Semiflexible Polymer Melts Research Articles

Validation and Refinement of Unified Analytic Model for Flexible and Semiflexible Polymer Melt Entanglement.

We combine molecular dynamics simulations and topological analyses (TA) to validate and refine a recently proposed unified analytic model [Hoy, R. S.; Kröger, M. Phys. Rev. Lett. 2020, 124, 147801] for the reduced entanglement length, tube diameter, and plateau modulus of polymer melts. While the functional forms of the previously published expressions are insensitive to the choice of the TA method and N e -estimator, obtaining better statistics and eliminating all known sources of systematic error in the N e -estimation alters their numerical coefficients. Our revised expressions quantitatively match bead-spring simulation data over the entire range of chain stiffnesses for which systems remain isotropic, semiquantitatively match all available experimental data for flexible, semiflexible, and stiff polymer melts (including new data for conjugated polymers that lie in a previously unpopulated stiffness regime), and outperform previously developed unified scaling theories.

Influence of chain stiffness on semiflexible polymer melts in two dimensions via molecular dynamics simulation

ABSTRACT Two-dimensional (2D) semiflexible polymers exhibit distinctive conformational and dynamical behaviour compared to the corresponding bulk polymers. Understanding the microscopic origins of this characteristic behaviour of 2D semiflexible polymers is critical for the practical applications of nanoscale polymer systems such as monolayer polymer films, polymer nanocomposites, and biological membranes. To this end, we used coarse-grained molecular dynamics simulation to investigate the behaviour of two-dimensional (2D) semiflexible polymers in a melt by varying the chain stiffness and chain length. Interestingly, the conformational and dynamical behaviour of semiflexible polymer chains was observed to depend on the chain stiffness.

Correction to Simulation Study of Entanglement in Semiflexible Polymer Melts and Solutions

Correction to Simulation Study of Entanglement in Semiflexible Polymer Melts and Solutions

Simulation Study of Entanglement in Semiflexible Polymer Melts and Solutions

Multiple scaling arguments have been proposed to describe how the entanglement molecular weight depends on polymer architecture and concentration. The Lin–Noolandi (LN) scaling argument, well suppo...

Structural Mechanism for Viscosity of Semiflexible Polymer Melts in Shear Flow.

The viscosities of semiflexible polymers with different chain stiffnesses in shear flow are studied via nonequilibrium molecular dynamics techniques. The simulation reproduces the experimentally observed results, giving a complete picture of viscosity as chain stiffness increases. Analysis of flow-induced changes in chain conformation and local structure indicates two distinct mechanisms behind a variety of viscosity curves. For polymers of small stiffnesses, it is related to flow-induced changes in chain conformation and, for those of large stiffnesses, to flow-induced instabilities of nematic structures. The four-region flow curve is confirmed for polymers of contour length close to persistence length and understood by combining the two structural mechanisms. Thus, these findings clarify the microscopic structures indicated by the macroscopic viscosity.

Isostaticity and the solidification of semiflexible polymer melts.

Using molecular dynamics simulations of a tangent-soft-sphere bead-spring polymer model, we examine the degree to which semiflexible polymer melts solidify at isostaticity. Flexible and stiff chains crystallize when they are isostatic as defined by appropriate degree-of-freedom-counting arguments. Semiflexible chains also solidify when isostatic if a generalized isostaticity criterion that accounts for the slow freezing out of configurational freedom as chain stiffness increases is employed. The configurational freedom associated with bond angles (θ) can be associated with the characteristic ratio C∞ = (1 + 〈cos(θ)〉)/(1 - 〈cos(θ)〉). We find that the dependence of the average coordination number at solidification [Z(Ts)] on chains' characteristic ratio C∞ has the same functional form [Z ≃ a - b ln(C∞)] as the dependence of the average coordination number at jamming [Z(ϕJ)] on C∞ in athermal systems, suggesting that jamming-related phenomena play a significant role in thermal polymer solidification.

