Polymer reversal rate calculated via locally scaled diffusion map

A recent study on the dynamics of polymer reversal inside a nanopore by Huang and Makarov [J. Chem. Phys. 128, 114903 (2008)] demonstrated that the reaction rate cannot be reproduced by projecting the dynamics onto a single empirical reaction coordinate, a result suggesting the dynamics of this system cannot be correctly described by using a single collective coordinate. To further investigate this possibility we have applied our recently developed multiscale framework, locally scaled diffusion map (LSDMap), to obtain collective reaction coordinates for this system. Using a single diffusion coordinate, we obtain a reversal rate via Kramers expression that is in good agreement with the exact rate obtained from the simulations. Our mathematically rigorous approach accounts for the local heterogeneity of molecular configuration space in constructing a diffusion map, from which collective coordinates emerge. We believe this approach can be applied in general to characterize complex macromolecular dynamics by providing an accurate definition of the collective coordinates associated with processes at different time scales.     

Active control of strong plasmon–exciton coupling in biomimetic pigment–polymer antenna complexes grown by surface-initiated polymerisation from gold nanostructures

Plexcitonic antenna complexes, inspired by photosynthetic light-harvesting complexes, are formed by attachment of chlorophylls (Chl) to poly(cysteine methacrylate) (PCysMA) scaffolds grown by atom-transfer radical polymerisation from gold nanostructure arrays. In these pigment–polymer antenna complexes, localised surface plasmon resonances on gold nanostructures are strongly coupled to Chl excitons, yielding hybrid light–matter states (plexcitons) that are manifested in splitting of the plasmon band. Modelling of the extinction spectra of these systems using a simple coupled oscillator model indicates that their coupling energies are up to twice as large as those measured for LHCs from plants and bacteria. Coupling energies are correlated with the exciton density in the grafted polymer layer, consistent with the collective nature of strong plasmon–exciton coupling. Steric hindrance in fully-dense PCysMA brushes limits binding of bulky chlorophylls, but the chlorophyll concentration can be increased to ∼2 M, exceeding that in biological light-harvesting complexes, by controlling the grafting density and polymerisation time. Moreover, synthetic plexcitonic antenna complexes display pH- and temperature-responsiveness, facilitating active control of plasmon–exciton coupling. Because of the wide range of compatible polymer chemistries and the mild reaction conditions, plexcitonic antenna complexes may offer a versatile route to programmable molecular photonic materials.

Excitons in pigment–polymer antenna complexes formed by attachment of chlorophyll to surface grafted polymers are coupled strongly to plasmon modes, with coupling energies twice those for biological light-harvesting complexes and active control of plasmon–exciton coupling.     

A molecular model for cohesive slip at polymer melt/solid interfaces

A molecular model is proposed which predicts wall slip by disentanglement of polymer chains adsorbed on a wall from those in the polymer bulk. The dynamics of the near-wall boundary layer is found to be governed by a nonlinear equation of motion, which accounts for such mechanisms on surface chains as convection, retraction, constraint release, and thermal fluctuations. This equation is valid over a wide range of grafting regimes, including those in which interactions between neighboring adsorbed molecules become essential. It is not closed since the dynamics of adsorbed chains is shown to be coupled to that of polymer chains in the bulk via constraint release. The constitutive equations for the layer and bulk, together with continuity of stress and velocity, are found to form a closed system of equations which governs the dynamics of the whole "bulk+boundary layer" ensemble. Its solution provides a stick-slip law in terms of the molecular parameters and extruder geometry. The model is quantitative and contains only those parameters that can be measured directly, or extracted from independent rheological measurements. The model predictions show a good agreement with available experimental data.     

