Regular and Inverse Polyampholyte Hydrogels: A Detailed

May 6, 2016 - Two series of polyampholyte (PA) hydrogels were prepared via reversible addition–fragmentation chain transfer polymerization, followed...
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Regular and Inverse Polyampholyte Hydrogels: A Detailed Comparison Anna P. Constantinou, Marios Elladiou, and Costas S. Patrickios* Department of Chemistry University of Cyprus P.O. Box 20537 1678 Nicosia Cyprus S Supporting Information *

ABSTRACT: Two series of polyampholyte (PA) hydrogels were prepared via reversible addition−fragmentation chain transfer polymerization, followed by removal of the protecting group of the acidic monomer repeating units (methacrylic acid, MAA), which were common in both series. One series bore (pyridin-2-yl)methyl methacrylate (2PyMMA) basic monomer repeating units, whereas the second series carried basic monomer repeating units of 2(dimethylamino)ethyl methacrylate (DMAEMA). The 2PyMMA-MAA combination in the first series had the peculiarity that the basic 2PyMMA units were more acidic than the MAA units, thus resulting in so-called “inverse PA” hydrogels. The DMAEMA-MAA pair in the other series led to “regular PA” hydrogels in which the basicity and acidity of the two types of units were in the conventional sense. The swelling and hydrogen ion equilibrium properties of the two series were explored and thoroughly compared to each other. The aqueous degrees of swelling of inverse PA hydrogels were found to be generally lower than those of the regular ones due to the more hydrophobic character of the 2PyMMA basic units employed in inverse compared to those (DMAEMA) in regular PA gels. The aqueous swelling pH-profiles in both series of PA hydrogels presented a minimum. However, this swelling minimum was deeper and wider in the case of inverse PA hydrogels, because of the greater hydrophobicity of the basic (2PyMMA) units and the greater difference in the effective pK values of the two types of units (2PyMMA-MAA) in inverse PA hydrogels, respectively. This larger separation of the pK’s in inverse PA gels was directly confirmed from the hydrogen ion titration curves of all the gels. Finally, the isoelectric points of inverse PA hydrogels possessed no detectable dependence on PA composition, which must be contrasted to the strong composition-dependence of the isoelectric points of regular PAs.

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INTRODUCTION Polyampholytes (PAs) represent a fascinating class of polyelectrolytes, which possess or may acquire both cationic and anionic charges.1 This leads to four possibilities, depending on how permanent each type of charge is. Among the four cases, the most interesting is the one where the values of both the positive and negative charge vary with pH (“doubly weak PAs”). Typical examples of chemical groups with such variable charge are amines and carboxylic acids for the cationic and anionic charge, respectively. As the charging procedure is pHsensitive, with the two types of groups becoming ionized in different pH ranges, PAs constitute a dually pH-responsive system. To this must be added the complexity arising from the electrostatic attraction between the two types of groups. This is so because the positively ionizable groups become highly charged always at pH values lower than their transition pH (known as the effective pK value), and the negatively ionizable groups get highly ionized always at pH values higher than their effective pK value, and also because the effective pK value of the former type of group is usually higher than that of the latter. Thus, within the pH range between the pK values of the two types of groups, positive and negative charges coexist, being attracted to each other electrostatically. Furthermore, the particular magnitude of each of the two types of charge and, consequently, the strength of their attraction vary with pH. This electrostatic attraction is maximized when the positive © XXXX American Chemical Society

charge becomes exactly equal to the negative charge. The pH where this happens is called the isoelectric point, pI, the pH of zero net charge.2 Now, what if the effective pK value of the positively ionizable groups becomes lower than that of the negatively ionizable ones? Or, alternatively, what happens if the effective pK value of the negatively ionizable groups rises above that of the positively ionizable ones? In other words, what is the result when the roles of the acidic and the basic groups in the PA are reversed? In this case, the dual pH-responsiveness at the two pK values is preserved. However, the possibility for strong electrostatic attraction is diminished, as positive charge exists only when the negative charge is zero, and negative charge exists only when the positive charge is zero. In the pH range between the two pK values, the charge is very small, changing very little with pH, and giving rise to only small attraction. At the pI of this type of PA, each type of group is practically uncharged. This also implies that the value of the pI varies very little with PA composition. We recently proposed the term “inverse PAs”3 to describe this type of polymers, which must be contrasted to “regular PAs”1 in which the acidic and the basic groups retain their traditional roles. Received: March 16, 2016 Revised: April 28, 2016

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DOI: 10.1021/acs.macromol.6b00538 Macromolecules XXXX, XXX, XXX−XXX

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Scharlau, Spain. Tetrahydrofuran (THF, 99.8%, both HPLC and reagent grade) was purchased from Fisher, U.K. Methods. The monomers (2PyMMA, THPMA, and DMAEMA) and the cross-linker (EGDMA) were (further) purified by passage through basic alumina columns to remove inhibitors and acidic impurities, followed by stirring over CaH2 for 72 h to achieve full drying via the neutralization of any remaining protonic impurities, and completed by vacuum distillation. The radical source, AIBN, was purified by two recrystallizations in ethanol, whereas the polymerization solvent, 1,4-dioxane, was purified by vacuum distillation over CaH2. Synthesis of the PA Gels. Reversible addition−fragmentation chain transfer (RAFT) polymerization7 was employed for the preparation of the precursor networks, with the MAA units in the protected THPMA form. The polymerization was performed at 70 °C and lasted 20 h. A 2:1 v/v mixture of 1,4-dioxane and triethylamine was used as the polymerization solvent, with triethylamine serving as an additive to protect THPMA from premature thermolysis to MAA at the rather elevated polymerization temperature (70 °C).6 The molar ratio of total monomer, i.e., basic monomer (2PyMMA or DMAEMA) plus acid precursor (THPMA) monomer, to CPDB chain transfer agent (CTA) was kept at 100:1, whereas the molar ratio of the basic to the acid precursor monomer was varied from 80:20 to 20:80, thus covering a broad range of (final) PA composition. The three homopolymer networks, i.e., those based on 2PyMMA, DMAEMA or THPMA, were also prepared, at the same monomer to CTA molar ratio of 100:1. The EGDMA to CTA molar ratio was kept at 4:1 for all networks, whereas the AIBN to CTA molar ratio was fixed at 0.63. Characterization of the PA Gels. After the polymerization, the networks were transferred to THF where they were left for 2 weeks to equilibrate and extract the sol fraction which was subsequently vacuum-dried and analyzed in terms of its composition using 1H NMR spectroscopy and in terms of its size using gel permeation chromatography (GPC). Subsequently, samples of the THFequilibrated networks were vacuum-dried, and their degrees of swelling (DS) in THF were calculated as their THF-swollen mass divided by their dry mass. The dried network samples were transferred into water, where a volume of excess (equivalent to three times the sum of the basic plus the THPMA units in the network sample) concentrated aqueous HCl (10 M) solution was added. The mixture was left for 3 weeks to ensure complete hydrolysis of the THPMA units. Afterward, the obtained PA gels were rinsed from the excess HCl by removing the supernatant acid solution and replacing it with fresh, deionized water. The water was renewed every day, for approximately 3 weeks (see Figure S1 in the Supporting Information). Although deprotection was performed hydrolytically rather than thermally, we explored the thermal response of all the original networks (before hydrolysis) using differential scanning calorimetry (DSC), and the results are provided in Figure S2 in the Supporting Information. For each hydrolyzed and rinsed hydrogel, 10−15 pieces were cut, and each was placed in a glass vial containing 5 mL of deionized water, where the appropriate quantity of mineral acid (HCl) or base (NaOH) was also added, so that, for each hydrogel, the pH range from ∼2 to ∼12 was covered. The gel pieces were allowed to equilibrate for 3 weeks, at which time the pH of the supernatant solution was measured using a Corning 30 portable pH-meter, and the swollen mass was determined gravimetrically; from the last measurement, the degree of swelling (DS) was calculated as the ratio of the swollen divided by the dried hydrogel mass. The “ionization increment” for each hydrogel piece was calculated as the ratio of the equivalents of HCl or NaOH added divided by the equivalents of MAA plus 2PyMMA or DMAEMA present in the sample, estimated from the dry mass and nominal network composition. Characterization of the Extractables. The composition of the (dried) sol fraction was characterized by 1H NMR spectroscopy using a 500 MHz Avance Bruker NMR spectrometer in deuterated chloroform, whereas the size of the sol fraction was characterized by GPC in THF using a Polymer Laboratories chromatograph. The chromatograph was equipped with a Polymer Laboratories ERC-

