Linear Rheology of Polyolefin-Based Bottlebrush Polymers

Two series of polyolefin-based bottlebrush polymers were synthesized using ring-opening metathesis polymerization of norbornenyl-functionalized macromonomers to compare the rheology of systems with and without the presence of side chain entanglement. The first series of bottlebrushes includes atactic polypropylene (aPP) branches with a fixed length of <i>M</i>_branch = 2.05 kg/mol and variable backbone degree of polymerization ranging from 11 to 732 (<i>M</i>_w = 22–1500 kg/mol). The branches in this series are shorter than the entanglement molecular weight of aPP (<i>M</i>_branch/<i>M</i>_e ≈ 0.5). The second series utilizes poly­(ethylene-<i>alt</i>-propylene) (PEP) branches with <i>M</i>_branch = 6.7 kg/mol and variable backbone degree of polymerization ranging from 13 to 627 (<i>M</i>_w = 89–4200 kg/mol). The branches in this series exceed the entanglement molecular weight of PEP (<i>M</i>_branch/<i>M</i>_e ≈ 3.5). The linear viscoelastic responses of each sample were examined using small-amplitude oscillatory shear measurements, and dynamic master curves were constructed by time–temperature superposition. Master curves of all aPP bottlebrush polymers exhibited relaxation spectra devoid of any entanglement plateau, despite their high molecular weight. Master curves of the PEP bottlebrushes displayed rubbery plateau regions at short relaxation times, indicating successful entanglement formation between PEP branches. The scaling behavior of the master curves reveals a transient power law scaling throughout the relaxation process. The dynamics are more Zimm-like at early relaxation times and become increasingly Rouse-like at long relaxation times. This transition arises due to the changing conformational character of bottlebrushes at different length scales. Finally, plots of the phase angle versus complex modulus revealed additional intrinsic characteristics of the bottlebrush polymers: (i) the presence of two distinct relaxation processes corresponding to the motion of side chains and of the backbone and (ii) the onset of thermorheological complexity as the glass transition temperature was approached. These two observations represent inherent material properties that are individually governed by the macromolecular architecture and by the monomer scale chemistry, respectively