Enhanced Microwave Synthesis: Fine-Tuning of Polyaniline Polymerization

Abstract

A series of energy-and time-efficient enhanced microwave syntheses (EMS) of polyaniline (PANI) have been performed and are discussed herein. The syntheses were performed at different microwave power levels while keeping the reaction system at a constant temperature of 24 ± 1 °C, with the samples extracted after 10 min of reaction. Molecular weights were determined with GPC (gel permeation chromatography) and showed that the molecular weight of the microwave-generated materials depends on applied power: the higher the power level, the greater the molecular weight. The chemical structure was investigated by FTIR and UV−vis spectroscopy, while the morphology was imaged using scanning electron microscopy (SEM). FTIR and UV−vis spectroscopy confirmed the formation of PANI, and SEM indicated the presence of a mixed morphology, with a prevalence of nanofibers with different aspect ratios. The conductivity of the samples (ca. 3−3.5 S cm −1 ) was found to be relatively independent of microwave power levels. Surface area measurements were carried out using Brunauer−Emmett−Teller (BET) method, with samples synthesized using EMS showing higher surface areas when compared with conventionally synthesized PANI. The fact that molecular weight depends on applied power implies that microwaveenhanced synthesis can be used to fine-tune PANI reaction conditions, directing it toward specific properties.

Microwave intensified synthesis of regular shaped sodium bisulfate crystal

“…On the other hand, the obtained n(HSO 4 -)s with linear structure is beneficial for orderly stacking in the continuous alternating electromagnetic field [20], resulting in oriented attachment layer by layer and preventing aggregation growth to irregular particles [20,40]. These are the possible reasons for rapid crystal growth accompanied with fine morphology.…”

“…However, the size of ʟ-Alanine crystals increase when utilizing microwave heating [15]. The consequences resulted from the nucleation and crystal growth in microwave were different because the properties of material such as structural and dielectric properties were different [20,21]. Therefore, the mechanism of microwave effects on crystal formation aroused ever-lasting interests and is significant in rational modulation of crystal morphology.…”

Self-assembly of nanostructures obtained in a microwave-assisted oxidative polymerization of aniline

“…The specific thermal effects include superheating phenomena, different distribution of heat compared with classical heating and different heating of phases present in the same system producing 'hot spots'. In order to explain the formation mechanism of nanostructured PANIs in MW, we hypothesized that two reaction stages existed, both affected by microwave irradiation: (a) nucleation and (b) chain growth (polymerization) [35]. The first stage is based on instantaneous local heating that develops conditions for homogeneous nucleation.…”

“…Besides this, PANI materials have been synthesised on a large scale using the EMS scheme [3,27]. Also, it has been shown that the EMS yields specific nanostructured materials which open possibilities for upscaled production of polymeric conducting nanostructures [3,35]. At the same time, various parameters such as a MW power, reaction time, concentrations of reagents, pH etc.…”

“…can be used to tune various chemical and physical properties of PANI such as molecular weight, morphology, conductivity etc. [35]. We have previously reported studies of the selfassembly of various nanostructures synthesized at relatively low temperatures and at initial medium pH based on a conventional aniline oxidative polymerization approach [18][19][20][21][38][39][40][41].…”