Small angle neutron scattering from complex systems

Systems with a hierarchical structure (complex systems) are ubiquitous in soft matter science. Scattering techniques have been used to unravel the structure of these complex systems. One of them, Small Angle Neutron Scattering (SANS), has been widely used in the study of polymeric materials due to a rare confluence of certain properties of both the radiation and polymeric systems. Two systems that satisfy the aforementioned criterion for complexity are (1) the structure of equilibrium swollen polymer networks and (2) the structure of self-associating triblock copolymers in solution.; A model to describe the different levels of structure exhibited in polymer networks swollen to equilibrium is described. The characteristic SANS pattern is attributed to stressed regions associated with the quenched topological constraints in the network. The model generalizes the idea of Pincus blobs, to non linear structural elements. Two characteristic lengths are identified, a gel tensile blob size and an extended structure length. Predictions are made relating these size scales to the system parameters. These predictions have been tested using SANS measurements.; Neutron scattering measurements were made on a triblock copolymer in solution as a function of temperature and concentration of several potassium salts. The self assembled aggregates of these triblock copolymers exhibit rich polymorphism, with the structure varying from spherical to cylindrical and finally the solution undergoing phase separation with an increase in temperature and/or salt concentration. The critical micelle transition temperature, the temperature of sphere to cylinder transition and the cloud point decrease with added salt concentration. These observations are used to argue that the evolution of the micellar structure and the phase separation are through gradual dehydration of the copolymer chains with an increase in either the temperature and/or the salt concentration. A quantity, L, is defined in terms of the structural parameters obtained from the fits to the SANS data. Relationships between the values of L and qualitative information about the structure of the micelle and the inter-micellar interaction are described in detail. It is argued that the value of L can be used to compare the effect of the different potassium salts on the micellization.