Discotic Liquid Crystals and PolymersIn Cylindrical Confinement:

Supramolecular Architecture of LC Nanorods and nanotubes

Bremen, Complex Materials Summer School

C. Stillings1, M. Rudisile1, M. Steinhart2, B. Brandl1, U. Gösele2, E. Martin1, G. Germano1, J. H. Wendorff1

1Department of Chemistry and Center of Optodynamics, Philipps-Universität Marburg

2Max Planck Institute of Microstructure Physics, Halle

MotivationRelevance: two-dimensional LC nanostructures are potential components

of miniaturized devices with specific electronic properties

Task: design of the supramolecular architecture of discotic nanorodsinside porous templates (C. R. Martin, Science 1994, 266, 1961)

Dependence of mesophasetexture on:

>> Interfacial energies>> Diameter>> Curvature

(>> Doping)

Specific motivation

Polymer dispersion controls optoelectronic properties

200 225 250 275 300 325 350 375 4000,0

0,1

0,2

0,3

0,4 30 wt% 15 wt% 10 wt% 5 wt% 1.2 wt% 0.56 wt% 0.27 wt%

Extin

ktio

nλ [nm]

250 275 300 325 350

0,00

0,01

0,02

0,03

0,04

Absorption

Mesophase Formation in 2D Geometric ConfinementStrategy:

Preparation of nanostructures by

Electrospinning

1.Variation of the polymer matrix

Wetting of ordered porous templates

1. Dispersed/nondispersed system

2. Modification of pore diameter >> diameter >> curvature

3. Modification of pore walls >> Interfacial energies

4. Doping of the LC phase

Simulation of structure formation

Discotic Model System: Ada-PBT

Discotic liquid crystal of the triphenylene type:

Ada-PBT

OBuBuO

O

OBu

OBuBuO

O

Spontaneous formation of columnsColumns form hexagonal super-

structures

High charge carrier mobility along long axis of columns(D. Adam et al., Nature 1994, 371, 141)

Performance depends on degree of order

A. Bayer, S. Zimmermann, J.H. Wendorff, Mol.Cryst. Liq. Cryst. 396, 1 (2003)

Dominant Reflections

(100) – intercolumnar distance

2Θ ~ 5°, very strong

(001) – intracolumnar distance

2Θ ~ 25 °, weak

Linear PDLC systems:

>> nanofibers

Electrospinning Coelectrospinning

Systems

Amorphous polymers

Polymethylmethacrylate (PMMA)

Polystyrene (PS)

- transparent

ii

→

il

Triphenylene AdaPBT

OBuBuO

O

OBu

OBuBuO

O

Electrospun PDLC fibers

REM studies

Electrospun PDLC fibers

Electron microscopical studies

(Rutheniumtetroxide stained)

Electrospun PDLC fibersMolecular dispersion in fibers

during electrospinning-kinetic effect

Phase separation and coarsening at elevated temperatures

>> Pronounced LC-Phase after annealing>> No columnar orientation along fiber axis

Core shell structures yet not consistently

Method: Template wetting

Contact Wetting Filling

• Melting the discotic onto the surface of ordered porous alumina

• Discotic LC infiltrates the template as an isotropic liquid

• Rapid quenching to glassy state

Highly Ordered Templates

Order by self-assembly; sharp pore size distribution

(H. Masuda, K. Fukuda, Science 1995, 268, 1466)

Pore diameters: 25 to 400 nm; pore depth: 100 µm

Laterally extendedmembranes having

aligned pores

Ada-PBT/PMMA

50 nm pore diameter>> nanorods

400 nm pore diameter>> nanotubes

(Rutheniumtetroxide stained); Mw(PMMA): ~ 250.000 g/mol

Triphenylene (Ada-PBT)

>> Nanorodscomplete filling of the pores

no defects

Ada-PBT Nanorods: Structure Model

Planar core phase, columnsgrow along long axes of

template pores

potential nanocables

Homeotropic anchoring at porewalls. Homeotropic shell at the

LC/wall interface surroundsplanar core phase

M. Steinhart, S. Zimmermann, P. Göring, A. K. Schaper, U. Gösele, C. Weder, J. H. Wendorff, Nano Lett. 2005, 5, 429

XRD: Θ/2Θ scans

XRD on alignedAda-PBT

nanorods:

Intensity of inter-columnar (100) peak

originates fromhomeotropic phase

Intensity of intra-columnar (001) peak

originates fromplanar phase

Ada-PBT nanorods: intra-columnar (001) peak withstrongly enhanced relative intensity

Dominant planar orientation(columns parallel to longaxes of template pores) !

Isotropic bulkAda-PBT

Pore diameter:400 nm

Uniaxial orientation of the columns along the pore axis

10 20 300

10000

20000

30000

Θ/2Θ Scans on Aligned Ada-PBT Nanorods

intracolumnardisk-disk distance

2Θ (°)

Cou

nts

10 20 300

15000

30000

45000intercolumnar (100) distance)

Modification of pore walls

Templates have pore walls consisting of alumina

>> polar surface

How does non-polar surface modification

influence texture of mesophase ?

