Question

Describe how T₁ and T₂ can be used in determining the structural features of polymers.  

Answer 1

Solution:

When an amorphous polymer is heated, the temperature at which the polymer structure turns “viscous liquid or rubbery" is called the Glass Transition Temperature, Tg or T1. It is also defined as a temperature at which amorphous polymer takes on characteristic glassy-state properties like brittleness, stiffness and rigidity (upon cooling).

This temperature (measured in °C or °F) depends on the chemical structure of the polymer and can therefore be used to identify polymers.

• Amorphous polymers only exhibit a Tg.
• Crystalline polymers exhibit a Tm or T2 (melt temperature) and typically a Tg since there is usually an amorphous portion as well (“semi”-crystalline).

The value of Tg depends on the mobility of the polymer chain, and for most synthetic polymers lies between 170 K to 500 K.

The transition from the glass to the rubber-like state is an important feature of polymer behavior, marking a region of dramatic changes in the physical properties, such as hardness and elasticity.

At Tg, changes in hardness, volume, percent elongation to break and Young’s modulus of solids are mainly seen.

Some polymers are used below their Tg (in glassy state) like polystyrene, poly(methyl methacrylate) etc., which are hard and brittle. Their Tgs are higher than room temperature.

. If the polymer is used as a load-bearing structural element, Tg defines an upper bound for its use temperature. For example, if certain polymer is used as part of the body of an airplane, one needs to ensure that the polymer will not be exposed to a temperature higher than its glass transition to keep it in its rigid state, since above Tg or T1 the polymer will soften. On the other hand, if the polymer is being employed as a flexible rubber, then Tg or T1 represents a lower bound on its use temperature. Failure to recognize this has led to tragic consequences.

Tg of polymers is often used for quality control and research and development. Also, it is an important tool used to modify physical properties of polymer molecules.

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