Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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13.2 Polyoleﬁ n-Based Nanocomposites 425

and are “ conﬁ ned ” (and thus are not extractable, with a higher T

than pristine

PO and the starting composite). Here, the Gordon – Taylor equation was employed

to calculate the amounts of both polymer phases by considering the T

of the

polymer in the nanocomposite (as determined by the T

of the free polymer), and

of the conﬁ ned polymer. This analysis provided results which, in terms of their

behavior, were in agreement with experimental data.

Interestingly, it was demonstrated that the unextractable polymer was respon-

sible for reductions in the oxygen permeability of LDPE and EPM/OLS nano-

composites prepared by using functionalized matrices [85] . In fact, by assuming

a negligible oxygen permeability through the polymer phase adsorbed onto

the surface or trapped inside the galleries of the OLS, these experimental

data were ﬁ nely correlated with the theoretical data predicted from mathemati-

cal models.

Besides the nature of the functional groups, it was also clear that the amount

of functional groups grafted onto the PO could inﬂ uence the ﬁ nal morphology of

the PO/OLS nanocomposite. In fact, the hydrophilicity of the compatibilizer was

seen to increase with the grafting level of MAH, thus improving the level of inter-

action between the polymer and the clay. In the case of the most common PO - g -

MAH compatibilizers, the level of grafting ranged typically between 0.1 and 2 wt%

[50, 85, 92, 93] . In the case of MAH - functionalized PO, there was seen to be a limit

value for the grafting level (ca. 0.1 wt%), for achieving a good improvement in

morphology. In the case of both PE and PP/OLS nanocomposites, if the grafting

level was higher than this limit value, a shift or even a disappearance of the basal

reﬂ ection peak of the OLS on the XRD diffraction pattern could be observed [51,

72, 77, 78, 92] .

In general, because of the low grafting percentage of polar groups onto the

common commercial compatibilizers, a large amount of functionalized polymer

is added to the polyoleﬁ n matrix in order to achieve the necessary polarity and

hence compatibility with the organoclay. Besides the degree of functionalization

( DF ), the molecular weight and structure of the compatibilizer, as well as its melt

viscosity and rheological properties, are responsible for the composite ﬁ nal mor-

phology [93, 94] . The best results are generally obtained by using a compatibilizer

for which the rheological properties are similar to those of the matrix; otherwise,

the miscibility between the two polymers might be compromised and the intercala-

tion/exfoliation process inhibited. A typical example is that of PP, for which a high

amount of compatibilizer is required to obtain an increase in the OLS basal

spacing, but this may cause a deterioration in the properties of the nanocomposite

due to the low molecular weight of the commonly used PP - g - MAH samples.

Unfortunately, functionalized PPs with high molecular weights are yet not avail-

able commercially. The conventional radical functionalization process with perox-

ide and MAH in the case of PP causes a dramatic decrease in the molecular weight

of the polymer, leading to severe damage of its rheological and mechanical proper-

ties. Notably, PP is very sensitive to degradation reactions when treated with

peroxides above its melting temperature, even in the presence of commonly used

maleate functionalizing agents [79 – 81, 94] .

426 13 Nanocomposites Based on Phyllosilicates

Consequently, conventional PP - g - MAH compatibilizers will in general have a

low DF and molecular weight. The problem of obtaining appropriately functional-

ized PP can be overcome by a new radical functionalization approach involving

the use of a furan derivative (BFA). This is added as coagent during the radical

functionalization of PP with MAH, which can yield a wide range of PP - g - MAH

samples with different DFs and molecular weights, by controlling the macroradical

formation and content [86, 87] . These new functionalized PP samples were recently

tested in the preparation of PP/OLS nanocomposites, both as a matrix or as a

compatibilizer of the system, the aim being to study the effects of both polarity

and chain structure/architecture on clay dispersion and the ultimate properties of

the corresponding nanocomposites. The results showed that PP - g - MAH with a

low molecular weight and a high DF ( > 2 mol%) had an excellent ability to disperse

