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Polyacrylamide hydrogels with trapped sulfonated polyaniline
European Polymer Journal 41 (2005) 1342–1349
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Polyacrylamide hydrogels with trapped sulfonated polyaniline Y. Tao *, J.X. Zhao, C.X. Wu State Key Laboratory of Fiber Modification, College of Materials Science and Engineering, Dong Hua University, Yanan Road West, 1882, Shanghai 200051, PR China Received 29 June 2004; received in revised form 9 October 2004; accepted 18 January 2005 Available online 17 February 2005
Abstract A new type of semi-interpenetrating network system consisting of a flexible polyacrylamide hydrogel with embedded stiff-chain linear polyelectrolyte, sulfonated polyaniline (SPANI), has been investigated. It is shown experimentally that the incorporation of stiff-chain polyelectrolyte inside the uncharged PAA network brings different swelling behavior from the pure PAA gel. Environmental pH values have remarkable influence to the swelling degree of this new gel system, the maximum swelling degree, appearing at the environmental pH value of about 12, can be 17 times greater than that at pH 1, and 12 times greater than that at pH 14. Release kinetics of SPANI has shown that SPANI is effectively retained by the gels in acidic or neutral environment, although they are not covalently attached to the network chains; but in alkaline environment, the entrapped SPANI molecules will be released into the surrounding solution gradually. The diffusion coefficient of SPANI is calculated from the experimental data. The experimental phenomena are tried to interpreted from aspects of aggregation and hydrogen bonding between SPANI chains. 2005 Elsevier Ltd. All rights reserved. Keywords: Polyacrylamide; Hydrogel; Polyaniline
1. Introduction The diffusion and release of solute from gel is an elemental question for the potential application of gel system such as Controlled drug delivery and membrane separation [1–4]. In order to satisfy these functions, effective pore size of the network should be responsive to the external stimulation of pH value, temperature, and electrical field, and thus the diffusion rate of solute from the gel could be controlled. Besides this, other fac-
* Corresponding author. Tel.: +86 21 6237 4607; fax: +86 21 623 73860. E-mail address: [email protected] (Y. Tao).
tors which influence the diffusion of solute is interaction between solute and network chains, as well as formation of Aggregates of Polyelectrolyte solute [10]. This kind of interaction, which is complicated and difficult to characterize, as well as its influence to the diffusion process, should be investigated as a important issue. The state of polyelectrolyte results from the interplay of two main force: repulsive electrostatic interactions which separate polyelectrolyte chains from each other, and attractive interactions between polymer units in poor solvent conditions which tend to make them condensate. For the gel system with trapped polyelectrolyte, direct electrostatic interactions become essential, along with the osmotic pressure of countions. The most important of them are the condensation of countions and the chain stiffening due to the repulsion of the similarly
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Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
charged units along the chains (characterized by an electrostatic persistent length) [5,6]. The aim of the present paper is to study the properties of a stiff-chain polyelectrolyte/flexible hydrogel system on the basis of a neutral polyacrylamide (PAA) gel with the embedded stiff-chain polyelectrolyte of sulfonated polyaniline (SPANI). The state of SPANI and its interaction with PAA network chains will be preliminarily studied from the swelling and release behavior of this new gel system.
2. Experimental 2.1. Synthesis and characterization of sulfonated polyaniline Polyaniline of emeraldine base (EB) form is synthesized by the traditional method [7]. Self-doping sulfonated polyaniline (SPANI) was synthesized according to Ref. [8]. In a typical procedure, EB (0.5 g) was sulfonated by dissolving EB Powder in 40 mL of 30% fuming sulfuric acid which was previously cooled to 5 C with constant stirring in ice-water. After addition of the polymer, the solution was stirred for 10 min in the ice bath. Subsequently, the solution was removed from the ice bath and allowed to reach room temperature while stirring was continued (5–10 min). The colour of the solution changed from dark purple to dark blue during 0.5 h at room temperature. The solution was then slowly added during 20 min to methanol (200 mL) to precipitate most of the product, the temperature during this step being hold between 10 C and 20 C via ice bath. The green powder was then collected on a buchner funnel and the precipitate cake was washed at least 10 times with 50 mL portions of methanol until the filtrate had a pH of 7 when tested by wet pH paper. It was then permitted to remain under suction for 10 min; the filter cake was then transferred on the filter paper to a vacuum desiccator and dried under dynamic vacuum for 24 h at room temperatures.
[
[
SO 3H H
H
N
N
+ SO 3 Na H
H
N
N
1343
The final product, sulfonated polyaniline (SPANI) can be dissolved in 0.1 M ammonium aqueous solution or 0.1 M NaOH solution, forming deep purple solution. Intrinsic viscosity measurement [9] of dilute emeraldine base was carried out to evaluate the weight average molecular weight of PANI from which SPANI was made, based on Mark–Houwink relation, ½g ¼ KM aw , here K = 1.2 · 104 and a = 0.77 [9]. From experiment data [g] = 0.31, the resulting Mw of emeraldine base is about 27,000 g/mol. Then the prepared SPANI is reduced by aqueous solution of hydrazine in order to estimate the Mw of SPANI by light scattering experiments. The resulting Mw of SPANI is about 16,000 g/mol, which show that polyaniline has undergone some degradation during sulfonating. Elemental analysis for SPANI gives a S/N atomic ratio of 0.48 indicating that approximately one –SO3H group is linked on the 2-position of the phenyl ring for every two phenyl rings. The structure of SPANI can be regarded as shown in Fig. 1. 2.2. Gel preparation. Polyacrylamide gels with trapped SPANI The gels were prepared by free-radical polymerization of acrylamide in aqueous solutions of SPANI with N,N 0 -methylene bis-acrylamide (BAA) as crosslinker. Ammonium persulfate (2.1 · 103 mol/L) and N,N,N 0 ,N 0 -tetramethyl ethylene diamine (TEMED) (2.1 · 103 mol/L) were used as initiator and accelerator. The stoichiometric concentrations of acrylamide monomer, SPANI, and BAA for different gel samples are listed in Table 1. The gel synthesis were carried out in cylindrical glass tubes at room temperature for 24 h. The pure polyacrylamide hydrogel as reference was prepared in the same way, except without adding of SPANI. In the following, the polyacrylamide gels which are synthesized in aqueous solutions of SPANI are marked with an asterisk (PAA1*–PAA6*). The reference polyacrylamide gels, which are prepared under similar condition but without adding of SPANI, are designated as PAA1–PAA6. SO 3H N
N
]n
SO 3 Na+ N
N
]n
Fig. 1. Structure of sulfonated polyaniline (SPANI), neutration by sodium hydroxide.
