Structure And Properties Investigation Of Polyurethane With Combined Soft Segments Composed By Polytetrahydrofuran And Polylactic Acid

Generally,the degree of microscopic phase separation of aliphatic polyurethane containing polyether soft segment is very small,resulting in the poor mechanical properties and cyclic tensile properties.Polylactic acid(PLA)is a bio-based degradable polyester material with high strength and good biodegradability and excellent biocompatibility.In this study,we have introduced PLA successfully into the main chain of polyurethane to improve the mechanical properties and cyclic tensile properties.In addition,the relationship between structures and properties of PLA-based polyurethanes are also invesigated.In this investigation,polydiol of poly(L-lactic acid)(PLLA)has been synthesized from L-lactide(L-LA).Then,the PLLA and polytetrahydrofurandiol(PTMG)have been used as the soft segment,while4,4’-dicyclohexylmethane diisocyanate(HMDI)and 1,4-butanediol(BDO)have been used as the hard segment for the synthesis of polyurethane via a two-step process.By infrared analysis(FT-IR),nuclear magnetic resonance spectroscopy(~1H-NMR),differential scanning calorimetry(DSC),thermogravimetric analyzer(TGA),etc.,we found that the Tg and Tm of the polyurethanes increased with the increase of PTMG segment lengths or PLLA/PTMG ratios,and the introduction of the PLLA chain reduced the thermal stability of the resulting polyurethane.We characterized the mechanical properties of polyurethanes with varying PTMG segment lengths and PLLA/PTMG ratios.The results showed that the increase of PTMG chain lengths was beneficial to the crystallization of PLLA chain and enhanced the mechanical properties of the resulting polyurethanes.The introduction of PLLA chain would enhance the elastic modulus and tensile strength of the polyurethanes,while the toughness of the material was reduced.When the cyclic tensile properties of polyurethanes with varying PLLA/PTMG ratios were characterized,the polyurethane containing 9%PLLA showed the most excellent properties.Through the microstructure analysis,it was found that the polyurethanes before stretching contained two types of hydrogen bonding,which were contained in the hard phase region and the amorphous phase region,respectively.During the stretching process,the previous one was stable and the latter one could be easily destroyed.At the same time,the PLLA crystal was broken and the PLLA chain oriented along the stretching direction,which improved the mechanical properties.This implied that the orientation of the PLLA chain in the stretching direction would cause stress hardening of the material.During the stretching process,the PLLA-based polyurethanes showed stress whitening at large strain.We found that the value of critical strain for stress whitening gradually decreased with the increase of PLLA/PTMG ratios,but the value of critical true stress remained about 100 MPa.The in-situ WAXS data showed that there was no crystal in the polyurethanes at large strain,and the PLLA chain highly oriented along the stretching direction.When the degree of orientation was around-0.17 which was not affected by PLLA/PTMG ratios,the stress whitening phenomenon appeared.The in-situ SAXS data showed that the scattering intensity in the meridian direction and the equatorial direction increased sharply when the phenomenon of stress whitening occurred.It indicated that the essence of stress whitening at large strain was the appearance of the cavity structure,and after removing the stress the scattering intensity decreased,suggesting the disappearance of the cavity structure.Finally,the specimens recovered to transparency after removing the stress.