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

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13.2 Polyoleﬁ n-Based Nanocomposites 415

shown to be adequate in offering a surplus enthalpy for promoting OLS dispersion

into a nonpolar matrix, as did a PO. In fact, the surfactant/silicate interactions

were more favorable compared to those of PO/silicates. Hence, the synthetic

approach was directed towards reducing the enthalpic interactions between the

clay and surfactant by using semi - ﬂ uorinated surfactants [52] as clay modiﬁ ers.

Although, in the case of PP/OLS composites this approach led to a signiﬁ cant

improvement in miscibility, this type of surfactant was seen to be much too expen-

sive for use in the mass production of PO nanocomposites.

A more general approach for improving the compatibility of a PO with OLS was

to introduce into the system a compatibilizer as the coupling agent. In general,

the compatibilizer was a functionalized PO, such as a maleic anhydride ( MAH ) or

maleate ester - grafted PO, an oligomeric - functionalized PO, or an ammonium -

terminated or OH - terminated PO, as well as a suitable block or random copolymer

(Table 13.4 ). These types of polymer or oligomer reﬂ ected some of the theoretical

calculations carried out to probe the interactions between polymers and clay

sheets, and were used substantially to design the ideal compatibilizer [70, 71] . It

became clear that for the simple penetration of polymer chains into the clay gallery,

the compatibilizer should contain a fragment which would be highly attracted by

the clay surface. In addition, it should incorporate a longer fragment that was not

attracted by the sheets, but rather would attempt to gain entropy by pushing the

layers apart. Subsequently, when the layers had separated, the surfaces would be

sterically hindered from coming into close contact. In this way the compatibilizer

could be seen to behave as a steric stabilizer for the PO/OLS colloidal suspension.

Thus, the role of the compatibilizer is not only to favor a layered silicate dispersion

and to enhance the polarity of the polymer matrix, but also to stabilize the ﬁ nal

morphology. The strong interactions between the compatibilizer and clay are not

only fundamental for the dispersion step, but also for maintaining the

morphology.

Initial attempts to improve the interactions between nonpolar PO and layered

silicates were carried out by melt - mixing the polymer and clay with modiﬁ ed oli-

gomers, so as to mediate the polarity [51, 55 – 57, 74] . Usuki et al . [55] were ﬁ rst to

report the preparation of PP/layered silicate nanocomposites using a functional

oligomer (PP – OH) with polar telechelic – OH groups as compatibilizer. In this

approach, PP – OH was intercalated between the layers of an OLS, after which the

PP – OH/OLS was melt - mixed with PP to obtain nanocomposites with an interca-

lated structure. Another typical example to be reported was that of PP/layered sili-

cate nanocomposites; these were prepared by Toyota, using PP oligomers modiﬁ ed

with approximately 10 wt% of MAH groups [51, 56] and clays exchanged with

stearyl ammonium cations. Wide - angle X - ray diffraction ( WAXD ) analyses and

TEM observations established the intercalated structure for all of these nanocom-

posites. Here, the driving force of the intercalation was considered to have origi-

nated from the interaction between the MAH group and the oxygen groups of the

silicate, through the formation of hydrogen bonding (Figure 13.3 ).

In general, the compatibilizer is a polymer in which the functional groups are

distributed randomly along the backbone, or functionalized at the chain end. Both,

416 13 Nanocomposites Based on Phyllosilicates

Table 13.4 Some examples of compatibilizers for the preparation of PO / OLS nanocomposites.

Types of compatibilizer for

PO/OLS systems

Examples Main characteristics Reference(s)

End - functionalized

oligomers (i.e., OH - ;

MAH - terminated)

OH end - functionalized PP oligomer

(by GPC) 20 000; OH value 54 mg KOH g

− 1

[52, 56, 58]

MAH end - functionalized PP oligomer M

(by GPC) 12 000 – 30 000. Acid value

7 – 52 mg KOH g

− 1

[52, 56, 58]

MAH end - functionalized PE oligomer M

5700. Acid value; weight fraction of polar

comonomer 0.01). This PE - g - MA has a low

anhydride content so that, on average, there is

one anhydride moiety per two molecules. It can

be considered as an end - functionalized PE

oligomer.

