Glass Transition

  • Assume that you have a polymer in the molten state, and you are cooling the polymer. As the temperature drops, it passes through the glass transition temperature , , and it's mechanical properties change from those of a rubber (elastic) to those of a glass (brittle.)

  • Below the glass transition temperature , the available polymer motions are limited, but above the glass transition, a motion that starts with one atom can pass through the chain and cause an effect 50 atoms away. I have read this in more than one text but have not located the author of this idea.
  • "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)
  • "A different interpretation is presented here in which these transitions are explained 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 , and the yellow line corresponds to temperatures above . The 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 , which is why the orange point is slightly above the intersection. The exaggerated plot in the orange box illustrates this point.

    An equation by Fox and Flory defines free volume, , in terms of K, the free volume at absolute zero, T, the temperature of system, and two alphas: , the cubic volume expansion coefficient when the polymer is in the glassy state and , the cubic volume expansion coefficient when the polymer is in the rubbery state.

    The Fox-Flory Equation for Free Volume


    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. Observe the graph below of specific volume vs. temperature, and how that for a slower cooling, the end result is a lower .


    This is strange...
    WHY WOULD COOLING AT A SLOWER RATE REQUIRE A LOWER TEMPERATURE TO "FREEZE IN" THE INTERMOLECULAR ASSOCIATIONS THAT GIVE THE POLYMER THE GLASSY STATE?
    Crosslinking and


    Chemical reactivity does take place below . 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 may climb as high as 60 deg C before the cure stops.

    As the curing thermoset resin approaches , the reaction becomes diffusion controlled. When the resin 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 sometimes they give different results.





    Last Update- September 22, 1995- wld