nanoscale particles for polymer degradation

In very few systems, the nanoparticulates have been incorporated into polymer as ‘nano-additives’ for both purposes: degradation and stabilization of polymers.. Key words: Polymer, nano

Accepted Manuscript

Title: Nanoscale Particles for Polymer Degradation and

Stabilization – Trends and Future Perspectives

Authors: Annamalai Pratheep Kumar, Dilip Depan, Namrata

Singh Tomer, Raj Pal Singh

Please cite this article as: Kumar AP, Depan D, Tomer NS, Singh RP, Nanoscale Particles

for Polymer Degradation and Stabilization – Trends and Future Perspectives, Progress

in Polymer Science (2008), doi:10.1016/j.progpolymsci.2009.01.002

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Nanoscale Particles for Polymer Degradation and Stabilization – Trends

and Future Perspectives

Annamalai Pratheep Kumar1, Dilip Depan1, Namrata Singh Tomer2 and Raj Pal Singh1∗

1Polymer Science and Engineering Division

National Chemical Laboratory, Pune-411008, India

E-mail: rp.singh@ncl.res.in

Telephone: +91-20-25902091, Fax: +91-20-25902615

2Department of Chemical & Biomolecular Engineering

Clemson University, Clemson, SC 29634-0909, USA

Abstract

The field of nanoscience and nanotechnology is extending the applications of physics, chemistry, biology, engineering and technology into previously unapproached infinitesimal length scales The polymer – nanoparticles / nanocomposites have been the exponentially growing field of research for developing the materials in last few decades and have been mainly focusing on the structure-property relationships and their development Since the polymer-nanocomposites have been the staple of modern polymer industry, their durability under various environmental conditions and degradability after their service life are also essential fields of research Thus, this article

is intended to review the status of worldwide research in this aspect Among various nanoparticulates, clay minerals and carbon nanotubes are more often used in enhancing physical, mechanical and thermal properties of polymers In very few systems, the

nanoparticulates have been incorporated into polymer as ‘nano-additives’ for both

purposes: degradation and stabilization of polymers The degradation and durability of polymers is reviewed in the presence of nanoparticles / nanocomposites under different environmental conditions Nanoparticle -induced biodegradation of polymers is also discussed

Key words: Polymer, nanoparticles, clay, carbon nanotubes, metal oxides,

nanocomposites, degradation and stabilization

1.3 Growth and Significance

2 Preparation and processing of polymeric nanomaterials

2.1 Nanocomposites based on layers

2.2 Nanocomposites based on nanotubes

2.2.1 Carbon nanotubes

2.2.2 Cellulose whiskers

2.2.3 Inorganic nanotubes/nanofibers

2.3 Nanocomposites using nanoparticles

2.3.1 Synthesis of nanoparticles: Nucleation and growth

2.3.2 Preparation methods

2.4 Characterization of nanoparticulates and nanocomposites

3 Degradability and durability of Polymers: Overview

4 Degradation of polymeric nanomaterials

4.1 Photo-degradation and stabilization

4.1.1 Nanocomposites based on nanolayers

4.1.2 Nanocomposites based on nanotubes / nanofibers

4.1.3 Nanocomposites based on nanoparticles

4.2 Thermal degradation and stabilization

4.2.1 Nanocomposites based on nanolayers

4.2.3 Nanocomposites based on nanoparticles

4.3 Biodegradation and stabilization

4.3.1 Nanocomposites of biodegradable polymers

5.1 Road to new polymer-nanoparticulates systems

5.2 Importance for studying durability /degradability

5.3 Need of new stabilizing systems

6 Conclusions

Acknowledgement

Reference

Abbreviations

AAGR Average Annual Growth Rate

AGU Anhydroglucose Unit

ATH Aluminium trihydroxides

CMC Critical Micelle Concentration

CNT Carbon Nanotubes

CVD Chemical Vapour Deposition

DMA Dynamo Mechanical Analysis

DNA Deoxyribose Nucleic Acid

DP Degree of Polymerization

DSC Differential Scanning Calorimetry

EPDM Ethylene-Propylene Diene Monomer

EVA Ethylene Vinyl Acetate Copolymer

FTIR Fourier Transform Infra Red

GPC Gel-Permeation Chromatography

HAP Hydroxyapatite

LDH Layered double Hydroxides

LDPE Low Density Polyethylene

LLDPE Linear Low Density Polyethylene

MMT Montmorillonite

MWCNT Multi-walled Carbon Nanotubes

NMR Nuclear Magnetic Resonance

NSP Nano scale particles

OATP Organo Attapulgite

OMMT Organically Modified Montmorillonite

PBT Polybutylene Terphthalate

PLA Polylactic Acid

PMMA Polymethyl Methacrylate

PVD Physical Vapour Deposition

SAXS Small Angle X-Ray Scattering

SEM Scanning Electron Microscopy

SPHERE Simulated photodegradation of high-energy radiant exposure

SWCNT Single-walled Carbon Nanotubes

TEM Transmission Electron Microscopy

Tg Glass Transition Temperature

TGA Thermogravimetric Analysis

TMDS Tetramethyl disiloxane

USAXS Ultra Small X-Ray Scattering Spectroscopy

UV Ultraviolet

WAXD Wide Angle X-Ray Diffraction

XPS X-Ray Photo Electron Spectroscopy

XRD X-Ray Diffraction

1 Introduction

The field of nanoscience and nanotechnology which deals with materials and structures having dimensions that measure up to billionth of a meter (nanometer) is extending the applications of physics, chemistry, biology, engineering and technology into previously unapproached infinitesimal length scales Now, at nanoscale one enters a world where physics and chemistry meet and develop novel properties of matter In chemistry, this range of sizes has historically been associated with colloids, micelles, polymer molecules, phase-separated regions of block copolymers and similar structures More recently, structures such as buckytubes, silicon nanorods, and compound semiconductor quantum dots have emerged as particularly interesting classes of nanostructures In physics and electrical engineering, nanoscience is most often associated with quantum behavior, and the behavior of electrons and photons in nanoscale structures Biology and biochemistry also have a deep interest in nanostructures as components of the cell; many of the most interesting structures in biology - from DNA and viruses to subcellular organelles and gap junctions can be considered as nanostructures [1-2] Recently, Whitesides [3] discussed the reasons for the fascination and growth of this inter- / multi-disciplinary research According to Braun et al [4], from 1980s, the growth of research papers dealing with the prefix called ‘nano’ is exponential It is an earlier indication of explosive growth of research and fascination on nanoscience and nanotechnology Not only in academia, in industries also, the impact of this field is significantly increasing such as in ceramics, chemical polishing agents, scratch-resistant coatings, stain-resistant trousers, cosmetics, sunscreens etc Thus, synthesis of various nanoscale structures / particles has gained the interested for developing new nanomaterials and devices For example, the

clusters, nanoparticles, nanowires, long molecules as nanotubes and polynucleotides, and functional supramolecular nanostructures are currently considered as potential building blocks for nanotechnology and nanoelectronic devices and circuits

