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Materials > Nematic liquid crystals > Flashcards

Nematic liquid crystals Flashcards

1
Q

What is anisotropy?

A

Exhibiting properties with different values when measured along axes in different directions
l/b = measure of molecular anisotropy

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2
Q

What is molecular anisotropy?

A

term used to describe the shape of a molecule when one axis is significantly longer than another axis e.g. molecules can be rod-like or disk-like

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3
Q

What is crystal state

A

Molecules are all in fixed positions and unable to rotate

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4
Q

What is the Nematic phase

A

Molecules are roughly parallel/ pointing in same direction, determined by director, but still free to flow

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5
Q

What is an isotropic liquid

A

Molecules are free to rotate and tumble like a normal liquid. No intrinsic order.

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6
Q

What is the order parameter, S?

A

S = 1/2 (3Cos^2 ๐œƒ -1 )
Where ๐œƒ is the angle between long axis of molecule and director
S is temperature dependent
Nematic liquid crystal, typically S = 0.4 - 0.7

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7
Q

How are the molecules arranged if S = 1

A

The molecules are perfectly ordered, all molecules are aligned with the director

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8
Q

How are the molecules arranged if S = 0

A

The molecules are disordered, ie isotropic liquid

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9
Q

How can you measure the order parameter?

A

diamagnetism
NMR
Raman scattering

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10
Q

What is the effect of increasing the length of aliphatic chain R?

A

Can lead to microphase segregation due to separation of aromatic and aliphatic units

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11
Q

Why do cyanobiphenyls have a strong internal dipole moment

A

O = +M group
CN = -M group
๐œ‹ electrons are able to flow through aromatic system from the O to the CN group forming a strong internal dipole

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12
Q

How do quadrupolar interactions occur

A

Antiparallel pairs of molecules form with secondary dipoles between molecules stabilising the interaction
This cancels out the internal dipole

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13
Q

What is a dipole moment?

A

The product of magnitude of charge and the distance separation between the charges.
Symmetry can reduce dipoles to 0 or dipoles can reinforce each other.

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14
Q

Effect of chain length on behaviour of Alkylcyanobiphenyls

A

Short chain length = strong ๐œ‹-๐œ‹ interactions between aromatic units so higher mp
longer chain length alkyl interactions dominating raise mp again

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15
Q

Effect of size of ๐œ‹ unit on transition temp of nematic liquid crystal

A

As the length of the ๐œ‹ unit decreases the quadrupolar antiparallel overlap of ๐œ‹ units decreases in size and the transition temp of nematic liquid crystals increases

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16
Q

Effect of the cage ring unit on nematic liquid crystal transition temp

A

The introduction of a cage ring unit to the molecule increases the transition temp.
Raises the temp most when next to the aliphatic chain

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17
Q

What is the effect that Heteroatoms have on transition temp?

A

Heteroatoms make the molecule more linear as is reduces the steric clash of H atoms on the exterior of 2 connected benzene rings which usually causes the molecule to twist
Therefore increases the transition temp as molecules can pack closer together

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18
Q

What effect do the size of heteroatoms have on the transition temp?

A

Larger heteroatoms e.g. Sulfur decrease linearity of molecule because theyโ€™re much large than the carbon atoms and therefore lower transition temp
If one O and one S atom = highly twisted structure = even lower transition temp

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19
Q

What effect do linking groups between rings have?

A

If linking group disrupts the 2 blocks of aliphatic and aromatic then it will decrease the transition temp.
However if introduces +M group into aromatic region will increase dipole moment and transition temp

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20
Q

How to add acyl group to benzene ring?

A

RCOCl and AlCl3

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21
Q

How to reduce acyl group to alkyl

A

NH2NH2 and KOH

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22
Q

How to reduce NO2 group

A

H2 and Pd/C

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23
Q

COCH3 => COOH

A

Br2 and NaOH

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24
Q

Boronic Acid Coupling

A
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25
Q

Why are boronic acids good for coupling reactions?

A

1) Easily purified
2) Stable in air and to moisture
3) Can be stored
4) High yields
5) No homocoupling (react w itself)

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26
Q

Add COOH to difluorobenzene

A

n-BuLi
CO2
HCl

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27
Q

Add I to diflurorbenzene

A

N-iodosuccinimide
H+

28
Q

Add B(OH)2 to difluorobenzene

A

n-BuLi
B(OMe)2
HCl

29
Q

What are the 2 key physical properties that are important to liquid crystal display applications?

A

Birefringence
Dielectric anisotropy

30
Q

What is Birefringence, ฮ”n ?

A

A beam of light is split into two rays, which are orthogonally polarised with respect to each other. Birefringent materials essentially exhibit two refractive indexes. In the case of a liquid crystal these refractive indexes will vary with temperature.

31
Q

How to calculate Birefrigence?

A

ฮ”n = ne - no
ne = perpendicular birefrigence
no = parallel birefrigence

32
Q

What is dielectric anisotropy, ฮ”๐œ€?

A

Related to the flow of polarisable electrons through the molecule. This can be referred to as a positive dielectric anisotropy (flow is along the long axis of the molecule for a rod-like molecule) or as a negative dielectric anisotropy (flow is towards the short axis of the molecule)

33
Q

How to calculate dielectric anisotropy?

