Motion Of Polymer Research Articles

Neutron Spin Echo for the Exploration of Large Scale Macromolecular Dynamics

In soft materials the structures, the macroscopic mechanical and rheological properties and the phase changes are determined to high degree but thermal motion of the atoms and molecules. Most of the relevant dynamics takes place on mesoscopic lengths and time scales in between the picosecond atomic scale and the macroscopic frame. Offering the proper space time observation window, neutron spin echo (NSE) spectroscopy uniquely addresses these motions. In this short review we briefly present some key experimental results on the mesoscopic dynamics of polymer systems. We address the standard model of polymer motion, the Rouse model, the role of topological confinement as expressed in the reptation model and finally, processes limiting the confinement–we discuss contour length fluctuations of entangled chains. Very recently it became also possible to directly identify large scale internal dynamics of proteins by neutron spin echo. We report the results of these pioneering studies, which most likely will initi...

Effect of orientation in translocation of polymers through nanopores

The motion of polymers with inhomogeneous structure through nanopores is discussed theoretically. Specifically, we consider the translocation dynamics of polymers consisting of double-stranded and single-stranded blocks. Since only the single-stranded chain can go through the nanopore the double-stranded segment has to unzip before the translocation. Utilizing a simple analytical model, translocation times are calculated explicitly for different polymer orientations, i.e., when the single-stranded block enters the pore first and when the double-stranded segment is a leading one. The dependence of the translocation dynamics on external fields, energy of interaction in the double-stranded segment, size of the polymer, and the fraction of double-stranded monomers is analyzed. It is found that the order of entrance into the pore has a significant effect on the translocation dynamics. The theoretical results are discussed using free-energy landscape arguments.

Primitive chain network model for block copolymers

A coarse-grained molecular simulation for block copolymers in the entangled state is proposed as an extension of the primitive chain network model. Polymers are represented as a sequence of segments between consecutive entanglements, the latter being modeled as sliplinks with other chains. Each sliplink connects two chains only, i.e., entanglements are taken as ‘binary’. The resulting 3D structure is a network of primitive chains, which makes our model different from other sliplink single-chain models, where the link to other chains is ‘virtual’. The 3D nature of our simulation makes it similar to, though considerably more coarse grained than, conventional molecular dynamics simulations. Because of the 3D space assignment of the polymers, monomeric density fields can be defined, and interactions due to different chemistry of the monomers can be accounted for, similarly to density field calculations. Polymer motion is described both by the 3D motion of sliplinks, and by the 1D transport of monomers along the primitive chain, while network topological rearrangement occurs due to chain-end hooking and unhooking processes. Each kinetic equation accounts for elastic forces along the chains, field forces arising from density gradients, and thermal random forces. In this paper, the primitive chain network model was modified (i) in the procedure of network rearrangement to account for the different chemistries in the copolymer, and (ii) in some details of the kinetic equations whenever the boundary between blocks is involved. We report results for diblock copolymers where for simplicity all relevant properties of the two monomers are the same, except for the interactions. Simulations reasonably reproduce the micro-phase formation process and the phase diagram for well entangled copolymers with a low calculation cost.

Crossover from reptation to Rouse dynamics in a one-dimensional model

A simple one-dimensional model is constructed for polymer motion. It exhibits the crossover from reptation to Rouse dynamics through gradually allowing hernia creation and annihilation. The model is treated by the density matrix technique which permits an accurate finite-size-scaling analysis of the behavior of long polymers.

Conformational Entropy Mechanism for Periodic Motion of DNA under Constant-Field Gel Electrophoresis

Entropic elasticity of a single charged polymer undergoing gel electrophoresis is a fundamental theme of polymer statistical physics since the discovery of “periodic” behavior in constant field gel electrophoresis (CFGE). In the present work we address the problem numerically by two steps. In the first step, we carry out Brownian dynamics (BD) simulations on CFGE by solving semi-microscopic Langevin equations of a polymer consisting of beads separated by a mean distance much smaller than the Kuhn length. Results are analyzed based on coarse-graining over the Kuhn length scale. We show the averaged elongation–contraction motion involves asymmetric V-shaped configurations whose shorter arm length depends on the field and the temperature consistently with what is expected when the BD chain is described by the freely-jointed chain (FJC) model with a suitable Kuhn length. To our knowledge, this is the first numerical confirmation of the FJC model itself from a submicroscopic description of polymer motion. The ...

