Understanding the conformation effect on force-induced melting is important for developing advanced semicrystalline polymer materials. Here, two types of polymer single crystals, polycaprolactone (zigzag conformation) and poly­(l-lactic acid) (helical conformation), have been selected to study the conformation effect on force-induced melting by using the atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS). We find that the zigzag chains facilitate the stick–slip motion, and the single helical chain takes smooth motion during force-induced melting from the single crystals. Furthermore, we illustrate that the conformation acts on the force-induced melting by defining the interaction in between the adjacent stems. This SMFS study deepens our understanding on the relationship between the chain conformation and nanomechanical properties of polymer crystals.

Chain folding is a motif of polymer crystallization, which is essential for determining the crystallization kinetics. However, the experimental quantification of the chain folding remains a challenge because of limited instrumental resolution. Here, we quantify chain folding in solution-grown single crystals by using atomic force microscopy (AFM)-based single-molecule force spectroscopy. The fingerprint spectrum of force-induced chain motion allows us to decipher the adjacent and nonadjacent re-entry folding with spatial resolution of subnanometers. The average fractions of adjacent re-entry folds ⟨f⟩ are in the range 91−95% for polycaprolactone, poly-L-lactic acid, and polyamide 66, which is higher than the values determined by other classical technologies. The established single-molecule method is applicable to a broad range of crystalline polymer systems with different chain conformations or compositions.

Understanding the side-chain effect on the mechanical properties of conjugated-polymer nanocrystals is critical for the design of high-performance soft electronic devices. However, due to the complexity of the side-chain effect and hierarchical structure in bulk materials, the classical macroscopic characterizations mask the mechanical properties of nanocrystals. Here, we have measured the mechanical properties of crystalline poly­(3-alkylthiophene) (P3AT) nanofibers with different side-chain lengths (with 4, 6, 8, 10, and12 alkyl carbons per side chain) by the combination of thermal shape fluctuation analysis and single-molecule force spectroscopy (SMFS). We found that poly­(3-hexylthiophene) (P3HT, with 6 alkyl carbons) nanofibers exhibit a higher Young’s modulus and strength than poly­(3-butylthiophene) (P3BT, with 4 alkyl carbons), poly­(3-octylthiophene) (P3OT, with 8 alkyl carbons), poly­(3-decylthiophene) (P3DT, with 10 alkyl carbons), and poly­(3-dodecylthiophene) (P3DDT, with 12 alkyl carbons) nanofibers. Furthermore, we illustrated that the more pronounced J-aggregate in P3HT nanofibers increases the probability of the one-step unfolding in SMFS experiments producing high strength and high Young’s modulus. The effects of the regioregularity of the polymer chain and the width of the nanofiber on the Young’s modulus of P3AT nanofibers were also investigated. Our results explain how the side-chain length affects the nanomechanical properties of P3AT nanofibers and deepen our understanding of the relationship between the chemical composition and nanomechanical properties of conjugated polymers.

Correlating the structure with nanomechanical property of semicrystalline conjugated-polymer crystal is of essential importance for the performance improvement and design of flexible electronic devices. Although it is well-known that the semicrystalline conjugated-polymer crystal exhibits anisotropic structure owing to the π–π and layer stacking of highly coplanar conjugated backbones, the structure–nanomechanical property relationship is missing. Here, we investigated the axial mechanical anisotropy of the P3HT nanofiber by using thermal shape-fluctuation analysis and a three-point bending test based on atomic force microscopy. Our results show that Young’s modulus in the layer-stacking direction (E L) is 1–2 orders of magnitude greater than that in the π-conjugated backbone direction (E B). We attribute this mechanical anisotropy to the π-stacking of the P3HT backbone, but the layer stacking will decrease E L, which weakens the mechanical anisotropy. Moreover, we demonstrated that the P3HT nanofiber shows a loading-rate-independent Young’s modulus and deformation-dependent resilience in the layer-stacking direction.

Exploring the relationship between the polymer structure and property/ function is a long-lasting topic in polymer science since relevant research is critical for the rational design of polymer materials or the control of biological functions. Due to the complexity of the real material and biological systems, it is quite difficult to establish such a relationship using traditional ensemble measurements. Single-molecule force spectroscopy (SMFS), which can be used to manipulate individual polymer chains, is a powerful technology for investigating the inter-or intramolecular interactions, chain structures and their dynamic behaviors with piconewton force and subnanometer spatial precision at the single-molecule level. In this Perspective, focusing on atomic force microscopy (AFM)-based SMFS, we first highlight recent advances in the AFM-SMFS study of mechanical properties in polymer systems, including the structure−mechanics relationships in polymer single crystal, covalent mechanochemistry of polymer, and polymer−nanoparticle composites. Then we highlight the recent advancement in the exploration of higher-order structures and their dynamic conversion in DNA using SMFS. We emphasize the prominent role of the application expansion of SMFS, innovation of experimental methods and sample preparation. At the end, future challenges and opportunities on single-molecule manipulations of polymers are discussed. We hope that this Perspective will also attract further attention from the material, biological, and computational research community, which would speed up our understanding on the structure−property (function) relationship of polymers.