Thesis abstract

Thesis Title

(En) A multiscale analysis and design of the light-responsive liquid crystal polymer using nonlinear finite element shell model

(Ko) 비선형 쉘 유한요소모델에 기반한 광반응 액정 고분자의 멀티스케일 해석 및 설계


The term, liquid crystal polymer (LCP), broadly refers to a hybrid structure where short and rigid liquid crystal molecules are incorporated within long and flexible polymeric chain networks. Interestingly, the combination of these two classical components has been proven to generate the coupled behaviors that render the LCP as a novel, smart material

the stimuli-responsive phase change (e.g., thermotropic) of the chromophores is reflected to the conformation of the polymer and changes the macroscopic shape of the LCP. It was recently revealed that the LCP can also be reversely actuated by light, given that the chromophores contain light-sensitive structures such as azo-benzenes. In this way, many applications are envisaged and realized, including light-driven mechanical mechanisms such as actuators, sensors, propellers, and even tweezers.
However, much of our knowledge regarding these anomalous spontaneous mechanisms is largely driven by experiments and simple analytic models because of the complex interplays between distinctive physics: light-LC, phase change-polymeric conformation, and microstate-to-macroscopic deformation. Therefore, there is a dire need for a framework that considers these distinctive physics, as well as the interdisciplinary interactions that emerge at the vicinity. To this end, this dissertation proposes a multiscale analysis framework for the photomechanical behavior of LCP. This consists of nonlinear finite element analysis and in-silico experiments to advance our understanding of the microscopic nature of LCPs.
In the first part of the dissertation, the theoretical bases found in the proposed multiscale analysis are described in depth. The present work employs finite element analysis as the solution of the photomechanical system that is equivalent to finding stress-free configurations of the LCP structures under various internal stresses that are induced by light. Herein, uniaxial liquid crystal (LC) configurations are assumed
this encompasses rotational symmetry (i.e., nematic) as well as translational symmetry (i.e., smectic). Hence, a variationally consistent constitutive equation that couples the stimuli to the stress-strain relation is described. Furthermore, in contrast to existing finite element analysis on LCP, which assumes a global linearity to simplify the problem, two sources of the nonlinearity—geometric nonlinearity and nonlinear thermomechanical behaviors—are considered. First, geometric nonlinearity is included in the model because many of the observed light-induced deflections undergo a large displacement, yet their local strains remain in the infinitesimal range. An element independent corotational formulation is utilized to consider such nonlinearity, which is saliently beneficial for both the computation and further sensitivity analysis. A molecular dynamics simulation is also undertaken in order to reveal the unprecedented nonlinearity accompanied by phase change found in the crosslinked mesogens. The fidelity of the present multiscale solutions is examined with available experiments.
In the second part of the dissertation, the possible extension of the multiscale framework to the design of LCP photo actuations is exemplified by facilitating the multi-scale nature of the material, which is the combination of microscale properties, such as the local alignment of LC, and macroscopic properties, such as the shape of the LCP or the distributions of the stimuli. The proposed results are categorized into modifications of the extrinsic (post-crosslinking) variables and the intrinsic (pre-crosslinking) variables. The influence that each variable has on the deformation is described and discussed for the first time by examining the sensitivity towards the stimuli. With regards to the extrinsic variables, the various directions of the uniaxial orientation of the LCP are studied for the first time, and the resulting change in the light-induced principal curvature direction is shown. Envisaged by the possible high-fidelity light control, a light-patterning schematic is also proposed to achieve the desired shape change. A topology optimization method, which was originally devised to compute lightweight and load-sustaining structures, is employed to compute the discrete light patterns that drive the LCP to become a desired shape specified a priori. In view of the intrinsic variables, the distorted textures of nematic LC are examined, which are possibly obtained using novel alignment techniques. An LCP with twisted nematic configuration is studied and compared to existing works based on either analytic calculations or experiments. The arbitrary textures prescribed to the LCP surface are also simulated to show the exotic shape change that consists of many hills-and-valley configurations and to determine their ability to induce photo-generated instability.
In this regard, the proposed model could possibly provide an efficient and consistent framework in which to analyze LCP behavior with complex internal structures and combined stimuli. Hence, the design of novel mechanical elements driven by light is facilitated whenever large, complex, and precise manipulation is valued over load-carrying capability.