Polyamide/Carbon Nanoparticles Nanocomposites: A Review

Farzaneh Faridirad, Shervin Ahmadi, Mohammad BarmarIran Polymer and Petrochemical Institution, Tehran, Iran

This review is designed to be a comprehensive source forpolyamide (PA) nanocomposite research, including funda-mental structure/property relationships, manufacturingtechniques, and applications of these materials. This workpresents the scientific framework for the advances in PAnanocomposite containing carbon nanofiller, and differentmethods applied in order to synthesis them. This reviewfocuses on the scientific principles and mechanisms inrelation to the methods of processing and manufacturing.A comprehensive discussion on technology, modeling,characterization, processing, manufacturing, and applica-tions have been done. The processing and properties ofPA nanocomposites with carbon nanofillers are investigat-ed. In addition, the mechanical properties and morpholo-gy changes of PA with the incorporation of nanoparticlesare described. POLYM. ENG. SCI., 00:000–000, 2016. VC 2016Society of Plastics Engineers

INTRODUCTION

Polyamides contain a repeated ACONH group, within the

chain. They have high mechanical strength, high melting point,

and high resistance against corrosion and abrasion. Also they

have low density and high potential of forming complicated

structures from the molding injection. In the other hand, these

polymers are Nonconductive [1].

Polyamide (PA) is a well-known engineering thermoplastic

material that is widely used in industrial applications (e.g.,

fibers, films, textiles, and various molding products) for its

remarkable mechanical and thermal properties. However, these

advantages are accompanied by limitations such as moisture

absorption, notch sensitivity, relatively low impact strength, and

poor dimensional stability. Thus, modification of PA to improve

its physical properties and to introduce new properties has

drawn much attention [2–5]. Polymeric nanocomposites offer

new technological and economical benefits. The incorporation

of nanometer-scale reinforcement may dramatically improve the

selected properties of PA. These nanocomposites exhibit superi-

or properties such as enhanced mechanical properties, reduced

permeability, increased electrical conductivity, and improved

flame retardancy [6–9].

Mechanical enhancement of polymers by incorporation of

nanofillers has been a key research topic in the field of polymer

materials. Most of the researches have indicated the nanofillers

succeed in increasing the stiffness of polymers. The degree of

improvement mainly relies on the dispersion of nanofillers in

polymeric matrices and the strength of nanofiller-polymer inter-

actions [10–12]. However, enhancing the toughness of polymers

by nanofillers is still in debate. Although most of researches

have reported the deteriorated ductility of polymers combined

with nanofillers, some researchers found the toughening effects

of nanofillers in polymers under some specific circumstances

[13, 14]. They have attributed this enhancement in toughness to

different practical aspects like, (a) improving the mechanical

interlocking and adhesion between nanofiller and polymeric

matrices; (b) increasing the mobility of nanofillers in polymeric

matrices providing the testing temperature above glass transition

temperature; and (c) changing the crystalline phase resulting

from the addition of nanofillers. Intensive reviews have also

been prepared relating to polymer/nanofiller interaction, in an

attempt to understand the toughening mechanism in polymer

composites [15, 16].

Blends of PA with polyolefin are particularly attractive

because it is theoretically possible to couple the excellent mechan-

ical properties of the PA and the good processability and tough-

ness of the polyolefin. Nanocomposites based on polymer blends

of PA and polyolefin have been widely reported in the scientific

literatures. For example, the blending of Polyamide 6 (PA6) and

polypropylene (PP) has been attempted to achieve improvement

in the mechanical properties, printability, and barrier properties.

PA6 contributes the mechanical and the thermal properties,

whereas PP ensures good processability and insensitivity to mois-

ture. The polymer blend nanocomposites may lead to a new type

of high performance material that combines the advantages of the

polymer blends and polymer nanocomposites [17, 18].

The major amounts of works are related to Polyamide 6 and

Polyamide 12. Polyamide 6 (PA6) is a semicrystalline polymer

that exhibits excellent chemical stability and mechanical

strength properties, and it is also competitively priced in com-

parison to other polyamides. The exceptional value and perfor-

mance in many products are thus making PA6 the material of

choice for a number of consumer goods and industrial applica-

tions [19]. The main applications of PA6 are in the fibers, films,

and as the injection-molded engineering plastics. PA6 crystalli-

zes fast, usually up to 30–40%, providing a high modulus to the

material even above the glass transition temperature (Tg) [20].

Polyamide 12 (PA12) is made of laurolactam monomers. It is

crystallized in hexagonal form and gamma phase, in all conditions.

Its melting point is about 172–1808C and its chemical resistance is

significant. The main application of PA12 is in plastic industry,

specially preparing films and sheets. Also it is used as cable coat-

ing, and in the hydraulic system of automotive industry [1, 21].

In this review, we have highlighted recent advances on the

synthetic strategies and properties of some polyamide Nanocom-

posites, which have been prepared by different carbon nanofil-

lers. The discussion is based on some recent examples of the

three main methods for polyamide nanocomposite polymeriza-

tion. The feasibility of functionalization or preparing the blend

nanocomposites has been studied too. At the end of this paper

some studies relating to nanocomposite modeling is presented.

While there are many researches working on the Polyamide 6

and Polyamide 12 composites and blends, the focus of this

review lies on these two polyamides’ Nanocomposites mainly.

Correspondence to: M. Barmar; e-mail: m.barmar@ippi.ac.ir

DOI 10.1002/pen.24444

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2016 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2016

CARBON NANOFILLER

Nanotechnology has been developed significantly over the

recent 20 years and the importance of this technology is in sensors,

biomedical, and many other applications. The progress in these

fields depends mostly on the ability of nanoparticles preparation,

with a wide range of size and different shapes. Therefore, discover-

ing the graphene, carbon nanotubes, and nanocomposites based on

these materials is one of the main issues in nanotechnology. Dis-

covering the polymeric nanocomposites by Toyota research team

provided a new perspective in material science [22].

The main property of the nanoparticles is the high aspect

ratio. According to the electrostatic forces among the nanopar-

ticles, they become close together and do not settle. Increasing

the aspect ratio causes the nanoparticles to be more active and

then, they tend to agglomerate, which is a problem to have an

efficient intercalation within the polymeric matrix, in the Nano-

composites [23]. A general comparison among different kinds of

the nanofillers is shown in Table 1.

The improvement of the Physical chemistry properties in the

nanocomposites depends on the distribution of the nanofillers

within the polymeric matrix and also the interfacial bonding of

nanofiller and matrix, which will be determined the final behavior

of the reinforced polymeric nanocomposites. Some nanofillers are

incompatible with the organic polymers, thus they do not provide

homogeneous nanocomposites. For instance, graphene oxide sheets

are oxygenized highly to improve the van der walls interactions

significantly and become more compatible with the organic poly-

mers. Surface modification of the nanofillers is one of the main

steps to obtain the molecular distribution in the polymeric matrix

[24]. Carbon nanotubes and graphene are known for their excellent

mechanical, electrical, thermal, and electronic transport properties.

Therefore, in this section they will be explained particularly.

Carbon Nanotubes

Carbon nanotubes (Fig. 1) have been discovered by Sumioli-

jima, in 1991 [25]. CNTs have high aspect ratios, with diame-

ters of 1–100 nm and lengths greater than several micrometers.

They show mechanical strength, electrical, and thermal conduc-

tivity, and distinctive physicochemical properties originating

TABLE 1. A comparison between different nanofillers.

Materials Tensile strength Thermal consuctivity (W/mk) at room temperature

Electrical

conductivity (S/m) References

Graphene 130 6 10 GPa (4.84 6 0.44) 3 103 to (4.50 6 0.48) 3 103 7,200 [23]

CNT 60–150 GPa 3,500 3,000–4,000 [22, 24]

Nanosized steel 1,769 MPa 5–6 1.35 3 106 [25]

Plastice (HDPE) 18–20 MPa 0.46–0.52 Insulator [26]

Rubber (natural rubber) 20–30 MPa 0.13–0.142 Insulator [27]

Fiber (Levlar) 3,620 MPa 0.04 Insulator [28]

FIG. 1. Graphene (top left) has a honeycomb structure. Graphite (top right) stack of graphene layers. Carbon nanotubes

are rolled cylinders of graphene (bottom left). Fullerenes (C60) are molecules consisting of wrapped graphene [25].

2 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

from their unique structure of the cylindrical graphene sheets.