Primitive chain network simulations for branched polymers

We present simulations of branched polymer dynamics based on a sliplink network model, which also accounts for topological change around branch points, i.e., for branch-point diffusion. It is well-known that, with the exception of stars, branched polymers may show a peculiar rheological behavior due to the exceptionally slow relaxation of the backbone chains bridging branch points. Though Brownian simulations based on sliplinks are powerful tools to study the motion of polymers and to predict rheological properties, none of the existing methods can simulate the relaxation of the bridge chains. The reason for that is lack of a rule for network topology rearrangement around branch points, so that entanglements between bridge chains cannot be renewed. Therefore, we introduce in this paper one possible branch-point mobility rule into our primitive chain network model. For star polymers, diffusion coefficients were calculated and compared with experiments. For both star and H-shaped polymers, diffusion was simulated both with and without the new rule, and the effect on linear viscoelasticity was also determined in one case.

Periodic orientational motions of rigid liquid-crystalline polymers in shear flow

The collective periodic motions of liquid-crystalline polymers in a nematic phase in shear flow have, for the first time, been simulated at the particle level by Brownian dynamics simulations. A wide range of parameter space has been scanned by varying the aspect ratio L/D between 10 and 60 at three different scaled volume fractions Lphi/D and an extensive series of shear rates. The influence of the start configuration of the box on the final motion has also been studied. Depending on these parameters, the motion of the director is either characterized as tumbling, kayaking, log-rolling, wagging, or flow-aligning. The periods of kayaking and wagging motions are given by T=4.2(Lphi/D)gamma(-1) for high aspect ratios. Our simulation results are in agreement with theoretical predictions and recent shear experiments on fd viruses in solution. These calculations of elongated rigid rods have become feasible with a newly developed event-driven Brownian dynamics algorithm.

Screening by Symmetry of Long-Range Hydrodynamic Interactions of Polymers Confined in Sheets

Hydrodynamic forces may significantly affect the motion of polymers. In sheet-like cavities, such as the cell's cytoplasm and microfluidic channels, the hydrodynamic forces are long-range. It is therefore expected that that hydrodynamic interactions will dominate the motion of polymers in sheets and will be manifested by Zimm-like scaling. Quite the opposite, we note here that although the hydrodynamic forces are long-range their overall effect on the motion of polymers vanishes due to the symmetry of the two-dimensional flow. As a result, the predicted scaling of experimental observables such as the diffusion coefficient or the rotational diffusion time is Rouse-like, in accord with recent experiments. The effective screening validates the use of the non-interacting blobs picture for polymers confined in a sheet.

A splitting technique for analytical modelling of two-phase multicomponent flow in porous media

In this paper we discuss one-dimensional models for two-phase Enhanced Oil Recovery (EOR) floods (oil displacement by gases, polymers, carbonized water, hot water, etc.). The main result presented here is the splitting of the EOR mathematical model into thermodynamical and hydrodynamical parts. The introduction of a potential associated with one of the conservation laws and its use as a new independent coordinate reduces the number of equations by one. The ( n) × ( n) conservation law model for two-phase n-component EOR flows in new coordinates is transformed into a reduced ( n − 1) × ( n − 1) auxiliary system containing just thermodynamical variables (equilibrium fractions of components, sorption isotherms) and one lifting equation containing just hydrodynamical parameters (phase relative permeabilities and viscosities). The algorithm to solve analytically the problem includes solution of the reduced auxiliary problem, solution of one lifting hyperbolic equation and inversion of the coordinate transformation. The splitting allows proving the independence of phase transitions occurring during displacement of phase relative permeabilities and viscosities. For example, the minimum miscibility pressure (MMP) and transitional tie lines are independent of relative permeabilities and phases viscosities. Relative motion of polymer, surfactant and fresh water slugs depends on sorption isotherms only. Therefore, MMP for gasflood or minimum fresh water slug size providing isolation of polymer/surfactant from incompatible formation water for chemical flooding can be calculated from the reduced auxiliary system. Reduction of the number of equations allows the generation of new analytical models for EOR. The analytical model for displacement of oil by a polymer slug with water drive is presented.

