Stereocomplex (SC) crystallites, formed between enantiomeric poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA), show a melting point 50 °C higher than that of PLLA or PDLA homocrystallites, which makes it possible for SC crystallites to be reserved in the melt of PLLA in asymmetric PLLA/PDLA blends and to act as a rheological modifier and a nucleation agent for PLLA. Herein, by a rheological approach, a transition from the liquid-like to solid-like viscoelastic behavior was observed for the SC crystallites reserved melt, and a frequency-independent loss tangent at low frequencies appeared at a PDLA concentration of 2.0 wt %, revealing the formation of SC crystallite network. By a delicately designed dissolution experiment, the structure of the formed network was explored. The results indicate that the network are not formed by SC crystallites connected directly with each other or by bridging molecules, but by the interparticle polymer chains which are significantly restrained by the cross-linking effect of SC crystallites. Nonisothermal and isothermal crystallization show that the reserved SC crystallites can accelerate remarkably the crystallization rate of PLLA due to heterogeneous nucleation effect. Besides, a special PDLA concentration dependence, e.g., the overall crystallization rate is almost independent of PDLA content for the blends with PDLA content higher than PDLA percolation concentration (2.0 wt %), was also observed. The increase of nuclei density for the blends containing PDLA from 2 to 5 wt % was estimated from POM observations. The result of an enhanced nucleation but an unchanged overall crystallization rate reveals the confining effect of the SC crystallite network on PLLA crystallization. This confining effect can be ascribed to the restrained diffusion ability of PLLA chains owing to the SC crystallite network.

Organic semiconducting donor–acceptor polymers are promising candidates for stretchable electronics owing to their mechanical compliance. However, the effect of the electron‐donating thiophene group on the thermomechanical properties of conjugated polymers has not been carefully studied. Here, thin‐film mechanical properties are investigated for diketopyrrolopyrrole (DPP)‐based conjugated polymers with varying numbers of isolated thiophene moieties and sizes of fused thiophene rings in the polymer backbone. Interestingly, it is found that these thiophene units act as an antiplasticizer, where more isolated thiophene rings or bigger fused rings result in an increased glass transition temperature (Tg) of the polymer backbone, and consequently elastic modulus of the respective DPP polymers. Detailed morphological studies suggests that all samples show similar semicrystalline morphology. This antiplasticization effect also exists in para‐azaquinodimethane‐based conjugated polymers, indicating that this can be a general trend for various conjugated polymer systems. Using the knowledge gained above, a new DPP‐based polymer with increased alkyl side chain density through attaching alky chains to the thiophene unit is engineered. The new DPP polymer demonstrates a record low Tg, and 50% lower elastic modulus than a reference polymer without side‐chain decorated on the thiophene unit. This work provides a general design rule for making low‐Tg conjugated polymers for stretchable electronics.

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.

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.

Intrinsic mechanical properties of sub-100 nm thin films are markedly difficult to obtain, yet an ever-growing necessity for emerging fields such as soft organic electronics. To complicate matters, the interfacial contribution plays a major role in such thin films and is often unexplored despite supporting substrates being a main component in current metrologies. Here we present the shear motion assisted robust transfer technique for fabricating free-standing sub-100 nm films and measuring their inherent structural–mechanical properties. We compare these results to water-supported measurements, exploring two phenomena: 1) The influence of confinement on mechanics and 2) the role of water on the mechanical properties of hydrophobic films. Upon confinement, polystyrene films exhibit increased strain at failure, and reduced yield stress, while modulus is reduced only for the thinnest 19 nm film. Water measurements demonstrate subtle differences in mechanics which we elucidate using quartz crystal microbalance and neutron reflectometry.