H-Bonded Amorphous Polymer Glass Nanofibers with Simultaneously Large Enhancement in Strength, Toughness, Ductility, and Thermal Stability

Understanding the direct effects of hydrogen bonding (H-bonding) on the mechanical response of glassy state polymers remains a challenge, largely owing to the use of semicrystalline polymers in such studies, wherein, separating the contributions due to the changes in degree of crystallinity and direct H-bonding effects on the observed mechanical properties is extremely difficult. This limitation is overcome by studying the mechanical behavior of amorphous, glassy polyvinylpyrrolidone (PVP), H-bonded with tannic acid (TA). Uniaxial tension experiments on individual submicron scale PVP fibers (0.6-1.0 mu m diameters) with different TA concentrations are conducted to capture their elasto-plastic deformation characteristics. Amorphous PVP-TA complexes exhibited some distinctive trends in elasto-plastic properties with increasing H-bond density, which are not observed earlier. These distinctive trends are discussed in terms of the competing effects of H-bond driven crosslinking and TA-induced reduction in entanglement density, and their differing roles during elastic and plastic deformation. The improvements in elasto-plastic properties (particularly ductility and toughness) are simultaneously accompanied by uniquely significant thermal stability (i.e., glass transition temperature, Tg) improvements, that can help improve the practical applicability, operational life, and performance of these materials. H-bonding is shown to significantly improve the mechanical properties of an already strong, tough, and glassy polymer, PVP, in the nanofiber form. It is shown that properties like strength, toughness, and thermal stability, which are commonly considered orthogonal, can all be simultaneously improved, thus opening new material design spaces for advanced applications.image