Time‐resolved luminescence anisotropy studies of the relaxation behaviour of polymers. 2. On the intramolecular mobility of poly(butyl methacrylate) in dilute solution

AbstractTime‐resolved anisotropy measurements (TRAMS) have been used to study the relaxation behaviour of poly(butyl methacrylate) (PBMA) in dilute solution both in toluene and dichloromethane. Various fluorescent labels (residues from copolymerisation of BMA with ca 0.3 mol% of acenaphthylene (ACE), 1‐vinylnaphthalene (VN), 1‐naphthylmethyl methacrylate, 9‐phenyanthrylmethyl methacrylate, and 9‐anthrylmethyl methacrylate (AMMA)) were used. It was found that, as expected, ACE and VN provided information on the segmental motion of PBMA. The fluorescence anisotropies of the various ester labels reflected the influence of motion, independent of that of the PBMA backbone, upon the observed relaxation behaviour. The motion of AMMA, in particular, could be resolved, with confidence, into components corresponding to segmental relaxation of both the polymer and ester group motion. Clearly, in this system, motion of the labelled ester is not fully cooperative with that of the polymer backbone, even at the high test frequencies to which these fluorescence measurements correspond. Copyright © 2005 Society of Chemical Industry

Direct observation of spatiotemporal dependence of anomalous diffusion in inhomogeneous fluid by sampling-volume-controlled fluorescence correlation spectroscopy

The direct observation of a spatiotemporal behavior of anomalous diffusion in aqueous polymer [hyaluronan (HA)] solution was achieved by fluorescence correlation spectroscopy (FCS) using a modified instrument, enabling continuous change of the confocal volume of a microscope, namely, sampling-volume-controlled (SVC) FCS (SVC-FCS). Since HA chains form a mesh structure with a pore size of about 10-40 nm, the observed diffusion coefficient (Dobs) is markedly dependent on the diffusion distance (L). By SVC-FCS, the curve of the distance dependence of diffusion coefficient was directly obtained as a continuous profile in L = 245-600 nm showing evidence of anomalous diffusion. On plotting Dobs against either of the sampling time (tauobs) or the diffusion distance (L), Dobs turnover was observed near the anomalous diffusion area. The appearance of this turnover is attributed to the nonuniform mesh structure that can be observed only by a fast observation and that should be dynamically averaged by polymer motions with large tauobs. This behavior is similar to that revealed in glass, colloidal systems, and gel solutions using dynamic light scattering, neutron scattering, and other techniques.

Bending modeling and its experimental verification for conducting polymer actuators dedicated to manipulation applications

Conducting polymer actuators are emerging new actuators with many promising features suitable to some cutting edge applications ranging from biomedical devices to micro/nano manipulation systems. These features are highly influenced by their interrelated mechanical, electrical and chemical properties. This makes their behavior complex, and difficult to understand and predict. In order to make use of their full potentials, there is an increasing need to investigate into their actuation mechanism in order to provide enhanced degrees of understanding and predictability. With this in mind, it is the object of this paper to characterize and model the bending motion of the strip-type fourth generation polypyrrole polymer (PPy) actuators, which operate in a non-liquid medium, i.e. in the air. After deriving a mathematical model approximately accounting for mechanical, electrical, and chemical properties and geometric parameters of the actuator, the model has been experimentally verified for two actuators with the dimensions of (20 mm × 1 mm × 0.16 mm) and (10 mm × 1 mm × 0.21 mm). Theoretical and experimental results are presented to demonstrate that the model is effective enough to predict the displacement output of the strip type-PPy actuator all along the edge of the actuator as a function of the applied voltage.

