Article pubs.acs.org/Macromolecules
Amine-Functionalized CO2 Responsive Triblock Copolymer MicellesA Small-Angle X‑ray Scattering Study Stefan Salentinig,*,† Phil Jackson,‡ and Adrian Hawley§ †
Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 381 Royal Parade, Parkville, Victoria 3052, Australia ‡ Department of Chemistry, University of Sydney, Building F11, Camperdown, New South Wales 2006, Australia § SAXS/WAXS Beamline, Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia S Supporting Information *
ABSTRACT: CO2 responsive colloids are of interest for the delivery of active molecules in both pharmaceutical applications and for gas treatment technologies, among others. Primary and secondary organic amines react with CO2 gas in aqueous solution to form ionic carbamates in an exothermic “CO2 sequestration” reaction. Several amines important in this context are 2-aminoethanol, 2,2′-iminodiethanol, and piperazine. We have used small-angle X-ray scattering at a high intensity synchrotron source to demonstrate that triblock copolymer micelles containing carbamate-forming amines change shape upon exposure to CO2. Modeling of the scattering data is used to elucidate the effects of exposure on micelle size and morphology. Electron density distribution within the micelles, derived from the SAXS data, established that the amines interact with the polymer micelles. The products of CO2 exposure, namely, carbamate, bicarbonate anions, and protons, modify the packing of the polymer chains, and occupy the volume within the polymer aggregates. Our findings contribute to the detailed understanding and optimization of liquid based CO2-responsive systems.
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INTRODUCTION Stimuli-responsive colloids alter their structure and properties in response to a variety of external factors, such as pH,1−3 light,4,5 ultrasound,6 and temperature,7,8 and are studied in many fields including targeted drug delivery and fuel processing (extraction, cleaning). A further stimulus is CO2, which is ubiquitous in biochemical systems, the natural environment and in engineering applications.9 The presence of CO2 can alter the properties of certain surfactants and polymers containing reactive moieties such as amine, amidine and guanidine.9−12 “Smart” vesicles and micelles based on these materials have been considered for use in areas such as drug delivery and viscosity control.9,13−17 In this study, we present evidence for CO2-tunable micelles based on the aqueous amines, 2-aminoethanol (MEA), 2,2′iminodiethanol (DEA), and piperazine (PZ) mixed with the triblock copolymer surfactant F127 [poly(ethylene oxide)− poly(propylene oxide)−poly(ethylene oxide)], (PEO99− PPO67−PEO99) in water. The structures of the amine molecules are shown in Scheme 1. The MEA, DEA and PZ amines are well-studied agents for CO2 capture:18 in the presence of CO2, the amine groups (RR′NH R,R′ = H−, HOC2H4− or −RR′− = −C2H4NHC2H4−) undergo reaction to form carbamate anions (RR′NCO2−) and a proton. The reaction is readily reversed upon heating, sparging with N2 gas or solution acidification.18−21 As a result of dipole−quadrupole interactions, polymers containing PEO groups have affinity for CO2 and exhibit © XXXX American Chemical Society
Scheme 1. Molecular Structure of Individual Components Used in This Study
selectivity in polymer membranes, which is desirable for applications such as gas scrubbing.22−24 Water is a good solvent for the PEO block, however, it is a poor solvent for PPO moieties at room temperature, because hydrophobic interactions tend to dominate. The conformational entropy of the polymer blocks is also an important parameter for the final micelle structure.25,26 The resulting aggregation behavior of triblock copolymers in water is well studied in the literature.25−29 F127 has been found to form micelles with a PPO core and PEO corona above the critical micelle concentration (cmc) of 0.7% w/v.28,30 The hydrophobic− hydrophilic interfacial area of such aggregates are the result of Received: December 22, 2014 Revised: March 11, 2015
