Polymer Clay Nano-composites Stefano Leporatti www.mdpi.com/journal/polymers Edited by Printed Edition of the Special Issue Published in Polymers Polymer Clay Nano-composites Polymer Clay Nano-composites Special Issue Editor Stefano Leporatti MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade Special Issue Editor Stefano Leporatti CNR Nanotec-Istituto di Nanotecnologia Italy Editorial Office MDPI St. Alban-Anlage 66 4052 Basel, Switzerland This is a reprint of articles from the Special Issue published online in the open access journal Polymers (ISSN 2073-4360) from 2017 to 2019 (available at: https://www.mdpi.com/journal/polymers/ special issues/polymer clay composites) For citation purposes, cite each article independently as indicated on the article page online and as indicated below: LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. Journal Name Year , Article Number , Page Range. 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Contents About the Special Issue Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Stefano Leporatti Polymer Clay Nano-composites Reprinted from: Polymers 2019 , 11 , 1445, doi:10.3390/polym11091445 . . . . . . . . . . . . . . . . 1 Yang Wu, Yongzhi Zhang, Junping Ju, Hao Yan, Xiaoyu Huang and Yeqiang Tan Advances in Halloysite Nanotubes–Polysaccharide Nanocomposite Preparation and Applications Reprinted from: Polymers 2019 , 11 , 987, doi:10.3390/polym11060987 . . . . . . . . . . . . . . . . 5 Xiandong Zhang and Guangshun Wu Grafting Halloysite Nanotubes with Amino or Carboxyl Groups onto Carbon Fiber Surface for Excellent Interfacial Properties of Silicone Resin Composites Reprinted from: Polymers 2018 , 10 , 1171, doi:10.3390/polym10101171 . . . . . . . . . . . . . . . . 23 Xiaohu Qiang, Songyi Zhou, Zhuo Zhang, Qiling Quan and Dajian Huang Synergistic Effect of Halloysite Nanotubes and Glycerol on the Physical Properties of Fish Gelatin Films Reprinted from: Polymers 2018 , 10 , 1258, doi:10.3390/polym10111258 . . . . . . . . . . . . . . . . 36 Vladimir Vinokurov, Andrei Novikov, Valentina Rodnova, Boris Anikushin, Mikhail Kotelev, Evgenii Ivanov and Yuri Lvov Cellulose Nanofibrils and Tubular Halloysite as Enhanced Strength Gelation Agents Reprinted from: Polymers 2019 , 11 , 919, doi:10.3390/polym11050919 . . . . . . . . . . . . . . . . . 50 Angelo Earvin Sy Choi, Cybelle Morales Futalan and Jurng-Jae Yee Fuzzy Optimization on the Synthesis of Chitosan-Graft-Polyacrylic Acid with Montmorillonite as Filler Material: A Case Study Reprinted from: Polymers 2019 , 11 , 738, doi:10.3390/polym11040738 . . . . . . . . . . . . . . . . . 61 Jinhui Liu, Di Li, Xiangshuai Zhao, Jieting Geng, Jing Hua and Xin Wang Buildup of Multi-Ionic Supramolecular Network Facilitated by In-Situ Intercalated Organic Montmorillonite in 1,2-Polybutadiene Reprinted from: Polymers 2019 , 11 , 492, doi:10.3390/polym11030492 . . . . . . . . . . . . . . . . . 77 Guiqing Shu, Jing Zhao, Xiu Zheng, Mengdie Xu, Qi Liu and Minfeng Zeng Modification of Montmorillonite with Polyethylene Oxide and Its Use as Support for Pd 0 Nanoparticle Catalysts Reprinted from: Polymers 2019 , 11 , 755, doi:10.3390/polym11050755 . . . . . . . . . . . . . . . . . 95 Chengcheng Yu, Yangchuan Ke, Xu Hu, Yi Zhao, Qingchun Deng and Shichao Lu Effect of Bifunctional Montmorillonite on the Thermal and Tribological Properties of Polystyrene/Montmorillonite Nanocomposites Reprinted from: Polymers 2019 , 11 , 834, doi:10.3390/polym11050834 . . . . . . . . . . . . . . . . . 106 Ji Zhou, Qiang Cai and Fu Xu Nanoscale Mechanical Properties and Indentation Recovery of PI@GO Composites Measured Using AFM Reprinted from: Polymers 2018 , 10 , 1020, doi:10.3390/polym10091020 . . . . . . . . . . . . . . . . 123 v Tingting Jiang, Guangxiang Chen, Xiangyang Shi and Rui Guo Hyaluronic Acid-Decorated Laponite R © Nanocomposites for Targeted Anticancer Drug Delivery Reprinted from: Polymers 2019 , 11 , 137, doi:10.3390/polym11010137 . . . . . . . . . . . . . . . . . 130 Jorge A. Ram ́ ırez-G ́ omez, Javier Illescas, Mar ́ ıa del Carmen D ́ ıaz-Nava, Claudia Muro-Urista, Sonia Mart ́ ınez-Gallegos and Ernesto Rivera Synthesis and Characterization of Clay Polymer Nanocomposites of P(4VP -co- AAm) and Their Application for the Removal of Atrazine Reprinted from: Polymers 2019 , 11 , 721, doi:10.3390/polym11040721 . . . . . . . . . . . . . . . . . 144 Alexandros K. Nikolaidis, Elisabeth A. Koulaouzidou, Christos Gogos and Dimitris S. Achilias Synthesis and Characterization of Dental Nanocomposite Resins Filled with Different Clay Nanoparticles Reprinted from: Polymers 2019 , 11 , 730, doi:10.3390/polym11040730 . . . . . . . . . . . . . . . . . 164 Cinzia Cristiani, Elena Maria Iannicelli-Zubiani, Giovanni Dotelli, Elisabetta Finocchio, Paola Gallo Stampino and Maurizio Licchelli Polyamine-Based Organo-Clays for Polluted Water Treatment: Effect of Polyamine Structure and Content Reprinted from: Polymers 2019 , 11 , 897, doi:10.3390/polym11050897 . . . . . . . . . . . . . . . . . 