Ind. Eng. Chem. Res. 1992,31, 213-218
Literature Cited Fleischer, M.; Wilcox, R. E.; Matzko, J. H. Microscopic Determination of the Nonopaque Minerals; U.S.Geological Survey Bulletin 1627; U.S.Government Printing Office: Washington, DC, 1984. Frazier, A. W.; Kim, Y. K. Redistribution of Impurities in Wet Process Phosphoric Acid; Chemical Engineering Bulletin No. Y-202; Tennessee Valley Authority: Muscle Shoals, AL, 1988. Frazier, A. W.; Kim, Y. K. Redistribution of Impurities in Commercial Wet Process Phosphoric Acid. Fert. Res. 1989,21,45-60. Frazier, A. W.; Lehr, J. R.; Smith, J. P. Precipitated Impurities in Wet-Process Phosphoric Acid. J. Agric. Food Chem. 1966, 14, 27-33. Frazier, A. W.; Scheib, R. M.; Thrasher, R. D. Clarification of Ammonium Polmhosphate Food Chem. 1972, - _ - Fertilizers. J. Anric. 20, 138-145. Frazier, A. W.; Lehr, 9. R.; Dillard, E. F. Chemical Behavior of Fluorine in the Production of Wet-Process Phosphoric Acid. ERV. Sci. Technol. 1977,11, 1007-1014. Frazier, A. W.; Waerstad, K. R.; Kim, Y. K.; Crim, B. C. Phase System Fe20,-K20-P206-H20 at 25 "C. Ind. Eng. Chem. Res. 1989,28, 225-230. Jameson, R. F.; Salmon, J. E. Complexes Involving Trivalent Iron
213
and Orthophosphoric Acid, Part 111. The System Ferric OxidePhosphoric Acid-Water at 25 "C. J. Chem. SOC.(Part I) 1954, 28-34. Kobayashi, E. Nitrogen-PhosphorusCompounds, Part XVIII. Phase Equilibrium of the System Fe2O3-PO4-Hz0and the Synthesis of Hydrogen Iron Phosphates. Kogyo Kagaku Zasshi 1970, 73 (8), 1797-1801. Larsen, E. S.; Berman, H. The Microscopic Determination of the Non-Opaque Minerals; U.S.Geological Survey Bulletin 848; U.S. Government Printing Office: Washington, DC, 1934. Lehr, J. R.; Brown, E. H.; Frazier, A. W.; Smith, J. P.; Thrasher, R. D. CrystallographicProperties of Fertilizer Compounds; Chemical Engineering Bulletin No. 6; Tennessee Valley Authority: Muscle Shoals, AL, 1967. Potts, J. M. Fluid Fertilizers; Chemical Engineering Bulletin No. Y-185; Tennessee Valley Authority: Muscle Shoals, AL, 1984. Vencato, I.; Mattievich, E.; Moreira, L. de F.; Mascarenhas, Y. P. The Structure of Ferric Oxonium Bis(Hydrogenph0sphate) Fe+++-H30+-2(P04Hj--.Acta Crystallogr. 1989,45,367-371.
Received for review April 29, 1991 Accepted August 20, 1991
An Application of Polyalcohols Prepared from Diols. 2.t Preparation of Redispersible Organosilica Sol Taketoshi Kito,* Makoto Yamaye,' Kohji Yoshinaga, Mitsuhiko Yoshimoto, Michio Komatu,s and Hiroyasu Nishidas Department of Chemistry, Kyushu Institute of Technology, Sensui-cho Tobata-ku, Kitakyushu-shi, 804 Japan
Hydrophobic organosilica sols have been prepared by chemical modification of a silica surface with polyol polymers having hydroxymethyl side chains. The sols were redispersible in common organic solvents, e.g., dimethoxyethane and chloroform, after complete evaporation of the solvent. For the modification, a simple functional transformation of the polymers into their Si-group-containing derivatives was conducted. A certain amount of a free polymer besides the polymers bound to the silica surface was required for redispersibility.
