toTg, up to about 5 orders of magnitude. Nevertheless, the diffusivity is
very much smaller than at high temperature. Figure 16.4b gives an extreme
example. Here the glass is formed by a polysaccharide that apparently leaves
relatively large pores for water to diffuse through. AtTgthe value ofDfor
water is only by a factor of about 100 smaller than in pure water. At the
glass transition, there is only a weak bend in the curve. On the other hand,
the diffusivity of the polymer is immeasurably small.
Altogether, the diffusivity of small molecules like water is greatly
reduced in a glass, but it is generally not negligible. Prediction of the value of
Dfrom theory is currently not possible.
Question
Is it possible that, in a mixture of two biopolymers containing, say, 30%water, one
of the polymers is in a glassy and the other in a rubbery (or liquid) state?
FIGURE16.4 Effect of temperature on diffusion coefficients (D) in systems near
the glass transition (Tg). (a) Experimental values and calculation according to the
WLF theory for diffusion of fluorescein in a sucrose–water system. (From results by
D. Champion et al. J. Phys. Chem. B 10 (1997) 10674.) (b) Diffusion of water in 81%
pullulan (a polysaccharide) in water. (From results by S. Ablett et al. In: J. M. V.
Blanshard, P. Lillford, eds. The Glassy States in Foods. Nottingham Press, 1993,
p. 189.)