OBTAINING OLIGOLACTIDE MODIFIED CELLULOSE/PLA BIODEGRADABLE COMPOSITE MATERIALS

ABSTRACT

In this work, the results of research on obtaining biodegradable composites of the product obtained by modifying the microcrystalline cellulose obtained from plant stems with oligomeric lactide and polylactide are presented. Polylactide was synthesized from lactic acid on AlCl₃ catalyst by ring-opening polymerization method. Microcrystalline cellulose obtained from plant stems (wheat straw) is modified in the presence of oligomers formed as waste (about 10-20%) in the polymerization reaction. Compared to pure microcrystalline cellulose, OLA-g-MCC-treated polymer was found to disperse better with PLA. The obtained materials were studied by FT IR spectroscopy and powder XRD methods. Incorporation of OLA-g-MCC has been shown to improve the crystalline properties of the materials.

АННОТАЦИЯ

В данной работе представлены результаты исследований по получению биоразлагаемых композитов продукта, полученного путем модификации микрокристаллической целлюлозы, полученной из стеблей растений, олигомерными лактидами и полилактидами. Полилактид синтезирован из молочной кислоты на катализаторе AlCl₃ методом полимеризации с раскрытием цикла. Микрокристаллическую целлюлозу, полученную из стеблей растений (соломы пшеницы), модифицируют в присутствии олигомеров молочной кислоты, образующихся в виде отходов (около 10-20%) в реакции полимеризации. Было обнаружено, что по сравнению с чистой микрокристаллической целлюлозой полимер, обработанный OLA-g-MCC, лучше диспергируется с PLA. Полученные материалы исследованы методами ИК-Фурье спектроскопии и рентгенофазового анализа. Было показано, что включение OLA-g-MCC улучшает кристаллические свойства материалов.

Keywords: polylactide; biodegradation; modified cellulose; degree of crystallinity; mechanical properties; biodegradable materials.

Ключевые слова: полилактид; биодеградация; модифицированная целлюлоза; степень кристалличности; механические свойства; биоразлагаемые материалы.

Introduction. Biodegradable composite materials based on poly(lactic acid) filled with various substances have been extensively researched in recent years. One of their main disadvantages is the low degree of crystallinity, which greatly affects the mechanical properties and strength of products based on them.

Many studies are being conducted to increase the mechanical properties of polylactide by adding fillers based on various substances. When using isothermal crystallization [1] to increase the crystallinity of polymers, the formation of spherulites of PLLA at high temperatures, and the formation of incorrect morphology at low temperatures was observed. The reason for this is that at high temperatures, the rate of formation of buds is low, and chain diffusion, on the contrary, is high. A way to increase the degree of crystallinity by adding a small amount of poly(D,L-lactide) (PDLLA) to the linear high molecular weight PLLA/PDLA blend has been proposed [2]. The addition of atactic PDLLA has been shown to enhance the occurrence of stereocomplex-type crystallization. Specific crystallization was observed at 20 wt% PDLLA with an average molecular mass of 1×10⁵ g/mol. Among the PLA enantiomers, it was observed that the intermolecular interactions are the best and the intermixing of the chains is improved. Another way to increase the degree of crystallinity of polylactide is the influence of shear-deformation forces, and the effect of factors such as conditions, cooling rate, molecular mass of the polymer, as well as the architecture of the macromolecule on the shear effect has been studied [3]. It has been shown that star polymers with a molecular weight of about 120 kg/mol have a higher degree of shear crystallization than their linear counterparts.

The properties of composites obtained by filling polylactide with microcrystalline cellulose have been studied [4]. Composites containing 35% microcrystalline cellulose (MCS) (PDLLA-MCC-35) were found to exhibit shape memory effect when exposed to water at 37 °C. It has been shown that microfibrillar cellulose from bamboo can be added as a reinforcing agent to improve the mechanical properties of biodegradable composites [5]. The obtained composite materials were analyzed by various methods, and it was observed that there were interactions between cellulose microfibrils and PL macromolecules. As a result, it was determined that the Young's modulus and strength limit of the composites increased to 3.1 GPa and 39 MPa, respectively, compared to pure polylactide resin (2.4 GPa and 33 MPa, respectively). Also, by acetylation of lignocellulose from bamboo, chemical compatibility was created for its interaction with polylactide macromolecules [6]. Composites containing 20% ​​acetylated cellulose by mass were found to have the highest strength (18.84 N/mm²).

