Interfacial reciprocal grafting by free radical reactions in polymer blends

Recent developments in the field of reactive compatibilization of polymer blends prepared by melt processing focus on the addition of low molecular weight compounds. This work deals with in situ compatibilization through the formation of graft or crosslinked copolymers at the interface. Mixtures of semicrystalline hydrocarbon polymers have been subjected to free radical reactivity, in a co-rotating twin screw extruder (ZSK 30) in a single step. The particular system, high density polyethylene and polyamide 6, was blended in the presence of a peroxide and a reactive bifunctional monomer, maleic anhydride. Because of a combined effect, the reaction appears to occur mainly at the interface, where the resulting grafted copolymer acts as an anchor for the final stabilization of the biphasic system. Different analytical techniques, such as differential scanning calorimetry, scanning electron microscopy and tensile testing, helped in characterizing the resulting blends and confirmed the high level of interfacial grafting and the expected improvement in mechanical properties.     

Phospholipid coated poly(lactic acid) microspheres for the delivery of LHRH analogues

The aim of this work was to develop alternative peptide‐loaded microspheres using liposphere formulation—a lipid based microdispersion system. This formulation represents a new type of lipid or polymer‐based encapsulation system developed for parenteral and topical drug delivery of bioactive compounds. Our strategy was to utilize the liposphere formulation to improve the entrapment efficiency and release profile of triptorelin and leuprolide [luteinizing hormone–releasing hormone (LHRH) analogues] in vitro. Peptides (2% w/w) were loaded into lipospheres contained of polylactic acid (PLA) or poly(lactic‐co‐glycolic acid) (PLGA) with several types of phospholipids. The effects of polymer and phospholipid type and concentration, method of preparation and solvents on the liposphere characteristics, particle size, surface and bulk structure, drug diffusion rate, and erosion rate of the polymeric matrix were studied. The use of L ‐PLA (M_w = 2000) and hydrogenated soybean phosphatidylcholine (HSPC) with phospholipid–polymer ratio of 1 : 6 w/w, was the most efficient composition that formed lipospheres of particle size in the range of 10 µm with most of the phospholipid embedded on the particles surface. In a typical procedure, peptides were dissolved in N‐methyl‐2‐pyrrolidone (NMP), and dispersed in a solution of polymer and phospholipids in a mixture of NMP and chloroform with the use of 0.1% poly(vinyl alcohol) (PVA) as the emulsified aqueous medium. Uniform microspheres were prepared after solution was mixed at 2000 rpm at room temperature for 30 min. Using this formulation, the entrapment efficiency of LHRH analogues in lipospheres was up to 80%, and the peptides were released for more than 30 days. Copyright © 2002 John Wiley & Sons, Ltd.     

Teacher and students’ joint production of a reversible fraction conception

Within a constructivist perspective, I conducted a teaching experiment with two fourth graders to study how a teacher and students can jointly produce the reversible fraction conception. Ongoing and retrospective analysis of the data revealed the non-trivial process by which students can abstract multiplicative reasoning about fractions. The study articulates a conception in a developmental sequence of iteration-based fraction conceptions and the teacher’s role in fostering such a conception in students.     

Biodegradable polycaprolactone–polyanhydrides blends

Biodegradable polymers based on a blend of polycaprolactone (PCL) and aliphatic polyanhydrides with various monomer lengths were prepared to obtain desired polymer blends for use as drug carriers. The physicochemical, mechanical, and drug‐release properties of these blends were investigated by various techniques to evaluate the uniformity degree of the polymer blends to establish their potential applications in drug delivery. The results demonstrated that the heat of fusion (ΔH) of the polyanhydride or the blend is increased in relation to the length of the aliphatic chain. However, the blends had different properties than pure polyanhydride, and the crystallization degree of the blends, as expressed by the ΔH, decreased in relation to the ΔH of the pure polyanhydride. Drug‐release studies from blends of PCL and aliphatic polyanhydrides demonstrated first‐order kinetics of the release rate. Polymer degradation was independent at the polyanhydride monomer length. On the basis of theoretical calculation of the interaction factor, a blend of PCL and poly(dodecanedeoic anhydride) was chosen for further elucidation of its thermal, mechanical, and degradation properties. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3781–3787, 2003     