On the other hand, synthetic polymeric materials are rapidly replacing more traditional inorganic materials, such as metals and natural polymeric materials (wood, fibers) As these synthetic materials are flammable, they require modifications to decrease their flammability through the addition of flame-retardant compounds Environmental regulations have restricted the use of some halogenated flame-retardant additives, initiated a search for alternative flame-retardant additives For this purpose, inorganic nanoparticles have become attractive since they can simultaneously improve both the physical, mechanical and flammability properties Thus, polymer nanocomposites, in last few decades, have become worldwide research interest for developing polymeric materials with improved / desired properties by incorporation of these nanoscale materials into polymer matrix Numerous research papers, patents and funding are generated out of this field Most of the efforts were mainly focused on the structure-property relationships and their development However, the usefulness of any materials depends on their durability in a particular environment in which they are used or their interaction with environmental factors [5] Since the polymer-nanocomposites have been the staple of modern polymer industry, their durability under various environmental conditions and degradability after their service life are also essential parts of research The clay mineral incorporated polymer nanocomposites have gained the fabulous attraction from the researchers Recently, the durability of polymer nanocomposites based

on layered silicates (clay minerals) under different environments (mainly under thermal

and photo-ageing) has been reviewed [6] Thus, this article is intended to review the status of worldwide research in the aspect that the nanoparticulates can be incorporated

into polymer as ‘nano-additives’ for both the purposes i.e degradation and stabilization

of polymers

1.1 Definitions

The terminologies very often applied in nanoscience and nanotechnologies are listed as follows;

Nanoparticles: Although not specifically describing nanoparticles, the above-mentioned

definitions imply a nanoparticle definition of particle less than 100 nm Those particles having (one or more) dimensions of 100 nm or less and physical and chemical properties should also be to differ measurably than those of the bulk material can be called as ‘nanoparticules’ [7, 8]

Nanocomposites: The composite materials, that combine one or more separate

components in order to improve performance properties, for which at least one dimension of the dispersed particles is in the nanometer range

Nanomaterials The development and use of nanoscale materials such as nanoparticles,

nanocomposites, nanopowder, nanocrystals etc

Nanoscience Nanoscience is the study of phenomena and manipulation of materials at

atomic, molecular and macromolecular scales, where properties differ significantly from those at larger scale

Nanotechnology Nanotechnology is the understanding and control of matter at

dimensions of roughly 1 to 100 nanometers, where unique phenomena enable novel applications Encompassing nanoscale science,

engineering and technology, nanotechnology involves imaging, measuring, modeling, and manipulating matter at this length scale Nanotechnologies are the design, characterization, production and application of structures, devices and systems by controlling shape and size at nanometer scale

Subdivisions of Nanoscience and Nanotechnology

Nanobiotechnology The design, synthesis or application of materials or devices or

technologies in the nanometer scale for basic understanding and / or treatment of disease

Nanomedicine Application of nanotechnology for treatment, diagnosis, monitoring,

and control of biological systems, which are needed at molecular level, has recently been referredto as "nanomedicine"

Nanophotonics: A novel optical nanotechnology, utilizing local electromagnetic

interactions between a few nanometric elements and an optical near

field This field deals with optical processes at the nanoscale, much smaller than the wavelength of optical radiation

Nanoelectronics: The development and use of electronics on a nanometer scale, whether

made by current techniques or nanotechnology which includes both molecular electronics and nanoscale devices resembling today's semiconductor devices

1.2 Various classifications of polymeric nanomaterials

1.2.1 Nanoparticles

As shown in Figure 1, nanoscale particles are classified into three categories depending

on their dimensions as follows [9-10];

a) Nanoparticles: When the three dimensions of particulates are in the order of nanometres, they are referred as equi-axed (isodimensional) nanoparticles or nanogranules or nanocrystals Example silica,

b) Nanotubes: When two dimensions are in the nanometer scale and the third is larger, forming an elongated structure, they are generally referred as ‘nanotubes’

or nanofibers / whiskers / nanorods E.g Carbon nanotubes (CNTs), cellulose whiskers

c) Nanolayers: The particulates which are characterized by only one dimension in nanometre scale are nanolayers / nanosheets These particulate is present in the form of sheets of one to a few nanometre thick to hundreds to thousands nanometres long Clay (layered silicates), layered double hydroxides (LDH)

These nanoscale particles (NSPs) can further be distinguished in three types as natural, incidental, and engineered nanoparticles depending on their pathway Natural nanoparticles, which are formed through natural processes, occur in the environment

nanoparticles occur as the result of manmade industrial processes (diesel exhaust, coal combustion, welding fumes, etc.) Sometimes, they are also called as waste or anthropogenic particles Mostly, both natural and incidental nanoparticles may have irregular or regular shapes Engineered nanoparticles most often have regular shapes, such as tubes, spheres, rings, etc Engineered nanomaterials can be produced either by milling or lithographic etching of a large sample to obtained nanosized particles (“top-down” approach), or by assembling smaller subunits through crystal growth or chemical synthesis to grow nanoparticles of the desired size and configuration (“bottom-up” approach)

Depending on practical applications, nanoscale particles regardless of engineered or natural ones, so far seem to fall into four basic categories [11] The group currently with the largest number of commercial nanomaterials is the metal oxides, such as zinc or titanium oxides, which are used in ceramics, chemical polishing agents, scratch-resistant coatings, cosmetics, and sunscreens A second significant group is nanoclays, naturally occurring plate-like clay particles that strengthen or harden the materials or make them flame-retardant A third group is nanotubes, which are used in coatings to dissipate or minimize static electricity (e.g., in fuel lines, in hard disk handling trays, or in automobile bodies to be painted electrostatically) The last group is quantum dots, used in exploratory medicine or in the self-assembly of nanoelectronic structures As it is not easy for every official source to find the same categorization useful, the U.S Environmental Protection Agency also divides engineered nanoparticles into four types They are carbon-based materials (nanotubes, fullerenes), metal-based materials

(including both metal oxides and quantum dots), dendrimers (nano-sized polymers built from branched units of unspecified chemistry), and composites (including nanoclays)

