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

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13.3 Poly(Ethylene Terephthalate)-Based Nanocomposites 435

Table 13.5 Montmorillonites used for the preparation of polyesters/MMT nanocomposites by intercalation in the melt.

Formula of the cation Commercial name Reference(s)

(a) Sodium montmorillonite Na

Cloisite Na

[122 – 124]

(b) Methyl, tallow, bis - 2 - hydroxyethyl,

quaternary ammonium

montmorillonite

T is Tallow ( ∼ 65% C18; ∼ 30% C16; ∼ 5% C14)

Cloisite

30 B

[123, 125]

ethyl] - 2.3 - dimethyl - 3H - imidazolium

montmorillonite

– [126]

(d) 1,2 - dimethyl - 3 - N - hexadecyl

imidazolium montmorillonite

– [124]

(e) 1,3 - Dioctadecyl imidazolium

– [127]

436 13 Nanocomposites Based on Phyllosilicates

Formula of the cation Commercial name Reference(s)

(f) Hexadecyl pyridinium

montmorillonite

– [127]

(g) Dimethyl, benzyl, hydrogenated

tallow, quaternary ammonium

montmorillonite

HT = hydrogenated tallow

Cloisite 10A

Dellite 43B

[123]

(h) Dimethyl, dehydrogenated tallow,

2 - ethylhexyl quaternary ammonium

montmorillonite

Cloisite 25A [122, 125]

(i) Dimethyl, dehydrogenated tallow,

quaternary ammonium

montmorillonite

Cloisite 20A (0.95 mg g

− 1

clay)

Cloisite 15A (1.25 mg g

− 1

clay)

Cloisite 6A

(1.4 mg g

− 1

clay)

Dellite 72T

[127]

[123, 125]

[125]

(l) Tributyl hexadecyl phosphonium

montmorillonite

– [127]

Table 13.5 Continued.

13.3 Poly(Ethylene Terephthalate)-Based Nanocomposites 437

the cation, sodium - MMT, methyl, tallow, bis - 2 - hydroxyethyl, quaternary ammo-

nium - MMT (Cloisite 30B) (Table 13.5 , row b), dimethyl, benzyl, hydrogenated

tallow, quaternary ammonium - MMT (Cloisite 10A) (Table 13.5 , row g) and dime-

thyl, dehydrogenated tallow, quaternary ammonium - MMT (Cloisite 15A) (Table

13.5 , row i) were compared.

As revealed by both TEM and WAXD characterization, the shear in the extruder

favored the exfoliation process, which resulted in both intercalated and exfoliated

morphologies. PET nanocomposites obtained by using clay with polar modiﬁ ers

showed intercalated and exfoliated morphologies, whereas tactoids were obtained

when only apolar modiﬁ ers were present. The polymer – clay interactions and the

extrusion conditions were sufﬁ cient to break the organized arrangement of the

natural MMT, so as to disperse it in the polymer matrix. The organic modiﬁ er also

appeared to sustain the exfoliated clay sheets ﬂ at since, in its absence, the intramo-

lecular interactions were stronger and the platelets tended to roll due to their high

ﬂ exibility. A comparison of these two reports [121, 122] showed contradictory

results, however. As the recycled and virgin PET had practically identical structural

characteristics (although the recycled form usually had a lower viscosity in the

melt), the comparison mainly highlighted the strong effect of shear stresses on

the morphology. In fact, the stronger shear stresses developed in the twin - screw

extruder were most likely sufﬁ cient to achieve an exfoliated/intercalated structure,

without using an organophilic modiﬁ er. In the study of Pegoretti et al . [121] , the

lower shear stresses of the injection - molding machine produced the opposite

result, as in these conditions the main driving forces were the interactions of the

hydrophobic chains with the polymer matrix, which were capable of producing a

primary disaggregation of the ﬁ ller.

The strong effects of processing conditions and rheological properties on mor-

phology were also reported by Krakalik et al . [123] , who prepared composites in a

co - rotating, twin - screw, microextruder by adding to recycled PET 5% (by weight)

of different modiﬁ ed MMTs (Cloisite 6A, 15A, 20A, 25A, and 30B) (Table 13.5 ).

