J. Phys. Chem. 1993,97, 29&292
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Specific Volume Phase Transition of Polymer Gels of the Same Composition but Different Overall Concentration. Focusing on a Particular Transition Point Seiji Katayama,' Fumiko Yamazaki, and Yukio Akabori Uniuersity of Shizuoka, School of Pharmaceutical Sciences, 52-1, Yada Shizuoka-City, 422 Japan Received: October 19, 1992
Three types of polymer gels of the same composition but different overall concentration were prepared as follows: 2-acryloyloxyethyl acid phosphate (AEAP) monopolymer gels, AEAP/acrylamide (Am) copolymer gels, and sodium acrylate (SAc)/Am/N,N'-methylenebisacrylamide (Bis) copolymer gels. When the gels were immersed in acetone-water mixtures, the gels all underwent discontinuous volume changes at a particular acetone composition. This specific transition behavior is considered consistent with the constant miscibility of each gel polymer with an appropriate acetonewater mixture, even when the gel concentration changes.
Polyelectrolyte gels' are known to undergo a reversible, discontinuousvolume change with changes in solvent composition, temperature, salt, and P H . ~ - ~This * behavior constitutes a volume phase transition between a swollen gel and a shrunken gel and has been generalized as a first-order phase transition. Volume phase transition has received attention in recent years because of its scientific and technological importance.23 According to the experimental results of volume phase transition observed for various types of polymer gels,23 most polymer gels immersed in acetone-water mixtures exhibiteddiscontinuous volume changes at various acetone compositions. The volume change at the transition point increased with an increase in ionic density, and the transition point shifted toward a higher level of acetone composition. This type of volume behavior has been defined as a pattern A volume phase transition. On the other hand, another type of volume behavior has been observed recently for the following polymer gels: 2-acryloyloxyethyl acid phosphate (AEAP) gels and 2.2'-bis(acry1amido)acetic acid (BAA) gels.24 In contrast to the volume phase transition of pattern A, that of pattern B was characterized by the following volume behavior: The gels immersed in acetonewater mixtures exhibited a volume change from a swollen to a shrunken state via a discontinuous volume change when the acetone composition increased, whereupon the discontinuous volume change at the transition point decreased with an increase in AEAP (or BAA) concentration, and the transition point shifted toward a lower level of acetone composition. In the course of the studies described above, discontinuous volume changes focused on a particular acetone composition were incidentally observed for the following gels: 2-acryloyloxyethyl acid phosphate (AEAP) mono polymer gels, AEAP/acrylamide copolymer gels and sodium acrylate/acry1amidelN.N'-methylenebisacrylamide copolymer gels. The purpose of this study is to clearly demonstrate such specific volume behavior occurring during volume phase transition and discuss the transition conditions in connection with an intermolecular interaction between the gel polymer and the acetonewater mixture (or miscibility of the gel polymer against the acetonewater mixture).
Experimental Seetion Acrylamide (Am), N,N'-methylenebisacrylamide (Bis), and acrylic acid (AJ were commercially obtained and used without further purification. 2-Acryloyloxyethyl acid phosphate (AEAP) was obtained from Daihachi Co. Ltd. Sodium acrylate (SAC) was obtained by reacting Ac with sodium carbonate and used as an ionic monomer after several repeated purifications. Three 0022-3654/58/2091-0290S04.00/0
types of gel samples were prepared in the present experiment. The first was a series of AEAP monopolymer gel samples produced by polymerization of AEAP solutionsof different concentrations. The second was a series of AEAP/Am copolymer gel samples produced by copolymerization of solutions containing equimolar amounts of AEAP and Am. The final was a series of gel samples which were produced by copolymerization of solutions containing SAC,Am, and Bis, where the molar ratio (SAc/Am/Bis) of each sample was maintained at 20/ 110/ 1. The actual concentrations of the individual gel samples are indicated in the footnotes for each figure. The sample preparation procedure of the gels was almost the same as in previous reports.ll.21 Mother liquors for each series of gel samples were first prepared and then aqueous solutions of desired concentrations were prepared by successive dilution of the mother liquors with distilled water. Gel samples were thus prepared by polymerization of the aqueous solutions in micropipettes at 50 OC for 1 h, after adding ammonium persulfate (initiator, 40 mg). The prepared gel samples were washed in flowing water for a few days and then immersed in acetonewater mixtures. After equilibrium was reached, the diameters of each cylindrical gel were measured. The gel volumes (V)were estimated by cubing the diameters.
