Dynamics of photoinduced orientational order of azo-dyes in polymer films

Dynamical and spectral behavior of photoassisted electrical poling and photoinduced anisotropy are studied in films of DR1-doped PMMA, DR1-PMMA copolymer and new DR1-Polyimide copolymer. The orientational mobility of DR1 is characterized both in ordering processes (angularly selective trans-cis photoisomerization) and in disordering relaxation processes. Mobility is increased by the photoisomerization and decreases slowly with time, after the end of optical pumping. The transient dichroism recorded simultaneously on six probe wavelengths shows a spectral inhomogeneity of the absorption band of DR1 and of the photoinduced anisotropy (particularly in polyimide films).     

Modulation Stark spectroscopy of holes: Stark effect on hole burning in the absorption spectra of octaethylchlorin in polymer film

The modulation Stark spectroscopy method is applied to study the main characteristics of holes burnt by a laser in inhomogeneously broadened absorption spectra of octaethylchlorin in thin polymer films at 4.2 K. The method used is characterised by high sensitivity, it allows us to detect holes with depth 10^?4 optical density units.     

Polarization and orientational order in polymer ferroelectric films based on vinylidene fluoride

Phase transitions and the development of orientational order are studied for three-dimensional polymer systems with the anisotropy of local intra- and interchain orientational-deformation interactions of chains with the dipole-type potential. In the proposed model of chains composed of elastically deformed segments with a fixed mean-square length (in the modified model of Gaussian subchains), there is a certain critical temperature at which the second-order phase transition from the isotropic state to the orientationally ordered state occurs. The temperature dependences of the parameter of the dipole order for thick films are calculated, and these dependences are compared with the corresponding dependences within the mean-field approximation according to the Ising model for ferromagnetics and within the Langevin continuum model for ferroelectric materials as well as with the experimental data on the thermal depolarization in the films based on the vinylidene fluoride-trifluoroethylene copolymer. The order parameter is calculated as a function of the film thickness (the length of chains) under certain boundary conditions imposed on film ends, and the calculated values are compared with the values predicted by the phenomenological theory and with the experimental data on the polarization distribution in the ferroelectric films based on vinylidene fluoride.     

Ultra-violet-visible spectroscopy of polymer films: reduction of scattering losses

An attachment to a single beam commercial spectrophotometer is described effectively eliminating the need for forward scattering corrections in transmittance measurements of films and of materials incorporated into films, in the visible-u.v. region.     

Dynamic ft-ir spectroscopy of polymer films and liquid crystals

The use of continuous-scan and step-scan Fourier transform infrared (FT-IR) spectroscopic techniques to study the dynamics of the response of polymer films and liquid crystals to external perturbations is described here. The first application of dynamic stepscan FT-IR deals with the response of various polymer films to sinusoidally modulated tensile strain. In these experiments, a small amplitude sinusoidal stress is applied to a polymeric film and the transition dipole responses are monitored as a phase lag with respect to the external perturbation. The degree of deformation is small enough to cause only linear reversible responses to the sample. The main advantage of the technique is that it can provide valuable information at the molecular level that can be used to interpret the macroscopic properties of the polymeric material under investigation. Results for heterogeneous polymers include semicrystalline high density/low density polyethylene blends and the micro-phase separated copolymer Kraton^r̀ are presented. In addition, continuous-scan stroboscopic FT-IR was used to explore the submolecular (functional group) contributions to the reorientation dynamics of the liquid crystal director in response to pulsed (DC) electric fields. For the nematic liquid crystal molecules, 4-pentyl-4'-cyanobiphenyl (5CB) and 4-pentyl-4'-cyanophenylcyclohexane (5PCH), the individual response of the rigid and floppy parts was examined.     

Dye doped polymer films: From supramolecular photochemistry to the molecular computer

A new concept of “molecular computing” based on spectral hole-burning, the interaction of molecular energy levels with an externally applied electric field and the interferometric properties of holography is presented. The molecular processor introduced here relies on the spectroscopic properties of a dye doped polymer film, which simultaneously can be used for image recording and for parallel processing of the stored information.     

