Functional Polymer Composites with Nanoclays: Volume 22 (Smart Materials Series) - Hardcover

Hardcover

ISBN 10: 1782624228 ISBN 13: 9781782624226

Publisher: Royal Society of Chemistry, 2016

Synopsis

This book will explain the great potential of polymer-clay nanocomposites and will bring together the combined physico-chemical, materials science and biological expertise to introduce the reader to the vibrant field of nanoclay materials.

About this title

About the Author

Katsuhiko Ariga has been working on supramolecular chemistry especially related with self-assembly processes, ultrathin films and molecular recognitions. Accomplishments in his researches contribute significantly developments of biomemetic systems as well as physicochemical understanding on interfacial phenomena. Especially, pioneer researches of layer-layer supramolecular assembly have been highly evaluated, and he is also recognized as an authority of supramolecular recognition at the interfaces.

From the Back Cover

Polymer-clay nanocomposites have flame-retardant, antimicrobial, anticorrosion and self-healing properties, they are biocompatible and environmentally benign. Multiple types of clay minerals may be exfoliated or individually dispersed and then used as natural nanoparticle additives of different size and shape for composite formation. Loading polymers with clays increases their strength, however, it is only recently that such composites were prepared with controlled nanoscale organization allowing for the enhancement of their mechanical properties and functionality. Edited by pioneers in the field, this book will explain the great potential of these materials and will bring together the combined physico-chemical, materials science and biological expertise to introduce the reader to the vibrant field of nanoclay materials. This book will provide an essential text for materials and polymers scientists in industry and academia.

From the Inside Flap

Polymer-clay nanocomposites have flame-retardant, antimicrobial, anticorrosion and self-healing properties, they are biocompatible and environmentally benign. Multiple types of clay minerals may be exfoliated or individually dispersed and then used as natural nanoparticle additives of different size and shape for composite formation. Loading polymers with clays increases their strength, however, it is only recently that such composites were prepared with controlled nanoscale organization allowing for the enhancement of their mechanical properties and functionality. Edited by pioneers in the field, this book will explain the great potential of these materials and will bring together the combined physico-chemical, materials science and biological expertise to introduce the reader to the vibrant field of nanoclay materials. This book will provide an essential text for materials and polymers scientists in industry and academia.

Excerpt. © Reprinted by permission. All rights reserved.

Functional Polymer Composites with Nanoclays

By Yuri Lvov, Baochun Guo, Rawil F. Fakhrullin

The Royal Society of Chemistry

Copyright © 2017 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78262-422-6

Contents

Chapter 1 Functional Nanocomposites Based on Fibrous Clays Eduardo Ruiz-Hitzky, Margarita Darder, Ana C. S. Alcântara, Bernd Wicklein and Pilar Aranda, 1, 
Chapter 2 Fibrillar Attapulgite–Rubber Nanocomposites Nanying Ning, Guangyu Chu, Yuxing Feng, Xiuchun Zhen, Ming Tian and Tung W. Chan, 54, 
Chapter 3 Rubber–Rectorite Composites with High Gas Barrier Properties Nanying Ning, Guangyu Chu, Yuxing Feng, Xiuchun Zhen, Ming Tian and Tung W. Chan, 100, 
Chapter 4 Design and Physicochemical Characterization of Novel Organic–Inorganic Hybrids from Natural Aluminosilicate Nanotubes A. Takahara and Y. Higaki, 131, 
Chapter 5 Surface Modification of Halloysite Jing Huang, Zhenghai Tang and Baochun Guo, 157, 
Chapter 6 Halloysite Based Smart Hybrid Nanomaterials for the Solubilization of Hydrophobic Compounds in Aqueous Media G. Cavallaro, G. Lazzara and S. Miloto, 187, 
Chapter 7 Halloysite and Related Mesoporous Carriers for Advanced Catalysis and Drug Delivery G. Cavallaro, G. Lazzara and S. Miloto, 207, 
Chapter 8 Application of Clay Materials as Nanocontainers for Self-Healing Coatings Dmitry G. Shchukin and Elena Shchukina, 223, 
Chapter 9 Flame Retardant Polymer–Halloysite Nanocomposites Zhenghai Tang and Baochun Guo, 245, 
Chapter 10 Polymer–Halloysite Composite Membranes for Ultrafiltration and Proton Exchange Applications Yatao Zhang, Jingtao Wang, Junyong Zhu, Yahua Liu and Jindun Liu, 271, 
Chapter 11 Rubber Functionalized with Halloysite Loaded with Antioxidants and Antibacterials Liqun Zhang and Jiajia Xue, 301, 
Chapter 12 Halloysite–Dopamine Hybrid Nanotubes to Immobilize Biomacromolecules Yifan Li, Yafei Zhao, Bing Zhang and Rongfeng Chen, 329, 
Chapter 13 Halloysite Clay Nanotubes for Long Acting Controlled Release of Drugs and Proteins Renata Minullina, Joshua Tully, Raghuvara Yendluri and Yuri Lvov, 354, 
Chapter 14 Biocompatible Electrospun Polymer–Halloysite Nanofibers for Sustained Release Yili Zhao, Zhe Wang, Mingwu Shen and Xiangyang Shi, 379, 
Chapter 15 Toxicological Evaluation of Clay Nanomaterials and Polymer–Clay Nanocomposites Ekaterina A. Naumenko and Rawil F. Fakhrullin, 399, 
Subject Index, 420, 