clay at the nanometric level, especially when used as the matrix of the correspond-

ing nanocomposite. Those samples characterized by a high DF value ( > 2 mol%)

and a branched structure/architecture produced nanocomposites with a lower

degree of exfoliation. However, those nanocomposites with a composition of

90/5/5 PP/compatibilizer (the functionalized PP)/organomodiﬁ ed layered silicate,

where the sample contained the compatibilizer characterized by a high DF; pre-

pared using a grafting procedure to avoid PP degradation reactions) provided the

best performance in terms of morphological and thermomechanical properties.

These results not only conﬁ rmed the important role of the DF, but also highlighted

the fact that the control of molecular weight and structure/architecture during

functionalization ensures a good compatibility of the compatibilizer with the PP

matrix, which in turn has a positive effect on the ultimate properties of the PP/

OLS. In particular, elongation at break point (which usually are poor for similar

systems) reached values in excess of 500%, with an excellent reproducibility [88] .

13.2.3

The One - Step Process

In addition to a correctly selected OLS and compatibilizer, it is also necessary to

utilize an effective mixing protocol (e.g., processing parameters such as mean

residence time, screw speed, etc.) capable of promoting both high shear stress and

shear rate, in order to assist the OLS dispersion into a PO matrix [82, 95] . It has

been shown that the residence time and kneading force applied during extrusion

can have a direct effect on the extent of the layered silicate exfoliation [96] . In the

case of LDPE/OLS nanocomposites, a correct balance between dispersive and

distributive actions, by ﬁ ne - tuning the screw proﬁ le, allowed an optimal nanocom-

posite morphology [82] . A good dispersion of clay layers was assessed by preparing

a master - batch PE - g - MAH/OLS, which highlighted the importance of the shear

stress for dispersing, intercalating, and exfoliating lamellae when the correct inter-

face interactions have been provided. A further improvement in dispersion degree

was observed during a second processing step, as the melt viscosity of the two

polymer components was accurately selected with the aim of improving their

compatibility.

13.2 Polyoleﬁ n-Based Nanocomposites 427

Besides the machine parameters, the method selected for feeding the

extruder – and therefore also the order of addition for components – can inﬂ uence

the morphology of the ﬁ nal material. In most reports, the best stack delamination

was observed when a master batch between the functionalized PO (compatibilizer)

and a large proportion of clay (10 – 50 wt%) was ﬁ rst prepared, followed by dilution

in the nonpolar PO matrix (master batch process). The advantage of this method,

compared to a direct melt blending of all ingredients together, is that it facilitates

the intercalation of the polar polymer chains between the clay stacks, thus obtain-

ing a form of pre - intercalated material.

One rarely discussed aspect is the possibility of obtaining, in a single step, both

functionalization of the PO and nanocomposite formation. Such a method would

be especially interesting because its successful application would not only avoid

selecting the compatibilizer but also save a preparative step. In fact, as noted above,

selection of the best compatibilizer may require special care, as this must be suf-

ﬁ ciently polar to allow interaction with the clay, but not so hydrophilic as to favor

phase separations. Not least, it should have similar rheological properties as the

matrix, in order to avoid any deterioration of the composite ’ s ultimate mechanical

properties.

Consequently, two different synthetic approaches were very recently compared

for the preparation of EPM and PP OLS nanocomposites [89] (Figure 13.9 ):

• The ﬁ rst approach is a classical procedure where the dispersion of OLS is

achieved by using, as the compatibilizer of the system or as a matrix, a previously

functionalized PO bearing carboxylate groups on the backbone (two - step

process, Method 1). This method involves a ﬁ rst step when the functionalized

PO is prepared. Frequently, the traditional approach for the preparation of

functionalized PO is by free radical grafting of polar unsaturated molecules.