Subsequently, these authors observed the partial exfoliation of Cloisite 25A, 30B,

and 10A, which was in good agreement with the results observed by both Pegoretti

et al . [121] and Calcagno et al . [122] .

Davis et al . [125] investigated the effects of melt - processing conditions on the

quality of PET/MMT nanocomposites prepared in a mini twin - screw extruder

at 285 ° C, and identiﬁ ed the need to use a highly stable ammonium salt in

modiﬁ ed MMT. In fact, the use N , N - dimethyl - N , N - dioctadecylammonium - MMT

led to the production of black PET nanocomposites as a result of ammonium

salt degradation under the processing conditions. The most dispersed, exfoliated

PET nanocomposite was achieved by melt - mixing at 21 radians per second for

2 min in a nitrogen atmosphere, after drying the polymer at 120 ° C, and the

clay at 150 ° C. Alternative mixing conditions, longer residence times, and higher

screw speeds resulted in poorer - quality nanocomposites. The alternative use of

a hexadecyl imidazolium salt, which resulted in an intercalated/exfoliated mor-

phology, allowed improvements in stability, as the imidazolium salt (which has

a decomposition temperature of 350 ° C) is stable under the conditions of

438 13 Nanocomposites Based on Phyllosilicates

processing. The subject of modiﬁ ed MMT stability was recently investigated by

Kim et al . [124] , who synthesized an imidazolinium salt with an aromatic group

and an hydroxyl group (Table 13.5 , row c), which was more stable than a

quaternary ammonium salt. Such treatment was found to provide a similar

dispersion with respect to Cloisite 15A and 30B, but with the advantage of a

higher thermal stability.

More recently, Stoefﬂ er et al . [126] used the surfactants alkyl phosphonium

(Table 13.5 , row l), alkyl pyridinium (Table 13.5 , row f) and dialkyl imidazolium

(Table 13.5 , row e) as intercalating agents for the preparation of highly thermally

stable organophilic MMTs, and compared these to commercial Cloisite 20A (Table

13.5 , row i) from the point of view of thermal stability. Although the thermal

stability of the former materials was improved with respect to the latter, a mass

spectrometric analysis of the volatile products showed the evolution of chlorometh-

ane from Cloisite 20A above 200 ° C, of pyridine from pyridinium - MMT above

250 ° C, and of tributyl phosphine and/or tributyl phosphine oxide from phospho-

nium - MMT above 250 ° C. Based on the toxicity of volatile products, the use of

imidazolium or phosphonium derivatives proved unsatisfactory. Rather, the most

interesting result obtained was the exfoliation of native sodium - MMT via the

establishment of proper extrusion conditions [122] .

In only one report [127] was the possibility considered of using a proper com-

patibilizer, namely a polyester ionomer (Figure 13.12 ), to improve the compatibil-

ity between an OMMT (Cloisite 20A) and PET. Here, the polyester ionomer was

efﬁ cient in promoting intercalation (or intercalation/exfoliation if the ionomer/

OMMT ratio was ≥ 3) of the OMMT in the PET matrix. Moreover, in terms of rheo-

logical behavior, the higher the ﬁ ller content and/or the degree of intercalation/

exfoliation of the OMMT, the more the nanocomposite behaved as a solid. With

regards to rheological behavior, the higher the content of ﬁ ller and/or degree of

intercalation/exfoliation of the OMMT, the more the nanocomposite behaved as

a solid because of the percolated structure formed by the OMMT layers, and the

more the storage and loss modulus, G ′ and G ″ , became independent of the fre-

quency at low frequencies.

Figure 13.12 Structure of the polyester ionomer (PETi) used by Vidotti et al . [127] . (x = 0.13).

