Macromolecular Research, Vol. 23, No. 1, pp 13-20 (2015) DOI 10.1007/s13233-015-3014-4
www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673
Micellar Packing of Pluronic Block Copolymer Solutions: Polymeric Impurity Effects Han Jin Park1, Gregory M. Treich1, Zachary D. Helming1, Joel E. Morgan2, Chang Y. Ryu*,1, Hee Sung Hwang4, and Gyoo Yeol Jung*,3,5 1
Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York 12180, USA Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, Troy, New York 12180, USA 3 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Korea 4 Institute of Environmental and Energy Technology, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Korea 5 School of Interdisciplinary Bioscience and Bioengineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Korea 2
Received March 17, 2014; Revised August 5, 2014; Accepted October 16, 2014 Abstract: Small angle X-ray scattering (SAXS), dynamic light scattering (DLS), and high performance liquid chromatography (HPLC) experiments are performed to support that the inter-micellar distance of Pluronic cubic structures in aqueous solutions is governed by the poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-PEO triblock copolymer concentration (not the overall polymer concentration) in the solutions. The “as-received (AR)” and “purified (Pure)” F108 solutions show a separate concentration dependence of body-centered cubic (BCC) lattice spacing, when the overall polymer concentration is used as a micellar packing parameter in aqueous solution. When the 22 wt% of non-micellizable polymeric impurities in the AR Pluronic F108 is taken into account, however, a universal concentration dependence of the BCC lattice spacing is observed, unifying results from both AR and Pure F108 solutions. When the PEO-PPO-PEO triblock copolymer concentration from the HPLC analysis is employed as an effective polymer concentration parameter, the universal relationship is observed to provide strong evidence that the polymeric impurities in AR F108 locate themselves in the less dense parts of the interstitial regions on the BCC lattice points, where were occupied by the triblock copolymer micelles. Although the polymeric impurities in AR F108 do not affect the actual triblock concentration dependence of the lattice spacing, they do shift the onset concentration of BCC micellar ordering. In the Pure F108, the onset of BCC packing occurs at the point where the nearest-neighbor radius (Rnn) in the BCC lattice is approximately equal to the hydrodynamic radius (Rh), indicating that lattice formation begins upon “hydrodynamic contact” between micelles. In the AR F108, the onset of packing occurs when Rnn/Rh is approximately 0.9, indicating that, in the presence of the polymeric impurities, micelles must be forced together beyond the point of hydrodynamic contact for the BCC packing. Keywords: block copolymers, micelles, pluronics, small angle X-ray scattering.
Introduction
Although the main component of as-received (AR) Pluronics is poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, it has been reported that low molecular weight (MW) polymeric impurities are present at levels of up to ~25 wt% in AR Pluronic samples.8,9 These polymeric impurities do not participate in the micellization of the PEO-PPO-PEO triblock copolymers in dilute aqueous solutions even at temperatures higher than the critical micelle transition temperature.10-12 In this report, we have used small angle X-ray scattering (SAXS) measurements to compare the micellar packing distance of AR and impurity-free “Pure” Pluronic samples to understand how low MW impurities affect the micellar packing when ordered cubic
Pluronic block copolymers are widely used as nonionic polymeric surfactants in a broad range of applications in therapeutics, cosmetics and nanotechnology, but they also serve as model amphiphilic macromolecules for the study of micellar self-assembly in solution.1-5 Commercially, Pluronics are produced by anionic polymerization of poly(propylene oxide) and poly(ethylene oxide) using alkali metal oxides including sodium hydroxide and potassium hydroxide.6,7 *Corresponding Authors. E-mails:
[email protected] or
[email protected] The Polymer Society of Korea
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structures of spherical micelles are formed. Micellar packing of Pluronics in aqueous solutions holds a key to controlling the size and structure of self-assembled hydrophobic and hydrophilic domains, which are important parameters to control the templated synthesis of mesoporous silica.13-15 Depending on the chemical composition, average molecular weight and chain architecture, Pluronics can self-assemble into spherical and non-spherical micelles in dilute aqueous solutions. It has been found that when the PEO block is much longer than the PPO block, as in F127, F68, and F108, spherical micelles are formed. At high concentrations in water, these micelles can order themselves into simple cubic, face-centered-cubic (FCC) and body-centered-cubic (BCC) lattices.16-27 When commercially available AR Pluronics are used, however, it is imperative to understand how the presence of the non-micellizable polymeric impurities affects the micellar packing and structures in the Pluronic solutions. For example, Mortensen et al.12 