Ethylene‐propylene copolymers: Degree of crystallinity and composition

X‐ray, differential scanning calorimeter, and density measurements of the degree of crystallinity have been made on ethylene—propylene copolymers which range in composition from 60 to 100 mole‐% ethylene. At 23°C the degree of crystallinity was found to vary from zero to approximately 75 wt‐% in a regular manner with composition. A systematic difference exists between the x‐ray and heat of fusion measurements if a simple two‐phase model is assumed in order to calculate the per cent crystallinity from heats of fusion. The apparent heat of fusion of the crystalline fraction detected by the x‐ray method is as low as 30 cal/g. This discrepancy increases with comonomer content. It is suggested that large surface or defect energies are present. The broad melting behavior of the samples is also consistent with such a description of the system.