Coupling of the electrical, mechanical and optical response in polymer,liquid-crystal composites [Elektronische Ressource] / presented by Lakshmi Meena Ganesan
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Coupling of the electrical, mechanical and optical response in polymer,liquid-crystal composites [Elektronische Ressource] / presented by Lakshmi Meena Ganesan

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University of PotsdamApplied Condensed-Matter PhysicsCoupling of the electrical, mechanical and opticalresponse in polymer/liquid-crystal compositesDissertationin partial fulifillment of therequirements of the degree ofDoctor of Natural Sciences(Dr.rer.nat.)in Applied Materials Physicssubmitted to theFaculty of Mathematics and Natural Sciencesof the University of Potsdampresented byLakshmi Meena GanesanPotsdam, March 4, 2010 Published online at the Institutional Repository of the University of Potsdam: URL http://opus.kobv.de/ubp/volltexte/2010/4157 URN urn:nbn:de:kobv:517-opus-41572 http://nbn-resolving.org/urn:nbn:de:kobv:517-opus-41572 to my mother Mangaleswari and brother Saravanan StatementLakshmi Meena Ganesan,student matric. no. 733464I, Lakshmi Meena Ganesan, formally submit my thesis “Coupling of the elec-trical, mechanical and optical response in polymer/liquid-crystal composites”in fulfillment of the requirements set forth by the Regulations for awarding the title “doc-tor rerum naturalium” (Dr. rer. nat.) in the Mathematics-Natural Science Faculty of theUniversity of Potsdam.I declare that the work presented in this thesis has not been submitted as an exercisefor a degree to any other university.Theworkdescribedhereinisentirelymyown,exceptfortheassistancementionedintheacknowledgmentsandcollaborativeworkmentionedinthelistofpublications.

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Publié le 01 janvier 2010
Nombre de lectures 20
Langue English
Poids de l'ouvrage 20 Mo

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University of Potsdam
Applied Condensed-Matter Physics
Coupling of the electrical, mechanical and optical
response in polymer/liquid-crystal composites
Dissertation
in partial fulifillment of the
requirements of the degree of
Doctor of Natural Sciences
(Dr.rer.nat.)
in Applied Materials Physics
submitted to the
Faculty of Mathematics and Natural Sciences
of the University of Potsdam
presented by
Lakshmi Meena Ganesan
Potsdam, March 4, 2010












































Published online at the
Institutional Repository of the University of Potsdam:
URL http://opus.kobv.de/ubp/volltexte/2010/4157
URN urn:nbn:de:kobv:517-opus-41572
http://nbn-resolving.org/urn:nbn:de:kobv:517-opus-41572 to my mother Mangaleswari and brother Saravanan Statement
Lakshmi Meena Ganesan,
student matric. no. 733464
I, Lakshmi Meena Ganesan, formally submit my thesis “Coupling of the elec-
trical, mechanical and optical response in polymer/liquid-crystal composites”
in fulfillment of the requirements set forth by the Regulations for awarding the title “doc-
tor rerum naturalium” (Dr. rer. nat.) in the Mathematics-Natural Science Faculty of the
University of Potsdam.
I declare that the work presented in this thesis has not been submitted as an exercise
for a degree to any other university.
Theworkdescribedhereinisentirelymyown,exceptfortheassistancementionedinthe
acknowledgmentsandcollaborativeworkmentionedinthelistofpublications. Thepresent
thesisworkwascompletedwithinthe“AppliedCondensed-MatterPhysics”(ACMP)group
at the Department of Physics in University of Potsdam.
Lakshmi Meena Ganesan
Potsdam, March 4, 2010
iAbstract
Micrometer-sized liquid-crystal (LC) droplets embedded in a polymer matrix may enable
optical switching in the composite film through the alignment of the LC director along an
external electric field. When a ferroelectric material is used as host polymer, the electric
fieldgeneratedbythepiezoelectriceffectcanorientthedirectoroftheLCunderanapplied
mechanical stress, making these materials interesting candidates for piezo-optical devices.