The mechanical strength of CNTs is much higher than that of

steel. CNTs are regarded as a suitable reinforcement material

for the composites because of their low density and high

mechanical strength. Moreover, the electrically conductive plas-

tics can be obtained by adding the small amount of CNTs.

To exploit these unique properties, studies of the composites

containing CNTs have been conducted to overcome the low

interfacial strength between CNTs and the polymers, and the

difficulty in dispersing CNTs in basic materials [26].

The electrical resistance of these nanofillers is reported about

1.5 3 1021 up to 1.2 3 1024 X/cm, which means that they

could be considered as the conductive materials [27]. CNTs

have emerged as potential conducting fillers due to their excep-

tional electrical properties and high aspect ratio (L/D). Thus, a

very high conductivity in the polymer/CNT nanocomposites can

be achieved by the presence of low amount CNT [28]. However,

due to the strong inter-tube van der Waals forces and the lack

of interfacial interactions with the polymer matrix, CNTs tend

to agglomerate to form the clusters (or insufficient deagglomera-

tion) and often manifest a higher electrical percolation threshold

with a lower effective L/D. Hence, an effective L/D is a key fac-

tor for the electrical percolation threshold in the polymer matrix.

In addition, adequate interfacial interaction between the CNTs

and the polymer matrix is another prerequisite for obtaining the

enhanced dispersions of CNTs. To this end, functionalization of

CNTs is one of the strategies employed to enhance their phase

adhesion with the polymer matrix [29].

Graphene

Graphene is the basic structural unit of some carbon allo-

tropes, including graphite, carbon nanotubes, and fullerenes

(Fig. 1). It is believed to be composed of benzene rings stripped

of their hydrogen atoms. The rolling up of the graphene along a

given direction can produce a carbon nanotube. A zero-

dimensional fullerene can also be obtained by wrapping-up the

graphene [30]. In 1940, it was established theoretically that the

graphene is the building block of the graphite. In 2004, Geim

and coworkers at Manchester University successfully identified

single layers of graphene and other 2-D crystals [31] in a simple

experiment, which were previously considered to be thermody-

namically unstable and could not exist under the ambient condi-

tions [32]. The promising mechanical, electrical, optical,

thermal, and the magnetic properties of graphene have led to the

creation of a new and exciting field of the fundamental science.

The simplest way of the preparing small samples of single-

or few layer graphene is through the mechanical cleavage from

either highly oriented pyrolytic graphite or good-quality natural

graphite [31]. Typically, this method produces a mixture of one-,

two-, and many-layer graphene flakes that have dimensions of the

order of tens of microns.

The rapid rise of interest in the graphene for using in appli-

cations that requires high volumes of the material, such as in

the composites, led to investigations into methods of the under-

taking large-scale exfoliation [33, 34]. One of the first success-

ful methods was the exfoliation and dispersion of the graphite

by using the organic solvents [35]. Depending on the levels of

agitation and purification suspensions with the large fractions of

graphene monolayers could be prepared.

NANOCOMPOSITES

The electrical conductive polymeric nanocomposites, pre-

pared from polymer and conductive nanofillers, such as natural

graphite, carbon black, and metal powders have been investigat-

ed in the recent decades significantly. These materials could be

used as antistatic coatings, the electromagnetic protection and

the resistant coating against corrosion. To improve the proper-

ties of polymeric material, many studies have been done to

introduce the nanofillers in the polymeric matrix. These nano-

composites have higher mechanical properties, they are stronger

barriers, and they can retard agitation more than pure polymers.

The nanoparticles could be the nanoclay, carbon nanotubes, sili-

ca, metal oxides, graphite oxide, and graphene. Among these

nanofillers, the nanoclay is more interesting due to its abun-

dance as an initial material. In fact, there are three kinds of

nanocomposites, based on the type of the nanofillers: (1) the

nanoclay/polymer nanocomposites (24%), (2) the metal oxide/

polymer nanocomposites (19%), and (3) the carbon nanopar-

ticles/polymer nanocomposites (15%) [36].

Characterization of the Nanocamposite

Earlier studies on the nanocomposites have suggested the

existence of three general states of the nanofiller dispersion on

short length scales: stacked, intercalated, or exfoliated, as shown

in Fig. 2 [37]. TEM and WAXS studies are perhaps the two

most common means by which the state of the dispersion can

be assessed. Immiscibility of the phases and/or insufficient exfo-

liation of the filler prior to mixing with the polymer can result

in large agglomerates consisting of the stacked nanofillers when

observed by TEM, which may also be suggested by the presence

of a diffraction peak corresponding to the interlayer spacing of

the nanofillers [38, 39]. The intercalated fillers retain a stacked

structure but with the increased interlayer spacing (on the order

of a few nanometers) [40]. An exfoliated morphology of the

FIG. 2. Three morphological states: (a) phase separated, (b) intercalated, and (c) exfoliated [37].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 3

nanofiller is thus usually desired as it provides higher aspect

ratio relative to stacked or intercalated nanofillers [41]. This

state of dispersion may be suggested by a scattering profile cor-

responding to that of the neat matrix polymer; however, the

multilayer intercalated nanofillers could actually be dispersed

(as observed by TEM) despite the absence of a diffraction peak.

Evaluating the homogenous dispersion is important. More-

over, the nanoparticle size and the functional group characteriza-

tion of these nanoparticles are very important. In this section,

some practical techniques will be studied.

Number and Size of Layers. XRD is used to investigate the

nanoparticle intercalation. Surface area is used as an indirect

intercalation factor; this means that the sheets with perfect

enough intercalation have higher surface area. AFM is a tool in

order to investigate the sheet dimensions, more accurately. AFM

is used in the contact and tapping mode in order to detect the

surface topology. SEM images present the qualitative perspec-

tive from three-dimensional structure of the filler plates. More-

over TEM images present some information about the particle

size, and make it possible to detach single and few layered

plates. Atomic bonding, atomic defects on functionalized plates

and also the presence of aliphatic areas containing carbon-

oxygen bonds, are recognizable through TEM photography with

high resolution [42–44].

Chemical Modification Characterization. The overall degree of

oxidization is determined through the standard elemental analy-

sis. The amount of oxygen on the surface is quantified and also

the kind of carbon bonds will be determined, through XPS.

NMR13C may be the most direct method in order to recognize

the oxygen groups. Also it is a quantitative method in order to

determine the conversion percentage [45–47].

Particle Dispersion Investigation. TEM presents the direct

images from the dispersion. Less thickness of the filler sheets

makes it difficult to recognize them in TEM images. Also the

rheology is another effective method to investigate the nanopar-

ticle dispersion within the nanocomposite [48, 49].

Rheology is a widely used evaluation method for detecting

the presence of the interconnected structures. This technique

seems to be relevant for the study of the dispersion state which

defines the nanostructure of the mixture between the conven-

tional, the intercalated, or exfoliated the nanocomposite. The

morphology change is detectable with the rheology analyzes by

the apparition of a shear-thinning behavior. This non-Newtonian

behavior can be attributed to the various factors such as the

change in the nanoparticles volume fraction, shape, and size or

size distribution. This decrease of viscosity is due to the reorien-

tation of the nanofillers in the direction of flow in the response

to the external applied shear. The degree of the shear-thinning

can then be used as an indicator of the exfoliation state of the

nanofillers inside the polymer matrix; a steeper slope can be

associated to an exfoliated mixture. The dispersion state of

nanoparticles in the matrix will be evaluated using both the rhe-

ology and the SEM analyses [37].

Nanocomposite Applications

The polymer composites comprising the nanofillers are often

investigated where the reinforcement of the polymer matrix is

achieved. While the reinforcement aspects are a major part of

the nanocomposite investigations reported in the literature,

many other variants and property enhancements are under active

study and in some cases commercialization. The advantages of

nanoscale particle incorporation can lead to a myriad of applica-

tion possibilities where the analogous larger scale particle incor-

poration would not yield the sufficient property profile for

utilization. These areas include the barrier properties, membrane

separation, UV screens, flammability resistance, polymer blend

compatibilization, electrical conductivity, impact modification,

and biomedical applications. Examples of the nanoparticle,

nanoplatelet, and nanofiber incorporation into the polymer

matrices are listed in Table 2 along with potential utility where

properties other than the mechanical property reinforcement are

relevant.

The barrier properties of the polymers can be significantly

altered by inclusion of the inorganic platelets with the sufficient

aspect ratio to alter the diffusion path of penetrant molecules as

illustrated in Fig. 3 [46].

