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.

Donor-acceptor (D-A) type semiconducting polymers have shown great potential for the application of deformable and stretchable electronics in recent decades. However, due to their heterogeneous structure with rigid backbones and long solubilizing side chains, the fundamental understanding of their molecular picture upon mechanical deformation still lacks investigation. Here, the molecular orientation of diketopyrrolopyrrole (DPP)-based D-A polymer thin films is probed under tensile deformation via both experimental measurements and molecular modeling. The detailed morphological analysis demonstrates highly aligned polymer crystallites upon deformation, while the degree of backbone alignment is limited within the crystalline domain. Besides, the aromatic ring on polymer backbones rotates parallel to the strain direction despite the relatively low overall chain anisotropy. The effect of side-chain length on the DPP chain alignment is observed to be less noticeable. These observations are distinct from traditional linear-chain semicrystalline polymers like polyethylene due to distinct characteristics of backbone/side-chain combination and the crystallographic characteristics in DPP polymers. Furthermore, a stable and isotropic charge carrier mobility is obtained from fabricated organic field-effect transistors. This study deconvolutes the alignment of different components within the thin-film microstructure and highlights that crystallite rotation and chain slippage are the primary deformation mechanisms for semiconducting polymers.

Understanding the mechanical behavior of glassy polymers at a fundamental molecular level is of critical importance in engineering and technological applications. Among various molecular parameters, cohesive interactions between polymer chains are found to play a key role in influencing the thermomechanical response of glass-forming polymers. Here, we employ atomistically informed coarse-grained molecular dynamics (CG-MD) simulations to study the mechanical properties of the polymer material in a glassy state. Built upon the recently developed “energy renormalization” (ER) coarse-graining approach, we take polycarbonate (PC) as a model system to systematically explore the shear response and dynamical heterogeneity of polymers under the influence of cohesive interactions. Our results show that the polymer with a larger cohesive interaction exhibits a greater shear modulus and a higher degree of dynamical heterogeneity, which is uncovered by evaluating the local molecular stiffness. This pronounced dynamical heterogeneity with increasing cohesive interactions is found to be closely correlated to the packing frustration at a molecular level, which can be quantified by the glass “fragility”, a measure of the relative strength of the temperature dependence of relaxation. Our findings highlight the critical role of cohesive interaction on the mechanical behavior of glassy polymers and provoke the idea of achieving a tailored design of polymer materials via molecular-level engineering.

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