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Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
Table 1 Composition of the gels Sample
Solvent
C 0M (g/L)
C 0BAA (mol% to AA)
C0 SPANI (g/L)
Equilibrium m/m0
PAA1*
Water Water Water Water Water Water Water Water Water Water Water Water
50 90 50 90 90 90 50 50 50 90 90 90
2% 1% 1% 1% 2% 2% 1% 0.6% 2% 1% 0.6% 2%
1.0 1.0 1.0 2.1 2.1 4.0 0 0 0 0 0 0
42.33 16.56 56.37 22.6 21.6 233.7 52.9 95.5 37.5 20.90 32.7 14.3
PAA2* PAA3* PAA4* PAA5* PAA6* PAA1 PAA2 PAA3 PAA4 PAA5 PAA6
In this table, the equilibrium swelling degree of PAA* and PAA gel is measured in distilled water of pH 7. The concentration of BAA is characterized as the molar ratio of BAA to monomer acrylamide (AA).
2.3. Gel swelling measurement
3. Results and discussion
The swelling degree of the gel samples was characterized by the ratio m/m0, where m is the mass of the gel sample swollen in aqueous solution and m0 is the mass of the dry gel. Gel samples were also swollen in a series of aqueous solution of different pH values, in order to study the influence of pH value to swelling equilibrium.
To incorporate charged chains in a polyacrylamide gel, the polymerization of acrylamide together with a crosslinking agent was performed in a solution containing neutralized SPANI. As a result of the network formation, most of the SPANI chains become trapped by the gel. Too much amount of polyelectrolyte SPANI seems disadvantageous to the forming of network. When the concentration of SPANI exceeding 4 g/L, very soft gel of poor mechanical strength is formed, implying that imperfect network formation. It can be assumed that too much SPANI would react with and annihilate the active radicals, because SPANI has reduction units on it. As Yue et al. [8] pointed out, SPANI (emeraldine base form) has sulfonation level, i.e. the atomic ratio of sulfur and nitrogen, of about 0.5 in average, independent of the sulfonation time over periods from 0.5 to 24 h. This means that only half the rings are sulfonated. This is consistent with the fact that only half the rings need to be sulfonated in order to produce the stable polysemiquinone form of the polymer. As Fig. 2 shows, at different pH values, SPANI can exist as polycations (in acid), polyampholytes (the SPANI can be self-doped by the sulfonate acid group on its chain), and polyanions (in alkali). In different states, the nature of interaction between SPANI chains differs, is determined by which is prevailing, attraction or repulsion.
2.4. Kinetic study of SPANI release To study the release of SPANI from the gel, the gel samples just after the preparation were put to swell in acidic, neutral and alkaline aqueous solution respectively. Unless otherwise specified, the volume of solution was 600 mL per 5 g of the wet gel. During the experiment the solution with the gel specimen was slightly agitated by a glass rod every day. Every three days, the solutions surrounding the gel are sampled and then the concentration of SPANI is measured by Perkin-Elmer Ambda Bio-40 UV–VIS spectrometer (Cole-Parmer L-97600-05 long wave lamp, 20 mW/cm2), at maxima absorption wavelength 315 nm. In all the experiments distilled water was used. The content of SPANI in the gel phase was evaluated by subtraction of the amount of SPANI in the surrounding solution from the initial amount of SPANI. The fraction of the SPANI in the gel was characterized by the molar ratio between SPANI and PAA repeat units in the gel, denoted as h. The initial molar ratio of SPANI and PAA repeat units in the system is designated as h0. The efficiency of the retaining of SPANI by the gel was characterized by the ratio Cg/C between the concentrations of SPANI in the gel, Cg, and in the surrounding solution, C.
3.1. Gels swollen in water Examination of the equilibrium swelling degree of the gels in water (Table 1) shows that the incorporation of the SPANI into the uncharged gel results in increase of swelling degree. This is a consequence of the osmotic pressure exerted by charged SPANI chains and their counterions, though it is not remarkable. As one might
Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349 polyanion
Na SO3
[
+ H
H
N
N
1345
Na + SO3 N
N
]n
+NaOH SO3
[
H
H
N
N
SO3 + N
+ N
H
H
]n
+HCl
polycation SO3H H
H
N
N
[
SO3H + N
+ N
H
H
]n
Fig. 2. Structure of SPANI under various environmental pH values.
expect, the swelling of PAA* gels increases also with the lowering of the crosslink density. 3.2. Gels swollen in aqueous solutions of different pH values Fig. 3 shows the equilibrium swelling degrees of several PAA* samples in solutions of different pH values. Obviously, for pure PAA gels the swelling degree is the same whether in water or in solutions of different pH values. The situation is different for PAA* containing stiff-chain SPANI. The swelling degree of these gels increase remarkably with equilibrium pH value increasing and reach a maximum at pH = 12, then level off. 220
PAA1* PAA2* PAA3* PAA4* PAA5*
equilibrium swelling ratio m/m0
200 180 160 140 120 100 80 60 40 20 0 0
2
4
6
8
10
12
14
pH value
Fig. 3. Dependence of equilibrium swelling degree of five PAA* gels on the environmental pH values (pH values are measured when gel samples have reached equilibrium state with surrounding solution).
For PAA2* sample the maximum swelling degree appearing at pH = 11.7 is as high as 200, about 17 times greater than at pH = 1, also 12 times greater than at pH = 14. The leveling off of swelling degree in low and high pH value region reminds us of the usual deswelling of polyelectrolyte gels in salt solutions. The remarkably excessive amount of acid at low pH values (pH < 7) or excessive amount of alkali at high pH values (pH > 12) acts as the same role as low molecular weight salts, which would shield the charges localized on the polyelectrolyte chains. Except for PAA3*, the equilibrium swelling degrees of all PAA* gel samples at low pH values (pH < 7) are almost the same as that at pH = 7, as shown in Fig. 3. At pH = 7, there is no any low molecular weight electrolyte in solution, and at pH values lower than 3, the concentration of HCl is considerably high enough to cause deswelling, if HCl acted as the same role as low molecular weight salt, such as NaCl. This implies that for PAA* samples, the SPANI chains have already assembled into compacted aggregates even in water, so the electrostatic screening of low molecular weight electrolyte can not cause further contraction. More abnormality occurred on PAA3* sample, whose swelling degree in solution of high pH values is greater than that in pure water. For PAA* samples, the remarkably increase of swelling degree at pH = 12 and then deswelling at even higher pH value suggests that, although not covalently attached to the network, the SPANI chains should interact strongly with elastically active network chains. The disintegration and reassemble of aggregates of SPANI caused by pH values should be the driving force for varying swelling degree of PAA* samples, which will be discussed about in the following.
