thermal expansion): refer to the figure on the top right. Here, heating rates of are common. The linear sections below and above Tg are colored green. Tg is the temperature at the intersection of the red regression lines. Summarized below are Tg values characteristic of certain classes of materials. Polymers Dry nylon-6 has a glass transition temperature of . Nylon-6,6 in the dry state has a glass transition temperature of about . Whereas polyethene has a glass transition range of The above are only mean values, as the glass transition temperature depends on the cooling rate and molecular weight distribution and could be influenced by additives. For a semi-crystalline material, such as polyethene that is 60–80% crystalline at room temperature, the quoted glass transition refers to what happens to the amorphous part of the material upon cooling. Silicates and other covalent network glasses Kauzmann's paradox As a liquid is supercooled, the difference in entropy between the liquid and solid phase decreases. By extrapolating the heat capacity of the supercooled liquid below its glass transition temperature, it is possible to calculate the temperature at which the difference in entropies becomes zero. This temperature has been named the Kauzmann temperature. If a liquid could be supercooled below its Kauzmann temperature, and it did indeed display a lower entropy than the crystal phase, the consequences would be paradoxical. This Kauzmann paradox has been the subject of much debate and many publications since it was first put forward by Walter Kauzmann in 1948. One resolution of the Kauzmann paradox is to say that there must be a phase transition before the entropy of the liquid decreases. In this scenario, the transition temperature is known as the calorimetric ideal glass transition temperature T0c. In this view, the glass transition is not merely a kinetic effect, i.e. merely the result of fast cooling of a melt, but there is an underlying thermodynamic basis for glass formation. The glass transition temperature: The Gibbs–DiMarzio model from 1958 specifically predicts that a supercooled liquid's configurational entropy disappears in the limit , where the liquid's existence regime ends, its microstructure becomes identical to the crystal's, and their property curves intersect in a true second-order phase transition. This has never been experimentally verified due to the difficulty of realizing a slow enough cooling rate while avoiding accidental crystallization. The Adam–Gibbs model from 1965 suggested a resolution of the Kauzmann paradox according