corresponding homopolymers. As in the case of random polymers, light absorption
can be diminished by using comonomers non-absorbing at the recording wavelength. Studies carried out in this type of materials have focused on series of block
copolymers with poly(methyl methacrylate) (PMMA) or polystyrene (PS) as amorphous transparent block and an azobenzene-containing block with liquid crystalline
character (Fig. 14b, c). The influence of the microstructure has been studied in detail.
Photoinduced anisotropy studies carried out in block copolymer films showing the
lamellar microstructure led to optical anisotropy values very similar to those
obtained in the homopolymer corresponding to the azobenzene block. This fact
supports the hypothesis that mesogen-to-mesogen interactions are preserved within
the blocks, and so they also keep the advantages of liquid crystalline photoresponsive systems (Frenz et al. 2004; Häckel et al. 2005) As a disadvantage,
lamellar morphologies, as well as cylindrical ones, present high levels of scattering
when compared to spherical morphologies, whose overall typical dimensions are
well below the wavelength of visible light. Studies carried out in homologous series
of photoresponsive block copolymers showed that systems with azobenzene-rich
spheres of sufficient radius (25–30 nm) led to photoinduced response similar to that
of homologous block copolymers with a lamellar microstructure and therefore to that
of the homopolymer (Berges et al. 2012). Despite the large surface to volume ratio in
these spheres, mesogen-to-mesogen interactions are able to build up large and stable
photoinduced values. This demonstrates that light-induced properties in these systems depend not only on the chemical nature of the monomers involved but also in a
very important way on the type of morphology and the size of the nanodomains.
Further dilution of the azobenzene content and therefore thicker films for this
application can be achieved with these same block copolymers by their direct
blending with amorphous non-absorbing polymers. An appropriate selection of
the molecular weight of the blocks and the amorphous homopolymer allows to
obtain a controlled microstructure of the blended material with no macrosegregation (Häckel et al. 2007). Well-defined spherical nanodomains of the azo
block have been obtained by this blending method with sphere size similar to that
of the parent block copolymer (Fig. 15). Photoinduced response experiments
carried in these materials demonstrate same performance in terms of optical
anisotropy (normalized to the azo content), enabling the preparation of films of
hundreds of microns and the recording and multiplexing of volume holograms on
them (Berges et al. 2012).
In the search of highly azo-diluted transparent materials for volume holography,
direct blending of materials has also been explored. A simple system consisting on a
small percentage (as low as 0.1 wt%) of a side chain liquid crystalline azopolyester
as the one in Fig. 7 and PMMA as an amorphous matrix has been explored for
volume holographic recording. Processing of this material from the melt lead to
nano-segregated azobenzene-rich regions; however, due to the low azopolyester
percentage, no macrophase segregation was identified. As a result, films with
thickness of several hundreds of microns with good optical transmission properties,
in which efficient and stable hologram were recorded with light pulses as short as
2 ms with this simplified material preparation method (Berges et al. 2013).
466
C. Sánchez-Somolinos
can be diminished by using comonomers non-absorbing at the recording wavelength. Studies carried out in this type of materials have focused on series of block
copolymers with poly(methyl methacrylate) (PMMA) or polystyrene (PS) as amorphous transparent block and an azobenzene-containing block with liquid crystalline
character (Fig. 14b, c). The influence of the microstructure has been studied in detail.
Photoinduced anisotropy studies carried out in block copolymer films showing the
lamellar microstructure led to optical anisotropy values very similar to those
obtained in the homopolymer corresponding to the azobenzene block. This fact
supports the hypothesis that mesogen-to-mesogen interactions are preserved within
the blocks, and so they also keep the advantages of liquid crystalline photoresponsive systems (Frenz et al. 2004; Häckel et al. 2005) As a disadvantage,
lamellar morphologies, as well as cylindrical ones, present high levels of scattering
when compared to spherical morphologies, whose overall typical dimensions are
well below the wavelength of visible light. Studies carried out in homologous series
of photoresponsive block copolymers showed that systems with azobenzene-rich
spheres of sufficient radius (25–30 nm) led to photoinduced response similar to that
of homologous block copolymers with a lamellar microstructure and therefore to that
of the homopolymer (Berges et al. 2012). Despite the large surface to volume ratio in
these spheres, mesogen-to-mesogen interactions are able to build up large and stable
photoinduced values. This demonstrates that light-induced properties in these systems depend not only on the chemical nature of the monomers involved but also in a
very important way on the type of morphology and the size of the nanodomains.
Further dilution of the azobenzene content and therefore thicker films for this
application can be achieved with these same block copolymers by their direct
blending with amorphous non-absorbing polymers. An appropriate selection of
the molecular weight of the blocks and the amorphous homopolymer allows to
obtain a controlled microstructure of the blended material with no macrosegregation (Häckel et al. 2007). Well-defined spherical nanodomains of the azo
block have been obtained by this blending method with sphere size similar to that
of the parent block copolymer (Fig. 15). Photoinduced response experiments
carried in these materials demonstrate same performance in terms of optical
anisotropy (normalized to the azo content), enabling the preparation of films of
hundreds of microns and the recording and multiplexing of volume holograms on
them (Berges et al. 2012).
In the search of highly azo-diluted transparent materials for volume holography,
direct blending of materials has also been explored. A simple system consisting on a
small percentage (as low as 0.1 wt%) of a side chain liquid crystalline azopolyester
as the one in Fig. 7 and PMMA as an amorphous matrix has been explored for
volume holographic recording. Processing of this material from the melt lead to
nano-segregated azobenzene-rich regions; however, due to the low azopolyester
percentage, no macrophase segregation was identified. As a result, films with
thickness of several hundreds of microns with good optical transmission properties,
in which efficient and stable hologram were recorded with light pulses as short as
2 ms with this simplified material preparation method (Berges et al. 2013).
466
C. Sánchez-Somolinos