Multistimuli-Responsive Emulsifiers Based on Two-Way Amphiphilic Diblock Polymers.

Diblock copolymers of poly(tert-butyl methacrylate) (PtBuMA) and poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) of four different block lengths were prepared by sequential two-step reversible addition-fragmentation chain transfer radical polymerization, followed by hydrolysis of the PtBuMA blocks to obtain poly(methacrylic acid)-b-PDMAEMA (PMAA-b-PDMAEMA). The effect of the PDMAEMA block length on the multistimuli-responsive amphiphilic features of both types of diblock copolymers was investigated as CO2-switchable emulsifiers for emulsification/demulsification of n-octane (an oil) in water in response to CO2/N2 bubbling. The amphiphilicity of PtBuMA-b-PDMAEMA was switched on, and the amphiphilicity of PMAA-b-PDMAEMA was switched off by CO2 bubbling at pH 12 and 25 °C to achieve emulsification/demulsification. A longer PDMAEMA block length in PMAA-b-PDMAEMA conferred more sensitive CO2-responsive amphiphilicity but reduced the extent of recovery of emulsification ability on N2 bubbling. This newly developed diblock copolymer system could potentially serve as a "multifunctional surfactant" for CO2-switchable emulsification/demulsification of oil-in-water and water-in-oil mixtures.

Introduction

The effects of pH, light, and temperature on the self-assembly of polymers in aqueous solutions have been extensively studied,[1−3] and gas stimuli have received increased attention in recent years.[4−6] The functions of polymers can be affected by adding gases into the polymer matrices, and most gases can be easily added and removed during the gas-bubbling process; thus, gas-responsive polymers have attracted a great deal of attention in both industry and research. Carbon dioxide (CO2) is a nonflammable, nontoxic, and easily removable gas that is inexpensive due to its abundance. CO2 has been reported to function as an environmentally friendly stimulus, for instance, for transformation of neutral and hydrophobic amidine groups or tertiary amine groups in water into charged and hydrophilic amidinium bicarbonate[4,7] or ammonium bicarbonate.[6,8] Thus, long chain alkyl amidines or tertiary amines can be transformed to amphiphilic molecules by CO2 bubbling and employed as emulsifiers in emulsion polymerization.[9,10] Such emulsification ability can be subsequently switched off by simply removing the CO2 by purging with an inert gas such as nitrogen, argon, or air.[4] Thus, using these CO2-triggered switchable emulsifiers, the resulting latexes can be easily destabilized by removing the CO2 by bubbling an inert gas and/or increasing the temperature to switch the active amidinium or ammonium bicarbonate emulsifier to a surface-inactive neutral compound, without the need for traditional approaches such as adding salts, strong acids, or alkalis. Furthermore, the aggregation and redispersion of the resulting latex particles can be reversibly controlled by alternating bubbling of N2 and CO2.[9,11−13]

Diblock copolymers that possess stimuli-responsive amphiphilicity have been also reported.[6,14,15] For instance, poly(N,N-dimethylaminoethyl methacrylate)-b-poly(N-isopropyl acrylamide) (PDMAEMA-b-PNIPAAm) exhibits temperature- and pH-triggered amphiphilicity under specific temperature and solution pH conditions. The temperature- and pH-responsive amphiphilic properties of these block copolymers in aqueous solution can lead to a variety of self-assembly processes; self-assembly was demonstrated to change from spherical micelles to vesicular structures with increasing PNIPAAm block length at fixed PDMAEMA block length.[16] Poly(N,N-diethylaminoethyl methacrylate)-b-poly(N-isopropyl acrylamide) (PDEAEMA-b-PNIPAAm) exhibits “schizophrenic” self-assembly in aqueous solution, depending on the polymer block length, solution pH, and temperature.[17] The schizophrenic self-assembly of diblock copolymers from vesicles to micelles in solution can be also controlled by bubbling CO2 at temperatures above or below the lower critical solution temperature (LCST) of the PNIPAAm block.[18] Therefore, these polymers have potential for a wide range of applications in various fields of engineering and medicine.[15,19,20]

Recently, we demonstrated that the amphiphilicity of PDMAEMA-b-PNIPAAm diblock copolymers with different PNIPAAm block lengths can be switched on/off by CO2/N2 bubbling at certain combinations of temperature and solution pH to enable the block copolymers to function as an emulsifier for oil-in-water emulsifications.[21] More importantly, phase transition studies and emulsion experiments clearly demonstrated that these copolymers possess dual CO2 and temperature responsiveness, which enabled efficient control of reversible emulsification processes under specific conditions. A longer PNIPAAm block length, which confers higher hydrophobicity to the PNIPAAm block at elevated temperatures (for instance, 40 °C), had a greater impact on the amphiphilicity of the block copolymers and their ability to function as an effective emulsifier. Our study[21] was the first example of CO2/N2-switchable diblock copolymer emulsifiers with reversibly controllable amphiphilic characteristics for achieving oil-in-water emulsification.