185 Yu Liang, Dexin Yang, Tao Yang, Ning Liang and Hao Ding The Stability of Intercalated Sericite by Cetyl Trimethylammonium Ion under Different Conditions and the Preparation of Sericite/Polymer Nanocomposites Reprinted from: Polymers 2019 , 11 , 900, doi:10.3390/polym11050900 . . . . . . . . . . . . . . . . . 201 Yidong Liu, Lingfeng Jian, Tianhua Xiao, Rongtao Liu, Shun Yi, Shiyang Zhang, Lingzhi Wang, Ruibin Wang and Yonggang Min High Performance Attapulgite/Polypyrrole Nanocomposite Reinforced Polystyrene (PS) Foam Based on Supercritical CO 2 Foaming Reprinted from: Polymers 2019 , 11 , 985, doi:10.3390/polym11060985 . . . . . . . . . . . . . . . . . 212 Elodie Bugnicourt, Nicola Brzoska, Esra Kucukpinar, Severine Philippe, Enrico Forlin, Alvise Bianchin and Markus Schmid Dispersion and Performance of a Nanoclay/Whey Protein Isolate Coating upon its Upscaling as a Novel Ready-to-Use Formulation for Packaging Converters Reprinted from: Polymers 2019 , 11 , 1410, doi:10.3390/polym11091410 . . . . . . . . . . . . . . . . 222 vi About the Special Issue Editor Stefano Leporatti , Ph.D., received his Master’s degree in Physics at University of Genoa and in 1999 obtained his PhD in Solid State Physics at Max Planck Institute of Colloids and Interface Science with Prof. Dr. Helmuth Mohwald. From 2001 to 2006, he was a Research Scientist at the Institute of Medical Physics & Biophysics, Universit ̈ at Leipzig, Leipzig (Germany). From 2006 to 2008 he was CNR Senior Researcher (University Associate Professor Equivalent) at the National Nanotechnology Laboratory (NNL) of CNR-INFM in Lecce, Italy (NanoBioMolecular Division of Prof. R. Rinaldi) and, since 2008, he has been their NanoCarriers and BioMechanics Group Leader. From 2015 he has worked at the newly constituted CNR Institute of Nanotechnology, CNR Nanotec in Lecce (Italy) as Primo Ricercatore CNR (Research Professor). vii polymers Editorial Polymer Clay Nano-composites Stefano Leporatti CNR Nanotec-Istituto di Nanotecnologia c \ o Campus Ekotecne via Monteroni, 73100 Lecce, Italy; stefano.leporatti@nanotec.cnr.it Received: 30 August 2019; Accepted: 3 September 2019; Published: 3 September 2019 Clay–polymer composite materials is an exciting area of research and this Special Issue aims to address the current state-of-the-art of “Polymer Clay Nano-Composites” for several applications, among them antibacterial, environmental, water remediation, dental, drug delivery and others. The original scope of the Special Issue was comprehensively devoted to the synthesis and characterization of polymer clay nano-composites employed for several applications, including nano-clay polymer composites and hybrid nano-assemblies. Furthermore, polymers can be loaded with clay nanoparticles creating novel composite nano-materials enhancing composite strength features. The issue is composed of 16 contributions, fifteen articles and one review. They can be conveniently divided into one group related to Halloysite-composites (four papers including one review), a second group which deals with Montmorillonite-composites (four papers) and a third group, which can be generically referred to Hybrid Clay Nano-Composites (eight contributions). Relative to the group of Halloysite-composites, in the review [ 1 ] Wu et al. summarized the recent progress toward the development of polysaccharide-HNTs composites, paying attention to the main existence forms and wastewater treatment application particularly. The purification of Halloysite Nanotubes (HNTs) and fabrication of the composites were also discussed. Furthermore, they reported the unique characteristics of polysaccharide-HNTs composites and reviewed the recent development of the practical applications. In particular they pointed out that (1) polysaccharide-HNTs composites have improved mechanical, thermal, and swelling properties and good biocompatibility. Therefore they are promising nano-fillers for high-performance polymer composites. (2) HNTs can be combined with polysaccharides by di ff erent methods (3) the degree of dispersion of HNTs and the interfacial interactions between polysaccharides and HNTs are key factors a ff ecting the performance of composites. (4) Polysaccharide-HNTs composites has shown promising potential for biomedical applications. Another contribution from Xiandong Zhang and Guangshun Wu [ 2 ] dealt with HNTs Carbon Fiber (CFs) composites. The authors achieved for the first time the chemical grafting of halloysite nanotubes (HNTs) with amino or carboxyl groups onto the CFs surface, which was aimed to enhance the composites interfacial strength. Functional groups of HNTs and fiber surface structures were characterized, as well as interfacial properties and anti-hydrothermal aging