Nonlinear Thermomechanics and Photomechanics of Liquid Crystal Polymers

• Other Authors: Wang, Hongbo (Engineer) (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Mechanical engineering
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-5256

Azobenzene liquid crystal polymers have emerged as a new type of adaptive material in recent years. The materials contain photo sensitive azobenzene mesogens that are embedded in the host polymer network which gives rise to unique field-coupled photomechanics. The complex constitutive relations of the materials are still poorly understood and of great interest to scientific communities. In this dissertation, a general nonlinear continuum framework is introduced to describe interactions between azobenzene liquid crystal evolution and mechanical deformation of polymer networks. A phase field model based a liquid crystal order parameter is developed to simulate the liquid crystal evolution. The phase field model is similar to the Landau-deGennes approach but does not use a unit liquid crystal order parameter. This is done to facilitate order-disorder material evolution. By considering rotational invariance, nonlinear coupling between the phase field model of liquid crystals and the mechanical model of polymer networks naturally forms. Based on the coupling, soft-elasticity and deformation induced liquid crystal reorientation of azobenzene liquid crystal elastomers are predicted and in good agreement with experiments. The nonlinear continuum framework is capable of incorporating more physics such as heat transfer and light propagation. Unlike phenomenological approaches commonly seen in modeling of adaptive materials, the coupling among different fields in this scheme naturally form and thus requires a small number of free parameters. The heat transfer equation is formulated based on the second thermodynamic law and Duhamel's law of heat conduction and couples with both liquid crystal microstructure evolution and a mechanical model of the polymer network. As an example, thermally induced liquid crystal phase transition and thermally induced spontaneous strain are modeled and reasonable qualitative results are obtained. Another very important extension of the nonlinear continuum scheme is the introduction of light absorption. The integration of the light model into the current scheme enables us to describe the unique photomechanics of azobenzene liquid crystal polymers. The governing equation of light attenuation through azobenzene liquid crystal polymers is derived from Maxwell's equations and a time averaged equation is also formulated to accommodate the time scale difference between liquid crystal evolution and oscillation of the electric field of blue and ultraviolet light. The coupling between liquid crystal evolution and the light attenuation equation varies when two possible photoisomerizations of azobenzene liquid crystals(trans-cis and trans-cis-trans) are considered. The integrated continuum model is applied on glassy polydomain azobenzene liquid crystal polymer films to simulate ultraviolet light induced trans-cis and blue light induced \emph{trans-cis-trans} photoisomerizations. The predicted mechanical bending caused by the two isomerizations quantitatively agrees with experimental data. The nonlinear continuum framework is relatively general and can be modified to model other field coupled materials such as tetragonal phase ferroelectric materials. The modifications include defining the polarization vector order parameter in ferroelectric materials and introducing an anisotropic free energy since only six possible directions are allowed for the polarization vector in tetragonal perovskite structures. The modified model predicts the anisotropic piezoelectric coupling in tetragonal phase lead zirconate titanate(PZT) surprisingly well, which is discovered recently in experiments. Another important part of this dissertation is to experimentally investigate light-emitting diode(LED) actuated blocked stress on glassy azobenzene liquid crystal polymers. Blue LEDs which emit almost monochromatic light have the potential to replace blue laser light sources to reduce weight, size and cost. The blocked stress induced by polarized blue LED light is dominated by thermal effects and the photochemical stress is only observed indirectly. The excessive thermal effects may be a consequence of uneven spatial distributions of LED luminous flux. Additional optics in combination with the LED may be necessary so that the light arriving at a specimen is evenly distributed and excessive thermal effects on certain regions of the specimen can be mitigated. It should also be pointed out that even though the LED tested is narrowband, the spectrum of the LED may be different from that of laser light sources. Thus quantification of the spectrum of the LED light may be important. Moreover, blocked stress relaxation of glassy azobenzene liquid crystal polymers is examined since polymers are well-known viscoelastic materials. Azobenzene liquid crystal polymers are elastic when the strain is small but viscosity exceeds elasticity when the strain increases. A standard linear solid model is used to fit stress relaxation and dynamical materials properties are also estimated. Discrepancy between the estimated results and other experimental measurements suggests that a better viscoelastic or viscoplastic model is needed to describe the observed nonlinear behavior of glassy azobenzene liquid crystal polymers. === A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Spring Semester, 2012. === October 19, 2011. === bending, blocked stress, Continuum mechanics, LED, Phase field, photo sensitive === Includes bibliographical references. === William S. Oates, Professor Directing Dissertation; John Collier, University Representative; Chiang Shih, Committee Member; Eric Hellstrom, Committee Member; Leon Van Dommelen, Committee Member.