Introduction Silica, an aggregate of Si02,has a hydrophilic surface and is dispersible in water to form a hydrosol. This is because the silica surface carries hydroxyl groups (silanol groups) in the range of 4.0-10.0 OH/nm2 (Iler, 1978) and strongly holds water molecules via hydrogen bonding (Lange, 1965). There have been many reports on the methods for dispersing silica particles into organic solvents, especially in order to prepare a hydrophobic silica. However, the solvents which have so far been employed for this purpose are such hydrophilic solvents as ethylene glycol, 1,6-hexanediol, and glycerol (Alexander, 1960; Stoesel, 1961; Pluta and VOSSOS,1972). Although silicate linkage formation by treating silica with an alcohol is another possible method for preparing a hydrophobic silica, the resulting modified silica is only wet in butyl alcohol (Broge, 1956). Recently, a method for preparing a silica sol dispersed in hydrophobic organic solvents such as toluene was reported (Ogihara and Shimizu, 1990). For this purpose, organosilicon compounds were used for treating the silica surface. An application of Polyalcohols Prepared from Diols. I.: Kito, T.; Ota, K.; Yamaye, M.; Yoshinaga, K. J. Appl. Polym. Sci 1988,
35,1593. Present address: Faculty of Engineering, Kyushu Kyoritsu University, Orio, Yahatanishi-ku, Kitakyushu-shi, Japan. *Present address: Catalysts & Chemicals Industry Co., Ltd., Kitaminato-machi, Wakamatau-ku, Kitakyushu-shi, Japan.
However, the possibility of developing redispersibility has not been described. Compounding silica into polymers is of practical interest for improving polymer properties. However, silica seems to be incompatible with common organic polymers due to hydrophilic nature of its surface. We previously reported poly01 polymers prepared from diols in the presence of a base such as potassium phenoxide (see eq 1: their average structure is shown) (Kito et al., 1985). mHO(CH2),PH
PhOK heat
H(-CH(CH2),2--],OH
I
+ ( m - 1)H20 (1)
CHZOH 1 (n)
Hereafter, the symbol n in parentheses stands for the carbon number of the starting diol and represents integers 6,8,10, and 12, unless otherwise mentioned. Pn-CH20H also represents l ( n ) . This polymer carries a number of hydroxymethyl side groups on the carbon main chain and are soluble in various solvents such as methanol, 1,2-dimethoxyethane(DME), tetrahydrofuran (THF), chloroform, pyridine, and hot benzene. Therefore, the polymer seems to be suitable for the purpose of treating silanol groups. And thus chemically modified silica particles possessing hydrophobic surfaces will be dispersible and miscible with a wide variety of solvents and polymers. In thispaper, we examined a new approach for preparing such hydrophobic organosilica sols.
0888-5885/92/2631-0213$03.00/00 1992 American Chemical Society
214 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
Experimental Section For compound numbers, refer to eqs 2-6. Proton and I3C NMR spectra were recorded on a JEOL JNM-FX-60 spectrometer and IR spectra on a Shimadzu IR-408 or JEOL JIR-5500 spectrometer. Preparation of Poly01 Polymer (Pn-CH,OH). The poly01 polymers were prepared according to the method reported previously (Kito et al., 1985). Polymerization of a diol in the presence of potassium phenoxide followed by addition of CHC13-THF to remove an insoluble product gave a solution of poly01 polymer. The solution was adjusted to pH 7 with aqueous HC1, and then the solvents were removed by distillation. The poly01 polymer (20 g) was dried by azeotropic distillation with benzene (200 mL). Determination of Molecular Weight. The following two methods were applied. (a) Acetic Anhydride Method. The hydroxyl number (H) per 1g of polymer 1 was determined by the literature method (Billmeyer, 1984). Approximately 0.1 g of 1 and 0.5 N KOH (1-2 mL when n is 10) were used. The molecular weight (M,) of l was calculated according to the following equation H = (m + l)/(Am + 18.016) then where
M , = (1- 18.016H)A/(Am - 1) + 18.016
