18 halloysite-based bionanocomposites biodegradable polymers ... cellulosic biofibers represent...

557

Vijay Kumar Thakur, Manju Kumari Thakur and Michael R. Kessler (eds.), Handbook of Composites from Renewable

Materials, Volume 7, (557–584) © 2017 Scrivener Publishing LLC

*Corresponding author: [email protected]

18

Halloysite-Based Bionanocomposites

Giuseppe Lazzara1*, Marina Massaro2, Stefana Milioto1 and Serena Riela2

1Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle Scienze,

Palermo, Italy 2Dipartimento STEBICEF, Sez. Chimica, Università degli Studi di Palermo, Palermo, Italy

AbstractScientific research has been invigorated by a new class of biodegradable materials as alterna-

tives to polymers derived from fossils. Such biomaterials can also offer economic advantages

because they are derived from renewable resources. Several biopolymers (gelatin, chitin, chito-

san, starch, pectin, cellulose and its modified versions, etc.) have been exploited to produce films

and formulations. Their use is limited because of fast degradation, predominant hydrophilic

character, and, in some cases, unsatisfactory mechanical properties. However, the properties of

these polymers can be improved by using inorganic fillers such as additives. Halloysite nanotube

is a promising green filler for this purpose.

Keywords: Biopolymers, halloysite nanotubes, HNT-biopolymers nanocomposites,

physicochemical properties, applications

18.1 Introduction

Nanocomposites are novel materials composed of polymer incorporating small amounts of nanosized fillers with improved properties.

In the past few years, particular attention has been devoted to alternatives of conven-tional petroleum-based polymers by biopolymers, such as polysaccharides, proteins, and biodegradable synthetic polyesters. The combination of a biodegradable fillers and a environmental friendly polymer forms the biocomposites.

Bionanocomposites are very promising materials since they exhibit interesting properties with preservation of the material biodegradability without eco-toxicity. Bionanocomposites represent the new generation of nanocomposites and are obtained from the combination of biopolymers and inorganic materials that have, at least, one nanometer scale size. The functionality of the final material depends on many factors,

558 Handbook of Composites from Renewable Materials-Volume 7

including type of materials, number of layers, their sequences, and preparation condi-tions. Depending on aspect ratio (length/diameter) and geometry, the various nano-fillers can be classified in (i) layered particles (e.g., clay), (ii) spherical (e.g., silica), or (iii) acicular ones (e.g., whiskers and carbon nanotubes). At the present, the most inten-sive researches are focused on layered silicates, increasing the mechanical properties of the resultant polymer–clay nanocomposites, and, among them, halloysite nanotubes are very emerging materials (Figure 18.1).

Halloysite is an abundantly available natural predominantly tubular nanomaterial (HNT) with chemical composition similar to that of kaolin. HNTs are novel natural nanomaterials with an unique combination of hollow tubular nanostructure, large aspect ratio, suitable mechanical strength, high perspectives in terms of functional-ity, biocompatibility and availability at low cost in large amounts. Halloysite-based nanocomposites have been studied for several decades owing to their physico chemical properties, including their tubular structures, ion exchange, and hydrophobicity. The emerging halloysite-based bionanocomposites are mainly addressed to biomedical applications and different short-term applications, e.g., agriculture, packaging, hygiene devices corrosion protection of implant alloys, and biosensors.

This chapter provides an overview of the recent progress achieved on the devel-opment of HNTs-biopolymer nanocomposites, in particular, those composed of polysaccharides polymers. Covalent and noncovalent functionalization modification methods for HNTs and different construction approaches for the HNTs-biopolymer nanocomposites are summarized first. Subsequently, the physicochemical prop-erties (such as mechanical and thermal responses to external stimuli) of HNTs-bionanocomposites are described in detail. Finally, we provide an outlook for the future development of HNTs-bionanocomposites in several applications, such as pharmaceutical (tissue engineering scaffolds, carrier and targeted delivery systems for drug, and wound dressing materials) and others where high-performance composites are considered.

20032005

20062007

20082009

20102011

20122013

20142015

0

50

100

150

200

250

Year

N°

of

pu

bli

cati

on

s

HNT

HNT/nanocomposites

HNT/Bionanocomposites

Figure 18.1 Data collected from SciFinder online database, with key words “halloysite nanotubes” and

include all the publications since 2003 regarding halloysite, halloysite/nanocomposites and halloysite/

bionanocomposites.

Halloysite-Based Bionanocomposites 559

18.2 Biodegradable Polymers

In the new millennium due to the increasing awareness concerning the human impact on the environment and the constant increase in the fossil resources cost, there is a growing interest to the development of efficient solutions to produce new environ-mental friendly materials. More attention has been devoted on the replacement of traditional plastics based on petroleum by materials based on biopolymers, such as polysaccharides, biodegradable polyester, or proteins (Pappu et al., 2015; Thakur & Kessler, 2015; Thakur & Thakur, 2014a–c, 2015; Thakur et al., 2014a–e). Different to petroleum-based plastics, biopolymers are decomposed by microorganisms and enzy-matic processes (ASTM standard D-5488-94d). Depending on the degradation condi-tions (aerobic vs anaerobic) and the medium, the material is promptly decomposed to form biomass, inorganic compounds, methane and/or carbon dioxide gases, and water. Moreover, biopolymers are very abundant in nature and, therefore, they can be poten-tially advantageous also from the economic view point. Biopolymers can be classified in four different categories as shown is Scheme 18.1.

Among all biopolymers, polysaccharides are the most abundant macromolecules in the biosphere. They are composed of multiple saccharide units joined to another one through glycosidic linkages and have a number of unique features. The diverse structures and properties of carbohydrates offer molecular and biological advantages for their use in the preparation of nanocomposites.

For this reason, in this chapter, we will take into account for the HNT-polysaccharide nanocomposites, with a special focus on cellulose, chitosan, starch, alginate, and pectin.

18.2.1 Cellulose

Cellulose is a natural polymer that can be obtained from natural sources such as cotton and wood pulp. For this reason, it is considered to be the most abundant biopolymer

From microorganism From bionanotechnology From petrolchemical

products

Biomass products

Polysaccharides Proteins,

lipids

Polyhydroxy

alkanoate

Polylactides Polycaprolactone

Polyesteramides

Aliphatic copolysters

Aromatic copolysters

Biopolymers

Starches

Lignocellulosic

products

Pectin, chitosan

and gums

Scheme 18.1 Classification of the main biodegradable polymers.


on earth. Cellulose is contained in plants for approximately one-third of their weight, whereas wood is made of cellulose for ca. half of its weight and cotton is almost pure cellulose (90%). Cellulose is a fairly rigid homopolymer with a linear backbone con-sisting of d-anhydroglucopyranose units linked together by -(1→4) glycosidic bonds obtained between C-1 and C-4 of glucose moieties (Figure 18.2).

