Self-assembly of ABA triblocks in ionic liquids provides a versatile route to highly functional physical ion gels, with promise in applications ranging from plastic electronics to gas separation. However, the reversibility of network formation, so favorable for processing, restricts the ultimate mechanical strength of the material. Here, we describe a novel ABA system that can be chemically cross-linked in a second annealing step, thereby providing greatly enhanced toughness. The ABA triblock is a poly(styrene-b-ethylene oxide-b-styrene) polymer in which about 25 mol % of the styrene units have a pendant azide functionality. After self-assembly of 10 wt % triblock in the ionic liquid [EMI][TFSA], the styrene domains are cross-linked by annealing at elevated temperature for ca. 20 min. The high ionic conductivity (ca. 10 mS/cm) of the physical ion gels is preserved in the final product, while the tensile strength is increased by a factor of 5.
A reproducible and accurate procedure, based on HPLC analysis, has been developed to determine simultaneously acibenzolar-S-methyl (CGA 245 704) and its acid derivative (CGA 210 007) in tomato leaves. The limit of detection and quantification of the method are 0.015 and 0.15 mg litre-1 for CGA 245 704 and 0.030 and 0.30 mg litre-1 for CGA 210 007. In tomato plants treated with 250 microM CGA 245 704, it was found that the inducer rapidly translocates from treated leaves (cotyledons, 1st and 2nd) to untreated leaves (3rd to 5th), with the maximum translocation (40% of the total quantity found) occurring 8 h after the treatment. CGA 245 704 residues decreased as time elapsed in both treated and untreated tomato leaves, reaching negligible values 72 h after treatment. The acid derivative, CGA 210 007, was formed in tomato plants as early as 2 h after CGA 245 704 treatment, albeit only in the treated leaves. CGA 210 007 residues decreased in treated tomato leaves with a trend similar to that observed for CGA 245 704. Treatment of tomato plants with CGA 245 704 or CGA 210 007 at 250 microM systemically protected the plants against Pseudomonas syringae pv tomato attacks, the causal agent of bacterial speak disease. Evidence of this were reductions in the degree of infection, the bacterial lesion diameter and the bacterial growth in planta. Since neither CGA 245 704 nor CGA 210 007 inhibited bacterial growth in vitro and the protection against bacterial speak of tomato was observed when the two compounds were completely degraded, the protection must be due to the activation of the plant's defence mechanisms.
Spiropyran molecular switches, in conjunction with transition metal ions, are shown to operate as reversible polymer cross-linkers. Solutions containing a spiropyran-functionalized polymer and transition metal ions underwent reversible thermally triggered (light-triggered) transient network formation (disruption) driven by the association (dissociation) of metal–ligand cross-links. Heat triggers metal-ion-mediated cross-linking via thermal isomerization of spiropyran to its open, merocyanine form, and exposure to visible light triggers dissociation of polymer cross-links. Cross-linking is found to depend on both the valence of the ion as well as the molar ratio of spiropyran to metal salt. We envision this to be a starting point for the design of many types of reversible, stimuli-responsive polymers, utilizing the fact that spiropyrans have been shown to respond to a variety of stimuli including heat, light, pH, and mechanical force.
The conjugation of acetochlor, alachlor, butachlor, dimethachlor, metolachlor, pretilachlor, and propachlor with glutathione nonenzymatically and enzymatically by action of the glutathione S-transferase (GST) was investigated. Shoots of corn (Zea mays), soybean (Glycine max), wheat (Triticum aestiuum), sorghum (Sorghum bicolor), redroot pigweed (Amaranthus retroflexus), and lambsquarter (Chenopodium album) were employed as enzyme sources for enzymatic tests. The nonenzymatic conjugation rates varied to the following order: propachlor > pretilachlor > alachlor > acetochlor > dimethachlor > metolachlor > butachlor. The presence of enzyme extracts induced significant conjugation increases that differed according to the enzyme source. Furthermore, within each enzyme source a different decreasing order of the conjugation rate, compared to the nonenzymatic reaction, was observed. Therefore, interferences of the molecular structure of the 2-chloroacetanilides also in the enzymatic mechanism of the reaction have been deduced. These findings and the kinetic parameters (Km and Vmai), determined for the GST enzymes of each plant, show that the molecular structure of the 2-chloroacetanilides, the catalytic efficiency of GST enzymes, their concentration in the protein bulk, and the protein content in plants are crucial factors in determining plant tolerance.
A key open question for transient polymer networks is the molecular reason underlying their elastic stiffening and viscous thickening nonlinearities, with the two prevailing conventional hypotheses being flow-induced (i) increase of the number density of elastically active network strands (i.e., bridging chains) and (ii) nonlinear elastic force–extension chain behavior. The objective of this work is to reliably infer, and theoretically justify, the mechanism driving the nonlinear response of unentangled reversible networks formed by semidilute aqueous solutions of poly(vinyl alcohol) (PVA) and sodium tetraborate (Borax), across a range of 11 compositions and Deborah number varying from ≃10–2 to ≃70. Weakly nonlinear perturbations from equilibrium are quantitatively analyzed via a strain stiffening transient network theory endowed with a single nonlinear parameter and a microscopic statistical mechanical theory for the deformation-induced change of local interchain packing due to conformational anisotropy and its influence on an effective transient cross-link density. As a result, network structuring through stretch-induced enhancement of interchain associations is shown to dominate the leading-order viscous and elastic nonlinearities over the entire range of compositions and frequencies considered. Furthermore, our theoretical approach allows for a quantitative rationalization for the magnitudes of the intrinsic nonlinearities that is in excellent agreement with observations.
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