Results and Discussion Figure 1 shows the volume behaviors of AEAP monopolymer gel immersed in acetonewater mixtures. The equilibrium volume of the AEAP monopolymer gel in distilled..water or mixtures of low acetone compositions decreased with an increase in AEAP concentration. This volume behavior is in marked contrast to that observed for conventional cross-linked gels with an ionizable group,23because equilibrium volumes of the gels were observed to increase with an increase in ionic density. Thus, the present experimental results could be explained as follows. AEAP is a bifunctional monomer with an ionizable group. Since a bifunctional group such as a cross-linker causes the gel to shrink and the ionizable group causes the gel to swell, the AEAP monop olymer gel should be influenced by the two competing effects at the same time. In this case, the former effect dominates the latter one, because an increase in gel concentration (or AEAP) actually resulted in a decrease in gel volume. In this manner, the increasein AEAP could be ultimatelyresponsible for the decrease in gel volume. The most dilutedAEAPmonopolymerge1(Sample E) exhibited swollen states in mixtures of low acetone composition and collapstd states in mixtures of high acetonecompositions. At 90% acetone, the gel underwent a discontinuous volume change. The other gel samples (D-A) also exhibited similar volume behavior. It is Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 291
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v/ vo Figure 1. Plot of the swelling ratio of 2-acryloyloxyethylacid phosphate (AEAP) gels immersed in acetone-water mixtures versus acetone composition. The AEAP gels were prepared by radical polymerization of AEAP solutions. The gels can be defined as different concentration gels, because they were prepared by varying only AEAP concentrations. Each gel sample contains the following amount of AEAP (A) 0.75, (B) 0.65, (C) 0.55, (D) 0.45, and (E) 0.40 M. ABCDE
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Figure 2. Plot of the swelling ratio of 2-acryloyloxyethylacid phosphate (AEAP)/acrylamide(Am) copolymer gels immersed in acetone-water mixtures versus acetone composition. The copolymer gels were prepared by copolymerizationof solutionscontaining equimolar amounts of AEAP and Am. The gels can be defined as different concentration gels, because the contents of AEAP and Am increase synchronously with an increase in the total gel concentration. Each gel sample contains the following amount of constituent AEAP (A) 0.5, (B) 0.4, (C) 0.3, (D) 0.25, (E) 0.20, and (F) 0.1 M.
noteworthy that all the gel samples underwent discontinuous volume changes at an acetone composition of 90%, and the equilibrium volume at 90%decreased gradually with an increase in AEAP. The above discontinuous volume behavior is clearly different from that observed for conventional cross-linked gels with an ionizable group.Z3 The specific transition behavior is considered as a characteristic structural property of the AEAP monopolymer gels as mentioned in the final section. Figure 2 shows the volume behavior of AEAP/Am copolymer gels immersed in acetonbwater mixtures. The gel samples all contain equimolar amounts of AEAP and Am regardless of the gel concentration. The equilibrium volume of the gel immersed in distilled water or mixtures of low acetone compositionsdecreased with an increase in gel concentration. The volume behavior is also in marked contrast to that observed for conventional crosslinked gels with an ionizable group. The volume behavior could also be accounted for by the specific function of AEAP incorporated in the gel as described above.
1
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v/ vo Figure 3. Plot of the swelling ratio of sodium acrylate/acrylamide/ N,N’-meth ylenebisacrylamidecopolymergels immersed in acetone-water mixture versus acetone composition. The gels can be defined as different concentration gels, because the relative molar ratio of constituent monomers in the gels remains constant at 201 lO:l, regardless of the gel concentration. The content of constituent Am in each gel sample is (A) 0.5, (B) 0.2, (C) 0.15, and (D) 0.1 M.
