( i 

Polymer-Layered Silicate 

Nanocomposites From Model 

Surfactants at High Coverage 

by Frederick L. Beyer, Arnab Dasgupta, 

and Mary E. Galvin 

Approved for public release; distribution is unlimited. 
The findings in this report are not to be construed as an 

official Department of the Army position unless so 

designated by other authorized documents. 

Citation of manufacturer’s or trade names does not 

constitute an official Cndorsement or approval of the use 

thereof. 

Destroy this report when it is no longer needed. Do not 

return it to the originator. 

1 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2742 Mav 2002 

Polymer-Layered Silicate 

Nanocomposites From Model 

Surfactants at High Coverage 

Frederick L. Beyer 

Weapons and Materials Research Directorate, ARL 

Arnab Dasgupta and Mary E. Galvin 

University of Delaware 

Approved for public release; distribution is unlimited. 
Abstract 

A series of experiments has been designed and conducted to test a recent model 

for the morphological behavior of polymer-layered silicate (PIS) 

nanocomposites, where a ITS nanocomposite is comprised of a homopolymer 

matrix, layered silicate clay mineral filler, and an organic surfactant modifier. 

For nearly complete exchange of naturally occurring interlayer cations with 

model surfactants synthesized for this study, the effect of surfactant length on 

the morphology of polymer-layered silicate nanocomposites has been examined. 

Here the surfactant and matrix homopolymer are both based on polystyrene and 

are therefore expected to be miscible. Small-angle x-ray scattering data show 

that there was no intercalation of the homopolymer into the modified layered 

silicate. This finding is consistent with autophobic dewetting such as that which 

occurs with densely grafted polymer brushes, based on high surfactant coverage 

within the layered silicate galleries. 

ii 
Acknowledgments 

The authors wish to thank Professor Darrin Pochan for assistance with 

small-angle x-ray scattering (SAX!3) data collection, Frederick Cox for matrixassisted laser desorption ionization (MALDI) analysis, and Professor Anna 

Balazs for helpful interpretation of the theoretical aspects of this work. In 

addition, Mary E. Galvin gratefully acknowledges funding from the American 

Chemical Society (ACS) Petroleum Research Fund and the U.S. Army. 

. 

. . . 

ill 
. 

INTENTIONALLYLEFTBLANK. 

iv 
Contents 

Acknowledgments . . . 111 

List of Figures vii 

List of Tables vii 

1. 

2. 

3. 

4. 

5. 

Introduction 

Experimental 

Results and Discussion 

Conclusions 

References 

1 

3 

5 

11 

13 

Distribution List 15 

Report Documentation Page 17 

V 
vi 
l 

List of Figures 

Figure 1. Representative TEM micrograph showing the separation of 

silicate layers after modification with PS-based sutfactant, sample 

M-7. No PS homopolymer is present in this sample. .. . . . . . . . . . . . . . . . . . . . . . . . . . 

Figure 2. SAXS data for modified clays alone and nanocomposites after 

annealing, showing negligible change in Bragg peak locations. The 

unchanged peak locations reveal no intercalation of the PS 

homopolymer into the PS-surfactant modified montmorillonite. . . . . . . . . . . . 

Figure 3. SAXS data for modified clays before and after annealing with 

PS homopolymer for the two longest surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

b........... 7 

........... 7 

........... 8 

List of Tables 

Table 1. Surfactant molecular characteristics. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-.............. 6 

Table 2. Characterization data for modified clays. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Table 3. Composition and morphological characterization data for 

PLS nanocomposites based on model PS-based surfactants and 

10,000 g/mol PS homopolymer. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 

Table 4. Calculations for comparison of coverage data with behavior 

predictions from autophobic dewetting theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Vii 
INTENTI~NMLY LEE BLANK. 

. . . 

Vlll 
1. Introduction 

Polymer-layered silicate (KS) nanocomposites have generated substantial 

interest in the materials community in the last decade, beginning with the 

publication of the findings from work at Toyota [l-3]. These works focused on 

the synthesis and physical properties of PLS nanocomposites based on nylon 6. 

The resulting materials exhibited substantively improved properties. Most of the 

subsequent work on PLS nanocomposites has also focused on using equally 

direct methods to produce materials with similarly improved physical properties 

and specific morphologies [4,5]. While much has been learned, one fundamental 

aspect of the behavior of these materials that has not been elucidated is the 

underlying thermodynamic relationships between the components in the system, 

which at equilibrium determine the morphological behavior of the final 

nanocomposite. 

