Boiling-Foaming Mechanisms of Polymeric Melts

is the nucleation mechanism in the bulk of these viscous molten polymer solutions  

In a polymeric system, it would be reasonable to examine the possibility that the free volume concept described earlier, which explains so well phenomena like molecular diffusion and viscosity, might perhaps also explain nucleation phenomena. The critical radius re of a stable bubble can be obtained from a simple mechanical-force balance, yielding the Laplace equation  

The term foaming comes from the fact that the melt is very viscous and, when the devolatilization process begins, the melt fills up with bubbles that appear as foam. Sometimes, as in foaming processes, low boiling-point additives are added to enhance the process. 

During World War II Harvey et al. investigated the well-known phenomenon of bubbles appearing in the blood and tissue of divers surfacing too quickly after deep diving. 

Alternatively, it can be argued, as did Harvey et al., that the contact angle between the liquid and the hydrophobic surface in this case can reestablish itself as the meniscus moves, due to absorption or release of vapor, and therefore it is unstable. This could explain the existence of relatively large, stable pockets of vapor and/or air dispersed within the melt. Lee and Biesenberger (24) estimated that pockets of the size of 1pm could exist in the melt, and these fall within the range of critical nuclei for bubble formation. When superheat is applied, the pressure within the pockets exceeds that in the liquid and the radius of curvature becomes positive, as in Fig. 8.9(b) and 8.9(c). 

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