Properties of polystyrene and polymethyl methacrylate copolymers of polyhedral oligomeric silsesquioxanes: A molecular dynamics study

AbstractMolecular dynamics simulations were carried out on copolymers of both styrene and methyl methacrylate with polyhedral oligomeric silsesquioxane (POSS) derivatives to identify the origin of the property changes imparted upon the chemical incorporation of POSS. Simulations were carried out on these hybrid copolymers and the parent homopolymers to elucidate the effect of the T8, T10, and T12 POSS cages. These POSS comonomers were derivatized with a single polymerizable function and 7, 9, and 11 nonpolymerizable hydrocarbon moieties, respectively. Glass transition temperatures (Tg) were computed from specific volume versus temperature plots. The packing of POSS units around the polymer backbone was analyzed via their radial distribution functions. The effect of POSS on polymer motion was analyzed through the mean square displacement function. The improvements in the elastic moduli upon incorporation of POSS were computed by employing the static deformation method. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 234–248, 2006

Light-induced random-walk motion in azo-polymers

We propose a model which accounts for the light-induced mass motion of polymers with azobenzene functions. To assess the implications of the model, we perform real-time AFM measurements of the deformation of such polymer layers under the effect of green laser light. A qualitative confrontation of the model with the experimental facts is quite satisfying. However, a quantitative approach displays that the model needs to be enriched with two other parameters : the influence of light polarization direction and the bleaching process. These two elements can easily be introduced into the model.

Inelastic neutron scattering for investigating the dynamics of confined glass-forming liquids

Inelastic neutron scattering was employed over recent years to investigate the influence of spatial confinement on the dynamics of glass-forming systems. We review the common phenomena observed by neutron scattering in such different confining hosts like porous glasses, molecular sieves, clays or free standing polymer films, which impose a spatial limitation to the motion of small organic molecules, oligomers or polymers. Near the glass transition temperature the mean squared displacements of the confined molecules show clear deviations from the bulk behavior. The observed increase or decrease of the mean squared displacements confirms the high relevance of the interface interaction near walls of confining media without excluding additional real confinement effects. We show a new comparison of the mean squared displacement for PDMS and PMPS in bulk and in different type of restricting geometries, which evidence a weak influence of the restricting geometry on the local methyl group motion, but a strong influence on the glass transition dynamics, if wall interactions are taken into account. Strong wall interaction is also supported by the intermediate scattering function, measured either by combining neutron backscattering and time-of-flight experiments to cover 3 decades in time from ns to ps or by neutron spin echo, which reveal above T g an increasing elastic fraction with decreasing pore size and a slowing down of the dynamics. Furthermore we show that a reduction of modes below the Boson peak frequency is a more general feature of confined glass-forming systems.

Molecular dynamics study of tethered polymers in shear flow

Single macromolecules can now be isolated and characterized experimentally using techniques such as optical tweezers and videomicroscopy. An interesting and important single-molecule problem is that of the dynamics of a polymer chain tethered to a solid surface and subjected to a shear flow. An experimental study of such a system was reported by Doyle et al. (Phys. Rev. Lett. 84, 4769 (2000)), and their results showed a surprising recirculating motion of the DNA chain. We explore this problem using molecular dynamics computer simulations with explicit hydrodynamic interactions. The dynamical properties of a Freely Jointed Chain (FJC) with Finitely Extensible Nonlinear Elastic (FENE) links are examined in similar conditions (i.e., confined between two surfaces and in the presence of a Poiseuille flow). We see the remarkable cyclic polymer motion observed experimentally, and we show that a simple cross-correlation function can be used to measure the corresponding period of motion. We also propose a new empirical equation relating the magnitude of the shear flow to the amount of chain deformation, an equation that appears to apply for both weak and strong flows. Finally, we report on packing effects near the molecularly flat wall, an associated chain-sticking phenomenon, and the impact of the chain hydrodynamic drag on the local fluid flow.

