Characterization by Atomic Force Microscopy of the Nanoheterogeneities Produced by the Radiation-Induced Cross-Linking Polymerization of Aromatic Diacrylates

Abstract

Atomic force microscopy was used in the tapping mode for studying the heterogeneities in radiation-cured acrylate networks via phase imaging of local viscoelasticity. Film samples were prepared by UV- or electron beam-initiated polymerization of a bisphenol-A-derived ethoxydiacrylate or of the epoxydiacrylate analogue under nitrogen. The top surface of the samples, nearly flat at the atomic level, was probed under moderate to hard tapping conditions. The phase images revealed the heterogeneous character of the network that can be interpreted in terms of local variations of cross-link densities. Appropriate image treatment was applied to determine a set of geometric descriptors for samples exhibiting monomer conversion levels ranging from 0.3 to 0.8. Rigid nodules exhibiting a mean characteristic dimension of ∼15 nm were observed very early in the cross-linking polymerization process. Those clusters initially embedded in a soft gel undergo limited evolution by growth and by aggregation up to a limiting size at higher conversion levels. Nucleation within the monomer-rich domains further continues up to ca. 50% conversion, together with limited growth by aggregation of adjacent particles. Polymerization then continues in interstitial domains, generating a stringy network with some isolated low conversion domains. These different features were observed for both aromatic diacrylate monomers and were qualitatively similar in EB- and UV-cured samples, without significant influence of the initiation mechanism and initiation rate on the nanostructure of the studied networks.