Glass transition theories

M. L. Roy, M. J. Pikal, E. C. Rickard, and A. Maloney, The effects of formulation and moisture on the stability of a freeze-dried monoclonal antibody-vinca conjugate A test of the WLF glass transition theory Dev. Biol. Standards, 74, 323 (1991). 

Fig. 3.4. Analysis of the degradation of KS1/ 4 hydrazide conjugate by the Williams-Landel-Ferry glass transition theory (Fig. 9 from [3.10]).

Nelson, K.A. and Labuza, T.P. 1993. Glass transition theory and the texture of cereal foods. In The Glassy State in Foods (J.M.V. BlanshardandP.J. Lillford, eds), pp. 513-517. Nottingham Univ. Press, Loughborough, Leicestershire. 

We therefore believe one of the most pressing problems in the field of free-volume theory and glass-transition theory to be the development of new concepts and obtaining of new parameters for the corresponding states. Miller37 as early as 1968 introduced the idea that the glass transition corresponds to the iso-relaxation state, which at molecular weights of polymers below the critical ones may be replaced by an iso-viscous state. 

On the other hand, some phenomenological distributions of relaxation times, such as the well known Williams-Watts distribution (see Table 1, WW) provided a rather good description of dielectric relaxation experiments in polymer melts, but they are not of considerable help in understanding molecular phenomena since they are not associated with a molecular model. In the same way, the glass transition theories account well for macroscopic properties such as viscosity, but they are based on general thermodynamic concepts as the free volume or the configurational entropy and they completely ignore the nature of molecular motions. 

It has been noted that although non-equilibrium situations exist in semi-moist food products, observed a values may predict microbial and biochemical activities fairly well (Chirife and Buera, 1994 Chirife and Buera, 1995 Chirife and Buera, 1996 Chirife et al., 1996 Cardona, 1997). The glass transition theory, on the other hand, remains inconclusive as more counter challenging data have been published (Chirife et al., 1996 Cardona, 1997 Buera et al., 1998). In this report, the investigation into water mobility has provided additional insights on the molecular dynamics of water (mobility) since water is the key solvent carrying nutrients and oxygen to cells. 

With today s knowledge of glass-transition theory, one might attempt to explain these results in terms of plasticization. All of the studied humectants would be expected to have some plasticization effect on the MCC matrix. Once again, there are two reasonable explanations for the effect of added glycols from each of the two main theories for food stability. These theories were explored further by our research group, as outlined below (Sherwin and Labuza, 2003). 

Nelson KA, Labuza TP. 1992. Relationship between water and lipid oxidation rates. In Angelo AJ St (Ed.), Water Activity and Glass Transition Theory, pp. 93-103. Washington, DC American Chemical Society. 

---