RESUMEN
Oligo(ethylene glycol) (OEG) side chains are widely used in donor-acceptor conjugated polymers (D-A CPs) and enable the polymers to dissolve and be processed in environmentally friendly and cost-effective nonchlorinated solvents, such as water. However, the OEG effect on the physical properties of D-A CPs has not been thoroughly studied and sometimes the results are controversial. In this study, two oligothiophene-isoindigo based conjugated polymers, P3TI and P4TI, are selected as model polymers to investigate the OEG effect. PnTI has octyl side chains on the oligothiophene unit and 2-hexyldecyl side chains on the isoindigo unit. The replacement of an alkyl side chain with OEG not only changes the optical and thermal properties but also the molecular arrangements of the polymers such as π-π d-spacing, crystallinity, and packing orientation. The domination of the crystallization behavior changes from the oligothiophene unit to the isoindigo unit when the bulky alkyl group is replaced by the flexible and linear OEG. The packing changes from edge-on to face-on orientation. The results are intriguing and provide new insights into this class of polymers.
RESUMEN
It is known that lecithin, a zwitterionic phospholipid, self-assembles into spherical reverse micelles in organic solvents. We have explored the effects of adding inorganic salts to lecithin organosols. Salts are insoluble in organic solvents, and hence their effects on reverse self-assembly have rarely been studied. Our studies show, however, that salts can indeed be dissolved in organic liquids in the presence of lecithin. More interestingly, salts of multivalent cations like calcium (Ca(2+)), magnesium (Mg(2+)), lanthanum (La(3+)), and cerium (Ce(3+)) greatly increase the viscosity of lecithin organosols and transform the samples into optically transparent organogels. In comparison, monovalent cations or transition-metal cations have negligible effect on reverse self-assembly. On the basis of data from small-angle neutron scattering (SANS), we show that gelation is accompanied by a nanostructural transition from spherical micelles to cylindrical micelles/filaments. The varying abilities of different cations to induce gelation is shown to correlate with their binding tendencies to the phosphocholine headgroups of lecithin. A two-component gelator such as lecithin/Ca(2+) could be attractive for applications due to its negligible cost and nontoxic nature. We demonstrate how such a gelator combination can convert a liquid fuel such as kerosene into a gel without the use of heat or shear. The same gel can also further be ungelled by addition of a few drops of alcohol.