応力-ひずみ-赤外吸收同時測定による低密度ポリエチレンの変形機構に関する研究

抄録

The rheo-optical techniques such as dynamic birefringence are very useful for the study of the relation between mechanical properties and the structure of high polymers, especially crystalline polymers. As a method in this line, an attempt has been made to measure the change in infrared absorption simultaneously with stress and strain for polymer films under stretching. As described in the previous paper¹⁾, the double beam type infrared spectrometer has been modified in such a way as to allow one of the beams go through a specimen mounted on stretcher similar to the Instron type tensile tester in its principle. The spectrometer has also been equipped with a quick return device for the motor in order to record the absorption in a rather narrow range of wave length repeatedly in a short time. In the present paper, the infrared dichroism of low-density polyethylene has been measured by means of this spectrometer.
The sample film used in this study was made by the inflation method, and is the same as that used in the previous paper¹⁾.
The dichroic ratio, D=A_⊥/A_||, where A_⊥ and A_|| are the absorbance (optical density/thickness) for polarized radiations whose electric vectors are perpendicular and parallel to the stretching direction, has been determined for the 720 and the 730cm^-1 bands. For the 730cm^-1 band, D increases slowly up to about 10% elongation, very rapidly thereafter up to about 30% elongation, and then slowly again up to about 70% elongation. D for the 720cm^-1 band, on the other hand, decreases slowly first, and then rapidly until it goes through minimum at about 30% elongation. Then it gradually increases. The orientation function for the α-axis, F_α, has been determined from D for the 730cm^-1 band, and the function for the b-axis, F_β, from D for the 720cm^-1 band, assuming that this band is primarily due to the crystalline phase. It is interesting to note that the change of these orientation functions with strain corresponds very well to the stress-strain diagram for the same film.
If we employ the current folded chain theory for the structure of crystalline polymers, this correspondence enables us to interprete the stress-strain curve very reasonably.
In the earlier stage of stretching up to the elastic limit, the spherulites or superstructures of, lamellae may deform elastically without appreciable orientation or unfolding of folded chains. In the neighborhood of the yield point, such spherulites or superstructures may split into smaller domains, and they can be oriented in the direction of stretching without appreciable unfolding up to 30% elongtion. Above this elongation the unfolding of folded chains in the oriented domains occurs predominantly, and the unfolded chains and crystals reformed tend to be oriented again in the direction of stretching.