Free-standing smectic liquid crystal elastomer films [Elektronische Ressource] / von Victor Aksenov

Victor Hugo

Sujets

• Physik

Informations

• Publié par: otto-von-guericke-universitat_magdeburg
• Publié le: 01 janvier 2010
• Nombre de lectures: 22
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Free-standing smectic liquid crystal elastomer films


Dissertation

zur Erlangung des akademischen Grades


doctor rerum naturalium
(Dr. rer. nat.)


genehmigt durch die Fakultät für Naturwissenschaften
der Otto-von-Guericke-Universität Magdeburg




von M. Sc. Victor Aksenov

geb. am 22.09.1979 in Blagoweshensk, Russland





Gutachter: Prof. Dr. Ralf Stannarius
Prof. Dr. Eugene Terentjev




eingereicht am: 31.07.2009

verteidigt am: 22.02.2010 Contents


Contents

Motivation ............................................................................................................................................ 2
Chapter 1 Introduction ......................................................................................................................... 5
1.1 Liquid crystals ...................................................................................................................... 5
1.2 Liquid crystalline polymers and elastomers ......................................................................... 9
1.2.1 Types of LC polymers .................................................................................................. 9
1.2.2 Conformation of a polymeric backbone in LC polymers and elastomers .................. 11
1.2.3 Orientation of LC polymers by external forces .......................................................... 13
1.2.4 Orientation of LC elastomers by mechanical deformation ........................................ 14
1.2.5 Anisotropy of the nematic mono-domain deformation .............................................. 16
1.2.6 Anisotropy of the smectic mmation 16
Chapter 2 Experimental methods ....................................................................................................... 20
2.1 Compounds and their phase behaviour .................................................................................... 20
2.2 Preparation of the free standing elastomer films ...................................................................... 21
2.3 Preparation of the free standing elastomer balloons ................................................................ 23
2.4 Optical microscopy .................................................................................................................. 25
2.5 Optical reflectometry ................................................................................................................ 26
2.6 X-ray diffraction ....................................................................................................................... 29
2.6.1 Experiments on non-oriented material .............................................................................. 29
2.6.2 Experiments on oriented samples ...................................................................................... 29
2.7 Polarized Fourier transform infrared spectroscopy .................................................................. 30
2.7.1 The Michelson Interferometer ........................................................................................... 30
2.7.2 Dependence of the IR light absorbance on a polarization angle. ...................................... 31
2.7.3 Experimental setup ............................................................................................................ 33
Chapter 3 Mechanical Properties ....................................................................................................... 34
3.1 Smectic elastomer balloons ...................................................................................................... 35
3.2 Uniaxial stretching of free-standing elastomer films ............................................................... 41
Chapter 4 Optical reflectometry measurements ................................................................................. 48
4.1 Cross-linking of free-standing films ........................................................................................ 48
4.2 Strain-induced compression of free-standing elastomer films ................................................. 52
4.3 Influence of crosslinking time on deformational behaviour of free-standing elastomer films 59
4.4 Conclusions .............................................................................................................................. 64
Chapter 5 X-ray measurements .......................................................................................................... 67
5.1 X-ray measurements of the temperature dependence of the smectic layer thickness .............. 67
5.2 Smectic layer compression in free-standing LCE films stretched parallel to the smectic layer
plane ............................................................................................................................................... 80
Chapter 6 Polarized FTIR spectroscopy measurements ..................................................................... 90
6.1 Polarized FTIR spectroscopy measurements of the mesogens orientation in deformed
elastomer film ................................................................................................................................. 91
Discussion and Conclusions ............................................................................................................. 100
Bibliography ..................................................................................................................................... 109
1Motivation


Motivation

Liquid crystalline elastomers (LCEs) combine rubber elasticity with anisotropic optical,
mechanical and electrical properties of liquid crystals. The polymeric network couples the
orientation of mesogenic groups with the mechanical deformation of the elastomer. For instance,
mechanical deformation of a nematic LCE, perpendicular to the director, can change the orientation
of mesogenic molecules [1, 2]. Deformation of a LCE parallel to the director can change the
orientational order parameter of mesogens [3], and vice versa, changing of the orientational order
parameter at the isotropic-smectic or isotropic-nematic phase transition can lead to a macroscopic
deformation [4-6]. Moreover, the orientation of mesogens in some LCEs can be changed by an
external electric field, and consequently the shape of these elastomers can be manipulated by an
electric field [7-9]. Even such LCEs have been synthesized in which the nematic-isotropic phase
transition can be induced by a light irradiation and their shape can be controlled just by a light
irradiation [10, 11].
It has to be noted that mechanical properties of LCEs are usually quite different from those
of conventional isotropic rubbers and highly anisotropic [3, 12-14]. An additional internal degree of
freedom in nematic elastomers, the rotation of the director, makes possible a so-called soft or semi-
soft elasticity [15-17], originally proposed by Golubovic and Lubensky [18]. An ideal soft
deformation can be performed without the change of the free-energy of an elastomer and hence at
zero stress. However, in reality, a very low but non-zero stress is required for this deformation, in
this case one can speak of a semi-soft elasticity [17, 19].
The deformation of a smectic elastomer parallel to the smectic layers is opposed by the
elastic modulus of the same order as in a conventional isotropic rubber [6, 12]. In this case smectic
layers don’t influence the elastic modulus and it’s defined only by the entropy elasticity of
polymeric chains μ ~ n k T, where n is the number of chain strands per unit volume. S B S
If deformation is perpendicular to the smectic layers, in addition to the elasticity of
polymeric chains, the elastic energy of a deformed elastomer has to include term which is enthalpic
in nature and proportional to the layer compression modulus. Experimental measurements confirm
that for some smectic elastomers, elastic moduli perpendicular to the layers can be one or two order
of magnitude higher than moduli parallel to the layers [6, 12]. But the anisotropy of elastic moduli
isn’t always so high, for some smectic elastomers the elastic moduli perpendicular to the layers just
four times larger than the moduli parallel to the layers have been measured [14, 20]. Motivation


Due to the coupling of microscopic parameters with the conformation of a polymeric
network and therefore with the shape of a sample, LCEs are perspective candidates for sensor [21]
and actuator applications and they even were proposed as materials for the creation of artificial
muscles [22-24]. These unique properties of LCEs made them an object of intensive investigations.
Elastic moduli, deformational behaviour and the temperatures of the phase transitions of LC
elastomers depend on many factors, such as; the density of crosslinkers [25], the temperature of
crosslinking (in which phase the polymeric network was crosslinked ), orientational order of the
mesogens, which in its turn depends on temperature and external forces such as electric and
magnetic fields and mechanical deformations.
The properties of nematic elastomers are much better understood than the propert