Films formed from polystyrene latex/clay composites: a fluorescence study.

This study reports a steady-state fluorescence (SSF) technique for studying film formation from surfactant-free polystyrene (PS) latex and Na-montmorillonite (SNaM) composites. The composite films were prepared from pyrene (P)-labeled PS particles and SNaM clay at room temperature and annealed at elevated temperatures in 10-min intervals above glass transition temperature ([T.sub.g]) of polystyrene. During the annealing processes, the transparency of the film improved considerably. Scattered light ([I.sub.s]) and fluorescence intensity ([I.sub.P]) from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of composite films was monitored by using photon transmission intensity, [I.sub.tr]. Scanning electron microscopy (SEM) was used to detect the variation in physical structure of annealed composite films. Minimum film formation temperature, [T.sub.0], and healing temperatures, [T.sub.h], were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. It was observed that both activation energies increased as the percent of SNaM was increased in composite films.

Keywords: Composite, void closure, interdiffusion, fluorescence, polystyrene latex, montmorillonite, atomic force, fluorescence spectroscopy, latexes, colloids, emulsions, clay, latex, water-based

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In the last decade, there has been growing interest in producing new materials by filling polymers with inorganic, natural, and/or synthetic compounds. These composite materials possess high heat resistance, mechanical strength, and impact resistance, or present weak electrical conductivity and low permeability for gases like oxygen or water vapor. Since the inorganic particles display rather macroscopic dimensions, and since there is mostly no interaction between the two mixed components at the interface between the two partners, the resulting composite materials can be seen as filled polymers.

The synthesis of nylon/clay nanocomposites via in-situ polymerization produced polymer/layered silicate, (1-4) where the addition of only a few percent of layered silicates improved the properties of the material significantly. Several procedures are known in the incorporation of layered silicate material into a polymer matrix in a fine dispersed manner. (5,6) The resulting composite materials display properties superior to those displayed by simply mixed components or filled polymers, which make them extremely interesting in the field of design and creation of new construction and packing materials.

The term latex film normally refers to a film formed from soft latex particles ([T.sub.g] below room temperature) where the forces accompanying the evaporation of water are sufficient to compress and deform the particles into transparent, void-free film. (7) However, latex films can also be obtained by compression molding of a film of dried latex powder composed of relatively hard polymers, such as polystyrene (PS) or poly(methyl methacrylate) (PMMA), that have [T.sub.g] above room temperature. Hard latex particles remain essentially discrete and undeformed during drying. The mechanical properties of such films can be evolved after all the solvent has evaporated by annealing process, which first leads to void closure and then interdiffusion of chains across particle-particle boundaries. (8)

Film formation from soft and hard latex dispersions can occur in several stages. In both cases, the first stage corresponds to the wet initial stage. Evaporation of solvent leads to the second stage in which the particles form a close packed array; here, if the particles are soft, they are deformed to polyhedrons. Hard latex, however, stays undeformed at this stage. Annealing of soft particles causes diffusion across particle-particle boundaries which leads the film to form a homogeneous continuous material. In the annealing of a hard latex system, however, deformation of particles first leads to void closure (7,9), and then after the voids disappear, diffusion across particle-particle boundaries starts, i.e., the mechanical properties of hard latex films evolve during annealing after all solvent has evaporated and all voids have disappeared.

After the void-closure process is completed, the mechanism of film formation is known to be the interdiffusion of polymer chains and the healing of the polymer-polymer interfaces. In general, when two identical polymeric materials are brought into intimate contact and heated at a temperature above the glass transition the polymer chains become mobile, interdiffusion of polymer chains across the interface can occur. After this process, the junction surface becomes indistinguishable. This process is called healing of the junction at which the joint achieves the same cohesive strength as the bulk polymeric material. The word interdiffusion in polymer science is used for the process of mixing, intermingling, and homogenization at the molecular level, which implies diffusion among polymer chains.

