•   Molecular Weight
•   Viscosity
•   Thermodynamics
•  Mechanical Properties
•  Modeling

 

• Home
• Introduction
• Concepts
• Equations
• Tour
• Library
• Site Map

A polymer below its glass transition temperature, T_g, is said to be in the "glassy state". As the temperature increases, it passes through the glass transition temperature, and it's mechanical properties change from those of a glass (brittle) to those of a rubber (elastic).

Below the glass transition temperature, the available polymer motions are limited to affecting only the nearby atoms, but above the glass transition, a motion that starts with one atom will pass through the chain and cause an effect on dozens of neighboring atoms.

"The glass temperature as usually observed occurs when the experimental time scale becomes comparable to the molecular relaxation time..." (see journal article below).

J. M. O'reilly, F. E. Karasz,
"Specific Heat Studies of Transition and Relaxation Behavior in Polymers"
J of Polym. Sci: Part C, No.14, 49-68 (1966)

"Secondary transitions are generally attributed to one or more relaxation processes, such as the rotation and/or oscillation of side chains, subgroups, and short segments of the main chain. The main or glass transition is thought to be due to the motion of longer segments of the main chain."

S. G. Turley, J. Kskkula
"A survey of Multiple Transitions by Dynamic Mechanical Methods"
J of Polym. Sci: Part C, No. 14, 69-87 (1966)

Andrews explains the glass transition in terms of the thermal breakdown of different types of intermolecular secondary bonding in the solid state.

R. D. Andrews
"Transition Phenomena and Solid-State Structure"
J. of Polym. Sci: Part C, No. 14, 261-265 (1966)

(Williams-196)



Above the glass transition temperature there is an increase to the slope of the specific volume vs. temperature function. In the above graph the red line corresponds to the temperatures below T_g, and the yellow line corresponds to temperatures above T_g. The T_g can be determined by locating where the two lines intersect. However, the change in the slope is gradual, as opposed to a sudden change at T_g, which is why the orange point is slightly above the intersection.

The phenomenon called "the glass transition" is somewhat of a misnomer since the term 'transition' implies an equilibrium phenomenon that is invariant to the speed of the heating or cooling. The graph below of specific volume vs. temperature shows that for a slower cooling, the end result is a lower T_g.

---

Crosslinking and T_g

Chemical reactivity does take place below T_g.

The crosslinking reaction of a thermoset polymer resin raises the glass transition temperature. This cure can raise the temperature as much as 40 degrees centigrade above the surrounding temperature at which the cure takes place. Said another way, if you cure at room temperature, the T_g may climb as high as 60 deg C before the cure stops.

As the curing thermoset resin approaches T_g, the reaction becomes diffusion controlled. When the resin T_g becomes sufficiently high such that no further cure occurs (assume there are unreacted groups still present that would react if they could), vitrification is said to have occured.

---

The glass transition can be measured using different techniques and they can give different results.

For poly(oxymethylene), dynamic mechanical analysis suggested 300 K but specific heat results suggested 160 K. Specific heat measurements are considered the technique of choice because there are no "steps" below the glass transition temperature. The 160 K measurement is considered to be the correct value.

See also:

Free Volume

The terminology "glassy state" may sound ad hoc, but it is used fairly frequently in the literature, as can be shown by a Google search on "glassy state" + polymers.