Macromolecular dynamics of conjugated polymer in donor-acceptor blends with charge transfer complex

Donor-acceptor blends based on conjugated polymers are the heart of state-of-the-art polymer solar cells, and the control of the blend morphology is crucial for their efficiency. As the film morphology can inherit the polymer conformational state from solution, the approaches for probing and controlling the polymer conformational state in the blends are of high importance. In this study, we show that the macromolecular dynamics in solutions of the archetypical conjugated polymer, MEH-PPV, is essentially changed upon addition of an acceptor 2,4,7-trinitrofluorenone (TNF) by using dynamic light scattering (DLS). We have observed four new types of the macromolecular dynamics absent in the parent polymer determined by the polymer and acceptor content. The MEH-PPV?:?TNF ground-state charge-transfer complex (CTC) is suggested to result in these dynamics. In the dilute polymer solution, the CTC formation leads to slower dynamics as compared with the pristine polymer. This is evidence of aggregates formed by intercoil links that are the CTCs involving two conjugated segments of different coils with acceptor molecules being sandwiched between them. At low acceptor content, the aggregates are not stable but at high acceptor content, they are. In the semidilute solution at low acceptor content, the dynamics becomes faster as compared with the pristine polymer that is explained by confinement of the coupled motions of entangled polymer chains. At high acceptor content, the dynamics is far much slower with a characteristic long-range correlation at the scale 3-5 μm that is explained by aggregation of polymer chains in clusters. One can expect that the DLS technique could become a useful tool to study the nano- and microstructure of donor-acceptor conjugated polymer blends to achieve controllable morphology in the corresponding blend films.     

Polymer adsorption onto a micro-sphere from optical tweezers electrophoresis

We explore the design and operation of an optical-tweezers electrophoresis apparatus to resolve polymer adsorption dynamics onto a single micro-sphere in a micro-fluidic environment. Our model system represents a broader class of micro-fluidic electrophoresis experiments for biosensing and fundamental colloid and surface science diagnostics. We track the adsorption of 100 kDa poly(ethylene oxide) homopolymer onto a colloidal silica sphere that is optically trapped in a crossed parallel-plate micro-channel. The adsorption dynamics are probed on the ～1 μm particle length scale with ～1 s temporal resolution. Because the particle electrophoretic mobility and channel electro-osmotic flow are exquisitely sensitive to the polymer layer hydrodynamic thickness, particle dynamics can be complicated by polymer adsorption onto the micro-channel walls. Nevertheless, using experiments and a theoretical model of electro-osmotic flow in channels with non-uniform wall ζ-potentials, we show that such influences can be mitigated by adopting a symmetrical flow configuration. The equilibrium hydrodynamic layer thickness of 100 kDa poly(ethylene oxide) on colloidal silica is ～10 nm at polymer concentrations ?10 ppm (weight percent), with the dynamics reflecting polymer solution concentration, flow rate, and polydispersity.     

Theory of glassy dynamics in conformationally anisotropic polymer systems

A mode coupling theory for the ideal glass transition temperature, or crossover temperature to highly activated dynamics in the deeply supercooled regime, T(c), has been developed for anisotropic polymer liquids. A generalization of a simplified mode coupling approach at the coarse-grained segment level is employed which utilizes structural and thermodynamic information from the anisotropic polymer reference interaction site model theory. Conformational alignment or/and coil deformation modifies equilibrium properties and constraining interchain forces thereby inducing anisotropic segmental dynamics. For liquid-crystalline polymers a small suppression of T(c) with increasing nematic or discotic orientational order is predicted. The underlying mechanism is reduction of the degree of coil interpenetration and intermolecular repulsive contacts due to segmental alignment. For rubber networks chain deformation results in an enhanced bulk modulus and a modest elevation of T(c) is predicted. The theory can also be qualitatively applied to systems that undergo nonuniversal local deformation and alignment, such as polymer thin films and grafted brush layers, and large elevations or depressions of T(c) are possible. Extension to treat directionally dependent collective barrier formation and activated hopping is possible.     

