Effect of Grafting Density on the Crystallization of Bottlebrush Polymers with Poly(ethylene oxide) Side Chains

The rate and extent of crystallization in branched macromolecules are sensitive functions of the branch length and spacing. This work investigates the extreme case of bottlebrush polymers, which comprise crystallizable polymeric branches (side chains) densely grafted to a linear polymer backbone. In particular, the effect of the grafting density (expressed as the percentage of backbone units bearing a side chain, z) on the crystallization behavior is investigated using a series of bottlebrushes with crystallizable poly(ethylene oxide) side chains and z = 40, 60, 80, and 100% (denoted PEOBB-z). Isothermal crystallization experiments, by differential scanning calorimetry, reveal that the crystallization rate is a non-monotonic function of z. The crystallization rate of PEOBB-80 is nearly equal to that of the analogous linear PEO, whereas for the other three bottlebrushes the crystallization rate is depressed and decreases with decreasing z. The crystallization exotherm for a given crystallization temperature and time, and the resulting peak melting point, follow the same trend. These results reflect the competition between increased chain stretching, decreased intermolecular overlap, and a lower fraction of non-crystallizable backbone units in the melt with increasing z.