A PERCOLATION VIEW OF NOVOLAK DISSOLUTION .3. DISSOLUTION INHIBITION

The dissolution of novolak films in aqueous alkali is controlled by th e diffusion of base through a thin penetration zone that forms at the interface with the developer solution. BaBe diffusion is a percolation process in which ions of the base migrate through the zone by steppin g from one hydrophilic Bite (phenol or phenolate) to the next. Dissolu tion inhibitors function by blocking some of the hydrophilic sites and thereby interrupting the diffusional pathways. Percolation theory sug gests a relation between the strength of inhibition and the percolatio n characteristics of the resin. The two are linked by the hydrophobic displacement volume of the inhibitor. The hydrophobic displacement vol ume depends not only on the space requirements of the inhibitor but al so on the mobility (or immobility) of the hydrophilic sites in the res in matrix; it is much smaller above the glass transition temperature o f the penetration zone than below it, and it is smaller in systems whe re some degree of motional freedom persists even below the glass trans ition of the zone. The hydrophobic displacement volume is the fundamen tal figure of merit of an inhibiting additive in a given base resin.