Average Chain Size Research Articles

Chemical composition of biodiesel produced in situ with Salvinia molesta DS Mitchell (Salviniaceae) by ethylic and methylic route

Macrophytes are aquatic plants that can cause environmental and economic damage due to their rapid growth in eutrophicated environments; however, this characteristic makes these biomasses promising alternatives for biodiesel production. Thus, this study aims to characterize and evaluate the chemical composition of the biodiesel produced from the macrophyte Salvinia molesta DC Mitchell (Salviniaceae). The biodiesel production was carried out in situ through the ethyl and methyl process. Attenuated total reflectance-Fourier transform infrared spectroscopy (FTIR-ATR) and gas chromatography with a flame ionization detector (GC-FID) were used to characterize the product. A commercial sample was also analyzed for comparison purposes. The biofuel produced with ethanol and methanol showed characteristic peaks between 900 to 1300 cm-1 and 1750 to 1735 cm-1 in the FTIR-ATR. Both samples showed less unsaturation degree compared to the commercial sample, with 34.44% of monounsaturated compounds (MUFA) and 36.73% of polyunsaturated compounds (PUFA) for methylic biodiesel, 34.79% of MUFA and 36.89% of PUFA for ethylic biodiesel, and 55.34% of MUFA and 24.14% of PUFA for commercial biodiesel. Samples produced by both routes showed similar chemical composition, with higher contents of saturated compounds than the commercial sample. The average chain size and the number of double bonds are smaller for S. molesta samples, 17.38 and 1.15 for S. molesta biodiesels and 17.65 and 1.41 for commercial biodiesel, respectively. The chemical composition of S. molesta biodiesel demonstrates the potential to be an alternative to commercial biodiesel.

Conformation of Network Strands in Polymer Gels.

Small angle neutron scattering was used to measure single chain radii of gyration of end-linked polymer gels before and after cross-linking to calculate the prestrain, which is the ratio of the average chain size in a cross-linked network to that of a free chain in solution. The prestrain increased from 1.06 ± 0.01 to 1.16 ± 0.02 as gel synthesis concentration decreased near the overlap concentration, indicating that the chains are slightly more stretched in the network than in solution. Dilute gels with higher loop fractions were found to be spatially homogeneous. Form factor and volumetric scaling analyses independently confirmed that elastic strands stretch by 2-23% from Gaussian conformations to create a space-spanning network, with increased stretching as network synthesis concentration decreases. Prestrain measurements reported here serve as a point of reference for network theories that rely on this parameter for the calculation of mechanical properties.

Polymorphism and Perfection in Crystallization of Hard Sphere Polymers

We present results on polymorphism and perfection, as observed in the spontaneous crystallization of freely jointed polymers of hard spheres, obtained in an unprecedentedly long Monte Carlo (MC) simulation on a system of 54 chains of 1000 monomers. Starting from a purely amorphous configuration, after an initial dominance of the hexagonal closed packed (HCP) polymorph and a transitory random hexagonal close packed (rHCP) morphology, the system crystallizes in a final, stable, face centered cubic (FCC) crystal of very high perfection. An analysis of chain conformational characteristics, of the spatial distribution of monomers and of the volume accessible to them shows that the phase transition is caused by an increase in translational entropy that is larger than the loss of conformational entropy of the chains in the crystal, compared to the amorphous state. In spite of the significant local re-arrangements, as reflected in the bending and torsion angle distributions, the average chain size remains unaltered during crystallization. Polymers in the crystal adopt ideal random walk statistics as their great length renders local conformational details, imposed by the geometry of the FCC crystal, irrelevant.

Sizes and shapes of simulated amorphous cis- and trans-1,4-polybutadiene

Controlling loss modulus can decrease tire rolling resistance. Deformation forces in elastomers depend on chain conformation. As a first step toward screening chemical effects on these, isolated single cis- and trans-1,4-polybutadiene chains of uncorrelated random conformations under unperturbed conditions are generated using Flory's Rotational Isomeric State approach, with discrete rotational states defined by Mark, Abe, and Flory. Averages and distribution of squared end-to-end distance, radius of gyration, and shape parameters are studied for chains ranging from 15 to 120 repeat units at 343 K and for the 50-unit chain at temperatures of 275 K–400 K. The calculated characteristic ratios are in good agreement with experimental and prior computed values. Only small absolute changes in chain size probability densities with temperature are found. A larger relative increase in probability density of unlikely larger chains and a smaller relative decrease in probability density of more likely smaller chains result in increased average chain size and characteristic ratios with increasing temperature. Within subdivisions of the size distribution, larger chains show an increase in characteristic ratio with temperature, while smaller and medium size chains show little change with temperature. This previously unreported effect is stronger for trans than for cis chains. Eigenvalues of the radius of gyration matrix quantify chain shapes along principal directions. Averaged shape measures differ between cis and trans chains, and most shape changes arise along the longest principal direction. Joint correlations between chain size and shape show they are mutually dependent.