In addition to CO2-switchable amidine and tertiary amine functional groups (described above) that switch from neutral to cationic, the COOH group of carboxylic acids can also function as the CO2-switchable functional group that switches from anionic to neutral.[22] At high solution pH, CO2 converts the carboxylates (anionic organic bases) of carboxylic acids from a more hydrophilic form (COO–) to the less hydrophilic COOH group. The direction of the CO2 switch for the neutral forms of bases (e.g., amines and amidines) is the opposite of that for the negatively charged forms of bases (e.g., carboxylates). Poly(methacrylic acid) (PMAA) in aqueous solution at high pH is an example of an anionic organic base. At high solution pH, the relatively low hydrophilic COOH groups of PMAA dissociate into relatively high hydrophilic carboxylate anions (COO–) of PMAA. These carboxylate anions can reversibly associate with protons to form relatively low hydrophilic COOH groups on bubbling CO2. Thus, at high pH, the relatively high water solubility of PMAA can be converted into a relatively low water solubility of PMAA by lowering the pH by bubbling CO2. Therefore, the direction of the CO2-induced switch in water solubility is opposite for PMAA and PDMAEMA; PDMAEMA converts from low to high water solubility on CO2 bubbling.

In this study, PMAA (containing COOH groups) was block-copolymerized with PDMAEMA (containing tertiary amine groups) via hydrolysis of the poly(tert-butyl methacrylate) (PtBuMA) block in PtBuMA-b-PDMAEMA, which was prepared by sequential two-step reversible addition–fragmentation chain transfer radical polymerization (Scheme ). The opposite directions of the CO2-induced switches in the hydrophilicity of PMAA and PDMAEMA enabled the preparation of versatile, multifunctional block copolymer emulsifiers. The potential of these multistimuli-responsive diblock copolymers with different PDMAEMA block lengths as versatile CO2-switchable emulsifiers were investigated for the emulsification of n-octane (an oil) in water in response to CO2/N2 bubbling. This report reveals that the amphiphilicity of these diblock copolymers can be switched on/off by CO2 bubbling at certain combinations of temperature and solution pH to achieve oil-in-water emulsification. This novel diblock copolymer system provides a potential route toward the fabrication of multifunctional polymeric emulsifiers for emulsion applications.

Scheme 1

Synthetic Procedures for PtBuMA-b-PDMAEMA and PMAA-b-PDMAEMA

Results and Discussion

Preparation of PtBuMA-b-PDMAEMA via RAFT

The chemical structures of all polymers were resolved by 1H NMR and FTIR spectroscopy. Figure shows the 1H NMR spectra of PtBuMA-CTA, PDMAEMA-CTA, and the four PtBuMA-b-PDMAEMA block copolymers prepared by RAFT. In spectra (B) to (E) in Figure , the peak at 1.44 ppm was assigned to the CH3 of the tert-butyl groups in PtBuMA, and the peak at 2.35 ppm was assigned to the CH3 of the tertiary amine in PDMAEMA. The intensity of the 1.44 ppm peak decreased from spectra (B) to (E), whereas the intensity of the 2.35 ppm peak increased from (B) to (E), indicating that four block copolymers of different molecular weights were prepared and the molar ratio of the DMAEMA/tBuMA units increased from (B) to (E). This indicates that the degree of polymerization of PDMAEMA increased from (B) to (E) since the degree of polymerization of PtBuMA in the four block copolymers was fixed. By calculating the areas of the characteristic peaks at 0.87 ppm for CTA, the CH3 peak at 1.44 ppm for PtBuMA, and the CH3 peak at 2.35 ppm for PDMAEMA, the molecular weight of the PtBuMA block was determined from the 1H NMR spectra of PtBuMA-CTA to be 7100 g/mol, corresponding to a degree of polymerization of 50. The molecular weights of the four PtBuMA-b-PDMAEMA block copolymers were 8300, 11,000, 18,600, and 48,900 g/mol, as determined from the corresponding 1H NMR spectra. The molecular weights of the PDMAEMA block were calculated to be 1200, 3900, 11,500, and 41,800 g/mol. The copolymer compositions and degrees of polymerization of the block copolymers obtained were PtBuMA50-b-PDMAEMA7, PtBuMA50-b-PDMAEMA24, PtBuMA50-b-PDMAEMA72, and PtBuMA50-b-PDMAEMA266.