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intraparticle structure (form factor) simultaneously. This analysis gives access to important parameters such as the effective interaction radius and effective charge in combination with particle size and shape (see SAXS Data Analysis section). Experiments were carried out immediately after the solutions were prepared. The vials were placed on a microbalance and each solution was sparged with CO2 gas separately at normal temperature and pressure, until CO2 saturation was achieved (no further increase in mass over a 10 min period). Sparging was performed with a glass capillary that was connected via a needle valve to the CO2 reservoir. SAXS measurements recorded changes in the colloidal structure with CO2 added. SAXS Measurements. SAXS measurements were performed at the SAXS/WAXS beamline at the Australian Synchrotron.39 An X-ray beam with a wavelength of 1.1271 Å (11 keV) was used. A sample to detector distance of 1015 mm gave the q-range 0.007 < q < 0.7 Å−1, where q is the length of the scattering vector, defined by q = (4π/λ) sin(θ/2), λ is the wavelength and θ the scattering angle. The 2D SAXS patterns were acquired within 1 s using a Pilatus 1 M detector with active area 169 × 179 mm2 and with a pixel size of 172 μm. All experiments were performed at T = 27 °C (hutch temperature at the beamline). For each curve, multiple frames were recorded and averaged manually after inspection for beam damage; no beam damage was observed in the samples. Two dimensional scattering patterns were integrated into the one-dimensional scattering function I(q) using the in-house developed software package scatterBrain. Scattering curves are plotted as a function of relative intensity, I, versus q. Water was subtracted as background from all scattering curves and Porod analysis at high q, above q-region containing the internal structure of the polymer corona, was used to determine the constant background that was then subtracted from the data. SAXS Data Analysis. The generalized indirect Fourier transformation method (GIFT) was used for a model free analysis of the scattering data.40−42 For particles of arbitrary shape with an electron density difference of Δρ(r) relative to the mean value, the pairdistance distribution function p(r) is given by p(r) = r2Δρ̃2(r) where Δρ̃2(r) is the convolution square of the electron density averaged for all directions in space. This averaging causes no loss of information in the case of particles with spherical symmetry. The p(r) is calculated from the scattered intensity I(q) using the equation43
the balance between the thermodynamics of solvation, dispersion and stronger coulomb forces in the case of ionneutral interactions.31 The addition of cosurfactants and cosolvents such as ethanol, glycerol, and alkanols was shown to significantly alter the aggregation behavior of the copolymers in solution.32−36 Salt ions also influence the packing and dimensions of triblock copolymer micelles as the overall micelle size increases according to the Hofmeister anion series.37 Thus, F127 is ideal for this study because of the potential for modification of the preferred curvature by cosolvents.32 At the molecular level MEA, DEA, and PZ are comparable in dimensions to the monomer units of the triblock copolymer. The molecular structure of the amines suggests that they could exhibit a range of interactions, for example, hydrogen bonding, with either their −OH or cationised amine group localizing at the ether oxygens of PEO or PPO group. MEA, DEA, and PZ are more hydrophobic than the PEO block and their octanol/ water partition coefficients (log P) are comparable to that of PPO, suggesting that they will tend to aggregate/localize within the PPO core (see Table 1).36 The log P value for the carbamate product will be more negative as a result of its ionic nature. Table 1. Octanol/Water Partition Coefficients (log P) for Components Used in This Study38a log P 36
ethylene glycol (for PEO) propylene glycol (for PPO)36 MEA DEA PZ
−1.93 −1.41 −1.31 −1.43 −1.50
a Note that the log P for the MEA, DEA, and PZ is comparable to the value of PPO.
Our hypothesis is that coulomb interactions, and to a lesser extent dispersion forces, between the amine (loaded and unloaded) and the occluding polymer moieties will influence the geometry of the micelles. The reaction following CO2 addition, which leads to the formation of carbamate ions and protons, further influences the micelle morphology as a result of charge repulsion between carbamate ions anchored within the polymer network. Our aim is to understand amine behavior within the triblock copolymer microstructures and how exposure to CO2 induces changes to the solvation of hydrophobic PPO and hydrophilic PEO segments. To the best of our knowledge, this is the first observation of CO2-induced transformations within aqueous triblock copolymer/amine nanostructures using small-angle Xray scattering (SAXS) on a high intensity synchrotron source.
