Recent Progress in Sulfur-Containing High Refractive Index Polymers for Optical Applications

The development in the field of high refractive index materials is a crucial factor for the advancement of optical devices with advanced features such as image sensors, optical data storage, antireflective coatings, light-emitting diodes, and nanoimprinting. Sulfur plays an important role in high refractive index applications owing to its high molar refraction compared to carbon. Sulfur exists in multiple oxidation states and can exhibit various stable functional groups. Over the last few decades, sulfur-containing polymers have attracted much attention owing to their wide array of applications governed by the functional group of sulfur present in the polymer repeat unit. The interplay of refractive index and various other polymer properties contributes to successfully implementing a specific polymer material in optical applications. The focus on developing optoelectronic devices induced an ever-increasing need to integrate different functional materials to achieve the devices' full potential. Several devices that see the potential use of sulfur in high refractive index materials are reviewed in the study. Like sulfur, selenium also exhibits high molar refraction and unique chemical properties, making it an essential field of study. This review covers the research and development in the field of sulfur and selenium in different forms of functionality, focusing on the chemistry of bonding and the optical properties of the polymers containing the heteroatoms mentioned above. The strategy and rationale behind incorporating heteroatoms in a polymer matrix to produce high-refractive-index materials are also described in the present review.

Sulfur-Rich Polyimide/TiO2 Hybrid Materials with a Tunable Refractive Index

High refractive index (RI) polyimide/titania nanoparticle hybrid materials were synthesized and characterized in this study. The polyimide synthesis took place via the conventional polycondensation process following the preparation of poly(amic acid), and the nanoparticles were incorporated using an in situ sol-gel process. Thin films of the polyimide/titania nanoparticle hybrids were prepared by optimizing the coating conditions using a spin coater. Thermal imidization of the nanoparticle containing poly(amic acid) films on Si wafers was completed in a temperature-controlled drying oven under a N2 atmosphere. Fourier transform infrared spectroscopy revealed the successful formation of inorganic bonds as well as imide linkages, and transmission electron microscopy results show well-dispersed nanocrystalline TiO2 nanoparticles of around 5 nm in the polymer matrix. Thorough optimization of the reaction time and concentration of TiO2 precursors enabled to achieve a titania content as high as 30% (wt). The RI of the resultant hybrid materials was found to be tunable according to the titania content, while the RI increased linearly with increasing titania content. A homogeneous hybrid material with a very high RI of 1.84 at 589 nm was achieved in this work for 30% (wt) TiO2.

High Refractive Index Polymer Thin Films by Charge-Transfer Complexation

High refractive index polymers are essential in next-generation flexible optical and optoelectronic devices. This paper describes a simple synthetic method to prepare polymeric optical coatings possessing high refractive indexes. Poly(4-vinylpyridine) (P4VP) thin films prepared using initiated chemical vapor deposition are exposed to highly polarizable halogen molecules to form stable charge-transfer complexes: P4VP-IX (X = I, Br, and Cl). Fourier transform infrared spectroscopy was used to confirm the formation of charge-transfer complexes. Characterized by spectroscopic ellipsometry, the maximum refractive index of 2.08 at 587.6 nm is obtained for P4VP-I₂. For P4VP-IBr and P4VP-ICl, the maximum refractive indexes are 1.849 and 1.774, respectively. By controlling the concentration of charge-transfer complexes, either through the halogen incorporation step or polymer composition through copolymerization with ethylene glycol dimethacrylate, the refractive indexes of the polymer thin films can be precisely controlled. The feasibility of P4VP-IX materials as optical coatings is also explored. The refractive index and thickness uniformity of a P4VP-I₂ film over a 10 mm diameter circular area were characterized, showing standard deviations of 0.0769 and 1.91%, respectively.

