Adverse effects of thermal dissociation of alkyl ammonium ions on

nanoclay exfoliation in epoxy–clay systems

Jonghyun Park, Sadhan C. Jana*

Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, University of Akron, Akron, OH 44325-0301, USA

Received 1 March 2004; received in revised form 19 August 2004; accepted 26 August 2004

Available online 21 September 2004

Abstract

It has been shown recently that storage modulus of intra-gallery epoxy plays a crucial role in producing exfoliated clay structures in epoxy-

nanoclay systems. In this study, the possibility of thermal dissociation of alkyl ammonium ions used as cation exchange agents of layered

silicate clays and its effects on plasticization of epoxy networks and the growth of storage modulus of intra-gallery epoxy were investigated.

It was found that at cure temperatures higher than the dissociation temperature, primary amines were generated from the thermal dissociation

of alkyl ammonium ions and the excess chloride salt, which reacted readily with the epoxy molecules and formed linear chains. In addition,

such reactions resulted in an excess of diamine curing agents, which in turn caused additional plasticization of epoxy networks and lowered

the values of intra-gallery storage modulus. In such cases, only intercalated epoxy composites were produced.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Epoxy–clay nanocomposites; Nanoclay exfoliation; Diamine curing agent

1. Introduction

In situ polymerization methods have been successfully

used in the development of polymer-clay nanocomposites

with exfoliated clay structures, e.g. polyamides [1–5],

epoxies [6–18], polyurethanes [19,20], cyanate esters [21–

24], and polymethylmethacrylate [25–28] to name a few. In

a recent study on epoxy–clay nanocomposite systems [11–

13], the role of elastic force—originating inside the clay

galleries from crosslinked epoxy networks—on exfoliation

of organically treated clay particles was highlighted. The

elastic force, measured in terms of storage modulus (G 0),

was found to be responsible for exfoliation; stronger elastic

force pushes out the outermost clay layers from the tactoids

against the opposing forces comprised of the electrostatic

attraction between the clay layers and the viscous force

offered by the extra-gallery epoxy [11]. The exfoliation

process continues until all clay galleries are exfoliated or the

cross-linked epoxy turns into gel or glass, whichever occurs

earlier. Since externally applied mechanical shear forces are

0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2004.08.075

* Corresponding author. Tel.: C1 330 972 8293; fax: C1 330 258 2339.

E-mail address: janas@uakron.edu (S.C. Jana).

absent during curing of epoxy, a majority of the clay

particles remain parallel even in an exfoliated state. In these

studies [11–13], exfoliated clay structures were produced

when the values of the ratio G 0/jh*j were high; intercalated

structures resulted for small values of G 0/jh*j. Here, G 0

refers to storage modulus of intra-gallery epoxy and jh*j

refers to the absolute value of complex viscosity of extra-

gallery epoxy [11,12]. Note that in these studies, the intra-

gallery material was emulated by mixing appropriate

amounts of epoxy, curing agent, and the alkyl amine

hydrochloride. The same alkyl amine hydrochloride was

used to treat the layered silicate clays. It was also found that

the hydrocarbon chains of the alkyl ammonium ions are

capable of plasticizing crosslinked epoxy networks, thereby

reducing the values of G 0/jh*j and altering the exfoliation

behavior [12]. Fig. 1 presents a case where the value of G 0/

jh*j ratio depended strongly on the nature of the curing

agents—benzenamine, 4,4 0-sulfonylbis (DDS) and Jeffa-

mine D230—even though exfoliated clay structures were

produced in both cases. The lower values of G 0 and much

reduced glass transition temperature (Tg) of DDS-cured

intra-gallery epoxy, e.g. 159 and 197 8C, respectively with

and without n-hexadecyl ammonium chloride [12] cannot

Polymer 45 (2004) 7673–7679

www.elsevier.com/locate/polymer

Fig. 1. Evolution of G 0 and jh*j during epoxy curing (a) using

JeffaminewD230 at 125 8C and (b) DDS at 200 8C. The values of G 0 were

obtained by adding n-hexadecyl ammonium chloride in epoxy-diamine

mixture and the values of jh*j were obtained without n-hexadecyl

ammonium chloride when epoxy was cured for the same duration. Solid

lines represent interpolation of data points.

