Glass transition temperature cohesive energy density

Material properties can be further classified into fundamental properties and derived properties. Fundamental properties are a direct consequence of the molecular structure, such as van der Waals volume, cohesive energy, and heat capacity. Derived properties are not readily identified with a certain aspect of molecular structure. Glass transition temperature, density, solubility, and bulk modulus would be considered derived properties. The way in which fundamental properties are obtained from a simulation is often readily apparent. The way in which derived properties are computed is often an empirically determined combination of fundamental properties. Such empirical methods can give more erratic results, reliable for one class of compounds but not for another. 

Hayes Method. A relationship between polymer structure, glass transition temperature and molar cohesive energy (cohesive energy density multiplied by the molar volume) was found by Hayes (9) ... 

On the other hand, many important properties of materials are intensive properties. The values of intensive properties are essentially independent of the amount of material present, provided of course that this amount is not zero. An intensive property can usually be expressed in terms of the quotient of a pair of extensive properties. For example, the density equals the molecular weight per repeat unit divided by the molar volume. The solubility parameter equals the square root of the cohesive energy density (defined as the cohesive energy divided by the molar volume). As shown in Chapter 1, the glass transition temperature (an intensive property) can often be estimated in terms of the molar glass transition function divided by the molecular weight of a repeat unit of the polymer. 

Table 3.3 Effect of Cohesive Energy Density on the Glass Transition Temperature...

The glass transition temperature, Tg, corresponds directly to polymer cohesive energy and its packing density. Tg depression can be used to predict influence of plasticizer on the Tg... 

The stress initiating the shear yielding Oy depends strongly on temperature and is additionally proportional to two parameters AT = Tg — T and to where Tg and T are the glass transition temperature and the temperature of the test, respectively, while d denotes the cohesive energy density. The reduced normalized yield stress was defined by ... 

Tbd Temperature of brittle-ductile transition Tg Temperature of glass transition Uch Bond energy of polymer chain Up Potential energy of the rubber particle a Coefficient of thermal expansion 8 Cohesive energy density Volume strain... 

Fig. 19. Correlation between the critical strain for crazing, and the product of the cohesive energy density (CED), the difference between the test temperature and the glass-transition temperature AT, and the elastic modulus E. Reprinted from Ref. 129, with permission from Elsevier.

Radical statistical copolymerization with acrylonitrile leads to SAN. They contain about 25% of the polar comonomer units homogeneously distributed along the chain because the reactivity ratios of each comonomer are lower than unity. These copolymers show an increase in the density of cohesive energy and thus an improvement in their mechanical properties as compared to those of PS homopolymers (for example, stress at break is 40 MPa for PS and 70 MPa for SAN). As compared to polystyrene, SAN also offers a higher glass transition temperature and a better resistance to solvents—hydrocarbons in particular. 

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