Mechanical and thermal performances of cellulosic materials are considered prom-ising and competitives with conventional plastics so that they are regaining interest as renewable resources to replace petroleum-based polymers (Delhom et al., 2010; Ma et al., 2011; Nishino et al., 2004). Therefore, actual application of cellulose reduces either the consumption of fossil resources or protects the environment.

However, the main limiting factor for cellulose manufactures raised in regenerating and processing that polymer owing to its large hydrogen-bond formation ability and its crystalline structure which limit the dissolution or regeneration in conventional sol-vents (Cao et al., 2009). To solve this problem, a number of synthetic methodologies was developed; one of them is the etherification of the cellulose that leads to biopolymers very promising into numerous applications. Examples of commercial cellulose ethers are hydroxypropylcellulose (HPC) and sodium carboxymethylcellulose. HPC is obtained by linking the propylene oxide to the anhydroglucose hydroxyl groups. There are differ-ent HPC on the basis of the molecular weight and substitution degree. HPC possesses high solubility in water although it is hydrophobically modified. HPC is employed for the consolidation of ancient papers improving significantly their physicomechanical properties over time. Filling porous structure of paper with nanoparticles can lead to materials with tunable properties and functions. Addition of very small amount of metal nanoparticles on the cellulose fibrous structure can generate new devices with antibacte-rial performance crucial for protecting paper-based documents over time (Giachi et al., 2010). It is well known that filling polymer matrices with natural nanoclays represents an efficient strategy to develop green composite materials with excellent properties.

Cellulosic biofibers represent another class of natural cellulosic biopolymers. Due to their interesting properties, including low cost and biodegradability, they can be used like biopolymers themselves as well as reinforcing fillers of polymeric matrices (Singha et al., 2008; Singha & Thakur, 2008; 2009a–c; 2010a–e; 2011a, b).

18.2.2 Chitosan

Chitosan is a linear polysaccharide constitutes of -1,4-linked-d-glucosamine (deacet-ylated unit) and N-acetyl-d-glucosamide (acetylated unit) in a random sequence. It is

OH

OH

OH

OH

O

O

O

n

O

O

HO

HO

Figure 18.2 Schematic representation of cellulose unit.


obtained from chitin, which is a constituent of crab and shrimp shells (Tang et al., 2009; Wang et al., 2005) (Figure 18.3). Its large availability makes chitosan an abundant natu-ral biomacromolecule behind only cellulose.

Due to its nontoxic nature, good compatibility with living tissues and antimicrobial activity, chitosan has been considered a good candidate to generate biomaterial in the recent scientific literature (Hong et al., 2011). In addition, it has good performance as adhesive and it easily forms films characterized by high water affinity, bio degradability, and sustainability (Ismail et al., 2011). Chitosan is readily soluble in aqueous acidic solu-tion, even dilute, and can be processed in various forms such as membranes (Jayakumar et al., 2011), films (Shen et al., 2007), fibrous mats (Jayakumar et al., 2011), and sponges (Aider, 2010). Among them, chitosan membrane is the most versatile, particularly for biomedical applications such as bone implantation, artificial skin, and tissue engineer-ing (Jayakumar et al., 2011; Wang et al., 2006) as well as for filtration applications such as waste water filtration and reverse osmosis (Brum; et al., 1995). However, chitosan has some limitations in terms of mechanical strength and thermal stability. The scarce water and gas barrier properties also restricts the applicability of chitosan as material (Hong et al., 2011; Tang et al., 2008). To solve these disadvantages, various fillers were employed for reinforcing chitosan matrix.

18.2.3 Starch

Starch is mainly extracted from tubers (potatoes, manioc, etc.) and cereals (wheat, corn, rice, etc.). It is stocked into seeds or roots and represents the main plant energy reserve. Starch is a promising polymer because it is abundant, cheap, and renewable. It is a homopolymer consisting of several glucose units linked by glycosidic bonds (Figure 18.4). Starch is largely produced by green plants that use this polymer to store

OH

NH2 NH2 NH2

OH

OH

OH

OO

O

n

OO

HO

HO HO HO

Figure 18.3 Schematic representation of chitosan unit.

OH

OH

OH

OH

OH

OH

OHO

OO

nO

O

HOHO

HO

HO

Figure 18.4 Schematic representation of starch unit.


energy. It is the most common carbohydrate in staple foods. Therefore, it is generally introduced in human diets with wheat, corn, potatoes, cassava, and rice.

Starch-based products are the most frequently used biomaterials in food, papermak-ing, packaging industries, beverages, and textiles as well as in medicine, pharmacy, and cosmetic (Balmayor et al., 2009; Saboktakin et al., 2011; Salgado et al., 2007).

Although starch shows a high capacity to form homogeneous films with excellent oxygen barrier properties, it exhibits some drawbacks, such as lacking mechanical properties and high-water vapor sensitivity which leads to high-water vapor perme-ability and retrogradation (Averous & Boquillon, 2004; Ghanbarzadeh et al., 2011).

18.2.4 Alginate

Alginate is a homopolymer formed by (1,4)-linked -d-mannuronate and -l- guluronate units linked in a linear manner (Figure 18.5). It is present in brown seaweed from which it can be obtained after extraction. The industrial applications of alginates are correlated to their ability to retain water and their gelling. Alginate gels find appli-cation mainly in pharmaceutical field, such as control release drugs, immobilization matrices for cells, or other materials and wound dressing. The wide use of alginate derives from the possibility to obtain hydrogels with spherical shape and controlled sizes in the millimeters and up to micrometers range via ionic interaction with Ca2+ and Ba2+ at room temperature. Alginate gel beads are efficient in the removal of dyes, such as victoria blue (Kumar & Tamilarasan, 2013), methylene blue (Ai et al., 2011), and crystal violet (Cavallaro et al., 2013c).

Finally, alginates can be used to produce biodegradable films for food packaging because of their unique colloidal and excellent membrane-forming properties.