CHAPTER 1

Functional Nanocomposites Based on Fibrous Clays

EDUARDO RUIZ-HITZKY, MARGARITA DARDER, ANA C. S. ALCÁNTARA, BERND WICKLEIN AND PILAR ARANDAa

1.1 Introduction

Since the beginning of the polymer age, silicates – like clays and other finely particulated solids such as silica, calcium carbonate and carbon black – have been incorporated as fillers at the micrometer dimension into plastics and elastomers with the aim of improving the mechanical and rheological properties of these polymers. Kaolinite was the clay mineral initially most widely used as a silicate filler of diverse polymeric matrices. More recently, swelling clays such as smectites have received great interest due to their ability to exfoliate, giving rise to elemental silicate platelets of 1 nm thickness, which represent a way to develop fillers at the nanometer dimension (nanofillers). In this context, the concept of clay delamination encompassed by its high dispersion in polymers, introduced by Fukushima and other researchers at the Toyota Central Laboratory almost three decades ago, represents a revolutionary idea, not only for the use of clays as reinforcing charges but also to introduce functionality in the resulting materials.

Very quickly, polymer-clay nanocomposites became a popular topic with a rapid increase of publications and registered patents and, in the last 20 years, about 13 000 articles and 200 patents have been published or registered, according to the ISI Web of Science data. Practically all these works make reference to polymer-clay nanocomposites belonging to the smectite layered clays family, i.e. 2:1 charged phyllosilicates, such as montmorillonite, hectorite and saponite natural clay minerals as well as analogous synthetic hectorites and fluoro-hectorites (e.g. LAPONITE[R] and the so-called "synthetic mica"). The smectite layers exhibit a high aspect ratio as each one is approximately 1 nm thick, while the diameter may rise to several microns or larger, as is the case with vermiculites. So, polymer-clay nanocomposites based on smectites have been extensively studied from basic aspects to applications, and a significant number of reviews have been published on this topic (see, for instance, ref. 5–11). Compared to smectites, other types of clay minerals, such as kaolinite, halloysite, imogolite, sepiolite and palygorskite, have barely been studied as nanofillers of polymers. However, as we report below in this chapter, the structural and textural characteristics of the fibrous clays could be of great benefit for the properties and applications of the novel polymer–clay nanocomposites derived from them.

The sepiolite and palygorskite fibrous clays also appear to be attractive nanofillers to reinforce polymer matrices. They do not exhibit intercalation capacity, but these two natural silicates offer interesting characteristics, such as microporosity and large specific surface area. Interestingly, the presence of hydroxyls (silanol groups, Si–OH) at their external surface allows easy functionalization by controlled modification based on chemical reactions, e.g. with coupling agents or by anchoring of nanoparticles (NPs). This behavior allows the preparation of a wide variety of polymer–clay nanocomposites provided with diverse functionality.

