Tailoring the mesomorphic structure and crystalline morphology via molecular
architecture and specific interactions:
from small molecules to long chains
Raluca I. Gearba
2005
Polymer Physics Laboratory Brussels, Belgium
Tailoring the mesomorphic structure and crystalline morphology via molecular architecture and specific interactions: from
small molecules to long chains
Raluca I. Gearba
Promotor:
Prof. Dimitri A. Ivanov
2005
List of abbreviations, acronyms and symbols
Techniques
AFM Atomic Force Microscopy
DSC Differential Scanning Calorimetry
MT-DSC Modulated Temperature Differential Scanning Calorimetry FT-IR Fourier Transform Infrared Spectroscopy
POM Polarized Optical Microscopy MAS Magic Angle Spinning NMR Nuclear Magnetic Resonance SAXS Small-angle X-ray Scattering WAXS Wide-angle X-ray Scattering TEM Transmission Electron Microscope Materials
PDES poly(di-n-ethylsiloxane) PDPS poly(di-n-propysiloxane) PE polyethylene
Pc phthalocyanine
Other notations in alphabetical order:
Colhd columnar hexagonal disordered phase Colh columnar hexagonal phase
Colho columnar hexagonal order phase Colrd columnar rectangular disordered phase Colob columnar oblique phase
ColL columnar lamellar phase LC liquid crystal
ND nematic discotic phase Ncol nematic columnar phase PSD power spectral density RT room temperature RMS root mean square s scattering vector 2D two-dimensional
Chapter 1: Introduction
1.1. General considerations ...2
1.2. Molecular architecture and columnar order...3
1.2.1. Discotic molecules ...5
1.2.2. Side-chain polymer liquid crystals ...8
1.3. Columnar order resulting from specific inter- and intra- molecular interactions ...10
1.3.1. Hydrogen bonds in columnar mesophase formation ...11
1.3.2. Dipolar interactions in columnar mesophase formation ...12
Aim of the work and outline ...14
Chapter 2: Experimental techniques 2.1. Differential Scanning Calorimetry. ...21
2.2. Polarized Optical Microscopy ...22
2.3. X-ray Diffraction experiments...22
2.3.1. D8 Advance Diffractometer ...23
2.3.2. Synchrotron radiation sources ...24
2.4. Atomic Force Microscopy ...27
2.4.1. Tapping Force AFM...28
2.4.2. High-temperature AFM...29
2.5. Transmission Electron Microscopy ...29
Chapter 3: Tailoring discotic mesophases: columnar order enforced with hydrogen bonds 3.1. Introduction ...34
3.2. Experimental Section...35
3.3. Results and Discussion ...37
3.4. Conclusions ...41
4.1. Introduction ...45
4.2. Experimental Section...46
4.3. Results and Discussion ...58
4.4. Conclusions ...60
Chapter 5: Templating crystal growth at the nano-scale with a monotropic columnar mesophase. 5.1. Introduction ...64
5.2. Experimental Section...65
5.3. Results and discussion ...66
5.4. Conclusions ...74
C
hapter 6: Mesomorphism, polymorphism and semi-crystalline morphology of poly(di-n-propylsiloxane) 6.1. Introduction ...786.2. Materials and methods 6.2.1 Materials...79
6.2.2. Methods ...80
6.2.2.1. Differential Scanning Calorimetry ...80
6.2.2.2. Density measurements ...80
6.2.2.3. X-ray diffraction ...80
6.2.2.4. X-ray data analysis and structure modelling...81
6.2.2.5. Atomic Force Microscopy ...81
6.2.2.6 Solid-state NMR ...82
6.3. Results 6.3.1. DSC measurements ...82
6.3.2. X-ray diffraction...84
6.3.2.1 Temperature-resolved powder diffraction ...84
6.3.2.2. X-ray fiber diffraction...85
6.3.3. NMR measurements ...91
6.3.4. AFM measurements ...93
6.3.4.1. Morphology of this shear-oriented layers in the crystalline and mesomorphic state ... 93
6.4. Discussion ...98 6.4.1. Structure of α crystal...98
6.4.2. Semicrystalline morphology and chain conformation
in the crystalline and mesomorphic state ...104 6.5. Conclusions ...105
Chapter 7: Role of columnar mesophase in the morphological evolution of polymer single crystals on heating: a combined atomic force microscopy and electron diffraction study
7.1. Introduction ...110 7.2. Materials and methods
7.2.1. Materials and sample preparation...111 7.2.2. Electron diffraction ...111
7.2.3. Calculation of electron diffraction patterns and
auto-correlation functions ...112 7.2.4. Atomic Force Microscopy...112 7.3. Results and Discussion
7.3.1. Single crystal morphology ...113 7.3.2. Selected area electron diffraction ...114 7.3.3. Following the melting of a PDPS single crystal with AFM ....121 7.3.4. Melting of a superposition of two PDPS single crystals ...125 7.3.5. Transforming a PDPS single crystal at ambient
temperature with the AFM tip ...127 7.4. Conclusions ...129
Summary ... 132 Acknowledgments
List of Publications
Chapter 1
INTRODUCTION
This chapter gives a brief introduction to the structure of columnar mesophases. It is organized as follows: The first section contains a general description of the main thermotropic mesophases together with some examples of mesogenic molecular architectures. The second section is specifically focused on the structure of columnar mesophases and on the types of molecules forming them. The stability of columnar mesophases formed by discotic molecules is analyzed with respect to the core size, lateral chain architecture and the presence of specific interactions. The section is concluded by a short overview on the side chain polymers, which can form columnar mesophases without having mesogenic moieties in the molecular structure. The third section includes a description of specific interactions used to reinforce the mesophase stability with an emphasis on hydrogen bonding and dipolar interactions. The last part of the chapter specifies the general aims of this work and gives the outline of the thesis.
1.1. General considerations
Liquid crystalline (LC) phases represent fascinating states of matter combining order and mobility at molecular and supramolecular level. The LC order can be influenced by such parameters as molecular shape, specific interactions and micro-phase segregation. Depending on the particular molecular structure one of these parameters can be dominant. The unique combination of order and mobility makes LC materials responsive to a variety of external fields (e.g. magnetic, electrical or mechanical). LC materials are characterized by improved processing characteristics, self- healing of structural defects and alignment control. They are therefore of great importance for numerous applications such as light emitting diodes,1 photovoltaic cells,2 and field effect transistors.3 To extend or optimise the applications of liquid crystals a detailed knowledge of the structure- properties relationship is required.
Liquid crystals are naturally self-assembling systems. In the mesophase, the molecules possess orientational order and/or one or two dimensional positional order, while maintaining their liquid character.
In the classical approach, the systems forming liquid crystalline phases can be classified in two groups:
a) non-amphiphilic molecules with anisometric shape and b) amphiphilic molecules.4
Typical examples of molecular architectures forming LC phases are shown in figure 1.1. The mesophase can be defined according to the positional and orientational correlation between the molecules. For example, when there is orientational order but no positional order, the mesophases are termed nematic liquid crystals. If positional order is lost in one or two dimensions, the mesophases can be smectic, columnar or cubic. In low molecular mass systems, the molecular shape is one of the major factors determining the mesophase type. For example, calamitic molecules favour the formation of layers (smectic phases), disk-like molecules self-organize in columns (columnar mesophases) and amphiphilic or dendritic molecules can form spherical or cylindrical aggregates which self-assemble in columnar or micellar cubic mesophases. In addition, specific interactions can dramatically influence the extent of order in these systems. This makes liquid crystalline materials ideal model compounds to study the effect of various interactions on the molecular organization.5,6
In the following, a detailed discussion on the systems forming columnar mesophases and their structural organisation will be presented. A special focus will be put on the columnar phases formed by discotic liquid crystals and side-chain polymer liquid crystals without mesogenic units in their structure.
