CONTROLLING MOLECULAR ORDER IN HAIRY-ROD LANGMUIR-BLODGETT-FILMS - A POLARIZATION-MODULATION MICROSCOPY STUDY

The interplay of molecular weight, layer thickness, and thermal anneal ing in controlling molecular order in ultrathin Langmuir-Blodgett film s is characterized with the use of polarization-modulation laser-scann ing 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(phthalo cyaninatosiloxane) (PcPS). images are presented for films as thin as t wo molecular layers (similar to 44 angstroms). Molecular alignment alo ng the transfer direction is much stronger for films of PcPS with simi lar to 25 repeat units (similar to 10 nanometers long) than for those with similar to 50 repeat units (similar to 20 nanometers long). Enhan cement of alignment by thermal annealing is also much greater for PcPS -25 than PcPS-50. Intimate interaction with the substrate suppresses i mprovement in alignment by annealing, evident by an anomalously small increase in anisotropic absorption of the first two layers.