Glass transition temperature, glassy and rubbery polymers

Amorphous polymers convert reversibly between the rubbery and glassy states as their temperature rises or falls. Below their glass transition temperature, amorphous polymers exist in a glassy state. Above their glass transition temperature they are rubbery. We can demonstrate this easily with a racquet ball, which is made of an amorphous polymer. At room temperature, as we all know, the ball bounces at this temperature it is in the rubbery state. If we immerse the ball in liquid nitrogen it becomes brittle and will shatter when we drop it, i.e., it has become a glass, If we were to allow the frozen ball to warm up to room temperature, it would become rubbery once more. We can freeze and thaw the same ball repeatedly with no loss of its properties at room temperature. 

Knot et al. (51) converted soybean oil to several monomers for use in structural applications. They prepared rigid thermosetting resins by using free radical copolymerization of maleates with styrene. The maleates are obtained by glycerol trans-esterification of the soybean oil, followed by esterification with maleric anhydride. They also synthesized several TAG-based polymers and composites and compared their properties. It was found that the moduli and glass transition temperature (Tg) of the polymers varied and depended on the particular monomer and the resin composition. They proposed that the transition from glassy to rubbery behavior was extremely broad for these polymers as a result of the TAG molecules acting both as cross-linkers as well as plasticizers in the system. 

Considerable effort has been made during the last two decades to develop a "microscopic" description of gas diffusion in polymers, which is more detailed than the simplified continuum viewpoint of Fick s laws. It has been known for a long time that the mechanism of diffusion is very different in "rubbery" and "glassy" polymers, i.e., at temperatures above and below the glass-transition temperature, Tg, of the polymers, respectively. This is due to the fact that glassy polymers are not in a true state of thermodynamic equilibrium, cf. refs. (1,3,5,7-11). Some of the models and theories that have been proposed to describe gas diffusion in rubbery and glassy polymers are discussed below. The models selected for presentation in this review reflect only the authors present interests. 

The mechanisms of gas diffusion are very different at temperatures above and below the glass-transition temperature, T, of the polymers, i.e., when the polymers are in their "rubbery" or "glassy" state, respectively (1,3-8). The difference in these mechanisms is reflected in the significant differences observed in the dependence of the diffusion coefficient, as well as of the permeability and solubility coefficients, on the penetrant gas pressure or concentration in polymers and on the temperature. 

This polymerization technique allows for the formation of copolymers in which the addition of relatively small quantities of comonomer may have a significant effect on the final properties of the polymer. This is particularly the case with the glass transition temperature, Tg, of the polymer. This is a physical transition, which occurs in the polymer when the amorphous structure of that polymer begins to change from a glassy to a rubbery state. At temperatures below a polymer s Tg, it will be relatively brittle, and will be unlikely to form a coherent film. 

Amorphous polymers generally exist as hard, rigid, glassy plastics below their glass-transition temperature Tg and as soft, flexible, rubbery materials above their Tg. Comparison of the densities of the glassy state (pg) and the rubbery state (pj) for various amorphous polymers indicates that Pg Pr [5]. 

The dependence of permeability, diffusion, and solubility coefficients on penetrant gas pressure (or concentration in polymers) is very different at temperatures above and below the glass transition temperature, Tg, of the polymers, i.e., for mbbery and glassy polymers, respectively. Thus, when the polymers are in the rubbery state the pressure dependence of these coefficients depends, in turn, on the gas solubility in polymers. For example, as mentioned in Section 61.2.4, if the penetrant gases are very sparsely soluble and do not significantly plasticize the polymers, the permeability coefficients as well as the diffusion and solubility coefficients are independent of penetrant pressure. This is the case for supercritical gases with very low critical temperatures (compared to ambient temperature), such as the helium-group gases, Ha, Oa, Na, CH4, etc., whose concentration in rubbery polymers is within the Heruy s law limit even at elevated pressures. 

EVA (ethylene vinyl acetate) is a copolymer which is available in various compositions of ethylene and vinyl acetate. At a content of 50 vinyl acetate or more the crystallinity has been vanished completely. Give qualitatively the nitrogen permeability at room temperature for a copolymer with 10%, 50% and 90% vinyl acetate respectively and indicate the character of the polymer in terms of rubbery and glassy, crystalline and amorphous (The glass transition temperatures of the pure polymers polyethylene and polyvinyl acetate are given in table II - 5). 

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