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Resonant Soft X-Ray Scattering of Tri-Block Copolymers Print
Wednesday, 30 May 2012 00:00

In principle, tri-block copolymers (tri-BCPs), consisting of three chemically distinct polymers covalently joined together at the ends of each polymer chain, can serve as scaffolds and templates for fabricating a vast number of nanostructures. While quantitatively understanding the details of the morphology and the manner in which the different blocks interact with surfaces and interfaces is critical to success, previous experiments have been few. Now, an international team from the United States, Korea, and Japan has succeeded in combining resonant soft x-ray scattering (RSoXS) at ALS Beamline 11.0.1 with transmission electron microscopy tomography (TEMT) and other techniques to unambiguously determine morphologies comprising two nested hexagonally packed arrays of nanoscopic, cylindrical microdomains in the bulk and a core-shell nanostructure in a thin film. Not only has this work revealed a new phase of ABC tri-block copolymer with complicated morphology, it has illustrated the importance of RSoXS as a unique, powerful tool for examining complex, multi-component systems that could not be characterized with conventional methods.

An X-Ray Probe
for Soft Materials

At the frontier of materials science and chemistry, so-called "soft" materials provide ideal platforms for constructing complex nanostructures possessing an enormous range of structural, chemical, electronic, magnetic, and other properties. Consider the case of block copolymers in which two or more chains of chemically different polymers (the blocks) are linked together. Through chemical trickery, the blocks can assemble themselves (self-assemble) into complex structures assuming an amazing array of morphologies and properties that make them candidates for applications in biomaterials, fuel cells and batteries, magnetic storage, and more. So far, so good, but the researcher needs to know what the structures are and how they relate to the properties in order to control them.

Conventional soft x-ray scattering (here soft refers to the relatively long x-ray wavelength) can provide some structural information, but it is not very detailed. To do better, Wang et al. have exploited the development of "resonant" soft x-ray scattering (RSoXS) at the ALS, a technique that overcomes the low electron-density contrast in soft materials by adding the elemental sensitivity of spectroscopy (through selecting the x-ray wavelength) to x-ray scattering. The result is the ability to image structures ranging in size from a few nanometers to a few microns. Not only has their work shown that RSoXS is ideal for studying thin films of soft materials, but it has revealed a new tri-block copolymer with complicated morphology.

By selectively removing one of the components or by phase-selective chemistries, polymer scientists today can use di-block copolymers (di-BCPs) in either bulk or thin-film forms as templates or scaffolds for synthesizing nanostructured materials that are finding applications in magnetic storage, dielectric insulation, thermoelectrics and separations media, to name a few. Yet, by simply attaching one more polymer that is chemically distinct from the other blocks to the end of the di-BCP and by incorporating specific functionality into the different blocks, researchers can enormously expand the range of well-defined designer-specified morphologies. Tri-BCPs and multi-block BCPs are being considered for use in applications ranging from fuel-cell membranes to hybrid biomolecular materials to structured electrolytes for lithum-ion batteries and supercapacitors.

Drawings of 12 possible morphologies

Varying morphologies of linear tri-block copolymers are the basis for designer soft materials. The 12 possible combinations shown here of block sequence (ABC, ACB, BAC), composition, and block molecular weights represent only a few of the possibilities, but they already illustrate the enormous parameter space for the creation of new morphologies. Illustration from F.S. Bates and G.H. Fredrickson, "Block copolymers—designer soft materials," Physics Today 52, 32 (1999).

 

While there have been extensive studies of di-BCPs, there have been far fewer of tri-BCPs, either in the bulk or in thin films, owing to the more demanding synthesis involved, the complexities of the morphologies, and the inability of available techniques to quantitatively distinguish the different blocks. The recent development of RSoXS, which combines the elemental and bonding sensitivity of soft x-ray spectroscopy with conventional x-ray scattering methods, provides a novel tool for unambiguously deciphering the complex morphologies of tri-BCPs and even more complex multi-block copolymers.

Tuning the x-ray photon energies to match the absorption spectrum of the different components in the tri-BCP isolates the scattering contributions from the different polymer blocks without any additional chemical labeling, enabling a glimpse into these complex morphologies with unprecedented detail. Moreover, by taking advantage of the inherent polarization of the synchrotron beam, the orientation of each block relative to the film interfaces and to each other can be determined independently, providing yet another key for unlocking structure–property relationships.

Molecular formula and data charts

Left: Molecular structure of the tri-BCP, poly(1,4-isoprene)-block-polystyrene-block-poly(2-vinylpyridine) copolymer. Right: Real (δ) and imaginary (β) part of the complex index of refraction (n) of poly(1,4-isoprene) (PI, blue solid line), polystyrene (PS, red dashed line), and poly(2-vinylpyridine) (P2VP, green dotted line), respectively.

In testing RSoXS on the tri-block copolymer IS2VP, which consists of poly(1,4-isoprene) (PI), polystyrene (PS), and poly(2-vinlypyridine) (P2VP) in the bulk and in thin films, the team found a novel bulk structure consisting of two nested, hexagonal arrays of P2VP and PI microdomains. The P2VP cylindrical microdomains were hexagonally packed on the larger of the two lattices and surrounded by six small cylindrical PI domains located at the interstitial sites of the P2VP lattice.

Illustrations of morphologies at 3 different energies

RSoXS patterns and schematic illustration of hexagonally packed cylindrical morphology of the IS2VP tri-block copolymer, as seen by x-rays at three x-ray energies (250, 280, and 284 eV, respectively) selected to isolate the scattering contributions from the three polymers. The image at 280 eV shows contributions from all three polymers, whereas those at 250 eV and 284 eV show only two contributions, owing to reduced contrast between two of the components at those energies.

The researchers complemented their RSoXS measurements with scanning force microscopy, transmission electron microscopy, small-angle x-ray scattering, and grazing-incidence small-angle x-ray scattering. For example, transmission electron microscopy tomography (TEMT) provided an independent assessement of the local structure of the IS2VP triblock copolymer in the form of a three-dimensional image of the morphology. Reconstructing the images at different tilt angles of the sample with respect to the incident electron beam yielded the 3D structure.

Tomography images

Three-dimensional TEMT images of the tri-BCP IS2VP. (a, b): PI cylinders stained with osmium tetraoxide vapor. (c, d): P2VP cylinders stained with 1,4-diiodobutane. Panels b and d were obtained by tilting a and c, respectively. The volumes of the reconstructed images of PI and P2VP microdomains are 67 x 67 x 30 nm3 and 53 x 53 x 21 nm3, respectively.

 


 

Research conducted by: C. Wang and A. Hexemer (ALS), D.H. Lee (Dankook University, Korea), M.I. Kim (University of California, Berkeley), W. Zhao and T.P. Russell (University of Massachusetts, Amherst), H. Hasegawa (Kyoto University, Japan), and H. Ade (North Carolina State University).

Research funding: Laboratory Directed Research and Development grant, Berkeley Lab; U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); and U.S. National Science Foundation. Operation of the ALS is supported by DOE BES.

Publication about this research: C. Wang, D.H. Lee, A. Hexemer, M.I. Kim, W. Zhao, H. Hasegawa, H. Ade, and T.P. Russell, "Defining the nanostructured morphology of triblock copolymers using resonant soft x-ray scattering," Nano Lett. 11, 39061 (2011).

ALS Science Highlight #248

 

ALSNews Vol. 331