An NMR Study On Nucleation Mechanism Of Synthesis Of Nanostructured Polyaniline In SDS Micelles

Nucleation,a common phenomenon throughout the nature,is the process with which the formation of new phase starts which determines the forthcoming growth pathways and the accompanied kinetics of the nuclei.Understanding the nucleation mechanism is essential for the identification of the anisotropic nature of the crystals and accordingly the formation mechanism of crystals with different nanostructures.Owing to transient processes and only trace amount of the nascent nuclei in the nucleation,detecting these processes experimentally becomes extremely difficult.Considering species such as clusters,nuclei as well as the reaction environment may all feel the changes in nucleation,abundant research has been performed in this respect.However,little attention has paid on solvent molecules in the reaction systems.In this thesis,we have focused on the water molecules in the hydration layers of the micelles to dig out the information in nucleation.Locating at the outer-most layer of the micelle,water molecules in the hydration layer of the micelle feel the external stimulation at first which then attend various structural arrangements initiated by the formation of the nuclei all the time,therefore potentially can be served as the most sensible probe for the whole process.Unfortunately the violent structural rearrangement in nucleation makes the desired signal decays so fast that only one average signal from the solvent can be obtained.Practically the micelle was kept standing still for long time so that the micelle in large sizes has the enhanced capability to withstand strong attacks from outside,elongating the wanted signal from decaying into the noisy background.In this way,the nucleation mechanism of polyaniline(PANI)in several types of micelles has been figured out.The micelles include spherical,rod-like,bilayered and bicontinuous ones composed of aniline and sodium dodecyl sulfate(SDS).The latter two are induced by camphor sulfonic acid(CSA)in different sample addition style.Major conclusions include:(1)In rod-like micelles,a nitrenium intermediate is found to be the original cluster in the nucleation.The nuclei are generated via sequential processes involving a translocation of the protonated tetramer to the aqueous bulk,dissociation of SDS micelles,and deprotonation to induce the confusion of the dissociated micelles and intermolecular packing of the oligomers.In addition the signal evolution of the water in hydration layer provides a rich family of information on aspects such as the nuclei sizes,the growth kinetics and so forth.(2)The transition from spherical to bilayered micelles through ellipsoid intermediate induced by CSA is investigated by NMR.Predominant procedures for the micellar transition include i)the head groups of SDS interact with protons from CSA at first which triggers SDS micelles transformed from spherical to ellipsoid(aniline and SDS exhibit characteristic chemical shift anisotropy corresponding to asymmetric character of ellipsoid micelle);ii)further protonation of aniline induces the micelle to turn into lamellar structures.In this way,aniline in bilayer is initiated to generate polyaniline nanosheets incorporating SDS bilayers in sandwich structures,beneficial for improving their capacitive behavior.(3)The transition from rod-liketo bicontinuous micelles and instantaneous nucleation in bicontinuous micelles for the formation of porous polyaniline are investigated.Three steps for the formation of porous polyaniline have been derived:i)aniline oligomers are generated and stabilized by water;ii)a nucleation occurs at the hydrocarbon/water interface;iii)the morphological transportation from rigid SDS framework to the product of polyaniline.The data of monitoring for the micellar transition indicates that solid CSA and phosphate can both induce the transition via H+…? interaction between proton and aniline with strong p-7c conjugation,and such p-? conjugation is further confirmed from 2D-NMR spectra.(4)The polymerization of aniline in spherical and rod-like SDS micelles is investigated viain-siti1H NMR experiments,and the morphologies and structures of products are characterized.After initiator added in reaction solution,reaction system is shaken to obtain uniform mixing.The data of monitoring indicates that:i)the protons released in reaction and the in-situ generated counterions induce the micellar transitions;ii)there are two ways for the association between proton and aniline,the H+…?(aromatic ring of aniline)interaction and the H+…NH2(lone pair electrons of aniline)interaction.The morphologies of products are nanofibres and nanosheets from the two types of micelles respectively,and the products from rod-like ones are of para-coupling dominated structure according to FT-IR spectrum.(5)The formation of interconnected nanostructured polyaniline in the ionic micelles composed of aniline,CSA and SDS is studied.The broadening of 1H NMR linewidths is induced by ionic association of protonated aniline and head group of SDS,and these SDS chains are all semi-rigid.The data of monitoring indicates that aniline oligomers are generated adjacent to hydrocarbon/water interface,and then translocate to the aqueous bulk and proceed in the growth period.At reletively high concentrations of SDS,the influences from nucleation(translocation of oligomers)are weakened and the control of nanostructure by micellar templates are improved.