Many researchers have studied the shape memory property of polyurethanes (PUs) by considering the melting temperature of the soft segment (for example as in PCL based PUs) as transition temperature. In this case, the polymer will not lose its flexibility (as the Tg is well below the Tm) when the shape fixing process is performed at T < Tm. However, if Tg is considered as the transition temperature, the material becomes brittle during the shape fixation process, as the polymer has to cool well below the Tg. Thus in the programmed shape (also called fixed shape or temporary shape) the material will lose its flexibility. In a real application which one (Tg or Tm) is better to consider as transition temperature?
It is not a question of which is better or worse, both Tm & Tg are thermal transitions that change the molecular system from one state to another. Before discussing the change, we ought to know that most polymers are either totally amorphous "i.e. non-crystalline" or semi-crystalline "where there are ordered & disordered domains". Fully crystalline polymers are rare.
Assume that there is a molten polymer which is subjected to cooling, there will be passage from a liquid state to a solid state. If the polymer is pure, there will be a sharp freezing point which is also melting point if the solid is heated. This thermal transition involves the "ordered domain" & is usually reported as < 3 oC range. In fact, DSC & DTA techniques will display sharp band for Tm.
The solid polymer is usually flexible " hence, described as being in the rubbery state". If further cooling is applied, there will be passage from the rubbery state into a glassy state "i.e. the polymer will look like glass in being, e.g. brittle & fragile". The glass-rubber transition (or the opposite) does not occur jump wise but over a wide range so the polymer chemist usually reports an average Tg . In the glassy state, there is virtual cessation of motion on molecular scale "some polymer books resemble that to frozen long unbroken spaghetti to clarify the picture ".
A continuous heating (or cooling) may be applied to a polymer sample to detect Tg & Tm several times as long as the polymer has "thermoplastic" behavior but this is not possible if the polymer has "thermosetting" behavior. A thermoset does not have "memory" & it is like a one-way street!
What polymer chemists usually put aside is the "softening" temperature which occurs before the melting point.
I may add that there is a wrong impression by some polymer students that Tg is either near zero or < 0 oC. Polymers, that are used as glass substitutes, have Tgs sometimes far above room temperature.
Dear Dr. Nizar Matar
Thank you for the nice explanation and giving your valuable time. Can we use any kind of crosslinked polymer for shape memory application (where the Tg can be used as transition temperature, and the crosslinks will provide a permanent shape)?
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