Phases, soft

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Thermoplastic elastomers that are hard polymer/elastomer combinations are often not truly synthesized. Instead, the two polymers that form the hard and soft phases are intimately mixed on high shear equipment. 

The soft phase is dynamically vulcanized, ie, cross-linked during mixing (32). 

The minimum service temperature is determined primarily by the Tg of the soft phase component. Thus the SBS materials ctm be used down towards the Tg of the polybutadiene phase, approaching -100°C. Where polyethers have been used as the soft phase in polyurethane, polyamide or polyester, the soft phase Tg is about -60°C, whilst the polyester polyurethanes will typically be limited to a minimum temperature of about 0°C. The thermoplastic polyolefin rubbers, using ethylene-propylene materials for the soft phase, have similar minimum temperatures to the polyether-based polymers. Such minimum temperatures can also be affected by the presence of plasticisers, including mineral oils, and by resins if these become incorporated into the soft phase. It should, perhaps, be added that if the polymer component of the soft phase was crystallisable, then the higher would also affect the minimum service temperature, this depending on the level of crystallinity. 

Oil resistance demands polar (non-hydrocarbon) polymers, particularly in the hard phase. If the soft phase is non-polar but the haid phase polar, then swelling but not dissolution will occur (rather akin to that occurring with vulcanised natural rubber or SBR). If, however, the hard phase is not resistant to a particular solvent or oil, then the useful physical properties of a thermoplastic elastomer will be lost. As with all plastics and rubbers, the chemical resistant will depend on the chemical groups present, as discussed in Section 5.4. 

Most of the thermoplastic elastomers can be produced in a wide hardness range without resort to additives. If it is practical to use soft and hard phases in any proportions, then the hardness range will be from that of the soft phase... 

The density of the polymer will clearly depend on the density of the soft phase (usually low), and the density of the hard phase (generally higher with crystallisable polar blocks) and the ratio of the soft and hard phases present. It will also clearly depend on the additives present and to some extent on the processing conditions, which may affect the crystalline morphology. 

Type Soft phase T, ro Hard phase Pg or T ro Oil resistance Hardness range Specific gravity... 

The morphology of the ABA-type linear block copolymers is strongly influenced by the volume fraction of the two components. For example, in PS-EB-PS-type block copolymer as the volume fraction of PS is increased, the shape of the dispersed PS phase changes from spherical (comprising body-centered cubic spheres of PS dispersed in continuous soft phase) to cylindrical form (hexagonal packed cylinders of PS) [10,133,134]. When the volume fraction of the two phases... 

The elasticity of thermoplastic polyurethane rubbers (which are also known as thermoplastic urethanes or TPUs) is a function of their morphology which comprises hard and soft phases. The hard phases consist of hydrogen bonded clusters of chain segments, which are linked by flexible chain segments that make up the soft phase. The hard blocks, which are the minor phase, exist as separate domains within a continuous matrix of the majority soft phase, as shown schematically in Fig. 25.9. 

Tor instance crystalline phase, amorphous phase, hard phase, soft phase, phases formed by different polymeric components in blends or block copolymers. 

Again, crystallinity may be replaced by hard phase fraction , soft phase fraction , or whatever designation applies better to die material that is studied. 

The Material of the Example. Poly(ether ester) (PEE) materials are thermoplastic elastomers. Fibers made from this class of multiblock copolymers are commercially available as Sympatex . Axle sleeves for automotive applications or gaskets are traded as Arnitel or Hytrel . Polyether blocks form the soft phase (matrix). The polyester forms the hard domains which provide physical cross-linking of the chains. This nanostructure is the reason for the rubbery nature of the material. 

In many cases the broader distribution can be attributed to the amorphous (or soft) phase. Even higher significance of the assignment can be achieved if the material is studied in time-resolved SAXS experiments during processing (under thermal load, mechanical load). Thus it is not always necessary to resort to secondary methods112 in order to resolve the ambiguity inherent to Babinet s theorem. 

Note 2 The interfacial interaction between hard and soft phase domains in a thermoplastic elastomer is often the result of covalent bonds between the phases and is sufficient to prevent the flow of the elastomeric phase domains under conditions of use. Note 3 Examples of thermoplastic elastomers include block copolymers and blends of plastics and rubbers. 

Figure 8. Stable state ofa hard-soft-hard lamellar structure. A soft phase with e = 0.01 is sandwiched between two hard phases. Each layer has 50 ML, and the periodic length is equal to 200 a.

Such materials essentially contain PS as the matrix polymer and uniformly dispersed in this matrix are elastomeric types of particles, which form the soft phase (3). The soft phase is essentially composed of poly(butadiene) or of block copolymers of butadiene and styrene. This soft phase can be also addressed as the impact modifier for PS. 

The monomers for HIPS fall into two groups, i.e., the monomers for the matrix resin and the monomers for the soft phase. Monomers are summarized in Table 9.1 and in Figure 9.1. 

The impact strength increases almost linearly with gel content and thus with the degree of crosslinking (17). Figure 9.3 shows the increase of the molecular mobility with the impact strength for ABS. For HIPS it is claimed that the situation is quite similar. The molecular mobility of the soft phase particles is determined by nuclear magnetic-resonance spectroscopy relaxation measurements (16). 

Although the intrachin energy contribution of the soft phase in polyurethanes is negative, the internal energy increases upon extension (Fig. 10). Moreover, the thermomechanical behaviour of the segmented block copolymers and a typical... 

From the molecular mobilities of the soft and hard phases, it appears that the increase in phase mixing caused by crosslinking, is more important in determining phase mobility than a simple increase in crosslink density. The fraction of protons in the hard phase (f) is relatively independent of crosslink density at 28 °C. However, f in linear and crosslinked PEU have different temperature behaviors (Fig. 12). Incurred PEU exhibits a single FID below —20 °C and again above 80 °C, when both phases are below and above T, respectively. In the region between approximately 10 and 75 °C, at which temperatures the hard and soft phase undergoes a transition, respectively, the fraction of protons in hard domains remains relatively constant. However, f in the crosslinked PEU decreases in a continuous fashion over a relatively... 

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