Detailed mechanism of aniline nucleation into more conductive nanofibers

• The carboxylate groups number determines nucleation type of aniline monomers.
• The higher number of nucleation points applies heterogeneous nucleation.
• The under cooling temperature can inhibit strong heterogeneous nucleation.
• 3D interconnected structure shows higher conductivity compared to 1D nanofibers.

A large number of studies have been focused on synthesis optimization of conductive hydrogels for medical and sensor approaches. Heterogeneous nucleation of aniline monomers is resulted from secondary nucleation that not only depends on number of reaction groups but also system temperature. In this study, 4 °C was examined as a critical temperature applying limited mode of heterogeneous nucleation. Our observations showed this undercooling condition can behave in a cooperative manner with the number of carboxylate groups. Herein, polyacrylic acid-based hydrogel was obtained by different ratios of acrylic acid (AA)/N,N′-methylenebisacrylamide (MBAA) as 18.77 and 14.03. The based hydrogels exposed carboxylate terminates as nucleation centers for aniline cations. The smaller ratio of AA/MBAA produced the based-hydrogels with the higher number of reaction points for aniline monomers. Accordingly, the higher nucleation number of corresponding monomers leads to larger ratio of aniline/Ammonium persulfate (APS) at growth phase. Therefore, secondary nucleation of aniline monomers was limited by undercooling condition at 4° C and 3D interconnected nanofibers resulted by limited heterogeneous nucleation. The corresponding nanofibers showed higher conductivity value compared to 1D nanofibers which was resulted as a function of homogeneous nucleation.

Based-hydrogel of higher concentration of acrylic acid (AA) as 1.5 M with 0.006 mol% N,N′-methylenebisacrylamide (MBAA) provides higher density of carboxylate terminates due to stronger crosslinking potency. The more density of reactive groups involves higher aniline monomers over nucleation phase. The higher nucleating aniline molecules enhances the ratio of aniline/ammonium persulphate (APS) leading into suppression of strong mode of secondary nucleation. In this manner, the limited heterogeneous nucleation occurs and the resulted nanofibers exposes 3D interconnected network. Consequently, the corresponding nanofibers demonstrate more conductivity as a function of higher electron density.This mechanism is opposite for based-hydrogel of lower concentration of AA as 1 M with 0.003 mol% MBAA. The low crosslinking ability reduces carboxylate groups for aniline monomers establishing of homogeneous nucleation. This type of nucleation produces 1D nanofibers with lower conductivity.Figure optionsDownload as PowerPoint slide