Controlling Molecular Order in "Hairy-Rod" Langmuir-Blodgett Films: A Polarization-Modulation Microscopy Study

The interplay of molecular weight, layer thickness, and thermal annealing in controlling molecular order in ultrathin Langmuir-Blodgett films is characterized with the use of polarization-modulation laser-scanning microscopy. The degree and direction of molecular alignment can be imaged rapidly and sensitively through the magnitude and orientation of linear dichroism in Langmuir-Blodgett films of rodlike poly(phthalocyaninatosiloxane) (PcPS). Images are presented for films as thin as two molecular layers (∼44 angstroms). Molecular alignment along the transfer direction is much stronger for films of PcPS with ∼25 repeat units (∼10 nanometers long) than for those with ∼50 repeat units (∼20 nanometers long). Enhancement of alignment by thermal annealing is also much greater for PcPS-25 than PcPS-50. Intimate interaction with the substrate suppresses improvement in alignment by annealing, evident by an anomalously small increase in anisotropic absorption of the first two layers.