Effect of Textured Surfactant Brushes on Polymer-Layered 

Silicate Nanocomposite Morphology 

by Mary K. Kurian, Arnab Dasgupta, Mary E. Galvin, 

and Frederick L. Beyer 

ARL-TR-3136 February 2004 

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Army Research Laboratory 

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ARL-TR-3136 February 2004 

Effect of Textured Surfactant Brushes on Polymer-Layered 

Silicate Nanocomposite Morphology 

Mary K. Kurian, Arnab Dasgupta, and Mary E. Galvin 

Department of Materials Science and Engineering 

University of Delaware 

Frederick L. Beyer 

Weapons and Materials Research Directorate, ARL 

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Effect of Textured Surfactant Brushes on Polymer-Layered Silicate Nanocomposite 

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* Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716. 

14. ABSTRACT 

The morphological behavior of a series of polymer-layered silicate nanocomposites (PLSNs) has been investigated. The goal 

was to probe the effect of “textured” silicate surfaces on PLSN morphology. The nanocomposites were fabricated by mixing 

montmorillonite clay that was carefully modified with tailor-made polystyrene (PS) surfactants into a PS homopolymer matrix, 

where the chemical similarity of the matrix polymer and surfactants ensures complete miscibility of surfactant and 

homopolymer. To examine the effect of silicate surface “texture,” clay was modified with combinations of long and short 

surfactants. The samples, characterized by small-angle x-ray scattering (SAXS), were then melt-annealed to allow the 

equilibrium morphology to develop. Based on the implications of the Balazs model and other work on the wetting behavior of 

polymer melts, textured surfaces were expected to promote the intercalation of the homopolymer matrix material into the 

modified clay. Extensive characterization of both the modified clays as well as the resultant nanocomposites clearly showed 

that the modified clays exhibited a high degree of order, but also that only phase-separated morphologies were formed in the 

corresponding nanocomposites. 

15. SUBJECT TERMS

polymer brush, layered silicate, small-angle x-ray scattering, morphology, polystyrene, montmorillonite 

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Mary K. Kurian 

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iii

Contents 

List of Figures iv

List of Tables iv

1. Introduction 1

2. Experimental 3

3. Results 5

4. Discussion 7

5. Conclusions 9

6. References 11

Distribution List 16 
iv

List of Figures 

Figure 1. SAXS data for the modified clays with “mixed” coverage and the corresponding 

annealed nanocomposites...........................................................................................................6 

Figure 2. Illustration for a bimodal surfactant brush of one possible outcome on longer 

surfactant conformation of the densely packed brush of shorter surfactant. .............................9 

List of Tables 

Table 1. Molecular weights for quaternary ammonium ion terminated PS surfactants..................4 

Table 2. Physical characterization data for the modified clays with “mixed” coverage. ...............6 

Table 3. Physical characterization data for the modified clays with “mixed” coverage and the 

corresponding PS-based nanocomposites..................................................................................7 
1

1. Introduction 

Polymer-layered silicate nanocomposites (PLSNs), as a class of materials, have generated 

significant research interest as a result of several important examples of dramatically enhanced 

properties including tensile strength, heat resistance, solvent resistance, and permeability (1–8). 

These properties were improved simply by the incorporation of small amounts of a suitable filler 

material such as montmorillonite clay mineral in the matrix polymer. 

The clay minerals typically used for these purposes have been thoroughly described elsewhere 

(9). In brief, they are phyllosilicates akin to mica and talc, and are composed of particles called 

tactoids which are stacks of separate crystalline layers. Within the interlayer spaces, which are 

called galleries, are found cations such as Na+

, which can be readily exchanged with cationic 

organic molecules to render the clays organophillic. 

PLSNs are dispersions of individual layers of a clay mineral in polymer matrices. This 

dispersion is normally achieved through one of three techniques: melt processing using 

extrusion (10–15), solution processing in a cosolvent (16–18) and in situ polymerization of the 

monomer in the presence of the silicate mineral (19–24). A significant impediment to the simple 

fabrication of PLSNs with well-dispersed clay layers is the hydrophilicity of the clay minerals. 

