Dielectric relaxation spectrometry

Various methods, including dilferential scanning calorimetry (DSC), dielectric relaxation spectrometry, and NMR, are known to be useful to determine molecular mobility of freeze-dried formulations [16,17]. The glass transition temperature (Tg) has been used as a measure of molecular mobility of lyophilized formulations, since it indicates the critical temperature of a-relaxation for amorphous polymer materials. Freeze-dried formulations containing polymer excipients can be considered to exhibit low molecular mobility without a-relaxation at temperatures below Tg. 

NMR is another useful means of measuring molecular mobility of freeze-dried formulations. The spin-lattice relaxation time in the laboratory frame (Ti) and the spin-spin relaxation time T2) of H, H, or have been used to represent the mobility of water and polymer molecules in freeze-dried cakes or aqueous polymer solutions [18-22]. In contract to DSC or dielectric relaxation spectrometry, NMR allows identification of the origin of molecular motion. Determining molecular mobility for each drug and excipient in a freeze-dried formulation is therefore possible when high-resolution solid-state NMR is used. 

Such PU molecular structure assumed, a priori, the possible manifestation of several dynamic modes within the glass transition, in particular, because of the different positions of segments within a PPG crosslink regarding rigid junctions. However, DMA and dielectric relaxation spectrometry (DRS) techniques exhibited only one broad relaxation region for PU glass transition, for instance, the asymmetric mechanical loss peak extending from —60°C to 50°C, with Tmax —30°C (Fig. 9). 

In Sections 1.7 and 4.8.3, we have studied the dielectric relaxation phenomena and dielectric spectroscopy, respectively. In dielectric spectrometry, the methodology allowed us to measure the capacity and, consequently, the real part of the complex dielectric constant. The imaginary part of the complex dielectric constant was calculated, in this case, with the help of the Kramers-Kronig... 

In addition to the bulk Tg, siower relaxation was assigned to polymer chains close to the polymer-filler interface, whose mobility was restricted by the physical interactions. The existence of an interfacial layer was proposed to explain the DSC results (showing a double step in heat capacity) and TSC/DDS measurements (distinguishing two well-defined dielectric relaxation processes). These results confirmed earlier studies by dynamic mechanical spectrometry, where a second tan 5 peak, observed at 50 to 100°C above the mechanical manifestation of Tg, was attributed to the glass transition of an interfacial polymer layer with restricted mobiUty [Tsagaropoulos and Eisenburg, 1995],... 

Samouillan et al. (2011) studied the dielectric properties of elastin at different degrees of hydration and specifically at the limit of freezable water apparition. The quantification of freezable water was performed by DSC. Two dielectric techniques were used to explore the dipolar relaxations of hydrated elastin dynamic dielectric spectroscopy (DDS), performed isothermally with the frequency varying from 10 to 3 x 10 Hz, and the TSDC technique, an isochronal spectrometry running at variable temperature, analogous to a low-frequency spectroscopy (10 to 10 Hz). A complex relaxation map was evidenced by the two techniques. Assignments for the different processes can be proposed by the combination of DDS and TSDC experiments and the determination of the activation parameters of the relaxation times. As already observed for globular proteins, the concept of solvent-slaved protein motions was checked for the fibrillar hydrated elastin (Samouillan et al. 2011). 

BordaUo HN, Zakharov BA, Boldyreva EV, Johnson MR, Koza MM, Seydel T, Fischer J (2012) Application of incoherent inelastic neutron scattering in pharmaceutical analysis relaxation dynamics in phenacetin. Mol Pharm 9 2434-2441 Bptker JP, Karmwar P, Strachan CJ, Cornett C, Tian F, Zujovic Z, Rantanen J, Rades T (2011) Assessment of crystalline disorder in cryo-milled samples of indomethacin using atomic pairwise distribution functions. Int J Pharm 417 112-119 Boutonnet-Fagegaltier N, Menegotto J, Lamure A, Duplaa H, Caron A, Lacabanne C, Bauer M (2002) Molecular mobihty study of amorphous and crystalline phases of a pharmaceutical product by thermally stimulated current spectrometry. J Pharm Sci 91 1548-1560 Bras AR, Noronha JP, Antunes AMM, Cardoso MM, Schdnhals A, Affouard Fdr, Dionfsio M, Correia NIT (2008) Molecular motions in amorphous ibuprofen as studied by broadband dielectric spectroscopy. J Phys Chem B 112 11087-11099... 

To determine the movements of the whole chain and those of subchains, various techniques are accessible and available to the experimenter, including dielectric spectroscopy and mechanical spectrometry. Dielectric techniques are suitable for the study of polymers in a wide range of frequencies (between 10 Hz and 10 ° Hz), while the mechanical dynamic characterization of polymers provides access to long relaxation times (>10 s) through creep and stress relaxation tests. 

---