Deformation induced morphological developments
The strain induced orientation of the PUs has been studied extensively by using a variety of experimental techniques [5, 21, 161, 162.169, 170, 174, 348–350].
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Deformation induced morphological developments
5.1 General considerations on strain induced orientation and crystallization developments There is nothing so stable as change (Bob Dylan)
The strain induced orientation of the PUs has been studied extensively by using a variety of experimental techniques [5, 21, 161, 162.169, 170, 174, 348–350]. Bonart was amongst the first to identify the orientation of the periodic microphase structure under tensile deformation. Bonart suggested that the hard domains are composed of laterally stacked HS. It was noted that in MDI polyether copolymers, the SS become fully extended along the stretch direction and begin to crystallize at strains of about 150%. In contrast, the HS were found initially to orient transverse to the stretch direction and ultimately to break up to allow orientation of the HS in the stretch direction [14]. Similar observations were made by us on DBDI based materials [67], as shown in the following section. The tensile alignment of initially randomly oriented SS chains exert a local torque force that orients the hard domains perpendicular to the direction up to 300% strain. As shown by Bonart, above 300% strain, the nature of the HS (crystalline or paracrystalline) strongly influences the deformation behaviour. For paracrystalline MDI ethylene diamine or MDI-hydrazine HS, further elongation resulted in the reorientation and restructuring of the hard domains along the stretching direction as the stress was transferred from the strain-induced crystalline (PTMO) or paracrystalline (mixed polyesters) SS. In the crystalline MDI-BDO hard domains (40wt% diisocyanate), the HS within the hard domains were found to orient along the equator up to 400% strain, which suggested that crystalline HS behave as inert fillers during deformation. Bonart also proposed that the HS reflections observed in diffraction patterns were due to the sterically-hindered lateral arrangement of hydrogen bonds between HS due to a pseudo crystalline lattice formation. The PUs mechanical behaviour has been investigated extensively and is intimately linked to the composition of the hard and soft domain and their resulting
C. Prisacariu, Polyurethane Elastomers, DOI 10.1007/978-3-7091-0514-6 5, c Springer-Verlag/Wien 2011
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5 Deformation induced morphological developments
morphology. The initial response to stress is elastomeric stretching of the SS matrix. As reported, strain-induced crystallization of the SS in the stretch direction occurs up to 200% elongation [4, 214]. A good connection between SS and HS due to covalent bonding can cause the hard domains to rotate as rigid units into the strain direction. At a critical strain, the hard domains shear yield. Restructuring of the hard domains occurs indicted by conservation of HS hydrogen bonding [141]. The irreversible deformation of the hard domains during the first loading is responsible for hysteresis in the stress-strain response and for the strain-softening which is typically observed when a PU is subjected to subsequent load/u
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