Crosslink connectivity inhomogeneities

A statistical theory is developed based on configurational restrictions which provides a general mechanism of formation of crosslink density inhomogeneities. The theory predicts that each intramolecular reaction enhances the probability that more intramolecular reactions take place leading to crosslinked regions which are poorly connected with the matrix or with each other. 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

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