A = 16 + 14.027n and n and m are given in eq 1. Calculated values were satisfactoryfor the determination of lower molecular weights, but not for that of higher ones, because, for the latter case, a small titration error caused considerable deviation of the values. For polymer 1(10), for example, an increment 0.01 mL of 0.5 N KOH solution corresponds to the molecular weight of 85 when m is 8, while it corresponds to 280 when m is 15. M , of 2 was calculated on the basis of values of 1. (b) Intrinsic Viscosity. Measurement was done in ethanol at 30 "C by using an Ubbelohde viscometer. Starting Sol. The methanol-silica sol (silica content 34 wt %; average diameter 10 nm) was supplied by Catalysts & Chemicals Industry Co., Ltd. (CCI). Preparation of Organosilica Sol Dispersed in DME. In a 200-mL flask with a dropping funnel was placed the methanol-silica sol (50 mL). Methanol was slowly removed by distillation. To the residue, dry DME (170 mL) was added dropwise through the dropping funnel at a rate that would keep the whole volume of the mixture constant until the distillate boiled at 85 "C (boiling point of DME). Excess DME was removed by distillation until the concentration became about 3 wt % silica, giving a cloudy organosilica sol dispersed in DME (DME-silica sol). No precipitates were formed when the sol was allowed to stand at room temperature for half a year. The mean diameter of 57 nm was obtained for silica dispersed in DME. THF-silica sol and chloroform-silica sol could not be prepared by this method. Modification of Silica with Poly01 Polymer 1. Silica was moditied with 1 by the following two methods. 'Typical examples are shown. Moles for polymer 1 are referred to the number of the hydroxymethyl groups contained in 1. Method a (The Alkyl Silyl Ether Method). (a-1) Formation of the C-0-Si Ether Linkage. In a flask with a mechanical stirrer, a mixture of l(8) (2.36 g, 0.0184 mol), diethoxydimethylsilane (9.08 g, 0.0614 mol), and dry THF (30 mL) was stirred under reflux. After 12 h, THF and unreacted diethoxydimethylsilane were removed by
distillation under reduced pressure to give a silyl-etherated polymer 2(8). 13CNMR data for 2(8) (CDC13): 6 -3.08* (Si-CH3), 18.4* (Si-OCH2CH3),25.8, 26.9,29.5,29.7, 29.9, 30.1, 31.0,32.6, 40.4,58.0* (Si-0-CH2CH3), 62.5,65.3. Values without an asterisk were attributed to carbons in the starting polymer. (a-2) Reaction of 2(8) with Silica. A mixture of 2(8) (0.55 g), DME-silica sol (7.0 g, containing 0.21 g of silica), and dry DME (15 mL) was stirred at 130 "C under reflux. After 24 h, DME was removed by distillation under reduced pressure to give product 3(8). The ratio of 2(8)/SiO, (g/g) was calculated to be 2.6. 13CN M R data for 3(8) (CDC13): 6 -3.03* (Si-CH,), 18.5* (Si-OCH,CH3), 25.9, 27.0,29.5, 30.0,30.2, 31.0, 32.7,40.4, 58.1* (Si-0-CH2CH3), 62.6, 65.3. Values without an asterisk were attributed to carbons in the starting polymer. Method b (The Silyl Carbon Method). (b-1) Introduction of the Allyl Ether Linkage. From a mixture of polymer l(12) (5.08 g, 0.0276 mol), sodium ethoxide (2.86 g, 0.0420 mol), and dry ethanol (70 mL), the ethanol was removed by distillation to dryness with stirring at 80 "C under reduced pressure. To the resulting residue were added allyl chloride (4.29 g, 0.0561 mol) and dry DME (50 mL), and the mixture waa heated at 80 "C for 22 h. The reaction mixture was filtered to remove the sodium chloride forped. From the filtrate, unreacted allyl chloride and DME were removed by distillation under reduced pressure. The residue was dissolved in benzene, washed with water, and dried by azeotropic dehydration with benzene to give 4(12). 13CNMR data for 4(12) (pyridine-d,): 6 26.6,27.5,30.0 (very strong), 30.6,31.9,33.8,35.7 (weak), 36.6 (weak), 41.5, 49.7, 62.2 (P12-CH2OH), 64.9 (terminal CH,OH), 67.7. 13C NMR data for 4(12) (CDCl,): 6 25.8 (weak); 26.2; 26.9; 29.7 (very strong); 30.1; 31.0; 31.5; 32.8; 38.4; 62.9; 70.5; 71.8 (with a shoulder), and 73.7 (these three signals were attributed to carbons adjacent to the ether linkage); 116.4, 116.6, and 135.1 (these three were attributed to olefinic carbons). (b-2)Addition of Ethoxydimethylsilane to 4( 12). In a flask were placed 4(12) (2.25 g), ethoxydimethylsilane (1.79 g, 0.0172 mol), dry DME (40 mL), and a 2-propanol solution of hydrogen hexachloroplatinate(1V) dihydrate (3wt %, 0.2 mL). The mixture was stirred under nitrogen atmosphere in an ice bath for 1h and at room temperature for 5 h. Filtration followed by vacuum distillation of DME and unreacted ethoxydimethylsilane gave 5(12). 