Langmuir 1995,11, 3369-3373
3369
Admicellar Polymerization of Polystyrene on Glass Fibers Sachin S. Sakhalkar and Douglas E. Hid* Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634-0909 Received January 30, 1995. In Final Form: May 31, 1995@ A preliminary investigation into an experimental technique, using surfactants, for producing organized thin polystyrene films on glass fibers has been performed. The thin-film polymerization process occurs in three steps, namely, the formation of surfactant admicelles, the partitioning of the styrene monomer into the admicelles, and the in situ polymerization of the absorbed monomer. Treated fibers have been observed using scanning electron microscopy to evaluate the film formation. The micrographs showed a nonuniform coating on the fiber surface. Fibers were also subjected to a THF extraction process that confirmed the formation of polystyrene. Experiments conducted also revealed that polymerization was not restricted to the admicelles and that some polymerization occurred in the supernatant as well.
Introduction The formation of thin films on solid surfaces has been the object of intense study in recent years because of a wide variety of possible applications of these films. The polymerization of thin films is carried out in a three-step process, which is based on the formation of micelle-like aggregates of physically adsorbed surfactants at a solidsolution interface. These surface aggregates are termed admice1les.l Just as micelles incorporate other molecules into their structure via the phenomenon known as solubilization, admicelles exhibit an analogous behavior, which is referred to as adsolubilization. The formation of admicelles followed by the adsolubilization of monomers into the admicelle and then polymerization of the monomers in situ constitutes the three-step process to construct a thin film on a solid substrate.2 The technique is reported to be quite versatile and is applicable to a variety of surface^.^ Various potential applications have been proposed for thin films formed by this technique, e.g., in the microelectronic industry, particularly for microlithographic applications for the manufacture of miniaturized circuit patterns on silicon wafers. Other uses include solid lubrication, corrosion inhibition, optical coatings, and surface-modified electrodes. In this research, the formation of a polystyrene film on the surface of glass fibers using two different cationic surfactants has been studied. The work reported here constitutes a preliminary investigation of one of many possible systems. However, the study does give valuable insight into the feasibility of the basic approach. Thin-Film Formation The thin-film polymerization process can be envisioned as occurring in three major steps (Figure 1) that are described briefly below:2 Step 1. Admicelle Formation. The aggregation of surfactants at solidlliquid interfaces to form bilayers (admicelles)through adsorption from an aqueous solution is a well-known To obtain admicelle formation, the most critical parameter to be manipulated
* To whom correspondence should be addressed. @
Abstract published inAdvance ACSAbstracts, August 1,1995.
(1) Harwell, J. H. Ph.D. Dissertation, The University of Texas at
Austin, 1983. (2) Wu, J.; Harwell, J. H.; O’Rear, E. A. Langmuir 1987, 3, 531. (3)Harwell, J. H.; O’Rear, E. A. In Industrial Applications of SurfactantsII;k s a , D. R., Ed.; Royal SocietyofChemistry: Cambridge, 1989. (4) Gaudin, A. M.; Fuerstenau, D. W. Trans AIME 1955, 202, 66. (5) Gaudin, A. M.; Fuerstenau, D. W. Trans AIME 1956,202, 958.
SOLUTION
d
SOLID SUBSTRATE
o
ADSORBED SURFACTANT
M
M
d
M
MONOMER PARTITIONING
3,
P ~ P P ~ P ~ P ~+ P~PP IN-SITU POLYMERIZATION
Figure 1. Thin-film polymerization process.
is the solution pH, relative to which the surface exhibits a net surface charge of zero (referred to as the point of zero charge or PZC). At pH values below the PZC, the surface becomes protonated and more positively charged; above the PZC, the surface is negatively charged. Consequently, anionic surfactants adsorb below the PZC and cationic surfactants above the PZC. In this study, glass fibers were used, which typically have a PZC in the acidic pH range. Since a cationic surfactant (dodecyltrimethylammonium bromide (DTAB)or cetylpyridinium chloride (CPC)) was chosen to form the bilayer, the pH of the surfactant solution was adjusted to a high pH level, (6)Somasundaran, P.; Fuerstenau, D. W. J.Phys. Chem. 1965, 70, 90. (7) Scamehorn,J. F.; Schechter,R. S.;Wade,W. H. J.Colloid Interface Sci. 1982, 85, 463.
