Crystalline Structure of Ethylene Copolymers and Its Relation to the Comonomer Content

Ethylene copolymers are a large group of polymer material with a lot of industrial applications [1]. They usually consist of two types of monomers. One is the ethylene unit and the other one is the comonomer unit like methyl methacrylate, vinyl acetate, vinyl alcohol, etc. These two monomer units distribute, randomly in most cases, along the polymer chain, forming ethylene segments with different length. Once the length of some ethylene segments exceeds a critical value, i.e. the so-called minimum crystallizable sequence length at a certain temperature [2], these segments tend to aggregate and form crystals, while the segments with the length smaller than the minimum crystallizable sequence locate in the amorphous region. The comonomer units, which often lack the ability of crystallization, are either expelled from the crystals or embedded in the crystals as defects when the volume of the comonomer unit is small. Similar to other semicrystalline polymers, the bulk samples of ethylene copolymers usually comprise the crystalline, the amorphous, and the interfacial region, which lies between the crystalline and amorphous regions. The structures and the dynamics of these different morphological regions, which have large impact on the macroscopic properties of the material, have been the topics with a lot of research interests.