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Highly Entangled α-Olefin Molecular Bottlebrushes: Melt Structure, Linear Rheology, and Interchain Friction Mechanism

López-Barrón, Carlos R., Tsou, Andy H., Hagadorn, John R., Throckmorton, Joseph A.
Macromolecules 2018 v.51 no.17 pp. 6958-6966
friction, melting, microstructure, molecular weight, polymerization, rheology, temperature, thermal expansion, viscoelasticity, viscosity, wide-angle X-ray scattering
Linear viscoelastic response and melt microstructure of ultra-high molecular weight poly(α-olefins) (UHMW PO) with bottlebrush architectures, from poly(1-hexene) to poly(1-octadecene) synthesized by metal coordinative insertion polymerization, were measured as a function of side-chain length, Nₛc. All these bottlebrush POs are highly entangled, with an average number of entanglements per chain, Z, greater than 50, which allows accurate determination of their rubbery plateau moduli, GN⁰, and their entanglement molecular weights, Mₑ. Their plateau moduli scale with their side-chain lengths as GN⁰ ∼ Nₛc–¹.⁴⁷, in agreement with the scaling theory for the dense bottlebrush limit that predicts GN⁰ ∼ Nₛc–³/². Melt structures of these bottlebrush poly(1-olefin)s and their melt structural changes with temperature were determined by wide-angle X-ray scattering. Concomitant with thermal expansion of these bottlebrush PO melts is a nonmonotonic change in backbone-to-backbone distance (d₁) and a monotonic increase in side-chain spacing (d₂). Both the melt-flow interchain friction coefficient and the viscosity of these UHMW PO bottlebrushes show a very strong dependence on d₂, characterized by two exponential decay regimes, with decay constants having an exponential dependence on Nₛc.