Advances in Material Nanosensitization: Refractive Property Changes as the Main Parameter to Indicate Organic Material Physical-Chemical Feature Improvements

In the current paper, the results of the sensitization process’ influence on the refractive organic materials’ features are shown. The correlation between the refractive properties and the intermolecular charge transfer effect of doped organic thin films are shown via estimation of the laser-induced change in the refractive index. The refractive parameter is shown for a model organics matrix based on a polyimide doped with fullerenes, carbon nanotubes, reduced graphene oxides, etc. A second harmonic of the Nd-laser was used to record the holographic gratings in the Raman–Nath diffraction conditions at different spatial frequencies. The laser-induced refractive index change was considered to be an indicator in order to estimate the basic organic materials’ physical–chemical characteristics. Additional data are presented for the liquid crystal cells doped with nanoparticles. The correlation between the content of the nanoobjects in the organics’ bodies and the contact angle at the thin film surfaces is shown. Some propose to use this effect for general optoelectronics, for the optical limiting process, and for display application.

A Comprehensive Review on Optical Properties of Polymer Electrolytes and Composites

Polymer electrolytes and composites have prevailed in the high performance and mobile marketplace during recent years. Polymer-based solid electrolytes possess the benefits of low flammability, excellent flexibility, good thermal stability, as well as higher safety. Several researchers have paid attention to the optical properties of polymer electrolytes and their composites. In the present review paper, first, the characteristics, fundamentals, advantages and principles of various types of polymer electrolytes were discussed. Afterward, the characteristics and performance of various polymer hosts on the basis of specific essential and newly published works were described. New developments in various approaches to investigate the optical properties of polymer electrolytes were emphasized. The last part of the review devoted to the optical band gap study using two methods: Tauc's model and optical dielectric loss parameter. Based on recently published literature sufficient quantum mechanical backgrounds were provided to support the applicability of the optical dielectric loss parameter for the band gap study. In this review paper, it was demonstrated that both Tauc's model and optical dielectric loss should be studied to specify the type of electron transition and estimate the optical band gap accurately. Other parameters such as absorption coefficient, refractive index and optical dielectric constant were also explored.

Bio-Composites Consisting of Cellulose Nanofibers and Na+ Montmorillonite Clay: Morphology and Performance Property

This paper reports the usage of cellulose nanofibers (CNFs) as a continuous nanoporous matrix and nanoclay (NC) as additive to fabricate hybrid films. CNF/Cloisite Na+ nanoclay composite films containing 10-50 wt % of NC were prepared for the study. The effects of NC incorporation and its content on mechanical, wettability and thermal degradation properties were investigated. The results showed that the film had a multilayer structure with gradually deposited CNT-NC hybrid on the filter paper Pure CNF films had higher moduli compared with those from the composite films, as the incorporation of NC decreased hydrogen bonding and networking ability of CNFs, especially at the high NC loading levels. The composite films demonstrated self-extinguishing ability when being exposed to the open flame. Composites with over 35 wt % NC did not burn because of the formation of a protective barrier containing ordered NC platelets. The addition of montmorillonite NC led to increased surface water contact angle, showing enhanced hydrophobicity of the material. During the film's thermal pyrolysis, the first process occurred between 100 and 200 °C, resulting mainly from the evaporation of absorbed water; the second, between 280 and 350 °C, indicated thermal decomposition of cellulose; and the slow third stage happened from the 350 to 600 °C, representing carbonization. The results demonstrate that the apparent activation energies for all the CNF/NC composites were higher than the pure CNF film. CNF/NC films fabricated in this process are a promising barrier material for packaging applications.

Preparation of High Refractive Index Composite Films Based on Titanium Oxide Nanoparticles Hybridized Hydrophilic Polymers

