CHAPTER 5

Nanofilled Thermoplastic�ThermoplasticPolymer Blends

Roberto Scaffaro and Luigi BottaDepartment of Civil, Environmental, Aerospace, and Material Engineering, University of Palermo,

Palermo, Italy

Chapter Outline5.1 Introduction 133

5.1.1 General Aspects of Nanofilled Polymer Blends 134

5.2 Interactions in Nanofilled Thermoplastic Polymer Blends 135

5.3 Kinetic Effects on the Morphology of Nanofilled Thermoplastic Polymer Blends 1375.3.1 Mixing Procedures 137

5.3.2 Viscosity 139

5.3.3 Mechanisms of Nanoparticle Migration 141

5.4 Compatibilizing Effect of Nanoparticles in Thermoplastic Polymer Blends 1435.4.1 Morphology Changes 143

5.4.2 Mechanisms of Compatibilization 149

5.4.3 Stability of the Morphology 152

5.5 Mechanical Properties 153

5.6 Conclusion 154

References 155

5.1 Introduction

A thermoplastic is defined as a polymer that softens or melts on heating, and returns to a

solid state on cooling. Multiple cycles of heating and cooling can be repeated allowing

reprocessing and recycling. Thermoplastic polymers are the class of polymers most used in

all industrial applications. Indeed, thermoplastic consumption is roughly 80% of the total

plastic consumption [1]. The most commonly used thermoplastic polymers include

polyethylene (PE) and ethylene copolymers, polypropylene (PP), polystyrene (PS),

poly(vinyl chloride) (PVC), polyamide (PA), polycarbonate (PC), poly(ethylene terephthalate)

(PET), poly(butylene terephthalate) (PBT), and poly(methyl methacrylate) (PMMA).

133Nanostructured Polymer Blends.

DOI: http://dx.doi.org/10.1016/B978-1-4557-3159-6.00005-5

© 2014 Elsevier Inc. All rights reserved.

Polymer blends based on thermoplastic polymers are probably the most diffused ones and

their morphology, properties, and durability have been widely studied [2�6].

Over the last two decades, there has been increasing activity toward using polymer blends

as matrices for nanoparticles either in fundamental research or industrial exploitation

[7�24]. The most used nanoscale fillers include layered silicates (e.g., montmorillonite),

nanotubes (mainly carbon nanotubes), metal oxides (e.g., TiO2, Fe2O3, Al2O3), metal

nanoparticles (e.g., Au, Ag), polyhedral oligomeric silsesquioxane (POSS), carbon

nanomaterials (e.g., graphene, carbon black), silica nanoparticles, and so on [25�35]. These

fillers have opened new perspectives since they vary by their shape factor (spheres,

platelets, fibers), surface energy, and ability to more or less disagglomerate or exfoliate

depending on the mixing conditions and on the surface treatments applied [29].

Concerning the polymer blend nanocomposites, the main subjects are principally related to

two important roles that the nanofillers can play in polymer blends. The first is

improvement of the various properties such as some mechanical, barrier, thermal, flame

retardancy, and electrical properties. The second is the modification of miscibility/

compatibility and morphology of polymer blends.

It is known that the performance and mechanical properties of immiscible polymer blends

strongly depend on their phase morphology and interfacial properties [2�4]. On this basis,

the main objectives of nanoparticle addition to immiscible polymer blends are how the

morphology of the blends is affected by nanoparticles and whether these additives can play

the role of compatibilizer in the blends. It should be noted that mechanism of action of

nanoparticles to modify the morphology, interfacial properties, and performance of

immiscible polymer blends relies on their localization, their interactions with polymer

components, and the way these additives disperse within the polymer blend [19].

The objective of this chapter is to review the research on nanofilled thermoplastic/

thermoplastic polymer blends, paying particular attention both to the distribution of

nanoparticles inside a binary polymer blend and to the effect of nanofillers on the

morphology and on the properties of thermoplastic polymer blends.

5.1.1 General Aspects of Nanofilled Polymer Blends

Recently, Fenouillot et al. [19] reviewed and discussed the distribution of nanoparticles

inside a binary polymer blend, emphasizing the mechanisms of segregation of the solid

particles in relation to the morphological specificities of such blends.

It is worth noting that generally polymers are not used as liquids but they are used as solids

with very low molecular mobility so that demixing of the phases during use cannot happen.

There is a need for stabilization only when the blend is viscous (molten), that is, during the

134 Chapter 5

mixing stage and during eventual further processing. In other words, in polymers, the

arrangement of the filler in one particular phase or at the interface is required either to

obtain the desired property and/or to inhibit coalescence during the time needed for

processing operations, which is generally a few minutes. Molten polymers need to be

stable during periods of time several orders of magnitude shorter than low viscosity fluid

emulsions.

Adding solid particles in polymer blends is a traditional technique in rubber and

thermoplastic processing. About 70% of polymer-based materials contain solid particles,

fillers of different sizes from a few nanometers to a micrometer. Originally, the purpose of

adding particles in elastomer blends was obviously an applicative objective such as

obtaining high electrical conductivity or improving mechanical properties. Since the typical

size of the classical fillers (calcium carbonate, talc, silica) was of the same order of

magnitude or greater than the size of the dispersed polymer phase, these particles were not

found to interfere significantly with the blend morphology. The exceptions were the “old”

nanosized fillers such as carbon black or fumed silica. For instance, works on carbon black

distribution in elastomer blends was reported 40 years ago [36�38]. These works were later

extended to other elastomers and thermoplastic polymers mainly with the aim of

minimizing the proportion of conductive fillers needed to induce electrical conductivity;

[39�54] and Huang [55] reviewed the works on the use of carbon black as a conductive

filler in polymer and polymer blends. Actually, the influence of fillers on blend

morphologies was reported far before the recent works on polymer nanocomposites and the

idea of compatibilizing a blend by using inorganic particles. From a material development

point of view, a requisite is to predict how additives will affect the performances of the

material, and undoubtedly the way these additives distribute inside the material is of crucial

importance. Thus, similar to fluid emulsions, a challenge in formulating polymer blend

nanocomposites is to control the blend morphology, that is, not only the shape and size of

the dispersed polymer domains and their interfacial interactions with the matrix, but also

the state of dispersion and the distribution of the fillers. It requires having identified and

classified the mechanisms involved in the particle movements and localization inside the

material if one aims at obtaining tailored morphologies.

