Morphology and packing behavior of model ethylene\propylene copolymers with precise Tmethyl branch placement

The packing behavior of ethylene\propylene (E\P) copolymers with precise methyl group placement on every 15th and 21st chain carbon atom (denominated HP15 and HP21) was investigated. Lamellar morphology with lamella thicknesses by far exceeding the distance between side groups gives strong evidence that the crystals contain pendant methyl groups as defects. The packing of chains in the lattice requires that adjacent E\P copolymer molecules stagger thereby forming a triclinic lattice with a hexagonal subcell of methylene groups. Defects inside the crystalline region are concentrated in planes oblique to the chain stems. The relationship between subcell and unit cell seems to be a function of the thermal history of the copolymer involving transient states. The atactic structure leads to conformationally disordered crystals (Wunderlichs CONDIS crystals) and enforces deviations from an all-trans stem conformation. These deviations shorten the chains and are stronger for HP15 than for HP21, a fact that has been supported by Raman spectroscopy performed on recrystallized samples of both materials. In fact, a Raman signature band found at 1,084 cm–1 for only HP15 indicates the presence of disordered chain conformations adjacent to singularities.