J. Phys. Chem. B 2007, 111, 13217-13220
13217
Temperature-Dependent Aggregation and Disaggregation of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) Block Copolymer in Aqueous Solution Xiangfeng Liang, Chen Guo,* Junhe Ma, Jing Wang, Shu Chen, and Huizhou Liu* Laboratory of Separation Science and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100080, China ReceiVed: June 26, 2007; In Final Form: September 6, 2007
Aggregation and disaggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEOPPO-PEO) block copolymers, Pluronics P103 and P104, in aqueous solutions during a heating and cooling cycle were investigated by dynamic laser scattering (DLS) and 1H NMR spectroscopy. Temperature hysteresis was observed by DLS when cooling the copolymer aqueous solutions because larger aggregates existed at temperatures lower than critical micellization temperature (CMT), but no temperature differences were observed by NMR. This phenomenon was explained as the forming of water-swollen micelles at temperatures lower than CMT during the cooling process.
Introduction
TABLE 1: Molecular Weight and Composition of P103 and P104 Pluronic Copolymers
Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, commercially available as Pluronics (BASF Corp.) and Poloxamers (ICI Corp.), are high molecular weight nonionic surfactants. They are an important class of surfactant and find widespread industrial applications such as detergency, dispersion, stabilization, foaming, emulsification, lubrication, and so forth.1,2 The aggregation properties of PEO-PPO-PEO copolymers in solution, as affected by concentration, temperature, and other additives, are important in understanding the action of copolymer in various applications.3-6 During the past two decades, the micellization of PEOPPO-PEO triblock copolymers has been studied extensively by various analytical methods, such as surface tension, light scattering, dye solubilization, FT-IR spectra, FT Raman spectra, NMR, SANS experiments, and so forth.5-14 A closer examination of the published work, however, reveals that some issues have not been satisfactorily resolved. Not like conventional, low molecular weight surfactant, a large difference is often noted between the CMC (or critical micellization temperature, CMT) values determined by different methods.9 The previous literature even gave contradictory results on whether the same copolymer with the same concentration could form micelles at certain temperature by the same method.15,16 McDonald and Wong15 reported that no aggregation was noted at concentrations of L64 up to 20% (w/v) at 25 °C. Al-Saden et al.16 observed that aqueous Poloxamer184 (Pluronic L64) with concentration from 8 to 20% could aggregate at 25 °C, and the aggregate sizes were larger than those at 35 °C. They suggested the open association type present at 25 °C and the closed association one at 35 °C. Brown et al.17,18 and Zhou and Chu5 also found that, at temperature below CMT, PEO-PPO-PEO block copolymers could form larger aggregates. In an attempt to resolve this predicament, temperature-induced aggregation and disaggregation of Pluronics P103 and P104 during a heating and cooling cycle were investigated. We carried out a careful * Corresponding authors. Telephone: +86-10-62642032. Fax: +86-1062554264. E-mail:
[email protected];
[email protected].
Pluronic
molecular weight
PPO percentage
general formula
P103 P104
4950 5900
3465 3540
(EO)17(PO)60(EO)17 (EO)27(PO)61(EO)27
experimental study including dynamic light scattering (DLS) and NMR under exact sample preparation and sufficient temperature control. The purpose of this work is to compare the aggregation and disaggregation of Pluronic polymers to get a thorough understanding of the micellization of PEO-PPOPEO block copolymer. Experimental Section Materials. PEO-PPO-PEO block copolymers Pluronics P103 and P104 were obtained as a gift from BASF Corp. and used as received. Information on the polymer molecular weight and composition is presented in Table 1. The reference 2,2-dimethyl2-silapentane-5-sulfonate sodium salt (g97%, DSS) was purchased from Sigma-Aldrich Chemical Corp. D2O (g99.9 atom % 2H) was purchased from CIL Corp. Sample Preparation. For the DLS experiments, Pluronics P103 and P104 aqueous solutions were prepared individually with Milli-Q water at 4 °C followed by filtration through 0.22µm Millipore filters directly into a dust-free light scattering cell. The cells were sealed to prevent any leakage of solvent. Solutions were in each case allowed to stand after filtering for 1 week before measurement. For the NMR experiments, the sample preparation is the same as described before.19 Laser Light Scattering (LLS). A commercial LLS spectrometer (ALV/DLS/SLS-5022F) equipped with a multi-τ digital time correlator (ALV5000) and a cylindrical 22 mW UNIPHASE He-Ne laser (λ0 ) 632.8 nm) was used. The samples were allowed to equilibrate for ca. 2 h at each measurement temperature before the LLS experiments were commenced. The CONTIN program supplied with the correlator was used.20,21 NMR Methods. The NMR experiment was conducted on a Bruker Avance 600 spectrometer. The sample temperature was
10.1021/jp074990n CCC: $37.00 © 2007 American Chemical Society Published on Web 10/31/2007
13218 J. Phys. Chem. B, Vol. 111, No. 46, 2007
Figure 1. Temperature dependence of relaxation time distribution function G(τ) of 1% (w/v) Pluronic P103 in one heating and cooling cycle. (A) Heating process. (B) Cooling process. (a) Fast relaxation mode. (b) Intermediate relaxation mode. (c) Slow relaxation mode.
