The effect of film thickness on the thermal rearrangement of an ortho- functional polyimide, derived from 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3'-dihydroxy-4,4'-diaminobiphenyl (NAB), to a polybenzoxazole structure was explored over the range from 50 nm to 100 mu m. The rate of thermal rearrangement of this ortho-functional polyimide was found to be strongly dependent on film thickness. A thin film of 56 rim experienced a much more rapid and extensive thermal rearrangement process than a thick film of 100 pm as tracked by thermogravimetric analysis, TGA. The conversion of the ortho-functional polyimide to polybenzoxazole commences at a lower temperature for thinner films than for thicker films. isothermal TGA experiments at 370 degrees C showed a much larger weight loss, or extent of rearrangement, at a given time for thin films compared to thick films. These observations are believed to reflect greater chain segmental mobility in the thin films, due to the proximity of the free surfaces, and perhaps the reduced diffusional resistance for removal of the evolving volatile compounds accompanying the rearrangement reaction. (C) 2013 Elsevier B.V. All rights reserved.