ma0c00266_si_001.pdf (960.61 kB)
Download fileLinear Viscoelastic Response of Unentangled Polystyrene Bottlebrushes
journal contribution
posted on 2020-05-12, 16:06 authored by Stelios Alexandris, Katerina Peponaki, Paraskevi Petropoulou, Georgios Sakellariou, Dimitris VlassopoulosWe 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 ≈ Mw,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.