The production protocol of internally skinned ultrafiltration membrane
s with no exterior skin layer was rationalized. The membranes were int
ended for use in bioreactor and filtration operations. The unique subs
tructure morphology was designed to have low resistance to liquid tran
sport and a large external surface area. The polysulfone and polyether
sulfone membranes that were produced had well-defined internal skin la
yers, integrally part of a substructure that consisted entirely of clo
sely packed and narrow-bore microvoids that extended from just below t
he internal skin layer of the membrane to the periphery. The regularly
shaped microvoids open into the membrane periphery as there is no out
er skin layer to bridge the cavities. The structures formed may be the
result of rapid demixing of the internal skin layer, followed by nucl
eation and growth of the polymer-lean phase, coupled with coalescence
at the membrane periphery.