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Flexible alkyl side chain in conjugate polymers (CPs) improves the solubility and promotes solution processability, in addition, it affects interchain packing and charge mobilities. Despite the well-known charge mobility and morphology correlation for these semi-crystalline polymers, there is a lack of fundamental understanding of the impact of side chain on their crystallization kinetics. In the present work, isothermal crystallization of five poly(3-alkylthiophene-2,5-diyl) (P3ATs) with different side-chain structures were systematically investigated. To suppress the extremely fast crystallization and trap the sample into amorphous glass, an advanced fast scanning chip calorimetry technique, which is able to quench the sample with few to tens thousands of K/s, was applied. Results show that the crystallization of P3ATs was greatly inhibited after incorporation of branched side chains, as indicated by a dramatic up to six orders of magnitude decrease in the crystallization rate. The suppressed crystallization of P3ATs were correlated with an increased π–π stacking distance due to unfavorable side-chain steric interaction. This work provides a pathway to use side-chain engineering to control the crystallization behavior for CPs, thus to control device performance. 

Abstract Extensive efforts have been made to develop flexible electronics with conjugated polymers that are intrinsically stretchable and soft. We recently systematically investigated the influence of conjugation break spacers (CBS) on the thermomechanical properties of a series n‐type naphthalene diimide‐based conjugated polymer and found that CBS can significantly reduce chain rigidity, melting point, as well as glass transition temperature. In the current work, we further examined the influence of CBS on the crystallization behaviors of PNDI‐C3 to C6, including isothermal crystallization kinetics, crystal polymorphism and subsequently time‐dependent modulus, in a holistic approach using differential scanning calorimetry, X‐ray scattering, polarized optical microscopy, atomic force microscopy, and pseudo‐free‐standing tensile test. Results demonstrate that increasing the length of CBS increases the crystallization half‐time by 1 order of magnitude from PNDI‐C3 to PNDI‐C6 from approximately 10³to 10⁴ s. The crystallization rate shows a bimodal dependence on the temperature due to the presence of different polymorphs. In addition, crystallization significantly affects the mechanical response, a stiffening in the modulus of nearly three times is observed for PNDI‐C5 when annealed at room temperature for 12 h. Crystallization kinetic is also influenced by molecular weight (MW). Higher MW PNDI‐C3 crystallizes slower. In addition, an odd–even effect was observed below 50°C, odd‐number PNDI‐Cxs (C3 and C5) crystallize slower than the adjacent even‐numbered PNDI‐Cxs (C4 and C6). Our work provides an insight to design flexible electronics by systematically tuning the mechanical properties through control of polymer crystallization by tuning backbone rigidity. 

Abstract Conjugated polymers are emerging as promising building blocks for a broad range of modern applications including skin‐like electronics, wearable optoelectronics, and sensory technologies. In the past three decades, the optical and electronic properties of conjugated polymers have been extensively studied, while their thermomechanical properties, especially the glass transition phenomenon which fundamentally represents the polymer chain dynamics, have received much less attention. Currently, there is a lack of design rules that underpin the glass transition temperature of these semirigid conjugated polymers, putting a constraint on the rational polymer design for flexible stretchable devices and stable polymer glass that is needed for the devices’ long‐term morphology stability. In this review article, the glass transition phenomenon for polymers, glass transition theories, and characterization techniques are first discussed. Then previous studies on the glass transition phenomenon of conjugated polymers are reviewed and a few empirical design rules are proposed to fine‐tune the glass transition temperature for conjugated polymers. The review paper is finished with perspectives on future directions on studying the glass transition phenomena of conjugated polymers. The goal of this perspective is to draw attention to challenges and opportunities of controlling, predicting, and designing polymeric semiconductors, specifically to accommodate their end use. 

Abstract Semiconducting donor–acceptor (D–A) polymers have attracted considerable attention toward 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 simulations. Informed from experimental results, a mass‐per‐flexible bond model is developed to capture such observation through a linear correlation betweenT_gand polymer chain flexibility. Using this model, a wide range of backboneT_gover 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 predictT_gand elastic modulus of future new D–A polymers. 

Abstract 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. 

ABSTRACT Thermomechanical properties of polymers highly depend on their glass transition temperature (T_g). Differential scanning calorimetry (DSC) is commonly used to measureT_gof polymers. However, many conjugated polymers (CPs), especially donor–acceptor CPs (D–A CPs), do not show a clear glass transition when measured by conventional DSC using simple heat and cool scan. In this work, we discuss the origin of the difficulty for measuringT_gin such type of polymers. The changes in specific heat capacity (Δc_p) atT_gwere accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δc_pfrom flexible polymer (0.28 J g⁻¹K⁻¹for polystyrene) to rigid CPs (10⁻³J g⁻¹K⁻¹for a naphthalene diimide‐based D–A CP). When a conjugation breaker unit (flexible unit) is added to the D–A CPs, we observed restoration of the Δc_patT_gby a factor of 10, confirming that backbone rigidity reduces the Δc_p. Additionally, an increase in the crystalline fraction of the CPs further reduces Δc_p. We conclude that the difficulties of determiningT_gfor CPs using DSC are mainly due to rigid backbone and semicrystalline nature. We also demonstrate that physical aging can be used on DSC to help locate and confirm the glass transition for D‐A CPs with weak transition signals. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019, 57, 1635–1644