Inherently Photohealable and Thermal Shape-Memory Polydisulfide Networks

Structurally dynamic polydisulfide networks that inherently exhibit both shape-memory and healable properties have been synthesized. These materials are semicrystalline, covalently cross-linked network polymers and as such exhibit thermal shape-memory properties. Upon heating above its melting temperature (T_m) films of the material can be deformed by a force. Subsequent cooling and removal of the force result in the material being "fixed" in this strained temporary shape through a combination of crystallinity and covalent cross-links until it is exposed to temperatures above the T_m at which point it recovers to its remembered processed shape. The incorporation of disulfide bonds, which become dynamic/reversible upon exposure to light or elevated temperatures, into these networks results in them being structurally dynamic upon exposure to the appropriate stimulus. Thus, by activating this disulfide exchange, the network reorganizes, and the material can flow and exhibit healable properties. Furthermore, exposure to light also allows the film's permanent "remembered" shape to be reprogrammed. Shape-memory experiments on these films show high degrees of both fixing and recovery (>95%), and photohealing experiments showed that the films were able to recover from a scratch whose depth is approximately half the thickness of the film. Using a combination of the thermal shape-memory behavior followed by photohealing allows wide scratches to also be efficiently healed.