To design inverse PAs, one needs to employ acidic units with uncommonly high pK values, or basic units with uncommonly low pK values, or both. An example of an acid with a high pK value is phenol (pK ∼ 9.95),4a whereas pyrimidine (1,3-diazine) and pyrazine (1,4-diazine) are bases with very low pK values, 1.104b and 0.37,4b respectively. Phenol is encountered in a natural amino acid, tyrosine, and in tyrosine-containing proteins. However, tyrosine’s phenol is not used in proteins for its physicochemical charging properties, but rather for its chemical reactivity as a reductant (in water−plastoquinone oxidoreductase, known as photosystem II) or as a phosphorylation site (leading to protein activation). In fact, proteins are regular PAs, mostly bearing common positively and negatively chargeable units, some of which form salt bridges facilitating protein folding and enhancing protein stability. In contrast, deoxyribonucleic acid (DNA) seems to have features of inverse PA, as two of its bases, adenine and cytosine, have amine groups with very low pK values,5 excluding the possibility of ion pair formation with the phosphodiester group, and allowing them to act independently, with the (charged) phosphodiester residues being at the exterior of the helix enhancing aqueous solubility, and the (uncharged) and hydrophobic nucleic bases hydrogen-bonding within the core of the helix. The aim in this investigation is to prepare a series of inverse PA hydrogels, and study in detail their swelling and ionization behavior, from which the pI values can be determined, and deduce the dependence of pI on copolymer composition. Furthermore, this study also includes the preparation and characterization of a series of the corresponding regular PA hydrogels for comparison. In designing the inverse PAs, we chose an amine monomer with an unusually low pK value, and an acid monomer with a standard (reasonably low) pK value. The same acid monomer was also used for the regular PAs of this investigation, together with an amine monomer with also a standard pK value. The study comprises the synthesis and characterization of ten PA hydrogels plus three polyelectrolyte homopolymer hydrogels, and represents a significant expansion of our recent preliminary investigation where only three PA hydrogels were prepared and studied.3 Another difference was the use of an easier-to-deprotect acid monomer precursor, whose deprotection was cleanly performed under mild acid hydrolysis conditions rather than thermolysis, thus avoiding the exposure of the monomer repeating units to thermal stress with the risk of (partial) pyrolysis. Another important development in the present study was the extraction of the hydrogen ion titration curves for all 13 hydrogels, which clearly indicated large differences in the effective pK values for the two types of units in the inverse PA system.

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EXPERIMENTAL SECTION

Materials. (Pyridin-2-yl)methanol (purity 98%), 3,4-dihydro-2Hpyran (97%), methacryloyl chloride (97%), methacrylic acid (MAA, 99%), 2-(dimethylamino)ethyl methacrylate (DMAEMA, 99%), ethylene glycol dimethacrylate (EGDMA, 98%), 2-cyanopropan-2-yl dithiobenzoate (2-CPDB, > 97%), 2,2′-azobis(isobutyronitrile) (AIBN, 95%), 1,4-dioxane (99%), basic alumina (≥98%), calcium hydride (CaH2, 90−95%), and 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH, 95%) were purchased from Aldrich, Germany. The monomers (pyridin-2-yl)methyl methacrylate (2PyMMA)3 and tetrahydro-2H-pyran-2-yl methacrylate (THPMA)6 were synthesized and purified in our laboratory as detailed in previous investigations.3,6 Deuterated chloroform (CDCl3, 99.8%), dimethyl sulfoxide (DMSO, ≥ 99.7%), and deuterated DMSO (DMSO-d6, 99.9%) were purchased from Merck, Germany. Triethylamine (≥99%) was provided by B

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Macromolecules 7515A refractive index detector and a Waters 515 isocratic pump, whereas a Polymer Laboratories PL-Mixed “D” column (bead size of 5 μm, and pore sizes of 100, 500, 103 and 104 Å) was used for sample analysis. The GPC system was calibrated against ten linear poly(methyl methacrylate) standards of narrow molecular weight distribution (molecular weights of 800, 2220, 6370, 12600, 23500, 41400, 89300, 201000, 392000, and 675000 g mol−1) supplied by Polymer Standards Service (PSS) GmbH, Mainz, Germany. Thermal Characterization. The thermal properties of the networks were investigated by DSC using a Q1000 differential scanning calorimeter from Thermal Analysis Instruments. Sample amounts of 0.5−2.0 mg were placed in hermetically sealed aluminum pans and were heated from 40 to 400 °C.