Study of orientational ordering in discotic liquid-crystalline thin films by using Fourier transform infra-red spectroscopy

A method of tilting a sample out of the normal incident plane of an infra-red beam has been developed for investigating the arrangement of the various segments of discotics in three dimensions. It is found that the discs of the molecules lie flat on the silicon windows and are arranged in the columns. It is also shown that the C-C stretching vibrations are randomly distributed along the disc. More precise assignment of some of the vibrational bands has been achieved with this method.     

Molecular weight effects in the third-harmonic generation spectroscopy of thin films of the conjugated polymer MEH-PPV

Thin spin-cast films of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were prepared from samples whose weight-average molecular weight (Mw) was varied in the range of 10-1600 kg/mol. We have characterized the films by means of transmission and reflection ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy to derive the linear optical constants, and third-harmonic generation spectroscopy with variable laser wavelengths to get the modulus and phase angles of the complex third-order nonlinear optical susceptibility chi(3). Increasing molecular weight yields films with significantly larger chi(3) values, absorption coefficients, and refractive indices. The chi(3) values of films from the largest and lowest Mw differ by a factor of 4, which is caused by chain orientation effects, local field effects, and changes of the effective conjugation length.     

Multi-dimensional spectroscopy of polymer films; surface and interfacial interactions

Behavior of macromolecules near surfaces and interfaces of polymeric thin films and coatings may play a vital role in numerous applications. Therefore, understanding of molecular level processes responsible for durability, adhesion, and many other macroscopic processes is of a particular importance. This presentation will focus on stratification processes in multi-component polymeric films, with particular emphasis to polymer-surfactant interactions in latexes, responsiveness of individual components during coalescence of water-borne polyurethanes, and behavior of thermoplastic olefins (TPO). The presence of macromolecular arrangements and interactions among various components near the film-air (F-A) and the film-substrate (F-S) interfaces can be effectively monitored using attenuated total reflectance (ATR) and step-scan photoacoustic (SS-PA) Fourier transform infrared (FT-IR) spectroscopy. Both approaches are capable of obtaining information from various surface depths and complement each other if one seeks molecular level information from 0 – 150 μm into the film. If one combines ATR and PA information with IR and/or Raman surface imaging, it is possible to obtain a 3-dimensional representation of polymeric films.     

Conformational analysis by UV spectroscopy: the decisive contribution of environment-induced electronic Stark effects

UV chromophores are frequently used as probes of the molecular structure. In particular, they are sensitive to the electric field generated by the molecular environment, resulting in the observation of Stark effects on UV spectra. While these environment-induced electronic Stark effects (EI-ESE) are already used for conformational analysis in the condensed phase, this work explores the potential of such an approach when performed at much higher conformational resolution in the gas phase. By investigating model alkali benzylacetate and 4-phenylbutyrate ion pairs, where the electric field applied to the phenyl ring is chemically tuned by changing the nature of the alkali cation, this work demonstrates that precise conformational assignments can be proposed based on the correlation between the conformation-dependent calculated electric fields and the frequency of the electronic transitions observed in the experimental UV spectra. Remarkably, the sole analysis of Stark effects and fragmentation patterns in mass-selected UV spectra provided an accurate and complete conformational analysis, where spectral differences as small as a few cm⁻¹ between electronic transitions were rationalized. This case study illustrates that the identification of EI-ESE together with their interpretation at the modest cost of a ground state electric field calculation qualify UV spectroscopy as a powerful tool for conformational analysis.

The correlation between experimental electronic transitions and calculated electric fields leads to precise conformational assignments and opens up the possibility of interpreting electronic spectra in a quantitative manner at the wavenumber scale.     

