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Light structuring of functional polymers

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If you have a question about this talk, please contact Dr. Z Chen.

The use of light to locally modify polymer morphology allows the implementation of structures with feature sizes in the micrometer range. Surface relief or three dimensional complex structures can be obtained by using light structuring techniques such as mask or holographic photolithographies or direct laser writing in the appropriate polymeric materials. These structures have demonstrated to be of great importance in the fabrication of optical elements and devices for information and communication technologies and more recently are being studied as scaffolds for cell and tissue growth with applications in biomedicine and regenerative medicine. The work within this area carried out in the Liquid Crystals and Polymers Group in Zaragoza will be presented in this talk through several examples.

Our work carried out in the field of polarization converters will be first presented. These systems are able to transform a linearly polarized beam into a radially polarized one and have potential application in tightly focused beam technologies such as optical storage, photolithography or microscopy. By properly rubbing planar polyimide films it is possible to build a LC cell able to implement this optical function. Light structuring of linear photopolymerizable polymers (LPPs) can also be used as an alternative to produce complex LC command surfaces to produce these elements. Based on liquid crystal technology, we can generate a simple, compact and mechanically robust polarization converter device.

The talk will then focus on our work on azobenzene containing polymers. These materials undergo isomerization-reorientation processes when they are irradiated with light in their absorption bands. Under irradiation with linearly polarized blue or green light (e.g.: 488 nm) a preferential orientation of the azobenzene units with their axes perpendicular to the polarization direction of the exciting light is induced. This results in macroscopic optical anisotropy making these polymers potential candidates to be used as optical storage media. Holographic patterned irradiation of films of these materials has been performed to record phase gratings based on these reorientation phenomena although surface relief gratings due to mass transport have also been found. Traditionally these studies have been limited to Raman Nath thin gratings due to the large optical absorption coefficient of azobenzene units at the recording wavelengths that hinders the effective sensitization of the deep layers of thick films. To fully exploit the advantages of volume holography thick polymeric films (in the range of 100 µm to 1 mm) with low optical absorption need to be prepared. In order to get a lower optical absorption, the azobenzene content has to be diluted. This can be achieved by copolymerization of the azo monomer with others that do not absorb at the wavelength of illumination. We have explored the optical response both random and block copolymers in which the azobenzene content is diluted being possible to obtain thicker films with suitable absorption at the recording wavelength. Block copolymers show nanosegregated morphologies in which azobenzene chromophores are in close interaction favoring in general the stabilization of the photoinduced orientation. The influence of the molecular architecture in the phase segregated morphology and in the photoinduced optical properties has been studied. Holographic recording and multiplexing of different holograms have also been performed in thick films of these materials.

References:

S.Hvisted, C. Sánchez, R. Alcalá; The volume holographic optical storage potential in azobenzene containing polymers, Journal of Materials Chemistry, 19, 6641-6648 (2009).

This talk is part of the Optoelectronics Group series.

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