Toward the Prediction and Control of Glass Transition Temperature for Donor–Acceptor Polymers

Abstract

Semiconducting donor-acceptor (D-A) polymers have attracted considerable attention towards the application of organic electronic and optoelectronic devices. However, a rational design rule for making semiconducting polymers with desired thermal and mechanical properties is currently lacking, which greatly limits the development of new polymers for advanced applications.Here, polydiketopyrrolopyrrole (PDPP)-based D-A polymers with varied alkyl side-chain lengths and backbone moieties are systematically designed, followed by investigating their thermal and thin film mechanical responses. The experimental results show a reduction in both elastic modulus and glass transition temperature (T g ) with increasing side-chain length, which is further verified through coarse-grained molecular dynamics (CG-MD) simulations. Informed from experimental results, a mass-per-flexible bond model is developed to capture such observation through a linear This article is protected by copyright. All rights reserved. 3 correlation between T g and polymer chain flexibility. Using this model, a wide range of backbone T g over 80 C and elastic modulus over 400 MPa can be predicted for PDPP-based polymers. This study highlights the important role of side-chain structure in influencing the thermomechanical performance of conjugated polymers, and provides an effective strategy to design and predict T g and modulus of future new D-A polymers.) The synthesis part was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a Discovery Grant (RGPIN-2017-06611), and by the Canadian Foundation for Innovation (CFI). M. U. O. thanks NSERC for a doctoral scholarship.

Molecular Origin of Strain‐Induced Chain Alignment in PDPP‐Based Semiconducting Polymeric Thin Films

“…In this work, four poly(diketopyrrolopyrrole‐co‐thiophene) (PDPPT)‐based D–A polymers with increasing side‐chain lengths from 2‐hexyl decyl (C2C6C8) to 2‐octyl dodecyl (C2C8C10), 2‐decyl tetradecyl (C2C10C12), and 2‐dodecyl hexadecyl (C2C12C14) were synthesized followed by previously reported procedures ( Figure a). [ 55 ] Their molecular weights and polydispersity were summarized in Table 1 . All four polymers were spun cast on the silicon substrate to fabricate thin films with a thickness around 100 nm, followed by tensile testing on the water surface using a previously reported pseudo‐free‐standing tensile tester, or film‐on‐water (FOW) tester.…”

“…All four polymers were spun cast on the silicon substrate to fabricate thin films with a thickness around 100 nm, followed by tensile testing on the water surface using a previously reported pseudofree-standing tensile tester, or film-on-water (FOW) tester. [55][56][57][58][59] Figure 1b plotted true stress (σ T ) -true strain (ε T ) curves of the four polymers, with optical images of PDPPT-C2C8C10 thin films at two strains highlighted to show the deformation behavior. It is also observed that polymers with shorter sidechain lengths are easier to deform along the width direction, i.e., a more evident Poisson effect (Figure 1b; Videos S1-S4, Supporting Information).…”

“…The degree of strain hardening could be represented by strain Reproduced with permission from. [55] Copyright 2020, John Wiley and Sons. c) Schematic of multimodal morphological characterization tools on tensile strained free-standing thin films.…”

“…a) Schematics of PDPPT polymers with different side-chain structures. b) True stress-strain curves for four polymers and representative optical images under different strain.Reproduced with permission from [55]. Copyright 2020, John Wiley and Sons.…”

Strain–Microstructure–Optoelectronic Inter-Relationship toward Engineering Mechano-Optoelectronic Conjugated Polymer Thin Films

“…Glass transition temperature of conjugated polymers is a dictating factor for the stability of microstructure and viscoelastic behavior of a MO thin film. Very recently, the glass transition of conjugated polymers, including P3ATs, has been characterized by rational prediction based on sorting elemental chemical structure by the level of rigidity and flexibility [ 129 , 130 ]. Glass transition temperature of a MO thin film needs to be controlled depending on a targeted operating temperature.…”