Single-Chain Conformation of Poly(alpha-olefins) in Dilute Solutions at the Crossover between Linear and Bottlebrush Architectures

CR Lopez-Barron and F Vargas-Lara and SH Kang, MACROMOLECULES, 54, 6854-6866 (2021).

DOI: 10.1021/acs.macromol.1c00725

The size and conformation of a series of poly(alpha-olefin) homopolymers in good solvent conditions were measured by a combination of small-angle neutron scattering (SANS), triple detector size exclusion chromatography (SEC), and MD simulations. The bottlebrush samples were prepared via organometallic coordinative insertion polymerization of 1-alkenes, with carbon numbers ranging from 6 to 18 carbons. A linear polyolefin, that is, polypropylene, is included in this study for comparison. SANS data for all the solutions are well described by the flexible cylinder model, from which the cylinder radius (R) and the Kuhn length (l(k)) are determined. These two quantities, also calculated by MD simulations, are monotonic increasing functions of the bottlebrush side chain length (N-sc). The dependences of the hydrodynamic radius (R-h) and the radius of gyration (R-g) with the molecular weight (M), measured by SEC and MD simulations, are well described by the power laws, Rh similar to M-upsilon h and R-g similar to M-upsilon g, respectively. Intriguingly, upsilon(h) is a nonmonotonic function of N-sc, whereas upsilon(g) increases monotonically with N-sc. We postulate that this is due to weakening of the intermolecular hydrodynamic interactions as N-sc increases, which also marks a transition from linear-like (for N-sc < 9) to brush-like chain architecture (for N-sc > 9). Taking the R-g data for all the bottlebrush polymers together, they cannot be described by the Kratky-Porod wormlike chain model. We propose an empirical parabolic function of the backbone molecular weight (N-bb) and N-sc that predicts the R-g values for all the bottlebrushes adequately.

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