Figure 1

1H NMR spectra of (A) PtBuMA-CTA, (B) PtBuMA50-b-PDMAEMA7, (C) PtBuMA50-b-PDMAEMA24, (D) PtBuMA50-b-PDMAEMA72, (E) PtBuMA50-b-PDMAEMA266, and (F) PDMAEMA-CTA in CDCl3.

Figure S1 shows the FTIR spectra of PtBuMA-CTA, PDMAEMA-CTA, and the four PtBuMA-b-PDMAEMA block copolymers prepared by RAFT. In spectra (A) and (B) in Figure S1, the peaks at 1367 and 1393 cm–1 corresponded to CH3 bending in the C–(CH3)3 group of PtBuMA, whereas the peaks at 2770 and 2820 cm–1 were assigned to CH3 stretching in the N–CH3 group of PDMAEMA. The variations in intensity of these peaks in Figure S1C–F suggested that four block copolymers of different molecular weights were successfully prepared, consistent with the 1H NMR results. GPC was performed to identify molecular weights and molecular weight distributions of the polymers. Figure shows the GPC curves for PtBuMA and the four PtBuMA-b-PDMAEMA block copolymers in DMF containing 20 mM LiBr as the mobile phase. The Mn, Mw, and dispersity (ĐM) values calculated from the GPC curves are presented in Table S1. The GPC data revealed that the ĐM values of the five samples prepared by RAFT were in the range of 1.24–1.53 (Table S1); this molecular weight distribution is relatively narrow compared to polymers prepared by conventional free radical polymerization. In this study, PtBuMA-CTA was first prepared by RAFT, followed by subsequent RAFT of PDMAEMA using PtBuMA as a macro chain transfer agent. In the GPC curves in Figure , PtBuMA-CTA had the longest retention time, and retention time decreased as the molecular weight of PDMAEMA in the copolymer increased, further indicating that four copolymers with different molecular weights were successfully prepared. The molecular weight rank of the four copolymers based on the GPC data and 1H NMR data was similar. However, GPC overestimated the molecular weights compared to 1H NMR. This could be attributed to the different features of the two analytical techniques. Thus, DSC analyses were performed over the temperature range from −20 to 160 °C to verify the structures. Figure shows the second DSC heating curves of pure PtBuMA, pure PDMAEMA, and the four PtBuMA-b-PDMAEMA block copolymers. The Tg values obtained from Figure are listed in Table S2. The Tg values of pure PtBuMA and pure PDMAEMA were determined to be 125.0 and 15.3 °C from curves a and f in Figure , respectively. This indicates that PtBuMA is a rigid polymer, whereas PDMAEMA is a soft polymer. Each of the four block copolymers appeared to have a single Tg located between the Tg values of PtBuMA and PDMAEMA, and Tg decreased as the molecular weight of PDMAEMA in the copolymer increased. The single Tg suggests that PtBuMA is compatible with the PDMAEMA in the block copolymer.

Figure 2

GPC curves for (a) PtBuMA50, (b) PtBuMA50-b-PDMAEMA7, (c) PtBuMA50-b-PDMAEMA24, (d) PtBuMA50-b-PDMAEMA72, and (e) PtBuMA50-b-PDMAEMA266.

Figure 3

DSC curves for (a) PtBuMA50, (b) PtBuMA50-b-PDMAEMA7, (c) PtBuMA50-b-PDMAEMA24, (d) PtBuMA50-b-PDMAEMA72, (e) PtBuMA50-b-PDMAEMA266, and (f) pure PDMAEMA.