Photoluminescent mechanism of a proton-transfer laser dye in highly doped polymer films

Photoluminescent properties in thin films of the proton-transfer laser dye, 2-(2-hydroxyphenyl)benzothiazole (HBT) were investigated, when it was doped into hole-transport polymer, poly(N-vinylcarbazole) and when it was codoped with hole-transport small molecule, N,N′-di(m-tolyl)-N,N′-diphenylbenzidine (TPD) into polystyrene. The more the doping concentration of HBT was raised up to about 40 wt%, the more its photoluminescent intensity was enhanced without showing excimer or exciplex emissions. The mechanism for such phenomena was discussed in connection with the excited-state intramolecular proton-transfer reaction of HBT.     

Time-resolved total internal reflection fluorescence spectroscopy of polymer films

Using total internal reflection, the possibility of a subnanosecond fluorescence spectroscopy for elucidating photophysical and photochemical processes of polymer surface is demonstrated. The thickness which can be studied under the present experimental conditions is of the order of 0.01 μm.     

Depth profiling of polymer films by confocal Raman spectroscopy

Confocal Raman microspectroscopy has many potential applications in the study of polymer-solvent interactions, including the determination of solvent and polymer-solvent complex depth profiles. This contribution focuses on preventing the formation of polymer-solvent complexes, using surface chemical modification of PVC films. While the surface-specific nature of the film modification is easily demonstrated,^[1] confocal Raman measurements clearly show the effects of film refractive index: the modifier depth profile shows a lack of symmetry and the film thickness is underestimated. A spectral normalisation method is described, and this is shown to result in a modifier depth profile which is in good agreement with data obtained by Raman microspectroscopy following physical cross-sectioning of a sample. Alternative techniques for Raman depth profiling are also discussed.     

Short-range orientational order and thermodynamic properties of polymer solutions

The concept of short-range orientational order was applied to the calculation of thermodynamic characteristics of polymer solutions in terms of the lattice model. It was shown that allowance for short-range order makes it possible to explain negative values of the entropy of mixing and the existence of a lower critical solution temperature in both polymer solutions and solutions of low-molecular-mass compounds. It was found that systems with lower critical solution temperature can exist even when the degree of orientational order in solutions slightly increases as compared with the corresponding values of this parameter in their components.     

p-Benzoquinone in aqueous solution: Stark shifts in spectra, asymmetry in solvent structure

Results from a simulation of p-benzoquinone (PBQ) in water is presented. An explicit solvent representation is used together with a multiconfigurational ab initio quantum chemical method. The electronic n --> pi* transitions are studied in aqueous solution and the two such transitions are both blue-shifted but to different degree. Both non-equilibrium and many-body effects are found to have decisive influence on the solvation: despite stronger hydrogen bonding between solute and solvent in an excited state than in the ground state, there is a blue-shift, and the solvent structure around the non-polar PBQ is asymmetric, which is argued to come from special many-body effects. The unusual result of strengthened hydrogen bonds in the excited state that follows from an excitation of a non-bonding electron on a proton acceptor, is explained by the near-linear Stark shift that is present in the transition.     

Stark absorption spectroscopy of indole and 3-methylindole

Indole and 3-methylindole (3-MI) doped into a polymethylmethacrylate (PMMA) film are studied by the Stark absorption (electroabsorption) spectroscopy. The 1La and 1Lb absorption bands are distinguished and the change in permanent dipole moment on 1La excitation is determined by a model fit to the measured absorption and electroabsorption spectra. Analysis of the spectra, measured at normal incidence and magic angle conditions, proved the essential role of the electric-field-induced orientation/alignment effects for polar indole and 3-MI molecules in the PMMA environment at room temperature.     

