New approach for the generation of initial configurations suitable for molecular dynamics studies of glassy polymers

Initial configurations suitable for molecular dynamics runs are usually assembled according to random values for the torsional angles of the molecules, and thus representing unrealistic conformations of the polymeric chains. In general, this would be acceptable if the system is allowed to run for periods of time long enough for the molecule to fully relax. However, in the current state of molecular dynamics runs, the 3D-periodic systems are usually allowed to run for 100 ps, which is too short for the polymeric system to relax. Alternatively, traditional rotational isomeric state approximation (RIS) could be used to generate the initial configurations of the polymeric chains. Unfortunately, RIS does not take into account the possible segment–segment overlap between atoms comprising the polymeric chains. In this work, we investigate the possibility of using the rotational isomeric state approximation to properly construct the initial configuration of 3D-periodic systems, without allowing any segment–segment overlap. In order to ensure that these configurations represent realistically the polymeric system, attempts were made to test the configurational properties of these systems against those determined experimentally. Further, these configurations were used to perform subsequent molecular dynamics runs in order to elucidate the effect of the molecular weight of poly(vinyl chloride) and temperature on some of the important thermodynamic properties such as self-diffusion coefficient, thermal pressure coefficient, heat capacity and dielectric constant.     

Self-assembly of aniline oligomers in aqueous medium

By dissolving branched or linear aniline oligomers in polar solvent and introducing their stock solution into an aqueous acidic medium, sheet-like as well as wire-like supramolecular structures with well-defined morphology were obtained, respectively. These oligomeric supramolecular structures were constructed via a post-synthetic precipitation process, indicating that aniline oligomers are capable of self-assembling in an aqueous medium, which is similar to the reaction medium of aniline chemical polymerization. Possible formation mechanisms of these supramolecular structures were proposed, i.e., sheet-like products were probably constructed by collapsed molecular chains of aniline oligomers with branched units through π–π stacking and hydrogen bonding, whereas formation of the wire-like products was attributed to “oriented-attachment” of collapsed molecular chains of linear aniline oligomers. The findings obtained in this study are supposed to provide useful clues for uncovering the formation mechanism of polyaniline micro-/nanostructures.     

Single Molecule Characterization of Amyloid Oligomers

The misfolding and aggregation of polypeptide chains into β-sheet-rich amyloid fibrils is associated with a wide range of neurodegenerative diseases. Growing evidence indicates that the oligomeric intermediates populated in the early stages of amyloid formation rather than the mature fibrils are responsible for the cytotoxicity and pathology and are potentially therapeutic targets. However, due to the low-populated, transient, and heterogeneous nature of amyloid oligomers, they are hard to characterize by conventional bulk methods. The development of single molecule approaches provides a powerful toolkit for investigating these oligomeric intermediates as well as the complex process of amyloid aggregation at molecular resolution. In this review, we present an overview of recent progress in characterizing the oligomerization of amyloid proteins by single molecule fluorescence techniques, including single-molecule Förster resonance energy transfer (smFRET), fluorescence correlation spectroscopy (FCS), single-molecule photobleaching and super-resolution optical imaging. We discuss how these techniques have been applied to investigate the different aspects of amyloid oligomers and facilitate understanding of the mechanism of amyloid aggregation.     

Side‐chain effect of octa‐substituted POSS fillers on refraction in polymer composites

To investigate the relationship between the chemical structures of the side chains of polyoctahedral oligomeric silsesquioxanes (POSS) fillers and the ability to decrease the refractive indices of the polymer composites, we examined the influence on the degree of polymer chain packing by the existence of the octa‐substituted POSS derivatives. The polymer composites containing methyl‐, ethyl‐, vinyl‐, isobutyl‐, octyl‐, octadecyl‐, cyclopentyl‐, and phenyl‐substituted POSS in poly(methyl methacrylate) (PMMA) were prepared. The packing coefficients of the PMMA composites containing POSS derivatives were evaluated from molar refractions and refractive indices of the films with the Lorentz–Lorenz equation. We found that the ethyl group shows the lowest values of the packing coefficients and a significant effect in reducing the refractive indices of the polymer composites. Finally, in summary, it was shown that POSS molecules can intrinsically offer to reduce a packing. In addition, less entanglement and smaller interactions between polymer chains and the substitution groups are favorable to release a packing, resulting in decreases in refractive indices. Our findings described here are the first efforts to quantitatively evaluate the ability of POSS fillers to lower refractive indices at the molecular level. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

RAFT polymerization kinetics: How long are the cross‐terminating oligomers?

