Crystallinity glass transition temperature, semicrystalline

Plastics can be divided according to their character into amorphous and crystalline. Crystallization is never complete and the so-called crystalline polymers are virtually semicrystalline ones. Examples of amorphous plastics are polystyrene, acrylonitrile-butadiene—styrene copolymers, styrene—acrylonitrile copolymers, polymethylmethacrylate, poly(vinyl chloride), cellulose acetates, phenylene oxide-based resins, polycarbonates, etc. Amorphous polymers are characterized by their glass transition temperature, semicrystalline polymers by both melting and glass transition temperatures. 

In contrast to the mature instrumental techniques discussed above, a hitherto nonexistent class of techniques will require substantial development effort. The new instruments will be capable of measuring the thermal (e.g., glass transition temperatures for amorphous or semicrystalline polymers and melting temperatures for materials in the crystalline phase), chemical, and mechanical (e.g., viscoelastic) properties of nanoscale films in confined geometries, and their creation will require rethinking of conventional methods that are used for bulk measurements. 

Although polymers exhibit both viscous and elastic responses at all temperatures, the elastic response is particularly strong at temperatures less than 50°C above the glass transition temperature, particularly for polymers well above their critical molecular weight. Polymers are often considered to have dominant viscous rheological responses if they are stressed at temperatures over 100 °C above the glass transition temperature for amorphous polymers or 100°C above the crystalline melting point for semicrystalline resins. 

Most addition polymers are thermoplastics that is, they are hard at room temperature but soften and eventually melt as they are heated. At low temperatures there is very little motion of the molecules and the polymer is glasslike and brittle. As the temperature of the polymer is raised, it passes through its glass transition temperature (Tg). Above J , more motion of the chains is possible and the polymer is a rubbery solid. Eventually, the polymer passes through its crystalline melting point (Tm) and melts to form a viscous liquid. Many semicrystalline polymers are most useful at temperatures between Tg and Tm. Both Tg and Tm increase as the crystallinity of the polymer increases and as the strength of the intermolecular forces between the polymer chains increases. The total intermolecular force increases as the length of the polymer chains increases. 

The heat performance of conventional ABS correlates in general with the glass transition temperature (Tg) of the rigid phase. Table 15.2 lists some typical rgs of amorphous polymers. Also listed are the crystalline melting points (Tm) for semi-crystalline polymers. Typically, the heat performance of a neat semicrystalline polymer under low load correlates with its Tm. 

Semicrystalline polymers are impact resistant if their glass transition temperatures are much lower than the test temperature. The impact strength of such materials decreases with increasing degree of crystallinity and particularly with increased size of supercrystallinc structures like spherulites. This is because these changes are tantamount to the progressive decrease in the numbers of tie molecules between such structures. 

Semicrystalline polymers must be considered two-phase mixtures of amorphous regions between lamellar crystals. It has been demonstrated that the yield stress increases with increasing crystallinity when the deformation process occurs at temperatures above the glass transition temperature of the amorphous phase and below, but close to, the melting... 

Aside textile and packaging applications the use of PET (Poly(ethylene Terephthalate) for structural applications is rather limited compared to equivalent polymers such as polyamides. Two main reasons can be given. Firstly, the high sensitivity of PET toward hydrolysis and its slow crystallisation kinetics constrain its processing. Secondly, its low glass transition temperature constrains its use if amorphous, whereas its weak impact resistance if semicrystalline constrains its use when crystallised. The industrial objective of this work deals with the latter of these points increasing the impact resistance of semi-crystalline PET. 

PVDF is semicrystalline. The glass transition temperature is ca. — 35°C. The crystalline lamellae generally represent about half of the material. Several crystalline phases have been reported. The structures of the a (nonpolar) and jS (polar) modifications are shown in Fig. 18.7. 

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