A differential scanning calorimetry study on poly(ethylene terephthalate) isothermally crystallized at stepwise temperatures: multiple melting behavior re-investigated

The multiple melting behavior of poly(ethylene terephthalate) (PET) was investigated with differential scanning calorimetry (DSC) by examining PET samples having been subjected to special schemes of crystallization and annealing treatment at multiple descending temperatures. Upon such step-wise annealing in decreasing temperatures, the existence of doublet melting peaks in addition to a series of multiple minor peaks in the PET has been demonstrated using carefully designed thermal schemes. Using the Hoffman theory, multiple lamellae populations, might be suggested to be simultaneously present in the PET subjected to such thermal treatments. However, direct experimental evidence has yet to be provided. The low-temperature minor crystals simply melt during normal scanning without having time enough to reorganize into higher-melt crystals. Nevertheless, the effect of scanning on non-isothermal crystallization does exist but is primarily confined to the temperature range much below the main melting region where the crystallization of polymer chains can progress at a reasonable rate. At higher temperatures near the main melting region, annealing for extended times is required in order to result in relative changes of the melting endotherms of the upper and lower peaks in the main melting doublet. In all we have shown that interpretations of the multiple melting phenomenon in semicrystalline polymers can be better refined.     

Fracture mechanisms in rigid core-shell particle modified high performance epoxies

The fracture mechanisms of a high performance epoxy system modified with two types of preformed rigid core-shell particles (RCSP) were investigated. The use of the preformed RCSP anables the control of the dispersion of the toughener phase in the epoxy, which, in turn, allows the mechanical properties of the modified epoxy to be optimized. The toughening effect via the RCSP modification is found to be as good as that via the core-shell rubber modification. The moduli andT _g of these RCSP-modified epoxies are virtually unaltered via the RCSP modification, when compared with the neat epoxy resin equivalent. The toughening mechanisms in these toughened systems appear to be predominantly crack deflection, crack bifurcation, and microcracking. Approaches for effective toughening of high performance polymers via rigid polymers are discussed.     

Multiple-locus variable number tandem repeats analysis for genetic fingerprinting of pathogenic bacteria

DNA fingerprinting has attracted considerable interest as means for identifying, tracing and preventing the dissemination of infectious agents. Various methods have been developed for typing of pathogenic bacteria, which differ in discriminative power, reproducibility and ease of interpretation. During recent years a typing method, which uses the information provided by whole genome sequencing of bacterial species, has gained increased attention. Short sequence repeat (SSR) motifs are known to undergo frequent variation in the number of repeated units through cellular mechanisms most commonly active during chromosome replication. A class of SSRs, named variable number of tandem repeats (VNTRs), has proven to be a suitable target for assessing genetic polymorphisms within bacterial species. This review attempts to give an overview of bacterial agents where VNTR-based typing, or multiple-locus variant-repeat analysis (MLVA) has been developed for typing purposes, together with addressing advantages and drawbacks associated with the use of tandem repeated DNA motifs as targets for bacterial typing and identification.     

Rigid-body dynamics in the isothermal-isobaric ensemble: a test on the accuracy and computational efficiency

We have developed a time-reversible rigid-body (rRB) molecular dynamics algorithm in the isothermal-isobaric (NPT) ensemble. The algorithm is an extension of rigid-body dynamics [Matubayasi and Nakahara, J Chem Phys 1999, 110, 3291] to the NPT ensemble on the basis of non-Hamiltonian statistical mechanics [Martyna, G. J. et al., J Chem Phys 1994, 101, 4177]. A series of MD simulations of water as well as fully hydrated lipid bilayer systems have been undertaken to investigate the accuracy and efficiency of the algorithm. The rRB algorithm was shown to be superior to the state-of-the-art constraint-dynamics algorithm SHAKE/RATTLE/ROLL, with respect to computational efficiency. However, it was revealed that both algorithms produced accurate trajectories of molecules in the NPT as well as NVT ensembles, as long as a reasonably short time step was used. A couple of multiple time-step (MTS) integration schemes were also examined. The advantage of the rRB algorithm for computational efficiency increased when the MD simulation was carried out using MTS on parallel processing computer systems; total computer time for MTS-MD of a lipid bilayer using 64 processors was reduced by about 40% using rRB instead of SHAKE/RATTLE/ROLL.     

Fluorine-Stabilized Sulfur–Carbon Multiple Bonds

Alkylidene and alkylidyne sulfur fluorides contain sulfur–carbon multiple bonds. In contrast to the sulfur ylides, alkylidene sulfur fluorides fulfill all the criteria for double bonds, i.e. they have short bond lengths, strong anisotropic distribution of electron density, and rotation about the C? S bond is restricted. Alkylidyne sulfur fluorides have especially short bond distances and, due to a high amplitude bending motion, appear to be more or less linear, depending on the physical state. The advantage of the C? S multiple bond systems in contrast with numerous others, e.g. those of phosphorus and silicon, is that they exist without steric stabilization. Moreover, the limits of the triple-bond principle are outlined: the prognosis for triple bonds between two elements of higher periods is poor, because carbene-like or fully bridged structures win in terms of stability.     

