J. Phys. Chem. 1996, 100, 1767-1770
1767
Swelling of Polymeric Resins in Organic Solvents Induced by Dialkyldithio-Containing Extractants Lev Bromberg† Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: June 20, 1995; In Final Form: September 15, 1995X
The phenomenon of swelling resulting in volume transitions of cross-linked poly(4-vinylpyridine) and poly(N,N-dimethylacrylamide) resins in organic solvents induced by bis(2-ethylhexyl)dithiophosphoric and bis(2,4,4-trimethylpentyl)dithiophosphinic acids is demonstrated. Observed volume transitions in resins are explained by the dithioacid-polymer complexation analogous to the surfactant-polymer association in aqueous media.
Introduction Volume transitions in polymeric resins (gels) have ushered in a surging interest because of numerous scientific and practical implications of the gels’ ability to change their volume in response to environmental stimuli.1 Overwhelming majority of these transitions, however, were observed in aqueous media. Examples of volume phase transitions of polymeric resins in organic solvents are limited to poly(3-alkylthiophenes) doped with iodine or poly[N-[3-(dimethylamino)propyl]acrylamide] doped with 7,7,8,8-tetracyanoquinodimethane2 and poly(4vinylpyridine) gels.3 These gels are ionizable in organic solvents due to either electric field2 or electron transfer agents3 and thus undergo discontinuous transitions. In this study we have furthered the concept of solventimpregnated resins in which gels swollen with an organic compound (extractant) are capable of selective binding either other organic compounds or metal ions.4 The basic problem we address here is whether the introduction of an extractant would change the structure of the resin itself. From this standpoint, we have chosen extractants, namely, bis(2-ethylhexyl)dithiophosphoric (DTPA) and bis(2,4,4-trimethylpentyl)dithiophosphinic (DTPiA) acids, which are already being used as metal-complexing agents5 and also are compounds with thoroughly studied hydrophobicity, acidic properties, and performance in various solvent- and polymer-solvent systems.6,7 It appeared that DTPA and DTPiA act as surfactants in organic solvents causing resins to swell, analogously to the surfactant effects previously observed only in the aqueous media.8 Experimental Section Materials. Bis(2-ethylhexyl)dithiophosphoric acid (DTPA) (92%) of the formula [CH3(CH2)3CH(CH2CH3)CH2O]2P(S)SH was obtained from Elco Co. and purified as described elsewhere.9 Bis(2,4,4-trimethylpentyl)dithiophosphinic acid (DTPiA) (80%) of the formula [(CH3)3CCH2CH(CH3)CH2]2P(S)SH was obtained from Cytec Industries, Inc., and was purified by flash chromatography on silica gel. N,N-Dimethylacrylamide (DMAAm, polymer constituent) (99%), 4-vinylpyridine (VP, polymer constituent) (95%), divinylbenzene (DVB; cross-linker, 80%, mixture of isomers), ethylene glycol dimethacrylate (EGDMA; cross-linker; 98%, benzoyl peroxide (initiator; 97%), N,N′-methylenebisacrylamide (BIS; cross-linker; 99+%), ammonium persulfate (APS; initiator; 98+%) N,N,N′,N′-tetram† Present address: Gel Sciences Inc., 213 Burlington Road, Bedford, MA 01730. X Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-1767$12.00/0
ethylethylenediamine (TEMED; 99.5+%), and octadecyltrimethylammonium bromide (C18TMAB) were all obtained from Aldrich Chemical Co. and were used as received. 2,2′-Azobis(2-methylpropionitrile) (Kodak; initiator) was repeatedly crystallized from acetone. All other chemicals and dry organic solvents used were obtained from commercial sources and were of the highest purity available. For preparation of the resins, radical polymerizations were carried out as follows. Poly(DMAAm) Resins. A mixture of thoroughly measured amounts of DMAAm and either EGDMA or BIS totaling 5.0 mL in a 20-mL vial sealed with a sleeve serum stopper was deaerated by N2 bubbling and, following quick addition of 10 µL of a freshly prepared saturated solution of APS in dry dimethyl sulfoxide and 10 µL of TEMED, was