**Preface to "Nanocomposites of Polymers and Inorganic Particles"**

In the last few years, significant efforts have been devoted to designing, fabricating, and exploit nanocomposite materials based on inorganic nanoparticles incorporated in a polymer matrix [1–5]. The extraordinary interest in such materials relies on the large range of properties that can arise from the synergic combination of the features of nanoparticles (NPs) and the host polymer. Indeed, the original size-dependent physical and chemical properties of nanomaterials (semiconducting, metals, oxides, and magnetic NPs) combined with the high processability, the defined chemistry, and the morphology of polymers and block copolymers finally turn out to be innovative materials with high technological impact in a variety of advanced application in photonic, optoelectronics, sensing, environmental, energy conversion, biological, and biomedical fields [6–11].

The contributions to this Special Issue cover all of the specific aspects of this topic, ranging from preparatory approaches, functionalization strategies of NPs and polymers, processing and integration of nanocomposites in additive manufacturing materials, and technological methodologies to obtain functional multiphase materials for advanced applications.

In composites, the use of fillers at the nanometric scale allows for taking advantage of the very small size, the extended surface area, and the tunability of the interpaticle distance as a function of the filler loading compared to conventional materials based on micro-sized particles. The use of such nanofillers can improve the stiffness of polymeric hosts and prevent the material from cracks delaying the natural failure of the material. However, the homogeneous dispersion of NPs within the polymeric host is not a straightforward outcome and is crucial for good preservation of the mechanical, morphological, and physicochemical properties of the nanocomposite, as the occurrence of micro-aggregation can catalyze the mechanical stress, reduce the effective surface area, and finally worsen the mechanical and rheological properties of the multiphase material. Thus, the interactions among NPs, and between NPs and the polymer matrix needs to be investigated and optimized to fabricate effective nanocomposite materials for advanced applications. In order to tune these interactions and then to disperse the fillers in a polymer matrix, grafting of the polymer to the nanoparticle surface is a technique often employed, able to improve the affinity between fillers and polymer matrices. In the first contribution to this Special Issue [12], the influence of graft length and graft density on the state of dispersion, crystallization, and rheological properties of poly(-caprolactone) (PCL)/silica (SiO2) nanocomposites were studied. Specifically, controlling the length and density of the grafted silica, different states of NP dispersion were found, ranging from small spherical aggregates to sheet-like microstructures depending on the matrix-to-graft molecular weight ratios.

Among the several methods for nanocomposite preparation, Demina and coworkers [13] developed an original approach based on near infrared light-activated photopolymerization of nanocomposites containing luminescent lanthanide-doped up-conversion NPs, effective both in oligomer bulk and on the NP surface in aqueous dispersion. In [14], interfacial polymerization in inverse (water-in-oil) nanoemulsion was exploited to prepare polyurea nanocapsules containing ionic liquid-modified magnetite NPs as multiphase systems with potential application in catalysis and biomedicine, such as for the targeted delivery of hydrophilic drugs. Possible walkthroughs for obtaining polymer nanocomposites with homogeneously dispersed inorganic nanoparticles, limiting the phase segregation and micro-aggregation phenomena, are chemical modification of the nanoparticle surface, proper functionalization of the polymer side-chains, or in situ synthesis of the NPs inside the host matrix. In order to introduce well-defined functionalities into polyolefin matrices, specifically UV absorption and antibacterial activity, Anzlovar et al. obtained nanocomposites ˇ through the deposition of surface-modified ZnO NPs on polyolefin granules and subsequent melt processing [15].

Besides great efforts devoted to the development of novel and original synthetic strategies to gain nanocomposite systems, a significant issue in such a intriguing field is the investigation of the mechanical, electrical, thermal, magnetic, and optical properties arising in the final composite, where the material performance has successfully benefited from the combination of peculiar features of all of the components constituting the system. An in-depth study of the mechanical properties and performance test of an epoxy resin matrix reinforced by the addition of "green"-synthesized oxidized MWCNTs is reported in [16]. The authors applied a vacuum casting approach for different geometries to limit the increase in viscosity resulting from adding CNTs to the epoxy resin; performing various mechanical tests that show improved fracture toughness, bending, and tensile properties with the addition of MWCTs; and finally, analyzing the strengthening mechanisms by SEM and in situ imaging by digital image correlation (DIC) of the fracture surface.

