Photoactive cross-linkers

After an efficient system has been found, the main open question is how to solidify the foams. One route could be to utilise water-soluble polymers or/and polymerise water-soluble monomers. Another route could be to switch to polymeric microemulsions. One would stepwisely replace the water by the preferred polymerisable hydrophilic monomer/polymer. By doing so one can be sure that the nanostructure of microemulsion remains unaffected. The generation of the nanofoam depends on the used monomer. A fixation via irradiation with fight would be preferable, when photoactive cross-linkers are... 

In order to illustrate this concept, five photoactive polysiloxane-based side-chain nematic LSCEs (EAZO-n, Fig. 18.5a) that contain 4,4-dialkoxyazobenzenes as photoactive cross-linkers, which hold spacers of different lengths, will be considered. Specifically, all of them are composed by the nematic mesogen 4-methoxyphenyl-4-(3-butenyloxy)benzoate (M40Me, 90 % mol), the isotropic cross-linker l,4-di-(10-undecenyloxy)benzene (VI, 5 % mol) and a 4,4-dialkoxyazobenzene as a light-sensitive cross-linker (AZO-n, 5 % mol). 

An intriguing report by Steinke et al. [ 17] describes the creation of molecularly imprinted anisotropic polymer monoliths . Optically transparent blocks of MIP using either methacrylic acid (MAA) or 2-(acrylamido)-2-methylpropanesulphonic acid (AMPSA) as functional monomers and TRIM (trimethylolpropane trimethacrylate 2-ethyl-2-(hydroxymethyl)-l,3-propanediol trimethacrylate) as the cross-linker were synthesised using the photoactive template Michler s ketone... 

Formation of a siloxane network via hydrosilylation can also be initiated by a free-radical mechanism (300-302). A photochemical route makes use of photosensitizers such as peresters to generate radicals in the system. Unfor-timately, the reaction is quite sluggish. Several complexes of platinum such as (jj-cyclopentadienyl)trialkylplatinum(rV) compoimds have been found to be photoactive. The mixture of silicone polymer containing alkenyl functional groups with silicon hydride cross-linker materials and a catalytic amoimt of a cy-clopentadienylplatinum(IV) compound is stable in the dark. Under UV radiation, however, the platinum complex imdergoes rapid decomposition with release of platinum species that catalyze rapid hydrosilylation and network formation (303-308). Other UV-active hydrosilylation catalyst precursors include (acetylacetonate)Pt(CH3)3 (309), (acetylacetonate)2Pt (310-312), platinum tri-azene compounds (313,314), and other sytems (315,316). 

Biotinylated copolymers of 3-undecylthiophene and 3-thienylmethanoI using streptavidin as a cross-linker protein is an electroactive matrix for the attachment of a photoactive protein, phycoerythrin. The biotinylation of the copolymer improves the film forming properties and results in a stable mono-layer. The phycoerythrin binding to the biotinylated copolymer monolayer can be monitored through fluorescence microscopy at the air-water interface [451]. 

Figure 34 Panel of mannose-based photocross-linkers. The cross-linking activity of photoactive a-mannosides was compared in a simple model system. The trifluoromethyl phenyl diazirine yielded the most effective cross-linking to a variety of amino acids.

---