There remains a lack of fundamental understanding in the role of backbone rigidity on the thermomechanical properties of conjugated polymers. Here, we provide the first holistic approach to understand the fundamental influence of backbone rigidity on an n-type naphthalene diimide-based conjugated polymer, denoted by PNDI-Cx, through insertion of a flexible conjugation break spacer (CBS). CBS lengths are varied from fully conjugated with zero alkyl spacer (PNDI-C0) to a seven-carbon alkyl spacer (PNDI-C7), with the CBS engineered into each repeat unit for systematic evaluation. Solution small-angle neutron scattering and oscillatory shear rheometry were employed to provide the first quantitative evidence of CBS influence over conjugated polymer chain rigidity and entanglement molecular weight (M e), demonstrating a reduction in the Kuhn length from 521 to 36 Å for fully conjugated PNDI-C0 and PNDI-C6, respectively, as well as a nearly consistent M e of ∼15 kDa upon the addition of CBS. Thermomechanical properties, such as elastic modulus and glass-transition temperature, were shown to decrease with an increasing length of CBS. An extraordinary ductility, upwards of 400% tensile strain before fracture, was observed for high-molecular-weight PNDI-C4, which we attribute to a high number of entanglements and disruption of crystallization. Furthermore, the deformation mechanism for PNDI-Cx was studied under strain through X-ray diffraction, polarized UV–vis spectroscopy, and atomic force microscopy. Overall, this work sheds light on the important role of backbone rigidity in designing flexible and stretchable conjugated polymers.

A scalable and green approach to manufacture semiconducting microfibers from polymer melts has been demonstrated. The polymer chains are highly aligned along the microfiber's long axis direction and exhibit highly anisotropic optical properties. In addition, the polymer microfibers show good flexibility and stretchability with a yield point around 10% under a reversible stress and can be stretched up to 180% without breaking. These features are desired for future flexible, stretchable, and conformable electronics. The origin of this stretchability is studied with diketopyrrolopyrrole derivatives using different conjugation break spacers and side chains. In addition, stretchable conducting microfibers can be obtained by doping with FeCl 3 , which are further evaluated as organic conductors and source/drain electrodes in organic field-effect transistors.

Thermomechanical properties of polymers highly depend on their glass transition temperature (T g). Differential scanning calorimetry (DSC) is commonly used to measure T g of 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 measuring T g in such type of polymers. The changes in specific heat capacity (Δc p) at T g were accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δc p from flexible polymer (0.28 J g−1 K−1 for polystyrene) to rigid CPs (10−3 J g−1 K−1 for 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 p at T g by 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 determining T g for 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

Complementary semiconducting polymer blends (c-SPBs) have been demonstrated as an effective approach to balance performance and processing of semiconducting polymers for organic field-effect transistors. All previously reported c-SPBs have been exclusively based on p-type polymers. In this report, we designed and synthesized naphthalene diimide (NDI) based matrix polymers and systematically studied n-type charge transport behaviors of their corresponding polymer blends. NDI-C m (m = 3–7) polymers displayed low melting points (55–105 °C) allowing for the lowest temperature melt-processing of organic transistors to date with mobilities up to 1.01 × 10–3 cm2 V–1 s–1. NDI-C m polymers were revealed to be nearly amorphous by GIXRD and thin film UV–vis which explain the lowered thermal transitions and observed poor charge transport. Utilizing a c-SPB with 5% fully conjugated P­(NDI2OD-T2), the transistor performance improved up to 100-fold of the pure matrix polymer despite the low crystallinity of NDI-C m thin films.

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 10 3 to 10 4 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.

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