Abstract
The aim of this work was to prepare sugar alcohol-based deep eutectic solvents (DES) and test them as starch plasticizers. Thermoplastic starch (TPS) films were obtained via a simple and convenient thermocompression method. Influence of starch/DES premixtures conditioning (preheating, storage time) on TPS properties was investigated. TPS/sorbitol (S)-based DES exhibited similar tensile strength (TS) (8.6 MPa) but twice higher elongation at the break (ε) (33%) when compared with TPS plasticized only with S. Extra treatment, i.e., heating or prolonged storage time, facilitated starch/DES plasticizing. Starch with selected DES was also extruded and the influence of preconditioning and extrusion rotational speed were subsequently studied on thermocompressed films. Extrusion at 100 rpm led to films with TS up to ca. 10 MPa and ε up to 52%. Some differences in film samples morphology obtained via two processing methods were observed. X-ray diffractograms revealed that extruded samples exhibited a V-type peak at 18.2°, with intensity depending on plasticizer total molecular size. Applied techniques (mechanical tests, XRD, Dynamic Mechanical Analysis (DMA), FTIR-Attenuated Total Reflection (ATR), and moisture sorption) indicated that S-based DES forms stronger interactions with starch than glycerol (G) only used as conventional plasticizer, thus leading to better mechanical properties and inhibited tendency to starch recrystallization (studied up to one year).
Highlights
Starch is one of the most abundant natural polymers containing two fractions of α-d-glucose unit: amylose and amylopectin
Thermocompression tests were performed for starch with deep eutectic solvents (DES) and with individual polyols (G, X, S, and M), as well as with CC, for comparison their plasticizing ability (Table 3)
In other works related to starch processing with multicomponent plasticizers [13,30], mechanical properties parameters of Thermoplastic starch (TPS) plasticized with S in G presence had values between these obtained for TPS/S and TPS/G. ε values of TPS materials with DES presented in this work are higher than for materials with individual components
Summary
Starch is one of the most abundant natural polymers containing two fractions of α-d-glucose unit: amylose and amylopectin. Bonds with molar mass of 105 –106 g/mol. Amylopectin is highly branched polymer linked with α(1–4) and α(1–6) bonds with molar mass 107 –109 g/mol. Reactivity of their hydroxyls leads to wide spectra of production possibilities of derivatives with different applications [2,3], but on the other, strong interaction between chains and glass transition temperature (Tg ) of unmodified starch near its degradation hinder its processing as conventional thermoplastic materials. The types of plasticizers mentioned above exhibit some drawbacks: G allows to polysaccharide recrystallization and, leads to stiffness of Polymers 2019, 11, 1385; doi:10.3390/polym11091385 www.mdpi.com/journal/polymers
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