Nematic elastomers: The influence of external mechanical stress on the liquid‐crystalline phase behavior
ABSTRACT The influence of external mechanical stress on the nematic-isotropic phase transformation of nematic elastomers was investigated. The experimental results of IR-dichroism measurements in the nematic phase and stress-optical measurements in the isotropic phase are in good agreement with the theoretical predicitions of the phenomenological Landau-de Gennes theory. This is for the first time that a significant influence of an external field on the nematic-isotropic phase transformation temperature and on the nematic order parameter S has been proved.
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ABSTRACT: Soft materials have attracted much scientific and technical interest in recent years. In this thesis, attention has been placed on the underpinning relations between molecular structure and properties of one type of soft matter - main chain liquid crystalline elastomers (MCLCEs), which may have application as shape memory or as auxetic materials. In this work, a number of siloxane-based MCLCEs and their linear polymer analogues (MCLCPs) with chemical variations were synthesized and examined. Among these chemical variations, rigid p-phenylene transverse rod and flat-shaped anthraquinone (AQ) mesogenic monomers were specifically incorporated. Thermal and X-ray analysis found a smectic C phase in most of our MCLCEs, which was induced by the strong self-segregation of siloxane spacers, hydrocarbon spacers and mesogenic rods. The smectic C mesophase of the parent LCE was not grossly affected by terphenyl transverse rods. Mechanical studies of MCLCEs indicated the typical three-region stress-strain curve and a polydomain-to-monodomain transition. Strain recovery experiments of MCLCEs showed a significant dependence of strain retentions on the initial strains but not on the chemical variations, such as the crosslinker content and the lateral substituents on mesogenic rods. The MCLCE with p-phenylene transverse rod showed a highly ordered smectic A mesophase at room temperature with high stiffness. Mechanical properties of MCLCEs with AQ monomers exhibit a strong dependence on the specific combination of hydrocarbon spacer and siloxane spacer, which also strongly affect the formation of ð-ð stacking between AQ units. Poisson s ratio measurement over a wide strain range found distinct trends of Poisson s ratio as a function of the crosslinker content as well as terphenyl transverse rod loadings in its parent MCLCEs. Ph.D. Committee Chair: Anselm C. Griffin; Committee Member: David M. Collard; Committee Member: John D. Muzzy; Committee Member: Mohan Srinivasarao; Committee Member: Satish Kumar01/2007;
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ABSTRACT: We develop a continuum theory for the mechanical behavior of rubber-like solids that are formed by the cross-linking of polymeric fluids that include nematic molecules as elements of their main-chains and/or as pendant side-groups. The basic kinematic ingredients of this theory are identical to those arising in continuum-level theories for nematic fluids: in addition to the deformation, which describes the trajectories of material particles, an orientation, which delineates the evolution of the nematic microstructure, is introduced. The kinetic structure of our theory relies on the precept that a complete reckoning of the power expended during the evolution of a continuum requires the introduction of forces that act conjugate to each operative kinematic variable and that to each such force system there should correspond a distinct momentum balance. In addition to conventional deformational forces, which expend power over the time-rate of the deformation and enter the deformational (or linear) momentum balance, we, therefore, introduce a system of orientational forces, which expend power over the time-rate of the orientation and enter an additional orientational momentum balance. We restrict our attention to a purely mechanical setting, so that the thermodynamic structure of our theory rests on an energy imbalance that serves in lieu of the first and second laws of thermodynamics. We consider only nematic elastomers that are incompressible and microstructurally inextensible, and a novel aspect of our approach concerns our treatment of these material constraints. We refrain both from an a priori decomposition of fields into active and reactive components and an introduction of Lagrange multipliers; rather, we start with a mathematical decomposition of the dependent fields such as the deformational stress based on the geometry of the constraint manifold. This naturally gives rise to active and reactive components, where only the former enter into the energy imbalance because the latter automatically expend zero power in processes consistent with the constraints. The reactive components are scaled by multipliers which we take to be constitutively indeterminate. We assume constitutive equations for the active components, and the requirement that these equations be consistent with the energy imbalance in all processes leads to the active components being determined by an energy density response function of the deformation gradient, the orientation, and the orientation gradient. We formulate the requirements of observer independence and material symmetry for such a function and provide, as a specialization, an expression that encompasses the energy densities used in the Mooney-Rivlin description of rubber and the Oseen-Zöcher-Frank description of nematic fluids.Journal of Elasticity 56(1):33-58. · 1.04 Impact Factor