Citric acid (CA)-modified hydrogels from corn starch and chitosan were synthesized using a semidry condition. This strategy has great benefits of friendly environment because of the absence of organic solvents and compatible with the industrial process. The hydrogel blends were prepared with starch/chitosan ratios of 75/25, 50/50, and 25/75. The thermal stability, morphology, water absorption, weight loss in water, and methylene blue absorption were determined. Multi-carboxyl structure of CA could result in a chemical cross-linking reaction between starch, chitosan, and CA. The cross-linking reaction between free hydroxyl groups of starch, amino groups of chitosan, and carboxyl groups of CA has been confirmed by attenuated total reflectance infrared (ATR-IR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) analysis. The water absorption properties of CA-modified hydrogel blends were increased significantly compared with the native starch and chitosan. Moreover, the hydrogel blends modified with CA showed good water resistance and gel content. The morphology study confirmed the complete chemical cross-linking and porous structure of hydrogel blends. The hydrogel blend with the starch/chitosan ratio of 50/50 presented powerful absorption of methylene blue as well as chemical cross-linking reaction and dense structure. In sum, the hydrogel blend comprising 50% starch and 50% chitosan has the potential to be applied for water maintaining at large areas, for example, in agriculture. K E Y W O R D S chitosan, citric acid, crosslinking, hydrogel, starch 1 | INTRODUCTION Hydrogels are extremely hydrophilic polymer networks, either synthetic (petrochemical based) or natural (biopolymers), which are usually stabilized through covalent bonds or noncovalent interactions among the macromolecule chains known as cross-linking. 1-4 Hydrogels will swell when placed in water and can retain an extensive fraction of water within their spatial structures without dissolving. 5 A gel structure forms within the polymer melt after cross-linking. Currently, a wide range of natural polymer hydrogels such as starch, chitosan, collagen, and cellulose have been described as suitable biomaterials for pharmaceutical and biomedical applications because of their biocompatibility, low-toxicity, and biodegradability. 6-10Starch and its derivatives have attracted many researchers for expanding biopolymer industries since they are cost-effective, renewable, abundant in nature, and biodegradable. 11-14 However, some limitations exist in the properties of starch including poor mechanical properties, high hydrophilicity, fragility, and poor dimensional stability. [15][16][17][18][19] To improve the quality and also broaden the application areas of starch, these drawbacks need to be reduced significantly.Chitosan, isolated from chitin, is another polysaccharide that is abundant in nature and available in large quantities. Chitosan is biocompatible, biodegradable, biofunctional, and antimicrobial. [20][21][22] Mo...

The poor mechanical properties and high water solubility of biodegradable thermoplastic starch (TPS) represent the main disadvantages of TPS in many applications. In this work, TPS film was prepared from a water solution of corn starch modified by 5 wt% dialdehyde starch (DAS) as crosslinking agent and 3 wt% montmorillonite (MMT) as reinforcing additive. Interactions occurring in the TPS films were investigated by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, XRD, DSC, dynamic mechanical thermal analysis (DMTA) and TGA. The results obtained fom FTIR spectroscopy and DSC suggest the formation of hydrogen bond interactions between the hydroxyl group of starch, DAS, the MMT layers and glycerol. DMTA indicated that the relaxation of films with DAS and MMT appears in a higher and broader temperature range due to the starch backbone stiffness; the extreme increase in the storage modulus confirmed the suggested interactions. The determination of the weight loss of the films in water indicated a significant increase of the water resistance of TPS due to incorporation of DAS and MMT. Changes in mechanical properties of the films containing DAS and clay were determined, showing a substantial increase in tensile strength from 2.7 to 6.7 MPa, while Young's modulus increased by 15 times for TPS modified with 5% DAS and 3% MMT. Therefore, the outcomes of this study confirmed that DAS is a suitable biomacromolecule crosslinker for starch and can significantly enhance TPS and TPS/MMT properties. © 2019 Society of Chemical Industry

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