5.2 Interactions in Nanofilled Thermoplastic Polymer Blends

The localization of nanoparticles in polymer blends should be linked to the balance of

interactions between the surface of the particles and the polymer components. As a

consequence, in the great majority of systems, the nanofillers distribute unequally between

the polymer phases. Uneven particle distribution depends on the balance of interfacial

energies and can be predicted by calculating the wetting parameter, ω12, according to

Young’s equation.

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 135

ω125γS22 2 γS21

γ12(5.1)

where γs-i is the interfacial tension between the particle and the polymer i and γ12 is theinterfacial tension between the two polymers. It represents the ability of the particle to be

wetted by polymers 1 and 2. Three cases exist: when ω12. 1 the particles are present only

in polymer 1; for value of ω12, 2 1 they are only found in polymer 2; and for other values

of ω12 (2 1,ω12, 1) the particles are concentrated at the interface between the two

polymers. The third situation corresponds to 9γs222 γs219, γ12 which is more likely to

occur in polymer blends with a high degree of incompatibility or when the differences in

the filler/polymer interactions are small.

Theoretically, the knowledge of the polymer�polymer and of the polymer�filler interfacial

tensions should be sufficient to anticipate the morphology. However, if experimental data

may be found for the polymer/polymer interface it is almost impossible to find it for

polymer/filler. Usually, they are estimated with the help of theoretical models such as the

well-known Owens�Wendt [56], Girifalco�Good [57,58], or Wu equations [59].

Such calculations, from the Owens�Wendt equation, have been performed for PE/PP, PE/

PMMA, and PP/PMMA blends filled with carbon black [60,61]; for poly(ethylene-co-vinyl

acetate) (EVA)/poly(L-lactic acid) (PLA)/carbon black [51]; and for silica particles

dispersed in PP/PS and PP/EVA blends [62,63]. However, it is important to highlight that

these authors calculated the surface tension at room temperature and that the melt surface

tension of polymers can be different from solid surface tension. Actually, the value at high

temperature is the relevant data that should be inserted in the thermodynamic model

(mixing is done at T. 150�C). Elias et al. corrected their data with the help of the

expression proposed by Guggenheim [64] but this is rarely done.

Even where some discrepancies between the predictions of these equations are significant,

the localization of the fillers was correctly predicted in many studies. Nevertheless, the

attempts to predict the preferred arrangement of the fillers inside a polymer blend are

sometimes useless and several studies reveal that the knowledge of the wetting parameter or

the estimation of the interactions are not sufficient to determine where the filler will be

located. Actually, a first problem is the quantitative determination of the surface tension of

the different components (polymers and filler) especially at high temperature. Some strong

discrepancies are reported for the surface tensions. Furthermore, it can be pointed out that

most of these systems are based on commercial products which contained some additives

(stabilizers mainly). These additives, even at low concentration, can considerably alter the

surface tensions of polymers. A second problem is that strong and preferential interactions

can be created at the filler/polymer interface through adsorption of the polymer chains or

their covalent grafting or also macromolecules’ intercalation inbetween clay platelets.

Finally, it must be emphasized that the localization of particles is determined by the

136 Chapter 5

thermodynamics of wetting only provided that thermodynamic equilibrium is attained. This

implies that the processing conditions have to be carefully considered and kinetic effects

induced by mixing procedure and mixing time have to be taken into account.

5.3 Kinetic Effects on the Morphology of Nanofilled ThermoplasticPolymer Blends

Kinetic effects are linked to the rate of the mixing process. When two polymers are blended

with a filler the final equilibrium morphology (shape and size of the polymeric phases), the

state of dispersion of the filler, and its distribution inside the blend are not immediately

attained because of the high viscosity. Several factors can influence the rate of

establishment of such equilibrium.

5.3.1 Mixing Procedures

The order of addition of the components is of importance and can have a strong effect on

the kinetics and intensity of mixing because it has a direct influence on the medium with

which the filler will be in contact during the course of its incorporation. By “medium” we

mean the nature of the polymer in contact and its state of melting. The simplest procedure

and the most reported in the literature is the addition of the components all together in the

mixer. They are blended at a temperature high enough to ensure that the polymers will

transform into viscous fluids, however a stage where the materials are solid and then

progressively melted precedes the actual melt blending. The process is complex, involving

mixtures of solids and viscous fluids and the simultaneous evolution of the morphology of

the polymer blend together with the dispersion and migration of the particles inside the

molten material. With this first mixing procedure, if one polymer melts at a temperature

significantly lower than the other (melts first), the solid particles may be incorporated into

it preferentially, although the said polymer does not have the better affinity. Since the

obtained initial distribution will not be that corresponding to the thermodynamic

equilibrium, different scenarios are then possible where the filler will have or not have the

opportunity to migrate to the preferred phase or to the interface. A second alternative

consists of melting the two polymers and afterward adding the filler so that the particles do

not see any solid medium. Thirdly, it is possible to incorporate the filler into the first

polymer and then introduce the second polymer. Depending on these sequences of addition

of the components, the filler may also have to transfer from one phase to the other to reach

its equilibrium distribution and this involves particle displacement inside the blend. The

easiest way to highlight the existence of particle movement inside a blend is to incorporate

the solid particles in the polymer having the lower affinity and then to add the higher

affinity polymer.

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 137

Elias et al. selected a PP/PS/silica blend and a sequence of addition where the silica was

first mixed with PP and this composite was then mixed with PS [62]. They observed that all

the hydrophilic silica transfers from the PP with which it has lower affinity toward the PS

preferred phase where it concentrated. A few minutes of mixing were sufficient for the

transfer to occur.

