Chain Stretch Research Articles

Chain stretch and relaxation in transient entangled solutions probed by double-step strain flows

Non-reversing and reversing double-step strain flows on concentrated entangled polystyrene in diethyl phthalate or tricresyl phosphate were employed to characterize transient entanglement properties affecting subsequent chain stretch and relaxation. An extended Doi-Edwards tube theory for double-step strain flows was employed to retrieve the phenomenological stretch relaxation function following a second large probe strain imposed on specially selected time scales that permit a direct assessment of the modified chain stretch with varying transient entanglement structure. Compared with single-step strain result, the maximum mean-square segmental stretch was noted to reduce by as much as 33% and 48% for non-reversing and reversing flows, respectively, with a probe strain γ2=7.

Mechanical Unfolding of a Homopolymer Globule Studied by Self-Consistent Field Modeling

We present results of numerical self-consistent field (SCF) calculations for the equilibrium mechanical unfolding of a globule formed by a single flexible polymer chain collapsed in a poor solvent. In accordance with earlier scaling theory and stochastic dynamics simulations findings we have identified three regimes of extensional deformation: (i) a linear response regime characterized by a weakly elongated (ellipsoidal) shape of the globule at small deformations, (ii) a tadpole structure with a globular “head” coexisting with a stretched “tail” at intermediate ranges of deformations, and (iii) an uniformly stretched chain at strong extensions. The conformational transition from the tadpole to the stretched chain is accompanied by an abrupt unfolding of the depleted globular head and a corresponding jump-wise drop in the intrachain tension. The unfolding-refolding cycle demonstrates a hysteresis loop in the vicinity of the transition point. These three regimes of deformation, as well as the first-order li...

Biomechanical modeling of acetabular component polyethylene stresses, fracture risk, and wear rate following press‐fit implantation

Press-fit implantation may result in acetabular component deformation between the ischial-ilial columns ("pinching"). The biomechanical and clinical consequences of liner pinching due to press-fit implantation have not been well studied. We compared the effects of pinching on the polyethylene fracture risk, potential wear rate, and stresses for two different thickness liners using computational methods. Line-to-line ("no pinch") reaming and 2 mm underreaming press fit ("pinch") conditions were examined for Trident cups with X3 polyethylene liner wall thicknesses of 5.9 mm (36E) and 3.8 mm (40E). Press-fit cup deformations were measured from a foam block configuration. A hybrid material model, calibrated to experimentally determined stress-strain behavior of sequentially annealed polyethylene, was applied to the computational model. Molecular chain stretch did not exceed the fracture threshold in any cases. Nominal shell pinch of 0.28 mm was estimated to increase the volumetric wear rate by 70% for both cups and peak contact stresses by 140 and 170% for the 5.9 and 3.8 mm-thick liners, respectively. Although pinching increases liner stresses, polyethylene fracture is highly unlikely, and the volumetric wear rates are likely to be low compared to conventional polyethylene.

Modelling elongational and shear rheology of two LDPE melts

Experimental data of two low-density polyethylene (LDPE) melts at 200°C for both shear flow (transient and steady shear viscosity as well as transient and steady first normal stress coefficient) and elongational flow (transient and steady-state elongational viscosity) as published by Pivokonsky et al. (J Non-Newtonian Fluid Mech 135:58–67, 2006) were analysed using the molecular stress function model for broadly distributed, randomly branched molecular structures. For quantitative modelling of melt rheology in both types of flow and in a very wide range of deformation rates, only three nonlinear viscoelastic material parameters are needed: Whilst the rotational parameter, a 2, and the structural parameter, β, are found to be equal for the two melts considered, the melts differ in the parameter $f_{\rm max}^2 $ describing maximum stretch of the polymer chains.

