Diffusion induced phase separation with crystallizable nylons. I. Mass transfer processes for nylon 4,6

Abstract

Mass transfer during membrane formation by means of phase inversion for a polymeric system with both a solid-liquid and a liquid-liquid equilibrium was studied on the basis of the theory developed by Reuvers and Smolders. During the first moments of immersion in the coagulation bath, the concentrations at the interface between bath and film are governed by the virtual liquid-liquid equilibrium. This equilibrium no longer exists at a larger time scale. The interfacial concentrations as a result of the local liquid-liquid equilibrium during mass transfer are located deeply in the crystallization region or solid-liquid demixing area and after an induction time the solid-liquid phase separation (crystallization) takes place when membranes are formed with an initial polymer concentration of 20% or larger. The calculated initial concentration profiles show a shallow pattern in polymer content for the films with initial concentration of 20 and 25%. From the calculated initial concentration profiles an isotropic morphology in the final membrane can be expected. A steep increase of the polymer concentration at the interface was observed for the more concentrated films correlated with a skinned morphology in the final membrane.

Diffusion induced phase separation with crystallizable nylons. II. Relation to final membrane morphology

“…The propagation of the concentration profiles through the film is then hardly influenced by the already started phase separation. Since the composition path of the 20% solution [13] is already rather shallow it will become even more levelled out in time; the morphology of the final membrane can be thus expected to be quite homogeneous as indeed is observed, Two conditions can cause spherulites to stop growing. 1.…”

“…Almost immediately after immersion the growth of a layer consisting of crystalline units starts. In the concentration profiles [13] it can be observed that the interfacial polymer concentration is almost 50%. The concentrations at the growing crystal surface, as given by the crystallization line, gives an equilibrium concentration with a lower polymer content.…”

“…In the first part of this series the system: nylon 4,6, formic acid and water was studied [13] making use of the earlier developed knowledge of the thermodynamics of the system [10]. This paper will emphasize whether intermediate stages in membrane formation can be experimentally studied and related to final membrane morphology.…”

“…Photographs were taken at magnifications between 5000 and 20000 times. The presence and nature of solid particles present during membrane formation was studied as a function of coagulation time and of the distance from profile during membrane formation for this membrane is taken form the first part of this series [13]; the different stages are indicated with i to iv. spherulitic morphologies when polymers are crystallized from the melt.…”

“…4 and 5, respectively. The transmission curves during the formation of these membranes are shown as well [13].…”