Another example is presented in a series of papers by Gubbels et al. [41�42,65] which

introduced carbon black in polystyrene/polyethylene to obtain electrical conductivity. These

studies, although not the most recent, illustrate well some of the factors involved in the

distribution of nanofillers in polymer blends and how they can influence the material

properties. The authors first emphasize that the localization of the filler has a tremendous

influence on the value of the percolation threshold needed to observe significant

conductivity of the material. They compare four situations where the filler is dispersed: (1)

in an amorphous polymer (PS), (2) in a semicrystalline polymer (PE), (3) in a PS/PE 45/55

blend where the carbon is confined inside the PE phase, and (4) in the same PS/PE blend

but with the carbon localized at the interface. The second situation is better than the first

thanks to a segregation of the carbon black during the crystallization of the PE. Situation 3

is again better because a double percolation is created: percolation of the PE phase by

adequately tuning the proportion of the phases and percolation of the carbon black inside

the PE is observed at the concentration close to 3 wt%. Note that the percolation threshold

of carbon black homogeneously dispersed in a polymer matrix is close to 20 wt%.

As proved by transmission electron microscopy (TEM) images, the carbon particles are

confined at the interface in a two-dimensional space. Note that this concept of multiple

percolation was first introduced by Sumita et al. [66] and Levon et al. [67]. Such

cocontinuous system with the carbon at the interface can be produced by compression

molding PE and PS powder together with the filler. More interestingly, however, is that it

can be produced by taking advantage of the possible migration of filler from the less

interacting phase (PS) toward the more favorably interacting one (PE). In practice, carbon

black has been first mixed with the molten PS and then PE is added. Here, kinetic effects

are illustrated clearly: as the mixing time increases, the carbon particles will show the

tendency to transfer from one phase to the other by crossing the interface. The resistivity

passes through a minimum when the particles have accumulated at the interface, then

resistivity progressively increases when the carbon leaves the interface to concentrate in the

PE phase. If mixing is stopped at the right time, the particles will remain at the interface on

cooling the blend and the morphology is then quenched at a nonequilibrium state.

To summarize, the approach described consists of driving particles to the interface by

transferring them from one phase to the preferentially interacting one and taking advantage

of the temporary state where they are blocked and accumulate at the interface (kinetic

control). On the other hand, if the surface affinity of the particles is balanced with that of

138 Chapter 5

the two polymers by modifying chemically the filler surface, they will thermodynamically

be stabilized at the interface regardless of the mixing time (thermodynamic control) [42].

Thus, if a specific particle localization is desired, expensive filler surface treatments may be

avoided by taking advantage of the kinetic control of the morphology. It can be pointed out

that carbon black is a very interesting filler to study since the electrical properties of the

blends will provide information on its morphology (percolation of the phases) and also on

the position of the carbon particles and their degree of agglomeration. Also, the surface of

carbon black may be easily treated to modify its polarity. Nevertheless, one aspect

complicates the situation: the state of agglomeration of carbon black and thus the

percolation threshold may be altered depending on the nature and viscosity of the polymer

phase in which it is dispersing. As a consequence it is not always straightforward to draw

conclusions by considering only the electrical conductivity measurements.

Carbon nanotubes have also been considered to be attractive fillers for improving the

electrical conductivity of polymeric blend materials. The concept of double percolation has

been tested on several types of blends. The filler was found to concentrate inside the more

polar phase, polycarbonate [68,69], poly(ethylene terephthalate) [70], or polyamide [71].

5.3.2 Viscosity

In a viscous medium like polymer melts where the flow is laminar, the kinetic effects are

directly related to the shear viscosity of the phases; nevertheless its influence on the

particles localization is rarely studied systematically. Gubbels et al. [42] proposed that the

transfer of filler from one phase to the other is slower when the particles are originally

confined in the more viscous phase, but in their system the difference in behavior may also

be attributed to some difference in the thermodynamic interactions, so it is difficult to

conclude. In the study of Persson and Bertilsson, polyethylene/polyisobutylene (PE/PIB)

blends with different viscosity grades were filled with aluminum borate whiskers with a

complex blending sequence [72]. Although the whiskers used were not nanometric this

work is interesting because it comments on the relative importance of interaction strength

and viscosity effects. The whiskers had high energy surface while the PE/PIB interfacial

tension was low. The interaction difference was rather small, although PE should interact

slightly more with the filler than the PIB. Their first observation was that the whiskers

accumulated in the more viscous phase because the blend organized itself to minimize its

dissipative energy during mixing. They support their hypothesis by estimating theoretically

the viscosities of the filled blends, making the assumption of filler repartition in one

polymer or the other. Finally, they compared the PE/PIB blend with a polyamide/poly

(styrene-co-acrylonitrile) (PA/SAN) and observed that all the whisker particles resided in the

PA phase, although it was less viscous. This result was contradictory to their conclusions on

the effect of viscosity. They hypothesized that viscous distribution effects are weak and

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 139

dominate only when the difference of interactions between polymer A/filler and polymer B/

filler is small. By contrast when one of the polymers interacts much more favorably than the

other with the filler (PA in the PA/SAN blend) the thermodynamic interactions will

dominate the viscosity effect. These comments illustrate the difficulty encountered when

decoupling the effects of thermodynamic wetting from kinetic effects. In that sense, the work

of Feng et al. with PP/PMMA/carbon black blends is particularly interesting because the

authors use three types of PMMA with different molecular weights while PP does not

change [73]. Accordingly, the effect of the viscosity ratio was investigated. The calculation

of the wetting parameter predicted that carbon particles should be dispersed in the PMMA

phase. The three components were added at the same time in the mixer and PMMA was the

minor phase. The TEM observations demonstrated that the confinement of carbon black into

the PMMA phase was attained only when the blend with viscosity ratio was close to 1.

In the situations where PMMA is more viscous than the matrix, the particles remain at the

interface (for the medium viscosity PMMA) or in the PP phase (for the highest viscosity

PMMA). The results from Feng et al. disagree with those of Persson and Bertilsson, but

agree with Clarke et al. [74]. More recently, the work of Elias et al. concerns two types of

fumed silica with different hydrophilicities in PP/EVA 80/20 wt% blends [64]. The

viscosity of the EVA samples was varied with a very fluid EVA and a grade with a

viscosity close to that of PP. The morphologies obtained are depicted in Figure 5.1.

Figure 5.1TEM images of PP/EVA/silica. (a) PP/EVA-1/hydrophilic silica, (b) PP/EVA-2/hydrophilic silica, (c)

PP/EVA-1/hydrophobic silica, (d) PP/EVA-2/hydrophobic silica. Source: Reprinted from [64],Copyright (2008), with permission from John Wiley and Sons.

