Synand characterisation of silicone based mesophase forming materials.

The aim of this project was to synthesise and characterise a range of novel cyclic and linear mesogenic siloxanes. A synthetic strategy based upon the synthesis of linear side-chain siloxanes was adopted. End-functionalised linear siloxane precursors were synthesised via ring opening polymerisation reactions. The coupling of mesogen and siloxane was carried out via a hydrosilylation reaction in solution. Products were isolated by phase-separation and gel permeation chromatography (GPC).Thermotropic behaviour was studied using optical microscopy, differential scanning calorimetry (DSC) and, in some cases, X-ray diffraction. An overview of the lyotropic behaviour of the amphiphiles was obtained using the penetration technique. The principles explaining the thermotropic behaviour of conventional amphiphiles appear to be generally applicable to these amphiphilic siloxanes. Thus, the amphiphiles aggregate in the neat state. The shape of these aggregates is determined primarily by packing constraints. The amphiphiles undergo a step-wise melting process, with the non-polar and the oolar moieties dominating the low and the high temperature transitions, respectively. The behaviour of different molecules can be explained by reference to the nature of the respective polar and non-polar moieties.The aqueous lyotropic phase behaviour of NaD4 appears to be similar to that of sodium myristate. Thus, the effect of attaching amphiphiles to a siloxane chain is to extend the non-polar chain of the amphiphile by approximately three methylene units. The linear amphiphiles did not form any aqueous lyotropic mesophases. This was explained in terms of the reversed micelle structure, which was dictated by packing constraints. There was no mesophase behaviour in non-polar solvents due to the strong forces of attraction between polar groups. The model considerations applied to non-amphiphilic side-chain polymers appear to applicable to the non-amphiphilic cyclics studied here. Thus, suitable flexible spacer groups will decouple the motions of mesogenic side-chains from those of an oligomeric cyclic backbone such that the mesogens may align. However, more efficient spacers are required to decouple the motions and steric effects of the cyclic backbone than is the case with the equivalent linear backbones.