Solids in food materials are either completely amorphous or partially crystalline. Amorphous materials can exist in a glassy or rubbery state. Glass formation is in fact just a failure of crystallization (Turnbull and Cohen, 1958; Angel and Donnell, 1977). The glass can be viewed as a supercooled liquid, which is not in internal equilibrium, with modulus >109 N.m-2. The glassy state of matter and the glass transition itself have still remained unsolved problems in various disciplines of science and engineering. The existence of glassy or rubbery state is related to the molecular motion inside the material that is mostly governed by temperature, timescale of observation, plasticization, and other factors. A glassy material lies below the temperature at which molecular motions exist on the timescale of the experiment, and a rubbery material is above the temperature at similar conditions (Andrews and Grulke, 1999). A glassy material is formed when a melt or liquid is cooled below its crystalline melting temperature, Tm, at a faster rate to avoid crystallization. The change between rubbery liquid and glassy behavior is known as the glass transition, and the critical temperature, which separates glassy behavior from rubbery behavior is known as the glass transition temperature, Tg (Figure 4.1). This transition occurs with no change in order or structural reorganization of the liquid and is not a thermodynamic first-order process since there is no change in entropy, enthalpy, or volume (Haward, 1973). The transition is considered as a thermodynamic second-order phase transition, which implies a jump in the heat capacity or expansivity of the sample that occurs over a temperature range. The glass transition is the most important property of amorphous materials, both practically and theoretically, since it involves a dramatic slowing down in the motion of chain segments, which rarely one can observe in the static state. Glass transition leads to affect many physical properties including density, specific heat, heat flow, specific volume, mechanical modulus, viscosity, dielectric properties, and so on (Andrews and Grulke, 1999).
ASJC Scopus subject areas
- Agricultural and Biological Sciences(all)