Cohesive forces, glass transition temperature

Of the microphase-structure dependent physical properties of ionomers, perhaps the most widely studied are glass transition temperatures, (Tg), and dynamic mechanical response. The contribution of the Coulombic forces acting at the ionic sites to the cohesive forces of a number of ionomeric materials has been treated by Eisenberg and coworkers (7). In cases in which the interionic cohesive force must be overcome in order for the cooperative relaxation to occur at Tg, this temperature varies with the magnitude of the force. For materials in which other relaxations are forced to occur at Tg, the correlation is less direct. 

Since the transition from viscous to rubbery behavior occurs near the glass transition temperature, it is evident that the point on the peel force versus peel rate curve at which there is a change from cohesive to interfacial separation will depend on the Tg of the adhesive. For practical purposes it is desirable to formulate the PSA to have a Tg that will yield a maximum in rubbery or clean peel at the test method rate of pulling. For NR-tackifier blends, the target Tg... 

On the other hand, differential scanning calorimetiy (DSC) revealed that chitosan and starch film and their blends have two distinct reaction zones. First, the endothermic peaks around 100°C for chitosan and 150°C for starch correspond to the evaporation of water in films. A second transition was registered in the chitosan film approximately at 200°C, corresponding to its glass transition temperature (Tg). However, the Tg was not observed in the starch film because the addition of plasticizer, this additive reduced the intermolecular forces between potymer chains and the overall cohesion. Finalfy, a broad exothermic peak located around 250°C in the chitosan and blend films could be ascribed to the pol mer... 

The Tg or glass transition temperature is an important and measurable parameter giving information about the retrogradation behavior. Besides that, the Tg is also of great importance for the mechanical properties of the material. Beneath the glass transition temperature the material exists in an amorphous, frozen liquid structure with stiff and brittle behavior. Below its Tg the intermolecular bonds are not broken, due to the small amounts of room left for Brownian movement, as is stated in Eyring s free volume theory [6]. The specific volume increases relatively slowly with increasing temperature. When the material is heated up, at temperatures close to Tg the cohesive forces decrease drastically, the polymer expands, and the free volume increases to such an extent that there is... 

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