Научная статья на тему 'Effect of plant environment on decomposition of biodegradable materials based on poly-3-hydroxybutyrate and polylactide'

Effect of plant environment on decomposition of biodegradable materials based on poly-3-hydroxybutyrate and polylactide Текст научной статьи по специальности «Химические науки»

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Ключевые слова
WHEAT SEEDS / BIODEGRADABLE POLYMERS / NONWOVEN MATERIALS / POLY-3-HYDROXYBUTYRATE / POLYLACTIDE / DESTRUCTION

Аннотация научной статьи по химическим наукам, автор научной работы — Shibryaeva L., Tertyshnaya Yu., Solovova Yu., Levina N., Zhalnin E.

Samples of nonwoven material from biodegradable polymers poly-3-hydroxybutyrate and polylactide used as carriers of wheat seeds, accelerate their germination compared to control. The nonwoven fabric is prepared by electroforming the fibers in organic solvents. Paper filters are used as a control. The influence of germinated seeds on the thermal parameters of melting and physical and mechanical properties of biodegradable polymer carriers is shown. It is established that there is a relationship between the destruction of the polymer material and the rate of germination of seeds, as well as the development of the root system of plants. It is shown that the destruction of polymers proceeds through several mechanisms, which depend on the stage of seed germination. At the initial stage, the mechanism of hydrolytic degradation and enzymatic catalysis of polymers is considered. The process of polymer destruction by mechanical destruction under the influence of growing roots is discussed at the stage of root system development. The development of the root system leads to the appearance of microcracks, their fusion with the formation of holes in the polymer material. The kinetics and mechanism of destruction of the seed carrier polymer and the growth rate of the root system depend on the nature and structure of the polymer. Nature determines the direction of crack germination in the polymer carrier for polylactide along its surface, for poly-z-hydroxybutyrate in volume.

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Текст научной работы на тему «Effect of plant environment on decomposition of biodegradable materials based on poly-3-hydroxybutyrate and polylactide»

Рис.3. Зависимость извлечения золота из хвостов флотации до и после обжига при температурах 600

0С и с обработке серной кислотой

Следовательно, использование ацетилтиомо-чевины с целью выщелачивания золота из руд и хвостов флотации рентабельно. Помимо того, с применением ацетилтиомочевины извлечение золота во много раз выше, чем при цианировании.

СПИСОК ЛИТЕРАТУРЫ:

1. Лодейщиков В.В. Технология извлечения золота и серебра из упорных руд. В 2-х т. - Иркутск: ОАО «Иргиредмет», 1999, 786 с.

2. Захаров Б.А., Меретуков М.А. Золото: упорные руды. - М.: Руда и Металлы, 2013, 450 с.

3. Самихов Ш.Р., Зинченко З.А., Бобомуро-дов О.М. Изучение условий и разработка технологии тиомочевинного выщелачивания золота и серебра из руды месторождения Чоре. - Доклады АН. -Душанбе, 2013 -т. 56, № 4 с. 181-184.

4. Лодейщиков В.В., Панченко А.Ф. Основы технологии извлечения золота и сурьмы из комплексных руд. - Интенсификация процессов обогащения минерального сырья. - М.: Наука, 1981, с. 189-193.

5. Самихов Ш.Р., Холов Х.И., Зинченко З. А. Технология обогащения руд нижних горизонтов Джижикрутского месторождения. - Доклады АН РТ - 2017. Том 60. - №10 с. 533-538.

UDC 541.64:544.032:577.12

EFFECT OF PLANT ENVIRONMENT ON DECOMPOSITION OF BIODEGRADABLE MATERIALS BASED ON POLY-3-HYDROXYBUTYRATE AND POLYLACTIDE

Shibryaeva L.

D. Sc., Prof., Leading researcher N. M. Emanuel Institute of Biochemical Physics, Russian Academy Sciences

Federal Scientific Agroengineering Center VIM

Tertyshnaya Yu. Ph.D., Senior Researcher N. M. Emanuel Institute of Biochemical Physics, Russian Academy Sciences

Federal Scientific Agroengineering Center VIM

Solovova Yu. Ph. D., Junior researcher N. M. Emanuel Institute of Biochemical Physics, Russian Academy Sciences

Levina N. Senior researcher Federal Scientific Agroengineering Center VIM

Zhalnin E.

D.Eng, Prof., Head of Laboratory Federal Scientific Agroengineering Center VIM

Abstract

Samples of nonwoven material from biodegradable polymers poly-3-hydroxybutyrate and polylactide used as carriers of wheat seeds, accelerate their germination compared to control. The nonwoven fabric is prepared by electroforming the fibers in organic solvents. Paper filters are used as a control. The influence of germinated seeds on the thermal parameters of melting and physical and mechanical properties of biodegradable polymer carriers is shown. It is established that there is a relationship between the destruction of the polymer material and the rate of germination of seeds, as well as the development of the root system of plants. It is shown that the destruction of polymers proceeds through several mechanisms, which depend on the stage of seed germination. At the initial stage, the mechanism of hydrolytic degradation and enzymatic catalysis of polymers is considered. The process of polymer destruction by mechanical destruction under the influence of growing roots is discussed at the stage of root system development. The development of the root system leads to the appearance of microcracks, their fusion with the formation of holes in the polymer material. The kinetics and mechanism of destruction of the seed carrier polymer and the growth rate of the root system depend on the nature and structure of the polymer. Nature determines the direction of crack germination in the polymer carrier for polylactide - along its surface, for poly-z-hydroxybutyrate in volume.

Keywords: wheat seeds, biodegradable polymers, nonwoven materials, poly-3-hydroxybutyrate, polylactide, destruction.

