Memory Effects in Monodomain Liquid Crystal Elastomers Produced in Magnetic Fields

Magnetic fields have been used to produce macroscopically oriented side-chain liquid crystal polymer films which were subsequently cross-linked to form monodomain liquid crystal elastomers. The orientation process is shown to be complete in minutes, in a modest magnetic field while the relatively slow cross-linking reaction is complete in around 60 hours. Films thus obtained show a memory of the state of order at cross-linking even after prolonged annealing in the isotropic phase. We find that the uniform memory is lost when the elastomer contains > 50% free chains by incorporating a controlled percentage of uncross-linkable homopolymer. We have thus lowered the effective modulus and looked in more detail at the coupling between mesogen and polymer backbone.