Complete Dynamic Phase Diagram of a Supramolecular Polymer

Abstract

We revisit the equilibrium phase diagram of the much-studied model supramolecular polymer, 2,4-bis(2ethylhexylureido)toluene (EHUT) in nonpolar solvents and provide unambiguous evidence of a much richer behavior, characterized by four distinct regimes. Typically, two types of self-assembled structures are formed: tubes (filaments) at higher (lower) concentrations and lower (higher) temperatures. The tube structure forms viscoelastic solutions that had been characterized by rheology, however, without detailed analysis of the experimental signals. Here, we combine rheology and microrheology to establish the complete dynamic phase diagram of EHUT in dodecane. It still comprises two structures, tubes and filaments, with the transition temperature being almost constant over the examined wide concentration range, as confirmed with the help of complementary differential scanning calorimetry measurements. The tubes are found to exist in three dynamic states with increasing concentration, unentangled, partially entangled, and well entangled, which are separated by isolength lines. We present criteria for unambiguously identifying these phases and discuss their distinct concentration and temperature dependencies. The new, complete phase diagram may serve as a guide for investigating other supramolecular polymers with tunable rheology and, more importantly, providing insights into a universal description of one-dimensional self-assembled structures by linking this class of materials with the classic wormlike surfactant micelles, for which the partially and well-entangled regimes were recently elucidated.

Dynamics and Rheology of Supramolecular Assemblies at Elevated Pressures

“…The relative viscosity increment (η i = η 0 * /η s – 1), where the ratio of zero-shear complex viscosity to the solvent viscosity is the relative viscosity, can be extracted from the LVE data, and its pressure dependence for different temperatures and concentrations is depicted in Figure at elevated pressures for EHUT/dodecane (Figure a) and EHUT/cyclohexane (Figure b) solutions. This representation suggests that, for both solutions at constant pressure, an increase in temperature will reduce the relative viscosity increment, consistent with the change in LVE spectra of Figure and in agreement with our recent work . More importantly, there is a weak increase in η i with pressure up to about 60 MPa, which is attributed to a change in the average length ⟨ L ⟩ of the supramolecular polymers.…”

“…Toluene is slightly less apolar compared to dodecane and cyclohexane (its dielectric constant is 2.38) . In dodecane, which is of worse quality for the polar parts of EHUT, the tube structure is stabilized as opposed to toluene; consequently, some qualitative differences in dynamics exist. , Recently, we combined bulk and passive microrheology to perform a systematic characterization of the phase diagram of EHUT/dodecane at ambient pressure . The transition between tubes and filaments was found to take place at about 88 °C in the semidilute regime.…”

“…We attribute the changes in the LVE spectra to significant changes in the average length of the assemblies. The shapes of the spectra are so different because by increasing the pressure there is a change in ⟨ L ⟩ and a transition from unentangled to partially (weakly) entangled (PE) to well-entangled (WE) regimes . The well-entangled regime exhibits an extended plateau region, whereas the partially entangled regime is characterized by a weakly viscoelastic behavior; the latter has a weak plateau that decays faster with decreasing frequency than that of the well-entangled regime.…”

“…The well-entangled regime exhibits an extended plateau region, whereas the partially entangled regime is characterized by a weakly viscoelastic behavior; the latter has a weak plateau that decays faster with decreasing frequency than that of the well-entangled regime. Nonetheless, the PE regime remains clearly different from the Zimm-like unentangled regime which has well-separated moduli with G ″ > G ′ . An alternative representation of the LVE spectra in terms of the so-called Cole–Cole plot is discussed below.…”

“…At fixed pressure, increasing temperature promotes the UE state of tubes (and also the tube-to-filament transition) . The different states should reflect a different length distribution of the self-assemblies. ,, …”