1.2. Molecular architecture and columnar order
The formation of columns can be observed in different molecular or macromolecular systems and results from different kinds of interactions. In most cases, a column, which can be a molecular or supermolecular structure, has a central rigid core surrounded by a flexible part. This ensures the repulsion between the columns preventing the system from crystallization
Anisometric molecules
Amphiphilic molecule C16H33COO-K+ 1
2
3
Fig.1.1. Examples of systems forming mesophases. 1-rod-like molecule, 2-disk-like molecule, 3-linear amphiphilic polyelectrolyte (detergent).
and thus favouring the LC order. Although the most studied columnar phases are formed by disk-shaped molecules,7 it is now recognized that depending on the molecular structure and intermolecular interactions involved such phases can also be formed by dendrimers,8,9 main- or side-chain polymers with10,11 or without12-14 mesogenic moieties, polycatenar molecules15,16 and sanidic or board-like molecules17 or other systems driven by specific interactions such as hydrogen bonding or dipolar interactions.
Disk-shaped liquid crystals can self-organize in various columnar mesophases (figure 1.2).
Apart from the nematic phase18 (ND), which is not columnar, the other phases have a well-defined columnar structure. In these mesophases the disks are stacked on top of each other to form columns that can subsequently arrange on a 2D lattice. An exception to this rule is the nematic columnar phase (Ncol) the examples of which are rather scarce. In this phase short columns are formed which exhibit only orientational order. Generally, columnar mesophases are classified according to the symmetry of the 2D array, the orientation of the core with respect to the columnar axis and the
Organization within one column
tilted disordered ordered helical
B Columnar rectangular disordered Colrd
Columnar oblique Colob
P2gg P2mg P2mm
C
Nematic discotic ND
Nematic columnar NCol
Columnar hexagonal ordered
Colho
Columnar lamellar (ColL)
A
Fig.1.2. A: Main types of supramolecular organization of disk-like molecules;
B: examples of molecular packing within the column; C: 2D lattices for rectangular and oblique columnar mesophases.
degree of order within the column. According to the classification given by Destrade et al.,19 one can distinguish columnar hexagonal ordered (Colho) and disordered (Colhd) mesophases, rectangular disordered mesophases (Colrd) and columnar oblique mesophases (Colob). It should be pointed out that the order criterion is more a matter of convention.20 This particular question will be discussed in more detail in chapter 4 for the case of a group of phthalocyanine derivatives. Recently, the existence of a new columnar structure was suggested, termed lamellar columnar phases (ColL), which combines smectic and columnar order. Helical arrangement of disks in the column has also been proven to exist.21
1.2.1. Discotic molecules
An archetypal discogenic molecule has a rigid, planar core surrounded by flexible side chains. The cores are commonly constructed by a single phenylene22,23 group, several phenylene groups,23-26 metal ion complexes27-30 or self-assembled disk-shaped systems.31,32 The flexible side-chains such as alkyl chains are in most cases necessary to form columnar mesophases. They are linked to the core directly or via an ether, thioether, ester, alkanoyloxy or amide groups. The formation of columnar mesophases is favoured for molecules with a small ratio of thickness and lateral size, which can be achieved by using a flat aromatic core. Previously, numerous combinations of different cores and side chains have been investigated. Some examples of discotic molecules are given in figure 1.3. In remaining part of this section, we shall review the thermal behaviour and mesophase structure for each of the examples given in the figure. Historically, hexasubstituted benzenes were the first discotic liquid crystals to be reported22. Benzene-based discotic compounds of type 1 have a small core and therefore favour relatively unstable mesophases. The window of the mesophase stability is relatively narrow and rarely exceeds 50°C.33 However, a remarkable stability of the mesophase was observed by Matsunaga et al.5 for the molecules where the alkyl chains are linked to the core by amide groups forming hydrogen bonds.
The influence of hydrogen bonds on the stability of the mesophases will be discussed further in the text. Molecules of type 2 were the first examples of lower symmetry26 (two-fold axis) molecules forming columnar mesophases.
The stability of the mesophases formed by rufigallol derivatives is comparable to that of benzene derivatives. For this system, both enantiotropic and monotropic phases are observed depending on the linking group and the length of the lateral alkyl chains.34,35
Triphenylene derivatives 3 are the best known representatives of discotic liquid crystals, the most commonly studied derivatives being the hexaethers, hexathioethers and hexaesters. All the hexaether derivatives with side-chains longer than propyloxy form a hexagonal columnar mesophase with the mesophase stability decreased with increasing the side chain length.36 The interest in hexaethers arises from the fact that they are photoconductive materials with potential semiconducting properties when doped with an oxidant.37 Several hexylthioether derivatives have been studied so far.38 A special case is the hexylthioether molecule with six carbon atoms in the lateral alky chains, which shows a remarkable charge carrier mobility in the helical columnar mesophase.39 A very rich polymorphism is found for the triphenylene hexakisalkanoate derivatives.36,40 Interestingly, some asymmetrically substituted hexabenzoate ester derivatives of triphenylene tend to form nematic mesophases.41,42 This behaviour can be explained by the fact that the molecule is more rod- than disk-shaped.
The truxene core 4 is even larger than triphenylene and consists of four benzene rings linked together by methylene groups, which keep an almost
Fig.1.3. Examples of disk-shaped mesogens: 1-hexasubstituted benzene, 2- anthracene (rufigallol), 3-triphenylene, 4-truxene, 5-phthalocyanine, 6- coronene derivatives, R-different substituents.
planar structure by preventing rotation of the rings. Accordingly, the mesomorphic character of compounds based on a hexa-substituted truxene core is very high. The temperature window of the mesophases in many instances extends over more than 200°C.43 The mesophase behaviour of some hexaesters of truxene derivatives is quite odd. Derivatives with short alkyl chains (from C6 till C8) show columnar rectangular and hexagonal mesophases and a re-entrant monotropic nematic phase. For derivatives with longer alkyl chains the nematic phase is enantiotropic. In this case the chains probably fold back toward the core, thereby disrupting the columnar packing. An interesting behaviour is found in the case of alkoxybenzoate esters where apart from a re-entrant nematic phase44 (called initially inverted nematic phase by Destarde et al.43), a re-entrant isotropic phase is also found.45,46
Substituted phthalocyanines 5 represent a large class of materials, which are important for many practical applications. The interest in phthalocyanines can be explained by the following features: (i) their flat aromatic core is quite large and offers the possibility for extended π-electron delocalisation, (ii) the strong van der Waals interactions between the flat cores enhance the thermal stability of the mesophase and, (iii) the possibility of introducing a variety of atoms in the molecular cavity. Phthalocyanines with eight peripheral moieties display a wide range of columnar hexagonal and rectangular mesophase stability. For octa-alkoxysubstituted phthalocyanines, linear chain derivatives tend to form stable mesophases47,48 and incorporation of a metal atom increases the columnar mesophase stability47,49, which in some instances extends beyond the decomposition temperature. Chain branching leads in some cases to a more complicated phase behaviour.50-52 Some more details about the effect of substitution with different alkyl chain will be provided in chapter 3.
Upon further increase of the core size (6) the stability of the mesophase dramatically increases. Therefore, the hexabenzacoronenes derivatives form a columnar ordered mesophase with a remarkably wide window of the mesophase (~ 300°C).53-55 This high stability is accompanied by the highest charge carrier mobility among discotic liquid crystals.56As far as the role of side chains is concerned, it is worth mentioning that partial fluorination of the lateral alkyl chains greatly increases the stability of the mesophase.57 This behaviour can be attributed to the enhanced segregation between parts of the molecules.