PLS nanocomposites contain two basic components -a 2:l mica-type layered 

silicate clay mineral and a polymer matrix. Layered silicate clay minerals are 

composed of individual microscopic sheets -1 run thick [6, 7J. The individual 

sheets stack together to form overall macroscopic clay particles called tactoids. 

In a 2:l layered silicate the sheets have three layers: an inner layer of alumina 

octahedra sandwiched between outer layers of silica tetrahedra, in which 

isomorphous substitutions of different metals for aluminum and silicon create a 

net charge imbalance within the sheet. Hydrated cations present in the galleries 

eliminate this imbalance and through a cation exchange reaction, can be replaced 

with organic surfactants. 

With the exception of hydrophilic polar polymers such as poly (ethylene oxide 

[8]), exchange of the naturally occurring cations in the silicate galleries with 

small-molecule organic surfactant has been found to be helpful for the formation 

of PLS nanocomposites. PLS nanocomposites are most often fabricated by either 

in situ polymerization of the matrix monomer in the presence of the modified 

layered silicate, solution intercalation where both the matrix polymer and clay 

are dispersed in a common solvent followed by precipitation, or melt processing 

that typically involves mechanical mixing of the matrix and filler. All three 

techniques limit the polymer systems from which PLS nanocomposites can be 

made. In situ polymerization methods affect the molecular characteristics of the 

polymer matrix. Solution processing requires the availability of a cosolvent for 

the hydrophilic clay and the often hydrophobic polymer. Mechanical processing 

places limitations on the stability and physical properties of the polymer matrix 

and can adversely affect the silicate clay layers. Additionally, these processes do 

not produce materials definitively at their thermodynamic equilibrium. The only 

remaining fabrication technique, direct melt intercalation [S-lo], is difficult and 

1 
often unproductive. As a result, little information is available about the 

underlying thermodynamic relationships between layered silicate, surfactant, 

and matrix polymer. Understanding these relationships should allow the 

fabrication of PLS nanocomposites from materials chosen for their specific 

properties rather than formation of PLS nanocomposites from only materials that 

are conducive to one of these three techniques. 

Recent work by Balazs, Singh, and Zhulina has provided a theoretical framework 

for probing the behavior of these materials in terms of the physical characteristics 

of the constituent materials [ll]. The model calculates free energy as a function 

of the spacing between two surfaces representing sheets of layered silicate clay 

mineral. It incorporates the areal density of a surfactant on the silicate surface, or 

coverage, given as p (chains/lattice unit), the length of the surfactant, NG, and 

the length of intercalating homopolymer, N. Enthalpic interactions are given in 

terms of separate x values, where x is a Flory-Huggins-type interaction 

parameter. The parameter for surfactant and polymer is given as x for silicate 

surface and surfactant, xssuTf, and for silicate surface and polymer, xsWf. The 

model predicts a broad range of morphological behaviors depending on the 

enthalpic interactions between polymer and surfactant. A second result 

predicted is that the length of surfactant should influence the morphological 

behavior by affecting the degree to which the entropic penalty to an intercalating 

homopolymer is reduced; lengthening the surfactant should facilitate 

intercalation by providing a greater reduction in the entropy penalty to the 

intercalating homopolymer. 

In this work, a series of model ITS nanocomposite samples were fabricated and 

characterized to investigate the effect of surfactant length on nanocomposite 

morphology for the simplest case probed in the Balazs model. In that case, the 

surfactant and homopolymer have neither attractive nor repulsive enthalpic 

interactions o( = 0). The layered silicate montmorillonite was modified with a 

series of polystyrene @)-based surfactants spanning a wide range in molecular 

weight. Unlike most recent work with F’S nanocomposites, where solution and 

in situ polymerization fabrication techniques have been used to fabricate the ITS 

