Qijing
Chen
a,
Yuanyuan
Xu
a,
Xueteng
Cao
a,
Lianjie
Qin
b and
Zesheng
An
*a
aInstitute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, P. R. China. E-mail: an.zesheng@shu.edu.cn
bSchool of Environmental and Material Engineering, Yantai University, Yantai 264005, P. R. China
First published on 30th August 2013
A series of five core cross-linked star (CCS) polymers has been synthesized by RAFT-mediated heterogeneous polymerization in aqueous media. These CCS polymers consist of poly(MEAx-co-PEGAy) (MEA is 2-methoxyethyl acrylate and PEGA is poly(ethylene glycol) acrylate) of different compositions. They exhibit responsiveness to both temperature and salt. Addition of kosmotropes reduces the cloud point, and addition of chaotropes raises the cloud point. These CCS polymers can stabilize dodecane-in-water high internal phase emulsions (HIPEs) in the absence or presence of salts. Addition of salts has negligible effect on the maximum oil fraction, the size of oil droplets or the stability of the HIPEs. HIPEs with large oil fractions (up to 92 vol%) at low concentrations of CCS polymers (≤1 wt%) can be routinely obtained. When the oil volume fraction is larger than 74 vol%, gelled HIPEs are formed with long-term stabilities (more than three months). The temperature and salt responsiveness of the CCS polymers is transferred to the CCS-stabilized HIPEs. Addition of kosmotropes can enhance demulsification efficiency, while addition of chaotropes can decrease demulsification efficiency or enhance the thermal stability of the CCS-stabilized HIPEs. The emulsification–demulsification cycle can be successfully repeated four times. The facile and aqueous synthesis of CCS polymers and the fast response of the CCS-stabilized HIPEs will open up new opportunities for the preparation and exploitation of a range of smart soft materials.
Recently, much effort has been focused on responsive emulsions in order to generate “smart” soft materials by using stimuli-responsive emulsifiers.16–18 Responsive emulsions have been realized using small molecular surfactants,19–21 polymers4 and particles.9,10,12,22–24 One particularly studied area is on demand triggering of demulsification.9,12
High internal phase emulsions (HIPEs) with an internal phase volume fraction larger than 0.74 have attracted much recent attention due to their applications in various technological sectors.25,26 Traditionally, HIPEs are prepared with a large amount of surfactant (5–50 wt%).25 Several types of colloidal particles have been studied for the formation of HIPEs of both oil-in-water (o/w) and water-in-oil (w/o) types, including titania,27 silica,28,29 Fe3O4,30 polymer particles18,31,32 and microgels.33
Core cross-linked star (CCS) polymers have been actively studied for a wide range of applications.34–50 We have recently developed several CCS polymers and investigated their use as emulsifiers.51–53 These CCS polymers are prepared via reversible addition–fragmentation chain transfer (RAFT) cross-linking copolymerization in aqueous heterogeneous media, which is a novel strategy, in contrast to the prevailing synthesis in homogeneous solutions, for the efficient synthesis of well-defined CCS polymers.51–58 CCS polymers have many arms connected a central, highly cross-linked core that is smaller in size than the dimensions of the arms. These materials can be regarded as an intermediate between common soluble polymers and polymeric nanoparticles, in that CCS polymers have a high fraction of flexible arms and also possess a hydrodynamic diameter typically in the 10–50 nm range. The arm flexibility allows CCS polymers to adopt a favorable configuration at the interface, and the nanometre size provides sufficient energy (>kBT) to stabilize the interface. Importantly, the flexibility and nanometre size guarantee the fast responsiveness of the CCS polymers to external stimuli. These unique properties have led to their use as interfacial stabilizers, including dispersing agents for carbon nanotubes,38 stabilizers for nanoprecipitation process57 and emulsifiers in emulsions.51–53
We previously reported the first example of CCS-stabilized HIPEs that showed high stability at low concentrations of stabilizer and fast responsiveness to pH changes using a poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) CCS polymer.52 Despite the fact that the CCS polymer demonstrated high efficiency for the formation of HIPEs and endowed them with fast responsiveness, several important scientific questions still need to be addressed. For instance, are other CCS polymers also effective in stabilizing HIPEs? Can other types of responsiveness be incorporated into the CCS polymers and thus into the CCS-stabilized HIPEs? Constructive answers to these questions will significantly expand the scope and versatility of CCS-stabilized HIPEs and help to establish the importance of CCS-stabilized HIPEs in emulsion and emulsion-templating technologies. In this article, we attempt to address these questions by synthesizing a series of novel CCS polymers that are responsive to both temperature and salts, and investigating their effectiveness as stabilizers for HIPEs and triggered demulsification (Scheme 1). This study demonstrates the first example of temperature and salt dual responsive CCS-stabilized HIPEs.