Synthesis and characterization of [Cu(Phca2en)(PPh3)X] (X=Cl, Br, I, NCS, N3) complexes: crystal structures of [Cu(Phca2en)(PPh3)Br] and [Cu(Phca2en)(PPh3)I]

New mixed-ligand copper(I) complexes, [Cu(Phca₂en)(PPh₃)X], [Phca₂en = N,N′-bis(β-phenylci-nnamaldehyde)-1,2-diiminoethane and X=Cl (1), Br (2), I (3), NCS (4), N₃ (5)] have been synthesized and characterized by various techniques. ¹H and ¹³C-NMR and IR spectral data of these copper(I) complexes are compared with the free ligand to elucidate some structural features. The structures of [Cu(Phca₂en)(PPh₃)Br] (2) and [Cu(Phca₂en)(PPh₃)I] (3) have been determined from single-crystal data showing that the coordination geometry around copper atom is a distorted tetrahedron. Furthermore, these Cu(I) complexes exhibit supramolecular motifs of the type multiple phenyl embraces resulting from attractive interactions between phenyl rings of PPh₃ moieties. The presence of the C–H…Cu weak intramolecular hydrogen bonds, due to the trapping of C–H bonds in the vicinity of the metal atoms, is also reported.     

Investigation of the rigid amorphous fraction in Nylon-6

A three-phase model, comprising crystalline, mobile amorphous, and rigid amorphous fractions (χ _c, χ _MA, χ _RA, respectively) has been applied in the study of semicrystalline Nylon-6. The samples studied were Nylon-6 alpha phase prepared by subsequent annealing of a parent sample slowly cooled from the melt. The treated samples were annealed at 110°C, then briefly heated to 136°C, then re-annealed at 110°C. Temperature-modulated differential scanning calorimetry (TMDSC) measurements allow the devitrification of the rigid amorphous fraction to be examined. We observe a lower endotherm, termed the ‘annealing’ peak in the non-reversing heat flow after annealing at 110°C. By brief heating above this lower endotherm and immediately quenching in LN₂-cooled glass beads, the glass transition temperature and χ _RA decrease substantially, χ _MA increases, and the annealing peak disappears. The annealing peak corresponds to the point at which partial de-vitrification of the rigid amorphous fraction (RAF) occurs. Re-annealing at 110°C causes the glass transition and χ _RA to increase, and χ _MA to decrease. None of these treatments affected the measured degree of crystallinity, but it cannot be excluded that crystal reorganization or recrystallization may also occur at the annealing peak, contributing to the de-vitrification of the rigid amorphous fraction. Using a combined approach of thermal analysis with wide and small angle X-ray scattering, we analyze the location of the rigid amorphous and mobile amorphous fractions within the context of the Heterogeneous and Homogeneous Stack Models. Results show the homogeneous stack model is the correct one for Nylon-6. The cooperativity length (ξ_A) increases with a decrease of rigid amorphous fraction, or, increase of the mobile amorphous fraction. Devitrification of some of the RAF leads to the broadening of the glass transition region and shift of T _g.     

Symmetry numbers and chemical reaction rates

This article shows how to evaluate rotational symmetry numbers for different molecular configurations and how to apply them to transition state theory. In general, the symmetry number is given by the ratio of the reactant and transition state rotational symmetry numbers. However, special care is advised in the evaluation of symmetry numbers in the following situations: (i) if the reaction is symmetric, (ii) if reactants and/or transition states are chiral, (iii) if the reaction has multiple conformers for reactants and/or transition states and, (iv) if there is an internal rotation of part of the molecular system. All these four situations are treated systematically and analyzed in detail in the present article. We also include a large number of examples to clarify some complicated situations, and in the last section we discuss an example involving an achiral diasteroisomer.     

Screening of Cerebrospinal Fluid and Blood Sera of Multiple Sclerosis Patients for Oligoclonal Immunoglobulins by Capillary Electrophoresis

Summary Multiple sclerosis (MS) is an autoimmune disease characterized by the production of specific types of immunoglobulins into the central nervous system. These immunoglobulins appear as oligoclonal bands (OCBs) in agarose isoelectric focusing (IEF) of cerebrospinal fluid (CSF). Among the cases with clinically definite MS, up to 95% have oligoclonal IgG bands in their CSF. In this report, we describe a micellar electrokinetic capillary chromatography (MEKC) method for the separation of CSF and serum proteins. MEKC was performed using 25 mM borate buffer, pH 10, containing 25 mM SDS at 20 kV and normal polarity. High values of repeatability in migration times and of reproducibility in peak areas were obtained (R.S.D. values were less than 2%). Calibration graphs were linear up to 2000 mg L^–1. LOQ was 6.5 mg L^–1 and LOD determined as a signal to noise ration of 3:1 was 4.5 mg L^–1. Analysis of CSF and serum samples from patients with clinical definite MS and healthy individuals demonstrated the presence of two peaks migrating as -globulins in the CSF samples of patients. These peaks were absent from controls and the serum of the same patients. Correlation of the data obtained from IEF and MEKC analysis for 25 patients showed that the diagnostic sensitivity and specificity of MEKC were ca 89% and 92% respectively. The obtained results indicate that this MEKC method may be helpful for the diagnosis of multiple sclerosis. Capillary electrophoresis compared to flat bed IEF provides reproducible results, requires shorter analysis time, and allows direct quantitative determination.Presented at: International Symposium on Separation and Characterization of Natural and Synthetic Macromolecules, Amsterdam, TheNetherlands, February 5–7, 2003