Recently, confinement of polymers with different geometries has become a research hotspot. Here, we report the dramatic deviation of glass transition behaviors of poly(methyl methacrylate) (PMMA) ...confined in cylindrical nanopores with diameter significantly larger than chain’s radius of gyration (R g ). Fast cooling a PMMA melt in the nanopores results in a glass with one single glass transition temperature (T g). But two distinct T gs are detected after slow cooling the melt. The deviation in T g could be as large as 45 K. This phenomenon is interpreted by a two-layer model. During vitrification under slow cooling two distinct layers are formed: a strongly constrained interfacial layer showing an increased T g as compared to that of the bulk polymer and a core with a decreased T g. By thermal annealing experiments, we find that these two T gs are inherently correlated. In addition, the deviation of T g for PMMA confined in nanopores reveals a dependence on molecular weight.
Soft and conformable wearable electronics require stretchable semiconductors, but existing ones typically sacrifice charge transport mobility to achieve stretchability. We explore a concept based on ...the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. The fully stretchable transistors exhibit high biaxial stretchability with minimal change in on current even when poked with a sharp object. We demonstrate a skinlike finger-wearable driver for a light-emitting diode.
Molecular additives are often used to enhance dynamic motion of polymeric chains, which subsequently alter the functional and physical properties of polymers. However, controlling the chain dynamics ...of semiconducting polymer thin films and understanding the fundamental mechanisms of such changes is a new area of research. Here, cycloparaphenylenes (CPPs) are used as conjugated molecular additives to tune the dynamic behaviors of diketopyrrolopyrrole‐based (DPP‐based) semiconducting polymers. It is observed that the addition of CPPs results in significant improvement in the stretchability of the DPP‐based polymers without adversely affecting their mobility, which arises from the enhanced polymer dynamic motion and reduced long‐range crystalline order. The polymer films retain their fiber‐like morphology and short‐range ordered aggregates, which leads to high mobility. Fully stretchable transistors are subsequently fabricated using CPP/semiconductor composites as active layers. These composites are observed to maintain high mobilities when strained and after repeated applied strains. Interestingly, CPPs are also observed to improve the contact resistance and charge transport of the fully stretchable transistors. ln summary, these results collectively indicate that controlling the dynamic motion of polymer semiconductors is proved to be an effective way to improve their stretchability.
Conjugated carbon cyclic nanoring compounds are used as molecular additives to enhance the stretchability of semiconducting polymers without compromising mobility. The additives are shown to significantly decrease long‐range crystalline order, while short‐range ordered aggregates are well‐maintained. Fully stretchable transistors fabricated with the newly established polymer semiconductor/molecular additive blend films exhibit improved mobility retention under strain and after repeated applied strain.
Diketopyrrolopyrrole (DPP)-based donor–acceptor conjugated polymers, with increasing amount of weak H-bonding units, namely 2,6-pyridinedicarboxamide (PDCA), inserted as end groups in alkyl side ...chains were prepared and investigated. In contrast to previously reported DPP polymers containing PDCA units as conjugation breakers along the polymer backbone, PDCA in the alkyl side chains readily produced almost quantitative formation of intermolecular H-bonding even at low PDCA unit content (<10 mol %) as shown by Fourier transform infrared spectroscopy (FTIR). The efficient intermolecular H-bonding was further supported by the appearance of a pronounced vibronic shoulder in the UV–vis spectra and a reduction of interlamellar spacing (from 24.02 to 22.87 Å) compared to the neat DPP polymer. Increasing mol % of PDCA units in side chains of DPP conjugated polymers also has a clear effect on the thermal and mechanical properties of the films as investigated by dynamic mechanical analysis (DMA). Polymers with a high loading of PDCA showed a linear increase in both tan delta intensity and temperature at which softening of film cross-linking occurs. In particular, at a comparable mol %, polymers with PDCA units along the conjugated backbone showed a lower transition intensity and on average a 10–20 °C higher temperature required for H-bonding breaking. FTIR coupled with crack onset measurements showed that H-bonding breaking during tensile deformation happens only at strains close to crack onset. All these observations suggest that molecular engineering of conjugated polymers bearing H-bonding units has a strong influence on microstructure, thermal and mechanical properties of solution processed films, and final energy dissipation mechanisms in stretchable electronics applications.
Understanding crystal polymorphism is a long-standing challenge relevant to many fields, such as pharmaceuticals, organic semiconductors, pigments, food, and explosives. Controlling polymorphism of ...organic semiconductors (OSCs) in thin films is particularly important given that such films form the active layer in most organic electronics devices and that dramatic changes in the electronic properties can be induced even by small changes in the molecular packing. However, there are very few polymorphic OSCs for which the structure–property relationships have been elucidated so far. The major challenges lie in the transient nature of metastable forms and the preparation of phase-pure, highly crystalline thin films for resolving the crystal structures and evaluating the charge transport properties. Here we demonstrate that the nanoconfinement effect combined with the flow-enhanced crystal engineering technique is a powerful and likely material-agnostic method to identify existing polymorphs in OSC materials and to prepare the individual pure forms in thin films at ambient conditions. With this method we prepared high quality crystal polymorphs and resolved crystal structures of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), including a new polymorph discovered via in situ grazing incidence X-ray diffraction and confirmed by molecular mechanic simulations. We further correlated molecular packing with charge transport properties using quantum chemical calculations and charge carrier mobility measurements. In addition, we applied our methodology to a 1benzothieno3,2-b11benzothiophene (BTBT) derivative and successfully stabilized its metastable form.
