Technical field

The invention relates to a method of preparation of hierarchically structured self-reinforcing composite systems based on biopolymers of polylactic acid.

In addition, the invention relates to composite systems prepared by this method.

Background art

In recent years, biodegradable polymers, which are prepared from renewable raw material sources, have been the subject of ever-increasing interest. The advantages of these materials primarily include their effect on reducing the carbon footprint, environmental benefits resulting from their ability to biodegrade and the potential to be sustainable material systems of the future. Among the most studied biopolymers falling into this group are aliphatic polyesters, such as polyhydroxyalkanoates (PHA) and polylactic acid (PLA), which is also often called polylactide.

With intensive research into the production of second and third generation high molecular weight PLA using lignocellulose-based raw material sources and direct conversion of greenhouse gases to lactic acid using microorganisms, PLA will gain in importance.

Since lactic acid contains a chiral carbon atom, there are two optical isomeric forms of lactic acid monomer, referred to as L and D, i.e. , poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). Depending on the purity of the individual homopolymers, the stereochemistry of their polymer chains and the process conditions, an amorphous or semicrystalline polymer can be prepared at a glass transition temperature (T _g ) in the range of 50 °C to 60 °C and a melting temperature (Tm) in the range of 130 °C to 180 °C.

Under the conditions of industrial composting, PLA is subject to biodegradation and, thanks to its good biocompatibility and bioresorbability, it belongs to the most widely studied polymers in clinical biomedical studies. In recent years, it has attracted interest not only as a replacement for traditional petroleum-based synthetic polymers, but also as a special polymer in various applications.

On the other hand, PLA also exhibits some properties that limit its use in technical applications. These properties mainly include low impact resistance and high brittleness, which are related to the glassy state of PLA at normal room temperature. The low temperature resistance of PLA is mainly related to poor homonucleation of PLA and insufficient mobility of macromolecular chains during crystallization from melts. Since there is an ever-increasing interest in a long-term sustainable economy that is environmentally friendly, it is necessary to overcome the shortcomings mentioned above. Thus, many strategies have been implemented in the last decades to improve the utility properties of PLA. The most commonly used strategies include heterogeneous nucleation, plasticization using plasticizers, copolymerization and mixing (blending) with other polymers.

Another option to improve the utility properties of PLA is stereocomplexation, which was described for the first time in 1987 by Ikada et al. [1 ], The formation of a PLA stereocomplex (sc-PLA) between the optical isomers poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) leads not only to an increase in mechanical properties, but also to an increase in thermal and hydrolytic resistance. The interactions responsible for the formation of sc-PLA are weak multicentred hydrogen bonding interactions (-CH3 ■ ■■ O=C < and =CH ■ ■ ■ O=C <) between PLLA and PDLA enantiomers, which keep both helical coiled macromolecular chains with opposite configurations together [2], As a result of these interactions, sc-PLA crystallites have a melting temperature approximately by 50 °C higher than PLLA or PDLA homocrystallites when equimolar mixed.

The preparation of sc-PLA melt samples by blending (mixing) PLLA/PDLA followed by an additional crystallisation step in the presence of steam or supercooled water at a temperature of 120 to 200 °C is disclosed in international application W02016172011A1 . Application WO2014147132A1 describes the preparation of sc-PLA by mixing solutions or melts using specific nucleating agents. The production of sc-PLA based samples by injection molding of PLLA/PDLA mixtures (blends) under specific temperature and pressure conditions is disclosed in international application W02008104757A1 . International application W02009045877A1 deals with the preparation of fibers by melt spinning of PLLA/PDLA blends and subsequent heat treatment, where a high sc-PLA content is achieved. The preparation of nanofibrous membranes based on sc-PLA is the subject of international application WO2015155182A1. Nanofibers are obtained by a complex process of spinning the PLLA/PDLA blend from a melt, which is followed by heat treatment, washing, selective extraction, drying and hot pressing.

The preparation of stereocomplexed PLA nanofibers by electrostatic spinning was first described in 2006 by Tsuji et al. [3], Since then, numerous laboratory procedures for the preparation of sc-PLA nanofibers using various solvent systems under the action of direct electric current have been described. The preparation of a flat film from non-woven sc-PLA nanofibers using direct current and an injection needle electrode placed on a syringe is described, for example, in CN107142610B.

The concept of single-polymer composites, often referred to as "selfreinforcing" composites, was first described in 1975 by Capiati and Porter [4], who used oriented polyethylene (PE) fibers and PE powder with different melting temperatures in their research. Since then, various techniques have been developed to combine oriented fibers with a separate homogeneous phase of the matrix of the composite. The advantages of such self-reinforcing systems mainly include the ability to achieve an excellent interphase interface and, due to their relative chemical homogeneity, they increase their added value by their recyclability compared to composites based on heterogeneous components. The preparation of such polymer structures is dealt with in documents WO1 997033017A1 , WO1993011204A1 and W02009127864A1 .

The preparation of self-reinforcing P LA-based composites was for the first time described by Makela et al. [5], Li and Yao [6] later prepared self-reinforcing PLA composites by stacking amorphous PLA films and highly-oriented PLA fibers using hot pressing. The preparation of unidirectionally oriented self-reinforced PLA composites was addressed by Gao et al. [7], Self-reinforcing transparent stereocomplexed PLA nanocomposites were then prepared by Kurokawa and Hotta [8], Their study utilized electrostatic spinning to prepare sc-PLA nanofibrous nonwoven structure, which was subsequently inserted between two PLA films and hot-molded. The disadvantage of this process is that the nonwoven fabric lacks the orientation of the nanofibers in the loading direction of the composite and the resulting material therefore does not exhibit high mechanical properties.

Thus, neither the preparation nor the composition of practically applicable hierarchically structured self-reinforcing composite systems based on PLA biopolymers with uniform structure and uniform distribution of mechanical properties has been described so far.