A

ฮ”๐œ€ = ๐œ€parallel - ๐œ€perpendicular

34
Q

What influences the birefringence of a molecule?

A

More ๐œ‹ electrons the more interactions the molecule will have with light therefore increasing the birefringence, ฮ”n
The addition of a +M group to the ๐œ‹-system means it can donate electrons and increase birefringence
More polarisable the molecule = higher birefringence

35
Q

What is a disadvantage of high birefringence?

A

The higher the polarisability/ birefringence the more likely the compound will be coloured - not favourable for liquid crystal displays

36
Q

What Increases the dielectric anisotropy of a molecule?

A

Heteroatoms which make molecule more planar so electron flow along parallel axis is higher
Halogens substituted onto benzene ring decrease quadrupolar interaction increasing parallel flow of e-

37
Q

Equation for order parameter, S?

A

S = ฮ”๐œ€/3(๐œ€bar-3)

38
Q

Equation for mean permativity, ๐œ€bar.

A

๐œ€parallel + 2๐œ€perpendicular/3

39
Q

What decreases the dielectric anisotropy of a molecule?

A

Halogens or CN substituted onto benzene ring increase perpendicular e- flow so decrease dielectric anisotropy
Antiparallel pairing (quadrupolar interactions) decreases parallel flow of electons and decreases dielectric anisotropy
Replacing -M CN group with +M group = more electron density towards perp dipole
Increasing number of ๐œ‹ electrons flowing towards perp dipole

40
Q

How to calculate degrees of freedom of temp/press phase diagram

A

F = C - P +2
C = number of components
P = number of phases

41
Q

For a 2 component system how many degrees of freedom are there at the eutectic point?

A

1 degree of freedom according to the phase rule but for a liquid crystal device fix pressure at 1 atm so F = 0

42
Q

What considerations must be made for component selection for liquid crystal devices?

A

Stability - chemically
Appearance - colourless
Temperature range - wide
Low cost
Safe - device could break
Low viscosity - respond faster

43
Q

Physical property considerations

A

Dielectric anisotropy - ฮ”๐œ€ should have range -10 -> +50
Birefringence - ฮ”n should have range 0.03 - 0.5

44
Q

Why is cyano group not suitable for modern liquid crystal displays?

A

Too high conductivity
Instead have components that incorporate terminal or lateral fluoro subs

45
Q

What is the arrangement of chiral nematic liquid crystals

A

Form a helical arrangement due to the chirality
Gives an iridescent or coloured appearance

46
Q

What are optical isomers

A

Optical isomers are superimposable mirror images of the same molecule that contains a chiral carbon

47
Q

How to calculate the number of isomers of a molecule

A

2^(number of chiral centres)

48
Q

What effect do chiral molecules have on polarised light and how do we measure this?

A

Chiral molecules can rotate the plane of polarised light
We can measure the observed optical rotation, ๐›ผ.
Clockwise rotation (+) or (d)
Anticlockwise rotations (-) or (l)

49
Q

Equation for specific optical rotation

A

[๐›ผ] = ๐›ผ / c x l
C = concentration (gml-1)
L = pathlength dm

50
Q

What does pitch mean?

A

Length of the helix of a chiral nematic
Short pitch (blue)
Long pitch (red)

51
Q

Simplified Bragg equation for the reflection of light from a chiral nematic

A

๐›Œ = n x p x cos๐œƒ
n = mean refractive index
P = pitch
๐œƒ = angle from normal incidence

52
Q

What is a chiral dopant?

A

Molecule which when added in small amounts can make a chiral nematic mixture - imparts chirality into system

53
Q

When making a chiral nematic mixture what do we need to consider?

A

Compatibility with the host
Helical twisting power of the dopant
Bend or kink in molecule = greater HTP
Longer chain molecule = greater HTP

54
Q

How to modify the pitch of a chiral mixture once dopant added?

A

Add more host
Add opposite enantiomer of dopant
Add racemate dopant
Add new dopant (larger HTP)

55
Q

How to measure pitch in chiral nematics?

A

UV vis spectrometry
Cano wedge cell

56
Q

Formula to determine pitch using cano wedge

A

P = 2s(tan๐›ผ)
s = distance between defect lines
๐›ผ = wedge angle

57
Q

What happens if you plot pitch against 1/C

A

straight line y = mx + c
Can be used to determine what concentration is required for a specific pitch

58
Q

COCH3 to COOH

A

Br2
NaOH

59
Q

NH2 to OH

A

NaNO2
H2SO4

60
Q

Add B(OH)2 group

A

n-BuLi
B(OMe)3
HCl

61
Q

Add secondary alcohol

A

n-BuLi
RCHO
HCL

62
Q

Add CHO to difluorobenzene

A

n-BuLi
DMF

63
Q

COOH to CONH2

A

SOCl2
NH3

64
Q

CONH2 to CN

A

SOCl2

65
Q

R-Br replaced with B(OH)2

A

Mg, THF
B(OMe)3
HCl

Materials (5 decks)

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