I would like to add some words.  Tg is for the amorph regions of a semicrystalline polymer. At room temperature the molecule chains of this amorphous phase have the ability to "move".  Let's say they are in "liquid" state.  Crystall are embebed in this "liquid Phase"giving the material more stiffnes.   Amorph phase contributes to the viscous component of their viscoelastic properties.  In rubber and in thermoplastics  above Tg molecules are able to move based in rotations of atoms around their bounds (conformations), but in a rubber  they can not slide to each other like in a thermoplastic.  This make a big difference:  deformations in Rubber are elastic reversible: molecules are fixed to each others.  Thermoplastic above Tg are viscoelastic and deformations are not totally reversible depending on deformation levels and time scale. Molecules can slide additionally.

Dear Subhendu,

the main reason is the high degree of crystallinity. Regards

(Thermo)plastics and rubbers are distinguished by linear and by crosslinked structures. For that mattter, poyethylene can be made "rubber" by crosslinking. The typical rubbery deformation can be observed only above Tg, in the case of semicrystalline polymers above Tm. Epoxy resisn are brittle, but become rubbery above the Tg. The Tg value is determined by chain flexibility (segment length), on the main and side chain structure. Tg of polyethylene is low because of its high flexibility and the absence of side groups. Additionally PE is semicrystalline due to its realtively simple and symmetrical structure, teherfore it remains tough and exhibits plastic behavior between Tg and Tm.

Yes...Bouaziz is absolutely right. For rubbery materials, it should have amorphous characteristics in order to free  segmental motion of polymeric chain in addition to lower Tg value( below room temp.).

I would like to add some words.  Tg is for the amorph regions of a semicrystalline polymer. At room temperature the molecule chains of this amorphous phase have the ability to "move".  Let's say they are in "liquid" state.  Crystall are embebed in this "liquid Phase"giving the material more stiffnes.   Amorph phase contributes to the viscous component of their viscoelastic properties.  In rubber and in thermoplastics  above Tg molecules are able to move based in rotations of atoms around their bounds (conformations), but in a rubber  they can not slide to each other like in a thermoplastic.  This make a big difference:  deformations in Rubber are elastic reversible: molecules are fixed to each others.  Thermoplastic above Tg are viscoelastic and deformations are not totally reversible depending on deformation levels and time scale. Molecules can slide additionally.

Polyethylene is plastic based on consideration of its  thermal behavior i.e.it  softens when heated .;when it is cooled  returns to initial state.Rubbery materials exhibits  long range elasticity in cross-linked  state.

 I also agree with Dr Boo Aziz regarding crystalinity content