A PERCOLATION VIEW OF NOVOLAK DISSOLUTION .3. DISSOLUTION INHIBITION

The dissolution of novolak films in aqueous alkali is controlled by the 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 stepping from one hydrophilic Bite (phenol or phenolate) to the next. Dissolution inhibitors function by blocking some of the hydrophilic sites and thereby interrupting the diffusional pathways. Percolation theory suggests a relation between the strength of inhibition and the percolation characteristics of the resin. The two are linked by the hydrophobic displacement volume of the inhibitor. The hydrophobic displacement volume depends not only on the space requirements of the inhibitor but also on the mobility (or immobility) of the hydrophilic sites in the resin matrix; it is much smaller above the glass transition temperature of the penetration zone than below it, and it is smaller in systems where some degree of motional freedom persists even below the glass transition of the zone. The hydrophobic displacement volume is the fundamental figure of merit of an inhibiting additive in a given base resin.