Analysis of the cluster formation in two-component cylindrical bottle-brush polymers under poor solvent conditions. A simulation study

Two-component bottle-brush polymers, where flexible side chains containing N = 20, 35 and 50 effective monomers are grafted alternatingly to a rigid backbone, are studied by Molecular Dynamics simulations, varying the grafting density s \sigma and the solvent quality. Whereas for poor solvents and large enough s \sigma the molecular brush is a cylindrical object with monomers of different type occupying locally the two different halves of the cylinder, for intermediate values of s \sigma an axially inhomogeneous structure of “pearl-necklace” type is formed, where microphase separation between monomers of different type within a cluster takes place. These “pearls” have a strongly non-spherical ellipsoidal shape, due to the fact that several side chains cluster together in one “pearl”. We discuss the resulting structures in detail and we present a comparison with the single-component bottle-brush case.     

Static and dynamic properties of grafted ring polymer: Molecular dynamics simulation

The static and dynamic properties of a system of end-grafted flexible ring polymer chains grafted to a flat substrate and exposed to a good solvent are studied by using a molecular dynamics method. The monomers are described by a coarse-grained bead-spring model. Varying the grafting density ρ and the degree of polymerization or chain length N, we obtain the density profiles of monomers, study the structural properties of the chain (radius of gyration, bond orientational parameters, etc.), and also present the dynamic characteristics such as chain energy and bond force. Compared with a linear polymer brush, the ring polymer brush exhibits different static and dynamic properties for moderate or short chain length, while it behaves like linear polymer brush in the regime of long chain length.     

Stiffness and conformations of molecular bottle-brushes strongly adsorbed on a flat surface

A theory of the conformational behavior and analysis of the persistence length of molecular bottle-brushes strongly adsorbed on a flat surface in a good solvent is presented. For long enough side chains we have shown that the adsorption of a small fraction of side chains already leads to significant increase of the stiffness and drastic conformational changes of the brush in comparison with the 3D case. For a symmetrical distribution of the adsorbed side chains with respect to the backbone, the equilibrium conformation of the brush is close to a rod-like one due to the very large stiffness of the molecule. Under certain conditions the excluded-volume interactions of the side chains of an asymmetrical brush can lead to its bending or twisting in spiral. The obtained results are in good agreement with computer simulations. Received 13 April 2000 and Received in final form 18 August 2000     

Molecular dynamics and strengthening of liquid-crystal polymers

The role of large-scale molecular motion in the self-organization and strengthening of liquid-crystal polymer fibers is discussed. It is shown that, at high temperatures, these objects are oriented liquid-crystal melts in which macromolecules remain extended but execute high-frequency conformational motions without leaving the tube approximately 20 Å in diameter. This large-scale motion is referred to as quasi-segmental motion. During annealing, the chains involved in quasi-segmental motion can accomplish longitudinal displacements (reptate) over considerable distances. It is this reptation that favors spontaneous self-organization and, consequently, strengthening of liquid-crystal polymer fibers upon heat treatment. The role played by the quasi-segmental motion of rigid macromolecules in the strengthening of polymers of different types is compared with the role played by the segmental motion of flexible chains in this process.     

Unusual conformation of molecular cylindrical brushes strongly adsorbed on a flat solid surface

Conformational properties of comb-like polymers strongly adsorbed on a flat solid surface were investigated using computer simulation and scanning force microscopy. The computer simulation showed that the macromolecules with asymmetric distribution of the side chains relatively to the backbone are effectively in a collapsed state even under conditions of a good solvent. They formed peculiar helical superstructures which could be observed by scanning force microscopy of cylindrical brushes of polymethylmethacrylate on mica. Received 13 September 1999     

Study of Self-Diffusion of Silicone MQ Resins in Chloroform Solutions by Pulsed Field-Gradient NMR Spectroscopy

The self-diffusion of silicone MQ copolymers in solutions with chloroform was studied by pulsed field-gradient nuclear magnetic resonance over a wide range of macromolecular concentrations. The highly branched structure of the molecules reveals some characteristic features of the translational mobility. The studies were performed with the initial MQ copolymer as well as with the narrow-dispersed fractions obtained from it. The data indicate the differences in dynamics between fractions. The appearance of the species with different self-diffusivity with increasing of the polymer concentration in solutions was observed. Such behavior was attributed to the formation of aggregates of macromolecules in solution. The results show that the molecular weight influences the diffusion behavior of the MQ resins in solutions. The diffusion of the MQ resins reveals features characteristic for rigid macromolecular structures rather than flexible polymers. Differences in translational dynamics of MQ copolymers, linear flexible polymers and particle-like macromolecules are discussed.     

