Jamming concepts in cold polymeric glasses

The purpose of the present paper is to propose an idea to construct the volume function W for polymer glasses and calculate the volume of polymer glasses as a function of the compactivity X utilizing a statistical-mechanical method developed for semiflexible polymers. We also discuss the analogue for glasses of "tapping" experiments which show the validity of statistical mechanics and the entropy concept in powders.     

Structure inhomogeneities of medium-range order in fluorozirconate glasses

Images of cleaved surfaces were obtained for 53ZrF₄-20BaF₂-4LaF₃-3AlF₃-20NaF (ZBLAN) glasses and fused quartz using an NTMDT atomic force microscope. It is shown that the scatter of particle size depends on the cooling rate and is 21–48.5 Å for the cleaved surface of ZBLAN glass obtained by very fast cooling and 68–172 Å for the cleaved surface of ZBLAN after slow cooling. For cleaved fused quartz, the range is 25-18 Å.     

Some properties of intermediate regions in interpenetrating polymeric networks

The molecular mobility in an interpenetrating polymeric network (IPN) based on crosslinked polyurethane (1) and styrene–divinylbenzene copolymer (2) was studied by broad-line NMR spectroscopy and reversed gas chromatography. NMR and infrared investigations show the absence of chemical interaction between the two constituent networks. For an IPN in the temperature range ?196 to 120°C transition regions corresponding to the individual networks and to an intermediate region were found, the latter being characterized by an additional transition at 25–60°C (NMR spectroscopy) and 44–78°C (gas chromatography). The existence of an intermediate region, presumably of loosely packed structure, leads to a shift in the beginning of the temperature transition in IPN to lower temperature and to a decrease in activation energies of relaxation in comparison with the individual networks. The activation energies in IPN are decreased with increasing weight fraction of the second network.     

Shear band formation and mode II fracture of polymeric glasses

Mode I and II fracture studies were performed from quasistatic to low velocity impact rates on polymethyl methacrylate (PMMA) and polycarbonate (PC). Mode II tests used an angled double‐edge notched specimen loaded in compression. The shear banding response of PMMA is shown to be highly sensitive to rate, with diffuse shear bands forming at low rates and sharp distinct shear bands forming at high rates. As the rate increases, shear deformation becomes more localized to the point where Mode II fracture occurs. PC is much less rate dependent and stable shear band propagation is observed over the range of rates studied with lesser amounts of localization. A new theory is formulated relating orientation in a shear band to intrinsic material properties obtained from true‐stress true‐strain tests. In a qualitative sense the theory predicts the high rate sensitivity of PMMA. A kinematic limit for orientation within a shear band is also derived based on entanglement network parameters. Mode II fracture in PMMA is shown to occur at this kinematic limit. For the case of PC, the maximum impact rates were not high enough to reach the kinematic limit. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Kinetics of a second order reaction involving two intermediate second order reactions

We have studied the kinetics of a reaction in which starting materials A and B react to form the products C and D directly, as well as by way of two additional intermediate reactions. In the first intermediate reaction, A and C react to form D and E, and in the second intermediate reaction, B and E react to form 2C. There are three conservation laws, which effectively reduce the number of variables to two, resulting in nonlinear coupled first order differential equations for [A](t) and [B](t). These equations are readily solved by standard numerical procedures, for various values of the relative reaction rates and starting concentrations. Our results are used to fit experimental data for the case in which A and B are 1-acetyl-4-(1-ethoxycarbonyl-1-cyano)methylene-1,4-dihydroquinoline and water, for which the first and third reverse reactions can be neglected.     

