Chemistry Reference
In-Depth Information
It was also shown that it is possible to synthesize polymethacrylate liquid crystal polymers with
mesomorphic properties that contain ferrocenes with two flexible chains at the l,l
0
-positions [
59
].
Based on dilatometric measurements, a head-to-tail molecular arrangement of the monomeric units
occurs within the smectic A phase. Because of special electrochemical properties of ferrocene, these
materials are of interest for developing electroactive mesomorphic polymers. The structure of the
polymer can be shown as follows [
59
]:
O
O
O
x
O
O
O
O
6
O
O
Fe
Finkelmann reported synthesis of a novel cross-linked smectic-C main-chain liquid-crystalline
elastomer that was formed by polycondensation of vinyloxy-terminated mesogens, tetramethyldi-
siloxane, and pentamethyl-pentaoxapentasilicane. The introduction of the functional vinyloxy group
allows the synthesis of well-defined networks with good mechanical properties due to elimination of
side reactions as in the case of vinyl groups [
60
].
Large amplitude oscillatory shear is frequently capable of generating macroscopic alignment
from an initially random orientation distribution in ordered polymer fluids. Berghardt and
coworkers [
61
] reported that by combined rheological and in situ synchrotron X-ray scattering to
investigate of such induced alignment in smectic side-groups
liquid crystalline polymers. In all
cases, they found that shear promotes anisotropic orientation states in which the lamellar normal
tends to align along the vorticity direction of the shear flow (“perpendicular” alignment). Rheologi-
cal measurements of the dynamic moduli by them revealed that large amplitude shearing in the
smectic phase causes a notable decrease in the modulus. They also observed that increasing strain
promotes higher degrees of orientation, while increasing molecular weight impedes development of
smectic alignment.
Ahn et al. reported [
62
] preparation of a smetic liquid crystalline elastomer with shape memory
properties. Shape memory polymeric materials can recover their equilibrium, permanent shapes from
nonequilibrium, temporary shapes as a result of external stimuli, like heat or light. Such materials
have application in medical practice. Main-chain polynorbornenes were linked with three different
side-chains, cholesterol, poly(ethylene glycol), and butylacrylate.
pf
2.5 Orientation of Polymers
There is no preferred direction or arrangement in the manner in which the macromolecules align
themselves in a polymeric mass during crystallization. If, however, after crystallization an external
stress is applied, the crystalline material undergoes a rearrangement. From the X-ray diffraction patter
it is surmised that the chains realign themselves in the direction of the applied stress. Polymeric films
and fibers usually show considerable increase in strength in the direction of that stress. Oriented fibers
are considerably stronger along their length then perpendicularly to them. Generally, if the degree of
