Block Copolymer Elastomers for Stretchable Electronics.Acc Chem Res 2019; 52(1):63-72AC
As industrial needs for healthcare sensors, electronic skin, and flexible/stretchable displays increase, interest in stretchable materials is increasing as well. In recent years, the studies on stretchable materials have spread to various pivot components, such as electrodes, circuits, substrates, semiconductors, dielectric layers, membranes, and active nanocomposite films. The block copolymer (BC) elastomers have been playing considerable role in the development of stretchable materials. Since BCs are soft elastomers based on physical cross-links, they show differences in physical properties from normal elastomers formed with chemical cross-linking. BC elastomers does not require additional chemical cross-linking procedure, so they can be easily processed after dissolved in various solvents. Their viscoelasticity and thermoplasticity enable the BCs to become moldable and sticky. Although their unique physical properties may serve as disadvantages in some cases, they have been actively applied to create various stretchable electronic materials and their uses are expected to be enlarged more than ever. In this Account, we summarize recent successful applications of BCs for the stretchable electronic devices and discuss the possibility of further uses and the challenges to be addressed for practical uses. Studies on BC-based stretchable materials have focused initially on the fabrication process of stretchable conductors; mixing conductive fillers physically with BCs, infiltrating BCs in a conductive filler layer, and converting metal precursors into metal nanoparticles inside BCs. When conductive fillers with high aspect ratios, such as nanowires or nanosheets are used, the fillers can be infiltrated by the BCs after deposited. Since the contacts between the fillers are maintained during the infiltration process, even thin composite films possess high conductivity and stretchability. The metal precursor solution printing is suggested as a promising approach because it is compatible with traditional printing techniques without clogging the nozzles and allows high filler loading efficiency. When using a BC as a substrate, it is advisable to use a BC/PDMS double layer because of viscoelastic and thermoplastic properties of BCs. If BC/PDMS double layer is used with much thicker PDMS layer instead of viscoelastic BC alone, the double layer substrate can show a perfect elastomeric behavior, and the advantages of the BC substrate are preserved. Additionally, the use of conventional manufacturing techniques is important for commercialization of the stretchable devices. BC substrates having preformed microfibril network on their surfaces facilitate the fabrication of high-resolution circuitry by directly depositing metals through a mask on the substrate. Recent successes of fabricating stretchable organic transistors were obtained based on in situ phase separation of polymer semiconductors to form nanofibril bundles on the surface of a BC substrate. They have led to the achievement of high resolution transistor array printed in large area. BCs are expected to expand their applicability, including stretchable batteries, since they make it feasible to fabricate various hybrid nanocomposites, pore size-controlled membranes, and microstructured surfaces. However, it is necessary to secure long-term stability under heat, solvent, and UV; in addition, there is a need for the synthesis of functional BCs for use in stretchable implanted biomedical devices.