The Viscoelastic Response of Liquid Crystalline Fibers Formed By Bent-core Molecules

From Microscopic Ordering to Macroscopic Behavior

by Oliver Herbert Kress
Doctoral thesis
Date of Examination:2018-11-23
Date of issue:2019-11-20
Advisor:Dr. Tanya Ostapenko
Referee:Prof. Dr. Jörg Enderlein
Referee:Prof. Dr. Simone Techert
Persistent Address: http://dx.doi.org/10.53846/goediss-7733

 

 

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Abstract

English

A free-standing fluid fiber must suppress the radial fluctuations described by the Rayleigh- Plateau (R-P) instability. The stability of such structures are typically restricted to polymers or naturally-occurring spider silk. The first exception was published in 1987 when Van Winkle et al. reported that thin films formed by discotic mesogens tended to collapse into thin, stable strands [1]. Nearly two decades passed before the field of liquid crystals reported its second example of freestanding fibers. Thanks to the work of J´akli et al. in 2003, it is now known that the ferroelectric SmCsPf (B7) phase formed by certain mesogens known as bent-core liquid crystals (BLCs) is capable of spontaneously forming stable microfibers when subjected to uniaxial extension [2]. These fibers stabilize against R-P fluctuations by means of a layered (smectic) structure which provides the necessary radially-stabilizing compression modulus [3]. While previous studies focused primarily on measuring the static tension of both single fibers and fiber bundles to better understand their structure and stability, their dynamic response was not investigated in convincing detail until a 2016 publication by Salili et al. reported on their rupture and recoil behavior [4, 5]. In the present work, specially designed glass capillaries, piezo positioning, a custom heat stage and specialized imaging and control software were combined to develop a compact, nanoextensional rheometer currently undergoing final European patent approval. Experiments to investigate the strain- and strain-rate dependent stress response and the induced structural (phase) changes of these fibers were conducted alongside theoretical work to gradually formulate a generalized viscoelastic model. An improved understanding of the interplay between the viscous and elastic responses of these fibers – both in the bulk and at the surface – will help to uncover ways in which their complex internal geometry can be manipulated by external stimuli such as electric fields or temperature fluctuations in order to achieve actuation in the form of controlled length contraction and dilation. In addition, polymer stabilization and the use of BLCs containing an azo group to enhance the stability and actuation behavior under UV irradiation, respectively, are under continuing investigation. It is expected that such soft, self-healing fiber actuators could provide a key advancement to the fields of robotics, biomedical and cybernetic engineering.
Keywords: bent-core; liquid crystals; artificial muscles; soft actuators; viscoelastic; extensional rheology; nano-rheometer; liquid crystal fibers
 
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