Comb and Branch‐on‐Branch Model Polystyrenes with Exceptionally High Strain Hardening Factor SHF > 1000 and Their Impact on Physical Foaming

The influence of topology on the strain hardening in uniaxial elongation is investigated using monodisperse comb and dendrigraft model polystyrenes (PS) synthesized via living anionic polymerization. A backbone with a molecular weight of M$_{w,bb}$ = 310 kg mol$^{–1}$ is used for all materials, while a number of 100 short (SCB, M$_{w,scb}$ = 15 kg mol$^{–1}$) or long chain branches (LCB, M$_{w,lcb}$ = 40 kg mol$^{–1}$) are grafted onto the backbone. The synthesized LCB comb serves as precursor for the dendrigraft-type branch-on-branch (bob) structures to add a second generation of branches (SCB, M$_{w,scb}$ ≈ 14 kg mol$^{–1}$) that is varied in number from 120 to 460. The SCB and LCB combs achieve remarkable strain hardening factors (SHF) of around 200 at strain rates greater than 0.1 s$^{–1}$. In contrast, the bob PS reach exceptionally high SHF of 1750 at very low strain rates of 0.005 s$^{–1}$ using a tilted sample placement to extend the maximum Hencky strain from 4 to 6. To the best of the authors’ knowledge, SHF this high have never been reported for polymer melts. Furthermore, the batch foaming with CO$_{2}$ is investigated and the volume expansions of the resulting polymer foams are correlated to the uniaxial elongational properties.