Abstract
Starch, an abundant and low-cost plant-based glucopolymer, has great potential to replace carbon-based polymers in various materials. In order to optimize its functional properties for bioplastics applications chemical groups need to be introduced on the free hydroxyl groups in a controlled manner, so an understanding of the resulting structure-properties relationships is therefore essential. The purpose of this work was to study the multiscale structure of highly-acetylated (degree of substitution, 0.4 < DS ≤ 3) and etherified starches by using an original combination of experimental strategies and methodologies. The molecular structure and substituents repartition were investigated by developing new sample preparation strategies for specific analysis including Asymmetrical Flow Field Flow Fractionation associated with Multiangle Laser Light Scattering, Nuclear Magnetic Resonance (NMR), Raman and Time of Flight Secondary Ion Mass spectroscopies. Molar mass decrease and specific ways of chain breakage due to modification were pointed out and are correlated to the amylose content. The amorphous structuration was revealed by solid-state NMR. This original broad analytical approach allowed for the first time a large characterization of highly-acetylated starches insoluble in aqueous solvents. This strategy, then applied to characterize etherified starches, opens the way to correlate the structure to the properties of such insoluble starch-based materials.
Highlights
Starch is, after cellulose and hemicelluloses, one of the most abundant carbohydrates in plants.Its semi-crystalline granules are mainly composed of two glucopolysaccharides: amylose, a quasi-linear chain of d-glucosyl units linked in α-1,4, and amylopectin which is constituted by a complex arborescent arrangement of linear chains of d-glucosyl units linked in α-1,4 with about 5–6% of branching points in Molecules 2020, 25, 2827; doi:10.3390/molecules25122827 www.mdpi.com/journal/moleculesMolecules 2020, 25, 2827 α-1,6 [1]
In order to develop a new general strategy for the multiscale structural characterization of chemically modified starches insoluble in water and with high molar mass, we focused first on one acetylated sample obtained from waxy maize starch (WMS), i.e., without amylose
The degree of substitution (DS) of the acetylated waxy maize starch (AWMS) was determined by three different methods: titration as previously described [22], liquid-state 1 H-Nuclear Magnetic Resonance (NMR) in dimethylsulfoxide-d6 (DMSO-d6 ) and solid-state
Summary
After cellulose and hemicelluloses, one of the most abundant carbohydrates in plants.Its semi-crystalline granules are mainly composed of two glucopolysaccharides: amylose, a quasi-linear chain of d-glucosyl units linked in α-1,4, and amylopectin which is constituted by a complex arborescent arrangement of linear chains of d-glucosyl units linked in α-1,4 with about 5–6% of branching points in Molecules 2020, 25, 2827; doi:10.3390/molecules25122827 www.mdpi.com/journal/moleculesMolecules 2020, 25, 2827 α-1,6 [1]. After cellulose and hemicelluloses, one of the most abundant carbohydrates in plants. A way to improve its physicochemical properties is introducing functional groups on the free hydroxyl groups by esterification or etherification with a maximum theoretical degree of substitution (DS) of 3 [4]. The properties of such chemically modified starches depend on the starch structure, the DS, the type of grafted groups and their repartition on the macromolecules [5] but these relations are not well known because of structure analysis limitations
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