Phase transformations of spherical block copolymer micelles

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Chen, Liwen
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Electronic thesis
Chemical engineering
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Polymorphism is ubiquitous in nearly all crystalline materials, and control of polymorphism is important to obtain material properties for target applications. Therefore, understanding how polymorphs of materials change and how to access target polymorph by controlling thermodynamic and kinetic factors is the fundamental task in materials research. Block copolymer surfactants consist of covalently connected chemically distinct polymer blocks and aggregate into micellar structures in selective solvents. Advances in the polymer chemistry enables fine-tuning of the size and chemical properties of block copolymer surfactants, which leads to fabrication of various block copolymer micelles with properties and shapes based on solid design principles. The rich structural and property variabilities of block copolymer micelles make this material class as one of the most important model systems for exploring and understanding self-assembly of nanoscale particles. Packing structures of spherical micelles prepared with poly(1,2-butadiene-b-ethylene oxide) (PB-PEO) diblock copolymers in aqueous solutions were investigated using small angle X-ray scattering (SAXS) technique. Depending on the processing conditions, the PB-PEO spherical micelles were observed self-assembling into different close-packed structures, i.e., polytypes made by stacking two-dimensional hexagonal close-packed (2D-HCP) layers of block copolymer micelles in different stacking orders. In a 12.7 wt % solution of the PB-PEO diblock copolymer (Mn = 6.8 kg/mol and the weight fraction of the PEO block wPEO = 0.71), direct dissolution of the PB-PEO diblock copolymer produced face-centered cubic (FCC) crystals of the PB-PEO micelles. The micellar FCC structures become disordered by heating to 90 °C, and rapid temperature quenching of the disordered micelle solution to three different temperature, 40 °C, 25 °C, and 0 °C, produced FCC, randomly stacked hexagonal close packing (RHCP), and hexagonal close-packing (HCP) structures, respectively. The micellar HCP and RHCP structures are stable for at least a few weeks when maintained at the quenched temperature, but heating or cooling transformed these HCP and RHCP to FCC by grain-coarsening. Careful examination of the 2D SAXS patterns reveals that the formation of HCP and RHCP structures is related to the size of crystallites. This suggests the Laplace pressure from the finite size of crystal domains is likely the origin of the formation of the non-cubic close-packed structures of block copolymer micelles. As the crystal grains grow, the Laplace pressure becomes weak, and the micelles on close-packed lattices transform into the most stable close-packed structures: FCC. In a 23 wt % PB-PEO solution, careful thermal treatments lead to discover martensitic transformations of shear-aligned FCC crystallites of block copolymer micelles to HCP structures. It occurs by selectively sliding one specific set of 2D-HCP layers in the shear-aligned FCC crystallites among other equally possible layers. Consideration of the morphology of the shear-aligned FCC crystallites suggests that the selective martensitic shear transformation originates from different areas of available 2D-HCP layers for the martensitic shear transformation: the transformation chooses the 2D-HCP layers with the lowest sliding area, i.e., lowest frictional kinetic energy barrier. Both thermally induced diffusive and diffusionless transformations of model block copolymer micelles suggest that the size of crystal domains is a critical factor in the polymorphism of crystalline materials. In the diffusive nucleation and growth process, the size of crystal domains determines the type of crystal structures. In the diffusionless transformation, the size of crystal domains regulates the initiation and kinetics of diffusionless transformations. This finding stimulates further investigations of the effects of polymer concentration and cooling rates to the crystal structures of block copolymer micelles. Non-close packed structures of PB-PEO micelles are observed from the solutions with the tetrahydrofuran co-solvent, which is a non-selective solvent for both PB and PEO blocks. This thesis work reveals the importance of the size of crystal domains to the crystal structures and transformation kinetics and provides new understanding.
2019 May
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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