Chemical modification removes defects from LCDs

A new technique for lining up liquid crystals could pave the way to cheaper and simpler flexible displays. Liquid crystal displays (LCDs) – key components in laptops and flat panel TVs – are controlled by a network of transistors. Crystals change their optical characteristics in response to electrical signals to create text and images on screen. The panels are made in a complex, multiple-stage process – one of which is prone to defects. A polymer film (called the alignment layer) is applied to the two pieces of glass between which the liquid crystals operate. After this, the film must be rubbed to anchor the alignment of the crystals. The rubbing can damage some of the transistors and introduce dust to the device – reducing the manufacturing yield. Now researchers at the Georgia Institute of Technology in the US have eliminated the polymer rubbing step by adding side chains to the polymer molecules. Instead, they use the in-situ ‘photopolymerisation’ of alkyl acrylate monomers in the presence of nematic liquid crystals. This provides a cellular matrix of liquid crystalline droplets in which the chemical structure of the encapsulating polymer controls the liquid crystal alignment. Mohan Srinivasarao, a professor at Georgia Tech’s School of Polymer, Textile and Fibre Engineering, has used the technique – and a nematic material with negative dielectric anisotropy – to fabricate highly flexible liquid crystal devices that have high contrast and fast response times. There is no alignment layer. Control is obtained by varying the alkyl side chains and through copolymerisation of two dissimilar monofunctional acrylates. As well as simplifying the manufacture of liquid crystal devices, the self-aligning technique could be used in new types of diffraction gratings. “Small changes in the chemical nature of the polymer will change the alignment of the molecules at surfaces,” Srinivasarao told Eureka. “This can be done over a fairly large area, and is reproducible. This would be an alternative way of creating the alignment that is required in these devices.” Beyond simplifying the fabrication process and potentially increasing device yield, the technique also offers other advantages. Because devices are based on vertical alignment of the liquid crystals, their ‘off’ state can be made completely dark. Also, the liquid crystals provide strong binding between the two substrate surfaces, making the resulting display less sensitive to mechanical deformations and pressure – ideal for flexible displays that lack the structure provided by glass plates. While the technique offers advantages over existing manufacturing methods, Srinivasarao does not expect an immediate change in the way that laptop screens and LCD televisions are produced. “Existing manufacturing processes are mature, and changing them probably cannot be justified economically at the moment,” he says. Beyond applications in flexible displays, the researchers believe their approach could be applied to the next generation of display devices based on liquid crystals. “When we make this polymer, the molecules automatically generate the alignment,” says Srinivasarao. “We are looking at what is responsible for making that happen. We want to link the chemical nature of these polymeric materials to how the liquid crystal molecules behave at the surface.” Current displays use polyimides for an alignment layer because these materials are heat-resistant and can be used over a broad range of temperatures for extended periods. “Alkyl acrylates lack that robustness, so material improvements would be needed before they could be used to manufacture flexible displays,” he explains. “If we could show similar results – switching times faster than 30 milliseconds and high contrast ratios – with more robust polymeric materials, this approach would be viable.” Pointers: * Adding alkyl ‘side chains’ to liquid crystal molecules can eliminate the complex ‘polymer rubbing’ stage in the manufacture of liquid crystal displays * The technique could lead to simplified, lower cost flexible displays for laptops and LCD televisions, but could also be applied to diffraction gratings