NEUTRON STUDY OF STRUCTURE AND DYNAMICS OF RIGID POLYMERS

The structure and the dynamics in two different classes of rigid polymers; ionomers and rigid luminescent polymers, have been investigated predominantly by neutron techniques. Rigidity of the backbones of both polymers affects their properties. For ionomers, it affects transport pathways, therefore, their applications for energy and separation membranes. In luminescent polymers, it determines the conformations of the chains and their electro-optical response. One group of the polymers consists of rigid hydrophobic para-phenylene backbone decorated with pendant phenyl side chains functionalized with sulfonic acid (SPP). The other family of polymers consists of poly para-phenyleneethynylene (PPE) backbone that becomes conjugated upon conformational constraints. The dynamic processes in an ionic network of rigid ionomers, SPP, and in spontaneously formed complex fluids of PPE were explored using quasi-elastic neutron scattering. The structure of PPEs in a good solvent and their collapsed conformations in poor solvent were investigated using small angle neutron scattering and atomic force microscopy techniques. Dynamics study of the complex network of SPP has shown that the polymer molecules in the membrane are not dynamic on the time resolution of the measurement. Water molecules, however, a solvent relevant to the applications of SPP, are confined and exhibit local jump dynamics. It is interesting to point that the water molecules remain dynamic well below its bulk freezing temperature. We found the strong dependence of water dynamics with temperature and hydration levels. Dynamics study of complex fluids of dinonyl PPE revealed that side chains of this polymer remain dynamic in its entire phase diagram including gel phase, micellar phase, and molecular solution. Finally, we studied polydots, PPE, confined to nano-dimensions in poor solvent. We found that structures and stability of polydots depend on concentration of parent polymer solution, temperature, solvent quality and nature of the substituents on the PPE backbone. Specifically, neutral PPE in water forms oblate shape polydots at lower concentration and spherical shape structure at higher concentration. The polydots remain stable at lower temperature (\u3c 80oC) and start to open up at higher temperature (\u3e 80oC) when collapsed in ethylene glycol. The polydots of ionizable PPE are found to have remarkable temperature stability compared to neutral analog, due to the presence of ionizable groups on the surface of the polydots