J. F. Rabolt

Anisotropic orientation in molecular monolayers by infrared spectroscopy

Fourier transform IR measurements of Langmuir–Blodgett monolayers have been made with both grazing incidence reflection and transmission techniques. The reflection arrangement at incident angles from 85° to 88° has the electric field component perpendicular to the substrate surface while the standard transmission arrangement has the electric field component in the plane of the film. These techniques were used to explore the anisotropic arrangement of cadmium arachidate molecules perpendicular and parallel to the monolayer plane. The implication of these results for characterizing the orientational anisotropy of chemical groups in thin polymer films is also discussed. In addition, the question of uniaxial and biaxial orientation in the monolayer plane was addressed because of the importance of intermolecular packing on both the extent and rate of polymerization in unsaturated fatty acid monolayer components which form synthetic membranes.

‘Organic metals’: polypyrrole, a stable synthetic ‘metallic’ polymer

Highly stable, flexible films of polypyrrole with p-type conductivities of 100 Ω–1 cm–1 have been prepared by electrolytic oxidation of the appropriate pyrrole monomers; similarly prepared films of mixtures of pyrrole and N-methylpyrrole have conductivities between 5 × 10–3 and 100 Ω–1 cm–1 depending upon the composition.

Order–disorder transitions in Langmuir–Blodgett monolayers. I. Studies of two‐dimensional melting by infrared spectroscopy

A two step melting process in Langmuir–Blodgett (LB) monolayer assemblies of cadmium arachidate (CdA) has been studied by infrared spectroscopy in conjunction with thermal measurements. These techniques have allowed, for the first time, the investigation of detailed structural changes which occur during heating, melting, and annealing cycles. Prior to the melting point, a pretransitional disordering of the hydrocarbon tails was observed. This was followed by a catastrophic breakup of the cadmium lattice at a temperature corresponding to the melting point as measured by differential scanning calorimetry (DSC).

Vibronic intensity enhancement in the infrared spectra of heavily doped (CH)x and (CD)x

Fourier Transform IR transmission spectroscopy has been utilized to elucidate the nature of the polymer–dopant interaction in doped (CH)x. Significant changes in the IR spectrum occur after doping and several intense bands at 1385, 1295, and 832 cm−1 have been observed and their shifts studied upon deuteration. The origin of these intense features can be attributed to a vibronic activation and enhancement of the totally symmetric Raman active Ag modes in the IR spectrum.

Studies of chain conformational kinetics in poly(di‐n‐alkylsilanes) by spectroscopic methods 2. Conformation and packing of poly(di‐n‐hexylsilane)

Unoriented and highly oriented films of poly(di‐n‐hexylsilane) have been studied by Raman scattering, wide angle x‐ray diffraction and optical absorption measurements. From a comparison of group theoretical predictions with those bands observed in the Raman experiments and from the x‐ray layer line spacing from an oriented sample, a planar zig–zag conformation for the silicon backbone was deduced. X‐ray reflections were suitably indexed by a monoclinic unit cell containing two molecules. Polarized Raman studies on uniaxially oriented samples also revealed that the hexyl side chains are not orthogonal to the silicon backbone but may be slightly tilted in order to minimize intramolecular steric interactions.

Polymer chain organization and orientation in ultrathin films: A spectroscopic investigation

The effect of confinement on the crystallinity and chain orientation of ultrathin poly(di-n-hexylsilane) films has been investigated using UV absorption, fluorescence and IR spectroscopies. UV absorption measurements in a series of poly(di-n-hexylsilane) films having thicknesses between 50 and 3500 A have shown that, for thicknesses less than 500 A, the polymer backbone disorders and extensive crystallization of the films is hindered irrespective of molecular weight or surface hydrophobicity. Fluorescence studies showed that rapid energy transfer occurs from the disordered chain segments to the crystalline ones. The orientation of both the polymer backbone and side chains was probed with IR absorption and grazing incidence reflection measurements. The side chains are extended, although not completely in the all-trans conformation, with their carbon plane mostly perpendicular to the substrate. The backbone lies extended, with the polymer axis parallel to the plane of the film. The hexyl side-chains disorder in films less than 2000 A thick and this disordering occurs through the introduction of gauche bonds. Our findings suggest the possibility of using thickness to control the chain organization and morphology of a polymer thin film.

