Mean-square Radius Of Gyration Research Articles

A Semiflexible Polymer Translocation Through a Cylindrical Channel

In this study, translocation of a semi flexible polymer through a cylindrical channel have been investigated. A two-dimensional Monte Carlo simulation was employed, by utilizing the bond fluctuation method (BFM) to investigate the translocation processes of a chain length N. To surmount the entropic barrier, the middle monomers of the polymer have been positioned at the center of the pore, which is situated between the CIS and TRANS regions. Consequently, the static properties of a semi-flexible polymer by calculating the mean square end-to-end distance ‹R2› and the mean square radius of gyration ‹R<sup>g</sup>2› as functions of the chain length (N) have been examined. The mean square end-to-end distance and the mean square radius of gyration are proportional to the number of monomers N as ‹R2› ~ N1.496 and ‹R2g› ~ N1.505 correspondingly for a short cylindrical channel length L = 2, which aligns with the theoretically predicted. These finding indicates that the relationships between ‹R2› and ‹R<sup>g</sup>2› and the polymer chain size N are strongly influenced by the channel length L. The dynamic properties by analyzing the translocation time of the polymers also studied. Additionally, the relationship between the escape time τ and the polymer chain length N depends on the pore width W, which is equivalent to the diameter of the cylindrical channel. These research demonstrates that the escape time τ decreases as the width increases and escape time τ increases as the chain stiffness increases.

Dispersion, Dynamics, and Mechanics of Polymer Nanocomposites Filled with Rigid-Soft Janus Nanoparticles via Molecular Dynamics Simulation.

The introduction of nanoparticles (NPs) presents boundless possibilities for enhancing the performance of polymer nanocomposites (PNCs). Consequently, the design of novel NPs becomes of paramount significance for PNCs. In our study, we employ the dumbbell two-component model of Janus nanoparticles (JNPs) and design rigid-soft JNPs as fillers. Using coarse-grained molecular dynamics simulations, we systematically investigate the dispersion, dynamics, and mechanical properties of these novel PNCs. First, we determine the optimal dispersion conditions by studying rcutoff and εnp. The simulation indicates that when the interaction between polymer chains and JNPs is a repulsive potential, the JNPs tend to aggregate together, forming a cluster with soft NPs inside and rigid NPs outside. Conversely, under attractive interactions, JNPs show superior dispersion uniformity compared to the repulsive system, and as εnp increases, the dispersion improves. Then, the mean square displacement (MSD) indicates that JNPs effectively impede the mobility of polymer chains, with the degree of hindrance increasing as εnp grows; this effect is more pronounced in attractive systems. Comparing JNPs of different particle sizes, we find that smaller JNP systems exhibit higher temperature sensitivity. Furthermore, there exists a critical particle size (Dnp ≈ 5σ) under a constant filling fraction at which the NPs exert the most pronounced restriction effect on the polymer. Next, upon examining the mechanical behavior, we find that the rigid-soft JNPs demonstrate notable elasticity and variability compared to traditional NPs. This observation is confirmed through measurements of the bond orientation and mean square radius of gyration of the soft segments of JNPs. In summary, this research provides a comprehensive understanding of the intricate interplay among various factors, offering valuable insights for optimizing JNP dispersion and enhancing the mechanical properties of PNCs.

Molecular dynamics simulation of stretching-induced ductility for polystyrene

Some glassy polymers have a mechanical behavior that stretching can induce the brittle-to-ductile transition (BDT). For polystyrene (PS) film, after pre-stretching above glass transition temperature (Tg), the property at room temperature transforms to ductile, showing higher elongation at break and higher yield strength. In this work, we have investigated this phenomenon of stretching induced ductility by molecular dynamics (MD) analysis referring to our experimental findings. PS specimens in united-atom model with different chain lengths were loaded by deformation at two-step temperature setting, which was 393 K at first and then 300 K, to simulate tensile behaviors with various per-stretching conditions. The result has shown that as deformations beginning at 393 K and proceeding at 300 K, root mean square radius of gyration (Rg) and the degree of orientation (P2,x) reach the maximum value higher than solely stretching at 393 K or 300 K. Therefore, the stretching-induced BDT is attributed to changing of both chain conformation and coil configuration in polymer chains in stepwise stretching, which can change their coil configuration at the pre-stretching, adjusting their chain network in a more stretchable position, leading the whole polymer amorphous from brittle to ductile. The simulation result is accordant with our experiment and previous BDT theory to glassy polymers.

