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

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Amphiphilic Poly(Oxyalkylene) - Amines Interacting with Layered

Clays: Intercalation, Exfoliation, and New Applications

Jiang - Jen Lin , Ying - Nan Chan , and Wen - Hsin Chang

459

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

14.1

Introduction

During recent years, organic – inorganic hybrid materials have attracted a great deal

of research interest due to their promising industrial applications [1 – 3] . The suc-

cessful development of Nylon - 6/ montmorillonite ( MMT ) nanocomposites by

the Toyota research group [4, 5] , reﬂ ects the importance of utilizing mineral clays

for improving polymer properties [6 – 8] , including ﬁ re retardation and gas barrier.

One of the crucial issues for preparing these advanced polymers is to overcome

the problem of incompatibility between the hydrophilic smectite - clays and hydro-

phobic polymers. Organic modiﬁ cation is the common method to convert

the hydrophilic clay into an organophile [9 – 13] . Fine dispersion of the silicate

particles in polymer matrices was then achieved for a variety of hydrophobic nano-

composites including polypropylene [14] , polystyrene ( PS ) [15] , polyurethane [16] ,

polyester [17] , epoxy [18 – 20] , and polyimide [21] .

In the past, many reviews have been devoted to the subject of clay nanocompos-

ites [1 – 5, 21 – 25] . Signiﬁ cant progress in polymer engineering and the limitations

of polymer compatibility to the nanoscale clays in intercalated and exfoliated forms

are already well reviewed. Recent research developments on utilizing the layered

silicate clays have also diversiﬁ ed into the areas of biotechnology [26] , such as the

use of anionic clays for gene therapy. Actually, recent advances in biomedical

applications involve different types of nanomaterial, which can be generally clas-

siﬁ ed into three categories according to their geometric dimensions: spherical

(e.g., silver and titanium oxide particles) [27, 28] ; ﬁ bril - shape (e.g., carbon nano-

tube) [29] ; and sheet - like (e.g., natural and synthetic clays) nanomaterials. With at

least one dimension that falls into the nanometer scale, all of these inorganic

materials possess a high - aspect ratio surface area relative to their weight. For the

sheet - like clays, each thin layer platelet with a thickness of approximately 1 nm

(almost at the molecular level) and the lateral dimension of approximately 50 nm

to several micrometers provides an extremely large speciﬁ c surface ( > 500 m

− 1

Hence, the primary structure of the thin - layer clays is unique with respect to its

460 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays

geometric shape as well as surface ionic charge, which could be important

when interacting with biological materials such as proteins, nucleic acids, and

microorganisms.

In this chapter, we review the synthetic aspects of organic modiﬁ cations of

smectite clays, with particular attention being paid to reports on the uses of various

intercalation agents for the polymer – clay composite applications, without empha-

sizing their composite performance. Recent efforts on using high - molecular -

weight poly(oxyalkylene) - diamines to tailor the layered - silicate spacing is

particularly addressed [30 – 33] . The reaction proﬁ le of the intercalation via ion

exchange by different equivalents of organic salt to clay exchange capacity, as well

as the unconventional mechanisms involving metal - ion chelation and hydrogen

bonding, are also reviewed. With the incorporation of hydrophobic amine - salts,

the resulting organoclays may exhibit unusual colloidal properties [34, 35] as well

as an ability to self - assemble into rigid - rod nanoarrays [36 – 38] . Furthermore, the

method of exfoliating the layered - silicate clays into random platelets has been

recently reported [39 – 41] . This process involved the use of polymeric amines via

zigzag conformation or phase inversion mechanisms for the platelet randomiza-

tion. The tailored organoclays are found to be suitable for embedding biomaterials

such as protein [42, 43] . The aim of this chapter is to summarize the literature

activities in clay utilization, with emphasis placed on the organic modiﬁ cations

and the emerging research in the areas of biomedical applications.

