Rheological study of the gelation of cross-linking polyhedral oligomeric silsesquioxanes (POSS)/PU composites

The gelation process of N-Phenylaminomethyl-POSS/PU (polyurethane) nanocomposites during curing and at the stable state after curing was investigated by rheology. An increase in dynamic shear moduli, G′ and G″, was observed during the dynamic temperature ramps of the sample. In time-resolved mechanical spectroscopy (TRMS), G′ and G″ increased with curing time at constant curing temperatures over a wide of frequencies. The gelation time of the composites decreased with the rise of curing temperature or with the increase of POSS concentration. The relaxation exponent n at the critical gel was around 0.73. The gel stiffness S decreased as curing temperature increased or as POSS concentration increased. After the completion of the curing reaction, the critical concentration of POSS beyond which the gelation of POSS/PU composites would happen was found around 2.5 wt%. The viscoelastic properties of crosslinking POSS/PU fitted time–temperature-superposition well which implied the incorporation of multifunctional POSS into PU increased the homogeneity of crosslinking POSS/PU composites. Surprisingly, the modulus of the fully cured materials between 2.7 wt% and 6 wt% could also be supposed onto a master curve at high temperature, which implied self-similarity of network near the critical gel. The similar microstructure of POSS/PU at stable state was also confirmed by TEM. The network formation mechanism and the fine structure of the crosslinking POSS/PU were firstly investigated which would provide technical and theoretical basis for other hybrid crosslinking systems.