However, alginate hydrogel beads are soft materials owing to the presence of high amount of water (ca. 95%). For this reason, the beads may undergo to disruption pre-maturely when compression force occurs during manufacturing or application (Leung et al., 1998; Tal et al., 1997).

18.2.5 Pectin

Pectin is a biodegradable polymer and it has been proposed to develop materials within a green protocol with smart functionalities useful for several industrial purposes. It is a complex anionic polysaccharide mainly constituted of homogalacturonan (1,4 linked

-d-galacturonic acid and its methyl ester) (Figure 18.6). The polymeric chain pres-ents also Rhamnogalacturonan-I. On the basis of the methyl esterification degree (DE),

O O

O

OO

O

O

OO

mn

HO

HO

HO

HO

Na

Na

Figure 18.5 Schematic representation of alginate units.


these biopolymers are classified as high methoxy pectins (HM) and low methoxy pec-tins (LM). In detail, HM and LM pectins have a DE value higher and lower than 50%, respectively. The main use of pectin is like a gelling agent, a thickening agent, and a stabilizer in food. In the pharmaceutical industry, it is used to reduce gastrointestinal disorders and blood cholesterol levels. Other applications of pectin include its use in edible films, paper substitute, foams, plasticizers, and so on (Thakur et al., 1997).

18.3 Natural Inorganic Filler: Halloysite Nanotubes

Halloysite (HNT) has a chemical formula for its cell unit Al2Si

2O

5(OH)

4 × nH

2O that

corresponds to kaolinite, a natural alluminosilicate clay. It is a dioctahedral 1:1 clay mineral present in soils. It is formed by weathering of several types of igneous and nonigneous rocks; thus, it can be found mainly in wet tropical and subtropical regions and weathered rocks. Of course, each deposit is characterized by different purity grade, characteristic sizes, and hydration state.

The term ‘halloysite’ was employed for the first time by Berthier in 1826 and derived from Omalius d’Halloy, who found the mineral in Angleur, Liége, Belgium.

HNT has mainly a hollow tubular structure in the subnanometer range with an aspect ratio of ca. 20; the wall is consituted of 10–15 bilayers of aluminum and silicon oxide. Depending to the deposit, the halloysite dimensions can vary. Generally, HNTs have a length in the range 0.2–1.5 mm, while the inner and outer diameters of tubes are in the ranges 10–30 and 40–70 nm, respectively (Abdullayev et al., 2013; Abdullayev & Lvov, 2013; Lvov & Abdullayev, 2013).

The special feature of halloysite clay tubes is the different surface chemistry at the inner and outer surfaces. In contrast to other clays, most of the aluminol groups are positioned into the HNTs inner surface, whereas the external portions are pri-mary siloxanes while a few silanols/aluminols are exposed on the edges of the sheets (Figure 18.7). Dielectric properties of aluminum and silicon oxides are different. Similarly, they undergo to ionization in aqueous media in an opposite waygenerating tube with inner and outer surfaces oppositely charged. This charge separation occurs in water within a wide pH range from 3 to 8 (Veerabadran et al., 2007). Experimentally, the charge separation is predicted by comparing the negative and positive values for electrical zeta-potential of silica and alumina surfaces in water, respectively. The overall

O

OO

O

OO

O

O

n

HO

HO

OH

OH

OH

Figure 18.6 Schematic representation of pectin units.


zeta-potential of halloysite almost reflects the zeta-potential of silica. The typical physi-cochemical features of HNTs are summarized in Table 18.1.

Halloysite is efficiently removed from an organism with macrophages; and therefore, it is considered a healthy and biocompatible material. Biological cells and tissues were incubated in the presence of variable amounts of halloysite, and it was demonstrated that concentration up to 0.2 g/dm3 can be considered safe. Actually, this is a safe amount for inorganic inclusions. The toxicity of HNT was tested after 48-hours incubation with fibroblast and human breast cells (Vergaro et al., 2010). It emerged that it is nontoxic and it is much less harmful than ordinary sodium chloride salt though it may penetrate into the cell interior as showed by confocal microscopy experiments (Figure 18.8). It turned out that accumulation of HNTs within cells does not prevent their proliferation and they are uptaken into the cells surrounding the cell nucleus. Therefore, halloysite is a safe material with a low environmental impact and, finally, due to its shorter length than asbestos it fits the size window (between 0.5 and 1.5 mm) considered not toxic for silicate nanoparticles (Wiessner et al., 1989).

Si

15 nm

Al O H

Figure 18.7 FE-SEM Image of Halloysite on Si-Wafer (Left) and Schematic Illustration of Crystalline

Structure of Halloysite (Right). Source: (Ya et al., 2012) Copyright 2012. Reprinted with permission from

American Chemical Society.

Table 18.1 Physico-chemical features of HNTs.

Chemical formula Al2Si

2O

5(OH)

4.nH

2O

Length 0.2–2 mm

Outer diameter 40–70 nm

Inner diameter 10–40 nm

Aspect ratio (L/D) 10–50

Elastic modulus (theoretical value) 140 GPa (130–340 GPa)

Mean particle size in aqueous solution 143 nm

Particle size range in aqueous solution 22.1–81.6 m2/g (Pasbakhsh et al., 2013)

BET surface area 50–400 nm

Pore space 22.1–46.8%

Lumen space 11–395

Density 2.14–2.59 g/cm3

Average pore size 79.7–100.2 Å

Structural water release temperature 400–600 °C


18.3.1 Functionalization of HNTs

The modification of both surfaces of HNT plays a crucial role for the properties of HNT-polymers nanocomposites. Indeed, it is known that the functionalization improves the dispersion of the HNTs in solvents by enhancing their interfacial compatibility as well as the stress–strain transfer into a polymeric matrix.

Different inside/outside chemistry allows for selective treatment of halloysite lumen and different charges at the inner and outer surfaces provide enhanced loading of nega-tively charged molecules into the tubes. Modification of halloysite outer surface is usu-ally intended to improve clay dispersal in polymer matrix and enhancing the properties of HNTs-polymer nanocomposites. Since the HNT functionalization improves the dispersion and stress–strain transfer in a polymeric matrix, nanocomposites based on modified nanotubes exhibit enhanced thermal and mechanical properties.

Moreover, selective lumen modification via covalent bonds of functional groups could open up new applications based on molecular recognition, such as molecular storage, molecular separation, catalysis, and drug delivery. Therefore, HNTs find appli-cations as nanocontainers, sustained release, cosmetics, nanoreactors, and catalyst support.

