Glass-Transition Temperature and Stresses

At the a material change occurs from a solid rigid glassy state to a soft amorphous state. The polymer molecules are transformed from a somewhat ordered state to a random state of high molecular motion. The change in CTE at the can be two to five times greater than its value before 

The glass transition is a second-order transition, that is, the second derivative ofthe free energy function. In comparison, a first-order transition, such as the melting of a material, is the first derivative of Gibbs free energy. The equation used to determine the glass transition is  

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The plasticizer addition is a relatively simple technique. A miscible low-T, compound is added to the formulation, so as to produce a decrease in both the glass transition temperature and the yield stress, and a corresponding improvement in the fracture resistance. These drawbacks are very severe for thermosets, and generally this method is not used for toughening purposes. 

Flexibilizers generally cannot be used to counteract internal stress in high temperature adhesive because of their relatively low glass transition temperature and thermal endurance properties. However, most high-temperature adhesive systems incorporate metallic fillers (generally aluminum powder) to reduce the coefficient of thermal expansion and degree of shrinkage. 

It is realized that even at this higher temperature, organic materials of this nature are below their glass transition temperature and their utility is dependent upon a lack of brittleness and high impact resistance below this transition. The problem, therefore, is to find a curable elastomer that will resist, as much as possible, bend stresses and shock and vibration loading without exhibiting brittle failure. 

Two other issues that are important concerning the effect of confinement on the glass transition temperature and its associated dynamics arise from the interactions of the confinement itself with the fluid or (polymer) of interest. The first effect is simply that of macroscopic confinement effects. Hence, if the confining medium causes a mechanical stress to be applied to the sample, there could be, for example, a hydrostatic tension applied to the fluid, and this would result in an apparent change in... 

If the surface sodium ions in a soda-lime-silica glass are ion-exchanged with potassium ions, estimate the surface stress. Assume the exchange is performed below the glass transition temperature and the percentage ionic volume increase is estimated to be 4.2% (sodium to potassium). 

Example of internal stress due to cooling Parabolic temperature profiles form across the thickness of a warm, cooling sheet of amorphous plastic. Fig. 71. If the initial temperature is above the glass transition temperature and cooling is rapid (quenching), the internal stress distribution across the cross-section of the material is nearly parabolic as well. 

In adhesive joints, for example with epoxy resin adhesives, internal stress in the adhesive layer may result from different coefficients of expansion in the glued materials, whereby their moduli of elasticity are important factors. The glass transition temperature, and thus the curing temperature, also play a role. The reaction shrinkage of the resin is another source of internal stress. Suitable formulations with added fillers, oligomers or copolymers are among the measures taken to reduce these influences. 

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