Example of PDLC architectural window

1
2.3 Lyotropic Main Chain Liquid Crystalline
Polymers
Formed by dissolution of amphiphilic polymer
molecules in appropriate solvents.

• Factors affect the development of lyotropic
  solutions
• Structure of the polymer, which should be
  quite rigid
• Molar mass of the molecules
• Solvent, temperature and most importantly

• One of the most important groups of synthetic
  polymers is the aromatic polyamides
• Rigid structure from the ring systems coupled
  by the
• amide link
• Extended conjugation between the phenyl rings
  and
• the coupling unit with a trans conformation

2
Solvent Systems
For poly(L-glutamate)s dioxane, methylene
chloride For aromatic polyamides protonating
acids H2SO4, CF3SO3H, CH3SO3H strong aprotic
solvents hexamethylene phosphoramide (HMPA),
dimethylacetamide (DMAC), or N-mehtyl pyrrolidone
(NMP) with a small amount of LiCl or CaCl2

• Characteristic viscosity behavior
• Viscosity increases with the increase in polymer
  concentration
• Sharp decrease in the viscosity at a critical
  polymer concentration because of the formation of
  the oriented nematic domains.

The additional chain orientation in the direction
of the fiber long axis leads to a dramatic
enhancement of the polymer properties.
Alternatives to metal or carbon fibers.
3
(No Transcript)
4
(No Transcript)
5
2.4. Thermotropic Main Chain Liquid Crystalline
Polymers

• Many of thermotropic main chain liquid crystal
  polymers are polyesters that are synthesized by
  condensation reactions
• Interfacial polymerization
• High temperature solution polymerization
• Ester interchange reaction in the melt
• Commonly used monomer units
• Hydroxybenzoic acid
• p-Terephthalic acid
• 2,6-Naphthalene dicarboxylic acid
• 4,4-Biphenol

6
(No Transcript)
7
(No Transcript)
8

• Polymer prepared by condensation polymerization
  tends to be
• Very insoluble
• High melting point
• Mesomorphic transitions at high temperatures
• Difficult to process
• The melting points of the main chain liquid
  crystal polymers can be reduced in a number of
  different ways
• Incorporation of flexible spacer units
• Copolymerization of several mesogenic monomers
  of
• different sizes to give a random and more
  irregular structure
• Introduction of lateral substituents to disrupt
  the chain
• symmetry
• Synthesis of chains with kinks, such as
  unsymmetrically
• linked aromatic units

9
(No Transcript)
10
The use of flexible spacers is a popular approach

• Flexible units of varying length space the
  mesogens along the chain and reduce the overall
  rigidity.
• The bridging groups in the repeat units must be
  rigid to maintain the overall stiffness of
  mesogens, and they are unusually multiple bond
  units.
• Ester groups also serve this purpose,
  particularly when in conjunction with aryl rings
  where the conjugation leads to stiffening of the
  overall structure.
• Small variation in structure can lead to
  formation of different mesophase.
• When the number of methylene unit (n) in the
  spacer is
• odd, a nematic phase is observed.
• When n is even, a smectic mesophase results.

11
(No Transcript)
12
For polyesters with multiple rings but different
orientation of the ester units, the phase can
also be changed. The introduction of a flexible
spacer can lower the melting point and increase
the temperature rang in which the mesophase is
stable. Both Tm (melting point) and Ti
(isotropization temperature) decrease as n
increases. The polymers with spacers having an
even number of CH2 units usually have higher Tm
and Ti than those with an odd number. The long
rang ordering will tend to try and maintain the
orientation of mesogen parallel to the director
axis and this may be easier for even numbered CH2
unit spacers if they are in the all trans zig-zag
conformation.
13
Modification of poly(ethylene terephthalate) by
reacting the preformed polymer with
p-acetoxybenzoic acid.
14
Introducing a mesogenic unit to the structure at
the points where the two units combine, producing
a thermotropic liquid crystal polymer with a
flexible spacer. At 30 mol incorporation, a
nematic phase appears in the melt. The optimum
mechanical properties are obtained when 60 to 70
of the oxybenzoate is present in the chain (a
large increase in the tensile strength
accompanied by a corresponding decrease in the
melt viscosity). Introduction of a lateral
substituent into the mesogen will also lower Tm
and Ti, as the bulky side groups will tend to
force the chains apart thereby reducing the
intermolecular forces of attraction.
15
(No Transcript)
16
(No Transcript)
17
The introduction of kinks by using meta
substituted monomer or a crankshaft monomer such
as 6-hydroxy-2-naphthoic acid (HNA) can be
equally effective. Other strategies for
disrupting chain symmetry include the use of
cross-shaped molecules or discotic
mesogens. Plasticization of polyester with a
small liquid crystal molecule to lower the
processing temperature. Bridging groups other
than esters are also used.
18
(No Transcript)