Thermoplastics flexible-chain

Both polymers are linear with a flexible chain backbone and are thus both thermoplastic. Both the structures shown Figure 19.4) are regular and since there is no question of tacticity arising both polymers are capable of crystallisation. In the case of both materials polymerisation conditions may lead to structures which slightly impede crystallisation with the polyethylenes this is due to a branching mechanism, whilst with the polyacetals this may be due to copolymerisation. 

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. 

Thermoplastic tri-block copolymers are interesting since they possess novel properties different from those of the homo- or copolymers. The thermoplastic elastomers have many of the physical properties of rubbers, i.e., softness, resilience, and flexibility. The unique properties of this kind of copolymer are due to the microphase separation of the hard crystalline domains dispersed in a continuous amorphous matrix (Fig. 6). Such phase morphology provides a physical network of flexible chains cross-linked by crystalline microdomains. The advantages over natural vulcanized rubbers are that thermoplastic elastomers are readily soluble in an appropriate solvent and can be processed as thermoplastics [109],... 

High-strength and temperature-resistant polymers Styrene-butadiene-styrene, triblock polymer, thermoplastic elastomer Crystalline domains with rigid chains between them and cross-finking chains Rigid-chain domains in a flexible-chain matrix... 

At least two good ways out exist. One consists of the dispersion at the molecular level of rigid polymer molecules between flexible chains - the concept of molecular composites of Helminiak, Hwang e.a.(2, 4j. The other involves the use of polymer liquid crystals (PLCs). As discussed by Witt (5), compared to widely used engineering thermoplastics, PLCs show clear... 

One method of classifying plastics is by their response to heat. Thermoplasts, also known as thermoplastic polymers, soften and liquefy on heating and harden again when cooled. The process is reversible and can be repeated. On heating, the weak secondary bonds between polymer chains are broken, which facilitates relative movement between the chains. If the molten polymer is further heated until the primary covalent bonds also break, degradation of the thermoplast follows. Thermoplastic polymers are linear or exhibit branching with flexible chains and include polyethylene, polystyrene and polypropylene (Figure 4.10). 

An interesting form of cross-Hnking is found in what are called thermoplastic rubbers. These are block copolymers of a flexible rubber chain with a less flexible chain that is a glass at room temperature. Usually there are two blocks of hard polymer separated by a block of flexible polymer. These have already been described in Chapter 2. A typical example contains first a chain of polystyrene units connected to a chain of polyisoprene units, in turn connected to a second chain of polystyrene. Now polystyrene and polyisoprene are not miscible, so the material undergoes a phase separation into polystyrene and polyisoprene phases. 

The relationship simply indicates that for prediction purposes, the magnitude of complex viscosity is comparable with that of shear viscosity at equal values of frequency and shear rate. The relationship has been found to hold largely for flexible-chain thermoplastic melts, particularly in the lower and intermediate ranges of a> and V Cabining Eqs. (2.43) and (2.51), the following expression for the complex viscosity t based on a modification of the Carreau model is written... 

A new type of polymer-polymer composites, the microfibrillar reinforced composites (MFC) from thermoplastic polymer blends, was created about ten years ago. Unlike the classical macro-composites e.g., glass fiber-reinforced ones) and the in situ composites (TLCP rod-like macromolecules and mostly their aggregates as reinforcing elements), the MFC are reinforced by microfibrils of flexible chains. The microfibrils are created during the MFC manufacturing. 

Thermoplastics soften when heated (and eventually liquefy) and harden when cooled — processes that are totally reversible and may be repeated. On a molecular level, as the temperature is raised, secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of adjacent chains is facilitated when a stress is applied. Irreversible degradation results when a molten thermoplastic polymer is raised to too high a temperature. In addition, thermoplastics are relatively soft. Most linear polymers and those having some branched structures with flexible chains are thermoplastic. These materials are normally fabricated by the simultaneous application of heat and pressure (see Section 15.22). Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride). 

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