13C NMR for 5(12) (CDCl,): 6 -0.86, 0.23, 1.26, 14.3 (SiO-CH2CH3),18.5,23.5,29.7, 31.0,31.6,32.9,38.4, 40.6, 59.1 (SiO-CH,CH,), 63.0, 65.7, 70.6, 71.0, 71.8*, 73.7*, 116.5*, 116.7*, 135.3*. Values with an asterisk were attributed to unreacted 4(12) and l(12). (b-3) Reaction of 5(12) with Silica. A mixture of 5(12) (0.31 g), DME-silica sol (7 g, containing 0.21 g of silica), and dry DME (35 mL) was heated at 140 "C for 51 h. DME was removed by distillation under reduced pressure to give product 6(12). The calculated ratio of 6(12)/SiO2 k / g ) was 1.5. 13C NMR data for 6(12) (CDCl,): 6 -3.09, -2.06, -0.80, 0.23, 12.5, 14.3, 17.2, 18.5, 23.5, 25.9, 26.3, 27.0, 29.7, 31.1, 31.6, 32.7, 33.5, 33.8, 35.7, 38.4,40.5,48.6,58.5,57.8,59.4, 62.6, 63.0, 65.4, 70.6, 71.0, 71.8, 72.6, 73.7, 116.7 (with a shoulder), 135.2 (with a shoulder). The last two signals were attributed to the olefinic carbons in unreacted 4(12). Examination of Redispersibility. In the final stage of sol preparation, just before evaporation of DME (see a-2 in method a and b-3 in method b), all of the sols prepared were transparent. DME was removed from such
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 215 a sol (6 mL) by distillation under reduced pressure at room temperature and then fresh DME (6 mL) was added to the residue. If the resulting sol becomes transparent without any precipitation after a 24-h standing period, we assume that redispersibility in DME has been realized. In fact, such a sol was very stable and no precipitation occurred for at least half a year. Repetition of these procedures was examined for other dispersing solvents. Determination of the Amount of Polymer Bound to Silica. (a) A typical example was shown. A mixture of 3(10) (M, = 790, [77] = 0.062, 1.23 g), DME-silica sol (9.09 g, containing 0.30 g of silica), and DME (70 g) was stirred under reflux in a flask. Two 6-mL portions were occasionally sampled from the mixture. One portion was subjected to redispersibility. For the other, DME was removed under reduced pressure at room temperature, hexane (25 mL) was added, and then the mixture was vigorously stirred. Centrifugation filtration (4000 rpm, 15 min) gave a residue, which was weighed after evaporation of the solvent at 100 "C under reduced pressure. The amount of bound polymer was calculated from the weight difference between the starting polymer (g) and the residue (g). A cloudy reaction mixture in the initial stage became transparent as the reaction proceeded. However, a partially cloudy mixture in the course of this experiment became transparent when cooled to room temperature. No precipitates were formed during the reaction. This phenomenon was also observed in the experiments of a-2 in method a and b-3 in method b. Separately, another run was carried out using 300) (M, = 2140, [77] = 0.067, 1.76 g), and DME-silica sol (11.9 g). IR absorption peaks between 3650-3750 cm-', which are attributed to a silanol group (Koberstein et al., 1970, were observed for FT-IR spectra of all the polymers bound to silica. (b) A mixture of 3(10) (M, = 790,0.351 g), DME-silica sol (5.25 g, containing 0.20 g of silica), and DME (21 mL) was stirred under reflux for 2 h. The resulting sol, which did not show redispersibility, had the ratio of bound polymer/silica (g/g) of 0.27. The sol was allowed to stand a t room temperature for a week; the ratio remained practically unchanged (0.26). Other Methods for Obtaining a Redispersible Sol. (a) For the direct Method, in a flask with a mechanical stirrer, a mixture of polymer l(10) (0.674 g), DME-silica sol (9.0 g, containing 0.27 g of silica), and dry DME (10 mL) was gradually heated. Gelation started after approximately 2 h, when it reached the reflux temperature. (b) A mixture of DME-silica sol (0.81 g, containing 0.03 g of silica), l(10) (0.14 g), and DME (10 mL) was vigorously stirred at room temperature for 1 h, but a redispersible sol was not obtained. (c) Hexane (25 mL) was added to a portion (5 mL) of the redispersible sol obtained in the experiment shown in Figure 1. After centrifuging, a supernatant containing hexane, DME, and unbound polymer was separated to give a residue (0.060 g), which did not show redispersibility. This residue, however, took on redispersibility upon addition of a methylene dichloride or THF solution of polycarbonate (0.2 g in 50 mL). This redispersibility was reproducible on repeated runs. (d) A portion (5 mL) of a sol, which did not show redispersibility, in the experiment shown in Figure 1 was allowed to stand at room temperature. After 10 days, the sol took on redispersibility.