Transmission electron microscopy (TEM) has been the most common technique used to investigate the structure of the dried films. (10,11) A pattern of hexagons, consistent with face-centered cubic packing, is usually observed in highly ordered films. When these films are annealed, complete disappearance of structure is sometimes observed, which is consistent with extensive polymer interdiffusion. Freeze-fracture TEM (FFTEM) has been used to study the structure of dried latex films. (12,13) Small-angle neutron scattering (SANS) has been used to study latex film formation at the molecular level. Extensive studies using SANS have been performed by Sperling and coworkers (14) on compression-molded polystyrene film. A direct-nonradiative energy transfer (DET) method has been employed to investigate the film formation process from dye-labeled hard (15) and soft (16,17) polymeric particles. The steady-state fluorescence technique combined with DET has been used to examine healing and interdiffusion processes in the dye-labeled PMMA latex systems. (18-20) Recently, a UV-visible technique was used to study film formation from PMMA and polystyrene (PS) particles (21, 22) where the transmitted light intensity was monitored during the film formation process.

Intercalated composites formed by the incorporation of only one or two polymeric chains between the sheets of the minerals have been reported. (23,24) Composites with polyetheneoxide have been studied by simple mixing of the molten polymer with layered minerals. (25) The incorporation of montmorillonite sheets into polypropylene was reported where the compatibilizing ability of the functionalized oligomers was studied. (26) Recently, inorganic clay was investigated as a compatibilizer for immiscible poly(propylene/polystyrene blends), by using a wide angle X-ray diffraction method. (27)

[FIGURE 1 OMITTED]

In this work, the evolution of film formation from composites of Na-montmorillonite (SNaM) clay and surfactant-free PS latex was studied. The scattered excited light intensity ([I.sub.s]) from the film surface and fluorescence emission intensity ([I.sub.P]) from pyrene were monitored using steady-state fluorescence (SSF) technique. Films were prepared by annealing composites above the glass transition temperature of PS for 10-min intervals at temperatures ranging from 100[degrees] to 300[degrees]C. Transmitted photon intensity, [I.sub.tr], was also monitored to study the evolution of transparency. The increase in [I.sub.P]/[I.sub.s] ratio achieved by increasing the annealing temperatures was attributed to the void-closure process. The decrease in [I.sub.P]/[I.sub.s] ratio was attributed to the interdiffusion processes. [I.sub.s] intensity jumped at the single temperature called the void-closure temperature, [T.sub.v]. It was observed that [I.sub.tr] increased dramatically above the certain onset temperature called the minimum film formation temperature, i.e., onset temperature, [T.sub.0], for film formation. The maximum of [I.sub.P]/[I.sub.s] was interpreted as the healing point during the film formation processes.

EXPERIMENTAL

Materials

SNAM CLAY: The clay used here was an aqueous suspension of montmorillonite, consisting of plate-like shaped particles. The clay sample was obtained from the bentonite deposits in Enez, Turkey (Bensan Co.). A Philips PW 1040 model X-ray diffractometer instrument was used to determine the clay mineral types. The dominant clay mineral was found to be dioctahedral montmorillonite with minor amounts of illite and kaolinite. Quartz was always present in the clay fraction. The purified SNaM was prepared from the corresponding bentonites by various treatments. First, iron oxides were removed by sodium citrate and sodium dithionate buffering techniques. To remove the carbonate, the bentonite dispersion was mixed with NaCl/HCl solution and centrifuged several times, and then washed by water. Organic material was oxidized with hydrogen peroxide solution at 80[degrees]C. The < 2 [micro]m fraction was separated by sedimentation using Stoke's law. The dispersion was then sodium-saturated, dialyzed, and freeze-dried. (28-30) The dried material was powdered in a ball mill. The layer charge, determined by the alkylammonium method, (31) was 0.283 (eq/mol). Montmorillonite is composed of stacks of alumina and silica sheets joined together. Two structural units are involved in the atomic latices of montmorillonite. The unit cell of montmorillonite consists of a combination of one octahedral and two tetrahedral sheets. Clay mineral crystals carry a charge arising from the isomorphous substitution of certain elements in their structure for other ions of a different valance. In the tetrahedral sheet, [Si.sup.4+] can be replaced by trivalent cations ([Al.sup.3+] or [Fe.sup.3+]), or divalent cations ([Mg.sup.2+] or [Fe.sup.2+]) can be replaced by [Al.sup.3+] in the octahedral sheet. In this case, a charge deficiency results that leads to a negative charge at the surface of the clay. The negative charge is compensated by the adsorption of exchangeable cations on the surface. The total amount of cations adsorbed by the clay, expressed in milliequivalents per hundred grams of dry clay, is called the cation exchange capacity (CEC). The CEC is high for sodium montmorillonite compared to the other clay minerals. The cation exchange capacity of montmorillonite is 80-150 meq/100 g. (32,33)

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The clay type of purified Na-montmorillonite was reanalyzed again after the purification procedure. X-ray diffraction (XRD) results show that the sample had only montmorillonite and that there was no other clay or non-clay mineral in the sample. The major elements of the sample were determined by using a RIGAKU 3070 model X-ray fluorescence (XRF) instrument and the rock standards of the Geological Survey of Japan. The results of the chemical analyses of the sample are given in Table 1.

