Yielding of glassy polymers in the second quadrant of principal stress space

Second quadrant crazing and shear yielding studies were performed on glassy poly(methyl methacrylate) by means of combined torsion-tension loading. The results are in quantitative agreement with the shear and normal stress yielding criteria proposed by Sternstein and Ongchin. It is shown that four distinct regions of material response exist in the second quadrant and, depending on the stress state, 1) no crazing and no shear yielding, 2) crazing alone, 3) shear yielding alone, or 4) crazing and shear yielding can occur. An analysis of stress field induced brittle-ductile transitions is presented which is in agreement with other studies of high-pressure yielding.     

Rheological Behavior of Blends of Poly(Phenylene Sulfide) with a Thermotropic Liquid Crystalline Aromatic Poly(Ether Ester)

A new type of thermotropic liquid crystalline aromatic poly(ether ester) (PEE) was prepared from 1,3-bis(4′-carboxyphenoxy)benzene, 1,4-diacetoxybenzene, and p-acetoxybenzoic acid through a melt transesterification process. The rheological behavior of blends of poly(phenylene sulfide) (PPS) with PEE was studied using a high-pressure capillary rheometer with the shear rate range of 50 s^?1 to 3000 s^?1. The results show that according to the range of shear rate, the flow curves of PEE/PPS blends can be divided into three zones: a first shear-thinning zone (n < 1, “n” represents non-Newtonian indexes), a shear-thickening zone (n > 1), and a second shear-thinning zone (n < 1), and the former two zones are more obvious with the increase of PEE content or elevated temperature. In the second shear-thinning zone, the PPS melt is close to a Newtonian fluid at high temperature and high shear rate; meanwhile the non-Newtonian behavior of the PPS melt at high temperature is enhanced with the addition of PEE. The apparent viscosity of PPS melts sharply dropped after adding PEE, especially at relatively low temperature and low shear rate. The curve of apparent viscosity vs. shear rate starts to flatten out after adding PEE, suggesting that the addition of PEE lowers the sensitivity of PPS to shear rate. As the content of PEE increases, the activation energy of the viscous flow, ΔE_η, of PPS decreases, which means that adding PEE weakens the temperature sensitivity of the apparent viscosity of the PPS melt. It can clearly be seen that the addition of PEE is beneficial to the processing of PPS.     

Calcium isotope fractionation in liquid chromatography with crown ether resin in methanol medium

Breakthrough mode liquid chromatography was employed to investigate calcium (Ca) isotope fractionation in methanol medium. Highly porous silica beads, the inner pores of which were embedded with a benzo-18-crown-6 ether resin, were used as column packing material. Enrichment of heavier isotopes of Ca was observed in the frontal part of the respective Ca chromatograms. The values of the isotope fractionation coefficient (?) were in the order of 10^?3 at 25 °C. Use of methanol as solvent has little advantage over the aqueous system as far as the values of ? are concerned. However, a substantial improvement was observed concerning the adsorption capacity of the crown ether resin for Ca ions. Molecular orbital calculations supported the present isotopic results in a qualitative fashion.     

The permeation of gases through symmetric and asymmetric (Loeb-Type) cellulose acetate membranes

Significant differences have been observed in the steady-state permeation of gases through symmetric and asymmetric (Loeb-type) cellulose acetate membranes. The studies were made with O₂, N₂, Ar, Kr, Xe, and CO₂ in the temperature range from -5 to 85°C and at subatmospheric pressures. The differences in permeation behavior may reflect structural differences between the symmetric membranes and the dense surface layer (“skin”) of the asymmetric membranes. The overall mechanism of gas permeation through the symmetric membranes appears to be one of “solution-diffusion,” similar to that observed with many other nonporous polymeric membranes. In the case of the asymmetric membranes, this mechanism is probably modified by the presence of micropores or other imperfections in the dense surface layer. Cellulose acetate exhibits two second-order transitions in the presence of the penetrant gases, one between 60 and 70°C and the other near 15°C. The transitions were observed with both types of membranes.     

High pressure organic chemistry; XV. Non-catalyzed cleavage reaction of ethers with acyl halides

Abstract

A variety of cyclic and acyclic ethers are efficiently reacted with acyl chlorides or acyl bromides to afford the ether-cleaved compounds of ω-chloro-or ω-bromoesters under high pressure conditions. The reactivity of ethers is found to be roughly depend on the basicity of the ether oxygen atom.     