Introduction

The most important task of the agro-industrial complex of any contry is the introduction of innovative technologies aimed at increasing the yield of agricultural crops and the creation of environmentally friendly products. These technologies are based on non-traditional methods of cultivation and storage of agricultural products. Among them are the technologies of mulching soil, pelleting seeds of onions, carrots, tomatoes and other vegetables, based on the use of film materials from biodegradable compositions based on natural and synthetic polymers [1-4]. Recently, the technology of planting seeds in the soil on a polymer tape has been developed for grain crops in Russia [5,6]. The use of the tape involves the provision of environmentally friendly conditions for germination of seeds of grain crops, their protection from the effects of pathogenic systems, the creation of a microclimate favorable for seed germination and plant development.

This technology is extremely important for seed farms. It is supposed that the tapes filled with seeds together with mineral fertilizers can create conditions for production of elite grades of grain crops. The main requirement for the materials used for the manufacture of tapes-carriers of seeds, is their ability to provide conditions for the life of crops. First of all, such materials, in contact with germinating seeds and plants, should not only not violate the mechanism of their development, but also stimulate these processes. The materials must be predisposed to degrade under the action of biological fluids, oxygen, UV- radiation with a high speed with the formation of environmentally friendly products, not contaminating the soil. At the same time, it must have mechanical parameters that can ensure the planting of seeds in the carrier tape into the soil with the help of agricultural machinery. Today there is a problem of creation of a material for tapes -carriers of seeds capable to solve objectives.

The most suitable materials that can meet the requirements are biopolymers based on oxy-derived fatty acids, the so - called polyhydroxyalkanoates, the main advantages of which are their high physical and mechanical properties and environmental friendliness [7]. Poly-3-hydroxybutyrate (PHB) and polylactide

(PLA) were used for agricultural purposes in [8-10]. Properties of these polymers provide their wide application in agricultural and food industry, medicine and biology [11-16]. They undergo biodegradation in enzymatic catalysis under the action of bacteria and fungi [12]. It is important for the metabolism of plant cells that the stresses arising in polymer chains, causing the decay of bonds, initiate radical chain processes of oxidation of macromolecules [13]. The decomposition products of these biodegradable polymers are carbon dioxide and water. However, to date, there are no studies of the processes of biodegradation of polymers under the influence of the environment which created by germinating seeds and developing plants.

The aim of the work was to establish patterns of biodegradation of polymer materials based on PHB and PLA under the influence of germinating wheat seeds.

Experimental part

Poly-3-hydroxybutyrate (PHB) of German firm "Biomer" with molecular weight Mw=2,5*105 g/mol in the form of fine powder, d = 1,248 g/cm3; polylactide (PLA) of brand 4032D manufactured by "Nature Works" (USA) with Mw=1,7x105 g/mol in the form of granules, with d =1,27 g/cm3; mixtures of PHB with synthetic nitrile rubber (SNR) were used in the work. SNR of Russian production with 28 wt.% of nitrile groups, with the Mooney viscosity at 100o C - 45 c. u., PHB-SNR mixtures contained 30 wt.% rubber's were used.

Samples in the form of extruded films and nonwoven material were investigated. Films with thickness of 80 - 120 mcm was obtained by pressing PLA at 195-200, PHB at 150-175°C, with subsequent slow cooling. The nonwoven material was prepared from nanofibers obtained by electroforming by exposure to an electric voltage of 10-60 kV to an electrically charged jet of 6% solution of each of the polymers in chloroform or in a mixture of chloroform with dichloroethane in a ratio of 80:20 wt.% arising from the capillary nozzle [17,18].

We studied the germination, germination of seeds, growth and development of seedlings of spring wheat varieties "Athena" (Triticum aestivum) harvest in 2015 (Krasnodar region) in the laboratory. For seed

germination, they were placed in Petri dishes of 50 seeds in accordance with GOST 12038-84 [19] on moistened pads made of paper filters or polymer material. Samples were filled with distilled water so that moisture covered the seeds. Germinating seeds were kept in a thermostat at a temperature of 20±1 ° C, maintaining a constant level of humidity.

As a reference to monitor the germination of seeds and development of plants on the polymeric samples was used a substrate of disinfected paper filters brand Blue ribbon FM without impurities on the other 2642001-68085491-2011. After 1,2,3 days, the number of sprouted seeds was determined as a percentage in each batch of samples taken for analysis (at least 5 samples). Seed germination energy was defined as the percentage of normally sprouted seeds in the first three days. Germination-on the seventh day. During 14 days the dynamics of growth of wheat roots and seedlings was analyzed.

Experimental data were processed using dispersion and correlation analysis according to the programs "AGR0S-2.02". The accuracy of determining the germination parameters is not less than 1.5%.

The thermophysical characteristics of polymer materials were determined using a differential scanning calorimeter of the company ("Netzsch", Germany, model DSC-204 F1) at a heating rate of 10 deg/min in the temperature range of 30-200 ° C in argon current. A portion of the sample was varied in the range of 2^8 mg using DSC received values of the heats and melting points of PHB and PLA.

To accurately determine the parameters, corrections were introduced into the recorded values for the parameters of the melting peak indium (with the temperature and heat of melting of Tm=156.7°C and LHm=28.58 J/g). The heat of fusion was determined by the crystallinity of the PHB and PLA ratio

=(AHm / AH*m) x 100%, where AHm - the heat absorbed during the melting of the sample per unit mass of pure polymer, AH*m - the specific heat of fusion of crystals of PHB and PLA is 90 [20] and 106 J/g [21], respectively.

The accuracy of the melting temperature of the Tm is±1 ° C. Standard deviations of experimental areas of melting peaks of different samples (at least 10 samples) were within 10%.