[64]

End - functionalized

polymers (i.e., NH

- , OH - ,

COOH - terminated)

Linear end - functionalized polyethylene with

dimethyl ammonium chloride end groups

Narrow molecular weight distribution [74, 75]

Polyethylene with multiple quaternized amine

groups along the polymer chain

Narrow molecular weight distribution [75]

13.2 Polyoleﬁ n-Based Nanocomposites 417

Types of compatibilizer for

PO/OLS systems

Examples Main characteristics Reference(s)

Block copolymers Poly(ethylene - block - methacrylic acid)

Narrow molecular weight distribution [75]

Poly(ethylene - block - ethylene glycol)

Mn (575 mol g

− 1

); 0.20 molar fraction of polar

comonomer; PE - b - PEG is a block - copolymer, the

chains of which contain 33 methylene groups

and 2.6 ethylene oxide units per molecule on

average

[64]

Poly(propylene - block - methyl methacrylate)

around 200 000; molar fraction of MMA

units 1 or 5

[52]

Styrene - b - ethylene/butylenes - b - styrene block

copolymer

A triblock copolymer based on hydrogenated

polybutadiene (10 wt% ethyl branches,

= 37 000 g mol

− 1

) central block and

polystyrene (30 wt%, M

= 7000 g mol

− 1

)

[76]

Table 13.4 Continued.

418 13 Nanocomposites Based on Phyllosilicates

Types of compatibilizer for

PO/OLS systems

Examples Main characteristics Reference(s)

Random copolymers Poly(ethylene - co - vinyl alcohol)

0.69 weight fraction of polar comonomer [64, 76]

Poly(ethylene - co - methacrylic acid) M

68 000; 0.11 weight fraction of polar

comonomer.

[64]

PP - r - (PP - OH)

PP - r - (PP - MA)

These copolymers derive from a random

polypropylene copolymer which contains 1

mol% p - methylstyrene comonomer.

Mw around 200 000; molar fraction of polar

units 0.5

[52]

Table 13.4 Continued.

13.2 Polyoleﬁ n-Based Nanocomposites 419

Types of compatibilizer for

PO/OLS systems

Examples Main characteristics Reference(s)

Randomly functionalized

polyoleﬁ ns

Maleic anhydride (MAH) - functionalized

polyoleﬁ ns

Functionalization degree ranging between 0.1

and 2% by mol

[32, 48, 51 – 55, 72,

77 – 82]

Diethyl maleate (DEM) - functionalized polyoleﬁ ns

Functionalization degree ranging between 0.1

and 2% by mol

[32, 79 – 81, 83 – 85]

Maleic anhydride (MAH) and Butyl furanyl

propenoate (BFA) - functionalized polypropylene

Functionalized polypropylene having high

molecular weight

[86 – 89]

Table 13.4 Continued.

420 13 Nanocomposites Based on Phyllosilicates

Figure 13.3 Schematic representation of the dispersion

process of the organo layered silicate (OLS) in the

poly(propylene) (PP) matrix with the aid of a maleic anhydride

functionalized PP oligomers. Reproduced with permission

13.2 Polyoleﬁ n-Based Nanocomposites 421

simulation studies and experimental results [74, 90, 91] produced evidence that

end - functionalized polymers – unlike more functionalized polymer systems –

assumed a special conﬁ guration on the clay surface which allowed complete

exfoliation of the clay. In particular, in the case of the PP/OLS nanocomposites,

it was shown that by using an ammonium group - terminated PP (PP - NH

) as

compatibilizer, the terminal hydrophilic NH

groups would be attached by cation

exchange on the inorganic surfaces, while the hydrophobic high - molecular - weight

PP tails would aid exfoliation of the structure, though this would be maintained

even after further mixing with undiluted PP [74] . In contrast, side chain - function-

alized PPs or PP block copolymers were seen to form multiple contacts with each

of the clay surfaces, which resulted not only in the polymer chains being aligned

parallel to the clay surfaces, but also consecutive clay platelets being bridged, thus

promoting the production of intercalated structures. This bridging effect was

established, for example, by using side chain - functionalized polyoleﬁ ns character-

ized by large ( > 2 wt%) amounts of grafted polar groups [74, 90, 91] .

Without doubt, however, the most popular compatibilizers are POs functional-

ized with MAH by free radical processes. PP functionalized with MAH (PP - g -

MAH) was shown to intercalate into the inter - galleries of OLS, much like the

analogous oligomers [77] . In fact, a WAXD analysis of the PP/OLS nanocompos-

ites revealed an absence of any peaks representing dispersed OLS in the PP - g -

MAH matrix; hence, in this case also, the strong hydrogen bonds between the

MAH - grafted groups and the polar clay surface promoted dispersion of the layers.

Similar results were obtained for PE – PP rubber (EPM)/OLS nanocomposites

which had been prepared by melt blending EPM - g - MAH and OLS modiﬁ ed with

C18 alkyl ammonium chains, in a twin - screw extruder [78] . Conﬁ rmation of the

intense interactions between the functional groups of the compatibilizer and the

lamellae of OLS was obtained from experiments with nanocomposites that had

been prepared by blending an EPM functionalized with diethylmaleate ( DEM )

(with 1 mol% grafted functional groups) with increasing amounts (up to 20 wt%)

of OLS. The use of a functionalized, rather than virgin, EPM allowed (in the case

of 5 wt% OLS nanocomposites) the shift to be made from an intercalated morphol-

ogy to an almost completely exfoliated morphology [83] (Figure 13.4 ).