Coating of pore walls with PPX

Chemical vapour

deposition of PPX

Infiltration of Ada-PBT into PPX-coated templates

Poly(paraxylylene):non-polar; high chemical and mechanical stability;

very good insulator

XRD: Θ/2Θ scansXRD on aligned

Ada-PBT nanorods:

Intensity of inter-columnar (100) peak

originates fromhomeotropic phase

Intensity of intra-columnar (001) peak

originates fromplanar phase

Θ/2Θ scans of aligned AdaPBT nanorod in PPX pores

5 10 15 20 25 300

10000

20000

30000

40000

50000

Inte

nsity

/cou

nts

2Θ/degrees

5 10 15 20 25 300

1000

2000

3000

4000

5000

Inte

nsity

/cou

nts

2Θ/degrees

5 10 15 20 25 300

1000

2000

3000

4000

inte

nsity

/cou

nts

2Θ/degrees

Non-modified pore walls:

planar texture

400 nm

35 nm

Non-polar pore walls modified with PPX

>> No texture; bulk-like pattern

Doping of mesophase

Dopants incorporated into Ada-PBT columns

Dopants form charge-transfer complexes with Ada-PBT and influence structural and electronic properties of mesophase(M. Möller, V. Tsukruk, J. H. Wendorff, H. Bengs, H. Ringsdorf,

Liquid Crystals 1992, 12, 17)

Model dopants: nitrofluorenone derivativesSelected mixing ratio: 80 mol-% Ada-PBT, 20 mol-% dopant

Dopants: nitrofluorenone derivatives

Systematic variation of the substitution pattern

2,7-Dinitro-9-fluorenone 2-Nitro-9-fluorenone

ONO2O2N

ONO2O2N

NO2

ONO2O2N

NO2NO2

ONO2

2,4,5,7-Tetranitro-9-fluorenone 2,4,7-Trinitro-9-fluorenone

Θ/2Θ-Scans on aligned, doped Ada-PBT Nanorods - 400 nm

0,00

0,25

0,50

0,75

1,00

Rel

ativ

e in

tens

ity

Tetranitrofluorenone Trinitrofluorenone

10 20 300,00

0,25

0,50

0,75

1,00 Dinitrofluorenone

10 20 30

Nitrofluorenon

2Θ (°)

5 10 15 20 25 300

15000

30000

45000

Inte

nsity

(cou

nts)

2Θ (°)

In contrast to pure Ada-PBT nanorods: no dominance of intra-columnar (001) reflection!

Growth of planar core phase disturbed!

Pure Ada-PBT nanorodsDoped Ada-PBT nanorods

Planar texture suppressed

(001)

(100)

Θ/2Θ-Scans on aligned, doped Ada-PBT Nanorods - 35 nm

5 10 15 20 25 300

1000

2000

3000

4000

inte

nsity

/cou

nts

2Θ(°)

Pure Ada-PBT nanorodsDoped Ada-PBT nanorods

In contrast to pure Ada-PBT nanorods: planar texture less pronounced! Exception in nanorods with Dinitrofluorenon:

0,25

0,50

0,75

1,00TrinitrofluorenoneTetranitrofluorenone

10 20 300,00

0,25

0,50

0,75

1,00 NitrofluorenoneDinitrofluorenone

Rel

ativ

e in

tens

ity

2Θ (°)

10 20 30

Planar texture

XRD: Texture analysis

Ψ-scans representorientation distributionsof (100) and (001) latticeplanes with respect to

template surface

Θ and 2Θ adjusted to corresponding peak

maxima

Ψ−scan (100) - 400 and 35 nm pores

0 20 40 60 80-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e in

tens

ityΨ/degrees

Tetranitrofluorenon Trinitrofluorenone Dinitrofluorenone Nitrofluorenone pure AdaPBT

0 20 40 60 80-0,2

0,0

0,2

0,4

0,6

0,8

1,0

Rel

ativ

e In

tens

ity

Ψ/degrees

Ψ-Scans of inter-columnar (100) peaks (homeotropic phase):

pure Ada-PBT: no significant difference between 400 and 35 nm pores

400 nm: slight lower degree of orientational order in case of doped Ada-PBT

35 nm: no significant difference between pure Ada-PBT and doped nanorods

400 nm 35 nm

0 20 40 60 80-0.2

0.0

0.2

0.4

0.6

0.8

1.0 Tetranitrofluorenon Trinitrofluorenone Dinitroflurenone Nitrofluorenone pure Ada-PBT

Rel

ativ

e in

tens

ityΨ/degrees

0 20 40 60 80-0,2

0,0

0,2

0,4

0,6

0,8

1,0

Rel

ativ

e in

tens

ity

Ψ/degrees

Ψ−scan (001) - 400 and 35 nm

Ψ-Scans of intra-columnar (001) peaks (planar phase):

pure Ada-PBT: slightly lower orientational order in 35 nm pores

400 nm: doping leads to decrease in orientational order depending on the substitution pattern

35 nm: much lower influence of dopants on orientational order

400 nm 35 nm

Outlook I: MD-simulations

simulation of ~14.000 molecules >> Simulations of molecular shape rather than atomic detail

>> Neglecting adamantanoyl substituent

Cylindric cell, 20x molecular diameter

Molecule/molecule interaction:Angular dependent Gay Berne potential with Bates Luckhurst extension

Favouring homeotropic anchoring on wallsNeglecting surface curvature in exploratory simulations

J. G. Gay, B. J. Berne, J. Chem. Phys. 74, 3316 (1981)M. A. Bates, G. R. Luckhurst, J. Chem. Phys. 104, 6696 (1996)

Results of exploratory MD runs

10.000 time steps 25.000 time steps 275.000 time steps

0°

90°

>> formation of a hexagonal columnar core

>> good agreement with the previous experimental results structure model

future simulations will focus on varying curvature and surface interaction

Conclusions:

Polar pore walls: pronounced planar texture.

Non-polar pore walls: no apparent texture

Doping of the Ada-PBT nanorods suppresses planar texture in 400-nm pores, but not in 35-nm pores

Future work will focus on the influence ofthe thermal history on the mesophase formation

Morphology design by molecular dynamics simulations (MD)

>> Set of parameters can be systematically varied

Acknowledgement

Markus Rudisile

Silko Grimm, Kathrin Schwirnand Kornelia Sklarek

(MPI Halle)

for the preparation of templates

VW for financial support

… and for your attention !