have reported that 20 wt% aqueous solution of AR F127 exhibited FCC structures upon heating, whereas “impurity-free” Pure F127 solutions exhibited a FCC-to-BCC transition at the same concentration. To compare the micellar ordering in Pluronic block copolymer solutions with and without the non-micellizable polymeric impurities, we have chosen to focus on F108 Pluronics. Because both AR F108 and Pure F108 solutions consistently form BCC ordered structures, F108 can thus serve as a model Pluronic system that allows us to investigate the effect of the polymeric impurities on the micellar packing distance in solution within the same BCC ordered structure. We have also used dynamic light scattering (DLS) to measure the “hydrodynamic” size of spherical micelles in dilute solution for comparison with the “structural” size of nearest neighbor distances between micelles in a BCC lattice, obtained from the SAXS measurements on semidilute solutions. This work shows, for the first time, that it is the concentration of actual PEO-PPO-PEO triblock copolymer that determines the size of the BCC unit cell upon ordering. When the mass of impurities is taken into account, both AR F108 and Pure F108 solutions show the same “universal” concentration dependence of the BCC unit cell. However, although the impurities in AR F108 do not affect the size of the unit cell, they do affect the onset of BCC packing. It is the ultimate aim of this report to provide experimental evidence to support the hypothesis that the non-micellizable polymeric impurities (i) are located in the less-dense interstitial space of the BCC lattice and (ii) make it difficult for the micelles to order into BCC structures (i.e. require higher triblock copolymer concentrations to induce the onset of BCC ordering).
Experimental Materials. For liquid chromatography, high performance liquid chromatography (HPLC)-grade solvents, such as water 14
Table I. Summary of Average Molecular Weight, Average Chemical Composition and the Content of Non-Micellizable Polymeric Impurities in AR F108 and Pure F108 Samples
Mwa (g/mol)
ÐM
Average PEO Composition (wt(PEO)%)c
Content of Polymeric Impurities (wt%)d
AR F108
10,600
1.40
85%
22%
Pure F108 11,100
1.26
78%
< 2%
b
a
Mw=Weight average molecular weight measured by aqueous size exclusion chromatography using monodisperse PEO calibration standards (Mw ranges: 106 to 116,300 g/mol). bÐM =Molar mass dispersity = Mw / Mn.29 cMeasured by H NMR using D2O. dMeasure by interaction chromatography using methanol and water mixture and a cyano (CN)-modified silica column.28
and methanol, were obtained from Fisher Scientific and used as received. Pluronic F108 was purchased from Sigma-Aldrich and designated as “as-received” (AR) F108. Polymeric impurities were removed from AR F108 by a large scale purification method using silica slurry;28 the purified F108 sample was designated as Pure F108. The molecular weights, average PEO composition and the impurity contents of AR F108 and Pure F108 are summarized in Table I. The molar mass dispersity is decreased from 1.40 to 1.26 upon the removal of low molecular weight polymeric impurities from AR F108. In addition, the impurity removal in AR F108 changed the average PEO composition from 85 to 78 wt(PEO)%. This is because the polymeric impurities in AR F108 are mainly PEO homopolymers and PEO-rich PEO-PPO copolymers (See Figure 8). Interaction Chromatography and Eluent Gel Permeation Chromatography. An isothermal interaction chromatography (IC) was used to quantify the polymeric impurities in AR F108 and Pure F108 (See Figure 1(a)). The liquid chromatography instrument is equipped with a refractive index detector (KnauerTM Smartline 2300). Separations were carried out using a Nucleosil cyano (CN)-modified silica column purchased from Macherey-NagelTM (inner diameter=4 mm, length=250 mm, average pore size=120 Å). Column temperature were controlled using a home-built water jacket and a circulating bath (NeslabTM). The concentration and the volume of each injection for the isothermal IC analysis were 0.8 wt% and 20 L, respectively. Assuming that the dn/dc values for the polymeric impurities and triblock copolymers are the same, the area ratio of between the well-separated impurity and triblock copolymer peaks in the IC profiles from the refractive index detector allows us to quantify the wt% of the polymeric impurities in Pluronic samples before and after the large scale purification.28 Eluent gel permeation chromatography (EGPC) analysis10,11 was performed to confirm that the polymeric impurities in AR F108 do not participate in micellization above the critical micellization temperature. (See Figure 1(b)) An eluent of 0.2 wt% of AR F108 aqueous solution was used at a flow Macromol. Res., Vol. 23, No. 1, 2015
Micellar Packing of Pluronic Block Copolymer Solutions: Polymeric Impurity Effects
Figure 1. (a) Interaction chromatography profiles of as-received (AR) and purified (Pure) F108 Pluronics using cyano-modified silica column with a mixed solvent of methanol/water=60/40 (wt/wt) at 40 oC at 0.8 mL/min. (b) Eluent gel permeation chromatography (EGPC) profiles of AR F108 at 35 and 50 oC. The dotted lines represent the de-convoluted EGPC profiles of the PEO-PPO-PEO triblock and the non-micellizable polymeric impurities at 35 oC.
