(555a) Confinement Effects on the Crystallization of Perfectly Linear Polyethylene

Block copolymers consist of polymer chains containing two or more distinct polymer segments covalently bonded together. Repulsions between the blocks cause the polymer chains to self-assemble into various nanoscale morphologies, including spheres, cylinders, and lamellae. The phase behavior can become much more complex when one of the blocks is crystalline, with either melt morphology or crystallization dictating the final structure [1].

We sought to investigate the crystallization behavior of diblock copolymers of perfectly linear (unbranched) polyethylene (LPE) and polyvinylcyclohexane (PVCH) in a variety of morphologies, with LPE forming either the matrix or the discrete domains. Because the glass transition temperature of PVCH is higher than the crystallization temperature of LPE, the mesoscale melt morphology should be ?frozen? upon cooling, confining crystallization within or around glassy microdomains. Previous work on PVCH-polyethylene diblocks has employed hydrogenated polybutadiene (hPBD) as the ?polyethylene? block, but in that case, the ethyl branch defects in hPBD limit the crystal thickness and crystallinity [2,3]. With LPE-PVCH diblocks, confinement should directly influence both crystallinity and crystal thickness, as reflected in the melting temperature and enthalpy.

We developed a synthetic route to well-defined, LPE-containing block copolymers by combining two living polymerization techniques, ring-opening metathesis polymerization (ROMP) and anionic polymerization, followed by hydrogenation. The LPE precursor is synthesized through the ROMP of cyclopentene and terminated to yield a styrene-like endgroup. This endgroup is initiated anionically, so that the polycyclopentene acts as a macromonomer from which the anionic block, polystyrene, is subsequently grown. Polycyclopentene ? polystyrene (ROMP-anionic) diblocks synthesized by this approach are then catalytically hydrogenated to yield LPE-PVCH.

Through calorimetry and small-angle x-ray scattering, we observe that the glassy PVCH microdomains do indeed confine the LPE crystals, dictating their maximum thickness (melting point) and reducing the overall degree of crystallinity of the LPE block. Spherical and cylindrical microdomains of 20nm diameter do not allow the formation of the 30nm thick LPE crystals formed in the homopolymer. In these environments we observe crystals of 10-15nm in thickness with 40% crystallinity, half that observed for LPE homopolymer. Deep undercooling (45-50C) is required to crystallize the LPE within spheres or cylinders (surrounded by the glassy PVCH matrix), which indicates that each microdomain must be nucleated independently and homogeneously [3]. Lamellar morphologies impose lesser limitations on LPE-block crystallization. Because the crystal stems lie parallel to the microdomain interface, the lamellar spacing does not limit crystal thickness. Crystal thickness and required undercooling (15-20C) were found to be similar for lamellae-confined LPE crystals and LPE homopolymer, although confinement in lamellae reduced the LPE crystallinity to 65%, possibly due to an exclusion of the crystal stems from the microdomain interfaces.

[1] Y.L. Loo, R.A. Register, and A.J. Ryan, Macromolecules, 35, 2365 (2002).

[2] I.W. Hamley, J.P.A. Fairclough, N.J. Terrill, A.J. Ryan, P.M. Lipic, F.S. Bates, E. Towns-Andrews, Macromolecules, 29, 8835 (1996).

[3] Y.L. Loo, R.A. Register, A.J. Ryan, and G.T. Dee, Macromolecules, 34, 8968 (2001).