Article pubs.acs.org/Langmuir
Behavior of Temperature-Responsive Copolymer Microgels at the Oil/Water Interface Yaodong Wu,† Susanne Wiese,‡ Andreea Balaceanu,† Walter Richtering,‡ and Andrij Pich*,† †
Functional and Interactive Polymers, Institute of Technical and Macromolecular Chemistry, and DWI Leibniz Institute for Interactive Materials, Rheinisch-Westfaelische Technische Hochschule (RWTH) Aachen University, Forckenbeckstraße 50, 52056 Aachen, Germany ‡ Institute of Physical Chemistry, Rheinisch-Westfaelische Technische Hochschule (RWTH) Aachen University, Landoltweg 2, 52056 Aachen, Germany S Supporting Information *
ABSTRACT: Herein, we investigate the interfacial behavior of temperaturesensitive aqueous microgels on the toluene/water interface. Copolymer microgels based on N-vinylcaprolactam (VCL) and two acrylamides, Nisopropylacrylamide (NIPAm) and N-isopropylmethacrylamide (NIPMAm), with various copolymer compositions were used in this study. It is revealed that these copolymer microgels have the similar internal structure, regardless of the chemical composition. A classic kinetics of interfacial tension with three distinct regimes is found in the dynamic interfacial tension plots of copolymer microgels, which is similar to inorganic nanoparticles and proteins. The influences of the copolymer composition and the temperature on the interfacial behavior of microgels are investigated. The results show that the interfacial behavior of copolymer microgels at the toluene/water interface follows exactly the trend of the volume phase behavior of microgels but, on the other hand, strongly depends upon the chemical compositions of copolymer microgels. In contrast, with respect to the size range of microgels studied here (50−500 nm), the size of the microgel has no influence on the interfacial tension. Below the volume phase transition temperature (VPTT), the equilibrium interfacial tensions of all microgel systems decrease as the temperature increases. Above VPTT, the equilibrium interfacial tension remains at a certain level for poly(N-vinylcaprolactam) (PVCL)- and poly(Nisopropylmethacrylamide) (PNIPMAm)-rich microgel systems and increases slightly for poly(N-isopropylacrylamide) (PNIPAm)-rich microgel systems. The evolution of dynamic interfacial tension for microgel solutions against toluene at T < VPTT is faster than that at T > VPTT, because of the reduced deformability of the microgel with the increase of the temperature. The softer microgels with lower cross-linking degrees exhibit faster kinetics of reduction of interfacial tension compared to those with more cross-linked degrees, which strongly supports the deformation-controlled interfacial behavior of microgels.
1. INTRODUCTION Microgels have become increasingly important in polymer and colloidal science because of their attractive properties and potential applications. Thus far, a lot of work on the synthesis of microgel particles has been performed, and mostly, such colloids are prepared in an aqueous medium and consist of water-soluble polymers.1 Among these microgels, special attention has been focused on poly(N-isopropylacrylamide) (PNIPAm)-based microgels because of their temperaturesensitive behavior. Poly(N-vinylcaprolactam) (PVCL) is another temperature-responsive polymer and is considered to have better biocompatibility.2,3 PNIPAm and PVCL have the lower critical solution temperature (LCST), and their corresponding microgels have the volume phase transition temperature (VPTT) close to the LCST of linear polymers.4−7 The temperature sensitivity of these microgels facilitates the fabrication of “switchable” or “stimuli−responsive” materials, such as novel stimuli−responsive emulsions or colloidosomes.8−12 © 2014 American Chemical Society
The application of microgels in the synthesis of colloidosomes or emulsion stabilization is based on the adsorption of microgel particles at the oil/water interfaces. The oil/water interfacial tension is reduced when microgel particles are adsorbed on the interface, and this effect leads to the stabilization of emulsions.9,11−14 The microgel-stabilized emulsion is a new kind of particle-stabilized emulsion, referred to by Richtering et al. as “Mickering emulsion” compared to traditional “Pickering emulsion”.14 The stability of the microgel-stabilized emulsion is strongly influenced by the environment because of the temperature and pH sensitivity of microgels.9,11−16 Until now, the interfacial behavior of solid particles in the conventional Pickering emulsion system has been widely investigated, and some theoretical models have been developed; nevertheless, these theories are normally based Received: March 28, 2014 Revised: June 13, 2014 Published: June 13, 2014 7660
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Table 1. Monomer Amounts and Copolymer Composition in PVCL/Acrylamide Microgels and Transition Temperatures of Microgels Derived from the Interfacial Tension and DSC Measurements samplea
VCL (g)
NIPAm or NIPMAm (g)
VCL/acrylamide (mol/mol), measuredb
transition temperature by DSC (°C)27
transition temperature of γ (°C)
V/N (5:1) V/N (1:1) V/N (1:5) V/NM (5:1) V/NM (1:1) V/NM (1:2) V/NM (1:3) V/NM (1:5)
1.905 1.220 0.437 1.872 1.16 0.78 0.592 0.397
1.905 1.220 0.437 0.342 1.06 1.43 1.623 1.820
4.6 1.01 0.3 5.04 1.08 0.48 0.33 0.2
34.9 35.5 35.0 33.8 35.9 41.3 42.3 44.6
32.7 31.8 31.7 32.6 32.4 40.8 42.0 45.0
a
V/N and V/NM represent PVCL/NIPAm and PVCL/NIPMAm, respectively, and the ratios in parentheses are the expected monomer ratios of copolymer microgels. bMeasured molar ratio by NMR for PVCL/NIPAm and Raman spectroscopy for PVCL/NIPMAm.