With the noted exception of polymers such as poly(ethylene oxide), it is usually necessary to 

modify the clay surface with an organic surfactant (25–29). Numerous research efforts to 

understand how to disperse a layered silicate clay mineral into a hydrophobic polymer matrix 

through the use of organic modifiers, in most cases alkylammonium compounds, have been 

made. Hasegawa et al. (29) reported the preparation and mechanical characterization of 

polypropylene-clay hybrids made by melt processing a mixture of stearylammonium-exchanged 

montmorillonite, maleic anhydride-modified polypropylene oligomer, and homopolypropylene 

(29–31). Other researchers have also reported similar work on polypropylene-based hybrids 

(32, 33). Similar approaches have been employed to assist the dispersion of clay in 

thermosetting polymers where reports (34, 35) describe epoxy-clay nanocomposites prepared by 

the reaction of epoxide resin and a curing agent in the presence of alkylammonium ion 

exchanged forms of montmorillonite clays. The melt-intercalation of polystyrene (PS) (36–40) 

into the interlayer gallery region of long chain primary and quaternary alkylammonium 

exchanged clays has also been investigated previously. In all cases, the use of modifiers was 

seen to assist in the formation of intercalated and partially exfoliated nanocomposite materials 

due to easier and more efficient dispersion of the inorganic clay in the polymer matrices. 

The studies previously mentioned show that an important requirement for reaching the goal of 

homogeneous dispersion of the layered clay mineral in the polymer matrix is the promotion of 

favorable interactions between the matrix and the silicate surfaces, achieved through the use of 

the “compatibilizing” surfactants. This is a classical problem in the physical sciences, and 
2

various approaches to improving the adhesion between two materials utilizing means such as van 

der Waals forces, physical entanglements, and chemical bonds have been investigated (41). 

Most studies probe the compatibility of a polymer film with a surface such as a silicon wafer, 

where incompatibility results in dewetting. A standard solution involves altering the surfacemelt interaction by modifying the surface with suitable molecules. Reiter et al. (42) have 

reported on the dewetting of thin liquid films of PS from a surface modified with a polymer 

brush. They investigated the use of bimodal brushes on the surfaces formed by the incorporation 

of longer connector molecules into the polymer brush to promote wetting of a liquid film that 

was otherwise incompatible (43). Yerushalmi-Rosen and Klein (44) and Yerushalmi-Rosen 

et al. (45) have also conducted similar studies on the utilization of grafted molecules on a surface 

together with free polymer chains in the melt to promote the adhesion of an otherwise 

nonwetting, chemically similar liquid polymer film. In each of these studies, the complex 

behavior of the experimental system could not be definitely attributed to a particular 

phenomenon and was seen to depend on an interplay between a number of factors such as 

molecular weights of the brush, the connector, and the film polymer molecules, the density of 

grafting of the surface by the brush, and film thickness. Given the inherent incompatibility 

between the matrix polymer and the silicate at the molecular level, similar modification schemes, 

which have successfully been applied to promote adhesion between otherwise incompatible 

components, may provide routes to the successful formation of stable nanocomposite materials. 

Though there have been numerous reports on PLS nanocomposite fabrication by various 

techniques, there are few reports probing the effect of fundamental interactions between 

nanocomposite components on the final nanocomposite morphology. Previously, we reported on 

the investigation of morphology formation in a model nanocomposite system based on PS (46). 

