Cross-linking and surface properties of EHEC and Starch -a QCM-D study
The increase of the green house gases and the crude oil price has resulted in a willingness to change the raw material in many industries from petroleum-based polymers to natural polymers. One such natural polymer is starch. Today, starch is used in the glue industry in wallpaper pastes. To be able to use this kind of environmental friendly products in further applications the knowledge about their surface characteristics and cross-linking properties must increase.
In this thesis we have mainly used the surface technique Quartz Chrystal Microbalance with Dissipation monitoring (QCM-D). By combining the two surface measuring techniques, QCM-D and Surface Plasmon Resonance (SPR), not only the absorbed amount can be calculated but also the water content, the viscoelasticity properties and the conformation of polymers at surfaces.
In the first part of the thesis the surface properties and cross-linking abilities of the cellulose derivative Ethyl(HydroxyEthyl)Cellulose (EHEC) and the hydrophobic analogue HM-EHEC was studied in detail. These polymers were used because of their chemical similarities to starch but have higher water solubility. The second part involves cellulose derivates and starch cross-linked with three different methods. The first cross-linking process was carried out with a very reactive molecule, divinylsulphone (DVS), connecting the polymers with the hydroxyl groups. The other two cross-linking reactions involved modification of the polymer introducing new chemical properties, Imidazole and Glycidyl Methacrylate (GMA), enabling a cross-linking reaction by introducing metal ions and UV-light, respectively. The interaction between the ligand imidazole and the metal ion Cu2+ is also studied in detail. The cross-linking behaviour, in combination with the viscoelastic properties enabled conclusions about the molecular structures of the adsorbed polymers. For example it was shown that for EHEC at the hydrophobic surface and HM-EHEC at the hydrophilic surface the polymers adsorbed in an individual phase-separated manner, making an inter-polymer reaction unsuccessful. Due to the polydispersity in the samples the rate of adsorption was very slow. Initially small polymers adsorbed due to a faster flux to the surface but since larger polymers are less soluble there were a continuous exchange of polymer molecular weight at the surface as larger polymers adsorbed.
The study resulted in new materials with new functionalities, easy to cross-link at surfaces resulting in polymer layers and thin interfacial hydrogels. It is also shown that the surface technique QCM-D is a very usable method for studying polymers at interfaces.
Keywords: QCM-D, SPR, EHEC, HM-EHEC, GMA, Imidazole, Starch, Hydrogel, Viscoelastic properties.