Internal structure and dynamics of PNIPAM based microgels in bulk and adsorbed state at different internal crosslinker distributions
Stimuli-responsive microgels are a unique class of polymer structures which can undergo a fast response to an external trigger such as light, temperature or pH. It provides a wide potential application spectrum in optical devices, smart surface coatings, emulsion stabilisation etc. The most prominen...
| Summary: | Stimuli-responsive microgels are a unique class of polymer structures which can undergo a fast response to an external trigger such as light, temperature or pH. It provides a wide potential application spectrum in optical devices, smart surface coatings, emulsion stabilisation etc. The most prominent example of these microgels are poly(N-isopropylacrylamide)(PNIPAM) microgels.
During the last decades PNIPAM-based microgels were widely studied and serve nowadays as model systems for the investigation of the basic properties of microgels and the principles of supported transport of active substances, e.g. drug delivery. The responsive behaviour of the microgels is governed by the polymer network structure, i.e. the crosslinking of the polymer chains has a considerable influence on the physico-chemical and mechanical properties of the microgels. Especially the amount and the crosslinker distribution within a single microgel have a high relevance for the application as well as for the understanding of the microgel nature.
This thesis focuses on a detailed analysis of microgels in solution and at interfaces with scattering experiments. Neutron and X-ray scattering provide a unique insight into the structure and dynamics of microgels, especially at the interface with grazing incidence small angle neutron scattering and neutron spin-echo spectroscopy (GISANS and GINSES). New insights into the inner structure and dynamics have been gained with improved experimental conditions and data analysis.
This thesis is divided into the two main parts. In order to characterise the internal structure and dynamics of the PNIPAM microgels with respect to the crosslinker (N,N'-methylenebis(acrylamide), BIS) distribution, sophisticated analysis of the internal microgel (domain-like) structure and studies of its influence on the polymer dynamics in nanometer and nanosecond scales are presented in the first part of the thesis. In the context of smart polymer coatings, the influence of the confinement to the solid surface on the internal architecture and the thermoresponsiveness of the adsorbed PNIPAM microgels is investigated. It is shown that in contrast to the atomic force microscopy, the inhomogeneities of the polymer network of the adsorbed soft microgels can be achieved with surface sensitive neutron reflectometry and GISANS.
The second part of the thesis aims at the peculiarities of the scattering experiments of thin polymer layers under grazing incidence conditions. Simulation of the scattering signal within the Distorted Wave Born Approximation is presented in order to improve the analysis of the GINSES data and to simplify the initial planing and performance of the grazing incidence experiments.
The main impacts of this thesis are (i) an extended description of the complex internal structure, dynamics and the thermoresponsiveness of the PNIPAM microgel before and after adsorption onto a solid surface and (ii) demonstration the advantages of the investigation of the polymer systems at grazing incidence conditions in combination with numerical simulations. |
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