Hydrolysis of PtBuMA-b-PDMAEMA To Obtain PMAA-b-PDMAEMA Diblock Copolymer

Next, the PtBuMA block in the copolymers was further hydrolyzed with aqueous hydrochloric acid to yield PMAA-b-PDMAEMA diblock copolymer. The structures of the products obtained after hydrolysis were confirmed by FTIR spectroscopy and DSC. Figure shows the FTIR spectra of pure PMAA, pure PDMAEMA, and the four PMAA-b-PDMAEMA block copolymers obtained from hydrolysis of PtBuMA-b-PDMAEMA. A very broad peak at 3400–2400 cm–1, which overlapped the C–H stretching absorptions in Figure A, was attributed to the characteristic COOH stretching vibrations in PMAA. The peaks at 2775 and 2818 cm–1 are the characteristic CH3 stretching in the N–CH3 group of PDMAEMA (Figure B). The characteristic CH3 stretching peaks of pure PDMAEMA were shifted to lower frequencies in the PMAA-b-PDMAEMA block copolymers and increased in the intensity as the molecular weight of the PDMAEMA blocks increased (Figure C–F), perhaps due to formation of complexes of COOH groups and tertiary amines. Figure shows that PtBuMA-b-PDMAEMA of four different molecular weights was successfully hydrolyzed to obtain PMAA-b-PDMAEMA of four different molecular weights. The 1H NMR spectra of a PtBuMA-b-PDMAEMA block copolymer before and after hydrolysis in Figure S2 indicated that the heat treatment under acidic conditions did not noticeably affect structural characteristics of the PDMAEMA block. In other words, the PDMAEMA block survived the harsh acid treatment during hydrolysis of PtBuMA. To further verify the FTIR results, DSC was performed to examine the phase behavior and Tg of the block copolymers in the presence of the PMAA blocks. Figure shows the second DSC heating curves of pure PMAA, pure PDMAEMA, and the four PMAA-b-PDMAEMA block copolymers. The Tg values obtained from Figure are listed in Table S3. The Tg values of pure PMAA and PDMAEMA were 177.4 and 15.3 °C from curves a and f in Figure , respectively, indicating that PMAA is a very rigid polymer due to strong intermolecular hydrogen bonds between COOH groups, whereas PDMAEMA is a soft polymer. Each of the four block copolymers appeared to have single Tg, between the Tg values of PMAA and PDMAEMA, and Tg decreased as the molecular weight of PDMAEMA (which has a low Tg value) increased. The single Tg suggests that PMAA is compatible with PDMAEMA in the block copolymer. The significantly higher Tg of PMAA-b-PDMAEMA compared to that of PtBuMA-b-PDMAEMA proved that PtBuMA-b-PDMAEMA was hydrolyzed to PMAA-b-PDMAEMA, in confirmation of the previously discussed FTIR evidence.

Figure 4

FTIR spectra for (A) pure PMAA, (B) pure PDMAEMA, (C) PMAA50-b-PDMAEMA7, (D) PMAA50-b-PDMAEMA24, (E) PMAA50-b-PDMAEMA72, and (F) PMAA50-b-PDMAEMA266.

Figure 5

DSC curves for (a) PMAA50, (b) PMAA50-b-PDMAEMA7, (c) PMAA50-b-PDMAEMA24, (d) PMAA50-b-PDMAEMA72, (e) PMAA50-b-PDMAEMA266, and (f) pure PDMAEMA.

The surface tension values of aqueous solutions of the four PMAA-b-PDMAEMA copolymers were measured by the pendant drop method to investigate the surface activity of the copolymers as emulsifiers for emulsification of an oil-in-water mixture. As shown in Figure A, at 25 °C and pH 2, the surface tension of aqueous solutions of the PMAA-b-PDMAEMA block copolymers with four different molecular weights slightly decreased with the copolymer concentration. However, at 25 °C and pH 12, the surface tension of the PMAA-b-PDMAEMA aqueous solutions (Figure B)significantly decreased with the copolymer concentration up to 0.05 wt %, and then the surface tension leveled off above this concentration, indicating that the PMAA-b-PDMAEMA block copolymer exhibited higher surface activity in pH 12 solutions than in pH 2 aqueous solutions. The longer PDMAEMA block length appeared to lead to higher surface activity of the block copolymer aqueous solutions of pH 12. Collectively, at 25 °C and pH 2, PMAA and PDMAEMA are both water soluble, although PMAA has a relatively low solubility compared to PDMAEMA. The block copolymers exhibited obscured amphiphilicity under these conditions; thus, surface tension only slightly reduced as the concentration of block copolymer increased. At 25 °C and pH 12, PMAA is hydrophilic due to strong interactions between carboxylate groups and water, whereas PDMAEMA has a low hydrophilicity due to deprotonation of the tertiary amine groups. Thus, the block copolymers exhibited clear amphiphilicity under these conditions, and surface tension significantly decreased as the block copolymer concentration increased.