Investigation of medium range order in silicate glasses by infrared spectroscopy

A new decomposition method of infrared spectra which takes into account the configurational and dynamical origins of the disorder allows retrieving structural information on short and medium range orders in potassium silicate glasses. The distribution of tetrahedral units, the occupation of cation sites, the ratio of SiO bond ionicities involving bridging and non bridging oxygen, and a measure of the impact of low frequency floppy modes on the high frequency dynamics are byproducts of the modeling process. The composition dependence of two vibrational modes clearly identified as spectral components signing medium range order, shows that the disruption of the silicate network follows selective schemes. The 3D silica like network completely disappears in glasses with K₂O amounts greater than 11 mol% and above this threshold a progressive appearance of 2D silicate sheets is evidenced.     

Measurement of dynamic properties of polymeric glasses for different modes of deformation

Methods for measuring various storage moduli and damping factors of glassy polymers in the frequency range 10^?2 to 10^?7 Hz are critically reviewed. At frequencies between about 10^?2 and 200 Hz, components of the complex tensile modulus E¹ and complex Poisson's ratio ν¹ have been determined at 21 °C for polymethyl methacrylate (PMMA) and rigid polyvinyl chloride (PVC) using a non-resonance technique with bidirectional strain gauges. From these measurments, components of the complex shear modulus G¹ and bulk modulus K¹ have been evaluated and shown to vary with frequency in a manner consistent with data obtained by other methods. Molecular motions responsible for the broad secondary relaxation regions in PMMA and PVC are thus able to couple with both shear and dilatational stress fields.     

Charge ordering and intermediate range order in ammonium ionic liquids

Molecular dynamics simulations were performed for ionic liquids based on the bis(trifluoromethylsulfonyl)imide anion, [NTf(2)], and ammonium cations with increasing length of the alkyl chain and ether functionalized chain. The signature of charge ordering is a sharp peak in the charge-charge structure factor, S(qq)(k), whose intensity is barely affected for longer carbon chain in tetraalkylammonium systems, but decreases in ether functionalized ionic liquids. The first sharp diffraction peak (FSDP) and the corresponding intermediate range order (IRO) are observed in the total S(k) of ionic liquids containing ammonium cations with relatively long chains. The intensity of the FSDP is lower in the total S(k) of the ether derivative in comparison with the tetraalkylammonium counterpart of the same chain length. It is shown that the nature of the IRO is structural heterogeneity of polar and non-polar domains, even though domains defined by chain interactions in the ether derivatives become more polar. Charge correlation in the ether derivative is modified because cations can be coordinated by oxygen atoms of the ether functionalized chain of neighboring cations.     

Using fluctuation microscopy to characterize structural order in metallic glasses.

We have used fluctuation microscopy to reveal the presence of structural order on length scales of 1-2 nm in metallic glasses. We compare results of fluctuation microscopy measurements with high resolution transmission electron microscopy and electron diffraction observations on a series of metallic glass samples with differing degrees of structural order. The agreement between the fluctuation microscopy results and those of the other techniques is good. In particular, we show that the technique used to make thin specimens for electron microscopy affects the structure of the metallic glass, with ion thinning inducing more structural order than electro-polishing. We also show that relatively minor changes in the composition of the alloy can have a significant effect on the medium-range order; this increased order is correlated with changes in mechanical behavior.     

Local order around rare earth ions during the devitrification of oxyfluoride glasses

Erbium L(3)-edge extended x-ray absorption fine structure (EXAFS) measurements were performed on rare earth doped fluorosilicate and fluoroborate glasses and glass ceramics. The well known nucleating effects of erbium ions for the crystallization of cubic lead fluoride (based on x-ray diffraction measurements) and the fact that the rare earth ions are present in the crystalline phase (as indicated by Er(3+) emission spectra) seem in contradiction with the present EXAFS analysis, which indicates a lack of medium range structural ordering around the Er(3+) ions and suggests that the lead fluoride crystallization does not occur in the nearest neighbor distance of the rare earth ion. Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were performed, and results indicate that Er(3+) ions lower the devitrification temperature of PbF(2), in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF(2) has been investigated. Although Er(3+) ions could indeed act as seeds for crystallization, as experiments suggest, molecular dynamics simulation results corroborate the experimental EXAFS observation that the devitrification does not occur at its nearest neighbor distance.     