on hard latexes or inorganic particles.17−21 However, microgels are colloidal particles whose behavior falls between that of hard spheres and ultrasoft systems (e.g., polymer coils in solution).22,23 The assembly laws deduced from the interfacial behavior of hard particles at the oil/water interface seem not to be completely suitable for microgels, because soft microgel particles are highly deformable at the oil/water interface.16,24,25 With the temperature-sensitive component, N-isopropylacrylamide (NIPAm), and the pH-sensitive component, methacrylic acid (MAA), the interfacial behavior of PNIPAm-co-MAA microgels strongly depends upon the temperature and pH.9−12,15,24 The interfacial properties of PNIPAm-based microgels as a function of the temperature were studied by Monteux and co-workers. A minimum interfacial tension was found around VPTT of the microgel. They assumed that the decrease in the interfacial tension below VPTT came from the formation of denser layers of microgels at the interface with the increase of the temperature and that the increase in the interfacial tension above VPTT arose from loosely packed aggregates of microgels at the interface as the temperature increases.13 In comparison to the microgel with ionizable groups, neutral microgels should be more suitable for investigating the interfacial behavior because the lack of electrostatic interaction would make the system much more simplified. It is known that the variation of the temperature induces a change in the hydrophilic/hydrophobic balance of microgels, consequently leading to a size variation. However, both the size and hydrophilic/hydrophobic balance have a great influence on the interfacial behavior of colloidal particles.17,18 Which one of them is more imperative in the change in the interfacial tension of microgel systems with the temperature is still unclear. A recent paper from Li et al. reports the adsorption kinetics of PNIPAm microgels at the oil/water interface. It is demonstrated that the spreading of microgels at the oil/water interface, governed by microgel deformability, plays an important role in the decline rate of the interfacial tension.26 In this paper, we synthesize microgels based on copolymers of PVCL/NIPAm and PVCL/N-isopropylmethacrylamide (NIPMAm) with different chemical ratios and investigate the interfacial behaviors of these microgels. In our previous work,27 we demonstrated that, in PVCL/NIPAm and PVCL/NIPMAm microgels, there is a statistical distribution of the monomer units in the colloidal networks independent of the copolymer composition. This gives a straightforward possibility to study systematically the influence of the chemical structure of the polymer chains on the interfacial behavior of microgels. In
addition, the variation of the cross-linker concentration allows for the synthesis of copolymer microgels with variable crosslinking degrees. As far as we know the influence of the copolymer structure of microgels on the oil/water interfacial tension was not investigated before. The equilibrium interfacial tension and the time for reaching the equilibrium state can be considered as the thermodynamic aspect and the kinetic aspect, respectively. Dependences of these two aspects upon the temperature, chemical composition, and cross-linking degree of microgels are analyzed. The results indicate that both the chemical composition of copolymer microgels and the crosslinking degree have a governing influence on the deformationcontrolled interfacial behavior of microgels at the toluene/water interface. The former strongly affects the volume phase transition temperature, which determines the temperature dependence of the softness of microgels, and the latter directly influences the softness of mcirogels.
2. EXPERIMENTAL SECTION 2.1. Materials. N-Vinylcaprolactam (VCL) (Sigma-Aldrich, of 98% purity) was purified by a high-vacuum distillation at 80 °C. NIPAm and NIPMAm (Sigma-Aldrich, both of 97% purity) were used as received. The initiator, 2,2′-azobis(2-methylpropioamidine) dihydrochloride (AMPA), the cross-linker, N,N′-methylenebis(acrylamide) (BIS), and a surfactant, cetyltrimethylammonium bromide (CTAB), were obtained from Sigma-Aldrich and used as received. Toluene was purchased from Sigma-Aldrich and used without further purification. Deuterium oxide (D2O) was obtained from KMF GmbH and used as received. 2.2. Synthesis of Microgels. Microgels are synthesized according to the previously reported procedure.27 For copolymer microgels with different chemical compositions, appropriate amounts of VCL, NIPAm, and NIPMAm (see Table 1) and 0.06 g of BIS (3 mol %) were added to 145 mL of deionized water. To synthesize microgels with a constant chemical composition but variable cross-linking degrees, VCL (1.220 g) and NIPAm (0.993 g) [1:1 (mol/mol) VCL/ NIPAm] were used and the amount of BIS was varied (0.05, 0.25, 1, and 3 mol % of monomers). To obtain microgels with the same chemical composition and cross-linking degree but various sizes, VCL (0.397 g) and NIPMAm (1.820 g) [1:5 (mol/mol) VCL/NIPMAm] were used and the concentration of surfactant CTAB was varied (0.37 and 1.10 mM). The role of the surfactant in the microgel synthesis is to increase the colloidal stability of the precursor particles and subsequently lower the size of the primary particles as the number of the particles is increased. With the increase in the concentration of CTAB, more precursor particles survive from the aggregation and grow to a smaller size as the monomer amount is constant.28 For all synthesis, a double-wall glass reactor equipped with a stirrer and a reflux condenser was purged with nitrogen. The solution of the monomers was placed in the reactor and stirred for 1 h at 70 °C with 7661