The particle size distributions (PSD) of the bentonitic clays were determined by the sedimentation technique method. (29,30) A photocentrifugal particle size analyzer SACP4L (Shimadzu Corp.) was used for particle size measurements. The sample was put into distilled water and dispersed ultrasonically for 10 min under the power of 50 W. No dispersing reagents were used. The preliminary measurement of the instrument is in the range of 20 to 0.2 [micro]m. After adjusting to the proper concentration and 30 sec of ultrasonic vibration, we again measured the PSD of the sample in the range of 40 to 0.03 [micro]m. Particle size distribution of SNaM is given in Figure 1. The PSD measurements revealed that the size of 90% of the particles was less than 0.5 [micro]m. (34) Scanning electron microscope (SEM) micrographs were taken by a JEOL JSM microscope. The SEM photograph of the sample was taken after the samples were coated by carbon (Figure 2).

[FIGURE 5 OMITTED]

PS LATEX: Fluorescent polystyrene latex was produced via a surfactant-free emulsion polymerization process. The polymerization was performed batch-wisely using a thermostatted reactor equipped with a condenser, thermocouple, mechanical stirring paddle, and nitrogen inlet. The agitation rate was 400 RPM and the polymerization temperature was controlled at 70[degrees]C. Water (80 g), styrene (4.8 g), and the 0.012 g of fluorescent 1-Pyrenyl-methyl methacrylate (PolyFluor[R] 394) were first mixed in the polymerization reactor. When the temperature was constant (70[degrees]C), potassium peroxodisulfate (KPS) initiator (0.2 g) dissolved in a small amount of water (3 ml) was introduced in order to induce styrene polymerization. The polymerization was conducted over a period of 17 hr.

[FIGURE 6 OMITTED]

Four different films with 0, 3, 5, and 7% SNaM contents were prepared from the dispersion of PS/SNaM composites by placing the same number of drops on glass plates measuring 0.8 X 2.5 [cm.sup.2], where the water was allowed to evaporate. Then the samples were separately annealed above [T.sub.g] of PS, 105[degrees]C, for 10 min at temperatures ranging from 100[degrees] to 300[degrees]C. The temperature was maintained within [+ or -]2[degrees]C during annealing.

[FIGURE 7 OMITTED]

After annealing, each sample was placed in the solid surface accessory of a Perkin-Elmer Model LS-50 fluorescence spectrometer. Pyrene (P) was excited at 346 nm and scattered light and fluorescence emission were detected between 300-500 nm. All measurements were carried out in the front-face position at room temperature. Slit widths were kept to provide the resolution of 8 nm during all SSF measurements. The sample position, incident ([I.sub.0]) and scattered ([I.sub.s]) light and P emission intensity ([I.sub.P]) are shown in Figure 3a, where it is seen that [I.sub.s] is collected at a right angle with respect to incident light.

Photon transmission experiments were carried out using model DU 530 Life Science UV-Visible (UVV) spectrometer from Beckman. The transmittances of the films were detected between 300 and 400 nm. A glass plate was used as a standard for all UVV experiments and measurements were carried out at room temperature after each annealing process. The sample position and the transmitted light intensity, [I.sub.tr], are presented in Figure 3b.

[FIGURE 8 OMITTED]

Scanning electron micrographs were taken at 10-15 kV in a JEOL JSM microscope. A hummer VII sputtering system was used for gold coating of latex films. Figure 4 presents a SEM micrograph of a composite film with 5% SNaM before annealing, where it is seen that particles are monodispersed with the size of 1 [micro]m.

