Control of orientation

The relative orientation of the molecules with respect to the substrate surface or, in other words, the molecular planes parallel to the substrate can be controlled, to a certain extent, by varying external variables such as Dt, Tgub, the chemical and physical nature of the substrate, etc. In the following lines we find examples of the 

When Lsub is kept constant and Dt is varied, similar selectivity in the film orientation is observed. Thin CuPc films prepared on amorphous substrates using PVD at Tsab = RT and low Dt (0.1 nm s ) are oriented with their (100) crystallographic planes preferentially parallel to the substrate surface (standing). At higher D (10 nm s ), an additional second type of preferred orientation is observed with (110) planes oriented preferentially parallel to the substrate surface (lying) (Resel et al, 2000). 

Substrate-induced orientation selectivity has also been observed for thin films of FieZnPc grown in UHV from ML coverages to average thickness of about 20 nm. 

KCl and KBr substrates induce a distribution where the FieZnPc molecules lie parallel to the surface as opposed to NaCl and amorphous S102 where growth occurs in cofacial stacks of molecules predominantly with the molecular plane vertical to the surface (Schlettwein et al, 2000). 

The analysis of such patterns reveals that the microcrystals are preferentially oriented with their (021) planes, the contact planes, parallel to the substrate s surface. The interesting point is that, in order to satisfy such orientation, the hydrogen bonds of the dimers at the interface have to be broken and in addition some reorganization of the molecules is needed (see Fig. 5.6(g)). In conclusion, the molecule-substrate interactions are sufficiently strong (larger yuns and y nv values) to induce COO Aik bonds, where Aik represents sodium and potassium, but the growing crystals adapt their structure in order to crystallize in the known monoclinic bulk phase. 

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Control of oriented antibody layer by monolayer assemblying technique. 

Phthalocyanines (Pc) are attractive materials for their potential functions including the semiconductive behaviours in addition to the thermal and chemical stabilities. In particular, control of orientation of the Pc macrocycles in thin films is expected to provide novel molecular electronic devices. Previously, we have found that copper tetrakis(butoxycarbonyl) Pc is oriented nearly perpendicular to the surface and also the dipping direction in the LB films [46], while octa-alkyl Pc derivatives [H2Pc(R)8, CuPc(R)a R = CnH2n+i. n=7,9,11] take the orientation with Pc macrocycles nearly parallel to the plane of films deposited by the horizontal lifting method to form a non-alternating X-type film [47], as illustrated schematically in... 

However, there are some contradictory reports on the composition of the products of toluene alkylation or benzene dialkylation at high conversions. In some cases, compositions corresponding to the thermodynamic equilibrium between ortho, meta and para isomers were found, and in other cases, kinetic control of orientation, giving mostly the ortho + para substitution, prevailed. Consecutive isomerisation of the ortho and para isomers to the more stable meta isomer seems to be the cause of the disagreement. More active catalysts gave more meta derivatives than the less active ones [343] and increasing the temperature has the same effect [351]. 

Olszowka V, Kuntermann V, Boker A (2008) Control of orientational order in block copolymer thin films by electric fields a combinatorial approach. Macromolecules 41 5515-5518... 

Weakly activating groups next take control of orientation... 

Pegoretti pointed out that a possible route for introducing nanofillers into polymers, while retaining a certain control of orientation and improving filler dispersion, can be found in polymer fiber technology... 

The second section of the talk is concerned with liquid crystals that have defects or texture, for whose flow behavior a theory is not yet available. The effects of these upon orientation development during flow of LCP s is significant, because normal processing operations do not lend themselves to the techniques developed for the control of orientation and of texture of small molecule liquid crystals. Also the texture of LCP s seems to be more persistent, if for no other reason than the time scale necessary to affect it, than that of small molecule LC s. A number of physical problems related to texture are described in closing. 

Experimentally, both small molecule liquid crystals and LCP s display all sorts of director orientation patterns at high shear rates, and the existence of these Is of obvious Importance to control of orientation In processing (see Page 656 of Reference 2 for some discussion and references). It is conjectured that these may correspond to instabilities of the solutions of L-E theory. If feasible, a study of these instabilities - classification of patterns, prediction of conditions for occurrence, etc. - would be useful to relate to experimental observations. 

Eguchi, Y., Ogiue-Ikeda, M., and Ueno, S. Control of orientation of rat Schwann cells using an 8-T static magnetic field. Neurosci. Lett., 351, 130,2003. 

Precise control of orientation may be difficult in PPT systems, because the system is rather based on destruction of organized structure induced by a large topological change of photochromic dopant and self-organizing character of the LC phase. 

Sue, H.)., Li, C. K. (1998). Control of orientation of lamellar structure in linear low density polyethylene via a novel equal channel angular extrusion process,/. Mater. Sci. Lett., 17,853-856. 

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