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Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
3.3. Aggregation of SPANI chains
in acidic solution nuetral in alkali solution
90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10
ecules. Experimental evidence for hydrogen bonding in PAA2* samples has been given by infrared studies on a Perkin-Elmer Nicolet infrared spectrometer, shown in Fig. 4. According to Ref. [13], there are two bands associated with the N–H stretching vibrations-one band located at about 3190 cm1 (hydrogen bonded N–H stretching) and another band located at about 3440 cm1 (non-hydrogen bonded). When PAA2* samples were dipped in concentrated acidic solution to equilibrium, the 3190 cm1 band become more intense than the 3440 cm1 band; when dipped to equilibrium in pure water and alkali solutions, the 3190 cm1 band can be negligible compared with intense 3440 cm1 band. This fact implies that intensity of hydrogen bonding varies with pH values. Hydrogen bonding is a kind of strong force that can results in aggregates of SPANI chains. The various intensity of hydrogen-bonding can further explain the contraction in acidic environment and swelling in alkali environment. These specific results do not distinguish whether the hydrogen bonding is formed between adjacent SPANI molecules or between SPANI and PAA molecules. From the fact shown in the following that the PAA* samples release easily the SPANI molecules to the surrounding alkali solution, but can retain almost all the entrapped SPANI in acidic solutions and pure water, it would be deduced that interaction between SPANI and PAAM molecules is strong in acid and weak in alkali.
3.4. Hydrogen bonding in gel
3.5. Release of SPANI chains from the gel
Besides the electrostatic interaction, another interaction of SPANI chains, as several authors [12,13] have pointed out, should be hydrogen bonding between the imine and amine nitrogen sites on adjacent SPANI mol-
Being immersed in aqueous solutions, the PAA* gels should lose a part of the SPANI chains under proper condition, because of their diffusion to the surrounding solution. Many factors are found to influence this
%Transmittance
As is shown above, there is not obvious difference for the swelling degree of PAA* gels in pure water or in strong acidic solution. But for another interpenetrating network system of PAA/polystyrene sulfonate (PSS) [10], the equilibrium swelling degree decreases as much as 31% when moved from pure water to 0.2 M NaCl solution, which suggests collapse of PSS in salt solution. This discrepancy can be attributed to the partial condensation of stiff-chain SPANI in the PAA* gel. According to ManningÕs theory [11], the condensation of counterions in water at room temperature begins when the distance between charged groups is lower than ˚ . As the number of charged groups approximately 7.0 A varies at different pH values, the average charge spacing ˚ in acidic or neuof SPANI differs, in details, about 5 A ˚ in alkaline solution. So tral aqueous solution, and 10 A condensation of counterions occurs in acidic or neutral solution, and all the charged groups on SPANI chains are free in alkaline solution. In acidic or neutral solution, the condensation may be due to the aggregation of several SPANI molecules in a rod, and a certain fraction of counterions becomes condensed in the vicinity of the rod by a strong electric field. For the interaction between polyion chains, electrostatic repulsion should be considered, in the meanwhile, the factors of attraction, such as dipole–dipole interaction, and hydrophobic effect, also should be under consideration. If effect of attraction prevailing, polyion chains would exist in form of aggregates. Based on this opinion, it is easy to interpret experimental phenomena. In acidic or neutral aqueous solution, the SPANI is in the form of polyampholyte, there would be strong dipole–dipole interaction between SPANI molecules, so it is reasonable to consider that SPANI tend to join in aggregates. A single SPANI chain can cross over from one aggregate to another acting as crosslinks, The further aggregation of SPANI chains in concentrated acid solution would compensate the additional osmotic pressure of SPANI and their counterions. This would explain why the swelling degree of PAA* gels are almost the same as PAA sample, and the swelling degree of PAA* samples in pure water are almost the same as in concentrated acid solutions. Alternatively, in alkaline environment, the undoped SPANI becomes polyanion and repulsive interaction prevailing, so the aggregates of SPANI collapse and effective crosslinking decrease, leading to increase of swelling degree.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-1
Wavenumber(cm )
Fig. 4. Diffuse reflectance FTIR spectrum of dried PAA2* gel samples. Thick straight line: dipped with 0.1 M HCl solution, fine straight line: dipped with pure water, dashed line: dipped with alkali solution.
Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
chains are destroyed by the electrostatic repulsion between the similarly charged SPANI chains.
process: the crosslinking density of the gel, the concentration of acrylamide units and concentration of polyelectrolyte chains at the network preparation, and the acidity of the surrounding environment.
The fraction of SPANI released in the surrounding solution, h, and the ratio of concentration Cg/C of the SPANI chains in the gel and in the solution (PAA3* sample has approached final equilibration) are presented in Table 2. From this table it is shown that the gel retains a major part of the SPANI chains, the initial molar ratio between SPANI and PAA gel repeat units, h0, seems to has not remarkable influence to the retain of SPANI by gels. The fact that most of the SPANI chains do not diffuse to the external solution (pH < 7) but become trapped in the gel may be resulted from that the experimental time scale is short and the diffusion rate of SPANI chains is not high, since the average length of the rods (about 150 nm) is more than 10 times larger than the mesh size of the gel (12 nm). Besides, aggregation involving the network chains, can also leads to the effective immobilization of the SPANI chains. The increase of both the degree of crosslinking and the concentration of acrylamide units at the network formation do favor to the retaining of stiff SPANI chains, the effect of the concentration of the monomer units being more pronounced, implying increasing entanglement between PAA chains and SPANI chains. A twofold increase of the concentration of monomer (cf. PAA2* and PAA4* gels) leads to an increase of Cg by a factor of 5, while a twofold increase of the crosslinking density results in a increase of Cg by 2.8. The initial concentration of SPANI do increase the release rate from the increasing slope of concentration–time curves.
(A) Release in pure water and acidic solution. It is found experimentally that there is seldom SPANI released from gel when the PAA* gel is dipped in acidic or neutral aqueous solution. This can be explained with the extensive aggregation of SPANI in pure water or acidic aqueous solution. The aggregation of SPANI is enhanced by the strong interaction between proton-doped SPANI chains. It is implied that in pure water or acidic aqueous solution, all SPANI chains have joined aggregates, whose large size and strong interaction with PAA prevent them from releasing. (B) Release in alkaline solution. The influence of the addition of alkali (0.1 M NaOH) on the release of SPANI is illustrated in Fig. 5. It is shown that although few SPANI is released in both acidic medium and pure water, the release rate of SPANI is remarkably improved in 0.1 M NaOH solution. In acidic or neutral medium, there exists strong attractive interaction between SPANI chains which promotes aggregation and prevents them from releasing, so the addition of alkali must destroy aggregates of SPANI chains and single SPANI chains can diffuse through the tunnel out to the surrounding solution. In alkaline medium SPANI become polyanions because the sulfonate acid groups was neutralized and the imine group on the SPANI backbone is undoped, the aggregates of SPANI
alkalPAA1* alkaliPAA2* alkaliPAA3* alkaliPAA4* alkaliPAA5* alkaliPAA6*
0.011 0.010 0.009
concentration of solution (gram/liter)
1347
0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000 0
20
40
60
80
100
120
time(day)
Fig. 5. Time dependence of concentration of SPANI in solutions of pH 13 (time is measured from the PAA* gel is put into solution).