Liang et al.
Figure 3. Temperature-dependent half-weight width of the PO-CH3 signal (in hertz) of 2% (w/v) P104 in one heating and cooling cycle.
Figure 4. Temperature-dependent half-weight width of the EO-CH2 signal (in hertz) of 2% (w/v) P104 in one heating and cooling cycle. Figure 2. Temperature dependence of average hydrodynamic radius of Pluronic micelle in one heating and cooling cycle. (a) 1% Pluronic P103. (b) 2% Pluronic P103. (c) 2% Pluronic P104.
kept constant to within (0.1 °C by using a Bruker BCU-05 temperature control unit. For the experiment, the sample was allowed to equilibrate at the desired temperature for at least 15 min before measurement. To eliminate the temperature-induced shifts, DSS was directly added into the sample solutions as an internal reference. Results and Discussion Dynamic Light Scattering. Figure 1 shows the relaxation time distributions (G(τ) versus τ) for 1% (w/v) P103 at various temperatures during a heating (A) and cooling (B) cycle. Those for 2% (w/v) P103 and 2% (w/v) P104 are shown in Figures S1 and S2 in the Supporting Information. The fast relaxation mode corresponds to cooperative diffusion of chain segments inside each “blob”, the slow relaxation mode reflects long-range correlated concentration fluctuation similar to the internal motion of a transient network, and the intermediate relaxation mode occurring at certain temperatures corresponds to the diffusion of micelle.22,23 The temperature at which the intermediate relaxation mode appeared is determined as CMT. CMT values for 1% P103, 2% P103, and 2% P104 were obtained from Figures 1A, S1, and S2. It is similar to that in ref 24, as shown in Table 2. The
slow relaxation mode coexisted with the intermediate relaxation mode at a small temperature range. The area percentage of intermediate relaxation mode increased sharply with temperature, while that of both slow and fast relaxation modes diminished to zero sharply within 4°. The temperature range in which unimers and micelles coexist is called the transition region.25 In the cooling process, temperature dependence of relaxation time distributions for three relaxation modes was almost the same as that in the heating process at higher temperature range, while a big difference was shown at temperatures lower than CMT. The intermediate relaxation mode still existed in the solutions as the solution temperature decreased to CMT. The temperature at which the intermediate relaxation mode disappeared is determined as Td in the cooling process. Td values for three copolymer solutions are listed in Table 2. The differences between CMT obtained from the heating process and Td obtained from the cooling process were also listed in Table 2. There is a 3.3° difference for 1% P103, 5.3° difference for 2% P103, and more than 10.2° difference for 2% P104. Figure 2 shows variation of average hydrodynamic radii (RH) of the micelles for 1% P103, 2% P103, and 2% P104 versus temperature in one heating and cooling cycle. In the heating process, the hydrodynamic radius of micelle decreased with the increase in temperature.16,26 In the initial stage of the cooling, the size of micelle increased with the decrease in temperature,
Aggregation and Disaggregation of PEO-PPO-PEO
J. Phys. Chem. B, Vol. 111, No. 46, 2007 13219
TABLE 2: Physical Chemistry Data Obtained by DLS Experiment in One Heating and Cooling Cycle
1% P103 2% P103 2% P104
CMT (°C)
CMT in ref 2 (°C)
Td (°C)
CMT-Td (°C)
maximum RH in the heating process (nm)
maximum RH in the cooling process (nm)
21.5 20.3 21
19.5 18.0 20.2
18.2 16.1 10.2
10.5 12.5 13.9
18.5 24 20.3
and RH values were almost the same as those at the corresponding temperature in the heating process. Further decreasing temperature until CMT, micelles still existed and RH values continued to increase until Td. The maximum RH in the cooling process and the maximum RH in the heating process are listed in Table 2. 1H NMR Spectroscopy. 1H NMR spectra of 2% (w/v) Pluronic P104 in D2O solution were recorded at various temperatures in one heating and cooling cycle, as shown in Figures S3 and S4 in the Supporting Information. The triplet at ∼1.16 ppm is attributed to the protons of the PO-CH3 groups. The sharp peak at ∼3.7 ppm is the protons of the EO-CH2units.14 The temperature-dependent chemical shifts (δ) of the PO-CH3 and the EO-CH2- signals are presented in Figures 3 and 4. In the heating process, the chemical shifts of the POCH3 signal decrease slightly with the increase in temperature at lower temperatures. When the temperature is increased above a certain value, a sudden upfield shift occurs. The CMT can be determined at the first inflection point (denoted in Figure 3). The significant upfield shift experienced by PO-CH3 protons indicates that the PO blocks apparently reduce contact with water and form a hydrophobic microenvironment.14 At higher temperatures, the second inflection point (27 °C) is reached, after which the decrease of the chemical shifts of the PO-CH3 protons slows down, suggesting the end of micellization. In the cooling process, the reverse trend is observed with the decrease in temperature, and the same δ values could be obtained at the same temperatures as in the heating process. When the temperature is above, the chemical shifts of the PO-CH3 protons are increased with decreasing temperature. An abrupt downfield shift is observed in the temperature range 22.5-27 °C. The chemical shifts of the PO-CH3 signal increase slightly with the decrease in temperature lower than CMT (22.5 °C). The same δ values are also obtained for the EO-CH2- signal of P104 at the same temperature, when reached by a temperature