α-Olefin Copolymer Bottlebrushes: Microstructure, Thermal, Rheological, and Tack Properties

Melt and crystal microstructure, linear viscoelasticity, thermal behavior, and tack performance of a series of poly(1-hexene-r-1-octadecene) (PHOD) random copolymers, with bottlebrush architecture, are investigated as a function of comonomer fraction. Copolymers with 1-hexene content ≤ 86 mol % crystallize upon cooling, and their melting temperature and entropy of fusion decrease monotonically as 1-hexene content increases. SAXS and WAXS measurements reveal that the crystal structures of the copolymers are similar to those found in PαO homopolymers. The side chains of the PHOD copolymers crystallize into nematic-rotator phases, and the bottlebrush molecules pack into hexagonal lattices, which aggregate to form larger fibrillar bundles. In the melt, two scattering peaks are discerned, corresponding to two characteristic backbone-to-backbone distances, d1,OD and d1,H, associated with the octadecene- and hexene-rich domains, respectively. The rubbery plateau modulus is an increasing function of 1-hexene content and scales with the average side chain length as GN0∝⟨Nsc⟩−1.59, in close agreement with the scaling theory for the dense bottlebrush limit that predicts GN0∝⟨Nsc⟩−3/2. The tack performance of the copolymers improves as the 1-octadecene content increases. Plots of the tack parameters of both PHOD copolymers and PαO homopolymers as a function of ⟨Nsc⟩ or GN0 reveal a universal trend of increasing tack performance with increasing average bottlebrush diameter.