Langmuir 1992,8,687-690
687
Effect of Additives on Phase Transition of N-Isopropylacrylamide Gels Hiroshi Inomata, Shuichi Goto, Katsuto Otake, and Shozaburo Saito' Department of Molecular Chemistry & Engineering, Faculty of Engineering, Tohoku University, Sendai 980, Japan Received July 23, 1991.I n Final Form: October 16,1991 Effects of salts on volume phase transition of N-isopropylacrylamide gels and cloud points of poly(N-isopropylacrylamide)solution were measured. The experiments were performed with inorganic salts and tetraalkylammonium salts. The transition temperature for inorganic salts was dependent on anions rather than cations, and the change in transition temperature was linearly correlated with the viscosity B coefficient of anions. On the other hand, the transition temperature for tetraalkylammonium bromides strongly depended on alkyl chain length, which was suspected to result from an adsorption of the salts to the polymer segments.
I. Introduction N-Isopropylacrylamide (NIPA) gels have been known to undergo a thermoshrinking type of volume phase transition, in which the gels collapse in water in response to ascent in temperature.' This thermally induced volume phase transition corresponds to a lower critical solution temperature (LCST) type of phase transitionof an aqueous solution of poly(N4sopropylacrylamide) (PNIPA). These transitionsof NIPA gels and PNIPA solutions are currently of great interest and have been extensively studied by several researcher^.^-'^ To date, it has been pointed out that these transitions result from not only the formation of hydrogen bondings between the polymer networks and water molecules but also hydrophobic interactions. In a series of studies on NIPA gels, we have asserted that hydrophobic interaction plays an important role in the transition of NIPA gels and PNIPA aqueous solutions.l@l2 Recent publications of our groupll and Schild and TirreIl'3 revealed that the addition of nonelectrolytes such as alcohols lowers both the transition temperature of NIPA gel and the liquid-liquid phase transition temperature (cloud point) of PNIPA solution. We supposed that these results could be also explained by considering an effect of the additives on hydrophobic interaction. Namely, additives such as alcohols tend to immobilize water molecules around them and consequently weaken hydrophobic hydration, resulting in the promotion of hydrophobic interactions. In this work, electrolytes have been used as additives because they interact more strongly with water than nonelectrolytes and are considered to be suitable for detailed discussions of the effect of hydration structure on tran(1)Hirokawa, Y.;Tanaka, T. J. Chem. Phys. 1984,81, 6379. (2)Hirotau, S.J. Chem. Phys. 1988,88, 427. (3)Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (4)Prange, M.;Hooper, H.; Prausnitz, J. Macromolecules 1989, 35, 803. ( 5 ) Bae, Y. H.; Okano, T.; Kim, S. W. J. Polym. Sci., Polym. Phys. 1990, 2-89, 923. (6)Winnik, F. M.Macromolecules 1990, 23, 233. (7) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Macromolecules 1990, 23, 2415. (8) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (9)Kubota, K.; Fujishige, S.; Ando, I. J.Phys. Chem. 1990, 94, 5154. (10)Otake, K.;Inomata, H.; Konno, M.; Saito, S. J.Chem. Phys. 1989, 91, 1345. (11)Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (12)Inomata, H.; Goto, S.; Saito, S. Macromolecules 1990,23, 4887. (13)Schild, H. G.; Tirrell, D. A. J.Phys. Chem. 1990,34, 4352.
sition temperature of NIPA gels and the cloud point of PNIPA solutions.
