Langmuir 1994,10, 2588-2593
2588
In Situ Formation of Polystyrene in Adsorbed Surfactant Bilayers on Precipitated Silica John H. O'Haver,*yt Jeffrey H. Hamell,? Edgar A. O'Rear,? Linda J. Snodgrass,* and Walter H. Waddell$ School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019,and PPG Industries, Chemicals Group Technical Center, Monroeville, Pennsylvania 15146 Received February 4, 1994. I n Final Form: May 20, 1994@ Surfactant bilayers adsorbed on silica are used as the reaction site for the formation of ultrathin polymer films from coadsorbed or adsolubilized monomer. Results from the polymerization of styrene in cetyltrimethylammonium bromide (CTAB), in octylphenoxypoly(ethoxy)ethanol,and in methyltri(C8Cl0)ammoniumchloridebilayers show effective conversionof adsolubilizedstyrene monomer to polystyrene. The process has been demonstrated effectivewith amorphous precipitated silica substrates having a variety of surface areas and with two types of initiation schemes. The extractable polymer has been characterized using photoacoustic FTIR, UV, and gel permeation chromatography (GPC). Only approximately 25% of the polymer was extractable after refluxing for 4 h in tetrahydrofuran. The impact of the treatment on some industrially important surface properties of the modified silicas has also been determined. The process offers a potentially inexpensive method for modifylng chemical and physical surface properties of various substrates.
Introduction There has been considerable research on modifying the surface groups of inorganic powders such as silica, alumina, and titanium dioxide, in order to make them more compatible with the composite hosts in which they are mixed.l Currently, siloxane, titanate, and zirconate coupling agents are used extensively in industry for this purpose. These chemical coupling agents can impart significant improvements in important compositephysical properties. However, they also can be expensive, thus significantly increasing the cost of a product. The formation of ultrathin polymer films in adsorbed surfactant bilayers, called admicelles, has been investigated since the mid 1980s. Wu, Harwell, and O'Rear2 first investigated this phenomenon in the formation of polystyrene from styrene adsolubilized in anionic surfactant sodium dodecyl sulfate (SDS) admicelles on alumina. The system employed thermal initiation using a water-soluble persulfate. An in-depth study of the adsolubilization process and the conversion kinetics was made.2 Recently, the method has been extended by Lai3 who studied poly(tetrafluoroethy1ene) formation on alumina from admicelles of perfluorinated surfactants, and by Chen4 who formed polystyrene on titanium dioxide from admicelles of sodium dodecyl sulfate (SDS). Both systems also used water-soluble, thermally cleaved persulfate initiators. German et al. are currently doing work on encapsulating inorganic particles using emulsion polymerization both with and without coupling agent^.^-^ + University of Oklahoma.
* PPG Industries.
Abstract published in Advance A C S Abstracts, July 15, 1994. (1)Plueddemann, E. P. Silane Coupling Agents, Plenum Press: New
York, 1991. (2) Wu, J.; Hanvell, J. H.; ORear, E. A. Langmuir 1987,3,531. (3)Lai,C. Masters Thesis, University of Oklahoma, 1992. (4) Chen, H.Masters Thesis, University of Oklahoma, 1992. ( 5 ) German, A.L.; Jannsen, R. Q. F.; Derks, G. J. W.; van Herk, A. M. Paper presented at the Manchester Congress on Encapsulation, October 14-15, 1992. ( 6 )Caris, C. H. M.; Juijpers, R. P. M.; van Herk, A. M.; German,A.
L. Makromol. Chem. Macromol. Symp. 1990,35136,535-548. (7) Caris, C. H. M.; van Elven, L. P. M.; van Herk, A. M.; German, A. L. Br. Polym. J . 1989,21,133-140. (8) van Herk, A. M.; Janssen, R. Q.F.; Janssen, E. A. W. G.; German, A. L.Unpublished paper.