The most concentrated gel (sample A) exhibited swollen states in mixtures of low acetone composition and collapsed states in mixtures of high acetone composition, and the gel underwent a discontinuous volume change at 80% acetone. The other gel samples (B-F) also exhibited similar volume behavior. As a consequence,the gels all underwent discontinuousvolumechanges at an acetone composition of 808, whereupon the equilibrium volume at 80% decreased with an increase in gel concentration. The specific transition behavior is also considered as a characteristic structural property of the AEAP/Am copolymer gels as mentioned in the final section. The same volume behavior as described abovewas also observed for SAc/Am/Bis copolymer gels (Figure 3). The equilibrium volume of the gel immersed in distilled water or mixtures of high acetone compositions also decreased with an increase in gel concentration. This volume behavior was the same as that observed for the AEAP monopolymer gels and the AEAP/Am copolymergels. In general, an increasein Am (linear constituent) or Bis (cross-link constituent) causes the gel to collapse, while an increase in SAC(ionic constituent) results in swelling of the gel. Accordingly, a polymer gel having these constituents should be influenced by such competing effects simultaneously. As a result of the present experiment, the equilibrium volume of SAc/Am/ Bis copolymer gel actually decreased with synchronousincreases in each constituent. This suggests that the collapsing effect from Bis (or Am) dominates the swelling effect from SAC,although the gel concentration increases. This is the reason for the specific volume behavior observed for the SAc/Am/Bis copolymer gels and is essentially the same as that discussed above. The most diluted SAc/Am/Bis copolymer gel (sample D) exhibited swollen states in mixtures of low acetone compositions and collapsed states in mixtures of high acetone compositions, and at 45% acetone, the gel underwent a discontinuous volume change. The other concentratedgels (samples C-A) alsoexhibited the same volume behavior. As a consequence, the gel samples all underwent discontinuousvolume changes at the same acetone composition of 45%, whereupon the equilibrium volume at 45% decreased gradually with an increase in gel concentration. This specific transition behavior could also be considered as a characteristic structural property of the SAc/Am/Bis copolymer gels as discussed in the final section. Tojustify the aboveobservations,theaccuracy of concentrations of the acetonbwater mixtures, especiallynear the transition point, should be well-specified. In this experiment, several hundred grams of acetone and water were weighed to the order of 0.01
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292 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
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resemblance that each type of gel consists of the samecomposition but different overall concentration. Because of the structural resemblance, the present gels can be termed "different concentration gels". Accordingly, a series of different concentrationgel samples may be similarin chemical properties becausethey consist of the same constituent monomers but the molar ratio remains unchanged regardless of changes in the gel concentration. This suggests that intermolecularinteractions between the gel samples of various concentrations and an appropriate acetone-water mixture may remain unchanged. On the other hand, according to the theory of volume phase transition,*.4a discontinuousvolume change at a transition point should primarily be determined by a specific intermolecular interaction between a gel polymer and an acetone-water mixture, and then the interaction could be obviously associatedwith the miscibility of the gel polymer in the solvent mixture and also specified by Flory's X parameter.' For these reasons, the present gel samples may all undergo discontinuous volume changes at a particular acetone composition. This is a qualitative interpretation of the discontinuous volume change focused on a particular transition point. This study represents the first demonstration of the specific transition behavior characteristic of polymer gels of the same compositionbut different overall concentration. Further detailed examination is now in progress.
References and Notes
1
I
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3
Gel Concentration (molll) Figure 4. Plots of the transition acetone concentration (V/V%) for each gel versus gel concentration. Curves A - C are from AEAP gels, AEAP/ Am copolymer gels, and SAc/Am/Bis copolymer gels, respectively.
g by microbalanceand then the volume per cent (V/V%) of each mixture was estimated by dividing by each density. Therefore, concentrationsof the mixturescould all be determinedwith errors of less than 0.01 V/V%. On the other hand, the observations of gel volumes near the transition point was carried out using acetone-water mixtures of 2 V/V% intervals. The AEAP mono polymer gel samples all exhibited experimentallyswollen states ina mixtureof 89 V/V%acetoneandcollapsedstatesina mixture of 9 1 V/V% acetone. As a consequence, the acetoneconcentration at the transition point is determined with an experimental error of less than 2 V/V%. This could easily be examined by the plots of the phase transition acetone (V/V%) vs polymer concentration as shown in Figure 4A. The transition points observed for the AEAP/Am and SAc/Am/Bis copolymer gels (Figure 4B.C) can also be determined with the same experimental error of less than 2% in the same manner as previously mentioned. The reason why three types of gels all undergo discontinuous volume changes focused on a particular acetone composition regardless of changes in gel concentration is as follows. These gels have a chemical property in common. It is a structural
(1) Flory, P. J. In Principles of Polymer Chemistry; Cornell University
Press: Ithaca, NY, 1953; Chapter 9.
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