0743-7463/95/2411-3369$09.00/00 1995 American Chemical Society
Sakhalkar and Hirt
3370 Langmuir, Vol. 11, No. 9, 1995
arbitrarily selected to be pH 12. With sufficientsurfactant, this level of basicity should assure a negatively charged surface and the formation ofdense bilayers at the interface. Step 2. Monomer Adsolubilization. Under conditions favorable for the formation of admicelles on a solid surface and unfavorable for the presence of micelles in an aqueous supernatant, hydrophobic species are concentrated at the interface in a phenomenon called adsolubilization. Formally, adsolubilization can be defined as the excess concentration of a species at an interface that would not exist in the absence of admicelles. As a prelude to the film-forming polymerization reaction (step 31, the hydrophobic monomer (e.g., styrene in this case) adsolubilizes or partitions into the adsorbed surfactant aggregates of step 1. The organic liquidlike environment exhibited by the hydrophobicinteraction of the amphiphilic tail renders a favorable region to solubilize the organic styrene. Step 3. In-Situ Polymerization of Adsolubilized Monomer:Beyond the purpose of concentratingmonomer at the surface of the substrate, the admicelles function as reaction loci or a two-dimensional reaction solvent for polymerization. Initiators begin the formation of polymer, probably by mechanisms similar to those occurring in conventional emulsion techniques. In the case studied here, sodium persulfate, a water-soluble initiator, was used. System Characteristics. The adsolubilization of styrene in the admicelle has a cooperative effect in enhancing the adsorption of the cationid surfactant on the glass fibers. This is due to the similar hydrophobic interaction between the water-insoluble styrene species and the amphiphile tails. As a result of this interaction, the hydrophobic environment at the solid surface increases,which aids in the adsorption process. The amount of surfactant added to the aqueous mixture is controlled so that the final concentration of the surfactant does not exceed its critical micelle concentration (cmc) in salt solution (0.1M NaBr). The role of sodium bromide added in the system is to strengthen the counterion binding near the water-admicelle interface and to stabilize the bilayer structure by reducing the repulsive electrostatic force between the surrounding polar headgroups of the amphiphile molecules. The decreased repulsion causes the surfactant molecules to come closer together. As a consequence,there is a small increase in the close-packed adsorption density due to the more compressed double layer. The adsolubilizationof styrene and the counterion binding are both expected to impart added stability to the admicelle structure.
Experimental Details The adsorption of cationic surfactants on glass fibers was studied by measuring the surfactant concentration before and after the adsorption processusing a two-phase titration method.8 Concentrations of styrene in solution were measured using highperformance liquid chromatography (HPLC). The analytical column used was a reverse-phase column packed with a slurry of 5-pm silica gel silanated with a polymeric Cp,-bonded phase. The column (called econosphere Cle column) was obtained from AllTech Applied Science Laboratories. Styrene concentrations were determined by a Varian 2050 variable-wavelength W detector at a wavelength of 248 nm. Methanol was used as the carrier solvent. Concentrations obtained were reproducible to within 5%. Due to the initial difficultiesin controllingstyrene evaporation, 60 mL of solution and the glass fibers were contacted in 70-mL borosilicate glass bottles sealed with aluminum foil and Teflon-