Electro-optical and dielectric study of multi-walled carbon nanotube doped polymer dispersed liquid crystal films

The self-organising property of nematic liquid crystals (LCs) was used to align multi-walled carbon nanotubes (MWCNT) dispersed in them. MWCNT not only well integrate in the matrix but also, even at very low concentration, have a detectable effect on the LC properties that can be very attractive for display applications. In the present work, MWCNT were doped (0–0.5% wt/wt) in two different types of LCs. These MWCNT doped polymer dispersed LC (CPDLC) films were studied comprehensively using fundamental techniques. Polarising optical microscope (POM) and scanning electron microscope (SEM) techniques used for morphological study reveal that the LC droplet size remains unchanged with increase in MWCNT concentration. The electro-optical (EO) study performed by increasing voltage in steps of 10 V up to 100 V at an optimised frequency of 200 Hz and at temperature 25°C shows that the low MWCNT concentration films show good optical response than the higher one. The dielectric behaviour of CPDLC films in the frequency range 20 Hz to 20 MHz was investigated using precision impedance analyser. The obtained data were modelled with Debye and Cole-Cole methods to calculate relaxation time and distribution parameter (α). The zero value of α indicates Debye type relaxation processes.     

Interfacial effects on moisture absorption in thin polymer films

Moisture absorption in model photoresist films of poly(4-hydroxystryene) (PHOSt) and poly(tert-butoxycarboxystyrene) (PBOCSt) supported on silicon wafers was measured by X-ray and neutron reflectivity. The overall thickness change in the films upon moisture exposure was found to be dependent upon the initial film thickness. As the film becomes thinner, the swelling is enhanced. The enhanced swelling in the thin films is due to the attractive nature of the hydrophilic substrate, leading to an accumulation of water at the silicon/polymer interface and subsequently a gradient in concentration from the enhancement at the interface to the bulk concentration. As films become thinner, this interfacial excess dominates the swelling response of the film. This accumulation was confirmed experimentally using neutron reflectivity. The water rich layer extends 25 +/- 10 A into the film with a maximum water concentration of approximately 30 vol %. The excess layer was found to be polymer independent despite the order of magnitude difference in the water solubility in the bulk of the film. To test if the source of the thickness dependent behavior was the enhanced swelling at the interface, a simple, zero adjustable parameter model consisting of a fixed water rich layer at the interface and bulk swelling through the remainder of the film was developed and found to reasonably correspond to the measured thickness dependent swelling.     

Structure of polyfurane/perchlorate doped films by FTIR spectroscopy: Effect of the synthesis conditions

The structure of stable polyfurane films doped with perchlorate anions (PFu/ClO₄), electrogenerated at constant deposition overpotential, has been studied by FTIR spectroscopy. Electropolymerization overpotential, and monomer/electrolyte concentration ratio were the variables analyzed in this study as a mean of determining the synthesis conditions that would best produce more aromatic polymers (which would imply a larger extended π‐conjugated system). The spectra revealed a clear influence of both variables on the films generated, and although ring opening was detected, justified by the existence of aliphatic CH, OH, and CO stretching vibrations, the spectra of these electroactive films showed higher aromaticity than had been previously reported by others authors. To study the aromaticity level and ring rupture of the PFu/ClO₄ films, the experimental profile of the 1829–1345 cm⁻¹ FTIR region was fitted to theoretical function, and the resulting three bands were assigned and analyzed as a function of the synthesis conditions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 291–298, 2000     

Photoactive additives for cross-linking polymer films: Inhibition of dewetting in thin polymer films

In this report, we describe a versatile photochemical method for cross-linking polymer films and demonstrate that this method can be used to inhibit thin polymer films from dewetting. A bifunctional photoactive molecule featuring two benzophenone chromophores capable of abstracting hydrogen atoms from various donors, including C-H groups, is mixed into PS films. Upon exposure to UV light, the bis-benzophenone molecule cross-links the chains presumably by hydrogen abstraction followed by radical recombination. Photoinduced cross-linking is characterized by infrared spectroscopy and gel permeation chromatography. Optical and atomic force microscopy images show that photocrosslinked polystyrene (PS) thin films resist dewetting when heated above the glass transition temperature or exposed to solvent vapor. PS films are inhibited from dewetting on both solid and liquid substrates. The effectiveness of the method to inhibit dewetting is studied as a function of the ratio of cross-linker to macromolecule, duration of exposure to UV light, film thickness, the driving force for dewetting, and the thermodynamic nature of the substrate.