Dependence of the thermal conductivity of a polymer chain on its tension

The heat transfer along a finite polyethylene macromolecule with fixed ends has been simulated. It has been shown that the thermal flux arising in the chain essentially depends on the distance between its ends. The thermal flux along the chain increases due to its stretching and decreases when its ends become closer to each other. There is a critical value for the approach of the ends at which the coil-globule transition begins. Upon a closer approach, a globule in the chain is formed and, upon a smaller approach, the chain always freely bends in space. It is at this critical approach of the ends that the tension in the chain and heat transfer along it are the weakest. Making the ends more remote or closer, one can change the thermal flux by more than two orders of magnitude. This effect is connected with the strong dependence of the morphology of the polymer chain on its tension. In nanotechnologies, such a polymer molecule with a variable distance between its fixed ends can be used as a regulated phonon resistor, i.e., as a phonon rheostat.     

Polymer depletion interaction between a particle and a wall

The attractive depletion interaction between a spherical particle and a planar wall in a dilute solution of long flexible nonadsorbing free polymer chains is found to depend crucially on the particle to polymer size ratio . While the polymer-induced force between particle and wall decreases monotonically with increasing distance for large , for small it has a maximum at a distance of the order of the polymer size. For ideal chains we study the crossover from large to small behavior in full quantitative detail. Besides the free energy of interaction and the force, we also discuss the spatial variations of the densities of chain-ends and chain-monomers near the wall and particle. Two independent procedures, (1) solving directly the diffusion equation for the density of ends in terms of planar and spherical waves and (2) minimizing the Ginzburg-Landau functional of the “magnetic analog” of the polymer problem, are used to obtain results numerically for a broad range of ratios of the three lengths particle size, polymer size and distance of particle from the wall. Besides previously known cases, we find two more interesting limiting regions of the length ratios for which analytical results can be obtained. [2mm] Received 11 December 1998     

The effect of the size of polymer chain on the energy transfer between chromophores bound to the ends of the chain

The dependence of the luminescence quantum yield of linear polymer molecules that contain chromophores at the ends on the number of units of polymer chain is studied. Numerical calculations for the dipole-dipole energy transfer between chromophores show that an increase in the chain length by a factor of 2 leads to a several-fold increase in the luminescence quantum yield depending on the ratio of the Förster radius to the radius of polymer coil. An approximate formula is derived for the adequate analysis of the dependence at the Förster radius that is significantly less than the radius of polymer coil. The dependence of the luminescence quantum yield of linear polymer molecules on the length of the statistical segment (unit) at a constant total length of the chain is also studied.     

Luminescence kinetics of chromophores attached to the ends of a flexible polymer chain

We studied the effect of the size of a polymer molecule, of the type of its motion in solution, and of the homogeneous and inhomogeneous broadening of its spectra on the luminescence kinetics of chromophores attached to the ends of a polymer chain.     

Laser induced decomposition of a designed and a commercial polymer studied by ns-interferometry and shadowgraphy

Commercially available polymers often exhibit quite poor laser ablation properties for irradiation wavelengths around 248 nm. At these wavelengths, the absorption is due to photostable aromatic groups. Photolabile triazene polymers were developed to compare the influence of a photolabile group on the laser ablation process. The photochemically active triazene group has a strong absorption band at 332 nm, whereas the second absorption maximum at 220 nm is due to the photostable aromatic group. By irradiating at 308 nm and 193 nm, the influence of the photochemically active group on the ablation process can be studied. The etching of the triazene polymer starts and ends with the laser pulse. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than with 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer compared with polyimide. Received: 28 August 2002 / Accepted: 20 September 2002 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/310-4412, E-mail: thomas.lippert@psi.ch     

Adsorption of model charged proteins on charged surfaces with grafted polymers

The adsorption of model charged proteins on charged surfaces with and without grafted polymers is studied using a molecular approach. The ability of the polymer layer to reduce the amount of proteins adsorbed on top of the surface (primary adsorption) and at the same time to increase the adsorption of the proteins on top of the polymer layer (secondary adsorption) is presented. It is found that charging the free ends of the chains can result in an efficient way to enhance adsorption at the tip of the brush. Increasing the surface coverage of the polymers with charged free ends enhances the amount of proteins adsorbed at the tip of the polymer layer, while at the same time strongly reduces the number of proteins adsorbed directly onto the surface. The interplay between the attractive van der Waals protein-surface interactions, the steric polymer-protein interactions and the effect of the electrostatic interactions in determining the final adsorption is discussed. The manipulation of solution conditions to tune the amount of secondary adsorption is presented.     

