Comparative Study of Phase Behaviour and Structure of a Water Soluble Synthetic Polymer and an Elastin-like Peptide

This work concerns itself with the structure and phase behaviour of water soluble macromolecules. The molecules are studied in solutions of different concentrations and the solutions are also brought into a soft confinement. We will study the influence of different chain ends of a polymer on the phase boundaries and structure formation of the bulk solution as well as on the surfactant layer of a water in oil microemulsion in the droplet phase providing the soft confinement. It will also be discussed how increasing temperature and chain length of the polymer changes the stability and radius of the microemulsion droplets. Further we will analyse structural changes of a peptide in a buffer solution in bulk and the microemulsion soft confinement. DSC measurements are performed to establish the phase diagram of the water polymer mixtures and they reveal that an eutectic system is formed, regardless of the endgroup of the polymer. Structure formation of the polymer water mixtures is observed via WAXS and SAXS measurements, which reveal that upon crystallisation the polymer and water separate before freezing, instead of forming a solid solution or mixed crystal. For some concentrations of polymer the formed hexagonal ice crystal exhibits a preferred growth direction, that is independent of the endgroups. But there is also a concentration and temperature dependent preferred growth direction of the polymer crystal for one set of endgroups that is not found in the other set. Further it can be seen that adding water to the polymer increases the long period of the polymer crystal, by increasing the thickness of the crystalline phase instead of the amorphous phase. Only for high concentrations of water in the mixture there is an increase in the long period with increasing temperature. SANS measurements are used, to analyse the structural conformation of the peptide in the buffer solution. Here we find that the radius of gyration of the peptide increases with temperature and concentration hinting at a strong aggregation process, except for low concentrations of peptide. In those samples it is possible to see an inverse temperature transition of the peptide. The structural conformation of the peptide is more that of a flexible polymer chain than of a folded particle, but deviates from the flexible behaviour with decreasing concentration. To analyse the influence of polymer concentration, chain length and endgroups on the stability of the surfactant layer of the microemulsion droplet, BDS and SAXS measurements are employed. We find that the stability of the droplet depends highly on the number and size of the polymer chain, but not on the endgroups. There is an ideal polymer size and number of polymer chains inside the droplet at which the destabilisation or stabilisation of the droplet is maximal. The radius of the microemulsion droplets decreases with temperature regardless of their degree of stability. But stabilised droplets exhibit a slower decrease in radius than destabilised droplets. An attractive interaction between surfactant shell and polymer is found for both types of endgroups. Finally, we use SANS and SAXS measurements in order to study the conformation of the peptide and of the surfactant layer in the microemulsion droplet phase. The peptide does not destroy the droplet phase and seems to adsorb at the surfactant interface, just like the polymer. This stabilises the droplet, which still decreases in radius with increasing temperature.