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

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

Controlled surface initiated polymerization of N-isopropylacrylamide from polycaprolactone substrates for regulating cell attachment and detachment

Poly(ε-caprolactone) (PCL) substrates were modified with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes to direct and control cellular attachment and detachment. Prior to brush growth, the surface of PCL was activated by a diamine to allow for initiator coupling. Infrared spectra taken before and after cell culturing demonstrated the covalently attached nature of the PNIPAM brushes. PCL is a biocompatible polymer and to prove that the modifications described above did not change this characteristic property, a cell attachment/detachment study was carried out. The modified substrates showed a lower cell attachment when compared to PCL alone and to PCL films modified with the initiator. The possibility to detach the cells in the form of a sheet was proved using PNIPAM-modified PCL films by lowering the temperature to 25 °C. No relevant detachment was shown by the unmodified or by the initiator modified surfaces. This confirmed that the detachment was temperature dependent and not connected to other factors such as polymer swelling. These functionalized polymeric films can find applications as smart cell culture systems in regenerative medicine applications