140 Chapter 5

The wetting parameters predicted that the hydrophobic silica should localize at the interface

while the hydrophilic silica should distribute in the EVA.

Indeed, Figure 5.1(a) and (b) show that the hydrophilic silica particles distribute in the EVA

regardless of the EVA viscosity. On the other hand, hydrophobic silica reaches completely

the interface only in the case of the low viscosity EVA (Figure 5.1(c) and (d)). It is

necessary to consider that the three components are added at the same time in the extruder

and that EVA is melted before PP (70�C compared to 165�C for PP) so that the silica is

probably incorporated in EVA at the early stages of the mixing process. Yet, hydrophilic

particles simply remain in their preferred phase. Besides, hydrophobic particles have to

move from the inside of the EVA domains toward their surface to attain their equilibrium

position and this migration is found to be easier when the EVA domains are less viscous.

The viscosity effect was also reported by Clarke et al. [74] and Zhou et al. [75] for carbon

black particles. They showed that under normal processing conditions carbon black particles

tend to be preferentially incorporated into the lower viscosity polymer. They also finally

argued that the interfacial energy of the particle�polymer can be considered as the

dominant factor only when the viscosity ratio of both polymer phases is nearly 1.

These results illustrate the fact that viscosity can play a dominant role but descriptive

interpretations are difficult to propose. Several reasons may be put forward. Actually, it is a

real issue to disconnect viscosity effects from other influent phenomena such as

thermodynamic interactions and kinetic effects. Mixing is a dynamic process during which

two polymers are molten (sometimes at different temperatures), the morphology evolves,

and, in addition, fillers are displacing and dispersing in the respective phases and at the

interface. The local and temporal variations of viscosity are huge, rendering the analysis

almost impossible unless model conditions and blends are selected. This picture of the

problem is complicated by the fact that the observations of the blend morphology should

ideally be made all along the mixing process and not only after a given time. To be even

more particular, one must not forget that the state of agglomeration (or exfoliation) of the

filler is an additional (and evolutive) parameter that should also be considered and

quantified since it can alter drastically the viscosity of the phases and the mobility of the

blend interface.

5.3.3 Mechanisms of Nanoparticle Migration

Though the role of interfacial interactions on particles’ localization is abundantly discussed

in the literature, the ways by which nanoparticles transfer from one phase to the other, pass

the interface, or, more generally, move inside the blend at the molten state are almost never

mentioned. In other words, the experimental observations evidence the migration of

particles in polymer blends but the fundamental processes by which this migration occurs

are not discussed.

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 141

Elias et al. listed and discussed at least qualitatively these mechanisms [64]. The uneven

distribution of particles and their migration from one phase to the other implies first that the

particle approaches the interface. Three mechanisms can be involved:

1. Brownian motion of the particles (self-diffusion of the particles): If we assume that the

characteristic size of diffusing particle aggregates is about 100 nm, we find that the

time tD for a particle to diffuse on a distance equal to its radius is about 7500 s for

η5 2.5 � 103 Pa s (PP viscosity) and T5 473 K. This rough calculation shows that the

order of magnitude of the motion time is very large and consequently not compatible

with the mixing time, which is equal to a few minutes. Generally speaking, the high

viscosity of the molten polymer impedes the particle movement by Brownian motion.

Thus, the migration of particles cannot occur under static conditions in molten polymer.

By contrast, the time of diffusion in liquid emulsion is of the order of magnitude of

milliseconds. The time of diffusion from one phase to the interface (a distance of a few

μm) will be just a few seconds.

2. Particle movement induced by the shear: Actually, the inorganic particles and the

droplets of the dispersed phase are moving into the matrix so that numerous collisions

between particles and drops occur. These collisions may or may not end up with the

embedding of a particle into the dispersed domains. Nevertheless, it must be noticed

that solid particles are very small and rigid entities so that the drainage of the polymer

matrix film trapped between the drop and the particle should be greatly facilitated

compared to that involved when two dispersed polymer drops are colliding [76] or

when a large particle and a drop are colliding [77]. Thus, this contribution is most

likely to be a dominant one. A second situation exists where the filler is embedded in

the drops and tends to move toward the matrix. It is almost never discussed in the

literature. The migration of particles from the matrix to the drops is more easily intuited

but evidence exists that it is not the only possible migration type. The experiments of

Elias et al. demonstrated that the transfer of a silica particle from a drop to the matrix

was possible and proceeded rapidly [64]. Austin and Kontopoulou observed that

nanoclay migrates from the PP drops toward the maleated poly(ethylene-co-propylene)

matrix [78]. These features show that the velocity field inside the drop causes the solid

particles to move, possibly cross the interface, and eventually be transferred to the

matrix by a mechanism very similar to the particle�drop collision.

3. Particles trapped in the interdroplet zone during a collision between two dispersed

polymer drops: In this case, the coalescence of polymer droplets is playing a role in the

transfer of the solid from one phase to the other. It is difficult to know whether this

mechanism is significant or not since to our knowledge there is no literature on such a

phenomenon. Its efficiency should depend a lot on the size and deformability of the

drops. Highly deformable drops are probably able to trap the solid particles in the

interface more easily. Nevertheless, in polymeric emulsions the frequency of collision

142 Chapter 5

of polymer drops is very high so the contribution of this mechanism must be kept

in mind.

These three mechanisms are related to particle movements inside the molten heterogeneous

medium. Surface effects must also be considered. When the particles transfer from one

phase to the other, they have to cross the interface, which implies that the macromolecules

adsorbed on the filler surface must desorb progressively to be replaced by the other polymer

(the one with the better affinity). If the energy barrier for desorption is high, this process

may not be immediate, so that particles will reside at the interface for a certain period of

time [52]. The competitive adsorption of polymer blend melts on solid particles is rarely

studied in contrast with polymer solutions but must be considered when trying to present a

global description of the process because this step may possibly be the one limiting the

transfer [79�81]. This aspect is complicated by the possible modification of the surface

tension of the polymer at a few nanometers length scale by the presence of a high-energy

substrate (here the filler) [82]. Finally, although this point is also never discussed in the

literature, we can imagine that during mixing, high shear forces are able to shift the

adsorption/desorption equilibrium by extracting the particles from the interface.

