分子整流的实验与理论研究进展*

分子电子器件的思想始于20世纪70年代,分子整流的研究在30多年中取得了显著进展,包括分子结构设计、实验测量以及理论模拟。本文简述了分子整流的发展历程,介绍了被广泛研究的分子整流体系以及分子水平整流机理,包括D-σ-A型、D-π-A型、D-A型、构象转变和界面引起的整流,以及负微分电阻现象。最后提出了分子整流研究中存在的一些问题,并展望了分子整流的研究和发展方向。     

The influence of molecular weight of the donor polymer on the solid-state behavior of interchain EDA complexes

N-(2-hydroxyethyl)carbazolyl methacrylate (HECM) and N-ethyl-3-hydroxymethyl carbazolyl methacrylate (HMCM-2) were polymerized by group transfer polymerization to varying molecular weights of somewhat narrow molecular weight distribution. The thermal behavior of the homopolymers and of their EDA complexes with poly(β-hydroxyethyl-3,5-dinitrobenzoyl methacrylate) (PDNBM-2) was studied as a function of molecular weight. The T_g′s of both polymers and their miscible complexes increase steadily as molecular weight increases and then become constant at about M _n = 6000. Both the PHECM–PDNBM-2 and PHMCM-2—PDNBM-2 systems are thermally reversible miscible networks over all polymer molecular weights. Miscibility is thermodynamically controlled over the entire range of molecular weights in the first system and decomplexation does not occur below the decomposition temperature. However, miscibility is thermodynamically controlled in the second system when the molecular weight of PHMCM-2 is less than 5000, and kinetically controlled for higher molecular weights. The decomplexation temperature or LCST of the PHMCM-2–PDNBM-2 system occurs below the decomposition temperature and increases with decreasing PHMCM-2 molecular weight, in agreement with theoretical predictions on the dependence of LCST on polymer molecular weight.     

Cationic polymerization of styrene by acetyl perchlorate. II. Steric structure of the polymer and nature of the propagating species

Cationic polymerization of styrene initiated by acetyl perchlorate in CH₂Cl₂ yields a polymer having a bimodal molecular weight distribution. The high molecular weight and the low molecular weight portions of the polymer were separated by thin-layer chromatography, and the steric structure of these separated polymers was investigated by ¹³C NMR spectra. The high molecular weight polymer had a larger racemic dyad content than the low molecular weight material. From the dependence of the steric structure of the polymer on the polarity of a solvent, it was estimated that the propagating species producing the high molecular weight material was a loose ion pair or a free ion, and that producing the high molecular weight material was a loose ion pair or a free ion, and that producing the low one was a nondissociated species.     

Recoverable compliance of a series of bimodal molecular weight blends of polystyrene

A series of bimodal molecular weight polystyrene blends was prepared. One component had a molecular weight greater than M_c and one less. The recoverable compliances were measured, and the retardation spectra determined via direct inversion. The retardation spectra of the blends are interpreted on the basis of the behavior of the spectra of the components, as a function of composition. It was observed that the low molecular weight component acted as a solvent. The effect of blending on the high molecular weight spectrum was to shift the position of the first peak to longer times and that of the second peak to shorter times. The peak separation was found to be proportional to the 3.4 power of the product of the molecular weight and the volume fraction of the high molecular weight component.     

Nanofriction properties of molecular deposition films

The nanofriction properties of Au substrate and monolayer molecular deposition film and multilayer molecular deposition films on Au substrate and the molecular deposition films modified with alkyl-terminal molecule have been investigated by using an atomic force microscope. It is concluded that ( i ) the deposition of molecular deposition films on Au substrate and the modification of alkyl-terminal molecule to the molecular deposition films can reduce the frictional force; (ii) the molecular deposition films with the same terminal exhibit similar nanofriction properties, which has nothing to do with the molecular chain-length and the layer number; (iii) the unstable nanofriction properties of molecular deposition films are contributed to the active terminal of the molecular deposition film, which can be eliminated by decorating the active molecular deposition film with alkyl-terminal molecule, moreover, the decoration of alkyl-terminal molecule can lower the frictional force conspicuously; (iv) the relat     

