Microwave‐assisted green approach for graft copolymerization of l‐lactic acid onto starch

Abstract

Poly L-lactic acid grafted starch (St-g-PLA) copolymers were directly synthesized under microwave irradiation by using sodium hydroxide (NaOH) and stannous 2-ethyl hexanoate acting as a catalyst, without the use of toxic solvents. The product were characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance ( 13 C CP/MAS NMR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA-DTG). SEM analysis indicated that microwave heating had a considerable effect on the interfacial adhesion between PLA and starch. Thermogravimetric analysis (TGA-DTG) revealed that copolymers exhibited better thermal stability. Maximum PLA grafting was achieved with the following reaction conditions: 450W microwave power, monomer ratio of 1:5 and 0.4M of NaOH. This study demonstrates that it is possible to obtain St-g-PLA copolymers with better processing characteristics and smaller sizes via microwave-assisted synthesis. The applied procedure is an interesting "green" synthesis method for the production of biodegradable materials used in a diverse range of applications. V C 2015Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42937.

Synthesis, characterization, and antioxidant activity of caffeic‐acid‐grafted corn starch

“…These results indicated the semi‐crystalline nature of corn starch had changed into amorphous character after grafting with caffeic acid. The elimination of starch crystalline peaks in caffeic acid‐g‐starch was mainly due to the fact that the original starch granules were broken down during gelatinization . In addition, the intermolecular hydrogen bonding of starch might be greatly destroyed during grafting processes.…”

Fabrication of starch‐g‐poly(l‐lactic acid) biocomposite films: Effects of the shear‐mixing and reactive‐extrusion conditions

“…The appearance of two peaks at 1228 and 2994 cm −1 , which were due to the OH bending and CH aliphatic groups of the PLA fragment compared to starch, strongly showed that the grafting of the PLA chains onto starch was successfully carried out through the application of shear‐mixing and reactive‐extrusion methods [Figure (a)]. The grafted PLA molar fraction in the St‐ g ‐PLA copolymers was calculated with the absorption bands of starch (1227 cm −1 for St CH 2 peak) and LA units [1736 cm −1 for LA CO ester peak; Figure (a)] . It was clear that with the application of shear mixing and reactive extrusion, a higher grafted PLA molar percentage (maximum 62 mol % PLA) was achieved compared to the previous copolymerization methods .…”

“…Actually, this study was a continuation of our previous studies presenting processing under a high shear rate to enhance the miscibility between starch and PLA for the fabrication of highly dispersed graft copolymers and their nanocomposites without the addition of any additives and conventional solvents by means of a high‐shear reactor and a twin‐screw extrusion system . The thermal stability, morphologies, and mechanical and permeability properties of the films were studied as a function of the starch–LA ratio and nanoclay content variations.…”

“…reported maleic anhydride as a nontoxic compatibilizer to enhance the compatibility of PLA/starch blends during the extrusion process. Moreover, in our previous works, we reported the graft copolymerization of l ‐lactic acid (LA) onto starch [poly( l ‐lactic acid) grafted starch (St‐ g ‐PLA)] with a supercritical carbon dioxide (CO 2 ) medium and microwave irradiation methods without any toxic solvent or environmentally hazardous components. Recently, nanofillers have been used to improve the physical properties (thermal stability, permeability properties, etc.)…”

Modification of starch by graft copolymerization

“…Stannous octoate can also be used to graft poly( l ‐lactide) on gelatinized starch (water was removed before grafting reaction) with lactide or lactic acid (formation of l ‐lactide in situ in the reaction medium) as the substrate. This grafting reaction was performed under various conditions on gelatinized starch: microwave heating (GP = 62%) , supercritical carbon dioxide medium (GP = 21–52%) , ionic liquid as the solvent (GE = 30%) . In the previous studies, XRD analysis showed that the grafting reaction modified the crystallinity of native starch and polylactide.…”