The Performance Of PU Films And PU Fibers Prepared By A Two-step Process Of Mixed Solvent

Polyurethane(PU)is a well-known linear segmented block macromolecular material consisting of hard segment(HS)and soft segment(SS),whose performance mainly depends on the phase structure and microphase separation degree of them.The hydrogenbonding interactions between urethane groups with strong polarity and rigidity forms the HS of PU,and polyether or polyester segment with low polarity are drawn together to form the SS of PU.Recently,there has been an extensive interest in study of spider silk because of its high strength and high toughness,which is mainly due to its intriguing structures,including complete recrystallization region,paracrystalline region and amorphous region.It is investigated that the swelling behavior of PU in various solvents seems to decrease the hydrogen-bonding interactions of HS,leading to lower strength and higher elongation at break of PU.In present work,the toluene was used for swelling PU,and the three phase structure of PU was simulated comparing the molecular structures of PU and spider silk.The results show that the strength,the elongation at break and the work at breaking of PU prepared via this method were significantly increased compared with those of PU prepared using pure DMF as a solvent.In this thesis,the mixed solvent system of toluene and DMF was employed for the preparation of PU.The main contributions of this thesis are listed as follows:FFTIRstly,PU films were prepared by changing the ration of toluene and DMF in the mixed solvent system.The analysis of the scanning electron microscope pictures indicates the PU films prepared via this method have relatively dense cross sections and similar mechanical properties.Secondly,PU films were prepared by a two-step process of mixed solvent.The structure and properties of these PU films were studied by scanning electron microscopy,infrared spectroscopy,thermal property,dynamic thermal mechanical property and mechanical property.The analysis of the scanning electron microscope pictures indicates that a more smooth surface and dense cross section were obtained due to the increase of the ration of toluene.FTFTIR reveals the peak at 3300cm-1removed to high wavenumber and was also broadened,which indicates that the hydrogen-bonding interactions between HS weredecreased with the increase of the ration of toluene.The DSC and DMA results show that lowest glass transition temperature of PU films was obtained when the ration of toluene was30%,demonstrating that the SS of PU turned to be softer.The mechanical properties of PU films show that the highest stress,tensile strain and work of breaking of PU films were obtained when the ration of toluene was 30%,which were 27.72 MPa,2034.18% and 10.25 J,respectively.These are 6.45 times,3.18 times and 8.47 times of those of PU films prepared in pure DMF.ThFTIRdly,PU fibers were prepared by a two-step process of mixed solvent.The structure and properties of these PU fibers were studied by scanning electron microscopy and mechanical property.The analysis of the scanning electron microscope pictures indicates the PU fibers prepared via this method have relatively dense cross sections due to the increase of the ration of toluene.The mechanical properties results show that the highest tensile strain of PU fibers were obtained when the ration of toluene was 30%.