Entropic Interactions in Semiflexible Polymer Nanocomposite Melts

By employing molecular dynamics simulations, we explored the effective depletion zone for nanoparticles (NP) immersed in semiflexible polymer melts and calculated the entropic depletion interactions between a pair of NPs in semiflexible polymer nanocomposite melts. The average depletion zone volumes rely mainly on polymer chain stiffness and increase with chain stiffness increasing. In the semiflexible polymer nanocomposite melts, the entropic depletion interactions are attractive and anisotropic, and increase with chain stiffness increasing. Meanwhile, the attractive interactions between NPs and polymers can also affect strongly the entropic depletion interactions. For the semiflexible polymer nanocomposite melts in the athermal system, the entropic depletion interactions change from anisotropic to isotropic when the NP/polymer interactions increase. For NPs in the rodlike polymer melts, a mixture structure of contact/"bridging" aggregations for NPs is formed at a strong attractive NP/polymer interaction. Our calculations can provide an effective framework to predict the morphology of NPs immersed in semiflexible polymer melts.

Untangling the Effects of Chain Rigidity on the Structure and Dynamics of Strongly Adsorbed Polymer Melts

We present a detailed analysis of coarse-grained molecular dynamics simulations of semiflexible polymer melts in contact with a strongly adsorbing substrate. We have characterized the segments in the interfacial layer by counting the number of trains, loops, tails, and unadsorbed segments. For more rigid chains, a tail and an adsorbed segment (a train) dominate while loops are more prevalent in more flexible chains. The tails exhibit a nonuniformly stretched conformation akin to the “polydisperse pseudobrush” originally envisioned by Guiselin. To probe the dynamics of the segments, we computed the layer z-resolved collective intermediate dynamic structure factor, S(q,t,z), mean-square displacement of segments, and the second Legendre polynomial of the time autocorrelation of unit bond vectors, ⟨P2[n⃗i(t,z)·n⃗i(0,z)]⟩. Our results show that segmental dynamics is slower for stiffer chains, and there is a strong correlation between the structure and dynamics in the interfacial layer. There is no “glassy layer”, and the slowing down in dynamics of stiffer chains in the adsorbed region can be attributed to the densification and a more persistent layering of segments.

Entropy theory of polymer glass formation revisited. I. General formulation

A generalized entropy theory of glass formation is developed by merging the lattice cluster theory for the thermodynamics of semiflexible polymer melts at constant pressure with the Adam-Gibbs relation between the structural relaxation time and the configurational entropy. Since experimental studies have suggested that the relative rigidity of the chain backbone and the side groups is an essential parameter governing the nature of glass formation in polymers, we incorporate this rigidity disparity parameter, along with monomer structure, into our new theoretical description of the polymer fluid thermodynamics. Our entropy theory is compared with alternative theories that describe the rate of structural relaxation in glass-forming liquids in terms of an activated rate process.

Isotropic to nematic transition in semiflexible polymer melts

Isotropic to nematic transition in semiflexible polymer melts

Isotropic to nematic transition in semiflexible polymer melts

Monte Carlo simulations are reported for the isotropic to nematic phase transition in semiflexible polymer melts. The molecules are modelled as tangent hard sphere chains with a bending energy that can be used to tune the stiffness of the chains. In the rod limit an isotropic to nematic transition occurs at high pressures. For freely-jointed chains no nematic phase is found, but a fairly sharp transition to a nematic phase is observed when the stiffness is increased at fixed density for sufficiently high densities. In semiflexible chains the isotropic to nematic transition is accompanied by a discontinuous change in the chain dimensions. The simulations show that existing theories are not quantitatively accurate for the isotropic to nematic transition in semiflexible chains.

Effect of chain stiffness on the conformational properties, pair correlations, and equation of state of polymer melts

The effect of molecular stiffness on the local structure and thermodynamic properties of semiflexible polymer melts is investigated via Monte Carlo simulation. Contrary to what might be expected from simple packing arguments, increasing the stiffness tends to decrease the tendency for segments on different chains to layer against each other, although the value of the pair correlation function at contact does increase. The pressure is relatively insensitive to the stiffness of the molecules because an increase in the two-body contribution to the pressure is compensated for by a decrease in the three-body contribution. Comparison of self-consistent polymer reference interaction site model predictions to the simulations shows that the accuracy of the theory for the pair correlation function diminishes somewhat as the chains become stiffer.

Local structure of semiflexible polymer melts

Polymer RISM (reference interaction site model) theory is used to examine the intermolecular radial distribution function and isothermal compressibility of semiflexible polymer melts. Parametric studies as a function of chain length and chain stiffness reveal that as the characteristic stiffness is increased beyond three bond lengths, the radial distribution function and isothermal compressibility saturate, becoming relatively insensitive to chain length or stiffness