This title appears in the Scientific Report :
2019
Please use the identifier:
http://hdl.handle.net/2128/21635 in citations.
Dynamics and phase behavior of (non-)ideal liquid crystals under shear
Dynamics and phase behavior of (non-)ideal liquid crystals under shear
As the simplest form of polymeric materials, rodlike polymers provide a unique opportunity to test and review the theory of polymer dynamics. Liquid crystalline solutions of rodlike particles are of high industrial relevance and their flow behavior during processing strongly influences the propertie...
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Personal Name(s): | Lang, Christian (Corresponding author) |
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Contributing Institute: |
Weiche Materie; ICS-3 |
Imprint: |
2019
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Physical Description: |
162 |
Dissertation Note: |
Dissertation, KU Leuven, 2019 |
Document Type: |
Dissertation / PhD Thesis |
Research Program: |
Directed Colloidal Structure at the Meso-Scale Functional Macromolecules and Complexes |
Link: |
OpenAccess OpenAccess |
Publikationsportal JuSER |
As the simplest form of polymeric materials, rodlike polymers provide a unique opportunity to test and review the theory of polymer dynamics. Liquid crystalline solutions of rodlike particles are of high industrial relevance and their flow behavior during processing strongly influences the properties of the final products.While industrially used materials are mostly non-ideal in several aspects such as flexibility and polydispersity of the particles, the theory of polymer dynamics is best applicable only to ideal rods. The attempt of this thesis is to provide new insights into the effect of such non-ideality parameters on the flow behavior of rodlike suspensions.Rodlike bacteriophages are used in this work in order to formulate materials with well-defined system characteristics, which can be alternated in a controlled way to understand non-ideal suspensions of rods. These rodlike viruses form various liquid-crystalline phases in aqueous suspension. However, here we focus on isotropic suspensions which have neither positional nor orientational order in the equilibrium state.The dynamics and phase behavior of suspensions in the isotropic state are measured under flow by means of a combination of small angle neutron scattering with rheology and heterodyne dynamic light scattering under flow. Based on the experimental outcome, the theory of rodlike polymers is reviewed.In chapter 2, a revised theory for ideal rodlike particles is derived and tested in chapter 4. In chapter 5, we test the theory against the influence of non-ideality parameters to gain a deeper understanding of the nature of these influences. Particularly, new expressions for the rotational diffusion coefficient under tube dilation and a non-equilibrium pair-correlation function are derived to supplement the Fokker-Planck equation for rods.In chapter 5, it is shown theoretically as well as experimentally that particle morphology is one of the key influences on the flow behavior of rods. In this respect, length and flexibility are two counteracting parameters. With increasing length, the dynamics of the rods slow down significantly leading to higher zero shear viscosities, while an increase of particle flexibility has the opposite effect. Furthermore, the onset of shear thinning depends crucially on the particle length. An increase in length shifts the onset of shear thinning to smaller shear-rates. In section 5.3.2, we use the understanding of this length dependence to make our theoretical predictions quantitative by experimentally determining the prefactor of the rotational diffusion coefficient in the tube model for the first time. This is very useful, as it is the basis for the understanding of other phenomena studied here and reported in literature. Due to a morphological transition to a hairpin state, an increase in flexibility causes an increase of the viscosity in the intermediate and high shear-rate regime, such that under strong flow, higher length and higher bending rigidity are both promoting shear thinning.In sections 4.3.1 and 5.2, small amplitude oscillatory shear is used to demonstrate that the rotational diffusion and the particle flexibility crucially influences the quasi-linear flow behavior of rods. It is found that, not unlike polymers, rods of finite stiffness possess a relaxation time spectrum, see section 5.5.Extensional flow measurements are conducted to demonstrate the effect of flexibility in the highly non-linear flow regime, see section 5.4. It is found that an increase in particle flexibility leads to a decrease in extensional viscosity. The Trouton ratios of rodlike systems are shown to be comparatively large despite of low normal stresses.In section 5.6, it is demonstrated that the zero shear behavior of polydisperse rodlike particle suspensions does not involve higher complexities, while the shear thinning behavior becomes very complex and, therefore, cannot be understood by employing linear mixing rules in the governing equations for particle dynamics.Finally, it is shown in section 5.7 that a high enough length of rods is crucial for a gradient shear banding transition to occur. Also, it is demonstrated that none of the systems under investigation undergo stable gradient shear banding. |