BIODEGRADABLE POLYMER FILMS BASED ON CARBOXYMETHYL STARCH

ABSTRACT

In this research, sodium salt of carboxymethyl starch (Na-CMS) was obtained by chemical modification of corn starch in the solid phase. The influence of various factors on the degree of substitution of the resulting Na-CMS was studied. Na-CMS macromolecules were cross-linked with ions of multivalent metals, and film materials were obtained on the basis of the resulting polymer hydrogels. Previous studies have demonstrated that the transparency and elastic properties of polymer films are influenced by the type and concentration of metal ions used for macromolecular crosslinking.

АННОТАЦИЯ

В данной исследовательской работе была получена натриевая соль (Na-KMK) карбоксиметилкрахмала путем химической модификации кукурузного крахмала в твёрдой фазе. Изучено влияние различных факторов на степень замещения образующегося Na-KMK. Проведена сшивка макромолекул Na-KMK ионами многовалентных металлов и на основе образовавшихся полимерных гидрогелей получены пленочные материалы. Показано влияние природы и количества ионов металлов, использованных для сшивки полимера на прозрачность и эластичность полученных полимерных пленок.

Keywords: corn starch, carboxymethylation, carboxymethyl starch, degree of substitution, polymer films, transparency.

Ключевые слова: кукурузный крахмал, карбоксиметилирование, карбоксиметилкрахмал, степень замещения, полимерная пленка, прозрачность.

Introduction. Today, there is a strong focus worldwide on obtaining biodegradable materials (bioplastics) based on natural polymers. The reason is that currently nearly 450 million tons of polymer materials are produced worldwide, only 2% of which are biodegradable polymers, while the rest are synthetic polymers. Because most synthetic polymers do not break down into harmless substances under normal conditions, they are causing significant harm to the environment (water, soil). In recent years, this has become a global problem worldwide. For this reason, chemists are conducting research to replace certain portions of synthetic polymers with naturally biodegradable ones under natural conditions. Among naturally biodegradable polymers, starch has attracted considerable attention owing to its biodegradability and wide availability. It is a polymer produced in large quantities, with annual production exceeding one hundred million tons. Since it is mainly extracted from annual plants, its reserves are quickly replenished and under normal conditions it breaks down into harmless substances [1–5]. Therefore, research aimed at expanding the potential for using starch-based polymer films in food packaging, pharmaceuticals, and medicine is of pressing importance. Starch does not dissolve in water under ordinary conditions; at elevated temperatures it begins to break down rather than liquefy, which limits the possibility of producing film-like materials based on it. Therefore, in this work, the sodium salt of the water-soluble starch derivative, carboxymethyl starch (Na-CMS), was used to obtain polymer films. The aim of the research is the synthesis of the water-soluble starch derivative-sodium carboxymethyl starch (Na-CMS)-the preparation of polymer films based on it, and the study of their properties.

Methodology for conducting experiments

Synthesis of Na-CMS

In the work, the synthesis of sodium carboxymethyl starch was carried out via a solid-phase carboxymethylation reaction using starch isolated from local corn varieties. To this end, the starch was first activated by reaction with NaOH, and then carboxymethylation was carried out with the sodium salt of monochloracetic acid (Na-MCHA). The obtained product was dissolved and purified by recrystallization in ethanol, then dried to a constant mass. The exchange degree (ED) of the synthesised Na-CMS was calculated by the inverse titration method using the following equations [6]:

;       ; 

Here: ED – degree of substitution, 162 – molar mass of one glycosidic chain (grams per mole), nCOOH – number of COOH groups (mol), m_ED – mass of the dried carboxymethylated sample (g), 58 – increase in molecular mass per glucose unit due to the attachment of one carboxymethyl group; m_s – mass of the starch sample (g), W_water(%) – moisture content; V_b – volume of HCl spent to titrate the control sample (ml), V – volume of HCl spent to titrate the sample (ml), C_HCl – concentration of HCl used for titration (mol/l), 4 – the ratio between the total volume of the solution (100 ml) and the volume taken for titration (25 ml).

The IR spectra of the substances were obtained using a Nicolet iS50 spectrophotometer (Thermo Fisher Scientific) in the 4000–450 cm⁻¹ wave number range.

Na-CMS based polymer films were prepared by uniformly spreading a cast polymer solution onto a glass substrate and evaporating the solvent.

Results obtained and their analysis

Currently, there are several methods for synthesizing Na-CMS: in aqueous solution, in suspension (with organic solvents), and in the solid phase [7,8]. Typically, the  ED(exchange degree) of Na-CMS synthesized in solution or suspension is higher than that of the product obtained in the solid phase, because in these methods the reagents interact more effectively with each other. The solid-phase synthesis of Na-CMS is widely used in production due to its economic efficiency and the relatively simple nature of the synthesis technology. Therefore, in this work, the starch was carbosymethylated in the solid phase to synthesise Na-CMS. In the carboxymethylation reaction, the starch was first treated with NaOH to activate its functional groups (mercerization), and then this intermediate product was reacted with Na-MCHA (Figure 1).