rate of 1 mL/min, and 20 L of 0.4 wt% of AR F108 solution was injected for the EGPC. Two Polysep-GFC-P linear columns (diameter = 7.8 mm and length = 300 mm for each column), purchased from Phenomenex, were used in series for the EGPC analysis; the column temperature was controlled as described above. Dynamic Light Scattering. A 90 Plus nanoparticle size analyzer from Brookhaven Instruments with 35 mW solid state laser (wavelength, =659.0 nm) and avalanche photodiode detector at a fixed scattering angle at 90o was used. A dilute solution of AR F108 at 0.5 wt/v% in water was prepared, and the solution temperature was ramped from 20 to 50 oC with an increment of 2 oC and an equilibration time of 1 min at each temperature. The average hydrodynamic radius (Rh) and its standard deviation at each temperature were obtained from 10 repeated measurements, where the autocorrelation function of intensity fluctuations averaged over 30 s. The CONTIN analysis30,31 using the software package Macromol. Res., Vol. 23, No. 1, 2015
in Brookhaven particle solution software v2.3 was used for the average size analysis. The sampling time was 5 s, and the longest correlation time for the CONTIN analysis was 1 s. Small-Angle X-Ray Scattering. A Bruker-AXS Nanostar-U instrument, equipped with a copper rotating anode x-ray source (wavelength, =0.154 nm, 6 kW supply 0.1×1 mm filaments) operated at 50 kV, 24 mA, and Hi-STAR 2D multi-wire proportional detector (1024×1024 pixels), was used for small-angle X-ray scattering (SAXS) measurements. The X-ray beam was focused to a point by means of a Montel optic, and collimated by a 3-pinhole assembly. The beam path between focusing optic and the detector was maintained under vacuum (< 0.1 mBar). Flood-field and spatial calibrations of the detector were performed using a 55Fe source installed at the sample position. The sample-to-detector distance was 105.65 mm based on calibration using silver behenate (d(001)=5.8380 nm). The SAXS characterization of the polymer solution sealed in a glass capillary tube serves as a convenient method to measure the polymer concentration dependence of the micellar packing distance in F108 Pluronic solutions. The solution samples were prepared in Boron-rich capillary tubes (outer diameter= 1.5 mm, wall thickness=0.01 mm, Charles Super Co.) and sealed with synthetic wax (melting point=65 oC, Charles Super Co.). The sealed capillary tubes were placed in the evacuated sample chamber and temperature was maintained with a thermoelectric Peltier controller and sample holder from Materials Research Instruments (Karlsruhe, Germany). Samples were equilibrated for at least 15 min at the designated temperature after which data were typically acquired with a typical exposure time of 30 min. The SAXS data in the 2D scattering images were integrated over the full circle of azimuthal angle, , values with an increment of 0.01 degrees of scattering angle, . Finally, the intensity I(q) was plotted against q=4/ sin(/2).
Results and Discussion Universal Concentration Dependence of BCC Domain Spacing. To elucidate the effects of the non-micellizable polymeric impurities on the formation of ordered structures in commercially available Pluronic block copolymers, Pluronic F108 was chosen. Because of the high PEO content in asreceived and purified F108 (i.e. AR F108 and Pure F108, respectively), both preparations form spherical micelles, which become ordered into a cubic structures at high concentrations. SAXS intensity profiles from both AR F108 and Pure F108 solutions showed higher order peak patterns of q*: √2q*: √3q*: √4q*, indicating that the micelles ordered on a BCC lattice in both cases (q* represents the “main” peak from the (110) plane in BCC; See Figure 2) Therefore, Pluronics F108 can be used as a model system in which to study the effects of impurities on micellar packing without the potential complications of inducing different cubic ordered structures upon removal of the impurities.12 Figure 3 shows the temperature dependence of unit cell 15
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Figure 2. SAXS profiles of (a) AR F108 and (b) Pure F108 for various polymer concentrations in aqueous solutions at 45 oC.