13.4 Poly(Lactide) (PLA)-Based Nanocomposites 439

13.4

Poly(Lactide) ( PLA ) - Based Nanocomposites

Today, the development of renewable polymeric materials with excellent proper-

ties forms the subject of much active research interest worldwide [4] . Aliphatic

polyesters are among the most promising materials for the production of high -

performance, environment - friendly, biodegradable plastics [128] . Among these

materials, PLA is a renewable and biodegradable polyester, the thermomechanical

and gas - barrier properties of which prevent the replacement of traditional poly-

mers in many areas of application. Consequently, innovative methods must be

developed to modulate the properties of PLA, such as blending [129] , plasticization

[130] , and the preparation of nanocomposites [131] .

13.4.1

Overview of Preparation Methods

13.4.1.1 In Situ Polymerization

The preparation of some biodegradable polyesters, including PLA and PCL, is

often achieved via ROP from corresponding lactones [132, 133] . In particular, the

synthesis of PLA can be carried out starting either from the cyclic dimer, the lactide

(3,6 - dimethyl - 1,4 - dioxane - 2,5 - dione) by ROP, or by the condensation polymeriza-

tion of lactic acid or its derivatives.

In terms of molecular weight control, the living ROP of lactide yields a linear

relationship between monomer conversion and molecular weight and PLA with a

narrow polydispersity index ( PDI ; deﬁ ned as the ratio between the weight average

and number average molecular weights, M

). In contrast, the step - growth

condensation polymerization limits the practically accessible range of molecular

weights, and leads to a PDI of 2.

The beneﬁ ts of ROP in conjunction with a “ living ” method have enabled the

controlled synthesis of block, graft, and star polymers [134] , and this had led to

the present consensus that living ROP represents a powerful and versatile method

of addition – polymerization. In particular, coordination – insertion polymerization

has been used extensively for the preparation of aliphatic polyesters with well -

deﬁ ned structure and architecture. The most widely used initiators include various

aluminum and tin alkoxides and carboxylates. The covalent metal alkoxides or

carboxylates with vacant d - orbitals react as coordination initiators by interacting

with the oxygen of carbonyl group of the ester. These initiators are capable of

producing stereoregular polymers of narrow molecular weight distribution and

controlled molecular mass, with well - deﬁ ned end groups.

Both, Kubies et al . [135] and Lepoittevin et al . [136, 137] studied the ROP of ε -

caprolactone ( ε - CL ) onto modiﬁ ed MMTs. PCL - grafted layered silicate nanohy-

brids were thus prepared according to a controlled coordination – insertion

mechanism. For this purpose, MMT was previously modiﬁ ed by an exchange of

the constitutive Na cations by ammonium cations bearing one hydroxy function

440 13 Nanocomposites Based on Phyllosilicates

((2 - hydroxyethyl)dimethylhexadecylammonium), or simply trimethylhexadecylam-

monium [136] . In a successive study [137] , Cloisite 30 B (Table 13.5 , row b), modi-

ﬁ ed with methyl, tallow, bis - 2 - hydroxyethyl, quaternary ammonium, was also

employed.

The modiﬁ ed MMT was then treated with aluminum or tin (II or IV) com-

pounds [AlEt

, Bu

Sn(OMe)

and Sn(2 - ethylhexanoate)

] to obtain the respective

alkoxides, all of which are known to initiate the controlled polymerization of ε - CL,

by creating initiating species onto the MMT layers. The grafting of PCL onto the

MMT was achieved in the presence of ε - CL (Figure 13.13 ).

After polymerization, the molar mass of PCL, as determined by size - exclusion

chromatography ( SEC ) after a reverse ion - exchange reaction with LiCl in tetrahy-

drofuran ( THF ), was found to decrease with the increasing content of hydroxy

groups available at the clay surface. Indeed, the molar masses were 56 000, 47 000

and 28 000 for OH - deprived organomodiﬁ ed clay and clay containing 50 and 100%

monohydroxylated ammonium cations, respectively. This observation suggests, at

least qualitatively, that polymerization is initiated by the surface - anchored hydroxy

groups activated in Sn(II) alkoxides by reaction with Sn(2 - ethylexanoate)

