(731h) Reprocessable Polyhydroxyurethane Network Composites: Effect of Filler Surface Functionality on Reprocessability and Stress Relaxation Behavior

We discovered that polyhydroxyurethane (PHU) networks synthesized in the presence of catalyst (4-dimethylaminopyridine) from five-membered cyclic carbonates are intrinsically reprocessable with full property recovery via transcarbamoylation exchange reactions and reversible cyclic carbonate aminolysis. Through judicious choice of monomers, we demonstrated that PHU networks can be recycled multiple times with full property retention. Recently, we investigated the effect of nanofiller surface functionality on the reprocessability and the rearrangement process of PHU network composites. Silica nanoparticles (diameter ~80 nm) with superhydrophobic (SH-composite), hydroxyl group (OH-composite), or amine group (NH-composite) functionality were incorporated into the PHU network at 4 wt% loading. Depending on the surface functionality, the nanoparticles can participate in transcarbamoylation exchange reactions (in the OH-composite) or reversible cyclic carbonate aminolysis (in the NH-composite) or exhibit behavior like a dormant species (in the SH-composite). The SH-composite was reprocessed (at 140 °C with ~10 MPa pressure for 2 hr) with full property recovery associated with cross-link density as determined from the rubbery plateau modulus. In contrast, the OH-composite and NH-composite showed losses of ~10% and ~20%, respectively, in cross-link density after reprocessing. Stress relaxation experiments were performed on the PHU network composites to determine the effect of different nanoparticles on the rate and the apparent Arrhenius activation energy of stress relaxation. Compared with the neat network, the network composites showed slower stress relaxation, with the SH-composite being the slowest, followed by the OH-composite and the NH-composite. In contrast, there was no significant impact of the nanoparticle surface functionality on the apparent activation energy of stress relaxation.