nanocomposites [12-151, direct melt intercalation of a polymer melt was used to 

avoid formation of non-equilibrium structures. The modified clays were mixed 

with P!3 homopolymers, then annealed for a period of time estimated to allow the 

samples to approach equilibrium. The molecular weights of the surfactants and 

I?!3 homopolymer were chosen for direct comparison to the Balazs model. The 

resulting composites were then characterized to determine if there was any 

morphology effect correlated with the surfactant molecular weight. 
1 

2. Experimental 

Na-montmorillonite (SWy-2) layered silicate was obtained from the Missouri 

Clay Repository. Impurities were removed by dispersing the clay in deionized 

water and then centrifuging. The re making clean clay mineral was dried, then 

ground with a mortar and pestle into a powder. The cation exchange capacity 

(CEC) of th e c 1 a y was given as 76.4 meq/lOO g. The measurement technique and 

conditions by which CEC was determined are not specified. Montmorillonite 

has an internal surface area of -750 mz/g, and a total surface area of 800 m*/g 

[16]. For calculations of coverage, the more conservative total area was used. 

ES was chosen as the basis of both the surfactant and matrix materials. This 

eliminated attractive and repulsive enthalpic interactions between the surfactant 

and polymer, where by definition x = 0. Amine-terminated ES surfactants were 

prepared following a method developed by Quirk and Lee [17]. Amine end 

functionalization could be achieved in nearly quantitative yield. A peak at 6 

= 67.9 ppm in the ‘SC-NMR spectra of the samples scanned in acetone-d6 

confirmed the presence of the -N(CH+ carbons in the polymer. TLC in a 

mixture of ethyl acetate and hexane (3:7, v/v) was done to detect presence of 

unfunctionalized polystyrene in the samples. Amine-terminated ES could be 

separated from the unfunctionalized fraction by column chromatography using 

the same solvent mixture as an eluent. The amine-functionalized polymers were 

then quatemized quantitatively by refluxing overnight with an excess of 

dimethylsulphate in dry tetrahydrofuran (THF) and precipitated from a 

concentrated solution in methanol. In the molecular weight range of the samples 

studied, all amine-functionalized polymers show significant elution behavior in 

THF, while the quaternized samples do not elute at all. TLC of the 

dimethylsulphate-treated products in THF showed that the spot did not climb 

(elute) at all confirming absence of any amine-terminated ES in the samples. 

Quatemary ammonium [ - (CH$N+] end functionalized PS surfactants with 

number averaged molecular weights of 17,600, 11,200, 6800, 2700, and 

1735 g/mol were thus prepared. Molecular weights and molecular weight 

distributions were obtained from gel permeation chromatography (GPC) of the 

amine-functionalized F’S samples. Analyses were performed using THF as the 

solvent eluting at a flow rate of 1 mL/min through two PL gel 5-u mixed-D 

columns connected in series and equipped with a Hewlett-Packard (HP) 1047A 

refractive index (RI) detector. For the sample having a molecular weight of 

1735 g/mol, matrix-assisted laser desorption ionization (MALDI) spectra were 

obtained in a Bruker biflex-III instrument operating in reflector mode. 

3 
A goal of complete cation exchange of the IS-based surfactant with the naturally 

occurring interlayer cations was selected for simplicity. This is consistent with 

many commercially available modified layered silicates, although generally the 

surfactant in those materials are short-chain alkyl quaternary amines. Complete 

surfactant exchange was also hoped to minimize interactions between the 

hydrophobic homopolymer and hydrophilic clay sheets. The modification of 

montmorillonite with the quaternary amine-terminated ES surfactants was 

carried out by treating a dispersion of the clay in a 3:l (v/v) mixture of water and 

either acetone or THF, with a solution of the surfactant in the same solvent used 

for clay dispersion. The surfactant solution was introduced all at once into the 

clay dispersion, resulting in the immediate formation of a white precipitate. The 

precipitate was filtered and washed several times with THF to remove any 

unbound ES and was dried under vacuum overnight at 105 “C to remove 

residual solvent and water before x-ray characterization. Five surfactant 

molecular weights were ultimately examined, ranging from a short surfactant 

-16 repeat units long to a polymeric surfactant -167 repeat units long. 

Surfactant content and coverage of the silicate surfaces was determined from 

thermogravimetric analysis (TGA) of the modified clays using a TA Instruments 

Hi-Res TGA 2950. A small amount of each modified clay was heated from room 

temperature through 650 “C at a rate of 10 OC/min. The residue at 650 “C was 

taken as the clay content, and the weight loss was used to determine surfactant 

coverage. 