14.2

Chemical Structures of Clays and Organic - Salt Modiﬁ cations

14.2.1

Natural Clays and Synthetic Layered - Double - Hydroxide ( LDH )

Smectic clays are naturally abundant, with a well - characterized lamellar structure

of multiple inorganic plates, a high surface area, and ionic charges on the surface

[9 – 11] . The phyllosilicate clays of the 2 : 1 type, or smectites such as MMT, ben-

tonite, saponite, and hectorite, are conventionally utilized as catalysts [44 – 46] ,

adsorbents [47, 48] , metal - chelating agents [49] , ﬁ llers for polymer composites

[1, 3, 21] , and so on. The generic structure is composed of multiple layers of

silicate/aluminum oxide, for example, with layers of two tetrahedron sheets sand-

wiching an edge - shared octahedral sheet [21] . Counter metal ions populate the

composition with variation in isomorphic substitution of silicon or aluminum by

divalent metal ions such as Mg

, Ca

, or Fe

. These ionic charges are potentially

exchangeable through further ionic exchange with alkali metal ions such as Na

or Li

, as well as with organic ions. Although these layered silicates are hydrophilic

and swell in water, they often exist as aggregates in micrometer sizes from their

primary stack units. In the case of MMT, the primary stack structure possesses

multiple aluminosilicate plates of irregular polygonal shapes at average dimension

of ca. 100 nm × 100 nm × 1 nm for individual platelets [50] . For the ionic - exchange

14.2 Chemical Structures of Clays and Organic-Salt Modiﬁ cations 461

reaction, the divalent counter - cations in most natural clays could be exchanged

into different ions, including Na

, Cu

, Zn

, Mg

, Ca

and an acidiﬁ ed H

form

[51] . The replacement priority for these cations are: Al

> C a

> M g

> K

> N a

[46, 52] . According to this exchange order, organic quaternary ammo-

nium salts can replace Na

ions, but not with divalent cations in Mg

- MMT and

- MMT. Therefore, for most natural clays, the sodium ion exchange is neces-

sary to facilitate the subsequent organic ion intercalation.

Synthetic ﬂ uorine mica , which structurally is similar to sodium tetrasilicic

micas, is prepared from the Na

SiF

treatment of talc at high temperature [53, 54] .

The synthetic mica (Na

- Mica) is water - dispersible and generally used as an inor-

ganic thickener. This synthetic ﬂ uorinated mica has an average dimension of

300 – 1000 nm in 80 – 100 nm for MMT. Another class of synthetic clays, layered -

double - hydroxide d ( LDH s), can be prepared from the coprecipitation of inorganic

salts. The chemical structure is described as [Mg

(OH)

]CO

· 4H

O] in the

example of magnesium/aluminum hydroxides. Various metal hydroxides, includ-

ing Ni, Cu, or Zn for divalent and Al, Cr, Fe, V, or Ga for trivalent metal ions, and

anions such as CO

2−

, Cl

−

, SO

2−

, NO

−

, or other various organic anions, have been

reported [55 – 58] . These LDHs are classiﬁ ed as anionic clays that can be organically

modiﬁ ed through an anionic - exchange reaction, using substances such as carboxy-

lic acids, anionic polymers, organic phosphoric acids, and so on [58] . These syn-

thetic clays may have various applications, including heterogeneous catalysts,

optical materials, biomimetic catalysts, separation agents, and DNA reservoirs

[59 – 63] . Recently, Mg – Al LDHs were incorporated with poly(oxypropylene) -

bis - amindocarboxylic acid salt s ( POP - acid ) to result in a wide basal spacing of

92 Å [64] . This wide spacing, as well as the introduction of a hydrophobic POP

backbone, may open up new applications for this class of anionic clays.

14.2.2

Low - Molecular - Weight Intercalating Agents and X - Ray Diffraction d - Spacing

The common strategy for utilizing smectite clays is to alter their inherent

hydrophilic nature so that they become hydrophobic and organically compatible

with polymers. For the synthesis of polymer – clay nanocomposites, organic oniums

such as alkyl ammonium salts [22] are commonly used to intercalate the layered

minerals. The resultant organoclays are then suitable for the consequent process

of melt - blending with polymers and in situ polymerization. For example, sodium

montmorillonite (Na

- MMT), consisting of sodium ions on the silicate surface

( ≡ Si – O

−

), can be intercalated with organic onium salts. The quaternary alkyl

ammonium (R

−

) or alkyl phosphonium (R

−

) salts are the common inter-

calating agents because of their commercial availability. The incorporation of

organic intercalating agents also resulted in a silicate gallery expansion. For

example, the C

- alkyl quaternary salts may intercalate Na

- MMT, causing a layer

space expansion to 20 − 30 Å basal spacing from the pristine 12 Å clay gallery. It is