1.2.2. Side-chain polymer liquid crystals
Generally, LC polymers can be subdivided in side- and main-chain polymers. Figure 1.4 shows a schematic representation of these classes of polymers. Main-chain LC polymers are formed when the mesogenic units are part of the polymer backbone, whereas side-chain polymers have the mesogenic parts appended to the chain by a flexible bridge. In most cases, the mesogenic parts are calamitic or discotic. However, there is a large group of polymers forming LC mesophases, which do not have any mesogenic moieties in their chemical structure. In this case, the column is formed by the array of conformationally disordered polymer backbones arranged on a 2D lattice, with a generally hexagonal symmetry.
According to the classification given by Ungar,58 there are three types of macromolecules, which can form hexagonal columnar mesophases:
1. Flexible linear macromolecules such as polyethylene at high pressure,59,60 1,4-trans-polybutadiene,61 polytetrafluoroethylene;62,63
2. Flexible branched macromolecules: alkyl-polysiloxanes,64-66
polysilanes,67-69 polyphosphazenes,70,71 polyamides,72 polydialkysilylenes;
3. Rigid macromolecules with flexible side-chains such as cellulose.
The LC nature of the molecules in the first group is difficult to rationalize.
For the molecules belonging to the second group the amphiphilic character seems to be responsible for the existence of the columnar mesophases. The flexible inorganic backbone is built up by polar constituents and is framed by various apolar organic lateral chains. Several examples of such polymers are given in figure 1.5. In the mesophase the backbone is placed along the columnar axis (figure 1.6). The columnar organization exhibits long range lateral order and short range order along the column. Similarly to the
Side chain liquid crystal polymers
-
calamitic mesogenic unit-
discotic mesogenic unit-
polymer chain Main chain liquid crystal polymersFig.1.4. A schematic representation of the structure of side- and main- chain LC polymer having calamitic and discotic mesogens.
columnar phases formed by discotic molecules, the side groups form the periphery of the column. The main difference between these mesophases and the ones formed by discotic molecules is that there is connectivity in the chemical structure along the column axis.
Polysilanes 1 have a backbone containing only silicon atoms. It was found that the lattice parameter of hexagonal phase as well as the stability interval of the mesophase increases with the number of carbon atoms in the lateral chains73. Moreover, the conformation of the silane backbone presumably has a significant number of gauche defects. This kind of behavior seems to be also valid for poly(di-n-alkylsiloxanes) 2 and poly(di-n- alkoxyphosphazenes) 3. In this context, the introduction of a methylene group between two silicon atoms in the backbone 4 results in narrower columnar hexagonal mesophases starting from methyl to hexyl substituted polymers. This feature is more likely due to a low backbone flexibility and to the decreased incompatibility between the side chains and the backbone.
A particular case of LC polyamides is given by molecule 5.
For this compound a helicoidal conformation of the backbone is proposed. It is suggested that linking the alkyl chains to the stiff amide may reinforce the
Fig.1.5. Examples of polymers exhibiting columnar hexagonal mesophases:
1- poly(di-n-alkylsilanes), 2- poly(di-n-alkylsiloxanes), 3- poly(di-n- alkylsilylanemethylenes), 4- phosthazenes, 5-polyamines.
overall rigid helical structure. In addition, the helical structure should be stabilized by the hydrogen bonds between the amide groups. These materials are described as systems bridging the gap between the columnar mesophases formed by the discotic molecules and the columnar mesophases formed by linear polymers as described above.
A new example of polymer columnar mesophase was reported recently.6,74 This is an alternated copolymer of an olefin and a semi-fluorinated polyethylene. In this case the columnar order is supposed to be due to the different nature of the segments, which prevents the fluorinated blocks from forming individual layers.
1.3. Columnar order resulting from specific inter- and intra- molecular interactions
Specific or secondary interactions are used in nature to build complex structures with multiple levels of organization. This inspired scientists to try to mimic the nature and synthesize self-assembling organic molecules. The specific interactions include π-π interactions, hydrogen bonding, electrostatic interactions (ion-ion, ion-dipole, dipole-dipole) and hydrophobic or solvophobic effects (in liquids).
Attractive interactions between π-systems are known to play an important role in nature. They control the base-base interactions in the helical structure
Fig.1.6. Schematic representation of the columnar hexagonal packing for an poly(di- n-propylsiloxane) molecule.
of DNA, the tertiary structure of proteins, complexation in many host-guest systems and porphyrin aggregation. As in nature, hydrogen bonds5,75, dipolar interactions6,76 and charge transfer interactions are used to construct mesogenes or to stabilize a particular phase. In the next section the influence of hydrogen bonds and dipole interactions on the formation of columnar mesophases will be discussed.
1.3.1. Hydrogen bonds in columnar mesophase formation
Hydrogen bonding is one of the key interactions in the processes of molecular aggregation and recognition in nature. In this respect, the design of new materials with controlled nanostructures, properties and stability is a great challenge. For example, Lehn et al.77 reported cases of complementary base pairing formed by hydrogen bonds, Matsunaga et al.5 and Malthête et al.75 employed hydrogen bonding directed along the columnar axis to stabilize the mesophase. Hydrogen bonding is also used to self-assembly identical78-80 or different81-83 multiple precursors in dimeric structures which can subsequently give rise to columnar structures. For example, numerous studies were performed on 1,3,5 benzene triamides derivatives 1 (figure 1.7).
They were found to be linked by hydrogen bonds in columnar or layer-like superstructures depending on the length of lateral alkyl chains. At the same time, the hydrogen bonds stabilize the mesophases up to high temperatures5. It was shown by X-ray diffraction that in the case of compound 1b the hydrogen bonds rotate the molecules out of plane giving rise to a helicoidal structure84. In solutions, an equimolar mixture of right and left handed helical columns can be found. Since for such a small core size the molecular interactions are dictated by hydrogen bonds rather than π-π interactions, introduction of chiral centers in the lateral chains can bias the helix. Chiral amplification within mixtures of chiral molecule 1d and achiral molecule 1c was investigated to reveal the cooperativity within the stacks in accordance with the “sergeant and soldiers model”85 (1d was used as sergeant, and 1c as soldier). It was found that due to the chiral directionality of the amide groups the self-assembled columns of 1d has correlation over at least 80 molecules.
The chirality was thus greatly amplified, only one chiral disk over 200 achiral disks being sufficient to control the helicity of one column.
Aggregation via hydrogen bonding and formation of gels at high concentrations was also studied for the systems where both intra- and inter- columnar hydrogen bonds coexist.86,87 Two examples of molecular
architectures of self -assembled semi-discoid molecules 2 and 1,3,5- benzenetricarboxamide unit 3 are given in figure 1.7. The formation of supramolecular aggregates via hydrogen bonding is followed by the
assembly in columnar structures.
1.3.2. Dipolar interactions in columnar mesophase formation
The high charge mobility found in hexahexylsulfanyltriphenylene88 stimulated many efforts to understand the factors governing the order in the columnar mesophase. Thus, Ringsdorf et al.6,76 proposed that limiting the molecular rotation within the column leads to a decrease of the degree of freedom within the mesophase and therefore to a higher order. This can be Fig.1.7. Examples of LC molecules forming inter- and/or intra- molecular hydrogen bonds
achieved via unfavourable steric interactions. Several examples illustrating the role of dipole-dipole interactions in the stabilization of the mesophase are given in figure 1.8. They include triphenylene derivatives6,76,89,90 and a new discotic molecule.91 The influence of dipole-dipole interactions on the thermodynamic properties and structure of molecule 1a was studied by replacing one of the alkoxy group by substituents carrying strong dipole moments. Compared to the symmetrically substituted disk, compounds 1b and 1c melt at higher temperatures than reference compound 1a, while molecule 1c displays only a monotropic mesophase. It can be thus concluded that increasing the dipole moment of the substituent increases the tendency of the molecule to crystallize. A strong dipolar moment is induced also when the nitro substituent is attached directly to the triphenylene (HAT 6) core (molecule 2). Surprisingly, this substitution renders the material liquid crystalline at room temperature and the mesophase is stable over more than 100°C.89 In the case of molecule 3, the mesophase has been substantially stabilized with respect to the structurally related first generation stilbenoid.92 It is supposed that this behavior is due to the permanent dipole improving the antiparallel stacking.91 It was therefore found that the phase diagram of discotic LC materials can be strongly modified by dipole-dipole interactions.
