Effect of stiffness on the phase behavior of cubic lattice chains

Gran canonica Monte Carlo (GCMC) simulazioni assistite da tecniche di riponderazione istogramma sono stati utilizzati per studiare l'effetto della flessibilità catena sul comportamento di soluzione fase di cubi catene reticolari corti con 4-32 segmenti. Ciò è stato fatto variando un parametro di rigidità gradualmente fino alla media calcolata end-to-end distanza avvicinato la lunghezza totale. Per entrambe le catene flessibili e rigide si è riscontrato che la temperatura critica, ottenuta tramite mista analisi dei campi di dimensioni finite, aumentata lunghezza della catena e la densità critica trasferisce a valori più bassi, in accordo con le osservazioni sperimentali. Il estrapolato lunghezza della catena infinita di temperatura critica è stata maggiore per le catene rigide. Ciò è stato attribuito il numero maggiore di favorevoli contatti intermolecolari tra le catene polimeriche più lunghe e / o rigidi. Temperature critiche ottenute in questo lavoro sono in eccellente accordo con le simulazioni al computer precedenti e le previsioni teoriche. E 'stato inoltre accertato che le buste fase di catene flessibili scesa al di sotto ed entro le controparti rigide. Ad alte densità, lungo ( r = 16 e R = 32) e catene rigidi mostrato una tendenza a formare strutture ordinate densi che non sono stati osservati in catene completamente flessibili o corto. Confrontando i valori di Flory x 1 ed x 2 parametri, ottenuti da attacchi dei diagrammi di fase calcolate a diversi gradi di rigidezza catena, si è concluso che, quando la separazione di fase si verifica, imballaggio aste rigide porta ad un cambiamento minore entropia e meno processo endotermico. Questi risultati supportano l'idea che, in sistemi polimerici, un aumento della rigidità della catena dorsale è equivalente ad un aumento della lunghezza della catena

Effect of stiffness on the micellization behavior of model H4T4 surfactant chains

The micellization behavior of a series of model surfactants, all with four head and tail groups (H4T4) but with different degrees of chain stiffness, was studied using grand canonical Monte Carlo simulations on a cubic lattice. The critical micelle concentration, micellar size, and thermodynamics of micellization were examined. In all cases investigated, the critical micelle concentration was found to increase with increasing temperature as observed for nonionic surfactants in apolar or slightly polar solvents. At a fixed reduced temperature and increasing chain stiffness, in agreement with previous observations, it was found that the critical micelle concentration decreased and the average micelle size increased. This behavior is qualitatively consistent with that experimentally observed when comparing hydrogenated and homologous fluorinated surfactants. Thermodynamic considerations based on the analysis of the temperature dependence of the critical micelle concentration indicated that both effects could be interpreted as arising from an increased number of heterocontacts between hydrophobic portions of stiff surfactants and a lower entropic cost on packing rigid chains. Structural analysis that was also based on considering the inner micellar radial dependence of the surfactant head and tail site fraction distributions suggested that, on stiffening the molecular backbone, the resulting micellar aggregates grew, without appreciable deviations from spherical symmetry. Stiffer surfactants led to a slightly denser micellar core because of better packing

Preparation of pH sensitive poly(vinilydenefluoride) porous membranes by grafting of acrylic acid assisted by supercritical carbon dioxide

Aim of this work is to study the preparation of pH sensitive membranes for biomedical applications via thermal induced graft polymerization of acrylic acid (AA) on poly(vinilydenefluoride) (PVDF) assisted by supercritical carbon dioxide (scCO2). Using scCO2 as a solvent and swelling agent, the monomer and initiator (benzoyl peroxide) could diffuse faster and distribute more uniformly into the polymer matrix. A better control of the final molecular architecture should be achieved because bimolecular grafting reactions are accelerated with respect to chain degradation processes. Commercial hydrophobic PVDF porous matrixes were selected as model membranes. Grafted polymers were characterized by FT-IR spectroscopy, field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA) and water permeability test at different pH values. Preliminary results indicate that the grafting degree can be tuned by controlling the monomer concentration and the carbon dioxide density. Moreover, the water permeation of grafted membranes decreases considerably as pH increases from 1 to 7, contrary to virgin membranes whose water permeability was substantially pH independent. Research activity is going on to study the microstructure of the membrane and to optimize the level and distribution of poly(AA) grafts

Copolymerization of Vinylidene Fluoride and Acrylic Acid in Supercritical Carbon Dioxide

The free-radical copolymerization of vinylidene fluoride (VDF) and acrylic acid (AA) was carried out in supercritical carbon dioxide using both precipitation and dispersion techniques in the presence of an ammonium carboxylate perfluoropolyether compound. Formation of a copolymer was confirmed by variable contact time CP MAS NMR spectroscopy. Macromolecular matrices were significantly enriched in AA with respect to the initial feed composition and we found that the nonfluorinated monomer has a much higher reactivity ratio with respect to VDF. The cumulative concentration, crystallinity, and water affinity of the synthesized copolymers could be modified changing the initial feed composition, the density of the polymerization mixture, the specific interfacial area of the polymer phase, and the polymerization time. The solubility of the macromolecular product in water was greatly affected by its composition