noteworthy that the same organic quaternary salt may not exchange with natural

clays with divalent counter ions such as M

- MMT [65] (where M

= Mg

or Ca

462 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays

A large number of intercalating agents for modifying the cationic smectite clays

have been described, and it is possible to classify these according to their chemical

structures and organic functionalities (see Table 14.1 ). Low - molecular - weight alkyl

ammonium and phosphonium salts are the common intercalating agents, and

normally these may result in a widening of the basal spacing in the range of 13

to 50 Å . Thermally stable and reactive surfactant types of imidazolium salts (with

, C

and C

alkyl groups) were reported for the preparation of polystyrene/

MMT nanocomposites [93] . The general idea here is that the presence of alkyl

imidazolium may contribute to the thermal stability of the composites. Other

reactive cationic surfactants, such as vinylbenzyl dimethyldodecyl ammonium

chloride and surfactants with 2 - methacryloyl functionalities, have been reported

for poly(methylmethacrylate) ( PMMA ) and PS – clay nanocomposites. The purpose

of introducing a reactive functionality to the intercalating agent is to facilitate not

only a layer exfoliation but also ﬁ ne dispersion in the polymer matrices.

14.3

Poly(Oxyalkylene) - Polyamine Salts as Intercalating Agents,

and Their Reaction Proﬁ les

14.3.1

Poly(Oxyalkylene) - Polyamine Salts as Intercalating Agents

The uses of poly(oxyalkylene) - amines of M

2000 − 4000 g mol

− 1

for the intercalation

of Na

- MMT to prepare organoclays with a large d spacing was reported in 2001

[30] . Poly(oxyalkylene) - diamine s ( POA - diamine s) are commercially available poly-

ether amines that are produced from the amination of polyols such as polyethylene

glycols, polypropylene glycols, and their mixed poly(oxyethylene - oxypropylene) [99,

100] . Both, hydrophobic and hydrophilic types of POA - diamines, poly(oxypropylene) -

amine s ( POP - amine s) and poly(oxyethylene) - amine s ( POEamine s), respectively,

are available. By using the hydrophobic POP - diamines of 230, 400, 2000, and

4000 g mol

− 1

, Na

- MMT was intercalated into organoclays at varied X - ray diffraction

( XRD ) basal spacings (15.0, 19.4, 58.0, and 92.0 Å , respectively). The lamellar

expansion was found to be proportionally correlated with the POP molecular

weights, through the POP tethering with silicate surfaces or the ionic - exchange

reaction of the quaternary ammonium ions. Within the silicate conﬁ nement,

the POP backbones may aggregate through a hydrophobic phase separation

and consequently stretch out the basal spacing of the silicate interlayer gallery

(Figure 14.1 ).

In contrast, the hydrophilic POE - diamine of M

2000 g mol

− 1

( POE2000 ) resulted

in a spacing of 19.4 Å . The conclusion has been drawn that the intercalating agent

requires a hydrophobic nature in order to expand the ionically charged platelets.

The hydrophilic POE backbones or the – (CH

– structure, resulted in

the organics associating tightly with the silicate surface. Only the hydrophobic

POP backbone – (CH

CH(CH

)O)

– has shown an ability to generate a new

“ supporting ” phase for widening the gallery space.

14.3 Poly(Oxyalkylene)-Polyamine Salts as Intercalating Agents, and Their Reaction Proﬁ les 463

Table 14.1 Representatives of the various intercalating agents described in the literature.