1.2.2 Nanocomposites

The interest of scientists in applying nano-scaled fillers into polymer matrices is the attainment of potentially unique properties, as a result of nanometric dimensions In principle, any kind of material can be produced to appear in a nano-scaled shape and size, but in last few decades, none of these particles has gained as much attention as clay (esp layered silicates) and carbon nanotubes This is because of the fact that they can simultaneously improve both physical, mechanical properties and flammability, unlike conventional fillers, these nanosized particles have exhibited tremendous improvement in properties with very low amount of loading (upto 10 wt %) The question can be arisen

‘how do they differ from traditional composites and improve the properties drastically?’ The distinguishing characteristics between conventional fillers and nanoscale particles have been explained by Vaia et al [11] as shown in Figure 2

Over the conventional micro-composites, the nanoscopic dimensions and inherent extreme aspect ratios of the nanofillers result in six interrelated characteristics, which distinguish the obtained nanocomposites They are 1) Low percolation threshold (~0.1-2 vol%), 2) Particle-particle correlation (orientation and position) arising at low volume fractions (φC < 0.001), 3) Large number density of particles per particle volume (106-108particles/µm3), 4) Extensive interfacial area per volume of particles (103-104 m2/ml), 5) Short distances between particles (10-50 nm at φ ~1-8 vol%); and 6) Comparable size scales among the rigid nanoparticles inclusion, distance between particles, and the

relaxation volume of polymer chains The first two characteristics are not commonly observed for spherical nanoparticles because of the small / low aspect ratio Still, the composites based these particles are considered to be a critical bridge between the composites made from conventional micron-scale fillers and high-aspect ratio nanoparticles It should also be noted that the first two characteristics can manifest in spherical nanoparticles systems also, vis-à-vis innovative processing or proto-assembly of these nanoparticles However, the key concept of nanocomposites is not specifically embodied within the shape of the particle, but with the characteristics of the nanoparticle

to engineer and tailor morphology to achieve a desired property suite from the resultant composite materials To convey the origin and interrelation of these distinguishing characteristics, Figure 2a compares the dominant morphological scale of a microfiber (1

µm x 25 µm) filled polymer matrix to that of a nanofiber (1 nm x 25 nm) filled system at the same volume fraction of filler There are three main material constituents in any composite: the matrix, the reinforcement (fiber), and the so-called interfacial region The interfacial region is responsible for ‘communication’ between the matrix and filler and is conventionally ascribed properties different from the bulk matrix because of its proximity

to the surface of the filler It is explained in terms of the radius of gyration of the matrix (Rg), which is key spatial parameter to which the majority of the polymer’s static and dynamic properties can be ultimately related and has a value in a few tens of nanometers

As shown in Figure 2a, contrast to conventional fillers, in the nanoparticles filled system, the distance between particles comparable to the size of the interfacial region (10 nm) because of the increased number density of particles Thus, the relative volume fraction

of interfacial material to bulk is drastically increased as the size becomes smaller Figure

2b, summarizes dependence of interfacial area per volume fraction (µm-1 = m2/ml) of filler on the aspect ratio (α) of nano-scale particles with varying shapes The aspect ratio (α = H/R) is based on approximating particles as cylinders (area/volume = 1/H + 1/R) and is shown in logarithmic scale Aspect ratios greater than one correspond to rods (length/diameter) and less than one to plates (height/diameter) [11] It is worth to mention, for example, that fully exfoliated and dispersed high aspect ratio plates or rods, such as montmorillonite or SWNTs, generate internal interfacial area comparable to that

of macromolecular structures, such as dendrimers or proteins, and two to three orders of magnitude more than classic mineral fillers

1.3 Growth and Significance

Recently, a big window of opportunities has opened for polymer nanocomposites just to overcome the limitations of traditional micro-composites Although, the chemistry of clay minerals and composites based on some nano-scale particles are known for a several decades, the research and development of nanoscale-filled polymers has been skyrocketed in recent years, for numerous reasons First, unprecedented combinations of properties have been observed in some polymer nanocomposites For example, incorporation of isodimensional nanoparticles, into thermoplastics, increases the modulus, the yield stress and the ultimate tensile strength [12] A volume fraction of 0.04 layered silicates (MTS) in epoxy polymer improves the modulus below glass transition region temperature by 58% and the modulus in the rubbery region by 450% [13] The permeability was shown to decrease by an order of magnitude with 4.8% of layered silicates in poly-ε-caprolacotne [14] The discovery of carbon nanotubes is a second

reason Though, the carbon nanotubes has the history sicne1960s [15], in mid 1990s only,

it was shown by Ijima [16] that the carbon nanotubes can be made in the quantities required for the reinforcement of polymer and their evaluation The properties of these carbon nanotubes, particularly strength and electrical properties are significantly different from those of carbon black or graphite and lead to new class of composite materials Third, the significant development in preparing and processing of nanoparticles and nanocomposites (esp In-situ methods) has directed to very good control over the interface between the inter-phases (which is also difficult in conventional composites), morphology and ultimately the properties of such composites [17]

In application point of view, the nanocomposites have been studied for their improved flammability It is well known that commercially available, traditional flame-retardants such as aluminium trihydrate or halogen-containing compounds are highly effective However, the mechanical properties and processing of flame retardant polymers are often negatively influenced by the large quantities of halogen-free flame-retardants that are needed Conversely halogen-containing flame-retardants are under pressure in many countries because of environmental considerations The addition of many flame-retardants often increases the production of soot and carbon monoxide during combustion Nanocomposites have many advantages over these traditional flame-retardants Processing of nanocomposites is straightforward and as the nanocomposites contain no additional halogen, they are considered to be an environmentally friendly alternative In nanoscale particle filled polymer systems, the char formation, which insulates the base polymer from heat and forms a barrier, reduce the escape of volatile

gases from the polymer combustion, is explained to be responsible for improved flame retardancy Current estimates for launching payloads into space orbit stand at $10,000/lb ($22,000/kg) [18] Significant weight, and hence cost, reductions can be realized with the use of organic materials, but such materials tend to perform very poorly in the harsh space environment Organic polymers with uniformly dispersed nano-scale inorganic precursors may enable these materials to withstand the harsh space environment and be used as critical weight-reduction materials on current and future space systems Nanocomposites, as these types of materials are often referred to, have received much attention over the past decade as scientists search for ways to enhance the properties of engineering polymers while retaining their processing ease Unlike traditional filled polymer systems, nanocomposites require relatively low dispersant loadings (∼2 wt%) to achieve significant property enhancements Some of these enhancements include increased modulus, increased gas barrier, increased thermal performance, increased atomic oxygen resistance, resistance to small molecule permeation and improved ablative performance As a result of these enhancements, nanocomposites have the potential to play a significant role in future space systems Launch vehicles would greatly benefit from appropriately designed nanocomposites that could provide improved barrier properties and gradient morphologies enabling linerless composite cryogenic fuel tanks Self-rigidizing, self-passiviating nanocomposite materials could be used to construct space vehicle components that are both highly resistant to space-borne particles and resistant to degradation from electromagnetic radiation, while reducing the overall weight

of the spacecraft Nanocomposite materials also offer the unique opportunity for improved tailorability of physical and structural properties such as the coefficient of

thermal expansion (CTE), which would be especially useful in constructing large aperture telescopes and antennas using inflatable membranes [19] In an earlier estimation, Business Communications Company, Inc [20] reported the worldwide growth of polymer nanocomposites as summarized in Table 1 It can be noted that total worldwide market for polymer nanocomposites reached 11.1 million kg valued at $90.8 million in

