Viscoelastic and Rheo-Optical Behaviors of Ethylene-Isoprene Block Copolymers

Abstract

In the previous paper, the viscoelastic and rheo-optical properties of ethylene-propylene block copolymers were reported and compared with those of polyethylene-polypropylene blends, suggesting the effect of the introduction of a primary bond between the different crystalline polymers on their physical properties. In the present work, ethylene-isoprene (A-B type) block copolymers having different compositions have been synthesized and their viscoelastic and rheo-optical properties have been studied to clarify the nature of block copolymers composed of crystalline and amorphous polymers.The samples of block copolymers were prepared with the catalyst system of Al (C2H5)3 and TiCl4 in n-hexane. The compositions of the copolymers were determined from the ratio of the infrared absorbances of the 830cm-1 and 720cm-1 bands.The temperature dispersion curves of storage Young's modulus E' measured with“Vibron” Model DDV II at 110Hz greatly depend upon the composition. The decrease of E' with increasing temperature is much restrained by the presence of the polyethylene component, even at small contents. The loss modulus E versus temperature curves for copolymers show clear absorption peaks below 0°C, near-95°C (designated to γ-absorption peak due to polyethylene) and-45°C (designated to α-absorption due to glass transition temperature of polyisoprene). The temperature Tmax at which the absorption maximum is observed and the intensity of the former absorption are independent of the composition, supporting the idea that the γ-dispersion of polyethylene originates from the motion of the chain end. In the case of latter, the temperature Tmax is independent of the composition, but the intensity is dependent on the amount of polyisoprene.The birefringence of the block copolymers was measured simultaneously with the stress relaxation measurement at 25°C. The higher the content of polyethylene component, the longer is the time at which the strain-optical coefficient reaches its equilibrium value and the higher is the equilibrium value. This result may support the interpretation that the time dependence of strain-optical coefficient for polyethylene mainly depends on that of the orientation of the crystalline phase, which was concluded previously from the result of dynamic birefringence.