An examination of intermolecular and intramolecular hydrogen bonding in biomolecules by AM1 and MNDO/M semiempirical molecular orbital studies

The recent NMDO/M modification and parameterization of the MNDO molecular orbital method has been used to analyze intermolecular hydrogen bonding between amino acids and water, and intramolecular hydrogen bonding in monosaccharides. The results have been compared to AM1 calculations on the same systems. The MNDO/M calculations gave values which were similar to ab initio calculations with respect to the intermolecular interactions, but yielded significantly poorer results for the intramolecular interactions. The AM1 procedure performed better on the intramolecular interactions than the MNDO/M procedure, but frequently provided unfavorable three-centered hydrogen bonding geometries for the intermolecular interactions.     

Thermodynamic parameters of dilute solutions of a polymer in benzene at varying temperatures and concentrations

Dilatometric studies of solutions of four different mole fractions of poly(ethylene glycol) of average molecular mass 400 g mol^?1 dissolved in benzene have been performed to estimate molar volumes (V) and volume expansion coefficients (α) at different temperatures. From these, thermodynamic parameters appropriate to these solutions have been evaluated at different temperatures ranging from 299 to 328 K. These parameters have been employed to collect and discuss the information on molecular structure, molecular order, molecular packing, polymer segment extension, and molecular interactions.     

Studies on molecular interactions of β-cyclodextrin and antiulcer agent

The search for the lowest energy conformation of complex {β-cyclodextrin (β-CD)+chlorambucil} were carried out by molecular mechanics method. Theoretical calculations of molecular interactions of complex were carried out using the molecular orbital method. The correlation between energy changes and molecular structures are discussed. The large interaction energies calculated by the molecular orbital method bears out the inclusion phenomenon.     

Effect of molecular weight on the radial growth rates of polymer spherulites

Data existing in the literature for the spherulitic growth rate of molecular weight fractions of linear polyethylene, poly-(tetramethyl-p-silphenylene)siloxane, and trans-1,4-polyisoprene have been analyzed according to nucleation theory on taking into account the influence of chain length on the free energy of fusion. All three polymers display very similar behavior in that the interfacial free energy reaches an asymptotic value at high molecular weights, decreases as the molecular weight is lowered, and appears to also reach an asymptotic value at low molecular weights. Although the changes in the interfacial energy with molecular weight are quite distinct, the relative change is much less than has been previously reported when a molecular crystal analysis is used. The same general behavior observed points out the dominating influence of the chain-like character of the molecules in governing the growth rate.     

Relaxation characteristics and intrinsic birefringence and viscosity of polystyrene solutions for a wide range of molecular weights

A comparison is made between measurements on polystyrene solutions and the relaxation characteristics and intrinsic birefringence and viscosity given by the theory for the flexible Gaussian chain of variable number of segments and with internal viscosity and internal hydrodynamic interaction. This is done in order to determine the applicability of the theory to polymers over a wide range of molecular weights, including the low molecular weight range in which there may be conflict with the theoretical assumption of chains having a large number of segments. The longest, terminal relaxation time and the number of chain segments are determined from measurements of the frequency dependence of oscillatory flow birefringence while the intrinsic birefringence and viscosity are determined from steady flow measurements. The range of molecular weights studied is from approximately 900 at 10⁶. It is found that the segment weight is approximately 1000 and the number of segments is in direct proportion to the molecular weight for the range from 1 to 1000 segments. The terminal relaxation time has a molecular weight dependence of the type given by the theory but with better agreement for higher molecular weights. While the measured dependences of the intrinsic viscosity and birefringence are in agreement with theory for molecular weights greater than 5 × 10⁴, they deviate significantly for molecular weights below 1 × 10⁴. The ratio of the intrinsic birefringence to intrinsic viscosity, which in theory is a constant independent of molecular weight, is found to change at the lower molecular weights.     