Computer-aided simulation of the influence of collective effects on polymer-melt dynamics in a straight cylindrical tube: Observation of the onset stage of the corset effect

The results of the computer-aided simulation of the dynamics of a polymer melt consisting of Fraenkel chains in straight cylindrical tubes and in bulk are discussed. Two different models are studied. In the first model, the dynamics of the polymer melt is simulated via the molecular dynamics simulation. The interaction of unbound polymer segments is described by the Lennard-Jones potential, which excludes any chain crossing of macromolecules and generates collective acoustic waves. In the second model, which serves as a reference, the system is studied via the Brownian dynamics method, in which intermolecular interactions are allowed for phenomenologically via friction and stochastic Langevin forces. In this case, cooperative effects are absent and the effect of spatial confinements makes itself evident only in a narrow near-wall layer. For the two models under consideration, there is a significant difference in the decay of dynamic correlation functions C _????(t) = ??b _??(t)b _??(t)b _??(0)b _??(0)????b _?? ² b _?? ² ??^?1, where averaging is performed over all macromolecular segments and b _?? (t) is the component of the end-to-end-segment vector (?? ?? ?? = x,y, and the cylindrical axis of the tube is directed along the z axis). For the first model allowing for collective effects, the dynamics of decay of C _???? (t)functions is much slower than that for the melt in bulk, and for the second model, in which the presence of the tube leads only to spatial confinements for the polymer segments in the direct vicinity of walls. This difference indicates the fundamental significance of the collective effects in the dynamics of polymer melts confined in porous media. This phenomenon is the first computer-simulated evidence of the onset stage of the so-called corset effect, which was first observed experimentally with the use of NMR relaxometry.     

One-dimensional models of polymer dynamics. II. A dashpot-spring model

The assumption of Clark and Zimm that coupled dashpots and springs can be used to model the dynamics of polymer molecules is here applied to a model different from that of Clark and Zimm. The precise differences are given in the preceding paper. The dielectric relaxation spectrum of the model is computed in time and frequency domains. The relaxation spectrum can be fitted reasonably well by the empirical Williams–Watts and Havriliak–Negami functions. The best-fit Williams–Watts and Havriliak–Negami parameters are given as functions of the parameters of the model. The model is compared with several related models found in the literature and possible interpretations are given.     

Polymer chain diffusion in polymer blends: A theoretical interpretation based on a memory function formalism

The diffusion of polymer chains in miscible polymer blends with large dynamic asymmetry—those where the two blend components display very different segmental mobility—is not well understood yet. In the extreme case of the blend system of poly(ethylene oxide) (PEO) and poly(methyl methacrylate)(PMMA), the diffusion coefficient of PEO chains in the blend can change by more than five orders of magnitude while the segmental time scale hardly changes with respect to that of pure PEO. This behavior is not observed in blend systems with small or moderate dynamic asymmetry as, for instance, polyisoprene/poly(vinyl ethylene) blends. These two very different behaviors can be understood and quantitatively explained in a unified way in the framework of a memory function formalism, which takes into account the effect of the collective dynamics on the chain dynamics of a tagged chain. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1239–1245     

Mesoscale simulation of polymer reaction equilibrium: combining dissipative particle dynamics with reaction ensemble Monte Carlo. I. Polydispersed polymer systems

We present a mesoscale simulation technique, called the reaction ensemble dissipative particle dynamics (RxDPD) method, for studying reaction equilibrium of polymer systems. The RxDPD method combines elements of dissipative particle dynamics (DPD) and reaction ensemble Monte Carlo (RxMC), allowing for the determination of both static and dynamical properties of a polymer system. The RxDPD method is demonstrated by considering several simple polydispersed homopolymer systems. RxDPD can be used to predict the polydispersity due to various effects, including solvents, additives, temperature, pressure, shear, and confinement. Extensions of the method to other polymer systems are straightforward, including grafted, cross-linked polymers, and block copolymers. To simulate polydispersity, the system contains full polymer chains and a single fractional polymer chain, i.e., a polymer chain with a single fractional DPD particle. The fractional particle is coupled to the system via a coupling parameter that varies between zero (no interaction between the fractional particle and the other particles in the system) and one (full interaction between the fractional particle and the other particles in the system). The time evolution of the system is governed by the DPD equations of motion, accompanied by changes in the coupling parameter. The coupling-parameter changes are either accepted with a probability derived from the grand canonical partition function or governed by an equation of motion derived from the extended Lagrangian. The coupling-parameter changes mimic forward and reverse reaction steps, as in RxMC simulations.     