Fig.1.8. Examples of molecules having dipoles in the structure
Aim of the work and outline
The results described in this thesis were obtained on a variety of systems forming columnar mesophases ranging from small molecular weight compounds to polymer materials. The study aims at understanding the influence of molecular architecture and specific interactions on the supramolecular structure, as well as at understanding the role of columnar mesophases in crystal growth.
The experimental part of my Ph.D. thesis starts with chapter 2, in which the main experimental techniques used in this work are briefly introduced. The complementarity and applicability of these techniques to studies of LC systems is specifically addressed in the chapter. The following five chapters present the main body of experimental results of the thesis. The influence of molecular architecture on the formation of columnar mesophases is analysed by studying molecules with different molecular core architecture and symmetry such as triphenylene derivatives (C3-symmetry), metal-free phthalocyanine molecules (C4-symmetry) and flexible star- shaped molecules.
Our interest in triphenylene derivatives and, more specifically, in hexaazatriphenylene, was driven by the possibility to study the influence of hydrogen bonding on the structure of the mesophase. These results are presented in chapter 3.
In chapter 4 we report on phthalocyanine derivatives, which hold promise for various opto-electronic applications. Phtalocyanines forming columnar liquid crystalline phases at ambient temperature are most suitable for these applications due to such features as their improved processibility and self- healing of structural defects. In spite of numerous studies on phthalocyanine derivatives carried out to date, only a few derivatives were found to form liquid crystalline phases at ambient temperature. In the present study, the LC character of these molecules was tailored via the lateral chains architecture. It was found that branching introduced in the long alkyl chains side chains close to the core renders the material LC at room temperature.
We have found that these molecules probably present the first case of ordered non-hexagonal columnar mesophases. Since the majority of industrial applications require the fabrication of films, it was challenging to visualize the organization of films at the nanometer scale. Therefore, AFM images with columnar resolution obtained on thick spin-coated films are presented for the first time.
Chapter 5 treats the supramolecular organization of a star-shaped mesogen.
This system initially forms a monotropic columnar hexagonal mesophase
upon cooling from the isotropic phase. However, on annealing at ambient temperature, the structure was found to undergo a transition to a helicoidal crystalline phase. In this chapter we show how the mesophase pre-ordering allows to template the crystallization process on the nanometer scale, i.e., on the scale of an individual molecule.
In the last two chapters of the thesis the templated crystal growth is further studied for the case of a polymer material forming a columnar hexagonal mesophase without having any mesogenic groups in its chemical structure.
This is poly(di-n-propylsiloxane), PDPS. In chapter 6 the structures of the crystalline phases and columnar mesophase of PDPS are described. The mesophase-assisted crystallization is studied on PDPS samples crystallized in different conditions. It is observed that, depending on crystallization conditions, both extended-chain and folded-chain crystals were found. It should be emphasized that the formation of extended-chain polymer crystals is a rare feature in polymer science and has mostly been studied for high- density polyethylene crystallized in special conditions.
Chapter 7 is focused on the study of PDPS single crystals. A combination of Atomic Force microscopy and Electron Diffraction allows to explore the chain unfolding mechanisms and the interrelations between the structures of the crystal and the mesophase.
The thesis ends with a summary of the main results, the list of publications and acknowledgments.
References:
1. Seguy, I.; Jolinat, P.; Destruel, P.; Farenc, J.; Mamy, R.; Bock, H.; Ip, J.;
Nguyen, T. P. J. Appl. Phys. 2001, 89, 5442.
2. Lüssem G.; Wendorff, J.H. Polym. Adv. Technol., 1998, 9, 443.
3. Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R.H.;
MacKenzie, J.D. Science, 2001, 293, 1119 4. Tschierske, C. Mater. Chem., 2001, 11, 2647.
5. Matsunaga, Y.; Miyajima, N.; Nakayasu, Y.; Sakai, S.; Yonenaga, M. Bull.
Chem. Soc, Jpn, 1988, 61, 207.
6. Henderson, P.; Kumar, S.; Rego, J.; Ringsdorf, H.; Schuhmacher, P. J. Chem.
Soc., Chem. Commun., 1995, 1059.
7. Chandrasekhar, S. Liq. Cryst., 1993, 14, 3.
8. Cameron, J.H.; Facher, A.; Lattermann, G.; Diele, S. Adv. Mater., 1997, 9(5), 398
9. Percec, V. ; Johhansson, G. ; Ungar, G. ; Zhou, J. J. Am. Chem. Soc., 1996, 118, 9855.
10. Kumar, S.; Manickam, M.; Schonherr, H. Liq. Cryst., 1999, 26(10), 1567.
11. Wang, T.; Yan, D.; Zhou, E.; Karthaus, O.; Ringsdorf, H. Polymer, 1999, 39(19), 4509.
12. Papkov, V.S.; Il’ina, M.N.; Zhukov, V.P.; Tsvankin, D.Ja.;.Tur, D.R Macromolecules, 1992, 25, 2033.
13. Out, G.J.J.; Turetskii, A.A.; Mollen, M.; Oelfin, D. Macromolecules, 1995, 28, 596.
14. Seitz, M.; Plesnivy, T.; Schimossek, K.; Ringsdorf, H.; Fischer, H.; Uyama, H.; Kobayashi, S. Macromolecules, 1996, 29, 6560.
15. Guillon, D. ; Skoulios, A. ; Malthéte, J. Europhys. Lett., 1987, 3(1), 67.
16. Nguyen, H.T. ; Destrade, C. ; Malthéte, J. Adv. Mater., 1997, 9(5), 375.
17. Struijk, C.W.; Sieval, A.B.; Dakhorst, J.E.J.; Van Dijk, M.; Kimkes, P.;
Koehorst, R.B.M.; Donker, H.; Schaafsma, T.J.; Picken, S.J.; van de Craats, A.M.; Warman, J.M.; Zuilhof, H.; Sudholter, E.J.R. J. Am. Chem. Soc. 2000, 122, 11057.
18. Tinh, N.H. ; Destrade, C. ; Gasparoux, H. Phys. Lett., 1979, 25, 724.
19. Destrade, C.; Foucher, P.; Gasparoux, H.; Tinh, N.H.; Levelut, A.M.;
Malthete, J. Mol. Crys. Liq. Cryst., 1984, 106, 121.
20. This point will be specifically addressed in chapter 4 of the present document.
The distinction between the ordered and disordered mesophases will be made based on the observation of a diffuse peak in the X-ray diffraction pattern measured on oriented samples corresponding to the order of the molecules in the column.
21. Fotnes, E. ;.Heiney, P.A. ; W.H de Jeu, Phys. Rev. Lett., 1988, 61, 1202.
22. Chandrasekhar, S.; Sadashiva, B.K. ; Suresh, K.A. Pramana, 1977, 9, 471.
23. Chandrasekhar, S. Mol. Cryst. Liq. Cryst., 1981, 63, 171.
24. Nguyen, H.T. ; Gasparoux, H.; Destarde, C. Mol. Cryst. Liq. Cryst., 1981, 68, 101.
25. Destrade, C.; Nguyen, T. H. ; Gasparoux, H.; Malthtete, J. ; Levelut, A. M.
Mol. Cryst. Liq. Cryst., 1981, 71, 111.
26. Billard, J.; Dubois, J.C.; Vaucher, C.; Levelut, A.M. Mol. Cryst. Liq. Cryst., 1981, 66, 115.
27. Giroud- Godquin, A. M.; Billard, J. Mol. Cryst. Liq. Cryst., 1981, 66, 147.
28. Piechocky, C.; Simon, J.; Skoulios, A.; Guillon, D.; Weber, P.J. J. Am. Chem.
Soc., 1982, 104, 5245.
