Ultralong room temperature phosphorescence (URTP) emitted from pure amorphous organic molecules is very rare. Although a few crystalline organic molecules could realize URTP with long lifetimes (>100 ms), practical applications of these crystalline organic phosphors are still challenging because the formation and maintenance of high-quality crystals are very difficult and complicated. Herein, we present a rational design for minimizing the vibrational dissipation of pure amorphous organic molecules to achieve URTP. By using this strategy, a series of URTP films with long lifetimes and high phosphorescent quantum yields (up to 0.75 s and 11.23%, respectively) were obtained from amorphous organic phosphors without visible fluorescence and phosphorescence under ambient conditions. On the basis of the unique features of URTP films, a new green screen printing technology without using any ink was developed toward confidential information encryption and decryption. This work presents a breakthrough strategy in applying amorphous organic materials for URTP.

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Ultralong room temperature phosphorescence from amorphous organic materials toward confidential information encryption and decryption.

Herein we report a rational design strategy for tailoring intermolecular interactions to enhance room-temperature phosphorescence from purely organic materials in amorphous matrices at ambient conditions. The built-in strong halogen and hydrogen bonding between the newly developed phosphor G1 and the poly(vinyl alcohol) (PVA) matrix efficiently suppresses vibrational dissipation and thus enables bright room-temperature phosphorescence (RTP) with quantum yields reaching 24%. Furthermore, we found that modulation of the strength of halogen and hydrogen bonding in the G1-PVA system by water molecules produced unique reversible phosphorescence-to-fluorescence switching behavior. This unique system can be utilized as a ratiometric water sensor.

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Tailoring intermolecular interactions for efficient room-temperature phosphorescence from purely organic materials in amorphous polymer matrices

In this work, two series of nanocomposites of poly(vinylidene fluoride) (PVDF) incorporated with reduced graphene oxide (rGO) and poly(vinyl alcohol)-modified rGO (rGO-PVA) were fabricated using solution-cast method and their dielectric properties were carefully characterized. Infrared spectroscopy and atom force microscope analysis indicated that PVA chains were successfully grafted onto graphene through ester linkage. The PVA functionalization of graphene surface can not only prevent the agglomeration of original rGO but also enhance the interaction between PVDF and rGO-PVA. Strong hydrogen bonds and charge transfer effect between rGO-PVA and PVDF were determined by infrared and Raman spectroscopies. The dielectric properties of rGO-PVA/PVDF and rGO/PVDF nanocomposites were investigated in a frequency range from 10² Hz to 10⁷ Hz. Both composite systems exhibited an insulator-to-conductor percolating transition as the increase of the filler content. The percolation thresholds were estimated to be 2.24 vol % for rGO-PVA/PVDF composites and 0.61 vol % for rGO/PVDF composites, respectively. Near the percolation threshold, the dielectric permittivity of the nanocomposites was significantly promoted, which can be well explained by interfacial polarization effect and microcapacitor model. Compared to rGO/PVDF composites, higher dielectric constant and lower loss factor were simultaneously achieved in rGO-PVA/PVDF nanocomposites at a frequency range lower than 1 × 10³ Hz. This work provides a potential design strategy based on graphene interface engineering, which would lead to higher-performance flexible dielectric materials.

Improved dielectric properties of nanocomposites based on poly(vinylidene fluoride) and poly(vinyl alcohol)-functionalized graphene.

Biomaterials of PVA and PVP in medical and pharmaceutical applications: Perspectives and challenges.

Structural, optical, thermal and morphological studies were performed for pure PVA/PVP (50/50) blend and PVA/PVP blend filled with different concentrations of chitosan upto 40 wt% using casting method. The prepared films were investigated by different techniques. All techniques demonstrate the complexation between chitosan and the polymers blend. There were some changes in the IR absorption bands position and its intensities. UV–vis analysis revealed intensity of the shoulder around 273–280 nm increase with increasing chitosan content. Differential scanning calorimetry (DSC) showed thermal stability of the product samples and that has improved after filling chitosan increases. Scanning electron microscope (SEM) images of different concentrations of films revealed that large crystals characteristic of chitosan were also frequently present, when chitosan content increase upto 20 wt%, rough surfaces and crystalline structures are the dominant features. The X-ray diffraction (XRD) showed that the incorporation of chitosan into the polymeric matrix causes decreasing in the crystallinity degree of the samples and this indicates the complexation between the filler and the polymers blend. Thermogravimetric analysis (TGA) is used to study the samples in the temperature range of 30–500 °C. Figures show typical TGA thermograms of weight loss as a function of temperature for the present system.

Chitosan filler effects on the experimental characterization, spectroscopic investigation and thermal studies of PVA/PVP blend films

Microstructural studies on BaCl2 doped poly(vinyl alcohol)

Microstructural studies on BaCl2 doped polyvinyl alcohol (PVA) polymer films were carried out using density, PAL and dielectric measurements at room temperature. The positron annihilation studies on these samples shows considerable effect on the PVA microstructure due to doping and is understood by invoking the chemical reaction between Ba2+ ions of BaCl2 with OH groups of PVA via intra/inter molecular hydrogen bonding, which forms the complex. This complex formation modifies the free volume content in the amorphous fraction, and results in an enhancement of the polymer crystallinity. At higher dopant concentrations, the number of such complexes increases, and ends up with the formation of dopant aggregates or agglomerates leading to certain phase separation into a polymer-rich phase and a dopant-rich phase. These phase separations are thought to be due to the existence of two or more crystalline phases within the polymer matrix. The XRD study also supports this enhancement of PVA crystallinity due to doping. The electrical studies on the doped PVA reflects that the complex formation due to doping affects the microstructure and hence the dielectric properties including the dc and ac conductivities of the polymer. All of these observed results were analyzed and understood based on the microstructural modification of PVA as a function of dopant concentrations.

Effect of barium chloride doping on PVA microstructure: positron annihilation study

Growth and characterization of an organic NLO crystal: 1-(4-methylphenyl)-3-(4-methoxyphenyl)-2-propen-1-one

The influence of ceramic filler TiO2 on ionic conductivity of poly(vinyl alcohol) (PVA)/chitosan (CN) blend doped with lithium perchlorate (LiClO4) nanocomposite films were studied. The nanocomposite films were prepared by solution casting method and characterized with Fourier transform Raman spectroscopy analysis (FT-Raman), differential scanning calorimetry (DSC), and mechanical analyses. The temperature dependent ionic conductivity obeys Arrhenius rule and understood based on interpolaron hoping model. The nanoparticle size (N
 p), hopping distance (R
 o), exciton energy (E
 ex), and conductivity (σ) of the PVA/CN-Li nanocomposite with different TiO2 concentrations have been calculated using Brus equation. The maximum ac conductivity of 2.49 × 10−3 Scm−1 is observed for 10 wt% TiO2 doping level in PVA/CN-Li nanocomposite. The ionic conductivity and dielectric properties of the prepared composites with different temperature and applied pressures have been studied in detail to know the pressure sensing properties. The conduction mechanism of the composite can be represented by the correlated barrier hopping (CBH) model.

Rajashekhar F. Bhajantri

Papers

Microstructural studies on BaCl2 doped poly(vinyl alcohol)

Effect of barium chloride doping on PVA microstructure: positron annihilation study

Growth and characterization of an organic NLO crystal: 1-(4-methylphenyl)-3-(4-methoxyphenyl)-2-propen-1-one

Pressure sensitive dielectric properties of TiO2 doped PVA/CN-Li nanocomposite

Optical and microstructural studies on electron irradiated PMMA: A positron annihilation study