Linear rheology of bis-urea functionalized supramolecular poly(butylacrylate)s: Part I - weak stickers

We investigated the linear viscoelastic properties in the melt of a series of nearly monodisperse poly(n-butyl acrylate) (PnBA) chains center-functionalized with a bis-urea sticker group, able to self-associate by quadruple hydrogen bonding. All materials are viscoelastic liquids at 40 °C and their Newtonian viscosity varies from 100 to 5000 Pa s. However we show clearly that the viscosity changes non monotonously going through a minimum for a molecular weight (Mn) of 20 kg/mol and a sticker density of 1.1 wt%. We found two regimes: For Mn < 20 kg/mol, the viscosity increases with decreasing Mn, the linear viscoelastic properties change dramatically with temperature, forming a viscoelastic gel at 0 °C and flowing at 40 °C and the strength of the stickers association controls the size of molecular aggregates. However for Mn > 20 kg/mol, the viscosity increases with Mn, the rheology is controlled by the polymer dynamics and the stickers simply increase the terminal relaxation time relative to unfunctionalized PnBA of the same Mn, as expected from sticky reptation theory. Despite clear evidence of sticker-sticker interactions by FTIR, none of the materials self-assemble at 20 °C into structures with a long range order detectable by SAXS.

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