hngmuir 1993,9, 46-49
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Art i c 1es Effect of Surfactant on the Phase Transition of N-Alkylacrylamide Gels Naozumi Wada, Yoshinori Kajima, Yasuhiko Yagi, Hiroshi Inomata, and Shozaburo Saito’ Department of Molecular Chemistry & Engineering, Tohoku University, Sendai 980, Japan Received May 20,1992. In Final Form: September 28,1992 Swellingratios of N-n-propylacrylamide( “ P A ) andN-cyclopropylacrylamide(NCPA)gels in a sodium dodecyl sulfate aqueous solution and transition temperatures of poly(N-isopropylacryWde)and poly“ P A in severalsurfactantaqueous solutionswere measured. T h e swelling behaviors of the gels and the transitiontemperaturesof the polymer solutionsare a f f d by the addition of small amountof surfactants, resulting from the association of the surfactant and the polymer chain. The addition effect of surfactant is stronglydependent on the alkyl chain length of polymer and surfactant. The results in the present work reveal that hydrophobic interaction between polymer chain and surfactant is a chief driving force of the association for anionic surfactant/nonionicgel (polymer)systems.
Introduction There have been a number of investigations on interactions between nonionic water-soluble polymers and anionic surfactants. These results indicate that the polymerstend to form complexeswith anionicsurfactants and the complexeabehave like a “polyelectrolyte”because the binding of anionic surfactants converts nonionic polymers into charged species. The hydrophobicwater-soluble nonionic polymer poly(N-isopropylacrylamide) (poly-NIPA), whose aqueous solution exhibits a phase separation with a lower critical solutiontemperature (LCST),’ is known to form a complex with an anionic surfactant, sodium dodecyl sulfate, SDS, resulting in an increase in cloud point temperature due to electratetic repulsion of the surfactants bound to the polymer.2 Previously, we conducted swelling measurements of a NIPA gel in SDS aqueous solution and found that the swellingequilibriaof a NIPA gelwas much affected by the presence of SDS.3*4Recently, Schild and Tirrell studied an aqueous mixture of poly-NIPA and a series of sodium n-alkyl sulfatee with alkyl chain lengths in the range 1-12: They reported that surfactant aggregation elevated the LCST and thiseffect was stronglydependent on the length of the surfactant alkyl chain. These findings may be considered to result from hydrophobicinteraction between the polymer chain and the alkyl chain of surfactants. This discussion based on limited experimental data will be confirmedby studies on other nonionic polymer geldsurfactant systems. In this report, we have studied the effect of SDS on the swelling behavior of N-n-propylacrylamide (NNPA) gel and N-cyclopropylacrylamide(NCPA) gel, both of which are thermosensitivegels that are shrunk in water at higher temperatures similarly to a NIPA geL6 Furthermore, the change in transition temperature of aqueous solutions of poly-NNPA and poly-NIPA by the addition of a small (1) Heekine, M.; Guillet, J. E. Macromol. Sci. Chem. 1968, A2 (8), 1441. (2) l!Xwnf, J. J. Appl. Polym. Sci. 1978,22, 873. (3)Kajima, Y.;Yagi,Y.;Inomata, H.; Saito, S. R o c . 4th Symp. Polym. Gele (Japanere) 1991,4,64. (4) Inomata, H.; Goto, S.; Saito, S. hngmuir 1992,8, 1030. (6) Schild, H. G.;Tirrell, D. A. hngmuir 1991, 7,665. (6) Inomata, H.; Goto, 5.; Saito, 5. Macromolecules 1990, 29, 4887.
0743-7463/93/2409-0046$04.00/0
amount of severalsurfactants was measured and compared with the results of Schild and Tirrell.
Experiments Materials. NIPA monomer, provided by Eastman Kodak Co., was purified via recrystallization from a benzene/n-hexane mixture. NNPA or NCPA monomer was prepared from either n-propylamine or cyclopropylamineand acryloyl chloridevia the following reaction:’
CH,CHCOCl + NH,R
+ EhN
-
CH,CHCONHCH,CH,CH,
+ EhNHCl
R n-propyl, cyclopropyl
Potaseium persulfate (KPS),an initiatorfor gel synthesis,was recrystallized from a water solution. NJP-Methylenebis(acry1amide) (BIS), a cross-linker, and N,N,N’,”-tetramethyl-
ethylenediamine (TEMED),an accelerator, were special grade products of Tokyo Kasei Co., Ltd. Azobis(isobutyronitrile) (AIBN),an initiatorfor polymerization,was obtained from Wako Pure Chemical Co.,Ltd., and ueed without further purification. Sodium dodecyl sulfate(SDS)was obtained from Tokyoh i Co., Ltd.,and recryetabed twice from a methanol solution.Three aodium n-alkylsulfonates (C = 8,10,12,C designates the carbon number of alkyl chain in surfactant) and n-dodecyl amine hydrochloride (C = 12,cationic surfactant)were obtained from Tokyo Kasei Co., Ltd., and ueed as received. Critical micelle concentration (cmc)values of the surfactantsused are as followd SDS, 8.1 m M sodium n-alkylsulfonate, (C = 8) 140 mM, (C = 10) 42 mM, (C = 12) 9.8 mM, n-dodecylamine hydrochloride (cationic eurfactant), 13.8 mM.
