Combining nanocalorimetry and synchrotron nano-focus X-ray scattering to address fast structure formation processes
Extending conventional methods of physical-chemical characterization to smaller and smaller sample size is crucial for the application fields where the amount of available material is limited. For example, in pharmaceutics the cost of a new material synthesis can be prohibitive and, therefore, very small sample size is often all what is available to study the necessary structural and thermal characteristics. Handling very small sample sizes is also required in studies of thermal behavior of energetic materials that produce a deflagration reaction, with all the associated risks in terms of safety. However, to significantly shift the scale of the sample sizes downwards, it may be necessary to introduce completely new ideas into the instrumental design. For example, extending differential scanning calorimetry (DSC) for probing minuscule sample amounts has dramatically changed all the implementation of this conventional technique. The advent of nanocalorimetry has become possible because of the development of Si-based technologies allowing fabrication of novel MEMS-type sensors suitable for such sample sizes. One of the first commercially available MEMS sensors produced by the Xensor company (TCG 3880) reached a sensitivity of 1 nJ.K-1 and a time resolution of 5 ms . The approaches toward quantitative nanocalorimetric experiments were developed further by M. Merzlyakov  and C. Schick .
The active area of the nanocalorimetric sensor is fabricated on a thin free-standing membrane of silicon nitride, a material with a relatively low thermal conductivity. Heating of the nanocalorimetric sensor can be realized by applying voltage on specially designed heating resistances. The sensor temperature can be measured using an array of thermopiles assembled on the active area. Since the heating and cooling rates that can be employed are much higher than in the classical DSC, much smaller samples become amenable to study as compared to the classical DSC. Although the advent of nanocalorimetry has significantly broadened the field of applications of the technique, a combination of calorimetry with other methods of physical-chemical characterization remains very attractive: the calorimetric measurements alone cannot always reveal the mechanisms of complex processes occurring in the material.
In our previous works, we described our custom-built nanocalorimetric accessory allowing for a combination of nanocalorimetry with other characterization techniques such as optical microscopy in reflection and transmission [4,5]. One of main advantages of our instrument compared to the commercial nanocalorimeter (i.e., “Flash DSC 1” from Mettler-Toledo) is that its design is fully open for integrating it into other experimental platforms. This is critical if one plans to make this accessory compatible for example with micro- and nano-focus synchrotron X-ray diffraction in transmission. The first successful in-situ measurements using nanocalorimetry and nano- and micro-focus X-ray diffraction, which has been developed in the frame of our Mega-Grant project conducted at the Moscow State University, were performed on metal micro-particles  and nanostructured hybrid materials .
In the present contribution, we report on the first application of a combined nanocalorimetry / nanofocus X-ray scattering accessory for in-situ studies of the structure formation processes in semicrystalline polymers. We also present the first in-situ measurements using a combination of fast calorimetry and fast X-ray scattering realized with a brand new family of X-ray detectors. The X-ray acquisition rate record achieved for the moment for a typical polymer sample deposited on a nanocalorimetric sensor is as fast as 1 ms/ frame. This rate makes it possible for example to perform real-time fast heating/cooling experiments at rates up to 10,000 °C/s with simultaneous recording of the sample heat capacity. As will be shown in the presentation, such combination can be valuable in analyzing the details of the microstructural evolution during fast heating of the semicrystalline polymers. In particular, based on the results of combined nanocalorimetry / nanofocus X-ray scattering experiments, we were able to revisit the long-standing issue of the multiple melting behavior in semirigid-chain polymers . Moreover, the developed combination of nanocalorimetry / nanofocus X-ray scattering allows us to analyze such fundamental features of polymer crystallization as its departure from equilibrium, which has important implications for understanding of the polymer crystallization thermodynamics.
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- A.P. Melnikov, M. Rosenthal, A.I. Rodygin, D. Doblas, D.V. Anokhin, M. Burghammer, and D.A. Ivanov "Re-exploring the Double-Melting Behavior of Semirigid-Chain Polymers with an in-situ Combination of Synchrotron Nano-Focus X-ray Scattering and Nanocalorimetry", European Polymer Journal (published on the WEB). DOI: 10.1016/j.eurpolymj.2015.12.031