**3. 3D-printing of polymer nanocomposites, characterized by a tailored viscosity and biodegradability**

Nowadays there is no polymer processing treatment capable of competing with the SLS with respect to the fabrication flexibility and complexity of the 3D shapes obtained. The priority of such processing for the construction of new complex parts is clear. However, the advancement of polymer laser sintering technologies is obviously hindered by several factors, such as:

**1.** Insufficient repeatability and control of the process, which stems from a lack of funda‐ mental understanding. Indeed, the intricate relations between the polymer properties and processing conditions on the one hand, and the final microstructure of the material and/or physical properties of the object on the other hand, are far from being completely understood.

Polymer powdered materials are characterized by complex physical mechanisms in‐ volved in the heating processes by a laser radiation source followed by their coalescence and melting, and then by crystallization during cooling, therefore they require the advanced experimental techniques allowing to study them in real time and at the adequate space resolution [13, 14].

**2.** Difficulties in developing of polymer matrices with optimized parameters of the melting transition and melt viscosity.

Since for pure polymer matrices the fine-tuning of these parameters can hardly be achieved, the use of polymer with nanocomposite inclusions is very promissory.

The above-mentioned conclusions are based on the recent studies of the nanocomposite rheology [12–14]. Earlier, in most studies on the polymer nanocomposites, the filler particles of 10–20 nm and even bigger were used. But of highest interest is the case when the size of the particles is still lower than 10 nm. In this case, the particles size becomes commensurable with the polymer ball dimensions, individual polymer coils are approached and the interparticle half-gap is comparable with their gyration radius (*R*g) or even smaller than this [15]. This produces chain confinements and distortion that can result in the depletion-driven phase segregation, or unusual properties—if segregation can be avoided. Thus, it is well known that the melt viscosity of polymers can be significantly reduced by adding a small amount of fine nanoparticles. This phenomenon is explained by the increase in the free volume owing to adding nanoparticles that is confirmed in some cases by the decrease in the glass-transition temperature. It should be noted that the viscosity reduction was observed only in cases when the interparticles half-gap was smaller than the *R*g of the polymer coil.

The self-healing effect is another interesting example observed in a multilayered nanocompo‐ site polymer structure [16]. The study showed that nanoparticles dispersed in a polymer matrix migrated to cracks generated at the interface between the polymer and glass fiber layer. According to the results of computer simulations, nanoparticles in a polymer can segregate to the surfaces and into the cracks due to the polymer-induced "depletion attraction" between the particles and the surface. In this case, only the particles comparable by their size to the polymer *R*g were driven from the matrix to the surface in the crack area. Importantly, a homogeneous dispersion of the nanoparticles in the polymer matrix is a prerequisite for achieving the above-mentioned properties.

The surface enrichment with nanoparticles may result in the improvement of several practically important properties (i.e., friction, wear resistance, flame retardation, chemical resistance, touch feeling, and optical properties). This phenomenon was not taken into account in most studies of the polymer nanocomposites. In recent years, polymer composites are used increasingly for tribological applications, and the effectiveness in tribological performance of nanocomposites over microcomposites has been verified in many systems [17]. However, the specific role of nanoparticles in this case remains an open question. In particular, the tribological properties have rarely been related to the nanocomposite morphology, i.e., the distribution of particles in the polymer (bulk vs. surface). Yet, the improvements of properties can be attributed in this case to the surface enrichment with the nanoparticles.

The selection of new bioresorbable nanocomposites is feasible by the SLS approach. The polymer materials became widely used in various biomedical applications such as design of synthetic tissues and organs from biocompatible materials and stem cells, testing of new drug delivery systems, researches on tissues and organs in normal and abnormal states. The ability to form controllable regular structures from polymer molecules is used for the development of biomedical materials such as porous matrixes for tissue engineering and therapeutic agents' delivery [18]. The fabrication of polymer matrices from biodegradable polymers can facilitate the solution of environmental issues related to the 3D printing industry and promote its further development.

In spite of a great amount of works devoted to the development of polymer nanocomposites, including biocompatible and/or bioresorbable polymers, a clear insights into their structure property relations is still extremely deficient. This is partly due to the tendency of nanoparticles toward the agglomeration inside the polymer matrix. The fabrication of polymer-based matrix nanocomposites with a homogeneous distribution of particles sized from tens to hundreds of nanometers, affords to systematically study the influence of the ratio of interparticles distance to the polymer coil size on the melt rheology and performance properties of the nanocompo‐ sites.

Nanoparticles of different sizes are to be surface-modified and mixed with polymer, so that to ensure a homogeneous particles distribution. Earlier, we received mesocomposites based on PA/PC etc. containing different amounts of inert inclusions (including those of nanosizes) [3, 6]. The use of the nanoparticles is expected to enhance the control over the melt viscosity, crystallization kinetics (e.g., nucleation effect of the nanoparticles), and preferable formation of certain polymorphic modifications of PA/PC (e.g., γ-phase vs. α-phase), which is highly important for SLS processes [13]. The use of ultrasmall nanoparticles for obtaining control over the nanocomposite melt viscosity is original and is based on recent researches of the melt viscosity decrease due to the addition of such nanoparticles to the polymer matrix.