Experimental measurement and molecular dynamics simulation analysis of thermal aging performance in composite solid propellants

As a complex mixed system, the aging mechanism of high-energy composite materials involves the aging of each component itself, interactions between components, and transformation of microstructural and thermodynamic properties. The study of a single component or performance parameter has great limitations for elaborating the cognitive complexity of aging processes, making it difficult to reveal the underlying principles of physicochemical property transformation. Thus, it can only be applied as an empirical summary. In view of this, this study comprehensively utilized numerical analysis and experimental detection to explore the evolution of micromolecular conformation and changes in macro aging performance from four aspects of composite systems, including binder aging, plasticizer migration, bond interface debonding, and mechanical property transformation. There was good consistency observed between molecular simulation and thermal aging experiments, indicating that there was basically no post curing process for HTPB propellants using TDI as the curing agent. In the early stage of aging, scission chain was the main process and, in the later stage of aging, oxidation crosslinking the main process. Oxidative crosslinking increased the mean square radius of gyration and glass transition temperature of the binder system, reduced the peak value of radial distribution function, and revealed the reason for the change of propellant hardness and CC and CO double bond content. However, chain degradation increased the number of polar groups, enhanced intermolecular polarity, reduced the solubility parameter, and increased the free volume fraction of the mixed system and the dioctyl sebacate diffusion coefficient. This resulted in an increase in the tangent value of the HTPB propellant loss angle from 0.43 to 0.44° and a decrease in the glass transition temperature from 203.3 to 202.0 K. The binding energy at the interfaces of AP/HTPB and Al/HTPB increased first and then decreased, which was consistent with SEM images of bonding interfaces. The joint effect of binding energy and mechanical properties of the binder system promoted the transformation of the stress-strain curve of aging propellant.

Insight of the viscometric behavior of pullulan and curdlan derivative solutions: Effect of the nature and salt concentration

Increasing interest for polysaccharides investigation is based on their friendly behavior with environment and living organisms. The chemical structure, molar mass, chain conformation adopted in different media plays an important role for their performances in various practical applications. In this context, the viscometric behavior of two cationic pullulan and curdlan derivatives has been followed in water and salt solutions. The biopolyelectrolytes contain trimethylammonium propyl carbamate groups and trimethyl-N-(2-hydroxypropyl)ammonium ones attached to the backbone of pullulan and curdlan, respectively. The viscosity data have been interpreted in terms of the Wolf model. From the estimated values of the hydrodynamic parameters expressed by intrinsic viscosity [η], the hydrodynamic interaction parameter (b), etc., the dependence of the polyion conformation on the polyelectrolyte concentration and salt nature and concentration has been revealed; [η] decreased and b increased with increasing salt concentration and counterions valence. The viscosity data obtained in salt solutions allowed the estimation of the stiffness parameter (B), the expansion factors for the viscosity (αη) and for the root mean-square radius of gyration, αS, etc. and the establishment of counterions binding type as an “atmospheric ionic” one for the mononovalent counterions.

The suspending appearance of poly(acrylic acid)‐based rheology modifier in high‐content surfactant: The effect of polymer structure and molecular weight on the rheological properties of the complex systems