0743-746319412410-2588$04.50/0
In the present study the method has been expanded to explore further the range of suitable materials, including the following: (1)use of a new substrate, amorphous precipitated silica; (2)use of other surfactant types (i) water-soluble cationic surfactant cetyltrimethylammonium bromide (CTAB), (ii) water-insoluble cationic surfactant ADOGEN@ 464 (methyltri(a mixture of C8Cl0)ammonium chloride), and (iii)a nonionic surfactant, MACOL@OPlOSP (octylphenoxypoly(ethoxy)ethanolwith a n average of 10 mol of ethylene oxide); (3)use of other initiation schemes (i)thermal initiation with a virtually water-insoluble initiator 2,2'-azobis(2-methylpropionitrile) (AIBN)and (ii)redox initiation a t 5 "C and ambient conditions using a system similar to those used commercially in emulsion polymerization;12(4)first examination of the effect of the polymer surface modification process on a variety of industrially important substrate properties; (5)study of the molecular weight of the polymer formed by this method. The present research further demonstrates the generality of this method2 in forming polymer on a variety ofinorganic substrates using varying polymerization techniques and the capability of the process to effect important surface modifications ofthat substrate.
Ultrathin Film Formation The method used for the modification of inorganic powders by the formation of ultrathin polymer films consists of four basic steps (Figure 1). Step 1consists of admicelle formation by the adsorption of a surfactant bilayer onto the surface of the substrate. Adsorption is accomplished through the use of a suitable surfactant under appropriate system conditions. The choice of surfactant is influenced by the point of zero charge of the substrate, the chemical nature ofthe polymer to be formed, and the chosen polymerization initiator system. A study of the point of zero charge (PZC)for the substrate provides information on the pH ranges in which cationic or anionic surfactants may readily be utilized. The instability or (9) Rozmajzl, J. A.Macromolecular Synthesis Vol I; John Wiley and Sons: New York, 1977. (10)Iler, R. K. The Chemistry ofsilica; John Wiley and Sons: New York. - .... 1979. (11)Barr, T.;Oliver, J.; Stubbings, W. V. JSCI 1948,February. I
(12)Odian, G. Principles New York, 1991.
of
Polymerization; John Wiley and Sons:
0 1994 American Chemical Society
Formation of Ultrathin Polymer Films
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Langmuir, VoE. 10,No. 8,1994 2589 the present study, films were formed on several amorphous precipitated silicas having BET nitrogen surface areas ranging from 140 to 215 m2/g.
Experimental Section
Figure 1. Schematic of the fotir-step process for ultrathin film formation: (1)adsorption; (2) adsolubilization; (3) polymerization; (4) washing. dissolution of components in certain pH ranges may further define the range of useful operating conditions and, hence, surfactant type. Step 2 in the process is the solubilization of monomer into the admicelle, a phenomenon called adsolubilization. Many organic monomers are nearly insoluble in water. Thus, a t equilibrium, they preferentially partition into the hydrophobic interior of the admicelle. This process can occur after the formation of the admicelles or concurrently with surfactant adsorption. It is convenient experimentally to dissolve the monomer, and sometimes the initiator, in the surfactant feed solution prior to surfactant adsorption. In these cases, the presence of micelles promotes the solubility of the monomer in the feed solution, increases the rate of the adsolubilization of the monomer in the admicelles, and helps prevent the formation of emulsions. When this feed solution is contacted with the mineral substrate, the adsorption of the micelles is thought to carry the solubilized monomer onto the substrate surface. Step 3 is the in situ polymerization of the monomer. For free radical polymerization, this is accomplished by the generation of radicals capable of initiating the polymerization reaction. In some cases, the compatibility of the initiator system with anionic or cationic surfactants may also affect the choice of an appropriate surfactant and suitable reaction conditions. To reduce the change of competing polymerization in the bulk solution, a detailed study of adsorption and adsolubilization isotherms is made to ensure that the system equilibrates with the surfactant concentration in the bulk solution below the critical micelle concentration (cmc). Once the reaction has started, additional monomer from the bulk solution diffuses into the admicelle.13 Ifthe reaction is continued for a sufficient length of time, virtually all monomer can be converted to polymer. Step 4 is the washing of the treated powder to remove as much of the outer surfactant layer as possible in order to expose the polymer film. Because of the nature and size of the system components, the process can be used to form films upon a variety of substrate surface topographies. W u et a1.2 formed polystyrene films (i) on a very flat alumina surface that was formed by oxidizing vapor-deposited aluminum on glass slides and (ii)on an irregular-shaped alumina powder with a surface area of 90-100 m21g. Chen4 formed polystyrene films on titanium dioxide powder having a relatively low surface area of approximately 7 m2/g. In (13)Wu,J.; Hanvell, H. J.; O'Fkar, E. A. J. Phys. Chem. 1987,91, 623.