behaviors. Interfacial reinforcement mechanisms for untreated and modified CF composites were also compared and discussed. Morphology, mechanical properties, water resistance and optical properties of the Fish gelatin (FG) / glycerol (GE) / halloysite (HT) composite films were investigated by Qiang et al. in [ 3 ]. Interestingly, they showed that with increasing GE content, the elongation at composite breaks increased significantly, but their tensile strength (TS) and water resistance decreased. Their results indicated that the addition of GE greatly improved film flexibility with a decrease in the TS of the film. Moisture uptake and water solubility were also improved by the addition of GE into the FG matrix, indicating that the water-resistance of the film decreased due to the GE added. Furthermore, the presence of GE enhanced the dispersion of HTs in the FG matrix and thus enhanced the properties of the obtained composite films. In the last paper of this HNTs-composites group, Lvov et al. [ 4 ] wanted to answer the following question: What additives could be used to increase the strength of silica gels? To answer this, they prepared colloidal silica gels with various additives and they measured gel strength. It was found Polymers 2019 , 11 , 1445; doi:10.3390 / polym11091445 www.mdpi.com / journal / polymers 1 Polymers 2019 , 11 , 1445 that cellulose nanofibrils considerably increased the gel strength. Furthermore, cellulose nanofibrils could be produced from cheap industrial-grade cellulose with low-cost industrial chemicals. Therefore, cellulose nanofibrils produced from renewable sources and naturally occurring halloysite nanoclay could be used as complementary reinforcing agents. In the second group related to Montmorillonite-composites, an interesting article, authored by Choi et al. [ 5 ], reported the synthesis of a chitosan–montmorillonite nano-composite material grafted with acrylic acid based on its function in a case study analysis. Fuzzy optimization was used for a multi-criteria decision analysis to determine the best desirable swelling capacity (YQ) of the material synthesis at its lowest possible variable cost. A multi-objective fuzzy optimization showed an innovative approach to determine a solution for the best condition in the material synthesis. Therefore, this approach proved to be a practical method for examining the best possible compromise solution based on the desired function to adequately synthesize a material. Moreover, the incorporation of the criteria of the variable cost in terms of material usage and the cumulative uncertainty of the response successfully ensued essential compromise results in the decision-making process. The development of a sacrificial bond provided unique inspiration for the design of advanced elastomers with excellent mechanical properties, but it was still a big challenge to construct a homogeneous polar sacrificial network in a nonpolar elastomer. In this view, Liu et al. [ 6 ] proposed a novel strategy to engineer a multi-ionic network into a covalently cross-linked 1,2-polybutadiene (1,2-PB) facilitated by in situ intercalated organic montmorillonite (OMMT) without phase separation. Overall, their work showed the design of a uniform and strong sacrificial network in the nano-clay / elastomer nano-composite with outstanding mechanical performances under both static and dynamic conditions. Future work will be devoted to further improving the ionic crosslinking density and constructing a stronger sacrificial network to prepare shape memory or self-recovery materials and studying the dynamics of ionic crosslinking. In another article montmorillonites (MMT) were modified by intercalating polyethylene oxide (PEO) macromolecules between the interlayer spaces in an MMT-water suspension system [ 7 ]. Shu et al. chose MMT / PEO 80 / 20 composite as the support platform for immobilization of Pd species in preparing novel heterogeneous catalysts. Their results confirmed that Pd nanoparticles were confined within the interlayer space of MMT and / or dispersed well on the outer surface of MMT. This work o ff ers an alternative approach to the preparation of Pd heterogeneous catalysts with fairly good performances, and heterogeneous catalysts with fairly good performances, and could have broad prospects in both experimental and industrial applications. The e ff ect of doubly functionalized montmorillonite (MMT) on the structure, morphology, thermal, and tribological characteristics of the resulting polystyrene (PS) nano-composites were investigated by Yu et al. [ 8 ]. The modification of the MMT was performed using a cationic surfactant and an anionic surfactant or a silane-coupling agent to increase the compatibility with PS matrix. The nanocomposites prepared by a cationic surfactant and a silane-coupling agent exhibited the best thermal stability and tribological