2003 and projected to grow at an average annual growth rate (AAGR) of 18.4% to reach

$211.1 million by 2008

2 Preparation and processing of various nanocomposites

The incorporation of nanoscale particles into polymer can generally be done in four ways

as follows;

a) Solution method: It involves dissolution of polymers in adequate solvent with nanoscale particles and evaporation of solvent or precipitation

b) Melt mixing: In this method, the polymer is directly melt-mixed with nanoparticle

c) In-situ polymerization: In this method, the nanoparticles are first dispersed in

liquid monomer or monomer solution Polymerization is performed in presence of nanoscale particles

d) Template synthesis: This method is completely different from other methods In this method, using polymers as template, the nanoscale particles are synthesized from precursor solution

2.1 Nanocomposites based on layers

This class of nanocomposites can be congregated under the name of polymer-layered crystal nanocomposites, which are almost exclusively obtained by the intercalation of the polymer (or a monomer subsequently polymerized) inside the galleries of layered host crystals There is a vast array of both natural and synthetic layered crystalline fillers that are able, under specific conditions, to intercalate a polymer [21] Table 2 gives a glimpse

of possible layered nanoparticles, which are potential candidates for preparing polymer nanocomposites Among these layered nanoparticles, clay minerals based on phyllosilicates have extensively been for last few decades, most probably because the starting clay materials are easily available and because their intercalation chemistry has been studied for a long time In the family of phyllosilicates, 2:1 type layered silicates are most popular among industries and academia

Natural montomorillonite (MMT) is smectite (2:1) type clay minerals whose one layer contains one central octahedral sheet (of either aluminium hydroxide or magnesium hydroxide) condensed to two parallel tetrahedral sheets (of silica), via silica oxygen apices to octahedral hydroxyl planes These layers are attracted by van der Waals forces The negative charges that arise due to isomorphic substitution of cations (Al3+) on the surface of layers are compensated by hydrated cations (e.g Na+, K+, Ca2+ and Mg2+) These natural clay minerals are hydrophilic It is well established that these cations can

be replaced by organic cations such as alkyl-ammonium ions to render the organophilicity for lowering the surface energy of the silicate layers and improving the wetting with polymer chains [22-23] These organophilic montmorillonite (OMMT) clays whose surface energy is lowered and is more compatible with organic polymers, polymer

molecules may be able to intercalate within the galleries, under well-defined experimental conditions When these layered silicates are associated with a polymer, depending on the nature of the components used (layered silicate, organic cation and polymer matrix) and the method of preparation, different structure / morphologies of composites can be obtained [24-25]

a) Phase-separated composites: If the polymer is unable to intercalate between the

silicate sheets, the obtained composite can be considered as phase-separated composite Their properties stay in the same range as traditional microcomposites

Beyond this classical family of composites, three types of nanocomposites can be recovered (Figure 3)

b) Intercalated nanocomposites: When a single and / or more than one extended polymer

chain is inserted in a crystallographically regular fashion between the silicate layers resulting in a well-ordered multilayer morphology built up with alternating polymeric and inorganic layers, the obtained composites can be called as ‘intercalated nanocomposites’

c) Flocculated nanocomposites: conceptually this is same as intercalated nanocomposites

However, silicate layers are some times flocculated due to hydroxylated edge–edge interaction of the silicate layers

d) Exfoliated nanocomposites: When the individual silicate layers are completely and

uniformly dispersed in a continuous polymer matrix, an exfoliated or delaminated structure is obtained Usually, the clay content of an exfoliated nanocomposite is much lower than that of an intercalated nanocomposite

By following any of the above-mentioned ways, the resulting nanocomposite can exhibit different morphologies [26-27]

a) In solution-adsorption, it is well known that such layered silicates, owing to the weak

forces that stack the layers together can be easily dispersed in an adequate solvent Then, the polymer adsorbs onto the delaminated sheets Depending of the interactions between polymer and clay surface, the resultant structure may be either intercalated or exfoliated During solvent evaporation (or the mixture precipitated), if the sheets do not reassemble

or the polymer chains do not allow them to come closer, the resultant composites will be exfoliated nanocomposites In general, after solvent evaporation, the clay layers reassemble and form an ordered multilayer structure In the inter-gallery space, the adsorbed polymer chains do not coil but remain in almost fully extended state Thus, after solvent evaporation, generally an intercalated nanocomposites may form

b) In in-situ intercalative polymerization method, the layered silicate is swollen within

the liquid monomer (or a monomer solution) so as the polymer formation can occur in between the intercalated sheets Polymerization can be initiated either by heat or radiation, by the diffusion of a suitable initiator or by an organic initiator or catalyst fixed through cationic exchange inside the interlayer before the swelling step by the monomer

c) In melt intercalation method, the polymer is directly melt-mixed with the layered

silicate Under these conditions and if the layer surfaces are sufficiently compatible with the chosen polymer, the polymer can crawl into the interlayer space and form either an intercalated or an exfoliated nanocomposite In this technique, no solvent is required which is advantageous In order to optimize the interaction, the process may require trial

and error based experiments with different compatibilisers The experimental conditions should be established in order to abolish the coherence of the clay layers In this process usually the temperature should not increase beyond the decomposition temperature of the clay modifier

d) Template synthesis [28-29]: The clay minerals might also be synthesized inside the

polymer matrix The typical method involves preparing slurry of precursor clay gels (typically, 0.32 R-monovalent organic salt, 1.0 LiF, 5.3 Mg(OH)2, 8SiO2, nH2O) with polymer and refluxing for 2-3 days, and washing and drying This method is also called

as in-situ hydrothermal crystallization

2.2 Nanocomposites using nanotubes

In many systems, the chemical nature of the filler is often less important than the particle size and shape, the surface morphology and the extent of distribution within the polymer matrix The next class of nanocomposites is based on cylindrical/tubular nanoscale particles

