• S. Seiffert, W. Oppermann, K. Saalwächter.
  Hydrogel formation by photocrosslinking of dimethylmaleimide functionalized polyacrylamide.
  Polymer 48, 5599–5611 (2007). DOI 10.1016/j.polymer.2007.07.013.

  Abstract

  Polyacrylamide (PAAm) hydrogels are obtained in an efficient and controlled manner by means of photocrosslinking of linear PAAm chains which are functionalized with dimethylmaleimide (DMMI) groups. The reaction is conveniently performed in the presence of thioxanthone disulfonate as a triplet sensitizer. The fundamental investigation of the photoreaction on the basis of model compounds shows that the dimerization of DMMI groups in aqueous solution leads to asymmetric products instead of the expected cyclobutane derivatives. Nevertheless, crosslinking occurs in a well controlled manner without perceptible side reactions. The systematic analysis of the progress of the reaction by means of UV-vis spectroscopy indicates that the rate of dimerization is simply proportional to the concentration of sensitizer and the intensity of irradiation. The dimerization reaction can be interrupted at any intermediate stage by discontinuing the UV irradiation in order to study the system as it changes from a semi-dilute polymer solution to a fully crosslinked gel. The network formation was investigated macroscopically by rheology and microscopically by multiple-quantum NMR experiments. The results clearly indicate that the formation of active network strands occurs in proportion with DMMI conversion. The crosslinking efficiency varies markedly with concentration, but is surprisingly high (>60% at 80 g/L), while the length of the network chains seems to be independent of concentration.

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