29. Barbera, J. ; Cativiela, C.; Serano, J. L.; Zurbano, M. M. Adv. Mater, 1991, 3, 602.
30. Zhang, H.; Lai, C.K.; Swager, T. Chem. Mater., 1995, 7, 2067.
31. Ebert, E.; Wendorff, J.; Lattermann, G. Liq. Cryst., 1990, 7, 553.
32. Lattermann, G.; Straufer, G.; Brezesinski, G. Liq. Cryst., 1991, 10, 169.
33. Demus, D. Handbook of Liquid Crystals, vol. 2B, 1998, New York.
34. Carfugua, G.; Roviello, A.; Sirigo, A. Mol. Cryst. Liq. Cryst., 1985, 122, 151.
35. Carfugua, G.; Iannelli, P.; Roviello, A.; Sirigo, A. Liq. Cryst., 1987, 2, 611.
36. Destrade, C.; Tinh, N.H.; Gasparoux, H.; Malthete, J.; Levelut, A.M. Mol.
Cryst. Liq. Cryst., 1981, 71, 111.
37. Boden, N.; Bushby, R.J.; Clements, J.; Jesudson, M.V.; Knowles, P.F.;
Williams, G. Chem. Phys. Lett., 1988, 152, 94.
38. Gramsbergen, E.F.;. Hoving, H.J.; de Jeu, W.H.; Praefcke, K.; Kohne, B. Liq.
Cryst., 1986, 1, 397.
39. Adam, D.; Schumacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.;
Etzbach, K.H.; Ringsdorf, H.; Haarer, D. Nature, 1994, 371, 745.
40. Vauchier, C.; Zann, A.; LeBarny, P;. Dubois, J.C.; Billard, J. Mol. Cryst. Liq.
Cryst., 1981, 66, 103
41. Tinh, N.H.; Gasparoux, H.; Destrade, C. Mol. Cryst. Liq. Cryst., 1981, 68, 101
42. Tinh, N.H. ; Gasparoux, H. ; Destrade, C. Phys. Lett., 1979, 72A, 251.
43. Destrade, C.; Gasparoux, H.; Babeau, A.; Tinh, N.H.; Malthete, J. Mol. Cryst.
Liq. Cryst., 1981, 67, 37.
44. Tinh, N.H.; Malthete, J.; Destarde, C. J. Physique, 1981, L42, 417.
45. Warmerdam, T.;. Frenkel, D.; Zijlstra, R.J.J. Liq. Cryst., 1988, 3, 149.
46. Warmerdam, T. W.; Nolte, R.J.M.; Drenth, W.; van Miltenburg, J.C.; Frenkel, D.; Zijlstra, R.J.J. Liq. Cryst., 1988, 8, 1187.
47. van der Pol, J.F.; Neeleman, E.; Zwikker, J.W.; Nolte, R.J.M.; Drenth, W.;
Aerts, J.; Visser, R.; Picken, S.J. Liq. Cryst., 1989, 6, 577.
48. Sauer, T.; Wegner, G. Mol. Cryst. Liq. Cryst., 1988, 162B, 97.
49. Guillon, D.; Weber, P.; Skoulios, A.; Piechocki, C.; Simon, J. Mol. Cryst. Liq.
Cryst., 1985, 130, 223.
50. Schouten, P.G.; Warman, J.M.; de Haas, M.P.; van Notrum, C.F.; Gelinck, G.H.; Nolet, R.J.M.; Copyn, M.J.; Zwikkler, J.W.; Engel, M.K.; Hanack, M.;
Chang, Y.H.; Ford, W.T. J. Am. Chem. Soc. 1994, 116, 6880-6894.
51. Schouten, P. G.; Van der Pol, J. F.; Zwikker, J. W.; Drenth, W.; Picken, S. J.
Mol. Cryst. Liq. Cryst. 1991, 195, 291-305.
52. Ohta, K.; Watanabe, T.; Hasebe, H.; Morizumi, Y.; Fujimoto, T.; Yamamoto, I.; Lelievre, D.; Simon, J. Mol. Cryst. Liq. Cryst., 1991, 196, 13
53. Herwig, P.; Kayser, C.W.; Mullen, K.; Spiess, H.-W. Adv. Mater., 1996, 8, 510.
54. Samori, P.;. Keil, M.; Friedlein, R.; Birgeson, J.; Watson, M.; Mullen, K.;
Salaneck, W.R.; Rabe, J.P. J. Phys. Chem. B, 2001, 105, 11114.
55. Trunemann, A.F.; Ruppelt, D.; Ito, S.; Mullen, K. J. Mater. Chem., 1999, 9, 1055.
56. van der Craats, A.M.; Warman, J.M.; Fechtenkotter, A.; Brand, J.D.;
Harbison, M.A.; Mullen, K. Adv. Mater., 1999, 11, 1469.
57. Dahn, U.; Erdelen, C.; Ringsdorf, H.; Festag, R.; Wendorff, J.H.; Heiney, P.A.; Maliszewskyi, N.C. Liq. Cryst. 1995, 19, 759.
58. Ungar, G. Polymer, 1993, 34, 2050.
59. Bassett, D.C.; Turner, B. Nature, Phys. Sci., 1972, 240, 146.
60. Ungar, G.; Keller, A.; Polymer, 1980, 21, 1273.
61. Rastogi, S.; Ungar, G.; Macromolecules, 1992, 25, 1445.
62. Lovinger, A.J.; Furukawa, T.; Davis, G.T.; Broadhurst, M.G. Polymer, 1983, 24, 1225.
63. Weeks, J.J.; Ebby, R.K.; Clark, E.S. Polymer, 1981, 22, 1496.
64. Beatty, C.L.; Pochan, J.M.; Froix, M.F.; Hinman, D.D. Macromolecules, 1975, 8, 547.
65. Godovsky, Yu.K.; Makarova, N.N.; Papkov, V.S.; Kuzmin, N.N. Makromol.
Chem., Rapid Commun., 1985, 6, 443.
66. Godovsky, Yu.K.; Papkov, V.S. Adv. Polym. Sci.,1989, 88, 129.
67. Lovinger, A.J.; Schilling, F.C.; Bovey, F.A.; Zeigler, J.M. Macromolecules, 1986,19, 2660.
68. Koopmann, F.; Frey, H. Macromolecules, 1996, 29, 3701.
69. Frey, H.; Out, G.; Moller, M.; Greszta, D.; Matyjaszewski, K.;
Macromolecules, 1993,26, 6231.
70. Kojima, M.; Kenge, W.; Magill, J.H. Macromolecules, 1984,17, 1421.
71. Papkov, V.S.; Ii’ina, M.N.; Zhukov, V.P.; Tavankin, D. Ja.; Tur, D.R.
Macromolecules, 1992, 25, 2033.
72. Seitz, M.; Plesnivy, T.; Schimossek, K.; Edelmann, M.; Ringsdorf, H.;
Fischer, H.; Uyama, H.; Kobayashi, S. Macromolecules, 1996,29, 6560.
73. KariKari, E.K.; Greso, A.J.; Farmer, B.L.; Miller, R.D.; Rabolt, J.F.
Macromolecules, 1993, 26, 3937.
74. Percec, V. ; Schlueter, D. ; Ungar, G. Macromolecules, 1997, 30, 645.
75. Malthête, J.; Levelut, A.-M.; Liébert, L. Adv. Mater., 1992, 4, 37.
76. Rego, J.A.; Kumar, S.; Dmochowski, I.J.; Ringsdorf, H. J. Chem. Soc., Chem.
Commun., 1996, 1031.
77. Kimizuka, N.; Fujikawa, S.; Kuwahara, H.; Kunitake, T.; Marsh, A.; Lehn, J.- M.J. Chem. Soc., Chem. Commun., 1995, 2103.
