Influence of temperature on light scattering of poly-N-isopropylacrylamide hydrogels synthesized by two different cross-linking methods
The first part of the thesis aims at a careful inspection of the influence of temperature on the microstructure of PNIPA gels synthesized by free radical cross-linking copolymerization, FRC, in the range 10-27.5 °C, i.e. below LCST by means of static and dynamic light scattering methods. The total scattering intensity was divided into two parts: thermal scattering due to Brownian motion and static scattering due to topological or spatial inhomogeneity. Different methods of data evaluation, i.e. the non-ergodic and the partial heterodyne approaches, were carefully compared. Consistent results were obtained clearly demonstrating that both parts of the scattering rise markedly upon increasing the observation temperature from 10 to 27.5 °C thus approaching the lower critical solution temperature. While the temperature dependence of the thermal scattering component is well understood, the rise of the static component was attributed to the establishment of local swelling equilibrium in a gel whose cross-link density features some inhomogeneity. This means that with rising temperature the more densely cross-linked regions deswell at the expense of the less densely cross-linked ones. As a result, the scattering contrast is enhanced thus leading to a larger static scattering intensity. Many studies have shown that structures of networks obtained by free radical cross-linking copolymerization generally seem to be inhomogeneous in the q-range << 0.1 nm-1. An alternative approach to obtain homogeneous gels is UV induced photo cross-linking polymerization, PC. Cross-links are expected to be introduced randomly in space for photo cross-linked gels. The purpose of the second part of this study is to compare network structures i.e. inhomogeneity, of the free radical cross-linking PNIPA gels (FRC gels) prepared at 25 °C to photo gels (PC gels) synthesized by photo cross-linking polymerization of (N-isopropylacrylamid)-2-(dimethylmaleimido)- N -ethyl-acrylamide, poly(NIPA-co-DMMIAAm) at observation temperatures 25, 20, 15, 10, 30 °C (in that sequence). It is expected that photo gels starting from semi-dilute polymer solution are more homogenous than free radical gels prepared by free radical cross-linking copolymerization starting from monomers and cross-linkers. In contrast to the general assumption the results of this study showed that the formed PC gels are not more homogenous than their analogous FRC gels. On the other hand the strong dependence of the inhomogeneity on the Mw of the linear polymer precursors is revealed. Increasing Mw of a polymer and solutions as well increases the inhomogeneity of the PC gels.
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