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 !