performance, providing significant guidance for the future synthesis and application of the PS / OMMT nanocomposites in the oil and gas drilling engineering field to improve drilling fluid lubrication. The last part of Special Issue is composed of di ff erent papers, which can be collected in a common category namely “ Hybrid Clay Nano-Composites ”. Zhu et al. [ 9 ] have prepared Polyimide@graphene oxide (PI@GO) composites by way of a simple solution blending method. The nanoscale hardness and Young’s modulus of the composites were measured using nano-indentation through atomic force microscopy (AFM). They showed that relatively low GO content could remarkably improve the nanoscale mechanical properties of PI and they demonstrated that 2D nano-materials could improve the self-healing performance of polymer composites. In another paper Jiang et al. [ 10 ] conjugated hyaluronic acid (HA)—a natural polysaccharide that can specifically bind to CD44 receptors, onto laponite ® (LAP) nano-disks for the encapsulation and targeted delivery of the anti-cancer drug doxorubicin (DOX) to CD44-overexpressed cancer cells. Their results demonstrate that the HA-modified LAP nano-disks with high drug loading e ffi ciency, pH-sensitive drug release properties and CD44 targetability might be an e ffi cient nano-platform for cancer chemotherapy. Another interesting work dealing with the 2 Polymers 2019 , 11 , 1445 adsorption of Atrazine (ATZ) from aqueous solutions using nanocomposite materials, synthesized with two di ff erent types of organo-modified clays was written by Jorge A. Ram í rez-G ó mez et al. [ 11 ]. The structural, morphological, and textural characteristics of clays, copolymers, and nano-composites were determined through di ff erent analytical and instrumental techniques. They finally demonstrated that the synthesized nano-composites with higher molar fractions of 4VP obtained the highest removal percentages of ATZ. The article written by Alexandros K. Nikolaidis et al. [ 12 ] covers an interesting area of the application of clay nano-composites: dental materials. It focuses on the reinforcement of dental nano-composite resins with diverse organomodified montmorillonite (OMMT) nanofillers. The aim of this work was to monitor whether the presence of functional groups in the chemical structure of the nanoclay organic modifier may virtually influence the physicochemical and / or the mechanical attitude of the dental resin nano-composites. An enhancement of the flexural modulus was observed, mainly by using clay nanoparticles decorated with methacrylated groups, along with a decrease in the flexural strength at a high filler loading. This work can provide novel information about chemical interaction phenomena between nano-fillers and the organic matrix towards the improvement of dental restorative materials. In the following contribution, Elisabetta Finocchio’s group [ 13 ] modified a montmorillonite clay with three di ff erent aliphatic polyamines and deeply investigated the interaction mechanisms between clay and amines by di ff erent experimental techniques among them X-ray powder di ff raction (XRD), thermal analysis measurements (DTG), Fourier Transform Infrared Spectroscopy (FT-IR). Their experimental results showed that the amount of amines e ffi ciently immobilized in the solid phase could be enhanced by increasing the initial concentration of polyamines in the clay modification process, envisaging that polyamine-based organo-clays are promising materials for their proposed application in environmental remediation. Layered silicates are suitable for use as fillers in nano-composites based on a large aspect ratio, easy availability, and chemical resistance. Sericite is distinguished for its higher aspect ratio, higher resilience, and ultraviolet shielding and absorption. In this view, Liang et al. [ 14 ] studied the stability of the sericite intercalated by CTAB by changing di ff erent washing solvents, di ff erent temperatures, ultrasonic cleaning, and di ff erent solution conditions. Sericite / polymer nano-composites were produced with the stable intercalated sericite, and demonstrated excellent properties compared with pure epoxy resin. Altogether these results have suggested that stable intercalated sericite is a precondition for good adhesion between the sericite and epoxy resin, which gives rise to good nano-composite mechanical properties. In the contribution of Liu et al. [ 15 ] an attapulgite (ATP) / polypyrrole (PPy) nano-composite was developed employing the in situ polymerization method to produce the hierarchical cell texture for the PS foam based on the supercritical CO 2 foaming. The results showed that the nano-composite could act as an e ffi cient CO 2 capturer enabling the random release of it during the foaming process. Therefore, the in situ polymerized ATP / PPy nano-composite makes a