Sepiolite is a natural hydrated magnesium silicate with fibrous morphology, displaying a crystal structure consisting of talc-like ribbons arranged parallel to the fiber direction (c-axis) with Si12O30 (OH)4(H2O)24x8H2O as the ideal formula (Figure 1.1A). To a variable extent, isomorphous substitution of magnesium at the octahedral layer by trivalent metals, mainly Al(iii), provokes a charge deficiency in the structure that is compensated by extra-framework cations. This is the origin of the cation-exchange capacity (CEC) of sepiolite samples, which has been generally established in the 10–20 mEq 100 g-1 range, depending on the origin of the clay and always being about 4–5 times inferior to typical CEC values found in smectite clays. As often occurs in clays formed in lacustrine sediments, certain sepiolite samples show that hydroxyls located at the octahedral layers (mainly Mg–OH) can be partially substituted by fluorine. One of the most interesting features of this silicate is the existence of microporosity ascribed, as occurring in zeolites, to their backbone structural arrangement – in this case with discontinuity of the phyllosilicate layers. This characteristic results in an alternating distribution of structural blocks, each one composed of two tetrahedral silica sheets and a central sheet of magnesium hydroxide, which determine the presence of structural cavities (tunnels) that grow along the c-axis direction (Figure 1.1A). The cross-section of the sepiolite tunnels is 1.6 × 0.37 nm2, which determines a structural microporosity typically superior to 0.3 cm3 g-1 as determined from nitrogen adsorption isotherms. Palygorskite (Figure 1.1B) is a related silicate with a higher content of aluminum and shorter dimensions of the structural blocks than sepiolite, with tunnel dimensions of 0.64 × 0.37 nm2. The irregular arrangement of blocks and tunnels gives rise to fibers, as represented for instance in Figure 1.1C, and determines an elevated surface area in both silicates. Sepiolite and palygorskite exhibit BET (Brunauer-Emmett-Teller theory) specific surface area values in the order of 300 and 200 m2 g-1, respectively.

The roughness of fibers at the nanometric scale in both silicates favors the interaction with their surroundings, which makes these materials excellent adsorbents for many applications in diverse sectors such as industry, agronomy and environmental remediation. The peculiar morphology and textural characteristics of sepiolite and palygorskite make these silicates also appropriate as fillers in polymer matrices. The fiber length of sepiolite and palygorskite clays depends on their origin: for instance, sepiolite from Taxus basin deposits in Spain are in the micrometer dimensions, whereas fibers from Finland and China can be much longer. The high aspect ratio and the mechanical properties of sepiolite fibers make these silicates useful as fillers for the reinforcement of polymers. The elastic properties of individual sepiolite fibers have been measured by atomic force microscopy (AFM) (Figure 1.2), obtaining values of the modulus in the order of 10 GPa in bending mode. The abovementioned characteristics indicate that sepiolite and palygorskite could act as efficient fillers of polymers to improve their structural properties.

Fibrous clays are present in Nature as aggregates of the fibers, often forming interwoven bundles as observed by electronic microscopy techniques. Disaggregation can be conducted under mechanical or sonomechanical treatment of the fiber aggregates in water dispersion, which after drying, results in xerogels that in turn produce highly viscous aqueous dispersions useful for many applications. These processes only produce a small proportion of individualized fibers, and an entire defibrillation of sepiolite and palygorskite appears to be extremely difficult. However, it is evident that high disaggregation degrees can facilitate the fibrous clay dispersions into polymer matrices, leading to the corresponding clay nanocomposites.

The aim of this chapter is to show how sepiolite and palygorskite fibrous clays can be incorporated as efficient fillers into polymer matrices, not only to improve their mechanical properties but also to provide functionality to the resulting polymer-clay nanocomposites. To achieve this objective, diverse approaches may be needed as preliminary procedures, including the disaggregation of the bundles of fibers by physical methods. In some cases, as occurs in other types of clays used as nanofillers, it could also be necessary to modify the nature of the silicate surface in view of making it compatible with the polymer.

