Using scanning contactless conductivity to optimise photografting procedures and capacity in the production of polymer ion-exchange monoliths

Abstract

Capacitively coupled contactless conductivity detection (C⁴D) is utilised as a simple, rapid and non-invasive technique for the quantitative evaluation of the ion-exchange capacity of charged polymer monoliths in capillary format. A charged monomer, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) was photografted onto a 100 µm i.d. butyl methacrylate-co-ethylenedimethacrylate monolith in a number of discrete 10 mm zones. By varying the energy dose (J/cm²) during grafting of each zone, the grafting density and thus ion-exchange capacity could be precisely controlled. Ion-exchange capacity could be correlated with energy dose by measuring the conductive response of each grafted region using scanning C⁴D techniques. Repeatability of the scanning C⁴D method was excellent with % RSD values of 0.7% and 2.4% obtained for three replicate scans of the ungrafted and grafted regions of a single monolith, respectively. Repeatability of the photografting process on separate monoliths was also examined by comparison of C⁴D profiles. The spatial accuracy of photografting was probed using scanning C⁴D which could measure the conductive response of the monolith at measurement intervals as low as 1 mm along its entire length. Scanning C⁴D was also used for the real time visualisation of the equilibration of grafted zones to permit the optimisation of monolith washing procedures. Finally, scanning C⁴D was applied to the measurement of the ion-exchange capacity of butyl methacrylate-co-AMPS-co-ethylenedimethacrylate copolymers with a direct correlation between monolith conductive response and concentration of charged monomer in the polymerisation mixture. The longitudinal homogeneity of charge along the monolith was 0.3% RSD, demonstrating that the charged functional monomer was evenly dispersed throughout the bulk of the monolith. Ion-exchange capacity was cross validated chromatographically using breakthrough studies and found to closely correlate to within 1% of the measurements made by scanning C⁴D.