Study of viscoelastic behavior of polymer crystals with introduction of short chain branches

According to the hypothesis that short chain branches can be introduced into the crystalline phase, the viscoelastic behavior and fine structure of solution-grown crystals of methyl-branched polyethylene have been investigated as a function of the number of branches in ethylene-propylene random copolymers. In the temperature dispersion curve of these copolymers, the crystalline absorption associated with the initiation of disordered thermal motion of molecular chain segments within the crystal lattice (αc) can be observed at about 20–60°C (3, 5 Hz) as a triplet. By inspecting the absorptions of the various kinds of samples with different methyl branching, the high-temperature crystalline absorption can be separated into the αc absorption, the αc absorption associated with the defect region within the crystalline phase, and the αa absorption associated with the amorphous region. The fraction of the perfect crystalline region C can be evaluated from the absorption magnitude of the αc absorption by use of the corresponding absorption of linear polyethylene samples as a reference. When the number of methyl branches increases, the C region of the various samples decreases, and the defect region C' and the amorphous region A increase. Under the assumption that the C' region can be ascribed to the lateral region surrounding each mosaic crystal composing a lamella, the diameter of the mosaic crystal varies between 18 and 140 A for single crystals of samples prepared from xylene solution at 80°C. The effect of the introduction of methyl branches into the polyethylene crystal phase on the viscoelastic behavior and fine structure is discussed in terms of the number of branches.