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.