More general applications include: packaging, fuel cell, solar

cell, fuel tank, plastic containers, impellers, and blades for the

vacuum cleaners, power tool housing, and cover for the portable

electronic equipment such as the mobile phones and pagers

[46].

POLYAMIDE NANOCOMPOSITES BASED ON CARBONNANOFILLERS

Polyamides are an important class of the thermoplastic poly-

mers which have a wide range of the industrial and household

applications. These thermoplastic polymers are known for their

exotic properties, e.g., extreme toughness, abrasion resistance,

good chemical resistance, light weight, low water absorption,

good electrical insulation, etc. As it is seen in Fig. 4, there are

two polymorphic phases of Polyamide 12 (PA12), namely the aand the c phase, the more stable one being the c form [50].

Polyamides are universally used as matrix material for the

composites. The first polyamide nanocomposite has been

TABLE 2. Some nanofillers and their properties [65].

Nanofiller Property enhancement(s)

Exfoliated clay Flame resistance, barrier, compatibilizer

for polymer blends

SWCNT; MWCNT Electrical conductivity, charge transport,

Nanosilver Antimicrobial

ZnO UV adsorption

Silica Viscosity modification

Graphene Electrical conductivity, barrier, charge transport

FIG. 3. Barrier to permeation imposed by nanoparticles imbedded in a

polymeric matrix [46].

4 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

prepared by Toyota Company in 1987 which included 4 wt %

nanoclay and the polymer was Polyamide 6. Nowadays many

companies try to introduce Polyamide 12 to the automobile and

fuel industries. The common methods for preparing such compo-

sites are melt-compounding, in situ polymerization and grafting.

A significant improvement in the modulus, strength, and hard-

ness of PA6 has been found by adding 2 wt % of MWCNT.

Electrically conducting nonwoven PA6 membranes have been

made with MWCNT adsorbed on the surface [51]. CNTs also

influence the crystallinity of PA6 as detected by X-ray scatter-

ing and differential scanning calorimetry (DSC) studies [52].

Reports have been suggested that the nanofillers act as nucle-

ation sites for the formation of new crystalline domains [53].

Recently graphene-PA6 composites prepared by in situ ring

opening polymerization showed excellent improvement in

mechanical properties of the polymer, even at a low graphene

concentration of 0.1 wt % [54]. With PA6 electrical percolation

could be achieved at 2.5 wt % or higher CNT loading [55]. The

improvement of tensile properties has been obtained through

laser sintering of PA12/carbon nanofiber composites [56].

Recently, it has been reported that with carbon nanofibers in

PA12 the thermal and thermo-oxidative stabilities of the matrix

could be improved along with the stiffness of the material [57].

The composite of PA12 with a small amount of the functional-

ized expandable graphene caused a significant improvement in

the tensile strength, elongation to break, impact energy, and the

toughness of the polymer, although no significant improvement

in Young’s modulus was reported [58].

Figure 5 [58] shows the scanning electron microscopic

(SEM) images of the fractured surfaces of PA12 composites

with CNT and graphene. Several other polyamides have also

been used as the matrix material for composites and have prov-

en to be effective in enhancing the mechanical, electrical, and

thermal properties indicating a bright future for such materials.

Since the polyamides are polar, it is better to functionalize the

nanofillers, in order to improve the carbon nanoparticles com-

patibility with polymer. Another method is to start polymeriza-

tion from the surface of the carbon nanoparticles. In this case,

the nanoparticles would be held as an arm and therefore the sta-

bility of the nanoparticle structure increases, which omit the

structure changes due to the tension and nanoparticle replace-

ment. Also nanoparticle functionalization prevent from the

aggregation.

Preparation of Polyamide/Carbon Nanofillers Nanocomposites

There are four strategies to intercalate the nanofillers

between the polymeric matrixes:

a. In situ intercalative polymerization

This method is used to synthesis the polymeric nanocompo-

sites such as epoxy, PMMA, Polyamide 6, polyurethane,

PBT, polyaniline, and PE. The polymerization begins with

heat or radiation. The interlayer space increases by interacting

the monomers among the carbon nanoparticles, during in situpolymerization and leads to a perfect distribution of the nano-

particles in the polymeric matrix. This method provided a

covalence bonding between the nanofillers and the polymeric

matrix through different chemical reactions. The main prob-

lem of this method is increasing the viscosity by progressing

the polymerization and also, the imitation of manipulation

and the filer ratio addition. Moreover, some processes are

done in the presence of solvent and therefore removing the

solvent is another critical problem [59].

b. Interfacial polymerizations

The reaction is done in a two-phase system. The amine and

the diacid chloride are dissolved in water and an organic sol-

vent, respectively. The two solutions are placed in the same

FIG. 4. Schematic structure of PA12, (a) a phase and (b) c phase [50].

FIG. 5. SEM images of (a) PA12/CNT composite showing well dispersed of CNTs [58].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 5

beaker. Of course, the two solutions are immiscible, so there

will be two phases in the beaker. At the interface of the two

phases, the polymerization is occurred between the diacid

chloride and the diamine. But generally, it is not used com-

mercially because acid chlorides are a lot more expensive and

are much more toxic than other acids. And also, the fibers

produced by this method are not very strong, anyway [60].

c. Melt compounding polymerization

Melt compounding polymerization is an applicable method

for the thermoplastic polymers. It uses high temperature and

shear force for the nanoparticles to be distributed within the

polymeric matrix. The melt compounding consists of prepar-

ing the nanocomposites formulations by mixing in a molten

state. Melt intercalation takes place during the melt com-

pounding in an extruder, internal mixer, or a similar device.

Usually, the matrix polymer is melted first, and then com-

pounded with a compatibilizer and the intercalated nanofillers

under blanket of inert gas, e.g., N2. Alternatively, the poly-

mer is first mixed with a compatibilizer (e.g., its functional-

ized homolog) then compounded with the intercalated

nanofillers [61].

A thermoplastic polymer is mixed mechanically with the

graphite or graphene or modified graphene at the elevated

temperatures using the conventional methods, such as the

extrusion and the injection molding [62].

High temperature softens the polymeric matrix and makes

it possible for reinforcement phase to be dispersed easily. The

method does not need any toxic solvent but in the higher

amounts of the filler the nanocomposite preparation becomes

hard, because of increasing the viscosity [63]. Another prob-

lem of this method is the aggregation of nanoparticles during

the mixing, due to heavy shear forces [63]. A wide range of

the polymer nanocomposites, such as PP/EG [64], HDPE/EG

[38], PPS/EG [65], and PA6/EG [66], etc., have been pre-

pared using this method.

d. Selective laser sintering polymerization

The selective laser melting of the polymer powders is a

well-established technology for additive manufacturing appli-

cations, although there is still a deficit in basic process

knowledge. Considering the demands of series production, the

technique of selective laser melting of the polymers is faced

with various challenges concerning suitable material systems,

process strategies, and part properties [67]. According to the

work of Bai et al. [68] experimental and simulation investiga-

tion of a polymer nanocomposite for laser sintering was car-

ried out to examine the thermal influence of the CNT

nanofiller on the Polyamide 12 matrix during laser sintering

process.

In situ Polymerization of Polyamide Nanocomposites Based onCarbon Nanofillers

Polyamide Carbon Nanotube (CNT) Nanocomposites. Gao

et al. in 2006 prepared Polyamide 6/single-walled CNT

(SWCNT) through hydrolyzed in situ polymerization. They

investigated the effect of the functionalized SWCNT through

acid and amide groups, on the mechanical properties improve-

ment. In their research, different degrees of carboxylic acid

were applied in order to make a comparison among the different

modes of interfacial interactions, and obtain an appropriate

degree of functionality. The purity of the carbon nanotube used,

was about 80–90% [69]. Carbon nanotubes and caprolactam as

monomer were combined though ultrasonic. Then 6-

aminohexanoic acid was added to the suspension. After 6 h, the

prepared mixture was added to water and the polymeric com-

posite was settled. The fictionalization process was as Fig. 6

[69].

Strain test results showed that by increasing the carboxylic

acid group concentration, CNT-polyamide interaction increased

too. Using amide functionality made longer polymeric connec-

tions on CNTs, and therefore increased the flexibility and ductil-

ity of nanocomposites. According to SEM results (Fig. 7) [69],

CNTs in different concentrations had a homogenous distribution

in polymeric matrix and when there was no carboxylic group,

some microcracks were evidence, indicating a limited reinforce-

ment. As the ACOOH group density increased to 6.8%, the

cross section of the composite fiber became noticeably rougher

(Fig. 3c), which may result from the stronger SWNT polymer

interaction.