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Figure 1. Hydrodynamic radius of the (a) PVCL/NIPAm and (b) PVCL/NIPMAm microgels with different compositions at various temperatures. The values of V/N and V/NM represent the molar ratios of VCL/NIPAm and VCL/NIPMAm in copolymer microgels, respectively. The normalized radii (Rh/Rh,collapsed) are shown in Figure s3 of the Supporting Information. The cross-linker (BIS) concentration added during microgel synthesis was 3 mol %. purging with nitrogen. After that, the 5 mL water solution of the initiator (5 g/L) was added under continuous stirring, and the reaction was carried out for 8 h. After the synthesis, all microgel suspensions were dialyzed by a Millipore Labscale TFF system with a Pellicon XL filter (PXC030C50) for 4 days. During the dialysis process, the microgel solution was pumped continuously through the membrane with a pore size much smaller than the size of microgels to remove small organic molecules and oligomers. The wastewater drained away directly into the sewer, and fresh water was added to the dialyzed solution to keep the total volume constant. The polymerization technique used is the precipitation polymerization, which involves the formation of microgel particles by a homogeneous nucleation mechanism. The process is carried out above the VPTT of the microgels, where the particles are in the collapsed state. Three sets of microgel samples were synthesized with variable (a) molar ratios between VCL and NIPAm and between VCL and NIPMAm (Table 1), (b) cross-linking degree, and (c) size. Despite using NIPAm and NIPMAm without additional purification, we obtained nice monodisperse particles characterized by dynamic light scattering (DLS) and microscopy [atomic force microscopy (AFM) and transmission electron microscopy (TEM)]. In addition, the nuclear magnetic resonance (NMR) and Raman spectroscopy results indicated that the chemical composition of the copolymer microgels was very close to the expected composition and no impurities were identified in the microgel structure (previous results published elsewhere).27 In our synthesis protocols, the total concentration of monomers was kept constant to achieve a precise and reproducible synthesis. However, we changed the amount of crosslinker BIS in the reaction mixture to obtain microgels with variable cross-linking concentrations without considering the slight change of the total monomer concentration, because the amount of BIS used here was much smaller compared to the amounts of co-monomers, VCL and NIPAm. In the one-batch precipitation polymerization, such a small difference in the monomer concentration would not bring a big variation in the structure and property of microgels. 2.3. Characterization Methods. The copolymer composition of PVCL/NIPAm microgel samples was characterized with a 400 MHz Bruker L-S NMR spectrometer. The composition of PVCL/NIPMAm copolymer microgel samples was measured on a FT-Raman Bruker Optics spectrometer, model RFS 100/S, equipped with a Nd:YAG 1064 nm laser, operating at a power of 200 mV. For 1H NMR measurement, PVCL/NIPAm microgel samples were dried by lyophilization and dissolved in D2O. For Raman measurement, microgel samples were dried by lyophilization and the powder was inserted in an aluminum pan. 1H NMR and Raman spectra of copolymer microgels and the according calculations are shown in Figures s1 and s2 of the Supporting Information. Both NMR and Raman spectroscopies provide values of the copolymer compositions close to the expected compositions (Table 1). Considering the high conversion of the polymerization process and the narrow size distribution of obtained microgels, we believe that the determined
values are accurate and depict real copolymer composition of microgels with a reasonably small error range.27 The size of microgel particles was measured with an ALV/LSE-5004 light scattering (DLS) multiple tau digital correlator and electronics, with the scattering angle set at 90°. The samples were measured at different temperatures (from 20 to 50 °C), and the temperature fluctuations were below 0.1 °C. Prior to the measurement, microgel samples were diluted with water for chromatography from Merck Millipore, which was filtered through a 100 nm filter before use. The static light scattering (SLS) measurements were conducted on a Sofica with a wavelength of 633 nm and a Fica with a wavelength of 495 nm. The measured data were fitted with a model of polydisperse microgels.29,30 The dynamic interfacial tension of microgels at the toluene/water interface was measured through the pendant drop method on a DSA 100 tensiometer (Kruss, Germany) equipped with a thermostat. Aqueous dispersions of microgels were prepared with deionized water (Milli-Q Academic A-10 system, Millipore) previously saturated with toluene. A drop of toluene (previously saturated with deionized water) was formed in microgel dispersions, and then the dynamic interfacial tension was determined through rapid acquisition of the drop image, edge detection, and fitting of the Laplace−Young equation conducted by computer automation. Because toluene is lighter than water, we formed a toluene drop from the bottom to up using a curved needle. For the temperature dependence experiment, fresh interfaces were produced at each temperature. Differential scanning calorimetry (DSC) measurements were carried out with a PerkinElmer Pyris 1 DSC. Aluminum pans were used as sample holders. A nitrogen flow was employed during the measurement. Approximately 40 mg of 10 wt % microgel aqueous dispersion was used. The measurements were carried out over a temperature range from 25 to 55 °C, with a constant heating rate of 2 °C/min. For all samples, at least two heating runs and one cooling run were performed, and all transitions were fully reversible and appeared in each heating−cooling cycle.