Study by 1H NMR of the Influence of the Porosity and Solvent on the Conformations of Macromolecules at an Interface: Poly(Ethylene Oxide) Grafted on Silica

The ¹H NMR technique was used to study the behaviour of poly(ethylene oxide) chains grafted chemically on silica. A noticeable influence of the nature of the surface and solvent on the conformation of the grafted macromolecules was observed. For precipitated silica the chains adopted a disordered conformation whereas for pyrogenic silica they formed a more uniform layer. The grafting ratio and the solvent can both modify the conformations of the chains. The dependence of the relaxation times (T₁ and T₂) on the grafting ratios is discussed qualitatively from a phenomenological point of view. Different models are proposed for the two extreme grafting ratio levels in both the absence and the presence of solvent. Without solvent, for both grafting ratios, the macromolecules lie very flat on the silica and the layer is more organized, especially at high grafting ratio. In the presence of solvent the chains spread out in the solvent, adopt a more extended conformation and the local concentration of monomers at the surface decreases.     

Compressing a rigid filament: Buckling and cyclization

We study elastic properties of rigid filaments modeled as stiff chains shorter than their persistence length. By rigid filaments we mean that fluctuations around the optimal filament shape are weak and that low-order expansions (quadratic or quartic) in the deviation from the optimal shape are sufficient to describe them. Our main interest lies in the profiles of force vs. projected filament length, closure probability and weakly buckled states. Results may be relevant to experiments on self-assembled biological (microtubules, actin filaments) and synthetic (organo-gelators) filaments, carbon nanotubes and polymers grafted with strongly repelling side chains, some of which are discussed here.     

Hydrogen bond network relaxation in aqueous polyelectrolyte solutions: the effect of temperature

Dielectric spectroscopy data over the range 100 MHz–40 GHz allow for a reliable analysis of two of the major relaxation phenomena for polyelectrolytes (PE) in water. Within this range, the dielectric relaxation of pure water is dominated by a near-Debye process at ν = 18.5 GHz corresponding to a relaxation time of τ = 8.4 ps at 25?°C. This mode is commonly attributed to the cooperative relaxation specific to liquids forming a hydrogen bond network (HBN) and arising from long range H-bond-mediated dipole–dipole interactions. The presence of charged polymers in water partially modifies the dielectric characteristics of the orientational water molecule relaxation due to a change of the dielectric constant of water surrounding the charges on the polyion chain. We report experimental results on the effect of the presence of a standard flexible polyelectrolyte (sodium polyacrylate) on the HBN relaxation in water for different temperatures, showing that the HBN relaxation time does not change by increasing the polyelectrolyte density in water, even if relatively high concentrations are reached (0.02 monomol l(?1) ≤ C ≤ 0.4 monomol l(?1)). We also find that the effect of PE addition on the HBN relaxation is not even a broadening of its distribution, rather a decrease of the spectral weight that goes beyond the pure volume fraction effect. This extra decrease is larger at low T and less evident at high T, supporting the idea that the correlation length of the water is less affected by the presence of charged flexible chains at high temperatures.     

Accelerated simulations of aromatic polymers: application to polyether ether ketone (PEEK)

For aromatic polymers, the out-of-plane oscillations of aromatic groups limit the maximum accessible time step in a molecular dynamics simulation. We present a systematic approach to removing such high-frequency oscillations from planar groups along aromatic polymer backbones, while preserving the dynamical properties of the system. We consider, as an example, the industrially important polymer, polyether ether ketone (PEEK), and show that this coarse graining technique maintains excellent agreement with the fully flexible all-atom and all-atom rigid bond models whilst allowing the time step to increase fivefold to 5 fs.     