Materials. All materials were obtained commercially and used as received except for styrene which was used both as purchased and afier removing the 4-tert-butylcatechol inhibitor by passing the styrene through active alumina. Hexadecyltrimethylammonium bromide (CTAB) was obtained from Sigma Chemical (St.Louis, MO)at a purity of 99% and from Lancaster Synthesis (Windham, NH)at a purity of 99+%. Dodecyl sulfate, sodium salt (SDS) was purchased from Aldrich Chemical (Milwaukee, WI) a t 98% purity. Chloroform was a J. T. Baker (Phillipsburg, NJ) Analyzed Reagent of 100% purity. Alcohol was purchased from EM Science (Gibbstown,NJ) with an analysis of 95% ethanol and 5% methanol. Styrene was purchased from Fisher Scientific(Fair Lawn,NJ) at 99.6% purity and from Aldrich a t a purity of 99%. 2,2'-Azobis(2-methylpropionitrile)(AIBN) was purchased from Eastman Kodak (Rochester, NY)at 100% purity and from Pfaltz & Bauer (Waterbury, CT) a t 98% purity. The following were obtained from Aldrich: tert-butyl hydroperoxide (TBH) a t 70% purity; ferrous sulfate at 98+% purity; methoxychlor (1,1'-(2,2,2-trichloroethylidene)bis[4-methoxybenzene] a t 95% purity; and ethylenediaminetetraaceticacid tetrasodium salt (EDTA) a t 98% purity. Sodium formaldehyde sulfoxylate (SFS)was obtained from Pfaltz & Bauer (Waterbury, CT) a t 100% purity. Tetrahydrofuran (THF)was obtained from Fisher Scientific a t 99.9% minimum purity. ADOGEN 464 (ADOGEN) was obtained from Witco (Dubin, OH) as 85%active agent in 2-propanol. MACOL OPlOSP (MACOL)was obtained from PPG Industries (Pittsburgh, PA), as were the following amorphous, precipitated silicas: Hi-Si1 233, Hi-Si1 915, Hi-Si1 SBG, and Lo-Vel HSF. Methods. The adsorption of CTAB on the different amorphous precipitated silicas was studied by placing solutions of known concentration on predetermined quantities of silica and allowing them to equilibrate a t 30 "C. The feed solution was adjusted to pH 8 using sodium hydroxide. More basic solutions (pH > 81, though desirable in terms of the driving force for adsorption, were not used due to the increasing solubility of silica in alkaline solutions.1° The equilibrium supernatant CTAB concentration was determined by using a known SDS solution in a two-phase titration." The differencein feed and equilibrium concentrations was used to calculate the quantity of adsorbed surfactant. The adsorption studies of MACOL on silica proceeded in a similar manner except the feed solution was approximately pH 7; however pH has little effect on the adsorption of nonionic surfactants on silica. The equilibrium MACOL concentrations were determined using UV-vis adsorption spectrophotometry and standards of known concentrations. The adsorption of water-insoluble ADOGEN was studied by forming a slurry of silica in water and adding ADOGEN in small, known quantities until the solution became cloudy on settling, indicating that dispersed surfactant was present in the bulk solution. Adsorption was then calculated by dividing the moles of ADOGEN added by the grams of silica in the sample. The adsolubilization of styrene was determined by preparing feed solutions with known CTAB, ethanol, and styrene concentrations and contacting each with a silica sample of known mass in a sealed vial. The system was