1.2 Modifications of Fibrous Clays for Use as Nanofillers

As indicated above, raw fibrous clays are present as aggregates of fibers or laths that form interwoven bundles, leading to rigid particles. Their disaggregation, based, for instance, on micronization processes conducted at the industrial level, represents an essential route for applications as fillers and rheological additives for water-based systems. For many applications, such as those based on rheological properties, it is crucial that sepiolite or palygorskite are separated into individual fibers. This is the case for important uses such as drilling fluids, construction, asphalt and bitumen, paints and coatings, liquid animal feeds, mortars, fluid fertilizers or grease thickener. The micronized fibrous clays, e.g. Pangel® commercialized by TOLSA S.A., are also useful as filler for rubber and provide reinforcing characteristics with polar polymers. This type of material is produced by a patented milling process of the fibrous clays using humidified samples, leading to defibrillation and producing the commercially called sepiolite of rheological grade, capable of producing very viscous suspensions at relatively low concentrations compared with other clays. More recently, a new patented invention claims the replacement of the grinding step and wet disaggregation by treatments based on ultrasound irradiation.

The high viscosity developed by sepiolite samples conveniently disaggregated in water can be exploited as a so-called suspension capacity enhancer that can be applied in diverse systems. As an illustrative example, multiwalled carbon nanotubes (MWCNTs) can be maintained in aqueous suspension by assembly with sepiolite under ultrasound irradiation. These carbon nanotube (CNT) dispersions are extraordinarily stable, avoiding settling over very long periods of time, greater than a year (Figure 1.3). The resulting sepiolite–MWCNT materials can be easily isolated from the dispersions giving rise to the so-called hybrid buckypapers – similar to self-supported films of MWCNTs alone. These materials exhibit electrical conductivity as well as reinforcing ability when incorporated as fillers in polymers such as polyvinyl alcohol (PVA). In this way, functionalization of sepiolite introduced by means of physical processes represents an option for the further production of functional polymer–clay nanocomposites, in this case related to conducting nanocomposites.

As the external surface of fibrous clays is covered by silanol groups ([equivalent to] Si-OH), a useful approach to introduce chemical modifications is based on their reaction with coupling agents such as organosilanes and other reagents such as epoxides and isocyanates. Organosilane coupling agents containing Si-X groups (X = OR, Cl) can react with the silanol groups; the organic part remains grafted to the mineral surface through very stable siloxane bridges ([equivalent to]Si-O-Si[equivalent to]). In agreement with thermogravimetry (TG) and Differential Thermal Analysis (DTA) data, the grafted groups are very stable, being eliminated by heating at temperatures above 400 °C. In this way, the characteristic hydrophilic surface becomes organophilic and the fibrous clays can then be easily dispersed in low-polar polymers. Sepiolite has been functionalized by treatment with silanes containing unsaturated or thiol groups, such as vinyl and methacryloxy or 3-propylmercapto, respectively, giving organic derivatives of sepiolite capable of further copolymerization reactions. Surface modifications by treatment of sepiolite with alkyl and functional silanes in the form of aqueous gels have been recently reported. Other reagents used to functionalize sepiolite by grafting reactions contain epoxide and isocyanate groups, which react with silanol groups, remaining covalently attached to the silicate through [equivalent to]Si-O-C[equivalent to] bonds and showing lower stability than siloxane bridges. However, the most usual approach to modify the external surface of fibrous clays to reduce their elevated polarity is based on treatments with alkylammonium salts, leading to fillers showing good compatibility with low-polar polymer matrices.

Organoclays derived from sepiolite and palygorskite can be easily prepared by treatment with neutral or cationic surfactants in aqueous solution. In this last case, treatment with long-chain alkylammonium salts following an ion-exchange process is carried out in a similar way to that in montmorillonite and other smectite clay minerals. These organically modified fibrous clays are attaining great importance because they are commercialized not only for applications as nanofillers of polymers, like in epoxy systems and plastisols, but also in solvent-based paints, greases, solvent asphalt coatings, inks, foundry washes and adhesives.