11. Experimental Section 11.1. Preparation of Samples. The NIPA gel was prepared by free radical polymerization in water. Prior to the polymerization, NIPA monomer was purified by recrystallization from a benzeneln-hexanesolution. An initiator, was recrystallized from water. A cross-linkingreagent (NX-methylenebis(acry1amide)) and an accelerator (N,","-tetramethylethylenediamine) were special grade products of Tokyo Kasei Kogyo Co., LM., and used without further purification. The details of reaction conditions were described in our previous paper.12 The polymer (PNIPA) used was prepared as follows: the purified NIPA monomer was dissolved in a benzene/acetone mixture and polymerized in a sealed tube at 60-65 "C with azobis(isobutylonitrile) (AIBN) as an initiator, which was recrystallized from methanol. The detailed reaction conditions were described in our previous paper." The polymer waa then precipitated from the solution by the addition of n-hexane and fractionated in an acetoneln-hexanemixture at room temperature. Molecular weight of the polymer was determinedby GPC (M,= 7.6 X 104,MdM, = 2.4;pullulan standards: water; Shodex OHpak column). As additives, we used two types of electrolytes. One was inorganic salts such as NaC1, KC1, NaBr, KBr, NaI, KI, KNOa, KC103, and KBr03. Another was organic salts; six kinds of tetraalkylammonium bromide of differentchain lengths (NH& to N(C5H&Br). All the salts used were samples of the highest purity available from Wako and were not subjected to further purification. 11.2. Experimental Procedures. Transition temperatures of the NIPA gel and cloud points of the PNIPA solution were measured by thermal analysis with a differential scanning calorimeter (DSC, Seiko I. Inc., Ltd., DSC-100). In the DSC analyses, about 12-15 mg of a sample was used, and pure water was adopted as reference. The transition temperature determination technique was described in a previous paper." The swelling volume of the gel was also measured by calibrated scale photography, and the swelling ratio was determined according to Tanaka et al.14 111. Results and Discussion The experimental data of the DSC analyses are tabulated in Table I together with the experimental conditions. Our previous work1' experimentally showed that the transition of a NIPA gel in pure water occurs at a temperature close to the cloud point of a PNIPA aqueous solution and that (14)Amiya, T.; Hirokawa, Y.; Hirose, Y.; Li, Y.; Tanaka, T. J. Chem. Phys. 1987,86, 2375.
0 1992 American Chemical Society
Znomata et al.
688 Langmuir, Vol. 8, No. 2, 1992 Table I. Experimental Conditions and Results of DSC Measurements PNIPA Aqueous Solution
additive
additive concn, mol/g of soln 3.01 X 4.99 x 10.0 x 3.01 x 5.04 x 9.97 x 2.93 X
lo4 10-4 10-4 10-4 10-4 10-4
lo4
5.06 x 10-4 9.83 X W4 3.00 x 10-4 4.98 X 9.98 X 1.51 X 2.95 x 4.92 x 10.0 x 3.02 X
10”’
9.88 X
lo4
lo4 10-4 10-4 10-4
5.06 x 10-4
monomer unit concn, mol/g of s o h 1.24 X 1.25 x 1.24 x 1.24 x 1.24 X 1.24 x 1.24 x 1.27 X 1.26 x 1.25 X 1.25 X 1.25 x 1.25 x 1.24 X 1.25 x 1.27 x 1.24 x 1.24 X 1.24 X 1.24 x
lo4 10-4 10-4 10-4 lo4 10-4 10-4 lo4 10-4 lo4 lo4 10-4 10-4 lo4 10-4 10-4 10-4 lo4 lo4 10-4
3.02 x 10-4 2.99 x 10-4 3.00 x 10-4 3.00 x 10-4 2.99 X lo4 3.02 x 10-4 3.02 X 10-4 3.01 X lo4 3.01 X lo4 2.98 X lo4 3.00 x 10-4 3.01 x 10-4 2.97 X 10”’ 3.00 x 10-4 2.98 X 10”’ 2.99 X 10-4 2.99 x 10-4 3.01 x 10-4
transition temp, OC 31.2 27.1 24.6 17.7 27.1 24.3 18.0 29.0 27.3 22.9 29.2 27.3 22.4 31.4 31.4 30.9 28.0 31.3 30.6 27.5
0
35 -
additive concn, mol/g of soh
transition temp, OC
20 -
KI KNO3 KC103 KBr03
10.0 x 10-5 9.95 x 10-6 49.9 x 10-5 100 x 10-5 10.0 x 10-5 9.98 x 10-5 10.0 x 10-5 10.0 x 10-5 9.99 x 10-5 9.95 x 10-5 10.0 x 10-5
33.8 32.3 32.4 26.8 20.6 33.1 32.9 34.0 34.1 33.4 32.7 32.0
HdNBr (CHANBr (CnHdrNBr (CsHd4NBr (C4Hs)4NBr (C5H11)4mI
10.0 x 9.98 X 9.99 x 9.98 x 9.98 x 9.99 x
33.6 33.6 34.4 34.6 34.0 29.6
10-5 10-5 10-5 10-5 10-5 10-5 10-5
5.08 x 10-5 9.87 x 10-5 19.9 x 10-5
4.89 x 10-5 9.83 x 10-5 20.1 x 10-5 4.94 x 10-5 9.92 x 10-5
15
Figure 1. Effect of KCl on transition temperature of NIPA gel and cloud point of PNIPA solution.