The PL!S nanocomposites were fabricated by manually mixing the modified clay 

with a low polydispersity (PDI I 1.06), 10,000 g/mol, P!S standard from Pressure 

Chemical Company. No solvent was used in this process. In a small vial, a 

specific amount of modified clay was added to a specific amount of the 

commercial l?!3 homopolymer. The weight of modified clay added to a given 

weight of F’S homopolymer was adjusted so that in each sample, the weight of 

the montmorillonite clay mineral was -5% of the total sample weight. These 

physical mixtures were pressed into pellets with contact pressure, annealed 

under vacuum for 1 week at 125 “C, and cooled under vacuum to room 

temperature. An estimate of the time it might take a PS homopolymer of 

10,000 g/mol molecular weight to self-diffuse over a distance of 0.5 urn, half the 

lateral dimension of a large clay tactoid, was taken as a guide to the minimum 

annealing time necessary. Based on work done in the Rouse regime, diffusion 

coefficient of a 10,000 g/mol PS chain in a matrix of 10,000 g/mol PS was 

calculated to be 3 x 10-n cm2/s [X?]. Using the Einstein rule, a diffusion time of 

-24 hr (at 116 “C) is calculated for 0.5 urn. This estimate is conservative for three 

reasons: (1) A Rouse model of chain dynamics fit the data used while in the 

current work-Zimm behavior is expected since 10,000 g/mol is well below the 

entanglement molecular weight of F’S [l&20]; (2) The annealing temperature in 

this experiment is slightly higher at 125 “C than in reference [18]; and (3) The 

4 
i 

average particle size of Wyoming montmorillonite is likely much less than 1 pm 

in diameter. Balancing a concern for adequate annealing time was the possibility 

of degradation of the samples if left at elevated temperatures for too long. 

Therefore, an annealing period of 1 week, at 125 “C was used in this work. 

Small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) 

were used to characterize morphology. Most SAXS data were collected at the 

National Synchrotron Light Source at the Brookhaven National Laboratories, 

Upton, NY. Data were collected at a camera length of 208 cm and a wavelength 

of 1.605 A, using a Bruker AXS 2D CCD detector. Data for the unmodified 

montmorillonite were collected using a Siemens D5005 powder diffractometer, 

using Cu-K, radiation. Two-dimensional (2-D) data were corrected for detector 

dark current, detector sensitivity, and background scattering where Kapton film 

was used to support the sample. The isotropic 2-D data sets were then circularly 

averaged for examination of intensity as a function of scattering angle. No 

correction was made for differing absorptions based on varying clay content. 

TEM was completed using a JEOL 200CX operated at 200-kV accelerating 

voltage. Microtomed sections were prepared using a Leica FCS Ultracut 

microtome and a Microstar diamond knife. 

3. Results and Discussion 

The sample nomenclature used in this work describes the salient molecular 

characteristics of the surfactants and homopolymers used in each sample. Each 

sample is described with a label such as M-A-B, where “M” indicates that 

montmorillonite was the clay mineral, “A” is the approximate molecular weight 

of the surfactant modifier in kilograms per mole, and ‘9” is the approximate 

molecular weight of the ES homopolymer matrix, in kilograms per mole. For 

example, sample M-2-10 is composed of montmorillonite modified with the 

1735 g/mol surfactant, which is contained in a matrix of ES homopolymer 

10,000 g/mol in molecular weight. Sample M-2 refers to the modified 

montmorillonite alone, with no homopolymer matrix. 

Table 1 describes the molecular characteristics of the surfactants synthesized for 

this experiment. The polydispersity index (PDI) is given as M,/M,. As a result 

of the living anionic nature of the polymerization of the base polystyrene 

oligomer or polymer, the polydispersities of the oligomers, polymers, and the 

surfactant end product after quaternization are quite low. 
Table 1. Surfactant molecular characteristics. 

Table 2 lists the molecular and morphological characteristics of the modified 

clays. Weight-percent clay was determined using TGA, and from that measure, 

coverage and area/cation were calculated. With the exception of modified clay 

M-18, all coverages were in excess of 75%, with an average area per cation 

ranging from 1.9 to 3.2 nm*/cation. Such coverages are similar to those found in 

commercially available short-chain alkyl-modified silicate fillers. 

Table 2. Characterization data for modified clays. 