CCS polymer synthesis using P(MEA80-co-PEGA20) was conducted at 35 °C in water, using the redox initiator sodium ascorbate/potassium persulfate (NaAs/KPS), in the presence of the cross-linker PEGDA and a spacing monomer BA. The molar ratio of P(MEA80-co-PEGA20):
PEGDA
:
BA
:
NaAs
:
KPS was controlled at 1
:
5
:
10
:
0.1
:
0.1.
CCS polymer syntheses using P(MEA56-co-PEGA9) or P(MEA74-co-PEGA8) were conducted at 70 °C in 1:
1 water–ethanol using V-50 as the initiator, in the presence of the cross-linker HDDA. The molar ratio of (P(MEA56-co-PEGA9) or P(MEA74-co-PEGA8))
:
HDDA
:
V-50 was controlled at 1
:
10 or 20
:
0.1.
The yield of CCS polymer was calculated from GPC chromatograms as:
CCS polymer yield = area of CCS polymer/(area of CCS polymer + area of low molecular weight polymers). |
The synthesized CCS polymers are designated as S(Dh, CP), where Dh and CP represent the hydrodynamic diameter at 25 °C and the CP of the CCS polymer measured at a concentration of 0.5 wt%.
For detailed information on the synthesis of the CCS polymers, please refer to the ESI.†
CCS code | Arm polymer | Yielda (%) | Purityb (%) | M n (kg mol−1) | Đ | N arm | χ arm (%) | D h (nm) | PDIg | CPh (°C) |
---|---|---|---|---|---|---|---|---|---|---|
a CCS yield determined by GPC, CCS yield = ACCS/(ACCS + Alow molecular weight polymers).
b CCS purity after extensive ultrafiltration.
c Determined by triple-detection GPC.
d Determined by RI-GPC (PMMA standard).
e Average arm number per CCS polymer.
f Arm mass fraction.
g Determined by DLS at 25 °C.
h Determined by variable temperature DLS.
i Molar ratio of P(MEA80-co-PEGA20)![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
||||||||||
S(16 nm, 51 °C)i | P(MEA80-co-PEGA20) | 80 | 98.3 | 278.9 | 1.16 | 23 | 86 | 16 | 0.23 | 51 |
S(35 nm, 40 °C)j | P(MEA56-co-PEGA9) | 93 | 98.4 | 193.5 | 1.19 | 28 | 83 | 35 | 0.17 | 40 |
S(43 nm, 38 °C)j | P(MEA56-co-PEGA9) | 90 | 98.4 | 194.2 | 1.26 | 22 | 71 | 43 | 0.15 | 38 |
S(19 nm, 31 °C)j | P(MEA74-co-PEGA8) | 83 | 98.2 | 149.6 | 1.17 | 17 | 84 | 19 | 0.14 | 31 |
S(43 nm, 31 °C)j | P(MEA74-co-PEGA8) | 90 | 98.3 | 195.7 | 1.16 | 21 | 73 | 43 | 0.15 | 31 |
The CCS polymers were efficiently synthesized with various star yields (≥80%), which were purposely controlled, along with other synthetic parameters, as a means to adjust the final molecular weights and thus the sizes of the CCS polymers. In order to remove the low-molecular-weight species, the CCS polymers were purified by extensive ultrafiltration, and the purity of all the CCS polymers was higher than 98% as determined by GPC (Fig. 1). The purified CCS polymers were characterized by NMR, GPC and dynamic light scattering (DLS) (Table 1). The molecular weight (Mn) of the CCS polymers was in the range of 150–279 kg mol−1 and each CCS polymer had a low dispersity Đ (≤1.26). The number of arms per CCS polymer was in the range of 17–28. The hydrodynamic diameter (Dh) determined by DLS at 25 °C was between 16 nm and 43 nm. S(16 nm, 51 °C) has the largest Mn but the smallest Dh, which may seem contradictory. However, these results were fully reproducible from repeated measurements. It should be noted that the arms in S(16 nm, 51 °C) are composed of P(MEA80-co-PEGA20) and have the highest proportion of PEGA grafts. As a result, the arms in S(16 nm, 51 °C) have a higher molecular weight and more compact structure than those in other CCS polymers. In addition, both the core and the arms contribute to the size and molecular weight of the CCS polymers.
