Polymer Physics

local non-equilibrium states of polymers towards the equilibrium in the thermal

fluctuations of a liquid. If |qA|>|qB|, upon cooling, the liquid cooled with the rate

qAwill leave the equilibrium states earlier than the liquid cooled with the rateqB,

so it could not get enough time to relax; upon heating, the glass obtained with the

cooling rateqAwill spontaneously relax back into the equilibrium states before

reaching the glass transition temperature. Therefore, the cooling and heating

curves display clear hysteresis, as illustrated in Fig.6.15b. Such a relaxation

behavior belowTgis called thephysical agingof glassy polymer solid.

3. Thearelaxation peak in dynamic mechanical spectroscopy and dielectric
   relaxation spectroscopy of non-crystalline polymers also reflect the glass transi-
   tion phenomenon;


4. Solid-state NMR can also measure the glass transition phenomenon according to
   the different signals of solid and liquid;


5. Glass transition phenomena can be influenced by multiple conditions, such as
   the transition temperature, the pressure, the frequency, the composition of
   copolymers, the mixing concentration, and even the molecular weight etc.

6.3.2 Glass Transition Theories

So far, theoretical interpretations on glass transition are still extremely controversial
in the research field. Some people intend to relate the glass transition phenomenon to
the well-investigated phase transitions. If one looks at the heat capacityCpchanging
with temperatureT, for the equilibrium states under constant pressures, dH¼TdS,
Cp¼dH/dT¼TdS/dT, one obtains

DS¼

ð

Cp

T

dT (6.52)

Fig. 6.15 Illustration of (a) the heat-flow rate curve changing with temperatures to reflect the
glass transition phenomenon; (b) the hysteresis of volume or enthalpy

6.3 Glass Transition and Fluid Transition 111