supercritical CO 2 foaming desired candidate to replace the widely used fluorocarbons and chlorofluorocarbons as PS blowing agents. Finally, Bugnicourt et al. [ 16 ] investigated the e ff ect of various preparation methods on di ff erent production scales (pilot- and semi-industrial scale) on the barrier performance and morphological properties of the applied nano-composites. A nano-enhanced composition was converted into a so-called “ready-to-use” formulation by means of a solid-state pre-dispersion process using ball-milling. The preparation of a coating formulation using the ready-to-use granules and its up-scaling for roll-to-roll converting of a pilot- and semi-industrial scale was also successfully implemented. Transmission electron microscopy, scanning electron microscopy, as well as oxygen permeability measurements have been employed to characterize the e ff ects of both the production at various scales and ultrasound treatment on the morphology and barrier performance of the nano-composites. Authors concluded that the solid state pre-dispersion of the nano-platelets during the production of the ready-to-use formulation was the predominant process determining the ultimate degree of nanoparticle orientation and dispersion state. 3 Polymers 2019 , 11 , 1445 References 1. Wu, Y.; Zhang, Y.; Ju, J.; Yan, H.; Huang, X.; Tan, Y. Advances in Halloysite Nanotubes–Polysaccharide Nanocomposite Preparation and Applications. Polymers 2019 , 11 , 987. [CrossRef] [PubMed] 2. Zhang, X.; Wu, G. Grafting Halloysite Nanotubes with Amino or Carboxyl Groups onto Carbon Fiber Surface for Excellent Interfacial Properties of Silicone Resin Composites. Polymers 2018 , 10 , 1171. [CrossRef] 3. Qiang, X.; Zhou, S.; Zhang, Z.; Quan, Q.; Huang, D. Synergistic E ff ect of Halloysite Nanotubes and Glycerol on the Physical Properties of Fish Gelatin Films. Polymers 2018 , 10 , 1258. [CrossRef] [PubMed] 4. Vinokurov, V.; Novikov, A.; Rodnova, V.; Anikushin, B.; Kotelev, M.; Ivanov, E.; Lvov, Y. Cellulose Nanofibrils and Tubular Halloysite as Enhanced Strength Gelation Agents. Polymers 2019 , 11 , 919. [CrossRef] [PubMed] 5. Choi, A.E.S.; Futalan, C.M.; Yee, J. Fuzzy Optimization on the Synthesis of Chitosan-Graft-Polyacrylic Acid with Montmorillonite as Filler Material: A Case Study. Polymers 2019 , 11 , 738. [CrossRef] [PubMed] 6. Liu, J.; Li, D.; Zhao, X.; Geng, J.; Hua, J.; Wang, X. Buildup of Multi-Ionic Supramolecular Network Facilitated by In-Situ Intercalated Organic Montmorillonite in 1,2-Polybutadiene. Polymers 2019 , 11 , 492. [CrossRef] [PubMed] 7. Shu, G.; Zhao, J.; Zheng, X.; Xu, M.; Liu, Q.; Zeng, M. Modification of Montmorillonite with Polyethylene Oxide and Its Use as Support for Pd0 Nanoparticle Catalysts. Polymers 2019 , 11 , 755. [CrossRef] [PubMed] 8. Yu, C.; Ke, Y.; Hu, X.; Zhao, Y.; Deng, Q.; Lu, S. E ff ect of Bifunctional Montmorillonite on the Thermal and Tribological Properties of Polystyrene / Montmorillonite Nanocomposites. Polymers 2019 , 11 , 834. [CrossRef] 9. Zhou, J.; Cai, Q.; Xu, F. Nanoscale Mechanical Properties and Indentation Recovery of PI@GO Composites Measured Using AFM. Polymers 2018 , 10 , 1020. [CrossRef] 10. Jiang, T.; Chen, G.; Shi, X.; Guo, R. Hyaluronic Acid-Decorated Laponite ® Nanocomposites for Targeted Anticancer Drug Delivery. Polymers 2019 , 11 , 137. [CrossRef] [PubMed] 11. Ram í rez-G ó mez, J.A.; Illescas, J.; del Carmen D í az-Nava, M.; Muro-Urista, C.; Mart í nez-Gallegos, S.; Rivera, E. Synthesis and Characterization of Clay Polymer Nanocomposites of P(4VP-co-AAm) and Their Application for the Removal of Atrazine. Polymers 2019 , 11 , 721. [CrossRef] 12. Nikolaidis, A.K.; Koulaouzidou, E.A.; Gogos, C.; Achilias, D.S. Synthesis and Characterization of Dental Nanocomposite Resins Filled with Di ff erent Clay Nanoparticles. Polymers 2019 , 11 , 730. [CrossRef] [PubMed] 13. Cristiani, C.; Iannicelli-Zubiani, E.M.; Dotelli, G.; Finocchio, E.; Stampino, P.G.; Licchelli, M. Polyamine-Based Organo-Clays for Polluted Water Treatment: E ff ect of Polyamine Structure and Content. Polymers 2019 , 11 , 897. [CrossRef] 14. Liang, Y.; Yang, D.; Yang, T.; Liang, N.; Ding, H. The Stability of Intercalated Sericite by Cetyl Trimethylammonium Ion under Di ff erent Conditions and the Preparation of Sericite / Polymer Nanocomposites. Polymers 2019 , 11 , 900. [CrossRef] [PubMed] 15. Liu, Y.; Jian, L.; Xiao, T.; Liu, R.; Yi, S.; Zhang, S.; Wang, L.; Wang, R.; Min, Y. High Performance Attapulgite / Polypyrrole Nanocomposite Reinforced Polystyrene (PS) Foam Based on Supercritical CO 2 Foaming. Polymers 2019 , 11 , 985. [CrossRef] 16. Bugnicourt, E.; Brzoska, N.; Kucukpinar, E.; Philippe, S.; Forlin, E.; Bianchin, A.; Schmid, M. Dispersion and Performance of a Nanoclay / Whey Protein Isolate Coating upon its Upscaling as a Novel Ready-to-Use Formulation for Packaging Converters. Polymers 2019 , 11 , 1410. [CrossRef] [PubMed] © 2019 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http: // creativecommons.org / licenses / by / 4.0 / ). 