5.4 Compatibilizing Effect of Nanoparticles in ThermoplasticPolymer Blends

It is well known that the compatibilization of immiscible polymer blends is most often

achieved by adding block copolymers (or by in-situ synthesis of this copolymer) that play a

role similar to emulsifiers in liquid emulsions [3]. The localization of the copolymer at the

interface inhibits coalescence and results in a fine morphology that remains stable in spite

of a further processing step. It is important that interfacial adhesion is enhanced, enabling

good ultimate mechanical properties that, by contrast, are known to be particularly bad in

noncompatibilized blends. What is expected from an efficient compatibilization is the

reduction of the characteristic size of the polymer domains, their stabilization against

processing or annealing, and good mechanical properties.

5.4.1 Morphology Changes

The information found in the literature on the effect of the nanofiller on morphology is

clear: a decrease in size of the dispersed polymer domains is reported for thermoplastic/

thermoplastic polymer blends. Coarsening is sometimes observed, for instance by

Gahleitner et al., but difficult to interpret since the PA/PP blend of their study also

contained functional polymeric compatibilizers and mixing sequences were complex [83].

From a historical point of view, the compatibilization of immiscible blends by nanofiller

was first reported for carbon black particles dispersed in elastomers [37,38]. Then, Gubbels

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 143

et al. [42,65] showed that carbon black particles enlarge the composition range in which

cocontinuity was obtained. If the amount of carbon black is less than 2 wt%, the

morphology coarsens significantly but remains cocontinuous. Above 2 wt%, for instance at

5 wt% of carbon black, the morphology is stabilized. From a mechanism point of view, the

increase of the viscosity of the PE phase containing fillers is thought to be responsible for

the inhibition of the coalescence. Clarke et al. [74] showed that carbon black has a powerful

compatibilizing effect when the particles are present at the interface between natural rubber

and nitrile butadiene rubber phases. Regarding silica particles, Liu and Kontopoulou [84]

showed that the addition of nanosilica particles into a PP/poly(ethylene-co-octene) blend

decreased the size of the dispersed elastomer phase. Elias et al. [62] and Zhang et al. [85]

also observed for PP/PS blends a drastic reduction of the size of the PS phase

corresponding to a concentration of hydrophobic fumed silica at the interface (Figure 5.2).

With the help of viscoelastic analysis, the authors concluded that the stabilization

mechanism of PP/PS blend by hydrophilic silica is the reduction of the effective interfacial

tension, whereas hydrophobic silica acts as a rigid layer preventing the coalescence of PS

droplets. Zhang noticed that after passing an optimum morphology, increasing the mixing

time results in a late coarsening of the dispersed PS domains accompanied by a redispersion

of the silica into the PP matrix rather than at the interface. The compatibilization is thought

to be kinetically controlled in that case. However, the relatively long mixing time (20 min)

is also able to degrade the PP and subsequently modify the viscosity ratio of the blend so

that it is difficult to draw unambiguous conclusions from their results.

Laoutid et al. [86] investigated the effect of hydrophilic and hydrophobic nanosilica on the

morphological properties of PA6/PP blends prepared by extrusion compounding. They

found that the filler localization changes with the nature of silica nanoparticles and it

Figure 5.2Morphology of polypropylene/polystyrene 70/30 wt%. (a) PP/PS blend without silica. (b) Blendfilled with 3 wt% of hydrophobic silica. (c) Zoom of image (b) showing silica particles in the PP

phase and concentrated at the interface. Source: Reprinted from [62], Copyright (2007), with permissionfrom Elsevier.

144 Chapter 5

greatly affects the morphology and thus the properties of the blends. Depending on the

difference between the polymer/nanoparticle interfacial energy, different morphologies were

obtained. Hydrophilic nanosilica stays preferentially within the PA phase, whereas

hydrophobic nanosilica will mainly go to the PA/PP interface. More specifically,

morphology results showed that the equilibrium state of PA/PP blends filled by

hydrophobic nanosilica is mainly governed by thermodynamic criteria. Indeed, in the frame

of this study, it was observed that increasing extrusion time does not modify the blend

morphology, which remains stable. Nevertheless, to achieve a good morphology control, at

least 5 wt% of hydrophobic silica is needed. The mixing procedure also appears to be a key

parameter that contributes to the morphology formation. It was observed that the interfacial

confinement of hydrophobic silica nanoparticles prevents/refrains dispersed phase domains

from coalescence and we see an important size reduction of the dispersed PP phase

domains, as schematically shown in Figure 5.3.

Recent studies [9,18,23,78,83,87�100] have been reported on layered silicates acting as

compatibilizing agents in immiscible polymer blends. The refinement of the morphology

may be sometimes very drastic. For example, Si et al. and Ray et al., on PC/SAN and PC/

PMMA blends, respectively, showed that at the concentration of organoclay, around 5 wt%

of the blends normally immiscible depict the characteristics of a “miscible” blend with only

one alpha relaxation peak [9,92�94]. Furthermore, both polymer phases are hardly

distinguished in the TEM pictures. However, it is difficult to ascertain whether the blend is

miscible at the molecular scale; more likely the small size of the phases and their

immobilization by the strong interactions with the solid are the cause of this apparent

miscibility. Also, in the case of PC/PMMA blends, transesterification reaction between PC

and PMMA chains can be catalyzed by the clay as this aluminosilicate can produce Lewis

or Bronsted sites at high temperatures.

Not only the size but also the shape of the dispersed polymer drops can be altered, especially

in the presence of high shape factor particles. Si et al. observed that PMMA preferentially

Figure 5.3Graphical representation of hydrophobic silica nanoparticles’ action against coalescence in PA/PPimmiscible polymer blend. Source: Reprinted from [86], Copyright (2013), with permission from John

Wiley and Sons.