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Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
Table 2 Variation of swelling degree and proportion of retained SPANI during release Sample
Cg (g/L)
C (g/L)
Cg/C
m/m0 initial
m/m0 110 days
min/mout
h
h0
PAA1*
0.0013 0.00125 0.00147 0.00152 0.00113 0.01
0.28 0.39 0.10 2.0 2.0 0.01
215 310 71.0 1320 1770 1.0
42.3 16.6 56.4 22.6 21.6 233.7
100.9 59.2 +1 55.3 42.8 +1
1.37 1.44 0.809 13.2 15.4 0.017
44.8 87.7 51.9 42.9 42.9 42.9
357 643 357 320 320 160
PAA2* PAA3* PAA4* PAA5* PAA6*
Dependence of the swelling degrees of the PAA*1 and PAA*2 gels on time during release are shown in Fig. 6. As the stiff chains were released into the outside solution, the swelling ratio of the gel increases gradually. The swelling of PAA* is the consequence of two processes: first, SPANI is neutralized by NaOH and changes to polyanion, osmotic pressure in the gel increase due to electrostatic repulsion between polyanions; second, because aggregates of SPANI chains involving PAA primary chains act as effective crosslink, so the collapse of aggregates of SPANI chains decrease the effective crosslinking density. So swelling of PAA* gels favor release process. Another experiment is carried out in the opposite direction, a pure PAA gel was put into alkaline aqueous solution with the SPANI concentration 0.5 g/L, to let SPANI in the surrounding solution to diffuse into gel sample if possible. We can estimate the concentration of SPANI in the gel by color, the deeper the color, the more concentrated SPANI in the gel. After 60 days the concentration of SPANI in gel was found to be about 0.001 g/L, far below that of surrounding solution. The gel was light purple and transparent as pure PAA gel, in contrary, gel with in-site trapped SPANI was deep
110
PAA1* PAA2*
variation of swelling ratio
100 90 80 70 60 50 40 30 20 10 0
10
20
30
40
50
60
dipping time(days)
Fig. 6. Variation of swelling ratio of gels when SPANI releasing outsides. Upper is PAA1*, lower is PAA2*.
black and opaque. And there is not any colour gradient observed through cross section of gel. From this fact, it can be deduced that only the oligomer of SPANI can diffuse into the network, and the majority of SPANI in the surrounding solution can not diffuse into pure PAA gel. 3.6. Kinetics of the release of polyelectrolyte rods from the gel The kinetic curves of the release of polyelectrolyte from the gels are presented in Fig. 6. from these data we try to evaluate the diffusion coefficient of SPANI in the gel. With the time of diffusion going, diffusion layer is developed from surface of the gel to the interior of the gel. During the time when the diffusion layer has not developed throughout the whole gel, the variation of concentration of SPANI in the diffusion layer accord with FickÕs law, ocSPANI o2 cSPANI ¼D ot ox2 a boundary condition is that the bulk concentration of SPANI of the surrounding solution is equal to the surface concentration of the gel. In the early period of diffusion, the diffusion layer has not developed throughout the whole gel, so that the time dependence of bulk concentration of SPANI of the surrounding solution can be simplified as [14] rffiffiffiffiffi Ct Dt ¼2 Ce p where Ct is bulk concentration at time of t, Ce is equilibrium bulk concentration, D is the diffusion coefficient of SPANI in the gel. Table 3 is calculated diffusion coefficient of SPANI in the gel for PAA1*–PAA6*. Data in Table 3 reflects some regulation more or less. Magnitude of diffusion coefficient increases with Concentration of acrylamide, and decreases with concentration of BAA in polymerization. This is reasonable for gel system. Diffusion coefficient also decrease with the initial concentration of SPANI in gel, this can be attributed to the increase of interaction with concentration of SPANI.
Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
1349
Table 3 Diffusion coefficient D of SPANI in 0.1 M alkali solution (dimension of D: m2/day) Sample D
PAA1*
PAA2* 7
2.59 · 10
PAA3* 8
1.76 · 10
Another question is the thermodynamic equilibrium between single SPANI chains and aggregates of SPANI, if it was that. As neutralized by alkaline, the aggregation state of SPANI changed in two aspects: one is the change of compacted degree of stiff chains in aggregates, even disassembly of aggregation; the other is equilibrium between aggregates and dissociated stiff chains. Compared with that in acidic solution, the aggregates become looser and the relative amount of dissociated stiff chains increases, thus promoting the rate of release. Further investigation should be done lately.
4. Conclusion A new type of semi-interpenetrating network system consisting of a flexible polyacrylamide hydrogel with embedded linear polyelectrolyte, sulfonated polyaniline (SPANI), is successfully prepared. Incorporation of polyelectrolyte SPANI into the uncharged network results in pH-responsive swelling degree. It can be deduced that molecules of SPANI is almost all in the state of aggregation in the acidic or neutral aqueous solution, seldom SPANI chains is dissociated. The dependence of swelling degree on pH value results from that the interplay of two main force between SPANI chains: repulsive electrostatic interactions which separate polyelectrolyte chains from each other, and attractive interactions of pole–pole interaction, hydrogen bonding, and poor solution condition which tend to make them condensate. From the fact that uncharged network can responsive to pH value, it is obviously implied that SAPNI chains interact strongly with network chains, although they are not covalently attached. Release kinetics of SPANI in aqueous solutions of different pH
PAA4* 5
3.69 · 10
PAA5* 5
1.26 · 10
5.25 · 106
values has proven the deduce above, that SPANI chains are effectively retained by the gel in acidic or neutral environment; in alkaline environment, the aggregates of SPANI chains collapse and the entrapped single SPANI chains are able to release gradually. The rate of release is increased with the initial concentration of SPANI in the gel, which implies that the release of SPANI obey FickÕs diffusion law in some degree.