In that work, PS-based PLSNs were fabricated using direct melt-processing, a technique where a 

mixture of modified clay and homopolymer matrix materials are simply annealed together with 

no solvents or shearing to promote intercalation. To promote interactions between the silicate 

and the PS matrix, organically modified clays were prepared by exchanging most cation 

exchange positions in Na+ montmorillonite with custom-made, quaternary ammonium ionterminated PS surfactants of five different molecular weights. It was shown that the high levels 

of modification resulted in dense polymer brushes on the clay surfaces, preventing intercalation 

of PS homopolymer molecules; phase-separated morphologies were observed in all cases, 

irrespective of surfactant molecular weight. This can be understood within the framework of 

well-established theories of dewetting from dense polymer brushes (47, 48). Here, in a 

continuation of that work, we describe results on an additional method of silicate modification, 

and the corresponding effect on PLSN equilibrium morphology. 

In this scheme, termed “mixed coverage,” the silicate is modified using a mixture of PS-based 

surfactants to create a silicate surface grafted with a bimodal brush. This texturing of the surface 

is expected to allow intercalation of matrix molecules into the galleries by providing a favorable 

entropic gain to the intercalating polymer through interaction with the longer grafted molecules, 
3

and reduced enthalpic interaction with the clay surface. This concept of an increase in entropic 

gain with increasing surfactant length is also outlined in the theoretical model of Balazs and 

coworkers (49). This scheme of modification should allow a clearer interpretation of the effect 

of surfactant length on morphological behavior, without interference from the competing 

influences of a dense surfactant brush or enthalpic interactions with the silicate surface. 

2. Experimental 

The silicate mineral Na+ montmorillonite (SWy-2) was purchased from the Source Clay 

Repository at the University of Missouri. Ten grams of clay were dispersed in 1-L deionized 

water, and the heavier impurity fractions were allowed to settle to the bottom of the beaker. The 

resulting suspension was decanted and centrifuged to further make it free of impurities. This 

cleaned clay was vacuum dried and then powdered using a mortar and pestle. The cation 

exchange capacity (CEC) of SWy-2 was given as 76-meq/100-g clay, but measurement of the 

CEC for the cleaned clay, using standard techniques (50–52) gave a value of 67 meq/100 g, and 

this value along with the reported total surface area (9) (internal and external) of 800-m2

/g clay 

was used in all relevant calculations. Low polydispersity PS of approximate molecular weight 

10,000 g/mol was used as the matrix polymer in all nanocomposites and was purchased from 

Polymer Source Inc., Dorval, Quebec, Canada. 

Quaternary ammonium end-functionalized PS surfactants of five different molecular weights 

were synthesized via a modification of the living anionic polymerization technique reported by 

Quirk and Lee (53). Complete details of synthesis and characterization of the surfactants have 

been reported elsewhere (54). Briefly, PS surfactants synthesized by living anionic 

polymerization were amine end-functionalized in nearly quantitative yields by using a 

terminating agent such as 3-dimethylaminopropyl chloride. The success of the reaction was 

confirmed by the presence of the –N(CH3)2 carbons in (13) C nuclear magnetic resonance 

(NMR) spectra of the terminated product. The amine-functionalized PS product was separated 

from residual unfunctionalized PS via column chromatography, and the product quaternized by 

refluxing overnight with a 100-fold excess of dimethyl sulfate in dry tetrahydrofuran (THF) 

solvent. The quaternized product was precipitated into methanol, was filtered, and dried. 

Complete conversion of the amine end-functionalized PS to the quaternary ammonium endfunctionalized PS product was confirmed by thin-layer chromatography. Molecular weights for 

surfactants were measured using gel permeation chromatography (GPC) and in the case of the 

shortest surfactant (~2000 g/mol) by matrix-assisted laser desorption ionization (MALDI) mass 

spectrometry due to the inaccuracy of GPC in this low molecular weight range. GPC analyses 

were performed using THF as the solvent eluting at a flow rate of 1 mL/min through two PL gel 

5-μm mixed-D columns connected in series and equipped with an HP 1047A RI detector. Mass 

spectra were obtained on a Bruker Biflex III MALDI time-of-flight mass spectrometer using 
4

delayed extraction in both reflectron (<5k m/z) and linear modes and were an average of 

minimum 50 spectra. Calibration was performed with PS standards, and at higher masses with 

protein standards. All solutions were prepared in toluene at 4 mg/mL. Dithranol was used as a 

matrix, and silver trifluoroacetate, when needed, as the cationization salt. Analyte solutions were 

premixed at an 8:4:1 ratio of matrix: sample: salt and applied in 1-μL aliquots to the probe. 