• The second process is carried out by contemporaneously performing the

functionalization of the PO (by using maleic anhydride and/or diethylmaleate

and a peroxide as radical initiator) and the dispersion of the ﬁ ller (one - step

process, Methods 2 and 3).

The results obtained using the two - step process (Method 1) indicated that, by using

the functionalized PO as matrix, only a fraction of the polar groups on the poly-

oleﬁ n macromolecules would be effective in establishing a bonding with the sili-

cate surface. In order to facilitate the functionalized polymer – clay interactions, it

seems convenient to perform the functionalization during mixing of the PO with

the clay. In this way, the small polar molecules could more easily penetrate the

clay channel, with grafting to the organic macromolecules occurring successively.

Comparative experiments were also carried out according to the one - step proce-

dure, following two different routes – Methods 2 and 3 (see Figure 13.9 ).

In Method 2, the functionalizing reagents and organically modiﬁ ed montmo-

rillonite ( OMMT ) were premixed and added contemporarily to the molten polymer

in a Brabender mixer, such that functionalization and intercalation/exfoliation

could occur simultaneously. In Method 3, the functionalizing reagents (initiator,

428 13 Nanocomposites Based on Phyllosilicates

Figure 13.9 Different methods of preparation for compatibilized PO/OLS nanocomposites.

13.3 Poly(Ethylene Terephthalate)-Based Nanocomposites 429

DEM and, occasionally, MAH) were added to the molten polymer, with the OMMT

being added after a few minutes. In the former case, the operating conditions the

aim essentially was ﬁ rst to have intercalation of the monomer, followed by grafting

to the macromolecules. In the latter case, the procedure was essentially similar to

the two - step method (Method 1), although for kinetic reasons there was a degree

of overlapping within the time scale between the grafting of DEM to PO, and

intercalation.

The results obtained indicated that, in terms of the role played by the differ-

ent parameters (including the polyoleﬁ n chemical structure), the process was

very complex. In the case of the random ethylene/propylene copolymer (EPM),

the simultaneous addition of a functionalizing reagent and OMMT to the molten

polymer (Method 2) was the most effective approach, even if an optimal ratio

between the DF and clay basal spacing enlargement appeared to exist. In the

case of PP, however, the application of Method 2 did not provide the same

results, with a better dispersion being obtained via Method 3. Indeed, the polymer

functionalization was seen to be favored, and degradation controlled, by the

presence of clay. However, the best polymer chain intercalation was obtained

when degradation was signiﬁ cant during the functionalization process of PP,

indicating that a reduction in molecular weight which favored intercalation

would probably be more effective than the presence of a larger amount of

grafted polar groups.

13.3

Poly(Ethylene Terephthalate) - Based Nanocomposites

Poly(ethylene terephthalate) ( PET ), which is one of the most diffused polymers, is

used in a wide variety of applications, notably the packaging and textile industries.

The need to improve the stiffness, surface properties and/or gas barrier properties

of PET has led to a plethora of investigations, in both academia and industry, into

PET - based nanocomposites. Within this area, those nanocomposites which contain

phyllosilicates are considered superior for providing improved gas barrier proper-

ties, due mainly to the strong effect of conﬁ nement as the result of a high

surface : volume ratio (i.e., reducing chain mobility and permeability [35] ), as well

as to the enhancement of tortuosity [97] of the path required for small molecules

to permeate through a polymer ﬁ lm due to the presence of silicate lamellae. Flame

retardancy and gas barrier functions are much - required properties in the area of

textiles, the main aim being to develop fabrics with ﬂ ame self - extinguishing prop-

erties [98] . Consequently, many research groups have recently investigated PET/

phyllosilicate nanocomposites in an attempt to provide solutions to these impor-

tant technological problems [99 – 102] .

