Rheological Behavior of Precursor PPV Monolayers

The rheological behavior of different precursor poly(p-phenylene vinylene) (prec-PPV) monolayers at the air-water interface was investigated using an interfacial stress rheometer (ISR). This device nicely reveals a transition of the precursor poly(2,5-dimethoxy-1,4 phenylene vinylene) (prec-DMePPV) monolayer from Newtonian to elastic behavior with increasing surface pressure. The transition is accompanied by an increase in the modulus. This behavior coincides with the coagulation of different 2D condensed domains as revealed by Brewster angle microscopy (BAM). However, partly converted prec-DMePPV monolayers show elastic behavior even at low surface pressures, although a sudden increase of the moduli does occur. This phenomenon is attributed to enhanced hydrophobic interactions between the conjugated moieties in the partly converted polymers. The latter also explains the stretching behavior of the partly converted prec-DMePPV upon transfer in Langmuir-Blodgett-type vertical dipping. The increase of the moduli which is observed is much more gradual in the precursor poly(2,5-dibutoxy-1,4-phenylene vinylene), prec-DBuPPV, a monolayer which is in agreement with the expected expanded state of the latter monolayer.

Structural Study of Langmuir-Blodgett Monolayers and Multilayers of Amylose-Esters with Various Alkyl Chains

A series of amylose-esters with various side chain lengths was studied. Isotherms of the monolayers on the water surface were measured, and polarized IR measurements, small-angle X-ray scattering, and ellipsometric measurements were carried out in order to characterize the structure of the multilayers formed. Amylose-esters with short alkyl side chains appear to have a helical conformation on the water surface which can be transferred into multilayers. The amylose-palmitate ester, however, as an example of an amylose-ester with long alkyl side chains, can only be transferred into mulilayers when the degree of substitution is lower than about 2.3 and only then when the monolayer is in the liquid analogous state to some extent. Amylose-palmitate forms partly ordered Y-type multilayers with side-chains that have a preferential orientation perpendicular to the surface.

Dipping induced orientation in Langmuir–Blodgett multilayers of a flexible PPV precursor polymer

The Langmuir monolayers of a chloroform soluble dimethoxy substituted precursor poly(p-phenylenevinylene) (PPV) with a methoxy leaving group were studied. During storage in chloroform the precursor polymer is partly converted to PPV. Multilayer films could only be prepared of this partly converted precursor. When a monolayer of this polymer containing 5% PPV units was deposited, an orientation of the polymer chain along the dipping direction was found. Whereas a monolayer containing 14% PPV units showed no orientation in the dipping direction. It is assumed that a monolayer of the precursor containing 5% PPV units can be considered as a 2D flexible network and the obtained orientation in the dipping direction is due to stretching of this network during deposition.

In-Plane Orientation in Langmuir-Blodgett Multilayers of a Partly Converted Flexible Poly(p-phenylenevinylene) Precursor Polymer

The dimethoxy-substituted precursor poly(p-phenylenevinylene) (prec-DMePPV) was partly converted to dimethoxy-PPV by exposing the precursor solution in chloroform to daylight. The monolayer and transfer behavior of this partly converted precursor (pc-DMePPV) was studied with the Langmuir-Blodgett technique. In contrast to prec-DMePPV, an excellent transfer behavior was found for pc-DMePPV, and it was possible to build up multilayer films. The observed Z-type transfer behavior was peculiar, because the transfer ratio was higher than 1, and after deposition the monolayer expanded. Although the pc-DMePPV is not a rigid-rod-like polymer, an in-plane orientation of the chain in the transfer direction was found in the multilayers. We assume that, due to hydrophobic intra- and intermolecular interactions between the conjugated parts in the chains, the monolayer of pc-DMePPV can be considered as a 2D physical network, and the obtained orientation in the dipping direction is due to the stretching of this network during deposition. After each dip the monolayer relaxes, restoring the free volume again as observed by the expansion of the film. After full conversion to dimethoxy-PPV (DMePPV) by thermal treatment of the multilayer, the dipping-induced in-plane orientation was preserved.

Structural Study of Langmuir-Blodgett Mono- and Multilayers of Poly(β-hydroxybutyrate)

In this study poly(β-hydroxybutyrate) (PHB) was used as a spreading material to form LB monolayers at the air-water interface. Isotherms were found to show a transition at about 14 mN/m that is argued to be associated with a phase transition in the monolayer. Stable monolayers could be obtained at various surface pressures before as well as beyond the transition. Multilayers on substrates were investigated with FT-infrared techniques and were shown to have crystalline characteristics. Hysteresis experiments confirm the occurrence of irreversible processes in the monolayer during compression. Transmission electron microscopy pictures clearly show the structural changes that appear with increasing stabilization surface pressure of the monolayer. At large areas PHB exhibits an expanded monolayer behavior. Under the influence of surface pressure PHB is argued to change into a crystalline structure and eventually to form a bilayer of helical molecules which is reflected in the shape of the isotherm. S-shaped stabilization curves are argued to be the result of an accelerated bilayer formation process, as is confirmed by TEM pictures of this layer. Infrared external reflection spectroscopy of a PHB monolayer on the water surface gives clear indications that PHB already crystallizes at the air-water interface during compression, thus confirming our theorem.

Interdiffusion of Thin Polymer Layers Studied by External Reflection Infrared Spectroscopy

Polymer interdiffusion can be studied through analysis of the interface between two polymers, using techniques such as small-angle X-ray scattering and small-angle neutron scattering for systems where one component has been dispersed in the other and Rutherford backscattering forward recoil spectroscopy, secondary ion mass spectroscopy, neutron reflection, infrared microdensitometry, and ATR-infrared spectroscopy for bilayer samples. For these bilayer systems we have used an external reflection IR technique, evaluating the experimental spectra by comparison with simulated spectra of mixed and unmixed systems.

Langmuir-Blodgett Films of Metal Complexes of 4-(10,12-Pentacosadiynamidomethyl)pyridine: A Structural Investigation

Complex formation between 4-(10,12-pentacosadiynamidomethyl)pyridine and metal ions in the subphase results in stable Langmuir monolayers up to surface pressures of 35mNm-1. Electron microscopy pictures show a flat monomer monolayer before polymerization and a polymer monolayer exhibiting a more striated structure after polymerization. Multilayers of the amphiphile can only be built up after complexation with metal ions. X-ray photoelectron spectroscopy (XPS) measurements definitely confirm the presence of metal ions in the multilayers, and the molar ratio between metal and amphiphile is derived from the spectra. These multilayers are further characterized bymeansof small-angle X-ray reflection measurements and Fourier transform infrared (FT-IR) spectroscopy, which show that the amphiphile has a large tilt angle (α) with respect to the surface normal. The multilayers are polymerized by means of UV irradiation, and UV-vis spectroscopy is used to study the polymerization process. The structural changes during polymerization are deduced from small-angle X-ray reflection measurements and FT-IR spectroscopy. In all cases, the bilayer spacing decreases during the polymerization process, whereas in some cases the regular plane of the all-trans conformation of the alkyl chains is converted to an irregular one containing gauche conformations. For multilayers built up from a CuCl2-containing subphase, the whole distinct layer structure is destroyed during the polymerization process.