Figure 1. Scheme for the carboxymethylation of starch in the solid phase

In the initial studies, the starting reagents were reacted in the dry state by mechanical mixing. The results showed that the exchange degree of the Na-CMS synthesized by this method is relatively low (not exceeding 0.15). For this reason, in subsequent experiments, the reaction mixture was moistened by spraying various solvents, water, and alcohol during mechanical stirring. Moisturizing the reaction mixture with solvents leads to an increase in the product's ED. The obtained results are presented in Table 1.

Table 1.

The effect of initial reagent molar ratios and of the solvents used as solvating agents on Na-CMS in ED (reaction time 30 min, temperature 25 °C)

As can be seen from the results presented in Table 1, the type of solvent used to wet the reaction mixture has a significant effect on the ED of the resulting Na-CMS. When using alcoholic solutions, the ED of the resulting CMS is higher compared to water. It can also be seen that the molar ratios of the reactants have a significant effect on the ED of the Na-CMS. The synthesized Na-CMS was identified by analyzing its IR spectra (Figure 2).

Figure 2. IR spectra of starch and Na-CMS samples

As can be seen from Figure 2, the starch's IR spectrum has absorption peaks at 3292, 2927, Absorption peaks at 1148 and 1008 cm^–1 are characteristic of the O-N, S-N, S-O-S, and S-O groups in the polymer's glucoside ring. The peak in the 1336 cm^–1 region is characteristic of the polymer's sidechain S-O-N group. In the Na-CMS IR spectrum, new absorption peaks appear alongside the characteristic starch peaks. The peaks at 1696 cm⁻¹ and 1280 cm⁻¹ are characteristic of the S=O and S–O bonds in the carboxymethyl group, while the peak at 1198 cm⁻¹ is characteristic of the S–O–S bond of the ether group.

The water solubility of Na-CMS depends on its ED, and as its value exceeds 0.15, the polymer becomes water-soluble at room temperature. Aqueous solutions of Na-CMS are homogeneous, transparent, viscous, and remain stable even after long-term storage. Currently, there are several technologies for obtaining polymer films, all of which use polymer solutions or melts. Starch derivatives do not liquefy under heat, so to obtain films, a thick solution of the polymer was applied as a thin layer on a glass matrix and the solvent was evaporated. To reduce the solubility of the polymer films, the Na-CMS macromolecules were first crosslinked in the presence of polyvalent metal salts. Compounds such as CaCl₂, MgCl₂, BaCl₂, Ba(NO₃)₂, ZnSO₄, FeCl₃, and Al(NO₃)₃ were used to cross-link the polymer. In an aqueous solution, the complex formation reaction of Na-CMS macromolecules with polyvalent metal ions can be represented by the following scheme:

Figure 3. The folding mechanism of Na-KMK macromolecules in the presence of metal ions

As shown in the schematic in Figure 3, the –COONa groups in Na-CMS react with metal ions in the solution, and crosslinking of the macromolecules is observed. As a result, the crosslinking of the macromolecules leads to the formation of a true solution, a suspension, or a gel-like substance. It was determined that when the resulting precipitates are dried on a solid matrix, they form films. These films swell in aqueous solutions to a certain extent, but their dissolution is not observed.

Studies have shown that polymer films obtained by adding ZnSO₄ and Al(NO₃)₃ salts to an Na-CMS solution have the highest transparency, while polymers prepared with other salts exhibit lower transparency and increased brittleness. Figures 4 and 5 show photographs of polymer film materials obtained from Na-CMS doped with ZnSO₄ and MgCl₂ salts.

Figure 4. Photograph of the polymer film obtained by adding ZnSO₄ solution to the Na-CMS solution. a) Film obtained by adding 0.025 M ZnSO₄, b) Film obtained by adding 0.05 M ZnSO₄.

Figure 5. Photograph of the polymer film obtained by adding MgCl₂ solution to the Na-CMS solution. a) film obtained by adding 0.025 M MgCl₂, b) film obtained by adding 0.05 M MgCl₂

Studies have also shown that the salts used to cross-link the polymer macromolecules have a significant effect not only on the transparency of the resulting polymer but also on its swelling and mechanical strength. In all cases, as the concentration of the chelating metal ion in the polymer increased, the films' elongation decreased, but their brittleness increased. It was found that adding glycerin, urea, and other plasticizers in an amount of 0.5-2.0 wt% to the polymer films leads to an increase in their elasticity by 50-125%.

Conclusions

Using Na-CMS as the water-soluble precursor, biodegradable polymer films were prepared from starch under natural conditions. In this work, the synthesis of Na-CMS was carried out using the solid-phase carboxymethylation of starch. The effects of various factors-solvents, reaction duration, and reagent ratios-on the starch carboxymethylation reaction were investigated. To reduce the solubility of the polymer films, the Na-CMC macromolecules were crosslinked in the presence of polyvalent metal ions. Evaporating the water from the resulting polymer solutions at room temperature produced film-like materials with various properties. It was shown that the transparency, elasticity, and mechanical strength of these films depend on the amount and type of salts used as crosslinkers. It was found that the elasticity of the resulting polymer films can be increased by incorporating plasticizers into their composition.

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