PA Design. In designing the present inverse PA networks, we elected to combine a standard, weakly acidic monomer, bearing a carboxylic acid in the pendant, MAA (pK of monomer = 4.65;11 effective pK of MAA homopolymer =5.35−5.6512,13), with a very weakly basic monomer based on pyridine (pK of pyridine = 5.144c − 5.2514), 2PyMMA with a pK = 3.76; linear 2PyMMA homopolymer = 1.543), thus having the amine basic units being more acidic than the carboxylic acid. As a control, we also prepared another network series, of regular PAs, also with MAA acidic units, combined with a tertiary amine monomer, DMAEMA (pK of DMAEMA monomer = 8.44;15 linear homopolymer = 7−813,16,17), where the relative pK values are in the normal sense; we and others have utilized this particular monomer combination several times in the past for the preparation and characterization of PA linear polymers,8,18 star polymers19 and gels.6,20 However, most of the samples in the previous studies6,8,18−20 possessed a block structure, with the DMAEMA and MAA monomer repeating units located in separate blocks. As monomer distribution does affect ionization properties, we decided to also prepare DMAEMA-MAA random PA networks for this study, and directly compare them with the corresponding 2PyMMA-MAA random inverse PA networks which were the main focus in this investigation. It is noteworthy that the MAA units in both types of PA networks in the present investigation were introduced via the incorporation of THPMA units followed by their acid hydrolysis, a much milder process than the thermolysis employed in our previous studies3,8 which may partially pyrolyze the 2PyMMA3 and the DMAEMA18b units, or may partially convert the MAA units to anhydride.18a EGDMA served as the cross-linker for both PA network series. Synthesis of the PA Networks. Figure 2 illustrates the synthetic route followed for the preparation of the inverse and regular PA networks, through the RAFT terpolymerization of the appropriate basic monomer, 2PyMMA or DMAEMA, the precursor to the acidic monomer, THPMA, and the EGDMA cross-linker, and subsequent protecting group removal to yield the MAA acidic units. For each network series, five comonomer compositions were accessed, from 20 to 80 THPMA mol %, plus the two homopolymers. The molar ratio of the sum of the two comonomers to the CTA (nominal degree of polymerization) was 100 for all networks. The molar ratio of cross-linker to CTA was 4, previously shown to lead to high comonomer incorporation in to the networks.20 The theoretical structure of the networks is listed in Table 1, together with the results of the characterization of their sol fraction extracted in THF after their synthesis. Network Sol Fraction. The networks in both series exhibited low sol fractions, 8−14% for the 2PyMMA− THPMA−EGDMA series and 7.5−12% for the DMAEMA− THPMA−EGDMA series. This indicates successful incorporation of most monomer and cross-linker in the networks. 1H NMR spectroscopy indicated that the largest part of the extractables was copolymer and the smallest monomers. In most cases, the extractables were composed of 86−100% soluble copolymer whose composition was close to the nominal composition of the network. The molecular weights of the (co)polymers in the extractables were rather low, comparable to or lower than the molecular weight corresponding to a linear 100-mer estimated to be 16,800 g mol−1; the latter calculation was of based on the average molecular weight of THPMA, 2PyMMA and DMAEMA of 168 g mol−1 [=(170 + 177 +

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RESULTS AND DISCUSSION Inverse Polyampholytes. Doubly weak polyampholytes (PAs) are polymers with a rich solution behavior arising from the presence of weakly basic and weakly acidic groups, leading to dual pH-responsiveness, and resulting in the attractive interaction between the oppositely charged groups and the existence of an isoelectric point.1 This behavior has occasionally been enriched by adding other features, such as micellization with8 or without9 the addition of a hydrophobic segment, or upper critical solution temperature (UCST) thermoresponsive behavior.10 The present investigation does not aim to add extra functionalities to a PA system, but rather to modify existing ones. In particular, the intention here is to reverse the roles of the basic and the acidic groups, in order to obtain “inverse” PAs,3 in which the pK value of the basic units is lower than that of the acidic, as is schematically illustrated in Figure 1. An

Figure 1. Charging properties of regular and inverse polyampholytes (PAs) with reference to the pK values of their acidic (pKa) and basic (pKb) groups. The lowest and highest pK values in each system are designated as pK1 and pK2, respectively.

immediate implication of this is that, within the pH range intermediate between the two pK values of inverse PAs, the positive charge and negative charge are both close to zero and vary very little with pH, also resulting in the lack of coexistence of positive and negative charges (at least to a large extent) within this or any other pH range. This is in marked contrast with regular PAs where both types of charges largely coexist and vary within the pH range intermediate between the two pK values, also shown in Figure 1. The great charge variation in this pH range for regular PAs also leads to the strong dependence of their isoelectric points on their acid−base composition, especially around the equimolar composition. As charge variation in the pH range between the two pK values is very small in inverse PAs, the isoelectric point should only weakly depend on composition. One of the main aims in this investigation is to explore the isoelectric points of inverse PA gels and confirm their weak dependence on PA composition. C

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Figure 2. Synthetic route employed for the statistical RAFT terpolymerization of the 2PyMMA−THPMA−EGDMA and DMAEMA−THPMA− EGDMA combinations, followed by the acidic hydrolysis of the THPMA units to MAA units to obtain the inverse and regular polyampholyte networks, respectively. The 2PyMMA, DMAEMA, THPMA, MAA, and EGDMA units are shown in pink, red, blue, green, and black, respectively.

Table 1. Characteristics of the Extractables from the 2PyMMA−THPMA−EGDMA and DMAEMA−THPMA−EGDMA Networks Obtained Upon Rinsing Them in THF, Following Their Synthesis, Including Sol Fraction, Monomer and Polymer Percentage, and Polymer Composition and Molecular Weights composition of extractablesc THPMA (mol %)

GPC results

network structurea

overall weight fraction of extractables (%)b

polymer fraction (%)

in copolymer

in comonomer

Mp (g mol−1)

Mn (g mol−1)

Đ

2PyMMA100-co-THPMA0-co-E4 2PyMMA80-co-THPMA20-co-E4 2PyMMA67-co-THPMA33-co-E4 2PyMMA50-co-THPMA50-co-E4 2PyMMA33-co-THPMA67-co-E4 2PyMMA20-co-THPMA80-co-E4 2PyMMA0-co-THPMA100-co-E4 DMAEMA100-co-THPMA0-co-E4 DMAEMA80-co-THPMA20-co-E4 DMAEMA67-co-THPMA33-co-E4 DMAEMA50-co-THPMA50-co-E4 DMAEMA33-co-THPMA67-co-E4 DMAEMA20-co-THPMA80-co-E4 DMAEMA0-co-THPMA100-co-E4

8.01 13.22 9.60 9.75 11.35 13.65 12.10 7.52 7.55 8.42 9.30 10.57 9.09 12.10

90.9 92.5 85.7 94.8 87.9 90.0 63.7 98.0 100.0 96.7 94.4 96.9 90.5 63.7

0.0 14.4 31.0 19.0 57.5 60.7 100.0 0.0 37.9 34.2 42.6 53.0 62.3 100.0

0.0 29.3 35.7 59.6 53.7 53.0 100.0 0.0 0.0 30.3 53.6 51.6 100.0 100.0

5500 8400 17000 7400 7000 8700 8700 8200 8758 2960 4270 6570 7660 8660

3000 5400 13000 2400 2400 6400 6000 4400 3319 1470 2150 3510 4340 6020

2.15 1.75 1.77 3.17 3.44 1.65 1.58 1.99 2.67 1.54 1.60 1.60 1.89 1.58

a 2PyMMA, (pyridin-2-yl)methyl methacrylate; THPMA, tetrahydro-2H-pyran-2-yl methacrylate; DMAEMA, 2-(dimethylamino)ethyl methacrylate; E, ethylene glycol dimethacrylate (EGDMA). bCalculated as the ratio of the mass of the extracted polymer plus monomers divided by the theoretically expected maximum network mass estimated as the sum of the masses of the comonomers, cross-linker, and CTA. cDetermined using 1 H NMR spectroscopy in CDCl3.