References [1] Wallace DG, Rosenblatt J. Adv Drug Delivery Rev 2003;55(12):1631–49. [2] Chen L, Tian Z, Du Y. Biomaterials 2004;25(17): 3725–32. [3] Murdan S. J Control Release 2003;92(1–2):1–17. [4] Coviello T, Grassi M, Lapasin R, Marino A, Alhaique F. Biomaterials 2003;24(16):2789–98. [5] Ostanevich YuM. Macromol Chem Macromol Symp 1988;15:91. [6] Svengun DI, Feigin LA. X-ray and neutron small-angle scattering. Moscow: Nauka; 1986. [7] MacDiarmid AG, Chiang JC. Mol Cryst Liq Cryst 1985;121:187. [8] Yue J, Gordon G, Epstein AJ. Polymer 1992;33:4410. [9] Yang D, Adams PN, Mattes BR. Synth Metals 2001;119:301. [10] Philippova OE, Wargner G, et al. Macromolecules 1998;31:1168. [11] Manning GS. Acc Chem Res 1979;12:443. [12] Liao Y-H, Kwei TK, Levon K. Macromol Chem Phys 1995;196:3107. [13] Zheng W, Angelopoulos M, Epstein AJ, MacDiarmid AJ. Macromolecules 1997;30:2953. [14] Wang J-Sh. Principle of dying and finishing. Beijing: Textile Industry Press; 1984. p. 10.
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Polyacrylamide hydrogels with trapped sulfonated polyaniline Y. Tao *, J.X. Zhao, C.X. Wu State Key Laboratory of Fiber Modification, College of Materials Science and Engineering, Dong Hua University, Yanan Road West, 1882, Shanghai 200051, PR China Received 29 June 2004; received in revised form 9 October 2004; accepted 18 January 2005 Available online 17 February 2005
Abstract A new type of semi-interpenetrating network system consisting of a flexible polyacrylamide hydrogel with embedded stiff-chain linear polyelectrolyte, sulfonated polyaniline (SPANI), has been investigated. It is shown experimentally that the incorporation of stiff-chain polyelectrolyte inside the uncharged PAA network brings different swelling behavior from the pure PAA gel. Environmental pH values have remarkable influence to the swelling degree of this new gel system, the maximum swelling degree, appearing at the environmental pH value of about 12, can be 17 times greater than that at pH 1, and 12 times greater than that at pH 14. Release kinetics of SPANI has shown that SPANI is effectively retained by the gels in acidic or neutral environment, although they are not covalently attached to the network chains; but in alkaline environment, the entrapped SPANI molecules will be released into the surrounding solution gradually. The diffusion coefficient of SPANI is calculated from the experimental data. The experimental phenomena are tried to interpreted from aspects of aggregation and hydrogen bonding between SPANI chains. 2005 Elsevier Ltd. All rights reserved. Keywords: Polyacrylamide; Hydrogel; Polyaniline
1. Introduction The diffusion and release of solute from gel is an elemental question for the potential application of gel system such as Controlled drug delivery and membrane separation [1–4]. In order to satisfy these functions, effective pore size of the network should be responsive to the external stimulation of pH value, temperature, and electrical field, and thus the diffusion rate of solute from the gel could be controlled. Besides this, other fac-
* Corresponding author. Tel.: +86 21 6237 4607; fax: +86 21 623 73860. E-mail address: [email protected] (Y. Tao).
tors which influence the diffusion of solute is interaction between solute and network chains, as well as formation of Aggregates of Polyelectrolyte solute [10]. This kind of interaction, which is complicated and difficult to characterize, as well as its influence to the diffusion process, should be investigated as a important issue. The state of polyelectrolyte results from the interplay of two main force: repulsive electrostatic interactions which separate polyelectrolyte chains from each other, and attractive interactions between polymer units in poor solvent conditions which tend to make them condensate. For the gel system with trapped polyelectrolyte, direct electrostatic interactions become essential, along with the osmotic pressure of countions. The most important of them are the condensation of countions and the chain stiffening due to the repulsion of the similarly
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.01.006
Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
charged units along the chains (characterized by an electrostatic persistent length) [5,6]. The aim of the present paper is to study the properties of a stiff-chain polyelectrolyte/flexible hydrogel system on the basis of a neutral polyacrylamide (PAA) gel with the embedded stiff-chain polyelectrolyte of sulfonated polyaniline (SPANI). The state of SPANI and its interaction with PAA network chains will be preliminarily studied from the swelling and release behavior of this new gel system.
2. Experimental 2.1. Synthesis and characterization of sulfonated polyaniline Polyaniline of emeraldine base (EB) form is synthesized by the traditional method [7]. Self-doping sulfonated polyaniline (SPANI) was synthesized according to Ref. [8]. In a typical procedure, EB (0.5 g) was sulfonated by dissolving EB Powder in 40 mL of 30% fuming sulfuric acid which was previously cooled to 5 C with constant stirring in ice-water. After addition of the polymer, the solution was stirred for 10 min in the ice bath. Subsequently, the solution was removed from the ice bath and allowed to reach room temperature while stirring was continued (5–10 min). The colour of the solution changed from dark purple to dark blue during 0.5 h at room temperature. The solution was then slowly added during 20 min to methanol (200 mL) to precipitate most of the product, the temperature during this step being hold between 10 C and 20 C via ice bath. The green powder was then collected on a buchner funnel and the precipitate cake was washed at least 10 times with 50 mL portions of methanol until the filtrate had a pH of 7 when tested by wet pH paper. It was then permitted to remain under suction for 10 min; the filter cake was then transferred on the filter paper to a vacuum desiccator and dried under dynamic vacuum for 24 h at room temperatures.
[
[
SO 3H H
H
N
N
+ SO 3 Na H
H
N
N
1343
The final product, sulfonated polyaniline (SPANI) can be dissolved in 0.1 M ammonium aqueous solution or 0.1 M NaOH solution, forming deep purple solution. Intrinsic viscosity measurement [9] of dilute emeraldine base was carried out to evaluate the weight average molecular weight of PANI from which SPANI was made, based on Mark–Houwink relation, ½g ¼ KM aw , here K = 1.2 · 104 and a = 0.77 [9]. From experiment data [g] = 0.31, the resulting Mw of emeraldine base is about 27,000 g/mol. Then the prepared SPANI is reduced by aqueous solution of hydrazine in order to estimate the Mw of SPANI by light scattering experiments. The resulting Mw of SPANI is about 16,000 g/mol, which show that polyaniline has undergone some degradation during sulfonating. Elemental analysis for SPANI gives a S/N atomic ratio of 0.48 indicating that approximately one –SO3H group is linked on the 2-position of the phenyl ring for every two phenyl rings. The structure of SPANI can be regarded as shown in Fig. 1. 2.2. Gel preparation. Polyacrylamide gels with trapped SPANI The gels were prepared by free-radical polymerization of acrylamide in aqueous solutions of SPANI with N,N 0 -methylene bis-acrylamide (BAA) as crosslinker. Ammonium persulfate (2.1 · 103 mol/L) and N,N,N 0 ,N 0 -tetramethyl ethylene diamine (TEMED) (2.1 · 103 mol/L) were used as initiator and accelerator. The stoichiometric concentrations of acrylamide monomer, SPANI, and BAA for different gel samples are listed in Table 1. The gel synthesis were carried out in cylindrical glass tubes at room temperature for 24 h. The pure polyacrylamide hydrogel as reference was prepared in the same way, except without adding of SPANI. In the following, the polyacrylamide gels which are synthesized in aqueous solutions of SPANI are marked with an asterisk (PAA1*–PAA6*). The reference polyacrylamide gels, which are prepared under similar condition but without adding of SPANI, are designated as PAA1–PAA6. SO 3H N
N
]n
SO 3 Na+ N
N
]n
Fig. 1. Structure of sulfonated polyaniline (SPANI), neutration by sodium hydroxide.