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I(q) = 4π
∫0
∞
p(r )
sin(qr ) dr qr
(1)
and gives a real space representation of the overall shape of the particles.44 In the case of spherical geometry, deconvolution of the p(r) gives the radial contrast profile Δρ(r) in electron density relative to the mean value, which gives information about the internal structure of the scattering particles.45 Most spherical micelles have a core−shell type structure, with a hydrophobic core and a hydrophilic shell, which also contains bound solvent molecules. If there is a difference in average radial electron density between core and shell, the radius of the core and the thickness of the shell can be discerned directly from the radial contrast profile. In aqueous solution, hydrophilic head-groups with the associated counterions and hydrophobic hydrocarbon chains are represented by positive and negative values in the Δρ(r). In the case of monodisperse, homogeneous and spherical particles, the scattering intensity can then be expressed by
MATERIALS AND METHODS
Materials. Reagents: Charcoal-filtered water (R > 18 MΩ), MEA, DEA, PZ (>99% purity, Sigma Australia), F127 (BioReagent, Sigma Australia), and CO2 gas (food grade, BOC gas supplies). A 10 wt % F127 stock solution (∼0.008 mol/L) was prepared by slowly adding the powder to water with constant stirring. Solutions of MEA, DEA, and PZ (1 mol/L) were prepared by dissolving the amines in 5 mL of the F127 stock solution contained in a sealable 20 mL vial under nitrogen atmosphere. The resultant mole ratio of amine/PPOunits and amine/PEO-units in F127, calculated with the average molecular weight of F127 of 12600 g/mol, is ∼1.9 and 0.6, respectively. The high concentration of the polymer micelles was chosen to study interparticle interactions (structure factor) and
I(q) = NS(q)P(q)
(2)
with N being the number of particles and P(q) the form factor describing the intraparticle structure and interactions. Interparticle effects described by the structure factor S(q) can influence the scattering function at higher concentration (volume fraction). The GIFT method allows the separation of form and structure factor.40,46 This technique uses the model free approach for the p(r) described above, which corresponds only to the form factor. Simultaneously, a model for the structure factor is fitted to obtain a fit of eq 2 to the data. The effective interparticle structure factor for spherical particles B
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Figure 1. Experimental SAXS patterns (symbols) and fits (lines) for 10 wt % in F127 in water before and after sparging the solution with CO2 gas (a). The corresponding p(r) functions calculated from the fits using the generalized indirect Fourier transformation method are presented in (b). The corresponding parameters for the structure factor model are presented in Supporting Information Table SI1 with the S(q) presented in Supporting Information Figure SI1. interacting through hard sphere (excluded volume) interactions was calculated using the Percus−Yevick closure relation.47 In case of charged micelles after the reaction with CO2, the Ornstein−Zernike equation with the hypernetted-chain approximation for the closure relation was used.46 In this model, the salt concentration of 0.1 M was included to account for the CO2 loaded into the solution and the solubilization of some of the carbamate ions solubilized in water (see Supporting Information for more details). The radius of gyration was also calculated from the p(r) using
Rg =
∫ p(r)r 2 dr 2 ∫ p(r ) dr
Pchain(q) =
Further model-dependent analysis was undertaken based on the P(q) resulting from the model independent analysis with the GIFT method. The form factor of spherical block copolymer micelles was described using a spherical core−shell model with Gaussian chains attached to the core surface, dominating the higher q region of the scattering data in these systems.25,48,49 The total form factor of the micelles was calculated using25