Figure 6

Surface tension of aqueous solutions of (a, e) PMAA50-b-PDMAEMA7, (b, f) PMAA50-b-PDMAEMA24, (c, g) PMAA50-b-PDMAEMA72, and (d, h) PMAA50-b-PDMAEMA266 as a function of concentration at 25 °C. The aqueous solutions were adjusted to pH 2 in (A) and pH 12 in (B).

pH Responsiveness of PMAA-b-PDMAEMA

Prior to hydrolysis of PtBuMA to PMAA, PtBuMA is hydrophobic and insoluble in water. PtBuMA-b-PDMAEMA is only water soluble at very low pH, for example, pH 2, as the pH-responsive PDMAEMA is highly water soluble at pH 2. After hydrolysis, PMAA exhibits pH-responsive solubility in water, with low water solubility at low pH and high water solubility at high pH. This encouraged us to investigate the pH responsiveness of PMAA-b-PDMAEMA in water, as PMAA and PDMAEMA exhibit opposite responses to changes in pH. The two-way pH responsiveness of PMAA-b-PDMAEMA in water is associated with the switch from protonation to deprotonation in response to a change from low to high pH for both PMAA and PDMAEMA. However, the switch from protonation to deprotonation leads to opposite changes in water solubility of each polymer. The water solubility of PMAA increases and that of PDMAEMA decreases in response to a change from low to high pH.

The two-way pH responsiveness of PMAA-b-PDMAEMA in between pH 2 and 12 is demonstrated in Figure in which PMAA-b-PDMAEMA aqueous solutions appeared to be cloudy (an indication of formation of cation/anion complexes) in an intermediate pH range. The positive charges (cations) of PDMAEMA came from protonation of tertiary amine groups, and the negative charges (anions) of PMAA were from dissociation of COOH groups. The positive charges increased with decreasing pH values, whereas the negative charges increased with increasing pH values. The pKa values are around 7.5 to 7.8 for PDMAEMA[23,24] and near 5.5 for PMAA.[25] As solution pH increases above the pKa value of a polymer, deprotonation prevails over protonation for the polymer. As solution pH decreases below the pKa value of a polymer, protonation prevails over deprotonation for the polymer. For the same PDMAEMA and PMAA block lengths, therefore, the cloudy solution (complex formation) would occur at a pH near the middle value (around 6.5) between the two corresponding pKa values. The pH at which cloudy solution appeared would be higher than this middle value (around 6.5) as the PDMAEMA block length was longer than the PMAA block and would be lower than 6.5 as the PDMAEMA block length was shorter than the PMAA block as can be seen in Figure .

Figure 7

pH responsiveness of PMAA-b-PDMAEMA aqueous solutions at 25 °C. Each photo was taken 4 h after sonication.

CO2-Switchable PtBuMA-b-PDMAEMA and PMAA-b-PDMAEMA Emulsifiers

To investigate the potential of the PtBuMA-b-PDMAEMA and PMAA-b-PDMAEMA block copolymers as CO2-switchable emulsifiers, 0.5 wt % copolymer in 5 mL of deionized water at pH 2 or 12 and 25 °C was mixed with 1 mL of n-octane to form separated, two-phase solutions with the aqueous layer on the bottom and the oil (n-octane) on the top. The two-phase pH 2 and 12 mixtures were ultrasonically vibrated to investigate the ability of the block copolymers to emulsify n-octane in water to form a single-phase solution. The phase change was recorded 4 h after ultrasonic vibration to ensure phase stability. The added copolymer could serve as an emulsifier if the two-phase mixture became a homogeneous, single-phase solution. If the mixture remained as two separate phases, the copolymer could not serve as an emulsifier.

As shown in Figure , single-phase solutions formed in all four mixtures with PtBuMA-b-PDMAEMA at pH 2, indicating that the four block copolymers all served as an emulsifier at pH 2 and 25 °C. In contrast, the pH 12 mixtures remained as two-phase solutions, indicating that the four block copolymers cannot serve as an emulsifier at pH 12 and 25 °C. At pH 2, PDMAEMA is protonated and becomes water soluble, while PtBuMA remains hydrophobic, so the block copolymers are amphiphilic and could serve as an emulsifier. At pH 12, PDMAEMA is deprotonated and has low hydrophilicity, while PtBuMA remains hydrophobic, so the copolymers exhibited obscured amphiphilic characteristics and could not serve as an efficient emulsifier to emulsify the oil in water.

Figure 8

Phase changes for mixtures of 1 mL of n-octane and 5 mL of deionized water at pH 2 or 12 and 25 °C containing 2.5 wt % PtBuMA-b-PDMAEMA relative to n-octane. The pH 2 and 12 mixtures were ultrasonically vibrated for 1 min, and each corresponding phase change was recorded 4 h after ultrasonic vibration. The photos in the last two columns were taken 4 h after CO2 or N2 bubbling. After CO2 bubbling and subsequent N2 bubbling, the pH values of the pH 12 solution decreased to 5.4 and increased to 8.5, respectively.