. In the

absence of hydroxylated ammonium cations, polymerization is initiated by resid-

ual protic impurities (water, silanol, etc.). Polymerization initiation on the clay

surface and PCL growth in a “ grafting - from ” manner each have strong effects on

the morphology of the PCL/layered silicate nanocomposites, such that intercalated

or exfoliated morphologies were obtained. However, extensive exfoliation occurred

only when the silicate sheets were surface - modiﬁ ed with more than 25 wt% mono-

hydroxylated ammonium [136] . Below this value, partially intercalated/partially

exfoliated structures were found to coexist, and an intercalated structure was

obtained, starting from sodium - MMT.

A good correlation was obtained between morphology and thermal stability, as

the composites prepared with the clay that had been modiﬁ ed with hydroxyl

groups were more stable than those deprived of them. Moreover, these composites

were more stable than those containing sodium - MMT.

More recently, Chrissaﬁ s et al . [138] prepared PCL nanocomposites by following

a similar approach, but used Ti(OBu)

as the coordination – insertion catalyst.

Moreover, they performed ROPs not only onto sodium - MMT and Cloisite 20A

(Table 13.5 , row i), but also onto mica and fumed silica. In agreement with the

above ﬁ ndings, there was also a decrease in the molecular weight of PCL obtained,

compared to the pure material prepared in the absence of ﬁ llers. However, the

mechanical properties (tensile strength at break, elastic modulus, elongation at

break) of the nanocomposites were similar, or even slightly improved, compared

to those of the pure PCL.

It appears that only two reports have been made concerning the synthesis of

PLA using this method. For example, Paul et al . [139] synthesized PLA/layered

aluminosilicate nanocomposites by ROP, using two different types of OMMT

(Cloisite 30B and Cloisite 25A; see Table 13.5 , rows b and h) for the preparation

of nanocomposites. In a typical synthetic procedure, the clay was thoroughly dried

and placed in the polymerization vial. The lactide solution in dried THF was then

13.4 Poly(Lactide) (PLA)-Based Nanocomposites 441

Figure 13.13 Scheme of the synthesis of PCL nanocomposites

through in situ ROP onto montmorillonite modiﬁ ed with

ammonium salt with hydroxyl groups. Reproduced with

Society.

transferred to the vial under nitrogen, and the solvent eliminated under reduced

pressure. The polymerizations were conducted in bulk at 120 ° C for 48 h, after 1 h

of clay swelling in the monomer melt. When Cloisite 30B was used, the polymeri-

zation was coinitiated by AlEt

, while Sn(2 - ethylexanoate)

was used to catalyze the

polymerization of lactide in the presence of Cloisite 25A. The clay Cloisite 30B led

442 13 Nanocomposites Based on Phyllosilicates

to a fully exfoliated structure, whereas Cloisite 25A - based nanocomposites exhib-

ited an intercalated morphology.

In the Cloisite 30B nanocomposites, the growing polymer chains were directly

“ grafted ” onto the clay surface through the hydroxyl - functionalized ammonium

cations, yielding exfoliated nanocomposites with an enhanced thermal stability.

Notably, with increasing clay content the thermal stability was improved, with a

maximum effect at a clay loading of 5 wt%. With a further increase of ﬁ ller content,

a decrease in thermal stability was observed – an effect explained by the relative

extent of exfoliation as a function of the amount of organophilic MMT. Nonethe-

less, a decrease in PLA molecular weight was observed compared to that synthe-

sized in the absence of ﬁ ller.

A similar result was obtained more recently by Lee [140] , who modiﬁ ed the clay

surface (Cloisite 30B, Table 13.5 row b) by grafting low - molecular - weight PLA

chains (M

9400 – 21 600) through the in situ polymerization approach, following a

preliminary activation of the hydroxyl sites with Sn(2 - ethylexanoate)

. The com-

posite obtained was then melt - blended with a high - molecular - weight PLA matrix.

