MULTILAYER MODELING OF DIFFUSION OF WATER IN ACRYLAMIDE-GRAFTED ALIPHATIC POLYESTERS

A new mass transfer model for layered structures, based on an implicit multistep integration algorithm including concentration-dependent dif fusivities, has been developed and applied to systems of water diffusi ng in electron beam pre-irradiated, acrylamide-grafted aliphatic polye sters (poly(epsilon-caprolactone) and poly(1,5-dioxepan-2-one)). The m uch higher water solubility and water diffusivity in the amorphous cro sslinked poly(1, 5-dioxepan-2-one) than in the semicrystalline poly(ep silon-caprolactone) is explained as being due to the fact that crystal s are more effective crosslinks than the chemical crosslinks. Water so lubility in the grafted layer is a function of the concentration of po lyacrylamide, but it is believed to be limited by compressive swelling -induced stresses. The zero concentration water diffusivity in the gra ft layer passes through a maximum, whereas the graft-layer density pas ses through a minimum with increasing acrylamide concentration. It is suggested that this is because the accessible free volume for the pene trating water molecule is increased in the initial stages of grafting because the matrix polymer chains are separated by growing acrylamide graft chains. At later stages, the accessible free volume decreases be cause additional polyacrylamide chains are growing adjacent to the alr eady existing ones and thus fill up empty spaces.