An experimental evaluation of the behaviour of mono and polydisperse polystyrenes in Cross-Slot flow

The behaviour of a number of mono and polydisperse polystyrenes are probed experimentally in complex extensional flow within a Cross-Slot geometry using flow-induced birefringence. Polystyrenes with similar molecular weight (M w) and increasing polydispersity (PD) illustrated the effect of PD on the principal stress difference (PSD) pattern in extensional flow. Monodisperse materials exhibited only slight asymmetry at moderate flowrates, although increased asymmetry and cusping was observed at high flowrates. The response of monodisperse materials of different M w at various flowrates is presented and characterised by Weissenberg numbers for both chain stretch and orientation using a theory for linear entangled polymers. The comparison of stress profiles against Weissenberg number for each process is used to determine whether the PSD pattern observed is independent of M w and elucidate which relaxation mechanism is dominant in the flow regimes probed. For monodisperse materials, at equivalent chain orientation Weissenberg number (We τd), different molecular weight materials were seen to exhibit similar steady state PSD patterns independent of We τR (chain stretch We). Whilst no obvious critical Weissenberg number (We) was found for the onset of increased asymmetry and cusping, it was found to occur in the “orientating flow without chain stretch” regime.

Effect of Varying Salt Concentration on the Behavior of Weak Polyelectrolytes in a Poor Solvent

Using grand canonical Monte Carlo simulations of a polyelectrolyte chain where the charges are in contact with a reservoir of constant chemical potential given by the solution pH, we study the behavior of weak polyelectrolytes in a poor solvent. We address the influence of variable screening of the electrostatic interaction on the chain structure for rather poor solvents as well as in the close-to-Θ-point regime. For the latter case, we demonstrate that a fine tuning of the chain structure is attainable by varying screening. With growing screening length, that is, by reducing the salt concentration, the degree of charging is reduced and the pearl distribution can be shifted toward a smaller number of pearls. In addition, we find that pearl necklaces can also occur at high charging provided the screening is very strong. In the rather-poor-solvent regime, the position of the discrete transition from a highly charged, stretched chain to a weakly charged globule can be shifted by the variation of screening. B...

Subgel transition in diluted vesicular DODAB dispersions

We have characterized the lipid chain freezing in dilute aqueous vesicle dispersions of the cationic lipid dioctadecyldimethylammonium bromide (DODAB) using wide and small angle X-ray scattering, solid state NMR, DSC, turbidity and density measurements. The lipids freeze in two steps. Above 40 °C the chains are fluid and the lipids are in a so-called liquid-crystalline state. When cooling below 40 °C, the lipids form a gel phase where the chains stretch, the molecules are more densely packed and most molecular degrees of freedom are frozen, or at least dramatically slowed down. In the gel phase, the chain packing is still disordered, while the chain mobility is significantly reduced. From NMR data we further conclude that also the molecular rotational diffusion around the molecular long axis is quenched. Slow chain reorientation may occur, but then as individual reorientations of the separate chains. When cooling further below 36 °C, crystalline ordering of the chains is obtained, resulting in a further increased packing density. We refer to this state as the subgel phase. The transitions are reversible. However, the formation of the ordered subgel is very slow for temperatures near the melting point. In fact, the gel phase can be supercooled by almost 20 °C for considerable time. From analyzing this transition in terms of classical nucleation we obtain an estimate of the intra-bilayer interfacial tension between the gel phase and the growing subgel domains of 2 mN m−1.

Modeling the structure of a polydisperse polymer brush

Numerical self-consistent field theory is used to study the structural characteristics of a polydisperse polymer brush. We consider the relevant case of a Schulz–Zimm distribution and find that even a small degree of polydispersity completely destroys the parabolic density profile. The first moment (average height) of the brush increases with polydispersity, while the average stretching in the brush decreases. The density profiles of separate chain length fractions in a single polydisperse brush are also strongly influenced by polydispersity. Short chains are found to be compressed close to the grafting interface, whereas longer chains have a characteristic flower-like distribution. These longer chains stretch strongly (stem) when surrounded by smaller chains and decrease their stretching (crown) when only surrounded by longer chains. In line with approximate analytical models, our numerical exact results show that the polymer chains in the brush have localized end-point positions (no fluctuations) in strong contrast to the anomalously large fluctuations in the end-point positions of the homodisperse brush. Despite these effects, the scaling of average height with grafting density and number average chain length is unaffected by polydispersity. Many results that we have presented can be understood qualitatively from the bidisperse brush.

