Effects of Molecular Orientation and Crystallinity on Measurement by X-Ray Diffraction of the Crystal Lattice Modulus of Poly(vinyl alcohol) Prepared by Gelation/Crystallization from Solution

Abstract

The crystal lattice modulus of poly(vinyl alcohol) was measured by X-ray diffraction using films which were prepared by gelation/crystallization from solution and elongated in a silicon oil after evaporating solvent. The measured crystal lattice modulus of specimen with Young’s modulus 13–20 GPa was in the range 200–220 GPa. These values were lower than that for polyethylene measured by using ultradrawn films. To check the morphology dependence of the measured crystal lattice modulus of poly(vinyl alcohol), a numerical calculation was carried out by considering molecular orientation and crystallinity. In this calculation, a three-dimensional model was employed, in which oriented crystalline layers are surrounded by oriented amorphous phase and the strains of the two phases at the boundary are identical. The theoretical results indicate that the difference between the crystal lattice modulus as measured by X-ray diffraction and the intrinsic value becomes less pronounced and Young’s modulus of a sample becomes lower when a series coupling between crystalline and amorphous phases is predominant. A series of numerical calculations for the system with low crystallinity and orientational degree of amorphous chain segments indicates that the real value of the crystal lattice modulus is slightly higher than 200–220 GPa measured by X-ray diffraction.