Sample Preparation. Gel samples were prepared through radical polymerization in an ice bath? according to the method reported by Hirotau et aL8NNPA (NCPA,NIPA) monomer,and BIS were dissolved in distilled water, while KPS was dissolved in another dietilled and degassed water. The two eolutions were then cooled to 0 O C and mixed. To the mixture, the accelerator was then added. The concentrationsof the monomer,BIS,KPS, and TEMED were adjusted to ca. 700, 7, 30, and 0.012 mM, (7)Ito, S.Kobushi Ronbunshu 1989,46,2476. (8) Hirotsu, S.; Hirokawa, Y.;Tanaka, T.J. Chem. Phys. 1987, 87, 1392. (9)Mukerjee, P.;M p b , K. J. Critical Micelle Concentrotions of AqwoueSurfoctant System;NationelBureauofStandarda: Wahin@on, DC,1971. CQ 1993 American
Chemical Society
Langmuir, Vol. 9, No.1, lW 47
Effect of Surfactant on Gel Swelling 50 '
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Figure 1. Swellingequilibriaof N-n-propylacrylamide(NNPA) gel in SDS aqueous solution. The SDS concentration varies: 0, 0 [lVmoVg of solution] (pure water); A, 0.5 [lW moVg of solution]; 0, 1.0 [lWmol/g of solution]; V, 3.0 [lVmol/g of solution], 0,5.0 [lWmol/g of solution].
Figure 2. Swelling equilibria of N-cylropropylacrylamide (NCPA) gel in SDS aqueoue solution. The SDS concentration varies: 0 , 0 [lo6 mol/g of eolutionl (pure water); A, 0.5 [lV movg of solution]; 0, 1.0 [lad moVg of soluti.on1; V, 3.0 [lad movg of solution]; 0 , 5.0 [1W moVg of solutionl.
respectively. This pregel solution was then transferred into a vial containing glase tubes of 1.65 mm inner diameter. After the gelation completed, the gels were taken out of the tubes and immersed in water for a few days, to wash away any residual chemicals. These gels were then cut into 1.6-1.7mm height rods. Poly-NIPA and poly-"PA were prepared by the method reported in the previous paper.6 Monomer was dissolved in a benzene/acetone mixture (benzene/acetone = 9/1),which was degassed by bubbling nitrogen gas, and was polymerized in a sealed vial with AIBN as the initiator at 60-70 "C. The reaction solutionwaa d w l v e d in a small amount of acetone and polymers were precipitated with an excess amount of n-hexane. After drying the precipitant was dried, samplepolymers were obtained via careful fractionation in an acetoneln-hexanemixture at room temperature. Characterizationresults for poly-NIPA and poly" P A by GPC were aa follows: poly-NIPA, M, = 2.0 x 106, Mw/M,, 2.0; poly-"PA, M, = 3.0 X l@,M,/M,, = 1.1. Measurements of Swelling Ratio and Transition Temperature. The gel rods were immersed in an excess amount of aqueous solution containing surfactant. After swelling equilibrium was attained, the diameter of the gels, d, was measured by calibrated scale photography. The swelling ratio of the gels was determined from the ratio of the equilibrium gel diameter to the original diameter do (at preparation) as follows
and NCPA gels in SDS aqueous solutions, respectively. It can be clearly seen that the addition of SDStends to make both gels swell in water, as observed in the case of a NIPA gel.4 The results in Figures 1and 2 show that even in the SDSaqueoussolutions,theNNPAgel underwenta volume phase transition while the NCPA gel exhibited a continuous volume change in the temperature range studied. The swellingratios of both NNPA and NCPA gels became larger and the transition temperature of the " P A gel increased with the SDS concentration. This can be explained by surfactant binding to polymer networks, leading to an increase in the osmotic pressure caused by dodecyl sulfate ions (DS-) bound to the gel networks. The transition temperatures of gels can be determined from the swelling curves. The determined transition temperatures of the " P A gel are plotted in Figure 3 together with those of NIPA gel. However, the precise change in transition temperature could not be determined from the swelling measurements because their minimum temperature interval was 0.5 K. Therefore the transition temperatures of these gels in the low SDS concentrations were determined from the DSC analyses and are shown in Figure 4. These figures indicate that the transition temperatures of both gels are initially constant (slightly decreasing)and then increasesignificantlywith an increese in SDS concentration. At lower SDS concentration, the difference in transition temperature of NIPA and " P A gels was about 10K,just as that in pure water. A possible explanationfor this behavior is the following. Inthe range of low SDS concentration, the amount of SDS m o l d e s binding to the gel is so small that an increase in oemotic pressure is negligible and the transition temperature is determined mainly from the interaction between polymer segmentsand solventmolecules. Therefore the difference in transition temperature observed between NIPA and " P A is based on thestrength of hydrophobicinteraction. Above a certain SDS concentration, which should corm spond to a critical aggregationconcentration (CAC),6SDS molecules(thealkylgroupein SDS)cooperativelyassociate with the polymer network, inducing ionic repubions and increasing the osmotic pressure of the gel due to counterions. This consequently bringa about an incnuue in
swelling ratio VIVO= ( d / d d 3 where do ia 1.65 mm. Phaee transition temperatures of the gels and the cloud points of the polymer solutione were measured via thermal analyses with a differential "n@gcalorimeter (SeikoI. Inc., DSC-100). The determinationtechmque of transition temperature haa been described in our previous paper.*O The scanning rate was set at 0.6 Wmin for the gels and 1.0 K/min for the polymer solutione to obtain clear DSC thermograms. All of our experiments were performed at very low SDS concentrations below the critical micelle concentration (cmc).
Rerultr and Dircurrione A " P A gel was reportad to undergo a discontinuous volume change at 24 OC in pure water and a NCPA gel shrank continuouslyin water with increasing temperature.8 Figures 1and 2 show the swellingbehaviors of both NNPA (10) M e , K.; Inomata, H.;Konno, M.;Saito, 5. Macromolecules two, Is,283.
Wada et 01.
48 Langmuir, Vol. 9, No.1, 1993
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