lined neoprene septa held in place by polypropylenecaps. This arrangement allowed for the removal or addition of styrene via a syringe, thus minimizing the evaporation of styrene. The glass fibers were received from PPG Industries as %-in. chopped strands with approximately13%moisture and some binder. Since the end use of the fibers was to be as a reinforcement for roof shingles,the binder used for this applicationwas a silane coupling agent, a lubricant, and a thickening agent to increase the mix viscosity. Provided the material was not allowed to air dry, the binder on the strands could be removed easily by rinsing with water. This was indicated by the hydrophilic nature ofthe fibers aRer rinsing (Le., before the rinse, the fibers would agglomerate in solution, whereas after the rinse, the fibers would show a more hydrophilic nature and would disperse in solution). The washed fibers were then dried at elevated temperatures (80-90 "C)in a vacuum oven for 1h to drive off any moisture and other volatile substances. The dry fibers were then weighed and added to a glass bottle containing an aqueous solution. The basicity of the aqueous solution was adjusted to a pH 12 by addition of NaOH to the solution. The pH was measured using a Corning pH meter (Model 345). The electrode used was a highperformance glass combination electrode (Catalog No. 476390). The use of pH 12 did not appear to damage the fiber surface within the time period used for the experiments (no etching was observed in SEM micrographs of the exposed fibers). Once the fibers were uniformly dispersed, known amounts of surfactant, NaBr, and ethanol were added to the mixture. The addition of ethanol in the system causes an increased solubility ofthe waterinsoluble styrene in the surfactant solution. Ethanol, being a short-chained compound, is not likely to adsorb in the bilayer. In fact, the solubilization of alcohols in a micelle shows a significant effect only for a carbon number greater than 5 or 6.9 Without ethanol, the solubility of the styrene is so low that it is impossible to have enough styrene in the feed to saturate the surfactant bilayer. It has been shown that the ethanol does not measurably affect the surfactant bilayer formation at the concentrationsneeded to enhance the styrene solubility.2 Finally, knownvolumes (and hence knownweights) of styrene were added using a syringe through the septum to a sealed bottle containing the solution. The adsorption of the surfactant molecules and the adsolubilization of styrene in the admicelle layer were assumed to be complete after 4 days (no depletionin the concentrationof styrene and surfactant was observed after 4 days). Before injecting samples of the supernatant into the HPLC column, the fibers were allowed to settle to the bottom of the vial. A sample of the clear supernatant was drawn through the septum by a syringe and injected directly into the HPLC to determine the equilibrium styrene concentration. Atwo-phasetitration technique8wasused to determine surfactant concentrations in the aqueous supernatant. The indicator used for the titration was prepared by dissolving the potassium salt oftetrabromophenolphthalein ethyl ester in ethanol to make a 0.2 wt % solution. A phosphate buffer was used to maintain the solution being titrated at a pH of 7.0. 1,2-Dichloroethanewas used as the organic phase. A 5-10-mL portion of the aqueous supernatant solution containing the M tetraphenylcationic surfactant was titrated with 5 x borate solution as the titrant. A color change from sky blue to faint yellow in the organic phase resulted when the equivalence point was passed. A blank titration was unnecessary because the organic phase was yellow in the absence of a cationic surfactant. Once the quantitative analysis was done, a known amount of the initiator, sodium persulfate, was added to the solution while heating the bottle to 90 "C to initiate the polymerizationreaction (no precipitation of the surfactant was observed under the experimental conditions used). Reactions were carried out for a period of 120 min. The bottle was cooled down to about 50 "C under tap water and then immersed in an ice bath to quench the reaction. After quenching, the glass fibers, presumably coated with polystyrene,were filtered using a side-armedflask connected to a vacuum pump. The fibers were then washed several times using distilled water to rinse off any styrene and surfactant in the adhering liquid. The washed fibers were then placed in an oven and heated (heating rate ,.- 2 "C/min)to 60 "C and held at ~~
(8)Tsubouchi, M.; Mitsushio, H.; Yamasaki, N. A w l . Chem. 1981, 53,1957.
(9) Hoiland, H.; Ljosland, E.;
1984,101,467.
~
Backlund, S. J.Colloid Interface Sei.