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Table 1 Composition of the gels Sample
Solvent
C 0M (g/L)
C 0BAA (mol% to AA)
C0 SPANI (g/L)
Equilibrium m/m0
PAA1*
Water Water Water Water Water Water Water Water Water Water Water Water
50 90 50 90 90 90 50 50 50 90 90 90
2% 1% 1% 1% 2% 2% 1% 0.6% 2% 1% 0.6% 2%
1.0 1.0 1.0 2.1 2.1 4.0 0 0 0 0 0 0
42.33 16.56 56.37 22.6 21.6 233.7 52.9 95.5 37.5 20.90 32.7 14.3
PAA2* PAA3* PAA4* PAA5* PAA6* PAA1 PAA2 PAA3 PAA4 PAA5 PAA6
In this table, the equilibrium swelling degree of PAA* and PAA gel is measured in distilled water of pH 7. The concentration of BAA is characterized as the molar ratio of BAA to monomer acrylamide (AA).
2.3. Gel swelling measurement
3. Results and discussion
The swelling degree of the gel samples was characterized by the ratio m/m0, where m is the mass of the gel sample swollen in aqueous solution and m0 is the mass of the dry gel. Gel samples were also swollen in a series of aqueous solution of different pH values, in order to study the influence of pH value to swelling equilibrium.
To incorporate charged chains in a polyacrylamide gel, the polymerization of acrylamide together with a crosslinking agent was performed in a solution containing neutralized SPANI. As a result of the network formation, most of the SPANI chains become trapped by the gel. Too much amount of polyelectrolyte SPANI seems disadvantageous to the forming of network. When the concentration of SPANI exceeding 4 g/L, very soft gel of poor mechanical strength is formed, implying that imperfect network formation. It can be assumed that too much SPANI would react with and annihilate the active radicals, because SPANI has reduction units on it. As Yue et al. [8] pointed out, SPANI (emeraldine base form) has sulfonation level, i.e. the atomic ratio of sulfur and nitrogen, of about 0.5 in average, independent of the sulfonation time over periods from 0.5 to 24 h. This means that only half the rings are sulfonated. This is consistent with the fact that only half the rings need to be sulfonated in order to produce the stable polysemiquinone form of the polymer. As Fig. 2 shows, at different pH values, SPANI can exist as polycations (in acid), polyampholytes (the SPANI can be self-doped by the sulfonate acid group on its chain), and polyanions (in alkali). In different states, the nature of interaction between SPANI chains differs, is determined by which is prevailing, attraction or repulsion.
2.4. Kinetic study of SPANI release To study the release of SPANI from the gel, the gel samples just after the preparation were put to swell in acidic, neutral and alkaline aqueous solution respectively. Unless otherwise specified, the volume of solution was 600 mL per 5 g of the wet gel. During the experiment the solution with the gel specimen was slightly agitated by a glass rod every day. Every three days, the solutions surrounding the gel are sampled and then the concentration of SPANI is measured by Perkin-Elmer Ambda Bio-40 UV–VIS spectrometer (Cole-Parmer L-97600-05 long wave lamp, 20 mW/cm2), at maxima absorption wavelength 315 nm. In all the experiments distilled water was used. The content of SPANI in the gel phase was evaluated by subtraction of the amount of SPANI in the surrounding solution from the initial amount of SPANI. The fraction of the SPANI in the gel was characterized by the molar ratio between SPANI and PAA repeat units in the gel, denoted as h. The initial molar ratio of SPANI and PAA repeat units in the system is designated as h0. The efficiency of the retaining of SPANI by the gel was characterized by the ratio Cg/C between the concentrations of SPANI in the gel, Cg, and in the surrounding solution, C.
3.1. Gels swollen in water Examination of the equilibrium swelling degree of the gels in water (Table 1) shows that the incorporation of the SPANI into the uncharged gel results in increase of swelling degree. This is a consequence of the osmotic pressure exerted by charged SPANI chains and their counterions, though it is not remarkable. As one might
Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349 polyanion
Na SO3
[
+ H
H
N
N
1345
Na + SO3 N
N
]n
+NaOH SO3
[
H
H
N
N
SO3 + N
+ N
H
H
]n
+HCl
polycation SO3H H
H
N
N
[
SO3H + N
+ N
H
H
]n
Fig. 2. Structure of SPANI under various environmental pH values.
expect, the swelling of PAA* gels increases also with the lowering of the crosslink density. 3.2. Gels swollen in aqueous solutions of different pH values Fig. 3 shows the equilibrium swelling degrees of several PAA* samples in solutions of different pH values. Obviously, for pure PAA gels the swelling degree is the same whether in water or in solutions of different pH values. The situation is different for PAA* containing stiff-chain SPANI. The swelling degree of these gels increase remarkably with equilibrium pH value increasing and reach a maximum at pH = 12, then level off. 220
PAA1* PAA2* PAA3* PAA4* PAA5*
equilibrium swelling ratio m/m0
200 180 160 140 120 100 80 60 40 20 0 0
2
4
6
8
10
12
14
pH value
Fig. 3. Dependence of equilibrium swelling degree of five PAA* gels on the environmental pH values (pH values are measured when gel samples have reached equilibrium state with surrounding solution).
For PAA2* sample the maximum swelling degree appearing at pH = 11.7 is as high as 200, about 17 times greater than at pH = 1, also 12 times greater than at pH = 14. The leveling off of swelling degree in low and high pH value region reminds us of the usual deswelling of polyelectrolyte gels in salt solutions. The remarkably excessive amount of acid at low pH values (pH < 7) or excessive amount of alkali at high pH values (pH > 12) acts as the same role as low molecular weight salts, which would shield the charges localized on the polyelectrolyte chains. Except for PAA3*, the equilibrium swelling degrees of all PAA* gel samples at low pH values (pH < 7) are almost the same as that at pH = 7, as shown in Fig. 3. At pH = 7, there is no any low molecular weight electrolyte in solution, and at pH values lower than 3, the concentration of HCl is considerably high enough to cause deswelling, if HCl acted as the same role as low molecular weight salt, such as NaCl. This implies that for PAA* samples, the SPANI chains have already assembled into compacted aggregates even in water, so the electrostatic screening of low molecular weight electrolyte can not cause further contraction. More abnormality occurred on PAA3* sample, whose swelling degree in solution of high pH values is greater than that in pure water. For PAA* samples, the remarkably increase of swelling degree at pH = 12 and then deswelling at even higher pH value suggests that, although not covalently attached to the network, the SPANI chains should interact strongly with elastically active network chains. The disintegration and reassemble of aggregates of SPANI caused by pH values should be the driving force for varying swelling degree of PAA* samples, which will be discussed about in the following.