157)/3 g mol−1] multiplied by the nominal degree of polymerization of 100. This suggested that the extractables comprised early deactivated chains where only little EGDMA was incorporated. The number-average molecular weights, Mn, of the (co)polymers in the extractables from the 2PyMMA− THPMA−EGDMA series were higher than those from the DMAEMA−THPMA−EGDMA series, with the former ranging from 2400 to 13 000 g mol−1 and the latter ranging from 1500 to 6000 g mol−1. The molecular weight dispersities, Đ, of the former series were higher than that of the latter, with corresponding ranges being 1.6 to 3.4 and 1.6 to 2.7, possibly indicating a tendency of 2PyMMA to participate in branching side-reactions.21 However, the fact that the Đ values of several samples in both series were higher than 2.0 also suggests light branching originating from some EGDMA incorporation.

Degrees of Swelling in THF. Table 2 shows the degrees of swelling (DSs) in THF of all the original copolymer networks, precursors to the PA networks (i.e., before the hydrolysis of the THPMA units), presenting values from 5 to 9. THF is a good solvent for all three types of units, THPMA, 2PyMMA, and DMAEMA, which is the reason why the DSs in THF are relatively independent of network composition. However, the DSs in THF for the DMAEMA−THPMA−EGDMA series span a higher range, from 6 to 9, than those for the 2PyMMA− THPMA−EGDMA series spanning a range from 5 to 6, suggesting a better compatibility between DMAEMA and THF, as compared to 2PyMMA and THF, with the difference arising from the aromatic character of 2PyMMA. The presence of pyridine in polymers is known to render them sometimes less soluble in various solvents. The most characteristic example is D

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greater hydrophobic character of 2PyMMA compared to that of DMAEMA due to the greater number of carbon atoms and the presence of the pyridine aromatic ring in the former. The second reason was more subtle but probably equally important. This was the fact that DMAEMA is a stronger base than 2PyMMA, owing to the higher pK value of the units of the former (∼7.5)13,16,17 compared to those of the latter (∼2).3 Thus, during the rinsing of the hydrogels with water, the DMAEMA units could more efficiently compete with water for HCl, therefore preserving it within the network, and stabilizing the pH of the supernatant of the DMAEMA−MAA−EGDMA series in the range 2−4. In contrast, the very weakly basic 2PyMMA units could not keep HCl which was washed away with water, raising the pH of the supernatant of the 2PyMMA− MAA−EGDMA series to ∼6−8. With an effective pK value of the DMAEMA units of ∼7.5,13,16,17 the DMAEMA units in the DMAEMA−MAA−EGDMA series were fully charged at pH 2−3, leading to the observed high aqueous DSs in this pH range, with values from 10 to 28. In contrast, the 2PyMMA units, with an effective pK value of ∼1.54,3 were fully uncharged in the pH range between 6 and 8, which was the (second) reason for the low aqueous DSs observed, ranging from 2.4 to 4.0. Note that the lowest aqueous DS in water from the 2PyMMA series was exhibited by the 2PyMMA homopolymer, displaying a value of 2.4, whereas the other members of the series presented values of 3.4 or higher due to the presence of MAA units which were partially charged under these conditions. It is noteworthy, however, that within each series of PA hydrogels (excluding the respective polyelectrolyte homopolymer hydrogels), there was not any observable systematic effect of PA composition on the DSs in water. In the case of the inverse PA hydrogels, this trend may be attributed to the fact that the final pH in water for all these hydrogels was within the isoelectric region where these networks were collapsed. pH-Dependence of the Aqueous Degrees of Swelling. The following four figures display the main characterization results of this study which are the aqueous DSs of the PA gels as a function of pH. Figures 3 and 4 present the complete pH profiles of the DSs in water for the 2PyMMA−MAA−EGDMA and DMAEMA−MAA−EGDMA series, respectively, whereas Figures 5 and 6 illustrate a summary of the aqueous swelling behavior of the PA hydrogels by plotting the DSs at selected, characteristic pH values against hydrogel composition. pH-Dependence of the Aqueous Swelling of the Inverse PA Gels. Figure 3 plots the pH-dependence of the DSs in water for all the members of the 2PyMMA−MAA−EGDMA inverse PA hydrogel series, including those of the 2PyMMA and MAA homopolyelectrolyte hydrogels. The swelling profiles of the five inverse PA hydrogels exhibited minimum swelling at low to intermediate pH values, typically extending from pH ∼1.5 to ∼7.5. The drop of the DSs at lower pH was not gradual but abrupt, forming a characteristic trough (large flat region), consistent with the expectation that the degrees of ionization did not change much within this pH range (also see Figure 8). At pH values lower than 1.5, the DSs abruptly increased, a result of the ionization of the 2PyMMA units, leading to electrostatic repulsion between the polymer chains bearing protonated pyridine rings and to an osmotic pressure built by the accumulated chloride counteranions to the pyridinium cations. The DSs also increased at pH values higher than 7.5, with this increase being small for the PA hydrogels bearing MAA lower than 50 mol % but greater for the hydrogels richer

Table 2. Degrees of Swelling in THF of the 2PyMMA− THPMA−EGDMA and the DMAEMA−THPMA−EGDMA Network Series network structurea

degree of swelling in THF

2PyMMA100-co-THPMA0-co-E4 2PyMMA80-co-THPMA20-co-E4 2PyMMA67-co-THPMA33-co-E4 2PyMMA50-co-THPMA50-co-E4 2PyMMA33-co-THPMA67-co-E4 2PyMMA20-co-THPMA80-co-E4 2PyMMA0-co-THPMA100-co-E4 DMAEMA100-co-THPMA0-co-E4 DMAEMA80-co-THPMA20-co-E4 DMAEMA67-co-THPMA33-co-E4 DMAEMA50-co-THPMA50-co-E4 DMAEMA33-co-THPMA67-co-E4 DMAEMA20-co-THPMA80-co-E4 DMAEMA0-co-THPMA100-co-E4

5 5.4 5 6 6 5 6 9 6 8 6 7 8 6

± ± ± ± ± ± ± ± ± ± ± ± ± ±

1 0.6 2 1 3 2 3 1 1 2 1 4 1 3

a

2PyMMA, (pyridin-2-yl)methyl methacrylate; THPMA, tetrahydro2H-pyran-2-yl methacrylate; DMAEMA, 2-(dimethylamino)ethyl methacrylate; E, ethylene glycol dimethacrylate (EGDMA).

poly(4-vinylpyridine) which is insoluble in THF, a result of the large dipole moment of pyridine. Degrees of Swelling in Pure Water. Table 3 shows the DSs in water of all the ampholytic networks, after their Table 3. Degrees of Swelling in Water and Final pH in the Supernatant Solution for the Ampholytic Hydrogels of the 2PyMMA−MAA−EGDMA and the DMAEMA−MAA− EGDMA Series, Following THPMA Deprotection and Extensive Rinsing in Water network structurea 2PyMMA100-co-MAA0-co-E4 2PyMMA80-co-MAA20-co-E4 2PyMMA67-co-MAA33-co-E4 2PyMMA50-co-MAA50-co-E4 2PyMMA33-co-MAA67-co-E4 2PyMMA20-co-MAA80-co-E4 2PyMMA0-co-MAA100-co-E4 DMAEMA100-co-MAA0-co-E4 DMAEMA80-co-MAA20-co-E4 DMAEMA67-co-MAA33-co-E4 DMAEMA50-co-MAA50-co-E4 DMAEMA33-co-MAA67-co-E4 DMAEMA20-co-MAA80-co-E4 DMAEMA0-co-MAA100-co-E4

degree of swelling in water

final pH

± ± ± ± ± ± ± ± ± ± ± ± ± ±

6.20 8.40 7.64 6.30 7.61 6.23 6.53 3.97 2.95 2.69 2.43 2.05 1.91 6.53

2.4 3.4 3.5 3.9 4.0 3.8 3.4 22.2 28 25.9 20.7 9.5 24.0 3.4

0.5 0.2 0.3 0.2 0.3 0.2 0.2 0.6 2 0.6 0.6 0.5 0.7 0.2

a

2PyMMA, (pyridin-2-yl)methyl methacrylate; MAA, methacrylic acid; DMAEMA, 2-(dimethylamino)ethyl methacrylate; E, ethylene glycol dimethacrylate (EGDMA).