RESULTS AND DISCUSSION

The emission and scattered spectra of composite films annealed for 10 min at various temperatures are shown in Figure 5, where it is seen that both [I.sub.P] and [I.sub.s] first increased and then decreased upon annealing. [I.sub.tr], [I.sub.s], and [I.sub.P] intensities versus annealing temperatures are plotted in Figures 6, 7, and 8, respectively, for the films with 0, 3, 5, and 7% SNaM content. Upon annealing, the transmitted light intensity, [I.sub.tr], started to increase above a certain onset temperature, called the minimum film formation temperature, [T.sub.0], for all film samples. [T.sub.0] moved slightly to the higher temperature region as the SNaM content was increased. The sample with 7% SNaM content showed almost no transmittance by indicating that light transmission is completely blocked by clay particles in the composite film. Scattered light intensity, [I.sub.s], showed a sharp increase for 0 and 3% at the single temperature. However, [I.sub.s] intensity continuously decreased as the annealing temperature was increased for the films with 5 and 7% SNaM content. This behavior predicts that the surface of the composite film became smoother as the annealing temperature was increased. Here we have to mention that [I.sub.s] is the scattered light from the surface of the composite film. However, [I.sub.tr] goes all the way through the film. Fluorescence intensity, [I.sub.P], first presents an increase by reaching a maximum, then decreases with increasing annealing temperature. The sample with 7% SNaM content showed almost no fluorescence emission, predicting that clay particles block the photon diffusion in the composite film. The corrected [I.sub.P] intensities, i.e., [I.sub.P]/[I.sub.s], are plotted in Figure 9. The temperature where the maximum [I.sub.P]/[I.sub.s] ratio is reached is called the healing temperature, [T.sub.h], as will be explained in the following section.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

The increase in [I.sub.tr] above [T.sub.0] can be explained by the evaluation of transparency of the composite films upon annealing. Most probably the increase in [I.sub.tr] corresponds to the void-closure process, i.e., the polystyrene starts to flow upon annealing and voids between particles can be filled. The sharp increase in [I.sub.s] occurs at a certain temperature, below which light scatters isotropically because of the rough surface of the composite films. Annealing of the film at this temperature creates a flat surface on the film which acts like a mirror. As a result, light is reflected to the photomultiplier detector of the spectrometer. Further annealing makes the PS film transparent to the light and [I.sub.s] drops to its minimum.

On the other hand, the increase in [I.sub.P] above [T.sub.0] presumably corresponds to the void-closure process up to the [T.sub.h] point where the healing process takes place. A decrease in [I.sub.P] above [T.sub.h] can be understood by the interdiffusion processes between polymer chains at the junction surface. The behavior of [I.sub.P] can be explained with the schematic picture in Figure 10. In Figure 10a, at the early stage of film formation, the composite film possesses many voids which result in short mean free, <a>, and optical, s, paths of a photon. At this stage most of the light is scattered which yields low [I.sub.P]. Figure 10b presents a film in which, due to the annealing, interparticle voids disappear, which gives rise to a long mean free (<a>) and the longest optical (s) paths. As soon as the voids are filled, the healing process takes place and <a> and s get even longer than before. At this stage, [I.sub.P] reaches its maximum value. Finally, Figure 10c presents a partially transparent film with the longest <a> but shorter s values. This film has low [I.sub.P] and [I.sub.s] intensities. This picture can be observed slightly for the film with 7% SNaM content. For samples with higher SNaM content, the fluorescence method cannot be applied to study film formation process because the high opacity of the samples blocks the journey of the photon in the film and prevents fluorescence emission.

Void Closure

In order to quantify the behavior of [I.sub.P]/[I.sub.s] and [I.sub.tr] above [T.sub.0], a phenomenological void-closure model was introduced. Latex deformation and void closure between particles can be induced by the shearing stress which is generated by surface tension of polymer, i.e., polymer-air interfacial tension. The void-closure kinetics control the time for optical transparency and latex film formation. (35) In order to relate the shrinkage of spherical void of radius, r, to the viscosity of surrounding medium, [eta], an expression was derived and given by the following relation (35):

[FIGURE 11 OMITTED]

[dr]/[dt] = -[[gamma]/2[eta]](1/[[rho](r)]) (1)

where [gamma] is surface energy, t is time, and [rho](r) is the relative density. It has to be noted that, here, surface energy causes a decrease in void size and the term [rho](r) varies with the microstructural characteristics of the material, such as the number of voids, the initial particle size, and packing. Equation (1) is similar to one that was used to explain the time dependence of the minimum film formation temperature during latex film formation. (36,37) If the viscosity is constant in time, integration of equation (1) gives the relation as

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

t = [[2[eta]]/[gamma]][r.[integral].[r.sub.o]][rho](r)dr (2)

where [r.sub.o] is the initial void radius at time t = 0.