4 polymers Review Advances in Halloysite Nanotubes–Polysaccharide Nanocomposite Preparation and Applications Yang Wu 1 , Yongzhi Zhang 1 , Junping Ju 1, *, Hao Yan 1 , Xiaoyu Huang 2 and Yeqiang Tan 1, * 1 State Key Laboratory of Bio-fibers and Eco-textiles, Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong Province, School of Materials Science and Engineering, Qingdao University, Qingdao 266071, China; wy921170920@163.com (Y.W.); zyz18919@163.com (Y.Z.); yanhao1287476167@163.com (H.Y.) 2 Key Laboratory of Synthetic and Self-Assembly Chemistry for organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China; xyhuang@sioc.ac.cn * Correspondence: jujunping@qdu.edu.cn (J.J.); tanyeqiang@qdu.edu.cn (Y.T.); Tel. / Fax: + 86-571-8595-0961 (Y.T.) Received: 18 April 2019; Accepted: 22 May 2019; Published: 4 June 2019 Abstract: Halloysite nanotubes (HNTs), novel 1D natural materials with a unique tubular nanostructure, large aspect ratio, biocompatibility, and high mechanical strength, are promising nanofillers to improve the properties of polymers. In this review, we summarize the recent progress toward the development of polysaccharide-HNTs composites, paying attention to the main existence forms and wastewater treatment application particularly. The purification of HNTs and fabrication of the composites are discussed first. Polysaccharides, such as alginate, chitosan, starch, and cellulose, reinforced with HNTs show improved mechanical, thermal, and swelling properties. Finally, we summarize the unique characteristics of polysaccharide-HNTs composites and review the recent development of the practical applications. Keywords: halloysite nanotubes; polysaccharide; interfacial interactions; reinforcing; adsorption 1. Introduction Nanofillers recently have drawn extensive attention from academic and industrial fields due to their unique performance [ 1 ]. The traditional materials, such as black carbon, graphite, silica, and silicate, can significantly improve the mechanical properties, thermal stability, and permeability of various polymers [ 2 , 3 ]. Nowadays, clay mineral nanofillers with large aspect ratios, high strength, and relatively low density have attracted intense research interest [ 4 ]. Clay minerals, natural materials with proven biocompatibility and abundant storage, exhibit unique properties for various applications [ 5 ]. The majority of the research concerning clay minerals is devoted to kaolinite [ 6 ], montmorillonite [ 7 ], and illite [ 8 ]. In recent years, halloysite nanotubes (HNTs), 1D natural materials with a unique tubular nanostructure, large aspect ratio, biocompatibility, and high mechanical strength, have arisen as promising nanofillers to improve the properties of polymers [9,10]. Halloysite was first proposed by Berthier (1826) [ 11 ]. Raw halloysite, which is usually white, is exploited from natural sediments and is easily processed into powder. The sizes of halloysite depend on its specific geological deposit, as reported in the literature on the basis of microscopy [ 12 ] and scattering techniques [ 13 ]. It possesses several typical morphologies, such as spherical, sheet-like, and tubular particles due to the diversity of crystallization conditions and geological occurrence. Among them, the tubular structure is the most common and valuable [ 14 ]. The tubular structure is caused by lattice mismatch between adjacent silicone dioxide and aluminum oxide layers [ 15 ]. The molecular formula of HNTs is Al 2 Si 2 O 5 (OH) 4 · nH 2 O, where n represents hydration or dehydration. HNTs are hydrated when n equals 2 and are dehydrated when n equals 0 [ 16 – 19 ]. Compared with traditional Polymers 2019 , 11 , 987; doi:10.3390 / polym11060987 www.mdpi.com / journal / polymers 5 Polymers 2019 , 11 , 987 nanofillers, such as carbon nanotubes (CNTs) [ 20 ] and boron nitride nanotubes (BNNTs) [ 21 ], HNTs have a prominent advantage, which is that they are far less expensive [ 22 ]. The length of HNTs ranges from 100 to 2000 nm, with the inner diameter from 10 to 30 nm and the outer diameter from 30 to 50 nm. In terms of functional groups, HNTs contain a large amount of hydroxyl groups situated between layers and on the surface, respectively. Due to the multi-layer structure, most of the hydroxyl groups are inner groups. In addition, the inner surfaces of the HNTs are positively charged, while the outer surfaces are negatively charged [12,23]. The detailed data of HNTs are listed in Table 1 [14,24]. Table 1. The detailed data of halloysite nanotubes (HNTs) related to combination with polysaccharides. Molecular Formula Al 2 Si 2 O 5 (OH) 4 · nH 2 O Length 100–2000 nm Inner diameter 10–30 nm Outer diameter 30–50 nm Aspect ratio (L / D) 10–50 Young’s modulus of a