Supramolecular Mimic for Bottlebrush Polymers in Bulk

Abstract

A series of poly(tetrahydrofuran)s with molecular weights above entanglement molecular weight Me were synthesized, and one of their end-groups was functionalized with a supramolecular entity so that the corresponding polymers form a brushlike structure suitable for comparison with conventional irreversible bottlebrush polymers. To compare their relaxation mechanisms, linear rheology was employed and showed that a hierarchical relaxation, which is usually observed in bottlebrush polymers, occurs in these materials, too. The polymer chain segments close to the supramolecular backbone are highly immobilized due to strong association in the center of polymer brush and cannot relax via reptation mechanism, which is mainly responsible for linear entangled polymer relaxations. Therefore, disentanglement can take much longer through contour length fluctuations and arm retraction processes similar to covalent bottlebrush polymers and combs. The relaxed ends of polymers then act as solvent to let the remaining segments of the polymeric brush undergo Rouse-like motions (constraint release Rouse). At longer times, additional plateau appears, which can be attributed to the relaxation of the entire supramolecular bottlebrush polymer via hopping or reptative motions. With an increase of temperature, viscoelastic solid behavior turns into viscoelastic liquid due to reversible depolymerization of the supramolecular backbone of the bottlebrush polymer. The elastic modulus (G′ in the order of kPa) was much less than the values found for the entanglement plateau modulus of linear poly(tetrahydrofuran) (in order of MPa). This low modulus value, which exists up to very low frequencies (high temperatures), makes them a good candidate for supersoft elastomers.

A Critical Approach to Polymer Dynamics in Supramolecular Polymers

“…47 We have shown that the presence of a parallel G ′ and G ″ and subsequent plateau can even happen with polymers with one sticker (which intuitively cannot lead to a gel formation) due to CRR events. 34…”

“…In particular, the magnitude of the plateau modulus is in the range we described as jamming for similar polymers. 34 Colloidal jamming is neither a new phenomenon nor limited to supramolecular graft polymers. Hyperstars have been reported to show similar jamming due to impenetrable cores.…”

“…This regime, however, can be ascribed to CRR relaxation of the arms (see Figure 5). 34 We have recently shown that if the supramolecular backbone of the polymer brush imposes steric hindrance, the segments close to the backbone show colloidal jamming. If the molecular weight of the arms is large and mobile enough (high M n and low T g ), the outermost segments of the arms relax first via arm retraction mechanism.…”

“…d *, d π–π , and ξ are the lamellar periodicity, the stacking distance, and the correlation length, respectively. Reproduced with permission from ref (34).…”

Overcoming the Necessity of a Lateral Aggregation in the Formation of Supramolecular Polymer Bottlebrushes in Water

“…[ 1 ] Recently, more attention has been paid to introducing motifs capable of forming directional, supramolecular interaction forces such as π–π interactions or hydrogen bonds (H‐bonds) that can allow to direct the polymer assembly into elongated structures, such as cylinders, that would otherwise not be accessible. [ 2–7 ] In particular, the so‐called supramolecular polymer bottlebrushes (SPBs) have gained increasing interest due to their potential applications in areas such as nanomedicine, [ 8 ] rheology [ 9 ] , electronics, and photonics. [ 10,11 ]…”