<title>Dissolution of poly(p-hydroxystyrene): molecular weight effects</title>

Abstract

In the present study, dissolution rates of PPHS films were measured in aqueous base solvents and in 4-methyl-2-pentanone (MIBK) using laser interferometry. Molecular weights from 6,000 to 100,000 were employed. The films (0.5 to 2 micrometers thick) were spun on silicon wafers from MIBK solutions and baked 1 hour at 160 degree(s)C. The most striking feature of dissolution behavior is the peculiar way in which rate varies with molecular weight. Below a molecular weight of about 20,000, the rate decreases with molecular weight as one might expect. Above 30,000, the rate in aqueous base is much higher and seems to be almost independent of molecular weight. The behavior in MIBK, while more conventional, shows some of the same features. Another feature found with PPHS that is not usual with other polymers is a decrease in dissolution rate with increased thickness. Conventional wisdom would predict that retained solvents (after baking) in thick films would accelerate dissolution compared to thin films. Moreover, the interferograms for thick films show no pronounced change in dissolution rate as the film dissolves. Baking conditions and the pH of the aqueous base developer also affect the dissolution behavior.