Molecular Motion of Polymers under Supercritical Carbon Dioxide

为了深入了解超临界CO_2下聚合物的分子运动，我们在不同温度、不同频率f范围内对PMMA的介电损耗e“进行了现场测量。弯曲可能是由于T_g处运动方式的改变引起的，即T_g以上的运动为αβ过程，而Jig以下的运动为β过程。在CO2作用下，PMMA也表现出阿累尼乌斯行为，曲线也发生弯曲。随着CO_2压力的升高，弯曲温度向低温方向移动。这可能归因于CO_2增塑效应导致Jig随CO_2压力的升高而减小。有趣的是，T_g以下的f_&lt；max&gt；也随着CO_2压力的增加而增大，这表明塑化效应也加速了侧基旋转等局部运动。注入CO_2后，随着时间的推移，峰位置的移动趋于平缓，峰形变窄，说明PMMA中吸附的CO_2饱和，密度涨落随时间趋于均匀，分子运动的加速度趋于平缓。峰高随时间增加，峰位置保持不变。在相同f&lt；max和gt；的条件下，CO_2下的峰高比常压下大2倍。峰高的增加可以归因于链构象从无规卷曲到局部有序状态的变化，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.