The structure, color, and rheology of concentrated diblock bottlebrush polymer solutions

Wade, Matthew A.

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https://hdl.handle.net/2142/117747 Copy

Description

Title

The structure, color, and rheology of concentrated diblock bottlebrush polymer solutions

Author(s)

Wade, Matthew A.

Issue Date

2022-11-04

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

Rogers, Simon A

Doctoral Committee Chair(s)

Rogers, Simon A

Committee Member(s)

• Evans, Christopher M
• Harley, Brendan A
• Statt, Antonia

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Rheology
• Bottlebrush Polymers
• Diblock Polymers
• Photonic Crystals
• Direct Ink Writing

Abstract

Bottlebrush polymers are a class of highly branched polymers consisting of a single linear backbone with side chain polymers densely grafted along its length. The steric repulsion between the side chains forces the backbone bottlebrush polymer to elongate resulting in the inhibition of entanglements and the polymer exhibiting a much faster relaxation time then linear polymers of equivalent molecular weight. Due to the wide range of properties that can be accessed simply by changing the architecture and chemistry of a bottlebrush polymer, extensive studies have been conducted describing the rapid dynamics of this material, the assembly of this material into larger structures, and the use of this material in application requiring super soft, highly elastic materials. Of note are the photonic properties exhibited by diblock bottlebrush polymers as they rapidly assembly into highly uniform structures with characteristics on the order of the wavelength of light. This work seeks to explore these properties and develop these materials for applications in direct-ink-writing by developing structure-property-process relations between the microstructure, bulk color, and processing conditions. We first explore the fundamental structure and shape of bottlebrush polymers to lay the foundation that can be used to eventually characterize the structure-property-process relations exhibited by this material. We probe the fundamental shape through intrinsic viscosity and dynamic light scattering measurements. Upon characterizing the intrinsic viscosity and hydrodynamic radius as a function of molecular weight, we note that the scaling factors are lower than their linear counterparts suggesting that the bottlebrush molecule itself is more collapsed than the linear polymer. We also observe the transition from a star-like structure to a short-cylinder and eventually flexible cylinder with increasing backbone length. We explore the sensitivity of the bottlebrush polymer to slight changes in architecture noting how two bottlebrushes with equivalent molecular weight but different side chain dispersities exhibit different bulk rheological properties. This result highlights the weaknesses of size exclusion chromatography techniques when characterizing complex molecules while highlighting the sensitivity of rheology at identifying differences in the relaxation modes. Having established a basic understanding of how this material behaves in a single molecule environment, we begin to develop the structure-property-process relation for this material through a combination of rheology, neutron scattering, and imaging techniques. In this study, we focus on diblock bottlebrush polymers with poly-lactic acid side chains in one block and polystyrene side chains in the other block. These polymers were dispersed in toluene with a concentration of 175 mg/ml. By combining imaging, neutron scattering, and rheology, we are able to observe how the color of the sample changes under flow and related that back to the self-assembled microstructure. Through these studies we note that the sample appears green at quiescence and low shear rates, cyan at moderate shear rates, and indigo at high shear rates. These colors were found to correspond to a highly uniform lamellea, a compressed lamellae with a reduced characteristic length scale, and an incoherent lamellae with notable variation in the characteristic length scale. The initial decrease in lamellae spacing was found to correspond to the green to cyan color transition. While the lamellae continued to compress with increasing rate, the change in length scale is not sufficient to account for the change in reflect wavelength. Instead the development of incoherence’s and thus local smaller length scales is attributed to the cyan to indigo transition. Throughout these tests, the intramolecular spacing was found to remain consistent. As part of these studies we are also able to identify the orientation of the lamellae which we find remains relatively consistent until the highest tested shear rates where it rotates to align along the vorticity direction, resulting in the sample appearing colorless. The combination of this structural study with the imaging and rheological measurements thus allows us to define a structure-property-relation between the microstructure, structural color, and applied flow conditions. Having identified the ability to control the self-assembled microstructure and thus color of the sample with flow, we turn to investigating this material in the context of printing processes. Specifically we investigate the material’s shear memory and retention of color. By carrying out relaxation studies, we note that the material reverts to its quiescent state on the order of less than a second. As such, we begin to explore possible avenues to increase the relaxation time of the polymer solution and thus retain any shear induced color. We first explore the use of a UV activated crosslinking mechanism achieved by functionalizing the PLA block of the bottlebrush with -allyl groups. Proof of concept tests were carried out by curing this material while applying steady stress from 0 Pa to 62.9 Pa. Macroscopic images were captured under zero stress conditions after the sample had cured clearly demonstrate a color change from green to cyan, highlighting our ability to lock-in the shear induced microstructure. Measurements were also carried out to describe the time scale over which the UV-curable bottlebrush transitioned from a sol to a gel. Due to key assumptions in the Winter-Chambon criterion not applying to this material, we were unable to define an exact gel point. That said, estimates based on the G’, G" crossover indicate the material solidifies on a similar time scale to the color relaxation time. SPP analysis was applied to this system in an attempt to expand the range of frequencies that could be measured during gelation. Analysis of these results is still ongoing but should allow us to carry out gelation studies without having to worry about the mutation number. A secondary approach to increasing the relaxation time of the bottlebrush polymer solution was explored by doubling the concentration from 175 mg/ml to 350 mg/ml. With this concentrated bottlebrush solution, we observe features in the flow curve and amplitude sweep that are typically associated with yield-like behavior. We predicted that yield-like behavior would allow us to lock in the microstructure simply by imposing a shear-induced structure while the material was flowing and then releasing the material such that it reverts to a solid state with a theoretically infinite relaxation time. Proof of concept studies demonstrated that this was not the case as the color of the concentrated polymer solution reverted back to its quiescent state on the order of 5 s, highlighting how yielding is not an instantaneous process. To further explore this idea, we carried out rheological studies to decompose the strain into recoverable and unrecoverable components and calculate the fluid and solid components of the loss moduli. The resulting moduli demonstrated that the bottlebrush polymer was exhibiting fluid-like behavior at low strains, further reinforcing the idea that yielding in this material does not occur instantly at the yield stress. We investigate the structural changes and processes that occur during yielding by calculating the transient Deborah number for a number of steady oscillatory deformations with amplitudes from 5.62% to 1000%. From this dimensionless group, we were able to identify points within an oscillation where the bottlebrush polymer undergoes yielding and recovery. Future experiments are planned to investigate the structural changes that occur at these points in time. Overall, this thesis combines a number of different techniques and measurements in order to explore the structure-property-process relations exhibited by a bottlebrush polymer system. This work then explores avenues that can be taken to adjust the material and begin optimizing it for applications in direct-ink-writing. On a broader sense, this work describes the steps and studies that need to be performed in order to develop a material for applications in on-the-fly property control in which the microstructure of a material is adjusted by changing the processing conditions in order to get a specific bulk property.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

© 2022 Matthew A. Wade

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