the reasons: 1. For low molecular weight materials, modulus drops rapidly with increasing temperature. 2. For High molecular weight amorphous materials, modulus drops to a secondary plateau region called the rubbery plateau (polymer entanglement prevents chain slippage). With further increase temperature, the modulus drops rapidly again. This point makes the viscous flow region. 3. For Semicrystalline polymers, the behavior is similar to high-molecular weight amorphous polymers, except that the modulus in the secondary plateau is normally higher because of the restriction of crystalline regions for movement. At Tm, the crystalline region melts and the modulus drops in the viscous-flow region.