Shape memory polymers (SMPs) are stimuli-responsive materials that remember an original “permanent shape” that can be recovered from a temporary fixed shape by exposure to external stimuli such as heat, electricity, moisture, solvent, or a magnetic field. Shape fixing is most commonly observed by heating the polymeric material above a transition temperature that can be the glass transition (Tg), melting (Tm), or crystalline clearing temperature (Tcl). Recent work has demonstrated that polymeric materials with multiple transitions can exhibit two-way shape memory (2W-SM) behavior and so-called triple shape memory in systems possessing both chemical and physical crosslinking.
Directing shape adaptations with light equips a potential user with remote, wireless control — in addition to the potential for spatial control — with masking or holographic exposure. Prior reports of light-activated SMP leverage photo-induced changes to the crosslink density of the network to program and release a desired shape. One prior report specifies a system composed of an interpenetrating network containing up to 10 mol percent of cinnamic acid groups that photo-crosslink when subjected to UV light >260 nm for one hour to “photo-fix” a mechanically elongated shape. Subsequent irradiation with UV light <260 nm for one hour de-crosslinks and releases the fixed shape.
The invention described here relates to the employment of glassy, photo-responsive polymeric materials as optically fixable SMPs in which the shape fixing mechanism is due to non-reactive, photo-induced reconfigurations of the polymer network morphology.
The optically fixable SMPs are capable of rapid optical-fixing with short exposures (<<5 min) of eye-safe 442-nm light. The use of polymeric materials composed of covalently attached photochromic units, such as azobenzene, and the use of light — which is capable of inducing adjustments to the polymer network morphology — is the key to optical fixing.
Although the model material system is liquid crystalline, liquid crystallinity is not a critical requirement for observing shape fixing in these materials. Optically fixable SMPs have been demonstrated in high-performance, azobenzene-functionalized polyimides. The SMPs of the invention also includes the ability to combine thermal and optically fixable shape memory.