Melt Miscibility in Diblock Copolymers Containing Polyethylene and Substituted Hydrogenated Polynorbornenes

The thermodynamic interaction strengths between linear polyethylene (PE) and members of a family of hydrogenated polynorbornenes prepared by ring-opening metathesis polymerization can be tuned across a wide range via the choice of substituent appended to the polynorbornene backbone at the 5-position. Isopropyl and certain n-alkyl groups yield polynorbornenes that are highly miscible with PE, capable of producing symmetric diblock copolymers with homogeneous melts at molecular weights in excess of 100 kg/mol. In contrast, phenyl-substituted polynorbornenes are quite immiscible with PE, exhibiting microphase separation in the melt at diblock molecular weights as low as 10 kg/mol. Interaction strengths within this series of polymers do not quantitatively obey regular mixing; entropic contributions to the mixing energy arising from mismatches in free volume and chain stiffness cannot account for the observed deviations. Instead, the interactions can be satisfactorily described by an empirical mixing rule of the form X = (Δγ)^1.5, where X is the interaction energy density and γ is a pure-component quantity, operationally analogous to a solubility parameter, with a distinct value for each polymer. These empirical γ parameters are obtained by regression against the entire set of experimental pair interaction energies.