Extensional Strain Hardening in Highly Entangled Molecular Bottlebrushes.

The highly strained conformation in dense molecular bottlebrushes (DB) has profound effects on the dynamics of these type of macromolecules. Understanding of such effects in both their linear and nonlinear viscoelastic responses is crucial for their design and processing. The nonlinear response of poly(1-octadecene), a highly entangled α-olefin DB with linear side chains sixteen carbons long, is studied here and compared to the nonlinear response of a linear polyolefin (polypropylene) with equivalent linear viscoelastic response. We found that the DB shows remarkably larger extensional strain hardening (SH) and extensibility than the linear polyolefin. The strong SH is attributed to a strain-induced increase in friction drag between adjacent chains resulting from side-chain interdigitation and alignment perpendicular to the flow. The higher extensibility of DBs compared to linear counterparts has been predicted previously [Daniel et al., Nat. Mater. 15, 183 (2015)NMAACR1476-1122] and confirmed here for the first time in DB melts.