Results Dried silica particles, obtained by vacuum evaporation
-
h
ar
'4
1.0
2 a
^r; I!;:.-..-.; ,e-
rn
0.5
5
+;p
0
79Q
, O
0
a 0 5
1 0
REACTION
1 5 TIME
2 0 (h)
Figure 1. Relationship of bound 2/Si02 with reaction time.
of methanol from methanol-silica sol at room temperature, were not dispersed in methanol, DME, chloroform,water, or hydrochloric acid (0.5 N). (The particles in 0.5 N NaOH, however, formed a clear solution.) We conclude that such sols as this methanol-silica sol do not have redispersibility. DME-silica sol (an organosilica sol dispersed in DME) was prepared from methanol-silica sol by solvent exchange and used as the starting sol. This sol and methanol-silica sol were then examined in terms of stability upon the addition of water. Transparency was retained for both of the sols even after a 24-h standing period a t room temperature, without any precipitation. According to the DLVO theory (by Derjaguin-Landau and Verway-Overbeek groups), the stability of a colloidal dispersion is mainly controlled by two factors, that is, van der Waals attraction and electrostatic repulsion. When a polymer is adsorbed onto the colloid surface, steric repulsion consisting of entropic and osmotic ones is an additional controlling factor (Sato, 1986). Redispersibility may also be controlled in a similar manner. Chemical modification of the silica surface with polymer 1is effective in order to prevent aggregation. The modified silica particles cannot come close together due to steric repulsion, and they have a hydrophobic surface. This will make them redispersible into a variety of organic solvents. An attempt was made to introduce 1 onto a silica surface by simply heating a stirred mixture of l(10) and DMEsilica sol (about 3 w t %) at the rate of about 10 OC/min. Precipitation resulted when the mixture was heated to reflux temperature. The following two methods a and b were adopted as shown in eqs 2-6, where H0-SiO2 represents silanol groups on the silica surface. All the reactions were carried out in DME using DME-silica sol. method a Pn-CHzOH + (EtO)zSiMez Pn-CH20SiMe2(OEt)+ EtOH (2) 2(n) Pn-CHzOSiMe2(OEt) + HO-SiOz Pn-CH20SiMezOSi02 EtOH (3) 3(n) method b Pn-CH20Na + CHz=CHCH2C1 Pn-CHzOCHzCH=CHz + NaCl (4) 4(n) Pn-CH20CH2CH=CH2 + Me2SiH(OEt) Pn-CHzOCH2CHzCHzSiMez(OEt) (5) 5(n) Pn-CH20CH2CHzCHzSiMez(OEt)+ HO-SiOZ Pn-CH20CH2CHzCHzSiMe20SiOz + EtOH (6) 6(n)
-
-
+
-
-
216 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 Table I. Solubility of Polymer 2(n) and RedisDersibility of Modified Silica 3 by Method a redispersibility in given solventcsd expt no. type of materiala 2(n)/Si02,g/g reaction time,b h DME MeOH THF CHC1.q 0
24 12 24
0
0
X
X
0
0
X
X
X X X
0 0
0 0
A 0
9
X
X
X
9
0
0
A
PY
For structure, refer to eqs 2 and 3. For “reactiontime”, see a-2 of method a in the Experimental Section. DME, dimethoxyethane;Py, pyridine. Symbols: 0,redispersible, X, not redispersible; A, redispersible but cloudy. Precipitation occurred on a 24-h standing period. e In 13C NMR spectra, signals due to an -0Et group disappeared completely. Table 11. Solubility of Polymer 5 ( n )and Redispereibility of Modified Silica 6(n)by Method b rediswrsibilitv in given solventc*d -
expt no. 1 2
type of materiala 5(n)/SiOp,g/g reaction time,bh 1.67
51 51
1.4 1.67
72 48
9
1.67 2.5
36 24
10 11
2.5
72
e 4 5 6 7 8
1.4
I
DME
MeOH
THF
CHC13
PY
CsHB
0 0
X X X X X X X X X X X
0 0 0 0
0 A A
0
0
0 0 0 0
X
X
X
0 0
0 0
0 0
X X
X
X
X
0
0
A 0
0 0
0
X
X
X
X
0 0 X
0 0 X
0 0 X
X X
0 0 X X
“for structure, refer to eqs 5 and 6. bFor “reaction time”, see b-3 of method b in the Experimental Section. c-dSeefootnotes c and d, respectively, in Table I.