3. RESULTS AND DISCUSSION 3.1. Characterization of Microgels. The experimental data from NMR and Raman spectroscopy confirm that the synthesized microgels consist of cross-linked copolymer chains. The real molar ratios of VCL and NIPAm or NIPMAm in PVCL/NIPAm or PVCL/NIPMAm microgels were calculated from NMR and Raman spectra (Table 1). The original spectra are shown in Figures s1 and s2 of the Supporting Information. It was found that the experimentally determined monomer molar ratio and the theoretical monomer molar ratio in microgels are very close to each other. Our previous work shows that the obtained microgels exhibit no heterogeneity in the sense of distribution of monomer units in a copolymer 7662
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structure.27 The temperature dependence of hydrodynamic radii for microgels with different chemical compositions obtained by DLS is presented in Figure 1. PVCL and PNIPAm microgels have similar temperature trends and ranges of VPTT (32−34 °C), while poly(N-isopropylmethacrylamide) (PNIPMAm) microgels have a higher VPTT at around 45 °C. Above VPTT, the destruction of hydrogen bonds induces the increase in the hydrophobic interactions between polymer chains. This leads to the exclusion of water from the microgels and, consequently, the shrinking of microgels. For PVCL/NIPAm copolymer microgels, the VPTT remains at a constant range (32−34 °C). It has no significant change with copolymer composition and is similar to that of PVCL and PNIPAm homopolymer microgels. In contrast, the VPTT of PVCL/ NIPMAm copolymer microgels increases from 33 to 45 °C with the increase of the NIPMAm content. It falls between the VPTT of PVCL homopolymer microgels and that of PNIPMAm homopolymer microgels. The result clearly exhibits the thermosensitivity of copolymer microgels and the variation in VPTT with the monomer ratio. Because of the fact that no monomer carrying acidic or basic groups or ionic surfactant was used for microgel synthesis, we can consider that only a small amount of ionizable groups in the microgel structure originates from the initiator residues incorporated into polymer chains. In this situation, we consider the investigated microgels as purely temperature-sensitive colloids, where no pH-sensitive effects are present. Because of the different reactivity ratios of the monomers and the crosslinker, microgels synthesized have a radial density profile in the internal structure, where the segment density gradually decays at the surface.5,29,30 We also studied the copolymer microgels by static light scattering and fitted the results with a polydisperse microgel model, which has been successfully applied to PNIPAm-based microgels.29,30 The fitting results show that all of the copolymer microgels have fuzzy surfaces and dense cores (see Figure s4 of the Supporting Information). Such an internal structure is independent of the chemical composition. As far as we know, it is the first time that the radial density profile fitting model has been successfully applied to the internal structure of PVCL-based microgels. 3.2. Dynamic Interfacial Tension. 3.2.1. Concentration Dependence. Figure 2 shows the interfacial tension of toluene against aqueous solutions of PVCL/NIPMAm microgels at different concentrations. It is obvious that the interfacial tension decreases with time and eventually approaches an
equilibrium value. The time for reaching the equilibrium interfacial tension decreases with the increase in the concentration of microgels. The γ−log(time) plots, especially at low microgel concentrations, interpret the classical kinetics of interfacial tension with three distinct regimes, which is similar to the dynamic interfacial behaviors of polymer-grafted inorganic nanoparticles and proteins.20,31 The first stage is a kind of induction regime, where the interfacial tension decreases slightly. In the second regime, a sharp decline in the interfacial tension is observed. In the last regime, the interfacial tension reaches a quasi-equilibrium value. Generally, the decrease of interfacial tension can be explained by the adsorption of microgel particles to the toluene/water interface. In the early stage, microgels in the vicinity of toluene drop adsorb onto the interface, which induces the gentle reduction of interfacial tension. The diffusion of microgel particles from the bulk solution to the interface increases the interface coverage. After a certain period of time, the interface coverage reaches a critical value, and then a steep decline in the interfacial tension is observed and maintains the status for a short time from several seconds to tens of seconds. When the interface coverage approaches the maximum value, the interfacial tension reaches an equilibrium value. For microgels of lower concentrations, a longer time is needed for the adsorption of a sufficient amount of microgel particles from bulk solution to the toluene/water interface. Therefore, the induction time increases with the decrease of the microgel concentration. When the concentration of the microgel is very low (here lower than 0.0014 mg/mL), the induction time is so long that the toluene droplet becomes unstable before the second regime starts. The induction period is only found for dilute microgel solutions. In the measurable time scale, the induction time is difficult to be detected for concentrated microgel solutions, because the dynamic interfacial tension decreases very fast to a stable level.20,31 One possible explanation for the acceleration of the interfacial tension decay is that the capillary force between the particles at the interface that increases with the particle concentration makes the soft microgels easier to deform.26 It should be noted that, in the conventional Pickering emulsions, solid particles seldom lead to appreciable changes in oil/water interfacial tension.16,32,33 However, microgels are organic colloidal particles, and their deformation leads to a higher number of polymer segments adsorbed at the interface.26 This leads to a significant decrease in the interfacial tension. To obtain the equilibrium interfacial tension, an empirical equation (Hua−Rosen equation) is employed (eq 1) γ0 − γ(t ) γ(t ) − γeq
=
⎛ t ⎞n ⎜ ⎟ ⎝ t* ⎠
(1)
where γ0, γeq, and γ(t) are the initial interfacial tension, the equilibrium interfacial tension, and the interfacial tension detected at time t, respectively. t* is a semi-empirical parameter having the unit of time and corresponds to the time when γ(t) = (γ0 + γeq)/2, and n is a dimensionless constant. The Hua− Rosen equation has been successful for small molecular surfactant systems, macromolecular systems, and globular protein systems.34−36 We carried out the fitting of experimental data using eq 1 for samples with medium microgel concentrations, which had typical γ−log(time) decay. In Figure 2, we can see a good agreement between the fitting results and the experimental data.
