Modeling Elongational Rheology of Model Poly((±)-lactide) Graft Copolymer Bottlebrushes

The shear and elongational rheology of graft polymers with poly(norbornene) backbone and one poly((±)-lactide) side chain of length N_sc = 72 per two backbone repeat units (grafting density z = 0.5) was investigated recently by Zografos et al. [Macromolecules 56, 2406–2417 (2023)]. Above the star-to-bottlebrush transition at backbone degrees of polymerization of N_bb>70, increasing strain hardening was observed with increasing N_bb, which was attributed to side-chain interdigitation resulting in enhanced friction in bottlebrush polymers. Here we show that the elongational rheology of the copolymers with entangled side chains and an unentangled backbone can be explained by the Hierarchical Multi-mode Molecular Stress Function (HMMSF) model, which takes into account hierarchical relaxation and dynamic dilution of the backbone by the side chains, leading to constrained Rouse relaxation. In nonlinear viscoelastic flows with larger Weissenberg numbers, the effect of dynamic dilution is increasingly reduced leading to stretch of the backbone chain caused by side chain constraints and resulting in strain hardening. If the backbone is sufficiently long, hyperstretching is observed at larger strain rates, i.e. the stress growth is greater than expected from affine stretch.