The CPs of the CCS polymers, in the absence or presence of different salts, were measured by DLS. Fig. 2A illustrates the thermal response of the CCS polymers in water. Each CCS polymer exhibits a sharp response to temperature change upon heating, i.e., the observed Dh is significantly increased due to the aggregation of the CCS polymers, which become more hydrophobic upon heating through the CP. The CP value of the CCS polymers ranges from 31 °C to 51 °C and increases upon increasing the molar ratio of the more hydrophilic PEGA in the arm polymer.
In order to evaluate the effect of added salts on the CPs of the CCS polymers, we selected S(43 nm, 38 °C) and S(43 nm, 31 °C) as representative CCS polymers, because these two CCS polymers have the same hydrodynamic diameter but different CPs, which is convenient for comparing the effect of CP in triggered demulsification by added salts (vide infra). From Fig. 2B and C, we can see that addition of salts can significantly change the CPs, depending on the type and concentration of the added salts. For both CCS polymers, kosmotropes (Na2SO4 and NaCl) lower the CPs while chaotropes (NaClO4 and NaSCN) increase the CPs, and the magnitude of change in CP agrees well with the Hofmeister series.59
![]() | ||
Fig. 3 Photograph of emulsions stabilized by S(43 nm, 31 °C) (1.0 wt% in water) with indicated dodecane volume fractions. |
As shown in Table 2, the maximum oil fraction in stable HIPEs is as high as 92 vol% and the lowest CCS polymer concentration required for the formation of stable HIPEs containing 80 vol% of oil is as low as 0.3 wt%. While most CCS polymers show similar emulsifying efficiency, S(16 nm, 51 °C), possessing both the smallest Dh and the highest CP, both of which are unfavourable for stabilization, is less effective in forming HIPEs than the other CCS polymers as it requires a higher CCS polymer concentration to stabilize 80 vol% HIPE.
CCS polymer | CCS polymer concentration (wt%) | Oil fraction (vol%) | Lowest CCS concentrationb | |
---|---|---|---|---|
a The total volume of emulsion is 1.5 mL, dodecane is the oil phase, and CCS polymers are in the aqueous phase. b Lowest CCS concentration required to form a HIPE of 80 vol% oil. | ||||
1 | S(16 nm, 51 °C) | 1.0 | 88 | 1.0 |
2 | 2.0 | 90 | ||
3 | S(35 nm, 40 °C) | 0.3 | 87 | 0.3 |
4 | 0.4 | 91 | ||
5 | 0.5 | 90 | ||
6 | 1.0 | 89 | ||
7 | S(43 nm, 38 °C) | 0.3 | 86 | 0.3 |
8 | 0.5 | 92 | ||
9 | 1.0 | 88 | ||
10 | S(19 nm, 31 °C) | 0.4 | 90 | 0.4 |
11 | 0.5 | 88 | ||
12 | 1.0 | 88 | ||
13 | S(43 nm, 31 °C) | 0.3 | 90 | 0.3 |
14 | 0.5 | 92 | ||
15 | 1.0 | 89 |
The confocal laser scanning microscopy (CLSM) images shown in Fig. 4 confirm that the formed emulsions are oil-in-water (o/w) emulsions. The emulsions consist of highly dispersed oil droplets that are characteristic of common emulsions. Upon increasing the CCS polymer concentration from 0.3 wt% (Fig. 4A) to 0.5 wt% (Fig. 4B) to 1.0 wt% (Fig. 4C) for HIPEs containing 80 vol% of oil, the average diameter of the oil droplets in the HIPEs drops from 35 μm to 20 μm to 12 μm, suggesting that a higher concentration of CCS polymers can reduce the size of the oil droplets and thus create more interfacial areas. However, the size of the oil droplets is shown to be insensitive to the oil fraction when the CCS polymer concentration is maintained at 0.5 wt% but the oil volumetric fraction is increased from 30 vol% (Fig. 4D) to 50 vol% (Fig. 4E) to 80 vol% (Fig. 4B); the average diameter of the oil droplets remains essentially unchanged (about 20 μm), except that the oil droplets pack more tightly in the HIPE (80 vol%). These results suggest that when the CCS polymer concentration is kept constant the droplet size is mainly determined by the input energy of the homogenizing process, which is the same for all these samples.
