments exhibit a rapid early stage followed by a slower one. Both stages occur at rates several orders of magnitude faster in the u.v.-oxygen treated film. (7) Temperature dependence of etching rates of aged film in the region 7-55°C indicates an activation energy of 0.81 eV for general attack rate, 0.71 eV for track etching rate, and an increase in cone angle with temperature. Etching rates of Lexan® polycarbonate film appear to be similar to those of Makrofol® polycarbonate film. (8) Etching at low temperatures may be interrupted and resumed at the higher temperature-a process useful for obtaining pores of the micron-range size with small cone angles in an economical time. © 1979.