The object of the invention is to provide a method of preparation of hierarchically structured self-reinforcing composite systems based on polylactic acid biopolymers and such composite systems. These systems will significantly improve the mechanical properties and temperature resistance of traditional PLA- based products, increasing their applicability in replacing fossil based polymers. Other advantages of such systems can be seen especially in the reduction of the carbon footprint and the fact that these materials can be completely based on renewable raw materials with targeted recyclability or biodegradability.

List of non-patent literature:

[1 ] Ikada, Y., Jamshidi K., Tsuji H., and Hyon, S. H. Stereocomplex formation between enantiomeric poly (lactides), Macromolecules, 20(4), 904-906, 1987

[2] Sarasua, J. R., Rodriguez, N. L., Arraiza, A. L., and Meaurio, E., Stereoselective crystallization and specific interactions in polylactides. Macromolecules, 38(20), 8362-8371 , 2005

[3] Tsuji, H., Nakano, M., Hashimoto, M., Takashima, K., Katsura, S., and Mizuno, A. Electrospinning of poly (lactic acid) stereocomplex nanofibers. Biomacromolecules, 7(12), 3316-3320, 2006

[4] Capiati, N. J. and Porter, R. S. The concept of one polymer composites modelled with high density polyethylene. Journal of materials science, 10(10), 1671 -1677, 1975,

[5] Makela, P., Pohjonen, T., Tdrmala, P., Waris, T., and Ashammakhi, N. Strength retention properties of self-reinforced poly l-lactide (SR-PLLA) sutures compared with polyglyconate (MaxonR) and polydioxanone (PDS) sutures. An in vitro study. Biomaterials, 23(12), 2587-2592, 2002

[6] Li, R., and Yao, D. Preparation of single poly (lactic acid) composites. Journal of applied polymer science, 107(5), 2909-2916, 2008 [7] Gao, C., Meng, L., Yu, L., Simon, G. P., Liu, H., Chen, L., and Petinakis, S. Preparation and characterization of uniaxial poly (lactic acid)-based selfreinforced composites. Composites Science and Technology, 117, 392-397, 2015

[8] Kurokawa, N., & Hotta, A. Thermomechanical properties of highly transparent self-reinforced polylactide composites with electrospun stereocomplex polylactide nanofibers. Polymer, 153, 214-222, 2018.

Principle of the invention

The object of the invention is achieved by a method of preparation of hierarchically structured self-reinforcing composite systems based on biopolymers of polylactic acid (PLA), which include nanofibers based on poly(L- lactide) (PLLA) or based on an asymmetric or symmetrical blend of poly(L-lactide) and poly(D-lactide) (PDLA). Preferably, these nanofibers are prepared by spinning a solution, preferably, for example, by the method according to WO2014094694A1 , using alternating current, where an electric field for spinning is alternately created between a spinning electrode to which alternating voltage is applied and air and/or gas ions created and/or brought into its vicinity. In this process, a polymer dosing device known from W02019047990A1 and/or a spinning electrode in one of the embodiments known from W02017108012A1 can be used. During spinning with the use of alternating current, the nanofibers formed are carried in the direction of the gradient of the electric field away from the spinning electrode, forming a hollow plume (sleeve). This is then deposited according to the method of WO2016192697A2 on a supporting fibrous core which rotates around its own axis and/or balloons, creating a nanofibrous sheath on its surface. Another method of creating such a two-component linear structure is also known from WO2008149488A1 .

The formed two-component core yam 1. contains a fibrous core 2 on which a nanofibrous sheath 3 is deposited, the nanofibrous sheath 3 being formed by a nanofibrous plume wound in a helix. The fibrous core 2 consists of a monofilament (see Figure 1 ) or multifilament (see Figure 2) textile yarn made from PLA biopolymer. This yam unwinds and simultaneously rotates around its own axis and/or balloons in the working space of the device in which the electrically neutral nanofibrous plume is created. Upon contact of the plume with the fibrous core 2, the plume is transformed into a flat stripe of oriented nanofibers and spirally wraps around the fibrous core 2. The density of the nanofibrous sheath 3 thus formed can be controlled by the rate at which the solution is dispensed onto the surface of the spinning electrode and/or by the rate at which the continuous fibrous core 2 is unwound.

Both the fibrous core 2 and the nanofibrous sheath 3 are made from PLA biopolymer, wherein in the case of the fibrous core 2, the biopolymer usually has the same or lower melting temperature than in the case of the nanofibrous sheath 3, as will be explained below.

Before entering the working space and the path of the nanofibrous plume, the fibrous core 2 is preferably preheated to a desired working temperature in the range of 25 to 55 ± 1 °C.

The formed two-component core yarn 11 can, if necessary, be heat-fixed by passing through a die heated to a temperature of 55 to 100 ± 1 °C and then dried in a tempering chamber located behind the working space of the device.

The two-component core yam 1. thus formed is subsequently processed by standard textile techniques into strands, planar (2D) and spatial (3D) textile structures which can serve independently or as self-reinforcing elements of hierarchically structured biodegradable composite systems based on PLA biopolymers, or PLA copolymers, mixtures with other biopolymers, their combination, or with the addition of additives (plasticizers, nucleating agents, flame retardants, lubricants, UV stabilizers, antioxidants, dyes, antistatic agents, blowing agents, impact modifiers, etc.), fillers and/or discontinuous fiber reinforcement. As a rule, these modifications must not compromise the biodegradability of the resulting material research.

In the preparation of hierarchically structured self-reinforcing composite systems according to the invention, the two-component core yarn 1. is processed into a flat (2D) textile, e.g., fabric - see Figs. 2 and 4, and deposited in a PLA- based matrix 4 - see Figs. 3 and 5. All components of this system have a melting temperature equal to or lower than the nanofibrous sheath 3 of the used two- component linear structure. Analogously, a knitted fabric made from the two- component core yarn 1. can also be used.