The dependence of the viscosity of the polymer melt on temperature is affected by overcoming the forces of macromolecular interaction, which enables the segments of polymer chain to jump from one equilibration position to another. This process happens at temperatures at which free volume becomes large enough and is connected with the overcoming of the potential barrier. The Frenkel-Eyring theory produces the following relation for the temperature dependence of viscosity (38,39):

[eta] = [[[N.sub.o]h]/V]exp([DELTA]G/kT) (3)

where [N.sub.o] is Avogadro's number, h is Planck's constant, V is molar volume, and k is the Boltzmann constant. It is known that [DELTA]G = [DELTA]H - T[DELTA]S, so equation (3) can be written as

[eta] = A exp([DELTA]H/kT) (4)

where [DELTA]H is the activation energy of viscous flow, i.e., the amount of heat which must be given to one mole of material for creating the act of a jump during viscous flow, and [DELTA]S is the entropy of activation of viscous flow. Here, A represents a constant for the related parameters which do not depend on temperature. Combining equations (2) and (4), the following useful equation is obtained:

t = -[2A/[gamma]]exp([[DELTA]H]/[kT])[r.[integral].[r.sub.o]][rho](r)dr (5)

In order to quantify the above results, equation (5) can be employed by assuming that the interparticle voids are in equal size and the number of voids stay constant during film formation (i.e., [rho](r)[proportional][r.sup.-3]). The integration of equation (5) gives the relation

t = -[2AC/[gamma]]exp([[DELTA]H]/[kT])([1/[r.sup.2]] - [1/[r.sub.o.sup.2]]) (6)

where C is a constant related to relative density [rho](r). As we stated before, a decrease in void size (r) causes an increase in mean free path <a> of a photon, which then results in an increase in [I.sub.tr] and [I.sub.0P] intensities. This picture can also be visualized by Frenkel's neck formation model, (23) which takes into account identical contacting spheres under the influence of surface tension. Frenkel's model assumes that the displaced volume is redistributed uniformly such that the remaining surfaces keep their spherical shapes, but of larger radii, which offers a larger mean free path <a> of a photon during its journey in the latex film. On the other hand, it is well known that scattering intensity increases with volume squared (the 6th power of radius) of scattering object. (40) If the assumption is made that emission and/or transmitted light intensity, I, is inversely proportional to the 6th power of void radius, r, then equation (6) can be written as

t = -[[2AC]/[gamma]]exp([[DELTA]H]/[kT])[I.sup.1/3] (7)

Here, [r.sub.o.sup.-2] is omitted from the relation since it is very small compared to [r.sup.-2] values after void-closure processes start. Equation (7) can be solved for I to interpret the results in Figures 5-8 as

I(T) = S(t)exp(-[3[DELTA]H]/[kT]) (8)

where S(t)=([gamma]t/2AC)[.sup.3]. For a given time, the logarithmic form of equation (8) can be written as follows:

LnI(T) = LnS(t)-([3[DELTA]H]/[kT]) (9)

As it was already argued, the increase in [I.sub.tr] and [I.sub.P] originate due to the void-closure process, then equation (9) was applied to [I.sub.tr] and [I.sub.P]/[I.sub.s] between the range of [T.sub.0] and [T.sub.h] for all film samples except 7% SNaM content film. Figures 11 and 12 present the Ln[I.sub.tr] and Ln([I.sub.P]/[I.sub.s]) versus [T.sup.-1] plots from the left-hand side of the data in Figures 6 and 9, respectively, from which [DELTA][H.sub.tr] and [DELTA][H.sub.P] activation energies were obtained. The measured [DELTA][H.sub.tr] and [DELTA][H.sub.P] activation energies are listed in Table 1 where it is seen that activation energies increase by increasing SNaM content. Here, one may argue that the presence of clay in the latex system screens and/or delays the void-closure process where one mole of polymeric material needs more energy to accomplish a jump during viscous flow. One may also argue that small clay particles (less than 0.5 [micro]m) may affect the void-closure process, because they covered 90% of the whole clay region. Here it has to be noted that the measured activation energies for viscous flow were found to be different in different techniques, i.e., [DELTA][H.sub.P] values are found much lower than [DELTA][H.sub.tr] values. Since pyrenes are labeled onto PS chain, it is believed that [DELTA][H.sub.P] values are more realistic to interpret the viscous flow. On the other hand, [DELTA][H.sub.tr] values are produced from the turbidity created during viscous flow. In other words, [DELTA][H.sub.tr] values were obtained indirectly in comparison to [DELTA][H.sub.P] values.

