Effect of γ-irradiation on the molecular-topological structure of the polyethylene matrix of glass-reinforced plastic

The molecular-topological structure of polyethylene (PE) and a PE matrix in a glass-reinforced plastic (GRP) after γ-irradiation of the plastic was studied by means of thermomechanical spectroscopy. The four-block topological structure of unirradiated PE (one amorphous block and three crystalline phases with different initial melting temperatures) is transformed into a three-block structure in the GRP already at a minimal radiation dose of 25 kGy. The intermediate crystalline phase disappears under these conditions, the molecular relaxation characteristics in all topological blocks alter, and chemical branching points appear in the pseudonetwork structure of the amorphous matrix block.     

On the analysis of conformational dynamics in polymers with several rotational isomers

The ability of different correlation functions to shed some light onto the conformational dynamics of an amorphous polymer has been analyzed. The study has been performed on a polyethylene model polymer, which has been simulated at decreasing temperatures towards its glass transition, via the molecular dynamics technique. Three rotational isomers are allowed by the considered torsional potential. The correlation times associated with the evaluated transition rates have shown to be Arrhenius in nature, with activation energies resulting basically from internal rotation barriers. Overall torsional autocorrelation functions have been calculated. We have observed that they are dominated by slow events. Alternatively, a set of torsional autocorrelation functions associated with every isomeric state has been evaluated. Stretched exponential fits lead to correlation times that display Vogel-Fulcher temperature dependence.     

Coarse-grained polyethylene: 1. The simplest model for the orthorhombic crystal

The coarse-grained model of polyethylene and alkanes (the united-atom model, in which each CH₂ group is represented by a single bead) was proposed several decades ago. It is widely applied in molecular dynamics simulations. For different tasks, the models with different geometrical and force parameters are used. Until now, it was thought that the coarse-grained model of polyethylene cannot reproduce the orthorhombic crystalline phase, which is typical of this polymer. In the present study, we analyze the simplest coarse-grained model of polyethylene. In this model, the Lennard-Jones potential (6–12) is adopted for van der Waals interactions between the beads of different chains. Of the bonded interactions, only the “valence” bonds between beads and the “bond” and “torsion” angles are taken into account, whereas the cross terms between them are disregarded. We consider the model variation in which the bead (the force center with the mass of a CH₂ group) is displaced from the center of the carbon atom and all the interactions, both bonded and nonbonded, are defined by the positions of these beads. For this model, we find the area of geometrical parameters (the displacement value and the van der Waals radius of the bead) in which all the three known crystalline phases of polyethylene are at equilibrium at low temperatures. We choose the force field constants for the model so that its oscillation spectrum reproduces the low-frequency part of the inelastic neutron scattering spectrum of the orthorhombic polyethylene. It proved to be that this choice can be made unambiguously. We compare the dispersion curves in the terahertz range with experimental data on the Raman scattering and infrared spectroscopy, and discuss the advantages and disadvantages of the analyzed simplest coarse model.     

Crystallite size in highly drawn polyethylene

The size and distortion of crystallites in samples of linear polyethylene were determined before and after plastic deformation. A slowly cooled sample, a quenched sample, and highly drawn films (draw ratio 16) were investigated by different methods. Wide-angle x-ray patterns were analyzed to study the average size of the crystalline mosaic blocks and their distortion. In addition, the longitudinal crystal thickness (in the chain direction) was evaluated by two other approaches, determination of the long period, and the melting temperature of irradiated samples. The results show clearly that the size of the crystalline mosaic blocks changes substantially with drawing of polyethylene. Not only is the lateral crystal thickness affected, but the longitudinal crystal dimensions also change during the drawing process. By the three independent methods we find that the longitudinal crystal thickness after drawing is independent of the value for the undrawn samples, as was reported earlier by Peterlin. The change in crystallite size after drawing is accompanied by a large decrease in crystal volume to about 10% of the value for the undrawn sample. The degree of distortion in the crystals seems not to be affected by the deformation process. These experimental data can be considered evidence for high chain mobility and for the possibility of rearrangement of chain molecules during the process of plastic deformation.     

对高分子体系进行分子动力学模拟方法的研究

以单链聚乙烯为例,研究了多种对高分子体系进行分子动力学模拟的方法.结果表明,不同的力场条件可以导致明显不同的结果.在使用OPLS力场的真空条件下.在100K体系出现了玻璃化现象,在200和300K条件下体系出现了局部结晶现象,而在400和500K体系出现了熔化现象.在给定体系中加入周期性边界和在300K下进行分子动力学模拟,无序和结晶状态都具有一定的稳定性.     

