Viscoelastic properties of single poly(ethylene glycol) molecules.

The viscoelastic properties of single poly(ethylene glycol) (PEG) molecules were measured by analysis of thermally and magnetically driven oscillations of an atomic force microscope (AFM) cantilever/molecule system. The molecular and monomer stiffness and friction of the PEG polymer were derived using a simple harmonic oscillator (SHO) model. Excellent agreement between the values of these two parameters obtained by the two approaches indicates the validity of the SHO model under the experimental regimes and the excellent reproducibility of the techniques. A sharp minimum in the monomeric friction is seen at around 180 pN applied force which we propose is due to a force induced change in the shape of the energy landscape describing the conformational transition of PEG from a helical to a planar state, which in turn affects the timescale of the transition and therefore modifies the measured internal friction. A knowledge of the viscoelastic response of PEG monomers is particularly important since PEG is widely used as a linker molecule for tethering groups of interest to the AFM tip in force spectroscopy experiments, and we show here that care must be exercised because of the force-dependent viscoelastic properties of these linkers.     

Pyrene分子(小片石墨晶体)的电子传导特性

利用量子理论中基于Green函数的tight-binding方法,对pyrene分子的电子传导和电子流分布进行了理论研究。在考虑到界面耦合和Hopping积分的情况下,得出了电子透射谱和流分布的模拟结果。结果显示透射与电子的能量紧密相关;谱的振荡特征是能级量子化的结果;流分布有着特定的方向,并且在每一个原子点上符合Kirchhoff量子流守恒定律。另外还发现了桥接pyrene分子的正负能开关特性。     

Vinyl Polymerization. 274. Polymerization of Acrylonitrile Initiated by the System Tetramethyl Tetrazene and Co(II) Chloride

Abstract

The binary system of tetramethyl tetrazene (TMT) and Co(II) chloride was used as initiator of acrylonitrile (AN) in dimethylformamide. The initial rate of polymerization (Rp) was found to be expressed by Rp = k[TMT]^0.62[Co(II) chloride]^0.57 [AN]^2.00

The polymerization was confirmed to proceed via a radical mechanism. The over-all activation energy for the polymerization was estimated as 15.1 kcal/mole. On the basis of these results and the product analysis of the reaction between the catalyst components in the absence of monomer, the initiation mechanism of the polymerization is discussed.     

Polymerization of Acrylonitrile Initiated by the System Tetramethyl-2-tetrazene and p-Toluenesulfonic Acid

The polymerization of acrylonitrile (AN) initiated by tetramethyl-2-tetrazene (TMT) and p-toluenesulfonic acid (TSA) in dimethylformamide (DMF) was studied. The polymerization was confirmed to proceed through a radical mechanism. The initial rate of polymerization R was expressed by the equation: R_p = k[TMT]^0.6 [TSA]^0.46 [AN]^1.35. The overall activation energy for the polymerization was estimated as 20.7 kcal/mole. In the absence of monomer, the reaction of TMT with TSA was also studied kinetically by measuring the evolution of nitrogen. From these results and ESR measurement of the TMT/TSA system, a possible initiation mechanism is proposed.     

Electro-optic effect,propagation loss,and switching speed in polymers containing nano-sized droplets of liquid crystal

Polymers containing droplets of liquid crystal smaller than 100 nm, which have good transparency and easily form films, were prepared under various conditions to evaluate their potential as electro-optic materials for waveguide-type devices. By varying the liquid crystal concentration and the strength of the UV irradiation causing photo-induced phase separation of the droplets, we were able to control the droplet size and density. We have clarified how the birefringence generated in an applied electric field, switching speed, and optical loss of light propagating in the film depend on droplet size and density. Polymer materials having a large electro-optic effect (δn = 0.001 at 8 V μm^-1), low propagation loss (~2.5 dB cm^-1), and fast response time (~10 μs) have been developed.     

Control of the Orientation and Photoinduced Phase Transitions of Macrocyclic Azobenzene

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1 ) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be ?0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.     

Neomacrophorin I,II, and III,novel drimenyl cyclohexanes with hydroxylated butanoates from Trichoderma sp. 1212-03

Neomacrophorins I (1), II (2), and III (3) were isolated from the culture broth of Trichoderma sp. 1212-03, which was collected at Shirakami Mountainous area in Japan. Structural analyses disclosed that these resemble known macrophorins but possess axial-hydroxy group at C3 as well as different side chains at C7′. These are diastereomeric forms of macrophorins for 5′,6′-epoxide functionality. The NMR analyses suggested their relative configurational relationship between the C1–C15 drimene and C1′–C7′ epoxyquinone moieties. ECD spectral discussions verified them particularly for C5′,C6′-epoxyquinone (1), C5′,C6′-epoxysemiquinone (2 and 3), and 2″,3″-dihydroxybutanoate moiety in 1 and 2. The configuration of C3″-stereocenter of 3 was determined by chiral GC–MS after converting into methyl (S)-3″-hydroxybutanoate by basic of 3 methanolysis. Biological assays disclosed that 1 induces hyphal branching of Cochliobolus miyabeanus as well as cytotoxicity against human colorectal cancer COLO 201.