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What information does the Tg (glass transition temperature) tell about a polymer system?

The Tg of a resin system defines when a thermoset polymer goes from an amorphous rigid state to a more flexible state.  Tg is an abbreviation for Glass Transition Temperature.   The most important information that the Tg provides is:  What is the nature of the polymer at its service temperature?  Is it rigid and glassy or is it flexible and rubbery?

The normal state of most thermoset polymers is to be an amorphous solid at room temperature.  The arrangement of the polymer molecules is a random arrangement, meaning the polymer structure does not have a repeating arrangement of polymer chains.  An amorphous solid is different than a crystalline solid where the polymer molecules would be in a structured, repeating arrangement .  At temperatures below the Tg, the molecular chains do not have enough energy present to allow them to move around.  The polymer molecules are essentially locked into a rigid amorphous structure due to short chain length, molecular groups branching off the chain and interlocking with each other, or due to a rigid backbone structure.   When heat is applied, the polymer molecules gain some energy and they can start to move around.  At some point the heat energy is enough to change the amorphous rigid structure to a flexible structure.  The polymer molecules move freely around each other.  This transition point is called the glass transition temperature.

The Tg is accompanied by a change in the heat capacity of the material.  The polymer does not melt (unlike a crystalline polymer which will melt when heat is applied) but it does undergo a change in structure (from rigid to flexible) that produces a change in the heat capacity of the material. 

The graph shows how the sample heats up at one rate below the Tg and then heats up at a higher rate above the Tg.   The lower slope below the Tg is due to the lower heat capacity for the amorphous polymer.  Above the Tg, the rubbery, flexible polymer has a higher heat capacity.

For some thermoset polymers, the Tg can be below 25°C in which case the polymer will be a soft, flexible polymer at 25°C.  This is typically true with silicones as well as with flexible epoxies or urethanes.  They cure with a long flexible chain that does not require much heat to make it easy to move.  Rubbery, flexible materials at ambient temperature typically have a Tg in the 0°C to -150°C range. 

The use of a polymer above its Tg depends on what properties are needed for the polymer.  A flexible silicone or epoxy has its Tg below 0°C.  It can be used at room temperature and higher because the desired property is flexibility and elasticity.  Above the Tg, the polymer is in the rubbery state.  For a flexible silicone or epoxy, i.e. one used as a sealant or flexible adhesive, the ability to move as stresses are applied is a key feature that is only attained if the service temperature is above the Tg.

Another polymer might need to have strong physical properties when it is being used at an elevated temperature.  For this polymer the service temperature should be below the Tg.  This polymer is in the rigid, glassy state when it is being used at a temperature below the Tg.  This means the polymer has high strength in such areas as compression, tension, shear, etc.