Figure 2. Time dependence of the toluene/water interfacial tension with PVCL/NIPMAm (5:1) microgels of various concentrations at 24 °C. The red lines show the fitting results based on eq 1. 7663
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3.2.2. Temperature Dependence. It has been reported that the temperature has a considerable influence on the interfacial/ surface tension of small molecular surfactants, polymers, and colloidal particles.34,37−41 Normally, as the temperature increases, the diffusion rate increases.34 That means that faster adsorptions of solutes and/or dispersed substances occur at higher temperatures, reflected by the faster interfacial/surface tension decay rate. However, through the experimental results of interfacial tension of temperature-sensitive polymers, such as PNIPAm, a different temperature dependence of surface tension was observed.39,40 Below LCST, the rate of the surface tension reduction increases with the increase of the temperature, which is similar to the behavior of non-ionic surfactants.34,42 On the contrary, the surface tension decay rate decreases as the temperature increases above LCST. Kawaguchi et al. attributed the slowdown of surface tension decay to the aggregation of PNIPAm chains, which retarded the diffusion of polymers from the bulk solution to the interface.39 Zhang and Pelton supposed that the unfolding of PNIPAm at the air/water interface above LCST prolonged the time for reaching a steady-state surface tension.40 They adopted the same explanation to the similar phenomena observed in the case of PNIPAm microgels.41 The dynamic interfacial tension of toluene against aqueous microgel solutions was measured at various temperatures. At each temperature, the interfacial tension of the freshly formed interface was measured. As shown in Figure 3, the dynamic
Table 2. Dynamic Interfacial Tension Parameters of the Hua−Rosen Equation for PVCL/NIPMAm (5:1) Microgels of 0.15 mg/mL at Different Temperatures (Fitting in Figure 3) T (°C)
γeq (mN/m)
t* (s)
n
24.0 27.5 31.0 35.5 39.5 43.0
8.5 7.1 5.6 5.2 5.6 6.6
1.2 1.0 0.5 5.4 51.3 124.6
1.94 2.06 0.81 1.09 2.26 3.32
3.3. Equilibrium State of the Interfacial Tension. 3.3.1. Influence of Chemical Composition of the Microgel. It was reported that the size of nanoparticles strongly affects their interfacial behaviors.17,20 To figure out whether the size of microgel particles can influence the equilibrium interfacial tension, we synthesized a series of microgels with the same chemical composition but of different sizes. When different amounts of surfactant CTAB are added during the synthesis, the size of the microgel can be tuned.28 From Figure 4, we can
Figure 4. Time dependence of the toluene/water interfacial tension at room temperature (24 °C) with PVCL/NIPMAm (1:5) microgels (0.15 mg/mL) synthesized with different amounts of surfactant CTAB. The inset shows the sizes of microgels, which decrease with the increase in the CTAB concentration. The dynamic interfacial tension above VPTT (49 °C) is shown in Figure s5 of the Supporting Information. Figure 3. Time dependence of the toluene/water interfacial tension with PVCL/NIPMAm (5:1) microgels of 0.15 mg/mL at different temperatures. The red lines show the fitting results based on eq 1.
