Analysis of network structure of UV-cured acrylates by 1H NMR relaxation, 13C NMR spectroscopy, and dynamic mechanical experiments

The network structure of W-cured acrylates was analyzed by dynamic mechanical experiments, H-1 T-2 NMR relaxation, and C-13 NMR spectroscopy. Photocured mixtures of difunctional (poly(ethylene glycol) diacrylate) and monofunctional (2-ethylhexyl acrylate) were studied. The mean crosslink density and fraction of dangling chains were varied in these networks by changing the content of monofunctional acrylate. Examining networks by C-13 NMR spectroscopy, we observed that the conversion is high, and the contribution or the effect of side reactions that generate extra cross-links is negligible. The spatial distribution of cross-links is very heterogeneous in the cured samples; i.e., polyacrylate ziplike network junctions are interconnected by polyether chains. An increase in the content of monofunctional monomer causes a significant decrease in the cross-link density because the monofunctional monomer acts as a chain extender. NMR T-2 relaxation method shows good correlation with mechanical tests with respect to molar mass between cross-links, M-c. Obtained values of M-c are significantly smaller compared to molar mass of diacrylate. As anticipated, it is apparent that classical rubber elasticity theories are not applicable for characterization of this type of networks containing ziplike network junctions.