Order–disorder transitions in Langmuir–Blodgett films. IV. Structure of [F(CF2)8(CH2)10COO−]2 Cd2+ multilayers at ambient and elevated temperatures

Langmuir–Blodgett multilayered (6–12) films of a semifluorinated fatty acid, F(CF2)8(CH2)10COOH, have been deposited on transmission and reflection substrates and investigated using polarized infrared (IR) spectroscopy. The spectra revealed that the cadmium salt of this fatty acid was transferred and not the acid itself. Unlike its hydrocarbon analog, [F(CF2)8(CH2)10COO−]2 Cd2+ is not oriented normal to the surface but is inclined due to the packing constraints dictated by the considerably larger cross section of the fluorocarbon part of the molecule. At elevated temperatures, the orientation of the fluorocarbon chain is observed to change and this has been attributed to disordered conformations preferentially introduced in the hydrocarbon portion of the molecule due to its inherent flexibility relative to the rigid fluorocarbon helix.

Studies of chain conformational kinetics in poly(di‐n‐alkylsilanes) by spectroscopic methods. III. Fourier transform Raman studies of unsymmetric side chain substitution

Fourier transform Raman (FT‐Raman) spectroscopy has been used to acquire spectra of polysilane derivatives in the absence of resonance effects arising from electronic absorptions of these materials between 320 and 370 nm. The nonresonant FT‐Raman spectra allow detailed analysis of the conformation of and lateral interactions between alkyl side groups. Comparison of spectra for the series of unsymmetrically substituted compounds poly(n‐pentyl,n‐hexylsilane), poly(n‐heptyl,n‐hexylsilane), and poly(n‐octyl,n‐hexylsilane) with the symmetrically substituted analog poly(di‐n‐hexylsilane) reveals various degrees of intra‐ and intermolecular disorder due to mismatch of side chain length. The induced disorder is well correlated with transition temperatures and UV‐vis absorption maxima.

Molecular orientation in thin monolayer films by infrared spectroscopy

Abstract Fourier transform infrared spectroscopic measurements have been made on monolayer samples of cadmium arachidate in order to determine orientation and molecular packing on the surface. This was accomplished by using both grazing angle reflection methods, where the polarization of the infrared radiation is very close to being perpendicular to the surface, and transmission methods, where the incident optical electric field is polarized parallel to the surface. Hence these two methods are sensitive to molecular vibrations whose change in dipole moment lies along different directions. Our results showed that independent of the substrate, silver for the reflection experiments and silver bromide for the transmission experiments, the chains of the fatty acid salt (no evidence for any free acid was found) are oriented within a few degrees of the normal to the surface of the substrate. From a detailed analysis of the observed vibrational bands in the two orientations, combined with the known literature values and assignments, we were able to make a “complete assignment” of the observed bands. Our experimental results and conclusions will be presented.

Fourier Transform Infrared Study of the Polymer-Dopant Interaction in Polyacetylene.

Fourier transform infrared (FTIR) measurements of AsF5 doped (CH)x with dopant concentrations which vary over three orders of magnitude have been obtained in both the mid (4000–400 cm−1) and far (400–15 cm−1) infrared. The bandwidths of the strong bands characteristic of the doped polymer have been observed to be invariant with dopant concentration. However, the frequency positions of these new IR bands are found to be a sensitive function of the dopant level. In the far infrared, no evidence of the predicted soliton pinning mode was found for any of the dopant concentrations studied. The consequence of these results, in view of current theoretical models of the polymer‐dopant interaction, is discussed.