A PERCOLATION VIEW OF NOVOLAK DISSOLUTION .4. MECHANISM OF INHIBITOR ACTION

It is difficult to reconcile the large inhibition effects of some diazoquinones, and other additives, with a site-blocking model of dissolution inhibition; to produce an effect commensurate with the observed inhibition factors, one molecule of inhibition would have to disable up to 16 OH groups of the resin. The dual nature of OH groups which are simultaneously hydrogen donors and accepters provides an amplification mechanism that can accommodate this requirement. Phenolic OH groups form aggregates in solutions of low polarity, and such aggregates, termed ''phenolic clusters'', are also formed in the coating solutions of novolak films. When inhibitors with strong hydrogen acceptor (carbonyl) groups are introduced into the system, they form hydrogen bonds with some of the OH groups, and in so doing polarize these groups. The polarized groups, in turn, become stronger hydrogen accepters and cause a larger number of OH groups to collect in phenolic clusters. The coming together of hydrophilic sites in clusters disturbs the randomness of the percolation field and decreases the site connectivity. This lowers the diffusional flux of base through the matrix and with it the dissolution rate.