2.2.1 Carbon nanotubes

Carbon nanotubes (CNTs) are tubular derivatives of fullerenes They were first observed

in arc-discharge methods and exhibit properties, which are quite different from those of the closed cage fullerenes such as C60, C70, C76, etc Carbon nanotubes can be visualized as a sheet of graphite that has been rolled into a tube Unlike diamond, where a 3-D diamond cubic crystal structure is formed with each carbon atom having four nearest neighbors arranged in a tetrahedron, graphite is formed as a 2-D sheet of carbon atoms arranged in a hexagonal array In this case, each carbon is covalently bonded to three

neighboring carbon atoms through sp2 hybridization in such a way to form a seamless shell ‘Rolling’ sheets of graphite into cylinders forms carbon nanotubes [17, 30-32] The properties of nanotubes depend on atomic arrangement (how the sheets of graphite are

‘rolled’), the diameter and length of the tubes, and the morphology, or nano structure This rolling can be from one (single-wall carbon nanotubes, SWCNTs) or more (multi-wall carbon nanotubes, MWCNTs) cylindrical shells of graphitic sheets Thus, nanotubes exist as either single-walled or multi-walled structures, and multi-walled carbon nanotubes (MWCNTs) are simply composed of concentric single-walled carbon nanotubes (SWCNTs) The special topologies are responsible for the unique and interesting properties of carbon nanotubes Due to their high mechanical strength, capillary properties and remarkable electronic structures, a wide range of potential uses has been reported within the field of material science research Various technological applications of the CNTs are for developing robust and lightweight composites, conducting paints [33], fabrication of flat panel displays, supports for metals in the field

of heterogeneous catalysis [34], Li+ batteries, gas (hydrogen) storage devices [35], toxic gas sensors, electronic nanodevices, and for immobilization of proteins and enzymes [36] Many material science researchers are working on development of methods for the production of CNTs in large-scale to realize their speculated applications Primary synthesis methods for single and multi-walled carbon nanotubes include arc-discharge [30, 37], laser ablation [38] gas-phase catalytic growth from carbon monoxide [39], and chemical vapor deposition (CVD) from hydrocarbons [40-41] methods However, to realize speculated applications of carbon nanotubes (esp polymer nanocomposites), the production of CNTs in large quantities is required and the scale-up limitations of the arc

discharge and laser ablation techniques would make the cost of nanotube-based composites prohibitive During nanotube synthesis, impurities in the form of catalyst particles, amorphous carbon, and non-tubular fullerenes are also produced Thus, subsequent purification steps are required to separate the tubes The gas-phase processes tend to produce nanotubes with fewer impurities and are more amenable to large-scale processing The electric-arc discharge technique generally involves the use of two high-purity graphite rods as the anode and cathode [42] The rods are brought together under a helium atmosphere and a voltage is applied until a stable arc is achieved The exact process variables depend on the size of the graphite rods As the anode is consumed, a constant gap between the anode and cathode is maintained by adjusting the position of the anode The material then deposits on the cathode to form a build-up consisting of an outside shell of fused material and a softer fibrous core containing nanotubes and other carbon particles To achieve single walled nanotubes, the electrodes are doped with a small amount of metallic catalyst particles In Laser ablation technique [43], a laser is used to vaporize a graphite target held in a controlled atmosphere oven at temperatures near 1200 °C To produce single-walled nanotubes, the graphite target was doped with cobalt and nickel catalyst The condensed material is then collected on a water-cooled target

In chemical vapor deposition (CVD) technique, nanotubes are formed by the decomposition of a carbon-containing gas The gas-phase techniques are amenable to continuous processes since the carbon source is continually replaced by flowing gas In addition, the final purity of the as-produced nanotubes can be quite high, minimizing

subsequent purification steps One unique aspect of CVD techniques is its ability to synthesize aligned arrays of carbon nanotubes with controlled diameter and length [44] The single walled nanotubes produced by laser ablation and arc-discharge techniques have a greater tendency to form ‘ropes’ or aligned bundles [45] Multi-walled nanotubes are composed of a number of concentric single walled nanotubes held together with relatively weak van der Waals forces [46] The chirality of the carbon nanotube has significant implications on the material properties In particular, tube chirality is known

to have a strong impact on the electronic properties of carbon nanotubes Graphite is considered to be a semi-metal, but it has been shown that nanotubes can be either metallic

or semiconducting, depending on tube chirality [47] It opens up a window for producing nanocomposite materials with improved mechanical properties, flammability with desired electrical properties However, significant key factors for their commercialization remain

to be solved about purification, dispersion and bulk processing Any method out of three

methods a) direct melt mixing, b) solution mixing and c) in-situ polymerization can

generally be adopted to incorporate the carbon nanotubes In the processing of these nanocomposites, uniform dispersion and improved interfacial adhesion between nanotubes and polymer matrix are critical issues for controlling any properties Dispersion is primarily achieved by sonication of nanotubes in a solvent Chemical modifications such as surface modification with the aid of surfactants, functionalization

of end-caps and functionalization of sidewalls have also resulted in stable suspensions of nanotubes However, none of the methods is ideal for composite processing In order to optimize the dispersion, the process may require trial and error based experiments with different modifications

2.2.2 Cellulose whiskers

Another type of tube like nanoscale particles are based on biopolymer, cellulose, which is

a linear condensation polymer, consists of β-1-4-linked D-anhydroglucopyranose units (AGU) It is most abundant polymer on the planet earth, in a various living species from the worlds of plants, animals, and bacteria as well as some amoebas Depending on the sources, degree of polymerization (DP) ranges from 2500 to 15000 As it can be seen in Figure 4, the inter- and intra-molecular hydrogen bonds result the linearity, the stiffness, the rigidity, strength and ultimately to form thread like material called micro-fibrils that are apparently bound to form natural fibers [48] They are often referred to as cellulosic fibers, related to the main chemical component cellulose, or as lignocellulosic fibers, since the fibers usually contain a natural polyphenolic polymer, lignin also, in their structure [49] Potential applications of agro-fiber based composites in railways, aircraft, irrigation systems, furniture industries, and sports and leisure items are currently being researched [50-53] In composite systems, if the adhesion level between the filler and the matrix is not good enough, a diffusion pathway can preexist or can be created under mechanical solicitation The existence of such a pathway is also related to the filler connection and therefore to its percolation threshold The percolation threshold can be evaded by nano-sizing the fibers Earlier theoretical calculations on mechanical properties of fibers demonstrated that the young modulus of molecular chains of cellulose

is about 250 GPa (Figure 4) [54-55] When these chains are bound to form nanosized single fibril, their young modulus reduced to 200-150 Gpa [56-58], which is almost equal