formation through the acid hydrolysis of the THPMA units in the parent amphiphilic networks, and extensive rinsing with water. The table also lists the final pH values which the networks acquired after rinsing. Figure S1 in the Supporting Information shows the temporal evolution of the pH of the supernatant solution from the start of the rinsing until equilibration. The DSs in water of the PA gels of the DMAEMA−MAA− EGDMA series were much higher than those of the 2PyMMA− MAA−EGDMA series for two reasons. The first reason was the E

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Figure 3. Aqueous degrees of swelling for the inverse ampholytic hydrogels (2PyMMA−MAA−EGDMA series) as a function of pH. The vertical red, green and blue arrows indicate the values of the degrees of swelling taken as characteristic for acidic, isoelectric, and basic conditions, are plotted in Figure 5, and are further discussed with that figure.

Figure 4. Aqueous degrees of swelling for the regular ampholytic hydrogels (DMAEMA−MAA−EGDMA series) as a function of pH. The vertical red, green, and blue arrows indicate the values of the degrees of swelling taken as characteristic for acidic, isoelectric, and basic conditions, and are plotted in Figure 6 and further discussed with that figure.

in MAA. After this initial increase in the DSs, there was a reduction in the DSs at pH > 8, with the reduction being greater again for MAA-rich PA gels. The initial increase in the DS at pH ∼ 7.5 was due to the full ionization of the MAA units (inter- and intrachain electrostatic repulsion due to the negatively charged carboxylates and osmotic pressure arising from the sodium countercations to the carboxylates, in analogy with the increased swelling at low pH arising from the charging of the 2PyMMA units), whereas the subsequent decline at pH > 8 was probably due to the screening of the electrostatic repulsion by the added sodium hydroxide. The pH-profiles of

the aqueous swelling of the 2PyMMA and the MAA homopolymer hydrogels are also given in Figure 3, presenting large increases in swelling at pH < 2 and pH > 6, respectively, arising from the charging of the corresponding ionizable groups. The red, blue, and green arrows in the plots indicate for each hydrogel the characteristic DS values at acidic, basic and isoelectric pH that were taken and plotted against hydrogel composition in Figure 5. For this hydrogel series, the characteristic acidic DS was taken at the lowest pH value measured, the characteristic basic DS was obtained as the maximum DS in the pH range 7.5−10.5, and as isoelectric DS F

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pH-Dependence of the Aqueous Swelling of the Regular PA Gels. Figure 4 is analogous to Figure 3, but concerns the PA hydrogels of the DMAEMA−MAA−EGDMA series. The regular PA hydrogels of this series also presented a swelling minimum around the isoelectric pH, with the exception of the two PA hydrogels with extreme composition which were also mechanically weak, leading to large errors in the swelling measurements. Thus, the discussion will focus on the three PA hydrogels with more balanced composition, plus the two homopolymer hydrogels. The swelling minimum with respect to pH of the three regular PA hydrogels with more balanced composition was not as flat as that in the 2PyMMA−MAA− EGDMA series, but more curved, as previously observed with PA hydrogels with the same monomer repeating units but block architecture.6,20 This can be attributed to the variation of the degrees of ionization of both the DMAEMA and the MAA units within this pH range, as it was to be expected for regular PAs (also see Figure 1). The swelling maxima at extreme pH values were, in some cases, followed by a decline in the DSs at even more extreme pH values. The swelling maxima can, again, be attributed to the full ionization of the basic and acidic units at low and high pH, respectively, and the decline in swelling at the very extreme of pH can be attributed to the high ionic strength at these conditions. The pH-profiles of the aqueous swelling of the two homopolymer hydrogels are also given in Figure 4; as expected, no swelling minima were presented here, but only large increases in swelling at pH < 6 and pH > 6, corresponding to the ionization of the DMAEMA and MAA monomer repeating units in the respective homopolymer hydrogels. In Figure 4 too, colored arrows were used to indicate the DS values chosen as characteristic for the acidic, the basic and the isoelectric pHs, with the same color convention preserved as that in Figure 3. The isoelectric point was determined also from Figure 4 as the pH at the swelling minimum, and is plotted against hydrogel composition in Figure 7. Composition-Dependence of the Aqueous Swelling of the Inverse PA Gels. Figure 5 shows the influence of hydrogel composition, expressed as the MAA mol % content in the network, on the aqueous DSs at different characteristic pH values, acidic, basic, and isoelectric, as defined in the discussion of Figure 3 above, for the PA hydrogels of the 2PyMMA− MAA−EGDMA series. Figure 5 also includes the DSs for the two homopolymer hydrogels at acidic and basic pH values. The DSs at high (basic) pH increased with the MAA content because the MAA units were fully charged under these conditions, greatly contributing to swelling via the electrostatic repulsive forces and the osmotic pressure. At these conditions, the 2PyMMA units were fully uncharged, and they might contribute to swelling in a negative way via their hydrophobicity; thus, the particular trend, at basic pH conditions, of increasing DS with MAA content may also partly be due to the decreasing 2PyMMA content with increasing MAA content. At the other extreme, at low (acidic) pH, the DSs increased with decreasing MAA content. This was so because the now fully ionized 2PyMMA content increased as the MAA content decreased. However, the changes in the DSs at low pH across the whole range of hydrogel composition were less pronounced than those at high pH because of the hydrophobic character of the 2PyMMA units and the more hydrophilic character of the MAA units. Thus, the 2PyMMA-rich hydrogels with fully ionized (hydrophobic) 2PyMMA units at low pH did not swell as much as the corresponding MAA-rich hydrogels with fully

Figure 5. Composition-dependence of the aqueous degrees of swelling of the inverse ampholytic hydrogels (2PyMMA−MAA−EGDMA series) plus the two homopolymer hydrogels, at alkaline, acidic, and isoelectric pH.

Figure 6. Composition-dependence of the aqueous degrees of swelling of the regular ampholytic hydrogels (DMAEMA−MAA−EGDMA series) plus the two homopolymer hydrogels, at alkaline, acidic, and isoelectric pH.