see that the more CTAB is used in the synthesis, the smaller the microgels obtained. However, the changes in the microgel size have no impact on the interfacial tension of microgel solutions against toluene, both below and above VPTT. Thus, within the range of microgel size investigated here, the effect of the size on the interfacial tension can be ignored. It ensures that the comparison of the interfacial behaviors of microgels with different chemical compositions, which are also varied in size (as shown in Figure 1), is reasonable. The variation of the equilibrium interfacial tension (γeq) and the size of copolymer microgels with the molar ratio of acrylamide at room temperature are summarized in Figure 5. It is found that the size of PVCL/NIPAm microgels has no significant change with the change in the fraction of NIPAm (around 400 nm in Rh), while the size of PVCL/NIPMAm microgels increases slightly with the fraction of NIPMAm (from 350 to 450 nm in Rh).27 As synthesized in the presence of CTAB, the corresponding homopolymer microgels are much smaller than the copolymer microgels. Consequently, an irregular trend of size variation with the chemical composition
interfacial tension represents a similar trend versus time at different temperatures, which decays with time and finally approaches an equilibrium value, as discussed before. A good agreement between the fitting through eq 1 and the experimental data was also found in γ−log(time) plots at different temperatures. However, the equilibrium interfacial tension and the time for reaching the equilibrium status behave differently at different temperatures. The resulting Hua−Rosen parameters, describing the dynamic interfacial tension, from the fittings in Figure 3 are presented in Table 2. The lowest value of n was found at around 31 °C. A similar change of n with the temperature was found in the dynamic surface tension of linear PNIPAm solution.39 It was mentioned that the value of n might reflect the hydrophobic character of the surfactant, but there is no direct evidence to prove this. However, studies by others show that the temperature has no special influence on the value of n.40,41 7664
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Figure 5. Variation of γeq with the fraction of (a) NIPAm and (b) NIPMAm in copolymer microgels at 24 °C. The concentration of the microgel solution is 0.15 mg/mL.
Figure 6. Dependence of the interfacial tension and time to equilibrium upon the temperature for microgels with different ratios of (a and c) VCL/ NIPAm and (b and d) VCL/NIPMAm. V/N and V/NM represent PVCL/NIPAm and PVCL/NIPMAm, respectively, and the ratios in parentheses are the monomer ratios of copolymer microgels. The concentration of the microgel solution is 0.15 mg/mL.
is found. However, the values of γeq for PVCL/NIPAm and PVCL/NIPMAm systems increase with the fraction of NIPAm and NIPMAm in the copolymer. It does not follow the changing trend of size at all. For both PVCL/NIPAm and PVCL/NIPMAm copolymer microgel systems, the change in γeq is very slight as the fractions of NIPAm and NIPMAm increase from 0 to 0.5. When the fractions of NIPAm and NIPMAm increase from 0.5 to 1, the values of γeq increase significantly. In this work, most of the microgels were synthesized without CTAB, except the homopolymer PVCL, PNIPAm and PNIPMAm microgels, and PVCL/NIPMAm copolymer microgels varied in size. For these microgels, a small amount of CTAB was used. We believe that no CTAB residues are left in the solution after the 4 day dialysis with a continuous fresh water exchange. If there were CTAB residues left in the
solution, they would lead to the same interfacial behavior of PVCL, PNIPAm, and PNIPMAm microgels. However, they are different from each other (Figure 5). The PVCL/NIPMAm microgels of different sizes (synthesized in the presence of CTAB) and a control sample prepared without CTAB have the same interfacial behavior (Figure 4). From this, we conclude that CTAB has been successfully removed from microgel samples during the dialysis step. 3.3.2. Influence of the Temperature. The equilibrium interfacial tensions at different temperatures for PVCL/NIPAm and PVCL/NIPMAm microgels are presented in Figure 6. At the same temperature, γeq of PVCL homopolymer microgels is always lower than those of PNIPAm and PNIPMAm homopolymer microgels. The value of γeq for PNIPAm microgels decreases with the increase in the temperature first, reachs a minimum around VPTT, and then increases with the 7665
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exclusively the chemical structure of microgels. Considering the independence of γeq on the copolymer microgel size (Figure 5), it can be inferred that the chemical constitution of microgels has a dominant influence on the interfacial tension. It is believed that the reduction of interfacial tension is caused by the adsorption of microgel particles at the oil/water interface. At the early stage, the adsorption of nanoparticles is expected to be a diffusion-controlled process. For non-ionic nanoparticles, the process can be quantitatively described as the following equations through the short-time (t → 0) and longtime (t → ∞) approximations20,34,42
temperature slightly when the temperature is higher than VPTT. This kind of transition temperature for interfacial tension is consistent with the reported results about the interfacial tension of NIPAm-based microgels.13 For PVCL and PNIPMAm microgels, the values of γeq also decrease with the temperature below VPTT, however, stay at a plateau value above VPTT. The interfacial behaviors of PVCL/NIPAm or PVCL/NIPMAm copolymer microgels fall between those of PVCL and PNIPAm or PNIPMAm homopolymer microgels and depend upon the chemical composition of microgels. Panels c and d of Figure 6 show the kinetics of interfacial tension reduction as reflected by t* for microgels at different temperatures. We can see that the values of t* remain at a constant level below VPTT and increase with the temperature above VPTT. The kinetics of interfacial tension (t*) also depends upon the chemical compositions of copolymer microgels. The increase in t* for PVCL- and PNIPAm-rich microgels above VPTT is more obvious compared to that for PNIPMAm-rich systems. It means that, at a higher temperature (above VPTT), the reduction of interfacial tension for PVCLand PNIPAm-rich microgel systems is slowed to a larger extent compared to PNIPMAm-rich microgel systems. The intrinsic reason for the difference between the interfacial behavior of PVCL, PNIPAm, and PNIPMAm microgels observed here is still not understood. It is revealed in this paper that these copolymer microgels have the similar internal structure, regardless of the chemical composition (SLS results in Figure s4 of the Supporting Information), and that the size of microgels has no impact on the interfacial behavior (Figure 4). The probable reason for such a difference that we can think of is the variation in the chemical structure. However, it is still an open question of how the chemical structure has such an influence on the interfacial behavior observed here. Although the reason for that is still unknown, such a dependence of the interfacial behavior upon the chemical composition could be made use of in the application of microgels. 