immediately placed into a refrigerator and kept there at 4 °C for 24 h. A series of borosilicate glass Pasteur micropipets of 0.1-2.0-mm internal diameters had been inserted into the vial prior the liquid mixture addition. Resulting homogeneous, transparent polymeric networks were allowed to stay in a temperature-controlled bath at 70 °C for another 24 h. Bulk gels were cut into small pieces, washed with excess acetone, and dried under vacuum. Micropipets containing gels were gently cut into two parts in a manner allowing the gel to remain intact while one of the parts of the micropipet was removed by means of a flow of N,Ndimethylformamide (DMF) within it. Small cylindrical pieces of gels recovered from micropipets10 were kept in an appropriate solvent. Poly(DMAAm/VP) Resins. A mixture of thoroughly measured amounts of DMAAm, VP, and EGDMA totaling 5.0 mL in a 20-mL vial sealed with a sleeve serum stopper was deaerated by N2 bubbling and, following quick addition of 10 µL of a freshly prepared saturated solution of APS in dry dimethyl sulfoxide, was allowed to stay at 75 °C for 48 h in a temperature-controlled bath resulting in homogeneous, transparent polymeric networks which were treated as described above for poly(DMAAm) resins. Poly(VP) Resins. A mixture of thoroughly measured amounts of VP, DVB, and DMF, totaling 50 mL, in a 60-mL reactor sealed with a serum sleeve stopper, was deaerated by N2 bubbling and, following addition of 300 mg/mL benzoyl peroxide in DMF (200 µL), was kept at 75 °C for 96 h, resulting in a transparent, orange polymeric network. A series of micropipets of various diameters had been inserted into the reactor prior the liquid mixture addition. Gels recovered from micropipets as described above were cut into small cylinders and were kept in an appropriate solvent. © 1996 American Chemical Society
1768 J. Phys. Chem., Vol. 100, No. 5, 1996 Alternatively, a mixture of VP, DVB, and toluene totaling 50 mL, in a 60-mL reactor sealed with a serum sleeve stopper, was deaerated by N2 bubbling and, following addition of 300 mg/mL 2,2′-azobis(2-methylpropionitrile) in acetone (200 µL), was kept in a temperature-controlled bath at 75 °C for 96 h, resulting in an opaque gel which was cut into cubic-shaped pieces, washed with excess benzene, and dried under vacuum, resulting in a white, nontransparent polymeric network. Compositions of polymeric resins were characterized by initial monomer and cross-linker concentrations which are given throughout the text. Procedures. Two experimental techniques were employed to characterize the swelling of gels. Volume transitions of cylindrical gels synthesized, where possible, in micropipets were monitored at 20 °C in a transparent, temperature-controlled cuvette under a microscope using a microscalar to fit to the boundaries of the gel on a video monitor. The kinetics of the volume transitions was monitored, and only equilibrium data are further discussed. Volume transitions were characterized by d/d0, where d and d0 are the diameters of the gel in a given and reference solvent, respectively. Nontransparent, dry networks of microscopically irregular shape were cut into pieces of known weights (Wd) and placed into excess solvent, where they were kept at 20 °C in sealed vials for 10-15 days. The kinetics of the swelling was monitored by weighing the gel samples and, upon reaching constant weights, swollen gels were gently wiped up and weighed to give Ws. The equilibrium swelling degree was expressed by S ) (Ws/Wd - 1) × 100 and was obtained for each gel and solvent combination in triplicate. Maximum standard deviations for S were measured to be 11%. Results and Discussion Figure 1 demonstrates the phenomenon of swelling of poly(VP) resins in poor solvents, such as toluene and benzene,11 caused by DTPA and DTPiA. It can be seen that addition of only 0.3 mM dithioacid results in a 50-fold increase in the swelling degree and more than a 2-fold increase in d/d0. The smooth, continuous character of the volume