One especially important ﬁ eld of application is that of recycling post - consumer

PET [103] . The recovery of PET from post - consumer packaging products (e.g.,

beverage bottles) allows ﬂ akes to be obtained which have purity levels suitable for

reprocessing. Unfortunately, in many countries (e.g., Italy) the recovered material

430 13 Nanocomposites Based on Phyllosilicates

cannot be recycled via a bottle - to - bottle approach, as legislation decrees that post -

consumer polymers cannot remain in contact with food. Consequently, there is a

need to develop new materials based on post - consumer PET ﬂ akes, in order to

achieve an effective recycling. At present, blending these ﬂ akes with other poly-

mers, especially rubber [104 – 111] , to achieve toughened blends, and/or adding

ﬁ llers [112] , represent the most frequently investigated strategies for producing

materials with added value by utilizing post - consumer ﬂ akes. This ﬁ nal point is

especially important because, if recycling cannot be used to provide materials that

perform similarly to commonly used plastic materials, then landﬁ lling or energy

recovery using plastic wastes will become inevitable. Nanoﬁ llers – and in particular

phyllosilicate - based nanocomposites – may well provide the means of modulating

the mechanical, thermal, and ﬂ ame - retardancy properties of post - consumer PET,

thus broadening its ﬁ eld of application.

Today, both technological and environmental driving forces continue to attract

the attention of scientiﬁ c research towards PET – phyllosilicate nanocomposites.

13.3.1

In Situ Polymerization

The preparation of PET/MMT nanocomposites by in situ polymerization has

been the object of several investigations. In particular, Hwang et al . [113] reported

the preparation of PET nanocomposites containing pristine and organically modi-

ﬁ ed MMT by in situ polymerization, since this technique has been shown to

produce a homogeneous dispersion of MMT particles in the polymer matrix.

The polymerization of PET was carried out through polycondensation [114, 115] .

In an esteriﬁ cation tube, ethylene glycol and sodium or organically modiﬁ ed

MMT and a catalyst of transesteriﬁ cation (zinc acetate) were subjected to ultra-

sonication before the ester interchange reaction was initiated. Dimethyl tereph-

thalate was then added to the ethylene glycol slurry to obtain a homogeneously

dispersed system. The mixture was then heated to 210 ° C to initiate esteriﬁ cation

between the silicate layers in the clay. The ester exchange reaction was carried

out for 3 h while continuously removing methanol. Finally, polycondensation

was performed at reduced pressure, at temperatures between 180 and 285 ° C,

by adding antimony (III) oxide catalyst as polycondensation catalyst.

The degree of exfoliation was determined by TEM measurements at 20 nm

resolution; these showed a clear separation between the silicate layers of the

clay, enhancing differences between the composites prepared using either

sodium - MMT or OMMT which had been modiﬁ ed with dimethyl, benzyl,

hydrogenated tallow, quaternary ammonium. The TEM images revealed the

presence in 1% MMT composites of uniformly oriented sodium - MMT particles

with complete layer stacks containing an average of 66 silicate sheets per

particle, whereas particles in PET/OMMT were largely exfoliated, with a larger

proportion of intercalated PET, as evidenced by signiﬁ cantly smaller particles

composed of only four to ﬁ ve randomly oriented silicate sheets. Scanning

electron microscopy ( SEM ) images of the nanocomposites conﬁ rmed that the

13.3 Poly(Ethylene Terephthalate)-Based Nanocomposites 431

organophilic modiﬁ er had signiﬁ cantly improved the compatibility between

PET and MMT.

DSC analyses showed that the crystallization temperature of PET/sodium - MMT

during cooling gradually increased, while the degree of supercooling decreased

with clay loading. This suggested that the sodium - MMT had acted as a nucleating

agent and accelerated the crystallization rate of the PET matrix. In subsequent

heating runs, the nanocomposites exhibited two melting peaks compared to pure

PET. The lower melting temperature peak was attributable to the melting of

imperfect crystals, which formed as a result of the nucleation effect of sodium -

MMT during the cooling run. The higher melting temperature peak corresponded

to melting of the melt - reorganized crystal.

