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(isoprene-block-ferrocenyldimethylsilane) was studied using atomic force microscopy, transmission electron microscopy, and optical microscopy. Block copolymers with the organometallic (ferrocenylsilane) phase between 20 and 28 vol % were investigated. At these compositions the copolymers form cylindrical morphologies 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 thickness matches the interdomain spacing, a surface morphology consisting of hexagonally packed domains is obtained for the diblock containing 20 vol % of the organometallic phase, and a complete wormlike structure was observed for the diblock containing 28 vol % of the ferrocenylsilane phase. The diblock containing 24 vol % of the organometallic phase displays a mixed morphology at this thickness consisting of a wormlike structure and hexagonally packed domains. Hole formation, due to incompatibility between the film thickness and the domain spacing, was observed for films thinner than a single domain layer. The formation of islands, or holes, in thicker films was not observed because lattice distortions can relieve the stress that is generated by excess material. The crystallization of the PFS phase took place at room temperature and resulted in large hedritic structures over the whole surface of the diblock copolymer films. The microdomain morphology was completely destroyed in the films consisting of crystallized organometallic domains. From DSC measurements on bulk samples, the melting temperature and enthalpy of the diblock copolymers indicated well-advanced crystallization compared to the case of ferrocenyldimethylsilane homopolymers.