Linear Viscoelastic Response of Unentangled Polystyrene Bottlebrushes

We investigated the linear viscoelasticity of bottlebrush polystyrenes (PSs) with total molar masses ranging from 132 to 769 kg/mol, bearing short side chains (with molar masses of 5 and 7 kg/mol, well below the entanglement limit of PS). Their estimated length to diameter ratio was smaller than 1, corresponding to a globular conformation and conforming to the molar mass dependence of the radii and to recent computer simulation results. The master curves were constructed by means of time–temperature superposition and featured a hierarchical relaxation (glassy, side-chain, intermediate, and terminal regimes) along with the absence of a rubbery plateau, indicating that the entire macromolecules behaved as unentangled polymers, though with some distinct features. The analysis of the dependence of storage and loss moduli on oscillatory frequency revealed cooperative side-chain dynamics at intermediate frequencies because of their mutual repulsion and Rouse-like dynamics at low frequencies. The zero-shear viscosity scaled with the total molar mass of the bottlebrush as η₀ ≈ M_{w,bottlebrush} like Rouse chains; however, the respective dependence of the terminal flow time appeared to be stronger. The estimated values of the fragility index suggested that these unentangled bottlebrushes became stiffer with increasing length of the side chains but remained less stiff compared to linear PSs of the same total mass. These results are compared with and contrasted against bottlebrush data from the literature, suggesting universalities and distinct features likely attributed to chemical differences and calling for further investigations.