AbstractThe “all in one” multifunctional and concentrated daily chemical products are required by the market, such as the long‐term release of anti‐bacterial, medical, or perfume agents. These functions mainly rely on the microcapsules, which could encapsulate these functional and unstable molecules, and then release them in the long term. To stabilize these functional solid particles in a high‐content surfactant, it is necessary to add a suspending rheological modifier. The poly(acrylic acid) (PAA)‐based rheology modifier is one of the most common additives in the industry of daily chemicals, for thickening or suspending. In this paper, linear PAA, and branched PAA polymers (Carbomer 941 and Carbomer 940) were selected to study the suspending ability of PAA‐based rheology modifier in high‐content surfactant and to investigate the effect of PAA polymer structure and Mw on the complex solution's rheological properties. The yield and viscosity test of the complex solutions of linear PAA and high‐content surfactant revealed that the addition of linear PAA could lead to lower yield stress and viscosity, and failed to suspend the polystyrene (PS) model microspheres due to its smaller mean square radius of gyration. The branched PAA polymer, Carbomer 940 and Carbomer 941 were more stable than the linear PAA in these two kinds of surfactant. More specifically, Carbomer 940 with a higher Mw, would exhibit higher yield stress and viscosity than Carbomer 941 with a lower Mw in nonionic surfactant alcohol polyoxyethylene ether (AEO) solvent with content less than 5 wt%. When the content was higher than 5 wt%, Carbomer 941 presented higher yield stress and viscosity than Carbomer 940, and successfully suspended PS microspheres in the 20 wt% AEO solution. This could depend on the polymer chain's elongation state of these two PAA‐based branched rheology modifiers. The same trend happened in the appearance of Carbomer 940 and Carbomer 941 in alcohol ether sulphate (AES) solution. However, the electrostatic repulsive force between the AES and PAA‐based rheology modifier could further make the polymer chain curl up and restrain the suspending ability in a lower content surfactant solution. Therefore, this fundamental research could guide the application of the anionic polyelectrolytes in the concentrated daily chemical product for stable suspending particles.

Coarse-grained simulation to study the thermal conductivity of crystalline polyethylene

<p indent="0mm">As one of the most used polymer material, polyethylene (PE) has excellent mechanical and processing properties. However, the low thermal conductivity limits the application of polyethylene in field of electronic devices, which needs good thermal conductivity for heat management. However, recent studies report excellent thermal conductivity for PE fibres. It should be very interesting to check if it is possible to convert PE from low thermal conductivity materials to high ones. In this study, in order to make a deep understand on thermal conductivity of PE material, a coarse-grained model of crystalline PE was constructed, and molecular dynamics simulation methods were used to explore the effect of crystallization on the intrinsic thermal conductivity of PE. First, Boltzmann inversion method was applied to build the coarse-grained model of PE, which is suitable to mimic physical behaviours of PE material with varied crystalline degrees. The simulation data such as the bond distribution function, angular distribution function, and radial distribution function of the coarse-grained model are found consistent with those from all-atom model. Furthermore, various physical data calculated from coarse-grained model are compared with those from reported experimental studies, such as glass transition temperature, thermal expansion coefficient, mean square radius of gyration, diffusion coefficient and thermal conductivity. All above physical data of coarse-grained model fit well with those from literatures. It means our coarse-grained model is readily to mimic PE materials, especially in field of thermal properties. For example, the curve of thermal conductivities of a single PE chain varied along with chain length calculated from our coarse-grained model lies between results from previous theoretic predictions. Second, the coarse-grained PE model with various crystalline degree and same crystalline degree of varied crystalline particles sizes are systematically investigated. We paid much attention on effect of stretching on thermal conductivity of PE models, because stretching is an efficient way to alter alignment state of PE chains. The results show that thermal conductivity of the PE model increases along with the increasing of crystalline degree. During stretching, the PE models with higher crystalline degree are easily reconfigured to align chains along the stretching direction, which is the thermal transporting direction, that in turn causes an obviously increasing for thermal conductivity. At the same crystalline degree, PE model with larger crystalline particle size is easier to form a high thermal conductivity system under stretching comparing with those with small crystalline particle sizes. Order parameters represent alignment of PE chains are calculated to analysize this observation. To conclude, the coarse-grained PE model established in this paper can be used to study the thermal conductivity of complex crystalline systems. The physical properties calculated from coarse-grained PE model are found to fit well with experimental and theoretic data of PE materials. Our simulation explores the dependence of thermal conductivity on crystalline state of PE material, and provides a theoretical basis for designing and manufacturing the intrinsic high thermal conductivity polymer materials. The coarse-grained model is thought to be able to explore mechanical properties of crystalline PE materials in the future.