A PLA-based spinning solution is prepared by dissolving PLLA or a mixture of PLLA and PDLA in a suitable solvent system which contains dichloromethane (DCM), dimethyl sulfoxide (DMSO) and pyridine (PY). The concentration of the polymer in this solution is 5 to 15 wt. %, the ratio of solvents in the solvent system DCM : DMSO : PY is (3 to 7.5) : (1 .5 to 4) : (0.5 to 3.5).

In the case of using the mixture of PLLA and PDLA, solutions of these polymers in DCM are preferably prepared separately, and after their mixing and homogenization, DMSO and PY are added to them. The polymer solution thus prepared is homogenized and then spun using one of the methods of preparation of polymer nanofibers. A particularly advantageous method is spinning using alternating current, during which an electrically neutral plume of nanofibers is created, which moves in the direction of the gradient of the electric field away from the spinning electrode, and at the same time is caught on a monofilament or multifilament PLA yam rotating or ballooning around its longitudinal axis, on which it creates a uniform and continuous nanofibrous sheath 3. Thus, a two- component yarn 1_ is prepared.

From the thus prepared two-component core yarn 1., a flat textile, e.g., a fabric or knitted fabric of any type, is then prepared by a suitable textile technique and is subsequently incorporated into a PLA-based matrix 4. In a suitable procedure, at least one layer of this fabric is inserted between two plates of PLA polymer or co-PLA and together with them is exposed to elevated temperature (150 to 200 °C) and pressure (150 to 250 kN). In a preferred variant, this semiproduct is heated for 3 to 8 minutes to a temperature of 150 to 180 °C, at which the material of the plates melts and the flat fabric is saturated with this matrix 4, and then it is heated for 15 to 80 seconds to a temperature of 160 to 200 °C, at which the textile is saturated with the matrix 4 and the entire structure is fixed. However, due to the different melting temperatures, the core 2 and the sheath 3 of the two-component core yam 1_ do not melt, and consequently, the yam 1_ and its components retain their morphology and character. The result of this process is a hierarchically structured self-reinforcing composite system based on PLA biopolymers with different melting temperatures, which contains at least one layer of fabric prepared from the two-component core yam 1_ based on PLA deposited in a PLA or co-PLA matrix 4. The textile constitutes 5 to 35 wt. % of this composite.

In order to achieve the required mechanical properties of hierarchically structured self-reinforcing composite systems when multiple layers of fabric are used, these layers can be suitably oriented relative to each other, e.g., by rotating them by a suitable angle, e.g., 45 °, 90 °, or in any other way, or by combining textiles of different types.

In the enclosed drawings, Fig. 1 a shows a scheme of a two-component linear structure with a monofilament yam core, Fig. 1 b shows a scheme of a two- component linear structure with a multifilament yam core, Fig. 2 shows a scheme of a plain weave fabric made from a two-component linear structure with a monofilament yam core, Fig. 3 shows a scheme of a section through the hierarchically structured self-reinforcing composite system according to the invention with one fabric layer according to Fig. 2, Fig. 4 shows a scheme of a plain weave fabric made from a two-component linear structure with a multifilament yam core, Fig. 5 is a scheme of a section through the hierarchically structured self-reinforcing composite system according to the invention with one fabric layer according to Fig. 4.

Fig. 6 is an SEM image of one variant of PLLA nanofibers at a magnification of 25,000 times, Fig. 7 shows an SEM image of the two-component linear structure with the monofilament yam core and the sheath formed by the layer of PLLA nanofibers according to Fig. 6 at a magnification of 475 times, Fig. 8 shows an SEM image of a unidirectional fabric made from the two-component linear structure according to Fig. 7 at a magnification of 109 times, Fig. 9 shows the DSC curves of the individual components of the hierarchically structured selfreinforcing composite system according to the invention formed using the unidirectional fabric according to Fig. 8, Fig. 10a shows an SEM image of the fracture surface of this hierarchically structured self-reinforcing composite system at a magnification of 158 times, and Fig. 10b shows a detail of an interphase interface at the fracture surface in Fig. 10a.

Fig. 11 shows an SEM image of one variant of sc-PLA nanofibers at a magnification of 15,000 times, Fig. 12 shows an SEM image of the two- component linear structure with the multifilament yam core and the sheath formed by the layer of sc-PLA nanofibers according to Fig. 11 at the point of rupture at a magnification of 183 times, Fig. 13 shows an SEM image of fabric consisting of the two-component linear structure according to Fig. 12 at a magnification of 125 times, Fig. 14 shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite system according to the invention formed using the fabric according to Fig. 13, Fig. 15a shows an SEM image of the fracture surface of this hierarchically structured self-reinforcing composite system at a magnification of 630 times, and Fig. 15b shows a detail of the interphase interface at the fracture surface in Fig. 15a at a magnification of 2,520 times.

Fig. 16 shows an SEM image of a second variant of sc-PLA nanofibers at a magnification of 25,000 times, Fig. 17 shows an SEM image of the two- component linear structure with the monofilament yarn core and the sheath formed by the layer of sc-PLA nanofibers according to Fig. 11 at the section point at a magnification of 416 times, Fig. 18 shows an SEM image of unidirectional fabric made from the two-component linear structure according to Fig. 17 at a magnification of 85 times, Fig. 19 shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite system according to the invention formed using fabric according to Fig. 18, Fig. 20a shows an SEM image of the fracture surface of this hierarchically structured selfreinforcing composite system at a magnification of 349 times, and Fig. 20b shows a detail of the interphase interface at the fracture surface in Fig. 20a at a magnification of 4,320 times.

