Our simulations revealed that the microscopic picture of these samples is clearly different to the picture as previously discussed in literature: It was proposed that the linking of symmetric mixtures of two types (identical up to the reactive end-groups which react selectively with the opposite type of star) of four-arm stars leads to an ordering of the structure upon cross-linking. We found that this ordering is a rather minor correction, but the absence of the shortest dangling loops has a striking effect on the network properties for experiments close to the overlap concentration c*. This is because not only the dangling loop is fully inactive, but also the attached strands contribute less to the modulus of the network. The Figure shows the frequency of these shortest dangling loops in star polymer networks in which this type of defect is still possible. In effect, much more stable networks were produced as compared to previous attempts using the very same precursor stars as building blocks of the network.
In order to describe the change of the network properties as function of polymer concentration, we additionally developed a rate theory approach that allows to monitor the local connectivity of the star molecules. While networks made of one type of star were well captured by mean field models, the networks made of symmetric mixtures showed a clear effect of mixing both species.
These results demonstrate once again that understanding the cross-linking process and the resulting network structure are inevitable to model the network properties.
- K. Schwenke, M. Lang and J.-U. Sommer "On the Structure of Star-Polymer Networks" Macromolecules 44, 9464 (2011)