Calculation of molecular weights was performed using both the XMASS (Bruker) and Polymerix 

(Sierra Analytics) software packages. Surfactant nomenclature and properties are recorded in 

table 1. The nomenclature used is such that for any surfactant it provides information about its 

approximate molecular weight in thousands of grams per mole; for example, Q-2 is a quaternary 

ammonium ion terminated PS surfactant of approximate molecular weight 2000 g/mol. The low 

polydispersity indices (PDI) indicate the ability to control molecular weight afforded by the 

anionic synthesis technique used in this work. 

Table 1. Molecular weights for quaternary ammonium ion terminated PS surfactants.

Surfactant Mw 

(g/mol) 

Mn 

 (g/mol) 

PDI 

Q2 1820 1735 1.05 

Q3 2800 2700 1.04 

Q7 7000 6800 1.03 

Q11 11600 11200 1.04 

Q18 18100 17600 1.03 

The Na+ cations in montmorillonite were easily replaced by the quaternized PS surfactants by a 

simple solution-phase cation exchange procedure as follows. A measured amount of Na+ 

montmorillonite was dispersed in a 3:1 (v/v) mixture of acetone: water by stirring the mixture 

for 8 hr. The required amounts of surfactants needed to modify the clay were also dissolved in a 

similar solvent system. The surfactant solution was added to the clay suspension with rapid 

stirring, resulting in the immediate formation of a white precipitate. The mixed solution was 

further stirred for 8 hr. The precipitate was separated from the solvents by centrifuging, washed 

with THF to remove any unbound surfactant, and then dried under vacuum at 60 °C for 12 hr. 

Four different modified clays were thus made by treating a measured amount of Na+ 

montmorillonite with a solution containing one of the longer four (Q-3, Q-7, Q-11, and Q-18) of 

five surfactants in combination with the shortest surfactant (Q-2). The relative amounts of 

surfactants in the mixture were adjusted such that in the final modified clay, of the total available 

CEC sites 17% would be occupied by the longer surfactant and rest 83% by surfactant Q-2. This 

scheme of clay modification provides surfaces that are in effect “textured” due to differences in 

the lengths of the modifiers used.

The composition of each of the modified clays was determined via thermogravimetric analysis 

(TGA). A 5–10 mg sample was analyzed using a Perkin Elmer TGA 7 instrument by recording 

the weight loss of the sample as a function of temperature in the range of 30 to 900 °C. The 

amount of residue that remains at the end of the experimental run provides a measure of the 

inorganic or ash content in the material. 
5

Nanocomposites incorporating the modified clays were made by the following intercalation 

technique described previously as “direct melt intercalation” (55). Based on TGA results of the 

modified clays, amounts of modified clay and PS were measured such that the total inorganic 

content in the nanocomposite was 5% by weight and then gently mixed by hand. These mixtures 

were then pressed into pellets using only contact pressure. The pellets were annealed under 

vacuum at a temperature of 150 °C for 1 week. As shown previously (46), this time was 

calculated to more than an order of magnitude in excess of the length of time required for the 

diffusion of matrix homopolymer molecules to achieve their equilibrium morphology. The 

annealed samples were cooled to room temperature under vacuum and were observed to be fully 

coalesced solid samples. 

Small-angle x-ray scattering (SAXS) data were collected to characterize the separation between 

individual clay sheets, a measure of the degree of intercalation in these materials (56). X-rays 

were generated using a Rigaku Ultrax18 rotating anode generator operated at 2.4-kW power, and 

the CuKα radiation was monochromated using a pyrolytic graphite crystal such that λ = 1.5418 Å. 