Fig. 21 shows an SEM image of a third variant of sc-PLA nanofibers at a magnification of 5,200 times, Fig. 22 shows an SEM image of the two-component linear structure with the multifilament yarn core and the sheath formed by the layer of sc-PLA nanofibers according to Fig. 21 at the section point at a magnification of 611 times, Fig. 23 shows an SEM image of twill fabric made of the two-component linear structure according to Fig. 22 at a magnification of 84 times, Fig. 24 shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite system according to the invention formed using the fabric according to Fig. 23, Fig. 25a shows an SEM image of the fracture surface of this hierarchically structured self-reinforcing composite system at a magnification of 500 times, and Fig. 25b shows a detail of the interphase interface at the fracture surface in Fig. 25a at a magnification of 2,540 times. Fig. 26 is an SEM image of a fourth variant of sc-PLA nanofibers at a magnification of 25,000 times, Fig. 27 shows an SEM image of the two- component linear structure with the monofilament yarn core and the sheath formed by the layer of sc-PLA nanofibers according to Fig. 26 at the section point at a magnification of 191 times, Fig. 28 shows an SEM image of plain weave fabric made from the two-component linear structure according to Fig. 27 at a magnification of 124 times, Fig. 29 shows the DSC curves of the individual components of hierarchically structured self-reinforcing composite system according to the invention formed using the fabric according to Fig. 28, Fig. 30a shows an SEM image of the fracture surface of this hierarchically structured selfreinforcing composite system at a magnification of 405 times, and Fig. 30b shows a detail of the interphase interface at the fracture surface in Fig. 30a at a magnification of 3,100 times.

Fig. 31 shows an SEM image of a fifth variant of sc-PLA nanofibers at a magnification of 14,800 times, Fig. 32 shows an SEM image of a two-component linear structure with a multifilament yarn core and a sheath formed by a layer of sc-PLA nanofibers according to Fig. 31 at the section point at a magnification of 396 times, Fig. 33 shows an SEM image of twill fabric made from the two- component linear structure according to Fig. 32 at a magnification of 221 times, Fig. 34 shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite system according to the invention formed using the fabric according to Fig. 33, Fig. 35a shows an SEM image of the fracture surface of this hierarchically structured self-reinforcing composite system at a magnification of 821 times, and Fig. 35b shows a detail of the interphase interface on the fracture surface in Fig. 35a at a magnification of 2,550 times.

Examples of embodiment

Illustrative examples of the preparation of hierarchically structured selfreinforcing composite systems based on biopolymers of polylactic acid (PLA) and composite systems thus obtained are given below for clarity.

Before each technological processing, all PLA biopolymers, compounds, mixtures and fabrics are dried for at least 12 hours in a vacuum dryer at a temperature of 80 °C to prevent their degradation due to hydrolytic reactions. Additives added are similarly dried at a temperature of 50 °C. Parameters in the examples of the used solvents based on dichloromethane (DCM, CH ₂ CI ₂ ), dimethyl sulfoxide (DMSO, (CH ₃ ) ₂ SO) and pyridine (PY, C5H5N) correspond to the following specifications, only their concentrations differ: DCM has a specific mass of 1320 kg/m ³ , a molar mass of 85 g/mol and a chemical purity of 99.88 %; DMSO has a specific mass of 1100 kg/m ³ , a molar mass of 78.13 g/mol, and a chemical purity of 99.88 %. PY has a specific mass of 983 kg/m ³ , a molar mass of 79.1 g/mol and a chemical purity of 99.5 %.

PLA monofilament (trade name 6101, Perlon) was used as a fibrous core 2, with a diameter of 0.22 mm, a melting temperature Tm of 170 °C, a fineness of 48 Tex, or PLA multifilament (trade name PLA-DTY dtex 76 f 32, Trevira) was used as a fibrous core 2. This multifilament consisted of 32 elementary fibers having a fineness of 7.6 Tex and a melting temperature Tm of 180 °C.

PLA biopolymers modified with additives (e.g., plasticizers, impact modifiers, nucleating agents, lubricants) and with other biopolymers (e.g., polyester-based thermoplastic elastomer - TPE) were compounded on a twin- screw extruder to achieve the desired weight in the resulting material system. The matrix of the composite was prepared from plates of PLA biopolymer or sc-PLA having dimensions of 145 x 145 x 2 mm prepared by injection molding technology; the mold being cooled to a temperature of 20 °C. The resulting hierarchically structured self-reinforcing composite systems were subsequently prepared by inserting at least one layer of fabric between two PLA or sc-PLA plates followed by hot pressing on a heated press.

Table 1 shows some mechanical parameters of the individual components of the structured self-reinforcing composite system and of the structured selfreinforcing composite systems according to Examples 1 to 6 for comparison. Example 1

10 g of poly(L-lactide) (PLLA, with the trade name Luminy L130, Total Corbion PLA) with a weight average molar mass Mw = 180 kg/mol, a melting temperature Tm = 180 °C and an L-enantiomer content of at least 99 % was dissolved under constant stirring at a temperature of 23 ± 1 °C in 63 g of dichloromethane (DCM). 18 g of dimethyl sulfoxide (DMSO) and then 9 g of pyridine (PY) was further added to the thus obtained solution. DMSO has a higher boiling point than DCM, so it evaporates later during spinning and thus aids in the elongation and orientation of the nanofibers. Pyridine is added to the mixture to increase the conductivity of the resulting solution. The polymer solution prepared by this process, which contained 10 wt. % of PLLA and 90 wt. % of the solvent mixture (70 % DCM, 20 % DMSO, and 10 % PY) was homogenized under constant stirring for 12 hours and then spun by electric spinning.

Alternating current (AC) with a voltage of 10 kV and a frequency of 50 Hz was applied to the spinning electrode, while the polymer solution described above was supplied to its spinning surface at a rate of 10 mL/min. Spinning took place at a laboratory temperature of 23 ± 1 °C and a relative humidity of 35 %. During this process, a spatial hollow plume consisting of nanofibers with a diameter of 200 ± 62 nm was formed - see Fig. 6, which shows an SEM image of these nanofibers at a magnification of 25,000 times. The plume was fed into the working space of the device, where it was continuously wound onto a core made from PLA monofilament yam which was unwound at a speed of 60 m/min. In this manner, a two-component yam was prepared with a core consisting of PLA monofilament yam and a sheath formed by a layer of PLLA nanofibers, see Fig. 7, which is an SEM image of this yam at a magnification of 475 times, showing its structure. The two-component core yam thus prepared showed a fineness of 53 tex.