allowed to equilibrate at 30 "C for 48 h. Styrene adsolubilization was measured using a Tracor HPLC unit consisting of a 95L LC pump and 970A variable wavelength detector and controlled by aVarian 4270 integrator. The column was slurry-packed with a reverse-phase silica gel. The carriersolvent was 85%methanol and 15%deionized water. The thermal polymerization experiments were carried out by placing equilibrated samples containing water, surfactant, ethanol, styrene, silica, and AIBN in a 70 "C water bath for times varying from 10 min to 12 h. Reactions were carried on both stirred and unstirred systems. Samples were quenched in an ice bath. REDOX polymerizations were carried out by adding ferrous sulfate to equilibrated samples containing water, surfactant, ethanol, styrene, silica, TBH, SFS, and EDTA a t 5 "C. Polym-
2590 Langmuir, Vol. 10, No. 8,1994
OHaver et al.
Table 1. hoperties Tested, Testing Methods, and Equigment Used for Silica Analysis
property BET Nz surface area bibutyl phthalate absorption particle size mercury porosimetry total pore surface area pore diameter,50% % carbon
method ASTM D 3037 ASTM D 2414 ASTM F 622 ASTM D 4284
instrument Micrometrics ASAP 2400 Brabender Absorptometer E Coulter Multisizer I1 Quantachrome Autoscan 33
ASTM E 350
Leco 521 Analyzer
-F
h
-" B
B
a Im
E im
g
E
eY
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0 c
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4
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Equilibrium[CTAB](micromolar)
Figure 2. Adsorption isotherm for CTAB on Hi-Sil233.
erization times ranged from 10 min to 24 h. The reaction was quenched by the addition of methoxychlor dissolved in ethanol. After polymerization, the silica was allowed to settle and the supernatant was decanted. The treated silicas were placed in large carboys which were then filled with deionized water. The treated silica was allowed to settle and the wash water siphoned off, Filtered water was added and the process repeated until the wash water no longer foamed on agitation. The solid was then filtered and dried at 105"C for at least 2 h to remove any adsorbed water or unpolymerized styrene. Silica surface area determination, porosimetry properties, oil adsorption, pH, agglomerate particle size distributions,and percent carbon were determined for the untreated controls and all treated silicas (Table 1). The presence of extractable polystyrene was initially determined using a Bausch and Lomb Spectronic 1001 W spectrophotometer. Extracted polymer and polymer-silica composites were later analyzed by photoacoustic-FTIR (PA-FTIR),using a Mattson Cygnus 25 spectrophotometerwith a Rev. 8 board and First software. X-ray fluorescence spectra were obtained using a Kevex 770 spectrometer equipped with an energy-dispersive analyzer to determinebromine concentrationsby direct analysis. Extractionswere performed by boiling a 5-gsample ofthe treated silica in refluxing THF for 4 h. The slurry was cooled to room temperature,filtered,and rinsed with hot THF, and the polymer precipitated by addition of the filtrate to water. Differential scanning calorimetric measurements were performed using a Dupont Instruments 912 differential scanning calorimeter equipped with a TA Instruments Thermal Analyst 2100 controller.
