Effect of Dopant on the Nanostructured Morphology of Poly (1-naphthylamine) Synthesized by Template Free Method
© to the authors 2007
Received: 15 September 2007
Accepted: 30 November 2007
Published: 7 December 2007
The study reports some preliminary investigations on the template free synthesis of ascantly investigated polyaniline (PANI) derivative—poly (1-naphthylamine) (PNA) by template free method in presence as well as absence of hydrochloric acid (HCl) (dopant), using ferric chloride as oxidant. The polymerization was carried out in alcoholic medium. Polymerization of 1-naphthylamine (NPA) was confirmed by the FT-IR as well as UV–visible studies. The morphology and size of PNA particles was strongly influenced by the presence and absence of acid which was confirmed by transmission electron microscopy (TEM) studies.
KeywordsPoly (1-naphthylamine) Alcohol Transmission electron microscopy Morphology Nanostructure
Scientific and technological interest in studying nanomaterials has spurred to develop conducting polymeric nanostructures, using reliable and scalable synthetic methods to provide better performance of these materials in the established areas of corrosion, sensors, batteries, and EMI shielding [1–4]. Chemical oxidative polymerization of aniline is the traditional method for preparing polyaniline in bulk . In the aniline polymerization reaction, an acidic solution is needed to enhance the head-to-tail coupling between aniline monomers. Typically a strong mineral acid such as hydrochloric, sulphuric, nitric, perchloric or phosphoric acid is used at a high concentration (1.0 M) for the preparation of Polyaniline (PANI) . It has been reported that the diameter of the nanofibres so formed is strongly influenced by the dopant used in the polymerization . Polyaniline prepared by using HCl is highly aggregated and contains mostly irregularly shaped agglomerates which deteriorate the desired properties of the polymer. Substituted polyanilines continues to be an emerging research area of great interest since these polymers hold the potential to improve upon the properties of polyaniline. Scarce literature is available on the chemical synthesis of poly (1-naphthylamine) (PNA)—a polyaniline derivative. Moon et al.  first reported the chemical synthesis of poly (1-aminonaphthlaene) and poly (aminoanthracene) using H2O2/Fe2+ system. Shaffie et al.  carried out the chemical oxidative polymerization of poly (1-naphthylamine) using potassium persulphate, and the conductivity of the polymer was reported to be in the range of ∼0.83 Scm−1. Recently, Shan et al.  synthesized PNA via enzymatic polymerization using horse dish peroxidase. Surprisingly, none of the studies mentioned above have reported the nanoscale synthesis of PNA.
This study reports some preliminary investigations on the template free synthesis of nanostructured PNA with a view to obtain an agglomerate free nanostructured conducting polymer. The effect of hydrochloric acid on the agglomeration of PNA is investigated by spectral as well as morphological studies.
Chemicals: Naphthylamine (Loba Chemie, India) was purified prior to use. The monomer was sublimed at 120 °C and recrystallized in ethanol. Ethyl alcohol, cupric chloride,N-methyl pyrolidinone (NMP) (Qualigen, India) were of analytical grade and were used as such.
Synthesis of Poly (1-naphthylamine)
1-Naphthylamine (NPA) monomer (0.1 M) was dissolved in a mixture of ethyl alcohol (10 mL) and 1N HCl (10 mL) at room temperature. The solution was purged in nitrogen for 1 h. Cupric chloride (0.1 M) dissolved in ethyl alcohol (5 mL) was then added to the solution of 1-naphthylamine with slow stirring at 0 °C. A violet coloured dispersion appeared as polymerization progressed. The reactor flask was cooled to −5 °C to obtain a purple glassy phase under static conditions for 48 h between −5 and −7 °C. The purple-black-coloured glassy phase turned into suspension after holding for 30 min at room temperature. It was washed thoroughly with distilled water and methyl alcohol to remove oligomers, metal ions and other impurities. Further purification of the polymer was done through soxhlet extraction using methyl alcohol for a period of 16 h to remove oligomeric fractions and other impurities. Resulting powder was then dried under vacuum at 50 °C for 72 h. Similar procedure was adopted for the synthesis of PNA in ethanol medium without HCl.
FT-IR spectra of the powdered polymers were taken in the form of KBr pellets on spectrometer model Perkin Elmer 1750 FT-IR spectrophotometer (Perkin Elmer Cetus Instruments, Norwalk, CT, USA). UV–visible spectra were taken on Perkin Elmer lambda EZ-221 of the solutions of polymers prepared in NMP. Transmission electron micrographs were taken on Morgagni 268-D TEM, FEI, USA. The samples were prepared by depositing a drop of well diluted polymer suspension onto a carbon (1 0 0)-coated copper grid and dried in an oven at 55 °C for 2 h. Conductivity measurements were performed by standard four-probe method using Keithley DMM 2001 and EG&G Princeton Applied Research potentiostat model 362 as current source. Pressed pellets of polymers were obtained by subjecting the powder to a pressure of 50 kN. The error in resistance measurements under these conditions was less than 2%.
Result and Discussion
FT-IR Spectral Analysis
As compared to the above spectra, the FT-IR spectra of PNA synthesized in the presence of HCl, Fig. 1b, shows NH-stretching vibration peak centred at 3,370 cm−1 for a secondary amine. The absorption peaks of imines-stretching mode are observed at 1,654 and 1,638 cm−1 while the multiple peaks at 1,596 and 1,570 cm−1 are assigned to the N = Q = N, quinonoid ring, skeletal vibrations. The peaks at 1,508 and 1,452 cm−1 appear due to N–B–N, benzenoid ring, skeletal vibrations. The CN vibration shows up at 1,400 and 1,302 cm−1. The B–NH+ = Q and B–NH–B vibration peak is noticed at 1,156 cm−1. The presence of strong peak at 766 cm−1 is consistent with the polymerization of NPA through N–C(4) linkages while the peak at 790 cm−1 exhibits N–C(5) coupling between neighbouring PNA rings .
It can be concluded that the presence of acidic conditions strongly influences the conformation of the PNA chains. In absence of HCl, hydrogen bonding takes place between the PNA and ethanol. The PNA chains in this case, therefore, contain larger number of quinonoid units predominantly linked through N–C(4) linkages. This is evident from the spectra, Fig. 1a, which show the presence of more quinonoid vibration peaks than the benzenoid vibration peaks. However, all peaks in this case are well formed indicating a well-ordered conformation of PNA.
The synthesis of nanostructured PNA described is this article is very facile and robust which does not require any extra structural directing agents or template removing steps. Hydrochloric acid when used as a dopant plays a significant role in deciding the morphology of the nanostructure of PNA. The aggregation of nanoparticles can be prevented by avoiding the use of highly acidic dopants such as HCl as well as by using alcohol as a medium for polymerization of conducting polymers. These findings may provide valuable information in the template free synthesis of many other nanostructures. The investigations on influence of other parameters such as polymerization temperature, reaction time, mechanical agitation, and choice of oxidant, are under progress in our laboratory and will be published soon.
This work was funded by CSIR through grant No. 01/(1953)/04/EMR-II. The authors wish to thank the CSIR for its financial support.
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