Crowding of Polymer Coils and Demixing in Nanoparticle-Polymer Mixtures

The Asakura-Oosawa-Vrij (AOV) model of colloid-polymer mixtures idealizes nonadsorbing polymers as effective spheres that are fixed in size and impenetrable to hard particles. Real polymer coils, however, are intrinsically polydisperse in size (radius of gyration) and may be penetrated by smaller particles. Crowding by nanoparticles can affect the size distribution of polymer coils, thereby modifying effective depletion interactions and thermodynamic stability. To analyse the influence of crowding on polymer conformations and demixing phase behaviour, we adapt the AOV model to mixtures of nanoparticles and ideal, penetrable polymer coils that can vary in size. We perform Gibbs ensemble Monte Carlo simulations, including trial nanoparticle-polymer overlaps and variations in radius of gyration. Results are compared with predictions of free-volume theory. Simulation and theory consistently predict that ideal polymers are compressed by nanoparticles and that compressibility and penetrability stabilise nanoparticle-polymer mixtures.Comment: 18 pages, 4 figure

Fusion of secretory vesicles isolated from rat liver

Secretory vesicles isolated from rat liver were found to fuse after exposure to Ca2+. Vescle fusion is characterized by the occurrence of twinned vesicles with a continuous cleavage plane between two vesicles in freeze-fracture electron microscopy. The number of fused vesicles increases with increasing Ca2+-concentrations and is half maximal around 10–6 m. Other divalent cations (Ba2+, Sr2+, and Mg2+) were ineffective. Mg2+ inhibits Ca2+-induced fusion. Therefore, the fusion of secretory vesiclesin vitro is Ca2+ specific and exhibits properties similar to the exocytotic process of various secretory cells. Various substances affecting secretionin vivo (microtubular inhibitors, local anethetics, ionophores) were tested for their effect on membrane fusion in our system. The fusion of isolated secretory vesicles from liver was found to differ from that of pure phospholipid membranes in its temperature dependence, in its much lower requirement for Ca2+, and in its Ca2+-specificity. Chemical and enzymatic modifications of the vesicle membrane indicate that glycoproteins may account for these differences

Inflammatory bowel disease in children and adolescents in Italy: data from the pediatric national IBD register (1996-2003).

Abstract BACKGROUND: The purpose was to assess in Italy the clinical features at diagnosis of inflammatory bowel disease (IBD) in children. METHODS: In 1996 an IBD register of disease onset was established on a national scale. RESULTS: Up to the end of 2003, 1576 cases of pediatric IBD were recorded: 810 (52%) ulcerative colitis (UC), 635 (40%) Crohn's disease (CD), and 131 (8%) indeterminate colitis (IC). In the period 1996-2003 an increase of IBD incidence from 0.89 to 1.39/10(5) inhabitants aged <18 years was observed. IBD was more frequent among children aged between 6 and 12 years (57%) but 20% of patients had onset of the disease under 6 years of age; 28 patients were <1 year of age. Overall, 11% had 1 or more family members with IBD. The mean interval between onset of symptoms and diagnosis was higher in CD (10.1 months) and IC (9 months) versus UC (5.8 months). Extended colitis was the most frequent form in UC and ileocolic involvement the most frequent in CD. Upper intestinal tract involvement was present in 11% of CD patients. IC locations were similar to those of UC. Bloody diarrhea and abdominal pain were the most frequent symptoms in UC and IC, and abdominal pain and diarrhea in CD. Extraintestinal symptoms were more frequent in CD than in UC. CONCLUSIONS: The IBD incidence in children and adolescents in Italy shows an increasing trend for all 3 pathologies. UC diagnoses exceeded CD

I quesiti di Matematica INVALSI 2013 sulle percentuali: dall’analisi degli errori al ripensamento dell’azione didattica

Dopo una breve descrizione di una esperienza di ricerca/azione svolta con alcuni insegnanti di scuola secondaria di secondo grado e focalizzata su attività di problem solving, sviluppate in classe in modalità di collaborazione tra pari, si presentano quattro quesiti sulle percentuali tratti dalle prove INVALSI 2013 per le classi seconde delle scuole secondarie di secondo grado. Si presentano e commentano gli esiti ottenuti nelle classi del progetto e si confrontano con gli esiti nazionali. Si commentano in particolare le possibili strategie risolutive e gli errori più significativi emersi dall’analisi dei protocolli. Si propongono infine alcune osservazioni nella direzione di una didattica della proporzionalità diretta (e delle percentuali) che privilegi decisamente l’aspetto funzionale piuttosto che la classica teoria delle proporzion

Molecular Dynamic Study of the Structure and Dynamics of Polymer Melt at Solid Surfaces

We investigate the dynamic and static properties of a polymer melt near solid surfaces. The melt, composed of linear chains, is confined between two solid walls which one of the walls being repulsive whereas the opposite, attractive wall, is characterized by different degrees of roughness caused by an array of short perpendicular pillars with variable grafting density. We demonstrate that the conformations of chains in the melt at the interfaces do not depend on substrate/polymer interactions and practically coincide with the conformations of a single end-grafted chain under critical adsorption conditions, in agreement with Silberberg?s hypothesis. This agreement is corroborated by the analysis of the size distributions of trains, loops and tails of melt chains at the walls which are found to be perfectly described by distributions pertaining to end-grafted single chains at critical adsorption. The adsorbed amount at the attractive bottom surface is found to scale with √macromolecule length as Γ ∝ N regardless of adsorption strength. We also find that the pressure of the melt PN decreases as PN − P∞ ∝ N −1 (where P∞ is the extrapolated pressure for N → ∞) with growing length of the chains N whereas the surface tension γ at both walls is found to decline as γN ∝ N −2/3 . Eventually, a study of the polymer dynamics at the rough interface reveals that surface roughness leads to dramatic drop of the coefficient for lateral diffusion whenever the separation between obstacles (neighboring pillars) becomes less than ≈ 2Rg where Rg is the radius of gyration of chains in the bulk.Fil: Sarabadani, Jalal. Max Planck Institute for Polymer Research; AlemaniaFil: Milchev, Andrey. Max Planck Institute for Polymer Research; Alemania. Bulgarian Academy of Sciences; BulgariaFil: de Virgilis, Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Vilgis, Thomas A.. Max Planck Institute for Polymer Research; Alemani