Morphological analysis of photocured polyurethane acrylate networks. Correlation with viscoelastic properties

In this paper, we investigate the final morphology of photocured polyuretha ne acrylates based on polypropylene oxide by means of Transmission Electron Microscopy (TEM), Small Angle X-ray Scattering (SAXS), and dynamic mechani cal measurements. Two interrelated structural features on two different siz e scales can occur in these systems. TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 manometers, mostly consti tuted by clusters of small hard units (diacrylated diisocyanate) connected by polyacrylate chains. The bimodal shape of the dynamic mechanical relaxat ion spectra corroborates this two-phase structure. Besides, a suborganizati on of the reacted diisocyanate hard segments inside the polyurethane acryla te matrix is revealed by SAXS measurements, depending on the nature of the hydroxylacrylate used for the synthesis of the precursor. Finally, W-exposu re time is found to induce modifications on the viscoelastic properties of the final network, even at high double-bond conversion: this effect can be due to a postreaction and to an increase of the crosslinking density inside the hard segments domains. (C) 1999 John Wiley & Sons, Inc.