Application of positron annihilation lifetime spectroscopy (PALS) to the characterization of nanostructure in ordered self-assembled systems

Abstract

Positron annihilation lifetime spectroscopy (PALS) has long been used to characterize porosity in polymers and ceramics, and has been shown in past studies to be sensitive to conformational changes in self-assembled amphiphile systems. However, due to inconsistencies in experimental setup, sample configuration and data analysis, direct comparison between studies was often not possible. This has resulted in a limited knowledge of the response of the PALS parameters (oPs lifetime and intensity) to structure in these systems, and hence the full potential of PALS as a characterization technique in mesophase systems remains unrealised. The research undertaken in this thesis utilizes a systematic approach to gain more understanding of the potential of using PALS to characterize self-assembled amphiphile systems. This thesis addresses a number of classes of nanostructures in soft self-assembled materials including micelles, lamellar phases, non-lamellar liquid crystals and microemulsions. The micellar to hexagonal phase transition of four different micellar systems prepared using cationic, anionic and non-ionic surfactants were investigated using both PALS and small angle x-ray scattering (SAXS). It was found that in all four systems the hexagonal (H1) phase had a lower oPs lifetime than the micellar (L1) phase, suggesting restricted chain packing and reduced chain mobility in the H1 phase. A characteristic oPs lifetime of 3.5 ns was also obtained for the H1 phase for systems with C12 surfactants. Past PALS studies often investigated either bulk or dispersed lamellar phases without putting emphasis on stating the sample composition. However it was not known whether there is a difference in the behaviour of the oPs parameters between the dispersed and bulk forms. In this study both bulk and dispersed lamellar systems were investigated and it was found that the dispersed lamellar phases (liposomes) resulted in an overall larger oPs lifetime than the bulk systems. It was also found that PALS was able to indicate the temperature of the main phase transition (Tm) from the rigid lamellar phases to the fluid lamellar phase (Lα). In the bulk systems a characteristic oPs lifetime of 3.4 ns was determined for the fluid lamellar phase, which was independent of lipid chain length. PALS was also used to probe the phase transitions of more complex nanostructures found in the phytantriol/water system. PALS was able to differentiate between these structures in both the bulk and dispersed systems. This thesis is the first to use PALS to characterise dispersed hexagonal (hexosomes) and bicontinuous cubic (cubosomes) phases. The most significant change in the oPs lifetime occurred at the Q2Pn3m to H2 phase transition. Addition of increasing concentrations of vitamin E acetate resulted in a more gradual change in lifetime at this phase transition. PALS was used to characterize the w/o to bicontinuous to o/w phase transitions in two microemulsion systems. Phase boundaries were confirmed using electrical conductivity and viscosity measurements. A difference of 0.4 ns was observed across the three phase transitions. The research described in the thesis highlights the potential of using PALS to characterize nanostructured self-assembled amphiphile systems.