Hairy rods

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

FIGURE 5.7 Schematic Representation of typical, (partially) electroluminescent LC polymer architectures. (a) Rodlike structure, (b) Hairy-rod structure, (c) Combined main-chain-side-chain system, (d) Semiflexible segmented structure, (e) Semiflexible segmented structure with disklike mesogen. (After Weder, C. and Smith, P., Main-chain liquid-crystalline polymers for optical and electronic devices, in Encyclopedia of Materials Science and Technology, Buschow, K.H., Cahn, R.W., Flemings, M.C., Ilschner, B., Kramer, E.J., and Mahajan, S., Eds., Elsevier Science, New York, 2001.)... 

Dense substitution of side chains along a main chain gives brush-like macromolecules such as hairy rods [15], worm-like brushes [16-21] or, in the case of the side chains, further branching and monodendron jacketed chains [22-26]. 

Scheme 11 Chemical structure of hairy rod" poly(L-glutamate)s used to obtain photochromic monolayers and Langmuir-Blodgett films. 81,85-91 ...

LB films of hairy-rod azo-poly(L-glutamate)s [Scheme 11, compounds XIX and XX (n = 2 and 6)] have been used to prepare photoresponsive waveguides. 931 These... 

Sekkat et al. 93 have investigated the photobehavior under polarized light of spin-coated films obtained from a hairy-rod poly(L-glutamate) with azobenzene in the side chains (Scheme 11, compound XIX). It is known 96,971 that, in the presence of a linearly polarized pump light beam, azo molecules experience a cycle of photoisomerization reactions and align themselves perpendicularly to the pump beam polarization direction. For the above polypeptide it was shown that, in the trans cis photoisomerization cycle, the created cis state is aligned perpendicularly to the polarization of the pump beam. 95 ... 

Fig. 7.23 Self-assembled hairy-rod polymers based on metal coordination and ionic interaction.

G. Lieser, M. Oda, T. Miteva, A. Meisel, H.-G. Nothofer, U. Scherf, D. Neher, Ordering, Graphoepitaxial Orientation and Conformation of a Polyfluorene Derivative of the Hairy-rod Type on an Oriented Substrate of Polyimide, Macromokcules 2000, 33, 4490-4495. 

We close this section by examining the status of applications of these methods to polymer monolayers. Initially, ISR was used to probe the 2D nematic state of phthalocyaninatopolysiloxane, descriptively called a hairy rod , dispersed in eicosanol [ 149], and subsequently applied to a set of poly(f-butyl methacrylate) in the semi-dilute regime and beyond [150]. In the semi-dilute regime, the surface viscosity is found to scale linearly with molecular weight, which is in good accord with the results of Sacchetti et al. [134]... 

Fig. 29 Self-assembly of hairy-rod molecules. From left to right the interfacial tension becomes relatively more important compared to the elastic stretching of the side chains [225]...

Fig. 32 Scheme to arrive at solid films of aligned rodlike conjugated polymers from the self-assembled smectic phase of the corresponding hairy-rod supramolecules [234]... 

Macrotnolecules are formed by covalent bonding of a large number of monomeric units. This results in a wide variety of possible structural architectures, e.g. linear chains, branched chains, brush-like chains ( hairy rods ), star-like, tree-like ( hyperbranched ), perfectly branched ( dendritic ) structures (Figure 1) [14,15],... 

AZOBENZENES IN SUPRAMOLECUIAR ASSEMBLIES PHOTO-CONTROL OF THE STRUCTURAL AND OPTICAL PROPERTIES OF lANGMUIR-BLODGETT-KUHN MULTILAYERS OF HAIRY-ROD AZOPOLYGLUTAMATES... 

PHOTOISOHERIZATION AND PHOTO-ORIENTATION OF AZOBENZENES IN SUPRAMOLECULAR ASSEMBUES PHOTO-CONTROL OFTHE STRUCTURALAND OPTICAL PROPERTIES OF LANGMUIR-BLODGETT-KUHN MULTILAYERS OF HAIRY ROD AZO-POLYGLUTAMATES... 

PIG. 4.10 Schematic drawing of the hairy-rod polyglutamate douWe-tayer structure. P indicates the structure s periodicity. The trans isomer can be represented by a highly anlsometric, i.e., nondegenerate, transition dipole oriented along the alkyl side chains. 

Fischer, X., Menzel, H., and Stumpe, J, Photo-reorientation of azobenzene side groups of thermotropic hairy rod polyglutamate in LB multilayers. Supramol. Sci. 4, 543 (1997). 

The structure of mono- and multilayers of polymers 38 was investigated in detail. The polymers form domains when spread at the air/water interface. From the peak position of the azobenzene x-x -band, it can be concluded that the chromophores are strongly aggregated in the domains even if no surface pressure is applied. Furthermore, it was found that the hairy rods, which should have a symmetrical distribution of the side chains around the main-chain helix, are deformed by contact with the water surface and the applied surface pressure (see Figure 6.16). 

The structure with the deformed hairy rods is preserved when multilayers are deposited on solid supports, as evidenced by X ray reflectometry revealing a marked bilayered structure (see Figure 6.16). Photoisomeriza-tion of the a egated azobenzene chromophores in LBK films is possible and leads to significant structural changes. The bilayered structure is completely... 

Wiegand, G., Jaworek, T., Wegner, G., and Sackmann, E. Heterogeneous Surfaces of Structured Hairy-Rod Polymer Film Preparation and Methods of Functionalization. Langmuir 13, 3563 (1997). 

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