Shape Memory Polymers Could Program And Recover At Room Temperature

Macroporous Photonic Crystals (Credit: Max Planck Institute of Microstructure Physics)

Shape memory polymers (SMPs) are a class of polymer that changes its shape with external stimuli, such as heat. The ability to memorize its original shape makes SMPs an intriguing material for many applications – reconfigurable nano-optics, biomedical devices, smart fabrics, or self-deployable aerospace structures. Typical SMPs can be activated by direct or indirect (e.g., light absorption and alternating magnetic field) heating. However, heat-demanding shape memory cycle significantly limits the broad applications of SMPs (especially as reconfigurable nano-optical devices). The tedious heating-and-cooling cycle impedes the application of SMPs under ambient conditions.

A photonic crystal is regarded as a periodic nanostructure that could interact with light. The most notable phenomenon of a natural photonic crystal is the striking bluish color shown by the morpho butterfly wings. This color is not a result of pigmentation but instead due to the interaction between visible light and photonic crystal nanostructures existing on the butterfly wings. Advances in nanotechnology have enabled scientists to mimic and fabricate the nanostructures akin to those contributed to butterfly wing coloration. In addition, small changes of these nanostructures can lead to different optical properties and colors. Hence, it is highly desirable to design smart and tunable optical nanostructures for various device applications.

To achieve real-world applications in smart and reconfigurable nano-optical devices, Dr. Peng Jiang’s group at the University of Florida has been focusing on developing rigid SMPs that can be programmed and recovered at room temperature. The merit of all-room-temperature operation can add significant values to SMPs. We could expand the application to many areas that previously seem impossible. Regarding the material itself, a highly rigid shape memory polymer can enhance its durability. With these design goals, the authors developed a new SMP, combine with unique nanostructures (photonic crystals) on the SMP surface, when programmed or recovered, exhibit a striking and easily perceived change in optical properties (color).

In the recent article published in Advanced Functional Materials, the authors presented a novel shape memory polymer that exhibited nontraditional and all-room-temperature shape memory cycle by integrating the concepts of SMPs with macroporous photonic crystals. This study chose a highly rigid polyurethane-based SMP. The final polyurethane photonic crystal membrane comprises 300 nm macroporous nanostructure on its surface. Crucially, this permanent glassy/rigid macroporous nanostructure could be cold-programmed into temporary disordered configurations under ambient conditions by slowly evaporating organic solvents that imbibed in the interconnecting macropore. The deformed macropores can be recovered, at room temperature by exposing it to vapors and liquids of organic solvents. The cyclic deformation and recovery of the macroporous nanostructures enable us to change the perceived colors of the sample.

The authors found that the interaction between the solvents and SMPs are crucial in affecting both the deformation and recovery processes. Swelling solvents (e.g., ethanol) can trigger “cold” programming and SM recovery; however, nonswelling solvents (e.g., hexane) cannot. The experiments revealed that the dynamics of swelling-induced plasticizing effects dominated both ‘cold’ programming and recovery process. Good solvent molecules that have a positive affinity towards the polymer could diffuse into the walls of macropores at the nanoscopic scale. Subsequently, the capillary force induced by solvent evaporation could induce temporary structural deformation of the softened macropores.

Importantly, swelling tests showed that the new SMP material exhibits negligible swelling in alkanes (e.g., hexane). By using the unique room temperature recovery property, the authors explored an exemplary system of ethanol-hexane solutions as a gasoline analog. The material could selectively detect trace amounts of ethanol in hexane with a detection limit of 150 ppm by monitoring the apparent color changes associated with the SM recovery. This work facilitated a better understanding of the unique properties of the novel SMP and demonstrated a new type of chromogenic sensor that can be applied to detecting trace amounts of analytes in a spectrum of solution and/or vapor mixture systems.

This study, Programmable Macroporous Photonic Crystals Enabled by Swelling-Induced All-Room-Temperature Shape Memory Effects was recently published in the journal Advanced Functional Materials.

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