Inthiswork, polymer-dispersedliquidcrystals(PDLCs)arepreparedfrompoly(vinylidene
fluoride-trifluoroethylene) (P(VDF-TrFE)) and a nematic liquid crystal (LC). The anchor-
ing effect is studied by means of dielectric relaxation spectroscopy. Two dispersion regions
are observed in the dielectric spectra of the pure P(VDF-TrFE) film. They are related to
the glass transition and to a charge-carrier relaxation, respectively. In PDLC films con-
taining 10 and 60 wt% LC, an additional, bias-field-dependent relaxation peak is found
that can be attributed to the motion of LC molecules. Due to the anchoring effect of the
LC molecules, this relaxation process is slowed down considerably, when compared with
therelatedprocessinthepureLC.Theelectro-opticalandpiezo-opticalbehaviorofPDLC
films containing 10 and 60 wt% LCs is investigated. In addition to the refractive-index
mismatch between the polymer matrix and the LC molecules, the interaction between the
polymerdipolesandtheLCmoleculesatthedropletinterfaceinfluencesthelight-scattering
behavior of the PDLC films. For the first time, it was shown that the electric field gener-
ated by the application of a mechanical stress may lead to changes in the transmittance
of a PDLC film. Such a piezo-optical PDLC material may be useful e.g. in sensing and
visualization applications. Compared to a non-polar matrix polymer, the polar matrix
polymer exhibits a strong interaction with the LC molecules at the polymer/LC interface
which affects the electro-optical effect of the PDLC films and prevents a larger increase in
optical transmission.Kurzfassung
Mikrometer-große, in eine Polymermatrix eingebettete Fluss¨ igkristall-Tr¨opfchen k¨onnen
als elektro-optische Lichtventile fungieren, da die Ausrichtung der Flus¨ sigkristalle durch
ein externes elektrisches Feld ver¨andert werden kann. Wird nun ein ferroelektrisches Poly-
mer als Matrix verwendet, so kann das durch den piezoelektrischen Effekt erzeugte und
von der ¨außeren mechanischen Spannung abh¨angige elektrische Feld den Fluss¨ igkristall
ausrichten. Solche Materialien konnen¨ daher als piezo-optische Lichtventile eingesetzt wer-
den. Im Rahmen dieser Arbeit wurden PDLCs (polymer-dispersed liquid crystals) durch
Einbettung von nematischen Flus¨ sigkristallen in Poly(Vinylidenfluoride-Trifluorethylene)
(P(VDF-TrFE))erzeugt. DieWechselwirkungenanderGrenzfl¨achezwischenFluss¨ igkristall
und Polymer wurden mittels dielektrischer Spektroskopie untersucht. Im dielektrischen
Spektrum des reinen P(VDF-TrFE) wurden zwei Dispersions-Regionen beobachtet, welche
vom Glasub¨ ergang und einer Ladungstrag¨ errelaxation des Polymers herruhren.¨ PDLC-
Folien mit unterschiedlichen Anteilen von Flus¨ sigkristall-Tr¨opfchen (10 bzw. 60 Gewicht-
sprozente) zeigten beim Anlegen eines elektrischen Wechselfelds zus¨atzliche Relaxation-
seffekte, welche der Bewegung der eingebetteten Flus¨ sigkristall-Molekule¨ zugeordnet wer-
den konnten. Durch die Einlagerung der Flussigkr¨ istall-Molekule¨ weist die Struktur eine
Relaxation auf, die gegenub¨ er vergleichbaren Prozessen im reinen Flu¨ssigkristall deutlich
verlangsamt ist. Des weiteren wurde das elektrooptische und piezo-optische Verhalten der
mit 10 und 60 Gewichtsprozent Fluss¨ igkristall geladenen Folien untersucht. Die Licht-
streuung h¨angt dabei ab von der Fehlanpassung der Brechungsindizes von Polymerma-
trix und Flus¨ sigkristallen sowie von den Wechselwirkungen der Polymerdipole mit den
Flussigkrista¨ ll-Molekulen¨ an der Tr¨opfchenoberfl¨ache. Es konnte erstmalig gezeigt werden,
dass die Lichtdurchl¨assigkeit der PDLC-Folien durch eine externe mechanische Spannung
gesteuertwerdenkann. DieserEffektmachtdaspiezo-optischePDLC-Materialfur¨ dieVer-
wendunginOptik-undSensoranwendungeninteressant. ImVergleichmitunpolarenWirt-
spolymerenzeigenpolareWirtsmaterialieneinedeutlichst¨arkereWechselwirkungzwischen
den Fluss¨ igkristall-Molekule¨ n an der Polymer/Flussigkrista¨ ll-Grenzfl¨ache, welche den elek-
trooptischen Effekt beeinflusst und so die maximale Transmissionsa¨nderung reduziert.Acknowledgements
I would like to convey my profound thanks and gratitude to Prof. Dr. Reimund Gerhard
for giving me an opportunity to work in his group and for his support and encouragement.