Polymerization of Polystyrene on Glass Fibers
Langmuir, Vol. 11, No. 9, 1995 3371
Table 1. Critical Micelle Concentrations of DodecyltrimethylammoniumBromide (DTAB) and
Results and Discussion Adsorption and Adsolubilization. Two different surfactants were used in the experiments to form the admicelles,namely, dodecyltrimethylammoniumbromide (DTAB) and cetylpyridinium chloride (CPC). The concentrations of the surfactants were maintained at or below their critical micelle concentration (cmc)to prevent micelle formation in the aqueous supernatant and, consequently, to avoid any emulsion polymerization from taking place in solution. It is desired to restrict the polymerization reaction within the surfactant bilayer assemblies on the glass fiber surface. The cmc of each surfactant is listed in Table 1. In the experiments utilizing DTAB as the surfactant, an electrolyte (NaBr) having the same counterion as the surfactant was added to the system. The addition of an electrolyte results in a small increase in the adsorption of the surfactant. Hence, it is advantageous to use an aqueous solution having a high ionic strength when using DTAB as the surfactant. A 0.1 M NaBr solution was used in all of the experiments with this surfactant (system A). In the case of CPC though, its cmc in aqueous solution is lower (see Table 1). Addition of an electrolyte (NaC1) reduces the cmc by an order of magnitude so that it is not possible to have enough surfactant in the solution to obtain substantial interfacial adsorption without forming micelles in the solution. Therefore, no electrolyte was added in this case (system B). Once the bilayer formation is complete locally on the fiber surface, the styrene monomer in the supernatant preferentially partitions or adsolubilizesinto the admicelle to reach equilibrium. The adsorption and adsolubilization data for the two systems studied are shown in Table 2. The adsorption was carried out on 0.3 g of glass fibers in the presence of 0.5 M ethanol. It should be noted that a 95% ethanol solution was used in preference to absolute ethanol because 100%ethanol contains residual amounts of benzenelo that absorb UV light at about the same wavelengths as styrene and may lead to an inaccurate determination of styrene concentrations. The initial and final concentrations of styrene and surfactant are presented in Table 2. The amount of surfactant adsorbed suggests more than a bilayer coverage on the glass fibers (fiber surface area 2500 cm2/g;surfactant head 0.4 nm2/molecule11). It should be noted, however, that this is only an estimation and that it is difficult to obtain the
actual adsorption density in such complex systems. Moreover, some of the adsorbed surfactant may be washed off during the postreaction washing steps, thus making it difficult for an accurate estimation of the amount of surfactant remaining on the fiber surface. Once the equilibration waB complete,sodium persulfate was added to the system at a concentration of 0.01 M and the system heated to 90 "C to initiate the polymerization reaction. It has been postulated that the polymerization reaction proceeds by the onset of transfer of free radicals from the supernatant fluid (aqueousphase) to the interior of the admicelle (organic phase>.12The absorption of the radicals by styrene monomers inside the admicelle should produce oligomeric polymer chains in active centers of the admicelle. Once the oligomers are formed, more styrene monomers should absorbwithin the active centers of the admicelle due to the increased solubility of the styrene in the polystyrene being formed.2 Styrene concentrations in the supernatant were measured using HPLC during the course of the reaction (Figure 2). As expected, the amount of styrene decreased continually as the reaction proceeded. The final concentration of the styrene after the reaction was complete (reaction time of 2 h) and the total adsolubilization of styrene are shown in Table 3. Once the film-forming experiments were complete, the fibers were washed with distilled water and dried and then subjected to the THF extraction process as described above. The THF extract, with the extracted polystyrene from the surface of the fibers, was then subjected to a UV