J. Park, S.C. Jana / Polymer 45 (2004) 7673–76797674

be interpreted solely based on plasticization of crosslinked

epoxy network by the alkyl chains of n-hexadecyl

ammonium chloride. The structure of DDS (Fig. 2) is

much more rigid and therefore, crosslinked epoxy networks

in DDS-cured system should be much less vulnerable to

plasticization effects.

1.1. The nature of organic ammonium ions

The ammonium ions of primary [6–8,11–14,16,18] and

tertiary [10,11,15,17,18] amines have been used in the past

to treat layered silicate clays. In epoxy–clay systems, the

ammonium ions catalyze both epoxy homopolymerization

[6,14,15] and epoxy-diamine reactions [11,12], thereby

creating a disparity in the curing rates of epoxy inside and

outside the clay galleries. The curing rate increases with the

acidity of the ammonium ions, e.g. ammonium ions of

primary alkyl amines are more acidic than quaternary

ammonium ions [7]; nanoclays, treated with primary alkyl

ammonium ions, have been used to produce transparent

clay-epoxy nanocomposites [18]. Despite extensive use of

nanoclays treated with ammonium ions of primary amines, a

Fig. 2. Chemical structures of Epon 8

thorough study on potential detrimental effects of thermal

dissociation of such ammonium ions on clay exfoliation

behavior in clay-epoxy system is still lacking. The

ammonium ions of alkyl amines, e.g. CH3(CH2)15NH2

used by our laboratory [11,12] and their chloride salts, are

capable of undergoing thermal dissociation reactions to

generate primary amines as depicted in Fig. 3. The proton,

thus generated, has been shown to catalyze homopolymer-

ization of epoxies [8]. However, the role of free primary

amines on subsequent steps has not been investigated. The

free amines may react with the epoxide functionalities

present inside the galleries or may diffuse out of clay

particles and react with the epoxide functionalities in the

bulk epoxy (Fig. 4). Such reactions present two unsavory

situations. First, due to the presence of one –NH2 group per

primary amine molecule, the epoxy-primary amine reac-

tions yield only linear cured epoxy chains or chain

segments, which do not contribute much to G 0/jh*j ratio.

Second, appreciable epoxy-primary amine reactions cause

an imbalance in reaction stoichiometry between epoxy and

diamine curing agent. The excess diamine curing agent may

cause further plasticization of the epoxy networks. There-

fore, the objective this study was to investigate such thermal

dissociation and to evaluate the adverse effects of such

thermal dissociation on clay exfoliation.

2. Experimental

2.1. Materials, preparation, and characterization

The alkyl ammonium ion was derived from a primary

amine, n-hexadecylamine, CH3(CH2)15NH2 (Aldrich, m.p.

43–45 8C, b.p. 330 8C). The alkyl ammonium chloride salt

in solid state was obtained by evaporating an aqueous

solution of n-hexadecyl ammonium chloride, CH3(CH2)15

NH3CClK. Sodium montmorillonite clay, Cloisite NAC

(Clay 1), obtained from Southern Clay Products (Gonzales,

28, JeffaminewD230, and DDS.

Fig. 3. Thermal dissociation of (a) n-hexadecyl ammonium ion and (b) its

chloride salt. In (a), only one negative charge on clay particle is shown for

illustration purposes.

J. Park, S.C. Jana / Polymer 45 (2004) 7673–7679 7675

TX) was treated in our laboratory with n-hexadecyl

ammonium chloride to produce Clay 2 containing

129 mequiv. of ammonium ion per 100 g of clay. This is

in excess of a maximum of 92 mequiv./100 g of clay,

indicating that an excess amount of n-hexadecyl ammonium

chloride was adsorbed onto the clay surfaces. An aromatic

epoxy, diglycidyl ether of bisphenol A (DGEBA),

Eponw828 of Shell Chemical (Houston, TX), with epoxide

equivalent weight of 178–190, viscosity of 11–15 Pa s, and

specific gravity of 1.15 at 25 8C was used to produce clay-

epoxy composite. Curing agents were benzenamine, 4,4 0-

sulfonylbis (DDS, HT976) obtained from Ciba (Tarrytown,

NY) and JeffaminewD230 of Huntsman Corporation

(Houston, TX), which offered significant differences in

curing temperatures. DDS is a solid (m.p. 180 8C) and

JeffaminewD230 is a liquid at room temperature. The

structures of the ingredients are shown in Fig. 2.