Results At 22 "C, the adsorption of CTAB a t a feed pH of 8 (Figure 2) and MACOL (Figure 3) a t a feed pH of 7 on silica yielded isotherm shapes similar to those reported by Wu et a1.l for SDS on alumina. Maximum adsorption of CTAB on Hi-Si1 233, keeping the equilibrium bulk concentration of CTAB below the cmc, was found to be ~ 2 7 ,umoVg 0 of silica. The maximum adsorption of the nonionic surfactant on the Hi-Si1 233 was found to be xl70,umoYg of silica while keeping the bulk concentration of MACOL below the cmc. ADOGEN adsorption was measured as approximately 820 ,umol/g. Adsolubilization studies made using the CTAB system determined the maximum adsolubilization of styrene to be ~ 1 5 pmol/g 5 of silica. This measurement was deter-
om1
0 WI
0 mi
001
Equilibrium[MACOLOP l O S P ] ( M )
Figure 3. Adsorption isotherm for MACOL OP 10 SP on Hi-
Si1 233. mined for experimental runs in which surfactant adsorption was 267 ,umoYgof silica. This gives a ratio of adsorbed surfactant molecules to adsolubilized monomer molecules of approximately 2, which is comparable to earlier results for SDS and styrene on alumina.2 The KAS,as defined by Wu et aLY2is an equilibrium constant defined to be analogous to the partition coefficients used in solubilization studies
KAs = (adsolubilized styrene molecules per adsorbed surfactant moleculeY(equi1ibrium concentration of styrene in the supernatant) The value of K.Q in these studies a t 22 "C and with near optimum adsorption was approximately 400 M-l, compared to the average of 300 M-' for the SDS-styrenealumina system.2 Unlike emulsion polymerization, where typical monomer to initiator weight ratios in a successful formula6 might be 200, the AIBN system required a ratio of less than 4 to show measurable conversion of monomer to polymer. Essentially complete conversion of monomer occurs within 2 h a t 70 "C, determined by a mass balance around the treated silica and checking the supernatant styrene concentration a t different polymerization times. The REDOX system shows successful conversion with monomer to initiator weight ratios of approximately 100, but conversion of monomer continues for 12 h or longer as determined by total percent carbon measurements. In order to determine in a qualitative sense how firmly the polymer was attached to the surface of the silica, the polymer was extracted from a treated silica sample as usual. The unextracted treated silica, the extracted material, and the treated silica after extraction were all analyzed using PA-FTIR (Figure 4). The total carbon content was determined for the treated silicas before and after extraction and for the extracted materials (Table 2). With thorough washing it is assumed that the outer layer of the surfactant bilayer is removed and the polymer surface treatment exposed. This is supported by X-ray fluorescence measurements of a treated silica prepared using CTAB as the surfactant. Approximately 100 ppm
Langmuir, Vol. 10,No.8,1994 2591
Formation of Ultrathin Polymer Films
0.02
05
Te
53.85 c
n
0.01
L
0.1 -20
D 4000
0
20
40
A I
I
3500
3000
60
80
100
120
140
c4.04 160
Temperature ( C )
2500
2000
1500
1000
Wavenumbers
Figure 4. PA-FTIRof (A) polystyrene standard, (B) extracted material, (C) silica after extraction,and (D) treated silicabefore
extraction.
