Robust POSS-PEG networks. Nanostructure, viscoelasticity and shape memory behavior

Abstract

A detailed Å- and nm-scale analysis was carried out on polyhedral oligomeric silsesquioxane (POSS) photo-crosslinked with polyethylene glycol (PEG) at 10:90 wt% ratio. POSS increased up to two orders of magnitude the shear mechanical modulus thus producing robust molecular networks with one-way shape memory (SM) behavior. The mechanical reinforcement was driven by POSS nanoclusters which were revealed by high-resolution transmission electron microscopy (TEM). On the other hand, PEG molecular weight tuned POSS dispersion, i.e., average single or three units POSS dispersion was achieved by varying PEG molecular weight. This resulted in the shear elastic modulus increasing over one and two orders of magnitude, respectively. X-ray scattering wide- and small-angle showed that the robust mechanical reinforcement is unique to POSS as crystallinity and crystal size were not the relevant factors. The one-way SM behavior was thermally activated by the melting the PEG crystals and a temporary shape was fixed by the recrystallization of PEG. That is, the network locked in a temporary shape by PEG crystallization and soft actuation resulted from the crystals melting freeing the stresses that extended through the crystalline phase. These versatile and mechanically robust networks broaden the scope of applications in actuators, sensors, and responsive materials. • Well-defined nanoscopic hydrophobic entity to control the shape memory of a network. • POSS photo-crosslinked with PEG increased shear moduli over an order of magnitude. • Unique and remarkably simple approach enabling industrial application.