The two-component core yam thus prepared was subsequently made into a unidirectional fabric - see Fig. 8, which shows an SEM image of this fabric at a magnification of 109 times, in which both the cores and the nanofibrous sheaths of the individual two-component core yams are visible.

To prepare the matrix of the composite, two plates based on PLA copolymer were used (co-PLA, trade name Luminy LX175, Total Corbion PLA) with a weight average molar mass M _w = 163 kg/mol, a melting temperature Tm = 155 °C and an L-enantiomer content of 96 %. The plates were made by thermoplastic injection technology at temperatures ranging from 160 °C to 190 °C, and their dimensions were 145 x 145 x 2 mm.

Unidirectional fabric made from two-component core yam was inserted between these two PLLA plates, and the thus prepared structure was tempered for 5 minutes in a press in contact mode at a temperature of 160 °C. During the tempering process, the co-PLA plates melt and the co-PLA fabric is saturated with the matrix; however, there is no melting of the core and the sheath of the two-component core - see Fig. 9, which shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite, from which it is clear that only the material of the plates melted at a temperature of 160 °C. The temperature was then increased to 170 °C and after reaching it, pressing took place for 20 seconds with a clamp force of 160 kN. This was followed by non-isothermal cooling while maintaining the clamp force of 160 kN until a temperature of 50 °C was reached; subsequent cooling was already without force. The result of the process was a hierarchically structured self-reinforcing composite system based on PLA biopolymers with different melting temperatures. The unidirectional fabric layer in this case constituted 9 ± 1 % of the weight of this system.

Fig. 10a shows an SEM image of the fracture surface of the above- mentioned structured self-reinforcing composite system at a magnification of 158 times and Fig. 10b shows an SEM image of a detail of the interphase interface on the fracture surface at a magnification of 968 times. It can be seen from the two images that neither the core nor the sheath of the two-component core yarn melted during the heat treatment.

Example 2

7.2 g of PLLA same as in Example 1 was dissolved in 49 g of DCM under constant stirring at a temperature of 23 ± 1 °C.

0.8 g of poly(D-lactide) (PDLA, with the trade name Luminy D070, Total Corbion PLA) with a weight average molar mass Mw = 85 kg/mol, a melting temperature Tm = 180 °C and a D-enantiomer content of at least 99 % was dissolved in 6.2 g of DCM.

The polymer solutions thus prepared were mixed and to the mixture formed 23 g of DMSO and 13.8 g of PY was added.

The polymer solution prepared by this process, which contained 8 wt. % of PLA biopolymers (PLLA 90 % and PDLA 10 %) and 92 wt. % of the solvent mixture (60 % DCM, 25 % DMSO, and 15 % PY) was homogenized under constant stirring on a magnetic stirrer for 12 hours and then spun by electric spinning. Alternating current (AC) with a voltage of 20 kV and a frequency of 60 Hz was applied to the spinning electrode, while the polymer solution described above was supplied to its spinning surface at a rate of 15 mL/min. Spinning took place at a laboratory temperature of 23 ± 1 °C and a relative humidity of 35 %. During this process, a spatial hollow plume consisting of nanofibers with a diameter of 310 ± 43 nm was formed - see Fig. 11 , which shows an SEM image of these nanofibers at a magnification of 15,000 times. The plume was fed into the working space of the device, where it was continuously wound onto a core made from PLA multifilament yarn which was unwound at a speed of 50 m/min. In this manner, a two-component yam was prepared with a core consisting of PLA multifilament yam and a sheath formed by a layer of sc-PLA nanofibers (PLLA 90 % and PDLA 10 %) with a fineness of 10 tex - see Fig. 12, which shows an SEM image of this yam at a magnification of 183 times and in which the structure of this yam is visible.

From the two-component core yam thus prepared, plain weave fabric was then formed - see Fig. 13, which shows an SEM image of this fabric at a magnification of 125 times and in which both the cores and the nanofibrous sheaths of the individual two-component core yams are visible.

Two PLLA plates with a content of 15 wt % plasticizer - acetyl tributy I citrate (ATBC, CITROFLEX* A-4, Vertellus LLC) were used for the preparation of the matrix of the composite. The plates were made by thermoplastic injection technology at temperatures ranging from 170 °C to 200 °C, and their dimensions were 145 x 145 x 2 mm.

Between these two PLLA plates, two identical layers of plain weave fabric made from two-component core yam, rotated relative to each other by 90°, were inserted, and the thus prepared structure was tempered for 5 minutes in a press in contact mode at a temperature of 165 °C. During the tempering process, the PLLA plates melt and the PLLA fabric is saturated with the matrix; however, there is no melting of the core and the sheath of the two-component core - see Fig. 14, which shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite, from which it is apparent that at a temperature of 165 °C only the material of the plates melted. The temperature was then increased to 170 °C and after reaching it, pressing took place for 30 seconds with a clamp force of 170 kN. This was followed by non-isothermal cooling while maintaining the clamp force of 170 kN until a temperature of 50 °C was reached; the following cooling process was already without applying clamp force. The result of the process was a hierarchically structured self-reinforcing composite system based on PLA biopolymers with different melting temperatures. In this case, the layers of plain weave fabric constituted 13 ± 1 % of the weight of this system.

Fig. 15 shows an SEM image of the fracture surface of the above- mentioned hierarchically structured self-reinforcing composite system at a magnification of 630 times and Fig. 15b shows an SEM image of a detail of the interphase interface on this fracture surface at a magnification of 2,520 times. It can be seen from the two images that neither the core nor the sheath of the two- component core yarn melted during the heat treatment.

Example 3

6.4 g of poly(L-lactide) (PLLA, with the trade name Luminy L175, Total Corbion PLA) with a weight average molar mass M _w = 210 kg/mol, a melting temperature Tm = 180 °C and an L-enantiomer content of at least 99 % was dissolved under constant stirring at a temperature of 35 ± 1 °C in 40 g of DCM.

