Abstract
We develop an experimental approach to analyze the water distribution around a core-shell micelle formed by polystyrene-block-poly[styrene-g- poly(ethylene oxide (PEO)] block copolymers in aqueous media at a fixed polymeric concentration of 10 mg/ml through contrast variation small angle neutron scattering (SANS) study. Through varying the D2 O/H 2 O ratio, the scattering contributions from the water molecules and the micellar constituent components can be determined. Based on the commonly used core-shell model, a theoretical coherent scattering cross section incorporating the effect of water penetration is developed and used to analyze the SANS I (Q). We have successfully quantified the intramicellar water distribution and found that the overall micellar hydration level increases with the increase in the molecular weight of hydrophilic PEO side chains. Our work presents a practical experimental means for evaluating the intramacromolecular solvent distributions of general soft matter systems.
| Original language | English |
|---|---|
| Article number | 144912 |
| Journal | Journal of Chemical Physics |
| Volume | 133 |
| Issue number | 14 |
| DOIs | |
| State | Published - Oct 14 2010 |
Funding
The support of HFIR ORNL (Grant No. IPTS-2301) and NCNR NIST is greatly acknowledged. The research carried out at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. X.L. and E.L. acknowledge the financial support by U.S. Department of Energy under NERI-C Award No. DE-FG07-07ID14889 and U.S. Nuclear Regulatory Commission under Award No. NRC-38-08-950. C.Y.S. thanks the support from the CUNY PSC grants. We also thank the support from the Laboratory Directed Research and Development Program (Project ID No. 05272) of ORNL. Table I. Characteristics of PS-b-P(S-g-PEO) block graft copolymers. Sample Mw(kg/mol) PDI Repeating unit (PS) Number of PEO chains Number of repeating unit per PEO chain V PS a ( Å 3 ) V PEO b ( Å 3 ) SGEO3 371 1.08 166 46 174 25 431 459 555 SGEO4 234 1.01 166 46 107 25 431 302 560 a Reference 21 . b Reference 22 . Table II. The evolution of the statistical uncertainties of intramolecular structure parameters for SGEO3 and SGEO4 micelles as a function of scattering contrast. Sample D 2 O molar ratio γ (%) Core radius R 2 (Å) Micellar radius R 1 (Å) Contrast ratio ρ 2 ρ 1 ≡ ρ core − ρ solvent ρ shell − ρ solvent Standard deviation σ 2 (Å) Standard deviation σ 1 (Å) SGEO3 100 111.5 ± 0.3 183.9 ± 8.6 3.08 ± 0.49 16.5 ± 2.9 91.9 ± 5.2 90 112.3 ± 0.5 184.6 ± 12.8 3.14 ± 0.70 16.6 ± 3.8 96.5 ± 7.6 80 109.4 ± 0.9 184.7 ± 15.6 3.15 ± 0.90 16.2 ± 5.6 91.3 ± 9.4 70 113.3 ± 4.3 186.5 ± 276.5 4.84 ± 21.57 56.3 ± 23.0 139.1 ± 124.4 50 105.8 ± 5.7 187.8 ± 63.0 3.44 ± 3.81 21.3 ± 19.5 95.5 ± 36.8 SGEO4 100 88.8 ± 1.0 229.4 ± 4.9 12.73 ± 1.65 46.3 ± 2.2 90.9 ± 3.9 90 88.0 ± 2.3 235.4 ± 8.6 14.52 ± 3.67 49.4 ± 4.3 91.0 ± 7.4 80 91.1 ± 1.9 235.3 ± 8.9 13.58 ± 3.24 45.6 ± 4.4 86.8 ± 7.4 70 93.7 ± 3.5 233.3 ± 15.8 11.29 ± 4.67 44.2 ± 7.7 91.3 ± 12.8 50 120.0 ± 2448.3 212.0 ± 2248.9 4.14 ± 593.65 72.9 ± 4486.0 54.5 ± 991.7 Table III. Structural characteristics of SGEO3 and SGEO4 micelles obtained from SANS model fittings. Sample Core radius R 2 (Å) Micellar radius R 1 (Å) Standard deviation σ 2 (Å) Standard deviation σ 1 (Å) Radius of gyration R G (Å) SGEO3 111.5 ± 0.3 183.9 ± 8.6 16.5 ± 2.9 91.9 ± 5.2 158.1 ± 5.2 SGEO4 88.8 ± 1.0 229.4 ± 4.9 46.3 ± 2.2 90.9 ± 3.9 171.4 ± 3.0 FIG. 1. The molecular structure of the PS-b-P(S-g-PEO) block graft copolymers studied in this work. The polystyrene chains build up the backbone, and the PEO chains are attached to a part of the backbone as side brushes. The length of each segment corresponds to the number of its repeat units schematically. FIG. 2. The SANS intensity distribution I ( Q ) and the associated model fitting curves of (a) SGEO3 and (b) SGEO4 micellar solutions. Different colors are used to specify the experimental data corresponding to the solutions with different γ values. For the sake of clarity, in both cases the absolute value of I ( Q ) measured in pure D 2 O solution (red) are kept intact, while the others are scaled respectively by a factor of 0.1 (for 90% D 2 O , green), 0.01 (80% D 2 O , magenta), 0.001 (70% D 2 O , orange), and 0.0001 (50% D 2 O , blue). The good quantitative agreement between the experiment and model is clearly seen. It is important to note that the experimental uncertainties are on the order or smaller than the symbol size. FIG. 3. Square root of the ratio of SANS coherent scattering amplitude A to the micellar number density n s as a function of D 2 O molar fraction γ . The volume of the core and shell in the micelle can be unambiguously determined by the contrast matching point. FIG. 4. The neutron scattering length density profile of SGEO3 (solid lines) and SGEO4 (dashed lines) micelles extracted from the SANS model fitting. Colored lines demonstrate the density profile of the hydrated micelle ρ micelle ( r ) measured in the solution of different γ values, with the same colors used as in Fig. 1 for the same D 2 O molar ratio. The black line is the scattering length density contribution from the constituent polymer components ρ polymer ( r ) . FIG. 5. The radial distribution of the number density of intramicellar water as a function of the radial distance r . The stars present the values of hydration if R G is used to represent the micellar radius. The associated distribution of the total number of water inside the sphere with a radius r is given in the inset.
| Funders | Funder number |
|---|---|
| CUNY PSC | |
| Oak Ridge National Laboratory | |
| Scientific User Facilities Division | |
| U.S. Department of Energy | |
| Basic Energy Sciences | |
| Oak Ridge National Laboratory | |
| Laboratory Directed Research and Development | 05272 |
| E.L. Wiegand Foundation |