Saeed and Park [70] prepared Polyamide 6/multiwalled CNT

(MWCNT) nanocomposite, through hydrolyzed in situ polymeri-

zation. The effect of nanotubes functionalization on properties

and morphology was investigated, using two kinds of nanotubes.

The polymerization mechanism is shown in Fig. 8 [70].

Nitric acid was used in order to purify the nanoparticles. In

this process, carboxyl groups were formed in the contact place

of acid and nanotubes, homogenously. SEM results showed that

modified nanoparticles had better dispersion in the polymeric

matrix in comparison with the pure nanoparticles, due to better

compatibility with polymeric matrix. Four-point probe was used

in order to determine the electrical conductivity, for samples

with different MWCNTs concentrations. According to Table 3,

better electrical conductivity for samples with modified CNTs

has been seen, which indicated the p bonding destruction of

MWCNTs surface during purification. The investigated nano-

composites in this research showed better performance than

melt compounding method. By comparing the DSC results for

both nanocomposite and pure polymer, it was seen that melting

point transferred to higher temperatures (from 189 to 1958C)

which is because of the carbon nanotube that performed as the

nucleating agent and increased the crystallization velocity in

comparison with the pure Polyamide 6.

In another research which has been done by Yang et al. [71],

the MWCNT containing AOH groups, was modified using tolu-

ene diisosyanate (TDI). Polyamide 6/MWCNT nanocomposite

was synthesized through anionic ring opening in situ polymeri-

zation by functionalizing the MWCNT in the presence of TDI

FIG. 6. Functionalized SWCNT preparation through amide group [69].

6 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

as an activator. FTIR (Fig. 9) [71] and TGA analysis indicated

both the nanotubes and the polymers were presented within the

material isolated from the polymerization reaction. Unlike sim-

ple blends of polymers and nanotubes, the two components of

composites could not be separated from one another by exten-

sive filtration and washing, indicating that they are covalently

bounded. Analysis of these structures by TEM provided further

evidence for the formation of polymerized nanotubes. Addition-

ally, it was found that the grafted polymer weight proportion

could be roughly adjusted by the feed ratios of the monomer to

MWCNT initiator.

Laurolactam in situ polymerization was investigated by

H€ansch et al. in 2012, in order to synthesis Polyamide 12/

MWCNT nanocomposite in a microcompounder [72]. The oper-

ating conditions were optimized according to the electrical resis-

tance and MWCNT dispersion. N,N-ethylene bis-Astyaramyd

and sodium hydride were used as an activator and catalyst,

respectively. The amount of residual monomer was less than 1

wt %. The molecular mass of the prepared nanocomposite was

5,000–20,000 g/mol. Higher screw velocities made MWCNTs to

be dispersed better and therefore decreased the percolation and

electrical resistance threshold. The higher molecular weights led

to the higher amounts of melting viscosity, while less viscosity

is more beneficial during impressive molding test of samples.

Moreover, higher molecular weights increased the carbon nano-

tubes length. Also by comparing the in situ and melt

compounding polymerization results, it was obvious that the for-

mer method performed better, according to Fig. 10 [72].

Polyamide/Graphene Nanocomposites. In 2010, Xu and Gao

[54] presented an efficient method in order to prepare the Poly-

amide 6/graphene nanocomposite via in situ polymerization of

caprolactam in the presence of graphene. During condensation

polymerization, graphene oxide was reduced thermally to gra-

phene, simultaneously. Appropriate grafting of Polyamide 6

arms on graphene sheets were authenticated through XPS, FTIR,

TGA, and AFM analysis. Results showed that grafting was

about 78 wt % and AFM indicated the two-dimensional brush

morphology. The efficient grafting of polymeric chains led to a

homogenous dispersion of graphene in the polyamide matrix.

By a condensation reaction between the carboxylic acid groups

on GO and terminal amino ends of PA6 chains, the macromo-

lecular chains of PA6 were effectively grafted onto GO sheets,

accompanying with the reduction of GO to graphene. The

grafted graphene sheets showed good solubility in the solvents

of PA6 and performed acceptable compatibility with PA6

matrix. With enhanced interfacial interaction of the matrix, the

modified graphene impacted great reinforcements to PA6 fibers

as made by melt spinning. The Young’s modulus and tensile

strength of NG composite fibers were greatly improved even

though at very low containing of graphene, offering great prom-

ises for wider application of PA6 materials.

FIG. 7. SEM images of cross-sectional structure of composite with 0.5 wt % SWNT containing the following func-

tionalities: (a) 0.0% ACOOH, (b) 4.2% ACOOH, (c) 6.8% ACOOH, and (d) 4.2% ACONH2 [69].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 7

O’neil et al. in 2014 investigated the in situ anionic ring

opining polymerization of e-caprolactam in the presence of sin-

gle layer graphene oxide (GO) and reduced graphene oxide

(rGO) [73]. Graphene was reduced during the polymerization

along with functionalization by Polyamide 6 chains. In their

work, the in situ preparation and characterization of a number

of PA6/GO and PA6/rGO composites were reported, with a

focus on the properties of the graphene before and after poly-

merization. After polymerization, functionalization of graphene

sheets with PA6 was observed along with reduction of the oxy-

gen species. It is thought that reduction is mainly due to thermal

kinetics; however as previously detailed some oxygen may be

lost in bonding with the polymer. A small quantity of doubly

bound oxygen is shown to be retained by FTIR (Fig. 11) [73].

As the presence of even a few oxygen species throughout the

sample would be enough to create a degree of exfoliation

enough to increase the spacing of the layers. Graphene inclusion

FIG. 8. Polyamide 6/MWCNT nanocomposite preparation steps [70].

TABLE 3. Conductivity of pure-MWCNT/Nylon and modified-MWCNT/

Nylon films as a function of MWCNT amounts [77].

Sample Conductivity (S/cm)

5 wt % P-MWNT/nylon 3.15 3 1025

5 wt % A-MWNT/nylon —

7 wt % P-MWNT/nylon 1.44 3 1025

7 wt % A-MWNT/nylon 7.46 3 1025

FIG. 9. FTIR spectra of (A) MWCNTAOH, (B) isocyanate functionalized

MWCNTs, (C) caprolactam functionalized MWCNTs, and (D) PA6 func-

tionalized MWCNTs [71].

8 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

promoted nucleation of crystallites especially where polymer

chains were bound to the surface. Interfacial interaction seemed

much greater in composites using GO when compared to rGO due

to the amount of functionalization on f-GO sheets. Overall the

introduction of graphene into the polymer matrix has had no detri-

mental effects on the thermal properties of the polymer. This work

reported a simple method for the production of multifunctional

nanocomposites which could easily be produced on a large scale.

Ryo and Han [74] prepared PA 6 (Polyamide 6)/MWNT

nanocomposites by the melt-compounding method for evaluation

of the mechanical and electrical properties. Flexural testing con-

firmed the much higher flexural modulus of the PA 6/noncova-

lently functionalized MWNTs relative to the PA 6/pristine

MWNT nanocomposites. Furthermore, the PA 6/noncovalently

functionalized MWNT nanocomposites exhibited better electri-

cal properties due to preservation of the intrinsic structure of the

MWNTs as well as the uniform dispersion of the MWNTs.

Gao and M€uller-Plathe [75] explored the effect of grafting

density (chains per surface area) and grafting length (number of

monomers per grafted chain) on the interfacial and parallel ther-

mal conductivity of nanocomposites by employing a realistic

atomistic model for graphene and polyamide-6,6. The influence

of the chemical functionalization on the intrinsic thermal con-

ductivity of the nanosized graphene was considered. Moreover,

they analyzed the interfacial coupling between graphene and PA

and put forward two channels for the heat transfer for grafted

graphene. By combining the results with the effective medium

approximation (EMA), they obtained the overall thermal con-

ductivity of these nanocomposites for three geometries.

Laura Arboleda-Clemente et al. [76] designed new CNT

nanocomposites—with an immiscible blend of polyamides as a

matrix—that had good electrical conductivity and a low percola-

tion threshold. They used rheological tests and alternating cur-

rent (AC) measurements to determine the percolation threshold

of nanocomposites. They also conducted a detailed investigation

of the electrical conductivity frequency dependence in the sam-

ples. Furthermore, they used transmission electron microscopy

(TEM) to examine the morphology of the nanocomposites and

the CNT localization in the polymer matrix.