spectrophotometric study. A typical U V spectrum obtained is shown in Figure 3. From the spectrum, it is apparent that there is a strong peak at 260 nm, which corresponds to a peak of polystyrene and confirms the formation of polystyrene. Film Formation. A portion of the treated fibers, after being washed and dried, was observed in a scanning electron microscope (SEM) to check for the presence of a polymer film. The SEM used was a field emission SEM (Hitachi, Model S-4500), and the samples required only a slight sputter coating of gold to a thickness of 7 nm. Micrographs of the untreated and treated glass fibers are shown in Figures 4 and 5 , respectively. These micrographs are representative of both experimental systems. Contrary to expectations,the SEM micrographs of the treated glass fibers showed a nonuniform coating of the polymer on the fiber surface. In fact, it appears that there are small islands of polymer on the surface (note that the bright-colored regions in the figure are probably extraneous particles deposited on the surface). The existence of such islands on the fiber surface may not be unusual because similar islands have been reported by McDermott and co-workers.13 They studied the structure of an adsorbed surfactant layer at a crystal quartdaqueous solution interface using neutron reflection. One of the possible reasons for this nonuniform coating could be the unequal distribution of charge on the surface of the glass fibers. It is known that glass has silica and alumina as its major constituents along with other metal oxides. Hence, the glass fibers will have both silica and alumina moleculespresent on the surface in large proportions. Pure silica and alumina have points of zero charge at pH values of approximately 3 and 10, respectively. Therefore, at a pH of 12, the negative charge on those parts of the glass fibers having alumina on its surface may not be large enough to maintain a strong electrostatic attraction
(10)Williams, D. H.; Fleming, I. Spectroscopic Methods In Organic Chemistry, 2nd ed.; McGraw-Hill: London, 1973. (11) Jungermann, E. Cationic Surfactants; Marcel Dekker: New York, 1970.
(12) Wu, J.; Harwell, J. H.; ORear, E. A. J.Phys. Chem. 1987,91, 623. (13) McDermott, D.C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J . Colloid Interface Sci. 1994, 162, 304.
Cetylpyridinium Chloride (CpC)11J6at 25 "C solvent cmc, M H2O 1.6 x 0.1 M NaBr 4.2 10-3
surfactant DTAB16 DTAB" CPC16
H2O
9.0
10-4
that temperature for 1 h to remove any excess water and unconverted styrene monomer from the surface of the fibers. A portion of the fibers was used for SEM studies, and the remaining fibers were transferred to a tetrahydrofuran (THF) solution to extract the polystyrene from the surface ofthe fibers. The fibers were brought in contact with approximately 10 mL of THF solution in 20-mL vials. Mixing was performed by manually shaking each vial several times a day to bring the fibers in intimate contact with the extracting THF solvent. The extraction was assumed to be complete after 3 days. f i r 3 days, the THF solution was analyzed by UV spectrophotometry (Shimadzu, Model UV 3100), and the spectrum was compared to those of known samples of polystyrene dissolved in the Bame solvent.
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3372 Langmuir, Vol. 11, No. 9, 1995
Sakhalkar and Hirt
Table 2. Adsorption and Adsolubilization on 0.3 g of Glass Fiber after 4 days of Mixing“
a
system
surfactant
A B
DTAB CPC
final concn, pM surf. styr. 2038 f 145 2229 f 202 410 f 17 2215 f 97
initial concn, pM surf. styr. 2973 f 7 2912 900 f 8 2912
adsorbed, pmol surf. styr. 53 f 9 45 f 6 29 f 3 42 f 6
note: f denotes standard deviation.
2400m
2200
I
i”.
CFC
DTAB
I
0
30
60
90
120
Figure 4. Untreated glass fiber.
time (min)
Figure 2. Decay of supernatant styrene with reaction time. Table 3. Styrene Adsolubilization Data= system
A B
styrene concn, pM total styr. initial before rxn after 1 7 ~ 1 adsolubzn, Dmol 2912 2038f 145 1547f63 82 f 4 1780f82 2912 2215f97 68 f 5
w-
.
.
note: f denotes standard deviation.
Figure5. Glass fiber subjected to the three-step polymerization process (note that the bright-colored regions are probably some extraneous particles deposited on the surface).