The self-healing effect is another interesting example observed in a multilayered nanocompo‐ site polymer structure [16]. The study showed that nanoparticles dispersed in a polymer matrix migrated to cracks generated at the interface between the polymer and glass fiber layer. According to the results of computer simulations, nanoparticles in a polymer can segregate to the surfaces and into the cracks due to the polymer-induced "depletion attraction" between the particles and the surface. In this case, only the particles comparable by their size to the polymer *R*g were driven from the matrix to the surface in the crack area. Importantly, a homogeneous dispersion of the nanoparticles in the polymer matrix is a prerequisite for

The surface enrichment with nanoparticles may result in the improvement of several practically important properties (i.e., friction, wear resistance, flame retardation, chemical resistance, touch feeling, and optical properties). This phenomenon was not taken into account in most studies of the polymer nanocomposites. In recent years, polymer composites are used increasingly for tribological applications, and the effectiveness in tribological performance of nanocomposites over microcomposites has been verified in many systems [17]. However, the specific role of nanoparticles in this case remains an open question. In particular, the tribological properties have rarely been related to the nanocomposite morphology, i.e., the distribution of particles in the polymer (bulk vs. surface). Yet, the improvements of properties

The selection of new bioresorbable nanocomposites is feasible by the SLS approach. The polymer materials became widely used in various biomedical applications such as design of synthetic tissues and organs from biocompatible materials and stem cells, testing of new drug delivery systems, researches on tissues and organs in normal and abnormal states. The ability to form controllable regular structures from polymer molecules is used for the development of biomedical materials such as porous matrixes for tissue engineering and therapeutic agents' delivery [18]. The fabrication of polymer matrices from biodegradable polymers can facilitate the solution of environmental issues related to the 3D printing industry and promote its further

In spite of a great amount of works devoted to the development of polymer nanocomposites, including biocompatible and/or bioresorbable polymers, a clear insights into their structure property relations is still extremely deficient. This is partly due to the tendency of nanoparticles toward the agglomeration inside the polymer matrix. The fabrication of polymer-based matrix nanocomposites with a homogeneous distribution of particles sized from tens to hundreds of nanometers, affords to systematically study the influence of the ratio of interparticles distance to the polymer coil size on the melt rheology and performance properties of the nanocompo‐

Nanoparticles of different sizes are to be surface-modified and mixed with polymer, so that to ensure a homogeneous particles distribution. Earlier, we received mesocomposites based on PA/PC etc. containing different amounts of inert inclusions (including those of nanosizes) [3, 6]. The use of the nanoparticles is expected to enhance the control over the melt viscosity, crystallization kinetics (e.g., nucleation effect of the nanoparticles), and preferable formation of certain polymorphic modifications of PA/PC (e.g., γ-phase vs. α-phase), which is highly

can be attributed in this case to the surface enrichment with the nanoparticles.

achieving the above-mentioned properties.

development.

242 New Trends in 3D Printing

sites.

Consideration of the polymer matrices based on biodegradable polymers such as polycapro‐ lactone (PLC) is another interesting issue for the practical medicine [19, 20]. This last issue is of high importance for all the 3D printing technologies, as it can solve the problem of growing concerns related to the environmental threat of the 3D printing industry development. We were among those first who experimentally studied the optical characteristics of polymer systems (PC, PA, etc.), impregnated with metallic particles [21]. We used an advanced strategy of determination the beam part, which was subjected to absorbing, scattering, and transmis‐ sion. We offered original decisions for the characterization of thermophysical properties of such metal—polymer powdered mixtures also [22]. However, there is an obvious need to continue this work with nano-sized additives.

While studying the heat transfer and phase transformation processes, it is highly demanded to analyze the microstructural model of semicrystalline polymer, which is usually forming the nano-sized crystals (lamels) that have a very small depth but a significantly larger lateral size. The polymer structure formation under laser sintering is described in papers [23, 24]. However, for the comprehensive theoretical description of the process, the knowledge of a great number of parameters is demanded, such as temperature of phase transformation, enthalpy changing and thermochemical properties of materials. Moreover, rheological parameters of the polymer melt will influence the sintering process, which in detail described by the molecular dynamic theory [25]. Besides, the particles aggregation depends on viscoelastic properties of the polymer matrix, which are, in turn, crucially dependent on the environment conditions, as well as on degree of crystallinity and crystal texture of the material. The modeling of the melted particles aggregation behavior can be conducted by using simplified models of Frenkel, Maxwell, Kelvin-Voigt, or Bingham.

Therefore, the study of the nanoparticle influence on the final material properties gains special importance. Oxides nanoparticles are most widely used nano-sized additives for polymers and their synthesis and surface modification are well known. Nevertheless, nanoparticles aggregation is still hardly avoidable. It is known that presence of silicon dioxide nanoparticles in the polymer matrix increases the Young modulus but decreases its ultimate elongation, as to the data on the nanoparticles influence on others properties, the data about it is contradictory and inconsistent.