We have previously shown that CCS polymers adsorb onto the oil/water interface to stabilize the oil droplets. The adsorption of CCS polymers at the interface is expected to reduce the interfacial energy between dodecane and water. We performed interfacial tension measurements using the pendant drop method to assess the ability of the CCS polymers to reduce the interfacial energy. Table 3 shows the interfacial tensions for all five CCS polymers at 0.5 wt% CCS concentration and 25 °C. The interfacial tension is significantly reduced from 53.4 mN m−1 for dodecane/water to values of 8–12 mN m−1 in the presence of CCS polymers. Within experimental error, two extremes can be identified, with S(16 nm, 51 °C) showing the highest interfacial tension of 11.9 mN m−1 and S(43 nm, 31 °C) showing the lowest interfacial tension of 8.0 mN m−1. The interfacial tension seemed to be insensitive to the CCS polymer concentration; increasing the concentration of S(19 nm, 31 °C) from 0.3 wt% to 0.5 wt% to 1.0 wt% produced rather similar interfacial tensions of 9.5, 8.7 and 9.3 mN m−1, respectively. The insensitivity of interfacial tension to the CCS polymer concentration seems to be inconsistent with the CLSM results that show reduced droplet size with increasing CCS polymer concentration. Previously, Li and co-workers also observed that interfacial tension could not be directly correlated with the emulsifying behavior using other CCS polymers.62 Both our results and literature results suggest that CCS polymers play multiple roles rather than just lowering the interfacial tension. The viscoelasticity of CCS polymers at the interface that accounts for the coalescence stability of droplets should also be considered.
CCS | S(16 nm, 51 °C) | S(35 nm, 40 °C) | S(43 nm, 38 °C) | S(19 nm, 31 °C) | S(43 nm, 31 °C) |
---|---|---|---|---|---|
a Measured by pendant drop method for aqueous solutions of 0.5 wt% CCS polymers and dodecane. | |||||
Interfacial tension (mN m−1) | 11.9 | 9.6 | 8.8 | 8.7 | 8.0 |
Having shown by vial inversion that the HIPEs were stable against creaming and coalescence, the properties of the HIPEs were further quantitatively investigated by rheology. Fig. 5A shows the changes in storage modulus (G′) and loss modulus (G′′) during strain sweeps at a fixed frequency of 1.0 Hz and 25 °C for HIPEs (80 vol% oil) stabilized by S(43 nm, 38 °C) at different concentrations. A linear regime for G′ is observed for all the HIPEs at low strains (<1%), and G′ increases monotonously with increasing CCS polymer concentration. In this linear regime, G′ is always significantly higher than G′′, indicating the solid-like properties of these HIPEs under the applied conditions. All the HIPEs show a yield point (crossover of G′ and G′′) at which the gels turn into fluidic emulsion droplets. The yield point first increases when the CCS polymer concentration is increased from 0.3 wt% to 0.5 wt%, and then decreases when the CCS polymer concentration is further increased to 2.0 wt%, suggesting that a higher storage modulus is achieved at the expense of reduced elasticity of the HIPEs when the concentration of CCS polymers is increased beyond a critical value (0.5 wt%). Fig. 5B shows the changes in storage modulus, G′, and loss modulus, G′′, during frequency sweeps at a fixed strain of 1.0% and 25 °C for these samples. G′ is always higher than the corresponding G′′, and G′ is almost independent of the frequency applied, confirming again the solid-like properties of these HIPEs.