31.7 31.5 31.3 31.0 31.5 31.5 31.4 31.8 32.1 32.6 31.9 32.3 32.8 31.7 31.8 31.7 30.0 27.8
5.32 X 9.94 x 19.9 x 4.82 x 9.80 x 20.1 x 5.71 x 9.62 X 20.2 x
5 10 KCI Concentration [ 1 0 ~ ~ m d / g s o h ]
0
NaCl KCI
A
NaBr KBr
A
Gel ~
additive none (water) NaCl KCl NaBr KBr
NaI
10-5 lW5 10-5 10-5 10-5 10-5
the thermograms of the gel were similar to those of the PNIPA solution. To examine whether this similarityholds even in the presence of additives, we compare the effects of the additives on the phase transitions of the gel and the PNIPA solution. Figure 1shows experimental results for KC1 solutions. The degree of change in the transition temperature of the gel by the addition of KC1 was almost the same as that of the polymer solution. Figure 2 shows the dependence of the cloud point of the PNIPA solution
0
5
10
15
Salt Concentration I d /gsdn. I Figure 2. Cloud point of PNIPA solution with inorganic salts as functions of salt concentration. Monomer unit concentration of the polymer solution is 1.2 x 10-4 mol/g of solution.
on salt concentrations. The cloud point was strongly dependent on the anionic species, and almost independent of the cations (Na+ and K+) used in this study. For all salts except iodides, the addition of the salt lowered the transition temperatures. On the other hand an increase in the concentration of the iodide once raised the cloud point of the PNIPA solution and then lowered it in the range of high concentrations. This behavior of the PNIPA solution was similar to that observed with poly(viny1methyl ether) solution, which is also known to be a polymer that reveals cloud points in response to a rise in temperature.15J6 Figure 3 shows the swelling ratio of the NIPA gel for different salts (1X mol/g of sol). Lines in the (15) Home, R.; Almeida, J.; Day, A.; Yu,N. J. Colloid Interface Sei.
1971, %,IT.
Effect of Additives on Phase Transition of NZPA Gels
Langmuir, Vol. 8, No. 2, 1992 689 Table 11. Viscosity B Coefficients of Ions
ion Na+
BpLlmol
0.086 K+ -0.007 c1-0.007 Br-0.042 “Value in water at 25 O C . ,
.
