An Investigation into the Role of Morphology on the Performance of Molecularly Imprinted Polymers

A detailed investigation into the role of polymer morphology on imprint medium performance has been carried out with a view to identifying the key physical parameters relating to affinity. A systematic and comprehensive study was carried out by synthesising a series of 2 aminopyridine (2-apy) imprinted polymers. The amount of crosslinking monomer (ethyleneglycol dimethacrylate, EGDMA) and the nature of functional monomer (methacrylic acid (MAA), 4-vinylpyridine (4-VP) and methyl methacrylate (MMA)) were all varied. The change in composition influenced polymer morphology, as shown by nitrogen sorption, SEM, particle size distribution and solvent swell studies. The polymers containing 4-VP were found to produce less porous materials than corresponding MAA counterparts. As a result, the free space within the network of the 4-VP polymers was reduced and so the diffusion of template into and out of the polymers was restricted. This was reflected in the affinity assessment of the polymers where the template, 2-apy, and two structural analogues, 3- and 4-aminopyridine (3- and 4-apy) displayed no affinity for the polymers in solvents with a range of polarities. The porosity of the MAA polymers decreased with decreasing EGDMA amount, but was greater than corresponding 4-VP counterparts. The flexibility of the polymers formed from lower amounts of EGDMA combined with the number of free carboxyl groups and the swelling effect of the solvents on the polymers resulted in an increase in affinity, which was both specific and nonspecific in nature. A link between polymer composition and selectivity towards 3- and 4-apy was also identified, with the pKa of the binding analyte also contributing to the affinity. The MAA polymers were comprehensively examined using the Langmuir (LI), Freundlich (FI) and Langmuir-Freundlich (L-FI) isotherms and using affinity distribution (AD) spectra. The LFI was identified as the most appropriate model in analysing the binding behaviour of the polymers. A relationship between binding site number and strength and the morphology of the polymers was identified. This was the first report of the use of AD spectra generated from the LFI binding parameters used as a methodology to display the sensitivity of binding site number and strength to changes in composition and morphology. Thermal desorption GC-MS was identified as a powerful technique for the characterisation of molecularly imprinted polymers. Bleed composition analysis of the pre-treated polymers suggested that a fundamental difference in the degree of crosslinking between MIP and corresponding NIP polymers existed. The technique also correlated with the morphological assessment of the polymers, where the level of bleed and template removal from the 4-VP polymers was consistently less than corresponding MAA polymers. The affinity of the polymers was also determined and correlations with solution phase binding analysis were identified. The affinity of the MAA polymers increased with decreasing EGDMA amount, whilst the 4-VP polymers displayed limited affinity for the binding analyte (2-apy). A second area of the research involved the preliminary investigation into the use of modified polyvinyl alcohol (PVA) for use as molecularly imprinted polymers. Photosensitivity was introduced into the PVA by reaction with chromophores containing an aldehyde functionality. Polymer solutions in the presence and absence of the template molecule (2-apy) were coated onto an ATR (attenuated total reflectance) crystal. The polymers were crosslinked via a UV initiated [2+2]-cycloaddition reaction. Subsequent analysis via ATR-FTIR spectroscopy, suggested that a minimum level of crosslinking was required to maintain the integrity and stability of the films. There was no evidence of template (2-apy) diffusion into the polymers formed with a higher level of crosslinking. This was due possibly to template surface binding or that the polymer was too flexible to facilitate pre-concentration within the sensing region