High intrachain hole mobility on molecular wires of ladder-type poly(p-phenylenes)

We have studied the high-frequency (34 GHz) mobility of positive charge carriers on isolated ladder-type polymer chains in dilute solution. We find that the high-frequency mobility is limited by the chain ends on chains as long as 35 monomers. The intrachain motion of charge carriers can be described by one-dimensional diffusion between infinitely high reflecting barriers, representing the chain ends. Our data indicate that the intrachain mobility for ladder-type polymer chains is close to 600 cm(2)/V s. With this high mobility the ladder-type polymer is a promising candidate for future use as an interconnecting wire in molecular electronics.     

Monte Carlo simulations of an amphiphilic polymer at a hydrophobic/hydrophilic interface

Monte Carlo simulations have been carried out for an off-lattice model of an amphiphilic polymer at a hydrophobic/hydrophilic interface. The model system consists of a polynorbornene backbone with poly(ethylene oxide) (PEO) grafts modelled atomistically at an idealized interface between hydrophobic and hydrophilic regions, which are represented by external potentials. Results are presented for the distribution of PEO chain ends, and the density of PEO segments perpendicular to the surface. The latter is used to provide predictions for neutron reflectivity profiles normal to the surface as a function of the lateral confinement of the PEO grafts. At low surface coverage the simulation results are found to be in good agreement with experimental neutron scattering results from similar polymers studied at the water/air interface.     

Dynamics of an anchored polymer molecule under an oscillating force

We study the dynamics of a polymer of varying stiffness, pinned or grafted at both ends and subjected to an oscillatory forcing at an intermediate point. Via stochastic simulations, we find a crossover from a periodic limit cycle to an aperiodic dynamics as the polymer gets "stiffer." An analytical argument valid in the 2D grafted case shows that in such a case this aperiodic dynamics has some chaotic signatures.     

Linear polymer with hydrogen bonds as a nonequilibrium system

A two-temperature nonequilibrium model of a linear polymer with possible formation of hydrogen bonds between monomers is proposed. A molecule is considered to be in an adiabatic stationary state far from equilibrium. The dependence of the mean-square distance between the chain ends on the number of monomers is calculated with the simplest approach of Flory in case T > Θ.     

Two-dimensional polymer networks at a mixed boundary: Surface and wedge exponents

We provide general formulae for the configurational exponents of an arbitrary polymer network connected to the surface of an arbitrary wedge of the two-dimensional plane, where the surface is allowed to assume a general mixture of boundary conditions on either side of the wedge. We report on a comprehensive study of a linear chain by exact enumeration, with various attachments of the walk's ends to the surface, in wedges of angles and , with general mixed boundary conditions. Received: 20 October 1997 / Accepted: 13 May 1998     

Time dependence of the luminescence from a polymer membrane swollen in water: Concentration and isotopic effects

The effect of UV irradiation of the surface of a Nafion polymer electrolyte membrane swollen in water in the pump grazing incidence geometry has been experimentally investigated. The photoluminescence from the Nafion surface has been measured in the spectral range characteristic of this polymer. The photoluminescence signal from a polymer with a variable isotopic composition is found to be sensitive to swelling in water. The spectral absorption lines of dry and water-swollen Nafion samples are characterized. It is shown that the luminescence centers in the polymer are sulfonic acid groups located on the ends of perfluorovinyl ether groups, which form the teflon base. Measurements of the temporal dynamics of the luminescence of these groups have revealed an informationally important and significant dependence of the luminescence parameters on the degree of Nafion swelling. A pronounced and nontrivial dependence of these parameters on the content of heavy isotope D₂O in water is also found.     