Healing and Interdiffusion

As noted in the previous section, the decrease in [I.sub.P] was explained by the increase in transparency of latex film due to the disappearance of deformed particle-particle interfaces. As the annealing temperature is increased above [T.sub.h], some part of the polymer chains may cross the junction surface and particle boundaries start to disappear. As a result, [I.sub.P] decreases due to the shorter optical path, s, of a photon. In order to quantify these results, the Prager-Tirrell (PT) model (41,42) for the chain crossing density can be employed. These authors used de Gennes' "reptation" model to explain configurational relaxation at the polymer-polymer junction where each polymer chain is considered to be confined to a tube which executes a random back and forth motion. A homopolymer chain with N freely jointed segments of length L was considered by PT, which moves back and forth by one segment with a frequency, v. In time, the chain displaces down the tube by a number of segments, m. Here, v/2 is called the "diffusion coefficient" of m in one-dimensional motion. PT calculated the probability of the net displacement with m during time t in the range of n - [DELTA] to n - ([DELTA] + d[DELTA]) segments. A Gaussian probability density was obtained for small times and large N. The total "crossing density" [sigma](t) (chains per unit area) at junction surface then was calculated from the contributions [[sigma].sub.1](t) due to chains still retaining some portion of their initial tubes, plus a remainder, [[sigma].sub.2](t). Here the [[sigma].sub.2](t) contribution comes from chains which have relaxed at least once. Figure 13 shows the pictorial representation of [[sigma].sub.1] and [[sigma].sub.2] contributions at the particle-particle interface. The small segments represent the part of the polymer chain called minor chain. In terms of reduced time, [tau] = 2vt/[N.sup.2], the total crossing density can be written as

[sigma]([tau])/[sigma]([infinity]) = 2[[pi].sup.-1/2][[[tau].sup.1/2] + 2[[infinity].summation over (k=0)](-1)[.sup.n][[[tau].sup.1/2]exp(-[k.sup.2]/[tau])-[[pi].sup.-1/2]erfo(k/[[tau].sup.1/2])]] (10)

For small [tau] values, the summation term of equation (10) is very small and can be neglected, which then results in

[sigma]([tau])/[sigma]([infinity]) = 2[[pi].sup.-1/2][[tau].sup.1/2] (11)

This was predicted by de Gennes on the basis of scaling arguments. (43) It should be mentioned that the dependence on time, t, in equation (11) goes as [t.sup.1/4] at early times of healing. (44,45) In order to compare our results with the crossing density of the PT model, the temperature dependence of [sigma]([tau])/[sigma]([infinity]) can be modeled by taking into account the following Arrhenius relation for the linear diffusion coefficient:

v = [v.sub.o]exp(-[DELTA][E.sub.b]/kT) (12)

Here, [DELTA][E.sub.b] is defined as the activation energy for backbone motion depending on the temperature interval. Combining equations (11) and (12), a useful relation is obtained as

[FIGURE 14 OMITTED]

[sigma]([tau])/[sigma]([infinity]) = [R.sub.o]exp(-[DELTA][E.sub.b]/2kT) (13)

where [R.sub.o]=(8[v.sub.o]t/[pi][N.sup.2])[.sup.1/2] is a temperature independent coefficient.

