I'm using Reichardt's dye as a solvatochromic probe in acrylate monomers, homopolymers, and copolymers. In some acrylates, for example HEMA, the absorption can not be detected due to bleaching of the probe. I know the fact, that bleaching is caused by protonation. But this shouldn't be the case with HEMA, which was received as a pure sample. Only impurities of water can be detected with H-NMR.
Does anyone know how the bleaching is occuring?
Did you try monitoring the protonation process via atomic force microscopy?
AFM is a powerful tool in such measurements
First of all, thank you for your valuable answers and input regarding this topic. Yes it bleaches in a solution of the monomer HEMA and the polymer. Interestingly the bleaching in the polymer can be seen as the solvent (acetone or butanone) evapourates and is completed in case of the dried polymer. This process occurs fast because of the preparation method as films. Solvents and monomers used are free of impurities. Water impurities caused by humidity are removed by drying at vacuum conditions in case of the film respectively removed with molecular sieve in case of the monomer. I was having an interaction with OH-group of HEMA in mind, but a bleaching should then also be seen in combination with ethanol, or not? I'm agreeing with Matthew that an interaction of the double bond is not possible in case of the polymer. Furthermore the typical absorption of Dimroth-Reichardt's dye can be seen in other methacrylates as a solvent (or polymer).
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