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Author ORCID Identifier

https://orcid.org/0000-0002-3980-8491

Document Type

Campus-Only Access for Five (5) Years

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2020

Month Degree Awarded

February

First Advisor

Alfred J. Crosby

Subject Categories

Polymer and Organic Materials

Abstract

Glassy polymer thin films are essential in many applications, including osmotic membranes and flexible electronics. Thinner films will benefit these technologies by increasing energy efficiency and lowering the cost of materials. However, as thickness decreases, the polymer films become fragile, making the films experimentally challenging to handle and to quantitatively study the mechanical properties. Many studies have focused on the changes in chain mobility and entanglements, while there are few studies on the changes in mechanical properties. Moreover, these previous studies were all in the low strain regime. Therefore, the advancement of polymer thin film technologies has been restricted due to a limited understanding of how mechanical properties change as the film thickness decreases below the representative size scale of an individual polymer molecule. In this thesis, we quantified the mechanical properties of ultrathin polymer films and related the properties to changes in entanglements and mobility.

In 2015, a technique was introduced that directly measures the stress-strain response of ultrathin polymer films. In this method, a polymer film is held on a water surface between a flexible cantilever and a movable rigid boundary, allowing both force and imposed displacement on the film to be measured. We expanded this technique to measure both linear (low strain) and nonlinear (high strain) response of glassy polymer films (TUTTUT), specifically polystyrene (Chapter 2). We observed a transition in strain localization from crazing to shear deformation zones as the polystyrene film thickness changed from 30 nm to 20 nm. We associated the strain localization transition to an increase in average molecular mobility. In addition, we measured an embrittlement for polystyrene films less than the average size of a polymer molecule (~25 nm for the material tested), which we attributed to a reduction in entanglement density.

To further understand the role of entanglements on the mechanical properties, we studied how molecular weight and thickness are coupled to control the mechanical response of polystyrene thin films (Chapter 3). We observed a molecular weight independent thickness-transition in strain localization and elastic modulus, and a molecular weight dependent decrease in maximum stress. We developed a model to capture the role of the thickness and the number of entanglements per chain on the decrease in maximum stress.

We addressed the question about the influence of water on the mechanical properties of polystyrene thin films by developing a new method to measure the stress-strain response of freestanding polymer films, TUFF (Chapter 4). We compared the results to the previous liquid-supported measurements and observed a similar response in maximum stress and elastic modulus between the liquid supported and freestanding films. We identified that the liquid supporting layer leads to enhanced craze stability for ultrathin polystyrene films.

Lastly, we explored the mechanical response of ultrathin polymer nanocomposites (Chapter 5). We found that the maximum stress decreased, and elastic modulus was constant with increasing nanoparticle loading. The changes in maximum stress were also sensitive to the film thickness. This result suggests that the mechanical response in ultrathin films is sensitive to both polymer-nanoparticle and polymer-free surface entanglement interactions.

This thesis provides direct quantification of the failure and deformation response of ultrathin glassy polymer films. These findings expand the fundamental understanding of polymer physics of ultrathin glassy polymer films and provide new design insights for engineering polymer films with enhanced mechanical strength for membranes and flexible electronic applications.

DOI

https://doi.org/10.7275/hgsz-x246

Available for download on Monday, February 01, 2021

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