18.3.1.1 Functionalization of External Surface

The negative surface potential in a wide pH range endows the external siloxane surface of HNT to be modified by adsorbing specific cations. Lvov et al. adsorbed a monolayer of poly(ethyleneimine) (PEI) with a thickness of 54 nm and, then, alternately adsorbed HNT forming a thin film with approximately 14 sets of HNT-PEI monolayers. Due to the loosely packed HNT in the composite (ca. 50% is empty space), the material could be used to load and, subsequently, release guest molecules (Lvov et al., 2002).

(a) (b)

Figure 18.8 CLSM image of halloysite nanotubes intracellular uptake by MCF-7 cells and HeLa cells.

MCF-7 cell membrane (red, TRITC-labeled) with co-localized halloysite (HNT): yellow, merging from

FITC-labeled HNTs (a). Localization of HNTs (green, FITC labeled) outside HeLa nuclei (blue, Hoechst

staining) (b). Scale bar is 10 m. Source: Vergaro et al., 2010; Copyright 2010. Reprinted with permission

from the American Chemical Society.


It was reported that halloysite tends to adsorb organic molecules via electron trans-ferring interaction; within this issue, it was reported the functionalization of HNT outer surface with 2,5-bis(2-benzoxazolyl) thiophene (BBT), a molecule that can act as electrons donor, selected as the interfacial modifier for polypropylene (PP)/HNTs com-posites (Figure 18.9). The nanocomposites with BBT showed substantially enhanced tensile and flexural properties, which were attributed to better crystallinity of the nano-composites (Liu et al., 2008).

Veerabadran et al. elaborated a layer-by-layer assembly (LbL) by sequentially absorb-ing layers of polyelectrolytes of different molecular weight, with a final layer of silica nanoparticles. LbL shell assembly was applied to dexamethasone loaded halloysite to retard the drug release from the nanotubes. The loading of the drug was confirmed by porosity measurements that showed a reduction in the pore volume of the dexa-methasone-loaded HNT compared to the unloaded tube. The drug loading was esti-mated, by UV experiments, to be ca. 7 vol., which is approximately the lumen volume (Veerabadran et al., 2009).

The organosilanes grafting on the HNT external surface is the most common cova-lent functionalization. It is achieved, as first reported by Yuan et al. (2008), via con-densation between hydrolyzed silanes and the surface hydroxyl groups of the HNTs. Even though the siloxane surface of HNTs is generally established as nonreactive, some data indicate that the hydroxyl groups at defect sites are available for modification. The grafting reactions take place in either water/alcohol mixtures or toluene and also in solvent-free condition. Barrientos-Ramirez et al. reported the covalent grafting of 3-(2-aminoethylamino)-propyltrimethoxysilane on the HNT outer surface to develop new supported catalysts for polymerization of MMA. The obtained PMMA showed controlled molecular weight and a relatively low polydispersity (Barrientos-Ramírez et al., 2009).

Riela et al. reported the functionalization of HNT with 3-mercapto-propyltrime-thoxysilane and the subsequent functionalization with organic salts to synthetize a new class of materials that possess most of the properties and characteristics of organic salt,

Shear

Shear

Absence of

BBOT

With shear

and heat

Pre

sen

ce o

f

BB

OT

Upon crystallizing

Upon crystallizing

BBOT BBOT fibrillarHNTsKebab of PPPP chain

Figure 18.9 Schematic of the orientation and the crystallization structure formation of HNTs-PP

nanocomposites with or without an interfacial modifier BBT. Source: Liu et al., 2008. Copyright 2008.

Reproduced with permission from IOP Publishing, UK.


allowing them to be dispersed in physiological media. The obtained materials were employed as carriers for biological molecules such as cardanol and curcumin, showing good loading capacity and sustained release (Massaro et al., 2015a; Riela et al., 2014). Similar materials were employed as support for palladium nanoparticles and tested as catalyst in the Suzuki reaction between phenyl boronic acid and several aryl halide under microwave irradiation. The obtained results confirmed the good catalytic activ-ity of the material and recycling investigations showed that these catalysts were active at least for five cycles (Massaro et al., 2015b). Cavallaro et al. (2015a) proposed the modification of HNT outer surface by grafting a pH and thermoresponsive polymer, namely poly(N-isopropylacrylamide) in order to obtain a novel drug carrier for cur-cumin delivery and release upon proper stimuli (Figure 18.10).

18.3.1.2 Functionalization of the Lumen

The empty lumen of halloysite is a good miniature container for entrapping chemical agents such as pharmaceuticals, enzymes, biocides, and other chemically active agents, e.g., anticorrosion for protective coating, for processes which benefit from their sus-tained release.

Veerabadran et al. (2007) reported the use of halloysite nanotubes for encapsula-tion and successive slow release of three drugs: furosemide (antihypertension), dexa-methasone (corticosteroid), and nifedipine (anti-anginal). Abdullayev et al. (2009) exploited the charge difference in outer and inner surfaces of halloysite for loading benzotriazole (corrosion inhibitor) and the obtained material was mixed to paint coat-ings in the amount of 2–10 wt%. Yah et al. (2012b) immobilized a dopamine derivative onto the internal lumen surface of HNT. It was proved that Dopa binds selectively the internal surface of HNT through alumina–catechol bonds. Cavallaro et al. reported the noncovalent functionalization of HNT lumen by selective adsorption of anionic surfactants as sodium alkanoates. The supramolecular modification of HNTs with the anionic surfactant generates whether stable dispersions or hybrid materials with hydro-phobic lumen (Figure 18.11). Due to this structure, this material behaves like a sponge to entrap hydrophobic compounds (Cavallaro et al., 2012).

However, selective modification of interior remains a difficult task; currently, to the best of our knowledge, only one example is reported in literature about the covalent modification of inner surface. Lvov et al. reported the first selective covalent func-tionalization of HNT inner lumen with octadecylphosphonic acid without octadey-lphosphonic acid bonding on the siloxane outer tube surface. The adsorption studies showed that modified HNT adsorbs more ferrocene than its hydrophilic derivative

PNIPAAM

Functionalization Loading

25 °C, pH 7.4

Release

37 °C, pH 6.8

Figure 18.10 Schematic representation of temperature triggered loading and release of curcumin into

HNT modified with poly(N-isopropylacrylamide).