single HNTs 130 ± 24 GPa Elastic modulus 460 GPa Interlayer water removal temperature 400 ◦ C Water contact angle 10 ± 3 ◦ Specific surface area 22.1–81.6 m 2 / g Total pore volume 0.06–0.25 cm 3 / g Density 2.14–2.59 g / cm 3 Mean particle size in aqueous solution 143 nm Although the characteristics above generate excellent mechanical, thermal, and regenerable properties, the direct application of HNTs is limited. The drawbacks include di ffi culty in dissolving, brittleness, and low permeability [ 25 ]. With abundantly renewable sources and charming properties, including inherent biocompatibility, polysaccharides have attracted rising attention, and they have been widely applied to the medical [ 26 ], textile [ 27 ], and food fields, among others [ 28 , 29 ]. By preparing polysaccharide-HNTs composites, we can overcome these shortcomings. Due to the stable tubular morphology, charge distribution, the specific origin, and unique crystal structure, HNTs can be dispersed into single particles easily and the lumen diameter of HNTs fits well to macromolecule and protein diameters, causing the good combination between polysaccharides and HNTs [ 30 – 32 ]. The present research mainly focuses on alginate [ 33 , 34 ], chitosan [ 35 ], starch [ 36 ], cellulose [ 37 ], pectin, and carrageenan [38]. Although general properties of polysaccharide / halloysite nanotube composites and biomedical applications have been reviewed earlier by Liu et al. [ 39 ], we review the recent progress toward the development of polysaccharide-HNTs composites, paying attention to the main existence forms, wastewater treatment, and food packaging applications particularly. Through this review, we have a better understanding of unique characteristics of polysaccharide-HNTs composites, which can be helpful to the continuous expansion of their application in the future. 2. Preparation of Polysaccharide-HNTs Composites 2.1. Purification Raw halloysite has impurities, such as quartz, illite, and perlite, since it is exploited directly from natural deposits. Therefore, the aggregate nanotubes should be separated to purify the HNTs before use in practical applications [ 40 ]. The traditional method of purification is the dispersion-centrifugation-drying technique. Firstly, we slowly added HNTs powder into deionized water under heating and mild stirring conditions. Then, the solution was further processed by lavation with deionized water three times and centrifugation. Finally, the pure HNTs were obtained after desiccation [ 41 ]. Figure 1 showed FE-SEM (Left) photos of HNTs and schematic illustration of crystalline structure (Right) of HNTs. 6 Polymers 2019 , 11 , 987 Figure 1. FE-SEM Image of HNTs on Si-Wafer (Left) and Schematic Illustration of Crystalline Structure of HNTs (Right). (Reproduced from [ 42 ] with permission from American Chemical Society and Copyright Clearance Center, 2012). 2.2. HNTs / Polysaccharide Preparations and Formulations Using traditional processing techniques, HNTs can be mixed with most polysaccharides, such as alginate, chitosan, starch, cellulose, and carrageenan. The purpose of di ff erent fabrication methods is to enhance the interfacial interactions and dispersibility. In this section, we introduce the main existence forms of polysaccharide-HNTs composites. 2.2.1. Hydrogels The hydrophilic structure of hydrogels enables them to hold large amounts of water in the three-dimensional networks. Due to the characteristics of high hygroscopicity and low sti ff ness, hydrogels are usually described as soft and wet materials [ 43 ,44 ]. Chan et al. prepared a HNTs / alginate hydrogel and the e ff ects of HNTs on the physicochemical, thermal, mechanical, and mass transfer properties of alginate hydrogel beads were investigated in detail [ 45 ]. It was found that HNTs filled the interspace in the alginate matrix and allowed more e ffi cient load transfer. The HNTs were embedded in the layers of alginate hydrogel networks and they had little e ff ect on the size and on the shape of the alginate beads. The mechanism for enhanced mechanical strength could be attributed to physical interaction between the alginate and HNTs, and the mechanical strength could be improved at lower HNTs loading if chemical interactions were present. Zhou et al. reported alginate / HNTs composite hydrogels via solution mixing and subsequent cross-linking with calcium ions [ 46 ]. The static and shear viscosity of composite solutions increases with the increase of HNTs. The rheological behaviors of alginate / HNTs solutions were a shear thinning and fit with the power law model. Due to the good dispersion ability of HNTs, polysaccharides and HNTs are mixed easily via interfacial interactions, such as electrostatic and hydrogen bonding interactions, contributing to the formation of homogeneous composites and enhanced properties. Fourier-transform infrared spectroscopy (FTIR) and X-ray powder di ff raction (XRD) are applied to study the interfacial interactions between alginate and HNTs. As shown in Figure 2b, the peaks at 1419 cm − 1 shifted to higher wave numbers and no new peaks appeared in the composites, which indicated that hydrogen bond interactions occur between HNTs and alginate but no chemical reaction occurs. The XRD patterns of composites (Figure 2c) were very similar to HNTs no new di ff raction peak occurring, which suggested the crystal structure of HNTs was retained in the composites. 