Transient interfacial instability in bilayer polymer films as observed by neutron reflectivity studies

Fundamental processes associated with reactive blending were explored for a system composed of polyamide (PA) and deuterated polysulfone having a reac tive phthalic anhydride end group (R) (dPSU-R) diluted with a low molecular weight hydrogenous polysulfone (low-hPSU) by neutron reflectivity (NR). By preparing bilayer films composed of a mixed film of dPSU-R/low-hPSU and a PA film, an intriguing phenomenon was observed in NR studies during the the rmal annealing of the sample. This phenomenon can be described as a transie nt instability of the interface between the PA layer and the dPSU-R/low-hPS U layer during annealing at temperatures high enough to allow chemical reac tions to occur between the dPSU-R end groups and the terminal amino groups of PA, The signature for the interfacial instability was a transient disapp earance of the NR fringes followed by a subsequent recovery of the fringes during additional annealing at high temperatures. The driving force for thi s phenomenon is rapid interdiffusion of a small, low molecular weight fract ion of low-hPSU into the PA layer. A consequence of the rapid interdiffusio n is an attendant Kirkendall shift of the interfacial position as the PSU l ayer shrinks. The magnitude of this special Kirdendall effect accompanying the interfacial instability is directly controlled by the amount of the mis cible low molecular weight fraction of low-hPSU present in the normally imm iscible PSU/PA pair. (C) 2001 Published by Elsevier Science Ltd.