Dynamics of large semiflexible chains probed by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy was used to probe the dynamics of lambda-phage DNA in aqueous solution labeled with the randomly intercalating dye TOTO. The linear macromolecules (i). carry more than one chromophore and (ii). are larger than the waist of the focal volume. The correlation function decays significantly faster than expected for a stiff globule of corresponding size but is in good agreement with the dynamic model of semiflexible chains including hydrodynamic interactions. As the chromophore density is lowered the correlation time decreases in accordance with this model.     

Molecular dynamics simulations of end-grafted centipede-like polymers with stiff charged side chains

We use molecular dynamics simulations to investigate centipede-like polymers with stiff charged side chains, end-grafted to a planar wall. The effect of the grafting density and the Bjerrum length on the conformational behaviour of the brush is examined in detail. In addition, we make a comparison of centipede-like polyelectrolyte (CPE) brushes with neutral centipede-like polymer (NCP) and linear polyelectrolyte (LPE) brushes. At weak electrostatic interaction, the main chains of the CPE chains adopt a strongly stretched conformation, and the monomer density profiles of side chains exhibit a clear oscillatory behaviour. With increasing Bjerrum length, the CPE brush undergoes a collapse transition. Compared to the CPE brushes, the counterion condensation effect is stronger for the LPE brushes, regardless of whether the electrostatic interaction is weak or strong and of whether the grafting density is low or high. Additionally, it is shown that the architecture of the grafted chains makes a weak contribution to the counterion condensation at strong electrostatic interaction. We also find that the electrostatic repulsion between charged side chains can enhance the stiffness of the main chains and thus limit the range of movement of the free-end monomers.     

Mesogenic polybutadiene diols with thiol side-chain units: synthesis and thermal behaviour

Several types of the chiral thiols with two aromatic rings have been synthesised and grafted on polybutadiene diols backbone. The resulting functional polymers possess the OH end groups for the possible preparation of liquid crystal ordered networks. The thermal and mesomorphic properties of the synthesised side-chain units and resulting polymers have been studied by polarising optical microscopy, differential scanning calorimetry and X-ray scattering. Some of the resulting polymers possess a mesomorphic behaviour. The effects of the side-chain structure, number of the chiral groups and density of grafting on the polybutadiene diols have been studied.     

The Great Myths of Polymer Melt Rheology,Part I: Comparison of Experiment and Current Theory

Abstract

Star polymers consisting of poly(?‐caprolactone), (PCL), grafted onto third generation dendrimer, which had hyperbranched and dendron cores, were studied by polarized light microscopy together with reference linear PCL. The degree of polymerization of the PCL arms in the star polymers ranged between 14 and 81. The star polymers exhibited a greater tendency than the linear polymers to form spherulites. It is suggested that the preference of the star polymers for forming spherulites is due to the presence of amorphous material—dendritic cores and PCL cilia—between crystal lamellae that generates the necessary pressure to force the lamellae to diverge at lamellar branch points. The linear growth rate data followed a single crystallization regime. The fold surface free energy was higher for the star polymers than for their linear analogs. It is proposed that the presence of the large and rigid dendritic cores on the fold surfaces of the star polymer crystals increases the fold surface energy.     

Drift and diffusion of a confined semiflexible chain

We study the transverse and longitudinal linear response function of rigid chains subjected to an external force. Our main concern are stiff polymers confined in narrow pores with diameter less than their persistence length. We explicitly consider confinement in a transverse harmonic potential and generalize results by scaling arguments. Our results describe the drift of the filament under an external force, time evolution of the filament shape, and filament diffusion. Diffusion of a confined filament resembles the celebrated reptation process for flexible chains, albeit with distinct kinetic exponents. The limiting case of stiff free filaments is also mentioned.     