Functionality Intercalating agent Clay XRD ( Å ) Reference(s)

POA - diamine

salt

Poly(oxypropylene) - diamine (2000 M

Poly(oxypropylene) - diamine (POP -

amines, 4000 M

Poly(oxypropylene) - diamine (POP -

amines, 5000 M

)

ﬂ uorohectorite,

- MMT,

- Mica

46 – 92 [12, 30, 35]

Polymeric

amine

Amine - terminated PS surfactants,

Amine - terminated butadiene

acrylonitrile copolymers

- MMT 10 – 155 [66 – 68]

Alkyl - amine

salt

1 - Hexadecylamine and octadecylamine Na

- MMT 16 – 37 [69 – 76]

Amino acid 6 - Aminohexanoic acid,

12 - Aminododecainoic acid

- MMT 17 – 49 [12, 77]

Acid C

carboxylic acids and other acids,

2 - Acrylamido - 2 - methyl - 1 -

propanesulfonic acid

- or

- MMT,

- MMT

35,

Exfoliation

[65, 78 – 80]

Ammonium

salt

Hexadecyltrimethylammonium

bromide,

Dioctadecyldimethyl ammonium

bromide,

Vinylbenzyldimethyldodecylammonium

chloride,

Dialkyldimethylammonium salt from

hydrogenated tallow

- MMT 19 – 40 [69, 70, 81 – 86]

Phosphonium

salts

10 - [3,5 - bis(methoxycarbonyl)phenoxy]

decyltriphenylphosphonium bromide

Dodecyltriphenyl phosphonium

bromide,

Hexadecyltributyl phosphonium

bromide,

Tetraoctyl phosphonium bromide

- Mica,

ﬂ uorohectorite,

- MMT

24 – 32 [72, 87 – 92]

Imidazolium

salt

1,2 - Dimethyl - 3 - (benzyl ethyl isobutyl

polyhedral oligomeric silsesquioxane)

imidazolium chloride ( DMIPOSS ),

Dioctadecyl imidazolium,

1,2 - Dimethyl - 3 - hexadecyl imidazolium,

vinyl - alkyl - imidazolium (C

, C

and

)

- MMT 36 [93 – 96]

Stibonium

salt

Triphenylhexadecyl stibonium

triﬂ uoromethyl sulfonate

- MMT 20 [97]

Poly(ethylene

glycol)

PEG400 vs. POP - amine

N a

- MMT 17.7 [98]

a POP - amines: poly(oxyalkylene) - amines of M

230 – 4000.

464 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays

Figure 14.1 Schematic representation of Na

- MMT

intercalation by hydrophilic and hydrophobic

poly(oxyalkylene) - diamine salts of the same molecular

weight [31] .

It was noted that formation of the hydrophobic POP phase in the silicate gallery

could be understood by estimating the theoretical length of the fully stretched POP

backbone. Based on calculations of the bond lengths (1.54 Å for C – C and 1.43 Å

for C – O) and bond angles (109.6 ° and 112 ° , respectively), the theoretical length

(77 Å ) for POP2000 was longer than the value of 58.0 Å observed for those mole-

cules in conﬁ nement. Apparently, the POPs were aggregated into hydrophobic

phase but not fully extended at the molecular level, nor arranged in a tilting ori-

entation in the gallery conﬁ nement.

14.3.2

Critical Conformational Change in Conﬁ nement During the Intercalating Proﬁ le [31]

The intercalation proﬁ le was studied by varying the equivalent ratios (from 0.2 to

1.0) of POP2000 salt to the clay CEC (120 mEq 100 g

− 1

). Critical intercalation points

were apparent, as shown in Figure 14.2 , for the clay expansion with respect to the

POP intercalants. Before the critical point, the basal spacing expansion was similar

(19.0 − 20.2 Å ) and in the range of amine addition, from 0.2 to 0.8 CEC equivalent.

14.3 Poly(Oxyalkylene)-Polyamine Salts as Intercalating Agents, and Their Reaction Proﬁ les 465

Figure 14.2 Schematic representation of POP - diamine

forming a hydrophobic phase in a clay gallery [31] .

Figure 14.3 Critical points of poly(oxyalkylene) - diamine salts

intercalating montmorillonite at different CEC ratios [31] .

At 0.8 CEC equivalent, the POP addition expanded the d spacing suddenly to

58 Å in the case of POP2000 intercalation.

The critical concentration for the basal spacing expansion is generalized

for several hydrophobic polyether - amines, as summarized in Figure 14.3 .

The POP4000 and poly(oxybutylene)diamine of 2000 g mol

− 1

( POB2000 )

showed a critical point for the intercalation (Figure 14.3 ,

䉲

and

䊏

, respectively).