Upon bubbling CO2 for 5 min into the pH 12 mixture at 25 °C, the separated two-phase mixture reverted to a single phase, indicating that CO2 bubbling reduced the pH and allowed the block copolymer to serve as an emulsifier again. The mixture containing PtBuMA50-b-PDMAEMA266 did not appear to return to a homogeneous single phase; it is possible that 5 min of CO2 bubbling at 20 mL/min was not sufficient to confer water solubility to the high molecular weight of PDMAEMA (because of chain entanglements) in this copolymer. The emulsification ability of the copolymers was reduced after subsequent N2 bubbling for 30 min to remove CO2, as indicated by phase separation of the mixtures (Figure ). The removal of CO2 increased the pH, leading to deprotonation and reduction in the water solubility of PDMAEMA and the amphiphilicity of the block copolymers.

As can be seen in Figure , at pH 2 and 25 °C, the mixture containing PMAA50-b-PDMAEMA266 appeared to be a single-phase solution, whereas the mixtures containing the other three block copolymers appeared to have small portions of clear phases at the bottom, indicating that PMAA50-b-PDMAEMA266—with the highest molecular weight of the PDMAEMA block—could serve as a highly efficient emulsifier, whereas the other three block copolymers with shorter PDMAEMA block lengths could only serve as a fairly good emulsifier. On the contrary, at pH 12 and 25 °C, the three block copolymers with relatively low molecular weights of PDMAEMA served as better emulsifiers than PMAA50-b-PDMAEMA266, which could only serve as a fair emulsifier—perhaps due to the high molecular weight and thus high chain entanglement of PDMAEMA, which may hinder formation of micelles. These findings revealed that the amphiphilicity of the PMAA-b-PDMAEMA block copolymer is strongly pH- and molecular weight-dependent. The PMAA-b-PDMAEMA block copolymers could serve as an effective emulsifier at both pH 2 and 12 if the PDMAEMA block length was appropriate. In theory, at low pH (pH 2), PMAA was considered to have relatively low water solubility and PDMAEMA had relatively high water solubility. In contrast, at high pH (pH 12), PMAA was considered to have relatively high water solubility and PDMAEMA had relatively low water solubility. Additionally, water solubility could be affected by the molecular weights of the constituent blocks. Therefore, the emulsification ability of the PMAA-b-PDMAEMA block copolymers was determined by pH and the molecular weights of the constituent blocks.

Figure 9

Phase changes in mixtures of 1 mL of n-octane and 5 mL of deionized water at pH 2 or 12 and 25 °C containing 2.5 wt % PMAA-b-PDMAEMA relative to n-octane. The pH 2 and 12 mixtures were ultrasonically vibrated for 1 min, and the phase changes were recorded 4 h after ultrasonic vibration. The photos in the last two columns were taken 4 h after CO2 or N2 bubbling.

Upon bubbling CO2 into the pH 12 mixture for 3 min, the pH reduced to around pH 6 at which interpolyelectrolyte complexation was found for PMAA50-b-PDMAEMA7 and PMAA50-b-PDMAEMA24 (Figure ). The emulsification abilities of PMAA50-b-PDMAEMA72 and PMAA50-b-PDMAEMA266 were clearly reduced due to a reduced amphiphilicity at pH 6 (Figure ), whereas PMAA50-b-PDMAEMA7 and PMAA50-b-PDMAEMA24 were remained as a fairly good emulsifier at pH 6 (Figure ) likely due to the Pickering effect of the interpolyelectrolyte complex. Upon subsequent N2 bubbling for 30 min, the pH was back up to near pH 8 at which interpolyelectrolyte complexation was found for PMAA50-b-PDMAEMA72 and PMAA50-b-PDMAEMA266 (Figure ). The emulsification abilities of PMAA50-b-PDMAEMA72 and PMAA50-b-PDMAEMA266 were slightly recovered at pH 8 likely due to the Pickering effect of the interpolyelectrolyte complex, whereas those of PMAA50-b-PDMAEMA7 and PMAA50-b-PDMAEMA24 increased at pH 8 (Figure ) likely due to a stable Pickering effect of the interpolyelectrolyte complex formed from pH 6 and/or an increase in amphiphilicity. In PMAA50-b-PDMAEMA7 and PMAA50-b-PDMAEMA24, the short PDMAEMA block lengths could have a great extent of recovery of hydrophobicity and lead to an increase in amphiphilicity of the two block copolymers by bubbling N2 because of a fast removal of CO2, as compared to PMAA50-b-PDMAEMA72 and PMAA50-b-PDMAEMA266. This observation confirmed that the process of CO2/N2 gas bubbling can be applied to achieve manipulation and dispersion of the PMAA-b-PDMAEMA block copolymers at the water/oil interface.