Dynamics of semiflexible generalized scale-free polymer networks

We study the relaxation dynamics of a new type of hyperbranched polymer networks, which are constructed using a degree distribution specific to scale-free networks. The topology of the constructed networks is controlled through three parameters: γ, which measures the density of links in the network, the minimum allowed degree Kmin and the maximum allowed degree Kmax. The static properties of the polymers are investigated through the mean square radius of gyration. We study the dynamics in the framework of the generalized Gaussian structures model by monitoring in detail the influence of networks’ construction parameters, namely γ, Kmin, and Kmax, and the stiffness parameter q on the relaxation quantities: averaged monomer displacement, storage modulus and loss modulus. In the intermediate (time or frequency) regions of the dynamical quantities we encounter a scaling behavior for γ=2.5 and usually a non-scaling behavior for other values of γ. Switching on the stiffness parameter q one is able to destroy, create or enlarge some scaling regions for other combinations of the parameters. Another way to widen a scaling region is to slightly increase Kmin.

A Molecular Dynamics Simulation Study of Amine-Carboxyl Ionic Interactions and Their Distribution in a Polysiloxanes Network

The ionic crosslinking mechanism of polysiloxanes networks based on amine/carboxyl interactions was investigated by molecular dynamics and tensile performance experiments. The “one-chain-crosslinking” mechanism between crosslinkers and polymers was investigated based on the corresponding simulations. The experimental results showed that the tensile strength and elongation at break of amine-functional polysiloxanes (AFPs)/carboxyl-functional polysiloxanes (CFPs) (1.54 MPa@264%, respectively) were higher than that of amine and carboxyl-functional polysiloxanes (ACFPs) (0.66 MPa@45%). The simulation results indicated that the number of intrachain and interchain hydrogen bonds played an important role for improving the mechanical performance of the polymer networks. In addition, it was found that the orientation and entanglement of the polymer chains had a direct correlation with hydrogen bonding in the equilibrium state and during the stretching process. The mean square radius of gyration (R g2), the mean square end-to-end distance (R ee2), the orientation parameter, the entanglement parameter, and the mean square displacement (MSD) of the chains in the equilibrium state were calculated. The molecular dynamic simulations were verified by comparing the simulated results with the tensile and rheological experimental results of the AFPs/CFPs and ACFPs elastomers.

A new type of mixed vegetable insulating oil with better kinematic viscosity and oxidation stability

• The molecular dynamics model of mixed vegetable insulating oil with different proportion was established. • The effects of different mixing ratios on the kinematic viscosity of mixed oil were studied by molecular dynamics. • The difference of oxidation stability between palm fatty acid ester and rapeseed oil was studied by First-Principle. • A mixing ratio with good kinematic viscosity and oxidation stability was obtained. Natural ester is a new type of environment-friendly energy, and the research on its performance is crucial in order to be widely popularized and applied in electrical equipment. The mixing of different natural esters can ameliorate the performance handicap of natural esters. Therefore, two widely applied natural esters, rapeseed oil and palm fatty acid ester (PFAE) were selected in this paper. The kinematic viscosity and oxidation stability of mixed oil were analyzed on the basis of the establishment of the mixture of two oils with different mixing proportion by simulation. Results indicate that the kinematic viscosity of the mixed oil decreased with the addition of PFAE. The kinematic viscosity of the mixed oil decreased by 51.3% when the mixing ratio was 80% rapeseed oil and 20% PFAE. The addition of short chain oil molecules increases the mean square displacement, mean-square radius of gyration and free volume of the mixed oil. The oil molecules in the mixed system have fewer tangles and reduced internal friction between them. DFT calculation shows that numerous electron traps in PFAE provide a channel for the transition of valence electrons. The methyl group in PFAE has strong positive electrostatic potential and strong electron affinity, which makes the oxidation stability of PFAE lower than that of rapeseed oil. Therefore, 80% rapeseed oil and 20% PFAE are the best mixing ratio in this study. This ratio has the best improvement on the kinematic viscosity of the mixed oil and the least influence on the oxidation stability.

