Amorphous systems molecular mobility

Using the time-dependent aspect of state diagrams, Roos (2003) illustrated the effects of temperature, water activity, or water content on relaxation times and relative rates of mechanical changes in amorphous systems (Figure 36). This diagram can be considered as a type of mobility map, where mobility increases (relaxation time decreases) as temperature and/or water content/activity increases. Le Meste et al. (2002) suggested the establishment of mobility maps for food materials showing characteristic relaxation times for different types of molecular motions as a function of temperature and water content. 

Luck, W.A.P. 1981. Structures of water in aqueous systems. In Water Activity Influences on Food Quality (L.B. Rockland and G.F. Stewart, eds), pp. 407 134. Academic Press, New York. Ludescher, R.D., Shah, N.K., McCaul, C.P., and Simon, K.V. 2001. Beyond Tg Optical luminescence measurements of molecular mobility in amorphous solid foods. Food Hydro colloids 15, 331-339. Ludwig, R. 2001. Water From cluster to the bulk. Angewandte Chem. Int. Ed. 40, 1808-1827. Maclnnes, W.M. 1993. Dynamic mechanical thermal analysis of sucrose solutions. In The Glassy State in Foods (J.M.V. Blanshard and PJ. Lillford, eds), pp. 223-248. Nottingham Univ. Press, Loughborough, Leicestershire. 

It is interesting that the PEMA-PVdF blends are amorphous up to at least 50 wt % PVdF even though the Tg of the latter is 24°C. The crystallization of PVdF observed in the analogous PMMA blend does not occur under the same conditions with PEMA—PVdF. This suggests that there is a specific interaction between the fluoropolymer and the methacrylate polymer which is sufficient to "dissolve PVdF in the PMMA and PEMA, and that this specific interaction is superimposed on the conventional diluent-crystalline polymer interactions. The complexity of the rate processes involved with high molecular weight systems arising from molecular mobility makes it impossible to elucidate the nature of... 

Before proceeding further, it is appropriate to discuss some aspects of molecular mobility of amorphous solids as it affects stability. The temperature dependence of molecular motion in amorphous systems is described by the empirical Vogel-Tammann-Fulcher (VTF) equation ... 

An alternative approach to describe nucleation from the amorphous state utilizes the glass transition temperature (Tg) concept (Williams et al. 1955 Slade and Levine 1991). Based on this approach, molecular mobility below Tg is sufficiently limited to kinetically impede nucleation for very long times. Amorphous systems, at temperatures above Tg, nucleate at a rate depending on the temperature difference above Tg. Williams et al. (1955) suggested that the rate of nucleation increases rapidly at temperatures just above Tg according to a kinetic expression given by the WLF (Williams-Landel-Ferry) equation. 

The effect of diluents on the viscoelastic behavior of amorphous polymers is more complex at temperatures below T, i.e., in the range of secondary relaxation processes. Mechanical, dielectric and NMR measurements have been performed to study the molecular mobility of polymer-diluent systems in this temperature range (see e.g. From extensive studies on polymers such as polycarbonate, polysulfone and polyvinylchloride, it is well known that diluents may suppress secondary relaxation processes. Because of the resulting increase in stiffness, these diluents are called antiplasticizers . Jackson and Caldwell have discussed characteristic properties... 

The degree of molecular mobility (assessed as the average molecular relaxation time r) of amorphous systems in the region near Tg follows a non-Arrhenius temperature dependence. This so-called fragility (dr/dr at Tg) of amorphous materials is a defining characteristic. ... 

Because of their greater molecular mobility in the solid state, amorphous systems generally exhibit greater physical and chemical instability at any given temperature compared with their crystalline counterparts. 

Similar to amorphous polymeric systems, low molecular weight amorphous substances also exhibit a reduction in Tg as moisture content increases, " thereby leading to favorable conditions for recrystallization to occur. Indeed, low molecular weight amorphous solids possess sufficient molecular mobility well below In some systems with multiple solid states... 

Inhibition of events resulting from increased molecular mobility due to increased moisture absorption and a subsequent reduction in Tg can be accomplished by formulating such materials with amorphous substances of higher Tg. The net effect is to increase molecular interaction and raise the system Tg to a level where molecular mobility is again sufficiently low (high viscosity) such that the undesired property changes do not occur.f ... 

Manifestation of Molecular Mobilities in Amorphous Aqueous Systems The View from Different Experimental Techniques... 

---