Experimental Determination of Elastic Moduli of the Crystalline Regions in the Direction Perpendicular to the Chain Axis in Oriented Polymers

The elastic moduli of the crystalline regions in the direction of the chain axis (El) in some oriented polymers were previously determined by us by an X-ray diffraction method. They were found to range from 50×104 to 250×104kg/cm2; e.g., polyethylene (PE) and polyvinylalcohol (PVA), which are characterized by the fully-extended planar zig-zag chain configuration in the crystal, have such high El-value as 240×104 and 255×104kg/cm2 respectively, while isotactic polypropylene and polyoxymethylene, whose chain configurations in the crystal are helicoidally contracted, show much smaller El-values of about 50×104kg/cm2. Thus, we have concluded that the chain skeleton configuration in the crystal is chiefly responsible for the elastic modulus of the polymer crystalline region in the direction of the chain axis.In the present paper, results for measurements of the similar elastic modulus for PE and PVA in the direction perpendicular to the chain axis (Et) are described. The experimental technique was the same as previously described with the only difference that instead of a bundle of fiber or monofilament a drawn and heat set film was used. Stress was applied to the specimen in the direction perpendicular to the fiber axis and the shifts of 2θ angles of some equatorial reflections were measureIt was found that the Et-values for PE are as follows:PE: (110)…Et=4.3×104kg/cm2(200)…Et=3.2×104kg/cm2(020)…Et=3.9×104kg/cm2 (20°C)The Et-values are some 1/60 of the El-value and seem to be reasonable because it is the intermolecular cohesion forces that play roles in the extension of polymer crystal in the direction perpendicular to the chain axis. The Et-values for PE vary to some extent according to the planes employed, which is perhaps due to the anisotropy of the polyethylene crystal.Et-values for PVA are as follows:PVA: (101)…Et=9.0×104kg/cm2(200)…Et=8.9×104kg/cm2 (25°C)It should be noted that the values of PVA are greater by about two to three times than those of PE. This is reasonable because the intermolecular forces of PVA are much greater than those of PE due to the formation of hydrogen bonds between chain molecules.