The rheological measurements to characterize the percolation

state of the multiwall CNTs (MWCNTs) and their dispersion

within the polyamide (i.e., PA12/PA6) blends were conducted.

Solution Polymerization of Polyamide Nanocomposite Based onCarbon Nanofillers

Gong et al. [77] in 2015 functionalized graphene nanopar-

ticles by using PVA through steric interaction of activated car-

bodiimide, between activated carboxylic acid on graphene and

hydroxyl groups on PVA (Fig. 12) [77].

In this study, graphene oxide first dispersed in DMSO

through sonication, and then added to DCC, DMAP, and HOBT

solutions. Next PVA solution in DMSO was added. Then the

reaction mixture was settled in acetone. By repeating the proce-

dure steric graphene oxide with PVA was obtained. In order to

prepare nanocomposites, modified graphene oxide by PVA was

sonicated in 5% Polyamide 6-formic acid solution. Then the

mixture was settled in deionized water due to form nanocompo-

sites. AFM images showed that the dispersion within polymeric

matrix and adhesion to polymeric chains was increased by mod-

ifying the GO through PVA. Raman spectroscopy confirmed the

presence of covalence bounds between GO and PVA. SEM

images for both modified and unmodified graphene oxide/Poly-

amide 6 nanocomposites showed that for the pure graphene

FIG. 10. Comparison MWCNT dispersion in nanocomposites with 2 wt % MWCNT (a) melt mixed bulk PA12 and

(b) in situ polymerization [72].

FIG. 11. FTIR absorbance spectra showing, from bottom to top, GO, rGO,

fGO, and f-rGO [73].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 9

oxide, the nanoparticles aggregated and did not dispersed appro-

priately while the modified ones, were intercalated perfectly

and dispersed finely within the polymeric matrix. This event

can be justified as PVA chains prevent from reaggregation of

graphene oxide nanoparticles. The improved dispersion was

also obvious from TEM images through unclear interface

between nanoparticles and Polyamide 6 matrix. Iqbal and Ali

[78] synthesized carbon nanotubes by various methods. In sec-

ond phase, polyamide and polythiophene were prepared by solu-

tion method. Finally polystyrene/polyamide/polythiophene and

carbon nanotubes composites were also prepared by solution

method. Scanning electron microscopy was used to study sur-

face morphology of composites and it confirmed shapes of car-

bon nanotubes with polymer blend coating. The conductivity of

polymer nanocomposites was increased from 6.7 3 10216 to

6.8 3 1021 S/cm.

Melt Compounding Polymerization of Polyamide NanocompositesBased on Carbon Nanofillers

Rafiq et al. [58] investigated the effect of functionalized gra-

phene (FG) on mechanical properties of Polyamide 12 in 2010.

The results showed that by introducing a little amount of FG

(about 0.6%), a significant improvement in ultimate tensile

strength, elongation, impact strength, and stiffness was occurred.

This amount of nanofiller caused 175% improvement in effec-

tive dissipated energy of Polyamide 12. FG increased the cphase and made the polymer tougher. Premixing the material

was an appropriate approach in order to have a homogenous dis-

persion of FG within the polymeric matrix which was evidence

from SEM images. From FTIR analysis it was obvious that

ANH peaks shifted to lower amount caused from hydrogen

bonding between ANH groups of Polyamide 12 and AO group

in FG, which improved the strength of composite (Fig. 13) [59].

Crystalline structure of composites was investigated through

XRD. The average size of nylon crystals decreased significantly

by adding FG, due to nucleation capability of FG.

In 2011, Socher et al. [79] incorporated hybrid filler systems

consisting of MWCNTs and CB by melt mixing in the PA12

matrix, in order to investigate if the synergistic effects can be

achieved concerning electrical percolation threshold or disper-

sion of primary CNT agglomerates (Fig. 14) [79].

Regarding the dispersion of primary CNT agglomerates a

synergistic effect of both fillers was found. It was clearly indi-

cated the CB improved the MWCNT dispersion and reduced

especially the size of big primary nanotube agglomerates possi-

bly due to viscosity effects and internal friction. Interestingly,

the significant positive effect of CB in dispersing MWCNTs

was not reflected in lower electrical percolation thresholds, as

one would expect. The melting and crystallization behavior

were influenced by the nanofillers as reflected by increases in

FIG. 12. Functionalization the carboxylic acid parts by PAV [77].

FIG. 13. (a) FTIR spectra of the nylon 12 and the FG (0.6, 1, and 3 wt %)/nylon 12 composites. (b,c) Magnified

images for the peak belonging to stretching and bending vibration of ANH group, respectively [58].

10 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

the crystallization temperatures; however this effect was nearly

independent on the kind and the amount of the nanofillers.

Combined use of the CB to MWCNTs did not show additional

effects. Therefore, influences of crystallinity on electrical prop-

erties can be excluded.

The expectations of the synergistic effects of hybrid filler

systems of CB and MWCNTs on the electrical percolation

threshold could not be complied with the used systems. Howev-

er, such systems at high loadings did show higher conductivity

values than what was achievable in systems containing fillers of

only one type.

Due to make a comparison between CNT and graphene per-

formance, Chatterjee et al. [80] investigated the properties of

Polyamide 12 nanocomposites prepared by carbon nanotubes

(CNTs) and graphene nanopellets, via melt compounding poly-

merization. The nanoparticles were dispersed within the poly-

meric matrix using three forms, (1) without modification, (2)

modified through the carboxyl agent, and (3) modified through

surfactant. SEM images showed that modified CNT through sur-

factant had better dispersion, while graphene nanopellets, did

not show an acceptable dispersion, even after modification by

surfactant. Both nanoparticle acted as a nucleating agent and

increased the crystallization percentage. Modified CNT through

surfactant, increased the modulus up to 80%, whereas it was not

obtained for graphene. Generally, the best mechanical and elec-

trical properties related to modified CNT through surfactant.

In 2014, the rheological and electrical properties of compres-

sion and injection-molded PA12/MWNTs composites were

investigated, by Versavaud et al. [81].

Room temperature measurements evidenced a complete loss

of the conductivity through the thickness of the injection-

molded composites, with values close to the neat matrix ones.

High shear stresses break the MWNTs network during the injec-

tion and cooling was obviously too fast to allow later reforma-

tion. Better electric properties were measured through the other

directions, especially in the flow direction. Polarized Raman

spectroscopy revealed a slight orientational anisotropy of

MWNTs but only under the percolation threshold and mostly in

the composites skin, where the shear forces were assumed to

peak. In fact, the injection molding induced an overall macro-

scale elongation of the MWNTs clusters, as observed by optical

microscopy, whereas only a slight orientational anisotropy was

found locally, inside a cluster, by Raman measurements. The

shear rates applied during injection molding affected the micro-

structure of the aggregates. Hence, the final electrical properties

of the injected parts seemed to be mainly dominated by the

arrangement of the aggregates.

In order to compare and characterize the performance of

nylon 11 and 12—graphene nanocomposites, Jin et al. studied

about the melt compounding polymerization of nylon 11 and

12 in 2013 [82]. The results indicated that FG sheets were sig-

nificant effective for improvement of the nylons at very low fil-

ler loadings.TEM observation revealed that the sheets orientated

and dispersed uniformly with layer thickness about 10–30 nm

(Fig. 15) [82]. In particular, the ultimate tensile strength, elonga-

tion at break, fracture toughness and impact failure energy of

the nylon 12 were significantly increased by 35%, 200%, 75%,

and 85% respectively, when only 0.6 wt % FG was incorporat-

ed. However, for the nylon 11 the tensile properties and fracture

toughness were slightly improved compared to the nylon 12 and

a greater increase of 250% in impact strength at 1 wt % FG

loading was achieved. In comparison between two nylon sys-

tems, the elongation to break and fracture toughness of the

nylon 12 were observed to improve more effectively, and the

improvement of impact and barrier resistances of the nylon 11

were greater.

FIG. 14. Schema of cosupporting networked formed by MWCNTs and CB

[79].