I endow my deep sense of gratitude to Prof. Dr. Axel Mellinger for helping me at every
step of the project through his guidance, support and encouragement. I would like to
thank Merck Chemicals, Germany for providing the liquid crystals which were used in this
work. I would like to express my heart felt gratitude to Dipl. -Ing. Werner Wirges for
giving valuable advice during technical difficulties, suggestions and helping me to build
the piezo-optical measurement setup. I would like to thank Dr. Peter Frubing¨ for helping
me immensely with his valuable guidance throughout the project. I would like to thank
Dipl. -Ing. Andreas Pucher for his help to build the new experimental setup. I endow
my sincere thanks to all the ACMP group members for their extensive support and co
operation in completing the project. Especially I would like to thank Dipl. -Ing. Monika
Ehlert for the DSC measurements and Dr. Brigitte Tiersch for the SEM images. I would
like to convey my thanks to Sandra Zeretzke for her friendship, support and assistance
with the administrative work. My heart-felt thanks to Dr. Frank Jaiser for helping me
to do absorption measurements. I endow my sincere thanks to Dr. Denis Mc Carthy for
reviewing my thesis and to Matthias Kollosche for translating abstract. Last but not least,
I would like to thank Rosy, Debby, Lekha, Kavitha, Tong and Huly¨ a for their love, care
and support throughout my stay in Berlin.
iiContents
Glossary vi
1 Introduction 1
1.1 Liquid crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Liquid-crystal displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Polymer-dispersed liquid crystals (PDLCs) . . . . . . . . . . . . . . . . . . 4
1.4 Ferroelectric polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 PDLCs with ferroelectric hosts. . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Sample preparation 9
3 Differential scanning calorimetry (DSC) 12
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3 DSC diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4 StepScan DSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.5 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 Dielectric relaxation spectroscopy (DRS) 16
4.1 Types of polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.2 Models for ideal and real systems . . . . . . . . . . . . . . . . . . . . . . . 17
4.3 Dielectric relaxations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.4 Experimental setup and measurement . . . . . . . . . . . . . . . . . . . . . 19
5 Infrared (IR) spectroscopy 22
5.1 Dispersive infrared spectrometer . . . . . . . . . . . . . . . . . . . . . . . . 22
5.2 Fourier-transform infrared spectrometer . . . . . . . . . . . . . . . . . . . . 23
5.3 Attenuated total reflection infrared spectrometer . . . . . . . . . . . . . . . 24
5.4 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6 Thermally stimulated current (TSC) 26
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2 Global and windowing polarization method . . . . . . . . . . . . . . . . . . 26
iiiCONTENTS
6.3 Bucci equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.4 Experimental setup and measurement . . . . . . . . . . . . . . . . . . . . . 28
7 Electrical hysteresis 30
7.1 Electric poling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
7.2 Experimental setup and measurement . . . . . . . . . . . . . . . . . . . . . 32
8 Electro-optical behavior 34
8.1 Light scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
8.2 Light-sca models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.2.1 Anomalous diffraction approximation . . . . . . . . . . . . . . . . . 35
8.2.2 Rayleigh-Gans approximation . . . . . . . . . . . . . . . . . . . . . 35
8.3 Light scattering of high-droplet-density films . . . . . . . . . . . . . . . . . 36
8.4 Factors affecting light scattering . . . . . . . . . . . . . . . . . . . . . . . . 36
8.4.1 Refractive index of the droplet . . . . . . . . . . . . . . . . . . . . . 36
8.4.2 Droplet configuration . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8.4.3 Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8.5 Characterization of electro-optical behavior . . . . . . . . . . . . . . . . . . 38
8.6 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
9 Piezo-optical measurement 39
9.1 Conceptual background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
9.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
10 Results and Discussion 43
10.1 Thermal behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
10.1.1 Liquid crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
10.1.2 Comparison of commercial and laboratory-prepared
P(VDF-TrFE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
10.1.3 Comparison of P(VDF-TrFE) and PDLC . . . . . . . . . . . . . . . 44
10.2 Optical micrographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
10.3 Scanning electron microscope images . . . . . . . . . . . . . . . . . . . . . 47
10.4 Dielectric relaxation behavior . . . . . . . . . . . . . . . . . . . . . . . . . 48
10.4.1 Liquid crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
10.4.2 Commercial P(VDF-TrFE) . . . . . . . . . . . . . . . . . . . . . . . 51
10.4.3 Laboratory-prepared P(VDF-TrFE) . . . . . . . . . . . . . . . . . . 52
10.4.4 Comparison of commercial and laboratory-prepared
P(VDF-TrFE) films . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
10.4.5 PDLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
10.4.6 Comparison of P(VDF-TrFE) and PDLC film . . . . . . . . . . . . 56
10.4.7 Arrhenius plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
10.4.8 Influence of bias field on the PDLC film . . . . . . . . . . . . . . . 59
10.5 Fourier-transform infrared-spectroscopy . . . . . . . . . . . . . . . . . . . . 60
iv