This novel clay/PLA nanocomposite showed a high shear - thinning behavior when

the molecular weight of the grafted PLA was higher than the critical molecular

weight of chain entanglement.

Today, the scientiﬁ c investigations in this ﬁ eld are ongoing, with the in situ

polymerization process clearly warranting further investigation. In particular, the

ROP of lactide onto clay or organoclay may represent a promising approach for

obtaining nanostructured PLA/layered silicate composites, allowing excellent

control of the structural features of those materials prepared.

13.4.1.2 Intercalation in Solution

Recently, several studies have focused on the preparation of PLA - layered silicate

nanocomposites using intercalation from solution. Initially, Ogata et al . [141] dis-

solved the polymer in hot chloroform in the presence of OMMT, but observed that

only microcomposites were formed. Marras et al. [142] adopted a similar method

but, in contrast, obtained intercalated/exfoliated structures. The differences in

experimental procedure here consisted of sonication of the organophilic MMT

in chloroform, and of the solution containing both organophilic MMT and PLA.

In the latter report, an increase in the d

001

distance was cited as a function of

the organophilic MMT modiﬁ cation, expressed as percentage of the CEC, when

investigated over the range of 0 to 250%. The degree of organophilic modiﬁ cation

was controlled by an ion - exchange reaction, using controlled amounts of

hexadecylamine.

Krikorian and Pochan [143] prepared PLA nanocomposites using dichlorometh-

ane ( DCM ) as the polymer solvent and as the organophilic MMT dispersion

medium (Cloisite 30B, 25A and 15A; Table 13.5 , rows b, h, and i). These

authors obtained intercalated or exfoliated nanocomposites, depending on the

type of organophilic MMT used. Exfoliated nanocomposites were formed by

using Cloisite 30B; that is, when hydroxyl groups were present in the organic

modiﬁ er of the clay, due to the favorable enthalpic interaction between the

13.4 Poly(Lactide) (PLA)-Based Nanocomposites 443

hydroxyl groups and the C – O bonds in the PLA backbone. Cloisite 25A, which

showed the higher starting basal interlamellar distance, gave a d

001

distance in

the nanocomposites lower than Cloisite 30B. Hence, the starting basal interla-

mellar distance, which inﬂ uences the capability of the organophilic MMT to be

dispersed in a polyoleﬁ nic matrix, is a key factor but is less important in PLA -

based nanocomposites.

Chang et al . [144] reported the preparation of PLA - based nanocomposites with

different types of OMMT via solution intercalation using N , N - dimethylacetamide

( DMA ). The preparation method allowed the production of intercalated/exfoliated

nanocomposites, providing material with an improved stiffness and barrier to

oxygen compared to pure PLA.

13.4.1.3 Intercalation in the Melt

To date, intercalation in the melt has attracted more attention that other intercala-

tion processes, with different aspects being considered by various research groups

with regards to melt blending. These include: the effects of different organophilic

modiﬁ ers on morphology; the effects of the processing conditions; and the pos-

sibility of optimizing the processing and/or properties via blending or

plasticization.

Effect of Different Organophilic Modiﬁ ers D i et al . [145] mixed two different orga-

nophilic MMTs (Cloisite 30B, Table 13.5 , row b; and Cloisite 93A) in a PLA matrix

by using an internal mixer. Cloisite 93A contains dimethyl - dihydrogenated tallow

ammonium as modiﬁ er. It was observed that only the Cloisite 30B gave exfoliated

nanocomposites, which was in good agreement with results obtained by Krikorian

and Pochan [143] for nanocomposites obtained in solution. When Feijo et al . [146]

compared two commercial organophilic MMTs, namely Dellite 43B (Table 13.5 ,

row g) and Dellite 72T (Table 13.5 , row i), the former material interacted more

strongly with PLA, as conﬁ rmed by the better dispersion of the organoclay in the

PLA matrix when compared to the neat PLA/Dellite 72T composites, which

showed aggregates of only micrometric dimensions. Moreover, the PLA/Del-

lite43B nanocomposite exhibited a slightly higher thermal stability. Hence, replace-

ment in the modiﬁ er of a long alkyl chain (hydrogenated tallow) with an aromatic

ring renders the modiﬁ er more suitable for creating interactions between a modi-

ﬁ ed nanoﬁ ller and the PLA polymer matrix. This result could, however, be ascribed

to the higher polarity of the Dellite 43B ammonium salt.