Inference of R0 and Transmission Heterogeneity from the Size Distribution of Stuttering Chains

For many infectious disease processes such as emerging zoonoses and vaccine-preventable diseases, and infections occur as self-limited stuttering transmission chains. A mechanistic understanding of transmission is essential for characterizing the risk of emerging diseases and monitoring spatio-temporal dynamics. Thus methods for inferring and the degree of heterogeneity in transmission from stuttering chain data have important applications in disease surveillance and management. Previous researchers have used chain size distributions to infer , but estimation of the degree of individual-level variation in infectiousness (as quantified by the dispersion parameter, ) has typically required contact tracing data. Utilizing branching process theory along with a negative binomial offspring distribution, we demonstrate how maximum likelihood estimation can be applied to chain size data to infer both and the dispersion parameter that characterizes heterogeneity. While the maximum likelihood value for is a simple function of the average chain size, the associated confidence intervals are dependent on the inferred degree of transmission heterogeneity. As demonstrated for monkeypox data from the Democratic Republic of Congo, this impacts when a statistically significant change in is detectable. In addition, by allowing for superspreading events, inference of shifts the threshold above which a transmission chain should be considered anomalously large for a given value of (thus reducing the probability of false alarms about pathogen adaptation). Our analysis of monkeypox also clarifies the various ways that imperfect observation can impact inference of transmission parameters, and highlights the need to quantitatively evaluate whether observation is likely to significantly bias results.

Unusual Sensitivity of Closed-Loop Phase Behavior to Chain Size and Distribution

An anionically polymerized polystyrene-block-poly(n-pentyl methacrylate) (PS-b-PnPMA) was fractionated by interaction chromatography (IC), and the closed-loop phase behavior of the mother block copolymer and its fractions with different chemical compositions was investigated by depolarized light scattering and polarized optical microscopy. With increasing elution time the average chain size (or the average molecular weight) of the block copolymer decreased slightly, and the average chemical composition changed steadily from a PS volume fraction of 0.583 to 0.403, thereby producing five fractions collected over the different ranges of elution time. This is caused by the difference of relative interaction strength between the stationary phase and the PnPMA chains of PS-b-PnPMA, in which PnPMA chains tend to be adsorbed onto the stationary phase while PS chains behave oppositely. Over the range of accessible temperatures studied (110−250 °C) a variety of phase transitions were observed, including fully ordered state, a closed-loop (consisting of the lower disorder-to-order transition and upper order-to-disorder transition), and fully disordered state. The original, unfractionated block copolymer was disordered over the entire temperature range. The variety of observed phase transitions obtained from the fractionated block copolymers is assumed to be governed by the sensitivity of the closed-loop phase behavior to average chain size, average chemical composition, and the compositional broadness.

Regimes of Conformational Transitions of a Diblock Polyampholyte

The conformational properties of symmetric flexible diblock polyampholytes are investigated by scaling theory and molecular dynamics simulations. The electrostatically driven coil-globule transition of a symmetric diblock polyampholyte is found to consist of three regimes identified with increasing electrostatic interaction strength. In the first (folding) regime the electrostatic attraction causes the chain to fold through the overlap of the two blocks, while each block is slightly stretched by self-repulsion. The second (weak association or scrambled egg) regime is the classical collapse of the chain into a globule dominated by the fluctuation-induced attractions between oppositely charged sections of the chain. The structure of the formed globule can be represented as a dense packing of the charged chain sections (electrostatic attraction blobs). The third (strong association or ion binding) regime starts with direct binding of oppositely charged monomers (dipole formation), followed by a cascade of multipole formation (quadrupole, hexapole, octupole, etc.), leading to multiplets analogous to those found in ionomers. The existence of the multiplet cascade has also been confirmed in the simulations of solutions of short polymers with only one single charge (either positive or negative) in the middle of each chain. We use scaling theory to estimate the average chain size and the electrostatic correlation length as functions of the chain length, strength of electrostatic interactions, charge fraction, and solvent quality. The theoretically predicted scaling laws of these conformational properties are in very good agreement with our simulation results.

