Molecular mobility of amorphous N-acetyl-α-methylbenzylamine and Debye relaxation evidenced by dielectric relaxation spectroscopy and molecular dynamics simulations.

The present work focusses on the molecular mobility characterization of amorphous N-acetyl-α-methylbenzylamine (Nac-MBA) by Broadband Dielectric Relaxation Spectroscopy (DRS) coupled with Fast Scanning Calorimetry (FSC) and Molecular Dynamics (MD) simulations covering over 12 decades in the frequency range. This study reveals another example of a secondary amide that shows a very intense Debye-like contribution (almost 90% of the global dielectric intensity) in addition to the structural α-relaxation and secondary Johari-Goldstein ß-relaxation. The D- and α-relaxations are separated by about one decade (in frequency) and their relaxation times follow a near parallel temperature evolution (Vogel-Fulcher-Tammann-Hesse). The micro-structure of Nac-MBA has been investigated from MD simulations. It is shown that the intense Debye-like process emanates from the formation of linear intermolecular H-bonding aggregates (precursors of the crystalline structure) generating super-dipole moments.

Impact of chirality on the Glass Forming Ability and the crystallization from the amorphous state of 5-ethyl-5-methylhydantoin, a chiral poor glass former.

The investigation of the glassy state of 5-ethyl-5-methylhydantoin (i.e. 12H, a chiral Active Pharmaceutical Ingredient) was attempted by Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). This compound exhibits a high crystallization propensity for every enantiomeric composition. Nevertheless, glassy states of pure enantiomer or mixtures between enantiomers were successfully reached by FSC at cooling rates of: 1000 °C/s and 300 °C/s respectively, even though limitations on the sampling reproducibility were evidenced due to FSC sample size. The Glass Forming Ability (GFA) was proven to increase with the counter-enantiomer content. From the glassy state, pure enantiomer displayed a more pronounced crystallogenic character (with a crystallization occurring 36 °C below Tg during ageing) than that of the mixture between enantiomers. Ageing of amorphous 12H promotes a strong nucleation behavior in both samples but enantiopure 12H crystallizes upon ageing while scalemic 12H evolves towards the metastable equilibrium. Finally, potential new phase equilibria (previously not reported) in the enantiomeric phase diagram could have been highlighted by FSC by recrystallization from the amorphous state.

Glass transition of anhydrous starch by fast scanning calorimetry.

By means of fast scanning calorimetry, the glass transition of anhydrous amorphous starch has been measured. With a scanning rate of 2000Ks-1, thermal degradation of starch prior to the glass transition has been inhibited. To certify the glass transition measurement, structural relaxation of the glassy state has been investigated through physical aging as well as the concept of limiting fictive temperature. In both cases, characteristic enthalpy recovery peaks related to the structural relaxation of the glass have been observed. Thermal lag corrections based on the comparison of glass transition temperatures measured by means of differential and fast scanning calorimetry have been proposed. The complementary investigations give an anhydrous amorphous starch glass transition temperature of 312±7°C. This estimation correlates with previous extrapolation performed on hydrated starches.

Correlated and cooperative motions in segmental relaxation: Influence of constitutive unit weight and intermolecular interactions.

This work clarifies the notion of correlated and cooperative motions appearing during the α-relaxation process through the role of the molecular weight of the constitutive units and of the interchain dipolar interactions. By studying amorphous copolymers of poly(ethylene-co-vinyl acetate) with different vinyl acetate contents, we show that the correlated motions are not sensitive to the interchain dipolar interactions, in contrast to the cooperative motions, which increase with a strengthening of the intermolecular interactions for this sample family. Concerning the influence of the molecular weight m_{0}, the notion of "correlated motions" seems to be equivalent to the notion of "cooperative motions" only for low m_{0} systems.

Water barrier properties in biaxially drawn poly(lactic acid) films.

Crystallization is among the easiest ways to improve polymer barrier properties because of the tortuosity increase within the material and the strong coupling between amorphous and crystalline phases. In this work, poly(lactic acid) (PLA) films have undergone α' thermal crystallization or different drawing processes. Although no effect of α' thermal crystallization on water permeability is observed, the drawing processes lead to an enhancement of the PLA barrier properties. This work clearly shows that, in the case of PLA, the crystallinity degree is not the main parameter governing the barrier properties contrary to the crystalline and amorphous phase organizations which play a key role. X-ray analyses confirm that the macromolecular chain orientation in the amorphous phase is the main cause of the improvement of the drawn PLA water barrier property. This improvement is due to the orthotropic structure formation for sufficient draw ratios, particularly when using the Simultaneous Biaxial drawing mode. Moreover, independently of the draw conditions, the drawing process tends to reduce the plasticization coefficient. Consequently, the drawn material barrier properties are not much affected by the water passage.