Effect of molecular weight on the crystalline structure of polytetrafluoroethylene as-polymerized

Melting and crystallization behavior of polytetrafluoroethylene as polymerized in emulsion and suspension is shown to depend on molecular weight. DSC heating curves for virgin PTFE with low molecular weight below 3 × 10⁵ have a single peak, whereas curves for higher molecular weight samples have double peaks. With increasing heating rate the areas of higher melting peaks become larger than the lower melting peaks. The morphology of polymer exhibiting double melting peaks is mainly folded ribbons or granular particles. The phenomenon of double melting is explained on the basis of two different crystalline states which correspond to the “fold regions” and the “linear segments” in a folded ribbon. The melting temperature of virgin PTFE is almost constant at ca. 330°C for molecular weights below 1 × 10⁶, and rises as the molecular weight increases above 1 × 10⁶. The heat of melting of virgin PTFE is nearly independent of molecular weight. On the basis of these results, we propose a model for melting and crystallization of low and high molecular weight PTFE and for the crystal structure.     

Vibration—Rotation spectra of DCl in hexadecane solvents of different molecular shapes

Infrared rotation—vibration spectra are used to assess solvent perturbation of DCl rotation in six hexadecane isomers of different molecular structure: n-C₁₆, 2?, 4? and 6-methylpentadecane, 6-pentylundecane and 2,2,4,4,6,8,8-heptamethylnonane. The rotation is sensitive to small regions of the alkane surface whose dimensions are comparable to those of DCl, whereas properties such as the molecular optical anisotropy and correlation of molecular orientation reflect the whole molecular envelope.     

Gel Permeation Chromatography of Polyethylene Terephalate

Abstract

This paper describes the development of the gel permeation chromatography (GPC) technique for the measurements of cyclic trimer content, molecular weights, and molecular weight distribution of polyethylene terephthalate (PET), utilizing a solvent system of o-chlorophenol-chloroform. Mark-Houwink constants for this solvent system are also described.

The GPC technique was applied to the study of the cyclic trimer content, molecular weights and molecular weight distribution of a variety of commercial PET resins. The results indicate that the cyclic trimer content in PET is dependent on molecular weight, polycondensation process and catalyst system. Solidstate polymerized PET contains less cyclic trimer than PET made by the melt-phase process of the same molecular weight. The cyclic trimer content in solid-stated PET appears to be dependent on the conditions of solid-state polymerization.

The polydispersity index determine for a variety of PET samples is higher than the theoretically predicted value of 2.0; however, there is no systematic dependence on molecular weight or polycondensation process.     

Stereochemical applications of mass spectrometry 4. Energy-resolved study of the fragmentation of esters of maleic and fumaric acids

The fragmentation of the dimethyl and diethyl esters of maleic and fumaric acids have been studied as a function of the internal energy of the molecular ions using charge exchange techniques and metastable ion studies in combination with isotopic labelling. The dimethyl ester molecular ions show distinctive behaviours at both low and high internal energies, indicating that interconversion of the molecular ions does not occur. The fumarate molecular ion fragments by elimination of CH₂O and (CO₂ + CH₃) in the metastable ion time-frame, while the maleate ester fragments primarily by loss of CH₃O. At higher internal energies both molecular ions fragment primarily by loss of CH₃O but the fragment ion from the maleate ester shows a greater stability, presumably because it assumes the cyclic cationated maleic anhydride structure. The diethyl maleate and diethyl fumarate molecular ions show identical metastable ion characteristics; in addition the [COS]⁺· charge exchange mass spectra are very similar. These results indicate that low-energy molecular ions interconvert. At higher internal energies interconversion does not occur, and, although both moiecular ions fragment by loss of C₂H₅O, the resultsint fragment ions show different stabilities and fragmentation reactions.     