Table 3 gives the composition of the PLS nanocomposites fabricated using 

modified clays and I’S homopolymer. The weight of modified clay and weight of 

homopolymer in the sample were recorded at the time the samples were 

fabricated. The weight of clay mineral was calculated using TGA data for 

composition and the weight of modified clay in each sample. The total weight 

percentage of clay mineral is based on the total weight and the weight of the clay 

mineral in each sample. 

Table 3. Composition and morphological characterization data for PLS nanocomposites 

based on model PSbased surfactants and 10,000 g/mol E’S homopolymer. 

Weight 

Weight Modified Weight PS Weight-Percent Standard Gallery 

Sample Clay Clay Homopolymer Clay * Deviation d 

J-l) (A-1) (A 

Height 

63) 

n743 4.7 

i? A7 

k) k) 

M-2-10 0.013 0.024 

M-3-10 0.014 0.036 

M-7-10 0.0080 0.041 

M-11-10 0.014 0.121 

M-18-10 0.014 0.143 

-.-. 

0.25" 

f-l135 

&., 

AR -.*- *.- 

0.156 5.2 0.0472 ( 

0.150 5.1 0.0408 ( 

6 
l 

Figure 1 shows a TEM micrograph collected from modified clay sample M-7, 

showing clearly the separation of silicate layers resulting from modification of 

the clay surfaces with ES-based surfactant. TEM data were not collected for all 

the modified clays due to the consistency of SAXS data shown in Figures 2 and 3. 

Figure 1. Representative TEM micrograph showing the separation of silicate layers after 

modification with PS-based surfactant, sample M-7. No F’S homopolymer is 

present in this sample. 

/ ” ‘1 I “, I ‘, 

. 

M-24 

0 0.05 0.1 

q (l/A) 

cl.15 0.2 

Figure 2. SAXS data for modified clays alone and nanocomposites after annealing, 

showing negligible change in Bragg peak locations. The unchanged peak 

locations reveal no intercalation of the F’S homopolymer into the 

PS-surfactant modified montmorillonite. 

7 
0 0.05 0.1 0.15 02 

q WA) 

Figure 3. SAXS data for modified clays before and after annealing with Ps homopolymer 

for the two longest surfactiints. 

Figure 2 shows SAXS data collected for the modified clays and PLSnanocomposites M-2, M-2-10, M-3, M-3-10, M-7, and M-7-10. Figure 3 shows the 

SAXS data collected for the modified clays and PIS-nanocomposites M-11, 

M-11-10, M-18, and M-18-10. In both cases, the intensity data have been scaled 

for convenience of having several sets of data in each graph and are given as 

In(I); thus, the intensity units are arbitrary and not to scale. A Gaussian 

probability function with a sloping linear background was fit to the primary 

Bragg reflection. The spacing of the first order Bragg reflection, q*, was taken as 

the mean of the Gaussian peak. The standard deviation is reported as a measure 

of the breadth of the first order reflection. Tables 2 and 3 list q*, standard 

deviation, the. corresponding real space length d, where d = 2x/q*, and gallery 

height (d - 10A) for the modified clays and PLS nanocomposites, respectively. 

The intent of this work was to examine the effect on equilibrium morphological 

behavior of the relationship between surfactant length and intercalating 

homopolymer in a system in which the enthalpic interactions between surfactant 

and homopolymer are minimal. Specifically, when x = 0, does lengthening the 

surfactant affect the morphological behavior of this PLS nanocomposite? 

The SAXS data shown in Figures 2 and 3 reveal the morphological behavior of 

the PJS nanocomposites fabricated for this work. For each modified clay, the 

location of the primary Bragg reflection (given in Table 2) is shifted to lower 9 

than for unmodified clay, which when dry has a periodicity of -10 A. This 

indicates the success of the cation exchange procedure. This modification is 

8 
confirmed by TGA data that allow calculation of coverage based on the cation 

exchange capacity and molecular weight of the surfactant modifier. Very high 

exchanges and coverages were achieved for each surfactant. 

More important, however, is the lack of change in SAXS data between each 

modified clay sample and the corresponding PLS nanocomposite. The SAX 

data reflect the morphology of each sample unambiguously. For a sample where 

intercalation occurs, the SAXS data should show a shift to lower 4 of the primary 

Bragg reflection, relative to 4” for the modified clay only. Higher-order Bragg 

reflections would also be shifted for intercalation. For exfoliation, the SAXS data 

should show basically no Bragg reflections and only a peak representing a mean 

correlation length between individual dispersed silicate layers. However, the 

samples characterized in this work show essentially no change in the SAXS data, 

consistent with no change in the structure of the feature giving rise to x-ray 

contrast, the silicate layers. Therefore, no intercalation is occurring in these 

materials, and only phase-separated morphologies are formed. 