Molecular simulation of an initial stage of the ordered-structure formation of linear and ring polymers upon cooling from the melts

Monte Carlo (MC) simulation of the coarse-grained ring and linear polyethylene (PE) models were employed to study the early stage of the ordered-structure formation upon cooling from the melts. The simulations suggest that the ordered structure with the single domain can be formed for both linear and ring PE chains which have the same mean square radius of gyration in the molten state. Molecular and structural properties evaluated from the simulation are the fraction of trans conformation and the sequence length of trans state, the anisotropic change of chain dimension, the overall orientation correlation function, intra- and inter-molecular bond orientation correlation functions and intermolecular pair correlation function. All these properties reveal an ordering process as the sign of crystallization especially for linear chains whereas it is more difficult for ring polymer to form an ordered phase. For ring chains, the ordered structures were formed with multiple folded conformations and smaller magnitude for the fraction of trans conformation, chain orientation, degree of chain packing, intra- and inter-molecular bond ordering. The simulations suggest that PE chains are needed to stretch to some extent before the chain alignment can proceed. Compared to the linear polymers with comparable molecular size, ring PE chains tend to form the less-ordered crystalline structure, have higher melting temperature and exhibit slower crystallization rates.

Conformation and structure of ring polymers in semidilute solutions: A molecular dynamics simulation study

Scaling is one of the most fundamental concepts in theoretical polymer physics. For linear polymers in semidilute solutions under a good solvent condition, there has been a well established scaling behavior of mean-square radius of gyration of chains with the concentration, i.e., R g 2 ~ ϕ β , with β = −0.25, which receives strong support from experiments, computer simulations and scaling arguments. However, a clear-cut conclusion about the scaling behavior of dimensions of nonconcatenated and unknotted ring polymers in the semidilute solution with the concentration has not been achieved. Values of the corresponding scaling exponent β are diversely reported in literature, ranging from −0.25 to −0.59 and in contradiction within experimental, simulation and theoretical studies. As an endeavour for clarifying this issue, we carried out extensive molecular dynamics (MD) simulations based on the Kremer-Grest model of ring polymers with chain lengths N up to 5120 in semidilute solutions, and compared our results about ring conformations with almost all of experimental, simulation and theoretical data available in literature. Our MD simulation results lead to a conclusion that the mean-square radius of gyration of ring polymers scales with the concentration in semidilute regime as R g 2 ∼ φ − 0.59 ± 0.01 . This conclusion is in a good agreement with a previous finding by a lattice Monte Carlo simulation. Surprisingly, by looking into the experimental and other simulation results reported previously with sufficient caution, we tend to conclude that in reality almost all of their data can be described reasonably well by the same scaling law, R g 2 ~ ϕ − 0.59 . Furthermore, the scaling exponent obtained (−0.59) can be successfully explained by a scaling argument based on concept of correlation length. Finally, effects of the concentration on shape properties and packing behavior of ring polymers in the semidilute solutions have also been illuminated within our MD simulations. • Scaling behavior of ring dimensions in semidilute solutions with the concentration has been revisited. • Comparison of our results with experimental, and other simulation and theoretical results has been conducted. • Effect of concentration on shape and packing behavior of ring polymers in semidilute solutions has been illuminated.

Size and Dynamics of a Tracer Ring Polymer Embedded in a Linear Polymer Chain Melt Matrix

Using dynamic Monte Carlo simulations based on the bond-fluctuation model, we systematically investigate the static and dynamic properties of a tracer ring polymer in a melt composed of linear polymers. Our results reveal that the mean-square radius of gyration of a cyclic polymer is independent of the topological constraints between the ring and linear chains. The scaling exponent (ν) of the radius of gyration Rg ∼ NRν, where NR is the ring chain length, increases from 0.5 to 0.6 as the length of the linear matrix chains, NL, decreases, which lies between the two values reported by Iyer et al. [ Macromolecules 2007, 40, 5995] and Lang et al. [ Macromolecules 2012, 45, 7642]. We find that both the structural relaxation time and self-diffusion coefficient are nearly independent of NL for a short ring chain (NR = 20) and the scaling exponents of NL for the self-diffusion coefficient are between −1 and −2 for large NR (NR ≥ 100), which cannot be understood by existing mechanisms, including the restraint reptation mechanism, the once-threaded mechanism, and the constraint release mechanism. Furthermore, we propose a “touch-threading” mechanism, which could be regarded as an important supplement to the above mechanisms, to describe the structural relaxation and translational diffusion of a tracer ring in a linear melt. In addition, when NL is large, the structural relaxation and translational diffusion of the tracer ring are decoupled, resulting in a breakdown of the extended Stokes–Einstein relation. These results provide fundamental insights into the properties of the ring-linear blends.

