Date of Award

Spring 2021

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

Sarah E Morgan

Committee Member 2 School

Polymer Science and Engineering

Committee Member 3

Jason D Azoulay

Committee Member 3 School

Polymer Science and Engineering

Committee Member 4

Sergei Nazarenko

Committee Member 4 School

Polymer Science and Engineering

Committee Member 5

Derek Patton

Committee Member 5 School

Polymer Science and Engineering

Abstract

The past decades have witnessed a surging exploration of semiconducting polymers for the application of wearable and flexible organic electronic devices. Despite the increased amounts of molecular engineered polymers and their much-improved electrical performances, a systematic study of the structure-thermal/mechanical property-morphology relationship of semiconducting polymers is still less investigated.

To understand the thin-film mechanical properties, a pseudo-free standing tensile tester was self-built and utilized to obtain their real-time stress-strain behaviors through uniaxial stretching on top of the water surface. It also enables the first quantitative measurement of fracture energy on ultrathin polymeric films. Through multiple mechanical testing methods (i.e., strain-rate dependent tensile tests, stress-relaxation, hysteresis tests, etc.), we found surprising viscoelastic behaviors from recently emerged donor–acceptor (D–A) type diketopyrropyrrole (DPP)-based semiconducting polymers, despite their rigid polymer backbones. Such observation was later directly correlated with their sub-room temperature glass transition temperature (Tg).

Thus, it is vital to explore the structural origin of the low-Tg nature in D–A polymers and the Tg prediction guidelines. Both backbone- and side-chain engineered DPP-based polymers were synthesized to investigate their thermal and mechanical performances. A modified dynamic mechanical analysis (DMA) and alternating current (AC)-chip calorimetry were utilized to measure the bulk and thin-film Tg, respectively. Our findings suggested the low-Tg results from the high weight fraction of flexible side-chains (typically > 50%). Furthermore, we developed a predictive mass-per-flexible bond model that establishes a linear relationship between chain flexibility and polymer Tg.

Moreover, a detailed morphological analysis was performed on tensile-aligned DPP-based polymer thin films through experimental measurements and molecular modeling. Two primary strain-induced alignment mechanisms were addressed: highly oriented crystalline domains coupled with crystallographic slippage, and substantial chain slippage in the amorphous domain.

To boost the mechanical and electrical performance of organic electronic devices, a semiconducting polymer composite was engineered by incorporating a low-Tg butyl rubber elastomer as the matrix. A mechanically and electrically self-healable composite system was obtained through careful control over the multi-scale phase separation behaviors. Such a method is proved to be broadly applicable to both n-type and p-type D–A polymers.

ORCID ID

https://orcid.org/0000-0001-9815-7046

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