Dr Andrew Parnell
I am interested in studying structure and structural transitions in complex biological systems. The TEM image below shows in detail the components of a Bascillus Subtilis bacteria showing the flagellum (used for propulsion) and the main rod shaped part of the bacteria.
My PhD work was concerned with the study of conformational change of weak polyelectrolyte brushes, principally using scanning probe to examine the kinetics of the swelling / collapse response and neutron reflectivity to elucidate the volume fraction profile.
'An Atomic Force Microscopy image of a spin cast blend of PS and PI'
Watching phase separation in spin coated polymer blends Spin coating polymer blend thin films provides a method to produce multiphase functional layers of high uniformity covering large surface areas. This work recently published in ACS Nano has employed synchronized stroboscopic illumination to optically observe structure evolution in such blends during spin coating in real time. The aim was to understand the evolution as the rapidly spinning layer dries to then be able to reliably control the final morphology.The work gave insights to the very early stage evolution of thin film phase separation.
I spent a period working on a large TSB project with Unilever and ICI looking at ways in which to encapsulate beneficial molecules within polymer vesicles. I have a keen interest int this work and am currently working on some neutron experiments aimed at understanding how to maximise the degree of loading in these systems.
Light emitting polymers
Block copolymers are useful materials that form an array of complex structures depending on a number of factors.
Photonic block copolymers
Blends of block copolymers as anti-counterfeit materials Coloured materials were made using block copoylmers (an alloy of two different polymers). By mixing the block copolymers together, we were able to create any colour in the rainbow from two non-coloured solutions. This type of polymer then automatically organises itself into a layered structure, causing optical effects similar to opals. The colour also changes depending on the viewing angle see video. This system has huge advantage in terms of cost, processing and colour selection compared to existing systems. The image to the left shows the range of colours that can be made by mixing the two block copolymers in varying proportions. The work was published in Soft Matter 2011, 7, 3721-3725 . We carried out the small angle x-ray measurements Diamond light source This work has been covered extensively in the media Independent online, the BBC, the Engineer and ScienceDaily
Polymer solar cells
Layering / stratification in polymer solar cell blends Using neutron reflectivity, self-stratification in a model P3HT/PCBM blend was observed. The as-spun and solvent-annealed films show a depletion of PCBM near the top surface and enrichment of PCBM at the substrate (see figure). Depletion of PCBM at the cathode interface in a photovoltaic device could act as a barrier to efficient electron extraction. On thermal annealing, the PCBM depleted region is eliminated; an effect that partially explains the improvement of P3HT/PCBM devices on thermal annealing.
Depletion of PCBM at the Cathode Interface in P3HT/PCBM Thin Films as Quantified via Neutron Reflectivity Measurements Andrew J. Parnell, Alan D. F. Dunbar, Andrew J. Pearson, Paul A. Staniec, Andrew J. C. Dennison, Hiroshi Hamamatsu, Maximilian W. A. Skoda, David G. Lidzey, Richard. A. L. Jones Advanced Materials Volume 22, Issue 22, pages 2444–2447, June 11, 2010
Ultra small angle x-ray data for a highly ordered lamellae forming block copolymer