Ion transport in ionic liquid/poly(vinylidene fluoride) system under electric fields: A molecular dynamics simulation

Molecular dynamics simulations were applied to explore the ion transport in five ionic liquid/poly (vinylidene fluoride) (IL/PVDF) systems under different electric fields where the cation is 1-ethyl-3-methylimidazolium (EMI+) and the anions are Cl−, Br−, BF− 4, PF− 6 and trifluoromethanesulfonate (TfO−) separately, and each system contains about 40 wt% of IL. The structural properties were analyzed by the following calculations as electric fields increase. The pair correlation function reveals that the local spatial structure of ion pairs can be split into two types, which tend to disorder; the dihedral angle and mean square radius of gyration show that PVDF chains tend to straighten. The ion transport properties were studied by the ion-pair intermittent time correlation function and continuous time correlation function, the diffusion coefficient and mobility of anions and cations as electric fields increase. The simulation results illustrate that ion transport is intensified. The transference numbers of anions and cations show the relative contributions of anions to charge transport shows an upward trend as electric fields increase, and under an applied electric field, contribution of anions to charge transport is closely related to anions volume. In addition, the relationship between diffusion coefficient and the electric fields can be fitted into an exponential function, and the fitting parameters are strongly related to the local structure of the ion pairs, and the relationship between ideal conductivity and the electric fields has the same law. Specially, the anions mobility is determined by the anions volume under the stronger electric fields, and the smaller the anion volume, the greater the mobility.

Properties of Epoxy-POSS Composites at Low Temperature Based on Molecular Dynamics Simulation

The epoxy resin is an important part of superconducting devices, and nanoparticles can improve the properties of the epoxy resin at low temperature. However, the performance of epoxy/nanoparticles composites at low temperature is still unclear. In this work, pure epoxy and three epoxy-polyhedral oligomeric silsesquioxanes (POSS) composites models are proposed. The results of molecular dynamics (MD) simulation show that POSS nanoparticles can improve the mechanical properties (bulk modulus, Young's modulus and shear modulus) of the epoxy at low temperature. As for the mean square radius of gyration and the mean square displacement, different organic groups in POSS particles have different effects on the epoxy resin.

An Artificial Intelligence‐Based Motion Trajectory Prediction of Fibrous Matters

Understanding and predicting the motion behavior of fibrous matters is critical for studying the structure, performance, function, and assembly behavior of fibrous matters. However, due to the limitation of characterization technology on a space−time scale, the existing technologies cannot provide real‐time in situ tracking of entire dynamic processes of fibrous matters. To address this shortcoming, an artificial intelligence‐based codebase, named the artificial intelligence‐based fiber motion tracking (AI‐FMT), is proposed for analyzing, studying, and predicting motion behaviors of fibrous matters. The proposed AI‐FMT can automatically extract morphology information of fibrous matters from a large number of pictures or video streams, which serve as an input for machine learning to predict the motion pattern of fibrous matters. Using a finite‐element concept in understanding the point−coordinate relationship, the AI‐FMT greatly simplifies the architecture of the neural network during motion prediction. For the case of 928 trainable parameters, an average accuracy of 97.8% is achieved in predicting variations in morphological parameters, such as mean square radius of gyration, end‐to‐end distance, and curvature in the movement process of animal hair. The presented results can help to understand the structure, assembly, performance, and function of fibrous matters.

Statistical mechanics and thermodynamics for typical hyperbranched polymerization systems

<p indent="0mm">We summarize statistical thermodynamics of two typical polymerization systems to relate thermodynamic quantities to the properties of hyperbranched polymers (HBPs) such as average degree of polymerization (DP), radius of gyration, and branching density. An attempt is made to obtain the regulation mechanism of structure and properties of HBPs. Starting from the physical viewpoint, the isothermal compressibility has been associated with the weight average DP of polymers that is interpreted as the spatial correlation of monomers due to polymerization. The subchain is employed to derive the mean square radius of gyration and hydrodynamics radius, and a calculation is implemented in terms of the adjacent matrix of an HBP. Furthermore, the scaling behavior of the system is studied to give the generalized scaling law. The average properties of HBPs under the semi-batch mode are also presented by means of the generation function method. In addition, a Monte Carlo simulation method based on the growth generation is proposed, by which the spatial structure of an HBP can be easily analyzed in detail. As an application, the effect of cyclization, nonequal reactivity, and solvent quality on the polymerization is also carried out.

