Morphology and crystallization of thin films of asymmetric organic-organometallic diblock copolymers of isoprene and ferrocenyldimethylsilane

The morphology of thin films of asymmetric block copolymers of poly(isopren e-block-ferrocenyldimethylsilane) was studied using atomic force microscopy , transmission electron microscopy, and optical microscopy. Block copolymer s with the organometallic (ferrocenylsilane) phase between 20 and 28 vol % were investigated. At these compositions the copolymers form cylindrical mo rphologies in the bulk. Thin films, spin-cast on silicon wafers, possessed different morphologies depending on the composition and the film thickness. The block copolymers exhibit a surface morphology consisting of a wormlike pattern for film thicknesses exceeding the domain spacing. When the film t hickness matches the interdomain spacing, a surface morphology consisting o f hexagonally packed domains is obtained for the diblock containing 20 vol % of the organometallic phase, and a complete wormlike structure was observ ed for the diblock containing 28 vol % of the ferrocenylsilane phase. The d iblock containing 24 vol % of the organometallic phase displays a mixed mor phology at this thickness consisting of a wormlike structure and hexagonall y packed domains. Hole formation, due to incompatibility between the film t hickness and the domain spacing, was observed for films thinner than a sing le domain layer. The formation of islands, or holes, in thicker films was n ot observed because lattice distortions can relieve the stress that is gene rated by excess material. The crystallization of the PFS phase took place a t room temperature and resulted in large hedritic structures over the whole surface of the diblock copolymer films. The microdomain morphology was com pletely destroyed in the films consisting of crystallized organometallic do mains. From DSC measurements on bulk samples, the melting temperature and e nthalpy of the diblock copolymers indicated well-advanced crystallization c ompared to the case of ferrocenyldimethylsilane homopolymers.