FIG. 15. TEM images of nylon 11 (A) and 12 (B)/FG composites with 0.6 wt % FG content, respectively [82].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 11

POLYAMIDE BLEND-BASED NANOCOMPOSITESREINFORCED WITH CARBON NANOFILLERS

Most of the PA blend-based nanocomposites are prepared by

melt compounding, whereas few studies used the combination

of in situ polymerization followed by melt mixing.

Carbon nanofillers-polymer blends have attracted a large

amount of attention recently. Most researchers reported the

CNT-filled immiscible polymer blends usually shows excellent

electrical conductivity due to the selective distribution in one

phase even if the CNT content is very low. Additionally, alter-

ing the morphology of immiscible polymer blends is possible by

adding CNTs [83, 84].

Most of the research regarding CNT-reinforced PA blend-

based nanocomposites is focused on the morphological evolution

and electrical properties.

In 2009, Bose et al. [29] prepared co-continuous blends of

PA6 and ABS containing multiwall carbon nanotubes (MWNT)

using a conical twin-screw microcompounder. The electrical and

rheological percolation thresholds in PA6/ABS blends were 3–4

and 1– 2 wt % MWNT, respectively. A unique reactive modifier

was employed to facilitate the network like structure of the

MWNT and to restrict them in a specific phase. This morpholo-

gy was achieved by establishing specific interactions with the

delocalized “p-electron” clouds of the MWNT and the melt

interfacial reaction during melt mixing. A significant refinement

in the co-continuous structure was observed in the blends in

presence of modified MWNT. TEM investigations revealed a

uniform dispersion and the selective localization of the MWNT

in the PA6 phase of the blends in the presence of Na-AHA. A

similar observation was reported by Zhang et al. [85] for PA6/

PP/MWNTs nanocomposites and Liu et al. [4] for PA6/ABS/

MWNTs nanocomposites. According to Zhang et al. [85], the

MWNTs preferentially located in the PA6 phase, and a small

amount of the MWNTs bridged the PA6 and PP phases.

In 2011, Zonder et al. introduced CNT nanoparticles to a

blend of high density polyethylene and Polyamide 12 [86]. In

this work, three formulations were studied: (1) simultaneously

mixing of three elements, (2) premixing of CNT in Polyamide

12 phase, and (3) premixing of CNT in polyethylene phase.

Compared to PE, PA can better disperse CNT due to its polar

nature, which resulted in electrical and rheological percolation

at around 1.4 wt % CNT whereas PE is almost unaffected by

CNT addition. At a composition of 75/25 PA/PE, CNTs were

driven to the interface when simultaneously mixed with the pol-

ymers and when premixed in the PE phase which produced at

least four orders of magnitude decrease in the electrical resistivi-

ty compared to the PA pre mixing procedure. This result is

attributed to the formation of surface and volume percolating

network where the CNT coated PE domains interconnect

through the interphase and act as centers or junction to connect

the few CNTs that are dispersed throughout the volume of the

PA phase. Interfacial localization of CNT was also shown to

increase the blend’s solid like behavior at low frequencies.

This was explained by increased elasticity due to retardation

of shape relaxation of the PE domains and/or clustering of the

finer PE domains. SEM images (Fig. 16) [86], show that when

CNTs are premixed in the PE phase the final morphology is fin-

er, with smaller PE domains. By correlating rheological and

electrical resistivity measurements, the induced structural

differences were characterized by kinetically controlling particle

location in the blend.

Another work was done in 2012 by Yan et al. in which gra-

phene nanaoparticles was introduced to Polyamide 12/polyethyl-

ene blend [87]. In this research, polyethylene was grafted by

maleic anidride. Three formulations was investigated the same

as previous work. All samples had a same XRD pattern and

there was not any graphene diffraction, which confirms the

good dispersion of nanoparticles. By varying the compounding

sequences of PA12, graphene and POE-g-MA components, the

location of graphene in the ternary nanocomposites was tailored.

The distinct differences in electrical conductivity, storage modu-

lus, and glass transition temperatures for the ternary nanocom-

posites are attributed to the different localization of graphene.

Both high electrical conductivity and storage modulus were

obtained when most graphene sheets were located in PA12

matrix rather than in POE-g-MA phase.

Liu et al. [4] observed a homogeneous and selective disper-

sion of MWNTs in the PA6 phase, and also a significant mor-

phology refinement with reduced sizes of the ABS domains, and

a stabilized interface.

In 2012, Xiang et al. [83] investigated the effect of function-

alized multiwall carbon nanotubes (FMWCNTs) on the phase

morphology of immiscible PA6/HDPE blends. Adding small

amounts of FMWCNTs (<2.0 wt %) did not exert a profound

influence on the sea-island morphology of the nanocomposites.

However, a typical co-continuous morphology was detected

with moderate content of FMWCNTs (2.0 and 5.0 wt %). Fur-

ther increasing the FMWCNT content (10.0 wt %) induced

phase inversion.

In 2013, Madhukar et al. [88] demonstrated that uniform

PMMA dispersion is achieved by the addition of carboxylic

acid-functionalized single walled carbon nanotubes (SWCNTs-

COOH) in PA6/PMMA. The SWCNTs-COOH acted as a com-

patibilizer of PA6/PMMA by inducing hydrogen bonding

between PA6 and PMMA.

POLYAMIDE NANOCOMPOSITES MODELING

From the experimental point of view, it is a great challenge

to characterize the structure and to manipulate the fabrication of

polymer nanocomposites. The development of such materials is

still largely empirical and a finer degree of control of their prop-

erties cannot be achieved so far. Therefore, computer modeling

and simulation will play an ever-increasing role in predicting

and designing material properties, and guiding such experimen-

tal work as synthesis and characterization. For polymer nano-

composites, computer modeling and simulation are especially

useful in addressing the following fundamental issues:

1. The thermodynamics and kinetics of the formation of polymer

nanocomposites.

2. The hierarchical characteristics of the structure and dynamics

of polymer nanocomposites ranging from molecular scale,

microscale to mesoscale and macroscale, in particular, the

molecular structures and dynamics at the interface between

nanoparticles and polymer matrix.

3. The dependence of polymer rheological behavior on the addi-

tion of nanoparticles, which is useful in optimizing processing

conditions; and

12 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

4. The molecular origins of the reinforcement mechanisms of

nanoparticles in polymer nanocomposites [89].

Molecular Scale Methods

The modeling and simulation methods at molecular level

usually employ atoms, molecules or their clusters as the basic

units considered. The most popular methods include molecular

mechanics (MM), MD, and MC simulation. Modeling of poly-

mer nanocomposites at this scale is predominantly directed

toward the thermodynamics and kinetics of the formation,

molecular structure, and interactions.

Molecular Dynamics. MD is a computer simulation technique

that allows one to predict the time evolution of a system of

interacting particles and estimate the relevant physical properties

[90]. Specifically, it generates such information as atomic posi-

tions, velocities, and forces from which the macroscopic proper-

ties can be derived by means of statistical mechanics. MD

simulation usually consists of three constituents: (1) a set of ini-

tial conditions; (2) the interaction potentials to represent the

forces among all the particles; and (3) the evolution of the sys-

tem in time by solving a set of classical Newtonian equations of

motion for all particles in the system. The equation of motion is

generally given by

~Fi tð Þ5mid2~ri

dt2(1)

where ~Fi is the force acting on the ith atom or particle is the

force acting on the ith atom or particle at time t which is

FIG. 16. SEM micrographs of nanocomposites containing 0.75 wt % CNT prepared with different mixing proce-

dures, simultaneously (a,b), PE pre mixed (c,d), PA premixed (e,f) [86].

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 13

obtained as the negative gradient of the interaction potential U,

mi is the atomic mass and ~ri the atomic position A physical sim-

ulation involves the proper selection of interaction potentials,

numerical integration, periodic boundary conditions.

Monte Carlo. MC technique, also called Metropolis method

[91], is a stochastic method that uses random numbers to gener-

ate a sample population of the system from which one can cal-

culate the properties of interest. A MC simulation usually

consists of three typical steps. In the first step, the physical

problem under investigation is translated into an analogous

probabilistic or statistical model. In the second step, the proba-

bilistic model is solved by a numerical stochastic sampling

experiment. In the third step, the obtained data are analyzed by

using statistical methods. MC provides only the information on

equilibrium properties, different from MD which gives nonequi-

librium as well as equilibrium properties.

Microscale Methods

The modeling and simulation at microscale aim to bridge

molecular methods and continuum methods and avoid their

shortcomings. Specifically, in nanoparticle-polymer systems, the

study of structural evolution involves the description of bulk

flow and the interactions between nanoparticle and polymer

components.

