Transcript Introduction to Mesomorphism and Thermotropic Liquid Crystals

Introduction to Mesomorphism and
Thermotropic Liquid Crystals
Terminology
A Mesogen has mesomorphic properties, exhibits mesomorphism, and
is mesomorph.
Mesogenic is a material that is “mesogen-like” but to necessary
mesomorph.
A liquid crystal (LC) is a mesogen but not every mesogen is a liquid
crystal.
Thermotropic LCs show mesomorphism upon heating of the neat solid
material (pure compounds or mixtures).
Lyotropic LCs display mesophases when dissolved in a solvent (often
water) or a solvent mixture and their mesomorphism depends on the
concentration and temperature.
States of Matter
low temperature
solids
high temperature
liquids
gases
plasmas
MESOPHASES
liquid crystals
• 0-2D l.-r. positional order
• l.-r. orientational order
• macroscopic fluidity
?
soft & plastic crystal phases
• 3D l.-r. positional order
• reduced l.-r. orientational order
• macroscopic plasticity
low-dimensional solids (e.g. soft polymer glasses)
• no 3D l.-r. positional order
• no l.-r. orientational order
• macroscopic plasticity
?
Order of N, Sm, and Col Mesophases
director
^
n
orientational
order
no
positional
order
N
1D
positional
order
Colh
2D
positional
order
SmA
calamitic
discotic
Calamitic LCs
Molecules with an elongated rigid core
(one molecule axis is much longer than
the other two; high aspect ratio)
Discotic LCs
Disc-shaped molecules (one
molecule axis is much shorter
than the other two; low aspect ratio)
Nematic (N) LCs (Gr. for thread-like)
Discotic nematic (ND) mesophases
Smectic (Sm) LCs (Gr. for soap-like)
Discotic columnar (Col) mesophases
Main- & side-chain
calamitic LC polymer
Main- & side-chain
discotic LC polymer
Rigid-rod and board-shaped LC polymers
Chiralety and Mesophases
The mesophase is marked with an Asterisks if it contains one enantiomer of a
chiral molecule, which could be a dopant or the LC itself. However, a
mesophase containing chiral molecules does not necessary show chiral bulk
properties (e.g. racemic mixtures).
The chiral nematic phase (N*) is also called cholesteric phase.
Polymorphism of Smectic Mesophases
Non-tilted
(uniaxial)
Tilted
(biaxial)
In plane
order
Molecular
rotation
SmA
SmC
none
unhindered
Hexatic B
(or SmB)
Hexatic
F, I (or
SmF, I)
bond
orientational
order
unhindered
Crystal B
Crystal
G, F
positional
unhindered
Crystal E
Crystal
H, K
positional
hindered
Collings PJ and Michael H, Introduction to LCs, Taylor & Francis, 1997.
Structure of Chiral Mesophases
N* or cholesteric mesophase
A pitch can be as small as 100 nm,
as large as 10 mm, and varies with
temperature.
Chiral smectic C (SmC*) phases
Collings PJ and Michael H, Introduction to LCs, Taylor & Francis, 1997.
