Self-assembly and morphology of polydimethylsiloxane supramolecular thermoplastic elastomers

Functionalization of polydimethylsiloxanes (PDMS) polymers with hydrogen-bonding ureidopyrimidinone (UPy) groups leads to supramolecular thermoplastic elastomers. In previous studies, no lateral stacking of UPy dimers was observed in UPy-functionalized polymers, unless additional urethane or urea groups were built into the hard block. However, we have shown that when PDMS is used as the soft block, this lateral aggregation of UPy dimers does take place, since long fibers could be observed in the atomic force microscopy (AFM) phase image. Also in bulk, the presence of these interactions was proven by oscillatory shear experiments. We attribute this aggregation to the incompatibility of soft block and hard block, leading to phase separation. Moreover, we have shown that additional urethane or urea groups in the hard block do lead to materials with more fibers and higher melting points. For the UPy-urea functionalized PDMS even single fibers are observed with AFM when dropcasted from a very diluted solution. When the length of the soft block is increased, the morphology changes from fibrous to spherical.