Molecular Motion of Polymers under Supercritical Carbon Dioxide

为了深入了解聚合物在超临界co2作用下的分子运动，我们在CO_2作用下的不同温度下，在宽频率f范围内对PMMA的介电损耗e”进行了原位测量。在环境压力下，PMMA表现出阿伦尼乌斯行为，并在吉格附近发生弯曲。弯曲可能是由于T_g处运动模式的改变引起的；即T_g以上的运动为αβ过程，而Jig以下的运动为β过程。PMMA在CO2作用下也表现出Arrhenius行为，曲线也发生弯曲。随着CO_2压力的增加，弯曲温度逐渐降低。这可能是由于CO_2的塑化作用导致跳汰器随CO_2压力的增加而减小。有趣的结果是，T_g以下的f_<max>也随着CO_2压力的增加而增大，这表明塑化效应也加速了局部运动，如侧基旋转。注入co2后，峰值位置的移动趋于平稳，峰的形状随着时间的推移而变窄，表明密度波动随着时间的推移而均匀化，分子运动的加速度趋于平稳，这是由于PMMA中吸收的co2饱和所致。峰高随时间增加，而峰的位置保持不变。在相同的f_<max>条件下，CO_2条件下的峰高是常压条件下的2倍。峰高的增加可能是由于链的构象从随机线圈变为局部有序状态，从而加速了相邻链之间的协同运动。

In order to deeply understand the molecular motion of polymers under supercritical C0_2, we carried out in-situ measurements of dielectric loss e" in PMMA over a wide frequency f range at various temperatures under CO_2. The PMMA under ambient pressure shows an Arrhenius behavior and a bend at around Jig. The bend may arise from the change in the mode of motion at T_g ; i.e., the motion above T_g is the αβ process while that below Jig is the β process. The PMMA under CO2 also exhibits an Arrhenius behavior and the plots also bend. The bending temperature shifts to lower temperature with increasing CO_2 pressure. This may be ascribed to the decrease of Jig with increasing CO_2 pressure due to the plasticizing effect by CO_2. The interesting result here is that the f_<max> below T_g also becomes higher with increasing CO_2 pressure, suggesting that the local motion such as the side group rotation is also accelerated by the plasticizing effect. After injecting C0_2, the shift of the peak position was leveled off and the shape of the peak became narrower with time, suggesting that the density fluctuation is homogenized with time and the acceleration of the molecular motion is leveled off due to saturation of the absorbed CO_2 in PMMA. The peak height increases with time while the peak position remains constant. The peak height under CO_2 became 2 times larger than that under ambient pressure having the same f_<max>. The increase of the peak height may be ascribed to the acceleration of the cooperative motion between adjacent chains due to a change in chain conformation from random coil to a locally ordered state.