Brownian Dynamics. BD simulation is similar to MD simula-

tions [92]. However, it introduces a few new approximations

that allow one to perform simulations on the microsecond time

scale whereas MD simulation is known up to a few nanosec-

onds. In BD, the explicit description of solvent molecules used

in MD is replaced with an implicit continuum solvent descrip-

tion. Besides, the internal motions of molecules are typically

ignored, allowing a much larger time step than that of MD.

Therefore, BD is particularly useful for systems where there is a

large gap of time scale governing the motion of different

components.

The force in the governing Eq. 1 is replaced by a Langevin

equation:

Fi tð Þ5X

j 6¼i

FCij 2cpi1rfi tð Þ (2)

where FCij is the conservative force of particle j acting on parti-

cle i, g, and s are constants depending on the system, pi the

momentum of particle i, and z(t) a Gaussian random noise term.

Dissipative Particle Dynamics. DPD was originally developed

by Hoogerbrugge and Koelman [93]. It can simulate both New-

tonian and non-Newtonian fluids, including polymer melts and

blends, on microscopic length and time scales. Like MD and

BD, DPD is a particle-based method. However, its basic unit is

not a single atom or molecule but a molecular assembly (i.e., a

particle). DPD particles are defined by their mass Mi, position

ri, and momentum pi. The interaction force between two DPD

particles i and j can be described by a sum of conservative FCij ,

dissipative FDij , and random forces FR

ij [94].

Lattice Boltzmann. LB [95] is another microscale method that

is suited for the efficient treatment of polymer solution dynam-

ics. It has recently been used to investigate the phase separation

of binary fluids in the presence of solid particles. The LB meth-

od is originated from lattice gas automaton which is constructed

as a simplified, fictitious molecular dynamic in which space,

time, and particle velocities are all discrete.

The main feature of the LB method is to replace the particle

occupation variables ni (Boolean variables), by single-particle

distribution functions (real variables) and neglect individual par-

ticle motion and particle-particle correlations in the kinetic

equation.

Time-Dependent Ginzburg–Landau Method. TDGL is a micro-

scale method for simulating the structural evolution of phase-

separation in polymer blends and block copolymers. It is based

on the Cahn-Hilliard-Cook (CHC) nonlinear diffusion equation

for a binary blend and falls under the more general phase-field

and reaction-diffusion models [96]. In the TDGL method, a

free-energy function is minimized to simulate a temperature

quench from the miscible region of the phase diagram to the

immiscible region. Glotzer and coworkers have discussed and

applied this method to polymer blends and particle-filled poly-

mer systems [97].

Dynamic DFT Method. Dynamic DFT method is usually used

to model the dynamic behavior of polymer systems and has

been implemented in the software package [98]. The DFT, mod-

els the behavior of polymer fluids by combining Gaussian

mean-field statistics with a TDGL model for the time evolution

of conserved order parameters. However, in contrast to tradition-

al phenomenological free-energy expansion methods employed

in the TDGL approach, the free energy is not truncated at a cer-

tain level, and instead retains the full polymer path integral

numerically.

Mesoscale and Macroscale Methods

The observed macroscopic behavior is usually explained by

ignoring the discrete atomic and molecular structure and assum-

ing that the material is continuously distributed throughout its

volume. Generally speaking, the macroscale methods (or called

continuum methods hereafter) obey the fundamental laws of: (i)

continuity, derived from the conservation of mass; (ii) equilibri-

um, derived from momentum considerations and Newton’s sec-

ond law; (iii) the moment of momentum principle, based on the

model that the time rate of change of angular momentum with

respect to an arbitrary point is equal to the resultant moment;

(iv) conservation of energy, based on the first law of thermody-

namics; and (v) conservation of entropy, based on the second

law of thermodynamics.

Micromechanics. Since the assumption of uniformity in contin-

uum mechanics may not hold at the microscale level, microme-

chanics methods are used to express the continuum quantities

associated with an infinitesimal material element in terms of

structure and properties of the microconstituents.

a. Halpin– Tsai model.

Halpin and Tsai presented a simple analytical equation

14 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

adapted for a variety of reinforcement geometries, including

discontinuous filler reinforcement [99]

E

Em5

11fg/f

12g/f

(3)

where E and Em represent the Young’s modulus of the com-

posite and matrix, respectively, f is a shape parameter depen-

dent upon filler geometry and loading direction, /f is the

volume fraction of filler, and h is given by

g5Ef=Em21

Ef =Em1f(4)

where Ef represents the Young’s modulus of the filler. It

should be noted that as f ! 0 the Halpin-Tsai theory con-

verges to the inverse rule of mixtures (lower bound), i.e.,

1

E5

/f

Ef1

12/f

� �

Em(5)

Conversely, when f ! 1 the theory reduces to the rule of

mixtures (upper bound), i.e.,

E5/f Ef 1 12/f

� �Em (6)

Assumptions like firm bonding of filler and matrix, perfect

alignment of the platelets in the matrix, and uniform shape

and size of the filler particles in the matrix make it very diffi-

cult to correctly predict the nanocomposites properties. The

model has recently been modified to accommodate the effect

of incomplete exfoliation and misorientation of the filler, but

the effect of imperfect adhesion at the surface still needs to

be incorporated [100].

b. Mori-Tanaka model

The Mori-Tanaka average stress theory has also received

considerable attention in the literature [100]. It was derived

on the principles of Eshelby’s inclusion model for predicting

an elastic stress field in and around an ellipsoidal particle in

an infinite matrix [101].

c. Nicolais and Nicodemo

Nicolais and Nicodemo [102] suggested a simple model to

predict the tensile strength of the filled polymers described by

the equation:

r=r1512P1uP2 (7)

where P1 is stress concentration-related constant with a value

of 1.21 for the spherical particles having no adhesion with the

matrix and P2 is geometry-related constant with a value of

0.67 when the sample fails by random failure.

Tandon and Weng [103] suggested that the strain can be

predicted by the simple equation as:

ec=em512u13

f (8)

where ec and em are the yield strains of the composite and

matrix, respectively, and uf is the filler volume fraction. It

was assumed that the polymer breaks at the same elongation

in the filled composite as the bulk unfilled polymer does. It

was assumed that the polymer breaks at the same elongation

in the filled composite as the bulk unfilled polymer does. The

much lower experimental values (Fig. 17) [104] agree with

the lack of adhesion as suggested above and the strain hard-

ening of the confined polymer. It also indicates that the brit-

tleness increased on increasing the filler volume fraction.

Equivalent-Continuum and Self-Similar Approaches. Recently,

two methods have been proposed for modeling the mechanical

behavior of single walled carbon nanotube (SWCN) composites:

equivalent-continuum approach and self-similar approach [105].

The equivalent-continuum approach was proposed by Ode-

gard et al. [106]. In this approach, MD was used to model the

molecular interactions between SWCN–polymer and a homoge-

neous equivalent-continuum reinforcing element (e.g., a SWCN

surrounded by a cylindrical volume of polymer) was constructed

as shown in Fig. 18 [107]. Then, micromechanics are used to

determine the effective bulk properties of the equivalent-

continuum reinforcing element embedded in a continuous

polymer.

Finite Element Method. FEM is a general numerical method

for obtaining approximate solutions in space to initial-value and

boundary-value problems including time-dependent processes. It

employs preprocessed mesh generation, which enables the mod-

el to fully capture the spatial discontinuities of highly inhomo-

geneous materials. It also allows complex, nonlinear tensile

relationships to be incorporated into the analysis. Thus, it has

been widely used in mechanical, biological and geological sys-

tems. In FEM, the entire domain of interest is spatially discre-

tized into an assembly of simply shaped subdomains without

gaps and without overlaps. The subdomains are interconnected

at joints (i.e., nodes) [107].

Electrical conductivity and thermal conductivity are method-

ologically alike in terms of physical transport property, no mat-

ter in the context of a steady state or a transient state. There are

four general approaches to building up of models for effective

thermal conductivity of composites: effective medium

FIG. 17. Relative yield strain of PP nanocomposites plotted as a function

of inorganic volume fraction. The solid lines represent the fitting using the

theoretical equations, whereas the dotted line serves simply as a guide.