An experimental evaluation of the formation of an instability in monodisperse and polydisperse polystyrenes

This paper presents an experimental study on the formation of a polymer flow instability seen in a range of polystyrene melts flowing through contraction–expansion slit geometries. Seen for both monodisperse and polydisperse materials, it took the form of an oscillation in the principal stress difference (PSD) and velocity field perpendicular to the bulk flow. The instability originated downstream of the slit and overtime propagated back upstream such that both regions were unstable. For the monodisperse materials, flowrates were characterised using a theory for linear entangled polymers. This allowed a comparison of the flow regime, with respect to both chain stretch and orientation, and the onset of the instabilities. These are presented as stability maps for comparison with future numerical work. In all cases, no single flowrate, Weissenberg number or critical pressure drop was seen which triggered the instability. The variation of slit curvature and length was found to affect the onset of the flow instability, but not its general form. For all materials, an increase in polydispersity and decrease in molecular weight led to increased processability of the material, delaying the onset of the instability until higher flowrates.

Verification of branch point withdrawal in elongational flow of pom-pom polystyrene melt

According to tube model ideas, chain stretch at deformation rates below the inverse Rouse time of the chain, is only possible for polymer topologies with two or more branch points. The basic topologies, which embody this idea, are the H molecule with two side chains, and the pom-pom molecule with q>2 side chains at each end of the backbone. According to the pom-pom hypothesis, maximum chain stretch of the backbone is limited by branch point withdrawal, i.e., the side chains are drawn into the tube of the backbone as soon as the relative tension in the backbone reaches a value of q. This hypothesis, which has never been verified before, can now be tested by considering recent elongational experiments by Nielsen et al. [Macromolecules 39, 8844–8853 (2006)] on a nearly monodisperse polystyrene pom-pom melt with q=2.5. The analysis presented is based on the original integral version of the pom-pom model, and on the molecular stress function (MSF) model with strain-dependent tube diameter. The material strain measure determined from the experiments is found to be consistent with a constant maximum stretch, independent of the elongation rate, which is, however, significantly larger than q. To achieve quantitative agreement between experiment and modeling, (1) dynamic dilution of the backbone, which increases the tube diameter of the backbone and reduces equilibrium tension in the backbone, (2) finite extensibility effects, (3) stretch relaxation causing a transition from chain stretch to tube squeeze at lower strain rates, and (4) the dynamics of branch point withdrawal need to be considered. Integrating all of these features in a MSF stretch evolution equation with multiple time scales, the fundamental pom-pom hypothesis is confirmed.

Thermal relaxation and mechanical relaxation of rice gel

Rice gel was prepared by simulating the production processes of Chinese local rice noodles, and the properties of thermal relaxation and mechanical relaxation during gelatinization were studied by differential scanning calorimetry (DSC) measurement and dynamic rheometer. The results show that during gelatinization, the molecular chains of rice starch undergo the thermal relaxation and mechanical relaxation. During the first heating and high temperature holding processes, the starch crystallites in the rice slurry melt, and the polymer chains stretch and interact, then viscoelastic gel forms. The cooling and low temperatures holding processes result in reinforced networks and decrease the viscoelasticity of the gel. During the second heating, the remaining starch crystallites further melt, the network is reinforced, and the viscoelasticity increases. The viscoelasticity, the molecular conformation and texture of the gel are adjusted by changing the temperature, and finally construct the gel with the textural characteristics of Chinese local rice noodle.