Clay 2, thoroughly dried in vacuum oven at 80 8C for

48 h, was intercalated by epoxy at 90 8C for 6 h. The

resulting mixture was dried in a vacuum oven at 80 8C for

30 min, mixed with stoichiometric amount of curing agent

for 5 min at 60 8C, degassed in a vacuum oven for 5 min at

60 8C, and cured in an aluminum mold. The reaction

mixtures containing JeffaminewD230 were cured at 75, 100,

and 125 8C for 3 h and those containing DDS were cured at

180 and 200 8C for 3 h. Binary mixtures of epoxy-curing

agent, epoxy-n-hexadecylamine, and epoxy-dry n-hexade-

cyl ammonium chloride were prepared at 60 8C by mixing

the ingredients for 5 min. Ternary mixtures of epoxy, curing

agent, and dry n-hexadecyl ammonium chloride and epoxy,

n-hexadecyl amine, and dry n-hexadecyl ammonium

chloride were prepared first by mixing epoxy and n-

Fig. 4. Schematic of intra-gallery reaction with epoxy and diffusion to

extra-gallery by n-hexadecylamine.

hexadecyl ammonium chloride at 90 8C for 20 min, and

then mixing the other ingredient for 5 min at 60 8C. It was

noted that the extent of epoxy polymerization during sample

preparation was negligible. The reaction mixtures were

dried in vacuum oven thoroughly and subjected to thermal

scans and isothermal curing in Dupont differential scanning

calorimeter (DSC), model DSC-2910 under nitrogen

environment at a scan rate of 10 8C/min. Thermo-gravi-

metric analysis was carried out using TGA-2950 (TA

Instruments) under nitrogen atmosphere with a scan rate of

20 8C/min. The values of G 0 and jh*j of the reaction mass

during curing of epoxy with and without n-hexadecyl

ammonium chloride respectively were measured using

RMS-800 Rheometrics rheometer with 25 mm parallel

plate set up under oscillatory shear flow. A strain of 5%

and a frequency of 5 rad/s were used.

3. Results and discussion

3.1. Thermal dissociation of n-hexadecyl ammonium

chloride

The ammonium chloride, n-C16H33NH3CClK, was sub-

jected to thermal scans in DSC and TGA to investigate its

thermal dissociation patterns. The thermal scan in DSC

showed four endothermic peaks as in Fig. 5. The

endothermic peak at 71 8C appeared due to melting of an

impurity or free amines. The peak at 103 8C is assigned to

melting of the chloride salt. The peak at 156 8C corresponds

to thermal dissociation of the ammonium chloride, which

leads to formation of n-C16H33NH2 and HCl, as in Fig. 3(b).

It appears from Fig. 5 that thermal dissociation started at

w130 8C and continued until w162 8C, with some loss in

weight of the specimen, purportedly due to volatile

hydrogen chloride. The endothermic peak at 227 8C is due

to degradation of the hydrocarbon chain as the n-hexadecyl

ammonium chloride and its dissociated products were

thermally stable up to 205 8C (Fig. 6). Since n-C16H33NH2

Fig. 5. DSC thermal scans of n-hexadecyl ammonium chloride.

Fig. 6. Thermogravimetric analysis of n-hexadecyl ammonium chloride.

J. Park, S.C. Jana / Polymer 45 (2004) 7673–76797676

has a boiling point of 330 8C, the weight loss apparent in

Fig. 6 between 162 and 210 8C was due to volatile hydrogen

chloride molecules as the HCl content in n-hexadecyl

ammonium chloride—13.2 wt%—is the same as the loss of

materials seen in Fig. 6 between 103 and 210 8C.

