Molecular dynamics of the α-relaxation during crystallization of a low-molecular-weight compound: A real-time dielectric spectroscopy study

Abstract

Low-molecular-weight compounds often crystallizes to systems with 100% crystallinity. There are only a few examples where a small amorphous fraction, characterized by a glass transition, remains after long time crystallization from the melt. The crystallization of such a glass-forming low-molecular-weight compound was investigated in order to monitor the change of the molecular dynamics with increasing crystallinity by dielectric spectroscopy and differential scanning calorimetry (DSC). The measurement of the dielectric α-relaxation was performed in real time during isothermal crystallization above the glass transition. At high crystallinities (above 90%) a shift of the peak position and a broadening of the dielectric spectrum was observed. The calorimetric glass transition temperature shifts in the same region for about 15 K to higher temperatures. No direct information about the morphology of the samples is available at the moment but indirect measurements indicate a layerlike crystalline structure. Then the remaining amorphous fraction can be considered between the crystal layers and the observed changes in the relaxation behavior may be caused by spatial confinement in the order of nanometer.

Dielectric study of the molecular mobility and the isothermal crystallization kinetics of an amorphous pharmaceutical drug substance

“…For polymers such as poly(ethylene terephthalate)32,33 and poly(ether ether ketone)34 that contain rigid backbones, DDS spectra shift to a lower frequency (i.e., the mean relaxation time of the α a ‐relaxation mode increases) during the crystallization, indicating that molecular motions in amorphous phases are significantly restricted by the presence of crystallites. In contrast, for low molecular weight systems 35,36 or polymers with flexible chains, such as poly( L ‐lactic acid),37 the relaxation time of the α a ‐relaxation mode is unaffected by the degree of crystallinity until the crystallinity is high enough to constrain the mobility of amorphous phase. In the latter case, the confinement effect on the amorphous phase appears as the distances between crystallites become shorter than the size of cooperative rearranging regions (CCRs) specific of the amorphous phase at the considered crystallization temperature.…”

Evidence of pre-crystalline-order in super-cooled polymer melts revealed from simultaneous dielectric spectroscopy and SAXS

“…In contrary to most other dielectric experiments, which detect changes in morphology by following the so-called a-relaxation (dynamic glass transition) in time or temperature [30][31][32][33][36][37][38][39][40][41][42][43][44], we performed our experiments at high temperatures, where the dielectric signal in the frequency window of the measurement is not influenced by the a-relaxation or other local relaxations. Such experiments allow us to crystallize polymers at temperatures just below the melting point.…”

Real-Time Monitoring of Molecular Dynamics of Ethylene Glycol Dimethacrylate Glass Former

“…Although low molecular weight materials have different crystal morphology and crystal growth, under certain conditions, the evolving of a new relaxation process is also reported upon isothermal crystallization, e.g., sorbitol 19 and terephthalic acid dipropyl ester. 20 The present work envisages being a further contribution concerning crystallization behavior of low molecular weight glass formers. Moreover, the kinetics of isothermal crystallizations will be analyzed.…”