Analytical Study of Discrete Optical Breathers in Spiral Polymer Chain

Physico‐mechanical properties of polymers in solid state, in particular conditions of their structural transformations, are substantially defined by existence and mobility of elementary nonlinear excitations. The localized oscillatory excitations (breathers), which have been revealed earlier in crystalline polyethylene alongside with topological solitons, turn out to be significant for thermodynamic studies as well as for investigation of energy transfer. We consider analytically the breathers in more complex crystalline polytetrafluoroethylene. It is shown that introduction of adequate variables allows to construct the continuum model describing spatially localized excitations with several internal degrees of freedom. Characteristic parameters of breathers and conditions of their mobility are defined.

     

结晶聚合物的中间层甲苯和辛烷萃取聚乙烯样品的研究

采用不同实验方法：ＳＡＸＳ、ＮＭＲ和激光Ｒａｍａｎ光谱。对甲苯和辛烷萃取的聚乙烯样品的中间层及其它结晶结构参数进行了研究，结果表明三种方法测得的中间相厚度有所不同，但均反映出辛烷萃取的聚乙烯样品中间层厚度比甲苯萃取的大。从测试和计算结果得出，中间层厚度与其它结构参数相比不可忽略。结晶聚合物结构以“三相模型”代替传统的“两相模型”可望逐渐被人们接受。     

Impact of neighboring chains on torsional defects in oligothiophenes

Conjugated organic oligomers are central to the development of efficient organic electronic devices and organic photovoltaics. However, the torsional flexibility of many of these organic materials, in particular oligothiophenes, can adversely affect charge transfer properties. Although previous studies have examined the torsional flexibility of oligothiophenes, there have been only limited studies of the effects of interchain interactions on their torsional potentials. B97-D/TZV(2d,2p) was first benchmarked against a CCSD(T)/aug-cc-pVTZ torsional potential for bithiophene as well as SCS-MP2/TZVPP interaction energies for noncovalent sexithiophene (6T) dimers. The effect of neighboring chains on three distinct torsional modes of sexithiophene was studied using B97-D. Complexation with one or more neighboring chains has a dramatic effect on each of these torsional potentials. For example, for two stacked chains, alternated twisting motions are competitive with torsion about a single terminal dihedral angle, and in both cases we predict nonplanar global energy minima and large amplitude torsional motions at room temperature. In other words, the presence of a single neighboring chain induces significant deviations from planarity in oligothiophenes. However, in the environment of crystalline 6T, the trend in predicted torsional potentials match those of isolated chains, but the force constants associated with torsional motions increase by an order of magnitude. Consequently, although individual oligothiophene chains are torsionally flexible and model stacked dimers exhibit extreme deviations from planarity, in crystalline 6T these oligomers are predicted to adopt planar configurations with a steep energetic cost associated with torsional defects.     

Floquet engineering of topological states in realistic quantum materials via light-matter interactions

Electronic states in quantum materials can be engineered by light irradiation, which is greatly advanced by ab-initio computational predictions in realistic light-matter coupled systems. Here we review the most recent progresses from first principles computation in the light-driven Floquet steady states and transient dynamical states with topological electronic bands in real crystals. We first introduce the first-principles modeling approach, dubbed time-dependent Wannier scheme, for simulating real quantum materials under light irradiation. Then, we present a few examples of theoretically-predicted Floquet-Bloch electronic bands engineered by time-periodic light fields, which include the three types of Floquet-Dirac fermions in graphene and black phosphorus, the Floquet-Chern flat bands with an unprecedented high flatness ratio of band width over band gap in a Kagome material, and the Floquet conversion between bright and dark valley excitons in monolayer transition-metal dichalcogenides. Next, we show the ultrafast dynamical evolution of Weyl nodal points in orthorhombic WTe₂ driven by a time-aperiodic short light pulse, and discuss the connection between the Floquet and transient states engineered by light. After that, we introduce three prominent experiments, inspired by theoretical predictions, on the light-induced topological Floquet electronic bands in quantum crystalline materials. Finally, we make a brief summary and perspective on the engineering of topological electronic states through light-matter interactions.     