The surfaces of the polymer samples, the structure and thickness of the fibers of the nonwoven material were investigated using an optical microscope Axio with a thermal imager Z2m, Carl Zeiss with software; with 50x, 200x, 500x magnification in both transmitted and reflected light.

Physico-mechanical parameters, including tensile stress (F), relative elongation (e), breaking length (L) of PHB and PLA samples were determined on a tensile testing machine RM-3-1 according to GOST 25.061065-72. The speed of movement of the lower clamp is set 45±5 mm/min. Samples of seeds grown on substrates during 9 and 14 days were dried, cleaned from the root system of plants and cut in the form of blades. The parameters were calculated by load -elongation curves. To obtain each value, 2 batches

containing at least ten samples were used. Standard deviations of experimental parameters were within ± 20%.

The parameters of water absorption of a solution of amylase, extracted from the seeds and roots of wheat, nonwoven material and polymeric films of native seeds was determined according to GOST 4650-80 [22]. The tests were carried out on square-shaped samples of ~30x30 mm in size by at least 3 for each material. Before determining the parameters, the samples were dried at (50±2)°C for (24±1) hours, then cooled in the desiccator above the desiccant at (23±2) ° C. After cooling, the samples were weighed for 5 minutes.

After that, the samples were placed in vessels with distilled water and amylase extract. On 1 cm2 of the sample surface there was at least 10 cm3 of liquid. The liquid with the sample placed in it was stirred by rotating the vessel at least once a day. When determining the maximum degree of swelling in the water (to equilibrium), the equilibrium was considered achieved if the difference between the mass of the sample determined with an interval of 24 hours did not exceed 0.1 %. After that, the samples were removed from the vessels and placed on a clean filter and removed moisture from the surface. Then weighed on electronic scales. The degree of water absorption and absorption of amylase solution was calculated by the formula:

a = (mu-nio)/ nio . 100%, where nio - the initial mass of the sample, mn - the mass of the sample after saturation with water or a solution of amylase during time .

IR spectroscopy and optical microscopy were used to control the filling of amylase films. IR spectra were obtained using a Fourier transform spectrometer of the company Perkin-Elmer.

Results and discussion

Important indicators of the possibility of using biodegradable materials for the technology of growing wheat as substrate - carriers of seeds is the ability of the material to destruct in contact with germinating seeds and at the same time affect the rate of germination of seeds and the development of various organs of the plant. In order to establish the feasibility of using of materials, based on poly-3-hydroxybutyrate and polylactid, for carriers of seeds, were carried out laboratory studies of dynamics of germination of wheat seeds on their surface in an aqueous medium (in Petri dishes).

We used samples of extruded films and nonwoven material made of polymers PHB, PLA, and mixtures of PHB-SNR structures. In the course of the work the regularities of changes in thermophysical and physico-mechanical parameters of the above samples under the influence of germinating seeds and developing root system with seedlings were studied. With the aim of establishing a relationship between the kinetics of destruction of the polymer and the dynamics of plant development parameters of germination, energy of germination and biometric parameters of wheat germ that grew on the samples of polymer materials were compared with similar parameters obtained for seeds

germinated on paper filters, performing the role of control samples. The results are presented in table. 1.

Table. 1. Indicators of germination and biometric parameters of wheat seed seedlings of the "Athena" variety (Triticum aestivum).

Comparison of the parameters of the germination and biometric parameters of seedlings of wheat seeds (table. 1) demonstrates noticeable differences in the indicators of seeds sprouted on polymer carriers from the control samples. When comparing samples of different polymeric carriers, differences in the germination rates of seeds germinating on them were found. Moreover, these differences depend on the chemical composition and structure of the substrate. On substrates of nonwovens seed germination and development of the root system on them are significantly accelerated compared to the control substrates of paper filters, while the pressed films seed germination slows and plant growth is inhibited (table. 1). At the study of the dynamics of seed germination on carriers of nonwovens was found to increase the effect

of the impact on the seeds in the transition from the stage of ontogenesis with the formation of the root to the stage of growth and development of the root system and seedlings (table. 2). If at the stage of ontogenesis there is a tendency to increase the quantitative parameters (germination and energy of seed germination) in comparison with the control, then at the stage of growth there is a significant increase in biometric indicators (table. 2).

Table 2. The dynamics of germination of wheat seeds on different carriers

Figure 1 shows a significant increase of differences between the parameters of seed seedlings, root system and wheat germ grown on the surface of the paper filter and of nonwoven material PHB with increased time of germination of seeds. As can be seen, the rate of development of wheat on a biodegradable polymer substrate is accelerating, which can be explained by the factor of initiation of growth processes on the part of the polymer.

Fig. 1. Photos of samples of wheat seeds which sprouted in Petri dishes on the surfaces of substrates - carriers of seeds from paper filter (a, c, e) and nonwoven material from PHB (b,d,f) in the aqueous medium for 2 (a,b), 4

(c,d) and 7 (e,f) days

The study of changes in polymer substrates -carriers in the process of germination of seeds and seedlings in the dynamics of the latter showed that developing plants are not indifferent to the polymer substrates and have a significant impact on the structural, physical and mechanical parameters of the latter. The germination of seeds and the development of seedlings on a sample of polymer material lead to its destruction, as evidenced by the decrease in mass. The results of the study of reducing the mass of samples in the process of destruction as a result of contact with germinating seeds are presented in table 3.