transition observed suggests that a rather steady change of the gel affinity to the solvent occurred. The following explanation of the observed phenomenon may be forwarded. Poly(VP), with the very welldocumented nucleophilicity of its nitrogen atom,12 should strongly associate with either DTPA and DTPiA in an organic solvent. The acid in turn makes the complex more hydrophobic due to its exposed two 2-ethylhexyl groups (in the case of DTPA, Chart 1) or 2,4,4-trimethylpentyl groups (in the case of DTPiA). The complexed acid readily adsorbs organic solvents. As can be seen from Figure 2, addition of 0.3 mM DTPA overrides any effect of the change in the composition of the solvents and in fact nullifies solvent-induced volume transitions because the DTPA complexed with poly(VP) adsorbs equally well solvents, independently whether they are good (ethanol, DMF) or poor (benzene, toluene) for poly(VP). Would a gel composed of a polymer of a lesser basicity exhibit acid-induced volume transitions in organic solvents? In order to address this question, poly(DMAAm) resins of various degrees of cross-linking were subjected to different concentrations of either DTPA or DTPiA in toluene (poor solvent for poly(DMAAm); Figure 3). A striking effect of dithioacid concentration is observed: no or little increase of the swelling degree in the range from 1.7 to approximately 70 µM, and a dramatic increase of the swelling at [dithioacid] > 70 µM. As expected,13 a higher degree of cross-linking results in lower swelling. A different type of “response” of poly(VP) and poly-
Bromberg
Figure 1. Effect of dithioacids on the equilibrium swelling degree (A) and the diameter (B) of poly(4-vinylpyridine) gels. DTPA (1) or DTPiA (2) were dissolved in either toluene (A) or benzene (B) in various concentrations. Initial conditions of gel preparation: (A) [VP] ) 1.4 M, [DVB] ) 31 µM, synthesis in toluene; (B) [VP] ) 1.4 M, [DVB] ) 35 µM, synthesis in N,N-dimethylformamide. For other experimental conditions, see Experimental Section.
CHART 1
(DMAAm) gels to the dithioacid addition (compare Figures 1 and 3) implies different mechanisms of interactions of these polymers with dithioacids. By analogy with the complexes of poly(DMAAm) with poly(acrylic acid) proven to be stabilized by hydrogen bonding,14 we assume the following structure of poly(DMAAm-DTPA) interaction Via hydrogen bond formation: CHART 2
The difference in the character of the swelling of poly(DMAAm) and poly(VP) (Figures 1 and 3) can be quantitatively understood in terms of the different extent of interaction between these polymers and the acids. DTPA or DTPiA in these cases act as surfactants. Surfactant binding to a polymer usually occurs at a certain, well-defined surfactant concentration, called the critical aggregation concentration (cac).15 Cac is a measure
Swelling of Polymeric Resins in Organic Solvents
J. Phys. Chem., Vol. 100, No. 5, 1996 1769
Figure 4. Effect of copolymer composition on DTPA-induced volume transitions of poly(N,N-dimethylacrylamide-co-4-vinylpyridine) gels in toluene: (1) poly(N,N-dimethacrylamide), no 4-vinylpyridine added; (2) molar ratio VP:DMAAm ) 1:1; (3) molar ratio VP:DMAAm ) 9:1. Initial conditions of gel preparation: (1) [DMAAm] ) 9.7 M, [EGDMA] ) 50 mM; (2) [VP] ) 4.7 M, [DMAAm] ) 4.7 M, [EGDMA] ) 50 mM; (3) [VP] ) 8.4 M, [DMAAm] ) 0.93 M, [EGDMA] ) 50 mM.
Figure 2. Effect of DTPA on solvent-induced volume transitions of poly(4-vinylpyridine) gels. The solvent composition designates the volume percents of ethanol in toluene (A) or DMF in benzene (B). For conditions of gel synthesis, see the caption to Figure 1. (1) No DTPA was added; (2) DTPA was added resulting in a 0.3 mM concentration in each solvent composition.
Figure 3. Effect of dithioacids on the equilibrium swelling degree of poly(N,N-dimethylacrylamide) gels in toluene: (1, 2) effect of DTPA; (3) effect of DTPiA. Initial conditions of gel preparation: (1) [DMAAm] ) 9.7 M, [EGDMA] ) 10 µM; (2, 3) [DMAAm] ) 9.7 M, [EGDMA] ) 2.4 µM.