We extend a new model for the kinetics of reversible addition‐fragmentation chain transfer (RAFT) polymerization. The essence of this model is that the termination of the radical intermediate formed by the RAFT process occurs only with very short oligomeric radicals. In this work, we consider cross‐termination of oligomers up to two monomers and an initiator fragment. This model accounts for the absence of three‐armed stars in the molecular weight distribution, which are predicted by other cross‐termination models, since the short third arm makes a negligible difference to the polymer's molecular weight. The model is tested against experiments on styrene mediated by cyano‐isopropyl dithiobenzoate, and ESR experiments of the intermediate radical concentration. By comparing our model to experiments, we may determine the significance of cross‐termination in RAFT kinetics. Our model suggests that to agree with the known data on RAFT kinetics, the majority of cross‐terminating chains are dimeric or shorter. If longer chains are considered in cross‐termination reactions, then significant discrepancies with the experiments (distinguishable star polymers in the molecular weight distribution) and quantum calculations will result. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3455–3466, 2009     

Resolution of a nonionic surfactant oligomeric mixture by means of DOSY with inverse micelle assistance

DOSY is a recognized, efficient technique in the analysis of mixtures. It relies on the differences in self-diffusion coefficients, which are determined by the molecular size. Nowadays, efforts are directed towards devising matrices able to interact with the components of the mixture with differential affinity, and therefore capable to interfere with the diffusion processes and to display resolving power towards species of close, or even equal molecular weight, like isomers. Usually, commercial nonionic surfactants are mixtures of oligomeric species, since the head group, which is a short polyoxyehtylene chain, is somewhat polydisperse. The embedment of Igepal CA-520, 5 polyoxyethylene iso-octylphenyl ether, in an inverse microemulsion led to the separation of (1)H signals of the various oligomeric components. This ensued from the differential partitioning between the oil and the surface of the inverse micelles, which depends on the ethyleneoxide number (EON) of the head groups. Thus, it was possible to ascertain that the length distribution of the polyethyleneoxide chains is ingood agreement with the Poisson distribution theoretically predicted for the polymerization of ethylene oxide. The DOSY spectrum contributed to the assignment of the signals and afforded the partition degree, between the two environments, for each individual oligomeric species, providing further insight into nonionic inverse microemulsions, at present widely employed reaction media in the nanotechnological syntheses.     

Rouse's dynamics of networks with pendant chains

A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus G″(ω) measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1121–1130, 1999     

Translational diffusion of relatively short polystyrene chains in cyclohexane. II. An interpretation of hydrodynamic radius with the freely jointed chain model

The translational diffusion coefficients for three relatively short, nearly monodisperse polystyrene chains (with molecular weights 2000, 4000, and 9000) in liquid cyclohexane have been measured up to 523.2 K using the extended Taylor dispersion method. The resultant hydrodynamic radii are successfully interpreted with the freely jointed chain model corrected for the approximations intrinsic to Kirkwood's theory with Stokes' radius as the only adjustable parameter, which is found to decrease with chain length but approach a constant value for long enough chains. The experimentally observed hydrodynamic radii of polystyrene standards with molecular weights 17,500, 50,000 and 100,000 at 308.2 K conform quite well to the values predicted with the approach formulated in this work.     

Synthesis and properties of thermoplastic and dissolvable polysiloxanes containing polyhedral oligomeric silsesquioxane

There are many benefits associated with thermoplastic silicones, but very few examples exist: silicone resins or rubbers are normally thermosets. In this article, a facile and efficient approach was reported to prepare thermoplastic silicone by introducing a bulky side siloxane group. Monofunctional polyhedral oligomeric silsesquioxane (POSS), as the bulky siloxane group, was grafted onto the linear polysiloxane backbone via thiol–ene click reaction, endowing the liquid polysiloxane with thermoplastic nature. The POSS-grafted polysiloxane could be remolded by a hot-melting or solution casting process. It was worth noting that the novel thermoplastic silicone was composed of both linear siloxane main chains and siloxane side groups, which was distinctly different from previous researches on thermoplastic silicones consisted of siloxane main chains and organic side groups. Thermal analysis, rheological characterization and molecular dynamics simulation results revealed the thermoplastic properties of POSS-grafted polysiloxane depended on the bulky POSS's hindrance to the movement of the polymer backbone rather than the interaction between the organic side groups.     

