Search results for "PLASTICS"
showing 10 items of 2724 documents
Chiral polyisocyanates, a special class of helical polymers
2001
Abstract Polyisocyanates (nylon 1) offer a lot of fascinating properties due to their dynamic helical conformation, which gives rise to orientational phenomena (large persistence length) or chiral amplification. This paper summarizes new results on polyisocyanates that appeared after the review by Bur and Fetters [Chem Rev 76 (1976) 727]. It starts with an overview of new synthetic methods, and then describes chiral polyisocyanates, their properties and the theoretical concepts developed to understand the experimental findings. Thereafter concentrated solutions (gels and LC phases) as well as photochromic polyisocyanates are discussed in detail. Finally, solid phase composite materials from…
The Electrostatic Expansion of Linear Polyelectrolytes: Effects of Gegenions, Co-ions, and Hydrophobicity
1997
The molar mass and ionic strength dependence of the dimensions of hydrophobically modified poly(vinylpyridinium) cations are demonstrated to be almost perfectly described by a theoretical expression derived on the basis of only excluded volume considerations. Generally, the effective charge density of the polyions decreases significantly with increasing hydrophobicity and with increasing polarizability of the gegenions. Unexpectedly, the intrinsic excluded volume effect which becomes dominating at high ionic strength not only depends on the hydrophobicity of the polyion but also significantly increases with decreasing polarizability of the gegenions (i.e., if the iodide gegenions are replac…
Effect of Chain Stiffness on the Adsorption Transition of Polymers
2013
Polymers grafted with one chain end to an impenetrable flat hard wall which attracts the monomers with a short-range adsorption potential (of strength e) are studied by large scale Monte Carlo simulations, using the pruned–enriched Rosenbluth method (PERM). Chain lengths up to N = 25600 steps are considered, and the intrinsic flexibility of the chain is varied via an energy penalty for nonzero bond angles, eb. Choosing qb = exp(−eb/kBT) in the range from qb = 1 (fully flexible chains) to qb = 0.005 (rather stiff chains with a persistence length of about lp=52 lattice spacings), the adsorption transition is found to vary from about e/kBTc ≈ 0.286 to e/kBTc ≈ 0.011, confirming the theoretical…
Understanding the Multiple Length Scales Describing the Structure of Bottle-brush Polymers by Monte Carlo Simulation Methods
2011
Bottle-brush polymers contain a long flexible macromolecule as a backbone to which flexible side chains are grafted. Through the choice of the grafting density and the length of the side chains the local stiffness of this cylindrical molecular brush can be controlled, but a quantitative understanding of these phenomena is lacking. Monte Carlo simulation results are presented and discussed which address this issue, extractingmesoscopic length scales (such as the cross-sectional radius, persistence length, and contour length of these objects). Large-scale simulations of the bond fluctuation model are combined with simulations of the simple selfavoiding walk (SAW) model with flexibility contro…
Persistence Length of Cylindrical Brush Molecules Measured by Atomic Force Microscopy
2006
Mechanical properties of single cylindrical polymer brushes with polyisopropylacrylamide (PNIPAM) side chains deposited on mica were probed by atomic force microscopy. Visualization and stretching of individual molecules in aqueous solution clearly reveal the semiflexible nature of the cylindrical macromolecules. Imaging of the brushes on mica and inferring lp from a 〈R2〉 vs L plot results in an average persistence length of lp = 29 ± 3 nm, assuming the chains adopt their equilibrium conformation on the surface. Stretching experiments suggest that an exact determination of the persistence length using force extension curves is impeded by the contribution of the side-chain elasticity. Modeli…
Ejection of a Polymer Chain from a Nanopore: Theory and Computer Experiment
2010
We consider the ejection dynamics of a flexible polymer chain out of confined environment. This situation arises in different physical contexts, including a flexible synthetic polymer partially confined in a nanopore and a viral genome partially ejected from its capsid. We describe the chain release from confinement both analytically and by means of dynamic Monte Carlo simulation. We find two distinct regimes of ejection dynamics depending on whether the chain is fully or partially confined. Partially confined chains are ejected from a pore of length L and diameter D after a typical time τ ∝ L2D5/3, regardless of their contour length N. The process is driven by a constant force f ≈ 5kBT/D a…
Near-coincidence lattice method for the determination of epitaxy strains during oxidation of metals
2000
Abstract A theoretical method is proposed to evaluate the strains due to epitaxy between a metal and its oxide. Based on Bollmann’s approach of two adjoining grains, it uses the quantitative texture analysis of the two materials separated by the phase boundary. Our study of the Zr/ZrO 2 and Mo/MoO 3 systems reveals strong correlations between the criterion of best fit proposed by Bollmann and the orientation distribution function obtained by a quantitative texture analysis. The results of this study are used in a thermo-mechano-chemical simulation of the oxidation process of zirconium, which leads to this observation: two different zirconia orientations induce two different oxidation kineti…
Interdiffusion in blends of polystyrene and polymethylstyrene studied by light scattering after temperature jumps across the phase boundary
1992
Abstract We describe a simple light scattering set-up for measuring interdiffusion coefficients D in polymer blends by generating spinodal decomposition and subsequent dissolution after temperature jumps across the phase boundary. In blends of polystyrene and polymethylstyrene (random copolymer of 60% m-methylstyrene and 40% p-methylstyrene) D values were obtained between 10−11 and 10−15 cm2s−1 at temperatures up to 50 K above the upper critical solution temperature. The results are discussed in relation to tracer diffusion in the same system.
Microfluidic Synthesis of Actuating Microparticles from a Thiol-Ene Based Main-Chain Liquid Crystalline Elastomer.
2016
In this article the microfluidic synthesis of strongly actuating particles on the basis of a liquid crystalline main-chain elastomer is presented. The synthesis is carried out in a capillary-based co-flow microreactor by photo-initiated thiol-ene click chemistry of a liquid crystalline monomer mixture. These microparticles exhibit a deformation from a spherical to a rod-like shape during the thermal-initiated phase transition of the liquid crystalline elastomer (LCE) at which the particles’ aspect ratio is almost doubled. Repeated contraction cycles confirm the complete reversibility of the particles’ actuation properties. The transition temperature of the LCE, the temperature range of the …
ABC and BAC triblock copolymers - morphological engineering by variation of the block sequence
1997
Different morphologies and phase transitions of ternary triblock copolymers with different block sequences (ABC and BAC) are discussed qualitatively. The interplay between interfacial energies and elastic energies of the different blocks leads to various morphologies and enhances or prevents mixing of different blocks, depending on composition and topology.