The birefringences, optical losses, refractive indices and their temperature dependency over a range of 30-90 degrees C for the films of carboxyl-containing polyaryletherketones with systematically varied alkyl substituents were investigated in detail. The results showed that the optical losses at 1310 nm were mainly affected by the content of C-H bond, whereas that at 1550 nm were more related to the presence of O-H bond. Moreover, two methyls and two bulky isopropyls on the phenylene rings led to the distorted polymer chain, which significantly reduced the chain packing density and disturbed the alignment of polymer chains along the substrate surface, and consequently resulted into the polymer films low refractive indices and birefringences. Furthermore, after conversion of carboxylic group to acyl chloride group, using rigid rod-like hydroquione and twisted phenolphthalein as bisphenol crosslinkers, respectively, the transparent smooth polymer films with three-dimensional network structures were readily obtained at the curing temperature below 200 degrees C without needing any additional catalyst. The cured polymers exhibited excellent resistance to organic solvents and good thermal stability as well as low birefringence of 0.0019 and greatly decreased optical loss at 1550 nm by about 78%. (C) 2010 Elsevier Ltd. All rights reserved.