Morphologies of diblock copolymer thin films before and after crystallization

Spin-coated thin films of about 100 nm of low-molecular-weight hydrogenated poly(butadiene-b-ethyleneoxide) (PBh-PEO) diblock copolymers have been cry stallized at various constant temperatures. Crystallization has been observ ed in real time by light microscopy. Detailed structural information was ob tained by atomic force microscopy, mainly enabled by the large viscoelastic contrast between amorphous and crystalline regions. The behavior in thin f ilms is compared to the bulk properties of the polymer. Crystallization sta rted from an annealed microphase separated melt where optical microscopy in dicated a lamellar orientation parallel to the substrate. A small differenc e in the length of the crystallizable block produced significantly differen t crystallization behavior, both in the bulk and in thin films. For thin fi lms of the shortest diblock copolymer (45% PEO content) and for an undercoo ling larger than about 10 degrees, crystallization created vertically orien ted lamellae. These vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase co ntact line. Annealing at temperatures closer to the melting point or keepin g the sample at room temperature for several months allowed the formation o f a lamellar structure parallel to the substrate. A tentative interpretatio n based on kinetically caused chain folding and relaxation within the cryst alline state, with implications on general aspects of polymer crystallizati on, is presented.