Coarse-grained modeling and its application in bottlebrush block copolymer self-assembly

Pan, Tianyuan

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https://hdl.handle.net/2142/120246 Copy

Description

Title

Coarse-grained modeling and its application in bottlebrush block copolymer self-assembly

Author(s)

Pan, Tianyuan

Issue Date

2023-04-14

Director of Research (if dissertation) or Advisor (if thesis)

Sing, Charles E

Doctoral Committee Chair(s)

Sing, Charles E

Committee Member(s)

• Evans, Christopher M
• Leal, Cecilia
• Statt, Antonia
• Guironnet, Damien

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Coarse-grain model
• Polymer physics
• Molecular simulation
• Monte Carlo
• Brownian Dynamics
• Inter-molecular interaction

Abstract

Over the past two decades densely grafted polymers, called ‘bottlebrushes’, have emerged as an exciting subclass of branched macromolecules. Bottlebrush polymers have found utility as building blocks for a wide range of novel materials, e.g., photonic and phononic crytals, molecular pressure sensors, pressure sensitive adhesives, pH responsive surfaces, stimuli-responsive molecular brushes, and low-modulus elastomers capable of strain-hardening and sustaining large deformation. These applications make use of the unique properties of bottlebrush polymers, which emerge from the more extended conformations bottlebrushes exhibit compared to linear chains of the same length, as well as the considerable stiffening of the backbone or even the side chains at high grafting densities. The stiffening of the backbone causes reduced molecular entanglements in bottlebrush melts, and in suspensions, lyotropic ordering has been reported. These recent efforts to use bottlebrush polymers in functional and self-assembly applications point to the need to study large-scale macroscopic behavior, e.g. rheology or self-assembly, of bottlebrush suspensions and melts, and understand how to tune these properties using an expanded list of molecular architecture parameters (e.g. side chain length and density, backbone length). We are driven by the interest to computationally understand the fundamental material problems of bottlebrush polymers, so that we can provide more molecular insights and inform rational design in experiments. Despite sophistication in the modeling of individual bottlebrush polymers, it still remains a challenge to model systems composed of multiple bottlebrush molecules, since it is computationally expensive to simulate both large numbers of molecules as well as architectures with long backbone and side-chain lengths. In this dissertation, we present a series of model development efforts in bottlebrush coarse-grain modeling, which is one of the possibilities to address the challenge. In the first stage, we explore the possibility of using the Worm-like Cylinder (WLCy) model as a coarse-grained representation of a bottlebrush, in a way that parameterizes from molecular simulation rather than experimental observation. The latter, molecular model is based on a hybrid BD/MC simulation model developed by the authors, with properties that arise from resolving side chain structure and in quantitative agreement with experiment. We use a parameterization procedure that systematically shows that the WLCy model is consistent among a wide range of conformational and dynamic properties, and for a large number of polymer architectures. We also show that, by parameterizing the WLCy model from a computationally-expensive molecular simulation with explicit side-chains, we can develop a coarse-grained model of a bottlebrush that has implicit side-chains. This model is focused on dilute suspensions of bottlebrush polymers, with non-dilute systems requiring additional study of inter-bottlebrush interactions. However, this implicit side chain representation represents the first step to carrying out large-scale simulations of bottlebrush suspensions, which would otherwise be computationally intractable in more fine grained polymer simulations. In the next stage, we develop a new method for modeling bottlebrush block copolymer (BBCP) solution self-assembly that is capable of capturing both large molecular architectures and a large number of assembling molecules. This multi-chain simulation uses the Implicit Side-chain (ISC) representation which rooted in the efforts in the first stage, and with a number of modifications is capable of matching well with experimental observations for analogous systems. We first derive a form for the pairwise potential between two bottlebrush polymers in solution from scaling arguments, and we directly include this pairwise potential into our simulations. We show that simulated self-assembly behavior for symmetric BBCPs are consistent with scattering experiments. This method thus results in a model that can capture the microphase separation of BBCPs. This work thus sets the stage for performing simulations to understand the in- and out-of-equilibrium assembly in an increasingly broad range of bottlebrush systems, especially in solutions where the system is not well described by mean-field arguments and is known to be affected by far-from-equilibrium processing conditions. Finally, we investigate the interaction between two bottlebrushes, and establish a protocol for incorporating these interactions into a coarse-grained, implicit side-chain representation of bottlebrush polymers. We use the Explicit Side-chain (ESC) representation to calculate the potential of mean force (PMF), osmotic second virial coefficient and interpenetration function between two bottlebrushes. We then use a corresponding ISC representation that incorporates this molecular information, and show that the two models give near quantitatively matched PMF profiles and second virial coefficients. This can be useful for generating a coarse-grained potential of bottlebrushes, which is applicable in large-scale simulations of bottlebrush systems.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Tianyuan Pan

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