7 Polymers 2019 , 11 , 987 Figure 2. FTIR spectra ( a , b ) and XRD ( c ) pattern of HNTs, alginate, and alginate / HNTs composites. (Reproduced from [46] with permission from Elsevier and Copyright Clearance Center, 2017). The e ff ect of HNTs on the swelling ratios of the polysaccharide / HNTs composites were investigated in NaCl and water solution. Compared with pure sodium alginate (SA) hydrogel, the SA / HNTs composite hydrogels showed low swelling ratios with the same conditions for soaking time, which gradually decreased with the increasing HNTs loading. This result was attributed to the hydrophilic polymer content in the composite hydrogels decreasing with the addition of HNTs, and the water adsorption of HNTs was lower than SA. In addition, the HNTs used as physical crosslinking points for alginate through the hydrogen bond interactions can greatly improve entanglement of the alginate and lower the mobility of the chains, resulting in water absorption being greatly decreased [ 47 ]. Sinem et al. reported a cryogenic technique to modify HNTs. The inner and outer diameters and the surface area of HNTs were evidently increased without disturbing the inherent tubular structure and wall features. Then, modified HNTs were mixed with chitosan to prepared composite hydrogels, showing remarkedly improved mechanical and swelling properties compared with pure chitosan hydrogel [ 48 ]. Sharifzadeh et al. synthesized carrageenan / HNTs nanocomposite hydrogels via physical crosslinking. The chemical structure confirmed by FTIR spectroscopy revealed the formation of physical interaction between carrageenan and HNTs in the hydrogels. It was revealed that the thermal stability and swelling of the nanocomposite hydrogels had significantly been improved due to the incorporation of HNTs compared with the pure carrageenan hydrogel [ 49 ]. The reasons why HNTs can improve the thermal stability of composites are as follows. The degradation temperature of HNTs is approximately 400 ◦ C, which is higher than most of the polysaccharides. Then, the dispersed HNTs have a blocking e ff ect on mass and heat transfer. Besides, the polysaccharide chains and degraded products enter the inner cavity of HNTs, delaying mass transport and improving the thermal stability. However, the good dispersion of HNTs into the hydrogel is urgently needed for the hydrogel fabrications to broaden their application. The HNTs functionalized via di ff erent types of silane coupling agents were used as a way to improve HNTs dispersal in the polymer matrix. Sabbagh et al. prepared novel chitosan / crosslinked 8 Polymers 2019 , 11 , 987 oxidized starch hydrogels, which were embedded by modified or unmodified HNTs. Incorporation of HNTs significantly a ff ected the swelling behavior and thermal properties of the hydrogel. The increase of the amine groups in HNTs modified with silane reagents made them react with oxidized starch, resulting in good dispersion in the structure of the hydrogel [ 50 ]. Figure 3 illustrates the formation of the bio-nanocomposite hydrogel. Figure 3. Schematic description of bio-nanocomposite hydrogel formation. (Reproduced from [ 50 ] with permission from Elsevier and Copyright Clearance Center, 2017). The swelling ratio of the chitosan / HNTs hydrogel also decreased compared with the pure chitosan hydrogel, due to the introduction of HNTs content causing the chitosan to contract more [ 51 ]. The cellulose / HNTs composite showed a similar variation trend [52]. 2.2.2. Films Regenerated cellulose / HNTs nanocomposite films were fabricated in 1-butyl-3-methylimidazolium chloride ionic liquid by solution casting method. Figure 4 showed the cross-sectional FE-SEM images of the cellulose and 6 wt.% HNTs-filled nanocomposite films. The HNTs were well dispersed in cellulose due to good interaction between cellulose and HNTs. Young’s modulus and the tensile strength of nanocomposite films were improved by 100% and 55.3%, respectively, when the loading of HNTs was 6 wt.%, which was owing to tubular geometry and the higher sti ff ness of the HNTs. The addition of HNTs also improved the thermal stability and char yield of regenerated cellulose, but moisture absorption capacity of the nanocomposites in constant relative humidity was reduced due to the addition of HNTs [53]. Kim et al. reported transp