Effect of Processing Conditions Pluta [147] investigated the processing of PLA and

phyllosilicates by preparing, in a discontinuous laboratory mixer, nanocomposites

containing 3% by weight of Cloisite 30B. In particular, the blending time (6.5, 10,

20, and 30 min) and rotor rate (50 and 100 r.p.m.) were varied in order to establish

their effects on the morphology and properties of the nanocomposites. Molecular

weight changes of the PLA matrices induced by melt compounding were deter-

mined using SEC. Whilst the molecular weight was marginally reduced by increas-

ing the blending time, the level of dispersion of the ﬁ ller into the matrix was

444 13 Nanocomposites Based on Phyllosilicates

improved by increasing the blending time, as revealed by WAXD and TEM

characterization.

The behavior of PLA during melt processing depends on many factors, includ-

ing the grade of PLA used, the processing conditions applied (temperature, rota-

tion speed, residence time, atmosphere, drying efﬁ ciency of the components), and

the presence of additives and their chemical nature. Although pure PLA degrades

slightly during blending, it was noticed that the degradation of PLA was enhanced

by the presence of the nanoﬁ ller. This suggested important roles for both the

shearing forces during compounding and the interaction of PLA with hydroxy

groups of the organomodiﬁ er in determining the degradation of the PLA matrix.

Moreover, the change in rotation speed adopted during nanocomposite prepara-

tion, from 50 to 100 r.p.m., caused a decrease in molecular weight but had no

effect on the material ’ s morphology and/or properties. It was also found that an

increase in the degree of dispersion of the silicate layers led to a pronounced

modiﬁ cation of the physical properties of the nanocomposites, via an increase in

thermal stability, as revealed by TGA.

The rheological properties of the nanocomposites, as determined during a

dynamic frequency sweep, appeared to be very sensitive to the nanostructure

evolution. On investigating the rheological behavior of PLA during melt blending

with Cloisite 30B, the apparent viscosity ( η * ) of the unﬁ lled PLA was unchanged

at low frequencies (indicating a Newtonian behavior), but this was followed by a

shear - thinning response at a higher - frequency region. The η * of the nanocompos-

ites showed that, the higher the shear - thinning effect, the better the organoclay

dispersion in the PLA matrix. These observed trends allowed an identiﬁ cation of

the concentration of organoclay capable of providing a non - negligible interaction

between nanoplatelets, based on rheology experiments. Similar results were

obtained by Gu et al . [148] , who explained the same trend as a function of the

organophilic MMT content, by considering the formation of a “ percolating

network ” resulting from reciprocity among the strongly related sheet particles.

Because of this “ percolating network, ” the values of entanglement molecular

weight (M

), as calculated from master curves, were lower than that of pure PLA.

During such processing, Lewitus et al . [149] investigated the preparation and

dilution of different types of masterbatch. The study results indicated the possibil-

ity of preparing PLA/Cloisite 25A nanocomposites via the preliminary preparation

of a masterbatch which contained 20% by weight of the nanoﬁ ller and PLA. In

particular, the most suitable PLA was that with the same grade as the ﬁ nal

composite.

Control of Processability and Properties by Blending and Plasticization The addition

of a reactive compatibilizer (e.g., an ethylene copolymer functionalized with maleic

anhydride) during preparation in a discontinuous mixer of PLA/organophilic

MMT nanocomposites was monitored by Pluta et al . [150] . When polylactide - based

systems composed of an organoclay (Cloisite 30B; 3 – 10 wt%) and/or the compati-

bilizer were investigated, X - ray investigations revealed an exfoliated nanostructure

in a 3 wt% - nanocomposite. The degree of exfoliation of the organoclay was notice-