INITIAL BUBBLE-GROWTH IN POLYMER FOAM PROCESSES

Bubble nucleation and growth are the key steps in polymer foam process es. The thermodynamic specification of the critical cluster size for n ucleation has been commonly used as the starting point for bubble grow th analyses. To initiate the solution, a perturbation of the critical radius is required and the selection of this perturbation is somewhat arbitrary. Depending on the size and nature of the perturbation, signi ficant differences can arise in the predicted bubble growth dynamics. In the present study, we consider a cluster to be nucleated when it re aches a supercritical state where the thermodynamic fluctuations canno t drive it back to subcritical conditions. For this purpose, a critica l region is defined within which the thermodynamic fluctuations have s ignificant effects on cluster growth. The state of the cluster at the upper bound of the critical region then provides the initial condition s for the bubble growth analysis. A perturbation solution in terms of the Peclet number has been used to determine the state of the cluster at the upper bound of the critical region (at nucleation) as well as t he initial bubble growth stage. The predictions of the analytical solu tion compare well with the predictions from numerical solutions. Copyr ight (C) 1997 Elsevier Science Ltd.