Synthesis and Characterization A Novel Thermally Stable Schiff Base Oligomer: Investigation of Conductivity Properties

Yılmaz Baran N. JOTCSA. 2018; 5(2): 607-620. RESEARCH ARTıCLE Synthesis and Characterization of A Novel Thermally Stable Schiff Base Oligomer: Investigation of Conductivity Properties Nuray Yılmaz Baran* Aksaray University, Technical Vocational School, Department of Chemistry Technology, 68100, Aksaray, TURKEY Abstract: This paper presents the design of a novel Schiff base, 2,2'-{[1,2-di(pyridin-2yl)ethane-1,2-diylidene]bis(azanylylidene)}diphenol (2,2’-DBD), and its oligophenol, Oligo(2,2'-{[1,2-di(pyridin-2-yl)ethane-1,2-diylidene]bis(azanylylidene)}diphenol) O(2,2’-DBD), synthesized with NaOCl and O2 oxidants by oxidative polycondensation reaction in aqueous alkaline media. The effects of oxidant type, polymerization temperature, and time on oligomer yield were determined. Characterization of the structures of the synthesized (2,2’-DBD) and O(2,2’-DBD) were done by UV-Vis, FTIR and 1HNMR techniques. Also, thermal degradations of the monomer and oligomer were investigated TG-DTG analysis and it was determined that the oligomer thermally stable up to 1200 °C. Additionally, electrical conductivity of the oligomer was improved by doping with iodine at 20 °C and the conductivity of the oligomer reached to 9x10-4 S/cm by increasing 107 at the end of the 48 h doping time fold according to its undoped form. Keywords: Conductivity, oxidative polycondensation, oligophenols, Schiff base polymers. Submitted: November 22, 2017. Accepted: March 27 2018. Cite this: .Yılmaz Baran N. Synthesis and Characterization of A Novel Thermally Stable Schiff Base Oligomer: Investigation of Conductivity Properties. JOTCSA. 2018;5(2):607–20. DOI: http://dx.doi.org/10.18596/jotcsa.351460. *Corresponding author. E-mail: . Tel.:+90 382 2882028; Fax: +90 382 2882125. 607 Yılmaz Baran N. JOTCSA. 2018; 5(2): 607-620. RESEARCH ARTıCLE INTRODUCTION Schiff base polymers, also known as polyazomethines, have drawn attention of researchers to design thermally resistant (1) and semiconductive (2) materials. They are promising materials for several applications such as photorefractive holographic materials (PRHMs) (3), solar cells (4) (SCs), organic light emitting diodes (OLEDs) (5), and organic field effect transistors (OFETs) (6). Researchers have enhanced superior properties of the Schiff base polymers by adding different functional groups to their structures (7-11). One of these polymers are Schiff base polymers containing phenol groups. These polymers have excellent properties such as high thermal stability (12), bonding ability to metals (13), electrochemical (14, 15), antimicrobial (11, 16), semiconductive (11, 15), and superior optical (14) properties. Although several methods have been used (17, 18) to synthesize Schiff base polymers containing phenol groups, oxidative polycondensation method have been frequently preferred due to its superior advantages such as cheapness of the used oxidants (NaOCl, H2O2, O2), to synthesize polymers with high solubility, moderate reaction conditions and release of eco-friendly by products (NaCl and H2O) (12, 19). In this study, a novel Schiff base oligomer, O(2,2’-DBD), was produced with NaOCl, and O2 oxidants by oxidative polycondensation of 2,2’-DBD monomer in an aqueous alkaline media. The effects of oxidant types, polymerization temperature, and time on oligomer yield were investigated. UV-Vis, FTIR, 1H-NMR techniques were used for verifying of structures of 2,2’-DBD and O(2,2’-DBD). Additionally, thermal degradations of monomer and oligomer were determined by TG-DTG. Also, the changing of electrical conductivity of O(2,2’-DBD) were monitored by doping with iodine at 20 °C with increasing doping time. EXPERIMENTAL Materials 2-Aminophenol, 2,2’-pyridil, potassium hydroxide (KOH), hydrochloric acid (HCl, 37%), iodine, acetone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), methanol, ethyl acetate, ethanol, n-methylpyrrolidone, 1,4-dioxane, n-heptane and tetrahydrofuran were purchased from Merck Chem. Co. (Germany). Also, sodium hypochlorite (NaOCl, 15% aqu.) was supplied from Birpa Co. (Turkey). Method Synthesis of 2,2’-DBD Production of 2,2’-DBD monomer was carried out by condensation of 2-aminophenol and 2,2’pyridil (Scheme 1). Solution of 2,2’-pyridil (0,01 mol, 2.12 g) prepared in 40 mL of methanol 608 Yılmaz Baran N. JOTCSA. 2018; 5(2): 607-620. RESEARCH ARTıCLE was added on the solution of 2-aminophenol (0.02 mol, 2.18 g) in 15 mL methanol. The prepared mixture was continuously stirred at 40 °C for 24 h. After the reaction was completed, ice watersalt mixture was added on the obtained brown solution and the yellow product precipitated. Then it was filtered out and rinsed with cold methanol. The product was recrystallized from n-heptane for purification. (Yield: 86%; melting point: 147 °C.) Synthesis of O(2,2’-DBD) O(2,2’-DBD) was synthesized using NaOCl (15%), and air O2 oxidants by oxidative polycondensation reaction of 2,2’-DBD in aqueous alkaline media. Firstly, 2,2’-DBD (1 mmol, 0,394 g) was dissolved in aqueous solution of KOH (10%, 1 mmol) under nitrogen atmosphere at the temperature which the polymerization reaction would be carried out. Then, 1 mmol NaOCl was added drop by drop. Nitrogen was passed during the reaction. When aerial O2 was used to be an oxidant, after the dissolution of monomer was completed, nitrogen gas was discontinued. At the end of the desired polymerization time, the cooled polymerization solution to room temperature was precipitated by neutralizing with HCl (37%). The product was filtered, washed with hot water and methanol, and then dried in oven. Oligomer yield was determined using Equation 1: Yield (%) = 𝑊p 𝑊m × 100 Where Wp and Wm show the oligomer and initial monomer weights, respectively. Scheme 1. Synthesis of 2,2’-DBD and O(2,2’-DBD). 609 (1) Yılmaz Baran N. JOTCSA. 2018; 5(2): 607-620. RESEARCH ARTıCLE Characterization UV-Vis spectra of 2,2’-DBD and O(2,2’-DBD) were got in DMSO in a 260-700 nm wavelength range using Shimadzu UV-1700 PharmaSpec UV-Visible Spectrophotometer. FTIR spectra of the products were obtained by Perkin Elmer FTIR Spectrometer in the frequency range of 4000-650 cm-1. To clarify the molecular structures of the 2,2’-DBD and O(2,2’-DBD), 1H-NMR spectra were acquired from their solutions in DMSO by Bruker Avance 500 MHz NMR. Thermal degradations of the products were determined by TG/DTG techniques recorded in the temperature range of 30-1200 °C with a heating rate of 10 °C/min using EXSTAR S11 7300 thermal analyzer under nitrogen atmosphere with a platinum crucible. To determine molecular weight values of the oligomer, Shimadzu Prominence Gel Permeation Chromatography which was fitted out a Nucleogel GPC 103-5 VA300/7.7 column was employed (eluent: DMF, flow rate: 0.5 mL/min, temperature: 40 °C). Surface resistivity values of the oligomer were measured from the prepared sample pellets 2 mm thick and 1.3 cm in diameter under a hydraulic pressure (1687.2 kg/cm2) by two probe t (...truncated)