Effects of disordered interchain interactions on polaron dynamics in semiconducting polymers

Polaron dynamics in a system of two randomly coupled polymer chains is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of the Su-Schrieffer-Heeger model modified to include disordered interchain interactions and an external electric field. By analysing the polaron velocity statistically, we find that the polaron motion is determined by the competition between the electric field and the disordered interchain interactions. Polaron dynamics are classified into two types, weak-coupling dynamics and strong-coupling dynamics. It is found that the strength of interchain interactions is the dominant factor controlling charge propagation in weak-coupling dynamics, whereas the effects of disorder are dominant in strong-coupling dynamics. The charge carriers tend to have higher mobility for stronger interchain coupling, and interchain coupling disorder can be favorable for charge transport depending on the coupling strength and the electric field.     

Excluded volume effect on confined polymer translocation through a short nanochannel

We simulated the translocation process of a polymer chain from a source container to a drain container through a short nanochannel. We utilized the bond fluctuation model coupled with Monte Carlo dynamics in our simulations. The calculation results show that the excluded volume effect significantly affects the polymer's translocation time tau. This time depends nonmonotonically on the polymer length N. For a fixed nanochannel length, tau decreases when the polymer length increases. tau, however, increases when the polymer length exceeds a certain threshold. This observation differs from those predicated for a Gaussian chain. In this paper, we will further present our findings to explain this phenomenon. The knowledge we gain from this research can enhance the understanding of complex transport processes in many biological systems.     

Aging in a Laponite colloidal suspension: a Brownian dynamics simulation study

The authors report Brownian dynamics simulation of the out-of-equilibrium dynamics (aging) in a colloidal suspension composed of rigid charged disks, one possible model for Laponite, a synthetic clay deeply investigated in the last few years by means of various experimental techniques. At variance with previous numerical investigations, mainly focusing on static structure and equilibrium dynamics, the authors explore the out-of-equilibrium aging dynamics. They analyze the wave vector and waiting time dependence of the dynamics, focusing on the single-particle and collective density fluctuations (intermediate scattering functions), the mean-squared displacement, and the rotational dynamics. Their findings confirm the complexity of the out-of-equilibrium dynamical behavior of this class of colloidal suspensions and suggest that an arrested disordered state driven by a repulsive Yukawa potential, i.e., a Wigner glass, can be observed in this model.     

Structure and dynamics of polymer-grafted clay suspensions

The structure and dynamics of polymer-grafted two-dimensional silicate layers in solution were investigated. The geometry of the individual silicate layers was examined by looking at both polarized and depolarized light scattering from dilute solutions, while higher-concentration systems were used to study the interaction and dynamics of polymer-grafted silicate layers in suspension. The form factor for an oblate ellipsoid was used to fit the polarized intensity profile, and values of a approximately 80 nm and b approximately 380 nm for the semi-axes were obtained. The 80 nm value compares reasonably with the dimensions of the polymer brushes grafted on the surface of the silicate layers. The modulus of the grafted silicate in solution, as determined by Brillouin scattering, is of the order of 10 GPa. The cooperative diffusion mechanism, typical of interacting polymer chains, is suppressed due to the high polymer osmotic pressure. The osmotic pressure is also responsible for the weak interpenetration of the densely grafted polymer chains on the surface of the silicate layers. The scattering data indicates that the polymer-grafted nanoparticles move via collective diffusion and experience significant decrease in mobility above their overlap concentration.     