Residual stress delaying phase transformation in Y-TZP bio-restorations

Engineering favorable residual stress for the complex geometry of bi-layer porcelain-zirconia crowns potentially prevents crack initiation and improves the mechanical performance and lifetime of the dental restoration. In addition to external load, the stress field depends on initial residual stress before loading. Residual stress is the result of factors such as the thermal expansion mismatch of layers and compliance anisotropy of zirconia grains in the process of sintering and cooling. Stress induced phase transformation in zirconia extensively relaxes the residual stress and changes the stress state. The objective of this study is to investigate the coupling between tetragonal to monoclinic phase transformations and residual stress. Residual stress, on the surface of the sectioned single load to failure crown, at 23 points starting from the pure tetragonal and ending at a fully monoclinic region were measured using the micro X-ray diffraction sin² ψ method. An important observation is the significant range in measured residual stress from a compressive stress of ?400?MPa up to tensile stress of 400?MPa and up to 100% tetragonal to monoclinic phase transformation.     

Cure Behavior and Thermal Properties of Poly (Amide-Amidic Acid) Modified Diglycidyl Ether of Bisphenol-A/4,4′-Diaminodiphenylsulfone

Poly (amide-amidic acid) (PAA) was selected to modify diglycidyl ether of bisphenol-A (DGEBA)/4,4′-diaminodiphenylsulfone (DDS). The cure behavior was studied by means of nonisothermal differential scanning calorimeter (DSC) analysis, indicating that PAA played a role of catalyst during the process of the curing reaction. Results of Fourier transform infrared spectroscopy (FT-IR) analysis showed that the PAA acted as a co-curing agent when the PAA content was 3.2–38.4 phr and also as a modifier when the PAA content was 12.8–38.4 phr. The glass transition temperature (T_g ) decreased with the increase of PAA content. The thermal stability improved when the PAA content was 3.2–6.4 phr because of the catalytic effect of PAA. The flexural strength improved for the varying PAA content studied in this work, with the highest flexural strength being obtained when the PAA content was 6.4 phr. The fracture surface morphology was observed using scanning electron microscopy (SEM); the morphologies varied with changing content of PAA.     

Crystal structure and electrochemical property of one‐dimensional complex: [K(cis‐syn‐cis‐ dicyclohexyl‐18‐crown‐6)]2[Cu(mnt)2]

One metalorganic complex [K(cis‐syn‐cis‐ dicyclohexyl‐18‐crown‐6)]₂[Cu(mnt)₂] (mnt = maleonitriledithiolate) has been obtained by the reaction of dicyclohexyl‐18‐crown‐6 with K₂mnt and CuCl₂·2H₂O. The title complex has been characterized by elementary analysis, FT‐IR, UV‐Vis spectroscopy and x‐ray single crystal diffraction. The title complex crystallizes in the triclinic space group P1 with crystallographic data: a = 10.870(6) Å, b = 11.536(6) Å, c = 12.904(7) Å, α = 101.541(10)°, β = 110.573(9)°, γ = 99.441(9)°, V = 1435.2(13) ų, Z = 1, D_calcd = 1.350 g/cm³, F(000) = 615, R₁ = 0.0641, wR₂ = 0.1475. It displays one‐dimensional chain‐like structure formed by [K(cis‐syn‐cis‐ dicyclohexyl‐18‐crown‐6)]⁺ complex cations and [Cu(mnt)₂]^2‐ complex anions through N‐K‐N interactions. Its electrochemical behavior has also been studied by the cyclic voltammetry. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Carboxyl-functionalized Nanocellulose Reinforced Nanocomposite Proton Exchange Membrane

Nanocellulose(NCC) was prepared through the acid hydrolysis of microcellulose(MCC) and was reacted with maleic anliydride to obtain carboxyl-functionized nanocellulose(MA-NCCs). The presence of .OH and .SO3H on the surface of rod-like MA-NCC was confirmed by Fourier transfonn infrared spectrometry(FTIR). Sulfonated poly(aryl ether ether ketone ketone)(Ph-SPEEKK) was synthesized with bis(4-fluorophenyl-methanone) and 2-phenylhydroquinone as monomer. MA-NCC/Ph-SPEEKK nanocomposite membranes with different MA-NCCs content were prepared, and their properties were characterized. Compared with Ph-SPEEKK, MA-NCC/Ph-SPEEKK nanocomposite membrane showed better mechanical and thermal properties and higher proton conductivity. The proton conductivity of the composite membrane with 4%(mass fraction) MA-NCCs under 80℃ was 0.095 S/cm. And its tensile strength reached 30.3 MPa, which was 21.2% higher than that of Ph-SPEEKK pure polymer membrane.