Highly Stretchable, Soft, and Clear Viscoelastic Film with Good Recoverability for Flexible Display

The advancement of flexible electronic devices has prompted new material development for the display application. For flexible display, a suitable clear viscoelastic film (CVF) is essential to bond different layers in a display stack in order to improve the visualization and durability during the repeated folding process. However, it is challenging to integrate different properties in the CVF by overcoming many contradictory requirements, such as low modulus/glass transition temperature (T_g) and high adhesion or high recoverability and good stress-relaxation. In this work, a CVF was prepared using an interpenetrating polymer network (IPN) with bimodal chain length distribution, and it exhibited several favorable properties. The bimodal elastomer was composed of short-chain polyurethane (PU) and long-chain polyacrylate. The long-chain polyacrylate network provided a large amount of entanglement that conferred stretchability, adhesion, and stress-relaxation, whereas the short PU chain network acted as an entropy spring and contributed mostly to the recoverability. The experimental data suggested the presence of a hydrogen-bonding interaction and interlocked polymer chains between the two networks. When the components of the IPN are adjusted, the CVF can simultaneously achieve good stress-relaxation, high strain recovery at large strain (1000%), high toughness, clarity, and adhesion. Moreover, the CVF displayed low glass transition temperature (-57 °C) and low storage modulus (20 to 30 kPa at room temperature). To the best of our knowledge, this is the first report using the IPN concept to prepare a CVF with well-balanced properties.