New Insights on Solvent-Induced Changes in Refractivity and Specific Rotation of Poly(propylene oxide) Systems Extracted from Channeled Spectra.

Investigation of chiroptical polymers in the solution phase is paramount for designing supramolecular architectures for photonic or biomedical devices. This work is devoted to the case study of poly(propylene oxide) (PPO) optical activity in several solvents: benzonitrile, carbon disulfide, chloroform, ethyl acetate, and p-dioxane. To attain information on the interactions in these systems, rheological testing was undertaken, showing distinct variations of the rheological parameters as a function of the solvent type. These aspects are also reflected in the refractive index dispersive behavior, from which linear and non-linear optical properties are extracted. To determine the circular birefringence and specific rotation of the PPO solutions, the alternative method of the channeled spectra was employed. The spectral data were correlated with the molecular modeling of the PPO structural unit in the selected solvents. Density functional theory (DFT) computational data indicated that the torsional potential energy-related to the O1-C2-C3-O4 dihedral angle from the polymer repeating unit-was hindered in solvation environments characterized by high polarity and the ability to interact via hydrogen bonding. This was in agreement with the optical characterization of the samples, which indicated a lower circular birefringence and specific rotation for the solutions of PPO in ethyl acetate and p-dioxane. Also, the shape of optical rotatory dispersion curves was slightly modified for PPO in these solvents compared with the other ones.

Testing the Performance of the Azo-Polyimide Supramolecular Systems as Substrate for Sensors Based on Platinum Electrodes.

Azo-polyimide films with supramolecular structure were obtained by casting onto glass plates a mixture based on polyamidic acid and different quantities of azochromophore, followed by thermal treatment to realize the final azo-polyimide structure. The dielectric characteristics of the supramolecular structure of polymer films were investigated by broad-band dielectric spectroscopy measurements at different temperatures and frequencies. The free-standing films proved to be flexible and tough and maintained their integrity after repeated bending. The work of adhesion at the polymer/platinum interface was calculated after the evaluation of the surface energy parameters before and after plasma treatment. Atomic force microscopy was used to image the surface morphology, the evolution of the roughness parameters, and the adhesion force between the platinum-covered tip and the polymer surface, registered at the nanoscale with the quantity of the azo dye introduced in the system. The simulation of the columnar growth of a platinum layer was made to provide information about the deposition parameters that should be used for optimal results in the deposition of platinum electrodes for sensors.

Review on Optical Methods Used to Characterize the Linear Birefringence of Polymer Materials for Various Applications.

Optical polymers are recognized for their high transparency, raised flexibility, low cost, and good film-forming ability; hence, they introduce a multitude of benefits in a wide range of devices, such as information storage, displays, optical communications, and filters. Among the optical properties, birefringence is an essential parameter in practical cases that demand the control of the state of polarization of light. This review is focused on describing some fundamental and applicative aspects concerning the optical birefringence of the polymer materials. First, elementary notions depicting the phenomenon of light double refraction in macromolecular media are provided. Furthermore, the most relevant optical techniques to determine birefringence are reviewed by highlighting the working principle and mathematical basis for computing this parameter. Then, a series of investigations of optically birefringent polymers are described, summarizing the most utilized approaches to induce light double refraction in such materials. The selected results are analyzed in relation to the pursued applications. In the end, the future of this scientific domain is briefly presented by establishing the research paths that need further exploration. Moreover, the novel directions that could be formulated and might contribute to certain considerable advancements in the materials employed in the modern optical technologies are mentioned.

Polyimide-Derived Supramolecular Systems Containing Various Amounts of Azochromophore for Optical Storage Uses.

