Date of Award

Spring 3-11-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Polymer Science and Engineering

Committee Chair

Xiaodan Gu

Committee Chair School

Polymer Science and Engineering

Committee Member 2

Jason D. Azoulay

Committee Member 2 School

Polymer Science and Engineering

Committee Member 3

Sarah E. Morgan

Committee Member 3 School

Polymer Science and Engineering

Committee Member 4

Sergei Nazarenko

Committee Member 4 School

Polymer Science and Engineering

Committee Member 5

Song Guo

Committee Member 5 School

Mathematics and Natural Sciences

Abstract

In contrast to conventional silicon-based electronics, semiconducting polymers show great promise for emerging applications in soft, flexible, and ductile electronic technologies. This is due to their polymeric nature, tailorable structure, and sub-100 nm device thickness. Despite this mechanical novelty, there remains a poor understanding of their structure-property-processing relationships, which has hindered growth within the field. This dissertation elucidates these relationships through investigation of their thermomechanics, and morphological phenomena. This was accomplished through the following projects:

1) To demonstrate the impact of backbone rigidity on semiconducting polymer thermomechanics, we varied the backbone rigidity of an NDI-based polymer by inserting flexible methylene units of varying lengths along the backbone of the monomer unit. Incorporation of the spacer resulted in a vast reduction of the glass transition temperature (Tg) and profound improvements in ductility.

2)We developed a free-standing tensile technique that enabled the characterization of polystyrene and poly(3-hexylthiophene) films down to 19 nm and 80 nm, respectively. Confinement was demonstrated to impact yield stress and strain at failure of polystyrene films, while modulus was relatively unaffected, despite literature suggestion of a sub-room temperature Tg. We then compared water-supported and free-standing films to elucidate their interfacial influence on the observed mechanical performance.

3) Amide and urea moieties were incorporated into a DPP-based polymer to demonstrate the role of hydrogen bonding strength on thermomechanical performance. Amide and urea were discovered to minimize and promote crystallinity, respectively, which led to a 400% increase in strain at failure for the amide-containing polymer. This finding demonstrated that hydrogen bonding may dictate mechanical performance through control of the crystalline morphology, rather than energy dissipation.

4) Due to the semicrystalline nature of semiconducting polymers, it has been postulated that they may possess a rigid amorphous fraction (RAF) which may dictate their optoelectronic performance. To illuminate the existence and impact of the RAF on semiconducting polymer performance we established a spectroscopic ellipsometry method to fully characterize their temperature-dependent thickness, optical profile, and degree of anisotropy. All semicrystalline semiconducting polymers were observed to possess a RAF which strongly dictated their optoelectronic performance.

ORCID ID

0000-0002-0101-8507

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