to that of composite SWNT [59] This reduction is attributed to drop in crystallinity when

crystalline fibers are twined together and for instance, microfibers (15µm) have shown young modulus about 25 GPa with 48 % crystallinity Conventionally used natural fibers for reinforcement are second level of elastic modulus By defibrillation, the third and fourth level of modulus can be exploited Earlier, several authors [48, 60-62] have demonstrated the defibrillation (transverse cleavage) of microfibrils by hydrolysis After defibrillation, they are mostly produced as stiff rodlike particles called whiskers or nanofibers (Figure 5) Generally, their geometrical characteristics depend on the origin of cellulose microfibrils and acid hydrolysis process conditions such as time, temperature, and purity of materials The modulus of naturally occurring cellulose whiskers has been reported, where a value of 143 GPa was obtained [56], close to the experimentally derived value of the crystal modulus of 138 GPa [57] Eichhorn et al [58] have found the young modulus values to be ~114 GPa for a single filament of bacterial cellulose obtained after acid hydrolysis Recently, Dufresne et.al [51] reviewed the development and applications of nanocomposite materials using cellulose nanofibers

2.2.3 Inorganic nanotubes/nanofibers

The next tube-like nanoparticles are based on inorganic nanotubes Since they are mostly like filaments i.e they are discussed in the next section Carbon nanotubes produced from organometallic precursors can be used to further prepare gallium nitride nanowires, silicon nitride nanotubes, and boron nitride nanotubes [63-68]

2.3 Nanocomposites using nanoparticles

The nanoparticles with various shapes fall in this category The majority of routes for preparation of nanoparticulates have taken care to avoid what might be termed “runaway” into the macroscopic world The particle growth is further inhibited by limiting the supply of the constituents forming the material either by working at higher dilution or by controlling their delivery by a chemical decomposition

2.3.1 Synthesis of nanoparticles

The chemical growth of bulk or nanosized particles involves the precipitation of solid phase from solution In a typical system undergoing a crystallization process nuclei or small crystallites are generally held to have unfavorable surface energy compensated by lattice energy of the solid In supersaturated solution, which is not stable enough in energy, homogeneous nucleation occurs by the combination of solute molecules to produce nuclei in the absence of a solid interface, due to the force driven by its thermodynamics [68-69] The overall free energy change, ∆G, is the sum of the free energy due to the formation of a new volume and the free energy due to the new surface created (Figure 6) For spherical particles [70]

γ πr (S) T k π V

∆G=−4 3 B ln +4 2

Where V is the molecular volume of the precipitated species, r is the radius of the nuclei,

kB is the Boltzmann constant, S is the saturation ratio, and γ is the surface free energy per unit surface area

For nucleation, the activation energy is at a critical size (r*) where the free energy ∆G

further decrease their free energy for growth and form stable nuclei that grow to form particles Thus, the critical nuclei size (r*) can be

)ln(

3

2

S T k

V r

be obtained at this stage by either stopping the reaction (nucleation and growth) quickly

or by supplying reactant source to keep a saturated condition during the course of the reaction Nanoparticles are thermodynamically unstable for crystal growth kinetically To finally produce stable nanoparticles, these nanoparticles must be arrested during the reaction either by adding surface protecting reagents, such as organic ligands or inorganic capping materials, or by placing them in an inert environment such as an inorganic matrix

or polymers [67, 71] In general, for preparing monodisperse nanostructures, the process requires a single, temporally short nucleation event followed by slower growth on the existing nuclei [69] The majority of routes for preparation of nanoparticulates have taken care to avoid what might be termed “runaway” into the macroscopic world They are as

follows; 1 Sol Process: This process is a classical method of preparing nanoparticles In

a typical system, nanoparticles formed as colloidal particles from a continuous phase of

and the particle growth is based on controlled release of the anions and cations in homogeneous solutions The control over kinetics of the nucleation and growth can

result in monodisperse nanoparticles 3 Sol-Gel Process: The sol-gel method is based on

inorganic polymerization reactions The sol-gel process includes four steps: hydrolysis, polycondensation, drying, and thermal decomposition The precursors of the metal or nonmetal alkoxides are hydrolyzed using water or alcohols according to the hydrolysis

process 4 Hydrothermal / Solvothermal Synthesis: Hydrothermal synthesis is a common

method to synthesize zeolite / molecular sieve crystals This method exploits the solubility of almost all inorganic substances in water / solvent at elevated temperatures and pressures and subsequent crystallization of the dissolved material from the fluid The properties of the reactants, including their solubility and reactivity, also change at high temperatures These changes provide more parameters to produce different high-quality

nanoparticles and nanotubes, which are not possible at low temperatures 5 Reaction in confined space: It involves reactions in confined solids, or constrained on the surface or

micelles The micelles exist only as a small amount of solubilized hydrophobic or hydrophilic material The droplet size can increased to a dimension that is much larger than the monolayer thickness of the surfactant As the surfactant concentration increases further, micelles can be deformed and can change into different shapes as illustrated in

Figure 7 [72], which makes it possible to synthesize different nanoparticle shapes 6 Pyrolysis: Pyrolysis is a chemical process in which chemical precursors decompose

under suitable thermal treatment into one solid compound and unwanted waste evaporates away To get a uniform nanosized material, some modifications of the pyrolytic preparation procedure and reaction conditions such as slow reaction rate or

decomposition of the precursor in the inert solvent are needed 7 Vapor Deposition: The

vaporized precursors are introduced into a reactor and adsorbed onto a substance held at

an elevated temperature These adsorbed molecules will either thermally decompose or react with other gases / vapours to form crystals

2.4 Characterization of nanoparticulates and nanocomposites

Generally, the crystallographic interactions of nanoparticles have typically been established using X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) Due to its easiness and availability XRD is most commonly used to probe the nanocomposite structure [3, 7–9] and occasionally to study the kinetics of the polymer melt intercalation [36] By monitoring the position, shape, and intensity of the basal reflections from the distributed nanoparticulates (e.g silicate layers) the nanocomposite structure (intercalated or exfoliated) may be identified For example, in an exfoliated nanocomposite, the extensive layer separation associated with the delamination of the original silicate layers in the polymer matrix results in the eventual disappearance of any coherent X-ray diffraction, whereas, for intercalated nanocomposites, the finite layer expansion associated with the polymer intercalation results in the shifting of basal reflection corresponding to the larger gallery height TEM allows a qualitative understanding of the internal structure, spatial distribution and dispersion of the nanoparticles within the polymer matrix, and views of the defect structure through direct visualization [37] However, special care must be exercised to guarantee a representative cross section of the sample Both TEM and XRD are essential tools for evaluating nanocomposite structure However, recent simultaneous small angle X-ray scattering