3.4. Discussion on the Temperature Dependence of Interfacial Tension. We can easily find turning points on the γeq−T curves (Figure 6). Before the point, the value of γeq decreases monotonously with the increase in the temperature. Here, the turning point can be considered as the transition temperature of the interfacial behavior, and its exact position was defined as the intersecting point of two trend lines (approximate slopes). According to our previous work, the phase transition temperature of microgels can be determined by DSC.27 The transition temperatures determined by DSC and the γeq−T curves are summarized in Table 1. We can see that the interfacial behavior of microgels is highly consistent with the phase behavior of microgels. For PVCL/NIPAm microgels, both kinds of transition temperatures are not influenced by the monomer ratio and remain in a certain range, although the phase transition temperatures (34−36 °C) are slightly higher than the transition temperature derived from the interfacial behavior (31−33 °C). For PVCL/NIPMAm microgels, both transition temperatures depend upon the monomer ratio in copolymers. When the VCL/NIPMAm ratio is higher than 1, the transition temperatures are around 34−36 °C. However, if the VCL/NIPMAm ratio decreases from 1 to 0.2, the transition temperatures increase rapidly to 45 °C. The results indicate that the behavior of copolymer microgels with VCL/NIPMAm ratios above and below 1 is dominated by VCL and NIPMAm, respectively. The transition temperatures of γeq follow exactly the same trends of the VPTT of microgels, which stands for
⎛ Dt ⎞1/2 ⎛ dγ ⎞ γ(t )t → 0 = γ0 − 2RTc ⎜ ⎟ or ⎜ 1/2 ⎟ ⎝π ⎠ ⎝ dt ⎠ ⎛ D ⎞1/2 = − 2RTc ⎜ ⎟ ⎝π⎠ γ(t )t →∞ = γeq + RT Γ 2 ⎜⎛ π ⎟⎞ 2c ⎝ D ⎠
t→0
1/2 ⎛ dγ ⎞ RT Γ 2 ⎜⎛ π ⎟⎞ or ⎜ −1/2 ⎟ ⎝ dt ⎠t →∞ 2c ⎝ Dt ⎠
1/2
=
(2)
where γ0 is the initial interfacial tension, γeq is the equilibrium interfacial tension, c is the bulk concentration of nanoparticles, Γ is the interfacial density of nanoparticles, and D is the diffusion coefficient. For the adsorption of the non-ionic surfactant, a mechanism of mixed diffusion activation is suggested, where a linear relationship exists between ln(DT/ D0) and T−1.20,34,42 Here, DT is the diffusion coefficients of the particles at different temperatures and D0 is the particle diffusion coefficient at 24 °C. From the interfacial tension, to obtain the values of D, the interfacial density of particles Γ is needed, as seen in (dγ/dt1/2)t→0 = −2RTc(D/π)1/2 and (dγ/ dt−1/2)t→∞ = (RTΓ2/2c)(π/D)1/2. However, because the microgels are very soft and deformable, the size of the particles adsorbed at the interface is hardly known. As a consequence of that, the interfacial density of the microgel particle is not possible to obtain. Therefore, the relative value of DT/D0 is calculated. The plots of ln(DT/D0) versus T−1 were obtained from DLS and calculated through the approximations in eqs 2 (see Figure s6 of the Supporting Information). We found that the ln(DT/ D0)−T−1 plot of the light scattering data has a good linear dependence in the small range of T−1 investigated here. In comparison to that, the calculation based on eqs 2 shows a different result. When T < VPTT, the values of ln(DT/D0) depend upon T−1 linearly and are also located near the fitted line of light scattering results for both approximations. That means the change in diffusion coefficients derived from interfacial tension (through eqs 2) with the temperature is consistent with the change in the bulk solution (light scattering results). However, when T > VPTT, the values of ln(DT/D0)− T−1 calculated with eqs 2 deviate far away from the fitting line of light scattering results. It indicates that the adsorption of microgels onto the oil/water interface is not simply controlled by a diffusion mechanism, especially above VPTT. At the equilibrium state, a balance exists between the adsorption and desorption at the oil/water interface for the small molecular surfactant or the solid inorganic particles in the Pickering emulsion.20,34 The increase in the temperature can bring stronger thermal fluctuations that result in a weak particle attachment at the fluid interfaces.20,43 In our work, the oil drop 7666
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was freshly formed in each experiment. At the temperature above VPTT, the interfacial tension of the freshly formed droplet decays, as usual. That means that the adsorption of microgels onto the interface is steady. According to the literature about the microgel-stabilized emulsion, the increase of the temperature leads to the breaking of emulsion, where macroscopic floccules are observed.9,13 However, even in the broken emulsions, microgels are found to be adsorbed at the oil/water interface.44 Thus far, none has found significant desorption for microgel particles from the oil/water interface even upon an increasing temperature. Monteux and co-workers reported the same interesting phenomena for PNIPAm-based microgels, where the variation of the interfacial tension versus the temperature presents a minimum value around VPTT. They correlated the decrease in the interfacial tension below VPTT to the increasingly compact packing of microgel particles at the interface and the increase in interfacial tension above VPTT to the loose aggregates of microgels at the interface at high temperatures.13 It has also been revealed that soft microgels deform at the interface.14,15,24,25 At T < VPTT, the swollen microgels are able to deform easily. They can spread their dangling chains at the particle surface to cover the interface as much as possible. That is the reason some chain bridges were found between adjacent microgels in some reports.14,15,24 However, at T > VPTT, the microgel particles deform less. Without good spreading ability, a higher number of particles and a closer packing density are needed to reach the maximum coverage of the interface. Besides, the mobility at the high packing density might be much slower because of crowding or jamming of colloidal particles at the interface.45−47 Therefore, a strong increase of t* is observed at T > VPTT. The adsorption of microgels below and above VPTT is schematically shown in Scheme 1.