(ferrocenecarboxylic acid) as a consequence of its hydrophobic inner surface. Therefore, the octadecylphosphonic acid immobilized in the HNT lumen can behave like a sponge for physisorption increasing the adsorption capacity for hydrophobic molecules (Yah et al., 2012a).

18.3.2 Composites Structured with Halloysite

HNTs were detected to show good performance as new kind of nanofiller for reinforc-ing polymer matrix due to their unique features such as

low cost that makes them an inexpensive alternative to the carbon nanotubes; the aspect ratio (length/diameter) of HNTs can range from 8 to 50 (Pasbakhsh and Churchman, 2015), and it increases filler–polymer inter-actions; therefore, HNTs are potential competitors to silicate fillers;the possibility to functionalize both internal and external surfaces with simple modification processes increases their performance compared to other nanoparticles;the empty lumen of HNT allows encapsulation of chemical agents for a variety of applications like sustained release of active molecules, self-healing, and targeted drug delivery;HNTs are biocompatible, environmental friendly, and no toxic materi-als that have competitive advantages over other nanomaterials such as carbon nanotubes.

Recent studies showed that the introduction of HNTs in a polymeric matrix can cause a conspicuous increase in its thermal and mechanical performance; in addition, due to the cylindrical pore (lumen) that can be loaded with chemically and biologically

Anionic surfactant

Cationic surfactant

Unstable

aqueous dispersion

Hydrophylic cavity

available in apolar

media

Stable

aqueous dispersion

Oil solubilization in

aqueous media

Figure 18.11 Illustration of the hybrid surfactant/HNTs materials.


active substances, it is possible to introduce in polymers applications specific chemical inhibitors (antimicrobial, anticorrosion, flame-retardant, drugs, and microcrack self-healing) that can be released in a sustained manner.

18.4 Bionanocomposites

Nanoparticles dispersed in a polymer matrix generates nanocomposites. The first industrial application of these kind of materials goes back to the nylon 6/montomoril-lonite nanocomposite prepared by Okada in 1988 in Toyota laboratories.

Nowadays, these nanomaterials draw considerable research interest because of the improvement in physical, thermal, and mechanical properties compared to those of pristine polymers (Bordes et al., 2009).

It is well known that the performances are strongly dependent on the matrix/filler interactions so that the shape and the characteristic sizes of the nanoparticles play a crucial role. Nanotubes and isodimensional nanoparticles maximize the interactions between polymer and filler and, therefore, they favor the improvement of the perfor-mance with respect to the pristine polymer materials (LeBaron et al., 1999).

A sustained development impose to dedicate efforts toward the development of bio-polymer/nanoclay composite which could be used as biocompatible materials alterna-tive to plastics. The nanoclay role should be at overcoming the typical limitation of bioplastics in terms of their performance requirements for specific applications. This paragraph is focused on the formation, characterization, and structure–properties correlations of nanocomposites based on biopolymers and HNTs.

18.4.1 HNT-Biopolymer Nanocomposite Formation

The choice of preparation methods has mainly focused on improving nanotube dis-persion and enhancing interfacial interactions (Liu et al., 2014a). The most common methods for bionanocomposites formation are solution processing, spray dry, electro-spinning, melt processing, and ball milling (Table 18.2). Among them, the most popu-lar method used for producing HNTs-biopolymer nanocomposites is solution mixing because it is easily performed and suitable for small sample sizes. This process is carried out dispersing the nanotubes and polymer in a suitable solvent (including organic sol-vent and water) with ultrasonic treatment or rigorous stirring, respectively. Finally, the HNTs-biopolymer nanocomposites as film are obtained after casting or precipitation.

Chitosan (Arcudi et al., 2014; Liu et al., 2012b; 2013; Sun et al., 2010), alginate (Fan et al., 2013; Liu et al., 2012a; 2015), cellulose (Arcudi et al., 2014; Cavallaro et al., 2014b), and pectin (Cavallaro et al., 2011a) can help the HNTs dispersion in water and by casting these solutions bionanocomposite films with rather homogenous nanopar-ticles distribution are formed.

In the case of insoluble polymers, the above method, that involves the removal of solvent, cannot be used. A good alternative of solution processing is the melt one, that is particularly useful for thermoplastic and rubber materials. Starches, for example, can be produced by melt-extrusion, a continuous, more productive, easy, and economic process (Schmitt et al., 2015b).


18.4.2 Properties of HNTs-Biopolymer Nanocomposites

Studies on bionanocomposites based on pectins and nanoclays with different shape (HNTs, laponite nanodisks, and kaolinite sheets) evidenced that HNTs are good can-didates to form bionanocomposite films with enhanced performance (Cavallaro et al., 2011b). The prepared composites exhibited evident differences already in the macro-scopic aspect; for instance, HNTs addition generated compact films while laponite pro-duced a structural deterioration (Figure 18.12).

Table 18.2 Preparation methods and applications of HNT-bipolymers nanocomposites.

Biopolymers Instrumentation Applications References

Poly-lactic acid Electrospinning Drug delivery Qi et al., 2010; Xue et al.,

2015

Chitosan Solution casting Catalysis Sun et al., 2010

Chitosan Solution casting Tissue engineering Liu et al., 2012b

Alginate Solution processing Removal pollutant Liu et al., 2012a

Alginate Solution processing Drug delivery Fan et al., 2013

Chitosan Spray dry Drug delivery Wang et al., 2014; Wang

et al., 2013

Chitosan Solution mixing and

freeze-drying

Tissue engineering Liu et al., 2013

Chitosan Solution processing Removal pollutant Ma et al., 2014; Zhai et al.,

2013; Palantöken et al.,

2015; Peng et al., 2015

Chitosan Solution mixing and

freeze-drying

Wound healing Liu et al., 2014

Chitosan Solution casting Packaging Arcudi et al., 2014

Chitosan Solution casting Cultural heritage Cavallaro et al., 2014a

Cellulose Solution casting Cultural heritage Cavallaro et al., 2014b

Beeswax Solution casting Cultural heritage Cavallaro et al., 2015

Pectin Ball milling Food packaging Gorrasi, 2015

Pectin Solution casting Packaging Cavallaro et al., 2011

Poly-lactic acid Solution casting Food packaging Makaremi et al., 2015

Poly-lactic acid Melt-blending Automotive

applications

Notta-Cuvier et al., 2015

Alginate Solution mixing and

freeze-drying

Tissue engineering Liu et al., 2015

Poly-(ε-

caprolactone)

Electrospinning Drug carrier Xue et al., 2015

Starch Melt processing Biomedical

applications

Schmitt et al., 2015b

Starch Melt processing Drug delivery Schmitt et al., 2015a


Compared to conventional nanocomposites, it is worth nothing that HNTs-biopolymer systems still maintain interesting elastic properties at very large nanopar-ticle concentration even above 50%.

