The effect of the evaporation step on the occurrence of particles in poly(e
thylene-co-vinyl alcohol) (EVAL) membranes cast from DMSO solutions via the
dry/wet process were studied. The structure of the EVAL membranes can be c
hanged from an asymmetrical structure consisting of a dense skin layer and
finger-like macrovoids in the sublayer to a skinless and symmetric structur
e by constituent particles bonded to each other. From the analysis of the m
embrane formation mechanism, the competition of different phase separation
process during membrane formation is most important. The onset of the phase
separation may be either liquid-liquid demixing or solid-liquid demixing,
which determines the resulting membrane properties. Directly immersing the
casting solution into a water bath, liquid-liquid demixing is of considerab
le importance during the phase separation of the solutions. As a result of
the lower activation energy for nucleation, liquid-liquid demixing can prec
ede solid-liquid demixing even in cases where solid-liquid demixing is favo
red thermodynamically. By using the evaporation process, the phase separati
on proceeds slowly via solid-liquid demixing and thus leads to a particulat
e morphology in the membrane. This suggests that the evaporation step cause
crystallization of EVAL molecules from the casting solution to inhibit the
macrovoid formation. In addition, the duration cf the evaporation step is
shown to have a strong influence on the disappearance of particles. The res
ults presented here offer a qualitative basis for the development of membra
nes with a particulate morphology. (C) 1999 Elsevier Science Ltd. All right
s reserved.