Comparison of these data with the Balazs model indicates two possible 

explanations for the phase-separated morphologies observed. The Balazs model 

predicts that for a certain surfactant coverage across the silicate surface, ignoring 

unfavorable enthalpic interactions between silicate and intercalating 

homopolymer, various behaviors of free energy as a function of gallery height 

can be expected for differing enthalpic interactions (x) between surfactant and 

homopolymer. Specifically, for x = 0, with a surfactant 25 units long, a polymer 

100 units long, and a surface density of p = 0.04 chains/lattice site, the Balazs 

model predicts that an intercalated morphology wilI be formed. These 

conditions are met by sample M-3-10, with the exception of coverage. The Balazs 

model is a mean-field model with a lattice having a dimension equal to that 

of the Kuhn length of the polymer in question. For a E-based system, 

p = 0.04 (chains/l a tti ce area unit) corresponds to an area of 25 . (0.69 run)*, or 

11.9 nm*/surfactant [21]. In the case of sample M-3, the area/cation is calculated 

to be -2.2 1123112, which corresponds to p = 0.22. Thus, it is clear that in regard to 

coverage, the modified clays fabricated for this work fail outside the range 

considered in the Balazs model predictions. 

Additionally, Balazs et al. [ll] examined the effect of surfactant coverage on the 

free energy potential as a function of gallery height and thus equilibrium 

morphological behavior. For x = 0, a surfactant 25 units in length, a 

homopolymer 100 units in length, and a surface coverage of p = 0.12 or 

4.0 nm*/surfactant, they found the free energy potential in this case was shifted 

significantly toward behavior indicative of phase separation; for all gallery 

heights, the change in free energy was less, and the minimum was shallower. It 

is reasonable to expect that at coverages comparable to those for these samples 

(p = 0.22), the Balazs model would predict phase separation. 

9 
Finally, the phase separation of the modified clays from the PS matrix is 

consistent with autophobic behavior previously observed in polymer brush 

systems, such as flocculation of surfactant-coated colloidal particles [22], and 

described well theoretically [23, 241. In the nomenclature of these works, a 

polymer brush is densely grafted if &N > (N/P)2, where o is the grafting 

density, N is the degree of polymerization of the grafted brush, and P is the 

degree of polymerization of the polymer melt. Table 4 gives these values for the 

modified clays in this work and also for the conditions used in the Balazs model, 

where surfactant density is p = 0.04, the surfactant is 25 units long, and the 

polymer melt is 100 units long. Because the relationship above is a scaling 

relationship, it is most convenient to examine f(o;N,P) = (&‘)/(N/P)2, which 

should be greater than some constant. For the purposes of this work, it is 

sufficient to note whether a modified clay has a more densely grafted brush than 

the hypothetical modified clay considered in the Balazs model. Therefore, Table 

4 lists f calculated for each modified clay made for this work, normalized by a 

value fB. Here, fB is the value of f calculated for the hypothetical sample 

conditions used by Balazs et al. [ll]. In this manner, a sample that has a brush 

more dense than that in the Balazs model (fB), will be indicated by a value of 

f/fB=- 

Table 4. Calculations for comparison of coverage data with behavior predictions from 

autophobic dewetting theory. 

Sample 0 (nm”) N OJN (N/P)2 ( f 

Balazs Model 0.084 25 0.42 0.063 I 

I f/fB . 

6.7 I 1.0 

M-2-l 0 0.38 16 1.5 0.030 51 7.6 

M-3-10 0.45 25 2.3 0.073 31 4.6 

M-7-l 0 0.43 64 3.4 0.46 I 7.5 ) 1.1 

1 M-11-10 I 0.53 1 106 I 5.5 I 1.3 4.2 1 0.63 

M-18-10 1 0.31 ( 167 4.0 [ 3.1 1.3 1 0.19 

One finds that, except for the two longest surfactants, which are essentially equal 

to or longer than the matrix homopolymer, f/fB is greater than unity. h those 

samples, M-2, M-3, and M-7, and corresponding PLS nanocomposites, the 

density of the surfactant is at a minimum greater than that used in the Balazs 

model and in the case of the shortest surfactants, significantly so. Under these 

conditions, one would anticipate the existence of a net attractive force between 

particles (silicate layers), which would encourage them to flocculate. In the case 

of surfactant-modified layered silicates, this corresponds to phase separation 

with the homopolymer matrix (or a lack of deflocculation). Phase separation 

allows the best interaction between adjacent polymer brushes, or modified 

silicate surfaces. 