Modification by Method a. The results are shown in Table I. For preparation of polymer 2, the reaction was carried out until an IR absorption at 3300 cm-l almost disappeared. The 13C NMR spectrum for a mixture of untreated polymer 1 and diethoxydimethylsilane was compared with that for 2. Only slight differences were observed between the corresponding chemical shifts. For example, the chemical shifts of -CH20H in P10-CH20H and -CH20-Si in 2(10) were 64.928 and 64.600 ppms, respectively, giving a difference of only 0.328 ppm. However, the absorption at 3300 cm-’ almost disappeared in the IR of 2, suggesting the formation of 2. The solubilities of the 2 series are also shown in Table I. Under the conditions shown in Table I, 2 was used to prepare modified silica 3. 3 was redispersible in watermiscible solvents, such as DME, THF, and pyridine, and in water-immiscible solvents, such as chloroform, forming transparent sols depending on the reaction conditions. 3 showed no redispersibility in benzene and methanol, though methanol was a good solvent for the original polymer 1 and a dispersing solvent for the original silica. This strongly suggests that the silica modified by this method has acquired a hydrophobic property. Development of redispersibility for 3 depended on the polymer/silica ratio (g/g) and reaction time. As shown in Table I, the lower polymer/silica ratio (compare experiment 7 with experiment 8) and shorter reaction time (compare experiment 4 with experiment 5) gave poor results. In the IR spectra of redispersible silica, an absorption between 3650 and 3750 cm-l (attributed to silanol groups) did not disappear completely, suggesting that there remained some unreacted silanol groups on the modified silica surface, as shown in Figure 2. The molecular weight of polymer 1 used in these experiments was about 400-4500 (see the molecular weight determination in the Experimental Section). Therefore, the number of hydroxyl groups per poly01 polymer molecule is about 4-30.
Train type
boEt Tail
type
O:-CHzOS iMenFigure 2. Silica surface modified with the poly01 polymer.
Modification by Method b. As shown in eqs 4-6, method b consists of three processes. The reaction shown in eq 4 gave 4, whose structure was confirmed as follows. (a) The intensity of a 3300-cm-’ hydroxyl absorption in the IR spectrum was decreased considerably. (b) In the 13CNMR spectrum, a chemical shift for the hydroxylmethyl carbon of Pn-CH20H shifted to a lower field (for example, 62.822 to 71.856 ppm for n = 10) and three new signals appeared at around 71,135.1, and 116.6 ppms, due to three carbon atoms of -O-CH2CH=CH2 in this order (there were two distinguishablesignals at around 71 P P ~ ) . In the presence of hydrogen hexachloroplatinate(IV), ethoxydimethylsilane was added to the C=C-bond-containing 4 to form 5 with Si-C linkage. The solubility of 5(n) is shown in Table 11. Modified polymer 5(n)was converted into 6(n)according to the method shown in eq 6. The results were shown in Table 11. Method b gave a result comparable with method a in terms of the relation of redispersibility with the poly-
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 217 1,0000
i
0.
* @ @@]
0 0000
10 o
3ebo
3sb0
34b0
32bo
iobo
2abo
2sbo
UAVENUMBERS
Figure 3. FT-IR spectra of the samples obtained in the experiments shown in Figure 1 (M,= 790). Reaction times (h): (a) 21, (b) 5, (c) 2.
mer/silica ratio and reaction time. To obtain a good redispersibility,however, a longer reaction time wm required for method b than for method a. Bound Polymer/Silica Ratio. For 3(10)with varied molecular weights, the ratio of bound polymer/silica (g/g) (abbreviated hereafter as P/S ratio; Pl/S and P2/S ratios correspond to the P/S ratios for 2 0 0 ) with molecular weights of 790 and 2140, respectively) was plotted against reaction time (see Figure 1). In the figure, the samples taken over the range of a dotted line did not show redispersibility, while those over the range of a solid line did. IR Spectra of Silica Modified with 2(10). IR spectra (2500-4000cm-’) were shown in Figure 3 for modified silicas obtained in the experiment (Figure l),one with redispersibility (spectrum a) and the others without redispersibility (spectra b and c).
