Insights into the mechanism of molecular imprinting by immersion precipitation phase inversion of polymer blends via a detailed morphology analysis of porous membranes.
Porous membranes were prepd. by immersion pptn. phase inversion of cellulose acetate-sulfonated polysulfone (CA-SPS) blends with the compns. 85:15, 95:5 and 100:0 (wt. %), and molecularly imprinted polymer (MIP) materials, prepd. with the fluorescent dye Rhodamine B (RhB), and blanks, prepd. without the template RhB, were analyzed by scanning force microscopy (SFM), SEM and the gas adsorption isotherm method (BET). RhB binding data from solid phase extn. expts. allowed an estn. of imprinting efficiency as a function of blend compn.: 95:5 > 85:15 > 100:0. SEM revealed an asym. cross-section morphol. with nodules in the top layer and macrovoids in the support layer which indicated instantaneous demixing as the overall mechanism of polymer solidification. SEM at high resoln. and SFM enabled a detailed anal. of the top layer morphol., in particular the estn. of the nodule size. Overall, significant differences in pore structure between MIP and the blank, as a function of the polymer blend compn. were found; the magnitude of these differences, measured by SEM, SFM and BET, clearly correlated with the imprinting efficiency. For the CA-SPS 95:5 blend, the characteristic nodule size was much smaller for the MIP than for the blank. Hence, the fixation of imprinted sites occurred mainly in small polymer particles, which were formed during a very fast demixing upon contact with the non-solvent. Further, the addn. of the template to the CA-SPS blend soln. seemed to facilitate the demixing after contact with the pptn. bath water, presumably via a complexation of the RhB with the sulfonic acid groups of SPS.
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