Page 171 - Photoreactive Organic Thin Films

MIKHAIL V KOZLOVSKY, LEV M. BLINOV, AND WOLFGANG HAASE

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FIG. 5.3 Optical appearance of polymer films: (A) racemic polymer, P8 M; (B) chira! polymer, P8*M,

the fact that the racemic isomer, P8*M, forms only in the conventional Sm A
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phase. On the other hand, the absence of a pyroelectric effect and sponta-
neous polarization in the chiral polymer excludes completely any tilted
smectic structure.
At the same time, to our surprise, P8*M appears visually transparent and
nonbirefringent, both in bulk samples and in thin films. This is illustrated by
Figure 5.3, where the spot of chiral polymer can hardly be recognized
between two glasses in contrast to its racemic isomer, P8*M, We should stress
here that it is impossible to induce any birefringence in P8*M using shear
flow, electric field up to 15 V/um, or magnetic field up to 2.5 T.
P8*M is not the only polymer forming the isotropic smectic phase. To
date, we have observed formation of that phase for a half-dozen chiral poly-
methacrylates and polysiloxanes. Table 5.1 summarizes the chemical struc-
ture and phase behavior of synthesized side-chain homopolymers, which
carry chirally substituted side chains derived from asymmetric esters of
terephthalic acid and hydroquinone. Such a structure with alternating orien-
tation of carboxylic link groups seems to favour the formation of the IsoSm*
phase, whereas isomeric derivatives of /7-hydroxybenzoic acid, where all
carboxylic links have the same orientation, form only conventional Sm A and
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Sm C* phases. Molar mass of all the synthesized homo- and
1
5
copoly(meth)acrylates is within the range of 1 to 2-10 g-mol" ; the polysilox-
anes have the average degree of polymerization, p ~ 35.
It is worth noticing that the combination of properties of the IsoSrn*
phase can hardly be explained in terms of known LC phases. The ultrashort
pitch TGB-like structure suggested in reference 64 still remains the only struc-
tural model that can explain the observed lack of birefringence in the IsoSm*
phase.
The twist grain boundary (TGB) phases predicted by Renn and
66 67
68
71
Lubensky ' have been intensively studied in the few last years. "" The
general structure of the TGB phase is shown schematically in Figure 5.4.
Because the symmetry of the Sm A phase does not allow continuous helical
twisting, the chiral superstructure is realized in a step wise manner: Small
smectic grains rotate around a helical axis, while screw dislocations build the