I am working on the interaction of hybrid of polymer and small molecule. The small molecule shows strong crystallization behavior and has several crystal-crystal transition and crystal-liquid crystal trlansition while heating/cooling mentioned in some papers. When blending polymer into small molecule up to a certain ratio, the phase transitions of small molecule can't be observed after high temperature annealing in DSC. I originally considered that polymer may inhibit the low-temperature phase of small molecule and remain high temperature phase of small molecuule in room temperature, and thus the phase transition peaks are not observed in later runs of DSC. But the in-situ heating XRD of hybrid still show apparent d-spacing change in some certain temperatures which are the phase transition temperature of pure small molecule. I thought that if the small molecule is remained in HT phase, the LT phase transition should not happen, but the result of XRD and DSC make me confused.
Is there any explanation of the mismatch of DSC and XRD result? Or is there any other way to clarify the phase behavior of hybrid?
It is common for many polymers or biopolymers that XRD doesn't match with the DSC. DSC is not the absolute method.
There are always different morphologies in the crystalline segment of the polymeric blends or composites that physical and chemical properties are varied depending on their properties. For example, when using DSC to evaluate the crystallinity, the melting enthalpy is always considered while there are a bunch of small crystals that are not enthalpy driven. When using the XRD, XRD can entirely differentiate the crystalline and amorphous, thereby detecting tiny changes in the d-spacing or crystallinity. If I were you, I will go with XRD as it is more reliable tool in this case (does not necessarily mean data given by DSC are not reliable). To mitigate the DSC limitations, you can analyze samples using your second cycle which first cycle removes your thermal history because mostly crystals are formed in the more stable morphology in the second cycle, or you can induce the heat in the lower heating rate to melt the small crystals gently and bring them into the latent heat as you are seeking the transition phase. Note that these two methods won't surely overcome the situation but you can get some insights if the problems are attributed to the morphologies of crystals.
It is not that simple to interpret XRD data along with that of DSC or vice versa. The reason is that XRD was performed on a stable state whilst DSC on a changeable state of the sample understudy. Therefore, DSC can provide more useful information regarding mixtures of polymer and small molecule. From what you described, it is most likely that amorphous solid dispersion or solid solution of two components, i.e. small molecule and polymer, were sometimes made by heating/cooling in DSC in your case . But it is not pure regarding the physical forms of small molecule (can't specify with limited information provided).
You know DSC isn’t accurate method and you can see Tg and Tni in DSC; but in XRD you can see nematic or smectic phase, percentages of crystallinity and order parameter!
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