List two molecular characteristics that are essential for elastomers. (1) for elastomers to experience relatively large elastic deformations, the onset of plastic deformations must be delayed and (2) elastomers must have huge molecular weight. (1) elastomers must experience a rise in temperature when stretched and (2) the modulus of elasticity decreases with increasing temperature (1) elastomer must not easily crystallize and (2) the lowest temperature at which rubber-like behavior persists is between -50 and -90°c. (1) elastomers must be amorphous, having chains that are extensively coiled and kinked in the unstressed state and (2) there must be some crosslinking