Research On The Single-Chain Mobility In Semi-Crystalline Polymer Materials

Semicrystalline polymeric materials have gained significant attention due to their unique crystalline-amorphous network structures,which provides self-adjusting capabilities during deformation.These materials exhibit numerous advantages such as high strength,high modulus,lightweight,and cost-effectiveness,making them indispensable in various everyday applications.While significant research efforts have been devoted to studying the comparatively ordered crystalline regions,the characterization of the relatively disordered amorphous regions still poses challenges.Atomic force microscopy(AFM)-based single-molecule force spectroscopy(SMFS)has emerged as a promising technique for investigating the nanomechanical properties of polymer single crystals.It has successfully established mechanical fingerprints based on diverse polymer systems,laying the foundation for studying nanomechanical properties in more complex systems.In this study,we employed an AFM-based ambient SMFS method to explore the single-chain mobility within the crystalline-amorphous networks.Additionally,we investigated the factors influencing the single-chain mobility in semi-crystalline polymer materials.Finally,by linking the microscale structural features to the macroscopic mechanical properties of the bulk materials,we established a novel understanding of the relationship between material structure and performance.This work is organized into the following four sections:1.Exploration of the influence of chain dynamics on chain mobility in semi-crystalline polymer materials.Theα_c relaxation phenomenon dominated by chain dynamics has an important influence on the ultra-stretching properties of the semi-crystalline materials.We chose polyethylene single crystals as the object of study forα_c relaxation in the crystalline region,and two kinematic activation processes were resolved through three unfolding pathways.The chain flip and chain diffusion motions are accurately distinguished and quantified,and the dynamic distributions are plotted at different temperatures and stretching velocities.Theα_c relaxation was found to enhance energy dissipation,facilitate chain mobility,and improve material extensibility during the deformation process.2.The study of chain trajectories and chain mobility in polymer spherulites.Polymer spherulites,representing the most common condensed state structure in polymers,were chosen as the target of investigation.Utilizing the AFM-based SMFS technique,the behaviors of polymer chain were identified,and chain trajectories within the spherulites were reconstructed.The influence of different chemical compositions and chain conformations on chain mobility in the spherulites was studied.The results revealed that the enthalpic effect not only impacts the complexity of polymer chain trajectories but also influences the kinetics of cross-lamellea slipping.In addition to the synergistic strengthening effect governed by helical conformations,the chain trajectories and chain mobility of polymers are predominantly determined by the inherent enthalpic effect of the polymers themselves.Finally,the amorphous regions of nylon 6 and poly(lactic acid)spherulites were chosen as models to investigate their glass transition behavior,revealing its significant influence on chain mobility,material strength,and modulus.3.The study of the density and utilization of tie molecule and chain mobility in polymer fibers.Through AFM imaging,we determined the microfiber and shish-kebab structures at different scales within the fibers.We successfully extracted individual polymer molecules along the fiber axis and estimated the densities and utilization of tie molecules based on the study of chain behavior and trajectories within the crystalline-amorphous network of the fibers.Finally,the migration properties of single chains in polymer fibers are explored.4.The study of the molecular mechanisms underlying the mechanical properties of bulk materials.We conducted tensile tests on several polymers’bulk materials above their respective glass transition temperatures.By combining AFM characterization with the structural evolution during plastic deformation,we unveiled the microscopic mechanisms governing the macroscopic performance.The results revealed that the fundamental nature of chain mobility reflects the molecular chain’s ability to undergo motion during the deformation process,which ultimately determines the self-adjusting capabilities of the crystalline-amorphous network and significantly impacts the tensile properties of semicrystalline materials.