1 .6 g of PDLA same as in Example 2 was dissolved in 6.2 g DCM.

The polymer solutions thus prepared were mixed and 27.6 g of DMSO and 13.8 g of PY was added to the mixture obtained. The thus obtained polymer solution, which contained 8 wt. % of PLA biopolymers (PLLA 80 % and PDLA 20 %) and 92 wt. % of the solvent mixture (55 % DCM, 30 % DMSO and 15 % PY) was left to homogenize under constant stirring for 12 hours and then spun by electric spinning.

Alternating current (AC) with a voltage of 30 kV and a frequency of 70 Hz was applied to the spinning electrode, while the above-described polymer solution was supplied to its spinning surface at a rate of 20 mL/min. Spinning took place at a laboratory temperature of 23 ± 1 °C and a relative humidity of 35 %. During this process, a spatial hollow plume consisting of nanofibers with a diameter of 245 ± 71 nm was formed - see Fig. 16, which shows an SEM image of these nanofibers at a magnification of 25,000 times. The plume was fed into the working space of the device, where it was continuously wound onto a core made from PLA monofilament yam which was unwound at a speed of 40 m/min. In this manner, a two-component yarn was prepared with a core made from PLA monofilament yarn and a sheath formed by a layer of sc-PLA nanofibers - see Fig. 17, which shows an SEM image of this yarn at a magnification of 416 times and from which its structure is visible. The two-component core yarn prepared in this way showed a fineness of 57 tex.

The two-component core yam thus prepared was subsequently made into a unidirectional fabric - see Fig. 18, which shows an SEM image of this fabric at a magnification of 85 times, in which both the cores and the nanofibrous sheaths of the individual two-component core yarns are visible.

To prepare the matrix of the composite, two plates based on PLA copolymer (co-PLA, trade name Luminy 130, Total Corbion PLA) with biodegradable polyester-based thermoplastic elastomer (TPE, trade name NP EL 208-65A, NaturePlast) were used. The TPE content was 20 wt. %. The plates were made by thermoplastic injection technology at temperatures in the range of 160 °C to 190 °C, and their dimensions were 145 x 145 x 2 mm.

Between these two co-PLA plates, two identical layers of unidirectional fabric made from two-component core yarn, rotated relative to each other by 90°, were inserted, and the thus prepared structure was tempered for 3 minutes in a press in contact mode at a temperature of 170 °C. During the tempering process, the co-PLA plates melt and the co-PLA fabric is saturated with the matrix; however, there is no melting of the core and the sheath of the two-component core - see Fig. 19, which shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite, showing that at a temperature of 170 °C only the material of the plates melted. Subsequently, the temperature was increased to 175 °C, and after reaching it, pressing took place for 40 seconds with a clamp force of 185 kN. This was followed by non-isothermal cooling while maintaining the clamp force of 185 kN until a temperature of 50 °C was reached; the following cooling process was already without applying clamp force. The result of the process was a hierarchically structured self-reinforcing composite system with a crystalline structure based on PLA biopolymers with different melting temperatures. In this case, the layers of unidirectional fabric constituted 19 ± 1 % of the weight of the above-mentioned system.

Fig. 20a shows an SEM image of the fracture surface of the above- mentioned structured self-reinforcing composite system at a magnification of 349 times and Fig. 20b shows an SEM image of a detail of the interphase interface on the fracture surface at a magnification of 4,320 times. It can be seen from the two images that neither the core nor the sheath of the two-component core yarn melted during the heat treatment. Furthermore, the morphology of the matrix is visible in the two images, where TPE is dispersed in co-PLA in the form of spherical particles.

Example 4

4.9 g of PLLA same as in Example 1 was dissolved in 32 g of DCM under constant stirring at a temperature of 35 ± 1 °C.

2.1 g poly(D-lactide) (PDLA, with trade name Luminy D120, Total Corbion PLA) with a weight average molar mass M _w = 154 kg/mol, a melting temperature Tm = 180 °C and an L-enantiomer content of at least 99 % was dissolved under constant stirring in 14.5 g of DCM.

The polymer solutions thus prepared were mixed and to the mixture obtained, 27.9 g DMSO and 18.6 g PY was added. The thus prepared polymer solution, which contained 7 wt. % of PLA biopolymers (PLLA 70 % a PDLA 30 %) and 93 wt. % of the solvent mixture (50 % DCM, 30 % DMSO, and 20 % PY) was left to homogenize under constant stirring for 12 hours and then spun by electris spinning.

Alternating current (AC) with a voltage of 35 kV and a frequency of 75 Hz was applied to the spinning electrode, while the above-described polymer solution was supplied to its spinning surface at a rate of 25 mL/min. Spinning took place at a laboratory temperature of 23 ± 1 °C and a relative humidity of 35 %. During this process, a spatial hollow plume consisting of nanofibers with a diameter of 402 ± 37 nm was formed - see Fig. 21 , which shows an SEM image of these nanofibers at a magnification of 5,200 times. The plume was fed into the working space of the device, where it was continuously wound onto a core made from PLA multifilament yam which was unwound at a speed of 30 m/min. In this manner, a two-component yam was prepared with a core consisting of PLA multifilament yam and a sheath formed by a layer of sc-PLA nanofibers - see Fig. 22, which shows an SEM image of this yam at a magnification of 611 times, and from which its structure is visible. The two-component core yam thus prepared showed a fineness of 14 tex. From the two-component core yam thus prepared, twill fabric was then formed - see Fig. 23, which shows an SEM image of this fabric at a magnification of 84 times, in which both the cores and the nanofibrous sheaths of the individual two-component core yams are visible.

Two PLLA plates (trade name Luminy 130, Total Corbion PLA) containing a PLA-based impact modifier (masterbatch) were used to prepare the matrix of the composite. The impact modifier content was 10 wt. %. The plates were made by thermoplastic injection technology at temperatures ranging from 170 °C to 200 °C, and their dimensions were 145 x 145 x 2 mm.

