Transport properties of hyperbranched and dendrimer-like star polymers

Moisture transport properties were assessed by sorption and desorption meas urements on hydroxyl-functional hyperbranched polyesters based on 2,2-bis(m ethylol) propionic acid (bis-MPA) as AB(2)-monomer with ethoxylated pentaer ythritol. A series of these polymers with different molar masses were studi ed. 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 relaxat ion data. The zero concentration diffusivities were very small and comparab le with the values of fully amorphous poly(vinyl alcohol). Both the diffusi vity data and the stress relaxation data indicated that these materials wer e plasticized by moisture. Ethylene glycol transport properties were measur ed on a hexadecanoate(C16)-terminated bis-MPA hyperbranched polymer. The re placement of the hydroxyl groups with C16 paraffin tails resulted in an inc rease in solute diffusivity despite the fact that these polymers became sem icrystalline. The shape of the desorption curves indicated that continuous micropores were present and facilitated mass transport. The moisture transp ort properties were also measured on a series of semicrystalline dendrimer- like star poly(epsilon-caprolactone(s)). The low measured densities of thes e highly crystalline dendrimers confirmed that the porosity, as indicated b y the shape of the desorption curves, had to be discontinuous. The porosity in the (C16)-terminated and the poly(epsilon-caprolactone(s)) was impossib le 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.