Semicrystalline shape-changing polymers
We develop reversible shape-changing polymeric materials based on semicrystalline polymers. For example, it is commonly assumed that the substantial element of reversibly actuating soft polymeric materials, is chemical crosslinking, which is needed to provide elasticity required for the reversible actuation. On the example of melt spun and 3D printed Janus fibers, we demonstrate here for first time that crosslinking is not an obligatory prerequisite for reversible actuation of solid entangled polymers since the entanglement network itself can build-up elasticity during crystallization. Indeed, we show that not-crosslinked polymers, which typically demonstrate plastic deformation in melt, possess enough elastic behavior to actuate reversibly. The Janus polymeric structure bend because of contraction of the polymer and due to entanglements and formation of nano-crystallites upon cooling. Actuation upon melting is simply due to relaxation of the stressed non-fusible component. This approach opens perspectives for design of solid active materials and actuator for robotics, biotechnology and smart textile applications. The great advantage of our principle is that it allows design of self-moving materials, which are able to actuate in both water and air and, which are not crosslinked. We demonstrate application of actuating fibers for design of walkers, structures with switchable length, width and thickness which can be used for smart textile applications.
Explanation of the actuation mechanism of semicrystalline polymers fibers. (a-g) - optical snapshots of Janus fibers during actuation (a-c) – melting; (e-h) - crystallization. Time is given in the lower left corner of each image. Image (h) is overlap of (a-g) images. Intensity profiles along yellow line are shown under each snapshot; (i) – overlap of optical snapshots of fiber during actuation upon slow cooling down.; (j) - sketch of the semi-crystalline material during growth. On the left hand side, crystalline domains, orange squares, represent high-functional crosslinkers of size R0 in the elastic matrix, which is entangled (gray circles). During grow the crystalline phase shrinks, as compared to the melt-state (about 20%) which we illustrate by the hatched squares, as displayed on the right hand side. In order to illustrate the concept, we have separated here the growth and densification in two steps where the orange blocks correspond to the crystal domain, which virtually has the same density as the melt (v=1). Shrinking of the crystalline domains as compared to the corresponding melt volume creates a negative stress if the sample is fixed, here denoted by L0. The extension of a crystalline domain is denoted by R0 (without volume shrinkage) and R (taking into account volume shrinkage).
Ionov, Leonid; Stoychev, Georgi; Jehnichen, Dieter; Sommer, Jens Uwe
Reversibly Actuating Solid Janus Polymeric Fibers
in ACS Applied Materials & Interfaces vol. 9 (2017) issue 5. - pp. 4873-4881