The progress of digital technologies demands more speed and larger storage capacity. Optical storage systems have the advantage of being cheap, fast and capacious. This article explores the potential use of polyimide-based films as a recording medium for optical storage devices. The materials were designed through a host-guest approach that involves a cyano-containing polyimide precursor and an azochromophore combined in the following ratios: 1:0.25, 1:0.5, 1:0.75 and 1:1. After thermal treatment up to 200 °C, polyimide systems were formed with supramolecular structures constructed via hydrogen bonding as shown by molecular modeling and FTIR at around 3350 cm-1. The aspects arising from the variation of the azo-dye content in the polyimide samples and their impact on the vitrification temperature, colorimetric features, refractive index, band gap, non-linear optical susceptibility and birefringence were investigated for the first time. The thermal analysis indicated a slight decrease in the vitrification temperature from 190.84 °C for the sample without azo dye to 163.91 °C for the film containing the highest leading of azo dye. The morphology images revealed the occurrence of periodic structures in azo-derived materials exposed to a UV laser, which is accentuated by the addition of more azo dye molecules. Optical tests allowed observation of the increase in the dominant wavelength, refractivity and optical conductivity of the samples, produced by the incorporation of azochromophore and laser irradiation. The photo-generated birefringence increased from 0.014 (sample with 1:0.25) to 0.036 (sample with 1:1), which in combination with the created regular topography pattern, is essential for the use of these materials as recording media in optical storage applications.

Polyimide Layers with High Refractivity and Surface Wettability Adapted for Lowering Optical Losses in Solar Cells.

The performance of photovoltaics with superstrate configuration is limited by the rigidity and low refractivity of a classical glass cover. In this work, two polyimides (PIs) and two copolyimides combined in the main chain cycloaliphatic moieties, aromatic sequences, chalcogen atoms, and having/lacking fluorine atoms, are proposed as shielding covers for solar cells. The samples containing small cycloaliphatic moieties displayed high transmittance above 80% at 550 nm. The refractive index values under changeable wavelengths and temperatures were shown to influence the magnitude of the reflection losses. At the sample interface with the transparent electrode, optical losses were reduced (~0.26%) in comparison to the classical glass (~0.97%). The samples with the best optical features were further subjected to a surface treatment to render the self-cleaning ability. For this, a new approach was used residing in irradiation with the diffuse coplanar surface barrier discharge (DCSBD), followed by spraying with a commercial substance. Scanning electron microscopy and atomic force microscopy scans show that the surface characteristics were changed after surface treatment, as indicated by the variations in root mean square roughness, surface area ratio, and surface bearing index values. The proposed PI covers diminish the optical losses caused by total internal reflection and soiling, owing to their adapted refractivity and superhydrophobic surfaces (contact angles > 150°), and open up new perspectives for modern photovoltaic technologies.

A New Texturing Approach of a Polyimide Shielding Cover for Enhanced Light Propagation in Photovoltaic Devices.

The efficiency of photovoltaics (PVs) is related to cover material properties and light management in upper layers of the device. This article investigates new polyimide (PI) covers for PVs that enable light trapping through their induced surface texture. The latter is attained via a novel strategy that involves multi-directional rubbing followed by plasma exposure. Atomic force microscopy (AFM) is utilized to clarify the outcome of the proposed light-trapping approach. Since a deep clarification of either random or periodic surface morphology is responsible for the desired light capturing in solar cells, the elaborated texturing procedure generates a balance among both discussed aspects. Multidirectional surface abrasion with sand paper on pre-defined directions of the PI films reveals some relevant modifications regarding both surface morphology and the resulted degree of anisotropy. The illuminance experiments are performed to examine if the created surface texture is suitable for proper light propagation through the studied PI covers. The adhesion among the upper layers of the PV, namely the PI and transparent electrode, is evaluated. The correlation between the results of these analyses helps to identify not only adequate polymer shielding materials, but also to understand the chemical structure response to new design routes for light-trapping, which might significantly contribute to an enhanced conversion efficiency of the PV devices.

New Strategy for Inducing Surface Anisotropy in Polyimide Films for Nematics Orientation in Display Applications.

The operability of liquid crystal displays is strongly impacted by the orientation aspects of nematics, which in turn are affected by the alignment layer surface features. In this work, two polyimide (PI) structures are obtained based on a cycloaliphatic dianhydride and aromatic or aliphatic diamines with distinct flexibility. The attained PI films have high transmittance (T) for visible radiations, i.e., at 550 nm T > 80%. Here, a novel strategy for creating surface anisotropy in the samples that combines rubbing with a cloth and stretching via pressing is reported. Birefringence and atomic force microscopy (AFM) scans reveal that the generated orientation of the chains is affected by the chemical structure of the polymer and order of the steps involved in the surface treatment. Molecular modeling computations and wettability tests show that the PI structure and produced surface topography are competitive factors, which are impacting the intensity of the interactions with the nematic liquid crystals. The achieved results are of great relevance for designing of reliable display devices with improved uniform orientation of liquid crystals.