Free volume distribution in crosslinked polyurethanes

Free volume distribution was studied for polyurethane networks differing in the molecular mass of oligopropylene glycol taken in the synthesis. To estimate the free volume distribution experimentally, inversed gel chromatography was proposed. The integral and differential distribution of the free volume was constructed. Experimental data for the crosslinked polyurethanes showed the difference in the value of the free volume and its distribution due to the varied packing of oligomeric chains in the regions of the molecular mass below and above the critical molecular mass of entanglements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 899–905, 2000     

Effect of molecular weight on conformational characteristics of poly(3‐hexyl thiophene)

This research focused on the effect of molecular weight and investigated the conformational characteristics of poly(3‐hexyl thiophene) (P3HT). An integrated system consisting of a gel permeation chromatograph, a static light scattering unit, and a viscometer was used for the study. It also estimated the radius of gyration (R_g) values of unsubstituted poly(thiophene) (PT) chains computationally using rotational‐isomeric‐states modeling and compared them with the experimental data for P3HT. In the low molecular weight region (20,000 to 30,000), both chains had nearly the same R_g values, meaning that the effect of side‐chains is limited. At higher molecular weights, the P3HT chains expanded more than the PT chains. In the molecular weight region from 20,000 to 60,000, both characteristic ratio and persistence length showed considerable molecular weight dependence. Beyond a molecular weight of 60,000, the molecular weight dependence decreased, and these parameters approached constant values, 16 and 3 nm, respectively. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1273–1277     

Postgel properties in the statistical crosslinking of heterochains. II. Free‐radical crosslinking copolymerization

The heterochain crosslinking theory is applied to postgel behavior in the free‐radical crosslinking copolymerization of vinyl and divinyl monomers. In this context, the crosslinked polymer formation can be viewed as a system in which the primary chains formed at different times are combined in accordance with the statistical chain‐connection rule governed by the chemical reaction kinetics. Because the primary chains are formed consecutively, the number of chain types N must be extrapolated to infinity, N → ∞. Practically, such extrapolation can be conducted with the calculated values for only three different N values. The analytical expressions for the weight fraction and average molecular weights of the sol fraction are derived for the general primary chain length distribution function in free‐radical polymerization. Illustrative calculations show that the obtained results agree with those from the Monte Carlo method, and that the postgel properties in free‐radical crosslinking copolymerization systems could be significantly different from those in randomly crosslinked systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2342–2350, 2000     

Hydrolytic degradation of poly(d,l-lactide-co-glycolide 50/50)-di-acrylate network as studied by liquid chromatography-mass spectrometry

The soluble products of the hydrolytic degradation of photochemically cross-linked poly-(d,l-lactide-co-glycolide 50/50)-di-acrylate film were analysed at different stages to obtain insight into the complex (bio)degradation processes. Liquid chromatography-mass spectrometry analyses have been used to identify and quantify the various oligomeric and polymeric degradation products from the soluble fraction. The products were analysed directly after release and also after complete hydrolysis of the soluble fraction. The study shows a rapid release of residual photo-initiator followed by a gradual release of lactide/di-ethyleneglycol/glycolide oligomers with varying composition and chain length. The final stage of the sigmoidal weight loss profile reflects the release of polyacrylate chains with lactide/glycolide side chains. The molecular weights of the polyacrylate chains released increase with degradation time, which indicates that the release of these polyacrylate chains is determined by the number and type of ester-groups that must be degraded hydrolytically to dissolve these chains. The analysis of the soluble degradation products provides detailed insights in the chemical changes at the different stages of degradation; extraction, network attack, network penetration, bulk degradation, and finally release of persistent network fragments. Chromatographic and mass spectrometric techniques prove to be powerful tools to enhance the understanding of the hydrolytic degradation of chemically cross-linked acrylates.     

Exciton transfer integrals between polymer chains

The line-dipole approximation for the evaluation of the exciton transfer integral J between conjugated polymer chains is rigorously justified. Using this approximation, as well as the plane-wave approximation for the exciton center-of-mass wave function, it is shown analytically that J approximately L when the chain lengths are smaller than the separation between them, or J approximately L-1 when the chain lengths are larger than their separation, where L is the chain length. Scaling relations are also obtained numerically for the more realistic standing wave approximation for the exciton center-of-mass wave function, where it is found that for chain lengths larger than their separation J approximately L-1.8 or J approximately L-2, for parallel or collinear chains, respectively. These results have important implications for the photophysics of conjugated polymers and self-assembled molecular systems, as the Davydov splitting in aggregates and the Forster transfer rate for exciton migration decrease with chain lengths larger than their separation. This latter result has obvious deleterious consequences for the performance of polymer photovoltaic devices.     