The bionanocomposites characterization concerns the morphology and the structure of the material in the mesoscopic range monitored by SEM. Physicochemical features such as the thermal stability, transparency, wettability, water uptake, tensile, and dielec-tric properties are typically determined to highlight the performance of the material itself and to acquire indirect information on the nanoparticle distribution/organization within the polymer matrix.

18.4.2.1 Bionanocomposites Surface Morphology

The morphology imaged at the HNTs-biopolymer surfaces is highly dependent on the polymer nature. The SEM images of the HNTs-HM pectin and the HNTs-LM pectin nanocomposites show that agglomerations of nanotubes occurs only at high filler com-positions for HNTs-HM pectin system, while HNTs are well dispersed into the LM pectin matrix in a much wider concentration range (Cavallaro et al., 2011b). It appears that the forces between the nanotubes compete with the interactions between nanotube and pectin, which trigger the HNT-clusters formation. This phenomenon is retarded in LM pectin likely due to the larger amount of carboxylic groups in the polymer chain, which enhance electrostatic interactions with HNTs leading to an increase of the nano-tube net charge (Cavallaro et al., 2011b). A similar explanation was provided for HNTs stabilized by the adsorption of anionic surfactants into the lumen. Other polyanions such as alginate provided a random well dispersion of HNTs within the biopolymer matrix (Chiew et al., 2014).

Peculiar is the case of composites based on modified cellulose (hydroxypropyl cellu-lose, HPC). The nanoparticles are absent at the film surface and consequently it appears homogeneous. On the contrary, different morphology was observed in HNTs-LM pectin films which showed dispersed HNTs into the polymer matrix as evidenced in Figure 18.12.

The SEM micrographies (Figures 18.13 and 18.14) of the edges of the nanocompos-ites for high filler content show that the nanotubes are well compacted between thin layers of HPC forming a sandwich-like structure in the mesoscopic scale.

As concerns the polycations, the addition of HNTs into chitosan no significantly influenced the microporose structure of the biopolymer. However, the HNTs-Chitosan

(a) (b)

Figure 18.12 Photos of films formed by pectin filled with HNTs (a) or laponite (b). Adapted from ref.

(Cavallaro et al., 2011b).


composite is more uniform in terms of pores distribution and less collapse of the pore walls are observed. This can be attributed to the enhanced stiffness and modulus of chi-tosan in the presence of nanoparticles (Liu et al., 2013). The surface properties of HNTs-biopolymer composites can be investigated by water contact angle (ϑ) measurements. The wettability changes may reflect the variation of the microstructures of the films sur-face generated by the HNTs. In principle, one should consider two effects: (i) the HNTs surface concentration and (ii) the films surface roughness; therefore, one has to compare

HM pectin LM pectin

Cf = 10 wt% Cf = 10 wt%

Cf = 30 wt%

30 m 30 m

30 m 30 m

30 m 30 m

Cf = 30 wt%

Cf = 60 wt% Cf = 60 wt%

Figure 18.13 Scanning electron microscopy images of pectins/HNTs films.

Figure 18.14 Scanning electron microscopy images of cross-section of HPC/HNTs film 60 wt%.


the results with those from SEM images. In the case of HNTs-Pectin, the enhancement in the surface hydrophilicity was registered as a consequence of the enrichment of the nanofillers at the interface (Cavallaro et al., 2011a). Differently the sandwich like struc-ture for HNTs-HPC composites hardly changes the surface wettability (Figure 18.15).

18.4.2.2 Bionanocomposites Mechanical and Thermal Response

Thermal degradation features of polymeric matrices are generally investigated through thermo-gravimetric analysis (TGA) under inert (N

2, Ar) or oxidizing (O

2) atmo-

sphere. The thermoanalytical curves are generally discussed in terms of mass loss vs temperature trends. The temperatures for each decomposition step are defined from the maximum of the degradation rate vs temperature trends; alternatively, the tem-perature at which a certain amount of mass loss is approached (typically 5%) can be considered (Blanco et al., 2014). The composite residual mass at the higher investi-gated temperature provides information on the actual filler content and its homogene-ity within the polymer (Cavallaro et al., 2011b). The polymers thermal stabilization in the presence of nanofillers is generally observed (Du et al., 2006) and ascribed to the entrapment of the gases obtained from the polymer pyrolytic decomposition. In other words, inorganic nanoparticles added to polymeric matrices induce obstacle both mass and heat transports. For this reason, a nanocomposite with well-dispersed filler typically shows a reinforcement under thermal stress. The HNTs effect on biopolymeric matrices is not predictable a priori as it is strongly dependent on both filler content and biopolymer nature. If a nonionic biopolymer is considered such as HPC, the pres-ence of HNTs causes the thermal stabilization up to 20 wt% of filler content, while a further addition of HNTs enhances the thermal degradation that occurs at tempera-ture even 65 °C below degradation temperature (T

d) of pristine polymer (Figure 18.16).

Pectin

76°

HPC

67°

Pectin + HNT 60 wt%

43°

HPC + HNT 60 wt%

76°

Strong decrease

of water contact angle

Small increase

of water contact angle

Large increase surface

hydrophilicity

Small increase surface

hydrophobicity

Figure 18.15 Water contact angle and structure of bionanocomposites.


The enhancement of degradation process was correlated to the reduced mobility of the polymer moiety in the presence of large amount of HNTs as monitored by dielectric spectroscopy data (Cavallaro et al., 2011a).

Similar thermal degradation behavior was observed for HNTs-PEG 20000 compos-ites (Cavallaro et al., 2013a). Concerning the residual mass after the nanocomposites pyrolysis, it generally scales linearly with the inorganic nanoparticle contents (Cavallaro et al., 2011b; 2012; 2014b) but some exceptions showing much larger residual mass show the enhanced char formation in the presence of HNTs likely due to the entrap-ment within the nanotubes lumen (Makaremi et al., 2015).