Composite materials based on polylactide and cellulose have been extensively studied [7-15], and despite the fact that their properties have been improved in various ways, the main drawback for all cases is the insufficient mechanical strength of the materials, and relatively high cost.

Due to the fact that nanocellulose, which is hydrophilic, is aggregated with a less hydrophilic polymer - polylactide, there are certain limitations for improving mechanical properties. To solve this problem, chemical processing, polymer etching methods were used. Chemically modified nanocellulose retains hydrophobic groups and has a polarity compatible with hydrophobic polymers, resulting in their uniform distribution in the polymer matrix.

The closest work to the results of the research discussed in this work is presented in [16], where cellulose nanofibers were successfully grafted with L-lactide monomers. Compared to pure cellulose nanofibers, PLA-g-CNF has been shown to disperse well into a poly(lactic acid) matrix. The annealed PLA chains crystallized strongly, providing optimal nucleation centers for crystal growth. It was found that the crystallinity of the extruded composite fibers increased from ~6 to ~12% before heat treatment and up to ~28% after heat treatment. An increase in the degree of crystallinity led to an improvement in the mechanical properties of composite fibers.

From the foregoing, we can conclude that experiments on the modification of cellulose obtained from wheat stalks modified with oligolactide have not been carried out and their crystalline properties have not been studied. This paper presents experiments on the study of the crystalline properties of composite materials obtained from polylactide and cellulose, the latter, which is modified with an oligolactide of medium chain growth.

The scientific novelty of the work that, for the first time was carried out modification of cellulose with oligolactide and obtained based on them biodegradable composite materials with polylactide. It has been established that, with the introduction of oligolactide as a modifier into the cellulose molecule, the polarity of the chain of the cellulose macromolecule becomes less hydrophilic, which contributes to their attachment to the almost non-polar polylactide chains.

Experimental  

Synthesis of polylactide

A common method of obtaining polylactide is ring-opening polymerization [17-19]. In this work, an 80% solution of L-lactic acid (AO "Baza No. 1 Khimreaktivov", Moscow) was used for the synthesis of polylactide. The synthesis of polylactide was carried out in three steps according to the method presented in [18]. Zinc oxide, zinc chloride, aluminum chloride and their mixtures in different proportions were used as catalysts, and good results were obtained on the basis of aluminum chloride (tin octanoate was used in the above method).

Step 1: lactic acid was concentrated at 160 °C for 2 hours and a part of it was transferred to form lactide of lactic acid. Dewatering was carried out in the atmosphere of inert gas - nitrogen ("Generator nitrogen", Chromatek-Crystal) at normal pressure. At each stage, the products were monitored by IR-spectroscopic method (Bruker). The obtained IR-spectra were compared with the data of ATR-LIB-PHARMA-2-472-2.S01 and DEMILIB.S01 library databases, and substances were identified. By comparing the results of the IR spectrum, it can be concluded that lactic acid has turned into a cyclic lactide (Fig. 1), then into an oligomer (Fig. 2), and then into a polylactide (Fig. 3).

Figure 1. IR spectra of cyclic lactide (red) (remaining spectra  belong to L(+)lactic acid and its ethyl ester)

Figure 2. IR spectra of the oligomer (red) (remaining spectra belong to D- and L-lactic acid)

Step 2: in which lactide was collected in a condensing flask by driving under 0.02 MPa vacuum (vacuum pump, KNF, Germany) at 220 °C; the temperature of the collecting flask was kept at 90 °C for 4 hours, if the temperature is low, the lactide solidifies; then the solid lactide was removed from the flask, the product was washed with cold water, filtered and dried overnight at 40 °C.

Step 3: the lactide obtained in the last step was mixed with the catalyst - aluminum chloride (0.05-0.1% by mass) and turned into polylactide at 140 °C for four hours.

Figure 3. IR spectra of polylactide (red) (the blue spectrum was determined to be Poly(lactide) 

Polylactide yield was 83%. The remaining 17% of lactic acid was in the form of oligomers, and the product was in a liquid state. The molecular mass of the oligomer, when determined by the viscometric method, is between 450-750, which indicates that the value of n is on average 8-10.

Extraction of cellulose from the stem of cereal crops (wheat).