DSC curves obtained for the PET/organophilic MMT exhibited an increase in

the crystallization peak intensity during the cooling run at 0.5 wt% loading. This

peak decreased with increasing clay content, although it maintained a higher

intensity than that of neat PET. This indicated that the nucleating effect of organo-

philic MMT was lower than that of sodium - MMT. This may be attributable to

interference by the alkyl groups on the organophilic MMT surfaces with secondary

nucleation and diffusion of PET molecules, resulting in a decrease in the DSC

crystallization peak.

The dispersion of rigid, exfoliated MMT layers served to enhance the macroscale

stiffness of the composite material. Elongation at break values was drastically

decreased due to increased stiffness and the formation of microvoids around the

clay particles during tensile testing. The main factor contributing to the enhance-

ment of mechanical properties in PET/clay nanocomposites was not the quantity

of clay, but rather the degree of dispersion and exfoliation of the clay in the PET

matrix.

A very similar in situ polymerization procedure was adopted by Chang and Mun

[100] , who prepared PET nanocomposites by using dodecyltriphenylphosphonium -

modiﬁ ed MMT as a reinforcing ﬁ ller in the fabrication of PET hybrid ﬁ bers. The

group studied the properties of the composites obtained, for organoclay contents

ranging from 0 to 5 wt%, by applying different draw ratios. The modiﬁ ed MMT

displayed well - dispersed individual clay layers in the PET matrix, although some

particles appeared to be agglomerated at size levels greater than approximately

10 nm.

Ke et al . [116] also reported the properties of PET/MMT nanocomposites pre-

pared by in situ polymerization of PET onto organophilic - modiﬁ ed MMT. The

reﬁ ned clay, reduced to particles of 40 μ m average diameter, was made into a

slurry, and formed a solution with an intercalated reagent, which reacted directly

with PET monomers in an autoclave [117, 118] . Although, this preparation method

led mainly to intercalated nanocomposites, a similar nucleating effect with respect

to the report of Chang and Munn was observed.

An alternative in situ polymerization method was provided by Lee et al . [118] ,

who successfully polymerized ethylene terephthalate cyclic oligomer s ( ETCO s) to

form a high - molecular - weight PET in the presence of OMMT by employing the

advantages of the low viscosity of cyclic oligomers and lack of chemical emissions

432 13 Nanocomposites Based on Phyllosilicates

during polymerization (Figure 13.10 ). Here, the OMMT was prepared via the

cation exchange of sodium - MMT with N , N , N - trimethyl octadecylammonium

bromide.

The application of ring - opening polymerization ( ROP ) is usually hindered by

the limited efﬁ ciencies for synthesizing and isolating cyclic oligomers. Therefore,

various types of preparative methods have recently been developed. The present

authors followed the direct reaction of acid chlorides with glycols in dilute solution,

which provides high yields of ETCs [119] . Subsequently, ETC, OMMT and Ti(O -

1 - C

)

were dissolved in dichloromethane and the solution maintained at room

temperature for 12 h. The oligomers, OMMT and Ti(O - 1 - C3H7)4, dissolved in

dichloromethane, were maintained at room temperature for 12 h; the solvent was

Figure 13.10 Schematic representation of nanocomposites

formation by ring - opening reaction of cyclic oligomers

in - between silicate layers. Reproduced with permission from

13.3 Poly(Ethylene Terephthalate)-Based Nanocomposites 433

Figure 13.11 Schematic illustration of the preparation of

exfoliated PET nanocomposites excluding organic modiﬁ er by

means of the solvent – nonsolvent system. Reproduced with

then removed, dried under reduced pressure and polymerized in a high - vacuum

system at temperatures ranging from 240 to 310 ° C.

