Stimuli-responsive materials change their state in response to external triggers. Switching between different states enables information to be written, stored, and read, if the transition between the states exhibits hysteresis. Thermally responsive polymers exhibit an intrinsic hysteresis for the volume phase transition between the swollen and de-swollen solution state. Here, it is shown that this hysteresis can be used to realize bistability, remanence, and reversible write/read information storage. This is demonstrated for the simplest and most widely used thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), as well as for PNIPAM block copolymers, which widens the hysteresis window. The hysteresis is shown to be related to cluster domain assembly/disassembly during the phase transition. Information can be written thermally using a laser, or using heated or cooled pen tips on a thin-film backscattering display. The bistable state can additionally be switched by pH, enabling an AND logic gate function. Furthermore, an unusual memory state is discovered, where information is visible in the hysteresis window and invisible at higher temperatures, allowing encoded information to be hidden. Since hysteresis is a very common intrinsic phenomenon for responsive materials, this principle to encode and store information is potentially applicable to a broad range of responsive materials.
Keywords: information storage; responsive materials; thermal writing; volume phase transition.
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