Thixotropic behavior of metal-containing coordination polymers: Melt viscosity of neutral aliphatic polyesters with Zn carboxylates

The viscosity behavior of polymer melts containing complexes formed between the neutralized polyester poly(diethylene glycol-co-succinic acid) and Zn acetates is discussed. The melt viscosity of these materials increases with the concentration of metal ions, and shows strong thixotropy and shear thinning. This behavior is attributed to the formation of coordination bonds between the electron donor groups within the polyester chain, and empty coordination sites of the various Zn acetate salts. The coordination complexes were obtained in situ in the polymer melt, which contains well-dispersed ZnO, by adding an equimolar amount of CH3COOH. It is proposed that the shear applied to the polymer melt destroys the polar network of the coordination polymer at a rate that is greater than the rate of reformation of the coordination bonds for the sample returning back to equilibrium, following a shear deformation

Lattice Imaging of Self-Assembled Monolayers of Partially Fluorinated Disulfides and Thiols on Sputtered Gold by Atomic Force Microscopy

The structure of self-assembled monolayers (SAMs) of various fluorinated disulfides, perfluoroalkylamide thiols, and a mixed alkyl perfluoroalkylamide disulfide on sputtered gold was studied by atomic force microscopy (AFM). AFM, performed both in air and in ethanol, revealed the monolayer structure with molecular resolution on the polycrystalline gold substrates. For all partially fluorinated disulfides containing ester groups, a hexagonal lattice with a lattice constant of 5.8-5.9 Å was found. A mixed alkyl perfluoroalkylamide disulfide formed a hexagonal lattice of a slightly larger lattice constant (6.1 Å), whereas the lattice observed for fluorinated thiols containing an amide group was either hexagonal (5.7-5.8 Å) or distorted hexatic (5.6, 6.2, 5.6 Å), depending on the length of the perfluoroalkane segment and the imaging force. The observed deviation from hexagonal symmetry is attributed to the distorting effect of hydrogen bonding between neighboring amide groups within the monolayer. For short perfluoroalkane segments the distortion is observed at low imaging forces, whereas for long perfluoroalkane segments significantly higher imaging forces are necessary in order to observe the distortion. The force dependence of the measured lattice symmetries for different chain lengths suggests that the AFM tip penetrates into the SAM and probes at least partially the interior of the SAM

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))