Conclusions

PtBuMA was block-copolymerized with PDMAEMA via two steps of RAFT polymerization to form PtBuMA-b-PDMAEMA block copolymers, followed by hydrolysis of the PtBuMA blocks to obtain PMAA-b-PDMAEMA. PtBuMA-b-PDMAEMA exhibited clear amphiphilic features and could serve as an efficient emulsifier at pH 2 and 25 °C. At pH 12 and 25 °C, PtBuMA-b-PDMAEMA exhibited an obscured amphiphilic feature but could be switched to an efficient emulsifier by CO2 bubbling and lost its emulsification ability on N2 bubbling. PMAA-b-PDMAEMA block copolymers could serve as effective emulsifiers at both pH 2 and 12 if the PDMAEMA block length was appropriate. The amphiphilicity of PMAA-b-PDMAEMA at pH 12 and 25 °C could be reduced by CO2 bubbling to reduce its emulsification ability, and its emulsification ability could be recovered on N2 bubbling; the duration of bubbling required strongly depended on the PDMAEMA block length. A longer PDMAEMA block length in PMAA-b-PDMAEMA conferred more sensitive CO2-responsive amphiphilicity but a lower recovery of emulsification ability on N2 bubbling. This study offers significant potential for the fabrication of two-way, dual-responsive diblock copolymers with controllable structures that offer two-way changes in amphiphilicity, excellent CO2 and pH responsiveness, and reversible emulsification properties in both organic and aqueous media.

Experimental Section

Materials

Tert-butyl methacrylate (tBuMA, 97%, Tokyo Chemical Industry) and 2-(dimethylamino)ethyl methacrylate (DMAEMA, 98%, Sigma-Aldrich) were purified by distillation at reduced pressure. Azobis(isobutyronitrile) (AIBN, Aldrich) was recrystallized from methanol. 2-Cyano-2-propyl-dodecyl trithiocarbonate (CPDT, >97%, Strem Chemicals) was used as received. 1,4-Dioxane (J.T.Baker) was distilled prior to use. All other solvents were obtained at the highest purity available and used as received.

Two-Step Reversible Addition–Fragmentation Chain Transfer (RAFT) Radical Polymerization of PtBuMA-b-PDMAEMA Diblock Copolymer

We present the two-step synthetic approach for the preparation of PtBuMA-b-PDMAEMA diblock copolymer in Scheme . A solution of tBuMA monomer (17.5 g, 0.123 mol), CPDT (0.21 g, 6.15 × 10–4 mol) as a chain transfer agent, and AIBN (0.01 g, 6.15 × 10–5 mol) as an initiator in 1,4-dioxane (20 mL; molar concentration ratio, 2000:10:1) in a reaction vessel was degassed through three freeze/pump/thaw cycles and placed in a thermostat oil bath (70 °C) to initiate the reaction with stirring under N2 for 48 h. The reaction product was precipitated in MeOH/water (4:1, 400 mL), filtered, and vacuum dried at 60 °C for 2 days to obtain the tBuMA homopolymer (PtBuMA) (yield: 41.9 wt %).

DMAEMA comonomer of four different weights (for preparing four different PDMAEMA block lengths), PtBuMA (as a RAFT macro chain transfer agent), and AIBN were dissolved in 1,4-dioxane (20 mL) in the reaction vessel. The molar ratios of DMAEMA/PtBuMA/AIBN were 8000:10:1, 4000:10:1, 2000:10:1, and 1000:10:1. The DMAEMA molar concentration was 1 M. The mixture was degassed through three freeze/pump/thaw cycles and placed in a thermostat oil bath (70 °C) to initiate block copolymerization with stirring under N2 for 24 h. The reaction product was precipitated in n-hexane (400 mL) to remove unreacted DMAEMA by decanting the upper portion of the solution. The polymer solution in the lower portion was dissolved in MeOH (50 mL) and added to n-hexane (400 mL), and the upper portion of the solution was decanted again. After repeating the same purification step for the third time, the precipitate was dried under vacuum for 2 days to obtain PtBuMA-b-PDMAEMA block copolymers with PDMAEMA blocks of four different molecular weights. 1H NMR analysis of the copolymer compositions revealed the degrees of polymerization for the obtained block copolymers were PtBuMA50-b-PDMAEMA266, PtBuMA50-b-PDMAEMA72, PtBuMA50-b-PDMAEMA24, and PtBuMA50-b-PDMAEMA7.