The decrease in [I.sub.P]/[I.sub.s] in Figure 9 above [T.sub.h] is already related to the disappearance of particle-particle interfaces, i.e., as annealing temperature increased, more chains relaxed across the junction surface and, as a result, the crossing density increased. Now, if it can be assumed that [I.sub.P]/[I.sub.s] is inversely proportional to the crossing density [sigma](T), then the phenomenological equation can be written as

[FIGURE 15 OMITTED]

[FIGURE 16 OMITTED]

[I.sub.p](T)/[I.sub.s]([infinity]) = [R.sub.o]exp([DELTA][E.sub.b]/2kT) (14)

Logarithmic plots of [I.sub.P]/[I.sub.s] versus [T.sup.-1] are presented in Figure 14 for samples with 0, 3, and 5% SNaM contents, respectively. The activation energy of backbone motion, [DELTA][E.sub.b], is produced by the least squares fitting the data in Figure 14 to equation (14) and are listed in Table 2. It is understood that clay particles can interfere with the motion of a polymer chain across the junction; as a result, a single chain needs more energy to overcome this difficulty. It is also seen that [DELTA][E.sub.b] values are much higher than [DELTA][H.sub.P] and [DELTA][H.sub.tr] values. This result is understandable because a single chain needs more energy to execute diffusion across the polymer-polymer interface than could be accomplished by the viscous flow process.

All these results have shown that the presence of SNaM clay in a latex system blocks and/or delays the film formation steps. However, in order to understand the evaluation of composite film formation in more detail, SEM pictures have to be analyzed carefully. SEM results of a composite film with 5% SNaM content annealed at 130, 150, 200, and 230[degrees]C temperatures are shown in Figure 15a, b, c, and d, respectively. It can be seen that viscous flow for this film starts at about 150[degrees]C and is completed at 200[degrees]C. At 230[degrees]C annealing, the composite film completes its interdiffusion processes and forms a mechanically strong film. In order to observe the screening effect of the clay particles on the film formation process, a 20% SNaM content composite film was prepared, which has shown no fluorescence intensity and allows no transmitted light. The SEM pictures of 20% SNaM content film are presented in Figure 16a, b, c, and d for the annealing temperatures of 130, 170, 200, and 230[degrees]C respectively. One can see in Figure 16 that the film formation process of this composite film cannot be completed, i.e., the structure of PS latex particles keep their original shapes during and after the annealing processes. Even after the annealing of this composite film at 230[degrees]C, the characteristics of PS particles still can be seen.

In conclusion, this work has shown that composite film formation from PS latex and SNaM particles can be successfully performed if the SNaM content is kept below 7%. The spectroscopic techniques such as fluorescence and photon transmission can be used to study composite film formation up to this limit. Composite films formed with high SNaM content cannot be studied with these techniques. SEM results have shown that in extremely high clay content films (20% SNaM), void-closure and interdiffusion processes between PS particles do not occur.

Table 1 -- Chemical Analyses of Clay Sample

Sample    Si[O.sub.2]    [Al.sub.2][O.sub.3]    [Fe.sub.2][O.sub.3]

SNaM      62.66          19.31                  6.10

Sample    [Na.sub.2]O    CaO     [K.sub.2]O     MgO     MnO

SNaM      2.36           0.22    0.72           2.48    0.01

Sample    Ti[O.sub.2]    [P.sub.2][O.sub.8]

SNaM      0.78           0.04

Table 2 -- Experimentally Produced Activation Energies

Clay (%)                            0     3      5

[DELTA][H.sub.tr](kj.[mol.sup.-1])  35.5  47.2   77.3
[DELTA][H.sub.P](kj.[mol.sup.-1])   16.7  21.7   27.6
[DELTA][E.sub.b](kj.[mol.sup.-1])   99.1  89.0  352.0

[DELTA][H.sub.tr] -- activation energy of viscous flow (measured from
[I.sub.tr])
[DELTA][H.sub.P] -- activation energy of viscous flow (measured from
[I.sub.P])
[DELTA][E.sub.b] -- activation energy of backbone motion (measured from
[I.sub.P])

ACKNOWLEDGMENTS

We would like to thank Professor Mustafa Urgen and Tuncay Turutoglu (from the Metallurgical and Materials Engineering Department at ITU) for helping us to take SEM micrographs. One of us (OP) also thanks the Turkish Academy of Sciences (TUBA) for their partial support.

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Saziye Ugur, Ayse Alemdar, and Onder Pekcan ([dagger]) -- Istanbul Technical University*

* Department of Physics, 34469 Maslak, Istanbul, Turkey.

([dagger]) Isik University, Department of Physics, 34398 Maslak, Istanbul, Turkey.