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 145

adsorbs at the clay surface so that the particles are dispersed in the PMMA phase and at the

interface of the PS/PMMA blends [94]. The size of the PMMA domains is very small

(,1 mm) and, interestingly, the PMMA domains do not remain rounded like in the pure

polymer blend (Figure 5.4). The clay platelets design the contour of the drops and they

impose the interfacial curvature. The authors hypothesize that clay has to bend to hug the

surface of the drop. Thus, it exists a minimum size attainable for the dispersed polymer and

it is related to the rigidity modulus of the platelets. Si et al. estimate the minimum radius of

such drops surrounded by clay layers by expressing the free energy of the system corrected

by a factor that accounts for the bending energy of the platelets located at the interface [94].

Filippone et al. [101] showed that the effect of organomodified clay on the morphology

HDPE/polyamide-6 (PA6) blends depends on the blend composition and on the clay loading,

as schematically shown in Figure 5.5. Indeed, they investigate the gradual changes of the

microstructure of two blends of high-density polyethylene (HDPE) and polyamide 6 (PA6) at

opposite composition filled with increasing amounts of an organomodified clay. As the filler

tends to enrich preferentially the more hydrophilic polyamide phase, different effects on the

microstructure of the blends were noticed depending on whether the host PA6 represents the

major or the minor blend constituent. In the former case, an abrupt reduction of the average

size of the dispersed polyethylene inclusions was noticed even for low filler loadings. This

finding was mainly ascribed to the inhibition of coalescence ensured by the platelet-like

structure of the organoclay stacks, which act as a physical barrier that hinders the merging of

colliding droplets during the melt mixing. When the filler is confined inside the minor

constituent of the blend, two situations have been observed: (1) at low filler contents, the

organoclay causes a gradual refinement of the morphology, which, remains globular, and

(2) for filler loading higher than a critical threshold, the filled polyamide assembles into a

highly continuous structure finely interpenetrated with the major polyethylene phase. The

filler content at which such a morphological transition occurs corresponds to that at which a

rheological transition from a liquid- to gel-like behavior takes place in the filled polyamide,

Figure 5.4TEM images of PS/PMMA blends annealed at 190�C for 14 h. (a) PS/PMMA (70/30). (b) PS/

PMMA (63/27) filled with 10 wt% of organoclay. (c) Zoom of (b). Source: Reprinted with permissionfrom [94], Copyright (2006) American Chemical Society.

146 Chapter 5

suggesting a simplified mechanism according to which the filler seems to play a major role

in driving the spatial arrangement of the wetting polymer.

Some recent works have focused attention on the effects of carbon nanotubes (CNTs) on

altering the microstructure of polymer blends [102�105]. Thanks to their high aspect ratio

and conductive properties, CNTs can generate conduction paths, usually referred to as a

percolation network, inside an insulating matrix at very small concentration. The

localization of CNTs in immiscible polymer blends can reduce the percolation threshold

versus the one found in single-phase polymers. The localization can be either in a specific

phase or confined at the interface between the two polymers. The former is known as the

double percolation system.

Yan and Yang [105] found that because of the poor wettability of multiwalled carbon

nanotubes (MWCNTs) by PA6 and PS, MWCNTs were selectively located at the interface

of PA6 and PS in PA6/PS/MWCNTs polymer blends. The interface-localized MWCNTs

could act as compatibilizers, which resulted in the decreased size of PS domains in PA6

Figure 5.5Schematic illustration of the evolutions of the microstructure in the blends with polyamide as theminor (a�c) or major (d�f) phase as a result of the addition of organoclay. Source: Reprinted from

[101], Copyright (2010), with permission from Elsevier.

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 147

matrix and the delayed phase inversion of immiscible PA6/PS blends. The PA6/PS interface

was continuous in the MWCNTs filled (70/30) PA6/PS blends. Selectively located

MWCNTs at the continuous interface were connected with each other, which resulted in the

establishment of a MWCNT conductive pathway in (70/30) PA6/PS blends.

Zou at al. [102] investigated the possibility of manipulating the phase morphology of poly

(p-phenylene sulfide)/polyamide-6,6 (PPS/PA66) blends using MWCNTs. In particular, they

found that by adding a small amount of MWCNTs into blends, the morphology changes

from sea-island to cocontinuous structure. As the MWCNT content was increased, the

morphology came back to sea-island but with increased domain size (Figure 5.6).

Figure 5.6SEM micrographs of PPS/PA66 (60/40 w/w) blends with varying MWCNTs content. (a) Without

MWCNTs, (b) with 0.001 phr MWCNTs, (c) with 0.01 phr MWCNTs, (d) with 0.05 phr MWCNTs,(e) with 0.1 phr MWCNTs, (f) with 0.3 phr MWCNTs, (g) with 0.5 phr MWCNTs, and (h) with

1 phr MWCNTs. Source: Reprinted from [102], Copyright (2006), with permission from Elsevier.

148 Chapter 5

Moreover, the MWCNTs were found to be selectively located in the PA66 phase, and their

assembling determines the final morphology of PPS/PA66 blends. A dendritic-contacted

MWCNT network was formed at low load, which leads to the formation of a cocontinuous

structure, and isolated MWCNT aggregates were observed at high load, which leads to the

formation of sea-island morphology, as schematically reported in Figure 5.7.

5.4.2 Mechanisms of Compatibilization

Undoubtedly, numerous experimental works evidence the compatibilizing effect of

nanofillers on binary polymer blends; nevertheless several interpretations are proposed. The

work of Ray et al. [9,92�93] summarizes well the questions that arise when trying to

identify the mechanisms involved in the refinement of the morphology by nanofillers.

Actually, several phenomena can lead to morphology changes: (1) a reduction of the

interfacial energy, (2) the inhibition of coalescence by the presence of a solid barrier around

the minor polymer drops, (3) the changes of the viscosity of the phases due to the uneven

distribution of the filler, (4) the immobilization of the dispersed drops (or of the matrix) by

the creation of a physical network of particles when the concentration of solid is above the

percolation threshold, and (5) the strong interaction of polymer chains onto the solid

particles inducing steric hindrance.

The discrimination and classification of these potential mechanisms are very difficult due to

the lack of models and experimental works with the objective of separating the influential

parameters (thermodynamic effects, kinetic effects, particle localization, transfer of

particles). For instance, most of the conclusions on the impact of the distribution of the filler

Figure 5.7Schematic representation of the PPS/PA66 (60/40 w/w)/MWCNTs nanocomposites morphologychanging. (a) Low load MWCNTs (,0.5 phr) disperse evenly and form a percolating network. (b)High load MWCNTs (.0.5 phr) aggregate like clews. Source: Reprinted from [102], Copyright (2006),

with permission from Elsevier.

