Phase transitions of lyotropic liquid crystalline polymers: Effect of fluctuations and disorder
Abstract (Summary)When a solution of rod-like polymers is subjected to a reduction in solvent quality, it is theoretically predicted that a biphasic system will result. This is not what is observed however as the result is gelation; the formation of a three dimensional self-supporting polymer rich network. The lyotropic system of poly ($\gamma$-benzyl l-glutamate) (PBLG) in benzyl alcohol (BA) has been studied on a molecular level as the system is brought from the high temperature phases towards the gel in hopes of understanding the processes that occur that result in gelation. Low angle light scattering and small angle neutron scattering were used to monitor the configuration of the PBLG molecule as the system was brought from the isotropic to gel phase and to study the gel itself. Quasi-elastic neutron scattering was utilized to measure the local dynamics of the PBLG molecule as the system is brought from the cholesteric to the gel phase. The results show that an aggregation of PBLG molecules exists up to 80$\sp\circ$C in the isotropic phase and the size of the aggregate changes little as the gelation threshold is approached. As gelation occurs, the size of the aggregate increase substantially. The thermal history of the solution in the isotropic phase also affects the local structure of the resultant gel. Short annealing times result in an open structure similar to an aggregate that is formed via the clustering of clusters mechanism and longer annealing times allow the aggregate to relax and results in a denser, more compact structure. Quasi-elastic neutron scattering demonstrated that the local dynamics of the polymer are continuous and unhindered in the cholesteric phase, but become more constrained and jump like as the system enters the gel phase. The effect of quenched disorder on the nematic to isotropic transition of a liquid crystal has also been studied. Using Monte Carlo simulation and differential scanning calorimetry, it was seen that quenched disorder will interrupt the ability of the liquid crystal to correlate and this will lower the transition temperature, round and lower the heat capacity peak, and possibly change the order of the transition.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/1991