The sample to detector distance is 62.3 cm as calibrated using a silver behenate standard. Twodimensional SAXS data were collected using a Bruker Hi-Star area detector. All data were 

azimuthally averaged for analysis in terms of intensity as a function of q, the magnitude of the 

scattering wave vector, where q = 4π·sin(θ)/λ, with scattering angle, 2θ, and wavelength, λ. The 

length scale for a given value of q is simply 2π/q. 

3. Results 

The sample nomenclature used in this report incorporates information about the exact silicate 

modification scheme, the surfactants and the matrix polymer used in a particular sample. For 

example, M-(2, 3)m-PS is a nanocomposite sample where “M” stands for montmorillonite clay, 

“(2, 3)” indicates that surfactants Q-2 and Q-3 (with approximate molecular weights 2000 and 

3000 g/mol) were used to modify the silicate, “m” indicates mixed coverage, and “PS” indicates 

that PS homopolymer (always ~10,000 g/mol) was used as the matrix material. The modified 

clay alone without the matrix used in the above sample previously mentioned would be named 

M-(2, 3)m. 

Table 2 lists the physical characteristics of all modified clays. As described in the procedure for 

silicate modification, surfactant amounts in the modifying solution were adjusted to achieve a 

“mixed” coverage where ~83% of the CEC sites on the silicate surfaces were occupied by the 

surfactant Q-2 and the rest 17% by the longer surfactant. The feed compositions calculated 

based on the total amount of surfactants used to modify a measured amount of clay are recorded 

in table 2. TGA was used to determine the fractions of organic and inorganic matter in the 
6

Table 2. Physical characterization data for the modified clays with “mixed” coverage. 

 Feed Composition After Modification Surf. Coverage 

Sample Surfs. Clay 

(%) 

Surf. 

(%) 

Clay 

 (%) 

Surf. 

(%) 

Q2 

(%) 

Long 

(%) 

M-(2, 3)m Q2, Q3 44 56 46 54 83 17 

M-(2, 7)m Q2, Q7 36 64 38 62 83 17 

M-(2, 11)m Q2, Q11 31 69 32 68 83 17 

M-(2, 18)m Q2, Q18 25 75 26 74 83 17 

modified clays and comparison of these compositions with the feed compositions shows good 

agreement. This allows the calculation of the coverages by each of the surfactants in the 

modified clays. Knowledge of the composition of a modified clay also allows the calculation of 

the exact amount of matrix homopolymer required to make a nanocomposite which contains 5% 

by weight inorganic matter. 

Figure 1 reports the morphological characterization data collected using SAXS. The data are 

presented in pairs, where the lower set of each pair is from the modified clay without any 

homopolymer, and the upper set in each pair is from the corresponding PLSN, containing 

modified clay and PS homopolymer after direct melt-intercalation (5 weight-percent total 

inorganic content). The data are plotted on a log-linear scale and scaled vertically for clarity of 

presentation. Table 3 records the scattering vector q for the first-order Bragg reflection, the 

associated real space length d, where d = 2π/q and the corresponding gallery height h, where h = 

(d-10) Å, for each of the modified clays and nanocomposites. 

Figure 1. SAXS data for the modified clays with 

“mixed” coverage and the corresponding 

annealed nanocomposites. 

0.00 0.05 0.10 0.15 0.20 0.25 0.30

M-(2,3)m

M-(2,3)m-PS

M-(2,7)m

M-(2,7)m-PS

M-(2,11)m

M-(2,11)m-PS

M-(2,18)m

M-(2,18)-PS

Log (Intensity) (a.u.)

q (1/Å)
7

Table 3. Physical characterization data for the modified 

clays with “mixed” coverage and the 

corresponding PS-based nanocomposites. 

