Phase behavior

Block copolymers (BCPs) are composed of two or more incompatible homopolymer blocks that are covalently linked. These materials are of interest for applications in nanotechnology due to their ability to self-assemble into well-ordered nanostructures via microphase separation. The strucrural deatils are dependent on several important parameters including degree of polymerization (N), block ratio (f), and Flory-Huggins parameter (χ). Precise synthetic control over the molecular design and composition has produced a library of achievable morphologies, including periodic lamellae, cylinders, spheres, and bicontinuous structures. BCP self-assembly provides a facile route to templating nanostructured materials for many applications such as nanolithography, photonics, separations, and metamaterials.

Advances in synthesis have led to BCPs with alternative architectures, including multiblock, star, and comb block copolymers. Of recent interest are brush block copolymers (BBCPs), which are a class of very large macromolecules that have at least two different types of polymer side chains densely grafted onto distinct segments on a common polymer backbone. Coupling short side chains with high grafting density leads to strong steric repulsion of the polymer side chains, extension of the bottlebrush backbone and a lack of entanglements. These factors contribute to rapid self-assembly of well-ordered nanostructures. We are interested in understanding the relationship between molecular design of BBCP architecture (such as side chain length & chemistry, backbone conformation, and block composition) on the resulting morphologies and overall phase behavior. Recently, we found that long range ordering of well-ordered lamellar morphologies can be generated in a few minutes through the self-assembly of rationally designed brush BCPs with high molecular weights up to 1000 kg/mol. In addition, large domain spacing over 100 nm and grain sized on order of 1 mm³ was accessible. We use these morphologies to create functional materials (see applications).

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Rheology

The rapid self-assembly of BBCPs into well-ordered morphologies is a consequence of suppressed side chain entanglements and extended backbone conformations. BBCPs possess a naturally unentangled architecture in addition to microphase segregated structures analogous to conventional linear BCPs (LBCP). We investigate the linear viscoelastic behavior of well-ordered BBCPs with short side chain lengths through oscillatory shear rheology with the goal of establishing a link between structure and dynamic behavior. We measure dynamic moduli G’ and G” over a wide range of frequencies. The scaling relationships in dynamic data show distinct power law behavior analogous to “liquid-like” or “gel-like” materials. 

Precise control over the alignment and orientation of an ordered nanostructure is highly desirable for applications ranging from sensors to energy and optical materials. Over the past 30 years, shear has been used to orient and align block copolymers morphologies. The highly mobile nature of BBCPs in the melt suggests interesting response to controlled shear processing. We explore the role that applied shear plays on the directed alignment of phase segregated domains in bulk BBCP samples with lamellar morphology. The periodic structures were found to align at exceptionally low strain amplitudes and mild processing temperatures as confirmed by small-angle X-ray scattering (SAXS). Alignment over several mm3 is realized, which is critical for many practical applications in optics, metamaterials and other fields. 

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Related publications:

• Song, D.P., Li, C., Colella, N.S., Xie, W., Li, S., Lu, X., Gido, S., Lee, J.H. and Watkins, J.J., 2015. Large-volume self-organization of polymer/nanoparticle hybrids with millimeter-scale grain sizes using brush block copolymers. Journal of the American Chemical Society, 137(39), pp.12510-12513.
• Gai, Y., Song, D.P., Yavitt, B.M. and Watkins, J.J., 2017. Polystyrene-block-poly (ethylene oxide) bottlebrush block copolymer morphology transitions: Influence of side chain length and volume fraction. Macromolecules, 50(4), pp.1503-1511.
• Yavitt, B.M., Gai, Y., Song, D.P., Winter, H.H. and Watkins, J.J., 2016. High Molecular Mobility and Viscoelasticity of Microphase-Separated Bottlebrush Diblock Copolymer Melts. Macromolecules, 50(1), pp.396-405