Crystallization of Polymers Induced by Supercritical Fluids

采用高压金刚石ATR电池，用FT/IR光谱仪测量了超临界二氧化碳诱导聚丙交酯的结晶速率。在1266 cm^<-1>处的吸光度变化与x射线衍射（XRD）所得的结晶度有关。由于溶质在聚合物中的扩散速率通常很小，结晶是随溶质的吸附而发生的，因此很难从实验观察中提取结晶速率。然而，在这项工作中使用的ATR方法将只测量结晶速率，因为在该方法中聚合物样品可以更薄（约10 μm），并且吸附平衡在9秒内完成。通过对4种样品厚度（10、50、100、250 μm）的结晶速率进行测定。吸光度随100 μm和250 μm厚度的增加而减小，而在10 μm和50 μm厚度的样品中，吸光度随样品厚度的增加而变化。因此，我们选用了厚度为10 ~ 20 μm的样品。在CO_2处理压力为1和2 MPa时，XRD未观察到结晶现象，而FT/IR观察到结晶现象。这一观察结果意味着存在小晶的可能性，这是XRD无法检测到的。对CO_2处理后的样品进行了DSC测量，以确定CO_2的存在。结晶温度随处理压力的增大而降低。这一结果表明存在小晶。通过吸光度变化测定，明确了结晶机理。该机制包括两个步骤，首先小晶迅速形成，然后开始生长。

Crystallization rate of polylactide induced by supercritical carbon dioxide was measured with a FT/IR spectrometer equipped with a high-pressure diamond ATR cell. Absorbance change at 1266 cm^<-1> was related with crystallinity obtained from X-ray diffraction (XRD). It is hard to extract the crystallization rate from experimental observation, because solute diffusion rate in polymer is usually small and the crystallization follows sorption of the solute. The ATR method used in this work, however, will measure only the crystallization rate, because polymer sample can be thinner (about 10 μm) in this method and sorption equilibrium accomplished less than 9 s. The crystallization rate of four kinds of sample thickness (10, 50, 100, and 250 μm) was measured to make sure this. While the rate of the absorbance change decreased with increasing the thickness of the 100 and 250 μm sample, sample thickness dependence on the rate did not be observed in the sample thickness of 10 and 50 μm. Therefore, we used the samples of thickness from 10 to 20 μm. The crystallization did not be observed with XRD at CO_2 treatment pressure of 1 and 2 MPa, whereas the crystallization was observed with FT/IR. This observation means possibility of small crystallite existence, which can not be detected with XRD. DSC measurement of CO_2 treated sample was carried out to make sure the existence. The crystallization temperature decreased with increasing the increasing the treatment pressure. This result means the existence of small crystallite. The crystallization mechanism was made clear through the absorbance change measurements. The mechanism consists of two steps, at first small crystallite rapidly forms and then growth of the crystallite begins.