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 149

inside the biphasic material are drawn from the observation of the final morphology and do

not necessarily account for the displacements of the particles during the course of mixing.

A second illustration of the difficulties is linked to the viscosity evolution of the phases;

this may be very complex since it is related to the local filler concentration and state of

agglomeration or exfoliation (which can be time dependent). Despite these complications it

is nevertheless possible to extract basic knowledge from the diversity of the publications.

The reduction of the interfacial tension due to the distribution of the filler at the polymer/

polymer interface is often cited as a potential explanation for the compatibilization

[9,62,63,106]. The interfacial tension change is sometimes calculated with the help of

rheology. Actually, the interface between the two polymers can be seen as a more

complicated interfacial zone with filler/polymer 1, filler/polymer 2, and polymer 1/polymer

2 interfaces. The modification of the interfacial tension affects the breakup/coalescence

equilibrium in favor of the breakup and should lead to smaller drops. In addition, the filler

can form a rigid shell around the polymer drop, strongly modifying its deformation ability.

This is observed in low viscosity emulsions when the total coverage of the interface by

interacting solid particles is achieved.

As a second mechanism, many authors agree to assert that the definitive or temporary

localization of the nanofiller at the interface of a blend is one of the requisite mechanisms

to ensure a reduction of the size of the minor polymer phase. The similarity with liquid

emulsions is again highlighted as the particles accumulated at the interface build a solid

barrier preventing the fusion of the drops. A schematic description of the mechanism is

given in Figure 5.8(a1) and an example of the possible corresponding morphology is

depicted in Figure 5.8(a2). The TEM image from the study of Hong et al. [18] on PBT/PE/

clay has been selected to illustrate that mechanism because the embedding of the PE drops

by clay platelets appears well but other publications highlight the same phenomenon with

other fillers [9,90].

In situations where a polar polymer such as polyamide (PA), polyester, or maleated olefins

is employed for the matrix, the filler distributes in that phase due to favorable

polymer�particle interaction. An important reduction in the drop size is, nevertheless,

sometimes reported [23,78,89,95,107]. If the filler has a high aspect ratio (clay for instance)

it may be trapped in the matrix film between two colliding drops and slow down

coalescence. This phenomenon is illustrated in Figure 5.8(b1) and (b2) with an example of

the morphology obtained for a PA6/poly(ethylene-co-propylene) (EPR) where it appears

that two drops will hardly fuse [95]. On the other hand, with a lower aspect ratio filler, Liu

et al. pointed out that silica was distributed exclusively in the PP/PP-g-MA major phase of

their PP/PP-g-MA/poly(ethylene-co-octene) (POE) blend and only a slight improvement of

the morphology was pointed out [84]. In that case, an increase of the viscosity of the matrix

may be the cause of the morphology change since silica does not constitute sufficient

150 Chapter 5

blockage. With the same type of blend but filled with organically modified

montmorillonite, Lee et al. obtained an impressive reduction of the size of the POE phase

[107]. Note that the montmorillonite was supposed to distribute into the PP/PP-g-MA

matrix phase despite no clear evidence of this point. Thus, it seems that the presence of

particles inside the matrix may inhibit coalescence provided that no other factor dominates.

When rheological constrains are encountered, coalescence is not inhibited. Wu et al. [108]

showed that the presence of clay does not prevent the coalescence of the polyphenylene

sulfide (PPS) phase in PPS/PBT blend with the clay mainly located in the continuous PBT

phase. The elasticity and viscosity ratio, which considerably changes with the preferential

location of clay in PBT phase, combined with shear history actually controls the

morphological evolution. Coalescence should not be the only factor considered. The

accumulation of solid particles inside the polymer drops may cause a shift in the breakup/

coalescence equilibrium. The viscosity of the drop may rise to a point where breakup is

inhibited or even suppressed if the concentration of particles form a rigid network that

immobilizes it. In a PE/PBT blend in which PBT is the minor phase and the proportion of

clay is increased, the platelets show the tendency to reside in the PBT phase. Hong et al.

Figure 5.8Two of the possible mechanisms explaining coalescence inhibition in polymer blends containingnanoclay. (a1) Barrier of particles concentrated at the interface and (a2) actual example of thecorresponding possible type of morphology. (b1) Particles confined in the matrix acting asobstacles to coalescence and (b2) actual example of the corresponding possible type ofmorphology. Source: Reprinted from [19], Copyright (2009), with permission from Elsevier.

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 151

observed that the size of the dispersed domain increases [18]. They hypothesized that the

viscosity rise of the drops is such that the breakup process is made difficult.

Finally, a mechanism of coalescence inhibition by steric hindrance may be imagined with

fillers provided that the macromolecules are strongly adsorbed to the filler surface.

Figure 5.9 illustrates how the interface between two polymers may be stabilized by clay

platelets depending on the strength of interaction [100]. In the situation depicted in

Figure 5.9(a) the silicate is the coupling species between the polymers, and a layer of

polymer A/silicate/polymer B is present at the interface playing a role similar to a block

copolymer. Although very schematic, this diagram has the advantage of visualizing

different cases; experimental evidence of the type and amount of coupling is very difficult

to obtain.

5.4.3 Stability of the Morphology

The stability of the morphology is another important aspect in the development of new

materials. Thermoplastic polymer blends are produced by twin-screw extrusion which is an

efficient mixing process. Nevertheless, the stability of the morphology is required to ensure

constant properties that will not be deteriorated after a second extrusion or injection

molding step. This stability is not studied very often. Most of the time, the experiment

consists of annealing the samples and observing the morphology after several hours at high

temperature. The example of the PE/PS 45/55 cocontinuous blends of Gubbels et al. can be

again cited [41]. The effect of annealing the materials by compression molding at different

times and temperatures is emphasized. If the amount of carbon black is less than 2 wt%, the

morphology coarsens significantly but remains cocontinuous. Above 2 wt%, for instance at

5 wt% of carbon black, the morphology is stabilized. When the conductive particles are

concentrated at the interface, an additional effect of the annealing time is pointed out [42].

