The 'flexible display material' that is gaining attention in the next-generation display market has the advantage of being able to unfold and bend freely; however, there have been issues of screen distortion or decreased wearability due to the limitations of existing materials. This is due to the 'Poisson's ratio' phenomenon, which causes horizontal shrinkage when stretched in one direction. Domestic researchers have developed a new substrate to address this.
On the 23rd, Son Jeong-gon, a principal researcher from the Korea Institute of Science and Technology (KIST), and Professor Hong Yong-taek from Seoul National University announced that they developed a 'NANO structure aligned stretchable substrate' that dramatically lowers the 'Poisson's ratio.' The research results were published in the international journal 'Advanced Materials' last December.
The research team achieved this result by combining two core ideas. First, they aligned the internal nano structures using block copolymers linked by polymer blocks. This block copolymer is composed of rigid 'polystyrene (PS)' and flexible 'polyisobutylene (PIB),' and by arranging it in one direction, they maximized the elastic differences between the tensile direction and vertical direction, thereby reducing contraction. The Poisson's ratio of existing elastomers is about 0.4 to 0.5, but the researchers were able to lower this value to below 0.07, resulting in almost no vertical contraction of the substrate even when elongated, and significantly reduced screen distortion.
To evenly align the nano structures across the entire substrate, the researchers introduced a shear-rolling process. This method utilizes the velocity difference between the roller and the stage at high temperatures to apply uniform shear force. Through this process, they were able to maintain the transparency of thick substrates. Experiments confirmed that even when the substrate was stretched more than 50% in the horizontal direction, vertical contraction barely occurred.
The research team applied the developed substrate to actual devices to observe changes in pixel arrangement. Existing elastomer substrates exhibited distorted pixel spacing or vertical pixels sticking together when stretched by 50%. In contrast, the newly developed nano-structured aligned substrate has uniformly arranged pixels, preventing image distortion and maintaining transparency without wrinkles or rough surfaces.
The new stretchable substrate is expected to be a key material across various fields, including next-generation displays, wearable electronics, and solar cells. Additionally, the shear-rolling process applied in this study can also be used to mass-produce other block copolymers or polymer films.
Son Jeong-gon, the principal researcher, noted, "This study proposes a new method for developing a completely transparent stretchable substrate without distortion by precisely controlling the nano structure, and the shear-rolling technology for implementing this can be easily applied to mass production and industrialization." He mentioned, "Currently, research is underway to implement a display device that maintains no distortion during tension using this substrate."
Reference materials
Advanced materials (2024), DOI: https://doi.org/10.1002/adma.202414794