Water | Na2SO4 | NaCl | NaClO4 | NaSCN | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a CPs were measured for 0.5 wt% CCS polymer solutions. HIPEs were formed using 0.5 wt% CCS polymer solutions in the absence or presence of salts. | ||||||||||||||
S(43 nm, 38 °C) | Concentration (mol L−1) | 0.05 | 0.1 | 0.2 | 0.1 | 0.3 | 0.4 | 0.1 | 0.3 | 0.5 | 0.1 | 0.3 | 0.5 | |
CP (°C) | 38 | 34 | 29 | 21 | 36 | 34 | 31 | 46 | 46 | 45 | 46 | 49 | 50 | |
Oil fraction (%) | 90 | 92 | 89 | 89 | 91 | 89 | 90 | 91 | 91 | 91 | 91 | 89 | 90 | |
S(43 nm, 31 °C) | Concentration (mol L−1) | 0.03 | 0.05 | 0.1 | 0.1 | 0.3 | 0.5 | 0.1 | 0.3 | 0.5 | 0.1 | 0.3 | 0.5 | |
CP (°C) | 31 | 27 | 25 | 21 | 28 | 25 | 23 | 37 | 38 | 37 | 37 | 39 | 42 | |
Oil fraction (%) | 90 | 89 | 90 | 89 | 90 | 90 | 90 | 90 | 91 | 90 | 91 | 90 | 91 |
In the absence of salts, the HIPEs were heated at temperatures higher than their CPs such that the CCS polymers dramatically changed their hydrophilicity, leading to destabilization of the HIPEs. Representative photographs of the demulsification of HIPEs (80 vol% oil) stabilized by S(19 nm, 31 °C) (0.5 wt%) are shown in Fig. 9. Significant demulsification occurs upon heating the HIPE for 11 min at 30 °C (Fig. 9B), which is a temperature near the CP. Heating the HIPE at temperatures higher than the CP leads to faster demulsification (Fig. 9C). Complete demulsification is rapidly reached within 3 min at 50 °C (Fig. 9D), which is 19 °C higher than the CP. Immediately after demulsification, the CCS polymer was seen as flocculate at the oil/water interface, but they became soluble in the aqueous phase again when cooled to room temperature. The fast and effective demulsification is attributed to the high degree of flexibility of the thermally responsive poly(MEAx-co-PEGAy) arm polymers, which are attached to the small core of the CCS polymer at one end, leaving the distal end highly mobile.
Photographs of demulsification of HIPEs stabilized by other CCS polymers can be found in the ESI.† The results for the demulsification of HIPEs stabilized by different CCS polymers are listed in Table 5. The efficiency of demulsification is correlated with the size and CP of the CCS polymers. For HIPEs stabilized by CCS polymers with a smaller size and lower CP, it is easier to induce demulsification. Except for S(16 nm, 51 °C), which has the highest CP, complete demulsification can be readily realized within 6 min under mild heating conditions. In principle, the emulsification–demulsification process can be infinitely repeated because temperature is used as the external trigger and no additives are necessary which can otherwise accumulate in the solution. To illustrate this point, we successfully performed four cycles of emulsification–demulsification and there was no detrimental effect on the formation of HIPEs (Fig. S13, ESI†).