change in the opposite direction, as shown in Figure 4. In the heating process, the chemical shifts of the EO-CH2- signal of P104 show a slightly linear decrease with the increase in temperature, implying that the PEO blocks in the triblock copolymer experience a slight dehydration process, but still keep in contact with water. In the cooling process, the chemical shifts of the EO-CH2- signal of P104 show a slightly linear increase with the decrease in temperature, implying that the PEO blocks in the triblock copolymer experience a slight hydration process, but still keep in contact with water. Temperature-Dependent Aggregation and Disaggregation of PEO-PPO-PEO Block Copolymer. Figure 5 schematically summarizes the aggregation and disaggregation of PEO-PPOPEO block copolymer in aqueous solutions in one heating and cooling cycle. It is well known that both PEO and PPO blocks are significantly hydrated and PEO-PPO-PEO copolymers exist as individual unimers in aqueous solution at low temperatures. With increasing solution temperature, the PPO moiety becomes increasingly hydrophobic. PEO chains show a slight dehydration with increasing temperature but keep in contact with water. At CMT, copolymer chains associate to form micelles consisting of a dense PPO core and a water-swollen PEO corona.24,27,28
When the temperature is further increased, the slightly dehydrated PEO chains lead to the dehydrated corona, and thus the size of the micelle reduces, as shown in Figure 2. Although disaggregation of PEO-PPO-PEO block copolymer in the cooling process by NMR spectroscopy was mentioned in previous literature,29 a thorough investigation on disaggregation of the polymer with the decrease in temperature is still absent. In this work, NMR and DLS were applied to study the disaggregation of Pluronic micelles in the cooling process. The RH of micelles during disaggregation could be obtained from DLS results, while the microenvironmental change of PEO and PPO blocks during disaggregation could be obtained from NMR results. When the temperature is above CMT, the reverse trend is observed with the decrease in temperature for both the DLS and NMR results. It indicates that PEO-PPO-PEO block copolymer micelles are exactly temperature-dependent. The RH of micelles decrease with increasing temperature, accompanied by the dehydration of PEO block and PPO block. With decreasing temperature, the RH of micelles increases, accompanied by the hydration of PEO block and PPO block. When the temperature is below CMT, the RH of micelles continues to increase, accompanied by disaggregation. Because PPO block changes from a hydrophobic microenvironment to a hydrophilic one at temperatures lower than CMT, big micelles detected by the DLS experiments in the unimer region are considered as water-swollen micelles, which are composed of a water-swollen PPO core and a water-swollen PEO corona. At higher temperature, hydrophobic PPO chains stay as a “hydrophobic” conformation, stretched trans conformation.12,14 The hydrophobic interaction leads to interlacing of PPO chains in micellar core. Such micellar cores formed by interlacing PPO chains cannot be completely dissolved in the cooling process,
Figure 5. Schematic of aggregation and disaggregation of PEO-PPOPEO in water during one heating and cooling cycle.
13220 J. Phys. Chem. B, Vol. 111, No. 46, 2007 and even water becomes a good solvent (i.e., the temperature is lower than CMT). Conclusions Studies of aggregation and disaggregation of Pluronics P103 and P104 in aqueous solutions during a heating and cooling cycle have been carried out by DLS and 1H NMR spectroscopy. The hydrodynamic radius of Pluronic micelle can be determined from the diffusion coefficient via the Stokes-Einstein equation by DLS. In the heating process, the hydrodynamic radius of the micelle decreased with the increase in temperature. In the initial stage of the cooling, the size of the micelle increased with the decrease in temperature, and RH values were almost the same as those at the corresponding temperatures in the heating process. Further decreasing temperature until CMT, RH values continued to increase until a certain temperature lower than CMT. Big micelles coexist with unimer in the “unimer” region. The microenvironmental change of PEO and PPO blocks in aqueous solution could be obtained from chemical shifts (δ) of the PO-CH3 and the EO-CH2- signals in NMR spectra during a heating and cooling cycle. No temperature differences were observed by NMR in the heating and cooling process. In the cooling process, the reverse trend was observed with the decrease in temperature, and the same δ values could be obtained at the same temperature as in the heating process. When the temperature is above CMT, the reverse trend was observed with the decrease in temperature for both the DLS and NMR results. It indicates that PEO-PPO-PEO block copolymer micelles are exactly temperature-dependent. When the temperature was below CMT, the RH of micelles continued to increase while the PPO block changed from a hydrophobic microenvironment to a hydrophilic one. This phenomenon was explained as the forming of water-swollen micelles at temperatures lower than CMT during the cooling process. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20221603, 20676137, and 20490200) and the National High Technology Research and Development Program of China (863 Program) (No. 20060102Z2049). We thank Miss Jinkun Hao for her help in LLS measurements. Supporting Information Available: Temperature dependence of relaxation time distribution function G(τ) of 2%
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