Rheology of Poly(-olefin) Bottlebrushes: Effect of Self-Dilution by Alkane Side Chains

Bottlebrush polymers are combs with extremely high grafting density along their backbone chain. We consider the rheology of bottlebrush poly(alpha-olefins) with side chains ranging from 6 carbons [poly(1-octene)] to 16 carbons [poly(1-octadecene)] as investigated by Lopez-Barron et al. (J. Rheol. 2019, 63 (6), 917-926). We argue that the backbone chain of poly(alpha-olefins) is diluted by the unentangled alkane side chains and that the rheology of poly(alpha-olefin) bottlebrushes is equivalent to that of poly(1-methylethylene), i.e., atactic polypropylene diluted by a low-molecular-weight solvent. We show that this approach is in agreement with the decreasing plateau modulus of the poly(alpha-olefins) with increasing side chain length, and it allows to replace empirical correlations by a relation based on physical arguments. The specific strong transient strain hardening in elongational flow shows similar features as observed for entangled solutions of linear polymers and can be explained by the enhanced relaxation of stretch model if self-dilution of the poly(alpha-olefins) is taken into account. Due to the large difference between the disengagement time tau(d) and Rouse time tau(R), the orientation and stretch of backbone chains are well separated, and strain hardening starts after full orientation at Weissenberg numbers above Wi(R) = (epsilon) over dot tau(R) congruent to 0.3. The amount of strain hardening increases with increasing dilution, and at high Wi(R), all poly(alpha-olefins) are expected to reach the same steady-state elongational stress.