A supramolecular azopolymer system that shows good optical performance and stability for use as high density optical information storage media has been developed by Chinese scientists. A 20-fold increase in information density over conventional DVDs is now possible, they claim. 

Azobenzene-containing polymers (azopolymers) are light-responsive materials that show reversible trans-cis isomerisation when irradiated with light. They are used in optical information storage, optical switching and as optically anisotropic materials and diffractive optical elements. Azopolymers have several advantages over azodye-doped polymers such as ease of synthesis and lack of aggregation at high concentration, but their use in real applications is hindered by poor stability and optical performance. Bisazopolymers can be used to overcome some of these performance issues but themselves have poor solubility and film-forming properties. 

Now, Si Wu and colleagues at the University of Science and Technology of China in Hefei have prepared films by connecting two azobenzene groups via hydrogen bonding to form a new supramolecular bisazobenzene group. Wu used azopolymers with pyridine groups to act as hydrogen bonding donors and a small molecular azo compound with a phenol group as a hydrogen bonding acceptor to make the supramolecular bisazopolymer film. The resulting films are promising materials for devices as they have improved optical performances, stability, solubility and good film-forming properties. 

The team optimised the information density of the polymers using a direct laser writing system to record images on the supramolecular bisazopolymers films while changing the laser intensities and polarisations. Up to 20 times more information than a conventional DVD could be stored. 

'The azobenzene-hydrogen bonding-azobenzene unit is a new supramolecular structure' explains Wu. 'We have attempted to advance our knowledge of the azopolymers' photoresponsive properties by designing azopolymers with improved solubility and film-forming properties for optical recording media and diffractive optical elements,' he adds. 

Arri Priimägi, an expert in polymer-based optical materials at Aalto University in Helsinki, Finland, comments, '[This work] demonstrates that concepts from supramolecular materials science can have a profound impact on the performance of polymer-based optical materials which will trigger further research of this highly promising class of materials.' 

The team are now exploring many avenues to continue to improve the research, including rebuilding a new laser system to allow for in situ measurements on the fabrication process, the synthesis of new supramolecular azopolymers and investigating two-photon technology for recording images onto the polymers. 

Carl Saxton 

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