Abstract
Green chemistry is the design of chemical processes that reduce or eliminate negative environmental impacts. The use and production of chemicals involve the reduction of waste products, non-toxic components, and improved efficiency. Green chemistry applies innovative scientific solutions in the use of new reagents, catalysts and non-classical modes of activation such as ultrasounds or microwaves. Kinetic behavior and non-thermal effect of poly(amic acid) synthesized from (6FDA) dianhydride and (BAPHF) diamine in a low microwave absorbing p-dioxane solvent at low temperature of 30, 50, 70 °C were studied, under conventional heating (CH), microwave (MW) and ultrasound irradiation (US). Results show that the polycondensation rate decreases (MW > US > CH) and that the increased rates observed with US and MW are due to decreased activation energies of the Arrhenius equation. Rate constant for a chemical process activated by conventional heating declines proportionally as the induction time increases, however, this behavior is not observed under microwave and ultrasound activation. We can say that in addition to the thermal microwave effect, a non-thermal microwave effect is present in the system.
Highlights
Polyimides (PI)s possess the cyclic imide and aromatic groups in the main chain and they are some of the best engineering plastics, because of their high thermal and oxidative stability, their good mechanical properties and excellent chemical resistance to dilute acids and organic solvents.Polyimides have been used in nanofiltration [1], hydrogen purification [2], fuel cells [3], adhesive [4], and solar cells [5].PIs are usually prepared by the so-called two-step method in which a dianhydride and a diamine are allowed to undergo condensation polymerization to form poly(amic acid) precursor, and subsequently the precursor is converted thermally or chemically into the final polyimide, but the reaction usually takes a long time, 20 hours or more
The formation of the poly(amic acid) structure under our experimental conditions was confirmed by the presence of infrared (IR) absorption bands located at: 3500–2500 cm−1 which indicates the existence of -OH group (−COOH), and 1711 cm−1 carbonyl group C=O (−COOH stretching vibration), 1658 cm−1 carbonyl group C=O (−CONH stretching vibration), 1539 cm−1 (N−H bending), 1500 cm−1 aromatic ring (C−C stretching), 1242 cm−1 indicates the existence of C−N bonds
The synthesis and characterization of poly(amic acid) obtained from 6FDA and BAPHF monomers in p-dioxane solvent under conventional heating (CH), microwave (MW) and ultrasound irradiation (US) in a relatively low temperature range were studied to gain a better understanding of the accelerated reaction rates observed under US and MW conditions
Summary
Polyimides (PI)s possess the cyclic imide and aromatic groups in the main chain and they are some of the best engineering plastics, because of their high thermal and oxidative stability, their good mechanical properties and excellent chemical resistance to dilute acids and organic solvents.Polyimides have been used in nanofiltration [1], hydrogen purification [2], fuel cells [3], adhesive [4], and solar cells [5].PIs are usually prepared by the so-called two-step method in which a dianhydride and a diamine are allowed to undergo condensation polymerization to form poly(amic acid) precursor, and subsequently the precursor is converted thermally or chemically into the final polyimide, but the reaction usually takes a long time, 20 hours or more. To find effective green chemistry alternative methods for synthesis of PIs, new techniques can be explored with a view to decreasing the reaction time and reaction temperature. Among these new techniques, the implementation of microwaves (MW) and ultrasound (US) energy are an emerging field of research. Ultrasound (US) has been applied in synthesis of inorganic-organic nanocomposite of polyacrylamide (PAM) and gamma-zirconium phosphate (γ-ZrP) [6], isoxazoles [7], polymeric materials as polyamide-6 (Nylon 6) [8,9] and polymethacrylate [10]. The use of microwave energy for the assembly of polyimides has had great acceptance, for the synthesis of poly(piromellitidimides) by means of monomers of nylon-salt-type [17,18], polyimides with nonlinear optical properties [19,20,21,22], poly (ester-imide) [23], and poly (amide-imide) [24,25,26,27]
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