Synthesis of two AMPS-based polymerizable room temperature ionic liquids and swelling difference between their co-polymeric gels with HEMA

2.1 Materials

AMPS, BA, DMAEMA, ammonium persulfate (APS), HEMA and MBAm were commercially obtained from Sigma-Aldrich (Shanghai, China); they were used as received without further purification; the acetone was of analytical reagent grade.

2.2 Synthesis of AMPS-BA and AMPS-DMAEMA

A total of 20.00 g of acetone was put into a flask, followed by the addition of 5.00 g of AMPS. Under stirring, 1.77 g of BA or 3.80 g of DMAEMA was added to the flask in drops. When the suspended white powder of AMPS in acetone disappeared and the solution became clear, the stirring was stopped and the solution was filtered, followed by evaporation of the filtrate under a reduced pressure at room temperature, to produce a transparent and pale yellow liquid.

2.3 Characterizations of AMPS-BA and AMPS-DMAEMA

Differential scanning calorimetry (DSC) analysis was performed for AMPS-BA and AMPS-DMAEMA using the DSC1 instrument from Mettler Toledo. The measurement conditions were as follows: scanning rate, 10°C/min; nitrogen gas flow rate, 50 ml/min.

2.4 Synthesis of the gels

A total of 1.2 g of AMPS-BA or AMPS-DMAEMA, 2.8 g of HEMA, 8.0 mg of MBAm and 10.0 mg of APS were dissolved in 6.0 g of water successively to produce a reaction solution. The weight ratio of AMPS-BA or AMPS-DMAEMA with HEMA was varied without changing the dosage of MBAm and APS, to produce different reaction solutions with the aim of preparing gels with different compositions. The reaction solution was heated in a water bath at 50°C for 24 h. The resulting hydrogels were immersed in distilled water for 7 days, and the water was changed every 24 h to remove water-soluble materials, followed by drying of the hydrogels in an oven at 105°C to produce xerogels.

The weight of xerogels was measured; the yield of the xerogels (Y_x,%) was calculated as follows:

Yx=WxWm×100%

Where W_x (g) is the weight of the xerogels and W_m (g) is the weight of the monomers.

2.5 Absorbency of the gels for different liquids

The xerogel of poly(AMPS-BA-co-HEMA) or poly(AMPS-DMAEMA-co-HEMA) was immersed in water and various organic solvents to test the swelling characteristics. The absorbency of the xerogels for the liquids was measured by a gravimetric method. The samples were made to swell with distilled water and various organic solvents at room temperature until the swelling equilibrium was reached, followed by removal and blotting with filter paper to remove the overloaded liquids on the surface and then weighed. The absorbency of the xerogels for the liquids was calculated as follows:

Absorbency (Q,g/g)=Ww-WdWd

Where W_w (g) is the weight of the swollen gels and W_d (g) is the weight of the xerogels.

3.1 Synthesis of AMPS-BA and AMPS-DMAEMA

Being a strong organic acid, AMPS is insoluble in acetone; the white powder of AMPS was suspended in the solvent under stirring. After the addition of BA or DMAEMA to the solvent, the white powder of AMPS disappeared. A transparent and sticky pale yellow liquid was obtained after the evaporation of the solvent under a reduced pressure at room temperature; DSC data showed that the glass transition temperature of AMPS-BA was -47.7°C and that of AMPS-DMAEMA was -45.8°C, as shown in Figures 1 and 2, respectively. The reaction of AMPS with BA or DMAEMA in acetone to produce AMPS-BA and AMPS-DMAEMA RTILs is expressed in Scheme 2.

Figure 1

DSC characterization of AMPS-BA.

Figure 2

DSC characterization of AMPS-DMAEMA.

Scheme 2

Preparation of AMPS-BA and AMPS-DMAEMA.