RESULTS AND DISCUSSION Characterization of F127 Micelles. The SAXS profiles for 10 wt % F127 micelles in water before and after addition of CO2 are presented in Figure 1. The I(q) shows a structure factor peak around 0.03 Å−1 caused by interparticle interaction in the concentrated system and the scattering from the intramicellar dimensions at higher q values (smaller sizes in real space). A micelle volume fraction of 33% was calculated from the structure factor for all systems, see Supporting Information Table SI1. As the F127 concentration is 10 wt % in water, the observed volume fraction is caused by the strong hydration of the PEO chains in the corona of the polymer micelles. The effective interaction radius from the structure factor for the uncharged systems (prior to CO2 addition) is ∼100 Å. The corresponding pair distance distribution functions (p(r)) calculated from the I(q) using the generalized indirect Fourier transformation method are shown in Figure 1b. The p(r) indicates that the micelles were of approximate spherical shape with a diameter of 225 Å (from p(r) = 0). The corresponding radius of the micelles (at Dmax/2 of the p(r)) is in good agreement with the effective interaction radius from the structure factor. This size also agrees with previous reports of 200−250 Å diameters for F127 micelles at room temperature.51,52 When no amines are present, no significant change in shape or size could be observed in the F127 micelles upon sparging with CO2. This confirms the micelles are not active in
+ [Δρshell VshellA shell (q , R core , R total)]2 + 2Δρcore VcoreA sphere (q , R core)ΔρshellV A shell (q , R core , R total) shell
(4)
where p is the number of polymer chains, Vi and Δρi the volume and contrast of core, shell, and polymer PEO block. The scattering amplitude from the spherical core with radius Rcore is
3[sin(qR core) − qR core cos(qR core)] (qR core)3
(5)
and the scattering amplitude from the spherical shell surrounding the core of the micelle with total radius Rtotal is A shell (q , R core , R total) =
A sphere(q , R total)R total 3 − A sphere(q , R core)R core 3 R total 3 − R core 3
(7)
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Pm(q) = scale[[Δρcore VcoreA sphere(q , R core)]2
A sphere(q , R core) =
(qR g)4
with Rg being the root-mean-square ensemble average of the radius of gyration. The scattering data in this study is plotted on an arbitrary scale. The scattering length density of the core and shell, as well as core-radius from the GIFT analysis, was used. The maximum micelle dimension from GIFT analysis was determined as the starting values for Rtotal. The value for ΔρPEO was fixed to Δρshell and VPEO estimated at 10000 as the modeling is performed on arbitrary scale with focus on relative differences in excess electron density and polymer contribution only. The values for scale, Rtotal, Rg, and p were optimized to achieve the best possible fit to the P(q) obtained from GIFT analysis. The fits were further improved by optimizing Δρshell and Rcore around the calculated value from GIFT analysis.
(3)
+ p[(ΔρPEO VPEO)2 Pchain(q)]]
2[exp(− (qR g)2 ) − 1 + (qR g)2 ]
(6)
The smearing of the core and corona surfaces was included by multiplying the scattering amplitudes Asphere(q, Rcore) and Asphere(q, Rtotal) with the Gaussian function exp(−σi2q2/2). The value for σi2 was fixed at 0.05Rcore and 0.20Rshell for the core and shell, respectively. The effective single chain form factor was calculated using the form factor for Gaussian chains given by the Debye function50 C
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Figure 2. SAXS scattering patterns for F127-based micelles with MEA, DEA, and PZ (symbols) and calculated fits (full lines) before (a) and after CO2 loading (b). The I(q) are offset to allow better comparison. The calculated p(r) profiles are presented in panels c and d, respectively. The p(r) profiles were normalized to the same peak-maximum. The obtained parameters for the structure factor model are presented in Supporting Information Table SI1 with the S(q) presented in Supporting Information Figure SI1.
Figure 3. Radial scattering-length density difference Δρ(r) of the micelles resulting from deconvolution of the p(r) profiles for F127 and the F127− DEA system in panel a. The MEA and PZ system is presented in panel b. The corresponding p(r) and calculated fits are shown in Supporting Information Figure SI2.
action radius was ∼102 Å with a volume fraction of 33% for the micelles, see Supporting Information Table SI1. These values are in good agreement with those obtained for the F127 micelles in absence of the amines. The CO2 loading of the MEA, DEA, and PZ-F127 solutions led to additional, significant changes in the nanoscale particle structures and intermicelle interactions. The reaction of the amines with CO2 generates charged carbamate anions within the polymer micelles. Thus, the structure factor model for charged spheres, based on the Ornstein−Zernike equation, was used to analyze the data.46 The structure factor analysis provided a volume fraction of 33% and interaction radii of 107 Å for the MEA, 110 Å for the PZ, and 115 Å for the DEA containing micelles. These interaction radii are in reasonable agreement with the particle dimensions derived from the p(r) with Dmax/2 of the p(r) ≈ 110 Å, Figure 2.
