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X-ray diffraction and DSC analysis To gain an insight into the PFC behaviour of FLA-Cn solids, powder wide-angle X-ray diffraction (PXD) and differential scanning calorimetry (DSC) experiments were conducted on the ground and pristine solids. The PXD patterns of pristine FLA-C3 and FLA-C5 solids show sharp and intense reflections, indicative of their well-ordered microcrystalline structures (Fig. 6a). In contrast, the ground samples display broad and featureless diffractograms reflecting notable amorphous features. This indicates that grinding has induced the phase transition of FLACn solids between crystalline and amorphous states, which should be responsible for the PFC behaviour of FLA-Cn solids. The formation of amorphous state upon grinding was also confirmed by DSC experiments. As shown in Fig. 6b, no additional thermal transitions could be detected for all pristine solids before the isotropic melt transition (Tm); however, there is clearly one exothermic transition peak at the lower-temperature region for ground FLA-C3 and FLA-C5 solids. This broad exothermic peak (Tc) is ascribed to the cold-crystallization of amorphized FLA-Cn solids upon heating. Amorphized FLA-C3 and FLA-C5 solids have high Tc values, which result in their stable PFC behaviour at room temperature. It is known that the Tc is obviously lower than the Tm of a compound, and the ground FLA-C12 solid could cold-crystallize at or below room temperature since its isotropic melt point is only about 65 C. Thus, the amorphized FLA-C12 solid induced by grinding is in a thermodynamically and dynamically unstable morphology. This is reason why ground FLA-C12 solid exhibits spontaneously recovering fluorescence properties at room temperature. |
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X射线衍射和DSC分析 为深入了解FLA-Cn固体的PFC性能，用粉末广角X射线衍射（PXD）和差示扫描量热（DSC）对研磨和原始固体进行了实验。FLA-C3和FLA-C5固体的PXD图谱呈尖锐和强烈的反射，表明他们为秩序井然的微晶结构（图6a）。与此相反，研磨样品呈宽和无特征的衍射图谱，显示其非晶状态特征。这表明研磨引起了FLACn固体晶态和非晶态之间转换，这应该是FLA-Cn固体PFC性能的原因。研磨时引起非晶状态形成也由DSC实验证实。如图6b所示，原始固体在等向性熔体转变（Tm）前没有热转变; 而研磨FLA-C3和FLA-C5固体在较低温度区域有一清楚的放热转变峰。这种广泛的放热峰（TC）是由于非晶FLA-Cn固体在加热时发生的冷结晶所致。非晶化FLA-C3和FLA-C5固体具有高Tc值，导致其在室温下具有稳定的PFC性能。化合物的Tc值明显低于Tm值，研磨FLA-C12固体在室温或低于室温时可冷结晶，因为它的等向性熔点约仅为65℃。因此，研磨诱导的FLA-C12固体非晶化在热力学和动态学上是一种不稳定形态。这就是为什么研磨FLA-C12固体在室温下具有自发恢复荧光的性能。 |
2楼2014-10-21 10:50:21