Thermoplastic single-phase

Wallace J.G., Single phase melt processible rubber. Handbook of Thermoplastic Elastomer (Walker B.M. and Rader C.P., eds.). Van Nostrand Reinhold, New York, 1988, 141. 

Thermoplastic elastomers, e.g., polyurethane elastomers, have an entirely different structure that is heterogenic. They have an elastomeric matrix and phase-separated hard blocks, which act as embedded physical crosslinking sites. The hard blocks can be softened at elevated temperatures to obtain a single-phase melt that is easily processed. Upon cooling, the two-phase nature is recovered and the material becomes a solid again. 

In their processible mixture, these one-component products, also belonging to physically setting adhesives, consist of two components PVC (polyvinyl chloride-) particles and plasticizers (Section 9.2.9). The solid PVC particles are disperged in the high-viscosity plasticizer. The adhesive layer formation occurs by heating (120-180 °C), when the thermoplastic PVC swells and is thus able to absorb the plasticizer (no chemical reaction ). This process is called a sol-gel process. The formerly two-phase system (sol) is turned into a single-phase system (gel) by the inclusion of the plasticizer. 

We have found that a wide range of metal salts will coordinate and chelate with polyethers to give single-phase thermoplastics. A common occurrence (1,2) in these systems is for the glass transition (Tg) to be highly elevated—up to 140°C in some cases. This aspect of the subject of molecularly dispersed salts in polymers can be found in Ref. 2. 

Miscible blends of elastomers differ from corresponding blends of thermoplastics in two important areas. First, the need for elastic properties requires elastomers to be high molecular weight. This reduces both the kinetic rate and the thermodynamic driving force for the interdiffusion and thus formation of a miscible single phase of dissimilar elastomers. Second, elastomers are plasticized in conventional compounding with process oils. The presence of plasticizers leads to both a higher free volume for the blend components and a decrease of the endothermal interactions. 

The polymers used to produce blends may be thermoplasts, thermosets, or elastomers. A monophase system is often sought in blends of two elastomers after vulcanization, both components form a single-phase network. In the ideal case of an interpenetrating network (IPN), a network of one component is first formed. This network is then swollen with the monomer of the second component. After cross-linking polymerization of the second component, two mutually interpenetrating but still mutually independent networks are produced. 

Solvent welding and solvent cementing are widely used techniques for the joining of thermoplastic polymers. A small amount of solvent along the joint promotes interdiffusion of polymer chains between the substrates. This creates a permanent weld, with no additional phases or potentially weak interfaces. A solvent cement is a solution of the polymer being joined. This also promotes interdiffusion, and leaves a single-phase joint when the solvent has evaporated. 

Polymers in the category of engineering resins and plastics may be classified in several ways. They may be thermoplastic or thermoset. They may be crystalline or amorphous and they may be single phase or multiphase systems. This would allow for eight types of materials except that thermosets, because of their irregular cross-linked structure, are never crystalline. Single phase polymers do not have discemable second phase structures of different chemical composition. Thus homopolymers and random copolymers are single phase polymers, even if they are semicrystalline and so contain amorphous and... 

Much information can be obtained by microscopy of crystalline thermoplastics, whereas microstructural study of single phase amorphous materials is not usually of much practical interest. This is why most microscopy studies of single phase polymers relate to crystalline materials, and amorphous polymers are mostly described in multiphase systems. 

Multiphase or multicomponent polymers can clearly be more complex structurally than single phase materials, for there is the distribution of the various phases to describe as well as their internal structure. Most polymer blends, block and graft copolymers and interpenetrating networks are multiphase systems. A major commercial set of multiphase polymer systems are the toughened, high impact or impact modified polymers. These are combinations of polymers with dispersed elastomer (rubber) particles in a continuous matrix. Most commonly the matrix is a glassy amorphous thermoplastic, but it can also be crystalline or a thermoset. The impact modified materials may be blends, block or graft copolymers or even all of these at once. 

There is another group of polyurethanes that is chemically crosslinked with a crosslinker, either triol or polyamine or polyisocyanate. They are single-phase elastomers, and they display lower strengths than the thermoplastic urethanes. However, their properties are less temperature sensitive, and elastic recovery is generally considerably better (permanent set is smaller) than in TPUs. Their strength can be improved by adding proper fillers. Such systems are called cast systems since they are processed by casting... 

Polymer blends are physical mixtures of polymers and provide a means of combining the useful properties of the constituent components to achieve an economic or property advantage. There are a number of commercial thermoplastic blends such as PPO/PS, ABS/PC, PVC/PMMA, and so forth currently in use [28], as summarized in Table 1.7. In polymer blends, the individual polymers are chemically different and do not form covalent bonds as in copolymers. The blends are often characterized by their phase behavior as being either miscible or immiscible. Certain blends are completely miscible and form a single phase, whereas others form domains rich in one polymer dispersed within the matrix of the second polymeric component. The degree of thermodynamic compatibility... 

Teng K, Chang F. Single-phase and multiple-phase thermoplastic/thermoset polyblends 1. Kinetics and mechanisms of phenoxy/epoxy blends. Polymer 1993 34(20) 4291-9. 

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