Table. 3. The mass of samples of initial films and after germination of seeds in them

As can be seen from table 3, the samples of nonwoven material from PHB and PLA, being in direct contact with the plants for 9 days decreased in weight by 1.4 and 2.2 times respectively. It is important to note that the samples has been dried and prepared for out weighing to obtain these results. It was taken into account that after the experiment some part of the plant roots was directly in the material, however, even with its mass significantly decreased compared to the mass of the original sample (table. 3). The development of the root system on polymer carriers causes changes in their physical and mechanical properties. On the tensile testing machine were tested samples of nonwoven material PHB and PLA, the original and after the experiment, in the continuation of which the material was in direct contact with the plant. The analysis of the obtained data showed that after germination of seeds in

PHB the values of relative elongation, maximum load and breaking length are significantly reduced, in polylactide with a significant drop in the index of relative elongation, the maximum load and breaking length are increased by ~ 2.5-3 and 6 times, respectively (table. 4). I.e. is detected the dependence of the nature of the impact of growing roots on the polymeric carriers from their chemical structure.

Table. 4. Physical and mechanical characteristics of seed substrates of nonwoven PGB, PLA initial and after germination of wheat seeds in them.

Figure 2 shows photos of destructive samples of nonwovens PHB and PLA exposed to the root system and seedlings during wheat growth for more than 14 days. Figure shows a different picture of the destruction of polymers. Based on the picture of destruction of PHB and PLA, shown in Fig. 2 and data in table. 4, it can be assumed that the observed difference in the change in mechanical parameters of PLA and PHB samples under the action of developing plants is due to different mechanisms of biodegradation of these polymers. It is possible that a significant increase in the maximum load and breaking length in the PLA is due to the influence of the reinforcing layer that strengthens the matrix, which arose from the undeleted root residues due to the development of the root system along the surface of the carrier (Fig. 2). At the same time, the roots of PHB germination are carried out in the depth of the material, creating a "hole" in the polymer matrix (Fig.2).

Fig. 2. Photographs of the samples of substrates of seeds from nonwoven material PHB (a) and PLA (b) with

germinated in them root systems of wheat.

Change of thermophysical parameters of samples of nonwoven material under the influence of germinated seeds was investigated by DSC method. The melting thermograms of crystallites were compared for the initial films - seed carriers before

their planting and after germination of plants and substrates exposed to the growing root system. Fig. 3 presents an example of endothermic melting peaks for nonwoven PHB materials and the PLA before and after germination of seeds in them.

Fig.3. Endotherms of melting of samples of nonwovens PHB (1,2) and PLA (3,4,5), before (1,3) and after (2,4,5) germination of seeds in them within 9 (2,4) and 14 (5) days.

Endotherms characterizing the melting of crystallites of the initial carriers of PHB and PLA have one peak in the temperature range 150-190oC with a maximum temperature (Tmax) equal to 172.8°C in PHB and 166°C in PLA.

The nature of the change in the shape of the peaks in the samples after germinate the seeds were depends on the nature and structure of the polymer material and the time of seed germination (Fig. 3). The analysis of melting endotherms was carried out for 9 and 14 days of seed germination. The peak of melting of PHB is shifted to the low-temperature region by several degrees and a low-melting shoulder appears after 9 days (Fig. 3). Changes in the shape of PLA melting peaks depend on the time of root germination to a greater extent than that of PHB. For example, after 9 days of seed germination, there is a shift in the melting peak Tmax in the high-temperature region with the appearance of a low-melting shoulder nearby with Tmax of the initial sample (Fig. 3). After 14 days, the entire melting peak shifts to the low-melting region (Fig. 3). The change in the forms of melting peaks of PHB and PLA is accompanied by a drop in the melting heat, hence the degree of crystallinity (table. 5). The shift in melting temperatures and the decrease in the degree of

crystallinity indicate the destruction of crystal structures.

Table. 5. Thermophysical parameters of polymer samples of the substrates - carriers of the seed from nonwoven material

From the difference of the magnitudes of the fall of the heat of fusion of the crystallites in samples of non-woven materials PHB, PLA, prepared from solutions in chloroform and mixtures of chloroform-dichloroethane (table. 5), the influence of the substrate structure on the seed germination rate is clearly observed (table. 1,2). Thus, the degree of crystallinity in samples of PHB prepared from CHF and CHF with EDC after germination of seeds in them is reduced by 1.8 and 1.1 times, respectively, in the sample of PLA from CHF by 1.1 times, in the sample mixture of PHB+ SCN-by 3-3. 5 times (table. 5). The greatest drop in the degree of crystallinity of the substrate corresponds to the largest mass and length of roots and seedlings (table. 1) and the highest seed germination index (table. 2).

To the question about the mechanism of degradation of polymeric substrates under the action of growth of seeds of wheat

An important task, the solution of which depends on the use of the studied materials as seed carriers, is to establish the mechanism and kinetics of biodegradation of the carrier under the action of germinating seeds and developing plants. Based on the literature data on the physiology of plants, as well as from the above results obtained in our work, it can be argued that the biodegradation of the polymers, which occurring in contact with germinating seeds, is not described by a single mechanism. The latter can vary depending on the stage of plant growth and the parameters of the polymer-seed carrier.

