(670d) Molecular Dynamics Study of Hydrophilic-Hydrophobic Diblock Copolymer Self-Assembly: Phase Diagram, Vesicle Morphogenesis, and Shear Flow Dynamics
AIChE Annual Meeting
2022
2022 Annual Meeting
Materials Engineering and Sciences Division
Polymer Thermodynamics and Self-Assembly: Polymer-Molecular Interactions
Thursday, November 17, 2022 - 4:30pm to 4:45pm
We have extended CGMD simulations models, previously developed to study self-assembly in ionic and non-ionic surfactant solutions [Sangwai and Sureshkumar, Langmuir 27, 6628 (2011); Sambasivam et al., Phys. Rev. Lett. 114, 158302; Dhakal and Sureshkumar, J. Chem. Phys. 143, 024905 (2015)] to diblock copolymer solutions. Depending on the copolymer concentration, relative composition (PB to PEO ratio), and temperature, spherical and cylindrical micelles, linear and branched wormlike micelles, rigid and flexible bilayers, vesicles, as well as topologically complex and heterogenous morphologies are observed. Structures are characterized by calculating parameters such as the aggregation number, surface to volume ratio, packing parameter, end-to-end distances, persistence length, bending modulus, and energy associated with bonded and non-bonded interactions. Various morphologies are organized into a phase diagram. Structure transitions will be explained based on considerations of geometry and energetics considerations. The influence of local (interfacial) distribution of water and copolymers on phase transitions will also be discussed.
Simulations are used to explore the pathways by which vesicles form from an initially homogeneous copolymer solution. In one such pathway, spherical micelles are observed to merge and reorganize into rigid bilayer structures. These bilayer structures grow to form flexible lamellae. Further growth, curvature development and folding of a lamella cause the formation of a vesicle. The morphogenesis of lamella to vesicle transition is understood by tracking the aggregation number, end-to-end distance, and energy of interactions as a function of time. These results will be discussed in the context of classical molecular thermodynamics theories.
We also compute the details of molecular organization (e.g. the splay/tilt angle between adjacent molecules) and mechanical properties (e.g. Poisson ratio) of the bilayer and how they are influenced by the insertion of additives. Further, the mechanical deformation of bilayers and vesicles under homogeneous shear flow are studied by non-equilibrium MD simulations. Results of these simulations will be discussed in the context of the experiments mentioned in the first paragraph above.