(689g) Energy Saturation: An Alternative Mechanism for the Glass Transition

Treating monomer-monomer energetics of a tangent sphere chain model as a square well potential in a quasi-chemical approximation yields configurational energy saturation prior to reaching zero temperature. At saturation, a balance exists between repulsive and attractive forces that stabilize the liquid density, which thereafter becomes effectively temperature independent. These low temperature, force stabilized states are identified as ideal glass states. Energy saturation at the glass temperature (Tg) appears to obtain in 23 of 25 polymers analyzed. Among 14 polymers for which fragility indices have been measured, a good correlation between fragility and saturation degree was found; fragile polymers tend to be less saturated. The repulsion-attraction force balance raises the question as to whether it defines a unique repulsive state. To address this question, electron density was used to measure the strength of repulsive forces; global electron densities were calculated at Tg and averaged 0.61 mol/cc for 15 polymers that contain oxygen and 0.53 mol/cc for 7 hydrocarbon polymers. In comparing 2 polymers of similar structure at the same temperature above the Tg of either, one expects the polymer with the higher electron density to have the higher Tg.