3.2. Catalyzed and uncatalyzed epoxy-amine reactions

Let us now evaluate if n-hexadecyl ammonium ion

influenced epoxy-amine reaction rates as well as the peak

reaction temperatures. The thermal scans of epoxy-diamine

curing agent reactions with and without n-hexadecyl

ammonium chloride are presented in Fig. 7. Fig. 8 presents

data on epoxy-n-hexadecylamine reactions. Except in the

case of DDS, epoxy-amine reactions were accelerated due

to the presence of n-hexadecyl ammonium chloride, e.g. in

epoxy-JeffaminewD230 system, the exothermic peak

appeared at 102 8C when cured in the presence of

Fig. 7. DSC scans of curing of epoxy; (a) with JeffaminewD230, (b) with Jeffami

DDS and n-hexdecyl ammonium chloride. The weight ratio of epoxy to n-hexdec

ammonium chloride (Fig. 7(b)) compared to 124 8C in

uncatalyzed system (Fig. 7(a)). Such catalytic effect was not

observed in DDS-cured system (Fig. 7(c) and (d))—the

main peak at 223 8C did not change much when cured in the

presence of n-hexadecyl ammonium chloride. Instead, a

new exothermic peak appeared at 127 8C in addition to the

usual peak seen at 221 8C, Fig. 7(d). We attribute the peak at

127 8C in Fig. 7(d) to reactions between epoxy and n-

hexadecylamine derived from the dissociation of n-

hexadecyl ammonium chloride as depicted in Figs. 4 and

5. The rationale behind such assignment comes from the fact

that the area ratio between the curves (I) and (II) in Fig. 7 (d)

is 14:86, the same as the ratio of molar amounts of n-

hexadecyl ammonium chloride and DDS in the initial

mixture. Fig. 8 shows that uncatalyzed n-hexadecyl amine-

epoxy reactions had an exothermic peak at 130 8C, while the

same reactions, catalyzed by ammonium ion showed a peak

at 102 8C. A comparison of Fig. 7(d) with Fig. 8(a) reveals

that the exothermic peak at 127 8C is indeed due to epoxy-n-

C16H33NH2 reactions.

This is further confirmed from the inspection of DSC

scans of the mixture of n-hexadecyl ammonium chloride

and Eponw828 in the absence of DDS, as in Fig. 9. The two

endothermic peaks at 77 and 104 8C, were also observed in

the thermal scan of n-hexadecyl ammonium chloride, in Fig.

5 and attributed to melting of impurities (77 8C) and n-

hexadecyl ammonium chloride (104 8C). The area of the

exothermic peak at 130 8C, labeled III, in Fig. 9 is the same

as the area of peak I in Fig. 7(d). In view of this, in DDS-

cured system, the reactions between the primary amine

derived from n-hexadecylamine hydrochloride and epoxide

groups of Eponw828 occurred earlier and at much lower

temperature than that between DDS and Eponw828.

Consequently, the faster intra-gallery epoxy curing

newD230 and n-hexdecyl ammonium chloride, (c) with DDS, and (d) with

yl ammonium chloride was 10:1. (I) and (II) represent area under the peak.

Fig. 8. DSC scans of curing of epoxy. (a) with n-hexadecylamine and (b) n-hexadecylamine and n-hexadecyl ammonium chloride. The weight ratio of epoxy to

n-hexadecyl ammonium chloride was 10:1.

J. Park, S.C. Jana / Polymer 45 (2004) 7673–7679 7677

expected due to catalytic effect of the alkyl ammonium ion

was absent in DDS-cured system. In this case, DDS-epoxy

reactions started much after n-hexadecyl ammonium

chloride dissociated and epoxy-n-hexadecylamine reactions

were completed. In view of these observations, we can now

study why the Tg was so much reduced in DDS-cured

Eponw828 system in the presence of n-hexadecyl

ammonium chloride and why only intercalated clay

composites were produced in DDS-cured system when the

curing temperature was in the range of 170–190 8C.

3.3. Elasticity of crosslinked epoxy networks

Fig. 10 shows how the values of G 0 and jh*j changed

during epoxy curing using two curing agents, DDS and

JeffaminewD230. Recall that the intra-gallery composition

was emulated by mixing epoxy, curing agent, and 3.4 wt%

n-hexadecyl ammonium chloride and G 0 represents elas-

ticity of intra-gallery epoxy networks [11,12]. The viscosity

of extra-gallery epoxy jh*j was obtained by curing epoxy

with the diamine curing agent in absence of n-hexadecyl

ammonium chloride. In each case, epoxy was cured to the

same extent as monitored by DSC. The thermal dissociation

of alkyl ammonium chloride affected the results only in the

case of DDS-cured system at 180 8C. Tests using wide angle

Fig. 9. DSC scan of a mixture of n-hexadecyl ammonium chloride and

epoxy. The weight ratio of epoxy to n-hexadecylamine hydrochloride was

10:1.