A further relevant issue that needs to be resolved concerns the degradation properties of plastic materials and the environmental stability of polymer nanocomposites. Indeed, achieving the biodegradation of commercial plastics is an enormous environmental challenge due to the increased social demand for higher sustainability processes. The addition of nanofillers often improves the mechanical behavior of the polymeric host, although it sometimes can accelerate photodegradation processes of the matrix. In [17], CaCO<sup>3</sup> NPs were synthesized and modified by oleic acid to improve their interaction with a low-density polyethylene (LDPE) matrix. The effect of photoaging under UV irradiation on the structural (crystallinity percentage, c), chemical (carbonyl index, CI), and mechanical (Young's modulus) properties of composites were studied and compared with those of the pure polymer to assess if the presence of CaCO<sup>3</sup> NPs accelerates the photodegradation of the LDPE.

Examples of nanofillers that strengthen the mechanical behavior of polymeric matrices are very common in the panorama of nanocomposite materials. Besides such beneficial effects, the introduction of inorganic nanoparticles within polymeric chains can also induce specific and well-defined abilities, thus allowing for their use in widespread potential applications. Wang et al. [18] proposed a novel nanocomposite film with enhanced mechanical performance and antimicrobial properties for potential biomedical applications. Specifically, a multifunctional nanocomposite obtained by the in situ growth of Ag NPs on the surface of sericin/agar film showed high mechanical property, hydrophilicity, hygroscopicity, and stability and finally demonstrated an excellent antibacterial activity against *E. coli* and *S. aureus*. An additional example of the preparation of silk sericin-based nanocomposites and its application as antibacterial material was proposed by Ai et al. [19]. In their work, the authors reinforced the mechanical properties of sericin by blending with PVA and by grafting ZnO NPs with the support of polydopamine (PDA). The ZnO NP-added PDA-SS/PVA films were characterized by improved mechanical stability, tensile strength, and elongation at break and possessed excellent hydrophilicity and swellability, demonstrating a remarkable antimicrobial activity against *E. coli* and *S. aureus* to thus be effectively exploited as active nanocomposite film in antibacterial biomaterial applications.

The last contribution to this Special Issue reported on the preparation of spherical poly

(styrene-co-(2-hydroxyethyl methacrylate)) (PS/HEMA) opal structures and spherical TiO<sup>2</sup> inverse opal structures by electro-hydrodynamic atomization [20]. Specifically, titania NPs of relatively small size were assembled at the interstitial site of PS/HEMA NP, resulting in a spherical opal composite structure with potential application in a widespread area, such as reflective mode display, photo catalysis, solar cell electrode materials, and analytical systems.

These contributions are not exhaustive but represent an updated panorama of some of the infinite possibilities in the field of nanocomposite materials and can be an inspiration for new and advanced challenges in their preparation and application in different technological fields.

Funding: This research received no external funding.

Acknowledgments: The guest editors thank all of the authors that have contributed to this Special Issue and all of the reviewers for their evaluation of the submitted articles.

Conflicts of Interest: The authors declare no conflicts of interest.

#### **References**

[1] Mourdikoudis, S., Kostopoulou, A., LaGrow, A. P., Magnetic Nanoparticle Composites: Synergistic Effects and Applications. Adv. Sci. 2021, 8, 2004951, doi: https://doi.org/10.1002/advs.202004951

[2] Nandihalli, N.; Liu, C.-J.; Mori, T. Polymer based thermoelectric nanocomposite materials and devices: Fabrication and characteristics. Nano Energy 2020, 78, 105186, doi: https://doi.org/10.1016/j.nanoen.2020.105186.

[3] Loste, J.; Lopez-Cuesta, J.-M.; Billon, L.; Garay, H.; Save, M. Transparent polymer nanocomposites: An overview on their synthesis and advanced properties. Prog. Polym. Sci. 2019, 89, 133-158, doi: https://doi.org/10.1016/j.progpolymsci.2018.10.003.

[4] Khalid, K.; Tan, X.; Mohd Zaid, H.F.; Tao, Y.; Lye Chew, C.; Chu, D.-T.; Lam, M.K.; Ho, Y.-C.; Lim, J.W.; Chin Wei, L., Advanced in developmental organic and inorganic nanomaterial: a review. Bioengineered 2020, 11, 328-355, doi: 10.1080/21655979.2020.1736240.

[5] Pourhashem, S.; Saba, F.; Duan, J.; Rashidi, A.; Guan, F.; Nezhad, E.G.; Hou, B., Polymer/Inorganic nanocomposite coatings with superior corrosion protection performance: A review. Journal of Industrial and Engineering Chemistry 2020, 88, 29-57, doi: https://doi.org/10.1016/j.jiec.2020.04.029.

[6] Melinte, V.; Stroea, L.; Chibac-Scutaru, A.L., Polymer Nanocomposites for Photocatalytic Applications. Catalysts 2019, 9, doi: 10.3390/catal9120986.

