Transport properties of hyperbranched and dendrimer-like star polymers

Moisture transport properties were assessed by sorption and desorption measurements on hydroxyl-functional hyperbranched polyesters based on 2,2-bis(methylol) propionic acid (bis-MPA) as AB(2)-monomer with ethoxylated pentaerythritol. A series of these polymers with different molar masses were studied. For the first time, it is reported that sigmoidal sorption curves were successfully modelled using a time-dependent surface boundary concentration, where the relaxation time was obtained from the mechanical stress relaxation data. The zero concentration diffusivities were very small and comparable with the values of fully amorphous poly(vinyl alcohol). Both the diffusivity data and the stress relaxation data indicated that these materials were plasticized by moisture. Ethylene glycol transport properties were measured on a hexadecanoate(C16)-terminated bis-MPA hyperbranched polymer. The replacement of the hydroxyl groups with C16 paraffin tails resulted in an increase in solute diffusivity despite the fact that these polymers became semicrystalline. The shape of the desorption curves indicated that continuous micropores were present and facilitated mass transport. The moisture transport properties were also measured on a series of semicrystalline dendrimer-like star poly(epsilon-caprolactone(s)). The low measured densities of these highly crystalline dendrimers confirmed that the porosity, as indicated by the shape of the desorption curves, had to be discontinuous. The porosity in the (C16)-terminated and the poly(epsilon-caprolactone(s)) was impossible to eliminate by applying a high external pressure during crystallization. A collective view of all the results obtained confirm that the transport properties are primarily controlled by the hydroxyl group concentration. (C) 1999 Elsevier Science Ltd. All rights reserved.