Figure 7. Dynamic interfacial tension of toluene against the solutions of microgels with different cross-linker contents at 24 °C. The values in the legend represent the amount of cross-linker BIS used for microgel synthesis. The concentration of microgel solution is 0.15 mg/ mL.
synthesized with different amounts of CTAB highly overlap each other.
4. CONCLUSION In the present work, temperature-responsive copolymer microgels based on VCL and two acrylamides, NIPAm and NIPMAm, with various monomer ratios are synthesized. The interfacial behaviors of these copolymer microgels are investigated. A classic kinetics of interfacial tension with three distinct regimes, similar to the dynamic interfacial behavior of inorganic nanoparticles and proteins, is found in the dynamic interfacial tension plots of copolymer microgels. The equilibrium interfacial tensions are obtained through the data fitting using an empirical equation (Hua−Rosen equation). We look through the influences of the monomer ratio and temperature on the interfacial behavior of microgels. Because the copolymer microgels have the similar internal physical structure, regardless of the chemical composition, and the size of microgels has no impact on the interfacial behavior, the variation of the interfacial tension for different microgels should be derived from the chemical structure. The increase in the monomer ratio of NIPAm or NIPMAm in microgels leads to an increase in the equilibrium interfacial tension. Furthermore, a transition of the interfacial behavior of microgels around VPTT is observed, which is highly consistent with the phase behavior of microgels. Below VPTT, the equilibrium interfacial tensions of all microgel systems decrease as the temperature increases. Above VPTT, the equilibrium interfacial tensions remain at a certain level for PVCL- and PNIPMAm-rich microgel systems and increase slightly for PNIPAm-rich microgel systems. When the temperature is below VPTT, the interfacial behavior of microgels complies with a diffusion-controlled process. However, when the temperature is above VPTT, the diffusion-controlled mechanism seems not to be suitable for the interfacial assembly of microgels studied here. It is proposed that the deformability (softness) of microgels plays an imperative role in the interfacial behavior. Because of the reducing deformability of the microgel with an increasing temperature, the evolution of the dynamic interfacial tension for the microgel at T < VPTT is faster than that at T > VPTT. The dynamic interfacial tension results of microgel systems with different cross-linker densities strongly support this point. The study of the interfacial behavior in this work might provide a new insight for better understanding the properties of
Scheme 1. Adsorption of Microgels below and above VPTT
We also investigate the dynamic interfacial tension of toluene against the solutions of microgels with different cross-linking densities. As shown in Figure 7, we can obviously see that microgels with lower cross-linker concentrations, which are softer, have the faster kinetics of reduction of interfacial tension with time. It strongly supports the foregoing interpretation of the deformation-controlled interfacial behavior of microgels. Such a deformation-controlled mechanism could also explain the independence of the interfacial tension upon the microgel size (Figure 4). Assuming that the use of surfactant in the synthesis does not change the internal structure and the crosslinking concentration in microgels, the deformability of the copolymer microgels should remain the same. Therefore, the interfacial tension decays for PVCL/NIPMAm microgels 7667
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microgels at the fluid/fluid interfaces and future applications, related to the stabilization of emulsions or the fabrication of microgel-based capsules for drug deliveries.
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ASSOCIATED CONTENT
S Supporting Information *
NMR and Raman spectra of copolymer microgels (Figures s1 and s2), normalized radii of copolymer microgels (Figure s3), static light scattering profiles of copolymer microgels rich in VCL and acrylamides (Figure s4), dynamic interfacial tension of the microgel with different sizes above VPTT (Figure s5), and comparison of apparent diffusion coefficients derived from interfacial tension and detected by DLS (Figure s6). This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Larisa Tsarkova for her kind discussion in the interfacial tension results. Yaodong Wu thanks the China Scholarship Council for funding. Financial support of the Volkswagen Foundation and Deutsche Forschungsgemeinschaft within SFB 985 “Functional Microgels and Microgel Systems” is highly appreciated. Special thanks to Lindsey Weger for her kind help in the polishing of the English.
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NOTE ADDED AFTER ASAP PUBLICATION The last paragraph of the Introduction has been updated. The revised version was re-posted on June 26, 2014.
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