Question

define the glass transition temperature of a polymer , explain the difference in the glass transition temperature values between polystyrene (Tg = 100degrees celcius) and polyethylene (Tg = -20 degrees celcius)

Answer 1

A glass transition temperature (Tg) is the temperature at which a polymer turns from a ductile material to a hard, brittle material. Each polymer with an amorphous structure has its own unique glass transition temperature, which makes a given polymer better suited for some applications over others.

(FOR MORE UNDERSTANDING)-Have you ever left a plastic bucket or some other plastic object outside during the winter, and found that it cracks or breaks more easily than it would in the summer time? What you experienced was the phenomenon known as the glass transition. This transition is something that only happens to polymers, and is one of the things that make polymers unique. The glass transition is pretty much what it sounds like. There is a certain temperature(different for each polymer) called the glass transition temperature, or Tg for short. When the polymer is cooled below this temperature, it becomes hard and brittle, like glass. Some polymers are used above their glass transition temperatures, and some are used below. Hard plastics like polystyrene and poly(methyl methacrylate), are used below their glass transition temperatures; that is in their glassy state. Their Tg's are well above room temperature, both at around 100 oC. Rubber elastomers like polyisoprene andpolyisobutylene, are used above their Tg's, that is, in the rubbery state, where they are soft and flexible.

The glass transition temperature of polystyrene:

A round robin test was performed to determine the reliability of values for the glass transition temperatureTg as determined by DTA on polymers. Ten different instruments were involved. The test material was high molecular weight polystyrene. Values forTg (midpoint) were reported in the range 107°C±2 K. The respective heat flow curves differed considerably in shape. In the literature aTg of 100°C is often given for polystyrene. The discrepancy between this value and the value of 107°C found in the round robin test is due to three differences: the thermal history of the sample, the evaluation of the heat flow curves, and the effect of finite sample size.

Glass transition temperature of Polyethylene:

The unit cell of the polyethylene molecular structure was established to investigate the glass transition temperature (Tg). The properties of interest predicted by the molecular dynamics method were density, free volume, specific volume, radial distribution function, non-bond energy, torsion energy, mean squared displacement and modulus. The Tg of the polyethylene unit cell were determined by means of analyzing these properties. The simulated value of the Tg is about 200 K, which is in good agreement with available data of 195 K in the literature. The method used in this work can be used in studying glass transition of other polymers.