The increases in basal spacing, from 20 Å to 92 Å for POP4000, and from 20 Å to

54 Å for POB2000, were observed and attributed to the molecular length,

POP4000 > POP2000 > POB2000. The existence of a critical concentration was

explained by the POP hydrophobic phase generated in the layer gallery. For the

POE - backbone amine salt, there was no change in the critical basal spacing.

466 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays

14.3.3

Correlation between MMT d - Spacing and Intercalated Organics [32, 33]

The basal spacing enlargement can only be achieved by a hydrophobic aggregation

of the intercalating agents in the gallery. By taking in consideration the molecular

end - to - end length in the conﬁ nement, a linear correlation was found for the basal

spacing expansion. As the incorporated organics occupy the same volume as the

measured silicate spacing, the relationship between the interlayer spacing ( D ) and

the organic weight fraction ( R or w/w of POP - amine/MMT by weight) is expressed

by the following equation [32] :

t=×

(14.1)

where A is the surface area of MMT, t is the plate thickness, and ρ is the density

of the intercalant in the gallery. The XRD measurement is a function of the organ-

ics occupied in the gallery, and the relationship of gallery distance, volume, and

organic fraction is predictable.

14.4

Amphiphilic Copolymers as Intercalating Agents

14.4.1

Various Structures of the Amphiphilic Copolymers [34, 40, 101, 102]

In order to tailor the hydrophobic nature for organoclays, several research groups

have used amphiphilic copolymers as intercalating agents. For example, the copol-

ymers of POP - amine grafting on poly(propylene) ( PP ) [34, 40] , poly( styrene -

ethylene/butadiene - styrene ) ( SEBS ) [101] and poly( styrene - maleic anhydride )

( SMA ) [102] copolymers were reported. These copolymers, which comprised mul-

tiple amines as the pendant groups, were comb - like in shape and amphiphilic in

nature (Figure 14.4 ). The copolymers, after treating with HCl, were able to form

stable emulsions in water and to intercalate with Na

- MMT. The generation of an

emulsion at 670 nm diameter for the amine - grafted PP copolymer at ambient

temperature, and at 560 nm at 75 ° C in toluene/water, has also been reported [34] .

The ﬁ ne emulsion rendered the copolymers able to intercalate with Na

- MMT. The

pendant quaternary ammonium ions of the copolymers may undergo an ionic -

exchange reaction to afford the silicates of 19.5 Å spacing. The layered silicate

platelet gallery was widened and surrounded with hydrophobic copolymer back-

bones. The resultant silicate/copolymer hybrids resembled a single micelle struc-

ture containing a hydrophilic rigid silicate core and a hydrophobic organic corona.

The hybrids were dispersible in toluene and were found to be approximately

500 nm in size in the example of PP - POP2000/MMT.

14.4 Amphiphilic Copolymers as Intercalating Agents 467

Figure 14.4 Chemical structures of POA - diamines and the grafted PP copolymers [34, 40] .

The POA - diamine grafting onto SEBS - g - MA generated another class of comb -

like and amphiphilic polyamines. The particular SEBS – POA copolymer, with an

average of nine amine pendants, is capable of forming a ﬁ ne emulsion and inter-

calating with Na

- MMT to afford a wide range of XRD d spacing, from 17 Å to

52 Å . The high d spacing was due to SEBS backbone intercalation, while the POA

intercalation could result in only a low spacing. Thus, it was concluded that two

different intercalating modes were present, the conceptual description of which is

shown in Figure 14.5 .

The amphiphilic properties of the copolymers can be better controlled by using

poly( styrene - co - maleic anhydride s) ( SMA ) grafted with the POA - diamines. The

468 14 Amphiphilic Poly(Oxyalkylene)-Amines Interacting with Layered Clays

Figure 14.5 Schematic representation of copolymer/clay

hybrids in different forms of intercalation [101] .

copolymers with different ratios of styrene/MA monomers provided a wide range

of hydrophobic properties. The SMA - POP copolymers generated the intercalated

organoclays with 12.9 Å to 78.0 Å basal spacing. Two types of intercalation were

also observed for the SMA - POP intercalation.