Dafna Knani

Determination of plasticizers efficiency for nylon by molecular modeling

Polyamides are semicrystalline polymers useful in a wide range of applications in the plastics industry. Some applications require higher flexibility and workability of the polyamides, therefore, plasticizers are added to ease compounding and processing procedures and produce the desired product properties. The goal of this study was to estimate plasticizers efficiency in plasticizing Nylon 66/6 copolymer (molar ratio 80/20, respectively) using computational tools and to compare the calculated estimations to experimental results. Four plasticizers were studied: glycerin mono stearate, benzene sulfonamide, methyl 4-hydroxybenzoate (M4HB), and diethylhexyl phthalate. Plasticizers efficiency was determined by calculating cohesive energy density, solubility parameters, free volume and interaction intensities of pristine nylon, and the nylon–plasticizer blends. It was found that the efficiency of the plasticizers increases with the degree of interaction intensity between the plasticizer and polymer chains and that M4HB molecules cause the largest changes in free volume. This finding correlates with the experimental results, based on reduction of polymer glass transition temperature (Tg). The highest calculated plasticization efficiency was obtained for M4HB, for which the decrease in Tg was the most significant.

Molecular modeling study of CO2 plasticization and sorption onto absorbable polyesters

Drug delivery systems are often made of porous polymer matrices. One method used to prepare a foamed polymer matrix is the controlled expansion of saturated polymers process in which amorphous polymer is exposed to CO2 at high pressure with a significant lowering of glass transition temperature. This plasticizing effect allows us to process temperature-sensitive polymers at relatively low temperatures. In the present study, computational tools were applied to estimate plasticizing effect of CO2 and calculate CO2- and H2O-loading capacities for three absorbable polyesters: polycaprolactone and two copolymers of (poly-d,l-lactid-co-glycolid)-co-polyethylenglycol. Plasticization caused by CO2 was estimated by solubility parameter and radial distribution function at several CO2 concentrations and by enlargement of free volume detected by mean square displacement of helium atoms, calculated after dynamic simulation. It was found that the maximal values of the solubility parameter and density can serve as a tool to predict saturation concentration. The loading capacities of the biopolymers that were preloaded with CO2 molecules were significantly higher than those of the nontreated polymers. Similar results were obtained for H2O molecules loading.

Simulation of DBS, DBS-COOH, and DBS-CONHNH2 as Hydrogelators

The organic gelator 1,3(R):2,4(S)-dibenzylidene-d-sorbitol (DBS) self-organizes to form a 3D network at relatively low concentrations in a variety of nonpolar organic solvents and polymer melt. DBS could be transformed into a hydrogelator by introduction of hydrophilic groups, which facilitate its self-assembly in an aqueous medium. In this work, we have investigated the hydrogelators DBS-COOH and DBS-CONHNH2 and the organogelator DBS by molecular modeling. We have used quantum mechanics (QM) to elucidate the preferred geometry of one molecule and a dimer of each of the gelators and molecular dynamics (MD) to simulate the pure gelators and their mixtures with water. The results of the simulation indicate that the interaction between DBS-COOH molecules is the strongest of the three and its water compatibility is the highest. Therefore, DBS-COOH seems to be a better hydrogelator than DBS-CONHNH2 and DBS. Intermolecular H-bonding interactions are formed between DBS, DBS-COOH, and DBS-CONHNH2 molecules as pure substances, and they dramatically decrease in the presence of water. In contrast, the intramolecular interactions increase in water. This result indicates that in aqueous environment the molecular structure tends to be more rigid and fixed in the preferred conformation. The most significant intramolecular interaction is formed between O3 acetal and H-O6 groups. Due to the H-bonds, DBS, DBS-COOH, and DBS-CONHNH2 molecules form a rigid structure similar to that of liquid crystal forming molecules, which might explain their tendency to create nanofibrils. It was found that the aromatic rings do not contribute significantly to the inter- and intramolecular interactions. Their main role is probably to stiffen the molecular structure.