Syndiotactic α‑Olefin Molecular Bottlebrushes: Crystallization, Melting, and Hierarchical Microstructure

• Published in: Macromolecules Vol. 53; no. 10; pp. 3778 - 3788
• Main Authors: López-Barrón, Carlos R, Hagadorn, John R, Mattler, Sarah J, Throckmorton, Joseph A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26.05.2020
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.0c00420

The crystallization of α-olefin molecular bottlebrushes occurs via side-chain crystallization exclusively, which is fundamentally different from the common chain folding process occurring in linear polyolefin crystallization. However, the exact mechanisms of side-chain crystallization have not been explored to date. Herein, we report the melting and crystallization behaviors of a series of poly­(α-olefin) homopolymers with a syndiotactic backbone (sPαO) and side-chain lengths (N sc) ranging from 4 to 16 carbons. Only sPαOs with N sc ≥ 10 are able to crystallize into a hierarchical morphology, as revealed by a combination of SAXS, SALS and optical microscopy measurements. A monotonic increasing dependence on N sc of the thermal properties (melting temperature, enthalpy, and entropy of fusion) and of the crystallinity is observed on the sPαOs, which reveal that the odd–even effect on those properties (as observed on n-alkanes) does not operate in α-olefin bottlebrushes. The side chains in the crystallizable sPαOs arrange into a nematic rotator phase, similar to atactic poly­(1-octadecene) recently reported [ Macromolecules 2018, 51 (3), 872−883 ]. The bottlebrush molecules pack into fibril bundles made of several molecules packed into a hexagonal lattice. On a larger scale, density fluctuations give rise to a bicontinuous microstructure with characteristic size on the order of 1 μm.