Hybrid Dendronized Polymers as Molecular Objects: Viscoelastic Properties in the Melt

Abstract

Two homologous series of dendronized polymers (DPs) of the second and third generations, with different degrees of polymerization of the backbone, were synthesized combining two previously reported approaches. First, methacrylate-based DPs of the first generation were prepared via radical polymerization of the corresponding methacrylate macromonomers. As the side branches of such first-generation DPs can form hydrogen bonds and π−π stacks, they are referred to as "classic" (intermolecularly interacting) DPs. Second, the first-generation DPs so prepared were grafted with branched oligoethylene glycol groups to increase the size of the dendrons up to the second and third generations. Because of the different chemical structures of the outermost generations with respect to the inner one, these DPs are termed "hybrid" DPs. The glycol-based generations do not form intermolecular supramolecular associations, which so strongly control the aging dynamics and viscoelastic properties of the interacting DPs. Therefore, the series of hybrid DPs allow for investigating the dynamics of DPs for which synergistic effects because of supramolecular interactions and topology are reduced or absent. We find, at first glance surprisingly, that the loss of intermolecular peripheral interactions increases the equilibration time dramatically. Concerning the viscoelastic behavior of the hybrids of the second generation, the onset of global relaxation is observed at low frequencies. This is in contrast with supramolecular DPs having the same generation and the same backbone degree of polymerization, for which a clear plateau region was previously demonstrated. In addition, the low-frequency plateau of the elastic modulus increases upon increase of the degree of polymerization of the backbone, suggesting that one oligoethylene generation does not suffice to completely shield supramolecular interactions between the branches. Such a scenario is also supported by atomistic simulations. The hybrid DPs of the third generation display two distinct plateau regions of the storage modulus. The first one (at higher frequencies) is of the order of 10 6 Pa, and attributed to the interpenetration of the side branches, the second is of the order of 10 3 Pa. In contrast with the behavior of second-generation hybrid DPs, the degree of polymerization of the backbone has no effect on the low-frequency plateau of third-generation hybrid DPs. This suggests that supramolecular interactions do not contribute to the elastic plateau. Hence, we ascribe it to the entanglements of the entire hybrid DPs. The reported results are summarized in a table, which compares the properties of different DPs and provides the needed ingredients for tailoring the rheology of such hyperbranched polymers.

Multi-scale Structure and Dynamics of Dendronized Polymers with Varying Generations

“…In general, the time needed for the systems to reach the equilibrium exceeded 30 h at a temperature of approximately 70 °C above T g . This is along the lines of what has been previously observed for hybrid dendronized polymers 26 and is attributed to the minimization and equilibration of monomer density gradients in order to eventually find the spatial configuration corresponding to the local minima of the free energy.…”

“…It is interesting that such behavior was not reported with other series of dendronized polymers having different molecular structures of the dendrons and in particular different linking of the phenyl groups, pointing to the need to understand the influence of local arrangement on the dynamics. 18,26 Thus, we speculate that π−π interactions have a role to play in the solidlike mode, manifesting in the G2 spectrum before the glassy mode; however, the main driving force is interpenetration, since if the dendrons do not interpenetrate, π−π interactions would not be possible. Clearly, a more systematic study to assess this scenario is necessary and would undoubtedly involve spectroscopic measurements and related analysis, but it is beyond the scope of this work.…”

“…We measure their viscoelastic modes over the entire frequency range, including the terminal regime, which is not always accessible with denpols. 18,26 This enables us to obtain the complete picture of relaxation dynamics of such thick polymers and show that the topological interaction of the side chains dominates at lower generations and influences the relaxation of the system in a nonmonotonic manner. We find that the dynamics are very different from that of conventional entangled (sparsely) branched polymers like combs, where the smaller branches typically lead to shorter terminal relaxation times due to tube dilation effects.…”

“…Using this ROMP, wedge denpols having second-generation dendrons were synthesized, and their rheological properties were investigated . Denpols exhibit complex hierarchical relaxation processes, , which are still not fully understood, whereas the stress relaxation dynamics of their linear counterpart is well understood, for example, in the context of Rouse or Doi–Edwards models for one-dimensional objects, , rendering the development of models addressing the effects of molecular thickness on dynamics necessary. Despite these advances, there has been no systematic investigation yet on the role of DP and generation on the viscoelasticity of noninteracting, well-defined denpols.…”

Polymer Topology Reinforced Synergistic Interactions among Nanoscale Molecular Clusters for Impact Resistance with Facile Processability and Recoverability

“…[6] Benefiting from the highly developed synthetic techniques, the fabrication of polymer-MCs composites has been well-established. [7] Similar to the dendronized polymers featuring bulky branches, [8] the incorporation of MCs profoundly modulates the interdigitation of the polymer chains, which brings distinct mechanical properties to the nanocomposites. [9] Meanwhile, the abundant surface engineering of MCs allows the access to PNCs with hierarchical structures.…”