S. Seiffert, W. Oppermann, K. Saalwächter.
Hydrogel formation by photocrosslinking of dimethylmaleimide functionalized
Polymer DOI: 10.1016/j.polymer.2007.07.013.
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.