3.2 Synthesis of the gels

Poly(AMPS-BA-co-HEMA) and poly(AMPS-DMAEMA-co-HEMA) gels were prepared by free-radical aqueous solution co-polymerization of the RTILs, MBAm and HEMA. The resulting poly(AMPS-BA-co-HEMA) and poly(AMPS-DMAEMA-co-HEMA) hydrogels were transparent and glassy. The formation of the cross-linked networks of poly(AMPS-BA-co-HEMA) and poly(AMPS-DMAEMA-co-HEMA) is expressed in Schemes 3 and 4, respectively.

Scheme 3

Synthesis of the poly(AMPS-BA-co-HEMA) gels.

Scheme 4

Synthesis of the poly(AMPS-DMAEMA-co-HEMA) gels.

It has been found that the yields of both xerogels were between 99.35 and 100.8%, irrespective of the weight ratio of the polymerizable ionic liquids and HEMA, which means that all polymerizable materials became a part of the xerogels. The cross-linked networks were well formed, and small water-soluble polymers formed during the polymerization.

3.3 Swelling characteristics of the gels in water and organic solvents

The swelling characteristics of poly(AMPS-BA-co-HEMA) and poly(AMPS-DMAEMA-co-HEMA) gels in water and various organic solvents were investigated. It showed that, although the synthesized poly(AMPS-DMAEMA-co-HEMA) hydrogels were transparent and glassy, the xerogels did not swell in any liquids at all, irrespective of water or organic solvents. Further investigation showed that, without a dehydration process, the synthesized poly(AMPS-DMAEMA-co-HEMA) hydrogels did not swell in any liquids either. On the contrary, the poly(AMPS-BA-co-HEMA) gels exhibited superabsorbency not only in water, but also in various polar and non-polar organic solvents. The superabsorbency of the poly(AMPS-BA-co-HEMA) gels with a composition of 30% RTIL, 70% HEMA and 0.20% MBAm (calculated by the gross weight of the RTIL and HEMA) was systematically investigated. Water and various conventional organic solvents, such as alcohols, chlorinated methanes, amines and acetone, acetonitrile and dimethyl sulfoxide (DMSO), were examined because they are of significant importance commercially in wide varieties of applications. Surveying and studying new materials to highly imbibe these organic solvents will be of great potential importance. The dielectric constant (ε) and the absorbency by the poly(AMPS-BA-co-HEMA) gels of these solvents are shown in Table 1.

3.4 Influence of HEMA content on the swelling characteristic

Various poly(AMPS-BA-co-HEMA) and poly(AMPS-DMAEMA-co-HEMA) gels with different compositions were prepared by changing the ratio of the polymerizable RTILs and HEMA during the preparation of the reaction solutions, to investigate the influence of HEMA content on the swelling characteristics. It was found that poly(AMPS-DMAEMA-co-HEMA) gels did not swell in any liquids irrespective of the HEMA feeding, while the superabsorbency of poly(AMPS-BA-co-HEMA) gels was seriously influenced by the composition. The effect of RTIL/HEMA ratio on the superabsorbency of the resulting gels is shown in Figure 3. It was found that, without HEMA feeding, the cross-linked homo-polymer of AMPS-BA was not a transparent and glassy gel but a muddy mixture without any strength; moreover, it could not swell in any solvent. Cross-linked poly(AMPS-BA-co-HEMA) did not absorb any solvent either when the HEMA feeding was less than 20%. Increased HEMA feeding in the range of 20–60 wt.% resulted in an increased absorbency of the gels, while the absorbency of the gels decreased when the HEMA feeding continued to increase. The gels did not absorb methanol or benzyl alcohol when the HEMA feeding was more than 90 wt.%, but the gels could still swell in water and DMSO, which showed that the superabsorbency for many organic solvents of the poly(AMPS-BA-co-HEMA) gels originates from the polymerizable RTIL.

Figure 3

Effect of HEMA feeding on the absorbency of poly(AMPS-BA-co-HEMA) gels.

