A. Karim et al., ORDERING IN ASYMMETRIC POLY (ETHYLENE-PROPYLENE)-POLY (ETHYLETHYLENE)DIBLOCK COPOLYMER THIN-FILMS, The Journal of chemical physics, 100(2), 1994, pp. 1620-1629
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.