TEMPERATURE-DEPENDENCE OF THE ELASTIC-MODULUS OF CRYSTALLINE REGIONS OF POLYOXYMETHYLENE

The temperature dependence of the elastic modulus El of crystalline regions in the direction parallel to the chain axis of various polyoxymethylenes (POM) has been investigated by X-ray diffraction. An El value of 52 GPa is obtained for both hot-drawn (draw ratio λ=14 at 170°C) and annealed (170°C, 1h) POM (specimen modulus Yl=27 GPa) and drawn (λ=5 at 140°C) POM (Yl=6.4 GPa) at room temperature, which agrees well with that previously reported. However, the El value of POM (λ=34 under dielectric heating, MISELA (microwave selective adsorption), Yl=60 GPa) is 73 GPa at room temperature, which exceeds the Yl of MISELA and is higher than the El value of hot-drawn and annealed POM. The El values of both hot-drawn and annealed POM and MISELA are in accord with each other and 85 GPa is obtained at -150°C, but the temperature dependences of El of these POM differ much from each other. With increasing temperature, the El value decreased and reached a plateau at 0°C for hot-drawn and annealed POM (52 GPa), but for MISELA El reached a plateau of 73 GPa at -60°C. Further, El values decreased at 100°C for both samples. The lattice spacing for the (009) plane of each POM also decreases with increasing temperature in the temperature range studied. The thermal expansion coefficients change discontinuously in the ranges 0 to 100°C for hot-drawn and annealed POM and -60 to 100°C for MISELA. These temperature ranges agree with that where the temperature dependence of El changed, that is, thermal molecular motion in the crystalline regions changed. Consequently, the mobility of a chain molecule, i.e. incoherent thermal vibration, differs from hot-drawn and annealed POM to MISELA, and this causes the difference in temperature dependence of El for the two kinds of POMs. Further, the discrepancy in the El values for these samples at room temperature is in fact not because of the inhomogeneous stress distribution but because of the difference in the mobility of chain molecules in the crystalline regions at room temperature. © 1990.