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Author ORCID Identifier

https://orcid.org/0000-0002-7971-2267

AccessType

Open Access Dissertation

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2022

Month Degree Awarded

September

Abstract

Polymer thin films are used in many applications including packaging, electronics, and membranes where they can be freestanding or serve as coatings within a multilayer system. In an effort to reduce plastic waste and conserve energy, minimizing the thickness of these applied polymer films is necessary but requires an understanding of the mechanical properties and how they change as film thickness decreases. Polymer chains exhibit changes in mobility and entanglements when confined in thin film geometries. Utilizing custom-built instrumentation that can measure the complete stress-strain response of polymer films below 100 nm in thickness, this dissertation explores the physical changes in polymer molecules, specifically related to entanglements and morphology, in ultrathin geometries and relates them to the observed mechanical response. To systematically manipulate entanglements, polystyrene of varying chain lengths is blended in different ratios and the complete uniaxial stress-strain response is measured for 100 nm films on a liquid surface (Chapter 2). The strength of these macroscopic films is quantitatively compared to uniaxial extension in molecular dynamics simulations of similar blended glassy films. Based on these results a mean-field model relating the

mechanical response to the number of load-bearing entanglements within the systems is developed. Moving on to a more complex, phase-separated system, the effect of morphology on poly(styrene-b-2-vinylpyridine) films is measured in a freestanding state. While maintaining a constant volume fraction in the block copolymer, the morphology is altered through solvent vapor annealing in chloroform. Through uniaxial extension, a higher maximum stress is measured in the lamellar morphology compared to the cylindrical morphology and a similar elastic modulus is measured for the two morphologies. Values for these two mechanical properties in both morphologies are higher than for polystyrene and poly(2-vinylpyridine) homopolymers. These enhanced properties are related to the chain conformations within the two morphologies and residual stresses. However, softening of P2VP is observed in the presence of water. To explore this softening, the two morphologies of poly(styrene-b-2-vinylpyridine) are measured in uniaxial extension on water’s surface. Elastic moduli and maximum stresses are reported that are below what is measured for the homopolymer components. The cylindrical morphology is also stronger of the two phase-separated morphologies. Both morphologies exhibit increases in failure strain of 10x. A reduced complex shear modulus and glass transition temperature are measured for poly(2-vinylpyridine) in the presence of water. These electrostatic interactions between water and the poly(2-vinylpyridine) are responsible for the extreme ductility and weakened mechanical strength observed. Through this dissertation, the number of load-bearing entanglements within polystyrene blends is quantified and the mechanical response of a phase-separated block copolymer is measured in two environments examining the effects of morphology and expanding the knowledge of ultrathin film mechanics.

DOI

https://doi.org/10.7275/29696447

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