Reinforcing And Functioning Modificaiton On The Polymer Electrospun Membranes

Nanofiber/membranes prepared by electrospinning method are of unique physicalproperties, such as high aspect ratio of nanofiber, large specific area and excellentmorphology. Such membranes have been widely applied in the fields of filtration, tissueengineering scaffold, catalysis, sensors, and protective clothing. However, mechanicalstrength of the electrospun membranes is always lower than the corresponding castingfilms. As a result, the application of the electrospun membranes will be limited.Nowadays, there are many methods in reinforcing the electrospun membranes,including forced orientation of the fibers, enhancement of the interlocking of thenanofibers by solvent vapor treatment, and improvement of the tensile strength of thesingle fibers by using carbon nanotubes, whiskers or inorganic nanoparticles. In thisthesis, some novel methods and new materials were used to strengthen the electrospunmembranes. Besides, some new phenomena were also found.It has been reported that the tensile strength of PA6single fibers could be60-400MPa. However, the tensile strength of PA6electrospun membranes was less than8MPa.It was speculated that the slippage between the PA6nanofibers might be attributed tosuch phenomenon. To confirm this hypothesis, polyurethane (PU) was used to dosurface modification of the PA6nano-membranes. The results showed that theinterlocking of the PA6nanofibers was enhanced by the sticky PU, resulting greatlyimproved strength. Further more, the PA6electrospun membranes turned fromhydrophilic to super-hydrophobic. At the same time, the water vapor permeation abilitywas well preserved. In conclusion, it is a good method in preparation of water-proof andvapor permeation membranes with high strength.Polyhedral oligomeric silsesquioxane (POSS) is composed of a rigid core formedby Si-O-Si bonds and a periphery of organic substitute groups, which has great potentialand advantage in polymer reinforcement. In this work, the POSS was used as a novelnano-filler to modify PU electrospun membranes through physically blending. Since itforms a kind of polymer-particle dynamic network between POSS and the PU chains,the entanglements of PU chains inside the nanofibers are enhanced. Consequently, thePU chains can be stretched to their fully length when subjected to the external force. Asa result, not only the tensile strength of PU electrospun membranes was elevated, butalso the elongation at break was significantly improved. Such a phenomenon has been rarely reported in the field of reinforcing electrospun membranes.Layered double hydroxide (LDH) is a kind of sheet-like reinforcing material, witha large diameter to thickness ratio, huge surface area and high modulus. However, thereare only few reports on the LDH reinforcing electrospun membranes. In this work, thePU/LDH electrospun membranes were prepared by physical blending the twocomponents. The mechanical properties of the electrospun membranes with differentconcentrations of LDH were studied. The results showed that the introduction of LDHdid not reduce the spinnability of PU. Instead, the electrospinning process was smoother.It was found that the PU electrospun membranes could be strengthened by smallamounts of LDH. When the LDH content was only1wt%, the tensile strength of theelectrospun membrane increased to16.2MPa. The increment was about217%. Inaddition, the elongation at break of the electrospun membrane remained at a high levelof~165%.In this thesis, the electrospun membranes with better strength, toughness, andfunctionality were fabricated by surface modification, or physical blending with POSS,or physical blending with LDH. Subsequently, the structure and properties relationshipof the electrospun membranes was discussed in details. These modification methods notonly can be used in PA6and PU electrospun membranes reported here, but also can beapplied in the other polymer materials, or even can be applied in thenanofibers/membranes prepared by methods other than electrospinning. These researchworks are not only of good theoretical significance, but also of good practical values.