CCS polymer | Demulsification conditions |
---|---|
a The total volume of emulsion is 1.5 mL, the dodecane fraction is 80 vol%, and the CCS polymer concentration is 0.5 wt%. | |
S(16 nm, 51 °C) | 80 °C, 6 min, >90% demulsification |
S(35 nm, 40 °C) | 60 °C, 5 min, complete demulsification |
S(43 nm, 38 °C) | 60 °C, 5 min, complete demulsification |
S(19 nm, 31 °C) | 50 °C, 3 min, complete demulsification |
S(43 nm, 31 °C) | 60 °C, 6 min, complete demulsification |
In Fig. 10, rheological test results show that the storage modulus G′ of the HIPE first increases with increasing temperature due to the thermal expansion of the oil droplets, and then drops significantly when the temperature is well above the CP. After demulsification, both G′ and G′′ are smaller than 1 Pa, indicating that a highly fluidic liquid is formed after demulsification.
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Fig. 10 Temperature dependence of rheology tests of a HIPE (80 vol% oil) stabilized by S(19 nm, 31 °C) (0.5 wt%). Strain = 1%, frequency = 1 Hz. |
In order to elucidate the possible mechanisms of thermally triggered demulsification, interfacial tensions of water/hexadecane in the presence of CCS polymers were measured during the temperature ramping process. Surprisingly, the interfacial tensions (8–10 mN m−1) remained essentially unchanged at all temperatures, both below and above the CPs. It is evident from DLS measurements that the arms of the CCS polymers undergo a hydrophilic-to-hydrophobic transition when being heated past their CPs, leading to the formation of aggregates. The hydrophilic-to-hydrophobic transition should be equally expected for the CCS polymers in the interfacial tension measurements: the hydrophilic-to-hydrophobic transition should result in changes in wettability of the CCS polymers with respect to both phases, and the formation of aggregates should reduce the available concentration of the CCS polymers for interfacial stabilization. While the exact reason for this phenomenon is unclear at the moment, we tentatively suggest that this may be due to the difference in the samples for HIPEs and the interfacial tension measurements. In HIPEs, oil droplets are dispersed into a continuous aqueous phase containing CCS polymers, leading to the generation of a high interfacial area. In the interfacial tension measurements, only one drop of CCS polymer aqueous solution is formed in the oil phase using the pendant drop method. As such, the interfacial area created in the interfacial tension measurements is rather small, and can be sufficiently stabilized by a tiny amount of CCS polymer adsorbed onto the interface.
For HIPEs stabilized by S(43 nm, 38 °C) and prepared by Method I, addition of kosmotropes such as Na2SO4 led to more efficient demulsification, while addition of chaotropes such as NaSCN resulted in the opposite effect. For example, in the presence of 0.1 M Na2SO4, which lowered the CP from 38 °C to 29 °C, the time required for complete demulsification by heating at 60 °C was reduced from 5 min to under 2 min. On the other hand, in the presence of 0.5 M NaSCN, which raised the CP from 38 °C to 50 °C, the time required for complete demulsification by heating at 60 °C was prolonged from 5 min to 25 min. Similar effects were also observed for HIPEs stabilized by S(43, 31 °C) in the presence of either kosmotropes or chaotropes. In the presence of 0.05 M Na2SO4, which lowered the CP from 31 °C to 25 °C, the time required for complete demulsification was reduced even at a lower temperature, from 8 min at 60 °C to 2 min at 50 °C. In the presence of 0.5 M NaSCN, which raised the CP from 31 °C to 42 °C, the time required for complete demulsification by heating at 60 °C was prolonged from 8 min to 12 min.
Salts were also added after formation of HIPEs to trigger demulsification at a specific temperature via Method II. In order to avoid dramatic alteration of the relative oil/water volume in a HIPE, the volume of salt solution added to a HIPE accounted for only 2% of the total volume of liquid in the HIPE. For instance, addition of Na2SO4 solution (to reach a final concentration of 0.1 M) to a HIPE stabilized by S(43 nm, 38 °C) resulted in a reduction of the CP from 38 °C to 29 °C, and maintaining the temperature at 30 °C for 5 min resulted in complete demulsification. Addition of Na2SO4 solution (to reach a final concentration of 0.05 M) to a HIPE stabilized by S(43 nm, 31 °C) resulted in a similar effect, albeit demulsification was achieved after 4 min at 25 °C.
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, characterization of polymers, photographs of emulsion and demulsification. See DOI: 10.1039/c3py00942d |
This journal is © The Royal Society of Chemistry 2014 |