Disorder-to-Order Transition upon Heating in an All-Hydrocarbon Polynorbornene Diblock Copolymer

Most block copolymer chemistries, with appropriate molecular weights, undergo order-to-disorder transitions (ODT) upon heating. ODT behavior predominates because the Flory interaction parameter χ, characterizing the repulsive strength between components, is usually positive and decreases with temperature. We identify a new diblock copolymer chemistry, composed of hydrogenated poly(n-hexyl norbornene) and poly(cyclohexyl norbornene), which instead undergoes a disorder-to-order transition (DOT) upon heating. Block copolymers exhibiting a DOT are extraordinarily rare, and to our knowledge this is the first all-hydrocarbon species identified to do so. Demixing at high temperatures is not driven by a large free volume mismatch between components, the mechanism commonly invoked to explain DOTs in block copolymers or lower critical solution temperatures in blends. We compare the DOT polymer with a family of other polyethylene and norbornene-based diblocks, and find that the thermal expansion coefficient mismatch between blocks, related to the free volume mismatch, does not dictate the type of phase behavior. Instead, we attribute DOT behavior to an enthalpic effect, where the component solubility parameters do not scale with the reciprocal of density as expected for a simple liquid.