Figure 7. Composition-dependence of the isoelectric points, pI, of the inverse (2PyMMA−MAA−EGDMA, red open circles) and regular (DMAEMA−MAA−EGDMA, blue closed triangles) ampholytic hydrogels. The red continuous and blue dashed lines are theoretical predictions based on (effective) pK values of 2.0, 7.4, and 5.7 for the monomer repeating units of 2PyMMA (pKb2), DMAEMA (pKb1), and MAA (pKa), respectively.

was considered that in the middle of the swelling trough; similarly, the isoelectric point itself was taken as the pH in the middle of the swelling trough, and is plotted in Figure 7. G

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Figure 8. Hydrogen ion titration curves of the inverse ampholytic hydrogels (2PyMMA−MAA−EGDMA series) plus the two homopolymer hydrogels. The three horizontal dotted lines indicate the regions where the 2PyMMA and MAA groups are titrated, whereas the vertical red and green arrows indicate the pH of maximum buffering capacity (greatest slope) corresponding to the effective pK values of the 2PyMMA and the MAA units, respectively.

Isoelectric Points. Figure 7 summarizes the isoelectric points for both series of the PA hydrogels as determined from the swelling minima, and plots them against network composition. Whereas the pI values of the hydrogels from the DMAEMA−MAA−EGDMA series showed a pronounced dependence on the MAA content, undergoing a large decrease as the MAA percentage increased, those from the 2PyMMA− MAA−EGDMA series were constant, independent of composition. These observations were just as expected, fully confirming the hypotheses made for the present investigation. The figure also shows two curves calculated independently of the experimental pI measurements, based on the following equation:8,22,23

ionized (hydrophilic) MAA units at high pH. Finally, the DSs at the isoelectric points were lowest and independent of composition. At the isoelectric point, the net charge of a PA is zero, providing no electrostatic repulsion or osmotic pressure necessary for appreciable swelling. In the present case of inverse PAs, not only was the net charge equal to zero around the isoelectric point, but, furthermore, each of the two types of units, the acidic and the basic, were fully uncharged in this pH range (see Figure 1). Thus, there was no charge-related polarity at all (absence of zwitterions), leading to even lower swelling. To this must also be added the high hydrophobicity of the 2PyMMA units, which would result in further reduction of the aqueous swelling. Composition-Dependence of the Aqueous Swelling of the Regular PA Gels. Figure 6 presents the effect of hydrogel composition, expressed again as the MAA mol % content in the network, on the aqueous DSs at different characteristic pH values, acidic, basic and isoelectric, as defined in the discussion for Figure 5 above, for the PA hydrogels of the DMAEMA− MAA−EGDMA series, including the two homopolymer hydrogels. The trends were similar to those observed in Figure 5, and therefore, similar explanations may be given. One difference was the higher DSs at low pH, especially that of the DMAEMA homopolymer hydrogel as compared to its 2PyMMA counterpart, attributable to the greater hydrophilicity of the DMAEMA units as compared to the 2PyMMA units. Another difference was the higher DSs at the isoelectric points of the hydrogels of the present DMAEMA−MAA−EGDMA series compared to those of the previous series, which can be attributed again to the higher hydrophilicity of the DMAEMA units over that of the 2PyMMA units and also to the presence of oppositely charged ionic units at the isoelectric pH in the case of regular PAs (thus increasing polarity and hydrophilicity), and the absence of such units at the corresponding conditions in inverse PAs.

⎧ ⎡ ⎪1 1 − R pI = pKb + log⎨ ⎢ ⎪2⎢ ⎩ ⎣ R +

⎤⎫ ⎛ 1 − R ⎞2 4 pKa − pKb ⎥⎪ ⎜ ⎟ + ⎬ 10 ⎝ R ⎠ ⎥⎦⎪ R ⎭

(1)

where pKa and pKb are the effective pK values of the acidic and the basic monomer repeating units, respectively, and R is the molar ratio of the acidic to the basic monomer repeating units. The following effective pK values were used: pKa = 5.7 (MAA units); pKb = 2.0 (2PyMMA units) or 7.4 (DMAEMA units). The agreement between the experimental and calculated pI values observed in Figure 7 is reasonably good, especially given the method used for the experimental determination of the pI, and the selection of the effective pK values used for the theoretical calculations. Note that, due to the complex electrostatic interactions within PAs, both effective pK values are copolymer composition-dependent.13,24−28 However, since the electrostatic interactions average to zero at the PA isoelectric point (this is expected to be more absolute in the H

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Figure 9. Hydrogen ion titration curves of the regular ampholytic hydrogels (DMAEMA−MAA−EGDMA series) plus the two homopolymer hydrogels. The three horizontal dotted lines indicate the regions where the DMAEMA and the MAA groups are titrated, whereas the vertical red and green arrows were drawn at the position of the effective pKvalues of the DMAEMA and MAA units determined for the corresponding homopolymer gels.

experimental points flattened on the left). The second horizontal line was drawn one unit below it, and the third line was drawn in between the two previous lines. Similarly to the hydrogen ion titration of the 2PyMMA homopolymer gel, the third horizontal line intersects this hydrogen ion titration curve at a x-value equal to the effective pK value of the MAA units. For the four remaining inverse PA hydrogels, the first horizontal line was drawn at the point of minimum buffering capacity (minimum slope; greatest pH change for a given quantity of added titrant). The next line was drawn above this first line, according to the MAA content (e.g., 0.67 for the gel with 67 MAA units), and the last line was drawn below the first line according to the 2PyMMA content (0.33 for the gel with 67 MAA units). In this graphical construction, the two lower horizontal lines envelope the part of the hydrogen ion titration curve corresponding to the 2PyMMA units, while the two upper horizontal lines include the titration of the MAA units. The two colored (red and green) vertical arrows indicate the points of maximum slope, corresponding to the effective pK values of the 2PyMMA and MAA monomer repeating units. Examining first the results from the y-axes, there is good agreement between the amount of titrant and amount of ionizable groups in the hydrogel samples because the hydrogen titration curves span approximately a range of ∼1 (∼100%), as expected. Regarding the x-axis results, the effective pK values indicated by the red and green arrows were taken from the graphs and are listed in Table 4, together with the corresponding values extracted from the hydrogen ion titration curves for the DMAEMA−MAA−EGDMA series illustrated in Figure 9 and discussed next. Titration Curves of the Regular PA Gels. Figure 9 plots the hydrogen ion titration curves of the PA hydrogels in the DMAEMA−MAA−EGDMA series and the two relevant homopolyelectrolyte hydrogels. Similar to Figure 8, in each hydrogen ion titration curve in Figure 9, three horizontal

case of inverse PAs), this explains the meaningful predictions of eq 1 using the above, average, effective pK values. Hydrogen Ion Titration Curves. Figures 8 and 9 illustrate the hydrogen ion titration curves for all the PA hydrogels in the two series, plus the homopolymer hydrogels, calculated from the previously described swelling experiments. The calculations were based on the known amount of inorganic acid (HCl) or base (NaOH) added to effect each pH change, relative to the also known number of equivalents of acidic (MAA) and basic (2PyMMA or DMAEMA) monomer repeating units within each network sample. Titration Curves of the Inverse PA Gels. We first discuss Figure 8 concerning the hydrogen ion titration curves of the hydrogels in the 2PyMMA−MAA−EGDMA series. The x-axes of the plots represent the solution pH, whereas the y-axes correspond to the ratio of equivalents of titrant divided by those of the acidic (MAA) plus the basic (2PyMMA) units of the (co)polymer gel. The reason why the experimental points in the hydrogen ion titration curves extend beyond 1 unit of yaxis is because water (the solvent) titration also takes place, especially at extreme pH values, acidic ones (below pH ∼ 2.5) in most cases. Each of the graphs in Figure 8 has three horizontal dotted lines, drawn in the following way: For the 2PyMMA homopolymer gel and the PA gel plotted next to it, richest in 2PyMMA, the first horizontal line was drawn at the apparent end-point at high pH (where points on the right became flat). The second horizontal line was drawn one unit (in terms of the y-axis units) below this, and the third line was drawn in between the previous two lines. Thus, the third line should cross the hydrogen ion titration curve at the point of 50% ionization, and the x-value of this intersection point should correspond to the effective pK value of the 2PyMMA units. For the MAA homopolymer gel, the first horizontal line was drawn at the apparent end-point at low pH (where the I

