Dispersion and stability study of carbon nanotubes in pH and temperature responsive polymeric matrix: Experiment and dispersion-corrected DFT study

Abstract

We report the temperature and pH responsive dispersion of multi-walled carbon nanotubes (MWNTs) in poly(N-isopropylacrylamide) (PNIPAM) and poly-l-lysine hydrobromide (PLL) based polymeric matrix. An excellent dispersion of MWNTs in PNIPAM was observed below its lower critical solution temperature (LCST; 32 °C), however, aggregation of MWNTs observed above this critical temperature. The pH effect on MWNTs dispersion was also analysed, where at room temperature PLL/PNIPAM mixture demonstrates an excellent dispersion of MWNTs at pH 7 and 10, whereas, an agglomeration was observed at pH 4. PLL transforms from ionized coil to α-helix conformation with increase in pH (more globular shape), thus the solution demonstrates agglomeration of MWNT at low pH and well dispersed MWNTs at high pH. Density functional theory based analysis was also carried out to concurrent changes in the interfacial morphology, where PNIPAM shows better interaction with CNT than PLL including a higher strength of interaction. This can be attributed to the geometry curvature of PNIPAM that enables strong van der Waals interaction with the CNT surface, thus, PNIPAM may serve as a better dispersion agent than PLL. The quick responsive nature of PNIPAM and PLL shows notable changes with variation in temperature and pH, thus may be helpful to fabricate CNT based smart devices.