Figure 3. Temperature dependence of d(110)BCC for F108 aqueous solutions at different polymer concentrations.
spacing d(110)BCC= 2/q* for AR F108 and Pure F108 aqueous solutions at 15 and 20 wt% polymer concentrations, exhibiting the BCC ordered structures as determined by SAXS. Because of the temperature instability of the wax sealing in the X-ray capillary, the solution capillary temperature was kept under 60 oC for the study. Within the temperature window between 25 and 55 oC, it was found that temperature had less significant impacts on the change of d(110)BCC than the polymer concentration. More importantly, it was noticed that the d(110)BCC values of 20 wt% AR F108 and 15 wt% Pure F108 solutions are very similar, suggesting an important clue as to understand how the non-micellizable polymeric 16
impurity removal from AR F108 affects the BCC micellar packing in solution. Therefore, we have focused on the SAXS measurements to investigate the dependence of the d(110)BCC domain spacing on “polymer concentration” in AR F108 and Pure F108 solutions at a fixed temperature of 45 oC (Figure 4). The unit cell spacing of BCC ordered micelles, d(110)BCC= 2/ q*, in F108 aqueous solutions is more significantly influenced by “polymer concentration” than temperature. Because more spherical micelles have to be packed within the same volume of solutions at higher polymer concentrations, the distance between the BCC ordered micelles is further reduced. Figure 4(a) shows the overall polymer concentration (CPolymer in wt%) dependence on 2/q* of AR F108 and Pure F108 solution under isothermal condition at 45 oC. We have also included the 2/q* data from the solutions at low concentrations (open symbols in Figure 4), which do not exhibit BCC Bragg peaks, but still have a distinct SAXS peak maximum at q*. When CPolymer was employed as the effective measure of polymer concentration dependence on 2/q*, as in Figure 4(a), the concentration dependence curves for AR F108 and Pure F108 solutions appear quite different from each other. An important consideration to understanding the polymer concentration dependence of the BCC domain spacing in Figure 4(a) is the fact that AR F108 contains 22 wt% polymeric impurities (See Table I). As confirmed from EGPC experiments (Figure 1(b)), these polymeric impurities in AR F108 do not participate in the formation of block copolymer micelles.10-12 Because only 78 wt% of the gross weight in AR F108 are available for the micellization of PEO-PPOPEO triblock copolymers, the effective polymer concentration governing the micellar packing distance on a BCC latMacromol. Res., Vol. 23, No. 1, 2015
Micellar Packing of Pluronic Block Copolymer Solutions: Polymeric Impurity Effects
Figure 4. (a) Polymer concentration and (b) triblock copolymer concentration effects on 2 /q* from SAXS for F108 aqueous solutions at 45 oC for AR F108 and Pure F108 solutions. Solid and open symbols represent the q* data from solutions with and without the BCC Braggs peaks of q*: √2q*: √3q*: √4q* in SAXS profiles, respectively.
solutions. Once the BCC micellar packing distance is predetermined by the packing of the triblock copolymer micelles, the non-micellizable polymeric impurities in AR F108 would have to sequester themselves in the less-dense regions of the interstitial space in the BCC structure. Because the BCC unit cell has a volumetric packing factor of 0.68, there is actually a great deal of space to accommodate non-micellizable MW impurities in the interstitial sites of BCC structures. This interstitial sequestration of polymeric impurities in ordered Pluronic cubic structures had been similarly proposed by Pozzo and Walker,32,33 who successfully sequestered silica nanoparticles and proteins into the interstitial volume of the cubic structures of ordered Pluronic hydrogels using asreceived Pluronic F127. Onset of BCC Ordering Upon Hydrodynamic Contact. One of the main goals in this paper is to clarify if the nonmicellizable polymeric impurities play any role in the onset of BCC ordering in F108 solutions. Because Rh of the Pluronic copolymer micelles can be independently measured by DLS in dilute solution, it can be compared with the “nearestneighbor” radius, Rnn, which was obtained from SAXS measurements using a lattice structure parameter, aBCC. This comparison offers an important clue for understanding the closest packing distance between the triblock copolymer micelles upon formation of BCC structure in F108 solutions. The hydrodynamic radius, Rh, of a spherical micelle serves as a reference size that can be related to the length scale of Rnn associated with the onset ordering into BCC structures in F108 solutions. The triblock copolymer micelles of F108 exhibited Rh values of approximately 11.6 nm at 45 oC