Discussion On the basis of the experimental results, we summarize the observations as follows. (S-1)Samples with lower P/S ratios in Figure 1did not show redispersibility. In the earlier stage, however, even the samples having nearly saturated P/S ratios did not exhibit redispersibility. (S-2) Even some samples, which had shown no redispersibility upon sampling, became redispersible after being dowed to stand at room temperature for a longer period, for example, 10 days. We confirmed experimentally that the reaction shown in eq 3 did not take place at room temperature. A redispersible sol could not be obtained by a simple procedure of mixing 2 into the DME-silica sol, either. (S-3) When a suffkient amount of the polymer 200) was not present in the system, a redispersible sol could not be obtained even if the reaction was carried out for a long time (Tables I and 11). For redispersibility, free as well as bound polymer molecules should be present in the system. Polycarbonate was also used in place of the original polymer and found to be effective as a free polymer. (S-4)The higher the ratice of polymer/silica (g/g) in the reaction system, the shorter the time required for redispersibility. With the lower ratios, redispersible sols could not always be obtained (Tables I and 11). (S-5) An increase in the PIS ratio increased the transparency, indicating that the hydrophilic property of the silica surface was gradually being lost. 6-61 There remained unreacted silanol groups even in the sample showing redispersibility, as confirmed by the
FT-IR method. These observations are consistent with the following picture of the modified silica. The summary number for the basis of statements is described. When the solvent is removed by distillation from the silica sol, silica particles will come close together for possible aggregation. We believe that silica must possess a hydrophobic surface in order to get redispersibility in organic solvents. We obtained in this experiment a very stable sol, in which both unreacted polymer and polymer-coated silica particles were well dispersed into DME. However, some silanol groups on the silica surface remain unreacted after redispersibility had been attained (S-6). These groups are effectively covered with bound polymer, as shown in Figure 2. In the earlier stage, they still weakly adsorb free polymer molecules, which can be removed easily by centrifugal separation. With lower P/S ratios, the polymer molecules on the silica surface, both chemically bound and weakly adsorbed, may not be enough to separate silica particles from each other (S-l),because each silica particle should exist in the nearest range of interaction if the solvent has been removed. Even when the P/S ratio nearly reaches a saturated value, the amount of bound polymer still might not be sufficient for redispersibility in the initial stage. It is unlikely that free polymer will react with silanol groups on long standing, because the P/S ratio was kept nearly constant, as shown in Figure 1. We suggest, therefore, that a main process for redispersibility on standing will be a reaction between the ethoxy groups of the bound polymer and free silanol groups on the surface. Thus, the silica surface will become hydrophobic (S-5)to get redispersibility. Two types of binding modes of 2 to silica surface are possible; one is the train or loop type where a polymer chain lies on the silica surface and binds to at least two silanol groups, and the other is the tail type where a polymer chain bound to a single silanol group stands upward on the silica surface. A typical picture of the modified silica surface is depicted in Figure 2. The ratio of (PB/S)/(Pl/S),after redispersibility has been attained, may depend on the binding mode. If the former type of binding takes place, the ratio may be nearly independent of the molecular weight of 2, and if the latter type is the case, the ratio may be approximately proportional to the molecular weight of 2. Although the ratio of (PB/S)/(Pl/S) in Figure 1 is more than unity, it is too small to account for the binding mode of the tail type. Even if the tail-type binding is formed, it will interconvert to train or loop type during the reaction. In our experiment described in S-3,chemical modification of the silica surface with polymer 1, where nonpolar alkyl main chains are directed outward, is required for development of redispersibility. A redispersible sol could not be obtained by a simple mixing of 2 with the DMEsilica sol (5-2). However, the presence of bound polymer alone is not enough for getting a redispersible silica (5-3). Silica particles must be surrounded by a sufficient amount of free polymer (polymer 1or polycarbonate) as well as the bound one (5-4). Both polymers have neither ionic nor highly polar groups. Therefore, this phenomenon can be explained by “depletion stabilization”proposed by Feigin and Napper (Feigin and Napper, 1980). This theory states that uncharged colloidal particles are stabilized upon addition of a nonionic polymer, which is not adsorbed on the colloidal surface. Registry No. 3(6) (graft copolymer), 136881-81-7; 3(8)(graft copolymer), 136881-82-8;3(10) (graft copolymer), 136881-83-9; 6(12) (graft copolymer), 136881-84-0.
Ind. Eng. Chem. Res. 1992,31,218-228
218
Literature Cited Alexander, G. B. Silica organosols. U.S.Patent 2,921,913,1960. Billmeyer, F. W. Textbook of Polymer Science, 3rd ed.; Wiley: New York, 1984,p 478. Broge, E. C. Surface-modified siliceous particles. US. Patent 2;739,078, 1956. Feigin, R. I.; Napper, D. H. Stabilization of Colloids by Free Polymer. J. Colloid Interface Sei. 1980, 74, 567-571; Depletion Stabilization and Depletion Flocculation. Zbid. 1980, 75, 525-541. Iler, R.K. The Chemistry of Silica; Wiley: New York, 1978;Chapter 6. Kito, T.; Yoshinaga, K.; Hatanaka, N.; Emoto, J.; Yamaye, M. Novel Polyalcohols with Hydroxymethyl Side Chains from Base-Catalyzed Polycondensation of Diols. Macromolecules 1985, 18, 846-850.