Table 2. Total Carbon Content of Untreated Silica, Treated Silica, Treated Silica after Extraction, and the Extracted Material samnle
total % carbon
untreated silica treated silica treated silica after extraction extracted material
0 4.52 4.17 84.8
Table 3. GPC Molecular Weight Analysis of Extractable Polymer number average weight average dispersity samule mol wt mol w t wt av/no. av A 45 800 159 000 3.47 B 36 610 101 300 2.77 C 6 790 46 300 6.82 ~~
bromine is detected. With this assumption, and given surfactant adsorption of approximately 250pmoYg,further assuming 100% conversion of monomer to the polymer, and since polystyrene contains approximately 92.3% carbon, the theoretical carbon content of washed, treated silicas should be approximately 4.8% carbon before extraction. A mass balance on the extract shows it to consist of surfactant and polymer in roughly a 5 to 1ratio by mass. The molecular weight of the extracted polymer was analyzed by GPC. The results for three samples are given in Table 3. Samples A and B both used HiSil233 as the substrate and had a feed of 5500 pM of CTAB, 3500 pM of styrene, and 3000 pM of AIBN. Sample C had a feed of 3000 pM of CTAB, 2000 of pM of styrene, and 500 pM of AIBN, using HiSil915 as the substrate. All three were polymerized for approximately 1 h a t 70 "C. The results of a set of experiments determining the molecular weight of polymer extracted from treated HiSil 233 as a function of reaction time are shown in Table 4. All samples consisted of a feed of 5500 pM CTAB, 3500 pM styrene, and 3000 pM AIBN. The polymerizations were carried out in a water bath at 70 "C. After 1.25 h, the treated silica had a 2.7% carbon which increased to and remained at approximately 3.5%(f0.3%)carbon after 2.25 h. After solvent extraction, 2.3% (f0.2%)carbon was present. The extracted polymer was analyzed utilizing differential scanning calorimetry (DSC). All of the samples showed glass transition temperatures (T,)between 49 and 59 "C. The DSC of the 10.25-h polymerization sample is shown as representative of the group (Figure 5). DSC's ofthe treated silicas were not attempted because of uncertainty regarding effects of the silica surfacelpolymer interaction and polymerlsurfactant interaction layer on the Tg
Figure 5. DSC of extracted polymer, 10.25-hpolymerization
time, AIBN initiator.
Table 4. Molecular Weight and Tgof Extractable Polymer and % Carbon of Unextracted Treated Silicas versus Reaction Time
number reaction time, h
average
1.25 2.25 3.25 4.25 5.25 6.25 7.25 8.25 9.25 10.25
36610 4098 3707 3389 3251 2962 2874 2960 2940 2977
molwt
weight average
dispersity wt av/
molwt
no. av
Tp
carbon
101300 32000 33300 34000 30400 28200 33700 33600 33800 33900
2.8 7.8 9.0 10.0 9.4 9.5 11.7 11.3 11.5 11.4
58.6 52.3 51.3 55.6 54.3 54.0 50.4 51.8 50.0 53.9
2.7
%
3.4 3.2 3.6 3.4 3.6 3.7
Table 6. Effect of Ultrathin Film Treatment on the Surface Properties of Five Silicas mean total pore total pore oil particle surface area, surface area, absorption, silica size, pm BET Nz, m2/g Hg, m2/g mU100 g 3.85 200 221 228 Hi-Sil915 29.2 113 127 218 treated 915 11.8 161 195 Hi-Si1 SBG 97 156 treated SBG 16.7 141 166 200 Hi-Si1 233 15.1 18.0 100 140 167 treated 233 3.83 161 189 LO-VEL275 106 155 treated 275 10.1 159 165 LO-VELHSF 7.02 102 151 treated HSF 20.5 16.7 102 167 extracted, treated 233
Surface Effects The process was tested on a variety of precipitated amorphous silicas having different commercial uses. HiSi1233 is generally used as a reinforcing filler in tires and rubber products such as hoses, belts, and shoes; Hi-Si1 HSF and LO-VEL 275 are used as flatting agents in paints; Hi-Si1 915 is used as a reinforcing filler for silicone rubber products; Hi-Si1 SBG is employed as a filler for polyethylene in manufacturing separators for batteries. The formation of ultrathin polymer films affected a variety of the surface characteristics of the silicas, including mean particle size, total pore surface area, and oil absorption (Table 5 ) . Discussion The 270pmollg adsorption of CTAB represents a bilayer coverage of approximately 23% of the total pore surface area of Hi-Si1 233. Wu et aL2were able to obtain nearly 100%bilayer coverage on alumina using SDS, as estimated by head group packing density. The lower percent adsorption of CTAB can be due to several factors:
2592 Langmuir, VoE. 10,No. 8,1994
substrate geometry-higher surface area with correspondingly smaller pore size and lower driving force. CTAB, which has a longer alkane chain than SDS, should find it more difficult sterically to penetrate into the smaller pores on the precipitated silica. In addition, as the CTAB bilayer is forming, the longer alkyl chain length could sterically block pores nearer to the surface as bilayers from the opposite walls approach each other. Recent studies indicate that higher percentage coverages can be achieved by increasing the feed pH.14 It was expected that the adsorption of the nonionic surfactant onto silica would be somewhat lower than that of CTAB due to the larger hydrophobic group of MACOL and current theories on how nonionic surfactants align on the surface.15J6 In contrast, the adsorption ofADOGEN onto silica was surprising. The area per molecule occupied by ADOGEN is much larger than that of CTAB, and its overall structure with four alkyl chains should sterically inhibit its adsorption into smaller pores. One explanation is that ADOGEN may form a thick layer on the surface of the silica. The van der Waals forces between molecules seem to be sufficient, along with its insolubility in water, to drive a considerable amount of surfactant onto the surface of the silica. The amount of adsolubilized styrene in CTAB was similar to that obtained by Wu, et aL2 The higher value ofKM is possibly due to a lower bulk styrene concentration, since the present ratio of adsolubilized styrene to adsorbed surfactant was similar to reported values. Cationic surfactants tend to have a greater solubilizing power than anionics, and this may be a factor as well." It is not known why relatively high initiator to monomer ratios are necessary in the present experiments when using AIBN. One possible explanation is that ethanol used to dissolve the AIBN also adsolubilizes, and it may act to consume many of the radicals formed. An examination using water-soluble azo initiators is in progress to determine their effect on the system. This explanation is also supported somewhat by the results obtained by using the REDOX system, where small concentrations of initiator are successfully used. The relatively slow rate of conversion in the REDOX system is not surprising. The amount of ferrous sulfate introduced into the system is very small compared to the amount of AIBN used in corresponding experiments. However,the relatively large amounts of EDTA and SFS ensure that the ferric ion can be reduced several times, thus enabling it to repeatedly initiate polymerization. Another differentiating feature of the silica system compared to the alumina system is the inability to extract all of the polymer formed. Wu et a1.2 was able to accomplish a mass balance, extracting polymer and surfactant. The inability to extract all of the polymer in the present experiments may be due to the difference in structure, particularly the porous structure of amorphous precipitated silica. Silica consists of very large agglomerates, which consist of smaller agglomerates and/or aggregates. Aggregates consist of 10to 20 spherical primary particles of 10to 50 nm diameter which are formed during acidification of a metal silicate before precipitation. The 50-500 nm pores permeate throughout the agglomerated silica structure. If organic polymer chains are formed within the silica pores, it is possible that a physical interpenetrating network is formed by organic polymer (14) Soderlind, E.; Stilbs, P. Langmuir 1993,9,2024-2034. (15) Levitz, P.; Van Damme, H.; Keravis, D. J.Phys. Chem. 1984, 88, 2228. (16)Levitz, P.;Van Damme, H. J.Phys. Chem. 1986,90,1302. (17)Rosen, Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley and Sons: New York, 1989.
