Ind. Eng. Chem. Prod. Res. Dev. 1904, 23, 435-437
435
Synthesis and Solution Properties of Styrene-Vinyl-N-Alkylpyridinium Halide Block Polymers Frederick C. Schwab' Mobil Chemical Company, Edkon, New Jersey 08818
Israel J. Hellwell Mobil Research and Development Corporatbn, Prlnceton, New Jersey 08540
Polymeric thickeners typically used as mobility control agents in oil recovery processes suffer from a variety of deficiencies, such as shear instabllity and brine intolerance. Block polymers containing both hydrophobic and hydrophilic segments have been shown to form high molecular weight, micellar type structures resulting in highly efficient, shear stable thickeners. Anlonically polymerized diblock polymers of styrene and 2-vinyipyrldinium halides were used to study the effects of block structure on thickening ability and shear and brine stabili. I t was found that the low molecular weight poly(styrene-b-alkylpyridinium halide) polymers when dissolved in water behave as high molecular weight species. These materials appear to exist as an aggregated polystyrene core stabilized by a highly solvated polar region. These materials are highly effective, shear stable thickeners even at very low concentrations. They do,however, suffer from the typical brine on polyelectrolytes which results in coil collapse and a drastic reduction in viscosity.
Introduction Polymers such as hydrolyzed polyacrylamides and polysaccharides are commonly used as thickeners for the mobility control agent in oil recovery processes. These materials suffer from a variety of deficiencies such as shear instability, brine intolerance, and adsorption on formation surfaces. In principle, the shear viscosity loss encountered in mixing and pumping solutions of very high molecular weight polymers could be overcome by using low molecular weight polymers which are capable of association when dissolved. Such materials can form high molecular weight micellar type structures leading to high solution viscosities. It is well recognized that block polymers produced from monomers which form noncompatible blocks exist in a variety of spacial arrangements in both the solid (Molau, 1971; Meier, 1973; Bi And Fetters, 1975) and dissolved states (Vanzo, 1966). When such polymers contain both hydrophobic and hydrophilic blocks in the same molecule, they are capable of functioning as high molecular weight detergents that can associate into micellar type structures. Gallot and co-workers (Gallot and Selb, 1975) demonstrated that one such system, poly(styrene-b-4-vinyl-Nethylpyridinium bromide), gave micellar structures that had molecular weights ten times higher than the unassociated polymer molecules when dissolved in watermethanol mixtures. These high molecular weight structures were found to be due to the aggregation of the nonsoluble polystyrene blocks, which were stabilized by the water-soluble vinylpyridinium blocks. In this work we have utilized diblock polymers of styrene and 2-vinylalkylpyridinium halides as a model system to study the effects of block structure in the areas of thickening ability and shear and brine sensitivity. In addition, we have utilized fluorescence polarization techniques to study the micellar structure formed in solution. Experimental Section A. Polymer Synthesis. The synthesis of the diblock polymers, poly(styrene-b-2-vinylpyridine),was carried out via "living" anionic polymerization techniques by sequen0198-432118411223-0435$01.50/0
Table I. Quaternization. Block Polymers Dissolved in DMF and Quaternized under the Following Conditions quat agent temp, OC time, h HC1 amb" 1 CHJ 40 16 C2H5Br 55 80 CaH11Br 55 144 C1ZH25Br 80 I1 CMH3SBr 80 12 ICHzCOOH amb 1 Ambient temperature.
tial addition of monomers as follows. To a 1-L reaction flask maintained under anhydrous nitrogen was added the styrene monomer and 500 mL of anhydrous benzene. A small amount of a dilute phenanthroline solution was added as an indicator and the mixture was titrated with sec-butyllithium (s-BuLi) until an orange color was obtained. The amount of s-BuLi required to achieve a final molecular weight was added and the polymerization was allowed to proceed to completion at 50 "C. The active polystyryllithium solution was cooled to 0 "C and the 2-vinylpyridine monomer/benzene mixture (5050),which had been titrated with phenanthroline/s-BuLi, was added. The polymerization was allowed to proceed to completion at room temperature. After termination with methanol, the diblock polymers were isolated by precipitation in hexane. The homopolymerization of 2-vinylpyridine was carried out in a similar manner but in tetrahydrofuran at -78 "C. Figure 1gives the synthesis scheme for producing the diblock polymers. B. Quaternization. These diblock polymers were then quaternized with a variety of alkyl halides to give the corresponding alkylpyridinium salts. The reactions were carried out in dimethylformamide (DMF) at various temperatures and times depending upon the quaternizing agent involved. The quaternized polymers were isolated by stripping the DMF under vacuum and drying overnight at 50 "C in a vacuum oven (Table I). C. Preparation and Measurement of Polymer Solutions. The polymer solutions were prepared by slurrying 0 1984 American Chemical Society