Heterogeneous Electrocatalytic Oxygen Evolution Reaction by a Sol-Gel Electrode with Entrapped Na3[Ru2(μ-CO3)4]: The Effect of NaHCO3

The Na₃[Ru₂(μ-CO₃)₄] complex is acting as a water oxidation catalyst in a homogeneous system. Due to the significance of heterogeneous systems and the effect of bicarbonate on the kinetic, we studied the bicarbonate effect on the heterogeneous electrocatalyst by entrapping the Na₃[Ru₂(μ-CO₃)₄] complex in a sol-gel matrix. We have developed two types of sol-gel electrodes, which differ by the precursor, and are demonstrating their stability over a minimum of 200 electrochemical cycles. The pH increases affected the currents and k_cat for both types of electrodes, and their hydrophobicity, which was obtained from the precursor type, influenced the electrocatalytic process rate. The results indicate that NaHCO₃ has an important role in the catalytic activity of the presented heterogeneous systems; without NaHCO₃, the diffusing species is probably OH⁻, which undergoes diffusion via the Grotthuss mechanism. To the best of our knowledge, this is the first study to present a simple and fast one-step entrapment process for the Na₃[Ru₂(μ-CO₃)₄] complex by the sol-gel method under standard laboratory conditions. The results contribute to optimizing the WSP, ultimately helping expand the usage of hydrogen as a green and more readily available energy source.     

On the Galois groups of the 2-class towers of some imaginary quadratic fields

Let and knr,2 be the maximal unramified 2-extension of k. To show that knr,2/k is finite, Michael Bush gave 8 possible presentations of finite groups for G=Gal(knr,2/k). However, his methods did not further isolate G. We eliminate 4 of the possibilities, and explain how to isolate G, although carrying out the latter strategy is beyond current technological capabilities. We also discuss related examples.     

Evolution of Unit Fraction Conceptions in Two Fifth-Graders with a Learning Disability: An Exploratory Study

The literature seems limited in what is known about conceptual processes that underlie evolution of students with learning disabilities (SLD) conceptions of fractions. This exploratory study examines how a foundational scheme of unit fractions (1/n) may evolve through the mathematical activity of two fifth grade girls. We analyze data segments from episodes conducted during a teaching experiment grounded in the activity of iterating estimates of one person's equal share. Our findings include four distinct conceptual stages: (1) No Conception of the Nature of Adjustment to the Magnitude of a Unit Fraction, (2) Evolving Anticipation of the Nature of Adjustment but not of its Relative Amount, (3) Anticipation of the Nature of Adjustment with an Evolving Partial Amount, and (4) a Dual Anticipation of the Nature and Amount of Adjustment. Findings demonstrate each girl was able to use her constructed scheme to successfully solve and reason about novel problems. We discuss the need for more research to confirm the findings from this study, while offering a conjecture of the possibilities for more SLDs to advance their conceptions of fractions in future interventions.     

Studies on Electronic Effects in O‐, N‐ and S‐Chelated Ruthenium Olefin‐Metathesis Catalysts

A short overview on the structural design of the Hoveyda–Grubbs‐type ruthenium initiators chelated through oxygen, nitrogen or sulfur atoms is presented. Our aim was to compare and contrast O‐, N‐ and S‐chelated ruthenium complexes to better understand the impact of electron‐withdrawing and ‐donating substituents on the geometry and activity of the ruthenium complexes and to gain further insight into the trans–cis isomerisation process of the S‐chelated complexes. To evaluate the different effects of chelating heteroatoms and to probe electronic effects on sulfur‐ and nitrogen‐chelated latent catalysts, we synthesised a series of novel complexes. These catalysts were compared against two well‐known oxygen‐chelated initiators and a sulfoxide‐chelated complex. The structures of the new complexes have been determined by single‐crystal X‐ray diffraction and analysed to search for correlations between the structural features and activity. The replacement of the oxygen‐chelating atom by a sulfur or nitrogen atom resulted in catalysts that were inert at room temperature for typical ring‐closing metathesis (RCM) and cross‐metathesis reactions and showed catalytic activity only at higher temperatures. Furthermore, one nitrogen‐chelated initiator demonstrated thermo‐switchable behaviour in RCM reactions, similar to its sulfur‐chelated counterparts.