ORDERING IN ASYMMETRIC POLY (ETHYLENE-PROPYLENE)-POLY (ETHYLETHYLENE)DIBLOCK COPOLYMER THIN-FILMS

We have used neutron reflection and phase contrast microscopy to inves tigate the morphology and surface topology of thin films of nearly sym metric (f=0.55) and asymmetric (f=0.77), poly (ethylene-propylene)-pol y (ethylethylene) (PEP-PEE) diblock copolymers (f being the PEP volume fraction) and have identified three important differences in their or dering properties. First, annealed films of the asymmetric diblocks do not form the lamellar microstructure found in symmetric diblocks; the ir structure can instead be modeled in terms of the hexagonal packing of PEE cylinders observed in bulk small-angle neutron scattering measu rements. However, the cylinders show in-plane distortions, which we in terpret in the context of nonintegral layering. These distortions are amplified at the surfaces where the PEE assumes lamellarlike form. Sec ond, as-cast films of the asymmetric diblock are characterized by a mi crostructure lacking long-range order, pinned between strongly segrega ted PEE at both surfaces. These films can be equilibrated through anne aling, leading to the well-ordered structures described earlier. The c hanges with annealing are surprising given that both PEP and PEE are w ell above their glass transitions at room temperature. Finally, the bl ock asymmetry and the associated cylindrical structure in the interior are also manifested in the surface topology. Thin films of asymmetric PEP-PEE are smooth on a macroscopic scale unlike their symmetric coun terparts, which form islands on the surface to accommodate films of no nintegral lamellar thicknesses.