X-ray diffraction and transmission electron microscopy

revealed that, of the three cases presented in Fig. 10,

exfoliated nanocomposite was obtained only in the case of

epoxy cured at 100 8C with Jeffamine D230. This is not

surprising considering that the value of G 0 of intra-gallery

epoxy was approximately two orders of magnitude greater

than the value of jh*j of extra-gallery epoxy, e.g. for

jh*jw100 Pa-s, G 0w104 Pa. In the other cases considered in

Fig. 10, the values of G 0 were of the same order as jh*j.

Consequently, clay remained as intercalated tactoids [11].

In DDS-cured system at 180 8C, the thermal dissociation of

n-hexadecyl ammonium chloride took place readily at the

beginning of curing step. As a result, intra-gallery curing

reactions between epoxy and DDS were not catalyzed by the

alkyl ammonium ion; the values of G 0 of intra-gallery epoxy

and the values of jh*j of extra-gallery epoxy grew at

comparable rates. In addition, faster epoxy-n-hexadecyl-

amine reactions yielded linear epoxy molecules and an

Fig. 10. Evolution of G 0 and jh*j during epoxy curing. The values of G0

were obtained by adding n-hexadecyl ammonium chloride in epoxy-curing

agent mixture. The values of jh*j were obtained without n-hexadecyl

ammonium chloride in epoxy-curing agent mixture. Epoxy was cured for

the same duration while measuring G 0 and jh*j in each case.

J. Park, S.C. Jana / Polymer 45 (2004) 7673–76797678

excess of DDS, which in turn reduced the values of storage

modulus.

Fig. 12. DSC scans of epoxy-DDS mixtures (a) No clay, (b) 4 wt% Clay 1,

(c) 4 wt% Clay 2.

3.4. Epoxy–clay nanocomposites

Epoxy-diamine mixtures were separately cured in the

presence of 4 wt% clay and the results of thermal scan are

presented in Figs. 11 and 12. In each case, single exothermic

peaks were obtained, even in DDS-cured system (Fig. 12). It

was anticipated that such thermal scans would not reveal the

features seen in Figs. 7–10, as the clay content was small

and the amount of alkyl ammonium ions with treated clay

was even smaller, e.g. approximately 1 wt%. Consequently,

the absence of exothermic peak at w127 8C does not

indicate that the ammonium ions and excess salt—

129 mequiv./100 g vs. 92 mequiv./100 g maximum—did

not undergo dissociation as seen in Fig. 7(d).

These observations pose an interesting possibility for

epoxy-nanoclay systems. For clays treated with alkyl

ammonium ions of primary amines, there is a maximum

curing temperature beyond which the ammonium ions and

the excess ammonium chloride dissociate to yield primary

amines. Note that this dissociation temperature is much

lower than the thermal decomposition temperature of

hydrocarbon chains, as seen in Fig. 5. The primary amines

consume epoxies to produce linear polymers and cause

stoichiometric imbalance between epoxy and diamine

curing agents. The excess diamine curing agents then

exert plasticization effects in addition to the hydrocarbon

chains of the primary amine. This scenario was true in the

case of DDS and n-hexadecyl ammonium chloride at a

curing temperature of 180–200 8C. In view of this, a test of

compatibility between the alkyl ammonium ions and epoxy

curing conditions should be developed. In such a test, the

DSC thermal scans of the alkyl ammonium salt, as presented

in Fig. 5 for n-hexadecyl ammonium chloride, must be

carried out. The epoxy-diamine curing temperature should

Fig. 11. DSC scans of epoxy-JeffaminewD230 mixtures (a) No clay, (b)

4 wt% Clay 2, (c) 4 wt% Clay 1.

be lower than the dissociation temperature of the alkyl

ammonium salt.

4. Conclusions

This study established that proper consideration should

be given to the possibility of thermal dissociation of alkyl

ammonium ions in organically treated nanoclay while

developing clay-epoxy nanocomposites. The epoxy curing

temperature must be chosen lower than the thermal

dissociation temperature of the alkyl ammonium ions to

avoid poor nanoclay exfoliation behavior due to reduced

intra-gallery storage modulus and possible plasticization

effects.

Acknowledgements

This research was supported by National Science

Foundation in the form of CAREER grant (DMI-0134106)

to SCJ.

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