Modern Polymer Spect..

62 2 Segniental Mobility of Lipid Crystals and Liquicl-Cvystalliiie Po1jwiei.s
SIDE-CHAIN
p 2 ' m = main-chain spacer length
n = side-chain spacer length
P
"=: ESTERLINKAGE
R = azobenzene substituent
Design parameter
a) variation of the length of the methylene units in
main-chain rn = 4, 10, 11, 12 ,I4
and
side-chain n = 6, 8, 10
b) variation of the azobenzene substituent R
R=CN(a), NO, (b), OCH, (c), H(d)
Figure 2-31. General structure
and nomenclature (PnRm) of the
investigated SCLC polyesters used
for reversible optical data storage
(e.g., P6a12: polyester with 12
CH? groups in the acid part of the
main chain and six CH2-groups
in the side-chain spacer and a
CN-suhstitucnt on the mesogen).
Table 2-2. Phase behavior of P8a12 [96].
Structure 4 3 2 1
Transition temperature range 30-40 "C 46-54 C 59-63 "C 65-71 "C
2.6.1 Materials
The variations in chemical structure of the investigated SCLC polyesters and their
nomenclature is outlined in Figure 2-3 1. Differential-scaiining-calorimetry (DSC 1,
polarization-optical-microscopy (POM) and X-ray investigations [95-971 reveal a
rather complex phase behavior of this homologous polyester series, which strongly
depends on the length (m) of the main-chain. For the polyadipates (m = 4), for
example, an enantiotropic liquid-crystalline behavior is observed, involving disordered
smectic mesophases in the range from T, at about 10°C to the sniectic tf
isotropic transition (from about 55-70 "C, varying with the side chain spacer length
n = 6,8, 10). The longer homologs, with a main chain length of 10-14 niethylene
units could be classified as inonotropic LCPs, with polyinorphic character. Some
of them exhibit up to four first-order transitions, which partially overlap, involving
inany different organized phases. An exact phase assigninelit is very difficult. Candidates
with the highest potential for optical data storage are the polytetradecanedioates
(ni = 12) with 6 and 8 methylene groups in the spacer [95, 981. DSC and
X-ray investigations reveal for both polyesters two low-order mesomorphic phases
(structures 3 and 4) and two high-order mesomorphic or crystalline phases jstruclures
I and 2). The glass-transition temperature of both systems is about 25°C.
Representative of the discussed SCLC polyesters, the phase behavior of P8a12 is
detailed in Table 2-2.
The influence of azobenzene substituents with varying dipole moments has also