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3.3. Aggregation of SPANI chains
in acidic solution nuetral in alkali solution
90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10
ecules. Experimental evidence for hydrogen bonding in PAA2* samples has been given by infrared studies on a Perkin-Elmer Nicolet infrared spectrometer, shown in Fig. 4. According to Ref. [13], there are two bands associated with the N–H stretching vibrations-one band located at about 3190 cm1 (hydrogen bonded N–H stretching) and another band located at about 3440 cm1 (non-hydrogen bonded). When PAA2* samples were dipped in concentrated acidic solution to equilibrium, the 3190 cm1 band become more intense than the 3440 cm1 band; when dipped to equilibrium in pure water and alkali solutions, the 3190 cm1 band can be negligible compared with intense 3440 cm1 band. This fact implies that intensity of hydrogen bonding varies with pH values. Hydrogen bonding is a kind of strong force that can results in aggregates of SPANI chains. The various intensity of hydrogen-bonding can further explain the contraction in acidic environment and swelling in alkali environment. These specific results do not distinguish whether the hydrogen bonding is formed between adjacent SPANI molecules or between SPANI and PAA molecules. From the fact shown in the following that the PAA* samples release easily the SPANI molecules to the surrounding alkali solution, but can retain almost all the entrapped SPANI in acidic solutions and pure water, it would be deduced that interaction between SPANI and PAAM molecules is strong in acid and weak in alkali.
3.4. Hydrogen bonding in gel
3.5. Release of SPANI chains from the gel
Besides the electrostatic interaction, another interaction of SPANI chains, as several authors [12,13] have pointed out, should be hydrogen bonding between the imine and amine nitrogen sites on adjacent SPANI mol-
Being immersed in aqueous solutions, the PAA* gels should lose a part of the SPANI chains under proper condition, because of their diffusion to the surrounding solution. Many factors are found to influence this
%Transmittance
As is shown above, there is not obvious difference for the swelling degree of PAA* gels in pure water or in strong acidic solution. But for another interpenetrating network system of PAA/polystyrene sulfonate (PSS) [10], the equilibrium swelling degree decreases as much as 31% when moved from pure water to 0.2 M NaCl solution, which suggests collapse of PSS in salt solution. This discrepancy can be attributed to the partial condensation of stiff-chain SPANI in the PAA* gel. According to ManningÕs theory [11], the condensation of counterions in water at room temperature begins when the distance between charged groups is lower than ˚ . As the number of charged groups approximately 7.0 A varies at different pH values, the average charge spacing ˚ in acidic or neuof SPANI differs, in details, about 5 A ˚ in alkaline solution. So tral aqueous solution, and 10 A condensation of counterions occurs in acidic or neutral solution, and all the charged groups on SPANI chains are free in alkaline solution. In acidic or neutral solution, the condensation may be due to the aggregation of several SPANI molecules in a rod, and a certain fraction of counterions becomes condensed in the vicinity of the rod by a strong electric field. For the interaction between polyion chains, electrostatic repulsion should be considered, in the meanwhile, the factors of attraction, such as dipole–dipole interaction, and hydrophobic effect, also should be under consideration. If effect of attraction prevailing, polyion chains would exist in form of aggregates. Based on this opinion, it is easy to interpret experimental phenomena. In acidic or neutral aqueous solution, the SPANI is in the form of polyampholyte, there would be strong dipole–dipole interaction between SPANI molecules, so it is reasonable to consider that SPANI tend to join in aggregates. A single SPANI chain can cross over from one aggregate to another acting as crosslinks, The further aggregation of SPANI chains in concentrated acid solution would compensate the additional osmotic pressure of SPANI and their counterions. This would explain why the swelling degree of PAA* gels are almost the same as PAA sample, and the swelling degree of PAA* samples in pure water are almost the same as in concentrated acid solutions. Alternatively, in alkaline environment, the undoped SPANI becomes polyanion and repulsive interaction prevailing, so the aggregates of SPANI collapse and effective crosslinking decrease, leading to increase of swelling degree.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-1
Wavenumber(cm )
Fig. 4. Diffuse reflectance FTIR spectrum of dried PAA2* gel samples. Thick straight line: dipped with 0.1 M HCl solution, fine straight line: dipped with pure water, dashed line: dipped with alkali solution.
Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
chains are destroyed by the electrostatic repulsion between the similarly charged SPANI chains.
process: the crosslinking density of the gel, the concentration of acrylamide units and concentration of polyelectrolyte chains at the network preparation, and the acidity of the surrounding environment.
The fraction of SPANI released in the surrounding solution, h, and the ratio of concentration Cg/C of the SPANI chains in the gel and in the solution (PAA3* sample has approached final equilibration) are presented in Table 2. From this table it is shown that the gel retains a major part of the SPANI chains, the initial molar ratio between SPANI and PAA gel repeat units, h0, seems to has not remarkable influence to the retain of SPANI by gels. The fact that most of the SPANI chains do not diffuse to the external solution (pH < 7) but become trapped in the gel may be resulted from that the experimental time scale is short and the diffusion rate of SPANI chains is not high, since the average length of the rods (about 150 nm) is more than 10 times larger than the mesh size of the gel (12 nm). Besides, aggregation involving the network chains, can also leads to the effective immobilization of the SPANI chains. The increase of both the degree of crosslinking and the concentration of acrylamide units at the network formation do favor to the retaining of stiff SPANI chains, the effect of the concentration of the monomer units being more pronounced, implying increasing entanglement between PAA chains and SPANI chains. A twofold increase of the concentration of monomer (cf. PAA2* and PAA4* gels) leads to an increase of Cg by a factor of 5, while a twofold increase of the crosslinking density results in a increase of Cg by 2.8. The initial concentration of SPANI do increase the release rate from the increasing slope of concentration–time curves.