Reproduced from Ref. 104.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 15

approximation (EMA), micromechanical analogy, statistical

approach, and various modern numerical methods. Some of

these models are as follows, Parallel and Series Models, Max-

well’s Model (Maxwell-Garnett Equation), Fricke’s Model,

Hamilton-Crosser Model, Hashin’s Model, Nielsen’s Microme-

chanics Model, Equivalent Inclusion Method, Benveniste-Miloh

Model, Davis’ Model, Hasselman-Johnson Model, and Felske’s

Model [108].

To determine the threshold of the electrical conductivity per-

colation, a power law relation is used.

r / m2mcoð Þb0 (9)

where �o is the electrical conductivity, m is the nano mass frac-

tion, mco is the threshold of the electrical conductivity percola-

tion, and b0 is the critical exponent [109].

Some electrical and mechanical modelings have been done

during recent years. In 2003, Fornes and Paul [110] employed

the composite theories of Halpin-Tsai and Mori-Tanaka to better

understand the superior reinforcement observed for well-

exfoliated nanocomposites relative to conventional glass fibers

composites.

The two theories differ in regard to their treatment of filler

geometry; however, they both show analogous responses to how

composite modulus responds to filler aspect ratio, modulus, and

orientation.

In 2006, Dalmas et al. [111] presented a discrete approach to

modeling the electrical properties in 3-D fibrous networks. This

model took into account the intrinsic electrical properties of the

fibers and the geometrical characteristics of the fiber network.

Such a numerical simulation was a useful tool to understand the

link between such fibrous microstructures and their electrical

properties. Important parameters were highlighted such as fiber

tortuosity and fiber-fiber contact conductivity and found to

influence strongly the percolation and the electrical conductivity

in such structures. The model morphological parameters were

then adjusted to account for carbon nanotube-polymer nanocom-

posite materials. A good agreement was found between the sim-

ulated and experimental percolation thresholds. With only one

adjustable parameter, i.e., the contact resistance, the strong

influence of the processing conditions on the composite conduc-

tivity level and on the percolation critical exponent was investi-

gated with this modeling approach.

FIG. 18. Representative volume elements of molecular, equivalent-truss, and equivalent-continuum models [105].

16 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

In 2012, Bao et al. [112], simulated the electrical properties

of bulk nanocomposites with aligned CNTs. Compared with

existing percolation models; their effort was characterized by

two major improvements: (i) it accounted for both contact and

intrinsic resistances, and (ii) it applied a new method to effec-

tively recognize the connective network, where all periodically

connective paths were identified. Simulation predictions demon-

strated the effectiveness of the new approach with regard to

reduction in the size effect, while retaining the desired numeri-

cal accuracy.

With these two improvements applied in their Monte Carlo

simulations, they found that the highest conductivity occurred

when CNTs were partially aligned rather than perfectly or ran-

domly aligned. They also predicted that this optimal orientation

approached the isotropic case with the decrease in the concen-

tration of the CNTs. Results showed good agreement with the

experimental data. This led to believe that this model can be

used as a predictive tool to design the CNT concentration and

orientation to maximize the electrical conductivity in polymer

nanocomposites. This model is not limited to CNTs, but can be

generally applied to a wider range of percolating transport in

networks, nanocomposites and field transistors with 1-D conduc-

tive fillers, such as nanotubes, nanowires, and nanorods.

In 2015, Zare [113] presented a simplified technique for pre-

diction of modulus in the CNTs. The experimentally measured

modulus of many samples was correlated with the various pow-

ers of their CNT volume fractions. It was found that the experi-

mental moduli are well fitted to “u2/3” As a result, a high

potential for prediction of modulus in CNTs was suggested. To

calculate the modulus, one parameter that correlates the modulus

to “u2/3” was determined by experimental characterization of

modulus for only one prepared sample. Accordingly, the modu-

lus of CNTs was simply calculated at a large range of CNT

contents.

Similar procedure can be developed and examined for differ-

ent polymer nanocomposites. As known, the progress of simula-

tion techniques is much crucial to realize the structure-

processing-properties relationship for design and optimization of

advanced materials

COMMON CHALLENGES

Nanocomposite materials hold the potential to redefine the

field of traditional composite materials both in terms of perfor-

mance and potential applications. There is little doubt that poly-

mer nanocomposites have tremendous market potential both as

replacements for current composites and in the creation of new

markets through their outstanding properties. But developing the

processing-manufacturing technologies in terms of quantity and

value for commercialization will be one of the biggest

challenges.

For example, dispersion of nanoparticles or chemical compat-

ibility with matrix materials is the important issue. A homoge-

neous dispersion of nanoparticles in a polymer by using

compounding techniques is very difficult due to the strong ten-

dency of fine particles to agglomerate [114, 115]. At the same

time if it is subjected to force, there is a possibility of splitting

of agglomerate nanoparticle. Therefore, premature failure takes

place in the final product. Degassing is another critical problem

while processing a nanocomposite. The air trapped while

pouring the highly viscous material in the mold, initiates crack

and failure of specimen can take place under low strains [115].

The alignment of nanoparticles in the composite matrix can be

critical to maximize unidirectional properties such as strength,

modulus, and toughness [116]. As in the case for traditional

composites, it is even more challenging to determine the

strength, composition, and functionality of the interfacial region.

The goal of improving the carbon nanofiber matrix interfacial

adhesion issue and complete dispersion must be solved before

achieving the full potential of nanocomposites.

Han et al. [117] looked at the defective structures and prop-

erties of carbon nanotubes. They generated model configurations

of nanotubes. They found SWNTs to be relatively defect-free

whereas, MWNTs typically had more defects, such as topologi-

cal defects and structural defects. To improve dispersion and

compatibility in polymer matrices, nanotube is being functional-

ized. There are still some concerns remaining like whether func-

tionalization of a nanotube will affect the properties to improve

the final product [118]. But Wilkins et al. [118] suggest that in

the case of composites, there might be a limitation of using

these materials in aerospace application as matrix response with

the Scale up is needed to produce large quantities of nanomate-

rials for manufacturing purposes. There is still a lack of real-

time characterization methods, instrumentation, tools, as well as

a lack of affordable infrastructure. To move nanotechnology for-

ward, education is needed for both scientists and engineers in

academics and industry. At the same time, researchers must con-

tinue to prove the disruptive and confusing nature of this

technology.

Molecular dynamics simulation (MD) and theoretical analy-

ses are mainly based on certain assumptions that may not be

practically applied to a real situation in polymeric nanocompo-

sites [119]. The mechanical and dispersion properties and align-

ment of nanotubes are mainly involved in enhancing the

properties of polymeric nanocomposites. However, it is also

hard to achieve this without a good interfacial bonding between

nanoparticles and matrix. Presently, no reports are available to

obtain the best solution for these points. One of the most impor-

tant problems which is existed is that do the nanoparticles still

maintain their extraordinary mechanical, electrical, and thermal

properties if chemical bonding exist between the nanoparticle

and the matrix. To enhance the mechanical properties of

advanced composite materials, many works have to be deeply

investigated and this is definitely a challenging area for all peo-

ple working in the composites community [119].

CONCLUSIONS

As illustrated within this review, carbon nanofillers represent

an interesting method to extend and to improve the properties of

PA to prepare high-performance PA nanocomposites. The prop-

erties of PA nanocomposites are influenced by various factors

such as the compositions, morphologies, interfacial interactions,

nanofillers and processing methods. The superior properties

derived through the combination of PA and nanofiller appear to

be relevant in the development of materials for various applica-

tions. However, there are some challenges of particular note.

Some PA nanocomposites processing, such as, water injection

assisted melt compounding [120] and ultrasound assisted melt

compounding is potential and feasible, in order to overcome

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 17

agglomeration of nanofillers and have a uniform dispersion

within the polymeric matrix. Although the surface chemistry of

the nanofiller has a dramatic influence on their localization in

the PA and hence on the possible compatibilizing role and on

the final properties. In most cases, the localization of nanofiller

is not often accurate and the size and surface chemistry are not

well-controlled parameters; thus, nanofillers with desired surface

properties with controllable localization and dispersion in PA

should be further developed. This is of great importance for

nanoparticle induced morphology control. And finally suitable

surface modification of nanofiller and compatibilization tech-

nique must be selected in order to maximize the properties of

PA nanocomposites. Thus, the compatibilization mechanisms

contribute to a finer morphology of polymer nanocomposites. In

addition, the interaction of compatibilizer and nanofiller is

essential to control the preferential state of the nanofillers in the

selected polymer phase [121].

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