At the ontogenesis stage, the determining factor of germination of seeds, is the rate of grain swelling and water supply to the embryo [23,24]. Germination of seeds on the surface of the polymer carrier will inevitably depend on the ability of the carrier to provide the embryo with water, hence the process of swelling of the polymer in contact with the grain and the rate of diffusion of water through the polymer to its surface. At this stage, the beginning of the destruction of polymer substrates may be due to swelling and caused by hydrolysis of polymer macromolecules. Biodegradation of polyethers based on polylactide and polyhydroxybutyrate is mainly carried out by hydrolysis of ether bonds by reaction:

-COO- + H2O —> -COOH + HO-

On the other hand, it is known that in the course of biochemical processes developing in germinating wheat grains, enzymes are formed, the main of which is amylase. Amylase can be run from the seed through

the aleuronic layer into an aqueous medium in contact with the polymer. In this case, at the stage of seed germination with the formation of the embryos and root system, in contact with the surface of the polymer carrier in an aqueous medium, the conditions for the reaction of enzymatic hydrolysis of the polymer. As is known for the reaction of the formation of intracellular enzymes that lead to the growth of roots and seedlings, requires a lot of energy. This energy can be obtained by the destruction of biodegradable polymers. Because the observed acceleration of seed germination on substrates of nonwoven materials PHB and PLA compared to the control samples, it is possible to hypothesize about the existence of the mutual influence of the speed of seed germination and the decomposition of the polymeric substrate and its nature.

The essence of this effect is that the polymers in contact with the seeds in the aqueous medium, subjected to enzymatic hydrolysis under the action of amylase released from the seeds, are energy "food" for the development of biochemical processes of formation of enzymes that stimulate the germination of plant seeds. To test the above hypothesis, the kinetics of hydrolysis of polymer carriers in distilled water and aqueous amylase solution was studied. Amylase solutions were prepared by extraction of the enzyme from swollen seeds.

Kinetic curves were obtained that characterize the swelling of polymer samples of pressed PHB film and nonwovens PHB and PLA and hydrolytic degradation under the action of water, as well as enzymatic decomposition in an aqueous solution of amylase. These curves are shown in Fig. 4.

Fig.4. Kinetic curves of swelling of samples of nonwoven material PHB (1), PLA (2), extruded films PHB (3) in distilled water, sorption and hydrolytic degradation of nonwoven materials PHB (4) and PLA (5) in the water extract of amylase and the curve of swelling of wheat seeds in water (6). T=22oC.

As can be seen from the figure within the selected time interval, the kinetic swelling curves of the studied samples have a typical form of the process with an accelerated initial stage and a stationary site. The kinetics of swelling of nonwoven material in water is described by the equation [22]:

dx dv

k = - In ^^

где к - the rate constant of swelling a_ - degree of swelling by time J

(1) (2)

amax - maximum degree of swelling,

From these curves it can be seen that the studied samples differ in the values of the swelling rates at the initial site and the maximum degrees of swelling ( i„(i;,: ). J max of the pressed film PHB is significantly lower (40-60%) than that of nonwoven material. Lmax of the nonwoven material of PHB is greater (380%) than PLA (240%).

The swelling rate constants (k) for the initial stages of water saturation (3 days) of polymer samples, were estimated by equations (1) and (2), were equal ~0.088, ~0.1136, ~0.1144 h-1 for PHB films, nonwovens PHB and PLA, respectively. For comparison, in Fig. 4 the kinetic curve of swelling of seeds in the aqueous medium (curve 6) to the stage of their germination is presented. This curve includes the stage of seed swelling, passing into the stage germination with the appearance of roots and seedlings for 3 days. The constant swelling rate (before the emergence of seedlings), estimated for seeds similar to polymer substrates, was ~0.12 h-1, which is within the same range with the data obtained for polymer substrates of nonwoven materials.

Since the water diffusion coefficient of nonwoven PHB material (density p = 0,12 - 0,21 g/cm3 Dh2o = 4.0 10-10 cm2/s ) is higher than from the extruded film PHB (Dh2o is 3.6.10-11 cm2/s [25,26]) may be to conclude that for a friable structural organization of the nonwoven material is characteristic not only a greater degree of swelling of seeds, but and it increase of rate of diffusion of water towards its embryo, this is determines the growth rate germination of seed in contact with nonwoven material.

Kinetic curves presented for non-woven fibers of PHB and PLA after their stand in an aqueous extract of wheat seeds containing amylase (Fig. 4, curves 4.5), demonstrate a significant effect of the enzyme. (The appearance of amylase in the aqueous medium was recorded by changing the pH of the aqueous medium with swollen grain and its UV- spectrum). In place of hydrolytic destruction is enzymatic hydrolysis. It is important that these curves can distinguish the initial

stage characteristic of the swelling process in water with increasing sample mass. After the first stage, there is a loss of polymer mass, indicating enzymatic hydrolysis (curves 4,5). A particularly high rate of mass loss is observed in the sample of PHB (curve 5), in PLA this rate is lower (curve 4).

When comparing the kinetic curve of swelling of seeds in the aqueous medium (Fig. 4, curve 6) with a curve of the enzymatic hydrolysis of non-woven fibers (especially for PHB (Fig. 4, curve 4) reveals that the time corresponding to the beginning of the selection of the enzyme and the emergence of seedlings in the swelling of the seed (~3 days) corresponds to the beginning of the fall weight of the polymer on the curve of enzymatic hydrolysis. The penetration of water into the seed substrate material, leading to hydrolysis in water and enzymatic decomposition in the enzyme solution, initiates violations in its structure.

This follows from the comparison of the thermophysical parameters of melting of the crystal structures of the studied samples of the original nonwoven materials PHB and PLA, with the treated aqueous medium and enzyme extract of seeds obtained by DSC. The melting endotherms of polymer samples, after swelling in water for 144 hours (previously dried at room temperature to a constant weight), show a shift of the melting peak by several degrees towards high temperatures, with the appearance of a low-melting shoulder (Fig. 5).

This is due to the recrystallization of the polymer, which occurs under the influence of water molecules localized in the amorphous regions. From the melting endotherms of samples hydrolyzed in amylase, a significant destruction of the crystal structure of the polymer follows. This is evidenced by the shift of the maximum melting peak for PHB and PLA towards low temperatures ~to 10 degree (Fig. 5). At the same time, the melting heat reduction reaches 9% (table. 6). It is important to note that a significant decrease in the melting temperature, indicating the enrichment of the crystalline structures of low-melting fraction of crystallites characteristic of enzymatic hydrolysis.

