An ab initio and force field study on the conformation and chain flexibility of the dichlorophosphazene trimer

Ab initio molecular orbital calculations have been used to study the conformation, valence electron charge density, and chain flexibility of a dichlorophosphazene trimer (CH3[NP(Cl2)]3CH3). The calculations were carried out at the restricted Hartree-Fock level with the 6-31 G* basis set. The dichlorophosphazene trimer adopts a planar transcis conformation. The valence electron charge distribution indicates strong charge separations along the backbone of the molecule, and is in agreement with Dewar's island delocalization model for bonding in linear and cyclic phosphazenes. In order to determine the height of the torsional barrier (2,5 kcal/mol), the torsional potential of a central P-N bond of the trimer was studied with a rigid rotor scan and geometry optimizations of selected rotamers. The flexibility of the P-N-P bond angle contributes significantly to the chain flexibility. Based on the results of the ab initio calculations, an empirical force field for the dichlorophosphazene trimer was developed. The energy expression includes bond stretch, angle bend, electrostatic, van der Waals, and torsional potential terms. A relaxed scan with the force field achieves good agreement with the ab initio results for the torsional potential in the vicinity of the stable conformation, and an excellent agreement with the ab initio results on changes in the P2N2P3 bond angle and the N1P2 - N2P3 dihedral angle during a full rotation around the N2 - P3 bond

Well-defined side-chain liquid-crystalline polysiloxanes

A route to well-defined side-chain liquid-crystalline polysiloxanes (ratio of weight-to number-average molar masses Mw/Mn < 1.2 is reported. Anionic ring-opening polymerization of pentamethylvinylcyclotrisiloxane yielded a poly(dimethylsiloxane-co-methylvinylsiloxane) backbone. A flexible disiloxane spacer was used to connect 4-(ω-alkenyloxy)-4'-cyanobiphenyl mesogenic molecules to the vinyl groups which belong to the backbone, leading to a side-chain liquid-crystalline polysiloxane (SCLCP) which has its mesogens distributed regularly along the main chain. Preliminary measurements indicate an electro-optic switching time s = 1 min at 20°C and 7 s at 32°C (dc, 5 V/µm))

Superstability of Surface Nanobubbles

Shock wave induced cavitation experiments and atomic force microscopy measurements of flat polyamide and hydrophobized silicon surfaces immersed in water are performed. It is shown that surface nanobubbles, present on these surfaces, do not act as nucleation sites for cavitation bubbles, in contrast to the expectation. This implies that surface nanobubbles are not just stable under ambient conditions but also under enormous reduction of the liquid pressure down to &#8722;6MPa. We denote this feature as superstability.Comment: 5 pages, 2 figure

Morphology of extruded high-density polyethylene pipes studied by atomic force microscopy

Atomic force microscopy (AFM) was used to study the structure of extruded polyethylene (PE) pipe. During extrusion, the outer surface of the pipe was cooled with water. Two cross sections, parallel and transverse to the extrusion direction, were examined in order to spatially follow the structural development during extrusion. The morphology revealed was spherulitic, and the spherulites had a mostly banded appearance when viewed under the AFM. We were not able to distinguish an oriented skin layer at the surface of the pipe, either by AFM or polarizing microscopy. The changes in the pipe's structure resulting from the cooling conditions were found to be rather gradual, and no clearly defined zones were observed. A slight orientation towards the extrusion direction was detected only in the area of the pipe crystallized under the lowest degree of undercooling. Measured spherulitic size, band period, and lamellae thickness showed a gradual increase in their values from the cooled to the noncooled surface of the pipe. Transmission electron microscopy (TEM) was used to verify the band period and lamellae thickness measurements done by AFM

Flame treatment of low-density polyethylene: Surface chemistry across the length scales

The relationship between surface chemistry and morphology of flame treated low-density polyethylene (LDPE) was studied by various characterization techniques across different length scales. The chemical composition of the surface was determined on the micrometer scale by X-ray photoelectron spectroscopy (XPS) as well as with time of flight secondary ion mass spectrometry (ToF-SIMS), while surface wettability was obtained through contact angle (CA) measurements on the millimeter scale. The surface concentration of hydroxyl, carbonyl and carboxyl groups, as a function of the “number” of the flame treatment passes (which is proportional to the treatment time) was obtained. Moreover, a correlation was found with chemical composition and polarity, emphasizing the role of oxygen-containing functional groups introduced during the treatment. Carboxyl functional groups were specifically identified by fluorescent labeling and the results were compared with the ToF-SIMS data. In addition, atomic force microscopy (AFM) was used to evaluate changes in surface topography and roughness on the nanometer to micrometer length scales. After flame treatment, water-soluble low molecular weight oxidized materials (LMWOM), which were generated as products of oxidation and chain scission of the LDPE surface, agglomerated into small topographical mounds that were visible in the AFM micrographs. After rinsing the flame treated samples with water and ethanol, bead-like nodular surface structures were observed. The ionization state of flame treated LDPE surfaces was monitored by chemical force microscopy (CFM). The effective surface pKa values of carboxylic acid (–COOH) obtained by AFM were revealed by chemical force titration curves and the effective surface pKa values were found to be around 6.\ud \u