tice is the triblock copolymer concentration (CTriblock , wt%). The CTriblock can be obtained in the following equation. CTriblock = CPolymer×wtTriblock
(1)
, where wtTriblock represent the weight fraction of triblock copolymer in AR F108. In this case, wtTriblock=0.78, which had been independently measured by interaction chromatography.28 When CTriblock is used to describe the concentration dependence on the packing distance of BCC ordered micelles in F108 aqueous solutions (Figure 4(b)), it was found that there exists a “universal” dependence of 2/q* on polymer concentration in terms of CTriblock that allowed us to consolidate the SAXS results of AR F108 (wtTriblock=0.78) and Pure F018 (wtTriblock ~1.0). This “universal” dependence of 2/q* in term of CTriblock supports the hypothesis that the formation of BCC micellar structure is predominantly governed by the micellar packing of the PEO-PPO-PEO triblock copolymers into a BCC lattice whether or not there are polymeric impurities in the F108 Macromol. Res., Vol. 23, No. 1, 2015
Figure 5. Temperature dependence of hydrodynamic radius (Rh), measured by dynamic light scattering, for AR F108 in a dilute 0.5 w/v% solution in water. It was found that Rh=11.6 nm at 45 oC. The error bar represents the standard deviation value from 10 consecutive measurements of Rh using dynamic light scattering at the same temperature. 17
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Figure 6. Nearest neighbor radius, Rnn, dependence on the triblock copolymer concentration for (a) Pure F108 and (b) AR F108 solutions at 45 oC.
for a dilute 0.5 wt% solution of AR F108 in water (Figure 5). The discontinuous increase in Rh at temperature above 36 oC suggests the formation of block copolymer micelles, as the PPO block in F108 eventually becomes hydrophobic. Assuming that the micelles are uniformly ordered into the BCC lattice, the following equations can be applied for the calculation of Rnn.20,34,35 6 3 Rnn = ---------- = ------ aBCC 2q* 4
(2)
Figure 6 shows the dependence of Rnn on the CTriblock of (a) Pure F108 and (b) AR F108 solutions at 45 oC, at which both solutions formed the BCC structures. The nearest-neighbor distance in the BCC lattice of micelles has to be diminished upon the increase of CTriblock, because more micelles have to be closely packed within a given volume at higher triblock copolymer concentration. While the dependence of Rnn on the CTriblock overlaps the same trend, the presence of polymeric impurities in AR F108 makes it difficult for the block copolymer micelles to form the BCC ordered structure. Because the polymeric impurities in AR F108 interfere with 18
Figure 7. Schematic diagrams representing the micellar packing in the BCC on-set (110) plane of BCC structure at the on-set concentration of (a) CTriblock BCC on-set =11.0 wt% for Pure F108, when Rnn~Rh and (b) CTriblock =13.3 wt% for AR F108, when Rnn~0.9 Rh.
the BCC ordering of micelles, it was found that the onset triBCC on-set block concentration for the BCC ordering, CTriblock , was higher in AR F108 solution than Pure F108. As highlighted in Figure 5, the onset of BCC ordering required that CTriblock be increased from 11.0 wt% (Pure F108) to 13.3 wt% (AR F108) in the presence of the non-micellizable polymeric impurities. In the case of Pure F108 solutions, it was found that Rnn~Rh at BCC on-set the triblock concentration (CTriblock =11.0 wt%) for the onset of BCC ordering to occur upon the “hydrodynamic contacts” between the block copolymer micelles forming the densest (110) plane in the BCC structure. In contrast, the onset of BCC ordering in AR F108 solutions occurs at the triblock conBCC on-set centration (CTriblock =13.3 wt%), when Rnn~0.9 Rh. As previously shown in Figure 1, liquid chromatography analysis on AR F108 indicates that 22 wt% of the “as-received” product is in the form of the non-micellizable polymeric impurities, which are able to interfere with the BCC packing of micelles in semidilute solutions. A schematic diagram is provided in Figure 7 to show BCC on-set the difference in Rnn with respect to Rh at CTriblock in AR and Pure F108 solutions. The values of Rnn, measured from SAXS, Macromol. Res., Vol. 23, No. 1, 2015
Micellar Packing of Pluronic Block Copolymer Solutions: Polymeric Impurity Effects
are defined in the (110) plane that is the densest plane in the BCC structure. The Rh serves as a reference length scale of micelles that had been measured by DLS in dilute solution at the same temperature (45 oC). In the presence of the polymeric impurities that do not participate in the micellization, before the BCC ordering takes place in AR F108 solutions, the micelles must be brought closer beyond the hydrodynamic contacts (i.e. Rnn
