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Dielectric behavior and phase behavior of block copolymer PEO13-PPO30-PEO13 aqueous solution Wantong Li, Juan Wang, Man Yang, and Kongshuang Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00776 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018
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Dielectric behavior and phase behavior of block copolymer PEO13-PPO30-PEO13 aqueous solution Wantong Li#, Juan Wang#, Man Yang, Kongshuang Zhao* College of Chemistry, Beijing Normal University, Beijing 100875, China (#: Wantong Li. and Juan Wang contributed equally, *: Email address:
[email protected])
Abstract: Dielectric spectroscopy can be applied to study the structure and dynamics of block polymer. In this work, dielectric measurements of block copolymer Pluronic L64 solution are carried out in the frequency range between 40 Hz and 110 MHz with variable temperatures and concentrations. We analyze the phase behavior of the PEO13-PPO30-PEO13 (Pluronic L64) aqueous system according to the concentration/temperature dependence of direct current conductivity. The result indicates the sensitivity of the phase behavior and conductivity of the Pluronic L64 solution to temperature. Besides, two relaxations were observed: Relaxation 1 (0.5 MHz) is related to the gelation process, while relaxation 2 (5 MHz) is caused by the interface polarization. Based on relaxation 2, the volume fraction and permittivity of the particle were calculated. The formations of the block copolymer micelle and gel are monitored successfully by the temperature/concentration dependence of the dielectric parameters and the volume fraction. Key Words: Block copolymer Pluronic L64, Dielectric relaxation, Gelatinization
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INTRODUCTION The block copolymer can self-assemble into micelles in the selective solvent due to its amphiphilic nature. By controlling the structure, composition, and content of the block copolymers, it is possible to obtain morphology-rich aggregates and functional membranes1-4. These aggregates of different types also have important application in the areas of life sciences, drug delivery and release, and preparation of nanomaterials 1, 5-7 . Specially, the self-assembly of block copolymer in water media is of fundamental interest, so many important reports have been made more than a decade ago. For example, Lodge8 used the modified triblock copolymer surfactants to induce micellization and deformation; Alexandridis9 studied the effect of solvent polarity on micelle formation of amphiphilic alkyl-propoxy-ethoxylated surfactants. Recently, the interest of researchers has greatly increased because more and more basic and applied researches on block polymers have been reported. For example, Wang et al.10 reported the self-assembly of colloidal particles and the preparation of biomimetic nanocomposites by block polymer micelles; Pawar et al.11 prepared biocompatible block polymers with hydrophobic urea blocks and hydrophilic polyethylene glycol (PEG) blocks; Khullar et al. 12 reported the morphology and preparation method of block copolymer micellar nanoreactors. The basic properties such as the dynamics behavior and phase behavior have also been systematically studied13-15. Poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) triblock copolymer, as a non-ionic surfactant, has a strong application background16-20 in cell biology, medicine and industry materials and other fields21-25. Those are the reasons why PEO-PPO-PEO triblock copolymer (trade name Pluronic) have received increasing attention among a large number of block polymers. The research methods of block polymers mainly include electron microscopy techniques such as scanning electron microscopy, transmission electron microscopy and atomic force microscopy8,10,18; scattering techniques such as small angle x-ray scattering (SAXS) and small angle neutron scattering (SANS) et al18,26,27, and various spectroscopic techniques such as Fourier transform infrared spectroscopy28-30. Although electron microscopy and spectroscopy techniques can obtain some microstructural information, the requirements are relatively harsh for the research system. In particular, block polymers or their self-assembly complex systems may bring some difficulties when using the above methods. In addition, there are many studies on the macroscopic mechanical properties of block polymers such as viscoelasticity and rheology13,21,23,31. Alternatively, dielectric spectroscopy can be used to measure the interaction of electromagnetic fields and matter, has also been used to study the polymer structure and dynamics behavior. The validity of the dielectric spectroscopy for polymer solution systems has long been recognized and reported by a large number of researches32-35. In recent years, we also studied the chain conformation of polyelectrolyte solution36-38, the microstructure and phase transition dynamics of microgel suspension39-42 by dielectric spectroscopy. Dielectric studies on block polymers have also been reported for the past decade or so, and most studies have focused on the chain dynamics of block copolymers 43-45. However, the
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study on the micellization behavior of block copolymers in solution is relative less. Cametti et al.46combined the dielectric spectroscopy with standard electrokinetic theory to study the micellization behavior of temperature-sensitive block copolymers and obtained the microstructure of the micelles; Song et al.47monitored the sol-gel transitions of thermo-reversible triblock copolymer Pluronic F127(PEO65-PPO100-PEO65) aqueous solution with a concentration of 5-22% by dielectric spectroscopy and obtained the critical micelle temperature and micelle volume fraction. All of the above results show the validity of dielectric spectroscopy in studying the block polymer. Recently, Wu et al. used the mechanics spectrum to construct the phase diagram of the Pluronic L64 (PEO13-PPO30-PEO13) aqueous solution system. They studied the viscoelasticity of the system and the dynamics of the gelation process, and obtained the information about the internal structure with the change of temperature 20. In this work, the system composed of PEO13-PPO30-PEO13 block copolymer and water was measured in the temperature range of 10-40℃and the frequency range of 10-110 MHz. Dielectric behavior (such as conductivity, permittivity, dielectric increment et al.) of the Pluronic L64 solution at different composition and temperatures as well as its relationship with microstructure was obtained. The relaxation dynamics and gelation behavior induced by the temperature were also discussed.
