Flexible Conducting Composite Film with Reversible In-Plane Folding-Unfolding Property

Abstract

Flexible conducting films in the forms of bendability or stretchability are developed as a key component to enable soft electronics. With the requirements of miniaturization and portability of modern electronics, conducting film that can endure in-plane shrinkage is urgently needed but still remains challenging. Here, a new type of conducting film achieving reversible in-plane folding-unfolding function with large deformation by infusing conductive liquids into hierarchically structured polymer films consisting of both nanostructured polymer nanofibers and microstructured frames is reported. Nanostructured polymer nanofibers that can be completely wetted by the conductive liquids provide capillary forces to gain reversible in-plane folding-unfolding property, while the microstructured frames greatly enhance the extent during folding-unfolding process. Conductivity of the produced films can be significantly improved by rationally tuning the composition of infused conductive liquids, which always keeps high values during the folding-unfolding deformation. It is believed that this work may serve as the basis for robust fabrication of flexible conducting films with reversible in-plane folding-unfolding function, and can also put one-step forward of modern soft electronics.

Keywords: flexible conducting films; in-plane folding-unfolding; reversible deformation; soft electronics.

Figure 1

Design of conducting film with reversible in‐plane folding–unfolding function. a) A PVDF‐HFP nanofiber network with thickness of about 50 µm is fabricated by electrospinning. b) PLA frames with width of about 500 µm are introduced onto the PVDF‐HFP nanofiber network by 3D printing. c) The fabricated hierarchical polymer film can be completely wetted by [EMIm][NTf₂] within 60 s, which shows reversible in‐plane folding–unfolding function.

Figure 2

Evaluation of in‐plane folding–unfolding property of the IL‐infused electrospun PVDF‐HFP film. a–d) Macro‐ and micromorphology of the film during reversible folding–unfolding process. e) Effect of liquid film thickness (h) on folding behavior. When liquid film thickness is much smaller than the elastocapillary length (h << L _ec), no capillary walls can be formed and the folding process is energy‐unfavorable. For h ∼ L _ec, soft capillary walls are formed to generate limited folding ratio. For h >> L _ec, hard capillary walls are formed and the folding process becomes energy‐favorable. Further increasing the liquid film thickness would lead to an unstable composite film, and thus even a slight decrease of folding ratio. Thus, an optimal liquid film thickness of about 60 µm is chosen for further studies.

Figure 3

Engineering microstructured frames to improve in‐plane folding–unfolding performance. a) Dynamics during folding process. A force vertical to the folding direction makes the free edges adopt a shape of circular arc with central angle of β. Folding the film leads to an increase of β until 180°, after which the edge will slide along the fixed support. b) When the total length of free edges (L _free = 2L) is larger than that of fixed edges (L _fixed = 2W) (2L > 2W), the film can only achieve insufficient folding and eventually turns into a line. c) Introduction of parallel frames satisfying the requirement of 2L < 2W can ensure sufficient folding. d) Introducing triangle frames is a reliable strategy to achieve sufficient in‐plane folding because the total length of free edges is always smaller than that of the fixed edges (L < 2W).

Figure 4

Fabrication of highly conductive PVDF‐HFP/PEDOT:PSS/IL composite film with reversible in‐plane folding–unfolding performance. a) Preparation of highly conductive composite film by wetting the electrospun PVDF‐HFP film with mixture of PEDOT:PSS and ILs. b) The highly conductive composite film can light an LED, and the LED brightness keeps almost unchanged during the folding–unfolding processes. c) Sheet resistance of the film only fluctuates in a relatively small range, between 40 and 50 Ω sq⁻¹, during 1000 cycles of in‐plane folding–unfolding.