Modification with alkyl chains and the influence on thermal and mechanical properties of aromatic hyperbranched polyesters

All-aromatic hyperbranched polyesters with hydroxy endgroups were functionalized with aliphatic n-alkyl carboxylic acids. The length of the n-alkyl chain as well as the degree of modification were varied and the resulting, partially amphiphilic polymers were characterized by differential scanning calorimetry (DSC). With both increasing degree of modification and increasing length of the alkyl chain the glass transition temperature decreases due to reduced intermolecular hydrogen bonding. When the alkyl chains start to crystallize Tg of the hyperbranched polymers increases again. The mechanical properties of the former brittle hyperbranched polyester were improved by modification with C12 chains and a stable free standing film was obtained by compression molding. The film was investigated by means of dynamic mechanic analysis (DMA) and microscopy, exhibiting a low temperature thermal transition and phase separation within the scale of a light microscope. Furthermore melt rheology measurements were performed on the starting polymer and on the C12 modified product. The complex viscosity is reduced strongly by the modification of the aromatic hyperbranched polyester.