EXPERIMENTAL SECTION Materials and measurement paths The Pluronic L64 (PEO13-PPO30-PEO13), with molecular weight of 2900 is obtained from Sigma-Aldrich. Aqueous solutions of the Pluronic L64 copolymer with different concentrations were prepared by dissolving the copolymer in deionized water and then kept at 4 oC for seven days before dielectric measurements. The phase diagram and measurement points used are shown in Figure 1, where L1 and H are the micellar and hexagonal phase, respectively, and the detailed prepared process is in the reference 20. The measurements were carried out according to the six paths in the phase diagram.
Fig.1 Temperature-concentration phase diagram of Pluronic L64 aqueous solution (adapted from Ref. 20).
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The phase diagram contains the CMC (Critical Micelle Concentration)–CMT (Critical Micelle Temperature) line, percolation line, cloud point line; L1 is the micellar phase, H is the hexagonal phase, ■, ●, ▲, ▼, ◆, and ★ represent the variable temperature paths for Pluronic L64 at 10, 20, 30, 45, 50, and 55 wt. %, respectively.
Dielectric measurements Dielectric measurements of aqueous solutions of Pluronic L64 were carried out by a HP4294A precision impedance analyzer (Agilent Technologies) from 40 Hz to 110 MHz. The applied alternating current (AC) electric field was 500 mV, and the instruction manual of the instrument indicates that the uncertainty of dielectric measurements is ±0.04% within the present frequency range. All samples were measured at 10-50 °C. A measurement cell with concentrically cylindrical platinum electrodes was employed48, however, the measured raw data for capacitance Cx and conductance Gx contain the influence of the measuring cell, and therefore, the raw data obtained needs to be corrected. First, the cell constant (Cl = 0.072 pF), the stray capacitance (Cr = 0.716 pF), and the residual inductance (Lr = 5.23 × 10−8 F/S2) of the cell were calibrated with standard substances (air, ethanol and pure water), and then the original data were corrected according to the Schwan method49:
C =
G =
− C
(1)
(2)
where ω (= 2πf ) is angular frequency. The subscripts x and s represent the actual measured data and the corrected data, respectively. After that, the permittivity and conductivity of the sample are obtained after the conversion by using the formula: ε = C − C ⁄C , κ = G ε⁄C (ε0 = 8.8541×10-12 F/m, vacuum permittivity) Determination of dielectric parameters Under the effect of AC electric field, the dielectric properties of the material can be expressed by the complex permittivity ∗ : ε∗ ω = ε ω − j
! "#
!$ #
= ε ω − jε ω − j "
(3)
where ε'(ω) and ε"(ω) are, respectively, the real and imaginary parts of ε∗ , and
j = (−1)1/2 . κl is the low frequency limit conductivity (direct current (DC) conductivity), and it can be read from the platform of the κ~f graph. The relaxation strength ∆εi, the relaxation time τi (τi=1/2πf, fi0 is characteristic relaxation frequency), and relaxation time distribution parameter βi (0