A class of nonplanar conjugated compounds with aggregation-induced emission: structural and optical properties of 2,5-diphenyl-1,4-distyrylbenzene derivatives with all cis double bonds

We have studied the structural and optical properties of four 2,5-diphenyl-1,4-distyrylbenzene derivatives with all cis double bonds. These compounds belong to a class of nonplanar conjugated compounds possessing a typical Aggregation-Induced Emission (AIE) property that has no emission in solution but intense emission in crystal. The four molecules are packed in different stacking modes with different intermolecular interactions, resulting in different crystalline state photoluminescence (PL) efficiency. The torsional molecular configuration increases the intermolecular distances effectively in the crystalline state, which decreases the difference of the optical properties from the frozen isolated molecules to the crystalline state. The Stokes shifts of these compounds are very large and the PL spectra have only one broad emission band with poor structure, due to the relatively large configuration difference between the ground state and the first singlet excited state, and the abundant vibration energy levels of the torsional molecule with changeable conformation.     

Mechanical properties of oriented low‐density polyethylene with an oriented lamellar‐stack morphology

A quantitative study was undertaken of the anisotropy of low‐strain mechanical behavior for specially oriented polyethylene with controlled crystalline and lamellar orientation. The samples were prepared by the die drawing of injection‐molded rods of polyethylene and annealing. This produced a parallel lamellar structure for which a simple, three‐dimensional composite laminate model could be used to calculate the expected anisotropy. Experimental data, including X‐ray strain measurements of the lateral crystalline elastic constants, showed good quantitative agreement with the model prediction. The X‐ray strain measurements confirmed that the amorphous regions exert large constraints on the crystalline phase in the lateral directions, where an order of magnitude difference was found between the measured apparent lateral crystalline compliances in the lamellar‐stack sample and the expected values for a perfect crystal. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 755–764, 2000     

天然橡胶在单向拉伸下的结晶与取向

<正> 从研究聚乙烯由球晶结构转变到纤维结构的过程得知,聚乙烯纤维由一束平行排列的细纤维(microfibril)组成,细纤维则由垂直于纤维轴互相平行排列的折迭链块(foldedchain block)组成,折迭链块之间有链段相连.处于高弹态的天然橡胶(-78°—28℃),进行单向拉伸,可发生取向并同时结晶,形成纤维,但是它与聚乙烯的起始态不同,为阐明天然橡胶拉伸后是否也有类似聚乙烯纤维的结构单元,本文采用透射式电子     

Infrared dichroism of polyethylene crystallized by orientation and pressure in a capillary viscometer

Polarized infrared absorption spectra have been obtained by Fourier-transform spectroscopy for several crystalline and noncrystalline absorption bands of polyethylene crystallized by orientation and pressure in capillary viscometer. An analysis of data obtained at room temperature yielded degrees of crystallinity which are in good accord with values obtained from calorimetry and density measurements. The dichroism of the infrared absorption bands for the crystalline region revealed an extreme degree of orientation consistent with previous x-ray studies and also demonstrated that the degree of orientation is a good or better than that obtained from drawn polyethylene films with extension ratios of 20. Dichroism of bands from the amorphous phases revealed that the noncrystalline chain segments are in a comparatively relaxed state compared with results for drawn films having extension ratios of about 2 to 7. This is 1/10 to 1/3 the extension ratio of drawn polyethylene which shows maximum crystalline orientation. The results also indicated that the ratio of the GTG′ to GG segment conformations in the amorphous regions is larger than that of amorphous portions in unoriented polyethylene. The vinyl endgroups were shown to be highly oriented, while the main bulk of the amorphous polymer was fairly relaxed, i.e., of low orientation. It is concluded that the amorphous polyethylene state is strongly dependent on the nature of the crystalline–amorphous interface.     

原子-键电负性均衡方法融合进分子力场(ABEEM/MM)应用于蛋白质分子(Crambin)模拟

建立应用于多肽和蛋白质模拟的ABEEM/MM浮动电荷力场.利用该模型和参数,对实际蛋白质分子Crambin(植物种子中的一种小的蛋白质)进行模拟,得到了满意的结果,为其更广泛的应用开辟了道路.     