Differences in pressure and temperature transitions of proteins and polymer gels

Pressure-driven and temperature-driven transitions of two thermoresponsive polymers, poly(N-isopropylacrylamide) (pNIPAM) and poly(N-vinylisobutyramide) (pNVIBA)), in both a soluble linear polymer form and a cross-linked hydro-gel form, were examined by a dynamic light-scattering method and direct microscopic observation, respectively. Their behavior was compared with that of protein systems. Changes in some characteristic parameters in the time-intensity correlation functions of dynamic light-scattering measurement of aqueous solutions of pNIPAM at various pressures and temperatures showed no essential differences during temperature and pressure scanning and, as a whole, the motions of polymers in aqueous solutions were similar in two types of transitions until chain shrinkage occurred. The gels (cross-linked polymer gels) prepared from the thermoresponsive polymers also showed similar volume transitions responding to the pressure and temperature increase. In temperature transitions, however, gels showed drastic volume shrinkage with loss of transparency, while pressure-induced transition showed a slow recovery of transparency while keeping the size, after first transient drastic volume shrinkage with loss of transparency. At a temperature slightly higher than the transition under atmospheric temperature, so-called reentry of the volume change and recovery of the transparency were observed during the pressure-increasing process, which implies much smaller aggregation or non-aggregated collapsed polymer chains in the gel at higher pressures, indicating a certain mechanistic difference of the dehydration processes induced by temperature and pressure.

Viscoelastic properties of bis(phenyl)fluorene‐based cardo polymers with different chemical structure

AbstractViscoelastic properties of urethane and ester conjugation cardo polymers that contain fluorene group, 9,9‐bis(4‐(2‐hydroxyethoxy)phenyl)fluorene (BPEF), were investigated. As for the urethane‐type cardo polymers containing BPEF in the main chain, it had a high glass‐transition temperature (Tg), which was observed as the α dispersion on viscoelastic measurement, and its temperature depended on the chemical structure of the spacing unit, such as toluene diisocyanate (TDI), 4,4′‐methylene diphenyl diisocyanate (MDI), methylene dicycloexyl diisocyanate (CMDI), and hexamethylene diisocyanate (HDI). Moreover, the Tg of urethane‐type cardo copolymers with various cardo contents increased with an increase of cardo content. Owing to the increase of Tg of cardo polymers, another molecular motion can be measured at the temperature between the α and β dispersion that was assigned to the molecular motion of urethane conjugation unit around 200 K, and it was referred to as the αsub dispersion. The peak temperature of the αsub dispersion was influenced by the chemical structure of the spacing unit, but it did not change for the cardo polymer containing the same spacing unit. Consequently, it was deduced that the αsub dispersion was originated in the subsegmental molecular motions of the cardo polymers. Ester‐type cardo polymer had higher Tg in comparison with noncardo polymer that consisted of dimethyl groups (BPEP) instead of BPEF as well. The αsub dispersion was also measured at the temperature between the α and β dispersion, which was assigned to the molecular motion of ester conjugation unit, around 220 K. For ester cardo polymer, the γ dispersion was measured in a low‐temperature region around 140 K, and it was due to a small unit motion in the ester‐type cardo polymers, such as ethoxyl unit, C2H4O. Moreover, the intensity of the γ dispersion of noncardo polymer was higher than that of cardo polymer, which means the molecular motion was much restricted by the cardo structure of BPEF. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2259–2268, 2005

Additive equivalence in turbulent drag reduction by flexible and rodlike polymers

We address the additive equivalence discovered by Virk and co-workers: drag reduction affected by flexible and rigid rodlike polymers added to turbulent wall-bounded flows is limited from above by a very similar maximum drag reduction (MDR) asymptote. Considering the equations of motion of rodlike polymers in wall-bounded turbulent ensembles, we show that although the microscopic mechanism of attaining the MDR is very different, the macroscopic theory is isomorphic, rationalizing the interesting experimental observations.

Characteristic Periodic Motion of Polymers in Shear Flow

The motion of both free and tethered polymer molecules as well as rigid Brownian rods in unbound shear flow is found to be characterized by a clear periodicity or tumbling frequency. Periodicity is shown using a combination of single molecule DNA experiments and computer simulations. In all cases, we develop scaling laws for this behavior and demonstrate that the frequency of characteristic periodic motion scales sublinearly with flow rate.