Annealing results in a decrease of the percolation threshold. These data can be explained

considering that in the molten state and in the absence of shear the interfacial area of the

blend spontaneously decreases, that is, the tortuosity of the PE phase decreases. Since the

Figure 5.9Schematics of the compatibilization mechanism of layered silicates. (a) Both polymer A and Bhave strong interaction with silicates. (b) Neither polymer A nor polymer B has interaction withsilicates. (c) Polymer A has a strong interaction with silicates but polymer B has none. Source:

Reprinted from [100], Copyright (2007), with permission from John Wiley and Sons.

152 Chapter 5

particles do not leave the interface, the consequence is a local increase of the particle

concentration, which is in favor of a further decrease of the percolation threshold. Si et al.

demonstrated that the stability against annealing was greatly enhanced in the presence of

organoclay for PS/PMMA and PC/SAN blends [94].

5.5 Mechanical Properties

The other critical point in the compatibilization of immiscible polymer blends is the

improvement of the ultimate mechanical properties that is evidenced when the

compatibilizing agent is adequately selected. It is not such a surprise that the modulus is

increased by the addition of high rigidity fillers but stress and strain at break are much

more relevant criteria to consider than the modulus since they are affected by the interfacial

adhesion.

In fact, a remarkable enhancement of the elastic modulus characterizes the blend

HDPE/PA6/clay in spite of the poor interfacial adhesion that emerged from morphological

analyses [23]. The authors emphasized that such a result stems from the cocontinuous

morphology of the hybrid blend, with both the HDPE and the filled PA6 contributing to

mechanical strength without stress being transferred across the interface. On the other hand,

the low elongation at break exhibited by the sample HDPE/PA6/clay was explained

considering the probable loss of ductility of the organoclay-rich polyamide phase and the

poor interfacial adhesion that emerged from the microstructural analyses.

Entezam et al. [109] analyzed the influence of nanoclay localization within immiscible

PP/PET 90/10 and 10/90 blends containing 0, 1, and 5 wt% of an organoclay on mechanical

properties of the blends. They found that the organoclay localized at the interface of the

blends had no compatibilization effect on the interfacial adhesion reinforcement. This is

because of the lack of interaction between the organoclay and PP chains at the interface. As

the matrix phase controls the mechanical behavior, especially at high deformations, of

immiscible blends with the matrix�dispersed morphology, contrary to the PP-rich blend

nanocomposites, the tensile strength of the PET-rich blend nanocomposites is different and

lower than that of neat PET-rich blends because of the presence of nanoparticles within the

PET matrix phase. On the other hand, the clay nanoparticles localized within PET as the

matrix phase of the PET-rich blend are more efficient in improving the tensile modulus of

this blend with respect to that of the PP-rich blend.

Generally, the ultimate properties of nanofilled thermoplastic polymer blends are often

reduced as well as impact properties [9,84,85,107,109�111]. Nevertheless, Ray et al. [93]

showed that organoclay PC/PMMA (30/70 w/w) blends prepared by melt mixing the exhibit

greatly improved mechanical properties when compared with the same unfilled blend

sample. In particular, that elongation at break of blend systematically increases with the

Nanofilled Thermoplastic�Thermoplastic Polymer Blends 153

addition of organoclay. These results were explained and suggested that the clay plays an

important role in the mixing and compatibilization process between the immiscible phases

as indicated by the morphological analysis.

Heidary et al. [112] found that addition of the end-capped nanosilica to LLDPE/EVA

blends increases the Young’s modulus and tensile strength as well as elongation at break of

the nanocomposites with increasing nanosilica content. The unusual improvement of the

ultimate properties and, in particular, of the elongation at break, was attributed to the ability

of the silica nanoparticles to change in the morphology of the blends acting as a

compatibilizer.

Finally, Istrate et al. [113] evaluated the mechanical properties of compatibilized and

noncompatibilized polymer blends (PS/PP)/clay nanocomposites, prepared via melt

processing. As expected, the addition of clay into the compatibilized and the

noncompatibilized polymer blends enhanced the tensile and the flexural moduli. Moreover,

the addition of clay to the compatibilized polymer blend increased the impact strength of

the material by 52%. These enhancements were attributed to the ability of clay to attain two

significant functions: reinforcement and compatibilization.

5.6 Conclusion

At the thermodynamic equilibrium, nanofiller distribution in thermoplastic immiscible

polymer blends is governed by the minimization of the overall free energy generated by the

existence of three types of interfaces: polymer 1/polymer 2, filler/polymer 1, and filler/

polymer 2 interfaces. The nanoparticles may distribute unevenly among the phases or

possibly form a layer at the fluid�fluid interface which consequently reduces the shared

area between the polymeric phases.

Moreover, in viscous systems like polymers, the equilibrium distribution of the filler is

attained after a period of time (estimated to several minutes) during which solid particles

move inside the blend before reaching their position of better stability. Hence, if the molten

blend is quenched before an equilibrium is attained, the particles’ localization may be

different from that assumed by the theory. One can take advantage of this kinetic control to

produce nonequilibrium morphologies without the help of expensive and toxic filler surface

treatments. The kinetic effect is strongly influenced by the sequence of addition of the

components of the blend, by the viscosity evolution of the phases, and by the competitive

adsorption/desorption of the two thermoplastic polymers.

In a large majority of cases, thermoplastic polymer blends filled with nanoparticles show

better compatibility in terms of morphology size than pure blends. Actually, the

morphology is finer and more stable against annealing; likewise when block copolymers are

added to the blend. Among the different potential mechanisms of compatibilization, a

154 Chapter 5

decisive one is the temporary or permanent accumulation of the filler at the blend interface.

A solid barrier is formed that inhibits or prevents drop coalescence. In addition, the

concentration of solid at the interface lowers the apparent interfacial tension. Less marked

improvement is observed when the particles distribute exclusively inside the minor phase or

the matrix.

Finally, the benefits of introducing nanofillers on the ultimate mechanical properties of the

blends are not always demonstrated. Although it is evident and not surprising that the

modulus of nanofilled thermoplastic polymer blends is increased by the addition of

nanoparticles, on the contrary, the ultimate properties are often reduced as well as impact

properties.

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