nzhao Yu

Synthesis of novolac/layered silicate nanocomposites by reaction exfoliation using acid-modified montmorillonite

Novolac/layered silicate nanocomposites were synthesized by condensation polymerization of phenol and formaldehyde catalyzed by H-montmorillonite (H-MMT). Exfoliation of the clay was investigated by X-ray diffraction and transmission electron microscopy (TEM). It turned out that the intra-gallery condensation of phenol and formaldehyde played a predominant role in the exfoliation of MMT. Exfoliated clay plates with interlayer spacings of 10-30 nm were dispersed in the matrix uniformly as shown by the TEM micrograph.

Preparation of phenolic resin/silver nanocomposites via in situ reduction

Phenolic resin/silver nanocomposites were prepared by in situ reduction of silver nitrate in the resin, in which hexamethylenetetramine acted as both curing agent and reducing agent. Formed particles dispersed homogeneously in the matrix, while the curing regime exerted an effect on the particle size and the distribution. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Surface modification of epoxy resin by polyether-polydimethylsiloxanes-polyether triblock copolymers

Abstract In order to reduce static friction coefficient (μs), polyether–polydimethylsiloxane–polyether triblock copolymers (PER–PDMS–PER) were incorporated into epoxy resins as surface modifiers. It was found that longer PDMS segments in PER–PDMS–PER and higher modifier contents led to lower μs of modified epoxy resins upon stainless steel. The μs of silicone rubber- and PTFE-side surfaces were evidently lower than that of stainless steel-side surface. ESCA investigations indicated that a longer PDMS block in modifier resulted in a flatter PDMS gradient at surface and the degree of PDMS accumulation on PTFE- and silicone rubber-side surfaces were much higher than that on stainless steel-side surface.

Synthesis of MQ silicone resins through hydrolytic condensation of ethyl polysilicate and hexamethyldisiloxane

MQ silicone resins were synthesized through hydrolytic condensation of ethyl polysilicate and hexamethyldisiloxane. The structure of the MQ resins was characterized by NMR and GPC. The concentration of an acid catalyst was essential for the capping of functional groups with the monofunctional units. The acid-catalyzed resins had a narrow molecular weight distribution. After postcondensation catalyzed by a base, the molecular weight distribution became bimodal.

Influences of cefotaxime and carbenicillin on plant regeneration from wheat mature embryos

The influences of cefotaxime and carbenicillin on regeneration potential of wheat (Triticum aestivum L.) mature embryos were investigated. Filter-sterilized cefotaxime enhanced regeneration capacity although it did not affect the average number of shoots per explant. The highest regeneration capacity of 55.4 % was obtained on regeneration medium supplemented with 100 mg dm−3 cefotaxime. Filter-sterilized carbenicillin did not stimulate plant regeneration. However, higher concentration (100 mg dm−3) accelerated callus browning and inhibited the following regeneration. Autoclaved antibiotics at all tested concentrations showed detrimental effects on callus morphogenesis and plant regeneration.

Synthesis and Properties of Silphenylene-containing Epoxy Resins with High UV-stability

Two novel silphenylene-containing cycloaliphatic epoxy resins, 1,4-di [2-(3, 4-epoxycyclohexylethyl) dimethylsilyl] benzene (DEDSB) and 1,3,5-tri [2-(3, 4-epoxycyclohexylethyl) dimethylsilyl] benzene (TEDSB) were synthesized through in situ Grignard reaction and hydrosilylation, and characterized by FT-IR and 1H-NMR. They were colorless transparent viscous liquids. Methyhexahydrophthalic anhydride (MeHHPA) was used to cure the epoxy resins to give glassy solids with high optical clarity. Differential scanning calorimetry (DSC) results indicated that DEDSB and TEDSB showed similar curing reactivity. The cured TEDSB had a higher glass transition temperature, a higher storage modulus and a lower coefficient of linear thermal expansion than the cured DEDSB due to a higher crosslink density. The cured silphenylene-containing epoxy resins exhibited a much higher resistance to discoloration under UV irradiation than the commonly used epoxy resins diglycidyl ether of bisphenol-A (DGEBA). XPS analysis revealed that they were much less susceptible to photo-oxidation than DGEBA.

Studies on Polysiloxane-based Alicyclic Episulfide Resin

The polysiloxane-based alicyclic episulfide resin (PSCER) was synthesized through substitution of oxygen atoms in 1,3-bis[2-(3{7-oxabicyclo[4,1,0]hepyl})ethyl]-tetramethyldisiloxane with sulfur atoms using potassium thiocyanate (KSCN) as the reagent. The product was purified by column chromatography, and the by-products were isolated. It was found by 1H-NMR and 13C-NMR that the by-products were cycloolefins formed by desulfurization of the episulfide groups. PSCER was unstable; desulfurization took place easily, especially at high temperatures. The PSCER resin could not be cured with methylhexahydrophthalic anhydride (MeHPPA). When m-phenylenediamine or isophorondiamine was used as curing agent, PSCER showed a higher reactivity compared with the parent epoxide. However, the reactivity of the alicyclic episulfide was much lower than the polysiloxane based aliphatic episulfide for the steric hindrance of the six member ring. Desulfurization took place upon curing of the alicyclic episulfide resin, therefore the cured resin showed poor mechanical properties. Thermal stability of cured PSCER resin was also poor on account of low bonding energy of C-S and crosslinking density.

Rheology modification of PVC plastisols with poly(butylene terephthalate)-b-poly(tetramethylene glycol)

A few percent of poly(butylene)-b-poly(tetramethylene glycol) was able to turn a liquid plasticizer into a gel, and thus imparted yield stresses to the fluid. When the plasticizer contained as little as 2.5 wt % of the block copolymer, sag free plastisols were obtained. A reduction in tensile strength was found for the modified plastisols, while the elongation was not affected.