Hydrolysis of PtBuMA-b-PDMAEMA To Obtain PMAA-b-PDMAEMA Diblock Copolymer

Subsequently, the tert-butyl groups of the PtBuMA blocks in the copolymers were hydrolyzed with dilute hydrochloric acid to yield PMAA-b-PDMAEMA diblock copolymers, as shown in Scheme . Hydrolysis of the PtBuMA blocks of the copolymer (1 g) was conducted in the presence of aqueous hydrochloric acid (2 mL, 12 M) in 1,4-dioxane (15 mL) under reflux for 8 h, and then the reaction mixture was precipitated in ether (200 mL). After decanting the upper portion of the solution, the polymer solution in the lower portion was precipitated twice with ether (200 mL), and then the precipitate was dried under vacuum for 2 days to give PMAA50-b-PDMAEMA266, PMAA50-b-PDMAEMA72, PMAA50-b-PDMAEMA24, and PMAA50-b-PDMAEMA7. The block copolymers were subsequently dissolved in DI water (30 mL) and purified by dialysis (molecular weight cutoff: 6000–8000 g/mol) against water for 3 days to completely remove low-molecular-weight molecules. High-purity PMAA-b-PDMAEMA was obtained after vacuum drying at 60 °C for 2 days.

Characterizations

An 1H NMR (JNM-ECZ600R) spectrometer (600 MHz, JEOL, Japan) was used to quantitatively characterize the copolymer compositions and molecular weights of PtBuMA-b-PDMAEMA in CDCl3 solvent and PMAA-b-PDMAEMA in D2O solvent. Fourier transform infrared spectrometry (FTIR, Spectrum Two, PerkinElmer, USA) was used to qualitatively characterize the PtBuMA-b-PDMAEMA block copolymers after casting THF solutions onto KBr and the PMAA-b-PDMAEMA block copolymers after casting DMF solutions onto CaF2. The spectra were generated at room temperature at a resolution of 2 cm–1 and a sensitivity of 16 scans.

The molecular weights and dispersity (ĐM) of the synthesized PtBuMA-b-PDMAEMA copolymers were determined using a gel permeation chromatography (GPC) system equipped with a series of two columns (PLgel 10 μm Mixed-B, Polymer Laboratories, UK) and a refractive index detector. DMF containing 20 mM LiBr was the eluent at a flow rate of 0.6 mL min–1 at 35 °C.

The glass transition temperatures (Tg) of the copolymers were determined by differential scanning calorimetry (DSC, Q100, TA Instruments, New Castle, DE, USA); the cycle conditions were heating (20 °C/min) to 180 °C, cooling (10 °C/min) to −50 °C, and then heating at 20 °C /min to 180 °C to record Tg.

Measurements on pendant drops of pH 2 and 12 aqueous solutions of PMAA-b-PDMAEMA were conducted to determine the surface tension of the solutions as a function of copolymer concentration at 25 °C using a contact angle analyzer (Model 100SL, Sindatek Instruments Co., Taiwan). The pH responsiveness of 0.5 wt % PMAA-b-PDMAEMA in water was investigated by adjusting the pH of the solutions using HCl or NaOH to pH 2, 4, 6, 8, 10, or 12. A digital camera was used to record phase changes. The photos were taken 4 h after each pH change.

CO2-Switchable PtBuMA-b-PDMAEMA and PMAA-b-PDMAEMA Emulsifiers

To investigate the potential of PtBuMA-b-PDMAEMA and the PMAA-b-PDMAEMA block copolymers as CO2-switchable emulsifiers, 0.5 wt % of each copolymer in 5 mL of DI water was adjusted to pH 2 or 12 and then mixed with 1 mL of n-octane (oil phase). The emulsification abilities of the copolymers were examined after ultrasonic vibration of the two-phase mixtures using a sonicator (Sonifier 150D, Branson, Danbury, CT) at 6.5 W for 1 min at 25 °C and recorded using a digital camera. The pH 12 solutions were subsequently bubbled with CO2 for 5 min (for PtBuMA-b-PDMAEMA as an emulsifier) or 3 min (for PMAA-b-PDMAEMA as an emulsifier) to investigate the CO2 switchability of emulsification/demulsification, followed by N2 bubbling for 30 min to investigate the recovery of demulsification/emulsification. Photographs were taken 4 h after each CO2 or N2 bubbling. The bubbling flow rates were controlled at 20 mL/min.