Determination of the distribution of straight-chain segment lengths in crystalline polyethylene from the Raman LAM-1 band

It is shown how the shape of the longitudinal acoustic vibration observed in the low-frequency region of the Raman spectra of crystalline polymers can be used to obtain a quantitative distribution of lengths of straight-chain segments associated with polymer lamellas. The procedure is demonstrated for a “solid-state” extrudate of polyethylene and for a bulk-crystallized specimen of the same polymer. Equations relating the shape of the LAM-1 band to the shape of the distribution curve are given. The low intensity observed for the LAM-3 mode relative to LAM-1 is explained quantitatively without recourse to end effects. LAM-5 has been observed for bulk-crystallized polyethylene. For the extrudate we find the distribution of lengths of straight-chain segments to have a tail on the long-length side which is not present for the bulk-crystallized sample. The Raman technique is shown to provide new morphological data unattainable at present by other methods.     

A NOVEL GAS SENSOR FROM POLYMER-GRAFTED CARBON BLACK: EFFECTS OF POLYMER,CRYSTALLINE ORGANIC COMPOUND,AND CARBON BLACK ON ELECTRIC RESPONSE TO TETRAHYDROFURAN VAPOR

The responsiveness of electric resistance of three types of composites, polyethylene (PE)/PE-grafted carbon black (CB) (CB-g-PE), N,N'-dicyclohexylcarbodiimide (DCC)/CB-g-PE and PE/DCC/CB-g-PE, to THF vapor was investigated. To modify the CB compatibility in the composite, PE was grafted onto a CB surface. Although responsiveness of composite DCC/CB-g-PE was the most sensitive to THF vapor among the three composites, the composite film is too weak to be of practical use. On the contrary, the responsiveness and the sensitivity of PE/CB-g-PE were too low to be used in many fields. However, through modification by the addition of DCC, responsiveness was greatly improved, and the sensitivity was considerably higher than that of PE/CB-g-PE. The effects of CB-g-PE and DCC content in the composites on the responsiveness to THF vapor was studied in detail. It was found that the responsiveness of electric resistance to THF vapor was caused by the change of the crystalline structure of the composite. A crystalline model was proposed to explain the response of the composite to solvent vapor.     

短链支化对低分子量聚乙烯结晶及熔融行为的影响

研究了金属茂催化的低分子量支化聚乙烯和线性聚乙烯的结晶及熔融行为 ,发现支化聚乙烯的结构与线性聚乙烯相同为正交结构 ,但晶格略有膨胀 .支链的存在对熔融行为影响不大 ,两种聚乙烯的熔点均随结晶温度的升高而非线性增加 ,表现出低分子量样品的共同特征 .但支链的存在对结晶行为却有很大的影响 ,主要是由于支链的存在降低了晶体的结晶速率从而影响结晶过程 ,使得低分子量的支化聚乙烯的结晶行为与高分子量线性聚乙烯的结晶行为相似而与低分子量的线性聚乙烯不同 .动力学分析表明 ,低分子量的支化聚乙烯的结晶生长方式的转变温度比同等分子量的线性聚乙烯降低了约 2 0℃     

Infrared studies of drawn polyethylene. II. Orientation behavior of highly drawn linear and ethyl-branched polyethylene

Infrared dichroism is employed to study the orientation of chain molecules in linear and ethyl-branched polyethylene in the crystalline and noncrystalline regions during drawing and subsequent annealing. A crystalline (1894 cm^?1) and a noncrystalline (1368 cm^?1) band, as well as the bands at 909 cm^?1 and 1375 cm^?1 resulting from vinyl endgroups and methyl endgroups and sidegroups, are studied. For these bands relative orientation functions are derived and compared as a function of draw ratio and annealing temperature. It is shown that the relative orientation functions as derived from the dichroism of the noncrystalline, vinyl and methyl bands follow the same curve while the orientation function for the crystalline bands does not. These results support a two-phase model for partially crystalline polyethylene and additionally favor segregation of the endgroups and sidegroups in the noncrystalline component during crystallization. It is further shown that shrinkage occurs at the temperature at which the noncrystalline chain molecules start to disorient. From the dichroism of the methyl groups in ethyl-branched polyethylene, a value for the mean orientation of the noncrystalline chain molecules is calculated. We obtain for the orientation function of the noncrystalline regions at highest draw ratios (λ = 15–20), f = 0.35–0.57, while the chain molecules in the crystallites are nearly perfectly oriented (f ≈ 1.0). On the assumption that the noncrystalline component consists of folds, tie molecules, and chain ends, the different contributions of these components to the overall orientation are estimated. From these the relative number of CH₂ groups incorporated into folds, tie molecules, and cilia can be derived. Further, on the basis of a simple structural model, the relative number of chains on the crystal surface contributing to the different noncrystalline components and their average length are estimated.