Between these two PLLA plates, three identical layers of twill weave fabric made from two-component core yam, rotated relative to each other by 90 °, were inserted and the thus prepared structure was tempered for 4 minutes in a press in contact mode at a temperature of 170 °C. During the tempering process, the PLLA plates melt and the PLLA fabric is saturated with the matrix; however, there is no melting of the core and the sheath of the two-component core - see Fig. 24, which represents the DSC curves of the individual components of the hierarchically structured self-reinforcing composite, showing that at a temperature of 170 °C only the material of the plates melted. The temperature was then increased to 180 °C, and after reaching it, pressing took place for 30 seconds with a clamp force of 190 kN. This was followed by non-isothermal cooling while maintaining the clamp force of 190 kN until a temperature of 100 °C was reached. At this temperature, the composite crystallized isothermally for 2 minutes. Subsequently, non-isothermal cooling continued while maintaining the clamp force of 190 kN until a temperature of 50 °C was reached; the following cooling process was already without applying clamp force. The result of the process was a hierarchically structured self-reinforcing composite system with a crystalline structure based on PLA biopolymers with different melting temperatures. In this case, the layers of twill fabric constituted 21 ± 2 % of the weight of the above-mentioned system.

Fig. 25a shows an SEM image of the fracture surface of this structured self-reinforcing composite system at a magnification of 500 times and Fig. 25b shows an SEM image of a detail of the interphase interface on the fracture surface at a magnification of 2,540 times. It can be seen from the two images that neither the core nor the sheath of the two-component core yarn melted during the heat treatment. Furthermore, the morphology of the matrix with cavities caused by the addition of the masterbatch can be seen in both images.

Example 5

3.6 g of PLLA same as in Example 3 was dissolved in 25 g of DCM under constant stirring at a temperature of 35 ± 1 °C.

2.4 g of PDLA same as in Example 4 was dissolved under constant stirring in 17.3 g of DCM.

The polymer solutions thus prepared were mixed and 28.2 g of DMSO and 23.5 g of PY was added to the mixture obtained. The thus prepared polymer solution, which contained 6 wt. % of PLA biopolymers (PLLA 60 % and PDLA 40 %) and 93 wt. % of the solvent mixture (50 % DCM, 30 % DMSO, and 20 % PY) was left to homogenize under constant stirring for 12 hours and then spun by electric spinning.

Alternating current (AC) with a voltage of 40 kV and a frequency of 80 Hz was applied to the spinning electrode, while the above-described polymer solution above was supplied to its spinning surface at a rate of 30 mL/min. Spinning took place at a laboratory temperature of 23 ± 1 °C and a relative humidity of 35 %. During this process, a spatial hollow plume consisting of nanofibers with a diameter of 290 ± 29 nm was formed - see Fig. 26, which shows an SEM image of these nanofibers at a magnification of 25,000 times. The plume was fed into the working space of the device, where it was continuously wound onto a core made from PLA monofilament yam which was unwound at a speed of 25 m/min. In this manner, a two-component yam was prepared with a core consisting of PLA monofilament yam and a sheath formed by a layer of sc-PLA nanofibers - see Fig. 27, which shows an SEM image of this yam at a magnification of 191 times and from which its structure is visible.

From the two-component core yam thus prepared, plain weave fabric was then formed - see Fig. 28, which shows an SEM image of this fabric at a magnification of 124 times, in which both the cores and the nanofibrous sheaths of the individual two-component core yams can be seen.

To prepare the matrix of the composite, two PLLA plates (trade name Luminy 130, Total Corbion PLA) containing 0.3 wt.% of the nucleating agent - orotic acid monohydrate (OA, Merck) were used. The plates were made by thermoplastic injection moulding at temperatures ranging from 160 °C to 200 °C, and their dimensions were 145 x 145 x 2 mm.

Between these two PLLA plates, three identical layers of plain weave fabric made from two-component core yarn, rotated relative to each other by 45°, were inserted, and the thus prepared structure was tempered for 6 minutes in a press in contact mode at a temperature of 170 °C. During the tempering process, the PLLA plates melt and the PLLA fabric is saturated with the matrix; however, there is no melting of the core and the sheath of the two-component core - see Fig. 29, which shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite, from which it is visible that at a temperature of 170 °C only the material of the plates melted. The temperature was then increased to 180 °C, and after reaching it, pressing took place for 40 seconds with a clamp force of 200 kN. This was followed by non-isothermal cooling while maintaining the clamp force of 200 kN until a temperature of 90 °C was reached. At this temperature, the composite crystallized isothermally for 3 minutes. Subsequently, non-isothermal cooling continued while maintaining the clamp force of 190 kN until a temperature of 50 °C was reached; the following cooling process was already without applying clamp force. The result of the process was a hierarchically structured self-reinforcing composite system with a crystalline structure based on PLA biopolymers with different melting temperatures. The layers of twill fabric constituted 30 ± 1 % of the weight of the above-mentioned system.

Fig. 30a shows an SEM image of the fracture surface of the above- mentioned structured self-reinforcing composite system at a magnification of 405 times and Fig. 30b shows an SEM image a detail of the interphase interface on the fracture surface at a magnification of 3,100 times. It can be seen from the two images that neither the core nor the sheath of the two-component core yarn melted during the heat treatment. Furthermore, the morphology of the matrix with a nucleating agent based on orotic acid can be seen in the two images.

Example 6

2.5 g of PLLA same as in Example 1 was dissolved in 16.6 g of DCM under constant stirring at a temperature of 40 ± 1 °C. 2.5 g of PDLA same as in Example 4 was dissolved in 17.3 g of DCM under constant stirring.

The polymer solutions thus prepared were mixed and 33.3 g of DMSO and 28.5 g of PY was added to the mixture obtained. The thus prepared polymer solution, which contained 5 wt. % of PLA biopolymers (PLLA 50 % and PDLA 50 %) and 95 wt. % of the solvent mixture (35 % DCM, 35 % DMSO, and 20 % PY) was left to homogenize under constant stirring for 12 hours and then spun by electric spining.