(SAXS) and XRD studies yielded quantitative characterization of nanostructure and crystallite structure in some nanocomposites [38, 39]

3 Degradability and durability of Polymers: Overview

Degradation is an irreversible change, resembling the phenomenon of metal corrosion Chemical degradation of polymers is a very important phenomenon, which affects the performance of all plastic materials in daily life In practice, any change of the polymer properties relative to the initial or desirable properties is called “degradation” In this sense, degradation is a generic term for any number of reactions that are possible in a polymer [73] The degradation of polymers involves several physical and /or chemical processes accompanied by small structural changes which lead nevertheless to significant deterioration of the quality of the polymeric materials (i.e., worsening of its mechanical, electrical or aesthetic properties) and finally to the loosening of its functionality [73-74] Table 3 gives list of environmental factors, which causes the polymer degradation [75-83] Figure 8 shows the generally accepted pathways of degradation and stabilization where radical formation is initiating and vital step for polymer degradation [84-85] This

is based on an original scheme for autocatalytic oxidation of hydrocarbons The oxidation

of polymers begins during processing (mechano-oxidation), and the hydroperoxide formation during fabrication affects further the rate of thermal / photo-oxidation during subsequent use (aging and weathering) Knowledge of the degradation mechanism of polymer has led to development of more efficient stabilizers [86-87] for improving the product performance on one hand and development of sensitizers [87-88] to produce degradable plastics [88-90] to preserve the environment, on the other hand Thus,

degradation of polymers is like a double-edged sword: it has harmful aspects as well as beneficial aspects If unchecked it can play havoc with a polymers performance, if uncontrolled it can lead to safety hazards of fire and toxicity, but if properly harnessed it can be used for producing new and better materials [90-95]

For various applications, the service life of polymeric materials can be tuned by incorporating additives externally In order to induce and promote oxidation of polymers, the sensitizers, which transfer the energy to polymer or decompose itself to give singlet oxygen (1O2) molecule which can initiate the oxidation, are added Certain organic and inorganic compounds are excited in presence of light, heat, high-energy radiations, and transfer energy to polymers or free radicals [87-96] (Table 4.)

On the other hand, for the purpose of increasing durability of polymeric materials by protecting from environmental factors or by reducing the rate of degradation process, the substances called ‘stabilizers’ can be incorporated into polymer matrix Table 5 gives few examples of the conventional stabilizers [73, 97-105]

There have been always some disadvantages in using single additive system such as compatibility, migration with low molecular weight stabilizers (esp HAS), immobility with high molecular weight stabilizers, yellowing with phenolic antioxidants, reduced efficiency of organo-phosphites The combined effect of screeners, quenchers, UV absorbers and antioxidants, which are synergistic towards one another, can provide effective protection against degradation [84] To overcome the difficulties of evaporation and migration, the higher molecular weight or polymeric stabilizers can be introduced The polymeric stabilizers can be prepared by following three ways [73];

i) Grafting of stabilizer onto polymer

ii) Synthesis of polymerizable monomers anchored with stabilizer and

homo/co-polymerization

iii) By using photo-rearranging polymers as additives

These sensitizers or stabilizers are generally added during preparation and processing of polymeric materials as it is done mostly for preparing nanocomposites As we mentioned earlier, the nanoparticles are incorporated for their own primary functions The degradability and durability of polymers are discussed as primary and /or additional functions in the next section Figure 9, represents the various techniques available for following/ monitoring the degradation as well as stabilization

4 Degradation of polymer-nanoparticulate / nanocomposites systems

4.1.Photo-degradation and stabilization

The usefulness of these materials depends on their durability in a particular environment

in which they are used or their interaction with environmental factors [75, 76] Thus, the study of degradation and stabilization of polymer nanocomposites is an extremely important area from the scientific and industrial point of view and a better understanding

of degradation mechanism of these materials will ensure the long service life of the product Not enough attention has been given to the study of durability of polymer nanocomposites Since polymer - clay hybrids are now expanding to numerous outdoor applications; their exposure to ultraviolet (UV) light of solar spectrum and other environmental deterrents is an unavoidable fact

4.1.1 Nanocomposites based on nanolayers

4.1.1.1.Polyolefins

Polyolefins have several key applications in various sectors such as medical products, packaging, automotive, household products, electronic appliances, etc which constitute a significant part of the common economy The production volume of polypropylene was and is one of the fastest growing polymers in history Polypropylene (PP) has been increasingly used in medical disposable, rigid packaging with products like food products, where aesthetics on account of clarity and gloss are of prime considerations PP

is the lightest polymer and has wide range applications such as packaging, fiber, automobile industry, nondurable goods and construction materials The excellent mechanical properties (toughness), resistance to chemicals, low cost and processability are considered to be of great advantage The incorporation of nanolayers (e.g montmorillonite) into polyolefins (esp PE and PP) have exhibited higher heat distortion temperatures, enhanced flame resistance, increased modulus, better barrier properties and decreased thermal expansion coefficient

Photo-oxidation of polypropylene [106, 107], polyethylene [108] and EPDM [109] filled with montmorillonite has been studied and it is established that the nanocomposites degrade faster than the pristine polymers because of the degradation of the alkyl-ammonium cation exchanged in MMT and the catalytic effect of iron impurities of the organomonmorrilonite Iron (Fe3+) could catalyse the decomposition of the primary hydroperoxide formed by photo-oxidation of polymers [107, 109, 110] Regardless of nanocomposite formation, it was found that the rate of photo-oxidative degradation of both PE/MMT nanocomposite and PE/Mn+MMT (where Mn+ stands for multivalent transition metal cation) micro-composites is much faster than that of pure PE It was also

suggested by Qin et al [111] that the decomposition of ammonium ion might create acidic sites on layered silicates; meanwhile, the complex crystallographic structure and habit of clay minerals could also result in some active sites Thus, the reversible photo-redox reaction of transition metal cations has a catalytic effect in the degradation of the polymer matrix All these catalytic active sites can accept single electrons from donor molecules of polymer matrix and induce the formation of free radical upon UV irradiation The generation of free radical leads to the oxidization and break of molecular chain Thus, the materials suffer degradation and their mechanical strength decreases Earlier, Tidjani et al [112].studied the consequences of this photo-irradiation of PP (under UV light under atmospheric oxygen) on the thermal stability and fire retardant performance of the nanocomposites using thermo-gravimetry and cone calorimetry In addition, the compatibilizer like maleic-anhydride-grafted copolymer is used for better dispersion of clay in the preparation of nanocomposites of PP and PE On the other hand, these components can catalyse the photo-oxidation of the PP matrix [113] Consequently, an integrated catalysis mechanism is proposed [114, 115] The mechanism (Figure 10) of the photo-oxidation of the polypropylene component was not modified in the polymer-nanocomposite, but that the rates of oxidation were modified, leading to an unexpected decrease of the durability of the material The organic modifier (in content region 1 to 15 wt%) has also influenced the photostability of the sPP nanocomposite with and without compatibilizer The efficiency of UV stabilizers was also observed to decrease [116] Several compositions of s-PP and montmorillonite were exposed to UV-C radiation (254 nm) The effect of chemical or radiation-induced cold crystallization was confirmed for these nanocomposites by FTIR spectroscopy, DSC and DMA tools The

shifts in the glass transition temperature were found for the different compositions of clay