Crystal Structure of a Prolactin Receptor Antagonist Bound to the Extracellular Domain of the Prolactin Receptor

The crystal structure of the complex between an N-terminally truncated G129R human prolactin (PRL) variant and the extracellular domain of the human prolactin receptor (PRLR) was determined at 2.5A resolution by x-ray crystallography. This structure represents the first experimental structure reported for a PRL variant bound to its cognate receptor. The binding of PRL variants to the PRLR extracellular domain was furthermore characterized by the solution state techniques, hydrogen exchange mass spectrometry, and NMR spectroscopy. Compared with the binding interface derived from mutagenesis studies, the structural data imply that the definition of PRL binding site 1 should be extended to include residues situated in the N-terminal part of loop 1 and in the C terminus. Comparison of the structure of the receptor-bound PRL variant with the structure reported for the unbound form of a similar analogue ( Jomain, J. B., Tallet, E., Broutin, I., Hoos, S., van Agthoven, J., Ducruix, A., Kelly, P. A., Kragelund, B. B., England, P., and Goffin, V. (2007) J. Biol. Chem. 282, 33118-33131 ) demonstrates that receptor-induced changes in the backbone of the four-helix bundle are subtle, whereas large scale rearrangements and structuring occur in the flexible N-terminal part of loop 1. Hydrogen exchange mass spectrometry data imply that the dynamics of the four-helix bundle in solution generally become stabilized upon receptor interaction at binding site 1.

Molecular physics of a polymer engineering instability: Experiments and computation

Entangled polymer melts exhibit a variety of flow instabilities that limit production rates in industrial applications. We present both experimental and computational findings, using flow of monodisperse linear polystyrenes in a contraction-expansion geometry, which illustrate the formation and development of one such flow instability. This viscoelastic disturbance is observed at the slit outlet and subsequently produces large-scale fluid motions upstream. A numerical linear stability study using the molecular structure based Rolie-Poly model confirms the instability and identifies important parameters within the model, which gives physical insight into the underlying mechanism. Chain stretch was found to play a critical role in the instability mechanism, which partially explains the effectiveness of introducing a low-molecular weight tail into a polymer blend to increase its processability.

Entangled polymer orientation and stretch under large step shear deformations in primitive chain network simulations

Orientation and stretch of entangled polymers under large step shear deformations were investigated through primitive chain network simulations. In the simulations, entangled polymer dynamics is described by 3D motion of entanglements, 1D sliding of monomers along the chain, and creation/destruction of entanglements described by hooking/unhooking with surrounding chains at chain ends. In addition to the conventionally proposed relaxation mechanisms that are reptation, contour length fluctuations, and constraint release (both thermal and convective), the simulations also account for force balance among entanglement strands converging to an entanglement node, and nodes also fluctuate in space. Nonlinear step strain data for monodisperse polystyrene melts (Ferri and Greco, Macromolecules, 37:5931, 2006) were quantitatively reproduced by using the same two molecular-weight-independent parameters already adopted by Masubuchi et al. (J Non-Newt Fluid Mech, 149:87, 2008) to fit linear viscoelastic data of several monodisperse polystyrene melts. Analysis of the orientation tensor and of the chain stretch ratio indicates that the segment orientation and stretch realized in the simulation are quantitatively described by a simple three-chain model (Marrucci et al., Macromol Symp, 158:57, 2000a).

Crystallization and Dissolution of Flow-Induced Precursors

We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals (T(m)(0)=141.2 degrees C) and requires stretch of the longest chains of the MWD. After the application of a shear flow of 120 s(-1) for 1 s at 142 degrees C, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity. Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes.

Molecular arrangement and photopatterning of copolymer containing ketal-protected group in ultrathin nanosheets

