Structure of symmetric polyolefin block copolymer thin films

Abstract

The microstructure of thin films of nearly symmetric poly(ethylene–propylene)–poly(ethylethylene) (PEP-PEE) diblock copolymers (f=0.55, where f is the volume fraction of PEP) was characterized by neutron reflectometry (NR). A symmetric film structure in which the PEE block segregates preferentially to both interfaces is observed above and below the bulk order–disorder transition (ODT). Measurements at room temperature for several chain lengths, N, provide a real-space picture of the change in interdomain interfacial profiles associated with the crossover between the strong and weak segregation limits. The polymer/air and substrate/polymer interfaces are observed to induce an ordered microstructure even when the center of the film is disordered. The characteristic dimension of this near surface microstructure is larger than the corresponding bulk value for values of χN lying between those of the bulk Gaussian-to-stretched-coil and order–disorder transitions, where χ is the segment–segment interaction parameter. This behavior is attributed to the correlation of large amplitude composition fluctuations in the film with the interfaces. A mean-field behavior prevails for χN<(χN)GST,bulk, where some preferential segregation occurs at the interfaces, but the characteristic dimension once again matches that in the bulk.