<P> The configuration of the glass in this temperature range changes slowly with time towards the equilibrium structure . The principle of the minimization of the Gibbs free energy provides the thermodynamic driving force necessary for the eventual change . It should be noted here that at somewhat higher temperatures than T, the structure corresponding to equilibrium at any temperature is achieved quite rapidly . In contrast, at considerably lower temperatures, the configuration of the glass remains sensibly stable over increasingly extended periods of time . </P> <P> Thus, the liquid - glass transition is not a transition between states of thermodynamic equilibrium . It is widely believed that the true equilibrium state is always crystalline . Glass is believed to exist in a kinetically locked state, and its entropy, density, and so on, depend on the thermal history . Therefore, the glass transition is primarily a dynamic phenomenon . Time and temperature are interchangeable quantities (to some extent) when dealing with glasses, a fact often expressed in the time--temperature superposition principle . On cooling a liquid, internal degrees of freedom successively fall out of equilibrium . However, there is a longstanding debate whether there is an underlying second - order phase transition in the hypothetical limit of infinitely long relaxation times . </P> <Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2009) (Learn how and when to remove this template message) </Td> </Tr>

A. list three material that have a negative temperature coefficient