Dynamic polymer materials can be obtained by introducing supramolecular interactions between the polymer chains. Here we report on the preparation and mechanical properties of poly(methyl acrylate) (PMA) and poly(n-butyl acrylate) (PBA) funcionalized with ureidopyrimidinone (UPy) in the side chains. In contrast to the traditional UPy with a methyl group, the selected UPy motif contained a branched alkyl side chain, which enhances solubility, compatibility with the polymer matrix and potentially prevents stacking of UPy dimers. Low molar mass PMA and PBA were synthesized via Cu(0)-mediated radical polymerization and allyl bonds were introduced with different degrees of functionalization by stoichiometrically controlled transesterification with allyl alcohol. The allyl esters served as functional handles for UPy attachment via UV-initiated radical thiol-ene coupling. The PMA-UPy materials displayed a more glassy appearance, in contrast to the rubbery PBA-UPy polymer networks, associated to its higher glass transition temperature. The mechanical properties of the resulting hydrogen bonded polymer networks were assessed by thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical thermal analysis and tensile testing, followed by rheological analysis of the network dynamics. Furthermore, the effect of associative groups on the linear viscoelastic response is discussed based on a modified sticky Rouse model indicating the absence of significant aggregation or phase separation of the UPY units.

The synthesis of a supramolecular polymer network based on ureidopyrimidinone (UPy) is demonstrated starting from a low-molecular weight polybutadiene (PB) polymer, which was modified with UPy quadruple hydrogen-bonding motifs through UV-initiated thiol-ene chemistry under ambient conditions. Here, the UPy units contain a branched alkyl side chain rather than a methyl group, as is the case in most literature examples. The sterically demanding aliphatic 2-ethylpentyl side chain was introduced to prevent stacking of the UPy dimers and to enhance the compatibility of the UPy unit with the apolar PB polymer matrix. The UPy units were grafted onto PB with different functionalization degrees yielding materials with a relatively broad range of glass transition temperatures (T-g) and material properties, as evaluated by thermogravimetric analysis, differential scanning calorimetry (DSC), and dynamic mechanical thermoanalysis. Importantly, only the system with 13 mol % UPy functionalization showed some macroscopic phase separation as indicated by its partial opaque appearance, even though this was not detected by DSC. Previous reports on PB end functionalized with methyl-functional UPy revealed phase separation through DSC, indicating that the branched side chains indeed suppress the phase separation. The network dynamics were assessed by rheological measurements at different temperatures, which were subsequently fitted to the sticky Rouse model and creep experiments. We found that the sticky Rouse model satisfactorily fits the linear response of the systems in the terminal regime. However, significant discrepancy from theory still persists in the rubbery plateau regime. This deviation is attributed to the presence of structural defects in the systems. Altogether, this work demonstrates the importance of designing the supramolecular physical cross-linker unit with a branched aliphatic side chain to enhance the compatibility with the polymer matrix for the formation of supramolecular polymer networks.

We investigate the viscoelastic properties of double dynamic networks (DDNs) based on side-functionalized P nBA chains. One of these networks is highly crosslinked by metal-ligand junctions characterized by a fast association/dissociation dynamics, while the other network is sparsely crosslinked with slow dynamic covalent networks (DCNs). We first show that modulating the dynamics of the metallosupramolecular networks, by playing with the temperature, the density of reversible junctions, or the stress applied, has direct consequences on the local equilibration of the DCN. The latter takes place by a constraint release Rouse process at the rhythm of the association/dissociation of the metal-ligand junctions. Then, based on creep-recovery experiments, we investigate the ability of the DDNs to recover their initial shape after a creep test and show again the important role played by the metallosupramolecular network. In particular, the sample recovery strongly depends on the network connectivity, which is enhanced if a denser metallosupramolecular network is used as it reduces the possible creep of the double dynamic network and increases its elastic memory. The sample recovery also depends on the association-dissociation dynamics of the metallosupramolecular bonds as it fixes how fast the stretched DCN can come back to its equilibrium conformation and can recover its initial shape after a large deformation has been applied. Adjusting the dynamics of the weak network is thus a key process to govern the viscoelastic response of the slow network.

The Doi-Edwards tube model, coupled with relaxation mechanisms, such as reptation, contour length fluctuation, and constraint release, allows us to quantitatively predict the linear viscoelastic properties of entangled polymers. However, for nonlinear elongational flows, large discrepancies between theoretical predictions based on the tube model and experimental results still persist today. This is in particular obvious for the experimentally observed strong qualitative differences in extensional flow of entangled polystyrene (PS) melts and solutions despite having the same number of entanglements and exhibiting the same linear viscoelastic behavior. The cause of this non-universality is often attributed either to a monomeric friction reduction or to an interchain pressure effect. In this work, we investigate the changes in extensional flow behavior going from polymer solutions to the melt state. For this purpose, we measure with a filament stretching rheometer the nonlinear extensional responses of differently long PS chains, both in the melt state and diluted in short-chain matrices of the same polymer at varying concentrations. These concentrations have been chosen sufficiently high, such that the chains stay entangled. This allows us to discuss the influence of concentration and molar mass on the steady-state elongational viscosity to highlight scaling relations. The purpose of the present work is to conduct well-defined experiments to further investigate how the steady extensional viscosity of polymer solutions and blends varies with the concentration and with the molecular weight of the chains.