Fig.5. Endotherms of melting samples of nonwoven PHB material: initial (1) exposed to aqueous medium (2)

and amylase extract with pH=11 (3).

Table. 6. The thermophysical parameters of samples of nonwoven material PHB, treated with water and extract with amylase and enzymes

It is known that the rate of hydrolysis is different for materials with different chemical and physical structures and depends on the presence of crystalline regions in the polymer, access to which is difficult. Hydrolysis can occur mainly on the surface of the polymer material, and in its volume.

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Perhaps the hydrolytic destruction of nonwoven PHB and PLA material in amylase affects their molecular structure in different ways. Usually for polylactide hydrolysis acceleration in the depth of the product occurs in the case of pH drop caused by acidic degradation products. However, in our case, the observed decrease in the rate of falling weight of PLA in the medium of wheat seed extract containing amylase, the process takes place in an alkaline medium

at pH = 11. Saturation of polymer films with amylase was controlled by IR spectra. Visible decrease in the rate of PLA degradation, observed from the enzymatic hydrolysis curves (Fig.4, curves 4.5), can be explained by the surface process (erosion). A sharp drop in the mass of PHB indicates that the destruction process develops in the volume of the polymer.

The fact of different localization of hydrolysis of PHB and PLA confirms the comparison of the pictures presented on microphotographs (Fig. 6), demonstrating the surface of the samples of nonwoven materials of PHB and PLA, obtained after filling with amylase from the extract and after the last destructive processes of enzymatic hydrolysis (Fig. 6). In the PLA sample, small cracks and small depressions are observed along the entire surface. In PHB-clearly expressed significant depressions and pits, locally distributed in separate zones on the surface of the polymer (Fig. 6).

Fig.6. Microphotographs of samples of nonwoven material PHB (a, b) and PLA (c, d), initial (a, c) and subjected to enzymatic hydrolysis for 6 days under the action of an aqueous extract of amylase from germinating

seeds (b, d).

It should be noted such experimental results as: first, the quantitative relations between the rate constants of swelling of polymers and seeds in water (Fig. 4, curves 1,2,4,6); secondly, the ratio between the time of amylase release into the aqueous medium and the beginning of seed germination and the time corresponding to the process of enzymatic hydrolysis of the polymer substrate material (Fig.4 curves 4,6); third, the destruction of the crystal structures of the studied polymer samples under the action of enzymatic hydrolysis (Fig. 5); fourth, the data optical microscopy of the surface, reflecting the nature of the enzymatic hydrolysis of PHB and PLA in the seed extract (Fig. 6).

Apparently, these results confirm the possibility of implementing the above hypothesis about the nature of the stimulating effect obtained from polymer carriers at the initial stage of seed germination as an effect caused by enzymatic hydrolysis of the polymer substrate.

The possibility of the enzymatic hydrolysis of the carrier during the germination of seeds on it was evaluated by the effect on the nonwovens of PHB aqueous extract of enzymes isolated from sprouted roots. The extract was prepared from the mass of roots grown after 9 days of seed germination. The melting endotherms of PHB obtained after treatment with this extract for several days demonstrated the same

character of changes in the polymer melting parameters that was observed in the sample treated with amylase from seed extract. The differences were in a greater drop in heat and melting temperature - a greater shift of the peak to the low-temperature region (table. 6).

Comparison of melting parameters of polymeric carriers of seeds on the example of samples of nonwoven PHB material exposed to seed germination before the stage of root formation, with samples exposed to enzymatic hydrolysis, indicates the presence of polymer destruction in both cases. However, the mechanisms of destruction are different. If the of seed carrier, under the action of the root system, there is mainly a significant drop in crystallinity, i.e. amorphization of the polymer (table. 5), that the sample, which subjected to hydrolysis, changes the structure of the crystalline regions without

a significant drop in their volume. For example, samples of PHB with sprouted roots reduce the melting temperature of crystallites (Tmax) to ~4oC with a significant drop in the degree of crystallinity (from ~47 to 70%) (table 5). In the sample subjected to enzymatic hydrolysis, the decrease in Tmax reaches 8-10oC. At the same time, the degree of crystallinity varies by 9% (table. 6).

Comparison of the curves of temperature dependences for the degree of transformation of crystalline structures into melt during melting of nonwoven material PHB demonstrates the differences in shape of curves in samples, subjected to enzymatic hydrolysis and in the samples after formations in them of the root system ( Fig. 7). Obviously, this is due to different laws of melting of PHB, due to different mechanisms of destruction of polymeric materials.

Fig. 7. Curves of the temperature dependence of the degree of conversion the crystallites PHB into the melt during melting of the nonwoven material PHB for initial sample (1), for sample after germination into it of the root system (2) for samples which were subjected to a water environment (3) and amylase (4)

Apparently, the contribution into the destruction of the polymeric substrates - carriers of the enzymatic hydrolysis process, can be come to light at the stage of ontogenesis, at the stage of formation and development of the root system this mechanism is replaced by another. Analysis of photographs of the samples of substrates seeds obtained for the stage of root formation and growth (presented in Fig. 2) indicate the evident destruction of the polymer under the influence of germinated this roots. In this case, the roots of the plant create mechanical stresses in the polymer, which there are creation cracks, their fusion, leading to the formation of through holes. Indeed, the pictures (Fig. 2 a, b) which are demonstrate surface of seed carriers, obtained on the 9 the day, show the evident cracks and holes in the polymer (Fig. 2). It is important to note that in the samples of PHB are dominated by transverse holes, while in the PLA they are longitudinal, parallel to the surface of the film.

