Differential thermal analysis of high polymers. V. Ethylene copolymers

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Abstract
Melting and crystalline properties of methyl‐and ethyl‐branched polymethylenes—models for ethylene‐propylene and ethylene–α‐butene copolymers—containing up to 20 branches per 1000 carbon atoms have been investigated by differential thermal analysis (DTA). The melting‐point depression is practically linear with branch concentration. However, ethyl groups are more effective than methyl groups. Cooling at different rates during annealing produces a wider melting‐point spread in copolymers containing more branches. Again, copolymers containing ethyl groups are affected more than those having an equal number of methyl groups. Copolymer crystallinity estimated by DTA also decreases linearly with the number of branches; ethyl groups disrupt crystallinity more markedly than methyl groups do. The effect of colling rates on crystallinity increases with branch concentration. Melting and crystalline behavior of two commercial ethylene copolymers agree with patterns derived from the models. The thermogram of a crystalline ethylene‐propylene block copolymer consists of two separate melting peaks corresponding to crystalline regions formed by ethylene and propylene sequences. Total crystallinity of this copolymer was estimated and used in calculating the copolymer composition. Restrictions for crystallization in the block copolymer are apparent from the thermogram.

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