The solution shearing method has previously been used to tune the molecular packing and crystal thin film morphology of small molecular organic semiconductors (OSCs). Here, we study how the solution ...shearing method impacts the thin film morphology and causes structural rearrangements of two polymeric OSCs with interdigitated side chain packing, namely P2TDC17FT4 and PBTTT-C16. The conjugated backbone tilt angle and the thin film morphology of the P2TDC17FT4 polymer were changed by the solution shearing conditions, and an accompanying change in the charge carrier mobility was observed. For PBTTT-C16, the out-of-plane lamellar spacing was increased by solution shearing, due to increased disorder of side chains. The ability to induce structural rearrangement of polymers through solution shearing allows for an easy and alternative method to modify OSC charge transport properties.
The backbone of diketopyrrolopyrrole‐thiophene‐vinylene‐thiophene‐based polymer semiconductors (PSCs) is modified with pyridine (Py) or bipyridine ligands to complex Fe(II) metal centers, allowing ...the metal–ligand complexes to act as mechanophores and dynamically crosslink the polymer chains. Mono‐ and bi‐dentate ligands are observed to exhibit different degrees of bond strengths, which subsequently affect the mechanical properties of these Wolf‐type‐II metallopolymers. The counter ion also plays a crucial role, as it is observed that Py‐Fe mechanophores with non‐coordinating BPh4– counter ions (Py‐FeB) exhibit better thin film ductility with lower elastic modulus, as compared to the coordinating chloro ligands (Py‐FeC). Interestingly, besides mechanical robustness, the electrical charge carrier mobility can also be enhanced concurrently when incorporating Py‐FeB mechanophores in PSCs. This is a unique observation among stretchable PSCs, especially that most reports to date describe a decreased mobility when the stretchability is improved. Next, it is determined that improvements to both mobility and stretchability are correlated to the solid‐state molecular ordering and dynamics of coordination bonds under strain, as elucidated via techniques of grazing‐incidence X‐ray diffraction and X‐ray absorption spectroscopy techniques, respectively. This study provides a viable approach to enhance both the mechanical and the electronic performance of polymer‐based soft devices.
Incorporation of metal–ligand coordination bonds into polymer semiconductors is able to simultaneously improve thin film deformability and charge transport efficiency. Such metal‐coordination bonds prove to be both dynamic and reversible under external forces, indicating that PSCs with metallated mechanophores are promising candidates for future high performance mechanically robust electronics.
The mobility and glass transition temperature (T g) for polymers under nanoscale confinement differ substantially from the bulk. Whereas many studies have focused on the one-dimensional confinement, ...it has great significance to extend studies to higher geometries. Here, we systematically investigate the mobility by dipolar-filter sequence in solid-state NMR and T g by DSC for thiolated polystyrene (PS-SH) on gold nanoparticles. The increase in T g and signal suppression in NMR spectra clearly indicate that the surface confinement dominates molecular mobility as well as T g. The molecular weight of PS-SH and nanoparticles size show significant influence on the immobilization and T g. Our results can be fitted with a core–two shell model; the inner shell is under strong constraints while the outer shell with less confinement. This work is essential to better understand the confinement effect and also provides a step toward the ultimate desire to tailor the properties of nanomaterials.
<正>The problems to efficiently and robustly pack the objects together have been studied for centuries due to their mathematic aesthetics and direct applications to understand the myriad phase ...structures compacted from spherical metallic atoms to other polyhedral building blocks.For the simplest polyhedron,rigid tetrahedra were shown to form crystalline and quasi-crystalline phases by analytical calculations or computer simulations if only packing or shape
The perspective by Ediger and Forrest stated that, while we know that the dynamics of polymers in ultrathin films can be significantly altered by substrate interfaces, our understanding of how this ...depends on the polymer structure and the particular interfaces is rudimentary. Here, we show that fluorescence nonradiative energy transfer (NRET) is an extremely sensitive method for characterizing the interfacial adsorption of polystyrene onto silicon dioxide, even though their interaction is often suggested to be weak. We observed that tensile stress was generated in the supported film by substrate adsorption, which imposes constraints on molecular motion and prevents a reduction of the glass transition temperature (T g). Furthermore, our investigation suggests that modifying the surface chemistry of the substrate can change the film conformation and dynamics when the film is thinner than 40 nm.
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