Koberstein, E.; Lakatos, E.; Voll, M. Ber. Bunsen-Ges. Phys. Chem. 1971, 75, 1104-1114. Lange, K. R. The characterization of molecular water on silica surfaces. J. Colloid Sei. 1965, 20, 231-240. Ogihara, T.; Shimizu, T. Manufacture of silica sols dispersed in organic solvents. European Patent 372,124,1990. Pluta, L. J.; Vossos, P. H. Making silica organosols. U.S.Patent 3,699,049,1972. Sato, T. Physical-Chemical Properties of Suspension (I). Stability of Dispersions. Shikizai 1986, 59, 682. Stossel, E. Colloidal dispersions of SiOz hydrogels in polyols. US. Patent 3,004,921,1961. Received for reuiew April 10,1991 Revised manuscript received August 15, 1991 Accepted August 27, 1991
Size Effects on Solvent Diffusion in Polymers Dominique Arnouldt and Robert L. Laurence* Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003
Capillary column inverse gas chromatography (CCIGC) has been used for the measurement of diffusion coefficients in polymer-solvent systems a t conditions approaching infinite dilution of the volatile component. In a study of the effect of penetrant size and configuration on solvent diffusion in two polymers, poly(methy1 methacrylate) (PMMA) and poly(viny1 acetate) (PVAc), measurements of diffusion coefficients were made a t temperatures ranging from Tgto 70 K above Tg.Over 30 solvents were evaluated: alkanes, alkenes, aromatic hydrocarbons, and aliphatic esters. The effect of solvent size on the activation energy of diffusion, ED,is examined in the limit of zero mass fraction of solvent. The data allow discrimination between the conflicting theories describing the variation of EDwith solvent size, the ceiling-value hypothesis and the hypothesis based on free-volume theory, and suggest that the Vrentas-Duda free-volume theory offers a better description. It was shown that the flexibility and compactness of the diffusing species have a profound influence on the diffusive behavior. The results of the study of the effect of penetrant size and configuration on diffusion indicate that free-volume theory must be reexamined or replaced with a model which would provide an improved accounting of differences in the solvent geometry and flexibility.
Introduction The free-volume theory of transport developed by Vrentas and Duda (1977a,b) presents an expression for the self-diffusion coefficient of the solvent, D1, in the limit of zero mass fraction:
v2*.
jumping units per mole of jumping units, In other words, 5 is the ratio of the size of the hole required for a solvent molecule to jump to the size of the hole needed for the movement of a polymer jumping unit. In the early version of the theory, the entire solvent molecule was assumed to be able to perform the jump and the quantity 6 was defined as 5= = Q1*M1/V2*Mj2 (3)
v1*/v2*
The parameters Dol and (7Q2*5)/K12depend on the properties of the polymer-solvent system, the parameter K22- Tg2depends on the polymer properties alone, and T i s the temperature. The work described here sought to investigate the influence of the solvent on the parameters iTol-ana(YP2*5)/K12. The parameters (7V2*)/KI2 and K22are related to the Williams-Landel-Ferry (WLF) constants, C l g and C2g,of the polymer:
The quantity 5 is defined as the ratio of the critical molar volume of solvent, V1*,to the critical volume of polymer
* To whom correspondence should be addressed. 'Current address: GE Plastics, Inc., Beauvais Plant, B. P. #1 60134 Villers-St. Sepulchre, France. 0888-588519212631-0218$03.00/0
where Vl* and Q2* are the specific critical hole-free volumes (Vrentas and Duda, 1977b) of the solvent and the polymer, Ml is the molecular weight of the solvent, and is the molecular weight of a polymer jumping unit. k i t h the assumption that all solvent molecules move as single units, the apparent activation energy for diffusion in the limit of zero solvent concentration, E D , defined as
should increase indefinitely as the size of the solvent molecule increases. This prediction of the free-volume theory does not agree with experimental data (Chen and Ferry, 1968;Fujita, 1961; Kokes and Long, 1953; Meares, 1965) which suggested that the activation energy approaches a limiting value as the size of the solvent molecule increases. Another interpretation of these experiments (Fujita, 1961; Kokes and Long, 1953; Meares, 1965) is that the ceiling value is reached for solvent molecules as large 0 1992 American Chemical Society