78. Kleppinger, R.; Lillya, C.P.; Yang, C. J. Am. Chem. Soc.,1997, 119, 4097.
79. Paulus, W.; Ringsdorf, H.; Diele, S.; Pelzl, G. Liq. Cryst., 1991, 9, 807.
80. Ebert, M.; Kleepinger, R.; Soliman, M.; Wolf, M.; Wendorf, J.H.;
Lattermann, G.; Staufer, G. Liq. Cryst., 1990, 7, 553.
81. van Gorp, J. J.; Vekemans, J.J.A.M.; Meijer, E.W. J. Am. Chem. Soc.,2002, 124, 14759.
82. Palmans, A.R.A.; Vekemans, J.A.J.M.; Hikmet, R.A.; Fischer, H.; Meijer, E.W. Adv. Mater., 1998, 10, 873.
83. Goldman, D.; Janietz, D.; Schmidt, C.; Wendorff, J.H. J. Mater. Chem., 2004, 14, 1521.
84. Lightoot, M.P;. Mair, F.S.; Pritchard, R.G.; Warren, J.E. J. Chem. Soc., Chem.
Commun., 1999, 1945.
85. Brunsveld, L.; Schenning, A.P.H.J.; Broenen, M.A.C.; Janssen, H.M.;
Vekemans, J.A.J.M.; Meijer, E.W. Chem. Lett., 2000, 292.
86. Yasuda, Y.; Iishi, E.; Inada, H.; Shirota, Y. Chem. Lett., 1996, 575.
87. Hanabusa, K.; Kawakami, A.; Kimura, M.; Shirai, H. Chem. Lett., 1997, 191.
88. Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.;
Etzbach, K.H.; Ringsdorf, H.; Haarer, D. Nature, 1994, 371, 141.
89. Boden, N.; Bushby, R.J. ; Cammidge, A. N.; Headock, G. J. mater. Chem., 1995, 5, 2275.
90. Kettner, A.; Wendorff, J.H. Liq. Cryst., 1999, 26, 483.
91. Atthias, A.-J.; Cavalli, C.; Donnio, B.; Guillon, D.; Hapiot, P.; Malthéte, J.
Chem. Mater., 2002, 14, 375.
92. Meier, H.; Lehmann, M. Angew. Chem. Int. Ed., 1998, 37, 643.
Chapter 2
EXPERIMENTAL TECHNIQUES
In this chapter the main experimental techniques used in this work will be briefly introduced. At the beginning of the chapter the complementarity of these techniques and their relevance to the study of LC systems will be specifically addressed
In our studies, a combination of different experimental techniques has been used to characterize the phase behavior of liquid crystalline materials. They include direct space techniques such as Polarized Optical Microscopy (POM) and Atomic Force Microscopy (AFM), and reciprocal space techniques such as X-ray and Electron Diffraction. Differential Scanning Calorimetry (DSC) was employed to study the thermal transitions occurring in liquid crystalline systems during heating and cooling ramps.
Importantly, none of the above-mentioned techniques alone would suffice for a complete structural characterization of a LC system. For example, the temperatures and types of phase transitions can be conveniently studied by DSC. On the other hand, the type of the mesophase can also be identified by POM, whereby the defects of the structure give rise to the formation of characteristic textures well documented in the literature.1 However, we observed that, in some instances, the transitions, which are not detected by DSC, could still be observed in POM and vice versa. It is also clear that the phase characterization by means of POM alone is almost never unambiguous since different phases can display similar textures. Therefore, the use of electron and X-ray diffraction is necessary to identify different phases and obtain quantitative information on their structure. For single-crystal-like materials, the electron diffraction presents a great advantage over X-rays as it allows to combine the spatial resolution with a possibility to explore
different diffraction zones of the reciprocal space. By contrast, X-ray diffraction is less demanding with respect to sample preparation and also less destructive.
As far as the AFM technique is concerned, it is worth noticing that this is a non-destructive technique, which can provide information on the columnar structure in the range from several to some hundreds of nanometers. This fills the gap between the scale of classical X-ray diffraction and optical observations. To use this capacity of AFM, special efforts were directed to obtain AFM images with columnar resolution.
2.1. Differential Scanning Calorimetry
DSC experiments were performed with a Mettler-Toledo heat-flux DSC 822e2 under helium atmosphere.
Generally, DSC measures the power released or absorbed by material during temperature treatments that can include dynamic (i.e., heating or cooling ramps) or isothermal segments. The measurement is performed by comparing the temperature of the sample and that of the reference material.
The instantaneous heat flux is computed from this temperature difference using instrumental calibration constants. Standard samples like pure indium or zinc with known transition enthalpies and temperatures are used for the calibration.
Fig.2.1. Schematical drawing of a DSC measuring cell. The thermal sensor contains 56 (28+28) thermocouples. The temperature probe (Pt100) measures the temperature of the furnace.
The measuring cell of a calorimeter includes the sample and reference materials enclosed in a single furnace, as shown in figure 2.1. The DSC furnace is made of massive silver and is separated from the DSC sensor by a ceramic plate. The temperature of each of the two containers (pans) is measured by 28 thermocouples connected in series and located around each of them.
In studies of LC systems the measure of the enthalpy variation can allow to assign a given thermal event to a polymorphic crystal-crystal or to a mesophase-mesophase transition. This is based on the fact that the enthalpy variation associated with crystal melting is by far more important than the one corresponding to the mesophase-mesophase or mesophase-isotropic phase transitions. The assignment can become difficult when one deals with ordered mesophases, sometimes called “soft crystals”, which exhibit transition enthalpies comparable to the ones corresponding to crystal melting. Some examples of such systems will be provided in chapter 4.
The main advantage of DSC is that it is a fast and convenient tool to measure the temperatures and transition enthalpies in order to determine the phase diagram of the system and to study the kinetics of transitions as a function of heating/cooling rates or as a function of time.
2.2. Polarized Optical Microscopy
POM observations have been carried out using an Olympus Provis AX 70 microscope. To study the phase transitions as a function of temperature the microscope was equipped with a Linkam heating stage coupled to a temperature controller and a LN2 pump. The sample is placed between two glass slides and observed in polarized light.
POM is a routine technique used for the LC phase identification. The interpretation is based on the observation of optical textures that can be assigned to different LC phases.1 However, as mentioned in the introduction to this chapter, caution should be exercised because the identification of mesophases could be ambiguous. Another complexity of the technique is due to sometimes high melt viscosity hampering the texture formation. Thus, the quality of the optical texture is typically reduced with increasing the molecular weight of the mesogen.
2.3. X-ray diffraction experiments
X-ray diffraction experiments were performed using our laboratory facility (D8 Advance Diffractometer), as well as synchrotron radiation sources at the
European Synchrotron Radiation Facility (ESRF, Grenoble) and Deutsches Elektronen-Synchrotron (DESY, Hamburg).
2.3.1. D8 Advance Diffractometer
Figure 2.2 (top) shows a D8 Powder Advance diffractometer equipped with a wide-range MRI chamber. The machine operates in θ-θ geometry, the sample being fixed, while the source and detector being mounted on two arms of the goniometer. The measuring circle (cf. figure 2.2, bottom) has a radius of 575 mm.
Fig.2.2. View of a D8 Advance Diffractometer equipped with a wide-range MRI chamber (top); schematic representation of the main parts and optical elements of the setup (bottom).
The X-ray tube with a copper anode (wavelength λ=1.54Å) is operated at 40kV and 40mA. The optical system includes a Göbel mirror, which monochromatizes the beam and renders it parallel in one direction. The irradiated samples surface is controlled with the help of a knife edge collimator (KEC). The diffraction intensity is measured with a scintillation counter.