[7] Liu, S.-W.; Wang, L.; Lin, M.; Liu, Y.; Zhang, L.-N.; Zhang, H., Tumor Photothermal Therapy Employing Photothermal Inorganic Nanoparticles/Polymers Nanocomposites. Chinese Journal of Polymer Science 2019, 37, 115-128, doi: 10.1007/s10118-019-2193-4.

[8] Adnan, M.M.; Tveten, E.G.; Glaum, J.; Ese, M.-H.G.; Hvidsten, S.; Glomm, W.; Einarsrud, M.-A., Epoxy-Based Nanocomposites for High-Voltage Insulation: A Review. Advanced Electronic Materials 2019, 5, 1800505, doi: https://doi.org/10.1002/aelm.201800505

[9] Surmenev, R.A.; Orlova, T.; Chernozem, R.V.; Ivanova, A.A.; Bartasyte, A.; Mathur, S.; Surmeneva, M.A., Hybrid lead-free polymer-based nanocomposites with improved piezoelectric response for biomedical energy-harvesting applications: A review. Nano Energy 2019, 62, 475-506, doi:https://doi.org/10.1016/j.nanoen.2019.04.090.

[10] Kausar, A., A review of high performance polymer nanocomposites for packaging applications in electronics and food industries. Journal of Plastic Film & Sheeting 2019, 36, 94-112, doi: 10.1177/8756087919849459.

[11] Cantarella, M.; Impellizzeri, G.; Di Mauro, A.; Privitera, V.; Carroccio, S.C., Innovative Polymeric Hybrid Nanocomposites for Application in Photocatalysis. Polymers 2021, 13, doi: 10.3390/polym13081184.

[12] Eriksson, M.; Hamers, J.; Pe, T.; Goossens, H. The Influence of Graft Length and Density on Dispersion, Crystallisation and Rheology of Poly(-caprolactone)/Silica Nanocomposites. Molecules 2019, 24, 2106, doi:10.3390/molecules24112106.

[13] Demina, P.; Arkharova, N.; Asharchuk, I.; Khaydukov, K.; Karimov, D.; Rocheva, V.; Nechaev, A.; Grigoriev, Y.; Generalova, A.; Khaydukov, E. Polymerization Assisted by Upconversion Nanoparticles under NIR Light. Molecules 2019, 24, 2476, doi:10.3390/molecules24132476.

[14] Natour, S.; Levi-Zada, A.; Abu-Reziq, R. Magnetic Polyurea Nano-Capsules Synthesized via Interfacial Polymerization in Inverse Nano-Emulsion. Molecules 2019, 24, 2663, doi:10.3390/molecules24142663.

[15] Anzlovar, A.; Primo ˇ ziˇ c, M.; ˇ Svab, I.; Leitgeb, M.; Knez, ˇ Z.; ˇ Zagar, E. Polyolefin/ZnO ˇ Composites Prepared by Melt Processing. Molecules 2019, 24, 2432, doi:10.3390/molecules24132432.

[16] Singer, G.; Siedlaczek, P.; Sinn, G.; Kirner, P.H.; Schuller, R.; Wan-Wendner, R.; Lichtenegger, H.C. Vacuum Casting and Mechanical Characterization of Nanocomposites from Epoxy and Oxidized Multi-Walled Carbon Nanotubes. Molecules 2019, 24, 510, doi:10.3390/molecules24030510.

[17] Zapata, P.A.; Palza, H.; D´ıaz, B.; Arm, A.; Sepulveda, F.; Ortiz, J.A.; Ram ´ ´ırez, M.P.; Oyarzun, ´ C. Effect of CaCO3 Nanoparticles on the Mechanical and Photo-Degradation Properties of LDPE. Molecules 2019, 24, 126, doi:10.3390/molecules24010126.

[18] Wang, Y.; Cai, R.; Tao, G.; Wang, P.; Zuo, H.; Zhao, P.; Umar, A.; He, H. A Novel AgNPs/Sericin/Agar Film with Enhanced Mechanical Property and Antibacterial Capability. Molecules 2018, 23, 1821, doi:10.3390/molecules23071821.

[19] Ai, L.; Wang, Y.; Tao, G.; Zhao, P.; Umar, A.; Wang, P.; He, H. Polydopamine-Based Surface Modification of ZnO Nanoparticles on Sericin/Polyvinyl Alcohol Composite Film for Antibacterial Application. Molecules 2019, 24, 503, doi:10.3390/molecules24030503.

[20] Lim, J.-M.; Jeong, S. Fabrication of Spherical Titania Inverse Opal Structures Using Electro-Hydrodynamic Atomization. Molecules 2019, 24, 3905, doi:10.3390/molecules24213905.
