Nanoscale structure and single molecule diffusion in smart polymeric systems.

Soft nanotechnology requires the development and understanding of smart polymeric systems that respond to small changes in the surrounding environment. This thesis reports on the structure and dynamics in poly(methacn"lic acid) (PMAA) hydrogels and hyperbranched poly(N-isopropyl acrylamide) (HB-PNIPAM) in response to physical and chemical stimuli. Fluorescence correlation spectroscopy (FCS) has been utilized to study the diffusion of single dextran molecules labelled with fluorescein isothiocyanate within a PMAA hydrogel. Diffusion in pure water shows a temperature dependence described by Zimm dynamics, whereas the diffusion coefficient decreases with temperature in the hydrogel for which a model has been developed. Diffusion in PMAA hydrogel has revealed the mesh size dependence on temperature. The effect of pH and salt on the diffusion in PMAA hydrogel has also been considered. Introducing magnetic nanoparticles to hydrogels forms ferrogels the mesh of which is controlled by applied magnetic fields. The swelling, diffusion and release in PMAA ferro gel has been found to follow the same scaling theory developed in this work. Small angle neutron scattering (SANS) has revealed the structural behaviour of HB-PNIPAM as a function of temperature compared to its linear counterpart. These experiments have shown that water is a good solvent for HB-PNIPAM at low temperatures, while increasing the temperature leads to a gradual collapse of these polymers until they form spherical particles with sharp boundaries of the order of 24-40 nm in diameter, depending on the branching degree. This indicates that HB-PNIPAM shows no entanglements either as a function of temperature or branching degree. In contrast, linear PNIPAM showed a network-like behaviour above its collapsing temperature. Neutron spin echo experiments on HB-PNIPAM are described well by the Rouse model for unentangled chains and the self-diffusion of HB-PNIPAl\I by FCS follows Zimm behaviour, which is in agreement with SANS results. These studies have given a better understanding of the nanostructure and dynamics in the investigated polymeric systems, showing their usefulness as delivery systems for many biological and medical applications.