control samples of pga non woven fiber meshes were also heated under the same conditions as the plla pga composite membranes the previously mentioned temperatures correspond to the interior of the oven because of heat transfer limitations due to the covered glass petri dish containing the sample the temperature in the sample is expected to be slightly lower than the oven temperature the temperature accuracy of the oven was u00b1 1 u00b0 c table i______________________________________heat treatment conditions ofplla pga composite membranes t sub 1 t sub 1 t sub 12 t sub 2 t sub 2sample u00b0 c min min u00b0 c min ______________________________________1 195 90 6 230 52 195 90 7 235 53 195 90 9 240 54 195 90 7 235 7 55 195 90 7 235 10______________________________________ after heat treatment the plla matrix of a plla pga composite membrane was selectively dissolved in 8 ml of methylene chloride for 75 min the methylene chloride was changed every 15 min the bonded pga fibers were vacuum dried at 100 u03bcm hg for 24 hours and stored in a desiccator under vacuum until use a zoom macroscope model m420 wild heerbrugg heerbrugg switzerland was used to observe the structure of composite membranes before the dissolution of the matrix the total magnification of the micrographs was 40x the samples were coated with gold using a sputter coater model desk ii denton vacuum cherry hill n j the gas pressure was set at 50 mtorr and the current was 40 ma for a coating time of 75 s cell cultures were first.