Question

explain why Tm in HDPE is higher than Tg in comparison to PET and PE
HDPE Tm>>>Tg
PET    Tm >>Tg
PE      Tm>Tg

tm: melting temperature of a polymer
tg=glass transition temperature

Answer 1

HDPE polymers has very disordered chains than PET and Than in case of PE. Now let us try to understand this concept.

The glass transition temperature(T_g) value is highly depends upon the mobility of chain of polymer. It is found that the T_g value is exhibited by only Amorphous solids (highly disordered chain).

Well ordered polymers and semi ordered polymers are generally Crystalline or semi crystalline exhibits melt temperature or melting point (Tm) along with a glass transition temperature(T_g) value.

The Glass transition temperature of polymers- if we see at the molecular level then we found that the
the chains of the polymers are extremely disordered and therefore they are very amorphous in nature. So it
is required higher thermal energy or temperature to move the polymer units or sliding over
them and when a minimum temperature is attained by the polymer then the whole disordered
chain starts to move freely due to attainment of well orderness and this temperature is called melting point of the polymer which is very
greater than glass transition temperature .

Glass transition temperature for amorphous polymers is a significant property where as melting
point is a property of crystalline polymers.

Below the glass transition temperature amorphous solids are highly disordered and and the
chain of the polymer is blocked or locked within one another or the motion is freezed, slow down the movement of
chain in the polymer. They starts wiggling around or above the glass transition temperature.

Higher the value of Glass transition temperature there is more number of immobile chain in
the polymers molecule in case of well ordered crystalline polymers. There is a sharp melting point.
Below that melting point there is a well ordered chain in the polymer and above the melting point the
polymer change become highly disorder.