Rollable Transparent Glass‐Fabric Reinforced Composite Substrate for Flexible Devices

Recently, there has been considerable interest in fl exible displays, as they facilitate the fabrication of displays that are thin, lightweight, robust, conformable, and fl exible. [ 1 ] To enable a fl exible display, a fl exible substrate must be used as the fundamental starting component in place of a conventional glass substrate. In general, metal foils, ultra-thin glasses, and plastic fi lms are considered candidates for a fl exible substrate. [ 2 ] In particular, fl exible displays using plastic substrates based on organic polymers have been a major topic not only because these show outstanding fl exibility and optical clarity at the same time, but also because they offer the possibility of a reduction in cost, coupled with a roll-to-roll process and ink-jet printing technology. [ 3 ] Common examples of commercially available polymers are polyethylene terephthalate (PET), polyether sulfone (PES), polyethylene naphthalate (PEN), polycarbonate (PC), and polyimide (PI). [ 1 , 4 ] To replace conventionally used glass substrates, a plastic substrate must be equipped with the properties of glass, such as optical transparency, thermal/chemical stability, O 2 /H 2 O permeability, low birefringence (or retardation), dimensional stability, and a low coeffi cient of thermal expansion (CTE). [ 1 ] Among these properties, the low CTE of plastic substrates coupled with dimensional stability is arguably the most important requirement, as it is directly related to compatibility with all other necessary display layers to be integrated onto them. [ 1 ] Although there have been extensive studies of fl exible devices built on polymer substrates, such as electrophoretic displays and organic thin-fi lm transistors (OTFTs), little progress have been made even with high-temperature processed devices such as active-matrix liquid crystal displays (AMLCDs). [ 5 ] Major obstacles include the high CTEs of polymers insuffi cient for the display layer integrations. Moreover, polymers usually have low glass transition temperatures ( T g s) where abrupt CTE changes are accompanied. This greatly limits their practical application in terms of the process temperature. Even polymers with a high T g , e.g., PES, are known to have a CTE of ∼ 50 ppm K − 1 , much higher than the typically required level (less than 20 ppm K − 1 ). [ 1 ] Other highT g polymers such as polytetrafl uoroethylene (PTFE) and poly(ether ether ketone) (PEEK) also have signifi cant drawbacks for implementation into large-area plastic electronics, as they are unfavorable in terms of cost. PI has a low CTE of ∼ 20 ppm K − 1 and a high