0
Figure 3. Typical UV spectrum of a THF extract.
between the cationic surfactant molecules and the fiber surface. On the other hand, those portions with silica on the surface are able to attract enough surfactant molecules to form admicelles. However, similar island formation has been reported on surfaces of pure quartz.13 This suggests that, in addition to this charge effect, there could be other effects like surface roughness or cleanliness that could affect the coverage for certain types of cooperative interaction. This could occur even at very low levels of interaction and could lead to the presence of domains of surfactant on the fiber surface. Many workers have reported the presence of certain “disturbed layers” on a range of glasses, silicas, and
quartz.14J5 In fact, McDermott and co-workers13 have indicated the presence of inhomogeneities at the quartz surface that affect the uniformity of the adsorbed bilayer. Figure 5 shows the islands that were observed from polymerizationexperiments. Someof these islands formed are larger than those envisioned by the neutron reflection data (-90 A). However, certain patches of polymer shown in Figure 5 appear to be in a state of favorable wetting on the fiber surface, which is an indication of a surface phenomenon rather than an extraneous deposition phenomenon. Polymerization in the Supernatant Liquid. Experiments were also conducted to determine whether any polymerization takes place in the supernatant and, if so, whether the polymer adsorbs onto the fiber surface. To do this, the polymerization reaction was performed in an aqueous solution as before but without the presence of (14)Beilby,G.Aggregation and Flow in Solids; Macmillan: London, 1921. (15)Henderson, J. H.;Syers, J. K.; Jackson, M. L. Isr. J . Chem. 1970,8, 357.
Langmuir, Vol. 11, No. 9, 1995 3373
Polymerization of Polystyrene on Glass Fibers
nJ
h)
0 0
c Yl 0
0
m
ij I" t
L
1P
0
0
0 0
Figure 6. Glass fiber, sample 1, subjected to the alternate experimental procedure.
240
300
360
500
L20
g
tnm)
Figure 8. W spectrum of fiber, sample 1, subjected to the alternate experimental procedure. N
h)
$51
.j:
- 0
,
p
.
8 0
240
. . . 280
320 (nm)
. . 360
.
?
. . 400
8 3
Figure 7. Glass fiber, sample 2, subjected to the alternate experimental procedure.
Figure 9. W spectrum of fiber, sample 2, subjected to the a1ternate experimenta1 procedure.
fibers in the system. Once the 2-h reaction was complete, glass fibers were added to the system and the system was allowed to equilibrate for 4 days. Once the equilibration was complete, the fibers were filtered and subjected to the washing and drying process as before. A portion of the fibers was then subjected to THF extraction and the extract to W spectrophotometric studies, while the remaining portion of the fibers was observed in the SEM. To check reproducibility, this experiment was performed twice. The SEM photographs for the two experiments, representative of both experimental systems, are shown in Figures 6 and 7. From the micrograph of sample 2, a coating on the fiber surface is observed, although such a coating is not apparent in the micrograph of sample 1.In fact, the micrograph of sample 1indicates the presence of globules of polymer on the fiber surface. Hence, reproducibility of the experiment was not achieved. The W spectra obtained for the two samples are shown in Figures 8 and 9. The spectra are similar to the one obtained for Figure 3with a peak at 260 nm, corresponding to polystyrene. This demonstrates that there is indeed polymerization occurring in the supernatant and that the polymer, after being formed, deposits onto the glass fiber surface. The concentration of the polystyrene obtained in the two experiments was approximately the same, which
implies that although the coating obtained may not be reproducible, the amount of polymer formed may be reproducible. But more importantly, the concentration of the polystyrene formed through this procedure was much lower than that formed with the original experimental procedure. This indicates that a majority of the polymerization occurred within the surface aggregates.
Conclusions The formation of thin polystyrene films on glass fiber surfaces has been attempted. The work reported here is a preliminary study, and the results obtained highlight the complexities of the process. It appears that in the third step of the polymerizationprocess, polymer formation was not restricted to the surface aggregates as originally thought, buta fraction of the polymerization occurred in the aqueous supernatant as well. It is also clear that polymer formation definitely occurred, although the formation of a uniform polymericcoating was not achieved in this research. LA950066L (16) Rosen, M.J. Surfactants and Znterfacial Phenomena, 2nd ed.; Interscience: New York, 1989.
