Deposition and Photopolymerization of Phase-Separated Perfluorotetradecanoic Acid–10,12-Pentacosadiynoic Acid Langmuir–Blodgett Monolayer Films

Abstract

Mixed monolayer surfactant films of perfluorotetradecanoic acid and the photopolymerizable diacetylene molecule 10,12-pentacosadiynoic acid were prepared at the air-water interface and transferred onto solid supports via Langmuir-Blodgett (LB) deposition. The addition of the perfluoroacid to the diacetylene surfactant results in enhanced stabilization of the monolayer in comparison with the pure diacetylene alone, allowing film transfer onto a solid substrate without resorting to addition of cations in the subphase or photopolymerization prior to deposition. The resulting LB films consisted of well-defined phase-separated domains of the two film components, and the films were characterized by a combination of atomic force microscope (AFM) imaging and fluorescence emission microscopy both before and after photopolymerization into the highly emissive "red form" of the polydiacetylene. Photopolymerization of the monolayer films resulted in the formation of diacetylene bilayers, which were highly fluorescent, with the apparent rate of photopolymerization and the fluorescence emission of the films being largely unaffected by the presence of the perfluoroacid.

Insight into diacetylene photopolymerization in Langmuir‐Blodgett films using simultaneous AFM and fluorescence microscopy imaging

“…An AFM topography image of a mixed PF‐PCDA film after polymerization is shown in Figure A. As reported previously, the appearance of elevated features is evidence of photopolymerization of PCDA. As noted above, red phase PDA and blue phase PDA are the two main phases reported .…”

“…In this study, a combined AFM/fluorescence microscope (FM) was used to image LB films comprised of a mixture of PF and PCDA surfactants. As demonstrated in previous reports, these mixed surfactants are amenable to AFM imaging as they adhere well to a solid support, in this work, microscope cover glass. In addition, PCDA shows a 0.8 nm lower profile than PF, which is easily detectable by AFM, and when it is illuminated and polymerized, the photoproduct is several nanometers in height .…”

“…In addition to this information, the coupled AFM‐FM microscope also revealed that the product of illumination was primarily the red phase polymer (~80%) although there was a sizable fraction (~ 20%) of the blue phase polymer remaining. In addition to determining the extent of polymerization and the relative fraction of the two forms of polymer, AFM height differences could also be used to distinguish any residual bare glass substrate from unpolymerized PCDA (see reference). Thus, the correlated imaging approach could distinguish between PF, PCDA, bare glass substrate or either of the polymers, and the relative fraction of these materials.…”

“…After the deposition pressure was reached, the film was allowed to stabilize for 20 minutes before the glass substrate was pulled upward through the water/air interface. Film deposition was carried out at a constant surface pressure of 15 mN/m, which has been shown previously to yield high‐quality deposited monolayers . The film was left to dry at room temperature for 1 hour before further measurement in the AFM.…”

“…By using AFM and polarized fluorescence microscopy individually, fibers of polymer strands were characterized in topography images and analyzed for orientation. Fluorescence images and the emission spectrum of polymerized samples were reported . An important remaining question in these systems is if the entire photopolymer film is in the red phase, the blue phase, or a mixture of the two.…”