THERMAL-EXPANSION OF THIN DIBLOCK COPOLYMER FILMS

The thermal expansion of thin films of symmetric diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) was investigate d by X-ray reflectivity. The confinement of the copolymer to the subst rate, coupled with the multilayering of the copolymer where PS and PMM A layers are oriented parallel to the substrate, gives rise to unusual thermal expansion characteristics. The total thickness of the film in creases as 3 alpha(L), where alpha(L) is the linear thermal expansion coefficient of the copolymer. Unlike homopolymer films, the thermal ex pansion of an ordered block copolymer film results in an excessive str etching of the copolymer chains at the interface between the PS and PM MA layers. This excess stretching is a result of the confinement of th e junction points of the copolymer chains to the interfaces and the su ppression of the lateral expansion of the copolymer. When the stretchi ng of the chains becomes too high, relaxation occurs by transporting c opolymer chains to the surface. This is evidenced by a reduction in th e period of the multilayer. After the copolymer chains have relaxed, t he change in the multilayer period with temperature closely follows al pha(L).