Linear Rheology of Supramolecular Polymers Center-Functionalized with Strong Stickers

Abstract

We investigate the linear viscoelastic properties of a series of low polydispersity poly(n-butyl) acrylate chains center-functionalized with a triurea interacting moiety, able to self-associate by six hydrogen bonds. Depending on the molecular weight (M n ) of the side chains, the polymers can self-associate and form supramolecular structures in the melt state. For M n < 40 kg/mol, the polymers form large bundles of parallel rods which relax stress as large colloidal objects. For M n ≥ 40 kg/mol, the self-assembly of stickers form smaller and randomly oriented rods and the viscoelastic properties are mainly governed by the side chains relaxations, as observed for weak stickers. However, in both regimes, the side chains relax at short time scales, and then, the relaxation of fibrillar aggregates follow (by scission or diffusion process) at long time scales. The weak correlation between both relaxation modes makes the tuning of the linear viscoelastic properties relatively easy by varying independently the length of the side chains and the associating strength of the sticker. The good control of the molecular dynamics via the chemical structure makes the supramolecular center-functionalized polymers extremely interesting for materials applications where dissipative rheological behaviors are targeted over large frequencies ranges, such as vibration damping or adhesive applications. ■ INTRODUCTIONSupramolecular chemistry, defined as the reversible association of molecules through weak interactions, is at the center of molecular associations in living organisms and has been recently harnessed to form new materials exploiting the ability of molecules to self-assemble. 3−7 Most of these self-assembly processes are studied in solution but if the material is a fluid itself, they can be studied in the absence of solvent. This is typically the case for flexible polymers above their glass transition, where chains enjoy the long-range molecular mobility necessary for self-assembly, but can store significant elastic energy, a characteristic of solids, to result in interesting soft materials. One particular type of new soft material which has emerged is a reversible network of polymers, obtained by functionalizing flexible polymer chains with strongly interacting groups. 8−11 Seiffert and Sprakel,8 have recently reviewed the dynamics of these supramolecular structures and pointed out the complex interplay between the bond energy of individual stickers, the presence of large-scale supramolecular structures and the resulting rheological properties of the material. It is therefore essential to systematically characterize the rheology of well-defined model systems where the molecular weight of the polymer, the position of the sticker groups on the chain and the sticker concentration are systematically varied.Most past studies have focused on systems containing more than one sticker group per chain and which therefore tend to form reversible elastic networks. 10−14 However, for applications where dissipative properties may be more i...

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