Optical materials with high refractive index (n) have been rapidly improved because of urgent demands imposed by the development of advanced photonic and electronic devices such as solar cells, light emitting diodes (LED and Organic LED), optical lenses and filters, anti-reflection films, and optical adhesives. One successful method to obtain high refractive index materials is the blending of metal oxide nanoparticles such as TiO₂ and ZrO₂ with high n values of 2.1-2.7 into conventional polymers. However, these nanoparticles have a tendency to agglomerate by themselves in a conventional polymer matrix, due to the strong attractive forces between them. Therefore, there is a limitation in the blending amount of inorganic nanoparticles. In this paper, various hydrophilic polymers such as poly(N-hydroxyl acrylamide) (pHEAAm), poly(vinyl alcohol), poly(ethylene glycol), and poly(acrylic acid) were examined for preparation of high refractive index film based on titanium oxide nanoparticle (TiNP) dispersed polymer composite. The hydrogen bonding sites in these hydrophilic polymers would improve the dispersibility of inorganic nanoparticles in the polymer matrix. As a result, pHEAAm exhibited higher compatibility with titanium oxide nanoparticles (TiNPs) than other water-soluble polymers. Transparent hybrid films were prepared by mixing pHEAAm with TiNPs and drop casting the mixture onto a glass plate. The refractive indices of the films were in good agreement with calculated values. The compatibility of TiNPs with pHEAAm was dependent on the surface characteristics of TiNPs. TiNPs with the highest observed compatibility could be hybridized with pHEAAm at concentrations of up to 90 wt%, and the refractive index of the corresponding film reached 1.90. The high compatibility of TiNPs with pHEAAm may be related to the hydrophilicity and amide and hydroxyl moieties of pHEAAm, which cause hydrogen bond formation on the TiO₂ surface. The obtained thin film was slightly yellow due to the color of the original TiNP dispersion; however, the transmittance of the film was higher than 80% in the wavelength range from 480 to 900 nm.

Optically Isotropic, Colorless, and Flexible PITEs/TiO2 and ZrO2 Hybrid Films with Tunable Refractive Index, Abbe Number, and Memory Properties

A series of novel polyimidothioethers (PITEs) and the respective polymer hybrids of titania or zirconia with fantastic thermal stability and optical properties have been successfully prepared. These colorless PITEs with high transparency were synthesized by Michael polyaddition from commercially available dithiol and bismaleimides monomers. The PITE with sulfide and hydroxyl groups (S-OH) and the corresponding hybrid films declare ultra-lowest birefringence value of 0.002 and tunable refractive index (1.65–1.81 for S-OH/titania and 1.65–1.80 for S-OH/zirconia), implying large potential to the optical applications in the future. Moreover, the S-OH/zirconia hybrid films exhibit higher Abbe’s number and optical transparency than those of S-OH/titania system because larger energy band gap of ZrO₂. Furthermore, by adding titania and zirconia as electron acceptor into S-OH system, the charge transfer complex can be facilitated and stabilized caused by the lower LUMO energy level of hybrid materials. Consequently, the devices of memory prepared from these polymer films of hybrid showed interesting and adjustable memory behavior from DRAM, SRAM, to WORM at various titania or zirconia contents with a large ON/OFF ratio (10⁸), denoting that the memory devices derived from these highly transparent novel S-OH/TiO₂ and S-OH/ZrO₂ hybrid films are attractive for the electrical applications.

Modification of TiO2 Nanoparticles with Oleyl Phosphate via Phase Transfer in the Toluene-Water System and Application of Modified Nanoparticles to Cyclo-Olefin-Polymer-Based Organic-Inorganic Hybrid Films Exhibiting High Refractive Indices

Oleyl-phosphate-modified TiO₂ nanoparticles (OP_TiO₂) were prepared via phase transfer from an aqueous phase containing dispersed TiO₂ nanoparticles to a toluene phase containing oleyl phosphate (OP, a mixture of monoester and diester), and employed for the preparation of OP_TiO₂/cyclo-olefin polymer (COP) hybrid films with high-refractive indices. The modification of TiO₂ by OP was essentially completed by reaction at room temperature for 8 h, and essentially all the TiO₂ nanoparticles in the aqueous phase were transferred to the toluene phase. The infrared and solid-state ¹³C cross-polarization and magic-angle spinning (CP/MAS) NMR spectrum of OP_TiO₂ showed the presence of oleyl groups originating from oleyl phosphate. The solid-state ³¹P MAS NMR spectrum of OP_TiO₂ exhibited new signals at -1.4, 2.1, and 4.8 ppm, indicating the formation of Ti-O-P bonds. CHN and inductively coupled plasma analyses revealed that the major species bound to the TiO₂ surface was tridentate CH₃(CH₂)₇CH═CH(CH₂)₈P(OTi)₃. These results clearly indicate that the surfaces of the TiO₂ nanoparticles were modified by OP moieties via phase transfer. OP_TiO₂/COP hybrid films exhibited excellent optical transparency up to 19.1 vol % TiO₂ loading, and the light transmittance of the hybrid films with 19.1 vol % TiO₂ loading was 99.8% at 633 nm. The refractive index of these hybrid films rose to 1.83.

