Effect of metal cations on polydiacetylene Langmuir films.

Polydiacetylene (PDA) Langmuir films (LFs) are a unique class of materials that couple a highly aligned conjugated backbone with tailorable pendant side groups and terminal functionalities. The films exhibit chromatic transitions from monomer to blue polymer and finally to a red phase that can be activated optically, thermally, chemically, and mechanically. The properties of PDA LFs are strongly affected by the presence of metal cations in the aqueous subphase of the film due to their interaction with the carboxylic head groups of the polymer. In the present study the influence of divalent cadmium, barium, copper, and lead cations on the structural, morphological, and optical properties of PDA LFs was investigated by means of surface pressure-molecular area (π-A) isotherms, atomic force microscopy, optical absorbance, and Raman spectroscopy. The threshold concentrations for the influence of metal cations on the film structure, stability, and phase transformation were determined by π-A analyses. It was found that each of the investigated cations has a unique influence on the properties of PDA LFs. Cadmium cations induce moderate phase transition kinetics with reduced domain size and fragmented morphology. Barium cations contribute to stabilization of the PDA blue phase and enhanced linear strand morphology. On the other hand, copper cations enhance rapid formation of the PDA red phase and cause fragmented morphology of the film, while the presence of lead cations results in severe perturbation of the film with only a small area of the film able to be effectively polymerized. The influence of the metal cations is correlated with the solubility product (K(sp)), association strength, and ionic-covalent bond nature between the metal cations and the PDA carboxylic head groups.

Phase transition kinetics in Langmuir and spin-coated polydiacetylene films.

Thin films of 10,12-pentacosadiynoic acid were prepared using Langmuir and spin-coating techniques and polymerized using a controlled dosage of UV radiation. The radiation-induced phase transitions: from the colorless monomer, via the metastable blue phase, to the red polydiacetylene phase, and finally to degradation of the material, were monitored by optical absorbance spectroscopy. Deconvolution analysis of the absorbance curves allowed us to monitor quantitatively the dynamical changes in the chromatic properties of the films as a function of applied UV radiation dose. Several reaction kinetics models were applied in order to describe the phase transitions in the films. The results present the phase evolution in PDA and compare the kinetics for Langmuir films vs. spin-coated films. Polymerization directly at the air-water interface was found to be two-to-three orders of magnitude faster compared to solid-supported films of the same material. Moreover, we show that the data of the solid supported films is considerably better fitted when a reversible intermediate phase between the blue and the red phases is considered. Furthermore, a shift of the Raman active triple bond supports the presence of the intermediate phase.

Structural transitions in polydiacetylene Langmuir films.

Polydiacetylene (PDA) Langmuir films (LFs) were investigated directly at the air/water interface using in situ synchrotron grazing incidence X-ray diffraction, and ex situ transmissison electron microscopy and diffraction. The films were compressed and polymerized on pure water. A crystallographic model describes the structures and phase transitions of the unpolymerized (monomer) film, via the metastable (blue phase), to the fully stable PDA red phase as a function of irradiation dose. The monomer-to-blue-to-red chromatic phase transitions are accompanied by changes in the in-plane crystal structure and pendant chains packing arrangement from arced alkyl chains (in the monomer and blue phases) to near-vertical closely packed chains in the red phase. Notably, the characteristic linear strand morphology of PDA films can be explained as a direct result of the marked decrease in spacing between adjacent polymer chains upon transition from the blue to the red phase.