Shape-memory and self-healing polyurethanes based on cyclic poly(ε-caprolactone)
The structure–property relationship is a primary and eternal topic in polymer science. Cyclic topology, due to no chain ends, has been confirmed to have significant effects on polymer properties and performance. In this work, cyclic poly(ε-caprolactone) (PCL) with two definite hydroxyl groups on each polymer chain was prepared via successive ring-opening polymerization, chain end modification and bimolecular ring closure reaction, and was fully characterized using size exclusion chromatography, NMR, FTIR and matrix assisted laser desorption/ionization time of flight. Higher crystallinity of the cyclic PCL was found in comparison with its linear counterpart. Cross linked polyurethanes (PUs) were then prepared based on cyclic PCL, hexamethylene diisocyanate and tetrahydroxy functionalized crosslinkers incorporated with dynamic covalent bonds between furan and maleimide. The PUs made using cyclic PCL resulted in more compact networks than those made from linear polymers due to topology effects, which significantly lowered the crystallinity of the cyclic polymers in the PUs. It was found that the shape memory of the cross linked PUs was not dominated by the crystallinity but by the degree of cross linking. The PUs made from cyclic PCL gave a higher shape fixity ratio (∼95%) than those from linear PCL (∼80%), however, the recovery ratio of the cyclic PCL made PUs was slightly lower. The discrepancy in the shape memory performance was attributed to the different cross linked networks arising from topology effects. Self-healing tests showed that the cyclic PCL and linear PCL derived PUs showed good and approximate self-healing abilities. This work provides an unprecedented example for the exploration of the cyclic topology effects on the shape memory performance of bulk PU materials, providing a novel angle to elucidate the structure–property relationship of polymeric materials.