Liquid crystalline phases of anisotropic, polymer functionalized nanoparticles [Elektronische Ressource] / vorgelegt von Stefan Meuer

Stefan Meuer

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2008
• Nombre de lectures: 20
• Langue: Deutsch
• Poids de l'ouvrage: 18 Mo

Extrait

Liquid Crystalline Phases of
Anisotropic, Polymer
Functionalized Nanoparticles




Dissertation
zur Erlangung des Grades
„Doktor der Naturwissenschaften“
im Promotionsfach Chemie
am Fachbereich Chemie, Pharmazie und Geowissenschaften
der Johannes Gutenberg-Universität Mainz







vorgelegt von


Stefan Meuer
geboren in
Bad Soden am Taunus



Mainz, 2008

Die vorliegende Arbeit wurde unter Betreuung von XXXXXXXXX in der Zeit von
Oktober 2005 bis September 2008 am Institut für Organische Chemie der Johannes
Gutenberg-Universität Mainz angefertigt.






































Dekan: XXXXXXXXXXXXX
Erster Berichterstatter: XXXXXXXXXXXXX
Zweiter Berichterstatter: XXXXXXXXXXXXX

Tag der mündlichen Prüfung: Deutsche Zusammenfassung
Diese Arbeit beschäftigt sich mit der Polymerfunktionalisierung formanisotroper
Nanopartikel wie TiO Nanostäbchen oder Kohlenstoff Nanoröhren. Dies dient der 2
Solubilisierung und sterischen Stabilisierung in organischen Medien, da diese
ionenfrei hergestellt werden können, was eine Nutzung für nanoskopische,
elektrische Schaltkreise ermöglicht. Die Polymere wurden mittels der RAFT
(reversible addition-fragmentation chain transfer) Polymerisation mit engen
Molekulargewichtsverteilungen hergestellt. Im Detail wurden Ankergruppen in
Blockcopolymere und an der Alphaposition eingeführt, welche eine Anbindung an die
Nanopartikeloberfläche ermöglichen. Die Polymere wurden durch Variation der
verschiedenen Blocklängen für eine bestmögliche Adsorption optimiert. Die so
gewonnenen Polymer funktionalisierten Nanopartikel zeigten eine gute Löslichkeit in
organischen Medien und zeigten zudem eine lyotropes, flüssigkristallines
Phasenverhalten. Dies war aufgrund der Formanisotropie zu erwarten, zeigte jedoch
ebenfalls ein unerwartetes thermotropes Verhalten, welches durch die Polymerhülle
erzeugt wurde. Die Flüssigkristalle wurden eingehend mittels polarisierter
Mikroskopie und Differential Scanning Calorimetry (DSC) untersucht. Die
flüssigkristallinen Phasen aus Nanostäbchen und –röhren wurde dann zur
Orientierung der anisotropen Nanopartikel benutzt und es konnten makroskopisch
geordnete Proben hergestellt werden. Die Polymerhülle um die Nanopartikel
ermöglichte es ebenfalls diese in Polymerfilme einzuarbeiten und so
Nanopartikelverstärkte Kunststoffe herzustellen.

Schlagwörter:
Anisotrope Nanopartikel, TiO Nanostäbe, Kohlenstoff Nanoröhren, Polymer 2
Synthese, RAFT Polymerisation, Flüssig Kristall, Makroskopische Orientierung. English Abstract
This thesis focuses on the polymer functionalization of anisotropic nanoparticles like
TiO nanorods and carbon nanotubes. The polymer corona is used to stabilize the 2
nanoparticles sterically and to dissolve them in organic media. These media are
highly desired as they are intrinsically ion-free, which allows their use for the
fabrication of nano-scaled electronic devices. The polymers were synthesized with
the reversible addition-fragmentation chain transfer (RAFT) polymerization with low
polydispersities. Anchor units were incorporated in block copolymers or at the alpha
position of homoblock polymers, which allows the specific binding to the nanoparticle
surface. The polymers were optimized by variation of the block lengths for maximal
adsorption. The resulting polymer functionalized nanorods and –tubes were well
soluble in organic media and a lyotropic liquid crystalline behavior. This was
expected from their shape anisotropy, but they showed an additional, unexpected
thermotropic behavior induced by the polymer corona. The liquid crystals were
investigated by polarizing microscopy and differential scanning calorimetry (DSC).
The lyotropic liquid crystalline phases from nanorods and –tubes were used for the
orientation of the anisotropic nanoparticles and macroscopically oriented samples
were produced. The polymer corona did also allow to incorporate the nanoparticles
into polymers resulting in nanoparticles reinforced plastics.