New possibilities to modify sepiolite and palygorskite in view of their use as fillers for diverse polymer matrices are still open and novel developments in this sense may find applications in the future. One of them consists of the encapsulation of molecular species into the structural cavities of nanometric dimensions grown along the silicate fibers (tunnels), and the other one is based on the assembly of diverse NPs to the external surface of those clays. The presence of tunnels allows the access of small molecular species into the interior of the silicate by replacing the zeolitic water that is usually filling these structural cavities and is reversibly adsorbed and desorbed by heating or by vacuum exposure. Organic dyes such as methylene blue can penetrate at least partially into the tunnels of sepiolite and palygorskite. In fact, several centuries ago Maya people in Mesoamerica developed a technique to encapsulate indigo into palygorskite clay, giving rise to a pigment known as Maya Blue. It was used during the pre-Spanish period and later-on; its use was continued in Spain till the 17th century and until the 19th century in Cuba, where it was known as Havana Blue. This clay-dye hybrid material shows a remarkable stability against weathering and microbiological activity, also being resistant to thermal treatments and solvent extractions, attributed to the encapsulation of the dye molecules inside the silicate tunnels. Due to this quality, the existence of a strong interaction between the dye and the host silicate could be admitted, giving rise to the high stability of the Maya Blue pigment. Maya Blue analogs prepared by adsorption of molecular dyes such as methylene blue (MB) and methyl red (MR) by both sepiolite and palygorskite have been used for coloring polymer matrices, opening the way to introduce stable colors in polymer–clay nanocomposites. More recently, Ouellet-Plamondon et al. reported the functionalization of inorganic polymers (geopolymers) using sepiolite-dye hybrids inspired by Maya Blue. In this case, it is also confirmed that the encapsulation of MB and MR dyes into sepiolite leads to the stability of the pigment in the geopolymer matrix despite the chemical aggressivity of that system in showing strong alkalinity. The protection of the colors towards light and external reagents such as hydrogen peroxide and acids has been clearly shown in the resulting materials.

As indicated above, another possibility for introducing functionality in fibrous clays used as polymer fillers is based on the immobilization of diverse types of NPs that, in some cases, can be anchored on the silicate surface by interaction with the external silanol groups. This method could represent an excellent opportunity for the introduction of specific characteristics in fibrous clays-based nanocomposites such as antimicrobial activity, for instance by incorporation of Ag NPs, superparamagnetic properties, for instance by introduction of magnetite–maghemite NPs, or photoactivity by assembly of TiO2 NPs.

1.3 Functional Polymer–Fibrous Clay Nanocomposites

Research on polymer–clay nanocomposites is a discipline with a continuous increase in the number of publications since they were reported for the first time around 25 years ago. For a long time, the majority of the publications were related to smectite-based nanocomposites, however in recent years, the use of fibrous clays, sepiolite and palygorskite, has attracted increasing interest. Since 2014 until September 2015 more than 150 publications have been reported in the Web of Science, searched using "(sepiolite or palygorskite or attapulgite) and nanocomposite" for a total number of around 670 entries. The increasing interest in this type of nanocomposite may be related to the potentiality that the incorporation of fibrous clays offers in comparison to layered clays from the point of view of functionality. Actually, the most part of the interest in polymer-smectite-based nanocomposites has been directed to the improvement of mechanical properties of polymers. Additionally, the presence of delaminated clay nanosheets results of interest in the improvement of other properties such as the reduction of gas diffusion, as they introduce more tortuous paths which work as a barrier for gases. Hence, for a long time, fibrous clays have scarcely been employed for those purposes, but they may prove interesting for introducing other types of properties and functionalities.

(Continues...)

Excerpted from Functional Polymer Composites with Nanoclays by Yuri Lvov, Baochun Guo, Rawil F. Fakhrullin. Copyright © 2017 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2016
• Language: English
• ISBN 10: 1782624228
• ISBN 13: 9781782624226
• Binding: Hardcover
• Number of pages: 449
• Editor: Yuri Lvov