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respectively. Whereas the curves for inverse PA gels present two clearly separate sigmoidal branches, each corresponding to each of the two types of units, in the curves for regular PA gels, the titrations of the two types of units are hard to distinguish. This is so because the effective pK values of the two types of units are very close to each other. Table 4 shows how the pK value for each type of units changes from the monomer, to the linear polymer and to the polymer network. For instance, the pK value for MAA increases from 4.65 to 5.72 and to 7.09; i.e., the carboxylic acid group in MAA gradually becomes a weaker acid, as the monomer becomes linear polymer and then polymer network. The increased effective pK value of the MAA units in the linear polymer compared to the monomer is due to increased electrostatic interactions in the polymer, which make more difficult the removal of each subsequent proton (due to electrostatic attraction between the proton and the already negatively charged polyMAA). The higher effective pK value of MAA in the polymer network than in the linear polymer is due to the Donnan effect,29 according to which more protons are partitioned in the negatively charged gel phase than the corresponding supernatant solution (it is the pH of the supernatant solution, rather than that of the gel phase itself, that it is usually measured in gel titrations). The two basic monomers, 2PyMMA and DMAEMA, behave analogously to MAA, becoming weaker bases as they are transformed to linear polymers and then to polymer networks. Thus, one may appreciate that, although the pK values of the MAA and DMAEMA monomers differ by almost 4 units, the effective pK values of the MAA and DMAEMA units in their respective linear homopolymers differ by only 1.7 units, whereas this difference is reduced to 1.2 units when the comparison concerns the homopolymer networks. In contrast, the difference in the effective pK values of the MAA−2PyMMA pair is preserved high, at 5.4 units for the homopolymer networks, and 4.2 units for the linear homopolymers. It is noteworthy that the difference in the pK values of the MAA and 2PyMMA monomers is only 0.9 units. The divergence of the pK values and the increase of their difference in the linear polymers and polymer networks is another important characteristic of inverse PAs. To illustrate the effect of the relative values of pK’s of the two types of units in the hydrogen ion titration of PAs, Figure 10 presents the theoretical (simple calculation; no electrostatic interactions taken into account) hydrogen ion titration curves for two equimolar PAs, one inverse and one regular, using the

Table 4. Effective pK Values of the 2PyMMA, MAA and DMAEMA Monomer Repeating Units in the Ampholytic Hydrogels, in Linear Soluble Polymers, and in the Actual Monomers Themselves, As Determined Using Hydrogen Ion Titration network structurea

network

linear

monomer

2PyMMA100-co-MAA0-co-E4 2PyMMA80-co-MAA20-co-E4 2PyMMA67-co-MAA33-co-E4 2PyMMA50-co-MAA50-co-E4 2PyMMA33-co-MAA67-co-E4 2PyMMA20-co-MAA80-co-E4 2PyMMA0-co-MAA100-co-E4 DMAEMA100-co-MAA0-co-E4 DMAEMA80-co-MAA20-co-E4 DMAEMA67-co-MAA33-co-E4 DMAEMA50-co-MAA50-co-E4 DMAEMA33-co-MAA67-co-E4 DMAEMA20-co-MAA80-co-E4 DMAEMA0-co-MAA100-co-E4

1.74 − − − 7.82 7.82 7.09 5.90 − − − − − 7.09

1.54 − − − − − 5.72 7.39 − − − − − 5.72

3.76 − − − − − 4.65 8.44 − − − − − 4.65

a

2PyMMA, (pyridin-2-yl)methyl methacrylate; MAA, methacrylic acid; DMAEMA, 2-(dimethylamino)ethyl methacrylate; E, ethylene glycol dimethacrylate (EGDMA).

dotted lines were also drawn in a similar way. The hydrogen ion titration curve of the MAA homopolymer gel was identical to that displayed in Figure 8, and the three horizontal lines and the vertical green arrow were drawn in an identical fashion as well. For the DMAEMA homopolymer gel, the first horizontal line was drawn at the inflection point (point of maximum slope, where the pK was also identified using the red arrow). The second and third horizontal lines were drawn 0.5 units above and below the first horizontal line. The three horizontal lines for the five PA hydrogels of this family were drawn with reference to the pK values of the DMAEMA and the MAA monomer repeating units determined from the hydrogen ion titration curves of the two homopolymer gels, as follows. First, the two pK values determined from the homopolymer gels were indicated in the hydrogen ion titration curve of each PA hydrogel as red and green arrows; due to the proximity of their values, it was impossible to graphically determine them independently from the hydrogen ion titration curve of each regular PA hydrogel. The first horizontal line was drawn at a position intermediate between the two pK values, representing an average effective pK value. The second and third horizontal lines were drawn 0.5 units above and below this first horizontal line. As with the previous series, with this series there is too good agreement between the amount of titrant and the amount of ionizable groups in the hydrogel sample because the titration curves approximately span the expected range of 100%. The effective pK values of the DMAEMA and MAA monomer repeating units determined from the hydrogen ion titration curves of the corresponding homopolymer gels are displayed in Table 4, together with the values extracted from the hydrogen ion titration curves of Figure 8. Table 4 also records the pK values of the corresponding linear homopolymers and monomers. Comparison of the Titration Curves. There is a large difference in the shape of the hydrogen ion titration curves of inverse and regular PA hydrogels displayed in Figures 8 and 9,

Figure 10. Theoretically predicted hydrogen ion titration curves for an equimolar inverse (2PyMMA50-co-MAA50) and an equimolar regular (DMAEMA50-co-MAA50) polyampholyte. The following effective pK values were used: 5.7 (=pKa) for MAA, 2.0 (=pKb2) for 2PyMMA, and 7.4 (=pKb1) for DMAEMA. J

