The Influence Of Volume And Cis-trans Structure Of Chain Extender To Structure And Property Of Thermalplastic Polyurethane Elastomer

For thermoplastic polyurethane elastomers,the stability of shape dimension or the recoverability after large strain strongly depends on the stability of microstructures.Interestingly,the hierarchical nature of polyurethane structure has hardly been mentioned.The type of hydrogen bonding,packing style of hard segment,degree of microphase separation and distance between hard domains etc.all together decide the macroscopic behavior of polyurethane elastomer.Since the stability of these microstructures is affected by different factors,their failure behavior could be very different under large strain elongation.In this study,we utilize a multiscale investigation to explore the influence of volume and cis-trans structure in chain extender to structure and property of thermalplastic polyurethane elastomer.Firstly,we choose four chain extenders,1,3-propanediol(PDO),2-methyl-1,3-propanediol(MPO),2,2-dimethyl-1,3-propanediol(NPG)and 2,2,4,4-Tetramethyl-1,3-cyclobutanedio(CBDO),and use exactly the same diisocyanate 1,4-phenylene diisocyanate(PPDI)and soft segment poly(tetramethylene glycol)(PTMG)to prepare four model samples.Based on our results from computer simulation,these chain extenders have similar oxygen distance between the two hydroxyl groups,but their occupied volumes increase accordingly.As expected,when the volume of chain extender increases the degree of microphase separation decreases.Except for CBDO,the mechanical properties of the other three samples show systematic changes.During the tensile break,the stability of hydrogen bonding decreases with the increase of volume of chain extender in FTIR test,but the stability of hydrogen bonding in CBDO-PU is comparable with in PDO-PU.We also use SAXS to explore the variation of phase structure,except for the blurry pattern of CBDO-PU,the phase structure become more stable as the increase of volume of chain extender correspondingly to the macroscopic resilience.These trend may be related with the property of single molecular chain in single molecular force spectroscopy,as the volume of chain extender increases,the elastic coefficient and the length of Kuhn segment decreases and increases,respectively.But the molecular chain of CBDO-PU with the highest elastic coefficient and the lower length of Kuhn segment is an exception,which may be related with its ring structure.We completely separate cis-CBDO from trans-CBDO with purity of 99% by recrystallization based on above results.Then we choose six chain extenders,0%-cis,20%-cis,40%-cis,60%-cis,80%-cis and 100% cis-CBDO,and use exactly the same diisocyanate PPDI and soft segment PTMG to prepare six model samples.After that we studied how the ratio of cis/trans in chain extender influence on the crystallization,mechanical property and the change of phase structure during the tensile break.The results may be not consistent with the assumption that the property and structure regularly vary with the content of cis-CBDO.There may be two reasons,one is that the molecular weight of TPU samples and the actual content of hard segment in TPU is low in whole and therefore this results in the bad macroscopic property,another is that four methyl groups in CBDO make the stacking of hard segments difficult and bring about the weak cohesion between hard segments,so these samples are easy to be destroyed.