Salt and pH effect on thermoresponsive behavior of multiwalled carbon nanotube (MWCNT)/poly(N-vinylcaprolactam) dispersion

The influence of different salts and pH on the stability of the aqueous dispersion of multiwalled carbon nanotubes (MWCNTs) non-covalently modified by thermosensitive poly(N-vinylcaprolactam) (PVCL) was studied. The PVCL/MWCNT dispersion was prepared by adding MWCNTs into aqueous PVCL solution with the subsequent ultrasonication and centrifugation. The morphology of MWCNTs and MWCNT/PVCL composites was characterized by TEM. The aggregation temperatures of the MWCNT/PVCL dispersion and the lower critical solution temperature (LCST) of PVCL aqueous solution were determined for different salt types and concentrations as well as pH in the temperature range between 20 and 60 °C. Specific ion effect on LCST of PVCL was analyzed in terms of the Hofmeister series. In the presence of kosmotropes, both the MWCNT/PVCL dispersion aggregation temperature and the LCST of PVCL are well consistent with the Hofmeister series of ions. However, opposite trends were observed for LCST of PVCL and the MWCNT/PVCL dispersion aggregation temperature when adding chaotropic anions. The dynamic light scattering (DLS) technique was used to study the PVCL aggregation kinetics at a temperature above the LCST at different pH values. These results demonstrate that specific thermoresponsive properties of the MWCNT/PVCL dispersion at pH < 3 correlate with the behavior of the individual PVCL macromolecules in acid media above the LCST.