. .. ...
ion
B,OLlmol
I-
-0.069
NO3-
-0.046
(2103-
-0.024 0.006
salt COWenlrallOn
Naa
lxlO~mol/gl
2
E I-
a
28 lo-’
i 0
I
loo Swelling Ratio V/VO
I-] Figure 3. Effect of inorganic salts on swelling equilibria for NIPA gel. Salt concentration is 1 x lo-‘ mol/g of solution.
figure are smoothingcurves of the experimental data. The swelling behavior of the gel did not depend on the cationic but on the anionic species, as was seen in the transition behavior of the polymer solution. To explain this specific behavior, it might be necessary to consider the entropy change of the whole solution system, because the additives promote a formation of hydrate structure around them. Liquid water has a distinctive structure feature called an “iceberg”; a change in this structure is evoked by the presence of ionic solutes. For the structure of water in the presence of ionic solutes, Frank and Wed7 proposed a model which was derived from numerical analyses of the entropy of the hydration. According to the model, the ionic solute is surrounded by three concentric regions. The innermostregion (A region) is composed of water molecules which are immobilized through ionic-dipole interactions, the second (B region) one consists of water that is less icelike, namely more random in organizationthan “normal” water, and the third (C region) consists of “normal”water. In general, small and polyvalent ions have large A regions and are called structure makers. On the other hand, large and monovalent ions have small A regions and large B regions and are called structure breakers. It is known that the addition of structure breakers to water decreases the viscosity of the solutions and vice versa. Viscosity B coefficient (VBC)’* is generally considered to represent a measure of hydrate structure. An ion with a positive VBC is a structure maker and tends to enhance the hydrophobic interaction, while one with a negative value, a structure breaker, frees a part of the water molecules from the bulk “iceberg”and tends to stabilizethe hydrophobic hydration. Table I1 shows the VBC valueslgof the ions. Focusing on the anions according to the experimental results, the VBC values of Br- and I-, whose transition temperature depressions are smaller than that of C1-, are largely negative. Thus, the experimental results (Figures 2 and 3 and Tables I and 11)suggested that the VBC values of anions may be expected to correlate the degree of change in the transition temperatures. Figure 4 shows the (16)Huange, X.;Unno,H.; Akehata, T.;Hiraaa, 0.J. Chem.Eng. Jpn. 1988,21, 10.
(17)Frank, H.; Wen, W.-Y. Discuss.Faraday SOC.1957,23, 133. (18)Jones, G.;Dole, M. J. Am. Chem. SOC.1929,51, 2950. (19) Kaminsky, M. Discuss.Faraday SOC.1957,24, 171.
-0.1
-0.05
0
viscosity 9-coefficient[I/mol] Figure 4. Relationshipbetween viscosityB coefficientsof anions and AT, change in transition temperature of NIPA gel by an addition of potassium salt. AT is defined by the following equation and salt concentration 1 X lo-‘ mol/g of solution: AT = (Tt in pure water) - (Ttin salt solution),where Tt is transition temperature.
relationship between the VBC values of the anions and the change in transition temperature (AT) of the NIPA gel, which were determined from DSC analyses. As seen in Figure 4, the AT is almost proportional to the VBC value for the monovalent anions used in this work. As mentioned above, the VBC value is closely related to the degree of change in the transition temperature of the gel resulting from the addition of inorganic salts. This may be attributable to the fact that inorganic ions form hydrates through simple ion-dipole interactions. When electrolytes that have another interaction as well as the ion-dipole interaction are added, the additives would be expected to have different effects on the transition temperatures of the NIPA gel and the cloud points of the PNIPA solution. The tetraalkylammonium salts are known to show both a structure breaking effect resulting from ion-dipole interaction for N+ and a structure making one due to the hydrophobic interaction for alkyl groups. Namely, the structure breaking ability of the salt continuously changes into the structure making one with an increase in the chain length of alkyl groups. This was inferred by Frank and Evans from the entropies of hydrophobic hydration.2o Figure 5 shows the cloud points of PNIPA solution to which six kinds of tetraalkylammonium bromides were added. Some of the salts raise the transition temperature and the others lower them. The addition of the salt ( N H r Br), which has no alkyl groups, lowers the transition temperature. However, an increase in carbon number of the alkyl groups (CHa-C3H7) first raists the transition temperature but then lowers it above a certain chain length (CdH9, C6Hll). Figure 6 shows the equilibrium swelling curves of the gel with these salts. The addition of the tetraalkylammoniumsalts to the NIPA gel caused a similar change in the temperature of the volume phase transition to that in the cloud point of the PNIPA solution. (20) Frank, H.; Evans, M. J. Chem. Phys. 1945, 13, 507.
