Studies on the functional shape memory polyurethane nanofibers and nanofibrous nonwovens

Abstract

Fabricating shape memory polymer (SMPs) nanofibers is very important to the development of SMPs particularly in the nanoscales field. In this study, functional shape memory polyurethane (SMPU) nanofib ers are fabricated successfully using the electrospinning method from the SMPU/DMF solutions. In this thesis, the electrospinning parameters are investigated firstly for the fabrication of SMPU nanofibers; and the multi-jet electrospinning and co-axial electrospinning are also studied for the preparation of SMPU nanofibrous nonwoven. The results indicate that a low applied voltage results in better uniform nanofibers. The larger diameter nanofibers are formed at the higher feeding rate. The solution concentration plays a main role in affecting the diameter and morphology. The diameter increases with the increase of concentration linearly. The addition of ionic salt improves the electrospinability of spinning solution. Not only homogeneous SMPU nanofibrous nonwoven, but also SMPU composite nanofibrous nonwoven can be prepared using the multi-jet electrospinning. Additionally, coaxial electrospinning can be used to fabricate various core-shell nanofibers based on SMPU either as core materials or as shell materials. Accordingly, optimized electrospinning parameters and suitable electrospinning methods are provided to fabricate various SMPs nanofibers and core-shell nanofibers. Secondly, the unique properties of SMPU nanofib ers were investigated from thermal-properties, crystallization behaviors, thermal-shrinkage and thermal-induced shape memory effect, and water vapor permeability. The results show that the electrospun CLSMPU nanofibers have excellent crystallinabillity with high crystallinity and fast crystallization speed, e.g. only 1/26 recrystallization time of bulk film. Thermal-shrinkage occurs in the electrospun SMPU nanofibers. However, the thermal shrinkage can be eliminated with post thermal-treatment. Additionally, the unique morphology of nanofibers make the SMPU nanofibrous nonwoven have excellent shape memory properties, e.g. beyond 97% shape recovery. Importantly, the SMPU nanofibrous nonwovens have the excellent liquid moisture transfer properties and their water vapor permeability is also sensitive to the change of RH and temperature. These investigations not only enrich greatly our understanding about the relationship between morphology and structure of SMPs, but also it is very important to the applications of SMPs in the nanometer scale. Thirdly, by combining the unique properties of CLSMPU and PySMPU polymer, novel SMPU antibacterial nanofibers are functionalized successfully in this project. In addition to the good shape memory effect, the resulting PySMPU nanofibers, PySMPU/CLSMPU blended nanofibers and PySMPU-CLSMPU core-shell nanofibers all show excellent antibacterial activities against S. saureus and K. pneumoniae. Moreover, the shape recovery is improved greatly in the quaternized PySMPU nanofibers. The CLSMPU/PySMPU blended nanofibers and the CLSMPU-PySMPU core-shell nanofibers exhibit both good shape fixity and high shape recovery due to their high glass modulus of PySMPU material. Finally, antibacterial mechanism investigations show that the excellent antibacterial activity of PySMPU containing nanofibers is resulted from the combination of antibacterial activity of amido group in γ position of BINA unit and the high surface area per unit of nanofibers. Thus, this study provides a fascinating method to achieve good antibacterial activity in the PySMPU nanofibers by improving the surface area. Finally, some potential applications of SMPU nanofibers and PySMPU antibacterial nanofibers are explored and the future studies are suggested at the end of this thesis. Therefore, this study will promote the applications of nanofibers and SMPs in the medical and healthcare fields greatly. It will encourage the researchers to continuously develop the unique properties and explore the applications of SMPs in the nanometer scale with the electrospinning method.