436
n
CH,:
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 CH
BuLi __.c
'6
I
0 tCH,-CH~-,CH,-
0
CH Li + m C H , = C H
H6
-
-6 CH,-CH
4. 6, )
CH,
- C H-t,
A G'
Figure 1. Synthesis scheme for producing styrene/2-vinylpyridine block copolymers. Table 11. Effect of Block Styrene on Viscosity (cP) in Distilled Water ( T = Ambient; y = 1.83 8-l; Quaternizing Agent, CHJ) conc, d 1 0 0 mL % styrene 0.50 0.25 0.10 0.05 0 3.6 2.6 2.1 1.9 74 36 18 8 15 30 20 12 25 182 76 23 7
the polymer in distilled water in a blender, adding about 30% THF to effect solution and micellarization, and then stripping the THF off under vacuum. The concentrated polymer solutions (-0.5 to 0.8% w/v) were then diluted to the proper concentration with either distilled water or brine. Viscosities were run on a Model LV Brookfield viscometer with U.L. adapter at ambient temperature and various shear rates. D. Fluorescence Polarization. The degree of powas measured with an Elscint Model MV-1 larization (P) Analyzier (Elscint Inc., Hackensack, NJ) using fluorescent dyes (probes) specific to each block (Shinitzky and Barenholz, 1978). The nonpolar portion of the micelles (polystyrene) was probed using 1,6-diphenyl-1,3,5-hexatriene (DPH). The probe used for the polar region was 9-aminoacridine (AA). Results and Discussion Table I1 shows the effect of styrene on the viscosity of the polymers in distilled water. One can readily observe that the polymers containing as little as 8% block styrene show a very large increase in viscosity over the quatemized poly(2-vinylpyridine). This phenomenon is indicative of the association of the styrene portion of the molecule resulting in very high molecular weight micelles (Gallot and Selb, 1975). Reasonable viscosities were achieved at block polymer concentrations as low as 500 ppm. With the exception of the 0.25 g/100 mL solution containing the 15% styrene polymer, the achievable viscosity appears to depend more on the concentration of the block polymer than on its styrene content. This suggests that over the composition and molecular weight ranges studied, the styrene contents were sufficient to provide aggregation into micelles of roughly similar size. The effects of quaternizing agent on the thickening ability of the block polymers is given in Table 111. Up to the Cs derivative, the quaternized polymers behave in the expected manner; i.e., they are water soluble and good thickeners. At the 0.25 and 0.50 concentrations, there appears to be a dependence of thickening ability of the polymer on the size of the quaternizing agent used; i.e., the bulkier the quaternizing agent the more expanded the coil and hence, the higher the viscosity. However, the derivative containing Cs, while dispersible in water, gave very poor viscosities. The Clz and C16derivatives were not even soluble in distilled water. The Iack of water solubility in these samples could be due to the fact that they are not quaternized to a high enough degree because of steric
Table 111. Effect of Quaternizing Agent on the Viscosity (cP) in Distilled Water ( T = Ambient; y = 1.83 s-l; Polymer, 25% Styrene) concn, g/100 mL quat agent 0.50 0.25 0.10 0.05 HCl 104 38 10 4 CH31 182 76 23 7 253 C2H6Br 18 6 2 2 1.6 1.0 CBH17Br a a a a GzHzsBr C16H33Br
ICHzCOOH C8HI7Br/CH3I ~12H25Br/CHJ Not soluble.
a
a
a
a
-
128 26 1.6
28
8 6
2
10
1.6
Table IV. Effect of Brine on Viscosity (cP) (3% NaCl; T = Ambient; y = 7.34 s-l 25% Styrene) concn, g/IW mL quat agent 0.50 0.25 0.10 0.05 HCl 2.8 1.8 1.4 1.2 CHJ 2.8 1.4 1.5 1.4 C2H,Br/CH31 2.7 2.0 CBHI7Br/CH3I 1.9 1.6 1.2 1.2 ICHzCOOH 2.0 1.5 0.8
hindrance to reaction, or the molecule has become too hydrophobic in nature due to the long alkyl chains. To investigate these possibilities, the above nonsoluble derivatives were further quaternized with methyl iodide to improve water solubility. While the CH31improved the solubility of the previously quaternized materials, their thickening ability was considerably lower than the same polymer quaternized with CH31only. Apparently, the polymer molecule was made too hydrophobic by the use of long-chain alkyl halides (Cl2HZ5Brand C16H33Br),which resulted in a lower viscosity. Table IV gives the results of the effect of brine (NaCl) on the viscosity of the solutions as a function of concentration. Comparing Table IV with Table 111, a large salt effect is noted on all samples. This is to be expected from the behavior of a polyelectrolyte towards salt. What was hoped for, however, was that the bulky side groups from the longer chain alkyls would prevent some of the coil collapse simply by steric effects, thereby preventing such a large viscosity