Sample q 

(Å-1) 

D 

 (Å) 

h 

(Å) 

M-(2, 3)m 0.1191 53 43 

M-(2, 7)m 0.1191 53 43 

M-(2, 11)m 0.0818 77 67 

M-(2, 18)m 0.0551 114 104 

M-(2, 3)m-PS 0.1191 53 43 

M-(2, 7)m-PS 0.1191 53 43 

M-(2, 11)m-PS 0.0817 77 67 

M-(2, 18)m-PS 0.0551 114 104 

4. Discussion 

The underlying premise of this research effort, including previously published results (43), is to 

investigate the possibility that given the proper combination of matrix polymer, surfactant, clay 

and processing conditions, a PLSN material could be created in such a way that there would exist 

a thermodynamic driving force that would favor the formation of intercalated and exfoliated 

morphologies. As calculated by Balazs and coworkers, it may be possible to engineer a system 

that would have as its equilibrium morphology an intimate mixing of polymer and individual 

clay sheets (49, 57). Developing a system of this nature could significantly alter the economics 

of polymer-layered silicate nanocomposite materials because requiring the polymer matrix to be 

synthesized either in situ or forced into intercalation dramatically limits the feasibility of making 

PLSNs that can be commercialized. 

To that end, and as an extension of our previous efforts, the current series of polymer-layered 

silicate hybrid materials was fabricated. This series, containing clays modified simultaneously 

with two PS-based surfactants, was devised to explore the possibility of avoiding the 

morphological behavior of the series of PLSN materials fabricated for the earlier work. In that 

study (46), PS-based surfactants were used to modify montmorillonite clays as fully as possible, 

but the result was a “phase-separated” morphology, where no intercalation or exfoliation of the 

clay layers was observed. This was attributed to autophobic dewetting of the matrix polymer 

molecules due to formation of a dense polymer brush by the surfactant molecules on the silicate 

surfaces. To compensate for this behavior, a strategy for overcoming autophobic dewetting was 

adopted from an experiment where the adhesion between a PS-brush coated surface and an 

otherwise nonwetting PS liquid film was improved by using connector polymer molecules (43). 

Different amounts of these connectors, 5–14 times longer than the brush molecules, were 

anchored to the surface to form ‘bimodal brushes’ that stabilized the film by penetration of the 

connectors into the nonwetting film. The authors reported that the grafting of a small number of 
8

end-functionalized connector molecules into a dense polymer brush resulted in a reduced 

dewetting velocity for a subsequently deposited nonreactive homopolymer film, with longer 

connectors being more effective in reducing this velocity. Here, the longer surfactants were 

hoped to provide the entropic benefit to the intercalating polymer as predicted by the Balazs 

model, while the short surfactants would provide a screen between the hydrophobic PS and the 

hydrophilic silicate surfaces, a repulsive force expected to impede intercalation (39). The data 

shown in figure 1 indicate that the actual behavior of this series of samples was to remain in a 

phase-separated morphology, with no indication of intercalation or exfoliation of the layered 

silicate in the PS matrix. Examination of the SAXS data reveals that no significant change in the 

shape or location of the Bragg reflections occurred upon annealing, which would be expected if 

there were any significant changes in morphological structure. 

Several probable explanations for this morphological behavior are available. First, there are 

obvious differences between this experiment and that of Reiter et al. (43) regarding autophobic 

dewetting from a model polymer brush. Here, a natural clay mineral, with variations in 

composition, size, and texture was used, while the model experiment worked with a carefully 

prepared silicon wafer substrate. Here, all the surface modification occurred in one batch step, 

while the model experiment involved careful deposition of each component separately. In the 

model study, the polymer melt was observed in transition from a wetted state to a dewetted state, 

while in this experiment, only the transition in the opposite direction is of interest. 

