Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 84

PET

100

PVC

PEN

PS

10

PBMA

1

B 2 O 3

0 50 100 150 200

m

Fig. 3.19 Correlation of the parameters of variability (− Δ E ) and fragility ( m ). The m values are
from Table 3.1 (if there is more than one literature value, the mean value is used). PET poly(ethylene
terephthalate), PVC polyvinyl chloride, PS polystyrene, PEN poly(ethylene 2,6-naphthalate),
PBMA poly( n -butyl methacrylate). (Adapted from Vyazovkin et al. [ 44 ] with permission of Wiley)

3.4 Glass Aging

The glass is falling hour by hour, the glass will fall for ever
Louis MacNeice, Bagpipe Music

3.4.1 Background

When liquid is well above its T g , its relaxation time is significantly shorter than the
timescale of regular measurements, which typically last from minutes to hours. On
such timescale, any fluctuations of the molecular structure of liquid are relaxed, i.e.,
liquid is at equilibrium. When temperature drops below T g , the relaxation time of
liquid exceeds the experimental timescale, and the structural fluctuations do not have
enough time to relax. Instead, they become frozen kinetically as the nonequilibrium
glass phase (Fig. 3.20 ). This phase is driven thermodynamically to relax its structure
toward that of the equilibrium supercooled liquid. The relaxation can occur on heat-
ing (i.e., when temperature raises continuously) of a glass through the glass transition
temperature. In this case, the process is referred to as the glass transition (Sect. 3.3).
However, this relaxation also occurs on annealing (i.e., when temperature is held
constant) below the glass transition temperature. This process is called physical
aging [ 59 ]. Physical aging is accompanied by a change in mechanical, dielectric,
magnetic, and optical properties of a glass. Thermodynamically, the process is char-
acterized by a decrease in three thermodynamic parameters: the enthalpy, entropy,
and volume (Fig. 3.20 ). That is, physical aging is an exothermic process, during
which the glass becomes more ordered structurally and its volume shrinks.