The pattern of destruction of PHB and PLA corresponds to the nature of the root system. In the first polymer roots germinate in a perpendicular direction to the surface of the film, in the second - parallel to the surface. It can be assumed that the direction of growth and development of the root system in the nonwoven biopolymer material associated with the initiation of seed germination by enzymatic hydrolysis of the polymer carrier, developing on the surface or directed to the volume of the polymer. In turn, this may depend on the structure of the nonwoven material, the structure of their crystallites and amorphous regions, on the nature of the polymer, which determines the mechanism of its biodegradation [13, 27].

Besides the mechanisms of destruction of the polymer carrier of seeds by enzymatic hydrolysis and mechanical action, another mechanism of destruction is possible. Another process in which the biodegradable polymer carrier of seeds can destroy, affecting the stage

of plant growth is the ability of the polymer to participate in radical reactions [28]. Mechanical destruction that occurs in the polymer under the action of roots may rouse the appearance of free radicals.

In addition, the main direction of biochemical processes in the germinating seed, which is enzymatic hydrolysis of starch and lipids, which is in the mass of the seed, may be accompanied by the formation of an excess of free radicals that can destroy cell structures, therefore, lead to cell death.

Contact of germinating seeds with macromolecules of the polymer substrate through an aqueous medium can lead to the interaction of free radicals, which may pick out from the seed into water, with the functional bonds of the macromolecules contacting with it.

Resulting in the reaction of transfer of free valence (r*) from the cell molecule (MH) into the polymer macromolecule (RH), which is convert the kinetic chains of oxidation of cell molecules, protects them from destruction, i.e., the polymer carrier in relation to the cells acts as an antioxidant. At the same time, the oxidation process can be initiated in polymer macromolecules by the following reaction [28]:

MH ^ r*

RH+ r* —> R* + rH

Thus, oxidation of the polymer carrier significantly accelerates the destruction of its amorphous regions. It can be thought that the ability of the polymer matrix to break off the kinetic chains of the oxidative process in cells promotes the development of anabolic reactions of cell growth and, therefore, can accelerate the growth of the root system and plant germs. It is explains the acceleration of the development of wheat sprouts at a more advanced stage of plant growth on the substrate PHB (Fig. 1).

Ability to assess the contribution of enzymatic hydrolysis, mechanical degradation and radical process into summary process of biodegradation of polymer carriers during seed germination may by definite by comparing the energy parameters of melting crystal structures of polymers. That estimated calculation was made for nonwoven material PHB.

Known high sensitivity of wheat seeds to the energy effects, causing the flow of biochemical processes, in particular enzymatic processes. Reducing the energy barrier causing the participation of enzymes can occur due to the decomposition of macromolecules or decomposition of crystal structures.

In this work, the activation energy of crystallite melting Ea was estimated. For this purpose, the Kissinger equation (Kissinger) was used for thermograms obtained by heating the sample at a constant rate [29]. The equation is based on the dependence of the fixed temperature on the polymer heating rate:

Еa = [RT7 /(72 -7-1)] ln(V2/V0,

where Ea - activation energy, R- Universal gas constant,

(3)

T\, 72 - temperature in K, corresponding to heating rates of the sample V h V2.

Ea was determined with the help of equation (3). It were establish the change in the maximum melting point (Tmax), its shift according at the speed of heating in the range of 4 to 16 degrees/min, defined with respect to the standard (In). The obtained values Ea. are presented in table. 6. As can be seen from this table, enzymatic hydrolysis of PHB leads to a significant drop in the activation energy of crystallite melting. The low value of Ea indicates a high degree of defectiveness of the structure of the crystallites as a consequence of their destruction. It is important to emphasize the fact that Ea of the crystallites of PHB treated with root enzymes are lower than those treated with amylase (table 6). The value of Ea in a sample of PHB with sprouted roots is also reduced compared to the original polymer, and is close to the sample treated with water, which may indicate the process of polymer destruction occurring under the action of sprouted seeds, as a process mainly occurring in amorphous areas.

Conclusion

Analysis of the dynamics of changes in the parameters of wheat growth on polymer substrates shows that the process of seed germination and growth of the root system of the plant is autocatalytic and correlates with the destruction of the polymer material.

Based on the data obtained, it follows that the processes of destruction of polymeric materials stimulate seed germination and plant growth. In turn, the processes of seed germination are initiated degradation of polymers.

Destruction of the polymeric carrier of wheat seeds from biodegradable material includes several mechanisms depending on the stage of plant development. Comparison of processes of destruction of materials from PHB and PLA, occurring in the absence of contact with the plant, shows the acceleration of the destruction of the polymer under the action of extract of amylase, isolated from the seeds and enzymes isolated from the root system of wheat.

Comparison of the melting parameters of the samples of polymeric substrates after enzymatic hydrolysis with parameters of the samples obtained after germination of the root system lets make a guess that the effect of polymeric carrier on the rate of germination of wheat seeds is depend on process of enzymatic hydrolysis of the substrate at the stage of ontogenesis and radical processes of mechanodestruction and oxidation of the polymer at the stage of growth and development of plant.

The nonwoven materials PHB and PLA are most suitable in quality carriers of seeds. Ability of these polymers to show the stimulating action to the plant seeds depends on structural parameters, which define the ability to swell and chemical stability in an aqueous solution of enzymes and activity to oxidative destruction. Due to the different location of amorphous and crystalline regions in the fibers of nonwoven PLA compared with PHB changes the direction of growth the root system.

Table 1.

