Linear and Extensional Rheology of Model Branched Polystyrenes: From Loosely Grafted Combs to Bottlebrushes

Monodisperse comb polystyrenes (comb-PS) with loosely to densely grafted architectures up to loosely grafted bottlebrush structures were synthesized via anionic polymerization. This comb-PS series, named PS290-N-br-44, had the same entangled backbone, M-w,M-bb = 290 kg/mol, corresponding to a number of entanglements along the backbone Z(bb) congruent to 20, and similar branch length, M-w,M-br congruent to 44 kg/mol or Z(br) congruent to 3, but varied in the number of branches per molecule, N-br, from 3 to 190 branches. Consequently, the average number of entanglements between two consecutive branch points along the backbone (branch point spacing), Z(s), ranged from well entangled, Z(s) congruent to 5, to values that were far less than one entanglement, Z(s) congruent to 0.1. Linear viscoelastic data including the zero-shear rate viscosity, eta(0), diluted modulus, G(N,s)(0) and a new diluted modulus extracted from the van Gurp-Palmen plot, vertical bar G*vertical bar at delta = 60 degrees, were analyzed as a function of the M-w of the combs. Scaling of eta(0) versus M-w revealed three different regions for increasing Nbr or decreasing Z(s): (1) loosely grafted combs with Z(br) < Z(s) and eta(0) similar to exp(M-w), (2) densely grafted combs with 1 < Z(s) < Z(br) and eta(0) similar to M-w(-3.4) followed by eta(0) similar to M-w(-1) for 0.2 < Z(s) < 1, and (3) loosely grafted bottlebrushes with Z(s) < 0.2 and eta(0) similar to M-w(5) Mws. The relative maximum eta(0) corresponded to a comb-PS with Z(s) congruent to Z(br), and the relative minimum resulted from a comb-PS with Z(s) congruent to 0.2, which displayed almost the same eta(0) as the linear PS290. Strain hardening factors, SHF equivalent to eta(E,max)/eta(DE,max) measured in extensional experiments increased with increasing N-br and reached SHF > 200 for Hencky strains below epsilon(H) = 4, which is tremendously high and has to the best of our knowledge not been observed yet. Such a high strain hardening is of great fundamental and technical importance in extensional processes, e.g., foaming, film blowing, or fiber spinning.