A novel series of ketal-protected copolymer, poly( N-dodecylmethacrylamide-co-1,4-dioxaspiro [4.4] nonane-2-methyl methacrylate) (p(DDMA–DNMMA)), was synthesized and their monolayer and multilayer nanosheets were deposited by Langmuir–Blodgett (LB) technique. The surface behavior of the copolymer monolayer at the air/water interface was investigated by measuring the surface pressure ( π) and the static elasticity ( E s) as a function of mean repeat area ( A) isotherm. The results showed that p(DDMA–DNMMA) could form the stable, well-defined molecular orientation Langmuir monolayer on the water surface, which was further proved by the measurements of atomic force microscopy (AFM). The polymer main chain lies flat on a water surface and the side chains attached to the main chain stretch out of the angle of about 62°. The static elasticity ( E s)-surface pressure ( π) isotherm was used to achieve optimum conditions for LB films deposition, because it could give more detailed information on molecular interaction and motion than π– A isotherm. UV–visible absorption spectra indicated that a well-ordered layer-by-layer structure was successfully controlled in the LB films. Upon deep UV light irradiation, photochemical reaction could also occur in the ultrathin nanosheets and the positive-tone photopatterning was obtained followed by development with alkali aqueous solution. The p(DDMA–DNMMA) LB films also show higher resistance ability in the process of etching gold film. The morphology of the patterns on the mica substrate was investigated by AFM.

Effect of disorder on DNA electrophoresis in a microfluidic array of obstacles

The size-based separation of electrophoresing DNA chains of varying lengths has been experimentally achieved in microfluidic obstacle arrays. The separation is actuated by the occurrence of size-dependent chain-obstacle collisions and the subsequent formation of hooked chain configurations in the array. We investigate the role played by disorder in array geometry in determining chain dynamics in the array. As a prototypical example of a disordered post array, we select a self-assembled array of magnetic colloids, wherein the degree of disorder may be varied by varying the magnetic field strength under which the array is generated. We employ Brownian dynamics simulations of chain electrophoresis in the array to compute the mobility, dispersivity, chain-obstacle collision probability, and mean chain stretch in the device, and demonstrate the link between the orientational order of the array and the resulting chain dynamics.

Effect of chain density and conformation on protein adsorption at PEG-grafted polyurethane surfaces

Polyurethanes were modified using monobenzyloxy polyethylene glycol (BPEG) which possesses a bulky hydrophobic benzyloxy group at one end and a hydroxyl group at the other end as a preconstructed BPEG layer, and poly(ethylene glycol) (PEG) and monomethoxyl poly(ethylene glycol) (MPEG) with various chain lengths as fillers. Our objective was to investigate the effect of PEG graft density and conformation on protein adsorption at PEGlated surface. The graft density was estimated by a chemical titration method. The combination of ATR-FTIR, AFM and titration results provide evidences that the graft density can be increased by backfilling PEG or MPEG to a BPEG layer. However, fibrinogen and albumin adsorption significantly increased on all surfaces after PEG or MPEG backfilling. We conclude that the conformation of hydrophobic benzyloxy end groups of the BPEG layer plays a key role. The benzyloxy end groups of preconstructed PEG chains stretch to the surface after PEG backfilling, which possibly accounts for the observed increase in protein adsorption. The BPEG conformation change after backfilling with PEG or MPEG was also suggested by contact angles. Additionally, protein adsorption was slightly influenced by the length of filler, suggesting a change in surface morphology.

Anomalous Polymer Sedimentation Far from Equilibrium

A single flexible polymer in strong sedimentation fields is investigated using hydrodynamic simulations and scaling arguments. For short chains and small fields compaction is observed. For elevated fields or long chains the chain stretches and the sedimentation coefficient decreases, in agreement with ultracentrifuge experiments on linear as well as circular DNA. For very large fields a tadpole forms consisting of a compact leading head and a trailing stretched tail.

Compressive elasticity in polymer Couette flow

Accepted models of polymers in flow assume that the polymer chains stretch with the elastic energy stored as a reduced entropy of the chain. The first optical measurement of changes in the Polydiacetylene 4-Butoxycarbonylmethylurethane (4-BCMU) polymer Kuhn segment lengths in Couette flow are reported. This polymer in semi-dilute solution shows a reducing conjugated segment length with increasing flow rate. The visco-elasticity for this polymer is attributed to compressive entropy loss in the chain. This is a reversal of the physics assumed in models of polymer flow based on the Kuhn model. The restricted volume of the chain due to the presence of the neighbouring polymers results in compressive hydrodynamic forces on the chains in Couette flow at semi-dilute concentrations.