THERMODYNAMICS AND SHEAR FLOW DYNAMICS OF BLOCK COPOLYMER MORPHOLOGIES IN SOLUTION

Author

Senyuan Liu, Syracuse University

Date of Award

12-20-2024

Date Published

January 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Radhakrishna Sureshkumar

Second Advisor

Shalabh Maroo

Keywords

copolymer;molecular dynamics;nanovesicle;self assembly;shear-induced structure;Vesicle Morphogenesis

Subject Categories

Chemical Engineering | Engineering

Abstract

Block copolymers (BCPs) have been a focal point of research for several decades. They have been engineered to produce various morphologies by altering the combinations of chemically distinct polymer segments that interact selectively with their environment, such as in aqueous or organic solvents, or within a polymer matrix. The ability to tailor morphological features by manipulating polymer chain length, composition, and monomer chemistry has led to applications in advanced material manufacturing, catalysis, emulsification, environmental remediation, targeted drug delivery, gene therapy, and medical diagnostics. Consequently, understanding the relationship between copolymer architecture and the self-assembled morphologies at equilibrium, the pathways of structure evolution and the stability of equilibrium structures to perturbations in environmental variables such as flow shear and temperature, is crucial for designing polymers for diverse applications. This study employs CGMD simulations, focusing on prototypical amphiphilic copolymers composed of Poly(Butadiene) (PB) and Poly(Ethylene Oxide) (PEO). The behavior of single polymer chains in aqueous solutions is investigated. The dependence of the radius of gyration and configurational relaxation time of the copolymers on chain length L and PB/PEO ratio are studied. The study then examines the self-assembly of PB-PEO diblock copolymers. Across varying copolymer compositions (L and PB/PEO ratio), a diverse phase portrait of emergent morphologies is observed, including simple structures such as spherical micelles, vesicles (polymersomes), bilayer lamellae, linear wormlike micelles, and tori, as well as complex forms like branched micellar networks and composite aggregates. The topological and compositional details of these morphologies are quantitatively characterized. Further, the energetic and entropic metrics underlying morphology selection are analyzed. The simulation results align well with experimental observations, offering insights into the mechanisms that render morphological diversity in amphiphilic copolymer systems. The study also investigates how structural changes occur with variations in concentration and temperature. Further, a mechanism of vesicle formation by water diffusion into a spherical micelle is discovered. The study further focuses on the mechanism of vesicle formation by the self-assembly of AB and BAB type BCPs, where A and B represent the hydrophilic (PEO) and hydrophobic (PB) segments, respectively. For AB BCPs, evolution follows the order of spherical and rod-like micelles, wormlike structures, lamellae, cavities, and vesicles. BAB BCPs first form interconnected copolymer networks, then deform to lamellar cages, ultimately transitioning into stable vesicles. Molecular reorganization at constant aggregation number to reduce solvent accessible surface area (SASA) and consequently the unfavorable hydrophobic interactions is a common motif of vesiculation. Subsequently, this work investigates the behavior of stable BAB vesicles under shear flow. Non-equilibrium molecular dynamics simulations reveal how unilamellar triblock copolymer vesicles respond to external hydrodynamic forces. Their deformation mode is affected by Weissenberg number Wi, defined as the ratio of the time scale of vesicle shape fluctuations to the inverse shear rate. For Wi < 10, a spherical vesicle deforms into a flow-aligned ellipsoidal bilayer executing tank-treading motion. For Wi > 10, pronounced variations in bilayer thickness and polymer extension manifest along the contour of the elongated vesicle, which breaks up into lamellar fragments. Below a critical strain, the deformed vesicle upon flow cessation returns to initial spherical morphology. However, for larger strains, structure reorganization after flow stoppage results in the formation of a Novel Equilibrated Shear-Induced Structure (NoESIS), in which two vesicles are connected by a dynamic molecular bridge which can accommodate additional layers of copolymers leading to a reduction in the polymer-solvent interface area. At even larger strains, the deformed vesicle breaks apart after flow cessation and eventually reorganizes into a smaller vesicle and a composite vesicular structure.

Access

Open Access

Recommended Citation

Liu, Senyuan, "THERMODYNAMICS AND SHEAR FLOW DYNAMICS OF BLOCK COPOLYMER MORPHOLOGIES IN SOLUTION" (2024). Dissertations - ALL. 2046.
https://surface.syr.edu/etd/2046