Alternating current (AC) with a voltage of 50 kV and a frequency of 90 Hz was applied to the spinning electrode, while the above-described polymer solution was supplied to its spinning surface at a rate of 35 mL/min. Spinning took place at a laboratory temperature of 23 ± 1 °C and a relative humidity of 35 %. During this process, a spatial hollow plume consisting of nanofibers with a diameter of 480 ± 61 nm was formed - see Fig. 31 , which shows an SEM image of these nanofibers at a magnification of 14,800 times. The plume was fed into the working space of the device, where it was continuously wound onto a core made from PLA multifilament yarn which was unwound at a speed of 20 m/min. In this manner, a two-component yam was prepared with a core made from PLA multifilament yarn and a sheath formed by a layer of sc-PLA nanofibers - see Fig. 32, which shows an SEM image of this yarn at a magnification of 396 times and from which its structure is visible.

Subsequently, twill weave fabric was formed from the two-component core yarn thus prepared - see Fig. 33, which shows an SEM image of this fabric at a magnification of 221 times, on which both the cores and the nanofibrous sheaths of the individual two-component core yarns are visible.

Two PLLA plates (trade name Luminy 130, Total Corbion PLA) with a content of 0.5 wt. % of a lubricant based on N,N'-Ethylenebis(stearamide) (EBS, Merck) were used for the preparation of the composite of the matrix. The plates were made by thermoplastic injection technology at temperatures ranging from 180°C to 210°C, and their dimensions were 145 x 145 x 2 mm.

Between these two PLLA plates, four identical layers of twill weave fabric made from two-component core yarn, rotated relative to each other by 45°, were inserted, and the thus prepared structure was tempered for 5 minutes in a press in contact mode at a temperature of 170 °C. During the tempering process, the PLLA plates melt and the co-PLLA fabric is saturated with the matrix; however, there is no melting of the core and the sheath of the two-component core - see Fig. 34, which shows the DSC curves of the individual components of the hierarchically structured self-reinforcing composite, from which it is visible that at a temperature of 170 °C only the material of the plates melted. The temperature was then increased to 180 °C, and after reaching it, pressing took place for 60 seconds with a clamp force of 220 kN. This was followed by non-isothermal cooling while maintaining the clamp force of 220 kN until a temperature of 110 °C was reached. At this temperature, the composite crystallized isothermally for 1 minute. Subsequently, non-isothermal cooling continued while maintaining the clamp force of 220 kN until a temperature of 50 °C was reached; subsequent cooling was without force. The result of the process was a hierarchically structured self-reinforcing composite system with a crystalline structure based on PLA biopolymers with different melting temperatures. In this case, the layers of twill fabric constituted 27 ± 1 % of the weight of this system.

Fig. 35a shows an SEM image of the fracture surface of the above- mentioned structured self-reinforcing composite system at a magnification of 821 times and Fig. 35b shows an SEM image of a detail of the interphase interface on the fracture surface at a magnification of 2,550 times. It can be seen from the two images that neither the core nor the sheath of the two-component core yarn melted during the heat treatment.

Methods of measuring the properties of the resulting material systems:

The tensile test of two-component nanofibrous core yams according to Examples 1 to 6 was carried out according to ISO 2062, the standard is intended for threads on packages and determines the breaking strength and elongation at break of individual threads using an apparatus with a constant elongation rate. The results of the test are the values of modulus of elasticity in tension (Et), ultimate tensile strength (Om) and nominal relative elongation at break (stb). The sample was fixed in jaws spaced Io = 250 mm apart, which moved away from each other at a speed of 100 mm/min. The tensile test of the hierarchically structured self-reinforcing biocomposites according to Examples 1 to 6 was conducted according to ISO 527-4, the standard is intended for isotropic and orthotropic plastic composites reinforced with fibrous filler. The result of the test was modulus of elasticity in tension (Et), ultimate tensile strength (Om) and nominal relative elongation at break (stb). A preload of 2 N was used to measure the samples. To determine the modulus of elasticity in tension, a speed of 1 mm/min was used, and a speed of 5 mm/min was used to determine the ultimate strength and ultimate tensile strength.

The impact strength test of the hierarchically structured self-reinforcing biocomposites according to Examples 1 to 6 was carried out according to standard ISO 179-1 and complement ISO 179-1 Z1 fll by the Charpy method with a nominal hammer energy of 5 J.

Determination of the temperature of deflection of the hierarchically structured self-reinforcing biocomposites of Examples 1 to 6 under load (HDT) was performed according to ISO 75-2, method A using a bending stress of 1.8 MPa at a heating rate of 120 °C/h.

Transition temperatures and enthalpies, ratios of homo- and stereocomplex crystallites were obtained from differential scanning calorimetry (DSC) measurements according to ISO 11357, which describes thermoanalytical DSC testing methods. A heating rate of 10 °C/min was selected.

Below, Table 1 shows the mechanical properties of the two-component nanofibrous core yams used in Examples 1 to 6, and Table 2 shows the mechanical properties of the hierarchically structured self-reinforcing composite systems based on polylactic acid biopolymers according to Examples 1 to 6 created using these two-component nanofibrous core yams. able 1 (part 1/2) able 2 (part 1/6) able 2 (part 2/6) able 2 (part 3/6) able 2 (part 4/6) able 2 (part 5/6) able 2 (part 6/6)

Industrial applicability

Increasing the application possibilities of sustainable polymers in various industries in replacing traditional polymers that are based on notorious nonrenewable fossil resources. Possibilities of targeted recycling or biodegradation of prepared parts with the aid of industrial composting. The potential for use in biomedical applications or the transport industry due to good temperature resistance, biocompatibility and bioresorbability with the possibility of controlling the breakdown of hierarchical structures depending on the ratio of the individual isomers and the degree of crystallinity.