change upon the addition of salt. This, however, was not the case. Instead, the long-chain alkyls made the polymer molecule collapse in water due to the increased hydrophobicity of the chain and any effect of steric prevention could not be noticed due to this poor solubility. If coil collapse during salt addition can be overcome by steric hindrance, water-soluble quaternizing agents must be used. Figure 2 shows the effect of shear on the solution viscosity of one of the diblock polymers as a function of concentration. One can readily observe that the viscosity of the solution is essentially unchanged over three decades of shear, indicating a rather shear-stable system. By comparison, a typical oil field pumping situation may result in shear rates as high as 100000 s-l, three orders of magnitude higher than actually tested. Assuming linearity of the results in Figure 2, solution shear rates of 100000 s-l should still result in significant viscosities depending upon the concentration of the thickener. It should be noted, however, that the addition of mixed brines (those containing calcium ion) did not cause precipitation of the polymer up to 6%. This was to be expected since the polyelectrolyte is cationic in nature and resistant to calcium salt formation. The mixed brines,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 1000
I
h
21% Styrene
0.25%
O.A.= C z H s B r SOLVENT: D I s t l l l ~ dWater
F
t
l
j 10 y
100
,m-’
Figure 2. Effect of shear on solution viscosity. Table V. Fluorescent Polarization of Poly(styrene-b-N-methylpyridiniumiodide)-10% Styrene % Dolvmer % NaCl temn OC DP” Probe: l,B-Diphenyl-l,3,5-hexatriene (2 X 10” M) 0.10 0 15.5 0.398 0.10 0 24 0.405 0.10 0 36 0.400 0.10 0.1 24 0.409 0.10 1.0 24 0.415
Probe: 9-Aminoacridine (1-1.5 0.2 0 8 0.2 0 24 0.2 0 41 0.07 0 24 0.07 0.1 24 0.07 1.0 24 “DP = degree of polarization.
X
10”
M) 0.059 0.065 0.070 0.022 0.025 0.037
however, did cause coil collapse and a drastic reduction in viscosity. Results of the fluorescent polarization study are given in Table V. These findings show that the nonpolar portion of the micelle is in a higher condensed state (Shinitzky and Barenholz, 1978) than the polar portion as evidenced by the higher degree of polarization and is unaffected by temperature and low salt concentrations (0.15% NaC1).
23, No. 3, 1984 437
There appears, however, a discernable contraction (increase in degree of polarization) of the nonpolar aggregate at high salt concentrations. Since it is highly unlikely that the nonpolar aggregate is affected by the ionic strength, the observed contraction may be due to a transmission of the contraction of the polar micellar region upon the nonpolar core. The polar portion of the micelle appears to be affected both by the ionic strength of the medium and the temperature. Both cause a contraction of the polar region of the micelle, as evidence by an increase in the degree of polarization of the polar region. This is not an unusual phenomenon with respect to ionic strength, but the effect of temperature increase may be caused by dehydration, resulting in a contraction of the polar portion of the micelle. In conclusion, the low molecular weight poly(styreneb-alkylpyridinium halide) polymers when dissolved in water behave as high molecular weight species. These materials appear to exist as an aggregated polystyrene core stabilized by a highly solvated polar region. These materials are highly effective shear stable thickeners even at very low concentrations. They do, however, suffer from the typical effects of polyelectrolytes on viscosity. The use of long chain alkyl quaternizing agents to provide steric inhibition to coil collapse was unsuccessful due to a lack of water solubility imparted by these compounds. Acknowledgment It is a pleasure to acknowledge the technical assistance of Mr. Harris Yourison. Registry No. Poly(2-vinylpyridine) CH31 quaternized, 69253-95-8; (styrene).(2-vinylpyridine)(copolymer) HCl quaternized, 58067-77-9; (styrene).(2-vinylppidine) (copolymer) CH31 quaternized,60262-64-8; (styrene).(2-vinylpyridine) (copolymer) C2H& quaternized,58067-82-6;(styrene).(2-vinylpyridine)(copolymer) CaHI7Br quaternized, 90337-37-4; (styrene).(2-vinylpyridine) (copolymer) C12HzsBrquaternized, 57062-36-9; (styrene)-(2-vinylpyridine)(copolymer) C16H33Brquaternized, 90337-38-5; (styrene)+vinylpyridine) (copolymer) ICH2C02H quaternized,90337-39-6. Literature Cited BI, L. K.; Fetters, L. J. M8cromolecules 1975. 8, 98. Gallot, Y.; Seib, J. J. Polym. Scl. 8 1975, 73, 615. Meier, D. J. I n “Block and Graft Copolymers”, Burke, J. J.; Weiss, V., Ed.; Syracuse Unlverslty Press: Syracuse, NY, 1973; p 105. Moiau, G. E., Ed. ”Colloid and Morphological Behavior of Block and Graft Copolymers”; Plenum: New York, 1971. Shinltzky, M.; Barenholz, Y. 8ichim. Siophys. Acta 1978, 5 1 5 ( 4 ) ,367. Vanzo, E. J. JPolym. Sci. A - 1 1968, 4 , 1727.
Received for review February 8, 1984 Accepted May 21, 1984