absorbing aqueous CO2 or either of its stable, hydrated counterparts, CO32− or HCO3−. CO2 Responsive F127 Micelles with Selected Amines. The addition of 1 mol/L MEA, DEA or PZ to the F127 solution alters the morphology of the micelles (see Figure 2). The I(q) profiles between q ≈ 0.03−0.10 Å−1, describe the internal structure dimensions of the micelles and show a more defined minimum and maximum compared to the pure F127 micelles. This change in structure is caused by the incorporation of the amines. The PEO segments in the F127 micelles extend into the aqueous phase as Gaussian chains. Following addition of the amines, the packing of the PEO segments appears to become denser as shown by the deviation from the q−2 behavior after amine and CO2 addition in the Kratky plot, Supporting Information Figure SI2. From the structure factor calculation, the effective interparticle interD
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Macromolecules The calculated p(r) profiles give additional detail about the block copolymer micelle size and shape. Amine addition induces a shift in the maximum of the p(r) to larger dimensions. The overall micelle dimension (at p(r) = 0) remained relatively constant for all amines at r ≈ 220 Å, see Figure 2c. A large change in the p(r) profiles was found after CO2 addition. The resulting shape of these p(r) profiles, with evolution of a second maximum and minimum, indicates approximate spherical particles with different electron densities in the core and shell regions. Morphological Changes in the CO 2 Responsive Polymer Micelles. The deconvolution of p(r) with a convolution square root operation generates the averaged difference radial electron density distribution of the micelles relative to the mean. The PPO core of the micelles has a lower electron density than water while the PEO chains in the shell have a higher electron density than water. The point at which the electron density equals that of the solvent provides an estimated radius of the hydrophobic micelle core. When interpreting the excess electron density profiles, one has to consider that the bulk electron density might increase upon addition of the amines and CO2 due to the solvation of some of the species in the bulk. Figure 3 shows the averaged radial electron density profiles for all studied systems. The core radius of the F127 micelles increased from ∼22 to ∼33 Å after the addition of DEA and to ∼43 Å after CO2 loading, Figure 3a. A similar increase was also observed following CO2 addition to the MEA-F127 system (from 28 to 40 Å) and the PZ-F127 system (from 29 to 40 Å) presented in Figure 3b. Further model dependent analysis of the P(q) profiles obtained from GIFT was used to complement the information on the morphology of the micelles. According to the results presented above, the form factor for a spherical core−shell model with Gaussian chains attached to the core surface (eq 4) was selected to obtain the best possible fit to the P(q).25,48,49 This model provides characteristic parameters, such as core and shell radius and relative differences in excess electron density between the shell and core. The resulting fits presented in Supporting Information Figure SI4 show that all P(q) data in this study could be described relatively well with this model. The corresponding fitting parameters are summarized in Supporting Information Table SI2. The results from this model dependent analysis agree to the observations from the model independent GIFT analysis: The core-radius of the F127 micelles increases upon addition of MEA, DEA, and PZ. A further significant increase in the core radius of the amine containing micelles can be seen after the addition of CO2. The total radius of the polymer micelles from the model dependent fitting was calculated between 92 and 94 Å (see Supporting Information Table SI2). These values are comparable to the radius of ∼110 Å from the radial electron density profiles and the interaction radius of ∼100 Å from the interparticle interaction from the model independent GIFT analysis reported above. Deviations between the two modeling approaches can result from polydispersity in the micelle radius as well as gradual variations in the hydration of the PEO chains (and thus excess electron density) with distance from the core, which is not accounted for by the Pm(q) model. The relative contrast Δρshell/Δρcore is ∼−2 from the radial scattering length density profiles and the model depended fitting parameters for the F127 micelles. These values decrease to ∼−0.9 after DEA addition and −0.3 after CO2 sparging of
the DEA based micelles from both GIFT and model dependent analysis. The MEA and the PZ systems follow this trend (see Figure 3 and Table 2). The results suggest that these amines mainly contribute to the packing of the PPO blocks in the core and decrease the electron density there. Table 2. Ratios for Δρshell/Δρcore Calculated from the Radial Electron Density Profiles from the Model Independent GIFT Method Combined with Deconvolution (Figure 3) and the Corresponding Ratios from the Model Dependent Fitting of the P(q) from GIFT Using Eq 4 Δρshell/Δρcore F127 +MEA +DEA +PZ
