Transparent stretchable substrate without image distortion shows potential for next-generation displays

Stretchable display materials, which are gaining traction in the next-generation display market, have the advantage of being able to stretch and bend freely, but the limitations of existing materials have resulted in distorted screens and poor fit.

General elastomeric substrates are prone to screen distortion due to the “Poisson’s ratio” phenomenon, in which stretching in one direction causes the screen to shrink in the vertical direction. In particular, electronics that are in close contact with the skin, such as wearable devices, are at risk of wrinkling or pulling on the skin during stretching and shrinking, resulting in poor fit and performance.

A research team led by Dr. Jeong Gon Son of the Korea Institute of Science and Technology (KIST) and Professor Yongtaek Hong of Seoul National University have developed a nanostructure-aligned stretchable substrate that dramatically lowers the Poisson’s ratio. The work is published in the journal Advanced Materials.

The research is notable for its ability to reduce the Poisson’s ratio while maintaining transparency, solving the problems of screen distortion and light scattering at the same time.

The researchers achieved this by combining two key ideas. In the first, they utilized block copolymers, which are polymer blocks linked together to align the internal nanostructures. The block copolymer (SIBS) consists of a stiff polystyrene (PS) and a softer polybutylene (PIB), which can be arranged in one direction to maximize the difference in elasticity between the parallel and perpendicular directions to reduce shrinkage.

While conventional elastomers have a Poisson’s ratio of 0.4 to 0.5, the researchers have reduced it to a Poisson’s ratio of 0.07 or less, which means that there is almost no shrinkage perpendicular to the substrate, even in the stretching direction, and screen distortion is greatly reduced.

The second idea was to introduce a shear-rolling process to align the nanostructures evenly across the substrate. It uses speed differences between rollers and stages to apply a uniform shear force at high temperatures. This process allowed the nanostructures to be reliably aligned on thick substrates without compromising transparency.

In experiments, the researchers found that there was little longitudinal shrinkage, even when the substrate was stretched by more than 50% in the vertical direction.

The researchers applied the developed substrate to a real device and observed changes in the pixel arrangement. The conventional elastomeric substrate, when stretched by 50%, showed distortion with jagged spacing between pixels or stuck vertical pixels.

The nanostructure-aligned substrate, on the other hand, had an even arrangement of pixels, resulting in an unbroken image and transparency without wrinkles or rough surfaces.

The new stretchable substrate is expected to be used as a core material in various fields such as next-generation displays, wearable electronics, and solar cells. In addition, the shear rolling process used in this study can be applied to other block copolymers and polymer films, making it a suitable technology for processing large areas in a simple manner.

“This research proposes a new method to develop a distortion-free and completely transparent stretchable substrate by precisely controlling the nanostructure, and the shear-rolling process to implement it can be easily applied to mass production and industrialization,” said Dr. Jeong Gon Son of KIST.

“We are currently conducting research to realize a real display device with no distortion even when tensile by transferring display light-emitting devices using this substrate.”