Comparing intermediate range order for alkyl- vs. ether-substituted cations in ionic liquids

X-ray scattering data from four pairs of ionic liquids (ILs) are compared. The alkyl-substituted cations show a first sharp diffraction peak between 3 and 4 nm(-1) that is not observed for ILs having cations with ether- or hydroxy-substitutions. These observations indicate a significant difference in the intermediate range order for these liquids.     

Simulation of EPR and time resolved EPR lineshapes in partially ordered glasses

Simulation of magnetic resonance spectra of probes in partially ordered glasses requires in principle a numerical integration on the full set of three Euler angles omega=(alpha beta gamma) from a laboratory fixed to a molecule fixed reference frame. It is shown that it is possible to manage efficiently this problem by using the algebraic properties of the Wigner matrix elements. This analysis is applied to time resolved EPR (TREPR) spectra of a series of bis-adducts of C60 in the ordered glass of a nematic liquid crystal solvent. A paramagnetic triplet state is created by light excitation and TREPR spectra are obtained with the external magnetic field set parallel or perpendicular to the director n of the mesophase. The preferred orientation in the mesophase of the triplet state zero field tensor is determined.     

Structural phases in non-additive soft-disk mixtures: glasses, substitutional order, and random tilings

Relaxation of the additivity condition on the interaction length between unlike species in a binary mixture of soft disks opens up a rich variety of structures in both crystal and amorphous states with an associated diverse range of relaxation dynamics. We report on MD simulation studies of binary soft disks with negative deviations from additivity that include evidence of accumulation of crystal-like structures in metastable liquids prior to crystallization and the occurrence of a liquid to random-tiling transition.     

A simple method for analysis of consecutive reactions with a second order formation and a first order decay of an intermediate

A simple method is developed to evaluate rate constants from absorbance-time traces for a pair of consecutive reactions consisting of a second order formation and a first order decay of an intermediate. Initially, a first order profile is simulated utilizing the data near the end of the reaction. The difference between this simulated and observed profiles provides the absorbance-time data for the initial phase from which a second order rate constant is evaluated. These rate constants were used to simulate composite kinetic curves which were then compared with experimental curves. This method was used to test the reaction between cis-Pt(NH₃)₂(H₂O)₂ ²⁺ and a nonapeptide, ERFKCPCPT. The reaction proceeds through a cysteine coordinated intermediate formed in a second order process (first order with respect to each reactant). The intermediate is then converted to a product through a first order process, in which both cysteines are coordinated to platinum(II).     

纤维素热裂解反应机理及中间产物生成过程模拟研究

基于改进的B-S机理模型,通过求解物料内部和气相空间两段反应过程,对纤维素热裂解过程中一些化合物(活性纤维素、左旋葡聚糖(LG)、乙醇醛、丙酮醇等组分)的生成和演变情况进行了模拟。结果发现,自由水的脱除过程使物料前期升温速率发生了下降,并未影响热解期间温度分布以及反应过程。热裂解过程中,由于一次反应的强烈吸热,物料在长时间内局限于中温范围,其内部各组分质量浓度分布的区别主要体现出一次反应竞争能力的强弱。物料厚度的增加使热裂解时间延长,并加剧物料内部的二次分解。左旋葡聚糖和其竞争产物乙醇醛的生成出现一个大量生成、快速逃逸的过程,相比于左旋葡聚糖,乙醇醛质量浓度的积累具有更快的速度,体现出较高温度下的竞争优势。对于小尺寸反应物,挥发分二次反应主要发生在气相空间,随着气相停留时间的增加,其二次分解的程度提高,该效果随辐射源温度的提高而加剧。相比于LG产率随反应时间的快速下降趋势,高温下生物油产率的降低略显缓和,其变化主要是组分分布的改变,即从大分子结构降解为小分子结构。