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Title: Statistical models of elasticity in main chain and smectic liquid crystal elastomers
Author: Adams, J. M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2005
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In the strongly nematic state main chain liquid crystalline polymers have hairpin defects along their length. When these chains are cross-linked together they show unusual elastic properties experimentally. The elastic properties of a main chain elastomer are modelled here by calculating the stiffness of chains with hairpin defects and of those without. The dramatically different spring constants motivate a non-affine model for deformation of the resulting elastomer. The chains with hairpin defects are less stiff than those without and so take up more of the macroscopic strain. As the elastomer is stretched the macroscopic strain becomes more concentrated in the elastically weaker hairpinned chains, and so the rubber shows a plateau in its stress-strain curve. A mechanisms of developing a polarisation in chiral main chain LCE is analysed. In this mechanism the dipoles of the chiral monomers can be aligned by a shear deformation. It is shown that the polarisation of a pure LCE is zero in equilibrium due to rotation of the detector. The response of the director must be altered in a specific way in order to realise a non-zero result. Three methods of circumventing this result are explored: oscillating shear, pinning the detector with smectic layers, and using a mixture of chiral and non-chiral chains. Each of these methods is shown to produce a polarisation which is much larger per unit stress than that of quartz crystal. A fully non-linear model of elasticity in smectic A elastomers is developed from a phantom network model. The rigid constraints required by the layered smectic system are analysed from a geometric perspective. This model is then used to look for soft modes in biaxial smectic. A elastomers and smectic C elastomers. A general procedure for the calibration of soft modes is developed and specific examples of soft modes given.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available