Hybrid MC/RISM technique for simulation of polymer solutions: Monte Carlo+ RISM integral equations

A new (hybrid) method is reported for modelling complex macromolecular systems. The approach combines the traditional atomistic Monte Carlo (MC) computer simulation of flexible polymer chains with the numerical solution of the site-site Ornstein-Zernike-like (RISM) integral equations. The method is used for calculating properties of a linear polymer in dilute solution. Since the condensed-phase environment of a flexible macromolecule affects the equilibrium configuration probability distribution of the macromolecule, the site-site intramolecular correlation function and the intramolecular potential field are treated in a self-consistent manner. Briefly, the MC method is applied to generate the configurations of a single chain molecule. Using the coordinates of chain beads, the averaged intrapolymer correlation function is obtained. Then, solving the coupled RISM equations for a given density of solvent particles, we find the polymer-solvent correlation functions. This yields the medium-induced intrapolymer potential and the corresponding effective intramolecular energies, which are used in the standard Metropolis MC procedure. The structural properties of the polymer chain are computed by averaging over the statistically representative set of configurations. As a result of many such iterations, the intramolecular structure is determined self-consistently. Using the hybrid MC/RISM method, extensive studies have been made of static properties of flexible polymer chains surrounded by LJ particles with purely repulsive interactions between the particles and chain beads. Also, direct molecular dynamics simulations have been carried out and have demonstrated that the hybrid MC/RISM approach gives a quite accurate prediction for condensed-phase effects.     

Thermoluminescence characterization of functionalized grafted polymers and its application for radiation dosimetry at low doses

Functionalized polymers were prepared by radiation-induced graft copolymerization of binary monomer system acrylic acid/acrylamide (AAc/AAm) onto low-density polyethylene (LDPE) and polypropylene (PP) films using direct radiation-grafting technique. Sulfonation was carried out for the prepared grafted copolymers using concentrated sulfuric acid (97%) at 60 ^°C for 15 min. The grafted and sulfonated grafted films found to have good properties such as thermal stability and hydrophilic properties. The sulfonated grafted films found to have a better hydrophilic character than the grafted ones due to ionic character resulted by this conversion. The thermoluminescence (TL) characteristics of a set of grafted and sulfonated films have been studied with regard to their use as off-line dosimeters in radiotherapy. The structural characterization has been performed by means of infrared spectroscopy. Their TL responses have been tested with radiotherapy beams of ⁶⁰Co photons in the dose range 0.1–7 Gy. The dosimetric characterization has yielded a very good reproducibility and is independent of the radiation energy. The TL signal is not influenced by the dose rate and exhibits a very low thermal fading. Moreover, the sensitivity of the samples compares favorably with that of the standard TLD100 dosimeters. Finally, at the same dose, the TL response for LDPE-g-P (AAm/AAc) films is higher than the PP-$g$-P(AAm$/$AAc), and the sulfonated grafted films are more sensitive to radiation than the grafted ones.     

Optical properties of hybrid polymers as barrier materials

The development of high barrier films for the encapsulation of organic electronics devices onto flexible polymeric substrates is attracting a considerable scientific interest, since it is important to protect the organic semiconductor layers of these devices from corrosion due to atmospheric gas molecule permeation. The barrier layers for encapsulation consist of a sequence of inorganic and hybrid polymer thin films that are deposited onto flexible polymeric substrates, such as polyethylene terephthalate (PET). In addition to their barrier response, these multilayer systems should also exhibit high transparency and good adhesion between the hybrid polymer and inorganic layers. The knowledge of their optical properties and the correlation of the optical response with their structure and the final barrier response are of major importance since it will contribute towards the optimization of their functionality. In this work, the optical properties of hybrid polymers deposited onto silicon oxide inorganic thin films that were grown onto flexible polymeric substrates, have been investigated by the use of spectroscopic ellipsometry in a wide spectral region from the infrared to the visible-ultra violet. As it has been found, the increase of the solid content in the hybrid polymers is associated with a reduction in the refractive index values. This behavior can be correlated to a lower density of the hybrid polymer, and furthermore to a poor barrier response, due to the less cohesive inorganic-organic bonding network. Finally, from the investigation of the optical response of the hybrid polymers in the IR spectral region has revealed information on their bonding structure that has been discussed together with their barrier response.