2.3.2. Synchrotron radiation sources
To generate synchrotron radiation, electrons emitted by an electron gun are first accelerated in a linear accelerator (linac) and then transmitted to a circular accelerator to reach the desired energy level (figure 2.3). The latter is 6GeV for the case of ESRF (most brilliant synchrotron source in Europe) and 4.5GeV for DESY. The high-energy electrons are further injected into a large storage ring, where they circulate in a vacuum environment at a constant energy for many hours. Inside the storage ring the electrons pass through different types of magnets: bending magnets, wigglers and undulators.
By changing the electron trajectory the magnets cause them to emit synchrotron radiation. The synchrotron beams emitted by electrons in different regions of the storage ring are directed toward beamlines, which surround the storage ring. Every beamline is designed for a specific type of experiment. It typically includes (figure 2.4):
Fig.2.3. The electron racetrack in a synchrotron
1. Optical hutch houses the optical system used to tailor the experimental conditions such as the beam energy and the beam size.
2. Experimental hutch contains the sample environment, slit system and detectors.
3. Control hutch allows the experimentalist to collect the data.
In the following, we will provide some important details of the beamlines we used at DESY and ESRF.
X33 camera/D1 beamline, EMBL/DESY
The experimental setup is schematically shown in figure 2.5. The beamline is operated at a fixed wavelength λ=1.5 Å. The beam is focused on the SAXS detector by a series of successive segmented mirrors placed after the Si (111) monochromator. An ionization chamber is placed before the sample to monitor the intensity of the primary beam. The data are recorded with two position sensitive delay line readout detectors connected in series. The sample temperature is controlled with a Mettler FP-82 HT heating stage under nitrogen flux.
The norm of the scattering vector s (s = 2sinθ/λ, where θ is the Bragg angle and λ the wavelength) was calibrated with tripalmitin (4.06 nm repeat) in the small s region and benzoic acid (monoclinic unit cell with a = 5.511 Å, b = 5.158 Å, c= 21.714 Å, β= 97.42°) in the large s region.
Fig.2.4. Drawing showing different regions of a beamline
BM26B beamline at the ESRF
The ESRF Dubble beamline is operated at a variable wavelength. Figure 2.6 shows a view of the experimental hutch. One can see a 10 m chamber with a 2D SAXS detector visible in the left side of the figure. In our work, we used two different setups, which will be discussed in more detail in the experimental part of the corresponding chapters. The gas detector used in SAXS experiments is characterized by the image size 133mmx133mm and the spatial resolution of 250±5µm. The detector irregularities were corrected by measuring the pattern of a 55Fe source.
Fig.2.5. Experimental setup of the X33 camera, DESY, Hamburg.
Fig.2.6. A view of the experimental hutch on beamline BM26B at the European Synchrotron Facility in Grenoble (France).
The s-axis was calibrated with silver behenate (d001 = 58.38 Å), which has a large number of well defined diffraction peaks evenly distributed in the 1.5- 20.0° 2θ range.
2.4. Atomic Force Microscopy
AFM experiments were carried out with a commercial Multimode Nanoscope III (/Digital Instruments/Veeco Metrology Group) shown in figure 2.7 (left). AFM Images have been taken with a J type scanner (max.
scan size is 220x220 µm).
AFM technique is widely used to characterize a large variety of materials due to its applicability to non-conducting samples and its non-destructive character. At the same time, little sample preparation is required before imaging.
In AFM imaging in contact mode, a sharp tip is rastered over the sample surface. The feedback mechanisms enable the piezo-electric scanner to maintain either a constant tip-sample force or a constant height (figure 2.7, right) using an optical detection system. More specifically, a diode laser is focused onto the back of a reflective cantilever. As the tip scans the sample surface, the reflected beam is deflected vertically or horizontally. This deflection is measured by a photodiode array, generating the signal for the feedback loop (figure 2.7, right). In imaging soft materials the contact mode is not very useful as it often results in the sample deformation/destruction.
Fig.2.7. Multimode AFM (left) and schematic illustration of its main components (right)
Therefore, our experiments were carried out only using tapping mode, which is introduced in the following section.
2.4.1. Tapping force AFM
In Tapping Mode AFM, the cantilever is driven close to its resonance frequency using a small piezoelectric actuator positioned in the tip holder.
The oscillation amplitude of the cantilever is used as a feedback signal to measure the topographical variations of the sample surface. Tapping Mode inherently prevents the tip from sticking to the surface and cause damage during scanning. Unlike contact and non-contact modes, the tip has sufficient oscillation energy to overcome the tip-sample attractive forces. In phase imaging, the phase lag of the cantilever with respect to the driving signal is simultaneously measured using the Phase Extender Module. The phase lag is sensitive to variations in material properties such as adhesion and viscoelasticity. This makes the phase imaging a powerful tool for mapping local sample surface properties at high resolution.
An important issue to be mentioned is related to the resolution achievable with tapping mode AFM. Generally, it is limited by the apical probe geometry and the tip-sample contact area. The conventional AFM tips are typically made of Si3N4 or Si, and are assembled on the end of the cantilever (figure 8 left). Novel Hi’Res probes (Mikromasch) with ultrasharp whiskers grown on top of conventional tips allowed us to obtain high-resolution images of columnar structures presented in the following chapters. Electron microscopy micrographs of such a tip are given in figure 2.8 (right).
Fig. 2.8. STM and TEM images (left and right, respectively) of a novel ultra sharp AFM tip.
2.4.2. High-Temperature AFM
High temperature AFM work was carried out using a commercial heater accessory for the MultiMode AFM (figure 2.9). The design includes a plug- in heater for the sample and a resistive heater integrated in the tip holder.
The heating of the tip is necessary when the temperature of the sample exceeds approximately 100°C to avoid water condensation on the backside of the AFM probe. The sample temperature is measured with a thermocouple placed under the sample puck, while the temperature of the tip is determined from the shift of its resonance frequency, as described elsewhere.3,4 The temperature of the scanner is controlled by a water cooling system. The experiments were performed under helium or argon atmosphere.
2.5. Transmission Electron Microscopy
Electron Microscopy experiments were performed with a Phillips CM12 electron microscope (figure 2.10, top) equipped with a lanthanum hexaboride (LaB6) filament and operated at 120kV. For crystallographic work we used a tilting stage (sample inclination angle is ±45°) allowing to
Fig.2.9. A: Multimode microscope with the thermal accessory installed, B: temperature controller, C: main components of the thermal accessory assembled on top of the piezo-scanner
explore different zones of reciprocal space. The samples were deposited on a 100µm mesh grids coated with carbon. Image acquisition was performed with a CCD camera. The data were collected at the Institute Charles Sadron in Strasburg with the assistance of Dr. Bernard. Lotz.
Fig.2.10. Philips CM12 electron microscope (top) and illustration of the image formation using the bright field and diffraction modes.
A schematic drawing of a transmission electron microscope (TEM) is shown in figure 2.10 (bottom). An electron beam produced by an electron gun is accelerated by an electric field of approximately 200kV. Using condensor lenses, the electron beam is focused down to a spot of about 100 nm on the sample to be investigated. The first image, which is formed by the objective lens, is typically magnified 25 times, while the following lenses give the final image magnification of more than one million. An important advantage of TEM is its capability to combine observations in both the direct space (bright field mode) and reciprocal space (dark field or diffraction mode). The electron beam paths corresponding to the bright field imaging and diffraction mode are shown on the left and right of the bottom panel of figure 2.9, respectively. In the bright field imaging, the image of a thin sample is formed by the unscattered electrons that pass through the film without diffraction, the diffracted electrons being stopped by a diaphragm. The contrast in such an image is entirely due to the density variations in the sample. In the dark field imaging mode, the image is formed by the diffracted beam. The diffraction pattern can give information on the atomic structure of the sample.
References:
1. "Textures of Liquid Crystals ", I. Dierking, John Wiley & Sons, 2003.
2. "Operating Instructions- Mettler Toledo, DSC 822e Module", Mettler Toledo GmbH, 2002.