OHaver et al. with the silica. Therefore, the polymer would be unextractable. If the polymer chain becomes entrapped, the surfactant layer between it and the silica surface can become conceivably difficult to remove by washing. Recall that approximately 100 ppm bromide is detected in treated silica using CTAB as surfactant. Any entangled, unextractable polymer located near the entrance of a pore could inhibit the washing or extraction of material in the pore. The surface characteristics of the amorphous precipitated silica are thus permanently modified. Presence of suitable organic polymer should enhance dispersion of the silica in an organic composite matrix such as in elastomers used in tires, hoses, belts, and other products. It is this surface modification that sets the present process apart from existing technologies. Groups are not actually bonded to the silica surface, as is the case with silane coupling agents.' Neither are they merely lying on the silica surface, as would be the case if a polystyrene latex were to be poured onto the silica and simply mixed. The formation of a n integral polymer-silica composite has been achieved. A subsequent paper on this work discusses the evidence for positive silicahulk elastomer interaction and composite reinforcement. The total carbon content analysis supported the assumption that most of the polymer and the monolayer of surfactant below it remained on the silica after washing. The actual amount of carbon detected should be somewhat less than that associated with the polymer and the lower surfactant layer since some patches of surfactant and polymer, which are formed on the surface of the silica or a t the entrance of large pores, could indeed be removed during the washing process. The carbon content of the THF-extracted material is lower than that expected, possibly because of surfactant and THF remaining in the polymer extract have lower percent carbons than the polystyrene. The GPC data indicate the capability of producing a reasonable molecular weight polymer on silica by this process. The reduction in the molecular weight of the extractable polymer, as well as the increase in dispersity, was expected. As the chain length of the polymer increases, they should become more entangled in the surface and become more difficult t o extract. Therefore, after a certain degree of polymerization, only the shorter chains and those longer ones which are on or near the surface of the silica can be extracted. This would tend to eliminate most, but not all, ofthe higher molecular weight chains from the analysis after extraction. Since only a portion of the polymer is extractable, it cannot be ascertained that the extracted polymer is truly representative of the polymer formed upon the silica surface. The DSC results, specifically the glass transition temperatures of the extracted polystyrenes, were consistent with the results given in the literature18for lower number average molecular weight polystyrene. If the polymer chains are indeed sterically hindering transport into and out of the silica pores, then monomer diffusing into the admicelles during the reaction may only be added near the surface of the silica. This could have the effect to increase the molecular weight of the surface polymer by adding to existing radical chains. It could also have the effect of lowering the molecular weight of the surface polymer if it is initiated independently after diffusing in. The reason for the increase in mean particle sizes may be due either to the organic polymer forming process or the subsequent processing of the silica. We do not know (18)Tun, E. A. Thermal Characterization of Polymeric Materials; Academic Press: New York, 1981.
Formation of Ultrathin Polymer Films if the polymerization process itself is somehow linking the silica into larger aggregates or if the processing of the treated silica (consisting of drying followed by regrinding the filter cake to a powder in a sieve) results in a greater degree of agglomeration and thus an increase in particle size. It would not be surprising for a change in surface character to cause a change in aggregate size; however, the regrinding process used should not be expected to produce very fine particles. Decreases in surface area and oil absorption were both expected as the reaction product covers part of the surface of the silica and fills part of the pore volume.
Summary and Conclusions The ability of the four-step polymerization process to modify the important surface characteristics of inorganic substrates has been expanded to precipitated amorphous silica. The four-step process of adsorption, adsolubilization, polymerization, and washing has been demonstrated to produce silicas which have significantly different properties than the starting materials. The process has been shown to work for cationic, nonionic, and waterinsoluble cationic surfactants. A detailed examination of the polymer formed on the silica surface has been
Langmuir, Vol. 10, No. 8, 1994 2593 accomplished. Results show that a polymer of reasonable molecular weight can be formed with this process, although at higher conversions the higher molecular weight material may be unextractable. The extracted polystyrene polymer has spectroscopic and thermal characters consistent with its low molecular weight nature. The process has also shown that, on precipitated silica substrates a t least, the organic polymer treatment is quite firmly attached even though not chemically bonded. This demonstrates the uniqueness of this technique over existing technologies. It also suggests further investigation into the use of this process with other monomers and substrates. For example, forming a polymer that would have unsaturations could serve to cross-link the treated silica to a composite matrix, enabling it to behave in a similar manner as coupling agents. Unsaturated surfactants which could participate in the polymerization process could also be utilized.
Acknowledgment. Research performed a t the University of Oklahoma was supported in part by Oklahoma Center for the Advancement of Science and Technology (Award No. ARO-075) and the National Science Foundation Grant No. CTS-9812806.