Adsorption of polymers anchored to membranes

Polymers, which are attached to a membrane at one of their ends, exert an entropic pressure, which curves the membrane away from the polymers. It is shown that adsorption which arises from a short-ranged potential between the polymer and the membrane has a large influence on the curvature of the membrane, leading to a decrease of the entropically induced curvature. If one ignores the finite size of the anchor segment, the polymer-induced curvature does not change sign and vanishes in the limit of strong adsorption and a pure contact potential. If one includes the finite size of the anchor segment, the membrane bends towards the polymer for sufficiently strong adsorption. Received 20 October 2000     

Metastable tight knots in a wormlike polymer

Based on an estimate of the knot entropy of a wormlike chain we predict that the interplay of bending energy and confinement entropy will result in a compact metastable configuration of the knot that will diffuse, without spreading, along the contour of the semiflexible polymer until it reaches one of the chain ends. Our estimate of the size of the knot as a function of its topological invariant (ideal aspect ratio) agrees with recent experimental results of knotted dsDNA. Further experimental tests of our ideas are proposed.     

Statistics of ideal randomly branched polymers in a semi-space

We investigate the statistical properties of a randomly branched 3-functional N-link polymer chain without excluded volume, whose one point is fixed at the distance d from the impenetrable surface in a 3-dimensional space. Exactly solving the Dyson-type equation for the partition function Z(N, d )= N^-θe^γN in 3D, we find the “surface” critical exponent θ = , as well as the density profiles of 3-functional units and of dead ends. Our approach enables to compute also the pairwise correlation function of a randomly branched polymer in a 3D semi-space.     

Polyelectrolyte brushes with specific charge distribution: Mean-field lattice theory

A novel polymer system containing charged diblock polymers grafted onto a planar surface, simple salt and solvent has been considered in the framework of a mean-field lattice theory. On the basis of predicted volume fraction profiles of polymer segments, free ends, block junctions, and small ions, a detailed picture of the system has emerged. It is found that the structure of the polymer brush is decisively dependent of the relative charge of the blocks. For certain conditions, bimodal profiles appear which demonstrate the simultaneous presence of two types of chain conformations, one coiled and one stretched. Received 30 August 2000     

Anchoring of Polymers by Traps Randomly Placed on a Line

We study the dynamics of a Rouse polymer chain which diffuses in a three-dimensional space under the constraint that one of its ends, referred to as the slip-link, may move only along a one-dimensional line containing randomly placed, immobile, perfect traps. Extensions of this model occur naturally in many fields, ranging from the spreading of polymer liquids on chemically active substrates to the binding of biomolecules by ligands. For our model we succeed in computing exactly the time evolution of the probability P _sl(t) that the chain slip-link will not encounter any of the traps until time t and, consequently, that until this time the chain will remain mobile.     

Numerical study of the single fibre push-out test: Part III. Singularity of interface stresses

The singular behaviour at the free edges of the fibre-matrix interface is analysed for the fibre push-out test geometry based on the boundary element method. The fibre push-out test has been extensively used to measure the fibre-matrix interfacial properties in polymer, ceramic and metal matrix composites. There are two free edges in the fibre push-out specimen: one is at the loaded fibre end and the other at the supported fibre end. The singular stresses can be expressed as a function of singular exponent and singular stress intensity. It is shown that the singular exponents obtained at both fibre ends are characteristic of composite constituent properties, such as Young's moduli of fibre and matrix, and does not vary with specimen dimensions. The singular exponents are real and identical for the shear and radial stress components at fibre ends where the wedge angles are the same. The singular stress intensities are also implicit in material properties, and vary with specimen dimensions, such as fibre to matrix radius ratio, fibre aspect ratio and support hole size. An interfacial failure criterion is proposed here based on the average stress concept to determine the critical singular stress intensities in mode I and mode II loads.     

Preparation of well-defined polymer-silicon wafer hybrids via surface-initiated RAFT-mediated process

A simple method was developed for the immobilization of reversible addition-fragmentation chain-transfer (RAFT) initiators on the silicon surface. Well-defined polymer-silicon hybrids, including the tethered brushes of glycidyl methacrylate (GMA) polymer, poly(ethylene glycol) monomethacrylate (PEGMA) polymer and block copolymer on a silicon wafer, were prepared via surface-initiated RAFT living radical polymerization. The “living” chain ends were used as the macroinitiator for the subsequent synthesis of diblock copolymers.