To obtain a more detailed information on the degradation path alteration induced by the nanoparticles, the kinetics parameters should be investigated. To this aim, TGA experiments at variable heating rates are needed for a given sample. Friedman’s as well as Flynn–Wall–Ozawa (FWO) approaches allow the determination of activation energy (E

a) related to degradation process. These methods are model-free and nonisothermal.

An advantage of these procedures is the possibility to compute the activation energy values as a function of the extent of conversion ( ) without making any supposition on the degradation mechanism (Vyazovkin et al., 2014). Briefly, the following equation represents the integral FWO method

ln . .AE

P

E

PTa5 3305 1 0516 (Eq. 18.1)

where is the heating ramp and Γ( ) a function characteristic for a given mechanism depending on . A is the pre-exponential factor while T is temperature and R is the ideal gas constant. At a given value, E

a represents the slope of ln vs 1/T plot. Concerning

the Friedman method, one can write

ln lnd

dTAf

E

PTa (Eq. 18.2)

where f( ) is a function of that defines the polymer degradation mechanism. Based on eq. 18.2, the activation energy values are obtained from the slopes of the ln ( d /dT) vs

20

Td/°

C

40 60

Cf/wt%

80 1000

300

320

340

360

380

Figure 18.16 Degradation temperature as a function of the filler concentration for HPC/HNTs films.


1/T linear trends. The reliability of the Ea data is generally provided by the comparison

between results from the two methods. This procedure was used for HNTs-HPC and HNTs-Pectin composites showing the variation of activation energy for the degrada-tion process and therefore the increase in the energetic barrier for thermally stabilized composites.

The mechanical performance of nanocomposites is a key technological fea-ture. The resistance to stress of bionanocomposites are generally measured upon elongation under a linear force/stress ramp. It should be noted that biopolymer films may possess rather similar dynamic mechanical properties of many traditional plas-tics used for packaging; for instance, poly-vinyl-acetate presents elastic modulus and breaking stress/strain comparable to those of biofilms (Mark, 1999). On this basis, the HNTs loading onto biopolymers is a valuable route already if the mechanical performance are kept unaltered as capsules for the entrapments of active species are introduced.

Chitosan filled with HNTs, even in small amounts, determined an improvements in the elastic properties as the young modulus increased by ca. 20% (Arcudi et al., 2014; De Silva et al., 2013; Liu et al., 2012b). The functionalization of HNTs before their loading into the chitosan matrices exhibited even better performance, with an improvement of the modulus with respect to the pristine biopolymer of ca. 55% and a significant increase in the stress at breaking (Figure 18.17). This result was likely due to the microfibers of the functionalized nanotubes arranging themselves in a network (Arcudi et al., 2014). These results are rather intriguing if one considers that multi-walled carbon nanotubes improved the mechanical performance of chitosan in a simi-lar extent (Wang et al., 2005).

For nonionic biopolymer, such as HPC, the presence of HNTs caused an enhance-ment of the elastic modulus, whereas the stress at the breaking point was slightly decreased (Arcudi et al., 2014). Oppositely, microcrystalline cellulose-based fibers showed an increase in the stress at the breaking point due to the uniform orientation of the well-dispersed HNTs (Luo et al., 2014).

5

Str

ess

/MP

a

10 15

Strain/%

20

Chitosan + f-HNT

Chitosan + HNT

Chitosan

250

30

25

20

15

10

5

Figure 18.17 Stress-strain curves for chitosan bionanocomposites reinforced with pristine and modified

HNTs. Source: Arcudi et al., 2014; Copyright 2014. Reprinted with permission from the American

Chemical Society.


The presence of HNTs causes a slight deterioration in the mechanical performance of pectins (Cavallaro et al., 2011b). Very intriguing appears the performance at high HNTs loading (60 wt%), which shows an elastic modulus of pectins substantially increased.

The mechanical performance of bioplastics under controlled environmental con-ditions represents a task and the humidity could be a parameter to be seriously taken into account for natural polymers. Within this issue, although conditioning situ-ations are generally controlled, there are a few studies on mechanical tests under controlled relative humidity and temperature. These studies on chitosan and HNT-Chitosan composites showed that besides the comparable water absorption behav-ior, the presence of halloysite generates nanocomposites with a mechanical response more sensitive to humidity than the pristine polymer. This is in agreement with the humidity-induced plasticization effect produced by the nanotubes (Cavallaro et al., 2014a).

18.5 Applications of HNT/Polysaccharide Nanocomposites

HNT/polysaccharide bionanocomposites show excellent physicochemical properties due to the synergetic effect of HNT nanoparticles and biopolymers. The potential appli-cations of these materials are relevant in several fields (Cavallaro et al., 2015b; De Silva et al., 2015; Notta-Cuvier et al., 2015; Palantöken et al., 2015; Peng et al., 2015; Qi et al., 2010; Wang et al., 2013; Xue et al., 2015).

Bionanocomposite films based on chitosan reinforced with HNT were able to trap horseradish peroxidase (HRP). Besides the use as immobilization matrix, chitosan/HNT was proposed as binder to improve the adhesive features to an electrode sur-face. In particular, the obtained HNTs-Chitosan film could promote the direct electro-chemistry of HRP and catalyze the reduction of H

2O

2 (Sun et al., 2010). Therefore, this

hydrophilic HNTs-Chitosan bionanocomposite could give a new appealing platform for additional studies on redox behavior of proteins. The use of a direct electrochemis-try approach is crucial for the development of innovative biosensors.

Other work reports on HRP immobilized on biohybrid HNTs-Chitosan through cross-linking by glutaraldehyde that was used to remove chemicals from wastewater. It exhibited overall high removal efficiency and removal rate, which demonstrates that the HRP immobilized on HNTs-Chitosan systems could be promising in wastewater treatment (Zhai et al., 2013).

Good adsorbent to dye based on alginate beads reinforced with HNT, with excellent physical and chemical properties, were obtained by Liu et al. (2012a) This new system showed high removal efficiency of methylene blue (above 90%) even after 10 successive adsorption–desorption cycles.

Biopolymers are also usually used for controlled drug delivery. However, pristine biopolymers matrices suffer from some disadvantages such as the burst release of drugs and weak stability. Beside the improved mechanical properties, inorganic materials have been used to improve the swelling behavior, drug loading efficiency, and controlled release behavior of the pristine biopolymer matrices via electrostatic interactions and hydrogen bonding. Therefore, the combination of HNTs and biopolymer should be a feasible approach to prepare carriers for sustained drug release.


