Polymer diffusion on topographically patterned surfaces

• Main Author: Clarkson, Christopher G.
• Other Authors: Geoghegan, Mark
• Published: University of Sheffield 2015
• Subjects: 500
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.667478

Biological systems are often very efficient and the photosynthetic pathway of R. Sphaeroides is a highly efficient example. Controlling the diffusion of biological molecules leads, in part, to the high efficiencies. Using biologically inspired designs may lead to more efficient devices but require control of diffusion. This thesis presents possible mechanisms that can be used to facilitate this by examining diffusion, with fluorescence correlation spectroscopy, of polymeric analogues for parts of the photosynthetic pathway. Poly(ethylene glycol) (PEG) was diffused on patterned surfaces of poly(oligo[ethylene glycol]methyl ether methacrylate) (POEGMA) brush. Square grids of POEGMA mimic the architecture of the cell membrane and simulate the confinement in the cell membrane. A slow mode of surface diffusion is observed that corresponds to PEG trapped in the grid. The level of confinement is lower than in the cell membrane, (45.6±2.6)% rather than ~100%, but this is explained by the nature of the systems. Biological molecules are highly interactive while PEG and POEGMA are quite inert. This suggests that while the chemistry of a system is important, structure has a strong impact on diffusion regardless of the materials used. The responsiveness of the cytosol was modelled by poly(glycerol monomethacrylate)-blockpoly( 2-hydroxypropylmethacrylate) (PGMA-PHPMA) diblock copolymer nanoparticles in water and the diffusion was examined as a function of temperature and pH. PGMAPHPMA nanoparticles form cylindrical micelles (rH = 58 ± 18 nm) at ambient conditions, producing a freestanding gel. Lowering either the solution temperature, or increasing the pH, induces a morphological 'worm'-to-'sphere' transition. The spherical micelles (from rH = 16.3 ± 0.3 to rH = 6 ± 4 nm) are accompanied by degelation and an increase in diffusion coefficient of a factor between four (temperature) and eight (pH).