Date of Award

Summer 6-28-2023

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

Derek Patton

Committee Member 2 School

Polymer Science and Engineering

Committee Member 3

Sarah Morgan

Committee Member 3 School

Polymer Science and Engineering

Committee Member 4

Sergei Nararenko

Committee Member 4 School

Polymer Science and Engineering

Committee Member 5

Boran Ma

Committee Member 5 School

Polymer Science and Engineering

Abstract

Semiconducting conjugated polymers (CPs) open opportunities to develop a new generation of lightweight, printable, low-cost, deformable, and large-area electronic devices. CPs are characterized by complex chain dynamics and heterogeneous molecular structure with large degrees of conformational freedom.

Chain conformation (i.e., chain rigidity and local backbone planarity) dictates the delocalized electron cloud along the polymer backbone which greatly influences the optical and electronic properties of conjugated polymers. For the first time, we indicate that using variable-temperature scattering and spectroscopy characterizations allows confirmation of the single-chain state to measure the molecular chain conformations. We found a critical role of side chains and oligothiophene donor units in persistence length and optical absorption.

To understand the naked backbone conformations, neutron scattering and deuteration chemistry were utilized. Small-angle neutron scattering techniques with contrast-variation experiments are employed on conjugated polymers with deuterated side chains to decouple the backbone and side-chain scattering signals. For relative flexible poly(3-alkylthiophenes), the strong scattering signal from long and flexible alkyl side chains leads to a seemingly more rigid conjugated polymer. However, we found that for more rigid D-A type diketopyrropyrrole (DPP)-based semiconducting polymers, the backbone conformation is the same as the whole polymer chain.

Understanding heterogeneous molecular dynamics in conjugated polymers, which has significant influence in optoelectronic properties, is challenging. Here, we decoupled the backbone and sidechain dynamics in DPP-based copolymers with different side-chain lengths by the combination of quasi-elastic neutron scattering (QENS) and deuterium labelling. Interestingly, a transition was indicated by DMA and QENS at around 430-450 K, which is accompanied by backbone twist motion as indicated by temperature dependent Fourier transform infrared spectroscopy (FTIR). Above the transition temperature, the thiophene twisting motion induces localizing of the intrachain exciton, leading to a reduction of dc conductivity and a significant blue shift in optical absorption.

ORCID ID

0000-0002-4439-4414

Available for download on Saturday, May 31, 2025

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