It was hypothesized that the hydrogen bonds between the – OH groups present

on the OLS surface and the carbonyl groups of the functionalized EPM allowed

both optimization and stabilization of the clay dispersion (Figure 13.5 ). Most

likely, the compatibilizer was placed between the polar silicate layers and con-

temporarily mixed with the nonpolar polymer chains, thus creating a concentra-

tion gradient suitable for providing an exfoliated morphology and avoiding phase

separation.

The strong interactions between the OLS and the EPM - g - DEM were conﬁ rmed

by extracting the nanocomposites with hot toluene; a signiﬁ cant proportion of

the rubber proved to be insoluble (up to 55 wt% for 15 – 20 wt% OLS), and the

amount of nonextractable polymer was increased by raising the OLS loading [32,

84] . The occurrence of strong polymer/clay interactions was also proved by com-

paring the infrared ( IR ) spectra of the composite and residue (Figure 13.6 ). The

422 13 Nanocomposites Based on Phyllosilicates

Figure 13.4 Transmission electron microscopy images

of (a) ethylene propylene rubber (EPM)/organo - layered silicate

(OLS) and (b) EPM functionalized with diethyl maleate

(EPM - g - DEM)/OLS 5 wt% nanocomposites. Reproduced with

Figure 13.5 Effect of organophilic and polar interactions on

the morphologies of ethylene – propylene rubber (EPM) and

EPM functionalized with diethyl maleate (EPM - g - DEM)/

organo - layered silicate (OLS) composites.

C = O stretching band of the carbonyl ester group of the EPM - g - DEM, when

retained in the toluene - extracted residue, was signiﬁ cantly broad and was shifted

to lower wavenumbers compared to neat EPM - g - DEM. Moreover, the shoulder at

approximately 1715 cm

− 1

suggested a hydrogen - bond interaction between the EPM

functional groups and the silicate surface [85] .

13.2 Polyoleﬁ n-Based Nanocomposites 423

Figure 13.6 Attenuated total reﬂ ectance (ATR)

spectra of ethylene – propylene rubber

functionalized with diethyl maleate (EPM - g -

DEM) and residue to toluene extraction of a

PE - g - DEM/organo - layered silicate (OLS)

nanocomposite 10 wt% OLS. Note the

enlargement of the C = O stretching region.

Reproduced with permission from Ref. [84] ;

Evidence of the strong interaction between the EPM - g - DEM and the layered sili-

cate was also provided with differential scanning calorimetry ( DSC ) and dielectric

relaxation analyses. Initially, the insoluble toluene fraction was seen to have a glass

transition temperature ( T

) which was appreciably higher than that of the original

composite. Furthermore, the dielectric relaxation analysis of nanocomposites indi-

cated a reduction in both the dielectric strength and α - relaxation frequency as the

temperature was increased (Figure 13.7 ). As the dipolar relaxing units are the side

DEM grafted groups, this phenomenon was explained by considering a partial

immobilization of such groups on the silicate surface, as consequence of the

nanoconﬁ nement of polymer chains. The α - relaxation was shown to disappear

for the residue to the toluene extraction, while the cooperative dynamics of the

dipolar groups of EPM - g - DEM were strongly reduced as they were interacting with

the layers.

The amount of unextractable polymer, the existence of which conﬁ rms the

polymer/OLS interactions, and can be related to an adsorbed/interacting

conﬁ ned polymeric fraction, was correlated (using a rough geometric model) to

the volume occupied by trapped and intercalated macromolecules (Figure 13.8 ).

424 13 Nanocomposites Based on Phyllosilicates

This comparison between experimental and theoretical data provided evidence of

the existence of different interaction levels between the polymer and the organo-

philic clay, which together caused the formation of an unextractable polymer,

namely:

• polymer – surfactant interactions

• polymer – clay surface chemical and physical interactions

• intercalated polymer – bulk polymer interactions.

By applying this model, the PO/OLS nanocomposite can be considered as a system

containing at least two phases: (i) the polymer matrix, which does not truly interact

with the layers ’ surface (and thus is extractable with the same T

of pristine PO);

and (ii) a nanostructured phase containing polymer chains that strongly interact

Figure 13.7 Dielectric loss spectra of

nanocomposite organo - layered silicate (OLS)

20 wt% (closed symbols) and its residue to

solvent extraction (open symbols) above the

glass transition temperature. The same

symbol type corresponds to the same

measurement temperature. Reproduced with

Ltd.

Figure 13.8 Schematic model of polymer conﬁ nement.

Elsevier Ltd.