By: Anant Kumar Complex Fluids, NMR and Surfaces Group ...

By: Anant Kumar

Complex Fluids, NMR and Surfaces Group

CeFEMA, Instituto Superior Tecnico, Lisboa

What is a liquid crystal?

Liquid crystal (LC) is a state of matter that has properties between those ofisotropic liquids and those of crystalline solids.

Solid Liquid Crystal Isotropic Liquid

T

A. Kumar | Complex Fluids, NMR and Surfaces Group | CeFEMA-IST, Lisboa 2

These substances possess long-range molecular order but still flow like liquids.

A. Kumar | Complex Fluids, NMR and Surfaces Group | CeFEMA − IST, Lisboa 3

What is a liquid crystal?

Liquid crystal (LC) is a state of matter that has properties between those ofisotropic liquids and those of crystalline solids.

Solid Liquid Crystal Isotropic Liquid

T

NC

Link

ing

Gro

upR

ingR

ing

Sid

e

Chain

Sid

e

Chain

(a)

(b)

These substances possess long-range molecular order but still flow like liquids.


Applications of liquid crystals

Liquid crystals have manyapplications, but the primaryapplication is in display devices.

NC

Linking

GroupRing Ring

Side

Chain

Side

Chain

(a)

(b)

5CB liquid crystal, the most studiedand well known liquid crystal molecule.

Thermotropic Liquid Crystals

Molecular systems in which the phase transitions occur by changing thetemperature are called thermotropic.

These are the most widely used and extensively studied liquid crystals for displayapplications.

Lyotropic Liquid Crystals

Lyotropic liquid crystalline phases are formed in solution, and thus, the phasetransitions are generated by changing the concentration.

Types of liquid crystals


Hal

f P

itch

Nematic : In this phase, molecules exhibit only orientational order but no positional order.

Smectic : In this phase, the nematic-like orientational order is present along with somepositional order and the molecules align themselves in layers or planes.

Cholesteric (chiral nematic): is formed by chiral molecules. In this phase, the director forms acontinuous helical pattern.

Nematic (N)

Smectic A (SmA)

Smectic C (SmC)

Cholectric(N*)


Different liquid crystalline phases (Mesophases)


NT-B phase: A new type of nematic Phase

Recently, in LC dimers, formed by attaching two typical rod-like LC monomers via a flexiblechain having odd number of methylene groups, a novel nematic phase has been observed.

This new nematic phase is known as Nx phase or NT-B phase.

The twist-bend molecular configuration.

D.

A.

Pat

erso

n J

. Am

. C

hem

. So

c.20

16,

138

, 16

, 52

83-

528

9

NC

Linking

GroupRing Ring

Side

Chain

Side

Chain

(a)

(b)

N C

Linking

GroupRingRing

Side

Chain

Side

Chain

(a)

(b)

+

Rigid Units

Flexible linkage

What is our research focus?


To study the molecular dynamics of different liquid crystalline systems by theproton NMR relaxometry.

To identify the correlations between the molecular organizations (in differentphases of LCs) and the molecular motions (e.g. rotations, diffusion) through thedispersion of the T1 relaxation times as a function of temperature and the Larmorfrequency.

The study of the relaxation of magnetization of spins in our system.

Certain atomic nuclei demonstrate the ability to absorb and re-emit the radiofrequency energy, when placed in a magnetic field.

NMR spectroscopy technique utilizes a nuclear property known as spin.


What is NMR Relaxometry?

Nuclear spin: It is common practice to represent the nuclear spin as the total

angular momentum of a nucleus.


𝜷 alignment opposes the appliedmagnetic field and is higher inenergy than 𝜶 alignment.

P+

P+P+

P+

P+

P+

P+

P+

𝜷

𝜶Spin System with no appliedmagnetic field

∆𝐄

𝑩𝟎

Influence of applied magnetic field

The absorption of electromagnetic radiation causes to flip the spin state of lowerenergy nuclei to the higher energy state.


P+ P+

P+

𝜷

𝜶

∆𝐄

P+ P+

𝜷

P+ P+

P+

𝑩𝟎

Inversion-Recovery


We measure the time the nuclear magnetization (Mz) takes to return to itsequilibrium/initial state after an RF excitation, known as the relaxation time.

T1: Longitudinal relaxation time; T2: Transvers relaxation time

Schematic representation of NMR relaxometry technique


The LC compound under investigation is a symmetric dimer (DTC5C7) in which twofluorinated mesogenic units, consisting of triphenyl rings, are connected by a flexibleheptyl spacer and the terminal rings are linked to the pentyl chains.

The DTC5C7 compound

Fig. Molecular structure and the mesophase sequence of DTC5C7.


Iso 156.8 oC N 127.8 oC Ntb 96.6 oC SmX 77.4 oC Cr

This compound has been synthesized by our collaborators from the department of chemistry, University of Hull, UK

Key findings

NMR relaxometry results confirm the existence of two phase transitions in theDTC5C7 LC dimer through the frequency and temperature dependent data of T1.

The results are more pronounced in the low frequency range.

Based on the best model fits of the T1 dispersions, clear differences among themolecular organizations in the N, NTB, and Sm phase were detected.



We are going to investigate other related difluoroterphenyl (DTC5Cn)compounds and compare the results with those of DTC5C7 and CB9CBdimer.

To identify the similarities/differences in their properties, specifically in theNTB phase.

Future research

Acknowledgements:

Portuguese Science and Technology Foundation (FCT) through CeFEMAstrategic project UID/CTM/04540/2018.

European Union and FCT through Grant M-ERA-NET2/0006/2016 (CellColor)and the EPSRC (project EP/M015726/1).


By: Anant Kumar Complex Fluids, NMR and Surfaces Group ...

Documents

Transcript of By: Anant Kumar Complex Fluids, NMR and Surfaces Group ...