Δρshell/Δρcore after CO2 addition
GIFT
Pm(q)
GIFT
Pm(q)
−2.0 −1.0 −0.9 −0.9
−1.8 −0.9 −0.8 −0.7
−0.5 −0.3 −0.4
−0.5 −0.3 −0.4
To obtain further information on the position of the amines within the structure of the micelles, the ratio of the difference in scattering length-density between shell and core ΔΔρ(F127)/ ΔΔρ(F127+amine) and ΔΔρ(no CO2)/ΔΔρ(CO2) with ΔΔρ = Δρshell − Δρcore, was calculated from the profiles in Figure 3, and the parameters from the Pm(q) model in Supporting Information Table SI2. The resulting ratios agree reasonably well between both analysis methods (see Table 3). For the F127/F127 + amine systems before CO2 addition, this ratio is ∼0.7. The ratio for the change in core volume, Vcore(F127)/ Vcore(F127 + amine), is ∼0.5. The difference between these ratios for the contrast and the core volume before and after amine addition shows that the composition of the micelles changes. The amines localize in the PPO core, according to their log P value. H-bond interactions between the amines and the ether groups in the PPO blocks modify the packing of the polymer chains, which results in the increase in the micelle core volume. CO2 addition to these samples had only minor influence on the contrast difference between shell and core, as presented in Table 3. However, a significant increase of the PPO corevolume of the micelles was observed with the ratio for Vcore(no CO2)/Vcore(CO2) around 0.3. The increase in core volume is caused by the increase in volume of the amines upon reaction with CO2 to carbamates, and the charge repulsion between the carbamate ions. This leads to the modification in the packing of the PPO chains in the micelle core and increase in its polarity. The difference between the ratios for the contrast and core volume before and after CO2 addition indicates a change in composition of the micelles. The transfer of water molecules and cations into the micelle core, caused by the presence of the charged carbamate ions, might compensate the decrease in the electron density of the micelle core caused by the increasing volume. Some carbamate ions might migrate into the bulk phase, increasing the solvent electron density. This can cause the observed decrease in Δρshell and the more negative values in Δρcore. The overall dimensions of the micelles remain constant, suggesting a closer packing of the PEO chains in the micelle shell, Figure 3 and Kratky plot in Supporting Information Figure SI2. The solubility of PEO in water can be affected by salt ions perturbing the hydrogen bonds between water and PEO.53 The PEO oxygen sites are slightly basic and will E
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Table 3. Relative Change in Excess Electron Density Difference between Shell and Core and Ratio for Core Volumes of the Polymer Micelles from Model Independent GIFT Analysis and Model Dependent Analysis of the P(q) from GIFT Using Eq 4 [ΔΔρ(F127)]/ [ΔΔρ(F127 + amine)] MEA DEA PZ
[Vcore(F127)]/ [Vcore(F127 + amine)]
[Vcore(no CO2)]/[Vcore(CO2)]
Pm(q)
GIFT
Pm(q)
GIFT
Pm(q)
GIFT
Pm(q)
0.68 0.65 0.63
0.77 0.85 0.68
0.54 0.37 0.48
0.86 0.35 0.65
0.95 0.91 0.97
0.86 0.73 0.94
0.36 0.30 0.33
0.20 0.39 0.24
the −OH groups at the PEO block termini. This result has important implications for the design of triblock copolymer based colloidal CO2 capture and CO2 responsive delivery systems. The gas induced swelling of the core and shrinking of the shell can modify mass transfer of included guest molecules such as drugs from the micelles. Information on the intermolecular interaction between amines and polymer can also be used to design CO2 capture membranes based on these amines in PEO and PPO polymer networks.
participate in cation charge solvation along with occluded water, for example, C2H4O···X+(H2O)n···OC2H4, X = H+, Na+. The amines are cation-scavenging, and the anionic carbamates even more so. Thus, the amines and carbamates may also compete for the PEO solvated cations, which can result in the modified packing of the polymer chains as observed with SAXS. Some of the carbamate ions may migrate to the surface of the micelle and solvate the −OH termini of the PEO strands, as a result of their polarity. From steric considerations, one carbamate ion could solvate four PEO termini in two dimensions, which can also result in a more compact packing of the PEO strands. The excess electron density of the dehydrated PEO shell is expected to increase upon dehydration. However, as mentioned above, the migration of some of the carbamates into the bulk and the resultant increase in bulk electron density can compensate the excess electron density increase of the PEO shell. The Rg values of all systems, calculated from the p(r) using eq 3, confirm this observation. The increase in Rg values after CO2 addition is significant for every amine-F127 solution, but is most pronounced for the DEA-containing micelles, Table 4. This change indicates an increase in polymer density in the shell with respect to the swelling core when the overall size of the micelles is constant.