(A) Release in pure water and acidic solution. It is found experimentally that there is seldom SPANI released from gel when the PAA* gel is dipped in acidic or neutral aqueous solution. This can be explained with the extensive aggregation of SPANI in pure water or acidic aqueous solution. The aggregation of SPANI is enhanced by the strong interaction between proton-doped SPANI chains. It is implied that in pure water or acidic aqueous solution, all SPANI chains have joined aggregates, whose large size and strong interaction with PAA prevent them from releasing. (B) Release in alkaline solution. The influence of the addition of alkali (0.1 M NaOH) on the release of SPANI is illustrated in Fig. 5. It is shown that although few SPANI is released in both acidic medium and pure water, the release rate of SPANI is remarkably improved in 0.1 M NaOH solution. In acidic or neutral medium, there exists strong attractive interaction between SPANI chains which promotes aggregation and prevents them from releasing, so the addition of alkali must destroy aggregates of SPANI chains and single SPANI chains can diffuse through the tunnel out to the surrounding solution. In alkaline medium SPANI become polyanions because the sulfonate acid groups was neutralized and the imine group on the SPANI backbone is undoped, the aggregates of SPANI
alkalPAA1* alkaliPAA2* alkaliPAA3* alkaliPAA4* alkaliPAA5* alkaliPAA6*
0.011 0.010 0.009
concentration of solution (gram/liter)
1347
0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000 0
20
40
60
80
100
120
time(day)
Fig. 5. Time dependence of concentration of SPANI in solutions of pH 13 (time is measured from the PAA* gel is put into solution).
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Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
Table 2 Variation of swelling degree and proportion of retained SPANI during release Sample
Cg (g/L)
C (g/L)
Cg/C
m/m0 initial
m/m0 110 days
min/mout
h
h0
PAA1*
0.0013 0.00125 0.00147 0.00152 0.00113 0.01
0.28 0.39 0.10 2.0 2.0 0.01
215 310 71.0 1320 1770 1.0
42.3 16.6 56.4 22.6 21.6 233.7
100.9 59.2 +1 55.3 42.8 +1
1.37 1.44 0.809 13.2 15.4 0.017
44.8 87.7 51.9 42.9 42.9 42.9
357 643 357 320 320 160
PAA2* PAA3* PAA4* PAA5* PAA6*
Dependence of the swelling degrees of the PAA*1 and PAA*2 gels on time during release are shown in Fig. 6. As the stiff chains were released into the outside solution, the swelling ratio of the gel increases gradually. The swelling of PAA* is the consequence of two processes: first, SPANI is neutralized by NaOH and changes to polyanion, osmotic pressure in the gel increase due to electrostatic repulsion between polyanions; second, because aggregates of SPANI chains involving PAA primary chains act as effective crosslink, so the collapse of aggregates of SPANI chains decrease the effective crosslinking density. So swelling of PAA* gels favor release process. Another experiment is carried out in the opposite direction, a pure PAA gel was put into alkaline aqueous solution with the SPANI concentration 0.5 g/L, to let SPANI in the surrounding solution to diffuse into gel sample if possible. We can estimate the concentration of SPANI in the gel by color, the deeper the color, the more concentrated SPANI in the gel. After 60 days the concentration of SPANI in gel was found to be about 0.001 g/L, far below that of surrounding solution. The gel was light purple and transparent as pure PAA gel, in contrary, gel with in-site trapped SPANI was deep
110
PAA1* PAA2*
variation of swelling ratio
100 90 80 70 60 50 40 30 20 10 0
10
20
30
40
50
60
dipping time(days)
Fig. 6. Variation of swelling ratio of gels when SPANI releasing outsides. Upper is PAA1*, lower is PAA2*.
black and opaque. And there is not any colour gradient observed through cross section of gel. From this fact, it can be deduced that only the oligomer of SPANI can diffuse into the network, and the majority of SPANI in the surrounding solution can not diffuse into pure PAA gel. 3.6. Kinetics of the release of polyelectrolyte rods from the gel The kinetic curves of the release of polyelectrolyte from the gels are presented in Fig. 6. from these data we try to evaluate the diffusion coefficient of SPANI in the gel. With the time of diffusion going, diffusion layer is developed from surface of the gel to the interior of the gel. During the time when the diffusion layer has not developed throughout the whole gel, the variation of concentration of SPANI in the diffusion layer accord with FickÕs law, ocSPANI o2 cSPANI ¼D ot ox2 a boundary condition is that the bulk concentration of SPANI of the surrounding solution is equal to the surface concentration of the gel. In the early period of diffusion, the diffusion layer has not developed throughout the whole gel, so that the time dependence of bulk concentration of SPANI of the surrounding solution can be simplified as [14] rffiffiffiffiffi Ct Dt ¼2 Ce p where Ct is bulk concentration at time of t, Ce is equilibrium bulk concentration, D is the diffusion coefficient of SPANI in the gel. Table 3 is calculated diffusion coefficient of SPANI in the gel for PAA1*–PAA6*. Data in Table 3 reflects some regulation more or less. Magnitude of diffusion coefficient increases with Concentration of acrylamide, and decreases with concentration of BAA in polymerization. This is reasonable for gel system. Diffusion coefficient also decrease with the initial concentration of SPANI in gel, this can be attributed to the increase of interaction with concentration of SPANI.
Y. Tao et al. / European Polymer Journal 41 (2005) 1342–1349
1349
Table 3 Diffusion coefficient D of SPANI in 0.1 M alkali solution (dimension of D: m2/day) Sample D
PAA1*
PAA2* 7
2.59 · 10
PAA3* 8
1.76 · 10
Another question is the thermodynamic equilibrium between single SPANI chains and aggregates of SPANI, if it was that. As neutralized by alkaline, the aggregation state of SPANI changed in two aspects: one is the change of compacted degree of stiff chains in aggregates, even disassembly of aggregation; the other is equilibrium between aggregates and dissociated stiff chains. Compared with that in acidic solution, the aggregates become looser and the relative amount of dissociated stiff chains increases, thus promoting the rate of release. Further investigation should be done lately.
4. Conclusion A new type of semi-interpenetrating network system consisting of a flexible polyacrylamide hydrogel with embedded linear polyelectrolyte, sulfonated polyaniline (SPANI), is successfully prepared. Incorporation of polyelectrolyte SPANI into the uncharged network results in pH-responsive swelling degree. It can be deduced that molecules of SPANI is almost all in the state of aggregation in the acidic or neutral aqueous solution, seldom SPANI chains is dissociated. The dependence of swelling degree on pH value results from that the interplay of two main force between SPANI chains: repulsive electrostatic interactions which separate polyelectrolyte chains from each other, and attractive interactions of pole–pole interaction, hydrogen bonding, and poor solution condition which tend to make them condensate. From the fact that uncharged network can responsive to pH value, it is obviously implied that SAPNI chains interact strongly with network chains, although they are not covalently attached. Release kinetics of SPANI in aqueous solutions of different pH
PAA4* 5
3.69 · 10
PAA5* 5
1.26 · 10
5.25 · 106
values has proven the deduce above, that SPANI chains are effectively retained by the gel in acidic or neutral environment; in alkaline environment, the aggregates of SPANI chains collapse and the entrapped single SPANI chains are able to release gradually. The rate of release is increased with the initial concentration of SPANI in the gel, which implies that the release of SPANI obey FickÕs diffusion law in some degree.
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