Conformation of poly(methacrylic acid) in alcohol–water mixtures. III. Dependence on molecular weight

Poly(methacrylic acid) has been studied in 0.002N HCl–ethanol mixtures as a function of molecular weight. A different dependence on molecular weight is noted at different alcohol concentrations. Since the intrinsic viscosity passes through a series of extrema with changes in alcohol concentration, the dependence on molecular weight has been considered in two regions of alcohol concentration in particular. The region of the first minimum and the region of the second minimum (or overall maximum). In the region of the first minimum, intrinsic viscosity is proportional to M^½, just as in 0.002N HCl. The Huggins coefficient k′ is large (ca. 60) but drops to about 10 when the molecular weight exceeds 320,000. In the region of the second minimum the dependence on molecular weight is complex. Intrinsic viscosity is proportional to molecular weight both at low and at high molecular weight and thus indicates freely draining structures. There is a conformational contraction, however, at molecular weight about 320,000 leading from one type of structure to the other. The structure at higher molecular weight may involve a specially strong bond between specifically grouped segments in the chain. The positions of the extrema along the alcohol concentration axis are not molecular weight dependent, particularly above 320,000. Results available for molecular weight dependence in methanol agree well with this picture. The present results confirm prediction inherent in the model of Silberberg and Priel and Silberberg.     

Critical molecular weight for onset of non-newtonian flow and upper newtonian viscosity of polydimethylsiloxane

The flow curves of fractionated polydimethylsiloxanes of different molecular weights were obtained over a wide range of shear rates, from 3 × 10^?1 to 4.3 × 10⁶ sec^?1, by use of a gas-driven capillary viscometer designed to decrease the experimental error in high shear rate region. Non-Newtonian flow can occur at molecular weights below the critical molecular weight M_c for the entanglement of polymer chain. The critical molecular weight M′_c for the onset of the non-Newtonian flow is identical with that of the segment of viscous flow. For the polymer of molecular weights from M′_c to M_c, the upper Newtonian viscosity increases with an increase in molecular weight. Above M_c, the upper Newtonian viscosity is almost independent of the molecular weight.     

A novel scheme of heterogeneous polymerization of ethylene

The heterogeneous polymerization of ethylene initiated by radiation in tert-butyl alcohol was studied. The polymerization was carried out in a 100-ml reactor at 25–100°C and pressures of 200–300 kg/cm² in the presence of 50 ml of tert-butyl alcohol containing 7 wt-% water. The amounts of polymerized monomer, the average molecular weight of polymer formed, and the molecular weight distribution of polymer were measured at various stages of reaction and at various temperatures. The molecular weight distribution was found to be very much dependent on the reaction time and temperature. For the polymer formed at 50–60°C in the very early stages of reaction, the molecular weight distribution is unimodal, and in the intermediate stage a shoulder appears at a molecular weight higher than the first peak which increases as the polymerization proceeds; eventually a bimodal curve is formed. The bimodal distribution curves were analyzed to determine the fractions and average molecular weights of the each peak. On the basis of these data for the molecular weight distribution and kinetic behavior, a new scheme for the heterogeneous polymerization is proposed which indicates that the polymerization proceeds via propagating radicals in two different physical states, namely, loose and rigid states.     

Natural molecular shells as open subsystems of small molecules

We compare the first few natural molecular shells of several small molecules to the corresponding “restricted Hartree‐Fock,” “second‐order Møller‐Plesset,” and “local density approximation” molecular shells. Occupation probabilities of each molecular shell are computed ab initio, especially the single‐occupation and double‐occupation probabilities, that is, the probabilities that the molecular shell is occupied by exactly one electron or by exactly two electrons. We observe that among corresponding molecular shells, the natural molecular shell has the least single‐occupation probability and the greatest double‐occupation probability. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Symbolic representation of the molecular conformation of calixarenes

In calixarene chemistry there is a continuous search for new shaping units or substructures useful for molecular recognition or for the binding of ions. The problem of assigning their molecular conformations cannot be dealt with by the use of the dihedral angles as done until now, because this method contains intrinsic ambiguities. The new approach, proposed here, is based on the use of the set ofn pairs of torsion angles (conformational parameters) which involve the flexible part of the calixarene and it is free of ambiguities. Moreover, knowledge of the set of conformational parameters allows one to build straightforwardly three-dimensional molecular models. A symbolic representation of the molecular conformation of any calix[n]arene may be obtained by combining the Schöenflies symbol of the molecular symmetry together with the signs of the conformational parameters.