In the next experiments, experiments were carried out on the extraction of cellulose from cereal crops, mainly from the stem of the wheat plant. To do this, first, the material was crushed, poured over it with a 10% solution of sodium alkali and boiled for 4 hours. After cooling, the liquid was poured into a plastic container. The product was washed with water. Waste water contains lignin and alkali. The remaining product was mixed with water 1:10 and mixed well with a blender. For disposal, lignin in the waste was mixed with water in a ratio of 1:10, sprinkled on the garden soil 1-2 times a week and treated with a biodegrader. The liquid (lignin) poured into the compost mixture loses its toxicity in 5-6 months.

Modification of wheat straw cellulose with oligomeric lactide.

The yield of the reaction in polylactide synthesis is between 80-90%, and in each cycle 10-20% of the raw material is released in the form of oligomers. Since the degree of polymerization (molecular weight) of the oligomer is not so high, it is mainly in the form of a viscous liquid. As one of the ways to increase the interaction of wheat straw cellulose with polylactide, it was proposed to modify it with oligomeric lactide. In this case, 3-5% of the oligomer by weight of the resulting cellulose was mixed and heated at 60 °C for 2 hours with stirring. As a result, oligomer-conjugated microcrystalline cellulose (OLA-γ-MCC) was obtained.

The progress of the process was evaluated by IR-spectroscopic method. Below are the obtained FTIR results and the data obtained by comparative evaluation with its baseline data.

Figure 4. FTIR spectra of obtained composite (L-lactide ethyl ester, ethyl D-lactate)

FTIR results show that OLA-g-MCC has a pronounced band at 1735.15 cm^-1 compared to pure microcrystalline cellulose. This band belongs to the carbonyl (C=O) group, which belongs only to OLA, and indicates the successful coupling of OLA to microcrystalline cellulose. At the same time, the intensity of the band belonging to the hydroxyl (O-N) group at 3357.93 cm^-1 was observed to decrease compared to pure microcrystalline cellulose, and the intense bands at 1019.10 and 1124.29 cm^-1 correspond to C-O-C symmetric stretching and asymmetric stretching vibrations, which confirms the previously predicted results.

After that, pure polylactide and modified OLA-g-MCC were made composite for 30 min under constant intensive stirring by liquidizing polylactide at 170 °C.

From the above results, it can be concluded that the following type of bond was formed between oligomeric lactide and microcrystalline cellulose:

From the above results, it can be concluded that the following type of bond was formed between oligomeric lactide and microcrystalline cellulose:

Figure 5. Scheme for crosslinking cellulose with oligolactide

This in turn causes increased interactions between OLA-g-MCC and PLA and the formation of crystalline points.

Study of crystalline properties of PLA/OLA-g-MCC composite materials.

Crystalline properties of polylactide and filler-modified cellulose (OLA-g-MCC) composite materials directly affect their strength and mechanical properties. Therefore, their crystalline properties were investigated in further studies. The crystalline properties of the PLA/OLA-g-MCC composite materials obtained in the experiments were studied with a powder X-ray diffractometer (XRD-6100, Shimadzu, Japan) (Fig. 6).

Figure 6. Results of the powder XRD analysis of the starting materials and the obtained composite material (polylactide/starch composite was also taken for comparison): No. 4 (red) - polylactide/starch composite; No. 6 (blue) - PLA/OLA-g-MCC; No. 10 (green color) -OLA-g-MCC

The obtained results showed that the level of crystallinity of the PLA/OLA-g-MCC composite was higher than that of the original materials and other samples, and its crystalline properties were improved. Schematically it can be represented as follows (fig.7):

Figure 7. Interaction between the polar chain of cellulose and the non-polar chain of polylactide

As a result, the bond between the polar microcrystalline cellulose and the less polar polylactide is strengthened and the crystalline properties of the materials are improved.

Conclusions

Polylactide was synthesized from lactic acid under AlCl₃ catalyst by ring-opening polymerization method. Microcrystalline cellulose obtained from wheat straw was modified in the presence of oligomers formed as waste (about 10-20%) in the polymerization reaction. Compared to pure microcrystalline cellulose, OLA-g-MCC-treated polymer was found to disperse better with PLA. After modification of polar cellulose macromolecules with oligomers, it was found that interaction with PLA macromolecules of low polarity is improved, crystallites are formed on growth surfaces, and as a result, the degree of crystallization is high. It was observed that the properties of the materials improved after heat treatment due to the introduction of OLA-g-MCC and its synergistic effect.

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