Due to their low molecular weight and low viscosity, the ETCs are successfully

intercalated into the clay galleries, as evidenced by XRD analyses which showed a

down - shift of the basal plane peak of layered silicate, in good agreement with the

results of TEM investigations. The successive ROP of ETCs in - between silicate

layers yielded a PET matrix of high molecular weight, along with high disruption

of the layered silicate structure and a homogeneous dispersion of the latter in the

matrix. However, the coexistence of exfoliation and intercalation states of silicate

layers was revealed by morphological investigations. Interestingly, the highest

investigated polymerization temperature provided the nanocomposite with the

PET having the greatest molecular weight (11 000 mol g

− 1

13.3.2

Intercalation in Solution

To date, only one report has been made [120] detailing the preparation of PET/

phyllosilicate nanocomposites in solution. Exfoliated PET - layered silicate nano-

composites excluding and including organic modiﬁ ers were obtained by solution

methods in chloroform/ triﬂ uoroacetic acid ( TFA ) mixtures, with and without

solvent – nonsolvent systems, respectively. For this, the PET/OLS/chloroform/

TFA solution was added dropwise to the cold methanol to obtain PET nanocom-

posites excluding an organic modiﬁ er; the precipitated materials were then col-

lected by ﬁ ltration and dried in a vacuum oven (Figure 13.11 ). In contrast, the PET

434 13 Nanocomposites Based on Phyllosilicates

nanocomposite including an organic modiﬁ er was prepared by removing both

solvents from the prepared PET/OLS/chloroform/TFA solution.

Based on wide - angle X - ray diffraction (WAXRD) and high - resolution TEM anal-

yses, both types of composite were found to have an exfoliated structure that was

attributed to a sufﬁ cient dispersion of silicate in prepared solvents, regardless of

the sample preparation method.

The DSC results indicated that both types of nanocomposite had higher degrees

of crystallinity and shorter crystallization half - times than neat PET, since the dis-

persed silicate layers had acted as nucleating agents in both situations. However,

those nanocomposites prepared with the organic modiﬁ er exhibited a lower degree

of crystallinity and a longer half - time of crystallization than those without an

organic modiﬁ er. This difference in crystallization behavior between the two nano-

composites was ascribed to the organic modiﬁ er, which may have acted as an

inhibitor of crystallization.

13.3.3

Intercalation in the Melt

The most frequently reported method of intercalation has been within the melt,

mainly because it is signiﬁ cantly more economical and simpler than other

methods, and is also compatible with existing processes. In fact, melt processing

allows nanocomposites to be formulated directly using ordinary compounding

devices such as extruders or mixers, without the need to involve resin production.

Consequently, nanocomposite production can be shifted downstream, providing

end - use manufacturers with many degrees of freedom with regards to their ﬁ nal

product speciﬁ cations. Notably, melt processing is also environmentally friendly,

as no solvents are required. It also enhances the speciﬁ city of intercalation of the

polymer, by eliminating any competing host – solvent and polymer – solvent

interactions.

One topic which has been widely investigated is the inﬂ uence of an organo-

philic modiﬁ cation of nanoclay on the morphology of composites. Pegoretti et

al . [121] compared the effect of a nonmodiﬁ ed natural MMT clay (Cloisite - Na+)

and an ion - exchanged clay modiﬁ ed with quaternary ammonium salt (Cloisite

25A) (Table 13.5 , row h) in a recycled PET matrix. The Cloisite 25A is a MMT

modiﬁ ed with dimethyl, hydrogenated tallow, 2 - ethylhexyl quaternary ammonium

cations. Following preparation of the composites in an injection - molding

machine, subsequent TEM images of the composite fracture surfaces indicated

that the particles of Cloisite 25A were much better dispersed in the recycled PET

matrix than those of Cloisite - Na+. Moreover, wide - angle X - ray scattering ( WAXS )

measurements indicated the intercalation of recycled PET between the silicate

layers (lamellae) of the clay. In addition, improvements in mechanical properties

were more evident in Cloisite 25A nanocomposites than in those containing

Cloisite - Na+. More recently, the effects of different modiﬁ ed commercial MMTs

on the morphology of PET composites prepared in a twin - screw extruder were

investigated [122] . In particular, within a scale of increasing hydrophobicity of