In-situ WAXS study of the crystal structure of R-BAPB oligoimide during heating

In the process of this work, an oligoimide FT-BAPB-R-BAPB-FT was synthesized, the particles of which consist of numerous lamellas having a semi-crystalline structure. On the DSC thermogram of oligoimide powder, two endo peaks were observed at 281.1°C and 306.4°C. A unique WAXS experiment was performed in-situ by heating the reactor oligoimid powder in a heat chamber. There was no significant difference in the X-ray profiles when the sample was heated. It should be noted that the transformation of the crystalline cell does not occur until the sample melts. It was revealed that the degree of crystallinity decreases during heating. We believe that the low-temperature endopeak is associated with the melting of small crystallites and amorphization of the sample. However, a significant increase in crystal size was found to be a result of recrystallization. Thus, the high-temperature endopic corresponds to the melting of larger crystallites.


Introduction
It is found that typical semirigid-chain polymers such as poly(ethylene terephthalate), PET, poly(butylene terephthalate), (PBT), poly(trimethylene terephthalate), PTT, and poly(ether ether ketone), PEEK, demonstrate a very complex thermal behavior [1][2][3]. To date, based on the data obtained by the differential scanning calorimetry (DSC) method [4,5], it is known that a multiple melting peak can be observed upon heating and cooling of thermoplastic polymers. It is assumed that such double melting peaks appear on the basis of two different crystal growth mechanisms. Namely, these mechanisms correspond to the growth and melting of spherulite and epitaxial crystals. To study the growth of crystallites in partially crystalline polymers, it is very important to investigate the process of crystallite melting, which can affect the final polymer crystallinity. Nowadays, it is clear that conventional thermal analysis methods, such as DSC, cannot provide the full understanding of the nature of different thermal transitions.
In the present work, double endothermic behavior, i.e. the appearance of double melting peaks and the corresponding morphology of crystallites of model FT-BAPB-R-BAPB-FT oligoimide were investigated using scanning electron microscopy (SEM), DSC and in-situ wide-angle X-ray scattering

SEM
The morphology of the FT-BAPB-R-BAPB-FT oligoimide reactor powder was studied using a SUPRA 55VP scanning electron microscope (Carl Zeiss, Germany). The samples were coated with a thin layer of Pt.

DSC
The thermophysical properties of the obtained FT-BAPB-R-BAPB-FT powders were investigated by differential scanning calorimetry using the DSC 204 F1 instrument (NETZSCH, Germany). The tests on the oligoimide powder were carried out in the temperature range from 30 to 380°C at a heating rate of 10°C/min in an inert atmosphere (argon). NETZSCH Proteus® software was used to determine the the melting temperature T m .

In-situ WAXS
The fine crystalline structure of the FT-BAPB-R-BAPB-FT oligoimide powder was studied by the WAXS using a Rigaku Ultima IV (CoK α ) diffractometer equipped with a thermal camera. X-ray profiles were recorded online in the range from 150 to 320°C in increments of 5°C.

Results and Discussion
An SEM study of FT-BAPB-R-BAPB-FT oligoimide powder particles revealed that these particles have lamellae morphology (Figure 2). It is assumed that these lamellas have a partially crystalline structure and are most likely formed during synthesis. To confirm this assumption, a thermogram of FT-BAPB-R-BAPB-FT oligoimide powder was recorded by DSC.  . Moreover, it is clearly seen that the areas under these two endopeaks are not the same. It can be concluded that oligoimide lamellas consist of crystallites that melt in a special way. Thus, it should be assumed that the initial reactor oligoimide powder has two phases that melt at different temperatures (this difference is more than 20°C). Based on this, the following two hypotheses can be proposed: 1) the lamellas of oligoimide FT-BAPB-R-BAPB-FT consist of two types of crystallites having different crystal cells; 2) or crystallites are identical in terms of crystallography, but their sizes are very different (according to [8]). Thus, a very interesting phenomenon was discovered in the test material. For a detailed study of the nature of the observed thermodynamic behavior, X-ray diffraction profiles were recorded online while heating the reactor powder. Figure 4 represents the results obtained in these in-situ WAXS experiments. No significant difference in the X-ray profiles is observed throughout the heating of the sample, except that the WAXS reflections become narrower and better resolved. It should be noted that until the oligoimide powder melts, no transformation of the crystalline cell occurs. Consequently, these results give us the opportunity to assert that our second hypothesis was true. The sizes of crystalline regions and the degree of crystallinity at each temperature were calculated (see Figure 5). It turned out that heating the sample leads to a double increase in crystallite sizes and a gradual decrease in the degree of crystallinity (i.e., amorphization) during heating. 5 similar to Oswald recrystallization, which has been well studied for low molecular weight solids. Recrystallized large crystallites exhibit a higher melting point, which is very logical.

Conclusions
Summarizing all the results obtained, one can draw the following conclusions: • FT-BAPB-R-BAPB-FT reactor powder particles are composed of numerous lamellas that have a semicrystalline structure; • When heating the studied oligoimide, two endopes were detected at 281.1°C and 306.4°C using the DSC method; • A unique WAXS experiment was conducted in-situ by heating the reactor powder in a heat chamber. It was found that when the sample was heated, there was no apparent difference in the WAXS profiles, i.e. to the end of the low-temperature endopeak. At the same time, WAXS peaks become narrower and better resolved. It should be noted that the transformation of the crystalline cell does not occur until the sample melts; • It was found that the degree of crystallinity gradually decreases until the sample is completely melted. Thus, one can conclude that the low-temperature endopeak is associated with the melting of small crystallites and amorphization of the sample. On the other hand, a smooth increase in crystal size was found to be a result of recrystallization process attributed to the high-temperature endopeak to the melting of larger crystallites.