Highly transparent and flexible bio-based polyimide/TiO2 and ZrO2 hybrid films with tunable refractive index, Abbe number, and memory properties

The novel bio-based polyimide (4ATA-PI) and the corresponding PI hybrids of TiO2 or ZrO2 with excellent optical properties and thermal stability have been prepared successfully. The highly transparent 4ATA-PI containing carboxylic acid groups in the backbone could provide reaction sites for organic-inorganic bonding to obtain homogeneous hybrid films. These PI hybrid films showed a tunable refractive index (1.60-1.81 for 4ATA-PI/TiO2 and 1.60-1.80 for 4ATA-PI/ZrO2), and the 4ATA-PI/ZrO2 hybrid films revealed a higher optical transparency and Abbe's number than those of the 4ATA-PI/TiO2 system due to a larger band gap of ZrO2. By introducing TiO2 and ZrO2 as the electron acceptor into the 4ATA-PI system, the hybrid materials have a lower LUMO energy level which could facilitate and stabilize the charge transfer complex. Therefore, memory devices derived from these PI hybrid films exhibited tunable memory properties from DRAM, SRAM, to WORM with a different TiO2 or ZrO2 content from 0 wt% to 50 wt% with a high ON/OFF ratio (10(8)). In addition, the different energy levels of TiO2 and ZrO2 revealed specifically unique memory characteristics, implying the potential application of the prepared 4ATA-PI/TiO2 and 4ATA-PI/ZrO2 hybrid films in highly transparent memory devices.

Fabrication of Tunable, High-Refractive-Index Titanate-Silk Nanocomposites on the Micro- and Nanoscale

The combination of water-based titanate nanosheets dispersion and silk fibroin solution allows the realization of a versatile nanocomposite. Different fabrication techniques can be easily applied on these nanocomposites to manipulate the end form of these materials on the micro- and nanoscale. Easy tunability of the refractive index from n = 1.55 up to n = 1.97 is achieved, making it attractive for flexible, biopolymer-based optical devices.

Refractive index investigation of poly(vinyl alcohol) films with TiO2 nanoparticle inclusions

The refractive index (RI) of polymer nanocomposite of poly(vinyl alcohol) films with TiO(2) nanoparticle inclusions with low concentration up to 1.2 wt. % was investigated. Accurate refractometric measurements, by a specially designed laser microrefractometer, were performed at wavelengths 532 and 632.8 nm. The influence of TiO(2) concentration on the RI dispersion curves was predicted based on the well-known Sellmeier model. The theoretical analysis, in a small filling factor approximation, was performed, and a relation between the effective RI of the nanocomposite and weight concentrations of the TiO(2) nanofiller was derived. The experimental values were approximated by two different functions (linear and a quadratic polynom). The polynomial approximation yields better result, where R(2)=0.90.

Novel high refractive index positive-tone photosensitive polyimide for microlens of image sensors

A novel high refractive index positive-tone photosensitive polyimide (posi-PSPI) has been successfully developed. The posi-PSPI has a refractive index of 1.78 at 633 nm and good transparency (> 80% at 400 nm). It is prepared from an alkali—soluble transparent polyimide precursor, a diazo-naphthoquinone-type photoactive compound, a surface-modified high refractive index nanosol, and a UV absorbent. The posi-PSPI has a round shape because of the UV absorbent. Further, it displays good sensitivity as well as a good round pattern profile after thermal curing. In addition, the 1% weight loss temperature of the posi-PSPI is 407 °C, which indicates that the posi-PSPI has excellent thermal stability.