Indicators of germination and biometric parameters of wheat seed seedlings of the "Athena" variety (Triticum _aestivum)._

Sample Characteristics of the **Seed ** Massa, g *Energy **Length **Height

№ sample-substrate in a germina- Full plant root germinations root, cm plants, cm

Petri dish tion, % seeds', %

1 Control (filter paper) 86±2 0.156±0.008 0.035±0.002 80±2 7.8±1.0 117±2

2 PLA extruded film 88±2 0.114±0.008 0.020±0.001 50±5 5.6±1.5 118±5

3 PLA non-woven material 92±2 0.184±0.01 0.054±0.003 70±2 12.0±2.0 125±5

4 PHB extruded film 90±2 0.124±0.008 0.028±0.002 55±5 8.2±1.8 120±5

5 PHB non-woven material 96±2 0.178±0.01 0.050±0.003 96±1 10.4±1.5 135±5

6 PHB+SCN pressed film 96±2 0.160±0.01 0.048±0.002 94±1 9.4±2.0 125±5

7 PHB+SCN nonwoven 96±2 0.196±0.01 0.050±0.003 94±1 9.8±1.0 138±5

material

Note: *the germination energy, defined on the 3 day. ** data obtained on the 7th day of seed germination

Table 2.

The dynamics of germination of wheat seeds on different carriers

Characteristics of the The solvent The number of germinated seeds. Germination

sample material of the from which on the on the on the on the on the index, GI

substrate seed the obtained 1-st 2-nd 3-rd 4-th 7-th

nonwoven material day day day day day

Control - 0 5-7 35-40 43-44 43-45 0.60-0.62

PLA extruded film - 0 0 5-8 25-30 35-40 0.40-0.41

PLA non-woven material CHF 0 7-10 25-27 47-48 47-48 0.63-0.65

PHB extruded film - 0 3-5 5-7 24-26 39-41 0.44-0.46

PHB non-woven material CHF 0 5-7 45-48 47-49 47-49 0.69-0.72

PHB non-woven material CHF + EDC 0 6-7 45-48 48-49 48-49 0.70-0.71

PHB+SCN nonwoven

fabric CHF 0-2 46-47 47-48 47-49 47-50 0.81-0.82

PHB+SCN nonwoven CHF + EDC 2-3 10-46 45-49 47-49 47-50 0.72-0.81

fabric

Table 3.

The mass of samples of initial films and after germination of seeds in them_

Characteristics of the samples sample series № Sample mass, g

1 0.0189±0.002

PHB non-woven material initial 2 0.0182±0.002

1 0.0102±0.002

PHB after seed germination 2 0.0098±0.001

1 0.0494±0.005

PLA non-woven material source 2 0.0445±0.005

1 0.022±0.003

PLA after seed germination 2 0.0197±0.002

Table 4.

Physical and mechanical characteristics of seed substrates of nonwoven PHB, PLA initial and after germination

of wheat seeds in them

Characteristics of the sample Relative The maximum Breaking length, m

samples series № elongation, % load, H

PHB nonwoven material 1-2 2.4-3.9 0.9-1.4 406.3-454.7

initial

PHB after seed germination 1-2 1.0-1.1 0.5-0.9 201.0-364.7

PLA nonwoven

initial material 1-2 30.2-86.1 1.3-1.6 175.1-194.3

PLA after seed germination 1-2 6.6-16.3 3.7-4.0 1073.0-1129.0

Note: measurement Error of physical and mechanical parameters, ±20%

*Note: Tm1 - initial temperature of the melting peak, Tmax. Temperature at the maximum peak of melting, Tm2 - final melting peak temperature, AHm. - melting heat, x is the degree of crystallinity, x - the time of seed germination.

Table 6.

The thermophysical parameters of samples of nonwoven material PHB, treated with water and extract with am-

ylase and enzymes

Table 5.

Thermophysical parameters of polymer samples of the substrates - carriers of the seed from nonwoven material*

Characteristics of the sample material of the substrate seed Temperature, T, °ch AHm, J/g X % x, day

TM 1 Tmax Tm 2

PHB (CHF) initial 156.0 175.0 190.0 61.3 68.1 0

after seed germination 158.0 173.5 184.0 34.1 37.9 9

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PHB (CHF + EDC) initial 160.0 172.8 185.0 65.2 72.4 0

after seed germination 156.0 169.8 180.0 59.7 66.3 9

PHB+SCN (CHF) source 162.0 173.0 180.0 62.8 69.7 0

after seed germination 153.0 170.0 179.0 20.8 23.1 9

PHB+SCN (CHF + EDC)

initial 147.0 172.0 177.0 61.2 68.0 0

after seed germination 159.0 170.0 179.0 17.6 19.5 9

PLA (CHF) initial 152.0 168.9 174.0 41.0 38.6 0

after seed germination 156.0 170.2+ shoulder 164.0 174.0 36.0 34.0 9

162.5+ shoulder 160.0

After seed germination 152.0 168.0 35.0 33.0 14

Characteristicss samples The temperature of the maximum melting peak Tmax, 0C The heat of fusion AH, J/g Crystallinity degree X, % Activation energy of melting of crystallites of PHB Ea, kJ/mol

PHB nonwoven initial material 172.0-174.0 62.9 69.9 500±20

PHB after germination of seeds with the appearance of roots (more than 9 days) 170.0-174.0 33.2 36.9 470±30

PHB nonwoven material after saturation with water 170.0+ shoulder 164 60.0 66.7 450±60

PHB nonwoven material after saturation with amylase from an aqueous seed extract 164.0+ shoulder 160.0 57.1 63.4 209±30

PHB nonwoven material after the saturation of the enzyme from the aqueous extract of the roots 160.0+ shoulder 157.0 51.0 56.7 169±20

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