Polymorphism of Discotic Mesophases
uniaxial
uniaxial
biaxial
Discotic nematic (ND)
biaxial
uniaxial
Discotic hexagonal columnar
Colh or Dh
Colho ordered;
Colhd disordered
Discotic oblique columnar
Colob or Dob
(Colh plastic crystal)
Discotic rectangular and
tetragonal columnar
Colr or Dr; Colt or Dt
An ordered molecular arrangement within
a column requires a persistence length of
at least 20-30 molecules
Thermotropic mesomorphism
governed by molecular design
Microphase separation
Shape anisotropy
H H
O
O
N
F F
O
O
O
O
O
n
rods
hydro- and fluorocarbons
do not display mesomorphism
discs
hard-rod potential by Onsager:
Ann. New York Acad. Sci. 51 (1949), 627
not mesomorph
F F
H H
n
Intermolecular interactions
O
n
O
m
semifluorinated n-alkanes
display soft crystal phases
CN
dipole-dipole interaction
induced mesomorphism
Liquid Crystals of Unusual Shape
NC
CN
NC
CN
CN
CN
NC
O C O C 11H 23
HO
H 25C 12O
CN
NC
SmA layers
CN
NC
O
O C 12 H 25
NC
O
O
NC
O
NC
NC
O C 12 H 25 O C H
12 25
OO
O
NC
Triptycene derivative (Epitaxygen)
Norvez et al. Liq. Cryst. 14 (1993), 1389
O
O
NC
O
OO
O
O
O
O
CN
O O
O
O
O O
OO
O O O OO
O O O O O
O
O
O
O
Si
Si
O
Si O O
OO S i
O O
O
O
O Si
O
Si
Si O
Si
O
O
O
O
Si
O
O
O
O
O O Si
Si
Si
Si O
O
O
O O
O
S i OOO
O Si
Si
Si O OO
O
O
O
O
O
O O O O O
O
O O
OO
O
O
O
CN
O
O
CN
O
O
CN
CN
O
O
CN
O
O
CN
O O
NC
O
O
O
OO
O
O
CN
NC
CN
NC
Cl
CN
Cl
CN
NC
CN
CN
CN
CN
CN
Dendrimeric liquid crystal (SmA)
Lorenz et al., Adv. Mater. 8 (1996), 414
Cl
Discotic liquid crystal without side-chains (Colh)
Barberá at al., Angew. Chem. Int. Ed. 37 (1998), 296
Introduction to Lyotropic Liquid Crystals
Thermotropic LCs show mesomorphism upon heating of the neat solid
material (pure compounds or mixtures).
Lyotropic LCs display mesophases when dissolved in a solvent (often
water) or a solvent mixture and their mesomorphism depends on the
concentration and temperature. The term lyotropic stems from the Greek
word for solve.
The Hydrophobic Effect
The entropy of water is exceptionally high and the tetrahedral network of
water molecules, maintained by hydrogen bonds, can accommodate small
guest molecules in its cavities. Large solute molecules brake the hydrogen
bonds between adjacent molecules which is a enthalpy-costly process.
Most often, the entropy of mixing can not overcompensate the
enthalpy-loss for solved molecules that do not form hydrogen bonds. This
is why hydrocarbons show very low solubility in water. They are expelled from
the water phase.
All pictures taken from H. Stegemeyer, “Liquid Crystals”,
Topics in Physical Chemistry; Steinkopff Darmstadt, Springer New York, 1994
Molecular Structure of Surfactants
Examples for amphiphilic molecules, composed of a polar hydrophilic
headgroup and a hydrophobic tail.
Aggregation of Surfactants
solution
micellar “solution”
solid
CMC = Critical Micelle Concentration
Surfactant molecules start
aggregating in water at rather low
concentration. Both of the two
parts of the amphiphilic molecules
profit by gathering into locally
hydrophobic and hydrophilic
micro-domains.
Micelles, “finite” size dynamic
aggregates of 50 up to several
hundred molecules, can only exist
at temperatures above the Krafftpoint temperature (TK).
The normal micelle turns into a
reverse micelle at high surfactant
concentrations confining the
remaining water molecules to its
now hydrophilic interior.
Structures of micellar aggregates
and lyotropic mesophases of rodshaped surfactants.
Disc-shaped water soluble, but nonamphiphilic, ionic aromatic
pharmaceuticals also self-organize
into p,p-stacks that form lyotropic
mesophases (“chromonics”).
Phase Diagrams of Surfactants in Water
Isotropic normal/inverse micelles (L1/L2), Isotropic bicontinuous (sponge) (L3);
Hexagonal normal/inverse (H1/H2); Nematic rod-like (NC), disc-like (ND, NL);
Rectangular (R); Lamellar liquid chains (La), all-trans chains (Lb);
Cubic normal/inverse spherical (I1/I2), normal/inverse bicontinuous (V1/V2);
Two-component phases diagram
of C12EO6 in water
Three-component phases diagram of
sodium octanoate/water/decanol (cosurfactant)
Packing Considerations developed
by Israelachvili
The hydrophobic tail is geometrically
characterized by its volume v and
length lc.
The area occupied by the head
group at the interface is defined as ao
and includes the first hydration layer.
The ratio of v/lc to ao determines the
shape of the aggregates (micelles).
SDS = sodium dodecyl sulfate
CTAB = Hexadecyltrimethylammonium bromide