Second, despite the bimodal nature of the brushes in this experiment, it is still possible that the 

brush is still effectively dense. This would be important only if the long surfactant fraction were 

being ineffective in fostering wetting and intercalation, which could be the case. Figure 2 shows 

a schematic of this scenario. In figure 2a, a long surfactant is illustrated at a low coverage over a 

surface that is repulsive to the homopolymer. In this case, the surfactant will stretch away from 

the surface until the energy required to stretch is no longer less than that of the repulsive 

enthalpic interactions. This is the scenario for a typical surfactant modified clay. In the bimodal 

brush, the shorter surfactant at full coverage (83%) effectively replaces the silicate surface. As 

illustrated in figure 2b, the surface now can be thought of as being composed of the dense brush. 

When a small fraction of those surfactants are replaced with longer surfactants, the situation may 

become something like that illustrated in figure 2c. Now, the length of the longer surfactant is 

important, but the conformation of the longer surfactant must also be considered. If the longer 

surfactant is similar in length to the shorter surfactant, one has effectively a short surfactant at 

low coverage. As the difference between the longer and shorter surfactants increases, the 

effective surfactant becomes longer, but does this longer brush actually assist in the wetting and 

intercalation processes? It is possible that it does not because there is now no enthalpic repulsion 

causing the surfactant to stretch away from the effective surface. The effect of conformation is 

clear for at least two samples examined here. Samples M-(2, 3)m-PS and M-(2, 7)m-PS both 

show primary and secondary Bragg reflections at the same spacings, in addition to showing no 

evidence of intercalation. When compared to the data collected for the PLS nanocomposite 
9

Figure 2. Illustration for a bimodal surfactant brush of one possible 

outcome on longer surfactant conformation of the densely 

packed brush of shorter surfactant. 

fabricated in the previous study, where the surfactant was also Q-2, one finds that the gallery 

heights of the two PLSNs here are only slightly larger than those of clay modified solely with the 

Q-2 surfactant (sample M-2 in the previous report). That the gallery height does not change 

between M-(2, 3)m-PS and M-(2, 7)m-PS is remarkable given the 3-fold increase in surfactant 

molecular weight, and could only occur if the majority of the longer surfactant is remaining close 

to the dense surfactant brush formed by the Q-2 surfactant. This image also helps explain the 

significant breadth in the SAXS data for the M-(2, 7)m samples. The breath is consistent with 

disordering of the second kind (58), where fluctuations in the gallery height distort the Bragg 

reflections. The most readily found reason for this variation in gallery height would be a nonuniform distribution of longer surfactants where their effective length is sufficient to add volume 

in their locality unevenly between the silicate layers. As the longer surfactants become 

significantly longer, in samples M-(2, 11)m-PS and M-(2, 18)m-PS, the effect of the bimodal 

brush on the breadth and number of Bragg reflections is still evident. The Bragg reflections 

observed here are fewer and far broader than those observed for the fully exchanged clays. Still, 

no evidence of a change in morphology is observed, though, indicating that even these highly 

textured surfaces are not enough to cause intercalation to occur. 

5. Conclusions 

Polymer-layered silicate nanocomposites based on a PS matrix were prepared using 

montmorillonite clays that were modified with blends of PS-based surfactants. In these samples, 

17% of the cations available for exchange in the clay mineral were replaced with a long 

surfactant, while the remaining 83% of the cations were replaced with a short surfactant ~2000 

g/mol in molecular weight. Low molecular weight PS homopolymer was used as the matrix 

material to ensure miscibility and mobility. The incorporation of a small amount of longer 

surfactant molecules in a brush of short surfactants did not promote intercalation of the matrix 

polymer into the clay galleries, and only phase-separated morphologies were observed, 

regardless of surfactant length used. Though entropic gain is provided to the intercalating 
10

molecules through the longer surfactant chains, this compensation is insufficient to prevent the 

autophobic dewetting behavior favored because of the dense brush conditions that still exist on 

the silicate surfaces. 
11

6. References 

1. Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. Synthesis and Properties of 

Polyimide Clay Hybrid. Journal of Polymer Science, Part A: Polymer Chemistry 1993, 31, 

2493. 

2. Messersmith, P. B.; Stupp S. I. Synthesis of Nanocomposites. Organoceramics. Journal of 

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