1
$\begingroup$

I'm currently working through past exam papers in preparation for my upcoming Inorganic Chemistry exam and I got to this question. We really haven't looked at DSC graphs, so I'm not completely sure how to answer this. I've shown my attempt, but am looking for someone to confirm if I'm right, if not, where I went wrong.

Question: The DSC thermogram depicted below was run on a sample of a new compound suspected of possessing mesogenic properties. The thermogram was obtained at a scan rate of 10 °C min−1 and the solid line represents the heating scan, while the dashed line represents the cooling scan.

enter image description here

Interpret the DSC thermogram, naming and explaining the processes occurring at peaks (a) – (e), using the correct terminology. Does the DSC thermogram confirm that the new compound is a mesogen? If so, what can you say about the liquid crystal phase(s), and what name is given to this class of mesogen? What word describes this particular kind of thermal behaviour?

My answer: e = Crystallization, a = melting point (crystalline -> smectic), b = transition point (smectic -> nematic), d = (re)crystallization (nematic -> crystalline), c = melting point (crystalline -> nematic). Since a nematic phase is a subset of liquid crystal (synonym for mesogens), it confirms that the new compound is a mesogen. Molecules in the nematic phase are (somewhat) orientationally ordered (in contrast to the smectic phase which is (somewhat) orientationally and positionally ordered). I don't have an answer to "What word describes this particular kind of thermal behaviour?".

$\endgroup$
1
$\begingroup$

As example, Skoog / Leary's principles in instrumental analysis contained a section about DSC in the chapter of thermoanalysis. Your university library likely has some of them or other books about the topic, too.

Then, beside monitoring glas transitions, melting, decomposition (while heating) or solidification / crystallisation (while cooling) I suggest to attribute the other phase transitions with (in the best case: simultaneous) visual observation of the sample (heat stage microscopy, like here) at these critical temperatures. As these transitions take time, from a practical perspective, a heating rate of $\pu{10 K/min}$ is maybe rather quick.

After reading "liquid crystals" in the title, I thought a cholestic phase would be mentioned, too, but if your data support its absence, ok. On the other hand, one of PerkinElmer's brochures includes this representation

enter image description here

(source)

which might be useful to complement the structural description of your liquid crystals.

$\endgroup$
1
  • $\begingroup$ I appreciate your answer but I checked the textbook, specifically that chapter, and it essentially just describes how DSC works and wasn't able to help me answer the questions. It wasn't even able to guide me to the right answer. Thanks anyways. $\endgroup$ – LLCJ27 Jun 28 '17 at 4:54
0
$\begingroup$

I went to my professor to discuss the answer with him and this is the solution I obtained:

As the sample is heated from 0 °C nothing significant happens until around 90 °C when at peak (a) the melting point [crystal → liquid crystal (1)] occurs as signified by the large endotherm.

Heating continues until the tiny peak (b) indicates a transition point [liquid crystal (1) → liquid crystal (2).]

Further heating results in the medium-sized endotherm (c) at the clearing point [liquid crystal (2) → isotropic melt].

Upon cooling there is a super-cooling effect and an exotherm at (d) [isotropic melt → liquid crystal] and then the liquid crystal phase remains until the crystalline phase is formed again at (e) [liquid crystal → crystal].

The DSC confirms two mesogenic phases upon heating [between (a) and (b) and another between (b) and (c), and a single mesogenic phase on cooling [between (d) and (e)].

This is a thermotropic mesogen and as liquid crystal phases are observed on both heating and cooling it is enantiotropic.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.