[117] This chemi-crystallization may be due to the fragmentations of macromolecular

chains at the amorphous region of the specimen indicating the UV induced oxidation Mailhot et al [107], have found that the presence of stabilizers in the formulation has also significant effect on reducing the rate of oxidation of nanocomposites However, the efficiency of additives is greatly reduced and the enhanced degradation was observed even with the externally stabilized polyolefins (Figure 11) The influence of the organo-clay on the durability of the stabilized nanocomposite material is explained by following two arguments [106, 107]; i) Because of the decomposition / disappearance of antioxidant during the melt processing where the first phase of oxidation takes place, the antioxidant will be no longer available for protection of polymer matrix during its service-life ii) The adsorption of antioxidant on clay surface could also reduce the activity of phenolic antioxidants [118, 119] This relative instability of nanocomposite to photo-aging and antioxidant loss could constitute a major drawback for materials in the above-mentioned outdoor applications [120] In order to overcome this problem, it is very important to investigate effect of antioxidant and method of incorporation clay and antioxidant on the behavior of EPDM/clay nanocomposites upon UV exposure We have prepared EPDM - layered silicates nanocomposites using antioxidant and to investigate the effect of antioxidant on the photo-degradation of EPDM / clay nanocomposites with slightly modified methodology which employs the initial incorporation of antioxidant in EPDM matrix prior to melt processing as shown in the Figure 12 It was found that reduction in degradation rate was linear with the content of antioxidant The significant stabilizing efficiency of antioxidant was observed with the samples containing higher

content (1.5 %) of antioxidant and the stabilization is not remarkable with 0.1 - 0.5 % of antioxidant containing samples The changes on the surface morphology have also exhibited the reduced rate of degradation i.e the surface morphology of these nanocomposites can be retained by the use of antioxidant for many outdoor applications This methodology is able to prevent the sacrificial antioxidant loss, extends significantly the lifetime of the EPDM-Clay nanocomposites when they are subjected to photo-oxidative degradation

In contrast to montmorillonite, photo-oxidation rate of PP / LDH nanocomposites [121] is much lower than that of PP and PP/MMT samples, indicating that the PP / LDH nanocomposites have better UV-stability Figure 13 shows the carbonyl group evolution upon photo-irradiation for neat PP, PP/ MMT and PP/LDH nanocomposites The XPS and FTIR results also give possible evidence that the photo-oxidation mechanism of PP

in the PP / LDH materials is not modified compared with that of virgin PP This result is somewhat different from the PS/LDH nanocomposite photo-oxidation, where Peng et al.[122] observed the increased degradation rate than neat PS matrix The reason for the reduction in photo-degradation rate by LDH has yet to be examined

4.1.1.2.Polyacrylates

Poly(methyl methacrylate) (PMMA) is widely used in adhesives, automotive signal lights, lenses, light fittings, medallions, neon signs and protective coatings because of the excellent, optical (clarity), physical and mechanical (dimensional stability with high modulus) properties Photo-irradiation of PMMA nanocomposites study reveals that the acrylate linkages undergo a scission reaction thereby compromising the original

properties of the material Furthermore, these scissions produce a yellowing of the polymer matrix which can inhibit its use where optical clarity in important [123]

4.1.1.3.Polyesters

Injection-molded clay-filled poly (butylene terephthalate) nanocomposites and pure PBT were exposed to artificial weathering conditions for up to 36 weeks Using X-ray diffraction, the degree of crystallinity and crystal size were measured for samples exposed up to 6000 hrs At exposures less than 400 hrs, crystallinity and crystal size increased for the pure PBT samples At longer exposures, the crystallinity reached a plateau while the crystal size dropped after 1200 hrs UV-radiation penetrates ≈25 µm into the exposed surface for the nanocomposite and ≈50 µm for pure PBT For longer exposures the degradation is higher for the unfilled material than the filled grades The material service life significantly increased [124] Oxygen permeability rates in the polyester nanocomposites decreased with increasing clay content, as expected, and was satisfactorily reproduced using a tortuousity based model [125]

4.1.1.4.Polycarbonates

The photo-oxidation study of polycarbonate layered silicates nanocomposites reveal that the carbonate linkages undergo a scission reaction upon UV exposure, thereby compromising the original properties of the material Furthermore, these scissions produce a yellowing of the polymer matrix which can inhibit its use where optical clarity

is important [126] Under accelerated weathering The results reveal that the carbonate linkages undergo a scission reaction upon UV exposure The incorporation of the

layered silicates appears to increase the rate at which chain scission occurs Furthermore, these carbonate scissions produce a yellowing of the polycarbonate which can inhibit its use where optical clarity in important [126]

4.1.1.5.Polyamides

Nylon is an engineering thermoplastic, commercially made by anionic ring opening polymerisation of caprolactum and is used in filaments of toothbrushes, ropes and filaments for garments like raincoats and is also used in the automobile industry for self-lubricating gears and bearings Nylons were the first commercial polymer nanocomposites made by TOYOTA research group [127] and where prepared by typical methods [128, 129] These nanocomposites are mainly used in automotive industry and packaging industry thus the study of durability has been widely investigated Through the comparison of the IR spectra of PA6 and PA6/MMT nanocomposite during different times of UV irradiation, it is indicated that the rate of photo-oxidation of PA6/MMT nanocomposite is faster than that of PA6 [130]

4.1.2 Nanocomposites based on nanotubes

The preparation of nanocomposites of polymers (polyolefins, polyacrylates, polyesters, polyamides, polyurethanes and etc.) and CNTs has been the interest of researchers, because of their flame retardancy, mechanical and electrical property improvement The dispersion of CNTs is very difficult and it is achieved by functionalizing CNTs [131] The existing reports on polymer-CNT nanocomposites have been focusing mainly on the functionalization of CNTs, preparation and property developments Unfortunately or