Inomata et al.
690 Langmuir, Vol. 8, No. 2, 1992
I
(n-C+hiL,NBr
28
26tI
iI
I
0
1
,
,
,
,
,
,
,,;i ,
,
,
,
, , ,
loo
lo-' Swelliq
mtio
VIVO
[ -1
Figure 6. Effect of tetraalkylammonium bromide on swelling equilibria for NIPA gel. Salt concentration is 1 X 1o-J mol/g of solution.
I
1
Salt Concentration
2 [1 6' m ~ig-sdn l
3
1
Figure 5. Cloud point of PNIPA solution with tetraalkylammonium bromide as functions of salt concentration. Monomer unit concentration of the polymer solution is 3.0 X lo4 mol/g of solution. According to the relationship between VBC and the transition temperature of inorganic salts, the addition of tetraalkylammonium salts might be expected to lower the transition temperatures of the gel and the cloud points of polymer solution because their VBC values are generally large and positive.21 However, our experimental results did not confirm this expectation. The above experimental results might be explained by considering some kind of interaction between the alkyl chain of the salts and polymer networks. Since alkylgroups of the salts are hydrophobic, the hydrophobic interaction is probably induced between the alkyl groups and the isopropyl groups in the polymer segments. In addition, the hydrophobic interaction is considered to promote the ions to adsorb (attach) to the polymer segments. The adsorbed ions will act like fixed charges on the network, eventually raising the transition temperature. This behavior is similar to that reported for PNIPA-surfactant ~ y s t e m . ~ ~ - ~ ~ There are several reasons for the depression in transition temperature in the case of the salts with longer alkyl chain. Although, at this stage, our results do not provide enough (21)Kay, R. L.; Vituccio, T.; Zawoyslci, C.; Evans,D. J. Phys. Chem. 1966, 70, 2336. (22) Schild, H. G.; Tirrell, D. A. Langmuir 1990, 6, 1676. (23) Winnik, F. M.;Ringsdorf, H.; Venzmer, J. Langmuir 1991,7,905. (24) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991,7,912.
information to specify the reasons, we supposed that the main one could be an associationlaggregation between the alkyl chains of the ions bound to the polymer networks and the association acted as pseudo-cross-linking between the polymer networks.
IV. Conclusion In the present work, the effects of salts on the volume phase transition of the NIPA gel and the cloud point of the PNIPA solution were measured. In the experiments, two types of salts were used: inorganic salts and tetraalkylammonium salts with different chain lengths. The transition temperature for inorganic salts was dependent on anionic species rather than the cationic species, and a linear relationship was obtained between the viscosity B coefficient of anions and the degree of the change in the transition temperature of the gel. These results were explained from the structure making or breaking character of the inorganic ions. The transition temperature for the tetraalkylammonium bromides strongly depended on the alkyl chain length of the salts and might be explained by considering both the hydration structure and an adsorption of the salts to the polymer segments. This work demonstrated the importance of the role of the hydrophobic interaction for the volume phase transition in the NIPA gels and the cloud points of the PNIPA solutions in the presence of ionic additives. Registry No. NIPA, 25189-55-3; NaCl, 7647-14-5;KCl, 7447KBr, 7758-02-3; NaI, 7681-82-5; KI, 768140-7;NaBr, 7647-15-6; 11-@ HINBr, 12124-97-9; (CHS)~NB~, 64-20-0; (CzH&Br, 71-910;(CsH,)J3r, 1941-30-6; (C4H&Br, 1643-19-2; (CsHIl),Br, 86697-7.
