Self-Diffusion in Binary Blends of Cyclic and Linear Polymers

A lattice model is used to estimate the self-diffusivity of entangled cyclic and linear polymers in blends of varying compositions. To interpret simulation results, we suggest a minimal constraint release model for the motion of a cyclic polymer infiltrated by neighboring linear chains. Both the simulation and recently reported experimental data on entangled DNA solutions support the simple model over a wide range of blend compositions, concentrations, and molecular weights

Identification of Topological Constraints in Entangled Polymer Melts Using the Bond-Fluctuation Model

We propose an algorithm to locate individual entanglements along chains, equilibrated using the bond-fluctuation lattice model. The algorithm identifies entanglements as local deviations of the primitive path from the shortest possible path between beads on a chain that are on lattice sites. For well-entangled chains (number of beads, N ≥ 125), the average number of entanglements enumerated using the proposed method is in excellent agreement with the number of entanglements per chain inferred using the ensemble-averaged primitive path length 〈Lpp〉 and mean-squared end-to-end distance 〈R2〉 of the chains, namely Z = 〈Lpp〉2/〈R2〉. As an application of this method, we show that the elimination of an entanglement releases, approximately, one additional entanglement. This implies a value of α = 1.03 ± 0.02 for the “dilution exponent” relating entanglement density ρent to polymer concentration c via ρent ∝ c1+α and is consistent with the description of entanglements as binary contacts

Spontaneous CdTe → Alloy → CdS Transition of Stabilizer-Depleted CdTe Nanoparticles Induced by EDTA

CdTe nanoparticles stabilized by l-cysteine are chemically transformed into CdS nanoparticles of the same diameter via an intermediate CdTeS alloy without any auxiliary source of sulfur. The reaction is induced by ethylenediaminetetraacetic acid dipotassium salt dehydrate (EDTA), which was demonstrated experimentally to act as a catalyst by partially removing thiol stabilizers from the nanoparticle surface. It is hypothesized that addition of EDTA facilitates Te2- release, and oxidation of Te2- drives the nanoparticle transition process. Unlike many reports on reactions catalyzed by nanocolloids, this is likely to be the first observation of a catalytic reaction in which nanoparticles function as a substrate rather than a catalyst. It opens new pathways for the synthesis of novel nanoscale II−VI and other semiconductors and represents an interesting case of chemical processes in nanocolloids with reactivity increased by depletion of the surface layer of thiol stabilizers. This includes but is not limited to accurate control over the particle composition and crystallization rate. The slow rate of the CdTe → alloy → CdS transition is important for minimizing defects in the crystal lattice and results in a substantial increase of the quantum yield of photoluminescence over the course of the transition

Spontaneous Transformation of CdTe Nanoparticles into Angled Te Nanocrystals: From Particles and Rods to Checkmarks, X-Marks, and Other Unusual Shapes

CdTe nanoparticles spontaneously transform into the branched Te nanocrystals with the unique, highly anisotropic shape of checkmarks after partial removal of the stabilizers of l-cysteine. The Te checkmarks are made in a relatively high yield and uniformity; the length of the arms is ca. 150 nm, whereas the angle between the arms is 74°. Subsequent growth of the particle yields mothlike nanocrystals retaining geometrical anisotropy. Unlike the previous synthesis methods of branched nanocrystals, they are formed via a merger of individual rod-shaped crystallites. High-energy crystal faces on their apexes act as the sticky points causing the particles to join in the ends. This is the first demonstration of spontaneous transformation of binary semiconductor particles into highly anisotropic nanocolloids in an angled conformation. The end reactivity of starting Te rods can be used both for bottom-up fabrication of nanoscale electronics and relatively safe and nontoxic method of synthesis of Te-based optical and other materials