3. "Exploring the High-Temperature AFM and Its Use for Studies of Polymers"
Ivanov, D.A.; Daniels, R.; Magonov, S. Application Note published by Digital Instruments/Veeco Metrology Group (2001), pp. 1-12. Available on line at URL: http://veeco.com/APPNotes_PDFs/AN45%20HeatingStage.pdf
4. Ivanov, D.A.; Amalou, Z.; Magonov, S.N. Macromolecules 2001, 34, 8944.
Chapter 3
TAILORING DISCOTIC MESOPHASES:
COLUMNAR ORDER ENFORCED WITH HYDROGEN BONDS
Reproduced from: R. I. Gearba, M. Lehmann, J. Levin, D. A. Ivanov, M. H. J.
Koch, J. Baberá, M. G. Debije, J. Piris, Y. H. Geerts, Adv. Mat., 15, 1614-1618, 2003.
In this chapter we explore the influence of hydrogen bonds on the mesophase order and charge carrier mobility. The system chosen for this study is a triphenylene derivative, hexaazatriphenylene with alkyl lateral chains. It is demonstrated that the formation of hydrogen bonds between the molecules in the column results in a significant reinforcement of the mesophase order bringing about the smallest intra-columnar distance ever found in columnar liquid crystals (3.18Å). The hydrogen bond formation is also supported by FT-IR data, but the exact proportion of intra- and inter-molecular hydrogen bonds remains unknown. The charge-carrier mobility is higher for this derivative as compared to that of other non-hydrogen bonded triphenylene derivatives (0.02 cm2/Vs).
3.1. Introduction
Considerable scientific and technological effort has been recently devoted to discotic liquid crystals as functional materials for applications as light emitting diodes,1 photovoltaic cells,2 and field effect transistors.3 Such interest can be explained by the increased charge carrier mobility in discotics as compared to conventional conjugated polymers.4 For example, record values of the charge carrier mobility up to 0.5 cm2/Vs have been obtained for self-organizing hexabenzocoronene mesogens.5 Generally, the high charge carrier mobility results from the faculty of discotics to self-organize into columns thereby maximizing the overlap of frontier orbitals. Recently, a theoretical model was proposed, in which the coplanar distance and orientation of two aromatic cores are correlated with their frontier orbital splitting, the latter governing the charge carrier mobility.6 To improve further the charge transport properties along the columns, the intermolecular distance should be made as small as possible.7 The inter-core distances for typical semiconducting columnar mesogens range from 3.5 to 4.0 Å,8 which roughly corresponds to the sum of the van der Waals radii of carbon atoms.9 Smaller inter-core distances have been reported for a tricycloquinazoline core (3.29 Å)10 and for Lutetium phthalocyanine dimers (3.26 Å).11 The latter system having the smallest intracolumnar distance also reveals the highest charge mobility in a Colh phase, which is attributed to the additional attractive interaction mediated by the Lutetium ion.
Hydrogen bonds have been used as a tool to enhance the attractive interactions between the discotic mesogens12,13 or bisurea compounds.14 More specifically, it was observed that amide hydrogen bonds serve as non- covalent “clamps” in the crystal phase of hexacarboxamidohexaazatriphenylene (1a) (figure 3.1A) and that the resulting π - π distance is as small as 3.32 Å.15 Disk-shape molecules (1b,c) with six hexyl or decyl chains were synthesized as potential mesogens by Czarnik et al., but no thermotropic behavior or structural data were reported (these molecules decompose before the transition to the isotropic phase).16 Our combined interest in electron deficient mesogens17 and in discotics with high charge carrier mobility4 led us to reinvestigate this type of molecules.
We report here on discotic mesogen HAT-CONHR, 1d, for which the formation of hydrogen bonds results in the smallest inter-disk distance (3.18 Å) ever found in columnar liquid crystals, to the best of our knowledge.
3.2. Experimental Section
Materials: Synthesis of 2,3,6,7,10,11-Hexa(N-(n-dodecyl))carboxamido- 1,4,5,8,9,12-hexaazatriphenylene (HATCONHR, 1d): The procedure of Czarnik15 leads to 44% of a colourless solid after recrystallisation from trifluoroacetic acid. The product starts to decompose at 250 °C. 1H NMR (300 MHz, CDCl3/CF3COOH) δ = 0.92 (t, 18H, CH3), 1.20-2.00 (m, 120H, CH2), 3.79 (m, 12H, NCH2), 9.73 (broad, 6H, NH). 13C NMR (75 MHz, CDCl3/CF3COOH) δ = 13.8, 22.8, 27.0, 28.6, 29.4, 29.5, 29.6, 29.8, 32.1, 42.8, 142.2, 144.8, 164.8. FT-IR (KBr) ν~ [cm-1] = 3258, 3093, 2921, 2853,
Fig.3.1. A: Molecular structure of disk-shaped compounds 1a-d.
Hexaamide 1a exhibits a small inter-disk distance of 3.32 Å in the crystal phase; compounds 1b-d are potential mesogens due to their six lateral N- alkylamide chains. B: The molecular structure of 1d (left panel) illustrates various possibilities of intra- and intercolumnar hydrogen bond arrangement. Schematic model of the supramolecular disk assembly in the ordered hexagonal columnar phase, Colho, (right panel)
1656, 1579, 1471, 1274, 1237, 1223, 732, 658. MS (MALDI-TOF) m/z (%)
= 1524.6 (100, [M+Na]+). EA (C90H156N12O6 • H2O) calculated [%] C71.10, H 10.48, N 11.06; found [%] C 71.03 , H 10.54, N 11.16.
Methods: X-ray diffraction experiments were performed on the X33 camera of the European Molecular Biology Laboratory at the storage ring DORIS III of the Deutsches Elektronen Synchrotron (DESY), Hamburg. Diffraction patterns were collected in transmission in series of frames of 10s or 6s each with two position sensitive delay line readout detectors connected in series.18,19 The sample temperature was controlled with a Mettler FP-82 HT heating stage under nitrogen flux. The temperature program employed in the experiments included heating and cooling ramps at 10°C/min. The data were normalized to the intensity of the primary beam using the SAPOKO program (Svergun and Koch unpublished). The norm of the scattering vector s (s = 2sinθ/λ, where θ is the Bragg angle and λ the wavelength λ=1.5 Å) was calibrated with tripalmitin and/or dry calcified rat tail collagen in the small s- region and benzoic acid in the high s-region.
The error on the WAXS distances was estimated by comparing positions of 8 reflections of benzoic acid (monoclinic unit cell, a = 5.511 Å, b = 5.158 Å, c= 21.714 Å, β= 97.42°) obtained in successive measurements. In particular, the (113) reflection of benzoic acid at 3.209 Å is well suited to check the precision of the c parameter of the hexagonal lattice corresponding to the intra-columnar distance, which is approx. 0.3%.20
The pulse-radiolysis time-resolved microwave conductivity technique (PR- TRMC) and method of data analysis as applied to discotic materials have been previously described in detail.21 Briefly, powder samples are compressed into a Ka-band (ca 30 GHz) microwave cell. A uniform, micromolar concentration of charge carriers is produced in the sample by a 5 nanosecond pulse of 3 MeV electrons from a Van de Graaff accelerator.
Microwaves propagating through the sample are attenuated if the charge carriers are mobile, and this is monitored as a reduction in the microwave power reflected by the cell on irradiation. The sum of the one-dimensional, intra-columnar mobilities, Σµ1D, is calculated from the dose-normalized end- of-pulse conductivity (∆σeop/D) as:
Σµ1D = 3Ep[∆σeop/D]/Weop (1)
In equation (1), Ep is the average energy absorbed in electron volts per initial ionization event, and Weop is the probability that the initially-formed electrons and holes become localized on separate columnar stacks and do not decay within the pulse. The values taken for Ep and Weop were 25 eV and