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Advances in the Study of Synthetic Polyampholytes in Solutions. Adv. Polym. Sci. 1999, 144, 115−197. (c) Kudaibergenov, S. E. Polyampholytes: Synthesis, Characterization and Application; Plenum: New York, 2002. (d) Ciferri, A.; Kudaibergenov, S. Natural and Synthetic Polyampholytes, 1 Theory and Basic Structures. Macromol. Rapid Commun. 2007, 28, 1953−1968. (e) Kudaibergenov, S. E.; Ciferri, A. Natural and Synthetic Polyampholytes, 2 Functions and Applications. Macromol. Rapid Commun. 2007, 28, 1969−1986. (f) Lowe, A. B.; McCormick, C. L. Synthesis and Solution Properties of Zwitterionic Polymers. Chem. Rev. 2002, 102, 4177−4189. (g) Lowe, A. B., McCormick, C. L., Eds., Polyelectrolytes and Polyzwitterions: Synthesis, Properties and Applications; ACS Symposium Series, American Chemical Society: Washington, DC, Vol. 937, 2006. (h) Kudaibergenov, S. E.; Nuraje, N.; Khutoryanskiy, V. V. Amphoteric Nano-, Micro-, and Macrogels, Membranes and Thin Films. Soft Matter 2012, 8, 9302−9321. (2) Cantor, C. R.; Schimmel, P. R. Biophysical Chemistry; Freeman: New York, 1980. pp. 41−51, 676−682. (3) (a) Pafiti, K. S.; Elladiou, M.; Patrickios, C. S. “Inverse Polyampholyte” Hydrogels from Double-cationic Hydrogels: Synthesis by RAFT Polymerization and Characterization. Macromolecules 2014, 47, 1819−1827. (b) Elladiou, M.; Patrickios, C. S. 2-(Pyridin-2yl)ethanol as Protecting Group for Carboxylic Acids: Chemical and Thermal Cleavage, and Conversion of Poly[2-(Pyridin-2-yl)ethyl Methacrylate] to Poly(methacrylic Acid). Polym. Chem. 2012, 3, 3228−3231. (c) Elladiou, M.; Patrickios, C. S. ABC Triblock Terpolymers with Orthogonally Deprotectable Blocks: Synthesis, Characterization, and Deprotection. Macromolecules 2015, 48, 7503− 7512. (d) Elladiou, M.; Patrickios, C. S. Dimethacrylate Cross-linker Cleavable Under Thermolysis or Alkaline Hydrolysis Conditions: Synthesis, Polymerization, and Degradation. Chem. Commun. 2016, 52, 3135−3138. (4) Fasman, G. D., ed., Handbook of Biochemistry and Molecular Biology: Physical and Chemical Data; 3rd ed., CRC Press: Cleveland, OH, 1976; Vol. 1 (a) p 314; (b) p 332; (c) p 331. (5) Peacocke, A. Historical Article: Titration Studies and the Structure of DNA. Trends Biochem. Sci. 2005, 30, 160−162. (6) Pafiti, K. S.; Philippou, Z.; Loizou, E.; Porcar, L.; Patrickios, C. S. End-linked Ampholytic Conetworks: Synthesis by Sequential Reversible Addition-Fragmentation Chain Transfer Polymerization, and Swelling and Structural Characterization. Macromolecules 2011, 44, 5352−5362. (7) (a) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Living Free-Radical Polymerization by Reversible Addition-Fragmentation Chain Transfer: The RAFT Process. Macromolecules 1998, 31, 5559−5562. (b) Moad, G.; Rizzardo, E.; Thang, S. H. Living Radical Polymerization by the RAFT Process − A Third Update. Aust. J. Chem. 2012, 65, 985−1076. (c) Moad, G.; Rizzardo, E.; Thang, S. H. Living Radical Polymerization by the RAFT Process − A Second Update. Aust. J. Chem. 2009, 62, 1402−1472. (d) Moad, G.; Rizzardo, E.; Thang, S. H. Living Radical Polymerization by the RAFT Process − A First Update. Aust. J. Chem. 2006, 59, 669−692. (e) Moad, G.; Rizzardo, E.; Thang, S. H. Living Radical Polymerization by the RAFT Process. Aust. J. Chem. 2005, 58, 379−410. (8) Patrickios, C. S.; Hertler, W. R.; Abbott, N. L.; Hatton, T. A. Diblock, ABC Triblock, and Random Methacrylic Polyampholytes: Synthesis by Group Transfer Polymerization and Solution Behavior. Macromolecules 1994, 27, 2364. (9) Liu, S.; Armes, S. P. Polymeric Surfactants for the New Millenium: A pH-Responsive, Zwitterionic, Schizophrenic Diblock Copolymer. Angew. Chem., Int. Ed. 2002, 41, 1413−1416. (10) Zhang, Q.; Hoogenboom, R. UCST Behavior of Polyampholytes Based on Stoichiometric RAFT Copolymerization of Cationic and Anionic Monomers. Chem. Commun. 2015, 51, 70−73. (11) Alfrey, T., Jr.; Morawetz, H. Amphoteric Polyelectrolytes. I. 2Vinylpyridine − Methacrylic Acid Copolymers. J. Am. Chem. Soc. 1952, 74, 436−438.

same effective pK values as those employed in Figure 7. The rather small difference in the effective pK values in the two types of units in the regular PA led to the merging of the two sigmoids practically into a single one. In contrast, the greater difference in the effective pK values in the two types of units in the inverse PA allowed the two sigmoidal branches of its hydrogen ion titration curve to remain well resolved.

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CONCLUSIONS We presented a comprehensive study in which we prepared, using controlled radical polymerization, a number of inverse PA hydrogels covering a wide range of acid−base composition, as well as the corresponding regular PA hydrogels. Both classes of PA hydrogels displayed a minimum in their aqueous swelling pH-profiles, with that of the inverse PAs being deeper and broader, with the former due to the greater hydrophobicity of the basic units employed in the inverse PAs, and the latter due to the greater separation in the effective pK values of the two types of units in inverse PAs. Contrary to the great sensitivity of the isoelectric points of the regular PA hydrogels to their acid− base composition, the isoelectric points of the inverse PA hydrogels were composition-independent, manifesting the reversed roles of the acidic and basic units in this intriguing novel class of PAs. Future work may involve the preparation of mixed PA hydrogels whose complexity would be increased by combining all three types of units, bearing both (conventionally) basic monomers and unconventionally very weakly basic monomers, in addition to the acidic ones.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00538. Evolution of the pH profiles of all the hydrogels during rinsing, following hydrolysis, and the DSC thermograms of all the networks before hydrolysis of the THPMA units (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(C.S.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the European Regional Development Fund (ERDF) and the Republic of Cyprus (RoC) for cofunding this work through the Cyprus Research Promotion Foundation (CRPF) within the project NEA YPODOMH/ NEKYP/0311/27. We are also grateful to the A. G. Leventis Foundation, and the ERDF, the RoC and the CRPF (Project NEA YPODOMH/NEKYP/0308/02) for the establishment of the NMR infrastructure at the University of Cyprus. Finally, we thank our colleagues Dr. S. I. Mirallai and Prof. P. A. Koutentis at the University of Cyprus for making available to us their DSC equipment.

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REFERENCES

(1) (a) Bekturov, E. A.; Kudaibergenov, S. E.; Rafikov, S. R. Synthetic Polymeric Ampholytes in Solution. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1990, C30 (2), 233−303. (b) Kudaibergenov, S. Recent K

DOI: 10.1021/acs.macromol.6b00538 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b00538 Macromolecules XXXX, XXX, XXX−XXX