The steadily decreasing value of normalized f/fB also seems to reflect the 

prediction of the Balazs model that a longer brush can facilitate better interaction 

10 
between surfactants and the polymer matrix. That sample M-18-10 givesf/f81 but 

still forms a phase separated morphology, indicating that even flfB < 0.19 may 

include brushes of sufficient density to cause autophobic dewetting. This result 

may also be due to the lower coverage of M-18-10 in comparison to the other 

modified clays, allowing increased interaction between the hydrophilic silicate 

surface and the hydrophobic F’S matrix, a condition not expressly examined by 

Balazs et al. Ill]. 

A final consideration in the specific parameters used in the model and the 

applicability of the model parameters to a real system. One interaction not 

specifically taken into account is the silicate surface-surfactant interaction, 

included in the model as xssurf. The strongly hydrophilic surface of the clay layers 

and the hydrophobic PS surfactants would be expected to repel one another. 

This interaction is not considered in the specific predictions of the model for the 

hypothetical nanocomposite, as x ssurf is set to zero. Another example that would 

be more relevant at low coverages is the interactions between the silicate surface 

and the intercalating homopolymer, also a factor incorporated into the model, 

but not specifically investigated in the cited works. 

4. Conclusions 

A systematic investigation of surfactant length on the equilibrium morphological 

behavior in a model polymer-layered silicate nanocomposite system was 

conducted. E-based surfactants were synthesized at five different molecular 

weights and with low polydispersity, were quaternized with a trimethylamino 

endgroup and exchanged with the naturally occurring cations present in the 

layered silicate galleries. The modified clays were then combined with I?!3 

homopolymer and annealed to allow direct melt intercalation to occur if 

energetically favorable, unlike most other work with FS-based nanocomposites 

where solution or in situ polymerization fabrication techniques are used. At 

high levels of surfactant coverage of the layered silicate clay mineral surfaces, no 

intercalation of the modified clay minerals was observed using SAX!S; only phase 

separated morphologies were found. This behavior was found to be consistent 

with the predictions of the Balazs model and also with the general phenomenon 

of autophobic dewetting. 

11 
. 

i 

12 
5. References 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

Giannelis, E. P. “Polymer Layered Silicate Nanocomposites.” Advanced 

Materials, vol. 8, p. 29,1996. 

Kojima, Y., A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, 

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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Polymer-Layered Silicate Nanocomposites From Model Surfactants at High EMATO~ 

Coverage 

6. AUTHOR(S) 

Frederick L. Beyer, Arnab Dasgupta,* and Mary E. Galvin* 

7. PERFORMING ORGANlZATlON NAME(S) AND ADDRESS 

U.S. Army Research Laboratory 

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FDepartment of Materials Science and Engineering, 301 Spencer Laboratory, University of Delaware, Newark, DE 

19716 

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13. ABSTRACT(Maximum 200 words) 

A series of experiments has been designed and conducted to test a recent model for the morphological behavior oi 

polymer-layered silicate (PLS) nanocomposites, where a PLS nanocomposite is comprised of a homopolymer matrix 

layered silicate clay mineral filler, and an organic surfactant modifier. For nearly complete exchange of naturally 

occurring interlayer cations with model surfactants synthesized for this study, the effect of surfactant length on the 

morphology of polymer-layered silicate nanocomposites has been examined. Here the surfactant and matrix 

homopolymer are both based on polystyrene and are therefore expected to be miscible. Small-angle x-ray scattering 

lata show that there was no intercalation of the homopolymer into the modified layered silicate. This finding is 

:onsistent with autophobic dewetting such as that which occurs with densely grafted polymer brushes, based on high 

rurfactant coverage within the layered silicate galleries. 

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>olymer, layered silicate, montmorillonite, polystyrene, small-angle x-ray scattering 21 

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