Cyclopropyl Building Blocks for Organic Synthesis,Part 100. Advanced Syntheses of Cyclopropylideneacetates –Versatile Multifunctional Building Blocks for Organic Synthesis

A well reproducible and inexpensive preparation of the cyclopropylideneacetates 2 – 4 has been developed. The key intermediate 2‐(1′‐mesyloxycyclopropyl)acetic acid ( 8 ), produced either from methyl phenylacetate ( 1 ) or 3,3‐dimethoxypropionate ( 5 ‐Me) and 3,3‐diethoxypropionate ( 5 ‐Et) in a sequence of Kulinkovich reductive cyclopropanation, mesylation and oxidative cleavage or cleavage and oxidation, respectively, was either converted to the benzyl ester 11b , or chlorinated (brominated) via the in situ formed acid chloride. The α‐chloro‐ 12a and α‐bromo ester 12b were dehydromesylated by treatment with triethylamine to furnish methyl 2‐chloro‐2‐cyclopropylideneacetate ( 3 ‐Me) and the 2‐bromo analogue 4 ‐Me with an overall yield of 68% (65%, 68%) and 52% (49%, 51%) respectively, starting from 1 ( 5 ‐Me, 5 ‐Et). The parent benzyl cyclopropylideneacetate 2 ‐Bn was obtained by dehydromesylation of 11b with potassium t‐butoxide in t‐butyl methyl ether with an overall yield of 60% (57%, 9%) from 1 ( 5 ‐Me, 5 ‐Et).     

Dielectric relaxation and viscoelastic behavior of polyurethane–titania composites: dielectric mixing models to explain experimental results

Polyurethane–titania nanocomposites with varying composition are prepared through two different mixing methods. The effect of titania on dielectric, mechanical, and thermal properties is investigated for different composites. A variety of electrical tests (like impedance and dielectric constant) are performed on the resultant composites and it was found that the dielectric constant of composites increased significantly, whereas impedance (|Z|) decreased with the increase in titania concentration. The effect of temperature on dielectric properties was also studied and it was found that the dielectric constant increased up to a certain temperature and beyond that it decreased. From mechanical testing, it is observed that the properties depend on both the composition and mixing methods. The glass transition temperature (T _g) of soft and hard segments along with vulcanization temperature (T _v) observed from the differential scanning calorimetry (DSC) are found to shift with the incorporation of titania into the PU matrix. The viscoelastic behavior of the nanocomposites was studied by dynamic mechanical analysis (DMA), and increase in storage modulus (E′) was achieved through addition of titania to the PU matrix, especially in the low strain region, whereas some decrease was observed in the higher strain region. Finally, different dielectric models were compared with the experimental data and the best match was achieved by the Lichtenecker model, especially at 1 kHz, which can be used as a predictive rule for different volume contents of titania filler in the PU matrix.     

Development of poly(dimethylsiloxane)–titania nanocomposites with controlled dielectric properties: Effect of heat treatment of titania on electrical properties

Polymer–ceramic composites were prepared using poly(dimethylsiloxane) as base matrix and normal as well as heat‐treated titania as fillers. Dielectric and mechanical properties of the composites were measured and found that dielectric constant of the composites was increased dramatically with the addition of filler, whereas resistivity was decreased. Hardness and modulus were found to increase but tensile strength, % elongation at break, and tear strength were decreased with the filler loading. Neat titania contains some moisture (physisorbed and chemisorbed) as revealed from thermogravimetric analysis. Both electrical and mechanical properties of the composites were affected by filler heat treatment. Further, untreated titania contains Ti³⁺ and Ti⁴⁺, which on heat treatment, increases the concentration of Ti⁴⁺, as a result electrical properties were affected. Filler dispersion in the composites was studied by field emission scanning electron microscopy and high resolution transmission electron microscopy. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Polydimethylsiloxane–PbZr0.52Ti0.48O3 nanocomposites with high permittivity: Effect of poling and temperature on dielectric properties

Polydimethylsiloxane (PDMS)/lead zirconate titanate (PbZr_0.52Ti_0.48O₃, PZT)-based nanocomposites with high dielectric constant (permittivity, k) are prepared through room temperature mixing. The effect of PZT loading on electrical and mechanical properties of the PDMS–PZT composites is extensively studied. It is found that there is significant increase in permittivity with PZT loading and decrease in volume resistivity. All the composites have low dielectric loss compared to permittivity value. It is observed that there is increase in permittivity and decrease in volume resistivity of composites after poling, which is due to the dipolar polarization. It is found that both permittivity (ε′) and alternating current conductivity (σ_ac) are increased with temperature at low frequency (1 Hz) and decreased with temperature at high frequency (1 MHz). The above composites are sensitive to external pressure and can be used as pressure/force sensor. The tensile strength and % elongation at break decreases with PZT loading, which is due to the nonreinforcing behavior of PZT ceramic. PZT particles distribution and dispersion in PDMS matrix are observed through field emission scanning electron microscopy, high resolution transmission electron microscopy, and atomic force microscopy/scanning probe microscopy. Thermal stability of composites increased with the PZT loading which is due to higher thermal stability of PZT particles compared to PDMS matrix. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47307.     

Secure access privilege delegation using attribute-based encryption

International Journal of Information Security - Attribute-based encryption (ABE) is widely used for a secure and efficient data sharing. The predetermined access policy of ABE shares the data with...