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Rg before CO2 addition (Å)
Rg after CO2 addition (Å)
78 80 80 81
79 89 90 95
ASSOCIATED CONTENT
* Supporting Information S
Details on the interaction parameters for the structure factor model, the Kratky plot of the SAXS data, the fits obtained from the deconvolution of the p(r) and the detailed parameters for the model dependent analysis of the form factor. 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]. Author Contributions
All authors contributed to the preparation of the manuscript. S.S. and P.J. planned the experiments and interpreted the results. S.S. was responsible for the modeling and interpretation of the scattering data. A.H. was local contact at the SAXS/ WAXS beamline at the Australian Synchrotron.
Table 4. Rg Values for the F127-Based Micelles Calculated from the p(r) Profiles Using Eq 3 F127 F127 MEA F127 PZ F127 DEA
[ΔΔρ(no CO2)]/[ΔΔρ(CO2)]
GIFT
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr Graham Webster for the careful reading of the manuscript. The scattering studies in this paper were conducted at on the SAXS/WAXS beamline at the Australian Synchrotron. Part of this work was undertaken during S.S.’s and P.J.’s tenure at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Energy Technology Centre, Newcastle, Australia.
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CONCLUSION The calculation of form and structure factor from SAXS data with the model independent GIFT method, combined with model dependent form factor analysis, gives us insight into triblock copolymer micelle behavior in the presence of CO2 capture amines. The addition of short-chain amines MEA, DEA, and PZ to F127 micelles induced a swelling of the PPO core and shrinking of PEO shell. Excess electron density profiles calculated from SAXS data show that these amines preferably locate in the PPO core where they may anchor via hydrogen bonding with the ether groups in the polymer. Reactions following CO2 addition lead to carbamate anions and protons. The charge repulsion between carbamate ions, anchored at the PPO core, results in further increase of the PPO volume. The total size of the micelles remains constant, as a result of modified packing of the PEO shell caused by dehydration and possible interaction of the carbamate ions with
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REFERENCES
(1) Negrini, R.; Mezzenga, R. Langmuir 2011, 27, 5296−5303. (2) Salentinig, S.; Phan, S.; Darwish, T. A.; Kirby, N.; Boyd, B. J.; Gilbert, E. P. Langmuir 2014, 30, 7296−7303. (3) Salentinig, S.; Tangso, K. J.; Hawley, A.; Boyd, B. J. Langmuir 2014, 30, 14776−14781. (4) Tangso, K. J.; Fong, W.-K.; Darwish, T.; Kirby, N.; Boyd, B. J.; Hanley, T. L. J. Phys. Chem. B 2013, 117, 10203−10210. (5) Zhao, Y. Chem. Rec. 2007, 7, 286−294. (6) Munshi, N.; Rapoport, N.; Pitt, W. G. Cancer Lett. 1997, 118, 13−19. (7) Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2005, 21, 569−577. (8) de Campo, L.; Yaghmur, A.; Sagalowicz, L.; Leser, M. E.; Watzke, H.; Glatter, O. Langmuir 2004, 20, 5254−5261.
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DOI: 10.1021/ma502584p Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules (9) Jessop, P. G.; Mercer, S. M.; Heldebrant, D. J. Energy Environ. Sci. 2012, 5, 7240−7253. (10) Liu, Y.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Science 2006, 313, 958−960. (11) Jessop, P. G.; Heldebrant, D. J.; Li, X.; Eckert, C. A.; Liotta, C. L. Nature 2005, 436, 1102−1102. (12) Han, D.; Tong, X.; Boissière, O.; Zhao, Y. ACS Macro Lett. 2011, 1, 57−61. (13) Liu, H.; Zhao, Y.; Dreiss, C. A.; Feng, Y. Soft Matter 2014, 10, 6387−6391. (14) Feng, A.; Zhan, C.; Yan, Q.; Liu, B.; Yuan. Chem. Commun. 