Simulation study of the coil-globule transition of a polymer in solvent

Molecular dynamics simulations are used to study the coil-globule transition for a system composed of a bead-spring polymer immersed in an explicitly modeled solvent. Two different versions of the model are used, which are differentiated by the nature of monomer-solvent, solvent-solvent, and nonbonded monomer-monomer interactions. For each case, a model parameter lambda determines the degree of hydrophobicity of the monomers by controlling the degree of energy mismatch between the monomers and solvent particles. We consider a lambda-driven coil-globule transition at constant temperature. The simulations are used to calculate average static structure factors, which are then used to determine the scaling exponents of the system in order to determine the theta-point values lambdatheta separating the coil from the globule states. For each model we construct coil-globule phase diagrams in terms of lambda and the particle density rho. The results are analyzed in terms of a simple Flory-type theory of the collapse transition. The ratio of lambdatheta for the two models converges in the high density limit exactly to the value predicted by the theory in the random mixing approximation. Generally, the predicted values of lambdatheta are in reasonable agreement with the measured values at high rho, though the accuracy improves if the average chain size is calculated using the full probability distribution associated with the polymer-solvent free energy, rather than merely using the value obtained from the minimum of the free energy.

Analysis of shape transitions using molecular size descriptors associated with inner and outer regions of a polymer chain

In random polymers, all descriptors of average chain size have the same scaling behaviour with the number of monomers. However, the situation can be different in polymers with non-random structures, e.g. biopolymers with secondary structure. In this case, descriptors of molecular size associated with inner and outer regions of the polymer chain (where ‘inner’ and ‘outer’ are defined with respect to the center of mass) are not necessarily equivalent. Indeed, differences in behaviour between size descriptors can provide useful information on structure and conformation. In this work, we use the ratio of two distinct molecular size descriptors to classify polymer configurations in terms of local compactness. When applied to protein dynamics, the new size-ratio descriptor ρ is able to distinguish between unfolding transitions leading to configurations with uniform and non-uniform monomer distributions.

Conformation of a polymer chain near the solvent critical region. II. Monte Carlo simulation

Using the Monte Carlo (MC) calculations, we study the conformational behavior of a two-dimensional (2D) single flexible-chain polymer dissolved in a monoatomic solvent. It is shown that near the solvent critical region the polymer chain can contract. Such a behavior is observed if the radius of fluctuations of solvent density is smaller than the natural size of the N-unit chain with excluded volume interaction [R∼Nν, where ν≈3/(d+2) is the Flory–Edwards exponent]. On the other hand, the chain goes back to the initial swollen state when the solvent correlation length becomes larger than R. Under these conditions, the polymer chain is effectively confined in a large solvent droplet. We find that the strongly fluctuating solvent can induce significant conformational changes only if there is a rather strong attraction between polymer segments and solvent particles. In this case, the average chain size is a nontrivial function of polymer–solvent attraction: At rather weak affinity of polymer and solvent, the average chain size grows with the increase of this attraction; with further increase of affinity of polymer and solvent, the chain begins to contract. Thus, in the case of high affinity of polymer and solvent, the polymer chain can undergo the complex coil–globule–coil transition. In general, the results of the MC simulations are in reasonable agreement with those obtained from the self-consistent integral-equation calculations.

Characterization of the Group B Streptococcal Hyaluronate Lyase

Hyaluronate lyase is one of several proteins secreted by group B streptococci which are believed to contribute to strain virulence. Characterization of the purified enzyme revealed that it degrades hyaluronan by a mechanism different from that of other previously studied hyaluronidases. Instead of randomly cleaving hyaluronan chains leading to a continuous decrease in average chain size, the group B streptococcal enzyme initially yields primarily unsaturated disaccharides. The observation that most of the free reducing ends generated during group B streptococcal hyaluronate lyase digestion are present in the unsaturated disaccharide units supports the conclusion that they are released primarily from the ends of the hyaluronan chains. Furthermore, the experimental evidence is consistent with a made of action by which the enzyme initially makes a random cut in a hyaluronan chain and then processively moves along the chain releasing disaccharide units. Group B streptococcal hyaluronate lyase also slowly degrades chondroitin sulfate, and its desulfation greatly increases the reaction rate. A preferential cleavage of unsulfated residues is consistent with the observed extensive release of free chondroitin sulfate chains following very limited digestion of aggrecan from bovine nasal cartilage.