Anomalous Small-Angle X-ray Scattering Study of Structure of Polymer Micelles Having Bromines in Hydrophobic Core

Abstract

Anomalous small-angle X-ray scattering (ASAXS) near the absorption edge of bromine has been used to investigate the internal structure of polymer micelles comprising poly(4-bromostyrene)-block-poly(ethylene glycol)block-poly(4-bromostyrene) (PBrS-b-PEG-b-PBrS) in aqueous solution. The ASAXS analysis revealed that the hydrophobic core composed of PBrS can be regarded as a homogeneous sphere with a radius of 10.2 nm and a decaying electron density at the surface. The aggregation number of micelle directly estimated on the basis of ASAXS analysis well agreed with that determined by field-flow fractionation coupled with multiangle light scattering, confirming the reliability of ASAXS analysis. The small-angle X-ray scattering (SAXS) profile from whole PBrS-b-PEG-b-PBrS micelle was further analyzed by using the result of ASAXS analysis. The micelles could be well described by a core−corona model in which the monomer concentration of PEG in the shell decreased as R −4.2/3 , where R is the distance from the center of micelle. The index 4.2/3 is very close to that predicted by theory for star polymers (R −4/3 ), indicating that the PEG chains in the shell can be well described by the blob model in which chains are modeled as strings of blobs that extend radially. Thus, combination of ASAXS and conventional SAXS analyses permit to reveal precisely the internal structure and nature of the tethered chains of polymer micelles, and it could become a powerful tool for exploring internal structures of multiphase systems when they contain a probe atom for ASAXS.

Spatial distribution of hydrophobic compounds in polymer micelles as explored by anomalous small-angle X-ray scattering near Br K-edge

“…ASAXS analyses have been applied for visualization of spatial distribution of a targeted element in nano-sized particles, such as metal nanoparticles, polymer micelles, and so forth [10][11][12][13][14][15][16][17][18][19][20][21][22]. Therefore, by using ASAXS for polymer micelles incorporating model drug compounds labeled with a marker element, we expect to visualize the 4 spatial distributions of drug compounds in polymer micelles.…”

“…As written below, I(q) must be compared for the same value of q in ASAXS analyses, although the change in energy of incident X-ray led to a shift in the q-value assigned to a given pixel of the detector. Then, the SAXS profiles were interpolated by means of a cubic-spline method without any smoothing or corrections to correct for the q-shift [19][20][21].…”

Recent developments in polymer applications of synchrotron small‐angle X‐ray scattering

“…As an interesting example of physical accumulation, condensation of counterions onto micellar surfaces has been studied . As for chemical accumulation, so‐called tagged polymers were used where, for example, bromine atoms were chemically bonded to the targeted parts in a polymer chain, being applied to structural analyses of polymeric micelles having a core–shell interface with hydrophobic core …”

Study of the internal structure of polymer micelles by anomalous small-angle X-ray scattering at two edges

“…where F 0 2 (q) is the non-resonant form factor, which can be measured by conventional SAXS, V 2 (q) corresponds to the form factor of the resonantly scattering parts, and the second term is the cross term of the resonant and non-resonant terms. The V 2 (q) factor can be obtained by numerically solving the simultaneous equations of Pðq; EÞ measured for at least three different energies near the K edge of a focused element (Akiba et al, 2012;Goergk et al, 2004;Patel et al, 2004):…”

“…For polymer systems, ASAXS using the K edge of Br is expected to be useful because Br atoms as ASAXS markers can be easily introduced into hydrophobic organic molecules through C-Br covalent bonds and the K edge of Br is within the range of energy of general synchrotron light sources (Akiba et al, 2012;Sakou et al, 2011;Varga et al, 2006Varga et al, , 2010. In a previous study, we used ASAXS analysis near the Br K edge to investigate polymer micelles (Akiba et al, 2012;Sakou et al, 2011). We found that the scattering profile of the hydrophobic core could be extracted from that of whole micelles by localizing Br in the core.…”