Structural patterns of rhamnogalacturonans modulating Hsp-27 expression in cultured human keratinocytes

Polysaccharide extracts were obtained from chestnut bran (Castanea sativa), grape marc (Vitis vinifera) and apple marc (Malus spp.) and fractionated by size exclusion chromatography after endopolygalacturonase degradation. Compositional and linkage analyses by GC and GC-MS showed the characteristic rhamnogalacturonan structure with specific arabinan (apple marc) and type II arabinogalactan (chestnut bran, grape marc) side chains. Type II arabinogalactan rhamnogalacturonan from chestnut bran significantly stimulated the in vitro differentiation of human keratinocytes, giving evidence of a tight structure-function relationship. This molecule comprises short and ramified 3- and 3,6-beta- D-galactan and 5- and 3,5-alpha-L-arabinan side chains, but also contains significant amounts of t-Xyl and 4-Xyl with a characteristic 2:1 ratio. Enzymatic hydrolysis of this polysaccharide produced fragments of lower molecular weight with unchanged xylose content which conserved the same ability to stimulate human keratinocyte differentiation. It could be then speculated that dimeric xylosyl-xylose and/or longer oligomeric xylose side chains attached to a galacturonan and closely associated to hairy rhamno-galacturonan domains are essential patterns that could determine the biological activity of pectins.     

Model networks of end-linked polydimethylsiloxane chains. XI. Use of very short network chains to improve ultimate properties

Elastomeric networks were prepared by tetrafunctionally end-linking mixtures of various proportions of relatively long and very short polydimethylsiloxane (PDMS) chains. The former had a number-average molecular weight of 18,500 and the latter either 660 or 220 g mole^?1. The series of (unfilled) bimodal networks thus prepared were studied in elongation to the rupture point at 25°C, and in swelling equilibrium in benzene at room temperature. Elasticity constants characterizing the Gaussian regions of the stress–strain isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity. The isotherms also gave values of the elongation at which the modulus begins to increase anomalously because of limited chain extensibility, and values of the elongation and nominal stress at the point of rupture. These results were interpreted in terms of the known configurational characteristics of the constituent PDMS chains. Values of the energy or work required for rupture were used as an overall measure of the “toughness” of the networks. The very short chains were found to give a marked increase in toughness, through an increase in ultimate strength without the usual corresponding decrease in maximum extensibility. A variety of additional experiments will be required in order to elucidate the molecular origins of this important effect.     

Graft polymerization of acrylic monomers onto starch fractions. III. Effect of carbohydrate structure

Grafted PMMA was isolated by the acid hydrolysis method. Weight-average and number-average molecular weights were determined by gel permeation chromatography. The number-average molecular weight of the grafted chains was about 475,000 for amylopectin and 403,000 for amylose. The number of grafted chains (mmol) ranged from 2.4 × 10^?3 to 4.6 × 10^?3 for amylopectin graft copolymers and from 2.9 × 10^?3 to 6.8 × 10^?3 for those of amylose. These results were related to others obtained from ceric ion consumption studies. The values suggest that amylopectin, because of its complex structure, favors a higher consumption of ceric ion in homopolymerization reactions and inhibits the initiation reactions of the copolymerization.     

Random grafting statistics in graft distribution analysis

Graft distribution functions have been derived from random grafting statistics. Among the functions, the weight fraction of ungrafted backbone chains, the molecular weight distribution of the ungrafted backbone chains and the GPC apparent molecular weight distribution of the graft copolymer have been found to agree with experimental values determined for a graft copolymer system in which grafting was expected to be random. The other functions, which are not directly measurable, are therefore probably also correct. In analytical work the entire set of graft distribution functions may be computed for a graft copolymer system from the following experimental data: (1) molecular weight distribution of the starting backbone chains; (2) the chemical composition of the mixture of the graft copolymer and ungrafted backbone; (3) the graft side-chain molecular weight distribution, which may be assumed to be identical to that of the ungrafted homopolymer separable from the reaction mixture.     

An overview on the recent developments in reactive plasticizers in polymers

This review is about the reactive plasticizer. Plasticizers are small molecules with low molecular weight. These compounds typically have an esteric structure. The plasticizers reduce the glass transition temperature, and the viscosity of the polymer also enhances the flexibility and processability of polymer materials. The main problem of these additives is that, over time, they migrate from the polymeric matrix and exude to the surface of polymeric matrix. As a result, the physical and mechanical properties of the polymer are affected. Various strategies, such as increasing molecular weight of plasticizer, selection of oligomeric structure for plasticizer, and adding nanoparticles of minerals, have been investigated to reduce and eliminate migration. An approach that has recently been of great interest to researchers is the use of reactive plasticizers. In this approach, plasticizers covalently bond to the polymeric chains and prevent migration.     

Electronic band structure of model polydiacetylene chains

Calculations are reported of the electronic band structure of model polydiacetylene extended chains, with the large side groups represented by H or CCH, using the extended Hückel linear combination of atomic orbitals approximation. Predicted band gap widths are in good agreement with the results of an earlierπ-band calculation, and considerably smaller than experimental values.