2014, 50, 8958−8961. (15) Yan, Q.; Zhou, R.; Fu, C.; Zhang, H.; Yin, Y.; Yuan, J. Angew. Chem., Int. Ed. 2011, 50, 4923−4927. (16) Zhang, Y.; Feng, Y.; Wang, J.; He, S.; Guo, Z.; Chu, Z.; Dreiss, C. A. Chem. Commun. 2013, 49, 4902−4904. (17) Su, X.; Cunningham, M. F.; Jessop, P. G. Chem. Commun. 2013, 49, 2655−2657. (18) Lewis, T.; Faubel, M.; Winter, B.; Hemminger, J. C. Angew. Chem., Int. Ed. 2011, 50, 10178−10181. (19) Jackson, P.; Beste, A.; Attalla, M. Struct. Chem. 2011, 22, 537− 549. (20) Jackson, P.; Fisher, K. J.; Attalla, M. I. J. Am. Soc. Mass Spectrom. 2011, 22, 1420−1431. (21) Jackson, P.; Beste, A.; Attalla, M. I. Phys. Chem. Chem. Phys. 2012, 14, 16301−16311. (22) Okamoto, K.-I.; Fuji, M.; Okamyo, S.; Suzuki, H.; Tanaka, K.; Kita, H. Macromolecules 1995, 28, 6950−6956. (23) Yoshino, M.; Ito, K.; Kita, H.; Okamoto, K.-I. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1707−1715. (24) Lin, H.; Freeman, B. D. J. Membr. Sci. 2004, 239, 105−117. (25) Jensen, G. V.; Shi, Q.; Hernansanz, M. J.; Oliveira, C. L. P.; Deen, G. R.; Almdal, K.; Pedersen, J. S. J. Appl. Crystallogr. 2011, 44, 473−482. (26) Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805− 812. (27) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145−4159. (28) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414−2425. (29) Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5440− 5445. (30) Desai, P. R.; Jain, N. J.; Sharma, R. K.; Bahadur, P. Colloids Surf., A 2001, 178, 57−69. (31) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: Sydney, 1985. (32) Sarkar, B.; Ravi, V.; Alexandridis, P. J. Colloid Interface Sci. 2013, 390, 137−146. (33) Holmqvist, P.; Alexandridis, P.; Lindman, B. Langmuir 1997, 13, 2471−2479. (34) Alexandridis, P.; Yang, L. Macromolecules 2000, 33, 5574−5587. (35) Jain, N. J.; Aswal, V. K.; Goyal, P. S.; Bahadur, P. J. Phys. Chem. B 1998, 102, 8452−8458. (36) Ivanova, R.; Lindman, B.; Alexandridis, P. Langmuir 2000, 16, 9058−9069. (37) Manet, S.; Lecchi, A.; Impéror-Clerc, M.; Zholobenko, V.; Durand, D.; Oliveira, C. L. P.; Pedersen, J. S.; Grillo, I.; Meneau, F.; Rochas, C. J. Phys. Chem. B 2011, 115, 11318−11329. (38) Hansch, C.; Leo, A.; Hoeckman, D. Exploring QSAR: Hydrophobic, Electronic, and Steric Constants, 1st ed.; American Chemical Society: Washington, DC, 1995; Vol. 2, p 368. (39) Kirby, N. M.; Mudie, S. T.; Hawley, A. M.; Cookson, D. J.; Mertens, H. D. T.; Cowieson, N.; Samardzic-Boban, V. J. Appl. Crystallogr. 2013, 46, 1670−1680. (40) Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1997, 30, 431−442. (41) Weyerich, B.; Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1999, 32, 197−209.
(42) Fritz, G.; Glatter, O. J. Phys.: Condens. Matter 2006, 18, 2403− 2419. (43) Glatter, O. J. Appl. Crystallogr. 1977, 10, 415−421. (44) Glatter, O. J. Appl. Crystallogr. 1979, 12, 166−175. (45) Glatter, O. J. Appl. Crystallogr. 1981, 14, 101−108. (46) Fritz, G.; Bergmann, A.; Glatter, O. J. Chem. Phys. 2000, 113, 9733−9740. (47) Percus, J. K.; Yevick, G. J. Phys. Rev. 1958, 110, 1−13. (48) Pedersen, J. S. Adv. Colloid Interface Sci. 1997, 70, 171−210. (49) Pedersen, J. S.; Svaneborg, C. Curr. Opin. Colloid Interface Sci. 2002, 7, 158−166. (50) Debye, P. J. Phys. Chem. 1947, 51, 18−32. (51) Attwood, D.; Collett, J. H.; Tait, C. J. Int. J. Pharm. 1985, 26, 25−33. (52) Yu, G.-E.; Deng, Y.; Dalton, S.; Wang, Q.-G.; Attwood, D.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 2537− 2544. (53) Behrens, M. A.; Bergenholtz, J.; Pedersen, J. S. Langmuir 2014, 30, 6021−6029.
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DOI: 10.1021/ma502584p Macromolecules XXXX, XXX, XXX−XXX