Fan et al. reported the preparation of HNTs-sodium alginate/hydroxyapatite nano-composite beads by generation of hydroxyapatite (HA) in a nanosized regime. To this aim, the sol–gel transition of the HNTs-sodium alginate dispersion was used to pro-duce the nanoparticles in situ. The beads were used for load and release of diclofenac sodium. The combination of HNTs with a tubular structure and HA nanoparticles could limit the flexibility of the alginate polymer chains, which is the principal reason for the enhanced loading of the active pharmaceutical component. Moreover, a sus-tained release behavior is achieved (Fan et al., 2013).

Magnetic microspheres, 2-hydroxypropyltrimethyl ammonium chloride chitosan/Fe

3O

4/halloysite nanotubes/ofloxacin (HACC/Fe

3O

4/HNTs/OFL), for the controlled

release of OFL were synthesized by in situ cross-linking with glutaraldehyde in the spray-drying process. The magnetic microsphere was employed as carrier for ofloxa-cin. It was found that HNTs have remarkable effect on the roughness of the surface, which decreases the entrapment efficiency and accelerates the release of OFL though an increase in HNTs content is conducive to the adsorption of OFL into HNTs. However, the introduction of HNTs can improve the bioavailability of OFL in the gastro-retentive drug delivery system (Wang et al., 2014).

Due to its features, among them hemostatic and anti-infectional activities, chitosan can also be used as wound healing. Chitosan can accelerate the infiltration of poly-morphonuclear cells at the early stage of wound healing, followed by the fabrication of collagen by fibroblasts. The addition of HNTs aids in faster re-epithelialization and collagen deposition. These properties are attributed to the particular characteristics of HNTs and their combination with chitosan. The obtained results show that these advanced HNTs-Chitosan bionanocomposite sponges have many potential applica-tions for diabetic, burn, and chronic wound infections (Liu et al., 2014b).

Controlled loading and sustained release of 5-aminosalycilic acid into HNT was proved (Aguzzi et al., 2013). The introduction of 5-aminosalycilic acid/HNT within the thermoplastic starch generates a biopolymer that can transport the drug through the acidic medium of the stomach and that can release it in sustained manner in the colon. The development of an HNT-Starch bionanocomposite thus should be considered as a basis for further development of colon specific drug delivery formulations using hal-loysite nanotubes (Schmitt et al., 2015a).

Thanks to the high performance and biocompatibility, HNTs-Chitosan as well as HNTs-Alginate bionanocomposites have also possible applications in bone tissue engi-neering. In order to verify cell attachment and viability on the bionanocomposites, sev-eral studies were carried out.

Cell morphology results exhibited that cells can be attached and grown well on the chitosan-HNTs. The introduction of HNTs caused changes in surface nanotopography of chitosan and involved an increasing of roughness of the bionanocomposite. A sur-face more rough and the existence of Si element in the HNTs-Chitosan bionanocom-posite surface are beneficial to attachment of the cells (Liu et al., 2012b; 2013).

Within the Cultural Heritage applications, polymers were successfully used as con-solidants for art work materials. For example, poly(ethylene) glycols (PEGs) (Cavallaro et al., 2013b), poly(propylene) glycols (PPGs) (Donato et al., 2010), and cellulose ethers (Strnadova and Durovic, 2009) are efficient for the impregnation of waterlogged woods. Among the cellulose ethers, hydroxypropylcellulose (HPC) (Strnadova and Durovic,


2009) is employed for the consolidation of ancient papers improving significantly their physicomechanical properties over time. When HNTs are used in combination with HPC, the HNT/HPC mixture enhanced the elasticity and the strength to elongation of paper (Table 18.3). Moreover, the peculiar hollow tubular shape might be strate-gic for future advances where active species can be stored in a protected nanocavity and released for paper self- healing or long-term protection purposes (Cavallaro et al., 2014b). HNT-Chitosan composites were successfully proposed for dry cleaning proce-dures of art works (Cavallaro et al., 2014a).

In the past few years, the possibility of using bionanocomposites to develop new bio-degradable materials for packaging applications is rapidly growing. The development of innovative packaging materials where different functionalities are included in a single film layer is of considerable interest. Gorrasi reported the synthesis and characteriza-tion of bionanocomposites based on pectins and nano-hybrids of HNTs loaded with rosemary essential oil, as an antimicrobial agent, for potential application in packaging field. The halloysite presence improved mechanical and barrier properties allowed a much slower release of rosmarinic acid compared to the molecules directly dispersed in the pectin and a preliminary evaluation of antimicrobial activity indicated the poten-tial application of the obtained composites in the active food packaging, opening new aspects for pectins-antimicrobials as coating agents for the application in food packag-ing field (Gorrasi, 2015). By means of casting method, the addition of HNT was suc-cessful in controlling the thermal and mechanical properties of chitosan and pectin matrices (Arcudi et al., 2014; Cavallaro et al., 2011a).

18.6 Conclusions

This chapter presents the state of the art of bionanocomposites generated from the combination of halloysite and biopolymers. Both them are wide available, biocompat-ible and low-cost materials and therefore, recently studies have shown that halloysite can be used as reinforcing nanofiller in several biopolymer matrices.

Different methods such as solution casting or melt processing were successfully used to prepare halloysite-based bionanocomposites. The physicochemical characterization

Table 18.3 Stress at breaking obtained from DMA.

Treatment sr/MPa s

r/%a

Paper 11.9

HNT 1% 10.3 –16

HNT 2% 11.0 –8

HNT 2.8% 10.6 –12

HPC 4 % 15.7 24

HPC/HNT (RHNT/HPC = 0.24) 16.2 27

HPC/HNT (RHNT/HPC = 0.56) 9.3 –28

aChange in strength with respect to paper.


of the resulting bionanocomposites showed that the introduction of halloysite into the biopolymer matrix generated compact films with enhanced mechanical and thermal properties compared to the pristine biopolymer.

The biocompatibility of halloysite-based bionanocomposites make them good can-didates in several applications; in particular, many efforts were devoted to apply them in biomedical applications like drug carrier and delivery systems.

However, up to now only a limited number of studies were focused on the large-scale application of halloysite for synthesizing biopolymer nanocomposites showing high physicochemical performances and multifunctional attitude. Nevertheless, being that the industry strategy search for low-cost sustainable developments of nanomateri-als, the preparation of HNTs-bionanocomposites reflects an emerging research area still offering many opportunities for new applications and commercial purposes.

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