of the degree at which the surfactant molecule prefers to bind the polymer chain rather than to stay aggregated with other surfactant molecules. Thus, the free energy of the surfactant binding to a polymer is expressed by15 ∆Gb° ) RT ln cac. ∆Gb° is known to vary considerably between the different types of polymer-surfactant systems. In particular, the interaction between a polyelectrolyte and an oppositely charged surfactant, which is the analogue of the poly(VP-DTPA) system, is strongly favored, and a decrease of the cac relative to the critical micelle concentration (cmc) of the surfactant by several orders of magnitude is commonly observed.15 This interaction is frequently so strong that it can lead to the formation of waterinsoluble complexes which if redissolved in organic solvents, still remain in a form of polyelectrolyte-surfactant associates.16 The opposite is true in the case of the interaction of uncharged
polymers and nonionic surfactants (analogue of poly(DMAAm)dithioacid system). This interaction is suggested to proceed Via cooperative hydrogen bonding,15,17 resulting in the high cac values that are often close to the cmc.15,18 Thus, the earlier “response” of the poly(VP) gel to the dithioacid addition may be quantitatively explained by the dithioacid-poly(VP) binding occurring at much lower concentrations of the acid than in the corresponding poly(DMAAm) systems. It is interesting to observe that the copolymerization of VP and DMAAm leads to the mode of interaction of the resulting copolymer gel with DTPA which more closely resembles the behavior of the poly(VP) gel as the concentration of VP in the copolymer increases (Figure 4). Gel-dithioacid complexation discussed above results in a dependency of the gel swelling on polymer/dithioacid aggregation number (σ) as predicted by Fredrickson.19 Depending on the value of σ, the complex can adopt either flexible (coillike) or extended (rodlike) configurations. The coillike configuration is expected for relatively low σ, when the configurational statistics of the surfactant tails are essentially unperturbed by attachment to the polymer chain.19 At larger values of σ, the bound surfactant (DTPA or DTPiA) tails must stretch out normal to the gel polymer backbone to avoid contacts with other tails. The resulting polymer-dithioacid complex can be quite rigid, leading to an unfolded, but still coiled, state of the polymer chain between cross-links. Both conditions correspond to approximately an equal, low-swelling degree. Finally, however, when σ is increased beyond the point where the dithioacid chains begin to interact, the configuration of the polymer chain complexed with dithioacid changes from that of a coil toward an extended rod, thereby increasing the gel swelling. Since the rationale of the dithioacid-polymer interaction outlined above necessitates involvement of the SH group into the complexation with the polymers (see Charts 1 and 2), it may be predicted that the addition of a cationic surfactant which competes with polymers for binding of the thio group according to the scheme
∼N(CH3)+Br- + HS-DTPA S ∼N(CH3)+S--DTPA + HBr would lower the gel swelling. For verification of this hypothesis, the cationic surfactant, octadecyltrimethylammonium bro-
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Bromberg Michael G. Goldfeld for help with some experiments. This work is dedicated to the memory of Jakov Bromberg whose loving generosity in so many ways guided the author’s scientific path. References and Notes
Figure 5. Effect of C18TMAB on DTPA-induced volume transitions of poly(4-vinylpyridine) (1, 2) and poly(N,N-dimethylacrylamide) (3, 4) gels in toluene: (1, 3) no C18TMAB added; (2, 4) C18TMAB added, resulting in 5 mM concentration in each toluene/DTPA solvent composition. Initial conditions of gel preparation: (1, 2) [VP] ) 1.4 M, [DVB] ) 34 µM, synthesis in toluene; (3, 4) [DMAAm] ) 9.7 M, [BIS] ) 10 µM.
mide (C18TMAB), is added into DTPA-containing toluene where the gels have been equilibrium swollen (Figure 5). As predicted, this results in a decrease of the gel swelling, especially at higher DTPA concentrations. Thus, it appears that the observed DTPA-induced volume transitions of gels is the first encounter of the surfactant effect on gels in organic solvents. This effect, resembling that induced by surfactants in aqueous media,8 may lead to the development of superabsorbent gels capable of absorbing a nonaqueous liquid many times their dry weight. Conclusions The effect of bis(2-ethylhexyl)dithiophosphoric and bis(2,4,4trimethylpentyl)dithiophosphinic acids on the swelling of crosslinked poly(4-vinylpyridine) and poly(N,N-dimethylacrylamide) resins (gels) in organic solvents is demonstrated. It is shown that the acids increase swelling of the gels under study in organic solvents, independently of the quality of the solvent toward the polymer constituting the gel. Observed volume transitions in gels can be explained by the dithioacid-polymer complexation at certain concentrations of the acid. Analogy is drawn between the dithioacid-induced volume transitions and the surfactantinduced volume phase transitions in aqueous media causing the phenomenon of superabsorbency. Acknowledgment. The author is grateful to Professor Toyoichi Tanaka for insightful discussions and to Professor
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