Keywords:
Anisotropic nanoparticles, TiO nanorod, Carbon nanotube, polymer synthesis, RAFT 2
polymerization, liquid crystal, macroscopic orientation.



Table of contents
A Introduction 1
A.1 Liquid Crystals 1
A.2 Anisotropic Nanoparticles 7
A.2.1 TiO Nanorods 8 2
A.2.2 Carbon Nanotubes 10
A.3 Controlled Radical Polymerization 13
A.3.1 Atom Transfer Radical Polymerization 14
A.3.2 Nitroxide Mediated Polymerization 16
A.3.3 Reversible Addition-Fragmentation Chain Transfer Polymerization 18
A.4 References 22
B Objective of this Work 23
C Results & Discussion 27
C.1 General Principle of Polymer Functionalization 27
C.2 Polymer Functionalized TiO Nanorods 31 2
C.2.1 Impact of Block Lengths on the Adsorption Behavior 31
C.2.1.1 Publication in J. Mater. Chem. 2008, 18, 3050–3058 35
C.2.2 Mesogen Characterization and Lyotropic Phase Behavior 63
C.2.2.1 Publication in Macromolecules 2008, accepted 65
C.2.3 Thermotropic Phase Behavior and Macroscopic Orientation 91
C.2.3.1 Publication in Advanced Materials 2007, 19, 2073-2078 93
C.3 Polymer Functionalized Carbon Nanotubes 113
C.3.1 Impact of Polymer Architecture on the Adsorption Behavior 114
C.3.1.1 Publication for Macromolecules, in preparation 117
C.3.2 Dispersion Stability and Depletion Phenomena 135
C.3.2.1 Publication in Polymer 2008, submitted 137
C.3.3 Liquid Crystalline Phases 157
C.3.3.1 Publication in Chem. Commun. 2008, 3166-3168 159
C.3.4 Carbon Nanotube reinforced Polymers 173
C.3.4.1 CNT reinforced PMMA: Collaboration with the DKI 174
D Summary 181
E Acknowledgement 189
F Appendix 190

Introduction
A Introduction
A.1 Liquid Crystals

Liquid crystals are partially ordered, anisotropic fluids. They are thermodynamically
located between the three-dimensionally ordered solid crystal and the totally
unordered liquid state. They usually show ordering in one or two dimensions in space
while the other dimensions are unordered and give mobility to the system. For
instance, a liquid crystal (LC) may flow like a liquid, but have the molecules in the
liquid arranged and oriented in space. There are many different varieties of liquid
crystalline phases known according to their degree of order and the impact of
temperature and/or concentration. Liquid crystals were first found by the Austrian
botanical physiologist Friedrich Reinitzer in 1888, working at the German University
of Prague, who was extracting cholesterol from carrots to establish its chemical
formula. He investigated the properties of various derivatives of cholesterol and
found a strange behavior for cholesteryl benzoate. The substance showed two
“melting points”: around 145.5°C it melted into a c loudy liquid, and at 178.5°C it
melted again and the cloudy liquid became clear. Nowadays we know that the first
transition was from crystalline to liquid crystalline and the second one from liquid
crystalline to isotropic, also called the clearing point. At that time this phenomenon
was not understood and Reinitzer sent samples to Otto Lehmann in Karlsruhe. He
was a well known expert on crystallography and observed the same behavior
Reinitzer described. Lehmann developed the revolutionary concept that a new,
previously undescribed, state of matter was found and used the names “living crystal”
or “liquid crystal” for the first time. His concept was heavily attacked by the scientific
communities which argued that impurities induced this strange behavior. It took a
while until Daniel Vorländer from the University of Halle found that the molecular
1 Introduction
shape of a substance correlates to the existence of a LC. He revealed that among
different isomers of aromatic substances, the para substituted were the only ones
showing LC behavior.
The liquid crystal phase is also called a mesophase from the Greek word
“meso” – “in between” the classical crystal and liquid phases. The building blocks of
liquid crystals, called mesogens, are classified by their shape, as this is the origin of
the mesophase behavior. The shape has to be anisotropic which can be realized by
rod-like, disc-like and lath-like molecules. Liquid crystals formed by rod-like
molecules are called calamitic, by disc-like molecules discotic and by lath-like
molecules sanidic. Another important classification of liquid crystals is done by the
variable inducing the liquid crystalline phase. One is temperature and these liquid
crystalline phases are called thermotropic and the other is volume fraction in solution
and those are called lyotropic liquid crystals.
For the liquid crystalline behavior of rod-like particles many theories have been
1,2,3
developed by Onsa