3.5 Mechanism of the different swelling characteristics of poly(AMPS-BA-c0-HEMA) and poly(AMPS-DMAEMA-co-HEMA) gels

It has been found that cross-linked neutral hydrophilic polymers can swell in water to produce hydrogels, such as the cross-linked polymer of HEMA, which has been widely used in the manufacture of soft contact lenses (30–34). In the same way, cross-linked neutral lipophilic polymers can swell in certain organic solvents, such as the swelling of rubber in oil (35). The swelling of cross-linked neutral polymers in certain liquid(s) originates from the compatibility of the macromolecular backbones with the solvent(s); good compatibility of the solvent(s) with the macromolecular backbones will allow the penetration of solvent molecules into the polymer networks. The swelling degree is decided by the balance between the repulsive forces among the polymer chains and the contractile forces due to stretching of elastically active networked structures in the solvent(s), which is mainly influenced by the cross-linking density (36–39). Reducing the amount of cross-linkers increases the swelling ability, but stable networks cannot form if the cross-linking density is too low. Thus, both the compatibility of the polymer chains with the solvents and the cross-linking density of the networked structures play a key role in the swelling and collapse of neutral polymer gels.

Compared with the lightly cross-linked sodium acrylate, which can absorb water up to several hundred times its dried weight, which is why it is called a superabsorbent polymer (SAP), the swelling degree of cross-linked neutral polymers is too low. The mechanism of superabsorbency for water of SAPs has been extensively discussed, and it was widely believed that the reason for such swelling is the compatibility of the polymer chains with water, while the high swelling ability of such materials in water originates from the dissociation of ion pairs on the cross-linked polymer chains. The ions fixed on the polymer chains originating from the dissociation of the ion pairs produced an electrostatic repulsion, which resulted in the expansion of the cross-linked networks with the ability of hosting plenty of water, while the freely mobile counterions that are integral to these networks produce osmotic pressure, which induces the penetration of water molecules from the outside into the networks (40–44). As the ion pairs on the polymer chains cannot become dissociated, SAP does not exhibit superabsorbency in organic solvents. Rationalized by this principle, the incorporation of polymerizable RTILs, which are melting salts at room temperature completely composed of discrete cations and anions, into the cross-linked polymer chains ought to inhibit the aggregation of the ionic partners in both water and organic liquids, which, in turn, allows RTIL-based gels to behave as superabsorbent gels irrespective of their liquid environment. However, for solvents that are not compatible with the counterions of polymerizable RTILs (the ions that are not covalently connected to the macromolecular chains), the mobility of the counterions is confined, just like the effect of counterion solubility, instead of backbone solubility, to the stability of polyelectrolyte solutions described by Alexander-Katz and Leibler (45). Therefore, it was the solubility of the counterions as well as the macromolecular backbone of poly(AMPS-BA-co-HEMA) gels that resulted in the superabsorbency for both water and some organic solvents.

However, for poly(AMPS-DMAEMA-co-HEMA) gels, both the anion AMPS^- and the cation DMAEMA⁺ are polymerizable. As a result, there were no free mobile counterions in the polymer networks, and there were both anions and cations on the macromolecular chains. There was no expansion of the cross-linked networks because of the lack of electrostatic repulsion of the macromolecular chains, as well as no osmotic pressure because of the absence of free mobile counterions, which resulted in the non-swelling behavior of poly(AMPS-DMAEMA-co-HEMA) gels in any liquids. The swelling/non-swelling mechanism of poly(AMPS-BA-co-HEMA) and poly(AMPS-DMAEMA-co-HEMA) gels is expressed in Scheme 5.

Scheme 5

Swelling/non-swelling mechanism of the poly(AMPS-BA-co-HEMA) and poly(AMPS-DMAEMA-co-HEMA) gels. (A) Expansion of the poly(AMPS-BA-co-HEMA) networks; (B) non-expansion of the poly(AMPS-DMAEMA-co-HEMA) networks.