Thermoplastic Polyurethane Elastomers Under Uniaxial Deformation
The special rubber elastic properties of thermoplastic elastomers are the result of their two- or multi-phase nanostructure consisting of hard domains in a soft matrix. The hard domains form physical cross-links and make the material behave rubber elastic
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Thermoplastic Polyurethane Elastomers Under Uniaxial Deformation
The special rubber elastic properties of thermoplastic elastomers are the result of their two- or multi-phase nanostructure consisting of hard domains in a soft matrix. The hard domains form physical cross-links and make the material behave rubber elastic. The soft phase between two hard domains is called soft domain (Fig. 4.1).The formation of nanostructure during manufacture is initiated by phase separation, while the polymer melt is cooling down. Phase separation occurs, because the polymer chains are composed from at least two blocks of different chemical composition that are immiscible at the service temperature of the material. Block copolymers synthesized by living polymerization are characterized by uniform block lengths. Processing of such compounds may result in lattice-like nanostructures or even in photonic crystals, in which the soft and the hard blocks reside completely in different domains. This is different with thermoplastic polyurethanes (TPU). Along their chains a mixed sequence of soft segments and hard segments is found. Thus, even optimum process control only leads to domains of very diverse shape and size, the arrangement of which can rarely lead to lattice-like correlation. Moreover, soft domains may contain several hard segments. Consequently, the chemistry [1] and the processing conditions [2–5] define the nanostructure that, in turn, determines the material’s mechanical properties. In this chapter we study oriented injection-molded TPU materials during tensile test. In doing so, the focus is on the still little investigated [2–5] effect of processing conditions on the structure and the properties of the material.
4.1 Optical Microscopy Figure 4.2 presents optical micrographs of the injection-molded materials. The material processed at 205 ◦ C shows a high fraction of spherulitic material. In the material processed at 215 ◦ C the fraction is considerably lower. In the material made from the hottest melt no spherulites are detected. Generally spherulites grow during polymer A. Zeinolebadi, In-situ Small-Angle X-ray Scattering Investigation of Transient Nanostructure of Multi-phase Polymer Materials Under Mechanical Deformation, Springer Theses, DOI: 10.1007/978-3-642-35413-7_4, © Springer-Verlag Berlin Heidelberg 2013
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4 Thermoplastic Polyurethane Elastomers Under Uniaxial Deformation
Fig. 4.1 Schematic presentation of microstructure of TPU materials. TPU chains are formed by alternating sequences of hard and soft segments. Upon cooling from the melt state, TPU materials undergo phase separation resulting in a (multi)two-phase morphology. The hard domains act as physical cross-links. The soft matrix between two alternating hard domains is called soft domain
205°C
215°C
235°C
Fig. 4.2 Optical micrographs of cross sections (10 µm thickness, cryo-microtome − 70 ◦ C) from the central zone of injection molded test bars. Labels indicate the temperature of the injected melt
crystallization. Spherulites consist of alt
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