Quantum mechanical correlation functions, maximum entropy analytic continuation, and ring polymer molecular dynamics

The maximum entropy analytic continuation (MEAC) and ring polymer molecular dynamics (RPMD) methods provide complementary approaches to the calculation of real time quantum correlation functions. RPMD becomes exact in the high temperature limit, where the thermal time betavariant Planck's over 2pi tends to zero and the ring polymer collapses to a single classical bead. MEAC becomes most reliable at low temperatures, where betavariant Planck's over 2pi exceeds the correlation time of interest and the numerical imaginary time correlation function contains essentially all of the information that is needed to recover the real time dynamics. We show here that this situation can be exploited by combining the two methods to give an improved approximation that is better than either of its parts. In particular, the MEAC method provides an ideal way to impose exact moment (or sum rule) constraints on a prior RPMD spectrum. The resulting scheme is shown to provide a practical solution to the "nonlinear operator problem" of RPMD, and to give good agreement with recent exact results for the short-time velocity autocorrelation function of liquid parahydrogen. Moreover these improvements are obtained with little extra effort, because the imaginary time correlation function that is used in the MEAC procedure can be computed at the same time as the RPMD approximation to the real time correlation function. However, there are still some problems involving long-time dynamics for which the RPMD+MEAC combination is inadequate, as we illustrate with an example application to the collective density fluctuations in liquid orthodeuterium.     

Segmented polymers — A new class of doped non-conjugated polymers with enhanced electrical conductivity

A new class of segmented non-conjugated dopable polymers, built up from short conjugated blocks connected with flexible chains (spacers), has been proposed. After the redox reaction of doping these polymers exhibit properties similar to those of the fully conjugated polymers and increase considerably their electrical conductivity. A solid state polymer effect has been observed. It has been found that the conjugated building units (biphenyl, diphenyl ether and 1,3,4-oxadiazole) do not interact with the dopant when included in a low molecular weight substance. When these units are incorporated in a polymer chain they change their reactivity and the polymer can be doped. The doping process takes place only when the polymer is in the solid state and the nature of this phase is of considerable importance. The phenomenon observed, i.e. doping of non-conjugated polymers with segmented structure could be explained with a favourable arrangement of the conjugated blocks in the solid phase, leading to enhanced π-π - interaction (equivalent to extended conjugation). By the collective interaction of several conjugated blocks with the dopant the polymer is partially oxidized and charge carriers are formed. The result is enhanced electrical conductivity.     

高分子相分离的分子动力学模拟

用粗粒化分子动力学(MD)模拟方法从分子层次研究两组分聚合物共混体系相分离过程中的动力学. 在相分离初期, 相区尺寸不随时间增加而变化; 在相分离中期, 相区尺寸与时间有很好的标度关系, 标度指数(α=1/3)符合Lifshiz-Slyozov提出的以扩散为主导的蒸发-凝聚机理的标度预测; 在相分离后期, 体系实现宏观相分离, 相区尺寸不再随时间改变而变化. 体积分数小的高分子链尺寸在相分离过程中先收缩再扩张, 在实现宏观相分离后, 高分子链尺寸又回到本体状态尺寸.     

Modeling a tethered polymer in Poiseuille flow

We investigate the behavior of a tethered polymer in Poiseuille flow using a multiscale algorithm. The polymer, treated using molecular dynamics, is coupled to a solvent modeled by the stochastic rotation algorithm, a particle-based Navier-Stokes integrator. The expected series of morphological transitions of the polymer: sphere to distorted sphere to trumpet to stem and flower to rod are recovered, and we discuss how the polymer extension depends on the flow velocity. Backflow effects cause an effective increase in viscosity, which appears to be primarily due to the fluctuations of the free end of the polymer.     

Dissipative Particle Dynamics Simulations of Polymer Brushes: Comparison with Molecular Dynamics Simulations

Summary: The structure of polymer brushes is investigated by dissipative particle dynamics (DPD) simulations that include explicit solvent particles. With an appropriate choice of the DPD interaction parameters , we obtain good agreement with previous molecular dynamics (MD) results where the good solvent behavior has been modeled by an effective Lennard–Jones potential. The present results confirm that DPD simulation techniques can be applied for large length scale simulations of polymer brushes. A relation between the different length scales and is established.

Polymer brush at a solid–liquid interface.