Rheological mechanism of polymer nanocomposites filled with spherical nanoparticles: Insight from molecular dynamics simulation

It is very crucial to understand the rheological behavior of polymer nanocomposites (PNCs) on the molecular level, which is very important for their processing and application. Thus, in this work the reverse nonequilibrium molecular dynamics simulation is employed to explore the rheological property of PNCs by tuning the volume fraction of nanoparticle (NP), the polymer-NP interaction and the NP size. The shear viscosity (η~γ˙-m) exhibits a power law with the shear rate where m varies from 0.42 to 0.53 at the high shear rates. By adopting the Carreau-Yasuda model, the calculated zero-shear viscosity gradually rises with increasing the volume fraction of NP, the polymer-NP interaction or reducing the NP size. This is attributed to the strong adsorption of chains by NPs and the formed network, which leads to the retarded dynamics. In addition, both the first and second normal stress differences also show the power laws on the shear rate. The chains are gradually extended as the increase of the shear rate, which is characterized by the mean-square end-to-end distance and the mean square radius of gyration. Especially, the evolution process of the NP network and the polymer-NP network is analyzed to deeply understand the shear thinning behavior. The number of the direct contact structure of NPs increases while the number of the polymer-NP bridged structure is reduced. This is further proved by the increase of the formation probability of the NP network and the decrease of the polymer-NP interaction energy. Finally, the chain dynamics is found to be enhanced due to the shear flow. In summary, this work provides a further understanding on the mechanism of the shear thinning of PNCs on the molecular level.

Revealing the architecture and solution properties of polysaccharide fractions from Macrolepiota albuminosa (Berk.) Pegler

Macrolepiota albuminosa (Berk.) Pegler is abundant in active polysaccharides, but little is known about their structures and solution properties. In this study, water-extracted polysaccharides from M. albuminosa (MAWP) were purified into three fractions with structural heterogeneity, which was attributed to the diversity in molecular weight, monosaccharide composition and linkage patterns, further affecting their solution properties. Methylation and NMR analysis revealed MAWP-60p and MAWP-70 were a 3-O-methylated glucomannogalactan and a previously unreported glucomannogalactan, whereas MAWP-80 was elucidated as a branched galactoglucan. Besides, three fractions exhibited random coil conformation in aqueous solution, while MAWP-60p had the highest viscosity due to its highest molecular weight, mean square radius of gyration (Rg) and O-methyl group attached to the backbone. The molecular weight, monosaccharide composition and glycosidic linkages might be the major contributors to the flexibility, molecular size and stereochemistry of mushroom polysaccharide chains.

Conformations of ring polymers with excluded volume interactions

The influence of distance dependent excluded volume interactions (EVIs) on the conformational properties of ring polymers is investigated within the principles of Rouse–Zimm theory. This study characterizes the structural features, i.e., the mean square radius of gyration, static structure factor, scattering intensity, fractal dimensions, and the mean square intermonomer distances of ring polymers in dilute solutions. The excluded volume interactions between the nonbonded monomers are modeled by a harmonic potential, where the strength of these interactions is evaluated from Flory’s mean-field approach. The mean square radius of gyration of the rings with EVI follows a similar scaling relation as its linear analog with the degree of polymerization. Unlike linear polymers, the maximum in the Kratky plot of the static structure factor is observed as a result of an increased segment density due to the circularity constraint. EVI marks a structural transition from relatively compact conformation for the smaller rings to expanded conformations for larger ones. The fractal dimension of the rings with EVI lies between those of the random walk and the self-avoiding walk models of linear polymers in three dimensions. This theoretical model produces a relatively pronounced effect of EVI for larger rings. An understanding of the influence of EVI on conformations of the ring polymers at a single molecule level may be applied to various naturally existing cyclic biomolecules such as plasmids, cyclic proteins, and polysaccharides.